Core Helium Ignition Research Articles

Stripped-envelope stars in different metallicity environments

Massive stars that become stripped of their hydrogen envelope through binary interaction or winds can be observed either as Wolf-Rayet stars, if they have optically thick winds, or as transparent-wind stripped-envelope stars. We approximate their evolution through evolutionary models of single helium stars, and compute detailed model grids in the initial mass range 1.5−70 M⊙ for metallicities between 0.01 and 0.04, from core helium ignition until core collapse. Throughout their lifetimes some stellar models expose the ash of helium burning. We propose that models that have nitrogen-rich envelopes are candidate WN stars, while models with a carbon-rich surface are candidate WC stars during core helium burning, and WO stars afterwards. We measure the metallicity dependence of the total lifetimes of our models and the duration of their evolutionary phases. We propose an analytic estimate of the wind’s optical depth to distinguish models of Wolf-Rayet stars from transparent-wind stripped-envelope stars, and find that the luminosity ranges at which WN-, WC-, and WO-type stars can exist is a strong function of metallicity. We find that all carbon-rich models produced in our grids have optically thick winds and match the luminosity distribution of observed populations. We construct population models and predict the numbers of transparent-wind stripped-envelope stars and Wolf-Rayet stars, and derive their number ratios at different metallicities. We find that as metallicity increases, the number of transparent-wind stripped-envelope stars decreases and the number of Wolf-Rayet stars increases. At high metallicities WC- and WO-type stars become more common. We apply our population models to nearby galaxies, and find that populations are more sensitive to the transition luminosity between Wolf-Rayet stars and transparent-wind helium stars than to the metallicity-dependent mass loss rates.

The effects of diffusion in hot subdwarf progenitors from the common envelope channel

Diffusion of elements in the atmosphere and envelope of a star can drastically alter its surface composition, leading to extreme chemical peculiarities. We consider the case of hot subdwarfs, where surface helium abundances range from practically zero to almost 100 percent. Since hot subdwarfs can form via a number of different evolution channels, a key question concerns how the formation mechanism is connected to the present surface chemistry. A sequence of extreme horizontal branch star models was generated by producing post-common envelope stars from red giants. Evolution was computed with MESA from envelope ejection up to core-helium ignition. Surface abundances were calculated at the zero-age horizontal branch for models with and without diffusion. A number of simulations also included radiative levitation. The goal was to study surface chemistry during evolution from cool giant to hot subdwarf and determine when the characteristic subdwarf surface is established. Only stars leaving the giant branch close to core-helium ignition become hydrogen-rich subdwarfs at the zero-age horizontal branch. Diffusion, including radiative levitation, depletes the initial surface helium in all cases. All subdwarf models rapidly become more depleted than observations allow. Surface abundances of other elements follow observed trends in general, but not in detail. Additional physics is required.

The implications of a companion enhanced wind on millisecond pulsar production

The most frequently seen binary companions to millisecond pulsars (MSPs) are helium white dwarfs (He WDs). The standard rejuvenation mechanism, in which a low- to intermediate-mass companion to a neutron star fills its Roche lobe between central hydrogen exhaustion and core helium ignition, is the most plausible formation mechanism. We have investigated whether the observed population can realistically be formed via this mechanism. We used the Cambridge STARS code to make models of Case B RLOF with Reimers' mass loss from the donor. We find that the range of initial orbital periods required to produce the currently observed range of orbital periods of MSPs is extremely narrow. To reduce this fine tuning, we introduce a companion enhanced wind (CEW) that strips the donor of its envelope more quickly so that systems can detach at shorter periods. Our models indicate that the fine tuning can be significantly reduced if a CEW is active. Because significant mass is lost owing to a CEW we expect some binary pulsars to accrete less than the 0.1Msun needed to spin them up to millisecond periods. This can account for mildly recycled pulsars present along the entire Mc-Porb relation. Systems with Pspin > 30ms are consistent with this but too few of these mildly recycled pulsars have yet been observed to make a significant comparison.

Pulsations in B-type supergiants with masses $\boldsymbol {M<20}\,\mathbf {M}_{\odot }$ before and after core helium ignition

Evolutionary tracks and pulsational analysis of models with masses of 13-18 $M_\odot$ are presented. We address two important questions. The first one deals with one of the most unresolved problems in astrophysics, i.e., the existence of a blue loop after core helium ignition; the so called "to loop or not to loop" problem. We show that inward overshooting from the outer convective zone in the red giant phase is prerequisite for the development of the blue loop. Our second question concerns pulsational instability of models in the core helium burning phase. We present for the first time that models on the blue loop can have unstable modes driven by the $\kappa$ mechanism operating in the $Z-$bump. Contrary to post-main sequence models in the shell hydrogen burning phases, pulsational instability of the blue loop models depends mainly on effective temperature and metallicity is of secondary importance. Finally, we try to interpret the oscillation spectrum of the blue supergiant HD 163899, the only member of the SPBsg class, and to get some clue on the evolutionary status of the star.

The nature of millisecond pulsars with helium white dwarf companions

We examine the growing data set of binary millisecond pulsars that are thought to have a helium white dwarf companion. These systems are believed to form when a low- to intermediate-mass companion to a neutron star fills its Roche lobe between central hydrogen exhaustion and core helium ignition. We confirm that our own stellar models reproduce a well-defined period-companion mass relation irrespective of the details of the mass transfer process. With magnetic braking this relation extends to periods of less than 1d for a 1Msun giant donor. With this and the measured binary mass functions we calculate the orbital inclination of each system for a given pulsar mass. We expect these inclinations to be randomly oriented in space. If the masses of the pulsars were typically 1.35Msun then there would appear to be a distinct dearth of high-inclination systems. However if the pulsar masses are more typically 1.55 to 1.65Msun then the distribution of inclinations is indeed indistinguishable from random. If it were as much as 1.75Msun then there would appear to be an excess of high-inclination systems. Thus with the available data we can argue that the neutron star masses in binary millisecond pulsars recycled by mass transfer from a red giant typically lie around 1.6Msun and that there is no preferred inclination at which these systems are observed. Hence there is reason to believe that pulsar beams are either sufficiently broad or show no preferred direction relative to the pulsar's spin axis which is aligned with the binary orbit. This is contrary to some previous claims, based on a subset of the data available today, that there might be a tendency for the pulsar beams to be perpendicular to their spin.

Structure and evolution of pulsating hot subdwarfs

AbstractHot subdwarfs are evolved low‐mass stars that have survived core helium ignition and are now in (or recently finished with) the core helium burning stage. At the hot end of the Horizontal Branch (HB), many of these stars are multiperiodic pulsators. These pulsations have revealed details of their global and internal structure, and provide important constraints on the origin of hot HB stars. While many features of their structure deduced from seismic fits have confirmed what we expected from evolutionary considerations, there have been some surprises as well (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Determining the location of the tip of the red giant branch in old stellar populations: M33, Andromeda I and II

The absolute bolometric luminosity of the point of core helium ignition in old, metal-poor, red giant stars is of roughly constant magnitude, varying only very slightly with mass or metallicity. It can thus be used as a standard candle. Here, we review the main difficulties in measuring this location in any real data set and we develop an empirical approach to optimize it for tip of the red giant branch (TRGB) analysis. We go on to present a new algorithm for the identification of the TRGB in nearby metal-poor stellar systems. Our method uses a least-squares fit of a data adaptive slope to the luminosity function in 1-mag windows. This finds the region of the luminosity function that shows the most significant decline in star counts as we go to brighter magnitudes; the base of this decline is attributed as the location of the tip. This technique then allows for the determination of realistic uncertainties which reflect the quality of the luminosity function used, but which are typically ∼0.02 mag rms + ∼0.03 mag systematic, a significant improvement upon previous methods that have used the tip as a standard candle. Finally, we apply our technique to the Local Group spiral galaxy M33 and the dwarf galaxies Andromeda I and II, and derive distance modulii of 24.50 ± 0.06 mag (794 ± 23 kpc), 24.33 ± 0.07 mag (735 ± 23 kpc) and 24.05 ± 0.06 mag (645 ± 19 kpc) respectively. The result for M33 is in excellent agreement with the Cepheid distances to this galaxy, and makes the possibility of a significant amount of reddening in this object unlikely.

The effect of the heavy element abundance on the evolution of stars

view Abstract Citations (105) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The effect of the heavy element abundance on the evolution of stars. Alcock, C. ; Paczynski, B. Abstract Stars of mass 2, 3, 5, 7, and 10 solar masses and chemical compositions (X = 0.7, Z = 0.03), (0.7,0.01), (0.7,0.003), (0.7,0.001), and (0.7,0.0004) have been evolved past core helium exhaustion (except for the 2-solar-mass stars, which are evolved to core helium ignition). These results demonstrate how the evolution of massive stars is determined by their initial heavy-element abundance and should be useful in the study of galactic evolution. Publication: The Astrophysical Journal Pub Date: July 1978 DOI: 10.1086/156256 Bibcode: 1978ApJ...223..244A Keywords: Abundance; Computerized Simulation; Heavy Elements; Stellar Evolution; Stellar Mass; Galactic Evolution; Tables (Data); Astrophysics; Element Abundances:Stellar Evolution full text sources ADS |

Luminosity functions and the evolution of low-mass population I giants

Luminosity functions in terms of bolometric magnitudes are constructed for M67 and for two samples of old-disk field giants. These are compared with theoretical rates of evolution on the giant branch. M67 has too few stars to give a useful comparison. The field giants show good agreement with theory, and the number of stars at the ''clump'' suggests that core helium-burning is prolonged by overshoot with semiconvective mixing. The fuel consumption derived from the luminosity functions is consistent with core helium ignition at the theortically predicted core mass, and with a final core mass in agreement with observed white dwarf masses. Data are needed for a larger complete sample of field giants for the luminosity function to be better determined. Further details, especially for the variable M giants at the top of the giant branch, are needed for population syntheses of elliptical galaxies. (AIP)

Theoretical Evolution of Extremely Metal-Poor Stars

view Abstract Citations (32) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Theoretical Evolution of Extremely Metal-Poor Stars Wagner, Raymond L. Abstract Theoretical evolutionary sequences for stars in the mass range 0.65 to 2.5 M0 having the extremely metal- deficient compositions (X, Z) = (0.739, l0- ), (0.739, 10-6), (0.739, 10-8), (0.9999, 10-8) have been calculated using a modified version of the Paczyn'ski stellar-evolution code. The models have been evolved from the zero-age main sequence through the central density maximum accompanying core helium ignition. Selected models in each evolutionary sequence have been analyzed for secular and pulsational instability. The characteristics of extremely metal-poor stellar models were found to be somewhat sensitive to the heavy-element abundance even below Z 10- , and low-mass stellar lifetimes from the main sequence to central helium ignition were found to be significantly shorter than values commonly assumed for galactic evolution studies. The main-sequence models of lowest mass are pulsationally unstable with fundamental periods of about 0.67 hour. These models become pulsationally stable before central hydrogen exhaustion. The most massive stellar models for each composition become pulsationally unstable at core helium ignition, with the pulsation periods for the fundamental mode lying in the range 0.3-4.0 days. Evolutionary tracks and time-constant loci are presented for each composition. Subject headings: interiors, stellar - Population II stars - pulsation - stellar evolution Publication: The Astrophysical Journal Pub Date: July 1974 DOI: 10.1086/152952 Bibcode: 1974ApJ...191..173W full text sources ADS |

THE EVOLUTION OF SUPER-HELIUM-RICH 2 MSUN STARS

ABSTRACT Evolutionary calculations are presented for three 29M0 configurations, two having super- helium -rich compositions, (X, Y, Z) = (0.30, 0.69, 0.01) and (0.30, 0.695, 0.005), and one a moderately helium-rich composition (0.60, 0.39, 0.01). The Z = 0.005 configuration is evolved past core helium exhaustion, the others only past core helium ignition. Neither super-helium-rich star passes through a significant convective-envelope (Hayashi) phase, either before or after core helium ignition. Both stars become substantially brighter after shell hydrogen ignition and after core helium ignition. The configuration evolved through core helium exhaustion becomes very blue (up to Te = 56,800 K) and very bright (Mb0l -5). The structural evolution of these models is discussed with regard to general ideas on post-main-sequence evolution. Observable features of a hypothetical super-helium-rich cluster are also indicated. Key words: stellar evolution - helium abundance - super -helium -rich stars