86 research outputs found
Volatile depletion in the TW Hydrae disk atmosphere
An abundance decrease in carbon- and oxygen-bearing species relative to dust
has been frequently found in planet-forming disks, which can be attributed to
an overall reduction of gas mass. However, in the case of TW Hya, the only disk
with gas mass measured directly with HD rotational lines, the inferred gas mass
(0.005 solar mass) is significantly below the directly measured value
(0.05 solar mass). We show that this apparent conflict can be resolved
if the elemental abundances of carbon and oxygen are reduced in the upper
layers of the outer disk but are normal elsewhere (except for a possible
enhancement of their abundances in the inner disk). The implication is that in
the outer disk, the main reservoir of the volatiles (CO, water, ...) resides
close to the midplane, locked up inside solid bodies that are too heavy to be
transported back to the atmosphere by turbulence. An enhancement in the carbon
and oxygen abundances in the inner disk can be caused by inward migration of
these solid bodies. This is consistent with estimates based on previous models
of dust grain dynamics. Indirect measurements of the disk gas mass and disk
structure from species such as CO will thus be intertwined with the evolution
of dust grains, and possibly also with the formation of planetesimals.Comment: 8 pages, 4 figures; accepted by ApJL for publicatio
From infall to rotation around young stellar objects: A transitional phase with a 2000 AU radius contracting disk?
Evidence for a transitional stage in the formation of a low-mass star is
reported, intermediate between the fully embedded and the T Tauri phases.
Millimeter aperture synthesis observations in the HCO+ J=1-0 and 3-2, HCN 1-0,
13CO 1-0, and C18O 1-0 transitions reveal distinctly different velocity fields
around two embedded, low-mass young stellar objects. The 0.6 M(sun) of material
around TMC 1 (IRAS 04381+2517) closely follows inside-out collapse in the
presence of a small amount of rotation (~3 km/s/pc), while L1489 IRS (IRAS
04016+2610) is surrounded by a 2000 AU radius, flared disk containing 0.02
M(sun). This disk shows Keplerian rotation around a ~0.65 M(sun) star and
infall at 1.3 (r/100 AU)^-0.5 km/s, or, equivalently, sub-Keplerian motions
around a central object between 0.65 and 1.4 M(sun). Its density is
characterized by a radial power law and an exponential vertical scale height.
The different relative importance of infall and rotation around these two
objects suggests that rotationally supported structures grow from collapsing
envelopes over a few times 10^5 yr to sizes of a few thousand AU, and then
decrease over a few times 10^4 yr to several hundred AU typical for T Tauri
disks. In this scenario, L1489 IRS represents a transitional phase between
embedded YSOs and T Tauri stars with disks. The expected duration of this phase
of ~5% of the embedded stage is consistent with the current lack of other known
objects like L1489 IRS. Alternative explanations cannot explain L1489 IRS's
large disk, such as formation from a cloud core with an unusually large
velocity gradient or a binary companion that prevents mass accretion onto small
scales. It follows that the transfer and dissipation of angular momentum is key
to understanding the formation of disks from infalling envelopes.Comment: Accepted ApJ. 33 pages, including 10 B/W figures and 1 color figure.
Uses AASTe
Resolving the chemistry in the disk of TW Hydrae I. Deuterated species
We present Submillimeter Array (SMA) observations of several deuterated
species in the disk around the classical T Tauri star TW Hydrae at arcsecond
scales, including detections of the DCN J=3-2 and DCO+ J=3-2 lines, and upper
limits to the HDO 3(1,2)-2(2,1), ortho-H2D+ 1(1,0)-1(1,1) and para-D2H+
1(1,0)-1(0,1) transitions. We also present observations of the HCN J=3-2, HCO+
J=3-2 and H13CO+ J=4-3 lines for comparison with their deuterated
isotopologues. We constrain the radial and vertical distributions of various
species in the disk by fitting the data using a model where the molecular
emission from an irradiated accretion disk is sampled with a 2D Monte Carlo
radiative transfer code. We find that the distribution of DCO+ differs markedly
from that of HCO+. The D/H ratios inferred change by at least one order of
magnitude (0.01 to 0.1) for radii 70 AU and there is a rapid falloff
of the abundance of DCO+ at radii larger than 90 AU. Using a simple analytical
chemical model, we constrain the degree of ionization, x(e-)=n(e-)/n(H2), to be
~10^-7 in the disk layer(s) where these molecules are present. Provided the
distribution of DCN follows that of HCN, the ratio of DCN to HCN is determined
to be 1.7\pm0.5 \times 10^-2; however, this ratio is very sensitive to the
poorly constrained vertical distribution of HCN. The resolved radial
distribution of DCO+ indicates that {\it in situ} deuterium fractionation
remains active within the TW Hydrae disk and must be considered in the
molecular evolution of circumstellar accretion disks.Comment: 12 pages, 12 figures, accepted to Ap
Molecular outflows and 1000 AU structure of low mass YSO envelopes
We present the results of an observational study into
the molecular outflows and small scale ( ~ 1000 AU)
envelope structure of a sample of nine low mass young
stellar objects (YSOs) in Taurus. The characteristics
of the outflows are derived from ^(12)CO J = 3 - 2 mapping
with the James Clerk Maxwell Telescope, while
the envelopes are imaged in the HCO^+ 1-0, ^(13)CO 1-
0 and C^(18)O 1-0 emission lines with the Owens Valley
Millimeter Array. Using dust envelope continuum
fluxes at 1 mm as the basis for an evolutionary ordering,
a picture emerges in which the mass, extent, and
collimation of outflows decreases over time as the envelopes
become less massive, the opening angle of the
outflow cavity increases, and mass accretion through
the disk slows down. On 1000 AU scales the HCO+
and ^(13)CO/C^(18)O emission in the envelope is closely related
to the outflow cavity, often outlining the cavity
walls. In addition, the envelopes are clumpy, and two
sources appear surrounded by an incomplete ring or
torus, 1500-3000 AU in radius. The role of the outflow
in shaping the small scale molecular emission may
be passive (creating a low-opacity pathway for heating
radiation) rather than, or in addition to, active (compressing
and shock-heating the material)
Subarcsecond Imaging at 267 GHz of a Young Binary System: Detection of a Dust Disk of Radius Less than 70 AU around T Tauri N
The young binary system T Tauri was observed with the Owens Valley Millimeter Array in the 267 GHz continuum and HCO^+ J = 3-2 emission at 0".8 resolution, with the single-baseline interferometer of the James Clerk Maxwell Telescope-Caltech Submillimeter Observatory in the 357 GHz continuum and with the W. M. Keck Telescope at λ = 4 μm. The 267 GHz emission is unresolved, with a flux of 397±35 mJy, located close to the position of the optical star T Tau N. An upper limit of 100 mJy is obtained toward the infrared companion T Tau S. The 357 GHz continuum emission is unresolved, with a flux of 1.35±0.68 Jy. HCO^+ J = 3-2 was detected from a 2" diameter core surrounding T Tau N and S. Both stars are detected at 4 μm, but there is no evidence of the radio source T Tau R.
We propose a model in which T Tau S is intrinsically similar to T Tau N but is obscured by the outer parts of T Tau N's disk. A fit to the spectral energy distribution (SED) between 21 cm and 1.22 μm is constructed on this basis. Adopting an r^(−1) surface density distribution and an exponentially truncated edge, disk masses of 0.04±0.01 and 6×10^(−5) to 3×10^(−3) M_☉ are inferred for T Tau N and T Tau S, respectively. A 0.005-0.03 M_☉ circumbinary envelope is also required to fit the millimeter to mid-infrared SED
Detection of H_2 Pure Rotational Line Emission from the GG Tauri Binary System
We present the first detection of the low-lying pure rotational emission lines of H_2 from circumstellar disks around T Tauri stars, using the Short Wavelength Spectrometer on the Infrared Space Observatory. These lines provide a direct measure of the total amount of warm molecular gas in disks. The J = 2 → 0 S(0) line at 28.218 μm and the J = 3 → 1 S(1) line at 17.035 μm have been observed toward the double binary system GG Tau. Together with limits on the J = 5 → 3 S(3) and J = 7 → 5 S(5) lines, the data suggest the presence of gas at T_(kin) ≈ 110 ± 10 K with a mass of (3.6 ± 2.0) × 10^(-3) M_☉ (±3 σ). This amounts to ~3% of the total gas + dust mass of the circumbinary disk as imaged by millimeter interferometry, but it is larger than the estimated mass of the circumstellar disk(s). Possible origins for the warm gas seen in H_2 are discussed in terms of photon and wind-shock heating mechanisms of the circumbinary material, and comparisons with model calculations are made
Evidence for Multiple Pathways to Deuterium Enhancements in Protoplanetary Disks
The distributions of deuterated molecules in protoplanetary disks are
expected to depend on the molecular formation pathways. We use observations of
spatially resolved DCN emission from the disk around TW Hya, acquired during
ALMA Science verification with a ~3" synthesized beam, together with comparable
DCO+ observations from the Submillimeter Array, to investigate differences in
the radial distributions of these species and hence differences in their
formation chemistry. In contrast to DCO+, which shows an increasing column
density with radius, DCN is better fit by a model that is centrally peaked. We
infer that DCN forms at a smaller radii and thus at higher temperatures than
DCO+. This is consistent with chemical network model predictions of DCO+
formation from H2D+ at T<30 K and DCN formation from additional pathways
involving CH2D+ at higher temperatures. We estimate a DCN/HCN abundance ratio
of ~0.017, similar to the DCO+/HCO+ abundance ratio. Deuterium fractionation
appears to be efficient at a range of temperatures in this protoplanetary disk.
These results suggest caution in interpreting the range of deuterium fractions
observed in Solar System bodies, as multiple formation pathways should be taken
into account.Comment: accepted for publication in Ap
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