Arkadiusz Olech

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.

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.

Variable stars in the globular cluster M5: application of the image subtraction method

We present V-band light curves of 61 variables from the core of the globular cluster M5 obtained using a newly developed image subtraction method (ISM). Four of these variables were previously unknown. Only 26 variables were found in the same field using photometry obtained with dophot software. Fourier parameters of the ISM light curves have relative errors up to 11 times smaller than parameters measured from dophot photometry. We conclude that the new method is very promising for searching for variable stars in the cores of the globular clusters and gives very accurate relative photometry with quality comparable to photometry obtained by the Hubble Space Telescope. We also show that the variable V104 is not an eclipsing star as has been suggested, but is an RRc star showing non-radial pulsations.

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

ABSTRACT We present an analysis and interpretation of oscillation spectra for all 69 SXPhoenicis stars discovered in the field of the cluster. For most of the stars we havereliable absolute magnitudes and colors. Except of one, or perhaps two, objects, thestars are cluster members. Their pulsational behaviour is very diversified. Multiperi-odic variabilitywith at least part ofthe excited modes being nonradial is most commonbut there are also many cases of high amplitude, presumably radial mode, pulsators.In a number of such cases we have evidence for two radial modes being excited. Param-eters of radial mode pulsators are in most cases consistent with standard evolutionarymodels for stars in the mass range 0.9 ÷1.15M ⊙ . However, in four cases we haveevidence that the masses are significantly lower than expected. Three objects showfrequency triplets that may be interpreted in terms of rotational frequency splittingof l = 1 modes. Implied equatorial velocities of rotation are from 10 to over 100 km/s.Nearly all measured frequencies fall in the ranges predicted for unstable modes. Twocases of low frequency variability are interpreted as being caused by tidal distortioninduced by close companions.Key words: stars: SX Phe - stars: variables – globular clusters: individual: ω Cen

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

ABSTRA C T We present V- and B-band charge-coupled device (CCD) photometry of 35 RR Lyr stars in the Southern hemisphere globular cluster NGC 6362. Fourier decomposition of the light curves was used to estimate the basic properties of these variables. From the analysis of the RRc stars we obtain a mean mass Ma 0:531 ^ 0:014 M(; luminosity log L=L(a 1:656 ^ 0:006; effective temperature T effa 7429 ^ 20 and helium abundance Ya 0:292 ^ 0:002: The mean values of the absolute magnitude, metallicity (on Jurcsik’s scale) and effective temperature for 14 RRab stars with ‘regular’ light curves are: MV a 0:86 ^ 0:01;âFe=Haa 20:93 ^ 0:04 and T effa 6555 ^ 25; respectively. From the B 2 V colours, periods and metallicities of the RRab stars we estimate the colour excess for NGC 6362 to be equal to EOB 2 VUa0:08 ^ 0:01: Using this value we derive the colours of the blue and red edges of the instability strip in NGC 6362 to beOB 2 VU BE a 0:17 andOB 2 VU RE a 0:43: From the relations between the Fourier coefficients of RRab and RRc stars and their absolute magnitudes we estimate the apparent distance modulus to NGC 6362 to be Om 2 MUV a 14:46 ^ 0:10: From the mean value of log L=L( of the RRc stars we obtain 14:59 ^ 0:03: The V-band light curves of three of the RRc stars exhibit changes in amplitude of over 0.1 mag on the time-scale of around one week. Near the radial first overtone frequency we find one or two peaks, which strongly suggests that these variables belong to the newly identified group of non-radial pulsating RR Lyr stars.

Image-Subtraction Photometry of Variable Stars in the Field of the Globular Cluster NGC 6934

We present CCD BVI photometry of 85 variable stars from the field of the globular cluster NGC 6934. The photometry was obtained with the image subtraction package ISIS. 35 variables are new identifications: 24 RRab stars, five RRc stars, two eclipsing binaries of W UMa-type, one SX Phe star, and three variables of other types. Both detected contact binaries are foreground stars. The SX Phe variable belongs most likely to the group of cluster blue stragglers. Large number of newly found RR Lyr variables in this cluster, as well as in other clusters recently observed by us, indicates that total RR Lyr population identified up to date in nearby galactic globular clusters is significantly (>30%) incomplete. Fourier decomposition of the light curves of RR Lyr variables was used to estimate the basic properties of these stars. From the analysis of RRc variables we obtain a mean mass of M = 0.63 M⊙, luminosity log L/L⊙ = 1.72, effective temperature Teff = 7300 and helium abundance Y = 0.27. The mean values of the absolute magnitude, metallicity (on Zinns scale) and effective temperature for RRab variables are MV = 0.81, [Fe/H] = -1.53 and Teff = 6450, respectively. From the B-V color at minimum light of the RRab variables we obtained the color excess to NGC 6934 equal to E(B-V) = 0.09 ± 0.01. Different calibrations of absolute magnitudes of RRab and RRc available in literature were used to estimate apparent distance modulus of the cluster: (m - M)V = 16.09 ± 0.06. We note a likely error in the zero point of the HST-based V-band photometry of NGC 6934 recently presented by Piotto et al. Among analyzed sample of RR Lyr stars we have detected a short period and low amplitude variable which possibly belongs to the group of second overtone pulsators (RRe subtype variables). The BVI photometry of all variables is available electronically via anonymous ftp. The complete set of the CCD frames is available upon request.

Cluster AgeS Experiment (CASE): RR Lyrae stars from the globular cluster ω Centauri as standard candles

New photometry of RRab and RRc stars in ω Centauri is used to calibrate their absolute magnitudes M V as a function of (a) metallicity and (b) the Fourier parameters of light curves in the V band. The zero point of both calibrations relies on the distance modulus to the cluster derived earlier by the Cluster AgeS Experiment (CASE) project based on observations of the detached eclipsing binary OGLE GC17. For RRab variables, we obtained a relation of M V = (0.26 ′ 0.08)[Fe/H] + (0.91 ′ 0.13). A dereddened distance modulus to the Large Magellanic Cloud (LMC) based on that formula is μ 0 = 18.56 ′ 0.14 mag. The second calibration of M V , which is based on Fourier coefficients of decomposed light curves, results in the LMC distance of μ 0 = 18.51 ′ 0.07 mag.

Curious Variables Experiment (CURVE) CCD photometry of QW Serpentis in superoutburst and quiescence

We report extensive photometry of the dwarf nova QW Ser throughout its February 2003 superoutburst untill qui- escence. During the superoutburst the star displayed clear superhumps with a mean period of Psh = 0:07703(4) days. In the quiescence we observed a double humped wave characterized by a period of P= 0:07457(2) days. As both periods obey the Stolz-Schoembs relation with a period excess equal to 3:30 0:06%, the latter period is interpreted as the orbital period of the binary system

Variable stars in the field of the old open cluster Melotte 66

We report the results of photometric monitoring of the Melotte 66 field in BVI filters. 10 variables were identified with nine being new discoveries. The sample includes eight eclipsing binaries of which four are W UMa type stars and one star is a candidate blue straggler. All four contact binaries are likely the members of the cluster based on their estimated distances. 10 blue stars with U - B < -0.3 were detected inside a 14.8 x 22.8 arcmin 2 field centred on the cluster. Time series photometry for seven of them showed no evidence for any variability. The brightest object in the sample of blue stars is a promising candidate for a hot subdwarf belonging to the cluster. We show that the anomalously wide main sequence of the cluster, reported in some earlier studies, results from a combination of two effects: variable reddening occurring across the cluster field and the presence of a rich population of binary stars in the cluster itself. The density profile of the cluster field is derived and the total number of the member stars with 16 < V < 21 or 2.8 < M v < 7.8 is estimated conservatively at about 1100.

Alpha Cygnids - a possible July minor meteor shower

We present a comprehensive study of a possible -Cygnid meteor shower. Based on visual and telescopic observations made by Polish observers we estimate ba- sic parameters of the stream. Activity of -Cygnids lasts from around June 30 to July 31 with clear maximum near July 18 (solar longitude = 116:5). Maximal Zenithal Hourly Rates (ZHRs) are equal to 3:6 1:2. The struc- ture of the radiant analyzed by radiant software is most compact for geocentric velocity of the events equal to 41 km s 1 , and for the drift of the radiant (in units/day) equal to = +0 : 6 =+ 0 :2. Center of the radiant for the moment of maximum is = 302:5 =+ 46 : 3 .W e derive population index r equal to 2:550:14 from magni- tude distribution of 738 possible members of the stream. Comparing the velocity distributions of 754 possible - Cygnids and 4339 sporadic events by 2 and Kolmogorov- Smirnov tests we conclude that both distributions are dif- ferent with a probability very close to 1.0. Telescopic observations strictly conrm the results ob- tained from visual observations. The smallest values of 2 parameter were obtained for the geocentric velocity equal to 40 km s 1 and for the drift of the radiant (in units /day) equal to = +0 : 6 =+ 0 :2. The cen- ter of the radiant for moment of maximum is = 304:9 =+ 46 : 2 . In spite of making many photographic exposures we still have no photographic or video conrmation of the existence of this stream.