62 research outputs found
The Origin of Hot Subluminous Horizontal-Branch Stars in Omega Centauri and NGC 2808
Hot subluminous stars lying up to 0.7 mag below the extreme horizontal branch
(EHB) are found in the UV color-magnitude diagrams of omega Cen and NGC 2808.
Such stars are unexplained by canonical HB theory. In order to explore the
origin of these subluminous stars, we evolved a set of low-mass stars from the
main sequence through the helium-core flash to the HB for a wide range in the
mass loss along the red-giant branch (RGB). Stars with the largest mass loss
evolve off the RGB to high effective temperatures before igniting helium in
their cores. Our results indicate that the subluminous EHB stars, as well as
the gap within the EHB of NGC 2808, can be explained if these stars undergo a
late helium-core flash on the white-dwarf cooling curve. Under these conditions
the flash convection will penetrate into the stellar envelope, thereby mixing
most, if not all, of the envelope hydrogen into the hot helium- burning
interior. This phenomenon is analogous to the "born-again" scenario for
producing hydrogen-deficient stars during a very late helium-shell flash.
"Flash mixing" greatly enhances the envelope helium and carbon abundances and,
as a result, leads to an abrupt increase in the HB effective temperature. We
argue that the EHB gap in NGC 2808 is caused by this theoretically predicted
dichotomy in the HB morphology. Using new helium- and carbon-rich stellar
atmospheres, we show that the flash-mixed stars have the same reduced UV flux
as the subluminous EHB stars. Moreover, we demonstrate that models without
flash mixing lie, at most, ~0.1 mag below the EHB and hence fail to explain the
observations. Flash mixing may also provide a new evolutionary channel for
producing the high gravity, He-rich sdO and sdB stars.Comment: 8 pages, 5 figures, to appear in "Omega Centauri: a Unique Window
into Astrophysics" (Cambridge, August, 2001), ASP Conf. Ser., edited by F.
van Leeuwen, G. Piotto, and J. Hughe
The Discovery of Pulsating Hot Subdwarfs in NGC 2808
We present the results of a Hubble Space Telescope program to search for
pulsating hot subdwarfs in the core of NGC 2808. These observations were
motivated by the recent discovery of such stars in the outskirts of omega Cen.
Both NGC 2808 and omega Cen are massive globular clusters exhibiting complex
stellar populations and large numbers of extreme horizontal branch stars. Our
far-UV photometric monitoring of over 100 hot evolved stars has revealed six
pulsating subdwarfs with periods ranging from 85 to 149 s and UV amplitudes of
2.0 to 6.8%. In the UV color-magnitude diagram of NGC 2808, all six of these
stars lie immediately below the canonical horizontal branch, a region populated
by the subluminous "blue-hook" stars. For three of these six pulsators, we also
have low-resolution far-UV spectroscopy that is sufficient to broadly constrain
their atmospheric abundances and effective temperatures. Curiously, and in
contrast to the omega Cen pulsators, the NGC 2808 pulsators do not exhibit the
spectroscopic or photometric uniformity one might expect from a well-defined
instability strip, although they all fall within a narrow band (0.2 mag) of
far-UV luminosity.Comment: 5 pages, 1 table, 2 color and 2 grayscale figures. Accepted for
publication in The Astrophysical Journal Letter
The Dearth of UV-bright Stars in M32: Implications for Stellar Evolution Theory
Using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, we have obtained deep far ultraviolet images of the compact elliptical galaxy M32. When combined with earlier near-ultraviolet images of the same field, these data enable the construction of an ultraviolet color-magnitude diagram of the hot horizontal branch (HB) population and other hot stars in late phases of stellar evolution. We find few post-asymptotic giant branch (PAGB) stars in the galaxy, implying that these stars either cross the HR diagram more rapidly than expected, and/or that they spend a significant fraction of their time enshrouded in circumstellar material. The predicted luminosity gap between the hot HB and its AGB-Manque (AGBM) progeny is less pronounced than expected, especially when compared to evolutionary tracks with enhanced helium abundances, implying that the presence of hot HB stars in this metal-rich population is not due to (Delta)Y/(Delta)Z greater than or approx. 4. Only a small fraction (approx. 2%) of the HB population is hot enough to produce significant UV emission, yet most of the W emission in this galaxy comes from the hot HB and AGBM stars, implying that PAGB stars are not a significant source of W emission even in those elliptical galaxies with a weak W excess. Subject headings: galaxies: evolution - galaxies: stellar content - galaxies: individual (M32) - stars: evolution - stars: horizontal branc
RR Lyrae Stars in NGC 6388 and NGC 6441: A New Oosterhoff Group?
NGC 6388 and NGC 6441 are anomalies among Galactic globular clusters in that
they cannot be readily place into either Oosterhoff group I or Oosterhoff group
II despite their significant numbers of RR Lyrae variables. The mean pulsation
periods, , of their RRab variables, at 0.71 d and 0.76 d, respectively,
are even larger than for Oosterhoff II clusters. Moreover, Oosterhoff II
clusters are very metal-poor, whereas NGC 6388 and NGC 6441 are the most
metal-rich globular clusters known to contain RR Lyrae stars. The location of
the NGC 6388 and NGC 6441 RRab variables in the period-amplitude diagram
implies that the RR Lyrae stars in those two clusters are brighter than
expected for their metallicities. Our results therefore indicate that a
universal relationship may not exist between the luminosity and the metallicity
of RR Lyrae variables.Comment: Four pages, three figures. ApJ (Letters), in pres
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