Cluster AgeS Experiment. CCD photometry of SX Phoenicis variables in the globular cluster M 55

Free access article

Issue		A&A Volume 367, Number 1, February III 2001


Page(s)		148 - 158
Section		Formation, structure and evolution of stars
DOI		http://dx.doi.org/10.1051/0004-6361:20000349

A&A 367, 148-158 (2001)
DOI: 10.1051/0004-6361:20000349

Cluster AgeS Experiment. CCD photometry of SX Phoenicis variables in the globular cluster M 55

W. Pych¹, J. Kaluzny¹, W. Krzeminski^{1, 2}, A. Schwarzenberg-Czerny^{1, 3} and I. B. Thompson²

¹ Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warszawa, Poland
² Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, CA 91101, USA
³ Astronomical Observatory of Adam Mickiewicz University, ul. Sloneczna 36, 60-286 Poznan, Poland

(Received 28 June 2000 / Accepted 27 November 2000)

Abstract
We present CCD photometry of SX Phe variables in the field of the globular cluster M 55. We have discovered 27 variables, three of which are probable members of the Sagittarius dwarf galaxy. All of the SX Phe stars in M 55 lie in the blue straggler region of the cluster color-magnitude diagram. Using period ratio information we have identified the radial pulsation modes for one of the observed variables. Inspection of the period-luminosity distribution permits the probable identifications of the pulsation modes for most of the rest of the stars in the sample. We have determined the slope of the period-luminosity relation for SX Phe stars in M 55 pulsating in the fundamental mode. Using this relation and the HIPPARCOS data for SX Phe itself, we have estimated the apparent distance modulus to M 55 to be $(m-M)_V=13.86\pm0.25$ mag.

Key words: globular clusters: individual: M 55 -- (stars): blue stragglers: stars: variables: $\delta$ Sct -- stars: oscillations -- stars: population II

Offprint request: W. Pych,pych@sirius.astrouw.edu.pl

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