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We report the discovery of a super-Earth orbiting the star GJ 536 based on the analysis of the radial-velocity time series from the HARPS and HARPS-N spectrographs. GJ 536 b is a planet with a minimum mass M sin $i$ of 5.36 +- 0.69 Me with an orbital period of 8.7076 +- 0.0025 days at a distance of 0.066610(13) AU, and an orbit that is consistent with circular. The host star is the moderately quiet M1 V star GJ 536, located at 10 pc from the Sun. We find the presence of a second signal at 43 days that we relate to stellar rotation after analysing the time series of Ca II H&K and H alpha spectroscopic indicators and photometric data from the ASAS archive. We find no evidence linking the short period signal to any activity proxy. We also tentatively derived a stellar magnetic cycle of less than 3 years.
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Astronomy &Astrophysics manuscript no. GJ536_AA_vArxiv c
ESO 2016
November 8, 2016
A super-Earth orbiting the nearby M-dwarf GJ 536
A. Suárez Mascareño1,2, J. I. González Hernández1,2, R. Rebolo1,2,3, N. Astudillo-Defru4, X. Bonfils5,6, F. Bouchy4,
X.Delfosse5,6, T. Forveille5,6, C. Lovis4, M. Mayor4, F. Murgas5,6, F. Pepe4, N. C. Santos7,8, S. Udry4, A. Wünsche7,8,
and S. Velasco1,2
1Instituto de Astrofísica de Canarias, E-38205 La Laguna, Tenerife, Spain
2Universidad de La Laguna, Dpto. Astrofísica, E-38206 La Laguna, Tenerife, Spain
3Consejo Superior de Investigaciones Científicas, Spain
4Observatoire Astronomique de l’Université de Genève, Versoix, Switzerland
5Univ. Grenoble Alpes, IPAG, Grenoble, France
6CNRS, IPAG, Grenoble, France
7Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto,CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
8Departamento de Física e Astronomia, Faculdade de Ciências,Universidade do Porto,Rua Campo Alegre, 4169-007 Porto, Portugal
Written July-2016
We report the discovery of a super-Earth orbiting the star GJ 536 based on the analysis of the radial-velocity time series from the
HARPS and HARPS-N spectrographs. GJ 536 b is a planet with a minimum mass M sin iof 5.36 ±0.69 Mwith an orbital period of
8.7076 ±0.0025 days at a distance of 0.066610(13) AU, and an orbit that is consistent with circular. The host star is the moderately
quiet M1 V star GJ 536, located at 10 pc from the Sun. We find the presence of a second signal at 43 days that we relate to stellar
rotation after analysing the time series of Ca II H&K and Hαspectroscopic indicators and photometric data from the ASAS archive.
We find no evidence linking the short period signal to any activity proxy. We also tentatively derived a stellar magnetic cycle of less
than 3 years.
Key words. Planetary Systems — Techniques: radial velocity — Stars: activity — Stars: chromospheres — Stars: rotation — Stars:
magnetic cycle — starspots — Stars: individual (GJ 536)
1. Introduction
Several surveys have attempted to take advantage of the low
masses of M-dwarfs – and therefore of the stronger radial-
velocity signals induced for the same planetary mass – and closer
habitable zones to detect rocky habitable planets (Bonfils et al.
2013; Howard et al. 2014; Irwin et al. 2015; Berta-Thompson
et al. 2015a). While surveying M-dwarfs has advantages, it also
has its own drawbacks. Stellar activity has been one of the main
diculties when trying to detect planets trough Doppler spec-
troscopy. Not only it introduces noise, but also coherent signals
that can mimic those of planetary origin (Queloz et al. 2001;
Bonfils et al. 2007; Robertson et al. 2014). M-dwarfs tend to
induce signals with amplitudes comparable to those of rocky
planets (Howard et al. 2014; Robertson et al. 2014). While these
kinds of stars allow for the detection of smaller planets, they also
demand a more detailed analysis of the radial-velocity signals
induced by activity. In addition this low mass stars oer valu-
able complementary information on the formation mechanisms
of planetary systems. For instance giants planets are known to be
rare around M dwarfs, while on the other hand super-Earths ap-
pear to be more frequent (Bonfils et al. 2013; Dressing & Char-
bonneau 2013; Dressing et al. 2015).
In spite of the numerous exoplanets detected by Kepler
(Howard et al. 2012) and by radial-velocity surveys (Howard
et al. 2009; Mayor et al. 2011) the number of known small
rocky planets is still comparably low. There are around 1500
confirmed exoplanets and more than 3000 Kepler candidates,
but only about a hundred of the confirmed planets have been re-
ported on M-dwarfs and only a fraction of them are rocky plan-
ets. The first discovery of a planet around an M-dwarf dates back
to 1998 (Delfosse et al. 1998; Marcy et al. 1998). Since then
several planetary systems have been reported containing Nep-
tune mass planets and super-Earths (Udry et al. 2007; Delfosse
et al. 2013; Howard et al. 2014; Astudillo-Defru et al. 2015) even
some Earth-mass planets (Mayor et al. 2009; Berta-Thompson
et al. 2015b; Wright et al. 2016; Aer et al. 2016). However the
frequency of very low-mass planets around M-dwarfs is not well
established yet. In particular, as noted by Bonfils et al. (2013),
the frequency of rocky planets at periods shorter than 10 days
is 0.36+0.24
0.10, being 0.41+0.54
0.13 for the habitable zone of the stars.
On the other hand Gaidos (2013) estimated that the frequency
of habitable rocky planets is 0.46+0.20
0.15 on a wider spectral sam-
ple of Kepler dwarfs and Kopparapu (2013) gave a frequency of
0.24 for habitable planets around M-dwarfs. The three mea-
surements are compatible, but uncertainties are still big making
it important to continue the search for planets around this type
of stars in order to refine the statistics.
We present the discovery of a super-Earth orbiting the nearby
star GJ 536, which is a high proper motion early M-dwarf at a
distance of 10 pc from the Sun (van Leeuwen 2007; Maldonado
et al. 2015). Because of its high proper motion and its closeness
this star shows a secular acceleration of 0.24 m s1yr1(Montet
Article number, page 1 of 14
arXiv:1611.02122v1 [astro-ph.EP] 7 Nov 2016
A&A proofs: manuscript no. GJ536_AA_vArxiv
Table 1: Stellar parameters of GJ 536
Parameter GJ 536 Ref.
RA (J2000) 14:01:03.19 1
DEC (J2000) -02:39:17.52 1
δRA(mas yr1) -823.47 1
δDEC (mas yr1) 598.19 1
Distance [pc] 10.03 1
mB11.177 2
mV9.707 2
mVASAS 9.708 0
Spectral Type M1 3
Te[K] 3685 ±68 3
[Fe/H] -0.08 ±0.09 3
M?[M] 0.52 ±0.05 3
R?[R] 0.50 ±0.05 3
log g(cgs) 4.75 ±0.04 3
log(L?/L) -1.377 3
HK) -5.12 ±0.05 0
Prot 45.39 ±1.33 0
vsin i(km s1)<1.20
Secular acc. (m s1yr1) 0.24 4
References: 0 - This work, 1 - van Leeuwen (2007), 2 -
Koen et al. (2010), 3 -Maldonado et al. (2015), 4 - Calculated
following Montet et al. (2014).
Estimated using the Radius estimated by Maldonado et al.
(2015) and our period determination.
et al. 2014). Table 1 shows the stellar parameters. Its moderately
low activity combined with its long rotation period, of more than
40 days (Suárez Mascareño et al. 2015), makes it a very interest-
ing candidate to search for rocky planets.
1.1. Spectroscopy
The star GJ 536 is part of the Bonfils et al. (2013) sample and
has been extensively monitored since mid-2004. We have used
146 HARPS spectra taken over 11.7 yr along with 12 HARPS-
N spectra taken during April and May 2016. HARPS (Mayor
et al. 2003) and HARPS-N (Cosentino et al. 2012) are two fibre-
fed high resolution echelle spectrographs installed at the 3.6 m
ESO telescope in La Silla Observatory (Chile) and at the Tele-
scopio Nazionale Galileo in the Roque de los Muchachos Obser-
vatory (Spain), respectively. Both instruments have a resolving
power greater than R115 000 over a spectral range from 380
to 690 nm and have been designed to attain very high long-
term radial-velocity accuracy. Both are contained in vacuum ves-
sels to avoid spectral drifts due to temperature and air pressure
variations, thus ensuring their stability. HARPS and HARPS-N
are equipped with their own pipeline providing extracted and
wavelength-calibrated spectra, as well as RV measurements and
other data products such as cross-correlation functions and their
bisector profiles.
Most of the observations were carried out using the Fabry
Perot (FP) as simultaneous calibration. The FP oers the pos-
sibility of monitoring the instrumental drift with a precision of
10 cms1without the risk of contamination of the stellar spectra
by the ThAr saturated lines (Wildi et al. 2010). While this is not
usually a problem in G and K stars, the small amount of light
collected in the blue part of the spectra of M-dwarfs might com-
promise the quality of the measurement of the Ca II H&K flux.
The FP allows a precision of 1 ms1in the determination of
the radial velocities of the spectra with highest signal to noise
while assuring the quality of the spectroscopic indicators even in
those spectra with low signal to noise. Measurements taken be-
fore the availability of the FP where taken without simultaneous
1.2. Photometry
We also use the photometric data on GJ 536 provided by the
All Sky Automated Survey (ASAS) public database. ASAS (Po-
jmanski 1997) is an all sky survey in the Vand Ibands run-
ning since 1998 at Las Campanas Observatory, Chile. Best pho-
tometric results are achieved for stars with V 8-14, but this
range can be extended implementing some quality control on
the data. ASAS has produced light-curves for around 107stars at
δ < 28. The ASAS catalogue supplies ready-to-use light-curves
with flags indicating the quality of the data. For this analysis we
relied only on good quality data (grade "A" and "B" in the inter-
nal flags). Even after this quality control, there are still some high
dispersion measurements which cannot be explained by a regular
stellar behaviour. We reject those measurements by de-trending
the series and eliminating points deviating more than three times
the standard deviation from the median seasonal value. We are
left with 359 photometric observations taken over 8.6 yr with a
typical uncertainty of 9.6 mmag per exposure.
2. Determination of Stellar Activity Indicators and
Radial Velocities
2.1. Activity Indicators
For the activity analysis we use the extracted order-by-order
wavelength-calibrated spectra produced by the HARPS and
HARPS-N pipelines. For a given star, the change in atmospheric
transparency from day to day causes variations in the flux dis-
tribution of the recorded spectra that are particularly relevant in
the blue where we intend to measure Ca II lines. In order to min-
imize the eects related to these atmospheric changes we create
a spectral template for each star by de-blazing and co-adding ev-
ery available spectrum and use the co-added spectrum to correct
the order-by-order fluxes of the individual ones. We also correct
each spectrum for the Earth’s barycentric radial velocity and the
radial velocity of the star using the measurements given by the
standard pipeline and re-binned the spectra into a wavelength-
constant step. Using this HARPS dataset, we expect to have high
quality spectroscopic indicators to monitor tiny stellar activity
variations with high accuracy.
SMW Index
We calculate the Mount Wilson Sindex and the log10(R0
HK ) by
using the original Noyes et al. (1984) procedure, following Lo-
vis et al. (2011) and Suárez Mascareño et al. (2015). We define
two triangular-shaped passbands with full width half maximum
(FWHM) of 1.09 Å centred at 3968.470 Å and 3933.664 Å for
the Ca II H&K line cores, and for the continuum we use two
20 Å wide bands centred at 3901.070 Å (V) and 4001.070 Å(R),
as shown in figure 1.
Then the S-index is defined as
Article number, page 2 of 14
A. Suárez Mascareño et al.: A super-Earth orbiting the nearby M-dwarf GJ 536
Fig. 1: Ca II H&K filter of the spectrum of the star GJ536 with
the same shape as the Mount Wilson Ca II H&K passband.
Fig. 2: Spectrum of the M-type star GJ536 showing the Hαfilter
passband and continuum bands.
+β, (1)
where ˜
NRand ˜
NVare the mean fluxes in each passband,
while αand βare calibration constants fixed as α=1.111 and
β=0.0153 . The S index serves as a measurement of the Ca
II H&K core flux normalized to the neighbour continuum. As
a normalized index to compare it to other stars we compute the
HK) following Suárez Mascareño et al. (2015).
We also use the Hαindex, with a simpler passband following
Gomes da Silva et al. (2011). It consists of a rectangular band-
pass with a width of 1.6 Å and centred at 6562.808 Å (core),
and two continuum bands of 10.75 Å and a 8.75 Å wide cen-
tred at 6550.87 Å (L) and 6580.31 Å (R), respectively, as seen in
Figure 2.
Thus, the Hαindex is defined as
HαIndex =Hαcore
Fig. 3: Cross correlation function for GJ 536. Upper panels show
the CCF with the Gaussian fit (left) and our Gaussian plus poly-
nomial fit (right). Lower panels show the residuals after the fit
for the Gaussian fit (left) and for our fit. Blue lines show the fit
(upper panels) and the zero line (lower panels).
2.2. Radial velocities
The radial-velocity measurements in the HARPS standard
pipeline is determined by a Gaussian fit of the cross correlation
function (CCF) of the spectrum with a binary stellar template
(Baranne et al. 1996; Pepe et al. 2000). In the case of M-dwarfs,
due to the huge number of line blends, the cross correlation func-
tion is not Gaussian resulting in a less precise gaussian fit which
might cause distortions in the radial-velocity measurements and
FWHM To deal with this issue we tried two dierent approaches.
The first one consisted in using a slightly more complex
model for the CCF fitting, a Gaussian function plus a second
order polynomial (Fig. 3) using only the central region of the
CCF function. We use a 15 Km s1window centred at the min-
imum of the CCF. This configuration provides the best stability
of the measurements. Along with the measurements of the radial
velocity we obtain the FWHM of the cross correlation function
which we also use to track variations in the activity level of the
star. A second approach to the problem was to recompute the ra-
dial velocities using a template matching algorithm with a high
signal to noise stellar spectral template (Astudillo-Defru et al.
2015). Every spectrum is corrected from both barycentric and
stellar radial velocity to align it to the frame of the solar system
barycenter. The radial velocities are computed by minimizing
the χ2of the residuals between the observed spectra and shifted
versions of the stellar template, with all the elements contami-
nated by telluric lines masked. All radial-velocity measurements
are corrected from the secular acceleration of the star.
For the bisector span measurement we rely on the pipeline
results, as it does not depend on the fit but on the CCF itself.
The bisector has been since more than 10 years ago a standard
activity diagnostic tool for solar type stars. Unfortunately its be-
haviour in slow rotating stars is not as informative as it is for
fast rotators (Saar & Donahue 1997; Bonfils et al. 2007). We re-
port the measurements of the bisector span (BIS) for each radial-
velocity measurement, but we do not find any meaningful infor-
mation in its analysis.
Article number, page 3 of 14
A&A proofs: manuscript no. GJ536_AA_vArxiv
2.3. Quality Control of the Data
As the sampling rate of our data is not well suited for modelling
fast events, such as flares, and their eect in the radial velocity is
not well understood, we identify and reject points likely aected
by flares by searching for an abnormal behaviour of the activ-
ity indicators (Reiners 2009). The process rejected 6 spectra that
correspond to flare events of the star with obvious activity en-
hancement and line distortion. That leaves us with 140 HARPS
spectroscopic observations taken over 10.7 years, with most of
the measurements taking place after 2013, with a typical expo-
sure of 900 s and an average signal to noise ratio of 56 at 5500 Å.
We do not apply the quality control procedure to the HARPS-N
data as the number of spectra is not big enough.
3. Stellar Activity Analysis
In order to properly understand the behaviour of the star, our
first step is to analyse the dierent modulations present in the
photometric and spectroscopic time-series.
We search for periodic variability compatible with both stel-
lar rotation and long-term magnetic cycles. We compute the
power spectrum using a Generalised Lomb Scargle Periodogram
(Zechmeister & Kürster 2009) and if there is any significant pe-
riodicity we fit the detected period using sinusoidal model, or a
double harmonic sinusoidal model to account for the asymmetry
of some signals (Berdyugina & Järvinen 2005), with the MPFIT
routine (Markwardt 2009).
The significance of the periodogram peak is evaluated using
both the Cumming (2004) modification of the Horne & Baliu-
nas (1986) formula to obtain the spectral density thresholds for
a desired false alarm probability (FAP) levels and bootstrap ran-
domization (Endl et al. 2001) of the data.
Figure 4 shows the time series for the photometry (top panel)
and the three activity proxies (bottom panels) used for this analy-
sis. The periodograms of both the photometric and FWHM time
series show significant signals at 40 days, compatible with the
typical rotation periods of low activity M1 stars (Suárez Mas-
careño et al. 2016; Newton et al. 2016). On the other hand the
periodograms of the SMW and Hαindexes show long term and
short term significant signals. The short period signal is again at
40 days while the long term signal is close to 1000 days.
3.1. Long Term Magnetic Cycle
Analysing the SMW and Hαindexes time series we find the pres-
ence of a long term magnetic cycle of 3 years. Figure 5 shows
the periodograms of the time series of both indexes. We see a
well defined peak in the SMW index periodogram at 806 d and
several peaks going from 600 d to 1100 d in the Hαindex peri-
odogram implying the shape of the cycle is still not well defined
within our observations. Table 2 shows the periods of the best
fits for both time series using least squares minimization with
the period corresponding to the highest peak of the periodogram
as initial guess. Figure 6 shows the phase folded curves using
these periods. The two estimates dier significantly. This might
be because of a sub-optimal sampling to detect signals of long
periods. The detected periodicities might not be the true period-
icities, but apparent periodicities close to the real one, caused by
the sampling. This also makes us think that the uncertainties in
the cycle length are underestimated. The length of the signal is
shorter than the typical magnetic cycles measured in solar type
stars, but is within the range of known magnetic cycles in M-type
Fig. 4: Time series of the mV(upper panel) , SMW index (upper-
mid panel), Hαindex (lower-mid panel) and FWHM (lower
panel) time series. Grey dots show HARPS-S data, black aster-
isks show HARPS-N data.
Fig. 5: Periodograms of the mV(upper panel) , SMW index
(upper-mid panel), Hαindex (lower-mid panel) and FWHM
(lower panel) time series. Horizontal lines show the dierent lev-
els of false alarm probability. Red dotted line for the 10% of false
alarm probability, greed dashed line for the 1% and blue thick
line for the 0.1%. Several peaks arise with significances better
than the 0.1%.
stars (Suárez Mascareño et al. 2016). Both in the SMW and Hαin-
dexes it seems that the cycle shape shows a quick rise followed
by a slow decline, as it is the case in the Sun and many other main
sequence stars (Waldmeier 1961; Baliunas et al. 1995). Unfortu-
nately this cycle is not well covered in phase yet, making it di-
cult to properly characterise it. More observations are needed in
order to better constrain its period.
3.2. Rotation
The other activity signal expected in our data is the rotational
modulation of the star.It shows up at 43 d with a false alarm
probability smaller or close to the 1% in the four time series
(Fig. 5) that grow in significance after removing the long term
Article number, page 4 of 14
A. Suárez Mascareño et al.: A super-Earth orbiting the nearby M-dwarf GJ 536
Fig. 6: Phase folded fit for the isolated long period activity signal using double-harmonic sine curves. Left panel shows the SMW
index data using the 824 d signal while right panel shows the Hαsignal using the 1075 d signal. Grey dots are the raw measurements
after subtracting the mean value. Red dots are the same points binned in phase with a bin size of 0.1.
In the photometric lightcurve we measure a modulation of
43.33 ±0.06 d with an amplitude of 5.21 ±0.68 mmag. For the
SMW index we find a signal 43.84 ±0.01 d with an amplitude of
0.0628 ±0.0010 when doing a simultaneous fit with the 824
days signal from Table 6. In the case of the Hαindex we find
a signal 42.58 ±0.08 d with an amplitude of 0.0042 ±0.0010,
also when doing a simultaneous fit with the 1075 days signal.
The time series of the FWHM show a linear increase with time
of 2 ms1yr1, which might be related to a slow focus drift of
HARPS. After subtracting the linear trend we find again a peri-
odicity of 44.47 ±0.03 d period with an amplitude of 4.56 ±0.31
ms1. Figure 7 shows the phase folded fits of all the signals. The
SMW index and FWHM signals seem to be in phase, while the
photometric signal is shifted a quarter of phase. The uncertainty
in the Hαlong term fit makes it dicult to give it a unique phase
to the rotation signal. Table 2 shows the parameters for the four
Our measurement of 45.39 d strengthens the previous esti-
mation of Suárez Mascareño et al. (2015). Having such a clear
detection of the rotational modulation in that many indicators
over so many years supports the idea that activity regions in at
least some M-type stars are stable over long time spans (Robert-
son et al. 2015).
4. Radial-velocity Analysis
Our 152 radial-velocity measurements have a median error of
1.33 ms1which includes both photon noise, calibration and
telescope related errors. We measure a total systematic radial
velocity of -25.622 Kms1with a dispersion of 3.28 ms1. Fig-
ure 8 shows the measured radial velocities. An F-test (Zechmeis-
ter et al. 2009) returns a negligible probability that the internal
errors explain the measured dispersion, smaller than the 0.1%.
To search for periodic radial-velocity signals in our time-
series we follow a similar procedure as the one explained in
section 3.1. We search for periodic signals using a Generalised
Lomb Scargle Periodogram and if there is any significant period-
icity we fit the detected signal using the RVLIN package (Wright
& Howard 2012). We sequentially find the dominant component
Table 2: Magnetic cycle and rotation periodicities
Series Period (d) Amplitude FAP (%)
SMW Cyc 824.9 ±1.7 0.0684 ±0.0011 <0.1
HαCyc 1075.8 ±36.1 0.0046 ±0.0011 <0.1
mVRot 43.33 ±0.06 5.21 ±0.68 mmag <1
SMW Rot 43.84 ±0.01 0.0628 ±0.0010 <0.1
HαRot 42.58 ±0.08 0.0042 ±0.0010 <0.1
FWHMRot 44.47 ±0.03 4.56 ±0.31 ms1<1
<Rot. >43.87 ±0.80
The mean value is the weighted mean of all the individual mea-
surements. The error of the mean is the standard deviation of
the individual measurements divided by the square root of the
number of measurements.
Fig. 8: Radial-velocity time series. Grey dots show HARPS-S
data, black asterisks show HARPS-N data.
Article number, page 5 of 14
A&A proofs: manuscript no. GJ536_AA_vArxiv
Fig. 7: Phase folded curve using the rotational modulation for the ASAS light curve (upper left), SMW index (upper right), Hαindex
using a double-harmonic sine curve (lower left) and FWHM (lower right). Grey dots are the raw measurements after subtracting the
mean value. Red dots are the same points binned in phase with a bin size of 0.1. The error bar of a given bin is estimated using the
weighted standard deviation of binned measurements divided by the square root of the number of measurements included in this
bin. This estimation of the bin error bars assumes white noise, which is justified by the binning in phase, which regroups points that
are uncorrelated in time.
in the time series and remove them, until there are no more sig-
nificant signals.
Following this procedure we identify one signal with a false
alarm probability much better than the 0.1%, both using the
bootstrap and the Cumming (2004) estimates, corresponding to
a period of 8.7 d with a semi-amplitude of 2.47 ms1consistent
with circular (Fig. 9 shows the periodogram). Removing this sig-
nal leaves a 43.9 d signal with a semi amplitude of 2.86 ms1and
an eccentricity of 0.57, with a false alarm probability better than
the 0.1%. No more significant signals are found after removing
this two (Fig. 9). Fig. 10 shows the phase folded fits of both the
8.7 d and the 43.9 d signals.
We tested the available dataset for the three ways of calcu-
lating the radial velocity, obtaining virtually the same results in
every case. Results are shown for the Gaussian +polynomial fit
of the cross correlation function.
4.1. Origin of the periodic radial-velocity signals
Stellar activity can induce radial-velocity signals similar to those
of Keplerian origin. The inhomogeneities in the surface of the
star cause radial-velocity shifts due to the distortion of the spec-
tral line shapes which can, in some cases, create a radial-velocity
signal with a periodicity close to the stellar rotation and its first
For this star we have a rotation period of 45.39 ±1.33 d,
and two radial-velocity signals of 8.7 d and 43.9 d. The second
signal matches almost perfectly the rotation period of the star.
On the other hand we do not see in the time series of activity
indicators any signal close to the 8.7 d. This is the first evidence
of the stellar origin of the 43.9 d signal, and the planetary origin
of the 8.7 d signal.
As a second test we measured the Spearman correlation co-
ecient between the SMW , the Hαindex, the FWHM and the
radial velocities. We find a significant correlation between all
the indexes and the raw radial velocity, which almost disappears
Article number, page 6 of 14
A. Suárez Mascareño et al.: A super-Earth orbiting the nearby M-dwarf GJ 536
Fig. 9: Periodograms of the radial velocity. Upper panel shows
the raw periodogram, middle panel the periodogram of the resid-
uals after subtracting the 8.7 d signal and the lower panel the
periodogram of the residuals after subtracting the 43 d signal
present in the middle one. Red regions show the periods of the
measured rotation and magnetic cycle. Red dotted line for the
10% of false alarm probability, greed dashed line for the 1% and
blue thick line for the 0.1%.
when we isolate the 8.7 d signal, and get slightly increased when
isolating the 43.9 d signal (see Table. 3). This constitutes a sec-
ond evidence of the stellar origin of the 43.9 d signal, and of the
planetary origin of the 8.7 d one. Following this idea we sub-
tract the linear correlation between the radial velocity and every
of the three activity diagnostic indexes. When doing this we see
that the strength of the 8.7 d signal remains constant, or even
gets increased, while the significance of the 43.9 d gets reduced
in all cases (see Fig. 11), even getting buried in the noise after
correcting for the correlation with the Hαindex.
Keplerian signals are deterministic and consistent in time.
When measuring one signal, it is expected to find the signifi-
cance of the detection increasing steadily with the number of
observations, as well as the measured period being stable over
time. However, in the case of activity related signal this is not
necessarily the case. As the stellar surface is not static, and the
configuration of active regions may change in time, changes in
the phase of the modulation and in the detected period are ex-
pected. Even the disappearance of the signal at certain seasons is
possible. Fig. 12 shows the evolution of the false alarm probabil-
ity of the detection of both isolated signals, as well as the mea-
surement of the most prominent period when isolating them. The
8.7 d signal increases steadily with time, and once it becomes the
most significant signal it never moves again. On the other hand
the behaviour of the 43.9 d is more erratic, loosing significance
during the last observations.
Of the two significant radial-velocity signals detected in our
data it seems clear that the one at 8.7 d has a planetary origin,
while the one at 43.9 has stellar activity origin.
The shape of the activity induced radial-velocity signal
present in our data is evidently not sinusoidal. A double har-
monic sinusoidal, as in the case of the activity signals, is the best
fit model and the only one that does not create ghost signals after
subtracting it. The rotation induced signal is not in phase with the
rotation signals in the activity indicators. It appears to be 45
shifted from the signal in the SMW index and FWHM time series
Fig. 10: Top panel: Phase folded curve of the radial velocity us-
ing the 8.7 d period. Grey dots are the raw radial-velocity mea-
surements after subtracting the mean value and the 43.9 d signal.
Bottom panel: Phase folded curve of the radial velocity using
the 43.9 d period using a double-harmonic sine curve. Grey dots
and black asterisks are the raw radial-velocity measurements af-
ter subtracting the mean value and the 8.7 d signal. Red dots are
the same points binned in phase with a bin size of 0.1. The error
bar of a given bin is estimated using the weighted standard devi-
ation of binned measurements divided by the square root of the
number of measurements included in this bin. This estimation of
the bin error bars assumes white noise, which is justified by the
binning in phase, which regroups points that are uncorrelated in
as seen in Bonfils et al. (2007) and Santos et al. (2014). The un-
certainty in the phase Hαtime series makes it dicult to measure
a reliable phase dierence.
Finally an analysis of the spectral window ruled out that
the peaks in the periodogram are artefacts of the time sampling
alone. No features appear at 8.7 or 43.9 days even after mask-
ing the oversaturated regions of the power spectrum. Following
Rajpaul et al. (2016) we tried to re-create the 8.7 days signal
by injecting the PRot signal along with a second signal at PRot/2
at 1000 randomized phase shifts with a white noise model. We
were never able to generate a signal at 8.7 days, or any signif-
Article number, page 7 of 14
A&A proofs: manuscript no. GJ536_AA_vArxiv
Table 3: Activity - Radial-velocity correlations
Parameter Raw data 8.7 d signal 43.9 d signal
SMW vs VR0.292 (>3σ) 0.069 (<1σ) 0.345 (>3σ)
Hαvs VR0.338 (>3σ) 0.113 (1σ) 0.321 (>3σ)
FWHM vs VR0.356 (>3σ) 0.164 (1σ) 0.340 (>3σ)
Long term variations of activity indicators have been subtracted.
The parenthesis value indicates the significance of the correla-
tion given by the bootstrapping process.
Fig. 11: Periodograms for the radial velocity after removing the
correlation with the dierent activity diagnostic tools. From left
to right there is the periodogram for the original data, the peri-
odogram after detrending against the SMW index, against the Hα
index, and against the FWHM.
Fig. 12: Evolution of the false alarm probability of the detections
(upper panel) for the isolated signals, and stability of the detec-
tions (lower panel). Blue thick line shows the behaviour for the
8.7 d signals and red dashed line for the 43.9 d signal.
icant signal at periods close to 8.7 days. It seems very unlikely
that any of the signals are artefacts of the sampling.
4.2. GJ 536 b
The analysis of the radial-velocity time series and of the activity
indicators leads us to conclude that the best explanation of the
observed data is the existence of a planet orbiting the star GJ 536
at the period of 8.7 d, with a semi-amplitude of 2.5 ms1. The
best solution comes from a super-Earth with a minimum mass of
5.3 Morbiting at 0.067 AU of its star.
MCMC analysis of the radial-velocity time series
In order to quantify the uncertainties of the orbital parameters
of the planet, we perform a bayesian analysis using the code
ExoFit (Balan & Lahav 2009). This code follows the Bayesian
method described in Gregory (2005); Ford (2005); Ford & Gre-
gory (2007). A single planet can be modelled using the following
vi=γK[sin(θ(ti+χP)+ω)+esin ω] (3)
where γis system radial velocity; Kis the velocity semi-
amplitude equal to 2πP1(1 e2)1/2asin i;Pis the orbital pe-
riod; ais the semimajor axis of the orbit; eis the orbital ec-
centricity; iis the inclination of the orbit; ωis the longitude of
periastron; χis the fraction of an orbit, prior to the start of data
taking, at which periastron occurs (thus, χPequals the number
of days prior to ti=0 that the star was at periastron, for an or-
bital period of Pdays); and θ(ti+χP) is the angle of the star in its
orbit relative to periastron at time ti, also called the true anomaly.
To fit the previous equation to the data we need to specify
the six model parameters, P,K,γ,e,ωand χ. Observed radial-
velocity data, di, can be modelled by the equation: di=vi+i+δ
(Gregory 2005), where viis the modeled radial velocity of the
star and iis the uncertainty component arising from account-
able but unequal measurement error which are assumed to be
normally distributed. The term δexplains any unknown measure-
ment error. Any noise component that cannot be modelled is de-
scribed by the term δ. The probability distribution of δis chosen
to be a Gaussian distribution with finite variance s2. Therefore,
the combination of uncertainties i+δhas a Gaussian distribu-
tion with a variance equal to σ2
i+s2(see Balan & Lahav 2009,
for more details).
The parameter estimation in the Bayesian analysis needs a
choice of priors. We choose the priors following the studies by
Ford & Gregory (2007); Balan & Lahav (2009).The mathemati-
cal form of the prior is given in Table 1 and/or 4 of Balan & La-
hav (2009). In Table 4, we provide the parameter boundaries ex-
plored in the MCMC Bayesian analysis. ExoFit performes 100
chains of 10000 iterations each resulting in a final chain of 19600
sets of global-fit parameters.
We want to simultaneously model the stellar rotation and
planetary signals. For that we use the ExoFit to model two RV
signals and for the rotation signal we also leave the eccentric-
ity as a free parameter. The posterior distribution of the eccen-
tricity parameter for the rotation signal (not shown in Fig. 13)
gives a value of 0.47 ±0.26. In Fig. 13 we depict the posterior
distribution of model parameters, the six fitted parameter, the
semi-amplitude velocity, Krot and the period, Prot , of the rotation
signal, the derived mass of the planet, mpsin i, and the RV noise
given by the sparameter. Most of the parameters show symmet-
ric density profiles except for the eccentricity, e, the longitude
Article number, page 8 of 14
A. Suárez Mascareño et al.: A super-Earth orbiting the nearby M-dwarf GJ 536
of periastron, ω, the fraction χof the orbit at which the perias-
tron occurs. We note that the density profile of the rotation period
displays a tail towards slightly lower values although the rotation
period is well defined.
In Table 4 we show the final parameters and uncertainties ob-
tained with the MCMC bayesian analysis with the code ExoFit.
5. Discussion
We detect the presence of a planet with a semi-amplitude of 2.60
m s1that, given the stellar mass of 0.52 M, converts to m sin
iof 5.36 M, orbiting with a period of 8.7 d around GJ 536,
an M-type star of 0.52 Mwith a rotation period of 43.9 d that
shows an additional activity signal compatible with an activity
cycle shorter than 3 yr.
The planet is a small super-Earth with an equilibrium tem-
perature 344 K for a Bond albedo A =0.75 and 487 K for A=0.
Following Kasting et al. (1993) and Selsis et al. (2007) we per-
form a simple estimation of the habitable zone (HZ) of this star.
The HZ would go from 0.2048 to 0.3975 AU in the narrowest
case (cloud free model) and 0.1044 to 0.5470 AU in the broader
one (fully clouded model). This corresponds to orbital periods
from 46 to 126 days in the narrower case, and 17 to 204 days in
the broader one.
GJ 536 b is in the lower part of the Mass vs Period diagram
of known planets around M-dwarf stars (Fig. 14). The planet is
too close to the star to be considered habitable. For this star the
habitable zone would be from 20 days to 40 days.
GJ 536 is a quiet early M-dwarf, with a rotation period on
the upper end of the stars of its kind (Newton et al. 2016; Suárez
Mascareño et al. 2016). Its rotation induced radial-velocity sig-
nal has a semi-amplitude of 2.26 ms1and seems to be stable
enough to allow for a clean enough periodogram and to be cor-
rectly characterized. The phase of the rotation induced signal
seems to be advanced 45with respect to the signals in SMW
index and FWHM time series. There is a hint for a short ac-
tivity cycle shorter than 3 yr, which would put it in the lower
end of the stars of its kind (Suárez Mascareño et al. 2016), and
whose amplitude is so small that would need further follow-up
to be properly characterized. The radial-velocity signal induced
by this cycle is at this point beyond our detection capabilities.
Given the rms of the residuals there is still room for the de-
tection of more planets in this system, especially at orbital pe-
riods longer than the rotation period. Fig. 14 shows the upper
limits to the mass of those hypothetical companions. The stabil-
ity of its rotation signals and the low amplitude of the radial-
velocity signals with a magnetic origin makes this star a good
candidate to search for longer period planets of moderate mass.
A rough estimate of the detection limits tells us there is still room
for Earth-like planets (1 M) at orbits smaller than 10, super-
Earths (<10 M) at orbits going from 10 to 400 days, and even
for a Neptune mass planet (<20 M) at periods longer than 3
yr. Giant planets on the other hand are discarded except for ex-
tremely long orbital periods. The time-span of the observations
and the RMS of the residuals completely discards the presence
of any planet bigger than twice the mass of Neptune with an or-
bital period shorter than 20 years.
6. Conclusions
We have analysed 152 high resolution spectra and 359 photomet-
ric observations to study the presence of planetary companions
around the M-dwarf star GJ 536 and its stellar activity. We de-
tected two significant radial-velocity signals, at periods of 8.7
and 43.8 days, respectively.
From the available photometric and spectroscopic informa-
tion we conclude that the 8.7 d signal is caused by a 5.3 M
planet with semi major axis of 0.067 AU and equilibrium tem-
perature lower than 500K. The short period of the planet makes
it a potential transiting candidate. Detecting the transits would
give a new constraining point to the mass-radius diagram.
The second radial-velocity signal of period 43.8 d and semi
amplitude of 1.6 ms1is a magnetic activity induced signal re-
lated to the rotation of the star. We also found a magnetic cycle
shorter than 3 yr which would place this star among those with
the shortest reported magnetic cycles.
We have studied and set limits to the presence of other plan-
etary companions taking into account the rms of the residuals
after fitting both the planet and the rotation induced signal. The
system still has room for other low mass companions, but plan-
ets more massive than Neptune are discarded except at extremely
long orbital periods, beyond the habitable zone of the star.
This work has been financed by the Spanish Ministry project
MINECO AYA2014-56359-P . J.I.G.H. acknowledges financial
support from the Spanish MINECO under the 2013 Ramón y
Cajal program MINECO RYC-2013-14875. X.B., X.D., T.F.
and F.M. acknowledge the support of the French Agence Na-
tionale de la Recherche (ANR), under the program ANR-12-
BS05- 0012 Exo-atmos. X.B. and A.W. acknowledge fund-
ing from the European Research Council under the ERC
Grant Agreement No. 337591-ExTrA. This work was sup-
ported by Fundação para a Ciência e a Tecnologia (FCT)
within projects reference PTDC/FIS-AST/1526/2014 (POCI-
01-0145-FEDER-016886) and UID/FIS/04434/2013 (POCI-01-
0145-FEDER-007672). NCS acknowledge support by through
Investigador FCT contract of reference IF/00169/2012, and
POPH/FSE (EC) by FEDER funding through the program “Pro-
grama Operacional de Factores de Competitividade - COM-
PETE”. This work is based on data obtained HARPS public
database at the European Southern Observatory (ESO). This re-
search has made extensive use of the SIMBAD database, op-
erated at CDS, Strasbourg, France and NASA’s Astrophysics
Data System. We are grateful to all the observers of the follow-
ing ESO projects, whose data we are using: 072.C-0488, 085.C-
0019, 183.C-0972 and 191.C-087.
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Fig. 13: Posterior distribution of model parameters including the activity signal associated with the rotation period and the orbital
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Parameter Value Upper error Lower error Prior
Pplanet [d] 8.7076 +0.0022 0.0025 8.3 - 9.0
γ[ms125625] 1.17 +0.20 0.20 5.0 - +5.0
e0.08 +0.09 0.06 0.0 - 0.99
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χ0.88 +0.08 0.12 0.0 - 0.99
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Table 5: Full available dataset. Radial velocities are given in the Barycentric Reference Frame after subtracting the secular acceler-
ation. Radial-velocity uncertainties include photon noise, calibration and telescope related uncertainties.
BJD - 2450000 VrσVrFWHM BIS Span SMW index σSMW Hαindex σHαindex Flag
(d) (ms1) (ms1) (Kms1) (ms1)
3202.5590 -25616.5610 1.3488 4444.8091 -5.6623 1.0772 0.0077 0.4960 0.0005
3579.4972 -25622.4158 1.3573 4448.4550 -5.0540 1.2490 0.0073 0.5058 0.0005
3811.8370 -25620.2605 1.4564 4438.6871 -9.9863 1.1320 0.0078 0.5043 0.0006
3813.8047 -25621.7093 1.2228 4433.0342 -8.1469 1.1526 0.0053 0.5047 0.0004
4196.7394 -25621.5343 1.4416 4443.0970 -6.2410 1.0422 0.0074 0.5001 0.0006
4202.7156 -25621.6073 1.2951 4425.1466 -6.9079 1.0538 0.0059 0.5039 0.0005
4340.4836 -25623.6979 1.3024 4429.4257 -7.9120 1.1163 0.0070 0.5076 0.0005
4525.8756 -25618.1517 1.2817 4433.4956 -11.6982 1.1585 0.0063 0.5069 0.0005
4528.8393 -25622.4852 1.3126 4433.8369 -5.2346 1.1304 0.0064 0.5044 0.0005
4591.7914 -25625.2134 1.5565 4442.0880 -7.2876 0.9757 0.0090 0.4940 0.0007
4703.4993 -25621.8483 1.6802 4433.6780 -10.1303 1.1269 0.0107 0.5032 0.0008
5226.8854 -25621.1796 1.4632 4446.4284 -10.0405 1.2068 0.0081 0.4996 0.0006
5281.7491 -25623.1861 1.3854 4428.1838 -8.6441 1.0451 0.0068 0.4889 0.0006
5305.7265 -25617.9212 1.3201 4427.4995 -9.9302 1.0898 0.0084 0.5030 0.0005
5306.7140 -25617.4308 1.3665 4430.6156 -8.9673 1.1605 0.0092 0.5063 0.0006
5307.7196 -25616.7602 1.2588 4436.3939 -10.4465 1.2272 0.0087 0.5109 0.0004
5308.7013 -25614.7745 1.3191 4446.0321 -8.0523 1.1629 0.0088 0.5039 0.0005
5309.6925 -25615.3506 1.3031 4442.9390 -8.8002 1.2196 0.0092 0.5080 0.0005
6385.6469 -25626.3858 1.3821 4428.1616 -3.0622 0.8991 0.0101 0.4978 0.0007
6386.7448 -25624.9118 1.2594 4425.8662 -9.9115 0.8084 0.0078 0.4935 0.0007
6387.7815 -25624.7334 1.1527 4424.2671 -8.2783 0.8489 0.0073 0.4952 0.0006
6388.7254 -25623.3896 1.3647 4416.3803 -7.3567 0.8284 0.0087 0.4964 0.0007
6389.7264 -25621.3622 1.3006 4421.5546 -11.3118 0.8655 0.0085 0.5048 0.0007
6390.7371 -25620.4734 1.6898 4419.3411 -9.8647 0.8066 0.0105 0.4943 0.0008
6391.7497 -25620.8694 1.3286 4424.2490 -11.7541 0.9001 0.0085 0.5003 0.0007
6393.7913 -25623.1815 1.2910 4415.1907 -7.5922 0.8531 0.0089 0.4978 0.0007
6394.7750 -25624.4828 1.2851 4429.5659 -9.1806 0.8915 0.0088 0.4949 0.0007
6395.7000 -25626.0196 1.2787 4417.1293 -9.7255 0.9155 0.0089 0.4957 0.0007
6396.7103 -25623.1147 1.4092 4424.7395 -12.7127 0.9218 0.0094 0.4968 0.0008
6397.6863 -25621.0933 1.3000 4423.8606 -9.5686 0.9836 0.0095 0.5064 0.0007
6398.6799 -25618.8329 1.4313 4426.1457 -7.6371 0.9339 0.0099 0.4992 0.0008
6399.6958 -25619.5072 1.3309 4440.7943 -9.3043 1.0242 0.0097 0.5024 0.0008
6400.6899 -25617.6148 1.3276 4426.9125 -10.7879 0.9848 0.0094 0.5022 0.0008
6401.6532 -25624.7094 1.2229 4430.4209 -8.8857 1.0296 0.0087 0.5028 0.0007
6402.6436 -25623.9620 1.5248 4430.2528 -9.1200 1.0116 0.0108 0.5027 0.0008
6403.6245 -25622.1027 1.3310 4443.0225 -9.7811 1.0437 0.0098 0.5032 0.0007
6404.6425 -25623.0456 1.4532 4434.0050 -10.1067 1.0945 0.0109 0.5106 0.0008
6410.6262 -25626.0065 1.5706 4448.4592 -8.4057 0.9966 0.0115 0.5039 0.0008
6414.6393 -25623.0607 1.4853 4439.0697 -6.4884 1.0073 0.0104 0.5072 0.0008
6415.5922 -25622.4846 2.8093 4434.8839 -17.9962 0.8264 0.0184 0.5053 0.0014
6415.7332 -25621.0645 1.6543 4445.0213 -9.2692 1.0473 0.0124 0.5114 0.0009
6416.6954 -25620.1184 1.2661 4431.5240 -4.3249 0.9507 0.0089 0.5034 0.0007
6451.5800 -25616.3891 1.3405 4433.3694 -16.8458 1.0419 0.0096 0.5063 0.0007
6452.5545 -25618.6586 1.3803 4434.6138 -8.7892 1.0683 0.0081 0.5099 0.0007
6454.5556 -25623.8325 1.3937 4431.7379 -9.5482 1.0701 0.0103 0.5075 0.0008
6455.5374 -25628.9792 1.4965 4422.2962 -7.9232 0.9761 0.0106 0.5020 0.0008
6458.5877 -25622.2235 1.2855 4423.7546 -11.6472 1.0516 0.0093 0.5101 0.0007
6460.5668 -25621.7813 1.6473 4431.8028 -10.6556 0.9902 0.0115 0.5048 0.0008
6481.4839 -25619.0212 1.3733 4419.7151 -8.8134 0.9295 0.0089 0.5034 0.0007
6508.4718 -25627.1324 1.8094 4441.7103 -10.4168 0.8926 0.0119 0.5119 0.0010
6514.4694 -25623.2844 1.3027 4424.7201 -6.1677 0.8424 0.0087 0.5033 0.0007
6521.4589 -25619.8433 1.2696 4431.0487 -10.7298 0.8505 0.0081 0.5007 0.0007
6690.8780 -25624.1045 1.1926 4422.6617 -9.2806 0.9930 0.0082 0.5063 0.0006
6691.8339 -25624.6074 1.3498 4426.5396 -8.4985 1.1321 0.0107 0.5168 0.0007
6692.8139 -25625.0787 1.2900 4431.9081 -10.8969 1.0794 0.0099 0.5048 0.0006
Article number, page 12 of 14
A. Suárez Mascareño et al.: A super-Earth orbiting the nearby M-dwarf GJ 536
Table 5: Continued
BJD - 2450000 VrσVrFWHM BIS Span SMW index σSMW Hαindex σHαindex Flag
(d) (ms1) (ms1) (Kms1) (ms1)
6694.8640 -25624.0179 1.1386 4433.0913 -9.0442 1.0415 0.0081 0.5084 0.0006
6695.8790 -25622.2008 1.1906 4429.7997 -8.1605 1.0384 0.0085 0.5108 0.0006
6696.8539 -25622.8668 1.3253 4428.3856 -10.0882 1.2174 0.0107 0.5241 0.0007
6697.7981 -25625.2516 1.3466 4425.1099 -6.5195 1.0566 0.0105 0.5067 0.0007
6712.8127 -25613.5661 1.3395 4437.9343 -10.5794 1.5124 0.0112 0.5544 0.0008 Rejected
6713.8033 -25613.2583 1.3276 4450.3946 -9.6570 1.2983 0.0109 0.5214 0.0007
6715.7953 -25620.9555 1.3577 4436.1206 -6.6456 1.2225 0.0106 0.5213 0.0007
6720.8502 -25616.0066 1.2598 4432.3554 -9.5264 1.1898 0.0094 0.5123 0.0006
6723.8540 -25622.0386 1.2931 4439.7320 -11.2245 1.2372 0.0103 0.5198 0.0007
6724.7853 -25624.4497 1.2257 4440.9264 -10.5082 1.2255 0.0097 0.5205 0.0006
6725.7743 -25624.1936 1.2975 4439.1990 -10.9028 1.1683 0.0103 0.5152 0.0007
6725.8844 -25626.5410 1.3803 4440.6907 -7.2889 1.1309 0.0105 0.5138 0.0007
6726.7959 -25625.8960 1.1504 4442.3973 -7.2719 1.0898 0.0084 0.5101 0.0006
6727.8296 -25620.9225 1.1367 4430.0904 -10.2483 1.0773 0.0079 0.5083 0.0006
6728.8039 -25621.7110 1.1250 4434.7157 -8.6990 1.0786 0.0080 0.5062 0.0005
6729.7718 -25616.4237 1.4276 4437.5013 -12.1011 1.0759 0.0085 0.5096 0.0006
6730.8216 -25619.5981 1.3086 4433.6524 -10.6152 1.0424 0.0095 0.5111 0.0007
6732.7980 -25622.9502 1.4351 4421.7672 -4.3220 1.0392 0.0101 0.5106 0.0008
6737.8572 -25620.5708 1.3554 4428.3888 -5.4130 1.0060 0.0093 0.5102 0.0007
6738.8726 -25622.5574 1.2338 4431.8640 -7.8770 0.9898 0.0086 0.5047 0.0007
6739.8058 -25622.1678 1.1512 4425.2586 -9.5216 1.0135 0.0078 0.5075 0.0006
6740.8311 -25624.4843 1.0987 4428.1852 -6.4714 0.9862 0.0073 0.5061 0.0005
6741.7462 -25623.9200 1.1678 4429.0187 -9.0739 1.0092 0.0081 0.5038 0.0006
6742.8207 -25623.1636 1.1028 4426.1406 -9.8389 1.0343 0.0076 0.5094 0.0005
6743.7632 -25622.7840 1.2011 4432.8256 -6.7370 1.0186 0.0084 0.5069 0.0006
6745.7321 -25617.2597 1.1588 4423.0781 -11.4489 1.0638 0.0081 0.5124 0.0007
6746.8203 -25613.0541 1.2944 4428.6487 -8.3582 1.0296 0.0090 0.5158 0.0007
6752.8315 -25621.4802 1.3848 4432.6911 -7.1023 1.0633 0.0106 0.5210 0.0008
6754.8603 -25617.1072 2.0773 4434.8424 -14.7497 1.1186 0.0161 0.5213 0.0011
6755.8430 -25617.0817 2.0606 4444.2810 -9.2983 1.0480 0.0157 0.5182 0.0011
6755.8530 -25614.7730 1.9338 4449.5330 -8.2994 1.1561 0.0154 0.5220 0.0010
6756.8521 -25616.6832 1.0939 4440.9586 -9.8160 1.1383 0.0085 0.5169 0.0005
6757.8085 -25617.4805 1.4014 4441.8165 -9.7412 1.1327 0.0112 0.5181 0.0007
6758.8266 -25622.4824 2.2098 4438.0807 -15.1975 1.0999 0.0174 0.5266 0.0012
6759.8277 -25621.4838 1.3814 4438.3218 -7.9763 1.1324 0.0111 0.5192 0.0007
6760.8142 -25620.2626 1.3119 4448.0559 -8.2829 1.5703 0.0125 0.5642 0.0007 Rejected
6763.7243 -25620.4110 1.0500 4442.6357 -10.9225 1.1538 0.0073 0.5130 0.0005
6764.7765 -25618.1639 1.2340 4444.4456 -9.9255 1.1184 0.0092 0.5128 0.0006
6765.7208 -25619.0905 1.3628 4437.2341 -6.5105 1.1466 0.0078 0.5131 0.0006
6766.7265 -25621.2292 1.0945 4439.1904 -9.8447 1.2270 0.0081 0.5229 0.0005
6767.6534 -25623.9784 1.5745 4433.4716 -10.2058 1.1048 0.0115 0.5113 0.0008
6768.6678 -25625.3559 1.2107 4429.1243 -7.0854 1.0659 0.0087 0.5089 0.0006
6778.6271 -25624.2948 1.3732 4424.6235 -8.2880 0.9981 0.0077 0.5003 0.0005
6779.7560 -25623.0220 1.5571 4433.6733 -7.6866 0.9779 0.0094 0.5036 0.0007
6781.6011 -25621.5230 1.5651 4434.4575 -9.5425 0.9537 0.0096 0.5022 0.0007
6782.6156 -25621.6172 1.3793 4433.3780 -4.7485 1.1337 0.0085 0.5159 0.0005
6784.6137 -25625.6796 1.2493 4430.6030 -6.3606 0.9865 0.0089 0.5079 0.0006
6785.5546 -25623.9276 1.5364 4411.9798 -7.3440 0.9995 0.0118 0.5097 0.0008
6786.6679 -25628.0817 1.1219 4420.8351 -8.7321 0.9403 0.0073 0.5038 0.0005
6814.7183 -25618.4375 1.3822 4442.1718 -6.9710 1.0105 0.0109 0.5029 0.0007
6822.5823 -25625.6969 1.6325 4427.8930 -14.3619 1.0863 0.0119 0.5063 0.0009
6823.5834 -25627.6799 1.3314 4432.6413 -9.5886 1.0479 0.0097 0.5004 0.0007
6824.5777 -25623.2701 1.4221 4430.6359 -9.1015 1.0257 0.0097 0.4996 0.0007
6825.6520 -25622.3060 1.3593 4435.5327 -7.2742 1.0811 0.0105 0.5055 0.0007
6826.5764 -25621.9304 1.2330 4428.6392 -4.9112 1.3187 0.0094 0.5315 0.0007 Rejected
6827.5754 -25624.3321 1.1535 4432.4085 -6.5820 1.0300 0.0082 0.5024 0.0006
6828.6006 -25624.5384 1.1901 4429.2042 -10.7163 1.1152 0.0086 0.5094 0.0006
6838.5568 -25626.1804 1.1653 4430.8116 -10.7542 1.0408 0.0083 0.5163 0.0006
6839.5704 -25622.4896 1.4164 4425.8922 -8.3993 0.9890 0.0105 0.5122 0.0006
6840.5286 -25620.4583 1.4235 4421.9119 -7.9951 0.9953 0.0097 0.5107 0.0007
Article number, page 13 of 14
A&A proofs: manuscript no. GJ536_AA_vArxiv
Table 5: Continued
BJD - 2450000 VrσVrFWHM BIS Span SMW index σSMW Hαindex σHαindex Flag
(d) (ms1) (ms1) (Kms1) (ms1)
6841.6035 -25614.8073 2.5202 4438.8469 -2.9369 1.1631 0.0220 0.5331 0.0013 Rejected
6842.4896 -25617.9822 1.3193 4436.3770 -9.2662 1.0293 0.0094 0.5150 0.0007
6857.5388 -25625.2710 1.7446 4421.9564 -10.6323 1.0678 0.0125 0.5036 0.0009
6858.5182 -25622.6933 1.4250 4432.2060 -9.9027 1.0034 0.0099 0.4929 0.0007
6863.5169 -25622.6876 1.3877 4434.8193 -7.8466 1.0297 0.0103 0.5056 0.0007
6864.5176 -25624.8996 1.1102 4424.7214 -8.4713 0.9533 0.0077 0.4994 0.0005
6874.4791 -25620.6138 1.6580 4440.2229 -5.6786 1.0463 0.0121 0.5086 0.0009
7047.8603 -25624.7545 1.2191 4434.2142 -8.8214 1.0933 0.0090 0.5089 0.0006
7053.8561 -25621.7439 1.4441 4446.4398 -7.8204 1.0895 0.0103 0.5060 0.0007
7057.8269 -25622.0109 1.4588 4452.3988 -8.1500 1.5121 0.0123 0.5507 0.0008 Rejected
7058.8515 -25617.6495 1.8982 4442.7452 -17.1716 1.0415 0.0136 0.5044 0.0010
7079.8236 -25619.7617 1.2772 4436.6205 -11.9584 1.1289 0.0094 0.5148 0.0007
7080.8500 -25621.6358 1.4059 4427.7199 -5.2545 1.0410 0.0100 0.5063 0.0007
7082.8651 -25623.5924 1.1403 4434.4908 -7.9841 1.0022 0.0078 0.5025 0.0006
7085.7333 -25621.7323 1.2833 4430.9624 -8.7143 1.0902 0.0098 0.5114 0.0006
7114.8209 -25625.5802 1.4614 4410.2902 -14.3900 0.9577 0.0102 0.5028 0.0008
7115.7150 -25627.5907 1.3311 4428.5220 -8.7149 1.0657 0.0092 0.5075 0.0007
7116.7852 -25627.3237 1.2820 4424.2680 -7.4215 0.9395 0.0083 0.5014 0.0007
7142.7719 -25625.3593 1.2290 4428.5104 -8.7045 0.9850 0.0083 0.5065 0.0006
7147.7808 -25616.3909 1.3538 4441.0481 -9.6390 0.9792 0.0103 0.5038 0.0007
7148.7468 -25620.8954 1.2741 4429.2924 -10.0502 0.9918 0.0094 0.5001 0.0006
7202.5939 -25616.1481 1.6565 4441.1922 5.6209 0.9188 0.0097 0.4982 0.0008
7204.6007 -25623.6987 1.4010 4452.3792 2.2721 0.9045 0.0081 0.4978 0.0006
7211.5712 -25624.5994 1.3200 4447.6138 2.6096 0.8998 0.0075 0.4998 0.0005
7212.6084 -25624.9280 5.6243 4437.5059 -12.1776 0.4064 0.0213 0.5013 0.0024
7214.5883 -25625.9395 2.0090 4454.7852 5.6477 0.8619 0.0135 0.5093 0.0010
7238.5220 -25620.5551 1.4908 4444.8883 4.9186 0.9594 0.0104 0.5031 0.0006
7249.4828 -25623.8234 1.8006 4447.2116 4.2354 0.8713 0.0112 0.4972 0.0008
7448.8620 -25628.4475 1.4117 4457.4186 6.7720 0.9810 0.0103 0.5065 0.0007
7473.8467 -25621.6222 1.1138 4455.5312 0.5160 0.9392 0.0082 0.5075 0.0005
7476.8649 -25621.8943 1.1800 4459.2948 3.4832 0.9172 0.0083 0.5028 0.0006
7508.4799 -25624.9467 1.0813 4467.1596 -5.9534 1.0357 0.0107 0.5091 0.0005 HARPS-N
7508.5698 -25621.9227 1.0769 4464.6067 -7.1361 1.2192 0.0108 0.5287 0.0005 HARPS-N
7509.4759 -25625.1223 1.2681 4460.0000 -9.6134 0.9486 0.0111 0.4974 0.0005 HARPS-N
7509.5684 -25626.8176 1.4120 4462.4128 -8.3783 0.9415 0.0128 0.4969 0.0006 HARPS-N
7510.4709 -25621.9484 1.3119 4461.3527 -5.7705 0.9028 0.0117 0.4927 0.0006 HARPS-N
7510.5488 -25620.3344 1.2794 4467.5365 -5.4813 1.0562 0.0124 0.5053 0.0006 HARPS-N
7535.4250 -25625.4364 2.1712 4464.4853 -2.7902 0.9369 0.0239 0.5095 0.0012 HARPS-N
7536.4320 -25624.6383 1.1442 4463.1872 -8.4287 0.9829 0.0134 0.5065 0.0006 HARPS-N
7537.4339 -25619.0095 1.3012 4461.0889 -10.0375 0.9686 0.0148 0.5083 0.0007 HARPS-N
7537.5223 -25618.7979 1.1950 4464.6137 -6.3194 1.0328 0.0149 0.5105 0.0007 HARPS-N
7538.4145 -25618.5006 1.0871 4459.3290 -6.8755 0.9491 0.0118 0.5029 0.0006 HARPS-N
7538.5184 -25615.3666 1.0356 4470.6070 -7.3075 0.9111 0.0099 0.5017 0.0005 HARPS-N
Article number, page 14 of 14
... GJ536 hosts a super-Earth planet in an 8.7 day orbit (Suárez Mascareño et al. 2017a). Using additional RV data from the CARMENES survey, Trifonov et al. (2018) ...
Full-text available
The M dwarf stars are exciting targets for exoplanet investigations; however, their fundamental stellar properties are difficult to measure. Perhaps the most challenging property is stellar age. Once on the main sequence, M dwarfs change imperceptibly in their temperature and luminosity, necessitating novel statistical techniques for estimating their ages. In this paper, we infer ages for known eccentric-planet-hosting M dwarfs using a combination of kinematics and α-element enrichment, both shown to correlate with age for Sun-like FGK stars. We calibrate our method on FGK stars in a Bayesian context. To measure α-enrichment, we use publicly available spectra from the CARMENES exoplanet survey and a recently developed [Ti/Fe] calibration utilizing individual Ti i and Fe i absorption lines in the Y band. Tidal effects are expected to circularize the orbits of short-period planets on short timescales; however, we find a number of mildly eccentric, close-in planets orbiting old (~8 Gyr) stars. For these systems, we use our ages to constrain the tidal dissipation parameter of the planets, Q p. For two mini-Neptune planets, GJ 176 b and GJ 536 b, we find that they have Q p values more similar to the ice giants than to the terrestrial planets in our solar system. For GJ 436 b, we estimate an age of and constrain the Q p to be >10⁵, in good agreement with constraints from its inferred tidal heating. We find that GJ 876 d has likely undergone significant orbital evolution over its lifetime, potentially influenced by its three outer companions that orbit in a Laplace resonance.
Context. The relevance of M dwarfs in the search for potentially habitable Earth-sized planets has grown significantly in the last years. Aims. In our on-going effort to comprehensively and accurately characterise confirmed and potential planet-hosting M dwarfs, in particular for the CARMENES survey, we have carried out a comprehensive multi-band photometric analysis involving spectral energy distributions, luminosities, absolute magnitudes, colours, and spectral types, from which we have derived basic astrophysical parameters. Methods. We have carefully compiled photometry in 20 passbands from the ultraviolet to the mid-infrared, and combined it with the latest parallactic distances and close-multiplicity information, mostly from Gaia DR2, of a sample of 2479 K5 V to L8 stars and ultracool dwarfs, including 2210 nearby, bright M dwarfs. For this, we made extensive use of Virtual Observatory tools. Results. We have homogeneously computed accurate bolometric luminosities and effective temperatures of 1843 single stars, derived their radii and masses, studied the impact of metallicity, and compared our results with the literature. The over 40 000 individually inspected magnitudes, together with the basic data and derived parameters of the stars, individual and averaged by spectral type, have been made public to the astronomical community. In addition, we have reported 40 new close multiple systems and candidates ( ρ < 3.3 arcsec) and 36 overluminous stars that are assigned to young Galactic populations. Conclusions. In the new era of exoplanet searches around M dwarfs via transit (e.g. TESS, PLATO) and radial velocity (e.g. CARMENES, NIRPS+HARPS), this work is of fundamental importance for stellar and therefore planetary parameter determination.
Full-text available
Context. The High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph has been mounted since 2003 at the ESO 3.6 m telescope in La Silla and provides state-of-the-art stellar radial velocity (RV) measurements with a precision down to ∼1 m s ⁻¹ . The spectra are extracted with a dedicated data-reduction software (DRS), and the RVs are computed by cross-correlating with a numerical mask. Aims. This study has three main aims: (i) Create easy access to the public HARPS RV data set. (ii) Apply the new public SpEctrum Radial Velocity AnaLyser (SERVAL) pipeline to the spectra, and produce a more precise RV data set. (iii) Determine whether the precision of the RVs can be further improved by correcting for small nightly systematic effects. Methods. For each star observed with HARPS, we downloaded the publicly available spectra from the ESO archive and recomputed the RVs with SERVAL. This was based on fitting each observed spectrum with a high signal-to-noise ratio template created by coadding all the available spectra of that star. We then computed nightly zero-points (NZPs) by averaging the RVs of quiet stars. Results. By analyzing the RVs of the most RV-quiet stars, whose RV scatter is < 5 m s ⁻¹ , we find that SERVAL RVs are on average more precise than DRS RVs by a few percent. By investigating the NZP time series, we find three significant systematic effects whose magnitude is independent of the software that is used to derive the RV: (i) stochastic variations with a magnitude of ∼1 m s ⁻¹ ; (ii) long-term variations, with a magnitude of ∼1 m s ⁻¹ and a typical timescale of a few weeks; and (iii) 20–30 NZPs that significantly deviate by a few m s ⁻¹ . In addition, we find small (≲1 m s ⁻¹ ) but significant intra-night drifts in DRS RVs before the 2015 intervention, and in SERVAL RVs after it. We confirm that the fibre exchange in 2015 caused a discontinuous RV jump that strongly depends on the spectral type of the observed star: from ∼14 m s ⁻¹ for late F-type stars to ∼ − 3 m s ⁻¹ for M dwarfs. The combined effect of extracting the RVs with SERVAL and correcting them for the systematics we find is an improved average RV precision: an improvement of ∼5% for spectra taken before the 2015 intervention, and an improvement of ∼15% for spectra taken after it. To demonstrate the quality of the new RV data set, we present an updated orbital solution of the GJ 253 two-planet system. Conclusions. Our NZP-corrected SERVAL RVs can be retrieved from a user-friendly public database. It provides more than 212 000 RVs for about 3000 stars along with much auxiliary information, such as the NZP corrections, various activity indices, and DRS-CCF products.
The Transiting Exoplanet Survey Satellite TESS has begun a new age of exoplanet discoveries around bright host stars. We present the discovery of HD 1397b (TOI-120.01), a giant planet in an 11.54-day eccentric orbit around a bright (V = 7.9) G-type subgiant. We estimate both host star and planetary parameters consistently using EXOFASTv2 based on TESS time-series photometry of transits and radial velocity measurements with CORALIE and MINERVA-Australis. We also present high angular resolution imaging with NaCo to rule out any nearby eclipsing binaries. We find that HD 1397b is a Jovian planet, with a mass of 0.415 ± 0.020 M J and a radius of 1.026 ± 0.026 R J . Characterising giant planets in short-period eccentric orbits, such as HD 1397b, is important for understanding and testing theories for the formation and migration of giant planets as well as planet-star interactions.
A low-amplitude periodic signal in the radial velocity (RV) time series of Barnard's Star was recently attributed to a planetary companion with a minimum mass of ∼3.2 M · at an orbital period of ∼233 days. The relatively long orbital period and the proximity of Barnard's Star to the Sun raises the question whether the true mass of the planet can be constrained by accurate astrometric measurements. By combining the assumption of an isotropic probability distribution of the orbital orientation with the RV-analysis results, we calculated the probability density function of the astrometric signature of the planet. In addition, we reviewed the astrometric capabilities and limitations of current and upcoming astrometric instruments. We conclude that Gaia and the Hubble Space Telescope (HST) are currently the best-suited instruments to perform the astrometric follow-up observations. Taking the optimistic estimate of their single-epoch accuracy to be ∼30μas, we find a probability of ∼10% to detect the astrometric signature of Barnard's Star b with ∼50 individual-epoch observations. In case of no detection, the implied mass upper limit would be ∼8 M · , which would place the planet in the super-Earth mass range. In the next decade, observations with the Wide-Field Infrared Space Telescope (WFIRST) may increase the prospects of measuring the true mass of the planet to ∼99%.
We announce the discovery of two planetary companions orbiting around the low-mass stars Ross 1020 (GJ 3779, M4.0V) and LP 819-052 (GJ 1265, M4.5V). The discovery is based on the analysis of CARMENES radial velocity (RV) observations in the visual channel as part of its survey for exoplanets around M dwarfs. In the case of GJ 1265, CARMENES observations were complemented with publicly available Doppler measurements from HARPS. The datasets reveal two planetary companions, one for each star, that share very similar properties: minimum masses of 8.0 ± 0.5 M and 7.4 ± 0.5 M in low-eccentricity orbits with periods of 3.023 ± 0.001 d and 3.651 ± 0.001 d for GJ 3779 b and GJ 1265 b, respectively. The periodic signals around 3 d found in the RV data have no counterpart in any spectral activity indicator. Furthermore, we collected available photometric data for the two host stars, which confirm that the additional Doppler variations found at periods of approximately 95 d can be attributed to the rotation of the stars. The addition of these planets to a mass-period diagram of known planets around M dwarfs suggests a bimodal distribution with a lack of short-period low-mass planets in the range of 2-5 M . It also indicates that super-Earths (>5 M ) currently detected by RV and transit techniques around M stars are usually found in systems dominated by a single planet.
We use radial velocity observations to search for massive, long-period gas giant companions in systems hosting inner super-Earth (1-4 R_⊕, 1-10 M_⊕) planets in order to constrain formation and migration scenarios for this population. We consistently re-fit all published radial velocity datasets for a sample of 65 stars and find 9 systems with statistically significant trends indicating the presence of an outer companion. We combine these radial velocity data with AO images in order to constrain the allowed masses and semi-major axes of these companions. We quantify our sensitivity to the presence of long period companions by fitting the sample with a power law distribution and find an estimated occurrence rate of 39+/-7% for companions between 0.5-20 M_(Jup) and 1-20 AU. Half of our systems were discovered by the transit method and the other half were discovered by the RV method. While differences in RV baselines and number of data points between the two samples lead to different sensitivities to distant companions, we find that the occurrence rates of gas giant companions in each sample is consistent at the 0.5sigma level. We compare the frequency of Jupiter analogs in these systems to the equivalent rate from field star surveys and find that Jupiter analogs are more common around stars hosting super-Earths. We conclude that the presence of outer gas giant planets does not suppress the formation of inner super-Earths, and that these two populations of planets instead appear to be correlated with each other. We also find that the stellar metallicities of systems with gas giant companions are significantly higher than those without companions, in good agreement with the well-established metallicity correlation from RV surveys of field stars.
Full-text available
Context. Previous simulations predicted the activity-induced radial-velocity (RV) variations of M dwarfs to range from ~1 cm s ⁻¹ to ~1 km s ⁻¹ , depending on various stellar and activity parameters. Aims. We investigate the observed relations between RVs, stellar activity, and stellar parameters of M dwarfs by analyzing CARMENES high-resolution visual-channel spectra (0.5–1 μ m), which were taken within the CARMENES RV planet survey during its first 20 months of operation. Methods. During this time, 287 of the CARMENES-sample stars were observed at least five times. From each spectrum we derived a relative RV and a measure of chromospheric H α emission. In addition, we estimated the chromatic index (CRX) of each spectrum, which is a measure of the RV wavelength dependence. Results. Despite having a median number of only 11 measurements per star, we show that the RV variations of the stars with RV scatter of >10 m s ⁻¹ and a projected rotation velocity v sin i > 2 km s ⁻¹ are caused mainly by activity. We name these stars “active RV-loud stars” and find their occurrence to increase with spectral type: from ~3% for early-type M dwarfs (M0.0–2.5 V) through ~30% for mid-type M dwarfs (M3.0–5.5 V) to >50% for late-type M dwarfs (M6.0–9.0 V). Their RV-scatter amplitude is found to be correlated mainly with v sin i . For about half of the stars, we also find a linear RV–CRX anticorrelation, which indicates that their activity-induced RV scatter is lower at longer wavelengths. For most of them we can exclude a linear correlation between RV and H α emission. Conclusions. Our results are in agreement with simulated activity-induced RV variations in M dwarfs. The RV variations of most active RV-loud M dwarfs are likely to be caused by dark spots on their surfaces, which move in and out of view as the stars rotate.
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We aim to investigate the presence of signatures of magnetic cycles and rotation on a sample of 71 early M-dwarfs from the HADES RV programme using high-resolution time-series spectroscopy of the Ca II H & K and Halpha chromospheric activity indicators, the radial velocity series, the parameters of the cross correlation function and the V-band photometry. We used mainly HARPS-N spectra, acquired over four years, and add HARPS spectra from the public ESO database and ASAS photometry light-curves as support data, extending the baseline of the observations of some stars up to 12 years. We provide log(R'hk) measurements for all the stars in the sample, cycle length measurements for 13 stars, rotation periods for 33 stars and we are able to measure the semi-amplitude of the radial velocity signal induced by rotation in 16 stars. We complement our work with previous results and confirm and refine the previously reported relationships between the mean level of chromospheric emission, measured by the log(R'hk), with the rotation period, and with the measured semi-amplitude of the activity induced radial velocity signal for early M-dwarfs. We searched for a possible relation between the measured rotation periods and the lengths of the magnetic cycle, finding a weak correlation between both quantities. Using previous v sin i measurements we estimated the inclinations of the star's poles to the line of sight for all the stars in the sample, and estimate the range of masses of the planets GJ 3998 b and c (2.5 - 4.9 Mearth and 6.3 - 12.5 Mearth), GJ 625 b (2.82 Mearth), GJ 3942 b (7.1 - 10.0 Mearth) and GJ 15A b (3.1 - 3.3 Mearth), assuming their orbits are coplanar with the stellar rotation.
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We report the discovery of a system of two super-Earths orbiting the moderately active K-dwarf HD 176986. This work is part of the RoPES RV program of G- and K-type stars, which combines radial velocities (RVs) from the HARPS and HARPS-N spectrographs to search for short-period terrestrial planets. HD 176986 b and c are super-Earth planets with masses of 5.74 and 9.18 M$_{\oplus}$, orbital periods of 6.49 and 16.82 days, and distances of 0.063 and 0.119 AU in orbits that are consistent with circular. The host star is a K2.5 dwarf, and despite its modest level of chromospheric activity (log(R'hk) = - 4.90 +- 0.04), it shows a complex activity pattern. Along with the discovery of the planets, we study the magnetic cycle and rotation of the star. HD 176986 proves to be suitable for testing the available RV analysis technique and further our understanding of stellar activity.
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We investigate the photometric modulation induced by magnetic activity cycles and study the relationship between rotation period and activity cycle(s) in late-type (FGKM) stars. We analyse light-curves spanning up to 9 years of 125 nearby stars provided by the ASAS survey. The sample is mainly conformed by low-activity main sequence late A to mid M-type stars. A search is performed for short (days) and long-term (years) periodic variations in the photometry. We modelled with combinations of sinusoids the light-curves to measure the properties of these periodic signals. To provide a better statistical interpretation of our results we complement them with the results from previous similar works. We have been able to measure long-term photometric cycles of 47 stars. Rotational modulation was also detected and rotational periods measured in 36 stars. For 28 stars we have simultaneous measurements of both, activity cycles and rotational periods, being 17 of them M-type stars. From sinusoidal fits we measured both photometric amplitudes and periods. The measured cycle periods range from 2 up to 14 yr with photometric amplitudes in the range of 5-20 mmag. We have found that the distribution of cycle lengths for the different spectral types is similar. On the other hand the distribution of rotation periods is completely different, trending to longer periods for later type stars. The amplitudes induced by magnetic cycles and rotation show a clear correlation. A trend of photometric amplitudes with rotation period is also outlined in the data. For a given activity index the amplitudes of the photometric variability induced by activity cycles of main sequence GK stars are lower than those of early and mid-M dwarfs. Using spectroscopic data we also provide an update in the empirical relationship between the level of chromospheric activity as given by log(Rhk) and the rotation periods.
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We use archival HARPS spectra to detect three planets orbiting the M3 dwarf Wolf1061 (GJ 628). We detect a 1.36 Mearth minimum-mass planet with an orbital period P = 4.888d (Wolf1061b), a 4.25 Mearth minimum-mass planet with orbital period P = 17.867d (Wolf1061c), and a likely 5.21 Mearth minimum-mass planet with orbital period P = 67.274d (Wolf1061d). All of the planets are of sufficiently low mass that they may be rocky in nature. The 17.867d planet falls within the habitable zone for Wolf 1061 and the 67.274d planet falls just outside the outer boundary of the habitable zone. There are no signs of activity observed in the bisector spans, cross-correlation full-width-half-maxima, Calcium H & K indices, NaD indices, or H-alpha indices near the planetary periods. We use custom methods to generate a cross-correlation template tailored to the star. The resulting velocities do not suffer the strong annual variation observed in the HARPS DRS velocities. This differential technique should deliver better exploitation of the archival HARPS data for the detection of planets at extremely low amplitudes.
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We determine rotation periods of a sample of 48 late F-type to mid-M dwarf stars using time series high-resolution spectroscopy of the Ca ii H&K and H α chromospheric activity indicators. We find good agreement between the rotation periods obtained from each of these two indicators. An empirical relationship between the level of chromospheric emission measured by $\log _{10}(R^{\prime }_{\rm HK}$) and the spectroscopic rotation periods is reported. This relation is largely independent of the spectral type and the metallicity of the stars and can be used to make a reliable prediction of rotation periods for late K to mid-M dwarfs with low levels of activity. For some stars in the sample, the measured spectroscopic rotation periods coincide, or are very close, to the orbital periods of postulated planets. In such cases, further studies are needed to clarify whether the associated periodic radial velocity signals reveal the existence of planets or are due to magnetic activity.
At a distance of 1.295 parsecs, the red dwarf Proxima Centauri (α Centauri C, GL 551, HIP 70890 or simply Proxima) is the Sun's closest stellar neighbour and one of the best-studied low-mass stars. It has an effective temperature of only around 3,050 kelvin, a luminosity of 0.15 per cent of that of the Sun, a measured radius of 14 per cent of the radius of the Sun and a mass of about 12 per cent of the mass of the Sun. Although Proxima is considered a moderately active star, its rotation period is about 83 days (ref. 3) and its quiescent activity levels and X-ray luminosity are comparable to those of the Sun. Here we report observations that reveal the presence of a small planet with a minimum mass of about 1.3 Earth masses orbiting Proxima with a period of approximately 11.2 days at a semi-major-axis distance of around 0.05 astronomical units. Its equilibrium temperature is within the range where water could be liquid on its surface. © 2016 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
Stellar activity and rotation frustrate the detection of exoplanets through the radial velocity technique. This effect is particularly of concern for M dwarfs, which can remain magnetically active for billions of years. We compile rotation periods for late-type stars and for the M dwarf planet-host sample in order to investigate the rotation periods of older field stars across the main sequence. We show that for stars with masses between 0.25 and 0.5 solar masses (M4V to M1V), the stellar rotation period typical of field stars coincides with the orbital periods of planets in the habitable zone. This will pose a fundamental challenge to the discovery and characterization of potentially habitable planets around early M dwarfs. Due to the longer rotation periods reached by mid M dwarfs and the shorter orbital period at which the planetary habitable zone is found, stars with masses between 0.1 and 0.25 solar masses (M6V to M4V) offer better opportunities for the detection of habitable planets via radial velocities.
M-dwarf stars -- hydrogen-burning stars that are smaller than 60 per cent of the size of the Sun -- are the most common class of star in our Galaxy and outnumber Sun-like stars by a ratio of 12:1. Recent results have shown that M dwarfs host Earth-sized planets in great numbers: the average number of M-dwarf planets that are between 0.5 to 1.5 times the size of Earth is at least 1.4 per star. The nearest such planets known to transit their star are 39 parsecs away, too distant for detailed follow-up observations to measure the planetary masses or to study their atmospheres. Here we report observations of GJ 1132b, a planet with a size of 1.2 Earth radii that is transiting a small star 12 parsecs away. Our Doppler mass measurement of GJ 1132b yields a density consistent with an Earth-like bulk composition, similar to the compositions of the six known exoplanets with masses less than six times that of the Earth and precisely measured densities. Receiving 19 times more stellar radiation than the Earth, the planet is too hot to be habitable but is cool enough to support a substantial atmosphere, one that has probably been considerably depleted of hydrogen. Because the host star is nearby and only 21 per cent the radius of the Sun, existing and upcoming telescopes will be able to observe the composition and dynamics of the planetary atmosphere.
We re-analyse the publicly available radial velocity (RV) measurements for Alpha Cen B, a star hosting an Earth-mass planet candidate, Alpha Cen Bb, with 3.24 d orbital period. We demonstrate that the 3.24 d signal observed in the Alpha Cen B data almost certainly arises from the window function (time sampling) of the original data. We show that when stellar activity signals are removed from the RV variations, other significant peaks in the power spectrum of the window function are coincidentally suppressed, leaving behind a spurious yet apparently significant ‘ghost’ of a signal that was present in the window function's power spectrum ab initio. Even when fitting synthetic data with time sampling identical to the original data, but devoid of any genuine periodicities close to that of the planet candidate, the original model used to infer the presence of Alpha Cen Bb leads to identical conclusions: viz., the 3σ detection of a half-a-metre-per-second signal with 3.236 d period. Our analysis underscores the difficulty of detecting weak planetary signals in RV data, and the importance of understanding in detail how every component of an RV data set, including its time sampling, influences final statistical inference.