286 research outputs found
A Giant Outburst at Millimeter Wavelengths in the Orion Nebula
BIMA observations of the Orion nebula discovered a giant flare from a young
star previously undetected at millimeter wavelengths. The star briefly became
the brightest compact object in the nebula at 86 GHz. Its flux density
increased by more than a factor of 5 on a timescale of hours, to a peak of 160
mJy. This is one of the most luminous stellar radio flares ever observed.
Remarkably, the Chandra X-ray observatory was in the midst of a deep
integration of the Orion nebula at the time of the BIMA discovery; the source's
X-ray flux increased by a factor of 10 approximately 2 days before the radio
detection. Follow-up radio observations with the VLA and BIMA showed that the
source decayed on a timescale of days, then flared again several times over the
next 70 days, although never as brightly as during the discovery. Circular
polarization was detected at 15, 22, and 43 GHz, indicating that the emission
mechanism was cyclotron. VLBA observations 9 days after the initial flare yield
a brightness temperature Tb > 5 x 10^7 K at 15 GHz. Infrared spectroscopy
indicates the source is a K5V star with faint Br gamma emission, suggesting
that it is a weak-line T Tauri object. Zeeman splitting measurements in the
infrared spectrum find B ~ 2.6 +/- 1.0 kG. The flare is an extreme example of
magnetic activity associated with a young stellar object. These data suggest
that short observations obtained with ALMA will uncover hundreds of flaring
young stellar objects in the Orion region.Comment: 29 pages, 7 figures, accepted for publication in Ap
The mass-to-light ratio of rich star clusters
We point out a strong time-evolution of the mass-to-light conversion factor
eta commonly used to estimate masses of unresolved star clusters from observed
cluster spectro-photometric measures. We present a series of gas-dynamical
models coupled with the Cambridge stellar evolution tracks to compute
line-of-sight velocity dispersions and half-light radii weighted by the
luminosity. We explore a range of initial conditions, varying in turn the
cluster mass and/or density, and the stellar population's IMF. We find that
eta, and hence the estimated cluster mass, may increase by factors as large as
3 over time-scales of 50 million years. We apply these results to an hypothetic
cluster mass distribution function (d.f.) and show that the d.f. shape may be
strongly affected at the low-mass end by this effect. Fitting truncated
isothermal (Michie-King) models to the projected light profile leads to
over-estimates of the concentration parameter c of delta c ~ 0.3 compared to
the same functional fit applied to the projected mass density.Comment: 6 pages, 2 figures, to appear in the proceedings of the "Young
massive star clusters", Granada, Spain, September 200
From 10 Kelvin to 10 TeraKelvin: Insights on the Interaction Between Cosmic Rays and Gas in Starbursts
Recent work has both illuminated and mystified our attempts to understand
cosmic rays (CRs) in starburst galaxies. I discuss my new research exploring
how CRs interact with the ISM in starbursts. Molecular clouds provide targets
for CR protons to produce pionic gamma rays and ionization, but those same
losses may shield the cloud interiors. In the densest molecular clouds, gamma
rays and Al-26 decay can provide ionization, at rates up to those in Milky Way
molecular clouds. I then consider the free-free absorption of low frequency
radio emission from starbursts, which I argue arises from many small, discrete
H II regions rather than from a "uniform slab" of ionized gas, whereas
synchrotron emission arises outside them. Finally, noting that the hot
superwind gas phase fills most of the volume of starbursts, I suggest that it
has turbulent-driven magnetic fields powered by supernovae, and that this phase
is where most synchrotron emission arises. I show how such a scenario could
explain the far-infrared radio correlation, in context of my previous work. A
big issue is that radio and gamma-ray observations imply CRs also must interact
with dense gas. Understanding how this happens requires a more advanced
understanding of turbulence and CR propagation.Comment: Conference proceedings for "Cosmic-ray induced phenomenology in
star-forming environments: Proceedings of the 2nd Session of the Sant Cugat
Forum of Astrophysics" (April 16-19, 2012). 16 pages, 5 figure
KELT-20b: A Giant Planet With A Period Of P ~ 3.5 Days Transiting The V ~ 7.6 Early A Star HD 185603
We report the discovery of KELT-20b, a hot Jupiter transiting a early A star, HD 185603, with an orbital period of days. Archival and follow-up photometry, Gaia parallax, radial velocities, Doppler tomography, and AO imaging were used to confirm the planetary nature of KELT-20b and characterize the system. From global modeling we infer that KELT-20 is a rapidly rotating ( ) A2V star with an effective temperature of K, mass of , radius of , surface gravity of , and age of . The planetary companion has a radius of , a semimajor axis of au, and a linear ephemeris of . We place a upper limit of on the mass of the planet. Doppler tomographic measurements indicate that the planetary orbit normal is well aligned with the projected spin axis of the star ( ). The inclination of the star is constrained to , implying a three-dimensional spin–orbit alignment of . KELT-20b receives an insolation flux of , implying an equilibrium temperature of of ∼2250 K, assuming zero albedo and complete heat redistribution. Due to the high stellar , KELT-20b also receives an ultraviolet (wavelength nm) insolation flux of , possibly indicating significant atmospheric ablation. Together with WASP-33, Kepler-13 A, HAT-P-57, KELT-17, and KELT-9, KELT-20 is the sixth A star host of a transiting giant planet, and the third-brightest host (in V ) of a transiting planet
Use of alcoholic beverages in VA medical centers
BACKGROUND: Benzodiazepines are the first-line choice for the treatment of alcohol withdrawal syndrome. However, several hospitals continue to provide alcoholic beverages through their formulary for the treatment of alcohol withdrawal. While there are data on the prevalence of this practice in academic medical centers, there are no data on the availability of alcoholic beverages at the formularies of the hospitals operated by the department of Veteran's Affairs. METHODS: In this study, we surveyed the Pharmacy managers at 112 Veterans' Affairs Medical Centers (VAMCs) to ascertain the availability of alcohol on the VAMC formularies, and presence or lack of a policy on the use of alcoholic beverages in their VA Medical Center. RESULTS: Of the pharmacy directors contacted, 81 responded. 8 did not allow their use, while 20 allowed their use. There was a lack of a consistent policy across the VA medical centers on availability and use of alcoholic beverages for the treatment of alcohol withdrawal syndrome. CONCLUSION: There is lack of uniform policy on the availability of alcoholic beverages across the VAMCs, which may create potential problems with difference in the standards of care
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Active Damping of the E-P Instability at the Los Alamos Proton Storage Ring
A prototype of an analog, transverse (vertical) feedback system for active damping of the two-stream (e-p) instability has been developed and successfully tested at the Los Alamos Proton Storage Ring (PSR). This system was able to improve the instability threshold by approximately 30% (as measured by the change in RF buncher voltage at instability threshold). The feedback system configuration, setup procedures, and optimization of performance are described. Results of several experimental tests of system performance are presented including observations of instability threshold improvement and grow-damp experiments, which yield estimates of instability growth and damping rates. A major effort was undertaken to identify and study several factors limiting system performance. Evidence obtained from these tests suggests that performance of the prototype was limited by higher instability growth rates arising from beam leakage into the gap at lower RF buncher voltage and the onset of instability in the horizontal plane, which had no feedback
How does a low-mass cut-off in the stellar IMF affect the evolution of young star clusters?
We investigate how different stellar initial mass functions (IMFs) can affect the mass-loss and survival of star clusters. We find that IMFs with radically different low-mass cut-offs (between 0.1 and 2 M⊙) do not change cluster destruction time-scales as much as might be expected. Unsurprisingly, we find that clusters with more high-mass stars lose relatively more mass through stellar evolution, but the response to this mass-loss is to expand and hence significantly slow their dynamical evolution. We also argue that it is very difficult, if not impossible, to have clusters with different IMFs that are initially ‘the same’, since the mass, radius and relaxation times depend on each other and on the IMF in a complex way. We conclude that changing the IMF to be biased towards more massive stars does speed up mass-loss and dissolution, but that it is not as dramatic as might be thought
On the association of ULXs with young superclusters: M82 X-1 and a new candidate in NGC 7479
We investigate the spatial coincidence of ultra-luminous X-ray sources (ULXs)
with young massive stellar clusters. In particular we perform astrometry on
Chandra and HST data of two ULXs that are possibly associated with such
clusters. To date M82 X-1 is the only ULX claimed to be coincident with a young
massive stellar cluster. We remeasure the position of this source with a high
accuracy and find that the position of the X-ray source is 0.65 arcsec away
from the stellar cluster, corresponding to an offset significance of 3 sigma.
We also report the discovery of a new candidate, based on observations of NGC
7479. One of the ULXs observed in three X-ray observations is found to be
spatially coincident (within 1 sigma of the position error) with a young
super-cluster observed in the HST images. In the brightest state, the absorbed
luminosity of the ULX is a few times erg s, and in the
faintest state below the detection limit of times erg
s. The luminosity in the brightest state requires an accreting black
hole mass of at least 100 M assuming isotropic emission. However it
is possible that the source is contaminated by X-ray emission from the nearby
supernova SN2009jf. In this case the luminosity of the ULX is in a range where
it is strongly debated whether it is a super-Eddington stellar mass black hole
or an intermediate mass black hole. The colours of the host cluster indicate a
young stellar population, with an age between 10 and 100 Myr. The total stellar
mass of the cluster is M.Comment: To appear in MNRAS letter
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