621 research outputs found
Cluster AgeS Experiment. CCD photometry of SX Phoenicis variables in the globular cluster M 55
We present CCD photometry of SX Phe variables in the field of the globular
cluster M 55. We have discovered 27 variables, three of which are probable
members of the Sagittarius dwarf galaxy. All of the SX Phe stars in M 55 lie in
the blue straggler region of the cluster color-magnitude diagram. Using period
ratio information we have identified the radial pulsation modes for one of the
observed variables. Inspection of the period-luminosity distribution permits
the probable identifications of the pulsation modes for most of the rest of the
stars in the sample. We have determined the slope of the period-luminosity
relation for SX Phe stars in M 55 pulsating in the fundamental mode. Using this
relation and the HIPPARCOS data for SX Phe itself, we have estimated the
apparent distance modulus to M 55 to be (m-M)_V=13.86 +- 0.25 mag.Comment: A&A accepted, 11 figure
UBVI CCD Photometry of the Old Open Cluster Berkeley 17
Photometric UBVI CCD photometry is presented for NGC 188 and Berkeley 17.
Color-magnitude diagrams (CMDs) are constructed and reach well past the
main-sequence turn-off for both clusters. Cluster ages are determined by means
of isochrone fitting to the cluster CMDs. These fits are constrained to agree
with spectroscopic metallicity and reddening estimates. Cluster ages are
determined to be 7.0+/-0.5 Gyr for NGC 188, and 10.0+/- 1.0 Gyr for Berkeley
17, where the errors refer to uncertainties in the relative age determinations.
These ages are compared to the ages of relatively metal-rich inner halo/thick
disk globular clusters and other old open clusters. Berkeley 17 and NGC 6791
are the oldest open clusters with an age of 10 Gyr. They are 2 Gyr younger than
the thick disk globular clusters. These results confirm the status of Berkeley
17 as one of the oldest known open cluster in the Milky Way, and its age
provides a lower limit to the age of the Galactic disk.Comment: to appear in AJ; 28 pages, 9 figure
Empirical relations for cluster RR Lyrae stars revisited
Our former study on the empirical relations between the Fourier parameters of
the light curves of the fundamental mode RR Lyrae stars and their basic stellar
parameters has been extended to considerably larger data sets. The most
significant contribution to the absolute magnitude M_v comes from the period P
and from the first Fourier amplitude A_1, but there are statistically
significant contributions also from additional higher order components, most
importantly from A_3 and in a lesser degree from the Fourier phase phi_51. When
different colors are combined in reddening-free quantities, we obtain basically
period-luminosity-color relations. Due to the log T_eff (B-V, log g, [Fe/H])
relation from stellar atmosphere models, we would expect some dependence also
on phi_31. Unfortunately, the data are still not extensive and accurate enough
to decipher clearly the small effect of this Fourier phase. However, with the
aid of more accurate multicolor data on field variables, we show that this
Fourier phase should be present either in V-I or in B-V or in both. From the
standard deviations of the various regressions, an upper limit can be obtained
on the overall inhomogeneity of the reddening in the individual clusters. This
yields sigma_E(B-V)}< 0.012 mag, which also implies an average minimum
observational error of sigma_V > 0.018 mag.Comment: 14 pages, 11 figures, 11 tables, accepted in Astronomy & Astrophysic
Photometric study of the variable star population in the globular cluster NGC 6397
We present the results of a photometric survey for variable stars in the
central region of the nearby globular cluster NGC 6397.Time series photometry
was obtained for 30 variable objects. The sample includes 12 new objects, of
which 6 show periodic lightcurves and 2 are eclipsing binaries of unknown
period. Six variables possess certain and three possess likely X-ray
counterparts detected with the Chandra observatory. Among them four are
cataclysmic variables and one is a foreground eclipsing binary. The cataclysmic
variable CV2 exhibited a likely dwarf nova type outburst in May 2003. The
cataclysmic variable CV3 was observed at 18.5<V<20.0 during 5 observing runs,
but went into a low state in May 2003 when it reached V>22. We have found that
thelight curve of the optical companion to the millisecond pulsar PSRJ1740-5340
exhibits noticeable changes of its amplitude on a time scale of a few months. A
shallow eclipse with Delta_V=0.03 mag was detected in one of the cluster
turnoff stars suggesting the presence of a large planet or brown dwarf in
orbit.Comment: 8 pages, 9 figures, submitted to MNRAS on 21 April 200
The Clusters AgeS Experiment (CASE). IV. Analysis of the Eclipsing Binary V69 in the Globular Cluster 47 Tuc
We use photometric and spectroscopic observations of the eclipsing binary
V69-47 Tuc to derive the masses, radii, and luminosities of the component
stars. Based on measured systemic velocity, distance, and proper motion, the
system is a member of the globular cluster 47 Tuc. The system has an orbital
period of 29.5 d and the orbit is slightly eccentric with e=0.056. We obtain
Mp=0.8762 +- 0.0048 M(Sun), Rp=1.3148 +-0.0051 R(Sun), Lp=1.94 +- 0.21 L(Sun)
for the primary and Ms=0.8588 +- 0.0060 M(Sun), Rs=1.1616 +- 0.0062 R(Sun),
Ls=1.53 +- 0.17 L(Sun) for the secondary. These components of V69 are the first
Population II stars with masses and radii derived directly and with an accuracy
of better than 1%. We measure an apparent distance modulus of (m-M)v=13.35 +-
0.08 to V69. We compare the absolute parameters of V69 with five sets of
stellar evolution models and estimate the age of V69 using mass-luminosity-age,
mass-radius-age, and turnoff mass - age relations. The masses, radii, and
luminosities of the component stars are determined well enough that the
measurement of ages is dominated by systematic differences between the
evolutionary models, in particular, the adopted helium abundance. By comparing
the observations to Dartmouth model isochrones we estimate the age of V69 to be
11.25 +- 0.21(random) +- 0.85(systematic) Gyr assuming [Fe/H]=-0.70,
[alpha/Fe]=0.4, and Y=0.255. The determination of the distance to V69, and
hence to 47Tuc, can be further improved when infrared eclipse photometry is
obtained for the variable.Comment: 49 pages, 15 figures, submitted to A
RR Lyrae Variables in the Globular Cluster M5
We present V-band CCD photometry of 65 RR Lyr variables from the globular
cluster M5. We have estimated the basic physical parameters for 16 RRc stars
and 26 RRab stars using a Fourier decomposition of the light curves of the
variables. The mean values of mass, luminosity, effective temperature and
relative helium abundance for the RRc stars are measured to be , log L = 1.69 T_{eff}=7353 K and Y=0.28, respectively.
For the RRab variables the derived mean values of absolute magnitude,
metallicity and effective temperature are: M_V=0.81, [Fe/H]=-1.23 and
T_{eff}=6465 K. We find that the V amplitude of an RRab star for a given period
is a function of metal abundance rather than Oosterhoff type. We find
significant problems with the calibration of both the zero point and the scale
of the luminosities measured with the Fourier technique. The apparent distance
modulus derived from RRc stars is equal to and it is in good
agreement with recent determinations. On the other hand distance modulus
obtained from the sample of RRab stars (calibrated by the Baade-Wesselink
observations of field RR Lyr variables) is significantly smaller and equal to
Comment: accepted for publication in Astronomy and Astrophysics, 15 pages with
6 figure
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