ar X iv : a st ro - p h / 01 05 14 5 v 1 9 M ay 2 00 1 Hot Stars in Globular Clusters – A Spectroscopist ’ s View

Globular clusters are ideal laboratories to study the evolution of low-mass stars. In this work we concentrate on three types of hot stars observed in globular clusters: horizontal branch stars, UV bright stars, and white dwarfs. After providing some historical background and information on gaps and blue tails we discuss extensively hot horizontal branch stars in metal-poor globular clusters, esp. their abundance anomalies and the consequences for the determination of their atmospheric parameters and evolutionary status. Hot horizontal branch stars in metal-rich globular clusters are found to form a small, but rather inhomogeneous group that cannot be explained by one evolutionary scenario. Hot UV bright stars show a lack of classic post-AGB stars that may explain the lack of planetary nebulae in globular clusters. Finally we discuss first results of spectroscopic observations of white dwarfs in globular clusters. Subject headings: (Galaxy:) globular clusters: general – stars: horizontal-branch – stars: post-AGB – (stars:) white dwarfs 1. Historical Background Globular clusters are the closest approximation to a physicist’s laboratory in astronomy. They are densely packed, gravitationally bound systems of several thousands to about one million stars. The dimensions of the globular clusters are small compared to their distance from us: half of the light is generally emitted within a radius of less than 10 pc, whereas the closest globular cluster has a distance of 2 kpc and 90% lie more than 5 kpc away. We can thus safely assume that all stars within a globular cluster lie at the same distance from us. With ages in the order of 10 years globular clusters are among the oldest objects in our Galaxy. Contrary to the field of the Galaxy globular clusters formed stars only once in the beginning. Because the duration of that star formation episode is short compared to the current age of the globular clusters the stars within one globular cluster are essentially coeval. In addition all stars within one globular cluster (with few exceptions) show the same initial abundance pattern (which may differ from one cluster to another). As we know today that Galactic globular clusters are old stellar systems people are often surprised by the presence of hot stars in these clusters since hot stars are usually associated with young stellar systems. The following paragraphs will show that hot stars have been known to exist in globular clusters for quite some time: About a century ago Barnard (1900) reported the detection of stars in globular clusters that were much brighter on (blue-sensitive) photographic plates than they appeared visually: “Of course the simple explanation of this peculiarity is that these stars, so bright photographically and so faint visually, are shining with a much bluer light than the stars which make up the main body of the clusters.” In 1915 Shapley started a project to obtain colours and magnitudes of individual stars in globular and open clusters (Shapley 1915a). In the first globular cluster (M 3, Shapley 1915b) he found a double peaked distribution of colours, with a red maximum and a blue secondary peak. He noticed that – in contrast to what was known for field dwarf (i.e. main sequence) stars – the stars in M 3 became bluer as they became fainter.