On the Upper Mantle

Caloi, P.

doi:10.1016/s0065-2687(08)60375-1

Cited by 13 publications

(13 citation statements)

References 110 publications

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“…For our purposes here we note that neither kind of high temperature shell flash is, if they do occur in nature, is of sufficient duration to affect the UV output from a population. This is especially true for the hydrogen shell flash phenomena (also found by Caloi 1989;Castellani & Tornambè 1991, and HDP), which cause rapid brightening for short periods ( 1000 yr) at very high temperatures.…”

Section: Evolution Of Models With High Metallicitymentioning

confidence: 75%

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Ultraviolet Radiation from Evolved Stellar Populations. I. Models

Dorman¹,

Rood²,

O’Connell³

1993

ApJ

494

682

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This series of papers comprises a systematic exploration of the hypothesis that the far ultraviolet radiation from star clusters and elliptical galaxies originates from extremely hot horizontal-branch (HB) stars and their post-HB progeny. This first paper presents an extensive grid of calculations of stellar models from the Zero Age Horizontal Branch through to a point late in post-HB evolution or a point on the white dwarf cooling track. The grid will be used to produce synthesized UV fluxes for the interpretation of existing and future short wavelength (900-3000Å) observations. Our sequences have been computed for a range of masses which concentrates on models that begin their HB evolution very close to the hot end of the ZAHB. We have calculated tracks for three metal-poor compositions ([Fe=H] = 2:26; 1:48; 0:47 with [O=Fe] > 0), for use with globular cluster observations. We have also chosen three metal rich compositions (Z = 0:017 = Z , Z = 0:04; 0:06) for use in the study of elliptical galaxy populations. For each of the two supermetal-rich compositions, for which the helium abundance is unconstrained by observation, we have computed two sets of sequences: one assuming no additional helium, and a second with a large enhancement (Y HB = 0:29 and 0.36 for Z = 0:04), and (Y HB = 0:29 and 0.46 for Z = 0:06). For each set of sequences our lowest ZAHB envelope masses (M 0 env ) are in the range 0:002 < M 0 env < 0:006 M . We use the term 'Extreme Horizontal Branch' (EHB) to refer to HB sequences of constant mass that do not reach the thermally-pulsing stage on the AGB. These models evolve after core helium exhaustion into Post-Early Asymptotic Giant Branch (AGB) stars, which leave the AGB before thermal pulsing, and AGB-Manqué stars, which never reach the AGB. We describe various features of the evolution of post-HB stars, discussing the correspondence between slow phases of evolution at high temperature and the Early-AGB evolution. We note that the relationship between core mass and luminosity for stars on the upper AGB is not straightforward, because stars arrive on the ZAHB with a range of masses and subsequently burn different amounts of fuel. We determine from our models an upper bound to the masses of EHB stars, finding that it varies little for [Fe=H] < 0, but that it is sensitive to the helium abundance. We show that for each composition there is a range of M 0 env (at least a few hundredths M ) in which the models have a slow phase of evolution at high temperature. The duration of this phase is found to increase with the metallicity, but its luminosity is lower, so that total UV energy output is not significantly different from metal-poor sequences. The properties of very metal rich stars are, however, made uncertain by our lack of knowledge of the helium abundance for [Fe=H] > 0; the range of stellar masses in which high temperatures are attained for significant periods of time increases with Y . There is no intrinsic composition dependence of the peak UV output from evolved stars; the output from a ste...

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Section: Evolution Of Models With High Metallicitymentioning

confidence: 75%

“…As pointed out by Caloi (1989), for any given choice of metallicity and Y each class of sequences (AGB-Manqué, P-EAGB, etc) is present for some range of masses (cf. Ciardullo & Demarque 1978).…”

Section: Discussionmentioning

confidence: 99%

Ultraviolet Radiation from Evolved Stellar Populations. I. Models

Dorman¹,

Rood²,

O’Connell³

1993

ApJ

494

682

View full text Add to dashboard Cite

show abstract

“…It has sometimes been suggested that this lifetime ratio could be decreased by EHB models which include "breathing pulses" (e.g. Caloi 1989, Renzini & Fusi Pecci 1988, which are sudden increases in the size of the convective core near the end of the HB evolution. Whether or not breathing pulses actually occur, however, remains an open question (Dorman & Rood 1993).…”

Section: Discussionmentioning

confidence: 99%

“…The evolution of extreme horizontal branch (EHB) stars has attracted considerable interest in recent years due to their importance for understanding the hot stellar population in both metal-poor and metal-rich stellar systems (e.g. Caloi 1989, Castellani et al 1994). According to canonical theory, EHB stars retain only a thin (< 0.02 M ⊙ ) inert hydrogen envelope due to prior mass loss on the red giant branch (RGB), and spend their core-helium burning lifetime at high effective temperatures (20,000 K ≤ T eff ≤ 35,000 K).…”

Section: Introductionmentioning

confidence: 99%

Ultraviolet Imagery of NGC 6752: A Test of Extreme Horizontal Branch Models

Landsman

Sweigart

Bohlin

et al. 1996

The Astrophysical Journal

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We present a 1620Å image of the nearby globular cluster NGC 6752 obtained with the Ultraviolet Imaging Telescope (UIT) during the Astro-2 mission of the Space Shuttle Endeavour in 1995 March. An ultraviolet-visible color-magnitude diagram (CMD) is derived for 216 stars matched with the visible photometry of Buonanno et al. (1986). This CMD provides a nearly complete census of the hot horizontal branch (HB) population with good temperature and luminosity discrimination for comparison with theoretical tracks.The observed data show good agreement with the theoretical zero-age horizontal branch (ZAHB) of Sweigart (1996) for an assumed reddening of E(B-V) = 0.05 and a distance modulus of 13.05. The observed HB luminosity width is in excellent agreement with the theoretical models and supports the single star scenario for the origin of extreme horizontal branch (EHB) stars. However, only four stars can be identified as post-EHB stars, whereas almost three times this many are expected from the HB number counts. If this effect is not a statistical anomaly, then some non-canonical effect may be decreasing the post-EHB lifetime. The recent non-canonical models of Sweigart (1996), which have helium-enriched envelopes due to mixing along the red giant branch, cannot explain the deficit of post-EHB stars, but might be better able to explain their luminosity distribution.

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“…Also plotted are evolutionary tracks from Scho Ènberner (1993) for post-AGB stars (solid lines; core masses labelled in units of 1 M ( ), the zero-age EHB from Sweigart (1987, dashed line) and the zero-age main sequence (dot±dashed line). Loci showing how objects with a variety of masses evolve away from the EHB are also shown (Caloi 1989). Obviously, there are a number of plausible interpretations for the nature of this object.…”

Section: Discussionmentioning

confidence: 99%

Optical spectroscopy of the candidate luminous white dwarf in the young Large Magellanic Cloud cluster NGC 1818

Burleigh

Saffer

Gilmore

et al. 1999

Monthly Notices of the Royal Astronomical Society

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The definitive version is available at www.blackwell-synergy.com. Copyright Blackwell Publishing DOI : 10.1046/j.1365-8711.1999.03123.xAn optical spectrum of the Elson et al. (1998) candidate luminous white dwarf in the young LMC cluster NGC1818 shows conclusively that it is not a degenerate star. A model atmosphere fit gives Teff 31,500K and log g 4.4, typical of a garden-variety main sequence B star. However, if it is a true LMC member then the star is underluminous by almost three magnitudes. Its position in the cluster colour-magnitude diagram also rules out the possibility that this is an ordinary B star. The luminosity is, however, consistent with a ~0.5M??? post-AGB or post-EHB object, although if it has evolved via single star evolution from a high mass (7.6???9.0M???) progenitor then we might expect it to have a much higher mass, ~0.9M???. Alternatively, it has evolved in a close binary. In this case the object offers no implications for the maximum mass for white dwarf progenitors, or the initial-final mass relation. Finally, we suggest that it could in fact be an evolved member of the LMC disk, and merely projected by chance onto NGC1818. Spectroscopically, though, we cannot distinguish between these evolutionary states without higher resolution (echelle) data

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On the Upper Mantle

Cited by 13 publications

References 110 publications

Ultraviolet Radiation from Evolved Stellar Populations. I. Models

Ultraviolet Radiation from Evolved Stellar Populations. I. Models

Ultraviolet Imagery of NGC 6752: A Test of Extreme Horizontal Branch Models

Optical spectroscopy of the candidate luminous white dwarf in the young Large Magellanic Cloud cluster NGC 1818

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