86 research outputs found
Spectroscopy of Candidate Members of the Eta Cha and MBM12 Young Associations
We present an analysis of candidate members of the Eta Cha and MBM 12A young
associations. For an area of 0.7 deg^2 toward Eta Cha, we have performed a
search for members of the association by combining JHK_s photometry from 2MASS
and i photometry from DENIS with followup optical spectroscopy at Magellan
Observatory. We report the discovery of three new members with spectral types
of M5.25-M5.75, corresponding to masses of 0.13-0.08 M_sun by theoretical
evolutionary models. Two and three of these members were found independently by
Lyo and coworkers and Song and coworkers, respectively. Meanwhile, no brown
dwarfs were detected in Eta Cha down to the completeness limit of 0.015 M_sun.
For MBM 12A, we have obtained spectra of three of the remaining candidate
members that lacked spectroscopy at the end of the survey by Luhman, all of
which are found to be field M dwarfs. Ogura and coworkers have recently
presented four "probable" members of MBM 12A. However, two of these objects
were previously classified as field dwarfs by the spectroscopy of Luhman. In
this work, we find that the other two objects are field dwarfs as well.Comment: to be published in The Astrophysical Journal, 19 pages, 7 figure
Comparing Star Formation on Large Scales in the c2d Legacy Clouds: Bolocam 1.1 mm Dust Continuum Surveys of Serpens, Perseus, and Ophiuchus
We have undertaken an unprecedentedly large 1.1 millimeter continuum survey
of three nearby star forming clouds using Bolocam at the Caltech Submillimeter
Observatory. We mapped the largest areas in each cloud at millimeter or
submillimeter wavelengths to date: 7.5 sq. deg in Perseus (Paper I), 10.8 sq.
deg in Ophiuchus (Paper II), and 1.5 sq. deg in Serpens with a resolution of
31", detecting 122, 44, and 35 cores, respectively. Here we report on results
of the Serpens survey and compare the three clouds. Average measured angular
core sizes and their dependence on resolution suggest that many of the observed
sources are consistent with power-law density profiles. Tests of the effects of
cloud distance reveal that linear resolution strongly affects measured source
sizes and densities, but not the shape of the mass distribution. Core mass
distribution slopes in Perseus and Ophiuchus (alpha=2.1+/-0.1 and
alpha=2.1+/-0.3) are consistent with recent measurements of the stellar IMF,
whereas the Serpens distribution is flatter (alpha=1.6+/-0.2). We also compare
the relative mass distribution shapes to predictions from turbulent
fragmentation simulations. Dense cores constitute less than 10% of the total
cloud mass in all three clouds, consistent with other measurements of low
star-formation efficiencies. Furthermore, most cores are found at high column
densities; more than 75% of 1.1 mm cores are associated with Av>8 mag in
Perseus, 15 mag in Serpens, and 20-23 mag in Ophiuchus.Comment: 32 pages, including 18 figures, accepted for publication in Ap
The dusty disk around VV Ser
We have carried out observations at millimeter and centimeter wavelengths
towards VV Ser using the Plateau de Bure Interferometer and the Very Large
Array. This allows us to compute the SED from near infrared to centimeter
wavelengths. The modeling of the full SED has provided insight into the dust
properties and a more accurate value of the disk mass.
The mass of dust in the disk around VV Ser is found to be about 4 10^(-5)
Msun, i.e. 400 times larger than previous estimates. Moreoever, the SED can
only be accounted for assuming dust stratification in the vertical direction
across the disk. The existence of small grains (0.25--1 micron) in the disk
surface is required to explain the emission at near- and mid-infrared
wavelengths. The fluxes measured at millimeter wavelengths imply that the dust
grains in the midplane have grown up to very large sizes, at least to some
centimeters.Comment: To appear in Ap
Improved to Transformation Equations for Main Sequence Stars
We report improved transformation equations between the and
photometric systems. Although the details of the transformations
depend on luminosity class, we find a typical rms scatter on the order of 0.001
magnitude if the sample is limited to main sequence stars. Furthermore, we find
an accurate transformation requires complex, multi-color dependencies for the
bluer bandpasses. Results for giant stars will be reported in a subsequent
paper.Comment: 7 pages, 8 figure
SHARC-II Mapping of Spitzer c2d Small Clouds and Cores
We present the results of a submillimeter survey of 53 low-mass dense cores
with the Submillimeter High Angular Resolution Camera II (SHARC-II). The survey
is a follow-up project to the Spitzer Legacy Program ``From Molecular Cores to
Planet-Forming Disks'', with the purpose being to create a complete data set of
nearby low-mass dense cores from the infrared to the millimeter. We present
maps of 52 cores at 350 microns and three cores at 450 microns, two of which
were observed at both wavelengths. Of these 52 cores, 41 were detected by
SHARC-II: 32 contained one submillimeter source while 9 contained multiple
sources. For each submillimeter source detected, we report various source
properties including source position, fluxes in various apertures, size, aspect
ratio, and position angle. For the 12 cores that were not detected we present
upper limits. The sources detected by SHARC-II have, on average, smaller sizes
at the 2sigma contours than those derived from longer-wavelength bolometer
observations. We conclude that this is not caused by a failure to integrate
long enough to detect the full extent of the core; instead it arises primarily
from the fact that the observations presented in this survey are insensitive to
smoothly varying extended emission. We find that SHARC-II observations of
low-mass cores are much better suited to distinguishing between starless and
protostellar cores than observations at longer wavelengths. Very Low Luminosity
Objects, a new class of objects being discovered by the Spitzer Space Telescope
in cores previously classified as starless, look very similar at 350 microns to
other cores with more luminous protostars.Comment: Accepted for publication in the Astronomical Journal. 52 pages, 16
figures. See http://peggysue.as.utexas.edu/SIRTF/ for high-resolution figure
A Catalog of Background Stars Reddened by Dust in the Taurus Dark Clouds
Normal field stars located behind dense clouds are a valuable resource in
interstellar astrophysics, as they provide continua in which to study phenomena
such as gas-phase and solid-state absorption features, interstellar extinction
and polarization. This paper reports the results of a search for highly
reddened stars behind the Taurus Dark Cloud complex. We use the Two Micron All
Sky Survey (2MASS) Point Source Catalog to survey a 50 sq deg area of the cloud
to a limiting magnitude of K = 10.0. Photometry in the 1.2-2.2 micron passbands
from 2MASS is combined with photometry at longer infrared wavelengths (3.6-12
micron) from the Spitzer Space Telescope and the Infrared Astronomical
Satellite to provide effective discrimination between reddened field stars and
young stellar objects (YSOs) embedded in the cloud. Our final catalog contains
248 confirmed or probable background field stars, together with estimates of
their total visual extinctions, which span the range 2-29 mag. We also identify
the 2MASS source J04292083+2742074 (IRAS 04262+2735) as a previously
unrecognized candidate YSO, based on the presence of infrared emission greatly
in excess of that predicted for a normal reddened photosphere at wavelengths >5
microns
Interstellar Polarization in the Taurus Dark Clouds, Wavelength Dependent Position Angles and Cloud Structure Near TMC-1
We use polarimetric observations of two stars (HD29647, HD283809) in the
general direction of TMC-1 in the Taurus Dark Cloud to investigate grain
properties and cloud structure in this region. We show the data to be
consistent with a simple two-component model, in which general interstellar
polarization in the Taurus Cloud is produced by a widely distributed cloud
component with relatively uniform magnetic field orientation; the light from
stars close to TMC-1 suffers additional polarization arising in one (or more)
subcloud(s) with larger average grain size and different magnetic field
directions compared with the general trend. Towards HD29647, in particular, we
show that the unusually low degree of visual polarization relative to
extinction is due to the presence of distinct cloud components in the line of
sight with markedly different magnetic field orientations. Stokes parameter
calculations allow us to separate out the polarization characteristics of the
individual components. Results are fit with the Serkowski empirical formula to
determine the degree and wavelength of maximum polarization. Whereas lambda_max
values in the widely distributed material are similar to the average (0.55um)
for the diffuse interstellar medium, the subcloud in line of sight to
HD~283809, the most heavily reddened star in our study, has lambda_max approx.
0.73um, indicating the presence of grains about 30% larger than this average.
Our model also predicts detectable levels of circular polarization toward both
HD~29647 and HD~283809.Comment: 17 pages including 6 figures, LaTeX, to appear in the Astrophysical
Journal, vol 48
Observations of Stellar Objects at a Shell Boundary in the Star-Forming Complex in the Galaxy IC1613
The single region of ongoing star formation in the galaxy IC 1613 has been
observed in order to reveal the nature of compact emission-line objects at the
edges of two shells in the complex, identified earlier in H-alpha line images.
The continuum images show these compact objects to be stars. Detailed
spectroscopic observations of these stars and the surrounding nebulae were
carried out with an integral field spectrograph MPFS mounted on the 6m
telescope of the Special Astrophysical Observatory. The resulting stellar
spectra were used to determine the spectral types and luminosity classes of the
objects. An Of star we identified is the only object of this spectral type in
IC 1613. The results of optical observations of the multi-shell complex are
compared to 21cm radio observations. The shells harboring the stars at their
boundaries constitute the most active part of the star-forming region. There is
evidence that shocks have played an important role in the formation of the
shells.Comment: 10 pages, 5 PS and 1 color JPEG figur
SCUBA Mapping of Spitzer c2d Small Clouds and Cores
We present submillimeter observations of dark clouds that are part of the
Spitzer Legacy Program, From Molecular Cores to Planet-Forming Disks (c2d). We
used the Submillimetre Common User's Bolometer Array to map the regions
observed by Spitzer by the c2d program to create a census of dense molecular
cores including data from the infrared to the submillimeter. In this paper, we
present the basic data from these observations: maps, fluxes, and source
attributes. We also show data for an object just outside the Perseus cloud that
was serendipitously observed in our program. We propose that this object is a
newly discovered, evolved protostar.Comment: 37 pages, accepted to The Astronomical Journa
Crystalline Silicate Emission in the Protostellar Binary Serpens--SVS20
We present spatially resolved mid-infrared spectroscopy of the class
I/flat-spectrum protostellar binary system SVS20 in the Serpens cloud core. The
spectra were obtained with the mid-infrared instrument T-ReCS on Gemini-South.
SVS20-South, the more luminous of the two sources, exhibits a mid-infrared
emission spectrum peaking near 11.3 \micron, while SVS20-North exhibits a
shallow amorphous silicate absorption spectrum with a peak optical depth of
. After removal of the the line-of-sight extinction by the
molecular common envelope, the ``protostar-only'' spectra are found to be
dominated by strong amorphous olivine emission peaking near 10 \micron. We also
find evidence for emission from crystalline forsterite and enstatite associated
with both SVS20-S and SVS20-N. The presence of crystalline silicate in such a
young binary system indicates that the grain processing found in more evolved
HAeBe and T Tauri pre-main sequence stars likely begins at a relatively young
evolutionary stage, while mass accretion is still ongoing.Comment: Accepted for publication by The Astrophysical Journa
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