42 research outputs found
Period-Luminosity Relations for Type II Cepheids and their Application
JHKs magnitudes corrected to mean intensity are estimated for LMC type II
Cepheids in the OGLE-III survey. Period-luminosity (PL) relations are derived
in JHKs as well as in a reddening-free VI parameter. Within the uncertainties
the BL Her stars (P < 4 d) and the W Vir stars (P = 4 to 20 d) are co-linear in
these PL relations. The slopes of the infrared relations agree with those found
previously for type II Cepheids in globular clusters within the uncertainties.
Using the pulsation parallaxes of V553 Cen and SW Tau the data lead to an LMC
modulus uncorrected for any metallicity effects of 18.46 +- 0.10 mag. The type
II Cepheids in the second-parameter globular cluster, NGC 6441, show a PL(VI)
relation of the same slope as that in the LMC and this leads to a cluster
distance modulus of 15.46 +- 0.11 mag, confirming the hypothesis that the RR
Lyrae variables in this cluster are overluminous for their metallicity. It is
suggested that the Galactic variable kappa Pav is a member of the peculiar W
Vir class found by the OGLE-III group in the LMC. Low-resolution spectra of
OGLE-III type II Cepheids with P > 20 d (RV Tau stars) show that a high
proportion have TiO bands; only one has been found showing C_2. The LMC RV Tau
stars, as a group, are not co-linear with the shorter-period type II Cepheids
in the infrared PL relations in marked contrast to such stars in globular
clusters. Other differences between LMC, globular cluster and Galactic field
type II Cepheids are noted in period distribution and infrared colours.Comment: Accepted for publication in MNRAS, 11 pages (3 tables, 5 figures) +
online material (5 pages, 1 table
Comparing AMR and SPH Cosmological Simulations: I. Dark Matter & Adiabatic Simulations
We compare two cosmological hydrodynamic simulation codes in the context of
hierarchical galaxy formation: The SPH code GADGET, and the Eulerian AMR code
ENZO. Both codes represent dark matter with the N-body method, but use
different gravity solvers and fundamentally different approaches to
hydrodynamics. We compare the GADGET `entropy conserving' SPH formulation with
two ENZO methods: The piecewise parabolic method (PPM), and the artificial
viscosity-based scheme used in the ZEUS code. In this paper we focus on a
comparison of cosmological simulations that follow either only dark matter, or
also adiabatic baryonic gas. The dark matter-only runs agree generally quite
well, provided ENZO is run with a comparatively fine root grid and a low
overdensity threshold for mesh refinement, otherwise the abundance of low-mass
halos is suppressed. This is due to the hierarchical particle-mesh method used
to compute gravitational forces in ENZO, which tends to deliver lower force
resolution than the tree algorithm of GADGET. At comparable force resolution,
we find that the latter offers substantially better performance and lower
memory consumption than the present gravity solver in ENZO. In simulations that
include adiabatic gas dynamics, we find general agreement in the distribution
functions of temperature, entropy, and density for gas of moderate to high
overdensity, as found inside dark matter halos. However, there are some
significant differences at lower overdensities. We argue that these
discrepancies are presumably owing to differences in the shock-capturing
abilities of the different methods. In particular, ZEUS hydro leads to some
unphysical heating at early times in preshock regions. Overall, the GADGET
hydro results are bracketed by those for ENZO/ZEUS and ENZO/PPM. (abridged)Comment: 45 pages, 17 figures. ApJ, in press. This paper has been
significantly revised after referee comments and several sections have been
adde
SNSPH: A Parallel 3-D Smoothed Particle Radiation Hydrodynamics Code
We provide a description of the SNSPH code--a parallel 3-dimensional
radiation hydrodynamics code implementing treecode gravity, smooth particle
hydrodynamics, and flux-limited diffusion transport schemes. We provide
descriptions of the physics and parallelization techniques for this code. We
present performance results on a suite of code tests (both standard and new),
showing the versatility of such a code, but focusing on what we believe are
important aspects of modeling core-collapse supernovae.Comment: 41 pages including 8 figures, submitted to ApJ, version with high
resolution figures and test data can be found at http://qso.lanl.gov/~clf
Period-luminosity relations of type II Cepheids in the Magellanic Clouds
Period-luminosity relations (PLRs) of type II Cepheids (T2Cs) in the Small
Magellanic Cloud are derived based on OGLE-III, IRSF/SIRIUS and other data, and
these are compared with results for the Large Magellanic Cloud and Galactic
globular clusters. Evidence is found for a change of the PLR slopes from system
to system. Treating the longer period T2Cs (W Vir stars) separately gives an
SMC-LMC modulus difference of 0.39+-0.05 mag without any metallicity
corrections being applied. This agrees well with the difference in moduli based
on different distance indicators, in particular the PLRs of classical Cepheids.
The shorter period T2Cs (BL Her stars) give a smaller SMC-LMC difference
suggesting that their absolute magnitudes might be affected either by
metallicity or by age effects. It is shown that the frequency distribution of
T2C periods also changes from system to system.Comment: 12 pages, 10 figures and 6 tables, accepted for publication in MNRA
An alternative origin for debris rings of planetesimals
Core Accretion, the most widely accepted scenario for planet formation,
postulates existence of km-sized solid bodies, called planetesimals, arranged
in a razor-thin disc in the earliest phases of planet formation. In the Tidal
Downsizing hypothesis, an alternative scenario for formation of planets, grain
growth, sedimentation and formation of planetary cores occur inside dense and
massive gas clumps formed in the outer cold disc by gravitational instability.
As a clump migrates inward, tidal forces of the star remove all or most of the
gas from the clump, downsizing it to a planetary mass body. Here we argue that
such a clump may form not only the planetary core but also numerous smaller
bodies. As an example, we consider the simplest case of bodies on circular
orbits around the planetary core in the centre of the gas clump. Bodies smaller
than 1 km suffer a strong enough aerodynamic drag, spiral in and accrete onto
the solid core rapidly; bodies in the planetesimal size range lose their
centrifugal support very slowly. We find that planetesimals orbiting the
protoplanetary core closely remain gravitationally bound to it; these may be
relevant to formation of satellites of giant planets. Planetesimals on more
distant orbits within the host clump are unbound from the protoplanet and are
set on mildly eccentric heliocentric orbits, generically forming wide rings.
These may correspond to debris discs around main sequence stars and the Kuiper
belt in the Solar System. For the latter in particular, our hypothesis
naturally explains the observed sharp outer edge and the "mass deficit" of the
Kuiper belt.Comment: Submitted to MNRA
HST Snaphot Study of Variable Stars in Globular Clusters: Inner Region of NGC 6441
[Abridged] We present the results of a Hubble Space Telescope snapshot
program to survey the inner region of the globular cluster NGC 6441 for its
variable stars. A total of 57 variable stars was found including 38 RR Lyrae
stars, 6 Population II Cepheids, and 12 long period variables. Of the RR Lyrae
stars observed in this survey, 26 are pulsating in the fundamental mode with a
mean period of 0.753d and 12 are first-overtone mode pulsators with a mean
period of 0.365d. These values match up very well with those found in
ground-based surveys. Combining all the available data for NGC 6441, we find
mean periods of 0.759d and 0.375d for the RRab and RRc stars, respectively. We
also find that the RR Lyrae in this survey are located in the same regions of a
period-amplitude diagram as those found in ground-based surveys. Although NGC
6441 is a metal-rich globular cluster, its RR Lyrae more closely resemble those
in Oosterhoff type II globular clusters. However, even compared to typical
Oosterhoff type II systems, the mean period of its RRab stars is unusually
long. We also derived I-band period-luminosity relations for the RR Lyrae
stars. Of the six Population II Cepheids, five are of W Virginis type and one
is a BL Herculis variable stars. This makes NGC 6441, along with NGC 6388, the
most metal-rich globular cluster known to contain these types of variable
stars. Another variable, V118, may also be a Population II Cepheid given its
long period and its separation in magnitude from the RR Lyrae stars. We argue
that there does not appear to be a change in the period-luminosity relation
slope between the BL Herculis and W Virginis stars, but that a change of slope
does occur when the RV Tauri stars are added to the period-luminosity relation.Comment: 28 pages, including 9 figures and 8 tables, emulateapj5/apjfonts
style. Accepted by the Astronomical Journal. Approximate publication date
September 2003. We recommend the interested reader to download the preprint
with full-resolution figures, which can be found at
http://www.astro.puc.cl/~mcatelan/Pritzl.zi
Virtually impossible: limiting Australian children and adolescents daily screen based media use
Background: Paediatric recommendations to limit children’s and adolescents’ screen based media use (SBMU) to less than two hours per day appear to have gone unheeded. Given the associated adverse physical and mental health outcomes of SBMU it is understandable that concern is growing worldwide. However, because the majority of studies measuring SBMU have focused on TV viewing, computer use, video game playing, or a combination of these the true extent of total SBMU (including non-sedentary hand held devices) and time spent on specific screen activities remains relatively unknown. This study assesses the amount of time Australian children and adolescents spend on all types of screens and specific screen activities. Methods: We administered an online instrument specifically developed to gather data on all types of SBMU and SBMU activities to 2,620 (1373 males and 1247 females) 8 to 16 year olds from 25 Australian government and non-government primary and secondary schools. Results: We found that 45% of 8 year olds to 80% of 16 year olds exceeded the recommended < 2 hours per day for screen use. A series of hierarchical linear models demonstrated different relationships between the degree to which total SBMU and SBMU on specific activities (TV viewing, Gaming, Social Networking, and Web Use) exceeded the < 2 hours recommendation in relation to sex and age. Conclusions: Current paediatric recommendations pertaining to screen use exposure may no longer be tenable because screen based media are central in the everyday lives of children and adolescents. In any reappraisal of SBMU exposure times, researchers, educators and health professionals need to take cognizance of the extent to which screen use differs across specific screen activity, sex, and age
The Luminosities and Distance Scales of Type II Cepheid and RR Lyrae variables
Infrared and optical absolute magnitudes are derived for the type II Cepheids
kappa Pav and VY Pyx from revised Hipparcos parallaxes and for kappa Pav, V553
Cen and SW Tau from pulsation parallaxes. Phase-corrected JHK mags are given
for 142 RR Lyrae variables based on 2MASS data. RR Lyrae itself is overluminous
compared with LMC RR Lyraes at the classical Cepheid modulus (18.39) consistent
with a prediction of Catalan and Cortes. V553 Cen and SW Tau deviate by only
0.02 mag in the mean from the Matsunaga PL(K) relation for globular cluster
type II Cepheids with a zero-point based on the same LMC modulus. Comparing
directly these two stars with type II Cepheids in the LMC and in the Galactic
Bulge leads to an LMC modulus of 18.37\pm0.09 and a distance to the Galactic
Centre of 7.64\pm 0.21kpc. Kappa Pav may be a binary. V553 Cen and SW Tau show
that at optical wavelengths PL relations are wider for field stars than for
those in globular clusters (abridged).Comment: 29 pages, 13 figures, accepted for MNRA
Spermidine Promotes Human Hair Growth and Is a Novel Modulator of Human Epithelial Stem Cell Functions
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