982 research outputs found
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
A deeply embedded young protoplanetary disk around L1489 IRS observed by the submillimeter array
Circumstellar disks are expected to form early in the process that leads to
the formation of a young star, during the collapse of the dense molecular cloud
core. It is currently not well understood at what stage of the collapse the
disk is formed or how it subsequently evolves. We aim to identify whether an
embedded Keplerian protoplanetary disk resides in the L1489 IRS system. Given
the amount of envelope material still present, such a disk would respresent a
very young example of a protoplanetary disk. Using the Submillimeter Array
(SMA) we have observed the HCO 3--2 line with a resolution of about
1. At this resolution a protoplanetary disk with a radius of a few hundred
AUs should be detectable, if present. Radiative transfer tools are used to
model the emission from both continuum and line data. We find that these data
are consistent with theoretical models of a collapsing envelope and Keplerian
circumstellar disk. Models reproducing both the SED and the interferometric
continuum observations reveal that the disk is inclined by 40 which is
significantly different to the surrounding envelope (74). This
misalignment of the angular momentum axes may be caused by a gradient within
the angular momentum in the parental cloud or if L1489 IRS is a binary system
rather than just a single star. In the latter case, future observations looking
for variability at sub-arcsecond scales may be able to constrain these
dynamical variations directly. However, if stars form from turbulent cores, the
accreting material will not have a constant angular momentum axis (although the
average is well defined and conserved) in which case it is more likely to have
a misalignment of the angular momentum axes of the disk and the envelope.Comment: 11 pages, 13 figures, accepted by A&
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
A Resolved Molecular Gas Disk around the Nearby A Star 49 Ceti
The A star 49 Ceti, at a distance of 61 pc, is unusual in retaining a
substantial quantity of molecular gas while exhibiting dust properties similar
to those of a debris disk. We present resolved observations of the disk around
49 Ceti from the Submillimeter Array in the J=2-1 rotational transition of CO
with a resolution of 1.0x1.2 arcsec. The observed emission reveals an extended
rotating structure viewed approximately edge-on and clear of detectable CO
emission out to a distance of ~90 AU from the star. No 1.3 millimeter continuum
emission is detected at a 3-sigma sensitivity of 2.1 mJy/beam. Models of disk
structure and chemistry indicate that the inner disk is devoid of molecular
gas, while the outer gas disk between 40 and 200 AU from the star is dominated
by photochemistry from stellar and interstellar radiation. We determine
parameters for a model that reproduces the basic features of the spatially
resolved CO J=2-1 emission, the spectral energy distribution, and the
unresolved CO J=3-2 spectrum. We investigate variations in disk chemistry and
observable properties for a range of structural parameters. 49 Ceti appears to
be a rare example of a system in a late stage of transition between a gas-rich
protoplanetary disk and a tenuous, virtually gas-free debris disk.Comment: 11 pages, 6 figures, accepted for publication in Ap
Physical structure and CO abundance of low-mass protostellar envelopes
We present 1D radiative transfer modelling of the envelopes of a sample of 18
low-mass protostars and pre-stellar cores with the aim of setting up realistic
physical models, for use in a chemical description of the sources. The density
and temperature profiles of the envelopes are constrained from their radial
profiles obtained from SCUBA maps at 450 and 850 micron and from measurements
of the source fluxes ranging from 60 micron to 1.3 mm. The densities of the
envelopes within ~10000 AU can be described by single power-laws r^{-p} for the
class 0 and I sources with p ranging from 1.3 to 1.9, with typical
uncertainties of +/- 0.2. Four sources have flatter profiles, either due to
asymmetries or to the presence of an outer constant density region. No
significant difference is found between class 0 and I sources. The power-law
fits fail for the pre-stellar cores, supporting recent results that such cores
do not have a central source of heating. The derived physical models are used
as input for Monte Carlo modelling of submillimeter C18O and C17O emission. It
is found that class I objects typically show CO abundances close to those found
in local molecular clouds, but that class 0 sources and pre-stellar cores show
lower abundances by almost an order of magnitude implying that significant
depletion occurs for the early phases of star formation. While the 2-1 and 3-2
isotopic lines can be fitted using a constant fractional CO abundance
throughout the envelope, the 1-0 lines are significantly underestimated,
possibly due to contribution of ambient molecular cloud material to the
observed emission. The difference between the class 0 and I objects may be
related to the properties of the CO ices.Comment: 21 pages, 12 figures, accepted by A&
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
Interferometric view of the circumstellar envelopes of northern FU Orionis-type stars
FU Orionis-type objects are young, low-mass stars with large outbursts in
visible light that last for several years or decades. They are thought to
represent an evolutionary phase during the life of every young star when
accretion from the circumstellar disk is enhanced during recurring time
periods. These outbursts are able to rapidly build up the star while affecting
the circumstellar disk and thus the ongoing or future planet formation. In many
models infall from a circumstellar envelope seems to be necessary to trigger
the outbursts. We observed the J=10 rotational transition of CO and
CO towards eight northern FU Orionis-type stars (V1057 Cyg, V1515 Cyg,
V2492 Cyg, V2493 Cyg, V1735 Cyg, V733 Cep, RNO 1B and RNO 1C) and derive
temperatures and envelope masses and discuss the morphology and kinematics of
the circumstellar material. We detected extended CO emission associated with
all our targets. Smaller scale CO clumps were found to be associated with five
objects with radii of 20005000 AU and masses of 0.020.5 ;
these are clearly heated by the central stars. Three of these envelopes are
also strongly detected in the 2.7 mm continuum. No central CO clumps were
detected around V733 Cep and V710 Cas but there are many other clumps in their
environments. Traces of outflow activity were observed towards V1735 Cyg, V733
Cep and V710 Cas. The diversity of the observed envelopes enables us to set up
an evolutionary sequence between the objects. We find their evolutionary state
to range from early, embedded Class I stage to late, Class II-type objects with
very low-mass circumstellar material. The results reinforce the idea of FU
Orionis-type stars as representatives of a transitory stage between embedded
Class I young stellar objects and classical T-Tauri stars.Comment: 17 pages, 11 figures; accepted for publication in A&
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