scholarly journals Extreme Horizontal Branch Stars in Passively Evolving Early Type Galaxies

2012 ◽  
Vol 10 (H16) ◽  
pp. 120-120
Author(s):  
Fabiola Hernández-Pérez ◽  
Gustavo Bruzual
Keyword(s):  
Binary Star ◽  
Horizontal Branch ◽  
Population Synthesis ◽  
Early Type ◽  
Star Evolution ◽  
Horizontal Branch Stars ◽  
Uv Excess ◽  
Ehb Stars

AbstractWe study the effects of including binary star evolution in population synthesis models. We use the Hurley et al. (2002) code to compute binary star evolutionary tracks, and follow the procedure by Han et al. (2002), in particular, the two 2HeWD merger channel, to form EHB stars from a binary pair. We apply the resulting models to study UV excess ETGs.

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 “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.

scholarly journals Early-Type Stars

1995 ◽  
Vol 164 ◽  
pp. 119-127
Author(s):  
K.C. Freeman
Keyword(s):  
Main Sequence ◽  
Velocity Dispersion ◽  
Galactic Rotation ◽  
Horizontal Branch ◽  
Early Type ◽  
Horizontal Branch Stars ◽  
The Galaxy ◽  
Stellar Halo ◽  
Early Type Stars ◽  
Insight Into

Away from the young disk, several classes of early type stars are found. They include (i) the old, metal-poor blue horizontal branch stars of the halo and the metal-poor tail of the thick disk; (ii) metal-rich young A stars in a rapidly rotating subsystem but with a much higher velocity dispersion than the A stars of the young disk, and (iii) a newly discovered class of metal-poor young main sequence A stars in a subsystem of intermediate galactic rotation (Vrot ≈ 120 km s−1). The existence and kinematics of these various classes of early type stars provide insight into the formation of the metal-poor stellar halo of the Galaxy and into the continuing accretion events suffered by our Galaxy.

scholarly journals Rapid binary star evolution for N-body simulations and population synthesis

1997 ◽  
Vol 291 (4) ◽  
pp. 732-748 ◽  
Author(s):  
C. A. Tout ◽  
S. J. Aarseth ◽  
O. R. Pols ◽  
P. P. Eggleton
Keyword(s):  
Binary Star ◽  
Population Synthesis ◽  
Star Evolution

scholarly journals UV color-color relation of early-type galaxies

2012 ◽  
Vol 10 (H16) ◽  
pp. 130-130
Author(s):  
Chang H. Ree ◽  
Hyunjin Jeong ◽  
Kyuseok Oh ◽  
Chul Chung ◽  
Joon Hyeop Lee ◽  
...  
Keyword(s):  
Spectral Shape ◽  
Minority Population ◽  
Horizontal Branch ◽  
Early Type ◽  
Spectral Slope ◽  
Canonical Models ◽  
Data Support ◽  
Uv Excess ◽  
Uv Upturn ◽  
Red Sequence

AbstractThe ultraviolet (UV) color-color relation of early-type galaxies (ETGs) in the nearby universe (0.05 < z < 0.12) is re-examined with the latest GALEX GR6 and SDSS DR7 data. By drawing the FUV – NUV (as a measure of UV temperature) versus FUV – r (as a measure of UV amplitude) color-color diagram for the morphologically-cleaned, spectroscopically-cleaned sample of ~3700 quiescent ETGs, we find that the “old and dead“ ETGs consist of a well-defined sequence in UV colors, the “UV red sequence”, so that the stronger UV excess galaxies should have a harder UV spectral shape systematically. However, the observed UV spectral slope is too steep to be reproduced by the canonical models in which the UV flux is mainly controlled by age or metallicity parameters. The observed data support the helium enhancement scenario in which the UV spectral shape of UV upturn (FUV – NUV < 0.9; FUV – r ~ 6) galaxies may be governed by the minority population of helium-enhanced horizontal-branch (HB) stars.

scholarly journals Star Formation in Nuclei of Early-Type Galaxies

1987 ◽  
Vol 115 ◽  
pp. 642-642
Author(s):  
B. Rocca-Volmerange ◽  
B. Guiderdoni
Keyword(s):  
Star Formation ◽  
Wavelength Range ◽  
Massive Star ◽  
Massive Stars ◽  
Spectral Features ◽  
Population Synthesis ◽  
Early Type ◽  
Long Wavelength ◽  
Evolved Stars ◽  
Uv Excess

A far-UV population synthesis is performed with the IUE Spectral Atlas on absolute spectrophotometry for nuclei of early-type galaxies. Two types of stellar populations may be distinguished in gas-poor galaxies: a) the bulk of evolved stars mostly contributing in the 2000 A – 3000 A wavelength range as well as in the visible. The Long Wavelength Range IUE spectra appear to be excellent indicators of the turn off age in galaxy nuclei; b) a possible young massive star population which could be the origin of the far UV excess in most cases and which is essentially contributing in the 1200 A – 2000 A wavelength range. Spectral features of massive stars may be identified.

Horizontal-Branch Stars as Sources of the UV Upturn in Early-Type Galaxies

2004 ◽  
Vol 291 (3) ◽  
pp. 223-230 ◽  
Author(s):  
Suk-Jin Yoon ◽  
Young-Wook Lee ◽  
Soo-Chang Rey ◽  
Chang H. Ree ◽  
Sukyoung Ken Yi
Keyword(s):  
Horizontal Branch ◽  
Early Type ◽  
Horizontal Branch Stars ◽  
Uv Upturn

scholarly journals Non-variable horizontal-branch stars

1997 ◽  
Vol 189 ◽  
pp. 363-368
Author(s):  
Robert T. Rood
Keyword(s):  
Mass Loss ◽  
Physical Basis ◽  
Horizontal Branch ◽  
Loss Process ◽  
Horizontal Branch Stars ◽  
Theoretical Results ◽  
Branch Morphology

For 25 years our ignorance of the physical basis of this mass loss process has been the barrier to progress in understanding horizontal branch morphology. I review some recent observational and theoretical results which may be giving us clues about the nature of the mass loss process.

The IMF-SFH connection in massive early-type galaxies

2015 ◽  
Vol 11 (S315) ◽  
Author(s):  
I. Ferreras ◽  
C. Weidner ◽  
A. Vazdekis ◽  
F. La Barbera
Keyword(s):  
Age Distribution ◽  
Stellar Populations ◽  
Mass Function ◽  
Initial Mass Function ◽  
Population Synthesis ◽  
Early Type ◽  
Massive Galaxies ◽  
Stellar Initial Mass Function ◽  
The Many ◽  
The Imf

The stellar initial mass function (IMF) is one of the fundamental pillars in studies of stellar populations. It is the mass distribution of stars at birth, and it is traditionally assumed to be universal, adopting generic functions constrained by resolved (i.e. nearby) stellar populations (e.g., Salpeter 1955; Kroupa 2001; Chabrier 2003). However, for the vast majority of cases, stars are not resolved in galaxies. Therefore, the interpretation of the photo-spectroscopic observables is complicated by the many degeneracies present between the properties of the unresolved stellar populations, including IMF, age distribution, and chemical composition. The overall good match of the photometric and spectroscopic observations of galaxies with population synthesis models, adopting standard IMF choices, made this issue a relatively unimportant one for a number of years. However, improved models and observations have opened the door to constraints on the IMF in unresolved stellar populations via gravity-sensitive spectral features. At present, there is significant evidence of a non-universal IMF in early-type galaxies (ETGs), with a trend towards a dwarf-enriched distribution in the most massive systems (see, e.g., van Dokkum & Conroy 2010; Ferreras et al. 2013; La Barbera et al. 2013). Dynamical and strong-lensing constraints of the stellar M/L in similar systems give similar results, with heavier M/L in the most massive ETGs (see, e.g., Cappellari et al. 2012; Posacki et al. 2015). Although the interpretation of the results is still open to discussion (e.g., Smith 2014; La Barbera 2015), one should consider the consequences of such a bottom-heavy IMF in massive galaxies.

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