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Observability of Debris Discs around M-stars
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Abstract
Debris discs are second generation dusty discs formed by collisions of planetesimals. Many debris discs have been found and resolved around hot and solar-type stars. However, only a handful have been discovered around M-stars, and the reasons for their paucity remain unclear. Here we check whether the sensitivity and wavelength coverage of present-day telescopes are simply unfavourable for detection of these discs or if they are truly rare. We approach this question by looking at the Herschel/DEBRIS survey that has searched for debris discs including M-type stars. Assuming that these cool-star discs are "similar" to those of the hotter stars in some sense (i.e., in terms of dust location, temperature, fractional luminosity, or mass), we check whether this survey should have found them. With our procedure we can reproduce the % detection rate of M-star debris discs of the DEBRIS survey, which implies that these discs can indeed be similar to discs around hotter stars and just avoid detection. We then apply this procedure to IRAM NIKA-2 and ALMA bands 3, 6 and 7 to predict possible detection rates and give recommendations for future observations. We do not favour observing with IRAM, since it leads to detection rates lower than for the DEBRIS survey, with 0.6%-4.5% for a 15 min observation. ALMA observations, with detection rates 0.9%-7.4%, do not offer a significant improvement either, and so we conclude that more sensitive far-infrared and single dish sub-millimetre telescopes are necessary to discover the missing population of M-star debris discs.
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Context. We present the second Gaia data release, Gaia DR2, consisting of astrometry, photometry, radial velocities, and information on astrophysical parameters and variability, for sources brighter than magnitude 21. In addition epoch astrometry and photometry are provided for a modest sample of minor planets in the solar system.
Aims. A summary of the contents of Gaia DR2 is presented, accompanied by a discussion on the differences with respect to Gaia DR1 and an overview of the main limitations which are still present in the survey. Recommendations are made on the responsible use of Gaia DR2 results.
Methods. The raw data collected with the Gaia instruments during the first 22 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium (DPAC) and turned into this second data release, which represents a major advance with respect to Gaia DR1 in terms of completeness, performance, and richness of the data products. Results. Gaia DR2 contains celestial positions and the apparent brightness in G for approximately 1.7 billion sources. For 1.3 billion of those sources, parallaxes and proper motions are in addition available. The sample of sources for which variability information is provided is expanded to 0.5 million stars. This data release contains four new elements: broad-band colour information in the form of the apparent brightness in the GBP (330–680 nm) and GRP (630–1050 nm) bands is available for 1.4 billion sources; median radial velocities for some 7 million sources are presented; for between 77 and 161 million sources estimates are provided of the stellar effective temperature, extinction, reddening, and radius and luminosity; and for a pre-selected list of 14 000 minor planets in the solar system epoch astrometry and photometry are presented. Finally, Gaia DR2 also represents a new materialisation of the celestial reference frame in the optical, the Gaia -CRF2, which is the first optical reference frame based solely on extragalactic sources. There are notable changes in the photometric system and the catalogue source list with respect to Gaia DR1, and we stress the need to consider the two data releases as independent. Conclusions. Gaia DR2 represents a major achievement for the Gaia mission, delivering on the long standing promise to provide parallaxes and proper motions for over 1 billion stars, and representing a first step in the availability of complementary radial velocity and source astrophysical information for a sample of stars in the Gaia survey which covers a very substantial fraction of the volume of our galaxy.
Resolved observations of millimetre-sized dust, tracing larger planetesimals, have pinpointed the location of 26 Edgeworth-Kuiper belt analogs. We report that a belt's distance R to its host star correlates with the star's luminosity , following with a low intrinsic scatter of 17%. Remarkably, our Edgeworth-Kuiper belt in the Solar System and the two CO snow lines imaged in protoplanetary disks lie close to this R- relation, suggestive of an intrinsic relationship between protoplanetary disk structures and belt locations. To test the effect of bias on the relation, we use a Monte Carlo approach and simulate uncorrelated model populations of belts. We find that observational bias could produce the slope and intercept of the R- relation, but is unable to reproduce its low scatter. We then repeat the simulation taking into account the collisional evolution of belts, following the steady state model that fits the belt population as observed through infrared excesses. This significantly improves the fit by lowering the scatter of the simulated R- relation; however, this scatter remains only marginally consistent with the one observed. The inability of observational bias and collisional evolution alone to reproduce the tight relationship between belt radius and stellar luminosity could indicate that planetesimal belts form at preferential locations within protoplanetary disks. The similar trend for CO snow line locations would then indicate that the formation of planetesimals and/or planets in the outer regions of planetary systems is linked to the volatility of their building blocks, as postulated by planet formation models.
The New IRAM KID Arrays 2 (NIKA2) consortium has just finished installing and commissioning a millimetre camera on the IRAM 30-m telescope. It is a dual-band camera operating with three frequency-multiplexed kilo-pixels arrays of lumped element kinetic inductance detectors (LEKID) cooled at 150 mK, designed to observe the intensity and polarisation of the sky at 260 and 150 GHz (1.15 and 2 mm). NIKA2 is today an IRAM resident instrument for millimetre astronomy, such as intracluster medium from intermediate to distant clusters and so for the follow-up of Planck satellite detected clusters, high redshift sources and quasars, early stages of star formation and nearby galaxies emission. We present an overview of the instrument performance as it has been evaluated at the end of the commissioning phase.
This paper presents a study of circumstellar debris around Sun-like stars using data from the Herschel DEBRIS Key Programme. DEBRIS is an unbiased survey comprising the nearest ~90 stars of each spectral type A-M. Analysis of the 275 F-K stars shows that excess emission from a debris disc was detected around 47 stars, giving a detection rate of 17.1+2.6-2.3 per cent, with lower rates for later spectral types. For each target a blackbody spectrum was fitted to the dust emission to determine its fractional luminosity and temperature. The derived under- lying distribution of fractional luminosity versus blackbody radius in the population showed that most detected discs are concentrated at f ~ 10^-5 and at temperatures corresponding to blackbody radii 7-40 AU, which scales to ~40 AU for realistic dust properties (similar to the current Kuiper belt). Two outlying populations are also evident; five stars have exceptionally bright emission ( f > 5x10^-5), and one has unusually hot dust < 4 AU. The excess emission distributions at all wavelengths were fitted with a steady-state evolution model, showing these are compatible with all stars being born with a narrow belt that then undergoes collisional grinding. However, the model cannot explain the hot dust systems - likely originating in transient events - and bright emission systems - arising potentially from atypically massive discs or recent stirring. The emission from the present-day Kuiper belt is predicted to be close to the median of the population, suggesting that half of stars have either depleted their Kuiper belts (similar to the Solar System), or had a lower planetesimal formation efficiency.
We present new analyses of ALMA 12-m and ACA observations at 233 GHz (1.3 mm) of the Proxima Centauri system with sensitivities of 9.5 and 47 Jy beam, respectively, taken from 2017 January 21 through 2017 April 25. These analyses reveal that the star underwent a significant flaring event during one of the ACA observations on 2017 March 24. The complete event lasted for approximately 1 minute and reached a peak flux density of mJy, nearly a factor of brighter than the star's quiescent emission. At the flare peak, the continuum emission is characterized by a steeply falling spectral index with frequency, with , and a lower limit on the fractional linear polarization of . Since the ACA observations do not show any quiescent excess emission, we conclude that there is no need to invoke the presence of a dust belt at AU. We also posit that the slight excess flux density of Jy observed in the 12-m observations compared to the photospheric flux density of Jy extrapolated from infrared wavelengths may be due to coronal heating from continual smaller flares, as is seen for AU Mic, another nearby, well-studied, M dwarf flare star. If this is true, then the need for warm dust at AU is also removed.
Debris disks are tenuous, dust-dominated disks commonly observed around stars over a wide range of ages. Those around main sequence stars are analogous to the Solar System's Kuiper Belt and Zodiacal light. The dust in debris disks is believed to be continuously regenerated, originating primarily with collisions of planetesimals. Observations of debris disks provide insight into the evolution of planetary systems; the composition of dust, comets, and planetesimals outside the Solar System; as well as placing constraints on the orbital architecture and potentially the masses of exoplanets that are not otherwise detectable. This review highlights recent advances in multiwavelength, high-resolution scattered light and thermal imaging that have revealed a complex and intricate diversity of structures in debris disks, and discusses how modeling methods are evolving with the breadth and depth of the available observations. Two rapidly advancing subfields highlighted in this review include observations of atomic and molecular gas around main sequence stars, and variations in emission from debris disks on very short (days to years) timescales, providing evidence of non-steady state collisional evolution particularly in young debris disks.
Context. Millimetre-wave continuum astronomy is today an indispensable tool for both general astrophysics studies (e.g. star formation, nearby galaxies) and cosmology (e.g. cosmic microwave background and high-redshift galaxies). General purpose, large-field-of-view instruments are needed to map the sky at intermediate angular scales not accessible by the high-resolution interferometers (e.g. ALMA in Chile, NOEMA in the French Alps) and by the coarse angular resolution space-borne or ground-based surveys (e.g. Planck , ACT, SPT). These instruments have to be installed at the focal plane of the largest single-dish telescopes, which are placed at high altitude on selected dry observing sites. In this context, we have constructed and deployed a three-thousand-pixel dual-band (150 GHz and 260 GHz, respectively 2 mm and 1.15 mm wavelengths) camera to image an instantaneous circular field-of-view of 6.5 arcmin in diameter, and configurable to map the linear polarisation at 260 GHz.
Aims. First, we are providing a detailed description of this instrument, named NIKA2 (New IRAM KID Arrays 2), in particular focussing on the cryogenics, optics, focal plane arrays based on Kinetic Inductance Detectors, and the readout electronics. The focal planes and part of the optics are cooled down to the nominal 150 mK operating temperature by means of an adhoc dilution refrigerator. Secondly, we are presenting the performance measured on the sky during the commissioning runs that took place between October 2015 and April 2017 at the 30-m IRAM telescope at Pico Veleta, near Granada (Spain).
Methods. We have targeted a number of astronomical sources. Starting from beam-maps on primary and secondary calibrators we have then gone to extended sources and faint objects. Both internal (electronic) and on-the-sky calibrations are applied. The general methods are described in the present paper.
Results. NIKA2 has been successfully deployed and commissioned, performing in-line with expectations. In particular, NIKA2 exhibits full width at half maximum angular resolutions of around 11 and 17.5 arcsec at respectively 260 and 150 GHz. The noise equivalent flux densities are, at these two respective frequencies, 33±2 and 8±1 mJy s 1/2 . A first successful science verification run was achieved in April 2017. The instrument is currently offered to the astronomy community and will remain available for at least the following ten years.
The disk mass is among the most important input parameter for every planet formation model to determine the number and masses of the planets that can form. We present an ALMA 887micron survey of the disk population around objects from 2 to 0.03Msun in the nearby 2Myr-old Chamaeleon I star-forming region. We detect thermal dust emission from 66 out of 93 disks, spatially resolve 34 of them, and identify two disks with large dust cavities of about 45AU in radius. Assuming isothermal and optically thin emission, we convert the 887micron flux densities into dust disk masses, hereafter Mdust. We find that the Mdust-Mstar relation is steeper than linear with power law indices 1.3-1.9, where the range reflects two extremes of the possible relation between the average dust temperature and stellar luminosity. By re-analyzing all millimeter data available for nearby regions in a self-consistent way, we show that the 1-3 Myr-old regions of Taurus, Lupus, and Chamaeleon I share the same Mdust-Mstar relation, while the 10Myr-old Upper Sco association has a steeper relation. Theoretical models of grain growth, drift, and fragmentation reproduce this trend and suggest that disks are in the fragmentation-limited regime. In this regime millimeter grains will be located closer in around lower-mass stars, a prediction that can be tested with deeper and higher spatial resolution ALMA observations.
We present the first images of four debris disks observed in scattered light
around the young (4--250 Myr old) M dwarfs TWA 7 and TWA 25, the K6 star HD
35650, and the G2 star HD 377. We obtained these images by reprocessing
archival Hubble Space Telescope NICMOS coronagraph data with modern
post-processing techniques as part of the Archival Legacy Investigation of
Circumstellar Environments (ALICE) program. All four disks appear faint and
compact compared with other debris disks resolved in scattered light. The disks
around TWA 25, HD 35650, and HD 377 appear very inclined, while TWA 7's disk is
viewed nearly face-on. The surface brightness of HD 35650's disk is strongly
asymmetric. These new detections raise the number of disks resolved in
scattered light around M and late-K stars from one (the AU Mic system) to four.
This new sample of resolved disks enables comparative studies of heretofore
scarce debris disks around low-mass stars relative to solar-type stars.
We have analysed 18 ALMA continuum maps in Bands 6 and 7, with rms down to
7.8Jy, to derive differential number counts down to 60Jy and
100Jy at 1.3 mm and 1.1 mm, respectively. The area
covered by the combined fields is at 1.1mm and at 1.3mm. We improved the source extraction method by
requiring that the dimension of the detected sources be consistent with the
beam size. This method enabled us to remove spurious detections that have
plagued the purity of the catalogues in previous studies. We detected 50 faint
sources with S/N3.5 down to 60Jy, hence improving the statistics by a
factor of four relative to previous studies. The inferred differential number
counts are at a 1.1 mm flux Jy, and at a 1.3
mm flux Jy. At the faintest flux limits,
i.e. 30Jy and 40Jy, we obtain upper limits on the differential number
counts of and , respectively. Our results provide a new
lower limit to CIB intensity of 17.2 at 1.1mm and of
12.9 at 1.3mm. Moreover, the flattening of the integrated
number counts at faint fluxes strongly suggests that we are probably close to
the CIB intensity. Our data imply that galaxies with SFR
certainly contribute less than 50% to the CIB while more than 50% of the CIB
must be produced by galaxies with . The differential
number counts are in nice agreement with recent semi-analytical models of
galaxy formation even as low as our faint fluxes. Consequently, this supports
the galaxy evolutionary scenarios and assumptions made in these models.
We present far-infrared and submillimeter maps from the Herschel Space
Observatory and the James Clerk Maxwell Telescope of the debris disk host star
AU Microscopii. Disk emission is detected at 70, 160, 250, 350, 450, 500 and
850 micron. The disk is resolved at 70, 160 and 450 micron. In addition to the
planetesimal belt, we detect thermal emission from AU Mic's halo for the first
time. In contrast to the scattered light images, no asymmetries are evident in
the disk. The fractional luminosity of the disk is and its
mm-grain dust mass is 0.01 MEarth (+/- 20%). We create a simple spatial model
that reconciles the disk SED as a blackbody of 53 +/- 2 K (a composite of 39
and 50 K components) and the presence of small (non-blackbody) grains which
populate the extended halo. The best fit model is consistent with the "birth
ring" model explored in earlier works, i.e., an edge-on dust belt extending
from 8.8-40 AU, but with an additional halo component with an
surface density profile extending to the limits of sensitivity (140 AU). We
confirm that AU Mic does not exert enough radiation force to blow out grains.
For stellar mass loss rates of 10-100x solar, compact (zero porosity) grains
can only be removed if they are very small, consistently with previous work, if
the porosity is 0.9, then grains approaching 0.1 micron can be removed via
corpuscular forces (i.e., the stellar wind).
The spatially resolved AU Mic debris disc is among the most famous and
best-studied debris discs. We aim at a comprehensive understanding of the dust
production and the dynamics of the disc objects with in depth collisional
modelling including stellar radiative and corpuscular forces. Our models are
compared to a suite of observational data for thermal and scattered light
emission, ranging from the ALMA radial surface brightness profile at 1.3mm to
polarisation measurements in the visible. Most of the data can be reproduced
with a planetesimal belt having an outer edge at around 40au and subsequent
inward transport of dust by stellar winds. A low dynamical excitation of the
planetesimals with eccentricities up to 0.03 is preferred. The radial width of
the planetesimal belt cannot be constrained tightly. Belts that are 5au and
17au wide, as well as a broad 44au-wide belt are consistent with observations.
All models show surface density profiles increasing with distance from the star
as inferred from observations. The best model is achieved by assuming a stellar
mass loss rate that exceeds the solar one by a factor of 50. While the SED and
the shape of the ALMA profile are well reproduced, the models deviate from the
scattered light data more strongly. The observations show a bluer disc colour
and a lower degree of polarisation for projected distances <40au than predicted
by the models. The problem may be mitigated by irregularly-shaped dust grains
which have scattering properties different from the Mie spheres used. From
tests with a handful of selected dust materials, we derive a preference for
mixtures of silicate, carbon, and ice of moderate porosity. We address the
origin of the unresolved central excess emission detected by ALMA and show that
it cannot stem from an additional inner belt alone. Instead, it should derive,
at least partly, from the chromosphere of the central star.
The precise mechanisms that provide the non-radiative energy for heating the
chromosphere and the corona of the Sun and those of other stars constitute an
active field of research. By studying stellar chromospheres one aims at
identifying the relevant physical processes. Defining the permittable extent of
the parameter space can also serve as a template for the Sun-as-a-star. Earlier
observations with Herschel and APEX have revealed the temperature minimum of
alpha Cen, but these were unable to spatially resolve the binary into
individual components. With the data reported here, we aim at remedying this
shortcoming. Furthermore, these earlier data were limited to the wavelength
region between 100 and 870mu. In the present context, we intend to extend the
spectral mapping to longer wavelengths, where the contrast between stellar
photospheric and chromospheric emission becomes increasingly evident. ALMA is
particularly suited to point sources, such as unresolved stars. ALMA provides
the means to achieve our objectives with both its high sensitivity of the
collecting area for the detection of weak signals and the high spatial
resolving power of its adaptable interferometer for imaging close multiple
stars. This is the first detection of main-sequence stars at a wavelength of
3mm. Furthermore, the individual components of the binary alpha CenAB are
clearly detected and spatially well resolved at all ALMA wavelengths. The high
S/N of these data permit accurate determination of their relative flux ratios.
The previously obtained flux ratio of 0.44, which was based on measurements in
the optical and at 70mu, is consistent with the present ALMA results, albeit
with a large error bar. Given the distinct difference in their cyclic activity,
the similarity of their submm SEDs appears surprising.
The Herschel DEBRIS (Disc Emission via a Bias-free Reconnaissance in the Infrared/Submillimetre) survey brings us a unique perspective
on the study of debris discs around main-sequence A-type stars. Bias-free by design, the survey offers a remarkable data set
with which to investigate the cold disc properties. The statistical analysis of the 100 and 160 μm data for 86 main-sequence
A stars yields a lower than previously found debris disc rate. Considering better than 3σ excess sources, we find a detection rate ≥24 ± 5 per cent
at 100 μm which is similar to the debris disc rate around main-sequence F/G/K-spectral type stars. While the 100 and 160 μm
excesses slowly decline with time, debris discs with large excesses are found around some of the oldest A stars in our sample,
evidence that the debris phenomenon can survive throughout the length of the main sequence (∼1 Gyr). Debris discs are predominantly
detected around the youngest and hottest stars in our sample. Stellar properties such as metallicity are found to have no
effect on the debris disc incidence. Debris discs are found around A stars in single systems and multiple systems at similar
rates. While tight and wide binaries (<1 and >100 au, respectively) host debris discs with a similar frequency and global
properties, no intermediate separation debris systems were detected in our sample.
Using the SDSS DR7 spectroscopic catalog, we searched the WISE AllWISE
catalog to investigate the occurrence of warm dust, as inferred from IR
excesses, around field M dwarfs (dMs). We developed SDSS/WISE color selection
criteria to identify 175 dMs (from 70,841) that show IR flux greater than
typical dM photosphere levels at 12 and/or 22 m, including seven new stars
within the Orion OB1 footprint. We characterize the dust populations inferred
from each IR excess, and investigate the possibility that these excesses could
arise from ultracool binary companions by modeling combined SEDs. Our observed
IR fluxes are greater than levels expected from ultracool companions
(). We also estimate that the probability the observed IR excesses
are due to chance alignments with extragalactic sources is 0.1%. Using SDSS
spectra we measure surface gravity dependent features (K, Na, and CaH 3), and
find 15% of our sample indicate low surface gravities. Examining tracers of
youth (H, UV fluxes, and Li absorption), we find 3% of our sample
appear young, indicating we are observing a population of field stars
1 Gyr, likely harboring circumstellar material. We investigate age-dependent
properties probed by this sample, studying the disk fraction as a function of
Galactic height. The fraction remains small and constant to pc,
and then drops, indicating little to no trend with age. Possible explanations
for disks around field dMs include: 1) collisions of planetary bodies, 2) tidal
disruption of planetary bodies, or 3) failed planet formation.
τ Ceti is a nearby, mature G-type star very similar to our Sun, with a massive Kuiper Belt analogue and possible multiplanet
system that has been compared to our Solar system. We present Herschel Space Observatory images of the debris disc, finding the disc is resolved at 70 μm and 160 μm, and marginally resolved at 250 μm. The Herschel images and infrared photometry from the literature are best modelled using a wide dust annulus with an inner edge between
1 and 10 au and an outer edge at ∼55 au, inclined from face-on by 35° ± 10°, and with no significant azimuthal structure.
We model the proposed tightly packed planetary system of five super-Earths and find that the innermost dynamically stable
disc orbits are consistent with the inner edge found by the observations. The photometric modelling, however, cannot rule
out a disc inner edge as close to the star as 1 au, though larger distances produce a better fit to the data. Dynamical modelling
shows that the five-planet system is stable with the addition of a Neptune or smaller mass planet on an orbit outside 5 au,
where the radial velocity data analysis would not have detected a planet of this mass.
(Abridged) The radii of debris disks and the sizes of their dust grains are
tracers of the formation mechanisms and physical processes operating in these
systems. We use a sample of 34 debris disks spatially resolved in various
Herschel programs to constrain them. While we modeled disks with both warm and
cold components, we focus our analysis only on the cold outer disks, i.e.
Kuiper-belt analogs. The disk radii derived from the resolved images reveal a
large dispersion, but no significant trend with the stellar luminosity, which
argues against ice lines as a dominant player in setting the debris disk sizes.
Fixing the disk radii to those inferred from the resolved images, we model the
spectral energy distributions to determine the dust temperatures and the grain
size distributions. While the dust temperature systematically increases towards
earlier spectral types, its ratio to the blackbody temperature at the disk
radius decreases with the stellar luminosity. This is explained by an increase
of typical grain sizes towards more luminous stars. The sizes are compared to
the radiation pressure blowout limit that is proportional to
the stellar luminosity-to-mass ratio and thus also increases towards earlier
spectral classes. The grain sizes in the disks of G- to A-stars are inferred to
be several times at all stellar luminosities, in agreement with
collisional models of debris disks. The sizes, measured in the units of
, appear to decrease with the luminosity, which may be
suggestive of the disk's stirring level increasing towards earlier-type stars.
Many low-mass pre-main-sequence stars exhibit strong magnetic activity and
coronal X-ray emission. Even after the primordial accretion disk has been
cleared out, the star's high-energy radiation continues to affect the formation
and evolution of dust, planetesimals, and large planets. Young stars with
debris disks are thus ideal environments for studying the earliest stages of
non-accretion-driven coronae. In this paper we simulate the corona of AU Mic, a
nearby active M dwarf with an edge-on debris disk. We apply a self-consistent
model of coronal loop heating that was derived from numerical simulations of
solar field-line tangling and magnetohydrodynamic turbulence. We also
synthesize the modeled star's X-ray luminosity and thermal radio/millimeter
continuum emission. A realistic set of parameter choices for AU Mic produces
simulated observations that agree with all existing measurements and upper
limits. This coronal model thus represents an alternative explanation for a
recently discovered ALMA central emission peak that was suggested to be the
result of an inner "asteroid belt" within 3 AU of the star. However, it is also
possible that the central 1.3 mm peak is caused by a combination of active
coronal emission and a bright inner source of dusty debris. Additional
observations of this source's spatial extent and spectral energy distribution
at millimeter and radio wavelengths will better constrain the relative
contributions of the proposed mechanisms.
We present 1.3 mm ALMA Cycle 0 observations of the edge-on debris disk around the nearby, ~10 Myr old, M-type star AU Mic. These observations obtain 06 (6 AU) resolution and reveal two distinct emission components: (1) the previously known dust belt that extends to a radius of 40 AU and (2) a newly recognized central peak that remains unresolved. The cold dust belt of mass ~1 M
Moon is resolved in the radial direction with a rising emission profile that peaks sharply at the location of the outer edge of the "birth ring" of planetesimals hypothesized to explain the midplane scattered light gradients. No significant asymmetries are discerned in the structure or position of this dust belt. The central peak identified in the ALMA image is ~6 times brighter than the stellar photosphere, which indicates an additional emission process in the inner regions of the system. Emission from a stellar corona or activity may contribute, but the observations show no signs of temporal variations characteristic of radio-wave flares. We suggest that this central component may be dominated by dust emission from an inner planetesimal belt of mass ~0.01 M
Moon, consistent with a lack of emission shortward of 25 μm and a location 3 AU from the star. Future millimeter observations can test this assertion, as an inner dust belt should be readily separated from the central star at higher angular resolution.
Debris disks have been found primarily around intermediate and solar mass
stars (spectral types A-K) but rarely around low mass M-type stars. We have
spatially resolved a debris disk around the remarkable M3-type star GJ581
hosting multiple planets using deep PACS images at 70, 100 and 160 microns as
part of the DEBRIS Program on the Herschel Space Observatory. This is the
second spatially resolved debris disk found around an M-type star, after the
one surrounding the young star AU Mic (12 Myr). However, GJ 581 is much older
(2-8 Gyr), and is X-ray quiet in the ROSAT data. We fit an axisymmetric model
of the disk to the three PACS images and found that the best fit model is for a
disk extending radially from 25+/-12 AU to more than 60 AU. Such a cold disk is
reminiscent of the Kuiper Belt but it surrounds a low mass star (0.3 M_sol) and
its fractional dust luminosity L_dust/L_* of \sim 10^-4 is much higher. The
inclination limits of the disk found in our analysis make the masses of the
planets small enough to ensure the long-term stability of the system according
to some dynamical simulations. The disk is collisionally dominated down to
submicron-sized grains and the dust cannot be expelled from the system by
radiation or wind pressures because of the low luminosity and low X-ray
luminosity of GJ581. We suggest that the correlation between low-mass planets
and debris disks recently found for G-type stars also applies to M-type
stars.Finally, the known planets, of low masses and orbiting within 0.3 AU from
the star, cannot dynamically perturb the disk over the age of the star,
suggesting that an additional planet exists at larger distance that is stirring
the disk to replenish the dust.
The majority of debris discs discovered so far have only been detected
through infrared excess emission above stellar photospheres. While disc
properties can be inferred from unresolved photometry alone under various
assumptions for the physical properties of dust grains, there is a degeneracy
between disc radius and dust temperature that depends on the grain size
distribution and optical properties. By resolving the disc we can measure the
actual location of the dust. The launch of Herschel, with an angular resolution
superior to previous far-infrared telescopes, allows us to spatially resolve
more discs and locate the dust directly. Here we present the nine resolved
discs around A stars between 20 and 40 pc observed by the DEBRIS survey. We use
these data to investigate the disc radii by fitting narrow ring models to
images at 70, 100 and 160 {\mu}m and by fitting blackbodies to full spectral
energy distributions. We do this with the aim of finding an improved way of
estimating disc radii for unresolved systems. The ratio between the resolved
and blackbody radii varies between 1 and 2.5. This ratio is inversely
correlated with luminosity and any remaining discrepancies are most likely
explained by differences to the minimum size of grain in the size distribution
or differences in composition. We find that three of the systems are well fit
by a narrow ring, two systems are borderline cases and the other four likely
require wider or multiple rings to fully explain the observations, reflecting
the diversity of planetary systems.
Debris disks around main-sequence stars are produced by the destruction of unseen parent bodies. AU Microscopii (GJ 803) is a compelling object to study in the context of disk evolution across different spectral types, as it is an M dwarf whose nearly edge-on disk may be directly compared to that of its A5 V sibling β Pic. We resolve the disk from 8-60 AU in the near-IR JHK' bands at high resolution with the Keck II Telescope and adaptive optics, and develop a data reduction technique for the removal of the stellar point-spread function. We measure a blue color across the near-IR bands, and confirm the presence of substructure in the inner disk. Some of the structural features exhibit wavelength-dependent positions. Recent measurements of the scattered-light polarization indicate the presence of porous grains. The scattering properties of these porous grains have a strong effect on the inferred structure of the disk relative to the majority of previously modeled grain types. Complementing prior work, we use a Monte Carlo radiative transfer code to compare a relatively simple model of the distribution of porous grains to a broad data set, simultaneously fitting midplane surface brightness profiles and the spectral energy distribution. Our model confirms that the large-scale architecture of the disk is consistent with detailed models of steady state grain dynamics. A belt of parent bodies from 35-40 AU produces dust that is then swept outward by stellar wind and radiation. We infer the presence of very small grains in the region exterior to the belt, down to sizes of ~0.05 μm. These sizes are consistent with stellar mass-loss rates * 102 ☉
We report 24 and/or 70 μm measurements of ~160 A-type main-sequence stars using the Multiband Imaging Photometer for Spitzer (MIPS). Their ages range from 5 to 850 Myr, based on estimates from the literature (cluster or moving group associations) or from the H-R diagram and isochrones. The thermal infrared excess is identified by comparing the deviation (~3% and ~15% at the 1 σ level at 24 and 70 μm, respectively) between the measurements and the synthetic Kurucz photospheric predictions. Stars showing excess infrared emission due to strong emission lines or extended nebulosity seen at 24 μm are excluded from our sample; therefore, the remaining infrared excesses are likely to arise from circumstellar debris disks. At the 3 σ confidence level, the excess rate at 24 and 70 μm is 32% and ≥33% (with an uncertainty of 5%), considerably higher than what has been found for old solar analogs and M dwarfs. Our measurements place constraints on the fractional dust luminosities and temperatures in the disks. We find that older stars tend to have lower fractional dust luminosity than younger ones. While the fractional luminosity from the excess infrared emission follows a general 1/t relationship, the values at a given stellar age vary by at least 2 orders of magnitude. We also find that (1) older stars possess a narrow range of temperature distribution peaking at colder temperatures, and (2) the disk emission at 70 μm persists longer than that at 24 μm. Both results suggest that the debris disk clearing process is more effective in the inner regions.
We have executed a survey of nearby, main-sequence A-, F-, and G-type stars with the CHARA Array, successfully measuring the angular diameters of forty-four stars with an average precision of ~1.5%. We present new measures of the bolometric flux, which in turn leads to an empirical determination of the effective temperature for the stars observed. In addition, these CHARA-determined temperatures, radii, and luminosities are fit to Yonsei-Yale model isochrones to constrain the masses and ages of the stars. These results are compared to indirect estimates of these quantities obtained by collecting photometry of the stars and applying them to model atmospheres and evolutionary isochrones. We find that for most cases, the models overestimate the effective temperature by ~1.5%-4% when compared to our directly measured values. The overestimated temperatures and underestimated radii in these works appear to cause an additional offset in the star's surface gravity measurements, which consequently yield higher masses and younger ages, in particular for stars with masses greater than ~1.3 M
☉. Additionally, we compare our measurements to a large sample of eclipsing binary stars, and excellent agreement is seen within both data sets. Finally, we present temperature relations with respect to (B – V) and (V – K) colors as well as spectral type, showing that calibration of effective temperatures with errors ~1% is now possible from interferometric angular diameters of stars.
Circumstellar debris disks are important because of their connection to
planetary systems. An efficient way to identify these systems is through their
infrared excess. Most studies so far concentrated on early-type or solar-type
stars, but less effort has gone into investigating M dwarfs.
We characterize the mid-infrared photometric behavior of M dwarfs and search
for infrared excess in nearby M dwarfs taken from the volume-limited RECONS
sample using data from the WISE satellite and the 2MASS catalog. Our sample
consists of 85 sources encompassing 103 M dwarfs. We derive empirical infrared
colors from these data and discuss their errors. Based on this, we check the
stars for infrared excess and discuss the minimum excess we would be able to
detect.
Other than the M8.5 dwarf SCR 1845-6357 A, where the excess is produced by a
known T6 companion, we detect no excesses in any of our sample stars. The
limits we derive for the 22um excess are slightly higher than the usual
detection limit of 10-15% for Spitzer studies, but including the [12]-[22]
color in our analysis allows us to derive tight constraints on the fractional
dust luminosity L_dust/L_star. We show that this result is consistent with M
dwarf excesses in the mid-inrared being as frequent as excesses around
earlier-type stars. The low detection rate could be an age effect. We also
present a tentative excess detection at 22um around the known cold debris disk
M dwarf AU Mic, which is not part of our statistical sample.
There is still no clear detection of a mid-infrared excess around any old
(>30 Myr) main-sequence M dwarf. It is unclear whether this is due to a
different dust evolution around M dwarfs or whether this is an age effect
combined with the diffculties involved in searching M dwarfs for infrared
excesses. A significantly larger sample of well-studied M dwarfs is required to
solve this question.
Fomalhaut is a young, nearby star that is suspected to harbor an infant
planetary system, interspersed with one or more belts of dusty debris. We
present far-infrared images obtained with the Herschel Space Observatory with
an angular resolution between 5.7 and 36.7 arcsec at wavelengths between 70 and
500 micrometer. The images show the main debris belt in great detail. Even at
high spatial resolution, the belt appears smooth. The region in between the
belt and the central star is not devoid of material; thermal emission is
observed here as well. Also at the location of the star, excess emission is
detected.
We use a dynamical model together with radiative-transfer tools to derive the
parameters of the debris disk. We include detailed models of the interaction of
the dust grains with radiation, for both the radiation pressure and the
temperature determination. Comparing these models to the spatially resolved
temperature information contained in the images allows us to place strong
constraints on the presence of grains that will be blown out of the system by
radiation pressure. We use this to derive the dynamical parameters of the
system.
The appearance of the belt points towards a remarkably active system in which
dust grains are produced at a very high rate by a collisional cascade in a
narrow region filled with dynamically excited planetesimals. Dust particles
with sizes below the blow-out size are abundantly present. The equivalent of
2000 one-km-sized comets are destroyed every day, out of a cometary reservoir
amounting to 110 Earth masses. From comparison of their scattering and thermal
properties, we find evidence that the dust grains are fluffy aggregates, which
indicates a cometary origin. The excess emission at the location of the star
may be produced by hot dust with a range of temperatures, but may also be due
to gaseous free-free emission from a stellar wind.
We have gathered a sample of 112 main-sequence stars with known debris disks.
We collected published information and performed adaptive optics observations
at Lick Observatory to determine if these debris disks are associated with
binary or multiple stars. We discovered a previously unknown M-star companion
to HD 1051 at a projected separation of 628 AU. We found that 25+/-4% of our
debris disk systems are binary or triple star systems, substantially less than
the expected ~50%. The period distribution for these suggests a relative lack
of systems with 1-100 AU separations. Only a few systems have blackbody disk
radii comparable to the binary/triple separation. Together, these two
characteristics suggest that binaries with intermediate separations of 1-100 AU
readily clear out their disks. We find that the fractional disk luminosity, as
a proxy for disk mass, is generally lower for multiple systems than for single
stars at any given age. Hence, for a binary to possess a disk (or form planets)
it must either be a very widely separated binary with disk particles orbiting a
single star or it must be a small separation binary with a circumbinary disk.
The vertical distribution of dust in debris disks is sensitive to the number and size of large planetesimals dynamically stirring the disk, and is therefore well-suited for constraining the prevalence of otherwise unobservable Uranus and Neptune analogs. Information regarding stirring bodies has previously been inferred from infrared and optical observations of debris disk vertical structure, but theoretical works predict that the small particles traced by short-wavelength observations will be puffed up by radiation pressure, yielding only upper limits. The large grains that dominate the disk emission at millimeter wavelengths are much less sensitive to the effects of stellar radiation or stellar winds, and therefore trace the underlying mass distribution more directly. Here we present ALMA 1.3 mm dust continuum observations of the debris disk around the nearby M star AU Mic. The 3 au spatial resolution of the observations, combined with the favorable edge-on geometry of the system, allows us to measure the vertical thickness of the disk. We report a scale height-to-radius aspect ratio of = -h 0.031+0.004 0.005 between radii of ∼23 au and ∼41 au. Comparing this aspect ratio to a theoretical model of size-dependent velocity distributions in the collisional cascade, we find that the perturbing bodies embedded in the local disk must be larger than about 400 km, and the largest perturbing body must be smaller than roughly 1.8M. These measurements rule out the presence of a gas giant or Neptune analog near the ∼40 au outer edge of the debris ring, but are suggestive of large planetesimals or an Earth-sized planet stirring the dust distribution. Key words: circumstellar matter; planet–disk interactions; planetary systems; stars: individual (AU Mic); submillimeter: planetary systems
Millimeter observations of CO gas in planetesimal belts show a high detection rate around A stars, but few detections for later type stars. We present the first CO detection in a planetesimal belt around an M star, TWA 7. The optically thin CO (J = 3-2) emission is colocated with previously identified dust emission from the belt, and the emission velocity structure is consistent with Keplerian rotation around the central star. The detected CO is not well shielded against photodissociation, and must thus be continuously replenished by gas release from exocomets within the belt. We analyze in detail the process of exocometary gas release and destruction around young M dwarfs and how this process compares to earlier type stars. Taking these differences into account, we find that CO generation through exocometary gas release naturally explains the increasing CO detection rates with stellar luminosity, mostly because the CO production rate from the collisional cascade is directly proportional to stellar luminosity. More luminous stars will therefore on average host more massive (and hence more easily detectable) exocometary CO disks, leading to the higher detection rates observed. The current CO detection rates are consistent with a ubiquitous release of exocometary gas in planetesimal belts, independent of spectral type. © 2019. The American Astronomical Society. All rights reserved.
Measurements in the infrared wavelength domain allow direct assessment of the physical state and energy balance of cool matter in space, enabling the detailed study of the processes that govern the formation and evolution of stars and planetary systems in galaxies over cosmic time. Previous infrared missions revealed a great deal about the obscured Universe, but were hampered by limited sensitivity.
SPICA takes the next step in infrared observational capability by combining a large 2.5-meter diameter telescope, cooled to below 8 K, with instruments employing ultra-sensitive detectors. A combination of passive cooling and mechanical coolers will be used to cool both the telescope and the instruments. With mechanical coolers the mission lifetime is not limited by the supply of cryogen. With the combination of low telescope background and instruments with state-of-the-art detectors SPICA provides a huge advance on the capabilities of previous missions.
SPICA instruments offer spectral resolving power ranging from R ~50 through 11 000 in the 17–230 μm domain and R ~28.000 spectroscopy between 12 and 18 μm. SPICA will provide efficient 30–37 μm broad band mapping, and small field spectroscopic and polarimetric imaging at 100, 200 and 350 μm. SPICA will provide infrared spectroscopy with an unprecedented sensitivity of ~5 × 10 ⁻²⁰ W m ⁻² (5σ/1 h)—over two orders of magnitude improvement over what earlier missions. This exceptional performance leap, will open entirely new domains in infrared astronomy; galaxy evolution and metal production over cosmic time, dust formation and evolution from very early epochs onwards, the formation history of planetary systems.
We present the results of a Herschel survey of 21 late-type stars that host planets discovered by the radial velocity technique. The aims were to discover new disks in these systems and to search for any correlation between planet presence and disk properties. In addition to the known disk around GJ 581, we report the discovery of two new disks, in the GJ 433 and GJ 649 systems. Our sample therefore yields a disk detection rate of 14%, higher than the detection rate of 1.2% among our control sample of DEBRIS M-type stars with 98% confidence. Further analysis however shows that the disk sensitivity in the control sample is about a factor of two lower in fractional luminosity than for our survey, lowering the significance of any correlation between planet presence and disk brightness below 98%. In terms of their specific architectures, the disk around GJ 433 lies at a radius somewhere between 1 and 30au. The disk around GJ 649 lies somewhere between 6 and 30au, but is marginally resolved and appears more consistent with an edge-on inclination. In both cases the disks probably lie well beyond where the known planets reside (0.06-1.1au), but the lack of radial velocity sensitivity at larger separations allows for unseen Saturn-mass planets to orbit out to 5au, and more massive planets beyond 5au. The layout of these M-type systems appears similar to Sun-like star + disk systems with low-mass planets.
Prior to the 1990s, speculations about the occurrence of planets around other stars were based only on planet formation theory, observations of circumstellar disks, and the knowledge that at least one seemingly ordinary star had managed to make a variety of different planets. Since then, Doppler and transit surveys have revealed the population of planets around other Sun-like stars, especially those with orbital periods shorter than a few years. Over the last decade these surveys have risen to new heights with Doppler spectrographs capable of 1 m/s precision, and space telescopes capable of detecting the transits of Earth-sized planets. This article is a brief introductory review of the knowledge of planet occurrence that has been gained from these surveys.
Proxima Centauri, the star closest to our Sun, is known to host at least one terrestrial planet candidate in a temperate orbit. Here we report the ALMA detection of the star at 1.3 mm wavelength and the discovery of a belt of dust orbiting around it at distances ranging between 1 and 4 au, approximately. Given the low luminosity of the Proxima Centauri star, we estimate a characteristic temperature of about 40 K for this dust, which might constitute the dust component of a small-scale analog to our solar system Kuiper belt. The estimated total mass, including dust and bodies up to 50 km in size, is of the order of 0.01 Earth masses, which is similar to that of the solar Kuiper belt. Our data also show a hint of warmer dust closer to the star. We also find signs of two additional features that might be associated with the Proxima Centauri system, which, however, still require further observations to be confirmed: an outer extremely cold (about 10 K) belt around the star at about 30 au, whose orbital plane is tilted about 45 degrees with respect to the plane of the sky; and additionally, we marginally detect a compact 1.3 mm emission source at a projected distance of about 1.2 arcsec from the star, whose nature is still unknown.
Recently revealed differences in planets around M dwarf vs. solar-type stars could arise from differences in their primordial disks, and surveys of T Tauri stars find a correlation between stellar mass and disk mass. "Minimum" disks have been reconstructed for the Solar System and solar-type stars and here this exercise is performed for M dwarfs using Kepler-detected planets. Distribution of planet mass between current orbits produces a disk with total mass of ~0.009Msun and a power-law profile with index 2.2. Disk reconstruction from the output of a forward model of planet formation indicates that the effect of detection bias on disk profile is slight and that the observed scatter in planet masses and semi-major axes is consistent with a universal disk profile. This nominal M dwarf disk is more centrally concentrated than those inferred around the solar-type stars observed by Kepler, and the mass surface density beyond 0.02 AU is sufficient for in situ accretion of planets as single embryos. The mass of refractory solids within 0.5 AU is 5.6Mearth compared to 4Mearth for solar-type stars, in contrast with the trend with total disk mass. The total solids beyond 0.5 AU is sufficient for the core of at least one giant planet.
We present a search for debris discs amongst M-dwarf members of nearby, young (5-150 Myr) moving groups (MGs) using infrared (IR) photometry, primarily from the Wide Infrared Survey Explorer (WISE). A catalogue of 100 MG M-dwarfs that have suitable WISE data is compiled and 19 of these are found to have significant IR excess emission at 22m. Our search is likely to be complete for discs where the ratio of flux from the disc to flux from the star . The spectral energy distributions are supplemented with 2MASS photometry and data at longer wavelengths and fitted with simple disc models to characterise the IR excesses. There is a bimodal distribution -- twelve targets have , corresponding to and are likely to be gas-rich, primordial discs. The remaining seven targets have () and include three objects with previously known or suspected debris discs and four new debris disc candidates that are all members of the Beta Pic MG. All of the IR excesses are identified in stars that are likely members of MGs with age Myr. The detected debris disc frequency falls from 13 to 5 per cent to per cent (at 95 per cent confidence) for objects younger or older than 30 Myr respectively. This provides evidence for the evolution of debris discs on this timescale and does not support models where the maximum of debris disc emission occurs much later in lower-mass stars.
Trends in the planet population with host star mass provide an avenue to
constrain planet formation theories. We derive the planet radius distribution
function for Kepler stars of different spectral types, sampling a range in host
star masses. We find that M dwarf stars have 3.5 times more small planets
(1.0-2.8 R_Earth) than main-sequence FGK stars, but two times fewer
Neptune-sized and larger planets (>2.8 R_Earth). We find no systematic trend in
the planet size distribution between spectral types F, G, and K to explain the
increasing occurrence rates. Taking into account the mass-radius relationship
and heavy-element mass of observed exoplanets, and assuming those are
independent of spectral type, we derive the inventory of the heavy-element mass
locked up in exoplanets at short orbits. The overall higher planet occurrence
rates around M stars are not consistent with the redistribution of the same
mass into more, smaller planets. At the orbital periods and planet radii where
Kepler observations are complete for all spectral types, the average
heavy-element mass locked up in exoplanets increases roughly inversely with
stellar mass from 4 M_Earth in F stars to 5 M_Earth in G and K stars to 7
M_Earth in M stars. This trend stands in stark contrast with observed
protoplanetary disk masses that decrease towards lower mass stars, and provides
a challenge for current planet formation models. Neither models of in situ
formation nor migration of fully-formed planets are consistent with these
results. Instead, these results are indicative of large-scale inward migration
of planetary building blocks --- either through type-I migration or radial
drift of dust grains --- that is more efficient for lower mass stars, but does
not result in significantly larger or smaller planets.
We present 11.7 μm observations of nine late-type dwarfs obtained at the Keck I 10 m telescope in 2002 December and 2003 April. Our targets were selected for their youth or apparent IRAS 12 μm excess. For all nine sources, excess infrared emission is not detected. We find that stellar wind drag can dominate the circumstellar grain removal and plausibly explain the dearth of M dwarf systems older than 10 Myr with currently detected infrared excesses. We predict that M dwarfs possess fractional infrared excesses on the order of LIR/L* ∼ 10-6 and that this may be detectable with future efforts. © 2005. The American Astronomical Society. All rights reserved.
We present an improved estimate of the occurrence rate of small planets
around small stars by searching the full four-year Kepler data set for
transiting planets using our own planet detection pipeline and conducting
transit injection and recovery simulations to empirically measure the search
completeness of our pipeline. We identified 157 planet candidates, including 2
objects that were not previously identified as Kepler Objects of Interest
(KOIs). We inspected all publicly available follow-up images, observing notes,
and centroid analyses, and corrected for the likelihood of false positives. We
evaluate the sensitivity of our detection pipeline on a star-by-star basis by
injecting 2000 transit signals in the light curve of each target star. For
periods shorter than 50 days, we found an occurrence rate of 0.57 (+0.06/-0.05)
Earth-size planets (1-1.5 Earth radii) and 0.51 (+0.07/-0.06) super-Earths
(1.5-2 Earth radii) per M dwarf. Within a conservatively defined habitable zone
based on the moist greenhouse inner limit and maximum greenhouse outer limit,
we estimate an occurrence rate of 0.18 (+0.18/-0.07) Earth-size planets and
0.11 (+0.10/-0.05) super-Earths per M dwarf habitable zone. Accounting for the
cooling effect of clouds by doubling the insolation limit at the inner edge of
the habitable zone results in a higher occurrence rate of 0.27 (+0.16/-0.09)
Earth-size planets and 0.25 (+0.11/- 0.07) super-Earths per M dwarf habitable
zone.
Far-infrared Herschel images of the Eridani system, seen at a fifth of the Sun's present age, resolve two belts of debris emission. Fits to the 160 μm PACS image yield radial spans for these belts of 12-16 and 54-68 AU. The south end of the outer belt is 10% brighter than the north end in the PACS+SPIRE images at 160, 250, and 350 μm, indicating a pericenter glow attributable to a planet "c." From this asymmetry and an upper bound on the offset of the belt center, this second planet should be mildly eccentric (ec 0.03-0.3). Compared to the asteroid and Kuiper Belts of the young Sun, the Eri belts are intermediate in brightness and more similar to each other, with up to 20 km sized collisional fragments in the inner belt totaling 5% of an Earth mass. This reservoir may feed the hot dust close to the star and could send many impactors through the Habitable Zone, especially if it is being perturbed by the suspected planet Eri b, at semi-major axis 3 AU.
Debris disks have been found primarily around intermediate and solar mass
stars (spectral types A-K), but rarely around low-mass M-type stars. This
scarcity of detections in M star surveys can be confronted with the predictions
of the steady state collisional evolution model. First, we determine the
parameters of the disk population evolved with this model and fit to the
distribution of the fractional dust luminosities measured in the surveys of A-
and FGK-type stars observed by the infrared satellite Spitzer. Thus, in our
approach, we stipulate that the initial disk mass distribution is bimodal and
that only high-mass collisionally-dominated disks are detected. The best
determined parameter is the diameter of the largest planetesimals in the
collisional cascade of the model, which ranges between 2 and 60 km,
consistently for disks around A- and FGK-type stars. Second, we assume that the
same disk population surrounds the M dwarfs that have been the subjects of
debris disk searches in the far-infrared with Spitzer and at submillimeter
wavelengths with radiotelescopes. We find, in the framework of our study, that
this disk population, which has been fit to the AFGK data, is still consistent
with the observed lack of disks around M dwarfs with Spitzer.
We report in this paper the direct interferometric measurement of the
angular diameter of five nearby Vega-like stars: α PsA, β
Leo, β Pic, ɛ Eri and τ Cet. The near-infrared (K and H
bands) observations were conducted at the VLTI during the commissioning
period with the VINCI instrument and three different baselines ranging
from 66 m to 140 m. The five stellar photospheres are resolved and we
derive their angular diameters with a 1 to 2% accuracy, except for
β Pic (14%). We discuss the detectability and the influence of a
possible small amount of warm circumstellar dust on our measurements. In
addition, we have used the stellar evolution code CESAM (Morel
\cite{m97}) to compare the computed fundamental parameters to the
observed values (linear diameter, luminosity, temperature and chemical
abundance). As a result of the simulation, the age of the stars is
inferred and found to be in good agreement with previous estimates from
various other methods.
Fomalhaut is one of the most interesting and well-studied nearby stars, hosting at least one planet, a spectacular debris
ring and two distant low-mass stellar companions (TW PsA and LP 876−10, a.k.a. Fomalhaut B and C). We observed both companions
with Herschel, and while no disc was detected around the secondary, TW PsA, we have discovered the second debris disc in the Fomalhaut
system, around LP 876−10. This detection is only the second case of two debris discs seen in a multiple system, both of which
are relatively wide (≳3000 au for HD 223352/40 and 158 kau [0.77 pc] for Fomalhaut/LP 876−10). The disc is cool (24 K) and
relatively bright, with a fractional luminosity Ldisc/L⋆ = 1.2 × 10−4, and represents the rare observation of a debris disc around an M dwarf. Further work should attempt to find if the presence
of two discs in the Fomalhaut system is coincidental, perhaps simply due to the relatively young system age of 440 Myr, or
if the stellar components have dynamically interacted and the system is even more complex than it currently appears.
Motivated by recent observations of short-timescale variations in the
infrared emission of circumstellar disks, we propose that coronal mass
ejections can remove dust grains on timescales as short as a few days.
Continuous monitoring of stellar activity, coupled to infrared observations,
can place meaningful constraints on the proposed mechanism.
We report the mid- and far-infrared properties of nearby M dwarfs. Spitzer MIPS measurements were obtained for a sample of 62 stars at 24 μm, with subsamples of 41 and 20 stars observed at 70 and 160 μm, respectively. We compare the results with current models of M star photospheres and look for indications of circumstellar dust in the form of significant deviations of K-[24 μm] colors and 70 μm/24 μm flux ratios from the average M star values. At 24 μm, all 62 of the targets were detected; 70 μm detections were achieved for 20 targets in the subsample observed, and no detections were seen in the 160 μm subsample. No clear far-infrared excesses were detected in our sample. The average far-infrared excess relative to the photospheric emission of the M stars is at least 4 times smaller than the similar average for a sample of solar-type stars. However, this limit allows the average fractional infrared luminosity in the M-star sample to be similar to that for more massive stars. We have also set low limits (10-4 to 10-9 M⊕ depending on location) for the maximum mass of dust possible around our stars.
We present resolved Herschel images of circumbinary debris disks in the alpha
CrB (HD139006) and beta Tri (HD13161) systems. We find that both disks are
consistent with being aligned with the binary orbital planes. Though secular
perturbations from the binary can align the disk, in both cases the alignment
time at the distances at which the disk is resolved is greater than the stellar
age, so we conclude that the coplanarity was primordial. Neither disk can be
modelled as a narrow ring, requiring extended radial distributions. To satisfy
both the Herschel and mid-IR images of the alpha CrB disk, we construct a model
that extends from 1-300AU, whose radial profile is broadly consistent with a
picture where planetesimal collisions are excited by secular perturbations from
the binary. However, this model is also consistent with stirring by other
mechanisms, such as the formation of Pluto-sized objects. The beta Tri disk
model extends from 50-400AU. A model with depleted (rather than empty) inner
regions also reproduces the observations and is consistent with binary and
other stirring mechanisms. As part of the modelling process, we find that the
Herschel PACS beam varies by as much as 10% at 70um and a few % at 100um. The
70um variation can therefore hinder image interpretation, particularly for
poorly resolved objects. The number of systems in which circumbinary debris
disk orientations have been compared with the binary plane is now four. More
systems are needed, but a picture in which disks around very close binaries
(alpha CrB, beta Tri, and HD 98800, with periods of a few weeks to a year) are
aligned, and disks around wider binaries (99 Her, with a 50 yr period) are
misaligned, may be emerging. This picture is qualitatively consistent with the
expectation that the protoplanetary disks from which the debris emerged are
more likely to be aligned if their binaries have shorter periods.
Spectral Energy Distribution (SED) fitting in the far-infrared (FIR) is
greatly limited by a dearth of data and an excess of free parameters - from
galaxies' dust composition, temperature, mass, orientation, opacity, to heating
from AGN. This paper presents a simple FIR SED fitting technique joining a
modified, single dust temperature greybody, representing the reprocessed
starburst emission in the whole galaxy, to a mid-infrared powerlaw, which
approximates hot-dust emission from AGN heating or clumpy, hot starbursting
regions. This FIR SED can be used to measure infrared luminosities, dust
temperatures and dust masses for both local and high-z galaxies with 3 to 10+
FIR photometric measurements.
This fitting method is compared to infrared template SEDs in the literature
using photometric data on 65 local luminous and ultraluminous infrared
galaxies, (U)LIRGs. Despite relying only on 2-4 free parameters, the coupled
greybody/powerlaw SED fitting described here produces better fits to
photometric measurements than best-fit literature template SEDs (with residuals
a factor of ~2 lower). A mean emissivity index of beta=1.60+-0.38 and
mid-infrared powerlaw slope of alpha=2.0+-0.5 is measured; the former agrees
with the widely presumed emissivity index of beta=1.5 and the latter is
indicative of an optically-thin dust medium with a shallow radial density
profile, ~r^-0.5. Adopting characteristic dust temperature as the inverse
wavelength where the SED peaks, dust temperatures ~25-45K are measured for
local (U)LIRGs, ~5-15K colder than previous estimates using only simple
greybodies. This comparative study highlights the impact of SED fitting
assumptions on the measurement of physical properties such as infrared
luminosity (and thereby infrared-based star formation rate), dust temperature
and dust mass, for both local and high-redshift galaxies. [abridged]
Model calculations of circumsolar dust are conducted taking into account the increased ion drag due to the interaction of F-coronal dust with CMEs inside 8Rs. The choice of 8Rs is not arbitrary, after considering the severe plasma environment inside this region. Dust particles are allowed to spiral inward towards the Sun via the Poynting-Robertson drag, and the characteristics of the CMEs are applied in order to numerically compute the increased ion drag on F-coronal dust. The results show that the spiraling time would roughly be cut in half for dust particles inside 8Rs. Differences in the spiraling time due to the dependence of the magnitude of increased ion drag on dust particle size, creates separation in the heliocentric distance of the orbits of small (> 10 μm) vs. larger particles. This result could create conditions where a dust ring or rings may appear as a transient feature, which would explain former citing of such dust rings by observers and their absence by others.
In previous works of this series, we have shown that late B- and early A-type
stars have genuine bimodal distributions of rotational velocities and that late
A-type stars lack slow rotators. The distributions of the surface angular
velocity ratio \Omega/\Omega_crit (\Omega_crit is the critical angular
velocity) have peculiar shapes according to spectral type groups, which can be
caused by evolutionary properties. We aim to review the properties of these
rotational velocity distributions in some detail as a function of stellar mass
and age. We have gathered v sin i for a sample of 2014 B6- to F2-type stars. We
have determined the masses and ages for these objects with stellar evolution
models. The (Teff, log L/Lsun)-parameters were determined from the uvby-\beta
photometry and the HIPPARCOS parallaxes. The velocity distributions show two
regimes that depend on the stellar mass. Stars less massive than 2.5 Msun have
a unimodal equatorial velocity distribution and show a monotonical acceleration
with age on the main sequence (MS). Stars more massive have a bimodal
equatorial velocity distribution. Contrarily to theoretical predictions, the
equatorial velocities of stars from about 1.7 Msun to 3.2 Msun undergo a strong
acceleration in the first third of the MS evolutionary phase, while in the last
third of the MS they evolve roughly as if there were no angular momentum
redistribution in the external stellar layers. The studied stars might start in
the ZAMS not necessarily as rigid rotators, but with a total angular momentum
lower than the critical one of rigid rotators. The stars seem to evolve as
differential rotators all the way of their MS life span and the variation of
the observed rotational velocities proceeds with characteristic time scales
\delta(t)\sim 0.2 t_MS, where t_MS is the time spent by a star in the MS.
(Abridged) Searching for planets around stars with different masses probes
the outcome of planetary formation for different initial conditions. This
drives observations of a sample of 102 southern nearby M dwarfs, using a
fraction of our guaranteed time on the ESO/HARPS spectrograph (Feb. 11th, 2003
to Apr. 1st 2009). This paper makes available the sample's time series,
presents their precision and variability. We apply systematic searches and
diagnostics to discriminate whether the observed Doppler shifts are caused by
stellar surface inhomogeneities or by the radial pull of orbiting planets. We
recover the planetary signals corresponding to 9 planets already announced by
our group (Gl176b, Gl581b, c, d & e, Gl674b, Gl433b, Gl 667Cb and c). We
present radial velocities that confirm GJ 849 hosts a Jupiter-mass planet, plus
a long-term radial-velocity variation. We also present RVs that precise the
planetary mass and period of Gl 832b. We detect long-term RV changes for Gl
367, Gl 680 and Gl 880 betraying yet unknown long-period companions. We
identify candidate signals in the radial-velocity time series and demonstrate
they are most probably caused by stellar surface inhomogeneities. Finally, we
derive a first estimate of the occurrence of M-dwarf planets as a function of
their minimum mass and orbital period. In particular, we find that giant
planets (m sin i = 100-1,000 Mearth) have a low frequency (e.g. f<1% for P=1-10
d and f=0.02^{+0.03}_{-0.01} for P=10-100 d), whereas super-Earths (m sin i =
1-10 Mearth) are likely very abundant (f=0.36^{+0.25}_{-0.10} for P=1-10 d and
f=0.35^{+0.45}_{-0.11} for P=10-100 d). We also obtained
eta_earth=0.41^{+0.54}_{-0.13}, the frequency of habitable planets orbiting M
dwarfs (1<m sin i<10 Mearth). For the first time, eta_earth is a direct measure
and not a number extrapolated from the statistic of more massive and/or
shorter-period planets.
We present the Extended Hipparcos Compilation (XHIP), a database of all stars
in the New Reduction of the Hipparcos Catalog extensively cross-referenced with
data from a broad survey of presently available sources. The resulting
collection uniquely assigns 116,096 spectral classifications, 46,392 radial
velocities, and 18,549 homogenized iron abundances [Fe/H] to Hipparcos stars.
Stellar classifications from SIMBAD, indications of multiplicity from CCDM or
WDS, stellar ages from the Geneva-Copenhagen Survey III, supplemental
photometry from 2MASS and SIMBAD, and identifications of exoplanet host stars
are also included. Parameters for solar encounters and Galactic orbits are
calculated for a kinematically complete subset. Kinetic bias is found to be
minimal. Our compilation is available through the Centre de Donn\'ees
astronomiques de Strasbourg (CDS) as Catalog V/137B.