449 research outputs found
LkH 330: Evidence for dust clearing through resolved submillimeter imaging
Mid-infrared spectrophotometric observations have revealed a small sub-class
of circumstellar disks with spectral energy distributions (SEDs) suggestive of
large inner gaps with low dust content. However, such data provide only an
indirect and model dependent method of finding central holes. We present here
the direct characterization of a 40 AU radius inner gap in the disk around LkHa
330 through 340 GHz (880 micron) dust continuum imaging with the Submillimeter
Array (SMA). This large gap is fully resolved by the SMA observations and
mostly empty of dust with less than 1.3 x 10^-6 M_solar of solid particles
inside of 40 AU. Gas (as traced by accretion markers and CO M-band emission) is
still present in the inner disk and the outer edge of the gap rises steeply --
features in better agreement with the underlying cause being gravitational
perturbation than a more gradual process such as grain growth. Importantly, the
good agreement of the spatially resolved data and spectrophometry-based model
lends confidence to current interpretations of SEDs with significant dust
emission deficits as arising from disks with inner gaps or holes. Further
SED-based searches can therefore be expected to yield numerous additional
candidates that can be examined at high spatial resolution.Comment: 11 pages, 3 figures, accepted to ApJ
Empirical Constraints on Turbulence in Protoplanetary Accretion Disks
We present arcsecond-scale Submillimeter Array observations of the CO(3-2)
line emission from the disks around the young stars HD 163296 and TW Hya at a
spectral resolution of 44 m/s. These observations probe below the ~100 m/s
turbulent linewidth inferred from lower-resolution observations, and allow us
to place constraints on the turbulent linewidth in the disk atmospheres. We
reproduce the observed CO(3-2) emission using two physical models of disk
structure: (1) a power-law temperature distribution with a tapered density
distribution following a simple functional form for an evolving accretion disk,
and (2) the radiative transfer models developed by D'Alessio et al. that can
reproduce the dust emission probed by the spectral energy distribution. Both
types of models yield a low upper limit on the turbulent linewidth (Doppler
b-parameter) in the TW Hya system (<40 m/s), and a tentative (3-sigma)
detection of a ~300 m/s turbulent linewidth in the upper layers of the HD
163296 disk. These correspond to roughly <10% and 40% of the sound speed at
size scales commensurate with the resolution of the data. The derived
linewidths imply a turbulent viscosity coefficient, alpha, of order 0.01 and
provide observational support for theoretical predictions of subsonic
turbulence in protoplanetary accretion disks.Comment: 18 pages, 9 figures, accepted for publication in Ap
ALMA observations of the molecular gas in the debris disk of the 30 Myr old star HD 21997
The 30 Myr old A3-type star HD 21997 is one of the two known debris dust
disks having a measurable amount of cold molecular gas. With the goal of
understanding the physical state, origin, and evolution of the gas in young
debris disks, we obtained CO line observations with the Atacama Large
Millimeter/submillimeter Array (ALMA). Here we report on the detection of 12CO
and 13CO in the J=2-1 and J=3-2 transitions and C18O in the J=2-1 line. The gas
exhibits a Keplerian velocity curve, one of the few direct measurements of
Keplerian rotation in young debris disks. The measured CO brightness
distribution could be reproduced by a simple star+disk system, whose parameters
are r_in < 26 AU, r_out = 138 +/- 20 AU, M_*=1.8 +0.5 -0.2 M_Sun, and i = 32.6
+/- 3.1 degrees. The total CO mass, as calculated from the optically thin C18O
line, is about (4-8) x 10^-2 M_Earth, while the CO line ratios suggest a
radiation temperature on the order of 6-9 K. Comparing our results with those
obtained for the dust component of the HD 21997 disk from the ALMA continuum
observations by Mo\'or et al., we conclude that comparable amounts of CO gas
and dust are present in the disk. Interestingly, the gas and dust in the HD
21997 system are not co-located, indicating a dust-free inner gas disk within
55 AU of the star. We explore two possible scenarios for the origin of the gas.
A secondary origin, which involves gas production from colliding or active
planetesimals, would require unreasonably high gas production rates and would
not explain why the gas and dust are not co-located. We propose that HD 21997
is a hybrid system where secondary debris dust and primordial gas coexist. HD
21997, whose age exceeds both the model predictions for disk clearing and the
ages of the oldest T Tauri-like or transitional gas disks in the literature,
may be a key object linking the primordial and the debris phases of disk
evolution.Comment: 8 pages, 4 figures, accepted for publication in Ap
Stringent Limits on the Polarized Submillimeter Emission from Protoplanetary Disks
We present arcsecond-resolution Submillimeter Array (SMA) polarimetric
observations of the 880 um continuum emission from the protoplanetary disks
around two nearby stars, HD 163296 and TW Hydrae. Although previous
observations and theoretical work have suggested that a 2-3% polarization
fraction should be common for the millimeter continuum emission from such
disks, we detect no polarized continuum emission above a 3-sigma upper limit of
7 mJy in each arcsecond-scale beam, or <1% in integrated continuum emission. We
compare the SMA upper limits with the predictions from the exploratory Cho &
Lazarian (2007) model of polarized emission from T Tauri disks threaded by
toroidal magnetic fields, and rule out their fiducial model at the ~10-sigma
level. We explore some potential causes for this discrepancy, focusing on model
parameters that describe the shape, magnetic field alignment, and size
distribution of grains in the disk. We also investigate related effects like
the magnetic field strength and geometry, scattering off of large grains, and
the efficiency of grain alignment, including recent advances in grain alignment
theory, which are not considered in the fiducial model. We discuss the impact
each parameter would have on the data and determine that the suppression of
polarized emission plausibly arises from rounding of large grains, reduced
efficiency of grain alignment with the magnetic field, and/or some degree of
magnetic field tangling (perhaps due to turbulence). A poloidal magnetic field
geometry could also reduce the polarization signal, particularly for a face-on
viewing geometry like the TW Hya disk. The data provided here offer the most
stringent limits to date on the polarized millimeter-wavelength emission from
disks around young stars.Comment: 15 pages, 6 figures, accepted for publication in Ap
Planet gaps in the dust layer of 3D protoplanetary disks. II. Observability with ALMA
[Abridged] Aims: We provide predictions for ALMA observations of planet gaps
that account for the specific spatial distribution of dust that results from
consistent gas+dust dynamics. Methods: In a previous work, we ran full 3D,
two-fluid Smoothed Particle Hydrodynamics (SPH) simulations of a planet
embedded in a gas+dust T Tauri disk for different planet masses and grain
sizes. In this work, the resulting dust distributions are passed to the Monte
Carlo radiative transfer code MCFOST to construct synthetic images in the ALMA
wavebands. We then use the ALMA simulator to produce images that include
thermal and phase noise for a range of angular resolutions, wavelengths, and
integration times, as well as for different inclinations, declinations and
distances. We also produce images which assume that gas and dust are well mixed
with a gas-to-dust ratio of 100 to compare with previous ALMA predictions, all
made under this hypothesis. Results: Our findings clearly demonstrate the
importance of correctly incorporating the dust dynamics. We show that the gap
carved by a 1 M_J planet orbiting at 40 AU is visible with a much higher
contrast than the well-mixed assumption would predict. In the case of a 5 M_J
planet, we clearly see a deficit in dust emission in the inner disk, and point
out the risk of interpreting the resulting image as that of a transition disk
with an inner hole if observed in unfavorable conditions. Planet signatures are
fainter in more distant disks but declination or inclination to the
line-of-sight have little effect on ALMA's ability to resolve the gaps.
Conclusions: ALMA has the potential to see signposts of planets in disks of
nearby star-forming regions. We present optimized observing parameters to
detect them in the case of 1 and 5 M_J planets on 40 AU orbits.Comment: 15 pages, 21 figures, accepted by Astronomy & Astrophysics, a higher
resolution version of the paper is available at
http://www-obs.univ-lyon1.fr/labo/perso/jean-francois.gonzalez/Papers/Gaps_ALMA.pd
ALMA Hints at the Presence of two Companions in the Disk around HD 100546
HD 100546 is a well-studied Herbig Be star-disk system that likely hosts a close-in companion with compelling observational evidence for an embedded protoplanet at 68 AU. We present Atacama Large Millimeter/Submillimeter Array observations of the HD 100546 disk which resolve the gas and dust structure at (sub)millimeter wavelengths. The CO emission (at 345.795 GHz) originates from an extensive molecular disk (390 ± 20 AU in radius) whereas the continuum emission is more compact (230 ± 20 AU in radius), suggesting radial drift of the millimeter-sized grains. The CO emission is similar in extent to scattered light images indicating well-mixed gas and micrometer-sized grains in the disk atmosphere. Assuming azimuthal symmetry, a single-component power-law model cannot reproduce the continuum visibilities. The visibilities and images are better reproduced by a double-component model: a compact ring with a width of 21 AU centered at 26 AU and an outer ring with a width of 75 ± 3 AU centered at 190 ± 3 AU. The influence of a companion and protoplanet on the dust evolution is investigated. The companion at 10 AU facilitates the accumulation of millimeter-sized grains within a compact ring, ≈20-30 AU, by ≈10 Myr. The injection of a protoplanet at 1 Myr hastens the ring formation (≈1.2 Myr) and also triggers the development of an outer ring (≈100-200 AU). These observations provide additional evidence for the presence of a close-in companion and hint at dynamical clearing by a protoplanet in the outer disk
Spatially extended PAHs in circumstellar disks around T Tauri and Herbig Ae stars
Our aim is to determine the presence and location of the emission from
polycyclic aromatic hydrocarbons (PAHs) towards low and intermediate mass young
stars with disks using large aperture telescopes.
VLT-VISIR N-band spectra and VLT-ISAAC and VLT-NACO L-band spectra of 29
sources are presented, spectrally resolving the 3.3, 8.6, 11.2, and 12.6 micron
PAH features. Spatial-extent profiles of the features and the continuum
emission are derived and used to associate the PAH emission with the disks. The
results are discussed in the context of recent PAH-emission disk models.
The 3.3, 8.6, and 11.2 micron PAH features are detected toward a small
fraction of the T Tauri stars, with typical upper limits between 1E-15 and
5E-17 W/m^2. All 11.2 micron detections from a previous Spitzer survey are
confirmed with (tentative) 3.3 micron detections, and both the 8.6 and the 11.2
micron features are detected in all PAH sources. For 6 detections, the spatial
extent of the PAH features is confined to scales typically smaller than
0.12-0.34'', consistent with the radii of 12-60 AU disks at their distances
(typically 150 pc). For 3 additional sources, WL 16, HD 100546, and TY CrA, one
or more of the PAH features are more extended than the hot dust continuum of
the disk, whereas for Oph IRS 48, the size of the resolved PAH emission is
confirmed as smaller than for the large grains. For HD 100546, the 3.3 micron
emission is confined to a small radial extent of 12 +- 3 AU, most likely
associated with the outer rim of the gap in this disk. Gaps with radii out to
10-30 AU may also affect the observed PAH extent for other sources. For both
Herbig Ae and T Tauri stars, the small measured extents of the 8.6 and 11.2
micron features are consistent with larger (>= 100 carbon atoms) PAHs.Comment: 14 pages, 17 figures, accepted for publication in A&
Searching for a link between the magnetic nature and other observed properties of Herbig Ae/Be stars and stars with debris disks
Among the 21 Herbig Ae/Be stars studied, new detections of a magnetic field
were achieved in six stars. For three Herbig Ae/Be stars, we confirm previous
magnetic field detections. The largest longitudinal magnetic field, =
-454+-42G, was detected in the Herbig Ae/Be star HD101412 using hydrogen lines.
No field detection at a significance level of 3sigma was achieved in stars with
debris disks. Our study does not indicate any correlation of the strength of
the longitudinal magnetic field with disk orientation, disk geometry, or the
presence of a companion. We also do not see any simple dependence on the
mass-accretion rate. However, it is likely that the range of observed field
values qualitatively supports the expectations from magnetospheric accretion
models giving support for dipole-like field geometries. Both the magnetic field
strength and the X-ray emission show hints for a decline with age in the range
of ~2-14Myrs probed by our sample supporting a dynamo mechanism that decays
with age. However, our study of rotation does not show any obvious trend of the
strength of the longitudinal magnetic field with rotation period. Furthermore,
the stars seem to obey the universal power-law relation between magnetic flux
and X-ray luminosity established for the Sun and main-sequence active dwarf
stars.Comment: 21 pages, 16 figures, 7 tables, accepted for publication in A&
The surprisingly low carbon mass in the debris disk around HD 32297
Gas has been detected in a number of debris disks. It is likely secondary,
i.e. produced by colliding solids. Here, we report ALMA Band 8 observations of
neutral carbon in the CO-rich debris disk around the 15--30 Myr old A-type star
HD 32297. We find that C is located in a ring at 110 au with a FWHM
of 80 au, and has a mass of M.
Naively, such a surprisingly small mass can be accumulated from CO
photo-dissociation in a time as short as 10 yr. We develop a simple
model for gas production and destruction in this system, properly accounting
for CO self-shielding and shielding by neutral carbon, and introducing a
removal mechanism for carbon gas. We find that the most likely scenario to
explain both C and CO observations, is one where the carbon gas is rapidly
removed on a timescale of order a thousand years and the system maintains a
very high CO production rate of 15 M Myr, much higher
than the rate of dust grind-down. We propose a possible scenario to meet these
peculiar conditions: the capture of carbon onto dust grains, followed by rapid
CO re-formation and re-release. In steady state, CO would continuously be
recycled, producing a CO-rich gas ring that shows no appreciable spreading over
time. This picture might be extended to explain other gas-rich debris disks.Comment: accepted for publication in the Ap
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