62,935 research outputs found
Mid-Infrared Observations of Class I/Flat-Spectrum Systems in Six Nearby Molecular Clouds
We have obtained new mid-infrared observations of 65 Class I/Flat-Spectrum
(F.S.) objects in the Perseus, Taurus, Chamaeleon I/II, Rho Ophiuchi, and
Serpens dark clouds. We detected 45/48 (94%) of the single sources, 16/16
(100%) of the primary components, and 12/16 (75%) of the secondary/triple
components of the binary/multiple objects surveyed. The composite spectral
energy distributions (SEDs) for all of our sample sources are either Class I or
F.S., and, in 15/16 multiple systems, at least one of the individual components
displays a Class I or F.S. spectral index. However, the occurrence of mixed
pairings, such as F.S. with Class I, F.S. with Class II, and, in one case, F.S.
with Class III, is surprisingly frequent. Such behaviour is not consistent with
that of multiple systems among T Tauri stars (TTS), where the companion of a
classical TTS also tends to be a classical TTS, although other mixed pairings
have been previously observed among Class II objects. Based on an analysis of
the spectral indices of the individual binary components, there appears to be a
higher proportion of mixed Class I/F.S. systems (65-80%) than that of mixed
Classical/Weak-Lined TTS (25-40%), demonstrating that the envelopes of Class I/
F.S. systems are rapidly evolving during this evolutionary phase. We report the
discovery of a steep spectral index secondary companion to ISO-ChaI 97,
detected for the first time via our mid-infrared observations. In our previous
near- infrared imaging survey of binary/multiple Class I/F.S. sources, ISO-ChaI
97 appeared to be single. With a spectral index of Alpha >= 3.9, the secondary
component of this system is a member of a rare class of very steep spectral
index objects, those with Alpha > 3. Only three such objects have previously
been reported, all of which are either Class 0 or Class I.Comment: 31 pages, 4 figures, 6 table
Interpolation of nonstationary high frequency spatial-temporal temperature data
The Atmospheric Radiation Measurement program is a U.S. Department of Energy
project that collects meteorological observations at several locations around
the world in order to study how weather processes affect global climate change.
As one of its initiatives, it operates a set of fixed but irregularly-spaced
monitoring facilities in the Southern Great Plains region of the U.S. We
describe methods for interpolating temperature records from these fixed
facilities to locations at which no observations were made, which can be useful
when values are required on a spatial grid. We interpolate by conditionally
simulating from a fitted nonstationary Gaussian process model that accounts for
the time-varying statistical characteristics of the temperatures, as well as
the dependence on solar radiation. The model is fit by maximizing an
approximate likelihood, and the conditional simulations result in
well-calibrated confidence intervals for the predicted temperatures. We also
describe methods for handling spatial-temporal jumps in the data to interpolate
a slow-moving cold front.Comment: Published in at http://dx.doi.org/10.1214/13-AOAS633 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
A Combined Spitzer and Chandra Survey of Young Stellar Objects in the Serpens Cloud Core
We present Spitzer and Chandra observations of the nearby (~260 pc) embedded
stellar cluster in the Serpens Cloud Core. We observed, using Spitzer's IRAC
and MIPS instruments, in six wavelength bands from 3 to 70 , to detect
thermal emission from circumstellar disks and protostellar envelopes, and to
classify stars using color-color diagrams and spectral energy distributions
(SEDs). These data are combined with Chandra observations to examine the
effects of circumstellar disks on stellar X-ray properties. Young diskless
stars were also identified from their increased X-ray emission. We have
identified 138 YSOs in Serpens: 22 class 0/I, 16 flat spectrum, 62 class II, 17
transition disk, and 21 class III stars; 60 of which exhibit X-ray emission.
Our primary results are the following: 1.) ten protostars detected previously
in the sub-millimeter are detected at lambda < 24 microns, seven at lambda < 8
microns, 2.) the protostars are more closely grouped than more evolved YSOs
(median separation : ~0.024 pc, and 3.) the luminosity and temperature of the
X-ray emitting plasma around these YSOs does not show any significant
dependence on evolutionary class. We combine the infrared derived values of AK
and X-ray values of NH for 8 class III objects and find that the column density
of hydrogen gas per mag of extinctions is less than half the standard
interstellar value, for AK > 1. This may be the result of grain growth through
coagulation and/or the accretion of volatiles in the Serpens cloud core.Comment: 69 pages, 16 figures, accepted to ApJ. Higher Resolution Figures at:
http://www.cfa.harvard.edu/~ewinston
AKARI Far-Infrared Source Counts in the Lockman Hole
We report initial results of far-infrared observations of the Lockman hole
with Far-Infrared Surveyor (FIS) onboard the AKARI infrared satellite. On the
basis of slow scan observations of a 0.6 deg x 1.2 deg contiguous area, we
obtained source number counts at 65, 90 and 140 um down to 77, 26 and 194 mJy
(3 sigma), respectively. The counts at 65 and 140 um show good agreement with
the Spitzer results. However, our 90 um counts are clearly lower than the
predicted counts by recent evolutionary models that fit the Spitzer counts in
all the MIPS bands. Our 90 um counts above 26 mJy account for about 7% of the
cosmic background. These results provide strong constraints on the evolutionary
scenario and suggest that the current models may require modifications.Comment: 25 pages, 8 figures, Publications of the Astronomical Society of
Japan, in pres
Spin alignment of stars in old open clusters
Stellar clusters form by gravitational collapse of turbulent molecular
clouds, with up to several thousand stars per cluster. They are thought to be
the birthplace of most stars and therefore play an important role in our
understanding of star formation, a fundamental problem in astrophysics. The
initial conditions of the molecular cloud establish its dynamical history until
the stellar cluster is born. However, the evolution of the cloud's angular
momentum during cluster formation is not well understood. Current observations
have suggested that turbulence scrambles the angular momentum of the
cluster-forming cloud, preventing spin alignment amongst stars within a
cluster. Here we use asteroseismology to measure the inclination angles of spin
axes in 48 stars from the two old open clusters NGC~6791 and NGC~6819. The
stars within each cluster show strong alignment. Three-dimensional
hydrodynamical simulations of proto-cluster formation show that at least 50 %
of the initial proto-cluster kinetic energy has to be rotational in order to
obtain strong stellar-spin alignment within a cluster. Our result indicates
that the global angular momentum of the cluster-forming clouds was efficiently
transferred to each star and that its imprint has survived after several
gigayears since the clusters formed.Comment: 14 pages, 3 figures, 1 table. Published in Nature Astronom
How unique is Plaskett's star? A search for organized magnetic fields in short period, interacting or post-interaction massive binary systems
Amongst O-type stars with detected magnetic fields, the fast rotator in the
close binary called Plaskett's star shows a variety of unusual properties.
Since strong binary interactions are believed to have occurred in this system,
one may wonder about their potential role in generating magnetic fields. Stokes
V spectra collected with the low-resolution FORS2 and high-resolution ESPaDOnS
and Narval spectropolarimeters were therefore used to search for magnetic
fields in 15 interacting or post-interaction massive binaries. No magnetic
field was detected in any of them, with 0G always being within 2sigma of the
derived values. For 17 out of 25 stars in the systems observed at
high-resolution, the 90% upper limit on the individual dipolar fields is below
the dipolar field strength of Plaskett's secondary; a similar result is found
for five out of six systems observed at low resolution. If our sample is
considered to form a group of stars sharing similar magnetic properties, a
global statistical analysis results in a stringent upper limit of ~200G on the
dipolar field strength. Moreover, the magnetic incidence rate in the full
sample of interacting or post-interaction systems (our targets + Plaskett's
star) is compatible with that measured from large surveys, showing that they
are not significantly different from the general O-star population. These
results suggest that binary interactions play no systematic role in the
magnetism of such massive systems.Comment: 11 pages, accepted for publication in MNRA
The Field White Dwarf Mass Distribution
We revisit the properties and astrophysical implications of the field white
dwarf mass distribution in preparation of Gaia applications. Our study is based
on the two samples with the best established completeness and most precise
atmospheric parameters, the volume-complete survey within 20 pc and the Sloan
Digital Sky Survey (SDSS) magnitude-limited sample. We explore the modelling of
the observed mass distributions with Monte Carlo simulations, but find that it
is difficult to constrain independently the initial mass function (IMF), the
initial-to-final-mass relation (IFMR), the stellar formation history (SFH), the
variation of the Galactic disk vertical scale height as a function of stellar
age, and binary evolution. Each of these input ingredients has a moderate
effect on the predicted mass distributions, and we must also take into account
biases owing to unidentified faint objects (20 pc sample), as well as unknown
masses for magnetic white dwarfs and spectroscopic calibration issues (SDSS
sample). Nevertheless, we find that fixed standard assumptions for the above
parameters result in predicted mean masses that are in good qualitative
agreement with the observed values. It suggests that derived masses for both
studied samples are consistent with our current knowledge of stellar and
Galactic evolution. Our simulations overpredict by 40-50% the number of massive
white dwarfs (M > 0.75 Msun) for both surveys, although we can not exclude a
Salpeter IMF when we account for all biases. Furthermore, we find no evidence
of a population of double white dwarf mergers in the observed mass
distributions.Comment: 15 pages, 16 figures, accepted for publication in MNRA
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