3,780 research outputs found
A Physical Limit to the Magnetic Fields of T Tauri Stars
Recent estimates of magnetic field strengths in T Tauri stars yield values
--. In this paper, I present an upper limit to the
photospheric values of by computing the equipartition values for different
surface gravities and effective temperatures. The values of derived from
the observations exceed this limit, and I examine the possible causes for this
discrepancy
The effect of a strong external radiation field on protostellar envelopes in Orion
We discuss the effects of an enhanced interstellar radiation field (ISRF) on
the observables of protostellar cores in the Orion cloud region. Dust radiative
transfer is used to constrain the envelope physical structure by reproducing
SCUBA 850 micron emission. Previously reported 13CO, C17O and H2CO line
observations are reproduced through detailed Monte Carlo line radiative
transfer models. It is found that the 13CO line emission is marginally
optically thick and sensitive to the physical conditions in the outer envelope.
An increased temperature in this region is needed in order to reproduce the
13CO line strengths and it is suggested to be caused by a strong heating from
the exterior, corresponding to an ISRF in Orion 10^3 times stronger than the
"standard" ISRF. The typical temperatures in the outer envelope are higher than
the desorption temperature for CO. The C17O emission is less sensitive to this
increased temperature but rather traces the bulk envelope material. The data
are only fit by a model where CO is depleted, except in the inner and outermost
regions where the temperature increases above 30-40 K. The fact that the
temperatures do not drop below approximately 25 K in any of the envelopes
whereas a significant fraction of CO is frozen-out suggest that the
interstellar radiation field has changed through the evolution of the cores.
The H2CO lines are successfully reproduced in the model of an increased ISRF
with constant abundances of 3-5x10^{-10}.Comment: 11 pages, 10 figures. Accepted for publication in A&
Current Star Formation in the Ophiuchus and Perseus Molecular Clouds: Constraints and Comparisons from Unbiased Submillimeter and Mid-Infrared Surveys. II
We present a census of the population of deeply embedded young stellar
objects (YSOs) in the Ophiuchus molecular cloud complex based on a combination
of Spitzer Space Telescope mid-infrared data from the "Cores to Disks" (c2d)
legacy team and JCMT/SCUBA submillimeter maps from the COMPLETE team. We have
applied a method developed for identifying embedded protostars in Perseus to
these datasets and in this way construct a relatively unbiased sample of 27
candidate embedded protostars with envelopes more massive than our sensitivity
limit (about 0.1 M_sun). Embedded YSOs are found in 35% of the SCUBA cores -
less than in Perseus (58%). On the other hand the mid-infrared sources in
Ophiuchus have less red mid-infrared colors, possibly indicating that they are
less embedded. We apply a nearest neighbor surface density algorithm to define
the substructure in each of the clouds and calculate characteristic numbers for
each subregion - including masses, star formation efficiencies, fraction of
embedded sources etc. Generally the main clusters in Ophiuchus and Perseus
(L1688, NGC1333 and IC348) are found to have higher star formation efficiencies
than small groups such as B1, L1455 and L1448, which on the other hand are
completely dominated by deeply embedded protostars. We discuss possible
explanations for the differences between the regions in Perseus and Ophiuchus,
such as different evolutionary timescales for the YSOs or differences, e.g., in
the accretion in the two clouds.Comment: Accepted for publication in ApJ (56 pages, 13 figures; abstract
abridged). Version with full-resolution figures available at
http://www.astro.uni-bonn.de/~jes/paper120.pd
Managed delay for coronary artery bypass graft surgery: The experience at one Canadian center
AbstractObjectives. This study sought to assess the impact of delaying coronary artery bypass surgery at one Canadian academic tertiary referral center.Background. Universal access to medical services in Canada comes at the expense of waiting lists whose impact has been incompletely assessed.Methods. A prospective, observational study of all residents of Nova Scotia and Prince Edward Island accepted for bypass surgery between 1 April 1992 and 31 October 1992 was undertaken to determine 1) whether triage guidelines were being followed; and 2) the incidence of cardiac death, nonfatal myocardial infarction and worsening symptoms associated with delayed operation. The analysis had 90% power to detect a mortality rate of ≥3% (alpha 0.05).Results. Of 423 patients referred, 35% were triaged as urgent, 9.7% as semiurgent A, 39% as semiurgent B and 16.3% as elective, with no age or gender bias identified. Operation occurred at ≤1 week in 25%, ≤1 month in 47%, and >6 months in 1.4%. There were no nonfatal myocardial infarctions, but five cardiac deaths occurred (1.2%). Of 275 patients not initially classified as urgent, 12.4% required reclassification to higher priorities because of worsening symptoms: none had perioperative myocardial infarction or died. One in four patients queued longer than target waiting times. Only 4% of patients considered prioritization on the basis of medical need unfair, but 64% experienced at least moderate anxiety.Conclusions. This triage system equitably stratified patients to a queue. Deaths were rare and could not be attributed to the triage process. Patients with worsening clinical status were safely accommodated with earlier waiting times, but concerns remain regarding excessive waiting times and patient anxiety
Filamentary Star Formation in NGC 1275
We examine the star formation in the outer halo of NGC~1275, the central
galaxy in the Perseus cluster (Abell 426), using far ultraviolet and optical
images obtained with the Hubble Space Telescope. We have identified a
population of very young, compact star clusters with typical ages of a few Myr.
The star clusters are organised on multiple-kiloparsec scales. Many of these
star clusters are associated with "streaks" of young stars, the combination of
which has a cometary appearance. We perform photometry on the star clusters and
diffuse stellar streaks, and fit their spectral energy distributions to obtain
ages and masses. These young stellar populations appear to be normal in terms
of their masses, luminosities and cluster formation efficiency; <10% of the
young stellar mass is located in star clusters. Our data suggest star formation
is associated with the evolution of some of the giant gas filaments in NGC~1275
that become gravitationally unstable on reaching and possibly stalling in the
outer galaxy. The stellar streaks then could represent stars moving on
ballistic orbits in the potential well of the galaxy cluster. We propose a
model where star-forming filaments, switched on ~50~Myr ago and are currently
feeding the growth of the NGC~1275 stellar halo at a rate of ~2-3 solar masses
per year. This type of process may also build stellar halos and form isolated
star clusters in the outskirts of youthful galaxies.Comment: 15 pages, 10 figures, accepted for publication in MNRA
Turn-taking in cooperative offspring care: by-product of individual provisioning behavior or active response rule?
ABSTRACT: For individuals collaborating to rear offspring, effective organization of resource delivery is difficult because each carer benefits when the others provide a greater share of the total investment required. When investment is provided in discrete events, one possible solution is to adopt a turn-taking strategy whereby each individual reduces its contribution rate after investing, only increasing its rate again once another carer contributes. To test whether turn-taking occurs in a natural cooperative care system, here we use a continuous time Markov model to deduce the provisioning behavior of the chestnut-crowned babbler (Pomatostomus ruficeps), a cooperatively breeding Australian bird with variable number of carers. Our analysis suggests that turn-taking occurs across a range of group sizes (2-6), with individual birds being more likely to visit following other individuals than to make repeat visits. We show using a randomization test that some of this apparent turn-taking arises as a by-product of the distribution of individual inter-visit intervals ("passive" turn-taking) but that individuals also respond actively to the investment of others over and above this effect ("active" turn-taking). We conclude that turn-taking in babblers is a consequence of both their individual provisioning behavior and deliberate response rules, with the former effect arising through a minimum interval required to forage and travel to and from the nest. Our results reinforce the importance of considering fine-scale investment dynamics when studying parental care and suggest that behavioral rules such as turn-taking may be more common than previously thought. SIGNIFICANCE STATEMENT: Caring for offspring is a crucial stage in the life histories of many animals and often involves conflict as each carer typically benefits when others contribute a greater share of the work required. One way to resolve this conflict is to monitor when other carers contribute and adopt a simple "turn-taking" rule to ensure fairness, but natural parental care has rarely been studied in sufficient detail to identify such rules. Our study investigates whether cooperatively breeding chestnut-crowned babblers "take turns" delivering food to offspring, and (if so) whether this a deliberate strategy or simply a by-product of independent care behavior. We find that babblers indeed take turns and conclude that part of the observed turn-taking is due to deliberate responsiveness, with the rest arising from the species' breeding ecology
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