1998/09/26 by Adrienne M. Cool, Jonathan E. Grindlay, Haldan N. Cohn +3 · 4 citations
Physics and Astronomy · Social Sciences · #Astronomy and Astrophysical Research #Binary star #Blue straggler #Educational Leadership and Practices #Globular cluster #Horizontal branch #Star cluster #Stars #Stellar, planetary, and galactic studies #Subdwarf #Variable star #White dwarf #astro-ph
paper · pdf · doi:10.1086/311730
11 pages plus 3 eps figures; LaTeX using aaspp4.sty; to appear in The Astrophysical Journal Letters
arxiv created 1998/09/26 · openalex publication_date 1998/11/20 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
We present evidence that the globular cluster NGC 6397 contains two distinct classes of centrally concentrated UV-bright stars. Color-magnitude diagrams constructed from U 336 , B 439 , V 555 , and I 814 data obtained with the Hubble Space Telescope Wide Field Planetary Camera 2 reveal seven UV-bright stars fainter than the main-sequence turnoff, three of which had previously been identified as cataclysmic variables (CVs). Light curves of these stars show the characteristic "flicker" of CVs as well as longer term variability. A fourth star is identified as a CV candidate on the basis of its variability and UV excess. Three additional UV-bright stars show no photometric variability and have broadband colors characteristic of B stars. These nonflickering UV stars are too faint to be extended horizontal-branch stars. We suggest that they could be low-mass helium white dwarfs, formed when the evolution of a red giant is interrupted, due either to Roche lobe overflow onto a binary companion or to envelope ejection following a common-envelope phase in a tidal-capture binary. Alternatively, they could be very low mass core-He-burning stars. Both the CVs and the new class of faint UV stars are strongly concentrated toward the cluster center, to the extent that mass segregation from two-body relaxation alone may be unable to explain their distribution.