121 research outputs found
The nearby population of M dwarfs with WISE: A search for warm circumstellar dust
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.Comment: Accepted for publication in A&A, 15 pages, 7 figure
Can a planet explain different cavity sizes for small & large dust grains in transition disks?
Dissimilarities in the spatial distribution of small (μm-size) and large (mm-size) dust grains at the cavity edge of transition disks have been recently pointed out and are now under debate. We obtained VLT/NACO near-IR polarimetric observations of SAO 206462 (HD 135344B). The disk around the star shows very complex structures, such as dips and spirals. We also find an inner cavity much smaller than what is inferred from sub-mm images. The interaction between disk and orbiting companion(s) may explain this discrepanc
Shadows and cavities in protoplanetary disks: HD163296, HD141569A, and HD150193A in polarized light
The morphological evolution of dusty disks around young (few Myr-old) stars
is pivotal to better understand planet formation. Since both dust grains and
the global disk geometry evolve on short timescale, high-resolution imaging of
a sample of objects may provide important hints towards such an evolution. We
enlarge the sample of protoplanetary disks imaged in polarized light with
high-resolution by observing the Herbig Ae/Be stars HD163296, HD141569A, and
HD150193A. We integrate our data with previous datasets to paint a larger
picture of their morphology. We report a weak detection of the disk around
HD163296 in both H and Ks band. The disk is resolved as a broken ring structure
with a significan surface brightness drop inward of 0.6 arcsec. No sign of
extended polarized emission is detected from the disk around HD141569A and
HD150193A. We propose that the absence of scattered light in the inner 0.6
arcsec around HD163296 and the non-detection of the disk around HD150193A may
be due to similar geometric factors. Since these disks are known to be flat or
only moderately flared, self-shadowing by the disk inner wall is the favored
explanation. We show that the polarized brightness of a number of disks is
indeed related to their flaring angle. Other scenarios (such as dust grain
growth or interaction with icy molecules) are also discussed. On the other
hand, the non-detection of HD141569A is consistent with previous datasets
revealing the presence of a huge cavity in the dusty disk.Comment: 10 pages, 5 figures; accepted by Astronomy & Astrophysic
Searching for young Jupiter analogs around AP Col: L-band high-contrast imaging of the closest pre-main sequence star
The nearby M-dwarf AP Col was recently identified by Riedel et al. 2011 as a
pre-main sequence star (age 12 - 50 Myr) situated only 8.4 pc from the Sun. The
combination of its youth, distance, and intrinsically low luminosity make it an
ideal target to search for extrasolar planets using direct imaging. We report
deep adaptive optics observations of AP Col taken with VLT/NACO and Keck/NIRC2
in the L-band. Using aggressive speckle suppression and background subtraction
techniques, we are able to rule out companions with mass m >= 0.5 - 1M_Jup for
projected separations a>4.5 AU, and m >= 2 M_Jup for projected separations as
small as 3 AU, assuming an age of 40 Myr using the COND theoretical
evolutionary models. Using a different set of models the mass limits increase
by a factor of ~2. The observations presented here are the deepest
mass-sensitivity limits yet achieved within 20 AU on a star with direct
imaging. While Doppler radial velocity surveys have shown that Jovian bodies
with close-in orbits are rare around M-dwarfs, gravitational microlensing
studies predict that ~17% of these stars host massive planets with orbital
separations of 1-10 AU. Sensitive high-contrast imaging observations, like
those presented here, will help to validate results from complementary
detection techniques by determining the frequency of gas giant planets on wide
orbits around M-dwarfs.Comment: Accepted for publication in ApJ, 6 pages text ApJ style (incl.
references), 4 figures, 1 tabl
Resolved images of the protoplanetary disk around HD 100546 with ALMA
The disk around the Herbig Ae/Be star HD 100546 has been extensively studied
and it is one of the systems for which there are observational indications of
ongoing and/or recent planet formation. However, up until now no resolved image
of the millimeter dust emission or the gas has been published. We present the
first resolved images of the disk around HD 100546 obtained in Band 7 with the
ALMA observatory. The CO (3-2) image reveals a gas disk that extends out to 350
au radius at the 3-sigma level. Surprisingly, the 870um dust continuum emission
is compact (radius <60 au) and asymmetric. The dust emission is well matched by
a truncated disk with outer radius of 50 au. The lack of
millimeter-sized particles outside the 60 au is consistent with radial drift of
particles of this size. The protoplanet candidate, identified in previous
high-contrast NACO/VLT L' observations, could be related to the sharp outer
edge of the millimeter-sized particles. Future higher angular resolution ALMA
observations are needed to determine the detailed properties of the millimeter
emission and the gas kinematics in the inner region (<2arcsec). Such
observations could also reveal the presence of a planet through the detection
of circumplanetary disk material.Comment: 6 pages, 4 figures. Accepted in ApJ
Small vs large dust grains in transitional disks: do different cavity sizes indicate a planet?
Transitional disks represent a short stage of the evolution of circumstellar
material. Studies of dust grains in these objects can provide pivotal
information on the mechanisms of planet formation. Dissimilarities in the
spatial distribution of small (micron-size) and large (millimeter-size) dust
grains have recently been pointed out. Constraints on the small dust grains can
be obtained by imaging the distribution of scattered light at near-infrared
wavelengths. We aim at resolving structures in the surface layer of
transitional disks (with particular emphasis on the inner 10 - 50 AU), thus
increasing the scarce sample of high resolution images of these objects. We
obtained VLT/NACO near-IR high-resolution polarimetric differential imaging
observations of SAO 206462 (HD135344B). This technique allows one to image the
polarized scattered light from the disk without any occulting mask and to reach
an inner working angle of 0.1''. A face-on disk is detected in H and Ks bands
between 0.1'' and 0.9''. No significant differences are seen between the H and
Ks images. In addition to the spiral arms, these new data allow us to resolve
for the first time an inner cavity for small dust grains. The cavity size
(about 28 AU) is much smaller than what is inferred for large dust grains from
(sub)mm observations (39 to 50 AU). The interaction between the disk and
potential orbiting companion(s) can explain both the spiral arm structure and
the discrepant cavity sizes for small and large dust grains. One planet may be
carving out the gas (and, thus, the small grains) at 28 AU, and generating a
pressure bump at larger radii (39 AU), which holds back the large grains. We
analytically estimate that, in this scenario, a single giant planet (with a
mass between 5 and 15 Jupiter masses) at 17 to 20 AU from the star is
consistent with the observed cavity sizes.Comment: 11 pages, 6 figures; accepted for publication in A&
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