UV-(2000 A) imaging of globular clusters. I - The projected radial distributions and counts of the blue horizontal-branch stars in M3, M5, M13 and M92

Details UV-(2000 A) imaging of globular clusters. I - The projected radial distributions and counts of the blue horizontal-branch stars in M3, M5, M13 and M92 UV-(2000 A) imaging of globular clusters. I - The projected radial distributions and counts of the blue horizontal-branch stars in M3, M5, M13 and M92

Jun 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992a%26a...259..510l&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 259, no. 2, p. 510-521.

Astronomy and Astrophysics

Astronomy

18

Globular Clusters, Horizontal Branch Stars, Imaging Techniques, Ultraviolet Photometry, Ultraviolet Telescopes, Balloon-Borne Instruments, Radial Distribution, Ultraviolet Astronomy

Scientific paper

The metal-poor globular clusters M3, M5, M13 and M92 have been imaged at 2000 A with a balloon-borne, 40-cm aperture, f/3.8 telescope. All clusters show a rather narrow range of ultraviolet magnitudes, well above the limiting magnitude, essentially identified as the blue horizontal branch. A general trend is that the radial distributions of the blue horizontal-branch stars, observed over a decade of radius, are best represented by a law slightly steeper than the King's law representative of a cluster profile in the visible. The lack of measured counts in the central parts, in conjunction with small number statistics, prevent however, to draw a conclusion on the dynamics. Some observational aspects and possible contributors are reviewed. The model distributions are found consistent with aperture photometry of OAO-2 and ANS. The total number of blue horizontal-branch stars belonging to each cluster is calculated. For M92 and M13 which do not have a red horizontal branch, the calculated total numbers are found in very good agreement with the numbers inferred from an estimate of the death rate of a coeval stellar population of 15 Gyrs, assuming a typical HB lifetime of 10 exp 8 years; the predicted numbers seem also roughly consistent for M3, and rather high for M5. The reliable observed numbers of horizontal-branch stars in conjunction with counts of red giants done in the visible are used to provide an estimate of the helium content of M13 and M92.

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