arXiv:0803.1294v1 [astro-ph] 9 Mar 2008
Astronomy & Astrophysics manuscript no. rliseau
March 9, 2008
c ? ESO 2008
q1Eri: a solar-type star with a planet and a dust belt⋆
R. Liseau1, C. Risacher2, A. Brandeker3, C. Eiroa4, M. Fridlund5, R. Nilsson3, G. Olofsson3, G.L. Pilbratt5, and
P. Th´ ebault3, 6
1Onsala Space Observatory, Chalmers University of Technology, S-439 92 Onsala, Sweden
2European Southern Observatory, Casilla 19001, Santiago 19, Chile
3Stockholm Observatory, AlbaNova University Center, Roslagstullsbacken 21, SE-106 91 Stockholm, Sweden
4Departamento de F´ ısicaTe´ orica, C-XI, Facultadde Ciencias, Universidad Aut´ onoma de Madrid, Cantoblanco, 28049 Madrid, Spain
5ESA Astrophysics Missions Division, ESTEC, PO Box 299, NL-2200 AG Noordwijk, The Netherlands
6LESIA, Observatoire de Paris, F-92195 Meudon Principal Cedex, France
Received ; accepted
Context. Far-infraredexcess emissionfrommain-sequence starsisduetodust produced byorbitingminor bodies. Inthesedisks, larger
bodies, such as planets, may also be present and the understanding of their incidence and influence currently presents a challenge.
Aims. Only very few solar-type stars exhibiting an infrared excess and harbouring planets are known to date. Indeed, merely a single
case of a star-planet-disk system has previously been detected at submillimeter (submm) wavelengths. Consequently, one of our aims
is to understand the reasons for these poor statistics, i.e., whether these results reflected the composition and/or the physics of the
planetary disks or were simply due to observational bias and selection effects. Finding more examples would be very significant.
Methods. The selected target, q1Eri, is a solar-type star, which was known to possess a planet, q1Erib, and to exhibit excess emission
at IRAS wavelengths, but had remained undetected in the millimeter regime. Therefore, submm flux densities would be needed to
better constrain the physical characteristics of the planetary disk. Consequently, we performed submm imaging observations of q1Eri.
Results. The detected dust toward q1Eri at 870µm exhibits the remarkable fact that the entire SED, from the IR to mm-wavelengths,
is fit by a single-temperature blackbody function (60K). This would imply that the emitting regions are confined to a narrow region
(ring) at radial distances much larger than the orbital distance of q1Erib, and that the emitting particles are considerably larger than
some hundred micron. However, the 870µm source is extended, with a full-width-half-maximum of roughly 600AU. Therefore, a
physically more compelling model also invokes a belt of cold dust (17K), located at 300AU from the star and about 60AU wide.
Conclusions. The minimum mass of 0.04 M⊕(3 MMoon) of 1mm-sizeicy ring-particles isconsiderable, given thestellar age of>
These big grains form an inner edge at about 25AU, which may suggest the presence of an unseen outer planet (q1Eric).
Key words. Stars: individual: q1Eri (HD10647) – Stars: planetary systems: planetary disks – Stars: planetary systems: formation
During the end stages of early stellar evolution, dusty debris
disks are believed to be descendents of gas-rich protoplanetary
disks. These had been successful to varying degrees in build-
ing a planetary system. What exactly determines the upper cut-
off mass of the bodies in individual systems, and on what time
scales, is not precisely known. However, the presence of debris
around matured stars is testimony to the action of orbiting bod-
ies, where a large number of smaller ones are producingthe dust
through collisional processes and where a small number of big-
ger bodies, if any, are determiningthe topology(disks, rings and
belts, clumps) through gravitational interaction. The time evo-
lution of the finer debris is believed to be largely controlled by
non-gravitational forces, though. By analogy, many debris disks
are qualitatively not very different from the asteroid and Kuiper
Send offprint requests to: R. Liseau
⋆Based on observations with APEX, Llano Chajnantor, Chile.
belts and the zodiacal dust cloud in the solar system (Mann et al.
For solar-type stars on the main-sequence, which are known
to exhibit infrared excess due to dust disks, one might ex-
pect, therefore, a relatively high incidence of planetary sys-
tems around them. Surveying nearly 50 FGK stars with known
planets for excess emission at 24µm and 70µm, Trilling et al.
(2008) detected about 10-20% at 70µm, but essentially none at
24µm, implying that these planetary disks are cool (< 100K)
and large (> 10AU). However, in general, the conjecture that
the infrared excess arises from disks lacks as yet observational
confirmation due to insufficient spatial resolution. In fact, until
very recently, there was only one main-sequence system known
that has an extended, resolved disk/belt structure and (at least)
one giant planet, viz. ǫ Eri, a solar-type star at the distance of
only three parsec (Greaves et al. 1998, 2005). Its planetary com-
panion, ǫ Erib, has been detected indirectly by astrometric and
radial velocity (RV) methods applied to the star (Hatzes et al.
2000; Benedict et al. 2006), whereas attempts to directly de-
2 R. Liseau et al.: q1Eri: a solar-type star with a planet and a dust belt
tect the planet have so far been unsuccessful (Itoh et al. 2006;
Janson et al. 2007).
As its name indicates, the object of the present study, q1Eri,
albeit at a larger distance (D = 17.35 ± 0.2pc) and is, as such,
unrelated to ǫ Eri. The planet was discovered with the RV tech-
nique (for a recent overview, see Butler et al. 2006). These RV
data suggest that the semimajor axis of the Jupiter-mass planet
q1Erib is about 2AU (Table1). It seems likely that regions in-
side this orbital distance have been largely cleared by the planet,
whereas outside the planetary orbit, substantial amounts of ma-
terial might still be present.
In fact, IRAS and ISO data were suggestive of significant
excess radiationabovethephotosphericemissionat wavelengths
longwardofabout20µm.Zuckerman & Song(2004)interpreted
these data in terms of dust in a disk at the orbital distance of
30AU and at a temperature of about 55K. Chen et al. (2006)
fitted the far-infrared emission with the corresponding values
of 20AU and 70K, respectively. Trilling et al. (2008) derived
20AU and 60K. In their entire sample of more than 200 stars,
q1Eri (=HD10647) has by far the highest 70µm excess.
At mm-wavelengths, Sch¨ utz et al. (2005) failed to detect the
disk and assigned an upper limit to the dust mass of 6 MMoon.
This is unsatisfactory, as the proper characterization of the dust
around q1Eri would require valid long wavelength data. In the
following, observations of q1Eri at 870µm are described and
their implications discussed.
2. Observations and Data Reductions
APEX, the Atacama Pathfinder EXperiment, is a 12m diameter
submillimeter telescope situated at an altitude of 5100m on the
Llano Chajnantor in northern Chile. The telescope is operated
by the Onsala Space Observatory, the Max-Planck-Institut f¨ ur
Radioastronomie, and the European Southern Observatory.
TheLarge Apex BOlometer
Siringo et al. 2007) is a multi-channel bolometer array for
continuum observations with 60GHz band width and centered
on the wavelength of 870µm. The array, having a total field
of view of 11′, is spatially undersampled and we therefore
adopted spiral pattern observing as the appropriate technique
(Siringo et al. 2007). This procedure results in fully-sampled
maps with a uniform noise distribution over an area of about
8′. During the nights of August1-4, 2007, we obtained 32 such
individual maps, for about 7.5min each with central coordi-
nates RA=01h42m29s·32 and Dec=−53◦44′27′′·0 (J2000). The
LABOCA beam width at half power (HPBW) is 18′′·6 ± 1′′·0. We
focussed LABOCA on the planet Jupiter and the rms-pointing
accuracy of the telescope was 3′′to 4′′.
We reduced the data with the BoA software (Siringo et al.
2007), which included flat fielding, baseline removal, despiking
and iteratively removing the sky noise, and filtering out the low
frequencies of the 1/f-noise, with the cut-off frequency corre-
sponding to several arcminutes. The software also accounts for
the map reconstruction and the absolute calibration, using the
opacities determined from numerous skydips (zenith opacities
were in the range 0.1 to 0.3) and observations of the planets
Uranus and Mars. The final result is an rms-noise-weighted av-
erage map (Fig.1).
The final product of the reduction process is the 870µm im-
age presented in Fig.1, which shows the central 5′× 5′of
Fig.1. q1Eri observed at 870µm with the submm camera
LABOCA at the APEX telescope (HPBW∼18′′). Within the
positional accuracy, the star is at the origin of the image (see
Table2, referring to 01h42m29s·32, −53◦44′27′′·0, J2000.0) and
rms noise of the flux density, Fν, and increments are in steps of
1σ. Thecolourcoding,inunits ofJy/beam,is shownbythescale
bar to the right of the image, which has been smoothed with a
circular 18′′Gaussian. At the distance of the star, the space be-
tween two tick marks (=20′′) corresponds to 350AU.
Table 1. Physical properties of the star and its planet⋆
The star q1Eri
Spectral type and luminosity class
Effective temperature, Teff
Surface gravity, logg
The planet q1Erib
Semimajor axis, aorbit
4.4 (in cm s−2)
(> 1 − 2)Gyr
2.75 ± 0.15yr
2.0 ± 0.2AU
0.2 ± 0.2
0.9 ± 0.2 MJupiter
⋆See Butler et al. (2006) and references cited in the text.
the LABOCA map. The peak flux in the map is found at the
position of q1Eri and a few other pointlike features of low
intensity are also present, one of which is close to the star.
If not merely noise, these low signals could be due to extra-
galactic background sources, as the displayed number density
is consistent with that observed elsewhere (e.g., Lagache et al.
2005; Bertoldi et al. 2007; Ivison et al. 2007). Other, comple-
mentary observations (e.g., optical, IR, X-rays, radio interfer-
ometry) would be required for their identification.
R. Liseau et al.: q1Eri: a solar-type star with a planet and a dust belt3
Table 2. Physical properties of the q1Eri dust system
Peak offseta, (∆α, ∆δ)
Peak flux densitya, Fν(0, 0), λ = 870µm
Integrated flux density,
Position anglea, pa
Inclination angle, i
Fractional luminosity, Lbb/Lstar
Inner (outer)bTemperature, Tbb
Inner (outer)bRadius, rbb
Inner (outer)bWidth, ∆rbb
Inner (outer)bMinimum massc, Mdust
(+4′′, +3′′), (error: ±4′′)
(16.2 ± 0.8)mJy/beam
(39.4 ± 4.1)mJy, Fν≥ 2σ
37′′± 2′′(640 ± 35)AU
55◦± 4◦(north over east)
1.1 × 10−4
0.04 M⊕(0.15 M⊕)
aTwo-dimensional Gaussian fits with 1σ formal fitting uncertainties.
bAn outer dust belt is implied by the extent of q1Eri at 870µm.
cκ1011.5Hz= 2cm2g−1(ρ = 1.18gcm−3, amax= 1mm, n(a) ∝ a−3.5).
The derived flux densities of q1Eri are provided in Table2,
which presents the results from fitting the data to a two-
dimensional Gaussian function. The indicated errors are formal
fit errors only, based on 1σ rms values. In individual cases, e.g.,
pa, realistic errors could be twice as large. The error on the in-
tegrated flux density in Table2 also includes an uncertainty of
10% in the absolute calibration.
The 870µm source is at best only marginally resolved in the
North-South direction (formal fit result is 23′′± 1′′), whereas
it is clearly elongated in approximately the East-West direction
(37′′± 2′′). At the distance of q1Eri, this corresponds to a disk
diameter of 640AU and assuming a circular shape, these disk
dimensions yield an inclination with respect to the line of sight
of i ≥ 52◦, not excluding the possibility that the disk is seen
essentiallyedge-on(iclose to90◦).Theverticaldiskscale height
4.1. Physical conditions and the age of the system
The spectral type of q1Eri is slightly earlier than that of
the Sun (F8-9 V, Nordstr¨ om et al. 2004; Decin et al. 2000,
2003; Zuckerman & Song 2004; Chen et al. 2006), with the ef-
fective temperature being bracketed by the extremes 6040K
(Nordstr¨ om et al. 2004) and 6260K (Chen et al. 2006) , with
the mean of 6150K, i.e., essententially the value given by
Butler et al. (2006, 6105 K) (see Table1).
Literature estimations of likely ages for q1Eri span the
range 0.3 to 4.8Gyr (with an entire range of 0.0 to 7.0Gyr,
Decin et al. 2000; Zuckerman & Song 2004; Decin et al. 2003;
Nordstr¨ om et al. 2004; Chen et al. 2006). However, the level of
chromospheric activity (logR′
1.9Gyr (see Eq.15 of Wright et al. 2004). The star has also
been detected in X-rays with ROSAT (logLX = 28.3, J.Sanz,
private communication), yielding 1.2Gyr (Ribas et al. 2005;
Guinan & Engle 2007). This value is also consistent with the
stellar rotation period of about 10days (uncorrected for sini;
Ecuvillon et al. 2007). It is clear that the star is definitely on
the main-sequence and that the age of the system likely exceeds
HK= −4.7) suggests an age of
Fig.2. The fit to the SED of q1Eri (HD10647) is shown from
1000Å to beyond 1mm in red. The stellar photosphere is repre-
sented by a Kurucz ATLAS9 model atmosphere and the excess
emission by a single temperature blackbody curve (in black).
Also, shown in red, are Simbad, TIMMI2 11.9 µm, and IRAS
FSC data, in addition to ISO 60µm (Decin et al. 2000). Spitzer
data have been given higher weight and are shown in green,
viz. 8.5-13µm and 30-34µm (Chen et al. 2006), and 24µm
and 70µm (Trilling et al. 2008), respectively.The SEST-SIMBA
3σ upper limit at 1.2mm (Sch¨ utz et al. 2005) and our APEX-
LABOCA point at 870µm are shown in blue. The black dots
refer to the combined 60 K modified blackbody (β = 1) and
17K blackbody (see the text).
4.2. The nature of the emitting particles
The absence of spectral features in the 10 to 30µm region sug-
gests that the dust grains are considerably larger than 10µm
(Chen et al. 2006; Sch¨ utz et al. 2005). Remarkably, the spec-
tral energy distribution (SED) of the excess emission can be
fit by a single-temperature blackbody of 60K, from the in-
frared to the submm/mm regime (see Fig.2). The blackbody
character is determined by the LABOCA flux and independent
of the relative weights assigned to the mid- and far-infrared
data. The radial distance from the central star, at which a grain
has attained thermal equilibrium, is approximately given by
[(1−A)/(16πǫσ)(Lstar/T4)]1/2, where A and ǫ are the integrated
reflectivity and emissivity, respectively. For a blackbody this re-
duces to rbb= (Rstar/2)(Teff/Tbb)2, which for Tbb= 60K, yields
a minimumdistanceof25AU fortheq1Eridust (Table2).Taken
at face value, this would mean that the range in dust tempera-
tures is very limited: single values lower than 50K or as high
as 100K can be excluded (without giving higher weight to the
Spitzer data, Tbbbecomes closer to 70K). Therefore, rbb, is de-
termined to better than within a factor of two. For unit filling
factor, the blackbody emitting regions would appear to be con-
fined to a very narrow ring-like structure (see Table2).
The fractional luminosity is 1 × 10−4and the emission is
optically thin. The blackbody fit also implies that the emitting
particles have sizes largely in excess of 100µm (2πa > λ) and
4 R. Liseau et al.: q1Eri: a solar-type star with a planet and a dust belt
that these grains have grey opacities in the infrared to submm,
i.e., κ ? κ(λ). Given the available evidence, it is not possible,
however, to tell the actual sizes of the particles or their absolute
Miyake & Nakagawa(1993),
properties of dust that produces small values of the opacity
index, and presented opacities over a broad range in frequency
and particle size. Maximum opacity, maxκ ∼ 2cm2g−1, was
found for the size amax = 1mm at ν = 1011.5Hz (λ ∼ 1mm)
and for larger particles, κ decreases rapidly (as ∼ 1/√amax).
This assumes compact spheres of density ρ = 1.18gcm−3
(well-mixed silicates and water ice) and being distributed in
size according to n(a) ∝ ap, with p = −3.5. The adopted
density is consistent with values determined for Kuiper Belt
objects (Grundy et al. 2007, and references therein). The value
of κ1mmis not strongly dependent on p, as long as −4 ≤ p ≤ −2
(Miyake & Nakagawa 1993). In general, these results are in
agreement with other work (e.g., Kr¨ ugel & Siebenmorgen1994;
Stognienko et al. 1995).
For this maximum value of κ, the 870µm flux density
yields a minimum mass Mdust = FνD2/κνBν(Tdust) ≥ 3 MMoon
(0.04M⊕, see Table2). This minimum mass is larger than the
’blackbody mass’ one would infer from the effective area of the
blackbody (6.85 × 1026cm2) and for the same a and ρ, but con-
sistent with the (re-scaled) result of Sch¨ utz et al. (2005).
The fact that the 870µm source appears linearly resolved,
speaks against the narrow ring scenario, and the existence of a
more massive and colder belt (T = 17K at r = 300AU, say)
can at present not be excluded (see Fig.2). The relative width of
such a cold belt would seem less implausible, viz. ∆r/r ≥ 0.2
and, hence, its physical width would be at least 60AU. Also,
the equilibrium temperature of the dust would be esssentially
constant. With the same parameters as before, the mass would
scale simply as the ratio of the temperatures, yielding 13 MMoon.
Of course, at shorter wavelengths, the spectrum would have to
be steeper than the blackbody SED, implying values of β > 0,
where β parameterizes the frequency dependence of the opacity,
i.e., κν ∝ νβ. Together with the 17K blackbody, the SED can
be fit with a modified 60K blackbody with β = 1.0 longward of
100µm (Fig.2).Thismodelwouldphysicallybe moreattractive,
but by its ad hoc nature would be of course not unique, and bet-
ter constraints on the physical parameters would require a better
sampling of the SED.
4.3. Relation to the planet q1Eri b
An index of the order of −3.5, used by Miyake & Nakagawa
(1993), could indicate that the size distribution resulted from a
collisional cascade (for a discussion, see Th´ ebault & Augereau
2007). The observed absence of small debris (a of the order of
1µm or smaller) in the q1Eri disk suggests that its production
has ceased and that it had diminished on a short time scale com-
pared to the age of the system. Remarkably, the density param-
eter of Wyatt (2005) has a value that, given the age of q1Eri,
is atypically large (η0 ∼ 1000 for the correct form of Eq.7),
which may mean that the observed absence of warm material
is caused by radiation pressure blow-out, rather than by the ac-
tion of a planet, hindering the migration inward toward the star.
Anyway, at 2AU distance, the Jupiter-size planet q1Erib could
hardly have had any influence on particles out to the innermost
edge at 25AU and on orbits far beyond that (Wyatt 2005). The
existence of this belt of large grains may point to the presence
of another major planet. It would therefore seem important to
verify or to disprove the existence of q1Eric.
Below, our main conclusions from this work are summarized:
• Observations of the solar-type star q1Eri and its planet
q1Erib at 870µm revealed a source with peak emission at
the position of the star. The source appears extended to the
LABOCA beam, i.e., elongated in roughly the East-West di-
rection(640AU) but essentially unresolvedin theperpendic-
• At an age exceeding 1Gyr, the fractional luminosity of the
infrared excess is very high (≥ 10−4). The entire SED of this
excess emission, extending from>
single-temperature blackbody (Tbb= 60K).
• This would imply that the emitting regions are located at
about 25AU from the star and in addition, very limited
in spatial extent (ring-like). Exhibiting a grey opacity over
the entire wavelength range, the emitting particles must be
large (>> 100µm). Using the theoretically derived maxi-
mum grain opacity for 1mm-size icy particles, we estimate
a minimum mass of the dust belt of 0.04M⊕.
• It seems highly unlikely that the planet q1Erib at 2AU
would be responsible for the clearing of the region from
small dust particles interior to 25AU, but may hint at the
existence of another planet.
• Takingtheobservedextentofthe870µm sourceintoaccount
leads to an emission model in which an outer cold (17K)
dust belt needs to be included. This belt would be centered
on the radial distance of 300AU and have a width of 60AU.
This belt is inclined at i ≥ 52◦, possibly viewed at an angle
close to edge-on.
oldstar-planetsystem q1Eri providesyetanotherexampleof
the large diversity of such disks.
∼20µm to 1mm, is fit by a
Acknowledgements. We are indebted to the staff at the APEXfacility in Sequitor
and at the telescope site for their enthusiastic and skillful support during our
observing run. The thoughtful comments by the referee were much appreciated.
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