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 Horizontal-Branch Stars in the Log Teff, Log G Diagram

1978 ◽  
Vol 80 ◽  
pp. 209-220
Author(s):  
A. G. Davis Philip
Keyword(s):  
Observational Data ◽  
Globular Clusters ◽  
Computational Error ◽  
Evolutionary Models ◽  
Horizontal Branch ◽  
Atmospheric Parameters ◽  
Good Match ◽  
Horizontal Branch Stars ◽  
Color Data ◽  
Rms Error

A review of the available Strömgren four-color data concerning horizontal-branch (HB) stars is presented. Several observers have studied seven globular clusters (with [Fe/H] values from −2.2 to −1.3) and more than 100 HB stars in fields at high galactic latitudes over the last ten years at Kitt Peak, Cerro Tololo, Mt. Wilson and Steward Observatories. The predictions of model atmospheres (Kurucz, 1976) allow one to calculate atmospheric parameters such as θeff, log g, and MV(Philip, Miller and Relyea, 1976). These parameters then can be compared with the predictions of evolutionary models (Sweigart and Gross, 1976).A comparison of the observed data with evolutionary tracks indicates that within the rms error of observation and the computational error of the models the data and the tracks agree quite well. This matches the case for Population I stars, where a similar analysis shows a good match between observational data and evolutionary tracks (Philip,et al. 1977).

scholarly journals Pulsational instabilities driven by the ∈ mechanism in hot pre-horizontal branch stars

2018 ◽  
Vol 614 ◽  
pp. A136 ◽  
Author(s):  
Tiara Battich ◽  
Marcelo M. Miller Bertolami ◽  
Alejandro H. Córsico ◽  
Leandro G. Althaus
Keyword(s):  
Horizontal Branch ◽  
Instability Domain ◽  
Instability Strip ◽  
Long Period ◽  
Horizontal Branch Stars ◽  
Stellar Pulsations ◽  
Nuclear Burning ◽  
Stellar Models ◽  
The Moment ◽  
Radial Pulsations

Context. The ∈ mechanism is a self-excitation mechanism of stellar pulsations that acts in regions inside the star where nuclear burning takes place. It has been shown that the ∈ mechanism can excite pulsations in models of hot pre-horizontal branch stars before they settle into the stable helium core-burning phase. Moreover, it has been shown that this mechanism could explain the shortest periods of LS IV-14°116, a mild He-sdBV star. Aims. We aim to study the ∈ mechanism in stellar models appropriate for hot pre-horizontal branch stars to predict their pulsational properties and the instability domain in the log g − log Teff plane. Methods. We performed detailed computations of non-adiabatic non-radial pulsations on stellar models during the helium subflashes just before the helium-core burning phase. Computations were carried out for different values of initial helium composition, metallicity, and envelope mass at the moment of helium flash. Results. We find an instability domain of long-period gravity modes due to the ∈ mechanism in the log g − log Teff plane at roughly 22 000 K ≲ Teff ≲ 50 000 K and 4.67 ≲ log g ≲ 6.15. Consequently, we find instabilities due to the ∈ mechanism on pre-extreme horizontal branch stellar models (Teff ≳ 22 000 K), but not on pre-blue horizontal branch stellar models (Teff ≲ 21 000 K). The periods of excited modes range between ~200 and ~2000 s. Comparison with the three known pulsating He-rich subdwarfs shows that ∈ mechanism can excite gravity modes in stars with similar surface properties (He abundances, log g, log Teff), but in our models it is only able to excite modes in the range of the shortest observed periods. Conclusions. We predict a new instability strip for hot-subdwarf stars of which LS IV-14°116 could be the first inhabitant. Based on simple estimates we expect 1 to 10 stars in the current samples of hot-subdwarf stars to be pulsating by the ∈ mechanism. Our results could constitute a theoretical basis for future searches of pulsators in the Galactic field.

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.

Conservation laws

2018 ◽  
Author(s):  
Nathalie Deruelle ◽  
Jean-Philippe Uzan
Keyword(s):  
Dynamical System ◽  
Angular Momentum ◽  
Conservation Laws ◽  
Total Energy ◽  
Superposition Principle ◽  
Momentum Conservation ◽  
Conserved Quantities ◽  
Angular Momentum Conservation ◽  
Third Law ◽  
The Law

This chapter defines the conserved quantities associated with an isolated dynamical system, that is, the quantities which remain constant during the motion of the system. The law of momentum conservation follows directly from Newton’s third law. The superposition principle for forces allows Newton’s law of motion for a body Pa acted on by other bodies Pa′ in an inertial Cartesian frame S. The law of angular momentum conservation holds if the forces acting on the elements of the system depend only on the separation of the elements. Finally, the conservation of total energy requires in addition that the forces be derivable from a potential.

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.

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