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Variable Stars In NGC 5024: RR Lyrae & SX Phoenicis Stars


RR Lyrae variables are commonly found in globular clusters, observed as low-mass stars that have evolved away from the main sequence and are in the core Helium burning phase (Christy 1966; Pritchet & van den Bergh 1987). Like Cepheid variables, they obey a period-luminosity relation and are useful candidates as distance indicators (Oort & Plaut 1975; Saha et al. 1990; Dolphin et al. 2002) with typical periods of ~0.2 to ~1d with amplitudes in the optical of 0.3 up to 2 magnitudes and spectral types of A2 to F6. Most RRLyrae stars pulsate in the radial fundamental mode (RRab stars), the radial rst overtone (RRc stars) and, in some cases, in both modes simultaneously (RRd stars).

RR Lyrae stars were formerly known as “cluster variables” because of their strong (but not exclusive) association with globular clusters; conversely, about 90% of all variables known in globular clusters are RR Lyraes suggesting an overwhelming connection. They can be found at all galactic latitudes, as opposed to classical Cepheid variables, which are strongly associated with the galactic plane.

An SX Phoenicis variable is, like RR Lyraes, another type of variable star. These stars exhibit a short period pulsation behavior that varies on time scales of 0.03–0.08 days (0.7–1.9 hours). They have spectral classifications in the range A2-F5 and vary in magnitude by up to 0.7 (McNamara 1995). Unlike RR Lyraes and Cepheid variables, SX Phoenicis are not found exclusively in one location, found in both globular clusters and in the galactic halo. All known SX Phoenicis variables in globular clusters are blue straggler stars (Young-Beom et al. 2004). These are stars that appear more blue (having a higher temperature) than the main sequence stars in the same cluster that have similar luminosities (Santolamazza et al. 2001).

A recent study by Arellano-Ferro et al. (2011)  has led to the identification of two RR Lyraes and thirteen SX Phoenicis stars in the globular cluster NGC 5024 (or Messier 53), which lies 60,000 light years from the galactic centre in the Coma Berenices constellation. M53 is metal poor (Zinn 1985) containing some 45 RR Lyrae variables (Sawyer-Hoog 1973; Clement 1997). The poor metallicity of the cluster suggests it comprises mainly from Population II stars, young stars which have a low proportion of elements other than hydrogen and helium.

The identification of variable stars in globular clusters has some importance. As all the stars of a globular cluster can be considered to be at the same distance from us (i.e. within a small distance error) their absolute magnitudes differ from their visual magnitude by an approximately small amount which can be assumed constant (Shapley 1917; Sandage 1957). The main sequence stars in the globular cluster will fall along a line that is believed to be comparable to similar stars in our solar neighborhood. The accuracy of this assumption is confirmed by comparable results obtained by comparing the magnitudes of nearby short-period variables, such as RR Lyrae stars and cepheid variables, with those in the cluster.

By matching up these curves on the Hertzsprung-Russell (HR) diagram the absolute magnitude of main sequence stars in the cluster can also be determined. This in turn provides a distance estimate to the cluster, based on the visual magnitude of the stars. The difference between the relative and absolute magnitude, the ‘distance modulus’, yields a healthy estimate of the distance. Thus, forming excellent weapons in distance calculations, yet also providing significant calibrations for distances further into the local group.

Journal References:

  • McNamara, D. (1997) Luminosities Of SX Phoenicis, Large-Amplitude Delta Scuti & RR Lyrae Stars. Publications Pacific Astronomical Society, 109 pp.1221-1232.
  • Dékány, I.; Kovács, G. (2009) Globular Cluster M53: New Variables, Distance Measurement & Metallicity. Astronomy & Astrophysics507 (2) pp.803-815.
  • Arellano-Ferro, A. et al. (2011) Exploring The Variable Stars In The Globular Cluster NGC 5024 (M53): New RR Lyrae & SX Phoenicis StarsMonthly Notices Royal Astronomical Society, 415 (1) pp.1-21.

Suggested Further Reading:

  • Hellier, C. (2001) Cataclysmic Variable Stars: How & Why They VarySpringer-Praxis Publishing, London.

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