Core Helium Burning Research Articles

OH/IR stars in the Magellanic Clouds

A sample of IRAS point sources in the Magellanic Clouds has been searched for 1612 MHz maser emission and for infrared emission at JHKL'. Six point sources, all in the LMC, are found to be OH masers while 19 have strong infrared emission typical of evolved giant and supergiant stars with dense circumstellar shells. Near-infrared monitoring over ∼1700 days has yielded pulsation periods for nine of the sources. All sources are classified as AGB stars, core helium burning supergiants or preplanetary nebulae according to pulsation amplitude, luminosity, and redness

THE ORIGINS OF HOT SUBDWARF STARS

ABSTRACT High signal-to-noise optical spectrophotometry of a sample of filed subluminous B stars drawn largely from the Palomar Green ultraviolet-excess survey is analyzed with a new grid of model atmospheres and synthetic spectra. The stellar effective temperatures, surface gravities, and photospheric helium abundances are determined simultaneously from a dtailed analysis of hydrogen and helium absorption-line profiles. The derived temperatures and gravities place the subluminous B stars in the theoretical H-R diagram along and bounded below by theoretical sequences of the zero-age extended horizontal branch, lending strong support to the hypothesis that these stars are compsed of helium-burning cores of 0.5 solar mass overlain by very thin layers of hydrogen (< 0.02 solar mass). Various scenarios for their past evolutionary history are exmined in the context of their probable future evolution into white dwarfs of lower-than-average mass. The derived distances above the Galactic plane support a scale height for the population of 285 pc, consistent with the identification of their progenitor stars as members of the old disk population. Radial velocities of sdB and sdO stars are analyzed to infer their kinematic characteristics. The results for the sdB stars are inconclusive, but for the sdO stars the results are also consistent with the population belonging to the older part of the thin disk.

Self-consistent models of Wolf-Rayet stars as helium stars with optically thick winds

view Abstract Citations (47) References (55) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Self-consistent Models of Wolf-Rayet Stars as Helium Stars with Optically Thick Winds Kato, Mariko ; Iben, Icko, Jr. Abstract A helium star with a steady, optically thick wind is proposed as a first-order model for Wolf-Rayet stars. Using an enhanced opacity which is guided by results of recent opacity calculations, self-consistent solutions are obtained which are made up of an inner helium-burning core in hydrostatic equilibrium and an outer radiative envelope through which a steady state wind is accelerated. The surface is at optical depth near unity. The models are of homogeneous composition (primarily helium) and are of mass in the range 9-30 solar masses. In the most massive model, the momentum of the mass flow exceeds the photon momentum at the photosphere by a factor of 30. Terminal wind velocities are of order 1000 km/s; these velocities are lower limits, as we neglect line-driven wind acceleration above the photosphere. For an appropriate choice of the opacity enhancement law, models of different mass define a relationship between the mass-loss rate and the photospheric temperature which is consistent with observed data. A forbidden region is found in the low-temperature part of the H-R diagram where no thick wind solutions exist. Publication: The Astrophysical Journal Pub Date: July 1992 DOI: 10.1086/171583 Bibcode: 1992ApJ...394..305K Keywords: Stellar Models; Stellar Winds; Wolf-Rayet Stars; Helium; Hertzsprung-Russell Diagram; Optical Thickness; Photosphere; Stellar Cores; Stellar Envelopes; Astrophysics; STARS: CHEMICALLY PECULIAR; STARS: INTERIORS; STARS: MASS LOSS; STARS: WOLF-RAYET full text sources ADS |

Evolved stars in Omega Centauri. I - Radial distribution of blue subdwarfs

A U - V color-magnitude diagram containing over 5000 stars from 10 fields in Omega Centauri at or brighter than the main-sequence turnoff is presented. Completeness corrections are obtained as a continuous function of magnitude and radial distance are obtained and used to show that the blue subdwarfs are centrally concentrated with respect to subgiants and stars on the horizontal branch proper. The chance probability of this result is less than 1 percent. Since the blue subdwarfs probably consist of helium-burning cores of about 0.5 solar mass surrounded by a thin hydrogen envelope, mass segregation could not produce this result if these stars had evolved singly. Two kinds of possible precursor systems are considered: pairs of degenerate dwarfs which subsequently merge to form helium-burning stars, and moderately wide binaries in which mass transfer is initiated shortly before the helium flash is ignited.

S-processing in massive stars as a function of metallicity and interpretation of observational trends

The s-process in massive stars is analyzed as a function of metallicity. The nucleosynthesis occurring in both core helium and shell carbon burning is investigated by numerically modeling nuclear reaction networks, subject to the conditions provided by stellar models. The efficiency of the neutron-capture mechanism depends on the amount of enhancement that is assumed for the α-rich nuclei, that are observed to be overabundant with respect to iron in stars of metallicity lower than solar

Interaction Beyond the Main Sequence: One Recipe?

We investigate the different aspects that govern the interaction of post-main-sequence evolution of binaries, using a new, homogeneous set of computations. The set describes the evolution of both components through the phase of mass transfer, till the end of the core helium burning of the primary. The mass range is 9 to 40 Mo, the mass ratios are 0.9 and 0.6 (and introducing 0.99 as a newcomer). Both for small and large masses we discuss the consequences of non-conservative mass transfer. Using a simple concept for the fraction β of transferred matter, we look to its value at the onset of mass transfer for various mass ratios and periods. Exploration of the phase space of interacting binaries reveals the influence of the various parameters on the dimensions of the resulting systems after mass transfer. Special attention is given to binaries with mass ratios very close to one. Their secondaries evolve directly into yellow supergiants such as observed in the LMC.

Evolutionary sequences of stellar models of intermediate and high mass including convective core overshooting

Stellar evolution with parameterized overshooting from the convective core has been studied for the stellar mass range M/M(solar) = 3-30 and for the initial chemical composition range Xe = 0.650-0.739 (hydrogen) and Ze = 0.021-0.044 (metals). Evolutionary sequences run from the zero-age main sequence to the end of core helium burning, but emphasis is placed here on the core helium-burning phase. Convective core overshooting during the previous main-sequence phase leads, in most cases, to a shortening of the blue loop on the H-R diagram that forms when helium is being depleted in the core. On the other hand, convective overshooting from the helium-burning core has the opposite effect. A larger initial metals abundance also tends to shorten the blue loop. In an extreme case the blue loop can be fully suppressed, but in a marginal case the normal trend can be reversed.

Blue loops during core helium burning as the consequence of moderate convective envelope overshooting in stars of intermediate to high mass

New calculations of stellar evolutionary sequences without semiconvection in any phase have unexpectedly turned up, for stars in the mass range 3-30 solar masses, an irregular pattern of blue loops on the H-R diagram during the core helium-burning phase. Blue loops occur for 3 and 10 solar masses, but not for 5, 7, 15, and 30 solar masses. It is found, however, that the models near the bottom of the red supergiant branch are only marginallly stable against small inward displacements of the former base of the outer convection zone when it was deepest, for the stellar mass range 5-15 solar masses. As a consequence, convective envelope overshooting need only penetrate a distance of about 0.3 of a local pressure scale height to promote a delayed blue loop in these particular sequences. In conformity with previous work, the triggering mechanism for the blue loop appears to be proximity of the hydrogen-burning shell to the hydrogen discontinuity at the former base of the outer convection zone.

Stellar evolution and period changes in RR Lyrae stars

The observed secular period changes of RR Lyrae stars in five globular clusters have been compared with those predicted by synthetic models of the horizontal branch (HB). It is shown that most of the observed number distributions of the period change rate beta could be attributed to evolutionary effects, if the random observational error is of order + or - 0.07 days/Myr in beta as suggested by the observers. The model calculations indicate that the mean rate of period change in RR Lyrae stars in globular clusters depends sensitively on their HB type, as is the case for the period shift at a given effective temperature. The distinct bias toward positive period changes in M15 and Omega Cen is evidence that most HB stars in the Oosterhoff group II clusters pass through the instability strip from blue to red toward the end of their core helium burning phase. If confirmed by future observations, this is strong support for the Lee et al. (1990) explanation of the Sandage (1957) period-shift effect.

New Results in Evolution in the Upper HRD

A theoretical interpretation of the observed upper luminosity limit is suggested here. Staritsin (1989) considered the core hydrogen and helium burning stages in a 64 M⊙ star. Mixing in a semi-convection zone in a diffusion approximation (Staritsin, 1987) and mass loss by stellar wind (de Jager et al., 1988) were taken into account. During MS evolution the star looses half of its initial envelope. After MS evolution an intermediate convective zone appears. The hydrogen content in the shell source increases. As a result, the star burns helium in its blue supergiant stage. After the hydrogen content in the envelope has decreased to 10% of its mass, the star looses mass with Wolf-Rayet mass loss rates according to de Jager et al. (1988). The star has a WR character during 5% of its full life time.

Single-Lined Spectroscic Appearances During the Evolution of a Massive Binary System

We calculated the evolution of both components of a massive binary system with masses 26 MO and 23.4 MO, initial period 27.4 days, through a case B of mass transfer, up to the end of the core helium burning phase of the primary (and further to the end of the main sequence of the secondary). For the WC phase we adopted Langer's (1989) mass dependent mass loss rate Ṁ ~ M2.5). The luminosities were transformed to visual magnitudes, applying bolometric corrections for O stars of Code et al. (1976) and Flower (1977) and BC = − 4.5 for the WR star (Smith and Maeder, 1989). If conservative mass transfer is adopted, the WR star is about 1.2 to 2.0 magnitude less brighter in the visual than the O star during the WN phase (lasting 1.5 105 yr). The brightness difference reaches almost 3 magnitudes during the WC + 08 phase (lasting 4.3 105 yr), showing the system as a single-lined O-type (!) spectroscopic binary. During the WR phase the mass function of this system varies from 0.67 sin3i to 0.05 sin3i! Nonconservative mass transfer and (or) a smaller initial mass ratio may result in the inverse situation, a single-lined WR binary with a large mass function (De Greve et al., 1988). Some of the results are shown in table 1 below.

The Mass of WR Progenitors

A search through literature reveals four methods in order to derive the mass of WR progenitors, i.e.a. WR stars must be descendant from the most massive stars which share their galactic distribution,b. the computation of detailed evolutionary models of massive close binaries up to the WR phase, able to explain the observational constraints of these WR binaries,c. comparing the very narrow mass-luminosity relation of massive core helium burning stars predicted by evolution and estimated bolometric luminosities of WR members of stellar aggregates,d. the minimum mass of the progenitor of a WR member of a cluster equals the mass of the most luminous star (or the star with the earliest spectral type) in the cluster.

The Influence of WR Like Stellar Wind Mass Loss Rates on the Evolution of Massive Core Helium Burning Stars

Evolutionary computations of massive close binaries (MCB) including the effects of stellar wind (SW) and convective core overshooting predict that all massive primaries with ZAMS mass larger than 10 M⊙ start their core helium burning phase (CHeB) as bare helium cores; the hydrogen rich layers are removed on a timescale of the order of 104 yrs as a consequence of Roche lobe overflow (RLOF). The CHeB remnant after RLOF resembles closely a zero age CHeB star and its further evolution is entirely independent from its binary nature. Similarly as has been done previously by Vanbeveren and Packet (1979, A.&amp;A.80, 242), I have performed a phenomenological study on the evolution of massive hydrogen less CHeB stars including the effect of SW mass loss using updated M determinations of van der Hucht et al. (1986, A.&amp;A.168, 111). The SW mass loss rate formalism used in the computations is based on the following requirements:

Pre-White Dwarf Evolution: Up to Planetary Nebulae

AbstractThe main evolutionary phases having some interest for the formation of the remnant white dwarf are discussed, starting from the core helium burning phase, in the attempt of evaluating a theoretical relation between initial main sequence mass and final white dwarf mass. Several difficulties, mainly due (but not only) to uncertainties in the theory of mass loss, have been met, so that only a fiducial bona fide correlation can be drawn. The mass function of population I white dwarfs has probably a secondary maximum at M = 0.9 – 1 Me.

Disturbed Binaries: The Early Phases

AbstractThe disturbance of components and system parameters by mass transfer during and at the end of core hydrogen burning, as well as at the onset of core helium burning is reviewed. Emphasis is given to the influence of the evolution of the gainer and to the variation of the chemical composition of the surface.

Thermonuclear Explosion in Binaries: White Dwarf Helium Star

AbstractEvolutionary sequences of a binary system consisting of a helium star in the stage of core helium burning and a white dwarf are presented. Different ways of He burning in the envelope of a dwarf are discussed. Incomplete helium burning is discussed with relation to occurence of possible observational features.

A spectral dissection of the ultraviolet emissions of Capella

view Abstract Citations (32) References (53) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Spectral Dissection of the Ultraviolet Emissions of Capella Ayres, Thomas R. Abstract Phase-resolved observations of Capella (α Aurigae A: G5 III+G0 III) were conducted with the IUE in order to separate the spectrum of the core-helium-burning (CHeB) primary (Aa) from that of the fast-rotating, chromospherically active secondary (Ab). The ultraviolet emissions of the primary were compared to those of the four K0 giants of the Hyades and the field K giants β Cet and β Gem. In terms of C IV emission, the six reference CHeB giants fall into two distinct groups: the more active β Cet type; and the less active β Gem type. Capella Aa clearly belongs in the β Cet group. The similarity between the Capella primary and the two active Hyads - γ Tauri and θ1 Tauri - is striking. If Capella truly is coeval with the Hyades nucleus, then the age of the cluster might be less than half that commonly assumed. The UV spectra of the Capella secondary show evidence of mass outflow in the upper chromosphere, although the flow very likely is part of a circulation system like that thought to operate in magnetic active regions on the Sun. Publication: The Astrophysical Journal Pub Date: August 1988 DOI: 10.1086/166572 Bibcode: 1988ApJ...331..467A Keywords: Binary Stars; Spectrum Analysis; Stellar Radiation; Stellar Spectra; Ultraviolet Spectra; B Stars; Giant Stars; Helium; Iue; Open Clusters; Astronomy; CLUSTERS: OPEN; STARS: BINARIES; STARS: INDIVIDUAL CONSTELLATION NAME: ALPHA AURIGAE; STARS: LATE-TYPE; ULTRAVIOLET: SPECTRA full text sources ADS | data products SIMBAD (9) MAST (1) INES (1)

Radiatively driven winds in Wolf-Rayet stars - A unified core-atmosphere model

A sequence of chemically homogeneous, dynamical stellar models is constructed for different masses. Each model is computed by fitting onto a helium-burning core a dynamical optically thick radiative envelope and an expanding atmosphere in which line acceleration is taken into account. The fitting procedure uniquely determines all the properties of the configuration, including the mass-loss rate M(dot) and the terminal velocity v(infinity), as a function of the total mass M. Although the bulk of the acceleration takes place in the atmosphere and is mainly due to line driving, dynamics is found to be not completely negligible even below the thermalization radius where only the gradients of isotropic radiation and gas pressure are responsible for acceleration. The derived values of M(dot) and v(infinity) are in good agreement with the observational data for Wolf-Rayet stars. In particular, the M(dot) versus M relation derived by Abbott et al. (1986) for W-R stars in binary systems is well reproduced. 24 references.

Abundances in Cool Evolved Stars

Nature has filled the upper right quadrant of the Hertzsprung-Russell diagram with more varieties of peculiar stars and odd chemical compositions than even our most speculative observers and theorists could dream up. To bring some structure to this vast subject I will categorize the phenomena we observe according to our model of stellar evolution, dividing the stars among the first ascent of the giant branch and the core-helium burning phase, the asymptotic giant branch (double shell-burning) phase, and the post-AGB and pre-planetary nebula stars. The types of stars found in these three groups are summarized below.

RAPID EMISSION-LINE VARIATIONS IN THE UV SPECTRUM OF THE WOLF-RAYET SYSTEM HD 90657.

ABSTRACT The authors report IUE observations of the Wolf-Rayet binary system HD 90657 which indicate that a major change in the properties of the W-R wind occurred on time scales of less than 90 minutes. These are very rapid variations when compared to those resulting from atmospheric eclipse effects, which are also observed in this system. The rapid change is attributed to wind instabilities, possibly induced by pulsations of the underlying helium-burning core.