1,221 research outputs found
A note on a third order curvature invariant in static spacetimes
We consider here the third order curvature invariant
in static spacetimes
for which is conformally flat. We evaluate
explicitly the invariant for the -dimensional Majumdar-Papapetrou multi
black-holes solution, confirming that does indeed vanish on the event
horizons of such black-holes. Our calculations show, however, that solely the
vanishing of is not sufficient to locate an event horizon in
non-spherically symmetric spacetimes. We discuss also some tidal effects
associated to the invariant .Comment: 5 pages, 3 figures. Extra material available at
http://vigo.ime.unicamp.br/in
CASTRO: A New Compressible Astrophysical Solver. II. Gray Radiation Hydrodynamics
We describe the development of a flux-limited gray radiation solver for the
compressible astrophysics code, CASTRO. CASTRO uses an Eulerian grid with
block-structured adaptive mesh refinement based on a nested hierarchy of
logically-rectangular variable-sized grids with simultaneous refinement in both
space and time. The gray radiation solver is based on a mixed-frame formulation
of radiation hydrodynamics. In our approach, the system is split into two
parts, one part that couples the radiation and fluid in a hyperbolic subsystem,
and another parabolic part that evolves radiation diffusion and source-sink
terms. The hyperbolic subsystem is solved explicitly with a high-order Godunov
scheme, whereas the parabolic part is solved implicitly with a first-order
backward Euler method.Comment: accepted for publication in ApJS, high-resolution version available
at https://ccse.lbl.gov/Publications/wqzhang/castro2.pd
Ethical and compliance-competence evaluation: a key element of sound corporate governance
Motivated by the ongoing post-Enron refocusing on corporate governance and the shift by the Financial Services Authority (FSA) in the UK to promoting compliance- competence within the financial services sector, this paper demonstrates how template analysis can be used as a tool for evaluating compliance-competence. Focusing on the ethical dimension of compliance-competence, we illustrate how this can be subjectively appraised. We propose that this evaluation technique could be utilised as a starting point in informing senior management of corporate governance issues and be used to monitor and demonstrate key compliance and ethical aspects of an institution to external stakeholders and regulators
Women, know your limits: Cultural sexism in academia
Despite the considerable advances of the feminist movement across Western societies, in Universities women are less likely to be promoted, or paid as much as their male colleagues, or even get jobs in the first place. One way in which we can start to reflect on why this might be the case is through hearing the experiences of women academics themselves. Using feminist methodology, this article attempts to unpack and explore just some examples of
‘cultural sexism’ which characterise the working lives of many women in British academia.This article uses qualitative methods to describe and make sense of just some of those experiences. In so doing, the argument is also made that the activity of academia is profoundly gendered and this explicit acknowledgement may contribute to our
understanding of the under-representation of women in senior positions
Bolometric correction and spectral energy distribution of cool stars in Galactic clusters
We have investigated the relevant trend of the bolometric correction (BC) at
the cool-temperature regime of red giant stars and its possible dependence on
stellar metallicity. Our analysis relies on a wide sample of optical-infrared
spectroscopic observations, along the 3500A-2.5micron wavelength range, for a
grid of 92 red giant stars in five (3 globular + 2 open) Galactic clusters,
along the -2.2<[Fe/H]<+0.4 metallicity range. Bolometric magnitudes have been
found within an internal accuracy of a few percent. Making use of our new
database, we provide a set of fitting functions for the V and K BC vs. Teff and
broad-band colors, valid over the interval 3300K<Teff<5000K, especially suited
for Red Giants. No evident drift of both BC(V) and BC(K) with [Fe/H] is found.
Things may be different, however, for the B-band correction, given a clear
(B-V) vs. [Fe/H] correlation in place for our data, with metal-poor stars
displaying a "bluer" (B-V) with respect to the metal-rich sample, for fixed
Teff. Our empirical bolometric scale supports the conclusion that (a) BC(K)
from the most recent studies are reliable within <0.1 mag over the whole
color/temperature range considered in this paper, and (b) the same conclusion
apply to BC(V) only for stars warmer than ~3800K. At cooler temperatures the
agreement is less general, and MARCS models are the only ones providing a
satisfactory match to observations.Comment: 18 pages with 12 color figures and 12 tables. To appear in the MNRAS.
Full spectral catalog of stars available at
http://www.bo.astro.it/~eps/home.htm
Further empirical evidence for the non-linearity of the period-luminosity relations as seen in the Large Magellanic Cloud Cepheids
(abridged) Recent studies, using OGLE data for LMC Cepheids in the optical,
strongly suggest that the period-luminosity (PL) relation for the Large
Magellanic Cloud (LMC) Cepheids shows a break or non-linearity at a period of
10 days. In this paper we apply statistical tests, the chi-square test and the
F-test, to the Cepheid data from the MACHO project to test for a non-linearity
of the V- and R-band PL relations at 10 days, and extend these tests to the
near infrared (JHK-band) PL relations with 2MASS data. We correct the
extinction for these data by applying an extinction map towards the LMC. The
statistical test we use, the F-test, is able to take account of small numbers
of data points and the nature of that data on either side of the period cut at
10 days. With our data, the results we obtained imply that the VRJH-band PL
relations are non-linear around a period of 10 days, while the K-band PL
relation is (marginally) consistent with a single-line regression. The choice
of a period of 10 days, around which this non-linearity occurs, is consistent
with the results obtained when this "break" period is estimated from the data.
Long period Cepheids are supplemented from the literature to increase our
sample size. The photometry of these long period Cepheids is compared with our
data and no trend with period is found. Our main results remain unchanged when
we supplement our dataset with these long period Cepheids. By examining our
data at maximum light, we also suggest arguments why errors in reddening are
unlikely to be responsible for our results. The non-linearity of the mean
V-band PL relation as seen in both of the OGLE and MACHO data, using different
extinction maps, suggests that this non-linearity is real.Comment: 18 pages, 10 tables, 7 figures. MNRAS accepte
Searching for links between magnetic fields and stellar evolution. II. The evolution of magnetic fields as revealed by observations of Ap stars in open clusters and associations
The evolution of magnetic fields in Ap stars during the main sequence phase
is presently mostly unconstrained by observation because of the difficulty of
assigning accurate ages to known field Ap stars.
We are carrying out a large survey of magnetic fields in cluster Ap stars
with the goal of obtaining a sample of these stars with well-determined ages.
In this paper we analyse the information available from the survey as it
currently stands.
We select from the available observational sample the stars that are probably
(1) cluster or association members and (2) magnetic Ap stars. For the stars in
this subsample we determine the fundamental parameters T_eff, log(L/L_o), and
M/M_o. With these data and the cluster ages we assign both absolute age and
fractional age (the fraction of the main sequence lifetime completed). For this
purpose we have derived new bolometric corrections for Ap stars.
Magnetic fields are present at the surfaces of Ap stars from the ZAMS to the
TAMS. Statistically for the stars with M > 3 M_o the fields decline with
advancing age approximately as expected from flux conservation together with
increased stellar radius, or perhaps even faster than this rate, on a time
scale of about 3 10^7 yr. In contrast, lower mass stars show no compelling
evidence for field decrease even on a timescale of several times 10^8 yr.
Study of magnetic cluster stars is now a powerful tool for obtaining
constraints on evolution of Ap stars through the main sequence. Enlarging the
sample of known cluster magnetic stars, and obtaining more precise RMS fields,
will help to clarify the results obtained so far. Further field observations
are in progress.Comment: Accepted by Astronomy & Astrophysic
Open Issues on the Synthesis of Evolved Stellar Populations at Ultraviolet Wavelengths
In this paper we briefly review three topics that have motivated our (and
others') investigations in recent years within the context of evolutionary
population synthesis techniques. These are: The origin of the FUV up-turn in
elliptical galaxies, the age-metallicity degeneracy, and the study of the
mid-UV rest-frame spectra of distant red galaxies. We summarize some of our
results and present a very preliminary application of a UV grid of theoretical
spectra in the analysis of integrated properties of aged stellar populations.
At the end, we concisely suggest how these topics can be tackled once the World
Space Observatory enters into operation in the midst of this decade.Comment: 8 pages, 4 figures, accepted for publication in Astrophysics & Space
Science, UV Universe special issu
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