734 research outputs found
Dust rings and filaments around the isolated young star V1331 Cygni
We characterize the small and large scale environment of the young star V1331
Cygni with high resolution HST/WFPC2 and Digitized Sky Survey images. In
addition to a previously known outer dust ring (~30'' in diameter), the
HST/WFPC2 scattered light image reveals an inner dust ring for the first time.
This ring has a maximum radius of 6.5'' and is possibly related to a molecular
envelope. Large-scale optical images show that V1331 Cyg is located at the tip
of a long dust filament linking it to the dark cloud LDN 981. We discuss the
origin of the observed dust morphology and analyze the object's relation to its
parent dark cloud LDN 981. Finally, based on recent results from the
literature, we investigate the properties of V1331 Cyg and conclude that in its
current state the object does not show suffcient evidence to be characterized
as an FU Ori object.Comment: 15 pages ApJ preprint style including 3 figures, accepted for
publication in ApJ (Feb. 2007
Assessment of the Influence of Demographic and Professional Characteristics on Health Care Providers' Pain Management Decisions Using Virtual Humans
Disparities in health care associated with patientsâ gender, race, and age are well documented. Previous studies using virtual human (VH) technology have demonstrated that provider characteristics may play an important role in pain management decisions. However, these studies have largely emphasized group differences. The aims of this study were to examine dentistsâ and physiciansâ use of VH characteristics when making clinical judgments (i.e., cue use) and to identify provider characteristics associated with the magnitude of the impact of these cues (ÎČ-weights). Providers (N=152; 76 physicians, 76 dentists) viewed video vignettes of VH patients varying in gender (male/female), race (white/black), and age (younger/older). Participants rated VH patientsâ pain intensity and unpleasantness and then rated their own likelihood of administering non-opioid and opioid analgesics. Compared to physicians, dentists had significantly lower ÎČ-weights associated with VH age cues for all ratings (p0.69). These effects varied by provider race and gender. For pain intensity, professional differences were present only among non-white providers. White providers had greater ÎČ-weights than non-white providers for pain unpleasantness but only among men. Provider differences regarding the use of VH age cues in non-opioid analgesic administration were present among all providers except non-white males. These findings highlight the interaction of patient and provider factors in driving clinical decision making. Although profession was related to use of VH age cues in pain-related clinical judgments, this relationship was modified by providersâ personal characteristics. Additional research is needed to understand what aspects of professional training or practice may account for differences between physicians and dentists and what forms of continuing education may help to mitigate the disparities
A Methane Isolated Planetary Mass Object in Orion
We report on the discovery of a free-floating methane dwarf toward the
direction of the young star cluster sigma Orionis. Based on the object's
far-red optical and near-infrared photometry and spectroscopy, we conclude that
it is a possible member of this association. We have named it as S Ori
J053810.1-023626 (S Ori 70 is the abridged name). If it is a true member of
sigma Orionis, the comparison of the photometric and spectroscopic properties
of S Ori 70 with state-of-the-art evolutionary models yields a mass of 3
(+5/-1) Jupiter mass for ages between 1 Myr and 8 Myr. The presence of such a
low-mass object in our small search area (55.4 sq. arcmin) would indicate a
rising substellar initial mass function in the sigma Orionis cluster even for
planetary masses.Comment: Accepted for publication in the ApJ. Twelve pages, figures and tables
include
The Substellar Mass Function in sigma Orionis
We combine results from imaging searches for substellar objects in the sigma
Orionis cluster and follow-up photometric and spectroscopic observations to
derive a census of the brown dwarf population in a region of 847 arcmin^2. We
identify 64 very low-mass cluster member candidates in this region. We have
available three color (IZJ) photometry for all of them, spectra for 9 objects,
and K photometry for 27% of our sample. These data provide a well defined
sequence in the I vs I-J, I-K color magnitude diagrams, and indicate that the
cluster is affected by little reddening despite its young age (~5 Myr). Using
state-of-the-art evolutionary models, we derive a mass function from the
low-mass stars (0.2 Msol) across the complete brown dwarf domain (0.075 Msol to
0.013 Msol), and into the realm of free-floating planetary-mass objects (<0.013
Msol). We find that the mass spectrum (dN/dm ~ m^{-alpha}) increases toward
lower masses with an exponent alpha = 0.8+/-0.4. Our results suggest that
planetary-mass isolated objects could be as common as brown dwarfs; both kinds
of objects together would be as numerous as stars in the cluster. If the
distribution of stellar and substellar masses in sigma Orionis is
representative of the Galactic disk, older and much lower luminosity
free-floating planetary-mass objects with masses down to about 0.005 Msol
should be abundant in the solar vicinity, with a density similar to M-type
stars.Comment: Accepted for publication in ApJ. 19 pages, 3 figures include
HST/NICMOS Imaging of Disks and Envelopes Around Very Young Stars
We present HST/NICMOS observations with 0.1" (15 AU) resolution of six young
stellar objects in the Taurus star-formation region. The targets of our survey
are three Class I IRAS sources (IRAS 04016+2610, IRAS 04248+2612, and IRAS
04302+2247) and three low-luminosity stars (DG Tau B, Haro 6-5B, and CoKu
Tau/1) associated with Herbig Haro jets. The broad-band images show that the
near-infrared radiation from these sources is dominated by light scattered from
dusty circumstellar material distributed in a region 10 - 15 times the size of
our solar system. Although the detailed morphologies of the individual objects
are unique, the observed young stellar objects share common features. All of
the circumstellar reflection nebulae are crossed by dark lanes from 500 - 900
AU in extent and from less than 50 to 350 AU in apparent thickness. The
absorption lanes extend perpendicular to known optical and millimeter outflows
in these sources. We interpret the dark lanes as optically thick circumstellar
disks seen in silhouette against bright reflection nebulosity. The bipolar
reflection nebulae extending perpendicular to the dust lanes appear to be
produced by scattering from the upper and lower surfaces of the disks and from
dusty material within or on the walls of the outflow cavities. Out of five
objects in which the central source is directly detected, two are found to be
subarcsecond binaries. This mini-survey is the highest resolution near-infrared
study to date of circumstellar environments around solar-type stars with age <=
1 Myr.Comment: 34 pages, 4 figures; also available at
http://spider.ipac.caltech.edu/staff/brandner/topics/disks/disks.html ;
accepted for publication in AJ (March 1999 issue
Optical spectroscopy of isolated planetary mass objects in the sigma Orionis cluster
We have obtained low resolution optical spectra of 15 isolated planetary mass
objects (IPMOs) in the sigma Orionis cluster, and derived spectral types by
comparison with nearby M and L dwarfs. The spectral types are in the range late
M - mid L, in agreement with our expectations based on colors and magnitudes
for bona fide members. Therefore, most of these objects have masses below the
deuterium burning limit. About 2/3 show H(alpha) in emission at our spectral
resolution. From our spectroscopic and photometric data, we infer that three
IPMOs in this sample may be binaries with components of similar masses. These
results confirm that the substellar mass function of the sigma Orionis cluster,
in the form dN/dM, keeps rising in the planetary domain.Comment: Astronomy and Astrophysics, accepte
A search for hot massive extrasolar planets around nearby young stars with the adaptive optics system NACO
We report on a survey devoted for the search of exo-planets around young and
nearby stars carried out with NACO at the VLT. The detection limit for 28 among
the best available targets vs. the angular separation from the star is
presented. The non-detection of any planetary mass companion in our survey is
used to derive, for the first time, the frequency of the upper limit of the
projected separation planet-stars. In particular, we find that in 50 % of
cases, no 5 MJ planet (or more massive) has been detected at projected
separations larger than 14 AU and no 10 MJ planet (or more massive) has been
detected at projected separations larger than 8.5 AU. In 100 % of cases, these
values increase to 36 AU and 65 AU respectively. The excellent sensitivity
reached by our study leads to a much lower upper limit of the projected
planet-star separation compared with previous studies. For example, for the
Beta Pictoris group, (~ 12 Myr), we did not detect any 10 MJ planet at
distances larger than 15 AU. A previous study carried out with 4 m class
telescopes put an upper limit for 10 MJ planets at ~ 60 AU. For our closest
target (V 2306 Oph - d = 4.3 pc) it is shown that it would be possible to
detect a 10 MJ planet at a minimum projected separation from the star of 1 AU
and a 5 MJ planet at a minimum projected separation of 3.7 AU. Our results are
discussed with respect to mechanisms explaining planet formation and migration
and to forthcoming observational strategies and future planet finder
observations from the ground.Comment: 35 pages, 10 figures, pdf file, ApJ, accepted, see also
http://www.mpia.de/homes/masciad
Three-Dimensional Simulations of Jets from Keplerian Disks: Self--Regulatory Stability
We present the extension of previous two-dimensional simulations of the
time-dependent evolution of non-relativistic outflows from the surface of
Keplerian accretion disks, to three dimensions. The accretion disk itself is
taken to provide a set of fixed boundary conditions for the problem. The 3-D
results are consistent with the theory of steady, axisymmetric, centrifugally
driven disk winds up to the Alfv\'en surface of the outflow. Beyond the
Alfv\'en surface however, the jet in 3-D becomes unstable to non-axisymmetric,
Kelvin-Helmholtz instabilities. We show that jets maintain their long-term
stability through a self-limiting process wherein the average Alfv\'enic Mach
number within the jet is maintained to order unity. This is accomplished in at
least two ways. First, poloidal magnetic field is concentrated along the
central axis of the jet forming a ``backbone'' in which the Alfv\'en speed is
sufficiently high to reduce the average jet Alfv\'enic Mach number to unity.
Second, the onset of higher order Kelvin-Helmholtz ``flute'' modes (m \ge 2)
reduce the efficiency with which the jet material is accelerated, and transfer
kinetic energy of the outflow into the stretched, poloidal field lines of the
distorted jet. This too has the effect of increasing the Alfv\'en speed, and
thus reducing the Alfv\'enic Mach number. The jet is able to survive the onset
of the more destructive m=1 mode in this way. Our simulations also show that
jets can acquire corkscrew, or wobbling types of geometries in this relatively
stable end-state, depending on the nature of the perturbations upon them.
Finally, we suggest that jets go into alternating periods of low and high
activity as the disappearance of unstable modes in the sub-Alfv\'enic regime
enables another cycle of acceleration to super-Alfv\'enic speeds.Comment: 57 pages, 22 figures, submitted to Ap
The substellar population of Sigma Orionis: A deep wide survey
We present a deep I,Z photometric survey covering a total area of 1.12
deg^{2} of the Sigma Orionis cluster (Icompl=22 and Zcompl=21.5mag). From I,
I-Z color-magnitude diagrams we have selected 153 candidates that fit the
previously known sequence of the cluster. Using J-band photometry, we find that
124 of the 151 candidates follow the previously known infrared photometric
sequence of the cluster and are probably members. We have studied the spatial
distribution of these candidates and found that there are objects located at
distances greater than 30 arcmin to the north and west of Sigma Orionis that
probably belong to different populations of the Orion's Belt. For the 102 bona
fide Sigma Orionis cluster member candidates, we find that the radial surface
density can be represented by a decreasing exponential function (sigma =
sigma_0 e^{-r/r_0}) with a central density of sigma_0=0.23+/-0.03
object/arcmin^{2} and a characteristic radius of r_0=9.5+/-0.7 arcmin. From a
statistical comparison with Monte Carlo simulations, we conclude that the
spatial distribution of the cluster member candidates is compatible with a
Poissonian distribution and, hence, they are not mainly forming aggregations or
sub-clustering. Using near-infrared JHK-band data from 2MASS and UKIDSS and
mid-infrared data from IRAC/Spitzer, we find that 5-9 % of the brown dwarf
candidates in the Sigma Orionis cluster have K-band excesses and 31+/-7 % of
them show mid-infrared excesses at wavelengths longer than 5.8 microns, which
are probably related to the presence of disks. We have also calculated the
initial mass spectrum (dN/dm) of Sigma Orionis from very low mass stars (0.10
Msol) to the deuterium-burning mass limit (0.012-0.013 Msol). This is a rising
function toward lower masses and can be represented by a power-law distribution
(dN/dm = m^{-alpha}) with an exponent alpha of 0.7+/-0.3 for an age of 3 Myr.Comment: 30 pages, 11 figures, ApJ, in pres
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