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 ${\cal M}=0.54{\cal M}_\odot$, 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 $14.47\pm0.11$ 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 $14.27\pm0.04$Comment: accepted for publication in Astronomy and Astrophysics, 15 pages with 6 figure