396 research outputs found
A pulsational distance to Omega Centauri based on Near-Infrared Period-Luminosity relations of RR Lyrae stars
We present new Near-Infrared (J,K) magnitudes for 114 RR Lyrae stars in the
globular cluster Omega Cen (NGC 5139) which we combine with data from the
literature to construct a sample of 180 RR Lyrae stars with J and K mean
magnitudes on a common photometric system. This is presently the largest such
sample in any stellar system. We also present updated predictions for J,K-band
Period-Luminosity relations for both fundamental and first-overtone RR Lyrae
stars, based on synthetic horizontal branch models with metal abundance ranging
from Z=0.0001 to Z=0.004. By adopting for the Omega Cen variables with measured
metal abundances an alpha-element enhancement of a factor of 3 (about 0.5 dex)
with respect to iron we find a true distance modulus of 13.70 (with a random
error of 0.06 and a systematic error of 0.06), corresponding to a distance
d=5.5 Kpc (with both random and systematic errors equal to 0.03 Kpc). Our
estimate is in excellent agreement with the distance inferred for the eclipsing
binary OGLEGC-17, but differ significantly from the recent distance estimates
based on cluster dynamics and on high amplitude Delta Scuti stars.Comment: 24 pages, 7 figures, accepted for publication on The Astrophysical
Journa
On the distance and reddening of the starburst galaxy IC10
We present deep and accurate optical photometry of the Local Group starburst
galaxy IC10. The photometry is based on two sets of images collected with the
Advanced Camera for Surveys and with the Wide Field Planetary Camera 2 on board
the Hubble Space Telescope. We provide new estimates of the Red Giant Branch
tip (TRGB) magnitude, m_{F814W}^{TRGB}=21.90+-0.03, and of the reddening,
E(B-V)=0.78+-0.06, using field stars in the Small Magellanic Cloud (SMC) as a
reference. Adopting the SMC and two globulars, Omega Centauri and 47 Tucanae,
as references we estimate the distance modulus to IC10: independent
calibrations give weighted average distances of mu=24.51+-0.08 (TRGB) and
mu=24.56+-0.08 (RR Lyrae). We also provide a new theoretical calibration for
the TRGB luminosity, and using these predictions we find a very similar
distance to IC10 (mu~24.60+-0.15). These results suggest that IC10 is a likely
member of the M31 subgroup.Comment: 4 pages, 4 figures, ApJ Letters accepte
Star Counts in the Globular Cluster Omega Centauri. I. Bright Stellar Components
We present an extensive photometry on HB, RGB, and MSTO stars in Omega Cen.
The central regions of the cluster were covered with a mosaic of F435W, F625W,
and F658N-band data collected with ACS/HST. The outer reaches were covered with
a large set of U,B,V,I-band data collected with the [email protected] ESO/MPI telescope.
The final catalogue includes ~1.7 million stars. We identified ~3,200 likely HB
stars and ~12,500 stars brighter than the subgiant branch and fainter than the
RGB bumps. The HB morphology changes with the radial distance. The relative
number of extreme HB stars decreases from ~30% to ~21% when moving from the
center toward the outer regions of the cluster, while the fraction of less hot
HB stars increases from ~62% to ~72%. We performed a detailed comparison
between observed ratios of different stellar tracers and predictions based on
canonical evolutionary models with a primordial helium (Y=0.23) content and
metal abundances (Z=0.0002,0.001) that bracket the observed spread in
metallicity of Omega Cen stars. We found that the empirical star counts of HB
stars are on average larger (30%-40%) than predicted. Moreover, the rate of HB
stars is 43% larger than the MSTO rate. The discrepancy between the rate of HB
compared with the rate of RG and MSTO stars supports the evidence that we are
facing a true excess of HB stars. The same comparison was performed by assuming
a mix of stellar populations made with 70% of canonical stars and 30% of
He-enhanced stars. The discrepancy between theory and observations decreases by
a factor of two when compared with rates predicted by canonical He content
models, but still 15%-25% (Y=0.42) and 15%-20% (Y=0.33) higher than observed.
Furthermore, the ratio between HB and MSTO star counts are ~24% (Y=0.42) and
30% (Y=0.33) larger than predicted lifetime ratios.Comment: 54 pages, 17 figures,to be published in ApJ, see link at
http://stellari.wiki.zoho.co
Discovery of a rapidly pulsating subdwarf B star candidate in omega Cen
We report the discovery of the first variable extreme horizontal branch star
in a globular cluster (omega Cen). The oscillation uncovered has a period of
114 s and an amplitude of 32 mmags. A comparison between horizontal branch
models and observed optical colours indicates an effective temperature of
31,500+-6,300 K for this star, placing it within the instability strip for
rapidly oscillating B subdwarfs. The time scale and amplitude of the pulsation
detected are also in line with what is expected for this type of variable, thus
strengthening the case for the discovery of a new subdwarf B pulsator.Comment: 5 pages, 7 figures; accepted for publication in A&
Distance to Galactic globulars using the near-infrared magnitudes of RR Lyrae stars: IV. The case of M5 (NGC5904)
We present new and accurate near-infrared (NIR) J, K-band time series data
for the Galactic globular cluster (GC) M5 = NGC5904. Data were collected with
SOFI at the NTT (71 J + 120 K images) and with NICS at the TNG (25 J + 22 K
images) and cover two orthogonal strips across the center of the cluster of
\approx 5 \times 10 arcmin^{2} each. These data allowed us to derive accurate
mean K-band magnitudes for 52 fundamental (RR_{ab}) and 24 first overtone
(RR_{c}) RR Lyrae stars. Using this sample of RR Lyrae stars, we find that the
slope of the K-band Period Luminosity (PLK) relation (-2.33 \pm 0.08) agrees
quite well with similar estimates available in the literature. We also find,
using both theoretical and empirical calibrations of the PLK relation, a true
distance to M5 of (14.44 \pm 0.02) mag. This distance modulus agrees very well
(1\sigma) with distances based on main sequence fitting method and on kinematic
method (14.44 \pm 0.41 mag, \citealt{rees_1996}), while is systematically
smaller than the distance based on the white dwarf cooling sequence (14.67 \pm
0.18 mag, \citealt{layden2005}), even if with a difference slightly larger than
1\sigma. The true distance modulus to M5 based on the PLJ relation (14.50 \pm
0.08 mag) is in quite good agreement with the distance based on the PLK
relation further supporting the use of NIR PL relations for RR Lyrae stars to
improve the precision of the GC distance scale.Comment: 19 pages, 6 figures, accepted for publication in MNRA
Relative distances of Omega Centauri and 47 Tucanae
We present precise optical and near-infrared ground-based photometry of two
Globular Clusters (GCs): Omega Cen and 47 Tuc. These photometric catalogs are
unbiased in the Red Giant Branch (RGB) region close to the tip. We provide new
estimates of the RGB tip (TRGB) magnitudes--m_I(TRGB)=9.84+/-0.05, Omega Cen;
m_I(TRGB)=9.46+/-0.06, 47 Tuc--and use these to determine the relative
distances of the two GCs. We find that distance ratios based on different
calibrations of the TRGB, the RR Lyrae stars and kinematic distances agree with
each other within one sigma. Absolute TRGB and RR Lyrae distance moduli agree
within 0.10--0.15 mag, while absolute kinematic distance moduli are 0.2--0.3
mag smaller. Absolute distances to 47 Tuc based on the
Zero-Age-Horizontal-Branch and on the white dwarf fitting agree within 0.1 mag,
but they are 0.1--0.3 mag smaller than TRGB and RR Lyrae distances.Comment: 5 pages, 4 figures, accepted for publication by ApJ
On the anomalous red giant branch of the globular cluster Omega Cen
We present three different optical and near-infrared (NIR) data sets for
evolved stars in the Galactic Globular Cluster Omega Cen The comparison between
observations and homogeneous sets of stellar isochrones and Zero-Age Horizontal
Branches provides two reasonable fits. Both of them suggest that the so-called
anomalous branch has a metal-intermediate chemical composition (-1.1 < [Fe/H] <
-0.8) and is located ~500 pc beyond the bulk of Omega Cen stars. These findings
are mainly supported by the shape of the subgiant branch in four different
color-magnitude diagrams (CMDs). The most plausible fit requires a higher
reddening, E(B-V)=0.155 vs. 0.12, and suggests that the anomalous branch is
coeval, within empirical and theoretical uncertainties, to the bulk of Omega
Cen stellar populations. This result is supported by the identification of a
sample of faint horizontal branch stars that might be connected with the
anomalous branch. Circumstantial empirical evidence seems to suggest that the
stars in this branch form a clump of stars located beyond the cluster.Comment: Accepted for publication on ApJ. High resolution figures can be
downloaded from ftp://ftp.mporzio.astro.it/dist/monelli2
Tigecycline use in serious nosocomial infections: a drug use evaluation
<p>Abstract</p> <p>Background</p> <p>Tigecycline is a novel antibiotic with activity against multidrug resistant bacteria. The aim of this study was to assess the efficacy of tigecycline use in serious hospital-acquired infections (HAI)</p> <p>Case presentation</p> <p>Prospective observational study of tigecycline use was conducted in a 1500 beds university hospital. From January 1, 2007 and January 31, 2010, 207 pts were treated with tigecycline for the following indications: intra-abdominal, pneumonia, bloodstream and complicated skin and soft tissue infections and febrile neutropenia. The therapy was targeted in 130/207 (63%) and empirical in 77/207 (37%) patients. All bacteria treated were susceptible to tigecycline. Median duration of tigecycline therapy was 13 days (range, 6-28). Clinical success was obtained in 151/207 (73%) cases, with the highest success rate recorded in intra-abdominal infections [81/99 (82%)]. Microbiological success was achieved in 100/129 (78%) treated patients. Adverse clinical events were seen in 16/207 patients (7.7%):</p> <p>Conclusions</p> <p>Considering the lack of data on tigecycline for critically ill patients, we think that the reported data of our clinical experience despite some limitations can be useful for clinicians.</p
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