737 research outputs found
Maximum stellar mass versus cluster membership number revisited
We have made a new compilation of observations of maximum stellar mass versus
cluster membership number from the literature, which we analyse for consistency
with the predictions of a simple random drawing hypothesis for stellar mass
selection in clusters. Previously, Weidner and Kroupa have suggested that the
maximum stellar mass is lower, in low mass clusters, than would be expected on
the basis of random drawing, and have pointed out that this could have
important implications for steepening the integrated initial mass function of
the Galaxy (the IGIMF) at high masses. Our compilation demonstrates how the
observed distribution in the plane of maximum stellar mass versus membership
number is affected by the method of target selection; in particular, rather low
n clusters with large maximum stellar masses are abundant in observational
datasets that specifically seek clusters in the environs of high mass stars.
Although we do not consider our compilation to be either complete or unbiased,
we discuss the method by which such data should be statistically analysed. Our
very provisional conclusion is that the data is not indicating any striking
deviation from the expectations of random drawing.Comment: 7 pages, 3 Figures; accepted by MNRAS; Reference added
1318 New Variable Stars in a 0.25 Square Degree Region of the Galactic Plane
We have conducted a deep photometric survey of a 0.5 deg x 0.5 deg area of
the Galactic Plane using the WFI instrument on the 2.2-m ESO telescope on La
Silla, Chile. The dataset comprises a total of 267 R-band images, 204 from a 16
day observation run in 2005, supplemented by 63 images from a six week period
in 2002. Our reduction employed the new numerical kernel difference image
analysis method as implemented in the PYSIS3 code and resulted in more than
500,000 lightcurves of stars down to a magnitude limit of R ~ 24.5. A search
for variable stars resulted in the detection of 1318 variables of different
types. 1011 of these are eclipsing or contact binary stars. A number of the
contact binaries have low mass-ratios and several of the detached binaries
appear to have low-mass components. Three candidate contact binaries have
periods at the known cut off including two with periods lower than any
previously published. Also identified are 3 possible pre-main sequence detached
eclipsing binaries.Comment: 54 pages, 17 figures, 11 tables, accepted by A&A. Photometry will be
available through CD
The formation and evolution of planetary systems: Grain growth and chemical processing of dust in T Tauri systems
This paper is one in a series presenting results obtained within the
Formation and Evolution of Planetary Systems (FEPS) Legacy Science Program on
the Spitzer Space Telescope. Here we present a study of dust processing and
growth in seven protoplanetary disks. Our spectra indicate that the
circumstellar silicate dust grains have grown to sizes at least 10 times larger
than observed in the interstellar medium, and show evidence for a
non-negligible (~5 % in mass fractions) contribution from crystalline species.
These results are similar to those of other studies of protoplanetary disks. In
addition, we find a correlation between the strength of the amorphous silicate
feature and the shape of the spectral energy distribution. This latter result
is consistent with the growth and subsequent gravitational settling of dust
grains towards the disk mid-plane. Further, we find a change in the relative
abundance of the different crystalline species: more enstatite relative to
forsterite is observed in the inner warm dust population at ~1 AU, while
forsterite dominates in the colder outer regions at ~5 to 15 AU. This change in
the relative abundances argues for a localized crystallization process rather
than a radial mixing scenario where crystalline silicates are being transported
outwards from a single formation region in the hot inner parts of the disk.
Last, we report the detection of emission from polycyclic aromatic hydrocarbon
molecules in five out of seven sources. We find a tentative PAH band at 8.2
micron, previously undetected in the spectra of disks around low-mass
pre-main-sequence stars.Comment: 53 pages, 14 figures, 3 tables, ApJ accepte
Timescale of Mass Accretion in Pre-Main-Sequence Stars
We present initial result of a large spectroscopic survey aimed at measuring
the timescale of mass accretion in young, pre-main-sequence stars in the
spectral type range K0 - M5. Using multi-object spectroscopy with VIMOS at the
VLT we identified the fraction of accreting stars in a number of young stellar
clusters and associations of ages between 1 - 50 Myr. The fraction of accreting
stars decreases from ~60% at 1.5 - 2 Myr to ~2% at 10 Myr. No accreting stars
are found after 10 Myr at a sensitivity limit of Msun yr-1. We
compared the fraction of stars showing ongoing accretion (f_acc) to the
fraction of stars with near-to-mid infrared excess (f_IRAC). In most cases we
find f_acc < f_IRAC, i.e., mass accretion appears to cease (or drop below
detectable level) earlier than the dust is dissipated in the inner disk. At 5
Myr, 95% of the stellar population has stopped accreting material at a rate of
> 10^{-11} Msun yr-1, while ~20% of the stars show near-infrared excess
emission. Assuming an exponential decay, we measure a mass accretion timescale
(t_acc) of 2.3 Myr, compared to a near-to-mid infrared excess timescale
(t_IRAC) of 2.9 Myr. Planet formation, and/or migration, in the inner disk
might be a viable mechanism to halt further accretion onto the central star on
such a short timescale.Comment: Accepted for publicatio
Young women's use of a microbicide surrogate: The complex influence of relationship characteristics and perceived male partners' evaluations
This is the post-print version of the article. The official published version can be found at the link below.Currently in clinical trials, vaginal microbicides are proposed as a female-initiated method of sexually transmitted infection prevention. Much of microbicide acceptability research has been conducted outside of the United States and frequently without consideration of the social interaction between sex partners, ignoring the complex gender and power structures often inherent in young womenâs (heterosexual) relationships. Accordingly, the purpose of this study was to build on existing microbicide research by exploring the role of male partners and relationship characteristics on young womenâs use of a microbicide surrogate, an inert vaginal moisturizer (VM), in a large city in the United States. Individual semi-structured interviews were conducted with 40 young women (18â23 years old; 85% African American; 47.5% mothers) following use of the VM during coital events for a 4 week period. Overall, the results indicated that relationship dynamics and perceptions of male partners influenced VM evaluation. These two factors suggest that relationship context will need to be considered in the promotion of vaginal microbicides. The findings offer insights into how future acceptability and use of microbicides will be influenced by gendered power dynamics. The results also underscore the importance of incorporating men into microbicide promotion efforts while encouraging a dialogue that focuses attention on power inequities that can exist in heterosexual relationships. Detailed understanding of these issues is essential for successful microbicide acceptability, social marketing, education, and use.This study was funded by a grant from National Institutes of Health (NIHU19AI 31494) as well as research awards to the first author: Friends of the Kinsey Institute Research Grant Award, Indiana Universityâs School of HPER Graduate Student Grant-in-Aid of Research Award, William L. Yarber Sexual Health Fellowship, and the Indiana University Graduate and Professional Student Organization Research Grant
Large dust particles in disks around T Tauri stars
We present 7-mm continuum observations of 14 low-mass pre-main-sequence stars
in the Taurus-Auriga star-forming region obtained with the Very Large Array
with ~1.5" resolution and ~0.3 mJy rms sensitivity. For 10 objects, the
circumstellar emission has been spatially resolved. The large outer disk radii
derived suggest that the emission at this wavelength is mostly optically thin.
The millimetre spectral energy distributions are characterised by spectral
indices alpha = 2.3 to 3.2. After accounting for contribution from free-free
emission and corrections for optical depth, we determine dust opacity indices
beta in the range 0.5 to 1.6, which suggest that millimetre-sized dust
aggregates are present in the circumstellar disks. Four of the sources with
beta > 1 may be consistent with submicron-sized dust as found in the
interstellar medium. Our findings indicate that dust grain growth to
millimetre-sized particles is completed within less than 1 Myr for the majority
of circumstellar disks.Comment: 11 pages, 4 figure
Five supernova survey galaxies in the southern hemisphere. I. Optical and near-infrared database
The determination of the supernova (SN) rate is based not only on the number
of detected events, but also on the properties of the parent galaxy population.
This is the first paper of a series aimed at obtaining new, refined, SN rates
from a set of five SN surveys, by making use of a joint analysis of
near-infrared (NIR) data. We describe the properties of the 3838 galaxies that
were monitored for SNe events, including newly determined morphologies and
their DENIS and POSS-II/UKST I, 2MASS and DENIS J and Ks and 2MASS H
magnitudes. We have compared 2MASS, DENIS and POSS-II/UKST IJK magnitudes in
order to find possible systematic photometric shifts in the measurements. The
DENIS and POSS-II/UKST I band magnitudes show large discrepancies (mean
absolute difference of 0.4 mag), mostly due to different spectral responses of
the two instruments, with an important contribution (0.33 mag rms) from the
large uncertainties in the photometric calibration of the POSS-II and UKST
photographic plates. In the other wavebands, the limiting near infrared
magnitude, morphology and inclination of the galaxies are the most influential
factors which affect the determination of photometry of the galaxies.
Nevertheless, no significant systematic differences have been found between of
any pair of NIR magnitude measurements, except for a few percent of galaxies
showing large discrepancies. This allows us to combine DENIS and 2MASS data for
the J and Ks filters.Comment: 17 pages, 3 figures, 5 tables, published in Astrophysics, Vol. 52,
No. 1, 2009 (English translation of Astrofizika
Dust mass-loss rates from AGB stars in the Fornax and Sagittarius dwarf Spheroidal galaxies
To study the effect of metallicity on the mass-loss rate of asymptotic giant
branch (AGB) stars, we have conducted mid-infrared photometric measurements of
such stars in the Sagittarius (Sgr dSph) and Fornax dwarf spheroidal galaxies
with the 10-m camera VISIR at the VLT. We derive mass-loss rates for 29
AGB stars in Sgr dSph and 2 in Fornax. The dust mass-loss rates are estimated
from the and colours. Radiative transfer models are used to
check the consistency of the method. Published IRAS and Spitzer data confirm
that the same tight correlation between colour and dust mass-loss
rates is observed for AGB stars from galaxies with different metallicities,
i.e. the Galaxy, the LMC and the SMC.
The derived dust mass-loss rates are in the range 5 to
3 Myr for the observed AGB stars in Sgr dSph
and around 5 Myr for those in Fornax; while
values obtained with the two different methods are of the same order of
magnitude. The mass-loss rates for these stars are higher than the nuclear
burning rates, so they will terminate their AGB phase by the depletion of their
stellar mantles before their core can grow significantly. Some observed stars
have lower mass-loss rates than the minimum value predicted by theoretical
models.Comment: 12 pages, 9 figures, accepted for publication in MNRA
Computer simulation of syringomyelia in dogs
Syringomyelia is a pathological condition in which fluid-filled cavities (syringes) form and expand in the spinal cord. Syringomyelia is often linked with obstruction of the craniocervical junction and a Chiari malformation, which is similar in both humans and animals. Some brachycephalic toy breed dogs such as Cavalier King Charles Spaniels (CKCS) are particularly predisposed. The exact mechanism of the formation of syringomyelia is undetermined and consequently with the lack of clinical explanation, engineers and mathematicians have resorted to computer models to identify possible physical mechanisms that can lead to syringes. We developed a computer model of the spinal cavity of a CKCS suffering from a large syrinx. The model was excited at the cranial end to simulate the movement of the cerebrospinal fluid (CSF) and the spinal cord due to the shift of blood volume in the cranium related to the cardiac cycle. To simulate the normal condition, the movement was prescribed to the CSF. To simulate the pathological condition, the movement of CSF was blocked
The bipolar outflow and disk of the brown dwarf ISO217
We show that the very young brown dwarf candidate ISO217 (M6.25) is driving
an intrinsically asymmetric bipolar outflow with a stronger and slightly faster
red-shifted component based on spectro-astrometry of forbidden [SII] emission
lines observed in UVES/VLT spectra taken in 2009. ISO217 is only one of a
handful of brown dwarfs and VLMS (M5-M8) for which an outflow has been detected
and that show that the T Tauri phase continues at the substellar limit. We
measure a spatial extension of the outflow of +/-190mas (+/-30AU) and
velocities of +/-40-50kms/s. We show that the velocity asymmetry between both
lobes is variable on timescales of a few years and that the strong asymmetry of
a factor of 2 found in 2007 might be smaller than originally anticipated when
using a more realistic stellar rest-velocity. We also detect forbidden
[FeII]7155 emission, for which we propose as potential origin the hot inner
regions of the outflow. To understand the ISO217 system, we determine the disk
properties based on radiative transfer modeling of the SED. This disk model
agrees very well with Herschel/PACS data at 70mu. We find that the disk is
flared and intermediately inclined (~45deg). The total disk mass (4e-6 Msun) is
small compared to the accretion and outflow rate of ISO217 (~1e-10 Msun/yr). We
propose that this discrepancy can be explained by either a higher disk mass
than inferred from the model (strong undetected grain growth) and/or by an on
average lower accretion and outflow rate than the determined values. We show
that a disk inclination significantly exceeding 45deg, as suggested from Halpha
modeling and from both lobes of the outflow being visible, is inconsistent with
the SED data. Thus, despite its intermediate inclination angle, the disk of
this brown dwarf does not appear to obscure the red outflow component, which is
very rarely seen for T Tauri objects (only one other case).Comment: Accepted for publication at A&A; minor changes (language editing
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