6,108 research outputs found
Constraining the Sub-AU-Scale Distribution of Hydrogen and Carbon Monoxide Gas around Young Stars with the Keck Interferometer
We present Keck Interferometer observations of T Tauri and Herbig Ae/Be stars
with a spatial resolution of a few milliarcseconds and a spectral resolution of
~2000. Our observations span the K-band, and include the Br gamma transition of
Hydrogen and the v=2-0 and v=3-1 transitions of carbon monoxide. For several
targets we also present data from Keck/NIRSPEC that provide higher spectral
resolution, but a seeing-limited spatial resolution, of the same spectral
features. We analyze the Br gamma emission in the context of both disk and
infall/outflow models, and conclude that the Br gamma emission traces gas at
very small stellocentric radii, consistent with the magnetospheric scale.
However some Br gamma-emitting gas also seems to be located at radii of >0.1
AU, perhaps tracing the inner regions of magnetically launched outflows. CO
emission is detected from several objects, and we generate disk models that
reproduce both the KI and NIRSPEC data well. We infer the CO spatial
distribution to be coincident with the distribution of continuum emission in
most cases. Furthermore the Br gamma emission in these objects is roughly
coincident with both the CO and continuum emission. We present potential
explanations for the spatial coincidence of continuum, Br gamma, and CO
overtone emission, and explore the implications for the low occurrence rate of
CO overtone emission in young stars. Finally, we provide additional discussion
of V1685 Cyg, which is unusual among our sample in showing large differences in
emitting region size and spatial position as a function of wavelength.Comment: Accepted for publication in MNRA
Outflow 20--2000 AU from a High-Mass Protostar in W51-IRS2
We present the results of the first high angular resolution observations of
SiO maser emission towards the star forming region W51-IRS2 made with the Very
Large Array (VLA) and Very Long Baseline Array (VLBA). Our images of the water
maser emission in W51-IRS2 reveal two maser complexes bracketing the SiO maser
source. One of these water maser complexes appears to trace a bow shock whose
opening angle is consistent with the opening angle observed in the distribution
of SiO maser emission. A comparison of our water maser image with an image
constructed from data acquired 19 years earlier clearly shows the persistence
and motion of this bow shock. The proper motions correspond to an outflow
velocity of 80 km/s, which is consistent with the data of 19 years ago (that
spanned 2 years). We have discovered a two-armed linear structure in the SiO
maser emission on scales of ~25 AU, and we find a velocity gradient on the
order of 0.1 km/s/AU along the arms. We propose that the SiO maser source
traces the limbs of an accelerating bipolar outflow close to an obscured
protostar. We estimate that the outflow makes an angle of <20 degrees with
respect to the plane of the sky. Our measurement of the acceleration is
consistent with a reported drift in the line-of-sight velocity of the W51 SiO
maser source.Comment: 19 pages, 5 figures (including 3 color). Accepted for publication in
ApJ (April 1, 2001 issue
First L-band Interferometric Observations of a Young Stellar Object: Probing the Circumstellar Environment of MWC 419
We present spatially-resolved K- and L-band spectra (at spectral resolution R
= 230 and R = 60, respectively) of MWC 419, a Herbig Ae/Be star. The data were
obtained simultaneously with a new configuration of the 85-m baseline Keck
Interferometer. Our observations are sensitive to the radial distribution of
temperature in the inner region of the disk of MWC 419. We fit the visibility
data with both simple geometric and more physical disk models. The geometric
models (uniform disk and Gaussian) show that the apparent size increases
linearly with wavelength in the 2-4 microns wavelength region, suggesting that
the disk is extended with a temperature gradient. A model having a power-law
temperature gradient with radius simultaneously fits our interferometric
measurements and the spectral energy distribution data from the literature. The
slope of the power-law is close to that expected from an optically thick disk.
Our spectrally dispersed interferometric measurements include the Br gamma
emission line. The measured disk size at and around Br gamma suggests that
emitting hydrogen gas is located inside (or within the inner regions) of the
dust disk.Comment: Accepted for publication in Ap
Interferometric Observations of V838 Monocerotis
We have used long-baseline near-IR interferometry to resolve the peculiar
eruptive variable V838 Mon and to provide the first direct measurement of its
angular size. Assuming a uniform disk model for the emission we derive an
apparent angular diameter at the time of observations (November-December 2004)
of milli-arcseconds. For a nominal distance of kpc,
this implies a linear radius of . However, the data are
somewhat better fit by elliptical disk or binary component models, and we
suggest that the emission may be strongly affected by ejecta from the outburst.Comment: 12 pages, 1 two-part encapsulated postscript figure. Accepted by
ApJL. Added a table of observation
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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