117 research outputs found
An Extinction Study of the Taurus Dark Cloud Complex
We present a study of the detailed distribution of extinction in a region of
the Taurus dark cloud complex. Our study uses new BVR images of the region,
spectral classification data for 95 stars, and IRAS Sky Survey Atlas (ISSA) 60
and 100 micron images. We study the extinction of the region in four different
ways, and we present the first inter-comparison of all these methods, which
are: 1) using the color excess of background stars for which spectral types are
known; 2) using the ISSA 60 and 100 micron images; 3) using star counts; and 4)
using an optical (V and R) version of the average color excess method used by
Lada et al. (1994). We find that all four methods give generally similar
results, with important exceptions. To study the structure in the dust
distribution, we compare the ISSA extinction and the extinction measured for
individual stars. From the comparison, we conclude that in the relatively low
extinction regions studied, with 0.9 < A_V < 3.0 mag (away from filamentary
dark clouds and IRAS cores), there are no fluctuations in the dust column
density greater than 45% (at the 99.7% confidence level), on scales smaller
than 0.2 pc. We also report the discovery of a previously unknown stellar
cluster behind the Taurus dark cloud near R.A 4h19m00s, Dec. 27:30:00 (B1950)Comment: 49 pages (which include 6 pages of tables and 6 pages of figures
Accretion Disks Around Young Objects. III. Grain Growth
We present detailed models of irradiated T Tauri disks including dust grain
growth with power-law size distributions. The models assume complete mixing
between dust and gas and solve for the vertical disk structure
self-consistentlyincluding the heating effects of stellar irradiation as well
as local viscous heating. For a given total dust mass, grain growth is found to
decrease the vertical height of the surface where the optical depth to the
stellar radiation becomes unit and thus the local irradiation heating, while
increasing the disk emission at mm and sub-mm wavelengths. The resulting disk
models are less geometrically thick than our previous models assuming
interstellar medium dust, and agree better with observed spectral energy
distributions and images of edge-on disks, like HK Tau/c and HH 30. The
implications of models with grain growth for determining disk masses from
long-wavelength emission are considered.Comment: 29 pages, including 11 figures and 1 table, APJ accepte
Dust Stratification in Young Circumstellar Disks
We present high-resolution infrared spectra of four YSOs (T Tau N, T Tau S,
RNO 91, and HL Tau). The spectra exhibit narrow absorption lines of 12CO, 13CO,
and C18O as well as broad emission lines of gas phase12CO. The narrow
absorption lines of CO are shown to originate from the colder circumstellar
gas. We find that the line of sight gas column densities resulting from the CO
absorption lines are much higher than expected for the measured extinction for
each source and suggest the gas to dust ratio is measuring the dust settling
and/or grain coagulation in these extended disks. We provide a model of
turbulence, dust settling and grain growth to explain the results. The
techniques presented here allow us to provide some observationally-motivated
bounds on accretion disk alpha in protostellar systems
Sub-arcsec imaging of the AB Aur molecular disk and envelope at millimeter wavelengths: a non Keplerian disk
We present sub-arcsecond images of AB Auriga obtained with the IRAM Plateau
de Bure interferometer in the isotopologues of CO, and in continuum at 3 and
1.3 mm. Instead of being centrally peaked, the continuum emission is dominated
by a bright, asymmetric (spiral-like) feature at about 140 AU from the central
star. The large scale molecular structure suggests the AB Aur disk is inclined
between 23 and 43 degrees, but the strong asymmetry of the continuum and
molecular emission prevents an accurate determination of the inclination of the
inner parts. We find significant non-Keplerian motion, with a best fit exponent
for the rotation velocity law of 0.41 +/- 0.01, but no evidence for radial
motions. The disk has an inner hole about 70 AU in radius. The disk is warm and
shows no evidence of depletion of CO. The dust properties suggest the dust is
less evolved than in typical T Tauri disks. Both the spiral-like feature and
the departure from purely Keplerian motions indicates the AB Aur disk is not in
quasi-equilibrium. Disk self-gravity is insufficient to create the
perturbation. This behavior may be related either to an early phase of star
formation in which the Keplerian regime is not yet fully established and/or to
a disturbance of yet unknown origin. An alternate, but unproven, possibility is
that of a low mass companion located about 40 AU from AB Aur.Comment: 10 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
Spitzer Warm Mission Transition and Operations
Following the successful dynamic planning and implementation of IRAC Warm Instrument Characterization activities, transition to Spitzer Warm Mission operations has gone smoothly. Operation teams procedures and processes required minimal adaptation and the overall composition of the Mission Operation System retained the same functionality it had during the Cryogenic Mission. While the warm mission scheduling has been simplified because all observations are now being made with a single instrument, several other differences have increased the complexity. The bulk of the observations executed to date have been from ten large Exploration Science programs that, combined, have more complex constraints, more observing requests, and more exo-planet observations with durations of up to 145 hours. Communication with the observatory is also becoming more challenging as the Spitzer DSN antenna allocations have been reduced from two tracking passes per day to a single pass impacting both uplink and downlink activities. While IRAC is now operating with only two channels, the data collection rate is roughly 60% of the four-channel rate leaving a somewhat higher average volume collected between the less frequent passes. Also, the maximum downlink data rate is decreasing as the distance to Spitzer increases requiring longer passes. Nevertheless, with well over 90% of the time spent on science observations, efficiency has equaled or exceeded that achieved during the cryogenic mission
A search for evolved dust in Herbig Ae stars
We present observations of six isolated, pre-main-sequence, intermediate mass
stars selected for shallow spectra at submillimeter wavelengths at 1.3, 2.6,
7.0, and 36 millimeters from the IRAM PdBI and the VLA. We analyze the new
observations of these stars (HD34282, HD35187, HD142666, HD143006, HD150193,
HD163296) together with similar observations of three additional stars from the
literature (CQ Tau, UX Ori, TW Hya), in the context of self-consistent
irradiated disk models. Our aim is to constrain the wavelength dependence of
the dust opacity and the total dust mass in the disks. The shallow wavelength
dependence of the opacity is confirmed and for a few stars extended to
significantly longer wavelengths. For any plausible dust properties, this
requires grain growth from interstellar sizes to maximum sizes of at least a
few millimeters, and very likely to several centimeters or more. For four of
the stars (HD34282, HD163296, CQ Tau, TW Hya), the millimeter emission has been
spatially resolved, and the large disk radii ( AU) rule out that high
optical depths play a role. The mass of dust that has been processed into large
grains is substantial, and in some cases implies a disk mass comparable to the
mass of the central star.Comment: 9 pages, 3 figures, accepted for publication in A&
Spitzer/MIPS Observations of Stars in the Beta Pictoris Moving Group
We present Multiband Imaging Photometer for Spitzer (MIPS) observations at 24
and 70 microns for 30 stars, and at 160 microns for a subset of 12 stars, in
the nearby (~30 pc), young (~12 Myr) Beta Pictoris Moving Group (BPMG). In
several cases, the new MIPS measurements resolve source confusion and
background contamination issues in the IRAS data for this sample. We find that
7 members have 24 micron excesses, implying a debris disk fraction of 23%, and
that at least 11 have 70 micron excesses (disk fraction of >=37%). Five disks
are detected at 160 microns (out of a biased sample of 12 stars observed), with
a range of 160/70 flux ratios. The disk fraction at 24 and 70 microns, and the
size of the excesses measured at each wavelength, are both consistent with an
"inside-out" infrared excess decrease with time, wherein the shorter-wavelength
excesses disappear before longer-wavelength excesses, and consistent with the
overall decrease of infrared excess frequency with stellar age, as seen in
Spitzer studies of other young stellar groups. Assuming that the infrared
excesses are entirely due to circumstellar disks, we characterize the disk
properties using simple models and fractional infrared luminosities. Optically
thick disks, seen in the younger TW Hya and eta Cha associations, are entirely
absent in the BPMG.
Additional flux density measurements at 24 and 70 microns are reported for
nine Tucanae-Horologium Association member stars. Since this is <20% of the
association membership, limited analysis on the complete disk fraction of this
association is possible.Comment: Accepted for Ap
Large grains in the disk of CQ Tau
We present 7mm observations of the dusty disk surrounding the 10 Myr old 1.5
Msun pre-main-sequence star CQ Tauri obtained at the Very Large Array with 0.8
arcsecond resolution and 0.1 mJy rms sensitivity. These observations resolve
the 7mm emission in approximately the north-south direction, confirming
previous results obtained with lower resolution. We use a two-layer flared disk
model to interpret the observed fluxes from 7mm to 1.3mm together with the
resolved 7mm structure. We find that the disk radius is constrained to the
range 100 to 300 AU, depending on the steepness of the disk surface density
distribution. The power law index of the dust opacity coefficient, beta, is
constrained to be 0.5 to 0.7. Since the models indicate that the disk is
optically thin at millimeter wavelengths for radii greater than 8 AU, the
contribution of an optically thick region to the emission is less than 10%.
This implies that high optical depth or complex disk geometry cannot be the
cause of the observed shallow millimeter spectral index. Instead, the new
analysis supports the earlier suggestion that dust particles in the disk have
grown to sizes as large as a few centimeters. The dust in the CQ Tauri system
appears to be evolved much like that in the TW Hydra system, a well-studied
pre-main-sequence star of similar age and lower mass. The survival of gas-rich
disks with incomplete grain evolution at such old ages deserves further
investigations.Comment: A&A in press, 7 pages, 5 figure
IRAS 21391+5802: The Molecular Outflow and its Exciting Source
We present centimeter and millimeter observations of gas and dust around IRAS
21391+5802, an intermediate-mass source embedded in the core of IC 1396N.
Continuum observations from 3.6 cm to 1.2 mm are used to study the embedded
objects and overall distribution of the dust, while molecular line observations
of CO, CS, and CH3OH are used to probe the structure and chemistry of the
outflows in the region. The continuum emission at centimeter and millimeter
wavelengths has been resolved into three sources separated about 15 arcsec from
each other, and with one of them, BIMA 2, associated with IRAS 21391+5802. The
dust emission around this source shows a very extended envelope, which accounts
for most of the circumstellar mass of 5.1 Msun. This source is powering a
strong molecular outflow, elongated in the E--W direction, which presents a
complex structure and kinematics. While at high outflow velocities the outflow
is clearly bipolar, at low outflow velocities the blueshifted and redshifted
emission are highly overlapping, and the strongest emission shows a V-shaped
morphology. The outflow as traced by CS and CH3OH exhibits two well
differentiated and clumpy lobes, with two prominent northern blueshifted and
redshifted clumps. The curved shape of the clumps and the spectral shape at
these positions are consistent with shocked material. In addition, CS and CH3OH
are strongly enhanced toward these positions with respect to typical quiescent
material abundances in other star-forming regions.Comment: 41 pages, including 11 figures, accepted for publication in ApJ (July
1); available at http://www.am.ub.es/~robert/Papers.html#las
CI observations in the CQ Tau proto-planetary disk: evidence for a very low gas-to-dust ratio ?
Gas and dust dissipation processes of proto-planetary disks are hardly known.
Transition disks between Class II (proto-planetary disks) and Class III (debris
disks) remain difficult to detect. We investigate the carbon chemistry of the
peculiar CQ Tau gas disk. It is likely a transition disk because it exhibits
weak CO emission with a relatively strong millimeter continuum, indicating that
the disk might be currently dissipating its gas content. We used APEX to
observe the two CI lines at 492GHz and 809 GHz in the disk orbiting CQ Tau. We
compare the observations to several chemical model predictions. We focus our
study on the influence of the stellar UV radiation shape and gas-to-dust ratio.
We did not detect the CI lines. However, our upper limits are deep enough to
exclude high-CI models. The only available models compatible with our limits
imply very low gas-to-dust ratio, of the order of a few, only. These
observations strengthen the hypothesis that CQ Tau is likely a transition disk
and suggest that gas disappears before dust.Comment: 5 pages, 5 figures, accepted for publication in A&
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