W. Pych

BVRI Observations of the Optical Afterglow of GRB 990510

We present BVRI observations of the optical counterpart to GRB 990510 obtained with the Las Campanas 1.0 m telescope between 15 and 48 hr after the burst. The temporal analysis of the data indicates steepening decay, independent of wavelength, approaching asymptotically t-0.76±0.01 at early times (t 1 day) and t-2.40±0.02 at late times, with the break time at t0 = 1.57 ± 0.03 days. GRB 990510 is the most rapidly fading of the well-documented gamma-ray burst (GRB) afterglows. It is also the first well-observed example of a broadband break for a GRB optical counterpart. The optical spectral energy distribution, corrected for significant Galactic reddening, is well fitted by a single power law with ν-0.61±0.12. However, when the B-band point is dropped from the fit, the power law becomes ν-0.46±0.08, indicating a possible deviation from the power law in the spectrum, either intrinsic or due to additional extinction near the source or from an intervening galaxy at z = 1.62. Broadband break behavior broadly similar to that observed in GRB 990510 has been predicted in some jet models of GRB afterglows, thus supporting the idea that the GRB energy is beamed, at least in some cases.

SN 2005bf: a possible transition event between type Ib/c supernovae and gamma-ray bursts

We present ugriBV photometry and optical spectroscopy of the Type Ib/Ic SN 2005bf covering the first �100 days following discovery. The ugBV light curves displayed an unprecedented morphology among Type Ib/Ic supernovae, with an initial maximum some 2 weeks after discovery, and a second, main maximum about 25 days after that. The bolometric light curve indicates that SN 2005bf was a remarkably luminous event, radiating at least 6.3×10 42 erg s −1 at maximum light, and a total of 2.1 × 10 49 erg during the first 75 days after the explosion. Spectroscopically, SN 2005bf underwent a unique transformation

Cluster AgeS Experiment: The Age and Distance of the Globular Cluster ω Centauri Determined from Observations of the Eclipsing Binary OGLEGC 17

We use photometric and spectroscopic observations of the detached eclipsing binary OGLEGC 17 to derive the masses, radii, and luminosities of the component stars, and we then use these estimates to calculate the age and distance of the globular cluster ω Cen. Age versus turnoff mass and age versus luminosity relations from Girardi et al. yield two independent estimates of the age, 9.1 < t < 16.7 Gyr and 12.9 < t < 18.5 Gyr. The distance and distance modulus derived by use of the infrared versus surface brightness relation are d = 5360 ± 300 pc and (m - M)V = 14.05 ± 0.11. Distances derived from our infrared surface brightness versus color relation and the Teff versus B-V color relation of Sekiguchi & Fukugita disagree by about 10%. Major improvements in the accuracy in estimated age and distance can be made with better measurements of the masses of the components of OGLEGC 17.

RR Lyrae Variables in the Globular Cluster M55. The First Evidence for Nonradial Pulsations in RR Lyrae 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 RR Lyrae variables in this cluster to 15 stars. Five of the newly discovered variables are Bailey type RRc, and two are 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 M = 0.53 ± 0.03 M⊙, luminosity log L = 1.75 ± 0.01, effective temperature Teff = 7193 ± 27 K, and helium abundance Y = 0.27 ± 0.01. Based on the B-V colors, periods, and metallicities of the RRab stars, we estimate the value of the color excess for M55 to be equal to E(B - V) = 0.11 ± 0.03. Using this value we derive the colors of the blue and red edges of the instability strip in M55. The blue edge lies at (B - V)0 = 0.20 mag, and the red edge lies at (B - V)0 = 0.38 mag. We estimate the values of the visual apparent distance moduli to be 13.65 ± 0.11 from RRab stars and 13.91 ± 0.08 from RRc stars. The light curves of three of the RRc variables exhibit changes in amplitude of over 0.1 mag on the timescale 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 for these three stars that are clearly visible in the light curve after removing variations with the first overtone radial frequency. This is strong evidence for the presence of nonradial pulsations, a behavior common for δ Scuti stars but not yet observed among RR Lyrae variables.

THE CLUSTER 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 29.5d and the orbit is slightly eccentric with e = 0.056. We obtain Mp = 0.8762 ± 0.0048 M ☉, Rp = 1.3148 ± 0.0051 R ☉, Lp = 1.94 ± 0.21 L ☉ for the primary and Ms = 0.8588 ± 0.0060 M ☉, Rs = 1.1616 ± 0.0062 R ☉, Ls = 1.53 ± 0.17 L ☉ 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, [α/Fe] = 0.4, and Y = 0.255. The determination of the distance to V69, and hence to 47 Tuc, can be further improved when infrared eclipse photometry is obtained for the variable.

Cluster AgeS Experiment. Catalog of variable stars in the globular cluster omega Centauri

We present results of a photometric survey for variable stars in the field of the globular cluster ω Centauri. The observed region was centered roughly on the cluster core and covered 644 arcmin 2 . The cluster was monitored on 59 nights in 1999 and 2000. A total of 117 new variables were identified. Among them there are 17 RR Lyr-type stars, 35 SX Phe variables and 26 eclipsing binaries. A comprehensive catalog including all variable stars so far reported from the cluster field is presented. We list basic photometric properties and provide finding charts for a total of 392 objects. For 313 of them new BV light curves were obtained. The presented sample includes several interesting variables, such as SX Phe stars with extremely short periods of pulsation and several candidates for pulsating K giants. Optical counterparts to 9 X-ray sources detected by XMM and Chandra telescopes were identified: all of them are likely to be foreground variables not related to the cluster.

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

Radial Velocity Studies of Close Binary Stars. XI.

(m-M)_V=13.86\pm0.25

Cluster Ages Experiment (CASE): SX Phe stars from the globular cluster ω Centauri

mag.

The Clusters AgeS Experiment (CASE): RR Lyrae variables in the globular cluster NGC 6362

Radial-velocity measurements and sine-curve fits to orbital radial velocity variations are presented for 10 close binary systems: DU Boo, ET Boo, TX Cnc, V1073 Cyg, HL Dra, AK Her, VW LMi, V566 Oph, TV UMi, and AG Vir. With this contribution, the David Dunlap Observatory program has reached the point of 100 published radial velocity orbits. The radial velocities have been determined using an improved fitting technique that uses rotational profiles to approximate individual peaks in broadening functions. Three systems, ET Boo, VW LMi, and TV UMi, are found to be quadruple, while AG Vir appears to be a spectroscopic triple. ET Boo, a member of a close visual binary with Pvis = 113 yr, was previously known to be a multiple system, but we show that the second component is actually a close, noneclipsing binary. The new observations have enabled us to determine the spectroscopic orbits of the companion, noneclipsing pairs in ET Boo and VW LMi. A particularly interesting case is VW LMi, for which the period of the mutual revolution of the two spectroscopic binaries is only 355 days. While most of the studied eclipsing pairs are contact binaries, ET Boo is composed of two double-lined detached binaries, and HL Dra is a single-lined detached or semidetached system. Five systems of this group have been observed spectroscopically before: TX Cnc, V1073 Cyg, AK Her (as a single-lined binary), V566 Oph, and AG Vir, but our new data are of much higher quality than in the previous studies.