253 research outputs found
A pulsational approach to near infrared and visual magnitudes of RR Lyrae stars
In this paper we present an improved theoretical scenario concerning near
infrared and visual magnitudes of RR Lyrae variables, as based on up-to-date
pulsating models. On this basis, we revisit the case of the prototype variable
RR Lyr, showing that the parallax inferred by this new pulsational approach
appears in close agreement with HST absolute parallax. Moreover, available K
and V measurements for field and cluster RR Lyrae variables with known
reddening and metal content are used to derive a relation connecting the K
absolute magnitude to period and metallicity, as well as a new calibration of
the M_V-[Fe/H] relation. The comparison between theoretical prescriptions and
observations suggests that RR Lyrae stars in the field and in Galactic Globular
Clusters should have quite similar evolutionary histories. The comparison
between theory and observations also discloses a general agreement that
supports the reliability of current pulsational scenario. On the contrary,
current empirical absolute magnitudes based on the Baade-Wesselink (BW) method
suggest relations with a zero-point that is fainter than predicted by pulsation
models, together with a milder metallicity dependence. However, preliminary
results based on a new calibration of the BW method provided by Cacciari et al.
(2000) for RR Cet and SW And appear in a much better agreement with the
pulsational predictions.Comment: 11 pages, 9 postscript figures, accepted for publication on MNRA
The Meinunger "Nicht Rote" Objects
Four high-latitude slow variable stars have been noted by Meinunger (1972) as
"nicht rote" ("not red") objects and thus curious. We have previously reported
(Margon & Deutsch 1997) that one of these objects, CC Boo, is in fact a QSO.
Here we present observations demonstrating that the remaining three are also
highly variable active galactic nuclei. The most interesting object of the four
is perhaps S 10765 (= NGP9 F324-0276706), which proves to be a resolved galaxy
at z=0.063. Despite the rapid and large reported variability amplitude (~1.6
mag), the spectrum is that of a perfectly normal galaxy, with no emission lines
or evident nonthermal continuum. We also present new spectroscopic and
photometric observations for AR CVn, suggested by Meinunger to be an RR Lyrae
star despite its very faint magnitude (=19.4). The object is indeed one of
the most distant RR Lyrae stars known, at a galactocentric distance of ~40 kpc.Comment: Accepted for publication in Publications of the Astronomical Society
of the Pacific, Volume 111, January 1999; 14 pages including 4 figures and 1
tabl
Is There a Difference in Luminosity between Field and Cluster RR Lyrae Variables?
Recent Hipparcos results have lent support to the idea that RR Lyrae
variables in the halo field and in globular clusters differ in luminosity by
~0.2mag. In this Letter, we study the pulsation properties of RR Lyraes in
clusters with distances determined via main-sequence fitting to Hipparcos
parallaxes for field subdwarfs, and compare them with the properties of field
variables also analyzed with Hipparcos. We show that the period-temperature
distributions for field and cluster variables are essentially
indistinguishable, thus suggesting that there is no significant difference in
luminosity between them.Comment: 11 pages, including three embedded figures and one table. ApJ
(Letters), in pres
Systematics of RR Lyrae Statistical Parallax III: Apparent Magnitudes and Extinctions
We sing the praises of the central limit theorem. Having previously removed
all other possible causes of significant systematic error in the statistical
parallax determination of RR Lyrae absolute magnitudes, we investigate
systematic errors from two final sources of input data: apparent magnitudes and
extinctions. We find corrections due to each of ~0.05 mag, i.e., ~1/2 the
statistical error. However, these are of opposite sign and so roughly cancel.
The apparent magnitude system that we previously adopted from Layden et al. was
calibrated to the photometry of Clube & Dawe. Using Hipparcos photometry we
show that the Clube & Dawe system is ~0.06 mag too bright. Extinctions were
previously pinned to the HI-based map of Burstein & Heiles. We argue that A_V
should rather be based on new COBE/IRAS dust-emission map of Schlegel,
Finkbeiner & Davis. This change increases the mean A_V by ~0.05 mag. We find
M_V=0.77 +/- 0.13 at [Fe/H]=-1.60 for a pure sample of 147 halo RR Lyraes, or
M_V=0.80 +/- 0.11 at [Fe/H]=-1.71 if we incorporate kinematic information from
716 non-kinematically selected non-RR Lyrae stars from Beers & Sommer-Larsen.
These are 2 and 3 sigma fainter than recent determinations of M_V from main
sequence fitting of clusters using Hipparcos measurements of subdwarfs by Reid
and Gratton et al. Since statistical parallax is being cleared of systematic
errors and since the chance of a >2 sigma statistical fluctuation is <1/20, we
conclude that these brighter determinations may be in error. In the course of
three papers, we have corrected 6 systematic errors whose absolute values total
0.20 mag. Had these, contrary to the expectation of the central limit theorem,
all lined up one way, they could have resolved the conflict in favor of the
brighter determinations. In fact, the net change was only 0.06 mag.Comment: submitted to ApJ, 21 pages, 2 tables, 4 figure
Evidence for an Overluminosity of the Variable Star RR Lyr, and a Revised Distance to the LMC
We use theoretical models to establish a tight relationship for the absolute
magnitudes of RR Lyrae stars as a function of their periods and Stroemgren
pseudo-color c_0 = (u-v)_0 - (v-b)_0. Applying this to RR Lyr, and comparing
the result with the predicted average absolute magnitude for stars of similar
metallicity from the same models, yields an overluminosity of 0.064 +/- 0.013
mag in Stroemgren y (and thus similarly in V) for RR Lyr. Based on a revised
value for RR Lyr's trigonometric parallax, and on a newly derived reddening
value of E(B-V) = 0.015 +/- 0.020, we provide a corrected relationship between
average absolute magnitude and metallicity for RR Lyrae stars that takes RR
Lyr's evolutionary status fully into account for the first time. Applying this
relationship to the LMC, we derive a revised true distance modulus of (m-M)_0 =
18.44 +/- 0.11.Comment: 4 pages, 4 figures (emulateapj format). ApJ (Letters), in pres
ROTSE All Sky Surveys for Variable Stars I: Test Fields
The ROTSE-I experiment has generated CCD photometry for the entire Northern
sky in two epochs nightly since March 1998. These sky patrol data are a
powerful resource for studies of astrophysical transients. As a demonstration
project, we present first results of a search for periodic variable stars
derived from ROTSE-I observations. Variable identification, period
determination, and type classification are conducted via automatic algorithms.
In a set of nine ROTSE-I sky patrol fields covering about 2000 square degrees
we identify 1781 periodic variable stars with mean magnitudes between m_v=10.0
and m_v=15.5. About 90% of these objects are newly identified as variable.
Examples of many familiar types are presented. All classifications for this
study have been manually confirmed. The selection criteria for this analysis
have been conservatively defined, and are known to be biased against some
variable classes. This preliminary study includes only 5.6% of the total
ROTSE-I sky coverage, suggesting that the full ROTSE-I variable catalog will
include more than 32,000 periodic variable stars.Comment: Accepted for publication in AJ 4/00. LaTeX manuscript. (28 pages, 11
postscript figures and 1 gif
Structure of S. aureus HPPK and discovery of a new inhibitor
The first structural and biophysical data on the folate pathway enzyme and drug target, 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase (HPPK), from the pathogenic bacterium Staphylococcus aureus is presented. HPPK is the second essential enzyme in the folate biosynthesis pathway, responsible for catalysing pyrophosphoryl transfer from cofactor (ATP) to the substrate (6-hydroxymethyl- 7,8-dihydropterin, HMDP). In-silico screening led to the discovery of a substrate competitive inhibitor, San1, which was subsequently co-crystallised with HPPK. A 1.65 Ă
resolution x-ray structure showed this to bind at the pterin site sharing many of the key intermolecular interactions of the substrate. ITC and SPR measurements yielded an equilibrium binding constant, Kd, of ~13 ÎŒM for San1. An IC50 of ~12 ÎŒM was determined by means of a new convenient tri-enzyme-coupled spectrophotometric assay. ITC and SPR further showed that the San1 inhibitor has no requirement for magnesium or ATP cofactor for competitive binding to the substrate site. According to 15N heteronuclear NMR measurements, the fast motion of the pterin loop (L2) is partially dampened in the ternary complex between SaHPPK, HMDP and , -methylene adenosine 5-triphosphate (AMPCPP), but the ATP loop (L3) remains mobile on the ÎŒs timescale. In contrast, for the SaHPPK/San1/AMPCPP ternary complex, loop L2 becomes rigid on the fast timescale and loop L3 becomes more ordered which are supported by a large entropic penalty associated with San1 binding as revealed by ITC. Backbone assignments and chemical shift perturbations implicate the sulphur in San1 as a likely important loop L2/L3 stabilizing mediato
CCD Photometry of Galactic Globular Clusters. IV. The NGC 1851 RR Lyraes
The variable star population of the galactic globular cluster NGC 1851
(C0512-400) has been studied by CCD photometry, from observations made in the
B, V, and I bands during 1993-4. Light curves are presented for 29 variables,
seven of which are new discoveries. The behavior of the RR lyraes in the
period-temperature diagram appears normal when compared to clusters which
bracket the NGC 1851 metallicity. Reddening and metallicity are re-evaluated,
with no compelling evidence to change from accepted values. Photometry for
stars within an annulus with radii 80 and 260 arcsec agrees to better than 0.02
mag in all colors with extensive earlier photometry, to at least V = 18.5.
Instability strip boundary positions for several clusters shows a trend for the
red boundary to move to redder colors as the metallicity increases.Comment: 29 pages, 9 figures, accepted by A.
Observational Tests of the Mass-Temperature Relation for Galaxy Clusters
We examine the relationship between the mass and x-ray gas temperature of
galaxy clusters using data drawn from the literature. Simple theoretical
arguments suggest that the mass of a cluster is related to the x-ray
temperature as . Virial theorem mass estimates based on
cluster galaxy velocity dispersions seem to be accurately described by this
scaling with a normalization consistent with that predicted by the simulations
of Evrard, Metzler, & Navarro (1996). X-ray mass estimates which employ
spatially resolved temperature profiles also follow a scaling
although with a normalization about 40% lower than that of the fit to the
virial masses. However, the isothermal -model and x-ray surface
brightness deprojection masses follow a steeper
scaling. The steepness of the isothermal estimates is due to their implicitly
assumed dark matter density profile of at large radii
while observations and simulations suggest that clusters follow steeper
profiles (e.g., ).Comment: 25 pages, 10 figures, accepted by Ap
An Error Analysis of the Geometric Baade-Wesselink Method
We derive an analytic solution for the minimization problem in the geometric
Baade-Wesselink method. This solution allows deriving the distance and mean
radius of a pulsating star by fitting its velocity curve and angular diameter
measured interferometrically. The method also provide analytic solutions for
the confidence levels of the best fit parameters, and accurate error estimates
for the Baade-Wesselink solution. Special care is taken in the analysis of the
various error sources in the final solution, among which the uncertainties due
to the projection factor, the limb darkening and the velocity curve. We also
discuss the importance of the phase shift between the stellar lightcurve and
the velocity curve as a potential error source in the geometric Baade-Wesselink
method. We finally discuss the case of the Classical Cepheid zeta Gem, applying
our method to the measurements derived with the Palomar Testbed Interferometer.
We show how a careful treatment of the measurement errors can be potentially
used to discriminate between different models of limb darkening using
interferometric techniques.Comment: 24 pages, to be published on the Astrophysical Journal, vol. 603
March 200
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