292 research outputs found
Stellar Photometry of the Globular Cluster NGC 6229. I. Data Reduction and Morphology of the Brighter Part of the CMD
BV CCD photometry of the central (1.5 arcmin x 2.0 arcmin) part of the mildly
concentrated outer-halo globular cluster NGC 6229 is presented. The data
reduction in such a crowded field was based on a wavelet transform analysis.
Our larger dataset extends the previous results by Carney et al. (1991, AJ,
101, 1699) for the outer and less crowded fields of the cluster, and confirms
that NGC 6229 has a peculiar color-magnitude diagram for its position in the
Galaxy. In particular, NGC 6229's horizontal branch (HB) presents several
interesting features, among which stand out: a well populated and very extended
blue tail; a rather blue overall morphology, with (B-R)/(B+V+R) = 0.24+/-0.02;
a bimodal color distribution, resembling those found for NGC 1851 and NGC 2808;
and gaps on the blue HB. NGC 6229 is the first bimodal-HB cluster to be
identified in the Galactic outer halo. A low value of the R parameter is
confirmed, suggestive of a low helium abundance or of the presence of a quite
substantial population of extreme HB stars fainter than our photometric limit
(~ 2.5 mag below the RR Lyrae level in V). Twelve new possible variable stars
were found in the central part of the cluster. The morphology of the red giant
branch (RGB) also seems to be peculiar. In particular, the RGB luminosity
function ``bump'' is not a prominent feature and has only been tentatively
identified, on the basis of a comparison with a previously reported detection
for M3 (NGC 5272). Finally, we compare the properties of NGC 6229 with those
for other outer-halo globular clusters, and call attention to what appears to
be a bimodal HB distribution for the outer-halo cluster population, where
objects with very red or very blue HB types are much more frequently found than
clusters with intermediate HB types.Comment: 31 pages, LaTeX, uses AASTeX v4.0, 11 postscript figures and 7
postscript tables pasted into text. To appear in The Astronomical Journal
(Feb. 1997 issue
The Hottest Horizontal-Branch Stars in omega Centauri - Late Hot Flasher vs. Helium Enrichment
UV observations of some massive globular clusters uncovered a significant
population of very hot stars below the hot end of the horizontal branch (HB),
the so-called blue hook stars. This feature might be explained either as
results of the late hot flasher scenario where stars experience the helium
flash while on the white dwarf cooling curve or by the progeny of the
helium-enriched sub-population recently postulated to exist in some clusters.
Moderately high resolution spectra of stars at the hot end of the blue HB in
omega Cen were analysed for atmospheric parameters and abundances using LTE and
Non-LTE model atmospheres. In the temperature range 30,000K to 50,000K we find
that 35% of our stars are helium-poor (log(n_He/n_H) < -2), 51% have solar
helium abundance within a factor of 3 (-1.5 <= log(n_He/n_H) <= -0.5) and 14%
are helium-rich (log(n_He/n_H)> -0.4). We also find carbon enrichment in step
with helium enrichment, with a maximum carbon enrichment of 3% by mass. At
least 14% of the hottest HB stars in omega Cen show helium abundances well
above the highest predictions from the helium enrichment scenario (Y = 0.42
corresponding to log(n_He/n_H) ~ -0.74). In addition, the most helium-rich
stars show strong carbon enrichment as predicted by the late hot flasher
scenario. We conclude that the helium-rich HB stars in omega Cen cannot be
explained solely by the helium-enrichment scenario invoked to explain the blue
main sequence. (Abridged)Comment: 4 pages, 3 figures, uses aa.cls (enclosed), accepted as A&A Lette
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