L. Baldacci

RR Lyrae and Short-Period Variable Stars in the Dwarf Irregular Galaxy NGC 6822

We report the discovery of a large number of short-period variable stars in the dwarf irregular galaxy NGC 6822, based on deep time-series imaging carried out with the ESO Very Large Telescope. In particular, we found a modest population of RR Lyrae stars tracing the presence of an old stellar component in NGC 6822. Measurements of the average luminosity of RR Lyrae stars provide a new independent estimate of the distance to this galaxy based on a Population II indicator, (m - M)0 = 23.36 ± 0.17. In addition, our new data show a significant population of small-amplitude, short-period variable stars filling the instability strip starting at luminosities only a few tenths of a magnitude brighter than the RR Lyrae stars. Given the presence of an extended star formation in NGC 6822, the faint end of this distribution of short-period variable stars is likely to originate from a population of intermediate-age, metal-poor He-burning stars, younger and more massive than RR Lyrae stars.

Variable stars in the dwarf irregular galaxy NGC 6822: The photometric catalogue ,,

Deep B, V time-series photometry obtained with the ESO Very Large Telescope has been used to identify variable stars in the dwarf irregular galaxy NGC 6822. We surveyed a 6.8 × 6.8 arcmin area of the galaxy and detected a total number of 390 candidate variables with the optimal image subtraction technique (Alard 2000, A&AS, 144, 363). Light curves on a magnitude scale were obtained for 262 of these variables. Differential flux light curves are available for the remaining sample. In this paper we present the photometric catalogue of calibrated light curves and time-series data, along with coordinates and classification of the candidate variables. A detailed description is provided of the procedures used to identify the variable stars and calibrate their differential flux light curves on a magnitude scale.

Water vapour masers in long-period variable stars I. RX Boo and SV Peg

Context. Water vapour maser emission from late-type stars characterises them as asymptotic-giant-branch stars with oxygen-rich chemistry that are losing mass at a substantial rate. Further conclusions on the properties of the stars, however, are hampered by the strong variability of the emission. Aims. We wish to understand the reasons for the strong variability of H2O masers in circumstellar shells of late-type stars. In this paper we study RX Bootis and SV Pegasi as representatives of semiregular variable stars (SRVs). Methods. We monitored RX Boo and SV Peg in the 22-GHz maser line of water vapour with single-dish telescopes. The monitoring period covered two decades for RX Boo (1987–2007) and 12 years for SV Peg (1990–1995, 2000–2007). In addition, maps were obtained of RX Boo with the Very Large Array over several years. Results. We find that most of the emission in the circumstellar shell of RX Boo is located in an incomplete ring with an inner radius of 91 mas (15 AU). A velocity gradient is found in a NW–SE direction. The maser region can be modelled as a shell with a thickness of 22 AU, which is only partially filled. The gas crossing time is 16.5 years. The ring-like structure and the velocity gradient remained stable for at least 11 years, while the maser line profiles varied strongly. This suggests that the spatial asymmetry is not accidental, so that either the mass loss process or the maser excitation conditions in RX Boo are not spherically symmetric. The strong variability of the maser spectral features is mainly due to incoherent intensity fluctuations of maser emission spots, which have lifetimes of the order of 1 year. We found no correlation between the optical and the maser variability in either star. The variability properties of the SV Peg masers do not differ substantially from those of RX Boo. There were fewer spectral features present, and the range of variations was narrower. The maser was active on the >10-Jy level only 1990–1992 and 2006/2007. At other times the maser was either absent (<1 Jy) or barely detectable. Conclusions. The variability of H2O masers in the SRVs RX Boo and SV Peg is due to the emergence and disappearance of maser clouds with lifetimes of ∼1 year. The emission regions do not evenly fill the shell of RX Boo leading to asymmetry in the spatial distribution, which persists at least an order of magnitude longer.

Distance scale, variable stars and stellar populations in Local Group galaxies

We present an overview of our study of the short period variable stars in the Large Magellanic Cloud, and in the dwarf galaxies Fornax, Leo I, and NGC 6822. Light curves are presented for RR Lyrae stars, Anomalous Cepheids and, for the first time, for Dwarf Cepheids in the field and in the globular cluster #3 of the Fornax galaxy.

NGC 6822: short period variable stars, stellar population and distance scale

Results are presented on a study of the short period variable stars in the dwarf irregular galaxy NGC6822. We observed an almost uniformely populated classical instability strip from the Horizontal Branch up to the Classical Cepheids region. The main goal we achieved from the analysis of the faint sample is the first detection of RR Lyrae stars in this galaxy.

A continuous population of variable stars up to about 1.5 mag above the horizontal branch

Increasing samples of pulsating variable stars populating the classical instability strip from the horizontal branch to a few magnitudes brighter are being found in several Local Group galaxies, irrespective of the galaxy morphological type. We will review the observational scenario focusing in particular on the Anomalous Cepheids and related objects.