2007/09/30 by A. Dieball, C. Knigge, D. R. Zurek +4 · 3 citations
Physics and Astronomy · #Astrophysical Phenomena and Observations #Astrophysics and Star Formation Studies #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/522103
published as Astrophys.J.670:379-399,2007; Erratum-ibid.708:1772,2010 · 47 pages, including 14 figures, accepted for publication in ApJ, Table 7 corrected, final version including ApJ corrections
arxiv created 2007/11/16 · openalex publication_date 2007/11/20 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/31
We have obtained deep FUV and NUV images of the inner region of the dense globular cluster M15 with the HST ACS. The FUV - NUV color-magnitude diagram shows a well-defined track of horizontal branch stars, as well as a trail of blue stragglers and white dwarfs. The main-sequence turnoff is clearly visible at FUV ≃ 23.5 mag and FUV - NUV ≃ 3 mag, and the main-sequence stars form a prominent track that extends at least 2 mag below the main-sequence turnoff. As such, this is the deepest FUV - NUV color-magnitude diagram of a globular cluster presented so far. Cataclysmic variable and blue straggler candidates are the most centrally concentrated stellar populations, which might either be an effect of mass segregation or reflect the preferred birthplace in the dense cluster core of such dynamically formed objects. We find 41 FUV sources that exhibit significant variability. We classify the variables based on an analysis of their UV colors and variability properties. We find four previously known RR Lyrae and 13 further RR Lyrae candidates, one known Cepheid and six further candidates, six cataclysmic variable candidates, one known and one probable SX Phoenicis star, and the well-known low-mass X-ray binary AC 211. Our analysis represents the first detection of SX Phoenicis pulsations in the FUV. We find that Cepheids, RR Lyrae stars, and SX Phoenicis exhibit massive variability amplitudes in this wave band (several magnitudes).