80 research outputs found
Imaging the Disk around TW Hydrae with the Submillimeter Array
We present ~2"-4" aperture synthesis observations of the circumstellar disk surrounding the nearby young star TW Hya in the CO J = 2-1 and J = 3-2 lines and associated dust continuum obtained with the partially completed Submillimeter Array. The extent and peak flux of the 230 and 345 GHz dust emission follow closely the predictions of the irradiated accretion disk model of Calvet et al. The resolved molecular line emission extends to a radius of at least 200 AU, the full extent of the disk visible in scattered light, and shows a clear pattern of Keplerian rotation. Comparison of the images with two-dimensional Monte Carlo models constrains the disk inclination angle to 7° ± 1°. The CO emission is optically thick in both lines, and the kinetic temperature in the line formation region is ~20 K. Substantial CO depletion, by an order of magnitude or more from canonical dark cloud values, is required to explain the characteristics of the line emission
The Detection and Characterization of cm Radio Continuum Emission from the Low-mass Protostar L1014-IRS
Observations by the Cores to Disk Legacy Team with the Spitzer Space
Telescope have identified a low luminosity, mid-infrared source within the
dense core, Lynds 1014, which was previously thought to harbor no internal
source. Followup near-infrared and submillimeter interferometric observations
have confirmed the protostellar nature of this source by detecting scattered
light from an outflow cavity and a weak molecular outflow. In this paper, we
report the detection of cm continuum emission with the VLA. The emission is
characterized by a quiescent, unresolved 90 uJy 6 cm source within 0.2" of the
Spitzer source. The spectral index of the quiescent component is between 6 cm and 3.6 cm. A factor of two increase in 6 cm
emission was detected during one epoch and circular polarization was marginally
detected at the level with Stokes {V/I} % . We have
searched for 22 GHz H2O maser emission toward L1014-IRS, but no masers were
detected during 7 epochs of observations between June 2004 and December 2006.
L1014-IRS appears to be a low-mass, accreting protostar which exhibits cm
emission from a thermal jet or a wind, with a variable non-thermal emission
component. The quiescent cm radio emission is noticeably above the correlation
of 3.6 cm and 6 cm luminosity versus bolometric luminosity, indicating more
radio emission than expected. We characterize the cm continuum emission in
terms of observations of other low-mass protostars, including updated
correlations of centimeter continuum emission with bolometric luminosity and
outflow force, and discuss the implications of recent larger distance estimates
on the physical attributes of the protostar and dense molecular core.Comment: 14 pages. Accepted for publication in Ap
From Molecular Cores to Planet-forming Disks: An SIRTF Legacy Program
Crucial steps in the formation of stars and planets can be studied only at mid‐ to far‐infrared wavelengths, where the Space Infrared Telescope (SIRTF) provides an unprecedented improvement in sensitivity. We will use all three SIRTF instruments (Infrared Array Camera [IRAC], Multiband Imaging Photometer for SIRTF [MIPS], and Infrared Spectrograph [IRS]) to observe sources that span the evolutionary sequence from molecular cores to protoplanetary disks, encompassing a wide range of cloud masses, stellar masses, and star‐forming environments. In addition to targeting about 150 known compact cores, we will survey with IRAC and MIPS (3.6–70 μm) the entire areas of five of the nearest large molecular clouds for new candidate protostars and substellar objects as faint as 0.001 solar luminosities. We will also observe with IRAC and MIPS about 190 systems likely to be in the early stages of planetary system formation (ages up to about 10 Myr), probing the evolution of the circumstellar dust, the raw material for planetary cores. Candidate planet‐forming disks as small as 0.1 lunar masses will be detectable. Spectroscopy with IRS of new objects found in the surveys and of a select group of known objects will add vital information on the changing chemical and physical conditions in the disks and envelopes. The resulting data products will include catalogs of thousands of previously unknown sources, multiwavelength maps of about 20 deg^2 of molecular clouds, photometry of about 190 known young stars, spectra of at least 170 sources, ancillary data from ground‐based telescopes, and new tools for analysis and modeling. These products will constitute the foundations for many follow‐up studies with ground‐based telescopes, as well as with SIRTF itself and other space missions such as SIM, JWST, Herschel, and TPF/Darwin
The Spitzer c2d survey of Large, Nearby, Interstellar Clouds. V. Chamaeleon II Observed with IRAC
We present IRAC (3.6, 4.5, 5.8, and 8.0 micron) observations of the
Chamaeleon II molecular cloud. The observed area covers about 1 square degree
defined by . Analysis of the data in the 2005 c2d catalogs reveals a
small number of sources (40) with properties similar to those of young
stellaror substellar objects (YSOs). The surface density of these YSO
candidates is low, and contamination by background galaxies appears to be
substantial, especially for sources classified as Class I or flat SED. We
discuss this problem in some detail and conclude that very few of the candidate
YSOs in early evolutionary stages are actually in the Cha II cloud. Using a
refined set of criteria, we define a smaller, but more reliable, set of 24 YSO
candidates.Comment: 19 pages, 10 figures, in press Ap
The Spitzer c2d Survey of Large, Nearby, Interstellar Clouds. IV. Lupus Observed with MIPS
We present maps of 7.78 square degrees of the Lupus molecular cloud complex
at 24, 70, and m. They were made with the Spitzer Space Telescope's
Multiband Imaging Photometer for Spitzer (MIPS) instrument as part of the
Spitzer Legacy Program, ``From Molecular Cores to Planet-Forming Disks'' (c2d).
The maps cover three separate regions in Lupus, denoted I, III, and IV. We
discuss the c2d pipeline and how our data processing differs from it. We
compare source counts in the three regions with two other data sets and
predicted star counts from the Wainscoat model. This comparison shows the
contribution from background galaxies in Lupus I. We also create two color
magnitude diagrams using the 2MASS and MIPS data. From these results, we can
identify background galaxies and distinguish them from probable young stellar
objects. The sources in our catalogs are classified based on their spectral
energy distribution (SED) from 2MASS and Spitzer wavelengths to create a sample
of young stellar object candidates. From 2MASS data, we create extinction maps
for each region and note a strong corresponence between the extinction and the
m emission. The masses we derived in each Lupus cloud from our
extinction maps are compared to masses estimated from CO and CO
and found to be similar to our extinction masses in some regions, but
significantly different in others. Finally, based on our color-magnitude
diagrams, we selected 12 of our reddest candidate young stellar objects for
individual discussion. Five of the 12 appear to be newly-discovered YSOs.Comment: 15 pages, 17 figures, uses emulateapj.cls. Accepted for publication
in ApJ. A version with high-quality figures can be found at
http://peggysue.as.utexas.edu/SIRTF
The Spitzer c2d survey of large, nearby, interstellar clouds. I. Chamaeleon II observed with MIPS
We present maps of over 1.5 deg2 in Chamaeleon (Cha) II at 24, 70, and 160 μm observed with the Spitzer Space Telescope Multiband Imaging Photometer for Spitzer (MIPS) and a 1.2 deg2 millimeter map from SIMBA on the Swedish-ESO Submillimetre Telescope (SEST). The c2d Spitzer Legacy Team's data reduction pipeline is described in detail. Over 1500 24 μm sources and 41 70 μm sources were detected by MIPS with fluxes greater than 10 σ. More than 40 potential YSOs are identified with a MIPS and 2MASS color-color diagram and by their spectral indices, including two previously unknown sources with 24 μm excesses. Our new SIMBA millimeter map of Cha II shows that only a small fraction of the gas is in compact structures with high column densities. The extended emission seen by MIPS is compared with previous CO observations. Some selected interesting sources, including two detected at 1 mm, associated with Cha II are discussed in detail, and their SEDs are presented. The classification of these sources using MIPS data is found to be consistent with that of previous studies
The Spitzer c2d Survey of Nearby Dense Cores: IV. Revealing the Embedded Cluster in B59
Infrared images of the dark cloud core B59 were obtained with the Spitzer
Space Telescope as part of the "Cores to Disks" Legacy Science project.
Photometry from 3.6-70 microns indicates at least 20 candidate low-mass young
stars near the core, more than doubling the previously known population. Out of
this group, 13 are located within about 0.1 pc in projection of the molecular
gas peak, where a new embedded source is detected. Spectral energy
distributions span the range from small excesses above photospheric levels to
rising in the mid-infrared. One other embedded object, probably associated with
the millimeter source B59-MMS1, with a bolometric luminosity L(bol) roughly 2
L(sun), has extended structure at 3.6 and 4.5 microns, possibly tracing the
edges of an outflow cavity. The measured extinction through the central part of
the core is A(V) greater than of order 45 mag. The B59 core is producing young
stars with a high efficiency
From Molecular Cores to Planet-forming Disks: A SIRTF Legacy Program
Crucial steps in the formation of stars and planets can be studied only at
mid-infrared to far-infrared wavelengths, where SIRTF provides an unprecedented
improvement in sensitivity. We will use all three SIRTF instruments (IRAC,
MIPS, and IRS) to observe sources that span the evolutionary sequence from
molecular cores to protoplanetary disks, encompassing a wide range of cloud
masses, stellar masses, and star-forming environments. In addition to targeting
about 150 known compact cores, we will survey with IRAC and MIPS (3.6 to 70
micron) the entire areas of five of the nearest large molecular clouds for new
candidate protostars and substellar objects as faint as 0.001 solar
luminosities. We will also observe with IRAC and MIPS about 190 systems likely
to be in the early stages of planetary system formation(ages up to about 10
Myr), probing the evolution of the circumstellar dust, the raw material for
planetary cores. Candidate planet-forming disks as small as 0.1 lunar masses
will be detectable. Spectroscopy with IRS of new objects found in the surveys
and of a select group of known objects will add vital information on the
changing chemical and physical conditions in the disks and envelopes. The
resulting data products will include catalogs of thousands of previously
unknown sources, multiwavelength maps of about 20 square degrees of molecular
clouds, photometry of about 190 known young stars, spectra of at least 170
sources, ancillary data from ground-based telescopes, and new tools for
analysis and modeling. These products will constitute the foundations for many
follow-up studies with ground-based telescopes, as well as with SIRTF itself
and other space missions such as SIM, JWST, Herschel, and TPF.Comment: (22 pages, 10 figures, PASP in press
The genomes of two key bumblebee species with primitive eusocial organization
Background: The shift from solitary to social behavior is one of the major evolutionary transitions. Primitively eusocial bumblebees are uniquely placed to illuminate the evolution of highly eusocial insect societies. Bumblebees are also invaluable natural and agricultural pollinators, and there is widespread concern over recent population declines in some species. High-quality genomic data will inform key aspects of bumblebee biology, including susceptibility to implicated population viability threats. Results: We report the high quality draft genome sequences of Bombus terrestris and Bombus impatiens, two ecologically dominant bumblebees and widely utilized study species. Comparing these new genomes to those of the highly eusocial honeybee Apis mellifera and other Hymenoptera, we identify deeply conserved similarities, as well as novelties key to the biology of these organisms. Some honeybee genome features thought to underpin advanced eusociality are also present in bumblebees, indicating an earlier evolution in the bee lineage. Xenobiotic detoxification and immune genes are similarly depauperate in bumblebees and honeybees, and multiple categories of genes linked to social organization, including development and behavior, show high conservation. Key differences identified include a bias in bumblebee chemoreception towards gustation from olfaction, and striking differences in microRNAs, potentially responsible for gene regulation underlying social and other traits. Conclusions: These two bumblebee genomes provide a foundation for post-genomic research on these key pollinators and insect societies. Overall, gene repertoires suggest that the route to advanced eusociality in bees was mediated by many small changes in many genes and processes, and not by notable expansion or depauperation
“But the moment they find out that you are MSM…”: a qualitative investigation of HIV prevention experiences among men who have sex with men (MSM) in Ghana’s health care system
Abstract: The prevalence of HIV in Ghana is 1.3%, compared to 17% among men who have sex with men (MSM). There is limited empirical data on the current health care climate and its impact on HIV prevention services for Ghanaian MSM. The purposes of this study were to investigate (1) MSM’s experiences using HIV prevention resources, (2) what factors, including health care climate factors, influenced MSM’s use of prevention resources and (3) MSM self-identified strategies for improving HIV/sexually transmitted infection (STI) prevention among MSM in Ghanaian communities. Methods: We conducted 22 focus groups (n = 137) with peer social networks of MSM drawn from three geographic communities in Ghana (Accra, Kumasi, Manya Krobo). The data were examined using qualitative content analysis. Interviews with individual health care providers were also conducted to supplement the analysis of focus group findings to provide more nuanced illuminations of the experiences reported by MSM..
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