2,914 research outputs found
A Rotating Disk in the HH 111 Protostellar System
The HH 111 protostellar system is a young Class I system with two sources,
VLA 1 and VLA 2, at a distance of 400 pc. Previously, a flattened envelope has
been seen in C18O to be in transition to a rotationally supported disk near the
VLA 1 source. The follow-up study here is to confirm the rotationally supported
disk at 2-3 times higher angular resolutions, at ~ 0.3" (or 120 AU) in 1.33 mm
continuum, and ~ 0.6" (or 240 AU) in 13CO (J=2-1) and 12CO (J=2-1) emission
obtained with the Submillimeter Array. The 1.33 mm continuum emission shows a
resolved dusty disk associated with the VLA 1 source perpendicular to the jet
axis, with a Gaussian deconvolved size of ~ 240 AU. The 13CO and 12CO emissions
toward the dusty disk show a Keplerian rotation, indicating that the dusty disk
is rotationally supported. The density and temperature distributions in the
disk derived from a simple disk model are found to be similar to those found in
bright T-Tauri disks, suggesting that the disk can evolve into a T-Tauri disk
in the late stage of star formation. In addition, a hint of a low-velocity
molecular outflow is also seen in 13CO and 12CO coming out from the disk.Comment: 16 pages including 5 figure
The Structure of the DoAr 25 Circumstellar Disk
We present high spatial resolution (< 0.3" = 40\Sigma
\propto r^{-p}$ with p = 0.34, significantly less steep than a steady-state
accretion disk (p = 1) or the often adopted minimum mass solar nebula (p =
1.5). Even though the total mass of material is large (M_d = 0.10 M_sun), the
densities inferred in the inner disk for such a model may be too low to
facilitate any mode of planet formation. However, alternative models with
steeper density gradients (p = 1) can explain the observations equally well if
substantial grain growth in the planet formation region (r < 40 AU) has
occurred. We discuss these data in the context of such models with dust
properties that vary with radius and highlight their implications for
understanding disk evolution and the early stages of planet formation.Comment: ApJL in pres
The Masses of Transition Circumstellar Disks: Observational Support for Photoevaporation Models
We report deep Sub-Millimeter Array observations of 26 pre-main-sequence
(PMS) stars with evolved inner disks. These observations measure the mass of
the outer disk (r ~20-100 AU) across every stage of the dissipation of the
inner disk (r < 10 AU) as determined by the IR spectral energy distributions
(SEDs). We find that only targets with high mid-IR excesses are detected and
have disk masses in the 1-5 M_Jup range, while most of our objects remain
undetected to sensitivity levels of M_DISK ~0.2-1.5 M_Jup. To put these results
in a more general context, we collected publicly available data to construct
the optical to millimeter wavelength SEDs of over 120 additional PMS stars. We
find that the near-IR and mid-IR emission remain optically thick in objects
whose disk masses span 2 orders of magnitude (~0.5-50 M_Jup). Taken together,
these results imply that, in general, inner disks start to dissipate only after
the outer disk has been significantly depleted of mass. This provides strong
support for photoevaporation being one of the dominant processes driving disk
evolution.Comment: Accepted for publication by ApJL, 4 pages and 3 figure
A contracting circumbinary molecular ring with an inner cavity of about 140 AU around Ori 139-409
Sensitive and subarcsecond resolution ( 0.7\arcsec) CHOH(7
6) line and 890 m continuum observations made with the
Submillimeter Array (SMA) towards the hot molecular circumbinary ring
associated with the young multiple star Ori 139-409 are presented. The
CHOH(7 - 6) emission from the ring is well resolved at
this angular resolution revealing an inner cavity with a size of about 140 AU.
A LTE model of a Keplerian disk with an inner cavity of the same size confirms
the presence of this cavity. Additionally, this model suggests that the
circumbinary ring is contracting with a velocity of V 1.5 km
s toward the binary central compact circumstellar disks reported at a
wavelength of 7 mm. {\bf The inner central cavity seems to be formed by the
tidal effects of the young stars in the middle of the ring.} The ring appears
to be not a stationary object. Furthermore, the infall velocity we determine is
about a factor of 3 slower than the free-fall velocity corresponding to the
dynamical mass. This would correspond to a mass accretion rate of about
10 M/yr. We found that the dust emission associated with Ori
139-409 appears to be arising from the circumstellar disks with no strong
contribution from the molecular gas ring. A simple comparison with other
classical molecular dusty rings (e.g. GG Tau, UZ Tau, and UY Aur) suggests that
Ori 139-409 could be one of the youngest circumbinary rings reported up to
date. Finally, our results confirm that the circumbinary rings are actively
funneling fresh gas material to the central compact binary circumstellar disks,
i.e. to the protostars in the very early phases of their evolution.Comment: Accepted by MNRA
Protoplanetary Disk Structures in Ophiuchus II: Extension to Fainter Sources
We present new results from a significant extension of our previous high
angular resolution (0.3" = 40 AU) Submillimeter Array survey of the 880 um
continuum emission from dusty circumstellar disks in the ~1 Myr-old Ophiuchus
star-forming region. An expanded sample is constructed to probe disk structures
that emit significantly lower millimeter luminosities (hence dust masses), down
to the median value for T Tauri stars. Using a Monte Carlo radiative transfer
code, the millimeter visibilities and broadband spectral energy distribution
for each disk are simultaneously reproduced with a two-dimensional parametric
model for a viscous accretion disk. We find wide ranges of characteristic radii
(14-198 AU) and disk masses (0.004-0.143 M_sun), but a narrow distribution of
surface density gradients (0.4-1.1) that is consistent with a uniform value
= 0.9 +/- 0.2 and independent of mass (or millimeter luminosity). In
this sample, we find a correlation between the disk luminosity/mass and
characteristic radius, such that fainter disks are both smaller and less
massive. We suggest that this relationship is an imprint of the initial
conditions inherited by the disks at their formation epoch, compare their
angular momenta with those of molecular cloud cores, and speculate on how
future observations can help constrain the distribution of viscous evolution
timescales. No other correlations between disk and star properties are found.
The inferred disk structures are briefly compared with theoretical models for
giant planet formation, although resolution limitations do not permit us to
directly comment on material inside R = 20 AU. However, there is some
compelling evidence for dust evolution in the planet formation region: 4/17
disks in the sample show resolved regions of significantly reduced optical
depths within ~20-40 AU of their central stars.Comment: accepted in ApJ, 39 pages, 10 figure
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A data visualization and data mining approach to response and non-response analysis in survey research
Survey data based on self-selected samples are inherently subject to the threat of selection bias. In this study, both data visualization and data mining techniques were employed to examine whether nonresponse bias had affected a survey regarding 1:1 computing conducted at Arizona State University. Unlike conventional hypothesis testing, data visualization/EDA attends to pattern recognition instead of probabilistic inferences. In addition, unlike logistic regression, classification trees in data mining are capable of ranking independent variables in terms of their predictive power. In contrast to the findings of other studies, this study reveals that academic level, gender, and race were not identified as crucial factors in determining the response rate. Rather, the nature of the subject matter might be more important for science/engineering and law students seemed more interested in this technology-related survey. Accessed 17,302 times on https://pareonline.net from December 20, 2007 to December 31, 2019. For downloads from January 1, 2020 forward, please click on the PlumX Metrics link to the right
Disk Imaging Survey of Chemistry with SMA: II. Southern Sky Protoplanetary Disk Data and Full Sample Statistics
This is the second in a series of papers based on data from DISCS, a
Submillimeter Array observing program aimed at spatially and spectrally
resolving the chemical composition of 12 protoplanetary disks. We present data
on six Southern sky sources - IM Lup, SAO 206462 (HD 135344b), HD 142527, AS
209, AS 205 and V4046 Sgr - which complement the six sources in the Taurus star
forming region reported previously. CO 2-1 and HCO+ 3-2 emission are detected
and resolved in all disks and show velocity patterns consistent with Keplerian
rotation. Where detected, the emission from DCO+ 3-2, N2H+ 3-2, H2CO 3-2 and
4-3,HCN 3-2 and CN 2-1 are also generally spatially resolved. The detection
rates are highest toward the M and K stars, while the F star SAO 206462 has
only weak CN and HCN emission, and H2CO alone is detected toward HD 142527.
These findings together with the statistics from the previous Taurus disks,
support the hypothesis that high detection rates of many small molecules depend
on the presence of a cold and protected disk midplane, which is less common
around F and A stars compared to M and K stars. Disk-averaged variations in the
proposed radiation tracer CN/HCN are found to be small, despite two orders of
magnitude range of spectral types and accretion rates. In contrast, the
resolved images suggest that the CN/HCN emission ratio varies with disk radius
in at least two of the systems. There are no clear observational differences in
the disk chemistry between the classical/full T Tauri disks and transitional
disks. Furthermore, the observed line emission does not depend on measured
accretion luminosities or the number of infrared lines detected, which suggests
that the chemistry outside of 100 AU is not coupled to the physical processes
that drive the chemistry in the innermost few AU.Comment: accepted for publication in ApJ, 41 pages including 7 figure
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