The Blue Stragglers in the Core of NGC 6397

Details The Blue Stragglers in the Core of NGC 6397 The Blue Stragglers in the Core of NGC 6397

Jul 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996hst..prop.6697d&link_type=abstract

HST Proposal ID #6697

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Hst Proposal Id #6697 Hot Stars

Scientific paper

Six bright blue stragglers dominate the core of the globularcluster NGC 6397. They are the most massive objects detectedin this cluster. No other blue stragglers are found within onecore radius of the cluster center, and although a total of 26blue stragglers are known in NGC 6397, none of them is asbright as the six objects in the core. Photometric data andlow resolution spectroscopy that we obtained with the HST inthe UV classify them in the late-B to mid-A spectral typerange. Although a merger of two stars is likely to have givenbirth to these stars (in view of their mass, estimatedphotometrically), no direct indication exists on theirformation mechanism: both hypotheses of a binary merger (inwhich stars in full contact evolve into coalescence after masstransfer) as well as coalescence following a collision arecompatible with the available data. We, therefore, propose toobtain medium resolution FOS spectra to investigate the natureof these objects. From the spectral analysis of the Balmerjump and Balmer line profiles and from the known UV colors, wewill be able to determine T(eff) and logg. Knowing thedistance to the cluster, masses will accurately be determinedfor the first time, and will allow us to assess more reliablythe formation mechanism. To better understand theirevolutionary scenario, we will also compare their photosphericHe abundances (higher after a collision) and rotationalvelocities (higher in a merged binary) with those of a

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