Distance scale, variable stars and stellar populations in Local Group galaxies (original) (raw)
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Variable stars in nearby galaxies. II. Population I and II Cepheids in Field A of IC 1613
Astronomy and Astrophysics, 1999
The light curves of Cepheids and other variable stars in Field A of IC 1613, obtained with a CCD and no filter (Wh photometry), have been analyzed. It is possible to separate first overtone from fundamental mode population I Cepheids taking into account the pulsation amplitude, the shape of the light curve and the period. The expected separation is verified in the period-luminosity PL diagram. Light curve Fourier parameters have been compared with those of Magellanic Clouds and galactic Cepheids, in order to point out the effects of the very low metallicity of IC 1613 on the light curve shape. Population II Cepheids of IC 1613 can be discriminated from those of population I in the PL diagram, and, taking into account their color, from other red or blue variables. Their PL relation is consistent with that observed in globular clusters, nearby dwarf spheroidal galaxies and LMC. We have shown it is possible to apply the single-phase method for deriving standard photometry PL relations for population I and II Cepheids; therefore with just one accurate BVRI observation it is possible to use the population I Cepheids for distance determinations. Some unusual stars have been identified on the basis of periods, light curve shapes and colors; they appear to be pulsating stars laying on the extension of PL relation of known anomalous Cepheids. A firmer classification of these and other faint stars requires further deeper multicolor observations. Based on observations collected at ESO-La Silla}
Proceedings of the International Astronomical Union
Dwarf galaxies in the Local Group (LG) represent a distinct as well as diverse family of tracers of the earliest phases of galaxy assembly and the processing resulting from galactic harrassment. Their stellar populations can be resolved and used as probes of the evolution of their host galaxy. In this regard, we present the first reconstruction of the star formation history (SFH) of them using the most evolved AGB stars that are long period variable (LPV). LPV stars trace stellar populations as young as ∼ 30 Myr to as old as the oldest globular clusters. For the nearby, relatively massive and interacting gas-rich dwarf galaxies, the Magellanic Clouds, we found that the bulk of the stars formed ∼ 10 Gyr ago for the LMC, while the strongest episode of star formation in the SMC occurred a few Gyr later. A peak in star formation around 0.7 Gyr ago in both Clouds is likely linked to their recent interaction. The Andromeda satellite pair NGC147/185 show different histories; the main epoch...
Variable Stars and Galactic Structure
2013
Variable stars have a unique part to play in Galactic astronomy. Among the most important of these variables are the Cepheids (types I and II), the RR Lyraes and the Miras (O- and C-rich). The current status of the basic calibration of these stars in their roles as distance, structure and population indicators is outlined and some examples of recent applications of these stars to Galactic and extragalactic problems is reviewed. The expected impact of Gaia on this type of work is discussed and the need for complementary ground based observations, particularly large scale near-infrared photometry, is stressed.
Astronomy & Astrophysics, 2002
The frequency--period distribution and other properties of Cepheids in IC 1613 are discussed and compared with those of stars in our Galaxy (Milky Way), LMC, SMC, M31 and M33. Taking into account the observational limitations and related incompleteness, it is concluded that the frequency-period distribution of Cepheids in IC 1613 is similar to that of SMC; we suspect that a much larger number of stars exist in IC 1613 with a period of less than 2 d that have not yet been detected. A discussion of the deficiency of fundamental mode Cepheids with periods in the range 8 - 10 d in the Milky Way, M31 and M33 is reported. The present data are not sufficient to verify if this is produced by a real bimodal frequency--period distribution or whether depends on the lack of pulsating stars in such a period range due to pulsational stability reasons. Some arguments are presented in favor of a bimodal distribution that is a function of the average metallicity. The Milky Way, M31 and M33 have the two maxima located at the same periods, about 5 and 13 d, respectively. A comment on very long period Cepheids is also given.
Variable stars in nearby galaxies I. Search for Cepheids in field A of IC 1613
Astron Astrophys, 1999
The first results are presented of a four-year program dedicated to the CCD observations of Cepheids in the nearby galaxy IC 1613. The goal was to obtain good light curves for Fourier decomposition and to detect shorter period Cepheids. Since the program was carried out with a relatively small telescope, the Dutch 0.9 m at ESO-La Silla, the observations were performed without filter (white light), or Wh-band; the advantage of this technique is that the photon statistics correspond to that of V-band observations made with larger telescopes than 2 m and similar exposure time. The effective wavelength of the Wh-band is intermediate between that of V and R bands for stars of A-G spectral type, for back-illuminated CCD detectors, therefore the photometric characteristics of variable stars (e.g. amplitudes) are generally analogous to those obtained with the standard technique. Field A in IC 1613 has size 3.8'x3.8'. A total of 67 images were obtained and the reduction was performed with DAOPHOT. More than 2900 stars were measured, and for about 1700 stars there are from 67 to 24 Wh data points. Indications on the color of 739 bright stars were obtained also from V and R additional data. The analysis revealed the presence of about 110 variable stars. The detected population I Cepheids are 43; 9 Cepheids were already known from previous works, while most of the new stars have a short period P. We remark the following results: a) for stars with P >~ 5 d and sufficient phase coverage it is possible to perform good Fourier decomposition with resulting standard deviation of the fit of 0.02-0.04 mag; b) there are several Cepheids with relatively small amplitude, and most of them are (probable) first overtone mode pulsators; c) the faintest detected Cepheids have m_V ~ 23. No double-mode Cepheid has been found, probably because the precision and sampling of the data are not sufficient for the detection. Furthermore, at least 5 population II Cepheids and at least 8 eclipsing binaries have been observed. The other variable stars are probable long period, semiregular and irregular variables. A comparison with results of other massive CCD photometric projects dedicated to the detection of variable stars shows some advantages of the observations in white light for fully exploiting the capabilities of relatively small telescopes. A suggestion is made on how to use these results for distance determinations. Based on observations collected at ESO-La Silla}