651 research outputs found
A High Resolution Study of the Slowly Contracting, Starless Core L1544
We present interferometric observations of N2H+(1--0) in the starless, dense
core L1544 in Taurus. Red-shifted self-absorption, indicative of inward
motions, is found toward the center of an elongated core. The data are fit by a
non-spherical model consisting of two isothermal, rotating, centrally condensed
layers. Through a hybrid global-individual fit to the spectra, we map the
variation of infall speed at scales ~1400AU and find values ~0.08 km/s around
the core center. The inward motions are small in comparison to thermal,
rotational, and gravitational speeds but are large enough to suggest that L1544
is very close to forming a star.Comment: 11 pages, 2 figures Accepted for publication in Astrophysical Journal
Letter
DCO, DCN and ND reveal three different deuteration regimes in the disk around the Herbig Ae star HD163296
The formation pathways of deuterated species trace different regions of
protoplanetary disks and may shed light into their physical structure. We aim
to constrain the radial extent of main deuterated species; we are particularly
interested in spatially characterizing the high and low temperature pathways
for enhancing deuteration of these species. We observed the disk surrounding
the Herbig Ae star HD 163296 using ALMA in Band 6 and obtained resolved
spectral imaging data of DCO (=3-2), DCN (=3-2) and ND
(=3-2). We model the radial emission profiles of DCO, DCN and
ND, assuming their emission is optically thin, using a parametric model
of their abundances and radial excitation temperature estimates. DCO can be
described by a three-region model, with constant-abundance rings centered at 70
AU, 150 AU and 260 AU. The DCN radial profile peaks at about ~60 AU and
ND is seen in a ring at ~160 AU. Simple models of both molecules using
constant abundances reproduce the data. Assuming reasonable average excitation
temperatures for the whole disk, their disk-averaged column densities (and
deuterium fractionation ratios) are 1.6-2.6 cm
(0.04-0.07), 2.9-5.2 cm (0.02) and 1.6-2.5 cm (0.34-0.45) for DCO, DCN and ND, respectively.
Our simple best-fit models show a correlation between the radial location of
the first two rings in DCO and the DCN and ND abundance
distributions that can be interpreted as the high and low temperature
deuteration pathways regimes. The origin of the third DCO ring at 260 AU is
unknown but may be due to a local decrease of ultraviolet opacity allowing the
photodesorption of CO or due to thermal desorption of CO as a consequence of
radial drift and settlement of dust grains
Exploring DCO as a tracer of thermal inversion in the disk around the Herbig Ae star HD163296
We aim to reproduce the DCO emission in the disk around HD163296 using a
simple 2D chemical model for the formation of DCO through the cold
deuteration channel and a parametric treatment of the warm deuteration channel.
We use data from ALMA in band 6 to obtain a resolved spectral imaging data cube
of the DCO =3--2 line in HD163296 with a synthesized beam of
0."53 0."42. We adopt a physical structure of the disk from the
literature that reproduces the spectral energy distribution. We then apply a
simplified chemical network for the formation of DCO that uses the physical
structure of the disk as parameters along with a CO abundance profile, a
constant HD abundance and a constant ionization rate. Finally, from the
resulting DCO abundances, we calculate the non-LTE emission using the 3D
radiative transfer code LIME. The observed DCO emission is reproduced by a
model with cold deuteration producing abundances up to .
Warm deuteration, at a constant abundance of , becomes
fully effective below 32 K and tapers off at higher temperatures, reproducing
the lack of DCO inside 90 AU. Throughout the DCO emitting zone a CO
abundance of is found, with 99\% of it frozen out below
19 K. At radii where both cold and warm deuteration are active, warm
deuteration contributes up to 20\% of DCO, consistent with detailed
chemical models. The decrease of DCO at large radii is attributed to a
temperature inversion at 250 AU, which raises temperatures above values where
cold deuteration operates. Increased photodesorption may also limit the radial
extent of DCO. The corresponding return of the DCO layer to the
midplane, together with a radially increasing ionization fraction, reproduces
the local DCO emission maximum at 260 AU.Comment: 9 pages, 5 figures, accepted 7th July 201
A Keplerian Disk around the Herbig Ae star HD169142
We present Submillimeter Array observations of the Herbig Ae star HD169142 in
1.3 millimeter continuum emission and 12CO J=2-1 line emission at 1.5 arcsecond
resolution that reveal a circumstellar disk. The continuum emission is centered
on the star position and resolved, and provides a mass estimate of about 0.02
solar masses for the disk. The CO images show patterns in position and velocity
that are well matched by a disk in Keplerian rotation with low inclination to
the line-of-sight. We use radiative transfer calculations based on a flared,
passive disk model to constrain the disk parameters by comparison to the
spectral line emission. The derived disk radius is 235 AU, and the inclination
is 13 degrees. The model also necessitates modest depletion of the CO
molecules, similar to that found in Keplerian disks around T Tauri stars.Comment: 10 pages, 2 figures, accepted by A
SMA Imaging of CO(3-2) Line and 860 micron Continuum of Arp 220 : Tracing the Spatial Distribution of Luminosity
We used the Submillimeter Array (SMA) to image 860 micron continuum and
CO(3-2) line emission in the ultraluminous merging galaxy Arp 220, achieving a
resolution of 0.23" (80 pc) for the continuum and 0.33" (120 pc) for the line.
The CO emission peaks around the two merger nuclei with a velocity signature of
gas rotation around each nucleus, and is also detected in a kpc-size disk
encompassing the binary nucleus. The dust continuum, in contrast, is mostly
from the two nuclei. The beam-averaged brightness temperature of both line and
continuum emission exceeds 50 K at and around the nuclei, revealing the
presence of warm molecular gas and dust. The dust emission morphologically
agrees with the distribution of radio supernova features in the east nucleus,
as expected when a starburst heats the nucleus. In the brighter west nucleus,
however, the submillimeter dust emission is more compact than the supernova
distribution. The 860 micron core, after deconvolution, has a size of 50-80 pc,
consistent with recent 1.3 mm observations, and a peak brightness temperature
of (0.9-1.6)x10^2 K. Its bolometric luminosity is at least 2x10^{11} Lsun and
could be ~10^{12} Lsun depending on source structure and 860 micron opacity,
which we estimate to be of the order of tau_{860} ~ 1 (i.e., N_{H_2} ~ 10^{25}
cm^{-2}). The starbursting west nuclear disk must have in its center a dust
enshrouded AGN or a very young starburst equivalent to hundreds of super star
clusters. Further spatial mapping of bolometric luminosity through
submillimeter imaging is a promising way to identify the heavily obscured
heating sources in Arp 220 and other luminous infrared galaxies.Comment: ApJ. in press. 26 pages, 10 figure
Limits on Radio Continuum Emission from a Sample of Candidate Contracting Starless Cores
We used the NRAO Very Large Array to search for 3.6 cm continuum emission
from embedded protostars in a sample of 8 nearby ``starless'' cores that show
spectroscopic evidence for infalling motions in molecular emission lines. We
detect a total of 13 compact sources in the eight observed fields to 5 sigma
limiting flux levels of typically 0.09 mJy. None of these sources lie within 1'
of the central positions of the cores, and they are all likely background
objects. Based on an extrapolation of the empirical correlation between the
bolometric luminosity and 3.6 cm luminosity for the youngest protostars, these
null-detections place upper limits of ~0.1 L_sun (d/140pc)^2 on the
luminosities of protostellar sources embedded within these cores. These limits,
together with the extended nature of the inward motions inferred from molecular
line mapping (Lee et al. 2001), are inconsistent with the inside-out collapse
model of singular isothermal spheres and suggest a less centrally condensed
phase of core evolution during the earliest stages of star formation.Comment: Accepted to the Astronomical Journal; 12 pages, 1 figur
Envelope Structure of Starless Core L694-2 Derived from a Near-Infrared Extinction Map
We present a near-infrared extinction study of the dark globule L694-2, a
starless core that shows strong evidence for inward motions in molecular line
profiles. The J,H, and K band data were taken using the European Southern
Observatory New Technology Telescope. The best fit simple spherical power law
model has index p=2.6 +/- 0.2, over the 0.036--0.1 pc range in radius sampled
in extinction. This power law slope is steeper than the value of p=2 for a
singular isothermal sphere, the initial condition of the inside-out model for
protostellar collapse. Including an additional extinction component along the
line of sight further steepens the inferred profile. Fitting a Bonnor-Ebert
sphere results in a super-critical value of the dimensionless radius xi_max=25
+/- 3. The unstable configuration of material may be related to the observed
inward motions. The Bonnor-Ebert model matches the shape of the observed
profile, but significantly underestimates the amount of extinction (by a factor
of ~4). This discrepancy in normalization has also been found for the nearby
protostellar core B335 (Harvey et al. 2001). A cylindrical density model with
scale height H=0.0164+/- 0.002 pc viewed at a small inclination to the cylinder
axis provides an equally good radial profile as a power law model, and
reproduces the asymmetry of the core remarkably well. In addition, this model
provides a basis for understanding the discrepancy in the normalization of the
Bonnor-Ebert model, namely that L694-2 has prolate structure, with the full
extent (mass) of the core being missed by assuming symmetry between the
profiles in the plane of the sky and along the line-of-sight. If the core is
sufficiently magnetized then fragmentation may be avoided, and later evolution
might produce a protostar similar to B335.Comment: 38 pages, 7 figures, accepted to Astrophysical Journa
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