K. Kuppuswamy

Large-scale wind structures in OB supergiants: a search for rotationally modulated Hα variability ⋆

We present the results of a long-term monitoring campaign of the Hline in a sample of bright OB-supergiants (O7.5-B9) that aims at detecting rotationally modulated changes potentially related to the existence of large-scale wind structures. A total of 22 objects were monitored during 36 nights spread over 6 months in 2001-2002. Co- ordinated broad-band photometric observations were also obtained for some targets. Conspicuous evidence for variability in His found for the stars displaying a fea- ture contaminated by wind emission. Most changes take place on a daily time-scale, although hourly variations are also occasionally detected. Convincing evidence for a cyclical pattern of variability in Hhas been found in 2 stars: HD 14134 and HD 42087 (periodic signals are also detected in other stars, but independent confirmation is re- quired). Rotational modulation is suggested from the similarity between the observed recurrence time-scales (in the range 13-25 days) and estimated periods of stellar rota- tion. We call attention to the atypical case of HD 14134 which exhibits a clear 12.8-d periodicity both in the photometric and in the spectroscopic data sets. This places this object among a handful of early-type stars where one may observe a clear link between extended wind structures and photospheric disturbances. Further modelling may test the hypothesis that azimuthally-extended wind streams are responsible for the patterns of spectral variability in our target stars.

Exploring the variable stars in the globular cluster NGC 5024 (M53): new RR Lyrae and SX Phoenicis stars★

We report CCD V and I time series photometry of the globular cluster NGC 5024 (M53). The technique of difference image analysis has been used which enables photometric precisions better than 10 mmag for stars brighter than V ∼ 18.5 mag even in the crowded central regions of the cluster. The high photometric precision has resulted in the discovery of two new RR1 stars and 13 SX Phe stars. A detailed identification chart is given for all the variable stars in the field of our images of the cluster. Periodicities were calculated for all RR Lyrae and SX Phe stars and a critical comparison is made with previous determinations. Out of the four probable SX Phe variables reported by Dekany & Kovacs, the SX Phe nature is confirmed only for V80; V81 is an unlikely case while V82 and V83 remain as dubious cases. Previous misidentifications of three variables are corrected. Astrometric positions with an uncertainty of ∼0.3 arcsec are provided for all variables. The light-curve Fourier decomposition of RR0 and RR1 is discussed, we find a mean metallicity of [Fe/H] = ―1.92 ± 0.06 in the scale of Zinn & West from 19 RR0 stars. The true distance moduli 16.36 ± 0.05 and 16.28 ± 0.07 and the corresponding distances 18.7 ± 0.4 and 18.0 ± 0.5 kpc are found from the RR0 and RR1 stars, respectively. These values are in agreement with the theoretical period-luminosity relations for RR Lyrae stars in the I band and with recent luminosity determinations for the RR Lyrae stars in the Large Magellanic Cloud (LMC). The age of 13.25 ± 0.50 Gyr, for NGC 5024, E(B ― V) = 0.02 and the above physical parameters of the cluster, as indicated from the RR0 stars, produce a good isochrone fitting to the observed colour-magnitude diagram (CMD). The period-luminosity (PL) relation for SX Phe stars in NGC 5024 in the V and I bands is discussed in the light of the 13 newly found SX Phe stars, and their pulsation mode is identified in most cases.

Variable stars in the globular cluster NGC 7492 - New discoveries and physical parameter determination

Aims. We have performed a photometric V,R,I CCD time-series analysis with a baseline of about eight years of the outer-halo globular cluster NGC 7492 with the aim of searching for new variables and using these (and the previously known variables) to determine the physical parameters of interest for the cluster (e.g. metallicity, absolute magnitude of the horizontal branch, distance, etc.). Methods. We used difference image analysis to extract precise light curves in the relatively crowded star field, especially towards the densely populated central region. Several different approaches were used for variability detection that recover the known variables and lead to new discoveries. We determined the physical parameters of the only RR0 star using a light-curve Fourier decomposition analysis. Results. We found one new long-period variable and two SX Phe stars in the blue straggler region. We also present one candidate SX Phe star that requires follow-up observations. Assuming that the SX Phe stars are cluster members and using the period-luminosity relation for these stars, we estimate their distances as ∼25.2 ± 1.8 and 26.8 ± 1.8 kpc, and identify their possible modes of oscillation. We refined the periods of the two RR Lyrae stars in our field of view. We found that the RR1 star V2 is undergoing a period change and possibly exhibits the Blazhko effect. A Fourier decomposition of the light curve of the RR0 star V1 allowed us to estimate a metallicity of [Fe/H]ZW ∼− 1.68 ± 0.10 or [Fe/H]UVES ∼− 1.64 ± 0.13, a log-luminosity log(L/L� ) ∼ 1.76 ± 0.02, an absolute magnitude MV ∼ 0.38 ± 0.04 mag, and a true distance modulus of μ0 ∼ 16.93 ± 0.04 mag, which is equivalent to a distance of ∼24.3 ± 0.5 kpc. All of these values are consistent with previous estimates in the literature.

The unusually large population of Blazhko variables in the globular cluster NGC 5024 (M53)

We report the discovery of amplitude and phase modulations typical of the Blazhko effect in 22 RRc and 9 RRab type RR Lyrae stars in NGC 5024 (M53). This brings the confirmed Blazhko variables in this cluster to 23 RRc and 11 RRab, that represent 66% and 37% of the total population of RRc and RRab stars in the cluster respectively, making NGC 5024 the globular cluster with the largest presently known population of Blazhko RRc stars. We place a lower limit on the overall incidence rate of the Blazhko effect among the RR Lyrae population in this cluster of 52%. New data have allowed us to refin e the pulsation periods. The limitations imposed by the time span and sampling of our data prevents reliable estimations of the modulation periods. The amplitudes of the modulations range between 0.02 and 0.39 mag. The RRab and RRc are neatly separated in the CMD, and the RRc Blazhko variables are on averge redder than their stable couterparts; these tw o facts may support the hypothesis that the HB evolution in this cluster is towards the red and th at the Blazhko modulations in the RRc stars are connected with the pulsation mode switch.

A detailed census of variable stars in the globular cluster NGC 6333 (M9) from CCD differential photometry

We report CCD V and I time series photometry of the globular cluster NGC 6333 (M9). The technique of difference image analysis has been used, which enables photometric precision better than 0.05 mag for stars brighter than V ∼ 19.0 mag, even in the crowded central regions of the cluster. The high photometric precision has resulted in the discovery of two new RRc stars, three eclipsing binaries, seven long-term variables and one field RRab star behind the cluster. A detailed identification chart and equatorial coordinates are given for all the variable stars in the field of our images of the cluster. Our data together with the literature V-data obtained in 1994 and 1995 allowed us to refine considerably the periods for all RR Lyrae stars. The nature of the new variables is discussed. We argue that variable V12 is a cluster member and an Anomalous Cepheid. Secular period variations, double-mode pulsations and/or the Blazhko-like modulations in some RRc variables are addressed. Through the light-curve Fourier decomposition of 12 RR Lyrae stars we have calculated a mean metallicity of [Fe/H]ZW = −1.70 ± 0.01(statistical) ± 0.14(systematic) or [Fe/H]UVES = -1.67 ± 0.01(statistical) ± 0.19(systematic). Absolute magnitudes, radii and masses are also estimated for the RR Lyrae stars. A detailed search for SX Phe stars in the Blue Straggler region was conducted but none were discovered. If SX Phe exist in the cluster then their amplitudes must be smaller than the detection limit of our photometry. The colour–magnitude diagram has been corrected for heavy differential reddening using the detailed extinction map of the cluster of Alonso-Garcia et al. This has allowed us to set the mean cluster distance from two independent estimates; from the RRab and RRc absolute magnitudes, we find 8.04 ± 0.19 and 7.88 ± 0.30 kpc, respectively.

Constraining the parameters of globular cluster NGC 1904 from its variable star population

Aims. We present the analysis of 11 nights of V and I time-series observations of the globular cluster NGC 1904 (M 79). Using this we searched for variable stars in this cluster and attempted to refine the periods of known variables, making use of a time baseline spanning almost 8 years. We use our data to derive the metallicity and distance of NGC 1904. Methods. We used difference imaging to reduce our data to obtain high-precision light curves of variable stars. We then estimated the cluster parameters by performing a Fourier decomposition of the light curves of RR Lyrae stars for which a good period estimate was possible. Results. Out of 13 stars previously classified as variables, we confirm that 10 are bona fide variables. We cannot detect variability in one other within the precision of our data, while there are two which are saturated in our data frames, but we do not find sufficient evidence in the literature to confirm their variability. We also detect a new RR Lyrae variable, giving a total number of confirmed variable stars in NGC 1904 of 11. Using the Fourier parameters, we find a cluster metallicity [Fe/H]ZW = −1.63 ± 0.14, or [Fe/H]UVES = −1.57 ± 0.18, and a distance of 13.3 ± 0.4 kpc (using RR0 variables) or 12.9 kpc (using the one RR1 variable in our sample for which Fourier decomposition was possible).

Revisiting the variable star population in NGC 6229 and the structure of the horizontal branch

We report an analysis of new

Variable stars in the globular cluster NGC 288: [Fe/H] and distance

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Variable stars in the globular cluster NGC 7492 (Corrigendum) - New discoveries and physical parameter determination

and

A Search for Rotationally Modulated Halpha Variability in OB Supergiants

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