347 research outputs found
Gas and Dust Emission at the Outer Edge of Protoplanetary Disks
We investigate the apparent discrepancy between gas and dust outer radii
derived from millimeter observations of protoplanetary disks. Using 230 and 345
GHz continuum and CO J=3-2 data from the Submillimeter Array for four nearby
disk systems (HD 163296, TW Hydrae, GM Aurigae, and MWC 480), we examine models
of circumstellar disk structure and the effects of their treatment of the outer
disk edge. We show that for these disks, models described by power laws in
surface density and temperature that are truncated at an outer radius are
incapable of reproducing both the gas and dust emission simultaneously: the
outer radius derived from the dust continuum emission is always significantly
smaller than the extent of the molecular gas disk traced by CO emission.
However, a simple model motivated by similarity solutions of the time evolution
of accretion disks that includes a tapered exponential edge in the surface
density distribution (and the same number of free parameters) does much better
at reproducing both the gas and dust emission. While this analysis does not
rule out the disparate radii implied by the truncated power-law models, a
realistic alternative disk model, grounded in the physics of accretion,
provides a consistent picture for the extent of both the gas and dust.Comment: 9 pages, 2 figures, accepted for publication in Ap
A sensitive survey for 13CO, CN, H2CO and SO in the disks of T Tauri and Herbig Ae stars
We use the IRAM 30-m telescope to perform a sensitive search for CN N=2-1 in
42 T Tauri or Herbig Ae systems located mostly in the Taurus-Auriga region.
CO J=2-1 is observed simultaneously to indicate the level of confusion
with the surrounding molecular cloud. The bandpass also contains two
transitions of ortho-HCO, one of SO and the CO J=2-1 line which
provide complementary information on the nature of the emission.
While CO is in general dominated by residual emission from the cloud,
CN exhibits a high disk detection rate % in our sample. We even report CN
detection in stars for which interferometric searches failed to detect
CO, presumably because of obscuration by a foreground, optically thick,
cloud. Comparison between CN and o-HCO or SO line profiles and intensities
divide the sample in two main categories. Sources with SO emission are bright
and have strong HCO emission, leading in general to [HCO/CN].
Furthermore, their line profiles, combined with a priori information on the
objects, suggest that the emission is coming from outflows or envelopes rather
than from a circumstellar disk. On the other hand, most sources have
[HCO/CN], no SO emission, and some of them exhibit clear
double-peaked profiles characteristics of rotating disks. In this second
category, CN is likely tracing the proto-planetary disks. From the line flux
and opacity derived from the hyperfine ratios, we constrain the outer radii of
the disks, which range from 300 to 600 AU. The overall gas disk detection rate
(including all molecular tracers) is , and decreases for fainter
continuum sources.
This study shows that gas disks, like dust disks, are ubiquitous around young
PMS stars in regions of isolated star formation, and that a large fraction of
them have AU.Comment: 31 pages (including 59 figures
Dust continuum and Polarization from Envelope to Cores in Star Formation: A Case Study in the W51 North region
We present the first high-angular resolution (up to 0.7", ~5000 AU)
polarization and thermal dust continuum images toward the massive star-forming
region W51 North. The observations were carried out with the Submillimeter
Array (SMA) in both the subcompact (SMA-SubC) and extended (SMA-Ext)
configurations at a wavelength of 870 micron. W51 North is resolved into four
cores (SMA1 to SMA4) in the 870 micron continuum image. The associated dust
polarization exhibits more complex structures than seen at lower angular
resolution. We analyze the inferred morphologies of the plane-of-sky magnetic
field (B_bot) in the SMA1 to SMA4 cores and in the envelope using the SMA-Ext
and SMA-SubC data. These results are compared with the B_bot archive images
obtained from the CSO and JCMT. A correlation between dust intensity gradient
position angles (phi_{nabla I}) and magnetic field position angles (phi_B) is
found in the CSO, JCMT and both SMA data sets. This correlation is further
analyzed quantitatively. A systematically tighter correlation between
phi_{nabla I} and phi_B is found in the cores, whereas the correlation
decreases in outside-core regions. Magnetic field-to-gravity force ratio
(Sigma_B) maps are derived using the newly developed polarization - intensity
gradient method by Koch, Tang & Ho 2012. We find that the force ratios tend to
be small (Sigma_B <= 0.5) in the cores in all 4 data sets. In regions outside
of the cores, the ratios increase or the field is even dominating gravity
(Sigma_B > 1). This possibly provides a physical explanation of the tightening
correlation between phi_{nabla I} and phi_B in the cores: the more the B field
lines are dragged and aligned by gravity, the tighter the correlation is.
Finally, we propose a schematic scenario for the magnetic field in W51 North to
interpret the four polarization observations at different physical scales.Comment: Accepted for publication in ApJ. 10 pages. 7 figure
Molecular line radiative transfer in protoplanetary disks: Monte Carlo simulations versus approximate methods
We analyze the line radiative transfer in protoplanetary disks using several
approximate methods and a well-tested Accelerated Monte Carlo code. A low-mass
flaring disk model with uniform as well as stratified molecular abundances is
adopted. Radiative transfer in low and high rotational lines of CO, C18O, HCO+,
DCO+, HCN, CS, and H2CO is simulated. The corresponding excitation
temperatures, synthetic spectra, and channel maps are derived and compared to
the results of the Monte Carlo calculations. A simple scheme that describes the
conditions of the line excitation for a chosen molecular transition is
elaborated. We find that the simple LTE approach can safely be applied for the
low molecular transitions only, while it significantly overestimates the
intensities of the upper lines. In contrast, the Full Escape Probability (FEP)
approximation can safely be used for the upper transitions (J_{\rm up} \ga 3)
but it is not appropriate for the lowest transitions because of the maser
effect. In general, the molecular lines in protoplanetary disks are partly
subthermally excited and require more sophisticated approximate line radiative
transfer methods. We analyze a number of approximate methods, namely, LVG, VEP
(Vertical Escape Probability) and VOR (Vertical One Ray) and discuss their
algorithms in detail. In addition, two modifications to the canonical Monte
Carlo algorithm that allow a significant speed up of the line radiative
transfer modeling in rotating configurations by a factor of 10--50 are
described.Comment: 47 pages, 12 figures, accepted for publication in Ap
Sensitive survey for 13CO, CN, H2CO, and SO in the disks of T Tauri and Herbig Ae stars II: Stars in Oph and upper Scorpius
We attempt to determine the molecular composition of disks around young
low-mass stars in the Oph region and to compare our results with a
similar study performed in the Taurus-Auriga region. We used the IRAM 30 m
telescope to perform a sensitive search for CN N=2-1 in 29 T Tauri stars
located in the Oph and upper Scorpius regions. CO J=2-1 is
observed simultaneously to provide an indication of the level of confusion with
the surrounding molecular cloud. The bandpass also contains two transitions of
ortho-HCO, one of SO, and the CO J=2-1 line, which provides
complementary information on the nature of the emission. Contamination by
molecular cloud in CO and even CO is ubiquitous. The CN detection
rate appears to be lower than for the Taurus region, with only four sources
being detected (three are attributable to disks). HCO emission is found
more frequently, but appears in general to be due to the surrounding cloud. The
weaker emission than in Taurus may suggest that the average disk size in the
Oph region is smaller than in the Taurus cloud. Chemical modeling shows
that the somewhat higher expected disk temperatures in Oph play a direct
role in decreasing the CN abundance. Warmer dust temperatures contribute to
convert CN into less volatile forms. In such a young region, CN is no longer a
simple, sensitive tracer of disks, and observations with other tracers and at
high enough resolution with ALMA are required to probe the gas disk population.Comment: 18 pages, 5 figures, accepted for publication in A&
A Sub-millimeterwave ``Flare'' from GG Tau?
We have monitored the millimeter and submillimeter emission from the young
stellar object GG Tau, a T Tauri binary system surrounded by a massive
circumbinary disk. We find that between 1992 and 1994, the flux has increased
significantly at 800, 1100, and 1300 microns, resulting in a steepening of the
observed spectral energy distribution at those wavelengths. Such an increase
appears consistent with a modest increase in disk luminosity (a factor of two).
The increase in the effective disk temperature might arise from a slight change
in the disk heating processes. Alternatively, the flux increase may reflect a
sudden change in the underlying dust optical properties.Comment: 15 pages, AASTex v.4.0 format, four postscript figures, four tables,
to appear in The Astrophysical Journa
Disk Imaging Survey of Chemistry with SMA (DISCS): I. Taurus Protoplanetary Disk Data
Chemistry plays an important role in the structure and evolution of
protoplanetary disks, with implications for the composition of comets and
planets. This is the first of a series of papers based on data from DISCS, a
Submillimeter Array survey of the chemical composition of protoplanetary disks.
The six Taurus sources in the program (DM Tau, AA Tau, LkCa 15, GM Aur, CQ Tau
and MWC 480) range in stellar spectral type from M1 to A4 and offer an
opportunity to test the effects of stellar luminosity on the disk chemistry.
The disks were observed in 10 different lines at ~3" resolution and an rms of
~100 mJy beam-1 at ~0.5 km s-1. The four brightest lines are CO 2-1, HCO+ 3-2,
CN 2_3-1_2 and HCN 3-2 and these are detected toward all sources (except for
HCN toward CQ Tau). The weaker lines of CN 2_2-1_1, DCO+ 3-2, N2H+ 3-2, H2CO
3_03-2_02 and 4_14-3_13 are detected toward two to three disks each, and DCN
3-2 only toward LkCa 15. CH3OH 4_21-3_12 and c-C3H2 are not detected. There is
no obvious difference between the T Tauri and Herbig Ae sources with regard to
CN and HCN intensities. In contrast, DCO+, DCN, N2H+ and H2CO are detected only
toward the T Tauri stars, suggesting that the disks around Herbig Ae stars lack
cold regions for long enough timescales to allow for efficient deuterium
chemistry, CO freeze-out, and grain chemistry.Comment: 29 pages, 4 figures, accepted for publication in Ap
Dynamical Masses of T Tauri Stars and Calibration of PMS Evolution
We have used the high sensitivity and resolution of the IRAM interferometer
to produce sub-arcsecond 12CO 2-1 images of 9 protoplanetary disks surrounding
T Tauri stars in the Taurus-Auriga cloud (7 singles and 2 binaries). The images
demonstrate the disks are in Keplerian rotation around their central stars.
Using the least square fit method described in Guilloteau and Dutrey (1998), we
derive the disk properties, in particular its inclination angle and rotation
velocity, hence the dynamical mass. Since the disk mass is usually small, this
is a direct measurement of the stellar mass. Typically, we reach an internal
precision of 10% in the determinations of stellar mass. The over-all accuracy
is limited by the uncertainty in the distance to a specific star. In a distance
independent way, we compare the derived masses with theoretical tracks of
pre-main-sequence evolution. Combined with the mean distance to the Taurus
region (140 pc), for stars with mass close to 1 Msun, our results tend to favor
the tracks with cooler photospheres (higher masses for a given spectral type).
We find that in UZ Tau E the disk and the spectroscopic binary orbit appear to
have different inclinations.Comment: 32 pages, 5 figure
Massive Protoplanetary Disks in the Trapezium Region
(abridged) We determine the disk mass distribution around 336 stars in the
young Orion Nebula cluster by imaging a 2.5' x 2.5' region in 3 mm continuum
emission with the Owens Valley Millimeter Array. For this sample of 336 stars,
we observe 3 mm emission above the 3-sigma noise level toward ten sources, six
of which have also been detected optically in silhouette against the bright
nebular background. In addition, we detect 20 objects that do not correspond to
known near-IR cluster members. Comparisons of our measured fluxes with longer
wavelength observations enable rough separation of dust emission from thermal
free-free emission, and we find substantial dust emission toward most objects.
For the ten objects detected at both 3 mm and near-IR wavelengths, eight
exhibit substantial dust emission. Excluding the high-mass stars and assuming a
gas-to-dust ratio of 100, we estimate circumstellar masses ranging from 0.13 to
0.39 Msun. For the cluster members not detected at 3 mm, images of individual
objects are stacked to constrain the mean 3 mm flux of the ensemble. The
average flux is detected at the 3-sigma confidence level, and implies an
average disk mass of 0.005 Msun, comparable to the minimum mass solar nebula.
The percentage of stars in Orion surrounded by disks more massive than ~0.1
Msun is consistent with the disk mass distribution in Taurus, and we argue that
massive disks in Orion do not appear to be truncated through close encounters
with high-mass stars. Comparison of the average disk mass and number of massive
dusty structures in Orion with similar surveys of the NGC 2024 and IC 348
clusters constrains the evolutionary timescales of massive circumstellar disks
in clustered environments.Comment: 27 pages, including 7 figures. Accepted by Ap
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