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J/A+A/287/769       UV photometry of NGC 6397                 (Burgarella+ 1994)
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HST observations of the core of globular cluster NGC 6397
     Burgarella D., Paresce F., Meylan G., King I.R., Greenfield P.,
     Baxter D., Jedrzejewski R., Nota A., Albrecht R., Barbieri C.,
     Blades J.C., Boksenberg A., Crane P., Deharveng J.M., Disney M.J.,
     Jakobsen P., Kamperman T.M., Macchetto F., Mackay C.D., Weigelt G.
    <Astron. Astrophys. 287, 769 (1994)>
    =1994A&A...287..769B      (SIMBAD/NED BibCode)
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ADC_Keywords: Clusters, globular ; Stars, blue ; Photometry, ultraviolet
Keywords: stars: binaries: close - stars: blue stragglers -
          stars: population II - globular clusters: individual: NGC 6397 -
          ultraviolet: stars - stars: kinematics
Mission_Name: Hubble Space Telescope

Description:
    The core of the nearby and very concentrated globular cluster NGC 6397
    has been imaged through the f/96-F140W, f/48-F140W, f/96-F210M and
    f/48-F220W ultraviolet filters of the Faint Object Camera on board the
    Hubble Space Telescope down to an ultraviolet limiting magnitude of
    ~19. The most interesting objects in the field of view are six very
    bright centrally concentrated, blue stragglers observed for the first
    time in the UV. Using these and other data from ground-based
    observations, we have been able to deduce from a comparison with
    Kurucz's atmosphere models, temperatures of ~10000K and masses of
    ~1.6M_{sun}_ for the four brightest ones, which is remarkably close to
    twice the turn-off mass of NGC 6397. This finding supports the idea
    that two-star mechanisms (collisions, mergers) are at the origin of
    the blue stragglers in the core of NGC 6397. Since the central density
    is very high, collisions between main sequence stars are frequent,
    therefore providing the best formation mechanism. We have computed the
    number of such collisions in the core of NGC 6397 and found it to be
    of the same order as the number of bright blue stragglers observed
    there. Thanks to the HST resolution, we have also been able to resolve
    one object, previously classified as a yellow straggler, into a blend
    of a blue straggler and three redder stars.

Objects:
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       RA  (2000)  DE    Designation(s)
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     17 40.9    -53 41   NGC 6397
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File Summary:
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  FileName      Lrecl  Records   Explanations
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ReadMe             80        .   This file
table2             66      179   Positions, magnitudes and colors for the 179
                                  objects detected in NGC 6397
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Byte-by-byte Description of file: table2
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   Bytes Format Units   Label     Explanations
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   1-  3  I3    ---     Id        Identification number
   5- 10  F6.2  arcsec  Xpos      X position
  12- 17  F6.2  arcsec  Ypos      Y position
  21- 25  F5.2  mag     m140a     ? f/48 camera m_140w_ magnitude
  28- 32  F5.2  mag     m140-m220 ? f/48 camera m_140w_-m_220w_ colour
  35- 39  F5.2  mag     m140b     ? f/96 camera m_140w_ magnitude
  41- 45  F5.2  mag     m140-m210 ? f/96 camera m_200w_-m_210w_ colour
  48- 52  F5.2  mag     Vmag      ? V magnitude
  55- 58  F4.2  mag     B-V       ? B-V color index
  60- 66  A7    ---     Note      Notes (1)
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Note (1): BSS: Blue stragglers star with an identification number for stars
                detected by Lauzeral et al. (1992A&A...262...63L)
          BHB: Blue horizontal branch star
          YSS: Yellow stragglers star
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History: Prepared via OCR at CDS.
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(End)                           James Marcout, Patricia Bauer [CDS]  08-Nov-1996