1,626 research outputs found
Helium-rich EHB Stars in Globular Clusters
Recent UV observations of the most massive Galactic globular clusters show a
significant population of hot stars below the zero-age HB (``blue hook''
stars), which cannot be explained by canonical stellar evolution. Stars which
suffer unusually large mass loss on the red giant branch and thus experience
the helium-core flash while descending the white dwarf cooling curve could
populate this region. They should show higher temperatures than the hottest
canonical HB stars and their atmospheres should be helium-rich and probably
C/N-rich. We have obtained spectra of blue hook stars in omega Cen and NGC 2808
to test this possibility. Our analysis shows that the blue hook stars in these
clusters reach effective temperatures well beyond the hot end of the canonical
EHB and have higher helium abundances than canonical EHB stars. These results
support the hypothesis that the blue hook stars arise from stars which ignite
helium on the white dwarf cooling curve.Comment: LaTeX, 8 pages, 3 figures, uses Kluwer style files (included), to
appear in "Extreme Horizontal Branch Stars and Related Objects", Astrophysics
and Space Science, Kluwer Academic Publishers, proceedings of the meeting
held in Keele, UK, June 16-20, 200
The IDL astronomy user's library
IDL (Interactive Data Language) is a commercial programming, plotting, and image display language, which is widely used in astronomy. The IDL Astronomy User's Library is a central repository of over 400 astronomy-related IDL procedures accessible via anonymous FTP. The author will overview the use of IDL within the astronomical community and discuss recent enhancements at the IDL astronomy library. These enhancements include a fairly complete I/O package for FITS images and tables, an image deconvolution package and an image mosaic package, and access to IDL Open Windows/Motif widgets interface. The IDL Astronomy Library is funded by NASA through the Astrophysics Software and Research Aids Program
DECONV-TOOL: An IDL based deconvolution software package
There are a variety of algorithms for deconvolution of blurred images, each having its own criteria or statistic to be optimized in order to estimate the original image data. Using the Interactive Data Language (IDL), we have implemented the Maximum Likelihood, Maximum Entropy, Maximum Residual Likelihood, and sigma-CLEAN algorithms in a unified environment called DeConv_Tool. Most of the algorithms have as their goal the optimization of statistics such as standard deviation and mean of residuals. Shannon entropy, log-likelihood, and chi-square of the residual auto-correlation are computed by DeConv_Tool for the purpose of determining the performance and convergence of any particular method and comparisons between methods. DeConv_Tool allows interactive monitoring of the statistics and the deconvolved image during computation. The final results, and optionally, the intermediate results, are stored in a structure convenient for comparison between methods and review of the deconvolution computation. The routines comprising DeConv_Tool are available via anonymous FTP through the IDL Astronomy User's Library
Hot Populations in M87 Globular Clusters
We have obtained HST/STIS far- and near-UV photometry of globular clusters in
four fields in the gE galaxy M87. To a limit of m(FUV) = 25 we detect a total
of 66 globular clusters (GCs) in common with the deep HST optical-band study of
Kundu et al. (1999). Despite strong overlap in V- and I-band properties, the
M87 GCs have UV/optical properties that are distinct from clusters in the Milky
Way and in M31. M87 clusters, especially metal-poor ones, produce larger hot HB
populations than do Milky Way analogues. Cluster mass is probably not a factor
in these distinctions. The most metal-rich M87 GCs in our sample are near Z_sun
and overlap the local E galaxy sample in estimated Mg_2 line indices.
Nonetheless, the clusters produce much more UV light at a given Mg_2, being up
to 1 mag bluer than any gE galaxy in (FUV-V) color. The M87 GCs do not appear
to represent a transition between Milky Way-type clusters and E galaxies. The
differences are in the correct sense if the clusters are significantly older
than the E galaxies. Comparisons with Galactic open clusters indicate that the
hot stars lie on the extreme horizontal branch, rather than being blue
stragglers, and that the EHB becomes well populated for ages > 5 Gyr. We find
that 43 of our UV detections have no optical-band counterparts. Most appear to
be UV-bright background galaxies, seen through M87. Eleven NUV variable sources
detected at only one epoch in the central field are probably classical novae.
[Abridged]Comment: 70 pages, 25 figures (including 4 jpgs), 7 tables. To appear in AJ.
Full resolution version available at
http://www.astro.virginia.edu/~rwo/m87/m87-hotpops.pd
Hot HB stars in globular clusters - Physical parameters and consequences for theory. VI. The second parameter pair M 3 and M 13
We present the results of spectroscopic analyses of hot horizontal branch
(HB) stars in M 13 and M 3, which form a famous ``second parameter'' pair. From
the spectra and Stroemgren photometry we derived -- for the first time in M 13
-- atmospheric parameters (effective temperature and surface gravity). For
stars with Stroemgren temperatures between 10,000 and 12,000 K we found
excellent agreement between the atmospheric parameters derived from Stroemgren
photometry and those derived from Balmer line profile fits. However, for cooler
stars there is a disagreement in the parameters derived by the two methods, for
which we have no satisfactory explanation. Stars hotter than 12,000 K show
evidence for helium depletion and iron enrichment, both in M 3 and M 13.
Accounting for the iron enrichment substantially improves the agreement with
canonical evolutionary models, although the derived gravities and masses are
still somewhat too low. This remaining discrepancy may be an indication that
scaled-solar metal-rich model atmospheres do not adequately represent the
highly non-solar abundance ratios found in blue HB stars affected by diffusion.
We discuss the effects of an enhancement in the envelope helium abundance on
the atmospheric parameters of the blue HB stars, as might be caused by deep
mixing on the red giant branch or primordial pollution from an earlier
generation of intermediate mass asymptotic giant branch stars.Comment: 15 pages, 9 figures, LaTex, uses aa.cls (included), accepted for
publication in A&
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