14,154 research outputs found
The Physical Properties of the Red Supergiant WOH G64: The Largest Star Known?
WOH G64 is an unusual red supergiant (RSG) in the Large Magellanic Cloud
(LMC), with a number of properties that set it apart from the rest of the LMC
RSG population, including a thick circumstellar dust torus, an unusually late
spectral type, maser activity, and nebular emission lines. Its reported
physical properties are also extreme, including the largest radius for any star
known and an effective temperature that is much cooler than other RSGs in the
LMC, both of which are at variance with stellar evolutionary theory. We fit
moderate-resolution optical spectrophotometry of WOH G64 with the MARCS stellar
atmosphere models, determining an effective temperature of 3400 +/- 25 K. We
obtain a similar result from the star's broadband V - K colors. With this
effective temperature, and taking into account the flux contribution from the
aysmmetric circumstellar dust envelope, we calculate log(L/L_sun) = 5.45 +/-
0.05 for WOH G64, quite similar to the luminosity reported by Ohnaka and
collaborators based on their radiative transfer modeling of the star's dust
torus. We determine a radius of R/R_sun = 1540, bringing the size of WOH G64
and its position on the H-R diagram into agreement with the largest known
Galactic RSGs, although it is still extreme for the LMC. In addition, we use
the Ca II triplet absorption feature to determine a radial velocity of 294 +/-
2 km/s for the star; this is the same radial velocity as the rotating gas in
the LMC's disk, which confirms its membership in the LMC and precludes it from
being an unusual Galactic halo giant. Finally, we describe the star's unusual
nebula emission spectrum; the gas is nitrogen-rich and shock-heated, and
displays a radial velocity that is significantly more positive than the star
itself by 50 km/s.Comment: 25 pages, 5 figures; accepted for publication in The Astronomical
Journa
System Analysis for the Huntsville Operation Support Center, Distributed Computer System
HOSC as a distributed computing system, is responsible for data acquisition and analysis during Space Shuttle operations. HOSC also provides computing services for Marshall Space Flight Center's nonmission activities. As mission and nonmission activities change, so do the support functions of HOSC change, demonstrating the need for some method of simulating activity at HOSC in various configurations. The simulation developed in this work primarily models the HYPERchannel network. The model simulates the activity of a steady state network, reporting statistics such as, transmitted bits, collision statistics, frame sequences transmitted, and average message delay. These statistics are used to evaluate such performance indicators as throughout, utilization, and delay. Thus the overall performance of the network is evaluated, as well as predicting possible overload conditions
Deconvolution with Shapelets
We seek to find a shapelet-based scheme for deconvolving galaxy images from
the PSF which leads to unbiased shear measurements. Based on the analytic
formulation of convolution in shapelet space, we construct a procedure to
recover the unconvolved shapelet coefficients under the assumption that the PSF
is perfectly known. Using specific simulations, we test this approach and
compare it to other published approaches. We show that convolution in shapelet
space leads to a shapelet model of order
with and being the maximum orders of the intrinsic
galaxy and the PSF models, respectively. Deconvolution is hence a
transformation which maps a certain number of convolved coefficients onto a
generally smaller number of deconvolved coefficients. By inferring the latter
number from data, we construct the maximum-likelihood solution for this
transformation and obtain unbiased shear estimates with a remarkable amount of
noise reduction compared to established approaches. This finding is
particularly valid for complicated PSF models and low images, which
renders our approach suitable for typical weak-lensing conditions.Comment: 9 pages, 9 figures, submitted to A&
I want it all and I want it now. Challenging the traditional nursing academic paradigm
A recent Twitter chat facilitated by the @NurseEducToday socialmedia team provoked a particularly strong reaction among a range of contributors. The focus of the discussion – expectations of the nurse academic – resulted in a high level of engagement from several participants who clearly held strong views, which were surprisingly polarised. Here we explore aspects of this polarization; and what it might mean for nurses working in academia. Our aim is to reflect on what this dialogue might tell us about current thinking in the profession, specifically around how nurse academics see themselves, what they expect from self and others, and what they are prepared to do to meet these,often, self-generated expectations
Red Supergiants in the Andromeda Galaxy (M31)
Red supergiants are a short-lived stage in the evolution of moderately
massive stars (10-25Mo), and as such their location in the H-R diagram provides
an exacting test of stellar evolutionary models. Since massive star evolution
is strongly affected by the amount of mass-loss a star suffers, and since the
mass-loss rates depend upon metallicity, it is highly desirable to study the
physical properties of these stars in galaxies of various metallicities. Here
we identify a sample of red supergiants in M31 (the most metal-rich of the
Local Group galaxies) and derive their physical properties by fitting MARCS
atmosphere models to moderate resolution optical spectroscopy, and from V-K
photometry.Comment: Accepted for publication in the Astrophysical Journa
Bringing VY Canis Majoris Down to Size: An Improved Determination of Its Effective Tempeature
The star VY CMa is a late-type M supergiant with many peculiarities, mostly
related to the intense circumstellar environment due to the star's high
mass-loss rate. Claims have been made that would suggest this star is
considerably more luminous (L = 5 x 10^5 Lo) and larger (R=2800 Ro) than other
Galactic red supergiants (RSGs). Indeed, such a location in the H-R diagram
would be well in the "Hayashi forbidden zone" where stars cannot be in
hydrostatic equilibrium. These extraordinary properties, however, rest upon an
assumed effective temperature of 2800-3000 K, far cooler than recent work have
shown RSGs to be. To obtain a better estimate, we fit newly obtained
spectrophotometry in the optical and NIR with the same MARCS models used for
our recent determination of the physical properties of other RSGs; we also use
and from the literature to derive an effective temperatures. We
find that the star likely has a temperature of 3650 K, a luminosity L = 6 x
10^4 Lo, and a radius of 600Ro. These values are consistent with VY CMa being
an ordinary evolved 15 Mo RSG, and agree well with the Geneva evolutionary
tracks. We find that the circumstellar dust region has a temperature of 760 K,
and an effective radius of approximately 130 AU, if spherical geometry is
assumed for the latter. What causes this star to have such a high mass-loss,
and large variations in brightness (but with little change in color), remains a
mystery at present, although we speculate that perhaps this star (and NML Cyg)
are simply normal RSGs caught during an unusually unstable time.Comment: Accepted by the Astrophysical Journa
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