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Astron. Astrophys. 349, 55-69 (1999)

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Variable stars in nearby galaxies *

I. Search for Cepheids in field A of IC 1613

E. Antonello, L. Mantegazza, D. Fugazza, M. Bossi and S. Covino

Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate, Italy (elio,luciano,fugazza,bossi,covino@merate.mi.astro.it)

Received 10 May 1999 / Accepted 28 June 1999

Abstract

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 3:08x3:08. 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 [FORMULA] 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 [FORMULA]. 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.

Key words: stars: oscillations – stars: variables: Cepheids – stars: variables: general – galaxies: individual: IC 1613 – galaxies: Local Group – galaxies: stellar content

* Based on observations collected at ESO-La Silla

Send offprint requests to: E. Antonello

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© European Southern Observatory (ESO) 1999

Online publication: August 25, 1999
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