scholarly journals The Effects of α-Enhancement on the Evolution of EHB Stars

2016 ◽  
Vol 04 (01) ◽  
pp. 20-32
Author(s):  
建坡 郭
Keyword(s):  
Ehb Stars

scholarly journals Rotational broadening and conservation of angular momentum in post-extreme horizontal branch stars

2018 ◽  
Vol 613 ◽  
pp. A66
Author(s):  
G. Fontaine ◽  
M. Latour
Keyword(s):  
Angular Momentum ◽  
Rotation Speed ◽  
Momentum Conservation ◽  
Evolutionary Models ◽  
Horizontal Branch ◽  
Natural Consequence ◽  
Average Value ◽  
Horizontal Branch Stars ◽  
The Moment ◽  
Ehb Stars

We show that the recent realization that isolated post-extreme horizontal branch (post-EHB) stars are generally characterized by rotational broadening with values of V rot sini between 25 and 30 km s−1 can be explained as a natural consequence of the conservation of angular momentum from the previous He-core burning phase on the EHB. The progenitors of these evolved objects, the EHB stars, are known to be slow rotators with an average value of V rot sini of ~7.7 km s−1. This implies significant spin-up between the EHB and post-EHB phases. Using representative evolutionary models of hot subdwarf stars, we demonstrate that angular momentum conservation in uniformly rotating structures (rigid-body rotation) boosts that value of the projected equatorial rotation speed by a factor ~3.6 by the time the model has reached the region of the surface gravity-effective temperature plane where the newly-studied post-EHB objects are found. This is exactly what is needed to account for their observed atmospheric broadening. We note that the decrease of the moment of inertia causing the spin-up is mostly due to the redistribution of matter that produces more centrally-condensed structures in the post-EHB phase of evolution, not to the decrease of the radius per se.

scholarly journals Close binary EHB stars from SPY

2004 ◽  
Vol 291 (3) ◽  
pp. 321-328 ◽  
Author(s):  
R. Napiwotzki ◽  
C.A. Karl ◽  
T. Lisker ◽  
U. Heber ◽  
N. Christlieb ◽  
...  
Keyword(s):  
Close Binary ◽  
Ehb Stars

scholarly journals Binary Evolutionary Models

2008 ◽  
Vol 4 (S252) ◽  
pp. 349-357
Author(s):  
Z. Han ◽  
Ph. Podsiadlowski
Keyword(s):  
Globular Clusters ◽  
A Priori ◽  
Elliptical Galaxies ◽  
Ultraviolet Excess ◽  
Sdb Stars ◽  
Binary Model ◽  
Type Ia ◽  
Binary Evolution ◽  
Ehb Stars ◽  
Uv Upturn

AbstractIn this talk, we present the general principles of binary evolution and give two examples. The first example is the formation of subdwarf B stars (sdBs) and their application to the long-standing problem of ultraviolet excess (also known as UV-upturn) in elliptical galaxies. The second is for the progenitors of type Ia supernovae (SNe Ia). We discuss the main binary interactions, i.e., stable Roche lobe overflow (RLOF) and common envelope (CE) evolution, and show evolutionary channels leading to the formation of various binary-related objects. In the first example, we show that the binary model of sdB stars of Han et al. (2002, 2003) can reproduce field sdB stars and their counterparts, extreme horizontal branch (EHB) stars, in globular clusters. By applying the binary model to the study of evolutionary population synthesis, we have obtained an “a priori” model for the UV-upturn of elliptical galaxies and showed that the UV-upturn is most likely resulted from binary interactions. This has major implications for understanding the evolution of the UV excess and elliptical galaxies in general. In the second example, we introduce the single degenerate channel and the double degenerate channel for the progenitors of SNe Ia. We give the birth rates and delay time distributions for each channel and the distributions of companion stars at the moment of SN explosion for the single degenerate channel, which would help to search for the remnant companion stars observationally.

scholarly journals “Hot Helium Flashers” – The Road to Extreme Horizontal Branch Stars

2008 ◽  
Vol 4 (S252) ◽  
pp. 261-262
Author(s):  
O. Yaron ◽  
A. Kovetz ◽  
D. Prialnik
Keyword(s):  
Mass Loss ◽  
Mass Loss Rate ◽  
Cooling Curve ◽  
Horizontal Branch ◽  
The Road ◽  
Horizontal Branch Stars ◽  
Low Mass ◽  
Loss Efficiency ◽  
Ehb Stars ◽  
The Masses

AbstractObservational and theoretical investigations, performed especially over the last two decades, have strongly attributed the far-UV upturn phenomenon to low-mass, small-envelope, He-burning stars in Extreme Horizontal Branch (EHB) and subsequent evolutionary phases.Using our new stellar evolution code – a code that follows through complete evolutionary tracks, Pre-MS to cooling WD – without any interruption or intervention, we are able to produce a wide array of EHB stars, lying at bluer (Teff ≥ 20,000 K) and less luminous positions on HRD, and also closely examine their post-HB evolution until the final cooling as White Dwarfs.HB morphology is a complex multiple parameter problem. Two leading players, which seem to possess the ability to affect considerably positions of HB, are those of: 1.Helium abundance, and 2.mass-loss efficiency on the first giant branch. We focus here on the latter; thus, EHB stars are produced in our calculations by increasing the mass-loss rate on the RGB, to a state where prior to reaching core He flash conditions, only a very small H-rich envelope remains. The core flash takes place at hotter positions on the HRD, sometimes while already descending on the WD cooling curve. We show preliminary results for a range of initial masses (MZAMS = 0.8 − 1.1 M⊙) and for metallicities covering both populations I and II (Z = 0.01 − 0.001). The [M,Z] combinations have been chosen such that the masses would be above and close to typical MS turnoff masses (e.g. the estimation of MTO ≃ 0.85 for NGC 2808), and also so that the ages at HB are of order of 10 ± 5 Gyr.

scholarly journals AstroSat Study of the Globular Cluster NGC 2298: Probable Evolutionary Scenarios of Hot Horizontal Branch Stars

2021 ◽  
Vol 923 (2) ◽  
pp. 162
Author(s):  
Sharmila Rani ◽  
Gajendra Pandey ◽  
Annapurni Subramaniam ◽  
Chul Chung ◽  
Snehalata Sahu ◽  
...  
Keyword(s):  
Globular Cluster ◽  
Hubble Space Telescope ◽  
Spectral Energy Distribution ◽  
Outer Region ◽  
Spectral Energy ◽  
Horizontal Branch ◽  
Horizontal Branch Stars ◽  
Cooling Phase ◽  
Ehb Stars ◽  
Optical Color

Abstract We present the far-UV (FUV) photometry of images acquired with UVIT on AstroSat to probe the horizontal branch (HB) population of the Galactic globular cluster NGC 2298. UV-optical color–magnitude diagrams (CMDs) are constructed for member stars in combination with Hubble Space Telescope UV Globular Cluster Survey data for the central region and Gaia and ground-based photometric data for the outer region. A blue HB (BHB) sequence with a spread and four hot HB stars are detected in all FUV-optical CMDs and are compared with theoretical updated BaSTI isochrones and synthetic HB models with a range in helium abundance, suggesting that the hot HB stars are helium enhanced when compared to the BHB. The estimated effective temperature, radius, and luminosity of HB stars, using the best spectral energy distribution fits, were compared with various HB models. BHB stars span a temperature range from 7500 to 12,250 K. Three hot HB stars have 35,000–40,000 K, whereas one star has around ∼100,000 K. We suggest the following evolutionary scenarios: two stars are likely to be the progeny of extreme HB (EHB) stars formed through an early hot-flasher scenario, one is likely to be an EHB star with probable helium enrichment, and the hottest HB star, which is about to enter the white dwarf cooling phase, could have evolved from the BHB phase. Nevertheless, these are interesting spectroscopic targets to understand the late stages of evolution.

To Be or Not to Be: EHB Stars and AGB Stars

2018 ◽  
Vol 14 (S343) ◽  
pp. 357-359
Author(s):  
David A. Brown
Keyword(s):  
Globular Clusters ◽  
Agb Stars ◽  
Multiple Populations ◽  
Galactic Field ◽  
Sdb Stars ◽  
Strong Case ◽  
Binary Formation ◽  
Ehb Stars

AbstractThe formation of EHB stars is linked to the lives of AGB stars by indications that such EHB/sdB stars might form in globular clusters with multiple populations linked to AGB evolution. Observations of massive globular clusters, such as ω-Centauri (Bedin et al.2004, Piotto et al.2005) suggest that single EHB stars might form from He-enhanced progenitors (D’Antona et al.2005, D’Antona & Caloi 2008, Lee et al.2005) in environments enriched by AGB ejecta. The studies conducted by Han et al. (2002), Han et al. (2003), and Han et al. (2007) have been able to provide a strong case for the binary formation of EHB/sdB stars in the Galactic field, though binary formation channels in globular clusters is uncertain. Simulations presented here are an extension of the simulations of Han et al. (2002) and Han et al. (2003), for low metallicities to examine the binary EHB population in globular clusters.

scholarly journals A Survey for EHB Stars in the Galactic Bulge

2004 ◽  
Vol 291 (3) ◽  
pp. 247-252 ◽  
Author(s):  
Donald M. Terndrup ◽  
Deokkeun An ◽  
Angela Hansen ◽  
Ruth C. Peterson ◽  
Alistair R. Walker ◽  
...  
Keyword(s):  
Galactic Bulge ◽  
Ehb Stars

Discovery of high-velocity EHB stars in the globular clusterωCentauri (NGC 5139)

2015 ◽  
Vol 15 (10) ◽  
pp. 1639-1646
Author(s):  
Xin-Hua Gao ◽  
Shou-Kun Xu ◽  
Li Chen
Keyword(s):  
High Velocity ◽  
Ehb Stars

scholarly journals The UV Upturn in Elliptical Galaxies

1995 ◽  
Vol 164 ◽  
pp. 239-248
Author(s):  
H.C. Ferguson
Keyword(s):  
Galaxy Evolution ◽  
Open Cluster ◽  
Elliptical Galaxies ◽  
Asymptotic Giant Branch ◽  
Star Mass ◽  
Uv Emission ◽  
Stellar Component ◽  
Red Clump ◽  
Ehb Stars ◽  
Uv Upturn

The hot stellar component in elliptical galaxies offers clues to both stellar evolution and galaxy evolution. Current observations suggest that extreme horizontal branch (EHB) stars dominate the far-UV emission from galaxies with the strongest “UV upturns,” while post asymptotic giant branch (PAGB) stars are probably significant contributors for weaker galaxies. Spectra near the Lyman limit indicate that a rather narrow range of temperature (and hence EHB star mass) is required. However, other arguments suggest that most of the helium-burning stars in elliptical galaxies are in the red clump. The HB star mass distribution therefore appears to be strongly bimodal. Such bimodality is qualitatively reproduced by two radically different stellar population models, (those of Lee and Bressan et al.), both of which require that the galaxies be very old. However, the Galactic open cluster NGC 6791 also contains EHB stars and exhibits strong bimodality, indicating that old age may not necessarily be a requirement for the UV upturn phenomenon.

scholarly journals UV Bright Stars and Planetary Nebulae in Elliptical Galaxies

1995 ◽  
Vol 10 ◽  
pp. 493-497
Author(s):  
Henry C. Ferguson
Keyword(s):  
Uv Spectroscopy ◽  
Mass Function ◽  
Elliptical Galaxies ◽  
Asymptotic Giant Branch ◽  
Uv Emission ◽  
Luminosity Functions ◽  
The Galaxy ◽  
Low Mass ◽  
Red Giant ◽  
Ehb Stars

AbstractFar UV observations and optical studies of planetary nebula luminosity functions (PNLFs) offer complementary views of the late phases of stellar evolution in elliptical galaxies and spiral galaxy bulges. UV spectroscopy reveals that the hot stellar population is composite, with a mix of temperatures that varies from galaxy to galaxy. This changing mix is most likely due to changes in the relative numbers of stars that channel through the Post-Asymptotic Giant Branch (PAGB), Post-Early-AGB (PEAGB) and Extreme Horizontal Branch (EHB) phases of evolution. EHB stars appear to dominate the integrated λ < 2000 Å flux from galaxies with the strongest far-UV emission, but are too faint to resolve individually in even the nearest galaxies. Far UV images of M31 and M32 reveal a population of hot stars that are much brighter, but do not account for the majority of the far-UV flux. The sources detected are most likely low-mass PAGB stars (0.55 < M/M⊙ < 0.59). In contrast, the PNLF probes the PAGB star mass function at values greater than ∼ 0.6 M⊙. For a given galaxy the relative numbers of stars in these different branches of evolution are determined by the age and chemical evolution of the galaxy and by the physics of mass loss on the red giant branch. We review current constraints on the mass function of hot evolved stars in elliptical galaxies, highlight a few puzzles, and outline where future observations might contribute.

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