82 research outputs found
RR Lyrae - Theory vs Observation
The luminosities, effective temperatures and metallicities that are derived
empirically by Kovacs and Jurcsik from the light curves of a large number of
globular cluster and field RRab and RRc stars are compared to theoretical RR
Lyrae models. The strong luminosity dependence of the empirical blue and red
edges (Log L vs Log Teff diagram) is in disagreement with that of both
radiative and convective models. A reexamination of the theoretical
uncertainties in the modelling leads us to conclude that the disagreement is
irreconcilable.Comment: 6 pages, 5 figures (revised april 2000, revisions relatively minor
Hydrodynamical Survey of First Overtone Cepheids
A hydrodynamical survey of the pulsational properties of first overtone
Galactic Cepheids is presented. The goal of this study is to reproduce their
observed light- and radial velocity curves. The comparison between the models
and the observations is made in a quantitative manner on the level of the
Fourier coefficients. Purely radiative models fail to reproduce the observed
features, but convective models give good agreement.
It is found that the sharp features in the Fourier coefficients are indeed
caused by the P1/P4 = 2 resonance, despite the very large damping of the 4th
overtone. For the adopted mass-luminosity relation the resonance center lies
near a period of 4.2d +/- 0.2 as indicated by the observed radial velocity
data, rather than near 3.2d as the light-curves suggest.Comment: ApJ, 12 pages, (slightly) revise
Mode Switching Time Scales in the Classical Variable Stars
Near the edges of the instability strip the rate of stellar evolution is
larger than the growth-rate of the pulsation amplitude, and the same holds
whenever the star is engaged in pulsational mode switching. Stellar evolution
therefore controls both the onset of pulsation at the edges of the instability
strip and of mode switching inside it. Two types of switchings (bifurcations)
occur. In a soft bifurcation the switching time scale is the inverse harmonic
mean of the pulsational modal growth-rate and of the stellar evolution rate. In
a hard bifurcation the switching times can be substantially longer than the
thermal time scale which is typically of the order of a hundred periods for
Cepheids and RR Lyrae. We discuss some of the observational consequences, in
particular the paucity of low amplitude pulsators at the edges of the
instability strip.Comment: 5 pages, 3 figures, ApJ (in press
Turbulent convection: comparing the moment equations to numerical simulations
The non-local hydrodynamic moment equations for compressible convection are
compared to numerical simulations. Convective and radiative flux typically
deviate less than 20% from the 3D simulations, while mean thermodynamic
quantities are accurate to at least 2% for the cases we have investigated. The
moment equations are solved in minutes rather than days on standard
workstations. We conclude that this convection model has the potential to
considerably improve the modelling of convection zones in stellar envelopes and
cores, in particular of A and F stars.Comment: 10 pages (6 pages of text including figure captions + 4 figures),
Latex 2e with AAS Latex 5.0 macros, accepted for publication in ApJ
RR Lyrae Variables in the Globular Cluster M55. The First Evidence for Non Radial Pulsations in RR Lyr Stars
We present the results of a photometric study of RR Lyrae variables in the
field of the globular cluster M55. We have discovered nine new RR Lyrae stars,
increasing the number of known variables in this cluster to 15 objects. Five of
the newly discovered variables belong to Bailey type RRc and two to type RRab.
Two background RRab stars are probable members of the Sagittarius dwarf galaxy.
Fourier decomposition of the light curves was used to derive basic properties
of the present sample of RR Lyrae variables. From an analysis of the RRc
variables we obtain a mean mass of , luminosity , effective temperature K, and helium
abundance . Based on the colors, periods and metallicities
of the RRab stars we estimate the value of the color excess for M55 to be equal
to . Using this value we derive the colors of the blue and
red edges of the instability strip in M55. The blue edge lies at
mag and the red edge lies at mag. We estimate the values of the
visual apparent and dereddened distance moduli to be and
, respectively. The light curves of three of the RRc variables
exhibit changes in amplitude of over 0.1 mag on the time scale of less than a
week, rather short for the Blazhko effect, but with no evidence for another
radial pulsational frequency. However we do detect other periodicities which
are clearly visible in the light curve after removing variations with the first
overtone radial frequency. This is strong evidence for the presence of
non-radial pulsations, a behavior common for Scuti stars but not yet
observed among RR Lyr variables.Comment: submitted to Astronomical Journal, 33 pages with 11 figure
Consensus of German Transplant Centers on Hematopoietic Stem Cell Transplantation in Fanconi Anemia
Allogeneic hematopoietic stem cell transplantation (HSCT) is currently the only curative therapy for the severe hematopoietic complications associated with Fanconi anemia (FA). In Germany, it is estimated that 10â15 transplants are performed annually for FA. However, because FA is a DNA repair disorder, standard conditioning regimens confer a high risk of excessive regimen-related toxicities and mortality, and reduced intensity regimens are linked with graft failure in some FA patients. Moreover, development of graft-versus-host disease is a major contributing factor for secondary solid tumors. The relative rarity of the disorder limits HSCT experience at any single center. Consensus meetings were convened to develop a national approach for HSCT in FA. This manuscript outlines current experience and knowledge about HSCT in FA and, based on this analysis, general recommendations reached at these meetings
BVI time series data of the Galactic Globular Cluster NGC3201. I. RR Lyrae stars
We present Johnson BV, and Kron-Cousins I-band time series data collected
over three consecutive nights in a region of 13 arcmin^2 centered on the
Galactic Globular Cluster (GGC) NGC3201. The time sampling of current CCD data
allowed us to derive accurate light curves, and in turn mean magnitudes and
colors for a sample of 53 RR Lyrae. To overcome the thorny problem of
differential reddening affecting this cluster, we derived new empirical
relations connecting the intrinsic (B-V) and (V-I) colors of fundamental
() RR Lyrae to the luminosity amplitude, the metallicity, and the
pulsation period. The key features of these relations are the following: i)
they rely on stellar parameters which are not affected by reddening; ii) they
supply accurate estimates of intrinsic colors across the fundamental
instability strip and cover a wide metallicity range; iii) they were derived by
neglecting the RR Lyrae that are affected by amplitude modulation. Moreover,
the zero-point of the E(B-V) reddening scale was empirically checked using the
large sample of RR Lyrae in M3 (Corwin & Carney 2001), a GGC affected by a
vanishing reddening. According to these relations we estimated individual
reddenings for RR Lyrae in our sample and the main results we found are the
following: i) the mean cluster reddening based on E(B-V) color excesses is
=0.30\pm0.03. iii) According to current individual E(B-V) and E(V-I)
reddenings and theoretical predictions for Horizontal-Branch stars, we found
that the true distance modulus for this cluster is 13.32\pm0.06 mag. iv) The
comparison between present luminosity amplitudes and estimates available in the
literature discloses that approximately 30% of fundamental RR Lyrae are
affected by amplitude modulation (Blazhko effect).Comment: 36 pages, 7 tables, 18 figures, AJ in pres
Infrared Light Curves of Mira Variable Stars from COBE DIRBE Data
We have used the COBE DIRBE database to derive near- and mid-infrared light
curves for a well-defined sample of 38 infrared-bright Mira variable stars, and
compared with optical data from the AAVSO. In general, the 3.5 micron and 4.9
micron DIRBE bandpasses provide the best S/N light curves, with S/N decreasing
with wavelength at longer wavelengths. At 25 microns, good light curves are
only available for ~10 percent of our stars, and at wavelengths >= 60 microns,
extracting high quality light curves is not possible. The amplitude of
variability is typically less in the near-infrared than in the optical, and
less in the mid-infrared than in the near-infrared, with decreasing amplitude
with increasing wavelength. On average, there are 0.20 +/- 0.01 magnitudes
variation at 1.25 microns and 0.14 +/- 0.01 magnitudes variation at 4.9 micron
for each magnitude variation in V. The observed amplitudes are consistent with
results of recent theoretical models of circumstellar dust shells around Mira
variables. For a few stars in our sample, we find clear evidence of time lags
between the optical and maxima of phase ~ 0.05 - 0.13, with no lags in the
minima. For three stars, mid-infrared maximum appears to occur slightly before
that in the near-infrared,but after optical maximum. We find three examples of
secondary maxima in the rising portions of the DIRBE light curves, all of which
have optical counterparts in the AAVSO data, supporting the hypothesis that
they are due to shocks rather than newly-formed dust layers. We find no
conclusive evidence for rapid (hours to days) variations in the infrared
brightnesses of these stars.Comment: 16 pages, Astronomical Journal, in press, to be publishe
Biomarkers in T cell therapy clinical trials
T cell therapy represents an emerging and promising modality for the treatment of both infectious disease and cancer. Data from recent clinical trials have highlighted the potential for this therapeutic modality to effect potent anti-tumor activity. Biomarkers, operationally defined as biological parameters measured from patients that provide information about treatment impact, play a central role in the development of novel therapeutic agents. In the absence of information about primary clinical endpoints, biomarkers can provide critical insights that allow investigators to guide the clinical development of the candidate product. In the context of cell therapy trials, the definition of biomarkers can be extended to include a description of parameters of the cell product that are important for product bioactivity
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