337 research outputs found
Cluster AgeS Experiment (CASE): RR Lyrae stars from the globular cluster Omega Centauri as standard candles
New photometry of RRab and RRc stars in Omega Cen is used to calibrate their
absolute magnitudes M_V as a function of: a) metallicity; and b) the Fourier
parameters of light curves in the V band. The zero point of both calibrations
relies on the distance modulus to the cluster derived earlier by the CASE
project based on observations of the detached eclipsing binary OGLE GC17. For
RRab variables we obtained a relation of M_V=(0.26\pm 0.08)[Fe/H]+(0.91\pm
0.13). A dereddened distance modulus to the LMC based on that formula is
mag. The second calibration of M_V, which is based on
Fourier coefficients of decomposed light curves, results in the LMC distance of
mag.Comment: accepted for publication in MNRAS, 13 pages, 8 figure
A runaway collision in a young star cluster as the origin of the brightest supernova
Supernova 2006gy in the galaxy NGC 1260 is the most luminous one recorded
\cite{2006CBET..644....1Q, 2006CBET..647....1H, 2006CBET..648....1P,
2006CBET..695....1F}. Its progenitor might have been a very massive (
\msun) star \cite{2006astro.ph.12617S}, but that is incompatible with hydrogen
in the spectrum of the supernova, because stars \msun are believed to
have shed their hydrogen envelopes several hundred thousand years before the
explosion \cite{2005A&A...429..581M}. Alternatively, the progenitor might have
arisen from the merger of two massive stars \cite{2007ApJ...659L..13O}. Here we
show that the collision frequency of massive stars in a dense and young cluster
(of the kind to be expected near the center of a galaxy) is sufficient to
provide a reasonable chance that SN 2006gy resulted from such a bombardment. If
this is the correct explanation, then we predict that when the supernova fades
(in a year or so) a dense cluster of massive stars becomes visible at the site
of the explosion
On the equivalence of the Langevin and auxiliary field quantization methods for absorbing dielectrics
Recently two methods have been developed for the quantization of the
electromagnetic field in general dispersing and absorbing linear dielectrics.
The first is based upon the introduction of a quantum Langevin current in
Maxwell's equations [T. Gruner and D.-G. Welsch, Phys. Rev. A 53, 1818 (1996);
Ho Trung Dung, L. Kn\"{o}ll, and D.-G. Welsch, Phys. Rev. A 57, 3931 (1998); S.
Scheel, L. Kn\"{o}ll, and D.-G. Welsch, Phys. Rev. A 58, 700 (1998)], whereas
the second makes use of a set of auxiliary fields, followed by a canonical
quantization procedure [A. Tip, Phys. Rev. A 57, 4818 (1998)]. We show that
both approaches are equivalent.Comment: 7 pages, RevTeX, no figure
Gauge symmetry and W-algebra in higher derivative systems
The problem of gauge symmetry in higher derivative Lagrangian systems is
discussed from a Hamiltonian point of view. The number of independent gauge
parameters is shown to be in general {\it{less}} than the number of independent
primary first class constraints, thereby distinguishing it from conventional
first order systems. Different models have been considered as illustrative
examples. In particular we show a direct connection between the gauge symmetry
and the W-algebra for the rigid relativistic particle.Comment: 1+22 pages, 1 figure, LaTeX, v2; title changed, considerably expanded
version with new results, to appear in JHE
VI Photometry of Globular Clusters NGC6293 and NGC6541: The Formation of the Metal-Poor Inner Halo Globular Clusters
We present VI photometry of the metal-poor inner halo globular clusters
NGC6293 and NGC6541 using the planetary camera of the WFPC2 on board HST. Our
color-magnitude diagrams of the clusters show well-defined BHB populations,
consistent with their low metallicities and old ages. NGC6293 appears to have
blue straggler stars in the cluster's central region. We discuss the
interstellar reddening and the distance modulus of NGC6293 and NGC6541 and
obtain E(B-V) = 0.40 and (m-M)_0 = 14.61 for NGC6293 and E(B-V) = 0.14 and
(m-M)_0 = 14.19 for NGC6541. Our results confirm that NGC6293 and NGC6541 are
clearly located in the Galaxy's central regions (R_{GC} < 3 kpc). We also
discuss the differential reddening across NGC6293. The interstellar reddening
value of NGC6293 appears to vary by Delta E(B-V) ~ 0.02 -- 0.04 mag within our
small field of view.
The most notable result of our study is that the inner halo clusters NGC6293
and NGC6541 essentially have the same ages as M92, confirming the previous
result from the HST NIC3 observations of NGC6287.Comment: AJ, accepte
Child Psychosocial Adjustment and Parenting in Families Affected by Maternal HIV/AIDS
Child adjustment and parenting were examined in 23 9-through 16-year-old youth from families affected by maternal HIV infection and 20 same-age peers whose mothers were not infected. Children whose mothers were seropositive reported significantly more externalizing problems. Infected mothers reported less age-appropriate supervision/monitoring relative to non-infected mothers. Better mother-child relationship quality and less impairment in parental supervision/monitoring of age-appropriate youth behaviors were associated with fewer externalizing difficulties among the HIV-positive group only. Similarly, only among HIV-infected mothers was refraining from engaging in inconsistent disciplinary tactics associated with lower reports of internalizing and externalizing problems. These data highlight the promise of programs targeting parenting skills to prevent or ameliorate child difficulties
Metallicities and Radial Velocities of Five Open Clusters Including a New Candidate Member of the Monoceros Stream
Near infrared spectra of 133 red giant stars from ten Galactic open clusters
and two Galactic globular clusters spanning 2.2 dex in metallicity and 11 Gyr
in age are presented. We combine this sample with ten clusters from Cole and
collaborators to investigate the Ca II triplet line strengths and their
relation to cluster metallicity and position along the red giant branch. We
show that characterizing the stellar surface gravity using Ks band photometry
(relative to the horizontal branch) taken from the Two Micron All-Sky Survey
allows for metallicity measurements at least as precise as those derived using
V or I band data. This has the great advantage that uniform photometry and
reliable astrometry is available for a large number of clusters. Using Ks band
photometry also reduces the effect of differential reddening within a given
cluster. We find no significant evidence for age or metallicity effects to the
linear Ca II triplet - metallicity relationship over the small range in
magnitudes studied when homogeneous reference metallicities are used. We derive
the first spectroscopic metallicity and new radial velocity estimates for five
open clusters: Berkeley 81, Berkeley 99, IC 1311, King 2, and NGC 7044. King 2
has an anomalous radial velocity compared with the local disk population. We
discuss the possibility that it is part of the Monoceros tidal stream.Comment: 35 pages, 14 figures, 9 tables. Accepted for publication in MNRA
Multicolor observations of the afterglow of the short/hard GRB 050724
New information on short/hard gamma-ray bursts (GRBs) is being gathered
thanks to the discovery of their optical and X-ray afterglows. However, some
key aspects are still poorly understood, including the collimation level of the
outflow, the duration of the central engine activity, and the properties of the
progenitor systems. We want to constrain the physical properties of the short
GRB 050724 and of its host galaxy, and make some inferences on the global short
GRB population. We present optical observations of the afterglow of GRB 050724
and of its host galaxy, significantly expanding the existing dataset for this
event. We compare our results with models, complementing them with available
measurements from the literature. We study the afterglow light curve and
spectrum including X-ray data. We also present observations of the host galaxy.
The observed optical emission was likely related to the large flare observed in
the X-ray light curve. The apparent steep decay was therefore not due to the
jet effect. Available data are indeed consistent with low collimation, in turn
implying a large energy release, comparable to that of long GRBs. The flare
properties also constrain the internal shock mechanism, requiring a large
Lorentz factor contrast between the colliding shells. This implies that the
central engine was active at late times, rather than ejecting all shells
simultaneously. The host galaxy has red colors and no ongoing star formation,
consistent with previous findings on this GRB. However, it is not a pure
elliptical, and has some faint spiral structure. GRB 050724 provides the most
compelling case for association between a short burst and a galaxy with old
stellar population. It thus plays a pivotal role in constraining progenitors
models, which should allow for long delays between birth and explosion.Comment: 8 pages, 5 figures, 4 tables, accepted for publication in A&A, typo
fixe
Photometry of Magellanic Cloud clusters with the Advanced Camera for Surveys - I. The old LMC clusters NGC 1928, 1939 and Reticulum
(Abridged) We present the results of photometric measurements from images of
the LMC globular clusters NGC 1928, 1939 and Reticulum taken with ACS on HST.
F555W and F814W exposures result in high accuracy CMDs for these three
clusters. This is the first time that CMDs for NGC 1928 and 1939 have been
published. All three clusters possess CMDs with features indicating them to be
>10 Gyr old, including main sequence turn-offs at V~23 and well populated HBs.
We use the CMDs to obtain metallicity and reddening estimates for each cluster.
NGC 1939 is a metal poor cluster, with [Fe/H] = -2.10 +/- 0.19, while NGC 1928
is significantly more metal rich, with [Fe/H] = -1.27 +/- 0.14. Reticulum's
abundance lies between the two, at [Fe/H] = -1.66 +/- 0.12. All three clusters
are moderately reddened with values ranging from E(V-I) = 0.07 +/- 0.02 for
Reticulum and E(V-I) = 0.08 +/- 0.02 for NGC 1928, to E(V-I) = 0.16 +/- 0.03
for NGC 1939. We estimate the HB morphology of each cluster. NGC 1928 and 1939
possess HBs consisting almost exclusively of stars to the blue of the
instability strip. In contrast, Reticulum has an intermediate HB morphology,
with stars across the instability strip. Using a variety of dating techniques
we show that these three clusters are coeval with each other and the oldest
Galactic and LMC globular clusters, to within ~2 Gyr. The census of known old
LMC globular clusters therefore now numbers 15 plus the unique, somewhat
younger cluster ESO121-SC03. The NGC 1939 field contains another cluster in the
line-of-sight, NGC 1938. A CMD for this object shows it to be less than ~400
Myr old, and it is therefore unlikely to be physically associated with NGC
1939.Comment: 15 pages, 9 figures. Accepted for publication in MNRAS. For a version
with full resolution figures visit
http://www.ast.cam.ac.uk/~dmackey/acs1.html (recommended
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