Results 1 - 10 of 1692
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[en] It is shown that mass loss by stellar wind with rates observed in O, B-stars cannot change qualitatively their evolution in the core hydrogen-burning stage. The effects, that are usually attributed to the mass loss, can be explained by other causes: e.g., duplicity or enlarged chemically homogeneous stellar cores. (Auth.)
[en] Evolutionary computations are presented for massive stars between 20 solar masses and 100 solar masses with chemical abundances holding for the Small Magellanic Cloud, i.e. X = .76 and Z = .003. Mass loss by stellar wind is taken into account during core hydrogen burning. After core hydrogen burning some models are considered as members of close binary systems and are followed during their Roche lobe overflow stage according an early case B of mass transfer. During the core helium burning stage of the RLOF remnants mass loss rates comparable to WR stars are included in order to study the formation and the evolution of WR stars. Comparison with similar galactic computations (Vanbeveren, Packet, 1978) is made. (Auth.)
[en] We present new photometric and spectroscopic observations for 2M 1533+3759 (= NSVS 07826147), the seventh eclipsing subdwarf B star + M dwarf (sdB+dM) binary ever found. It has an orbital period of 0.16177042 days, or ∼3.88 hr, significantly longer than the 2.3-3.0 hr periods of the other known eclipsing sdB+dM systems. Spectroscopic analysis of the hot primary yields Teff = 29230 ± 125 K, log g = 5.58 ± 0.03, and log N(He)/N(H) = -2.37 ± 0.05. The sdB velocity amplitude is K1 = 71.1 ± 1.0 km s-1. The only detectable light contribution from the secondary is due to the surprisingly strong reflection effect, whose peak-to-peak BVRI amplitudes are 0.10, 0.13, 0.15, and 0.19 mag, respectively. Light-curve modeling produced several solutions corresponding to different values of the system mass ratio, q (M2/M1), but only one is consistent with a core helium burning star, q = 0.301. The orbital inclination is 86.06. The sdB primary mass is M1 = 0.376 ± 0.055 Msun and its radius is R1 = 0.166 ± 0.007 Rsun. 2M 1533+3759 joins PG 0911+456 (and possibly also HS 2333+3927) in having an unusually low mass for an sdB star. SdB stars with masses significantly lower than the canonical value of 0.48 Msun, down to as low as 0.30 Msun, were theoretically predicted by Han et al., but observational evidence has only recently begun to confirm the existence of such stars. The existence of core helium burning stars with masses lower than 0.40-0.43 Msun implies that at least some sdB progenitors have initial main-sequence masses of 1.8-2.0 Msun or more, i.e., they are at least main-sequence A stars. The orbital separation in 2M 1533+3759 is a = 0.98 ± 0.04Rsun. The secondary has M2 = 0.113 ± 0.017 Msun, R2 = 0.152 ± 0.005Rsun, and Teff2= 3100 ± 600 K, consistent with a main-sequence M5 star. If 2M 1533+3759 becomes a cataclysmic variable (CV), its orbital period will be 1.6 hr, below the CV period gap.
[en] Current uncertainty about the most massive observed stars has led to a reexamination of the most massive star that is stable against radial pulsation. The nuclear energy generation equations in the LNA linear, nonadiabatic code have been considerably improved, so that it is now appropriate to redo the study to determine the maximum mass of ZAMS star that will be stable against pulsation. 12 refs., 2 figs., 2 tabs
[en] Evolutionary tracks for a 30 M star with initial mass loss rates of (0.0, 1.0, 2.5, 5.0, 10.0) x 10-7 M'period on line'/yr have been calculated to core helium exhaustion or until the star reaches the giant branch. During the main sequence phase, the amount of luminosity increase is reduced as the mass loss rate is increased. The radii of the stars of the different tracks are almost independent of the mass loss rates to that the stars evolving with the higher mass loss rates are redder. The main sequence life-time of the tracks does not increase monotonically with mass loss rate, but due to the semiconvection-mass loss competition decreased for low mass loss rates. The mass-luminosity law, L=μ4β4M3 where μ is the average mean molecular weight in the star, M is the evolutionary mass and β is the mean ratio of gas pressure to total pressure defined by the Eddington quartic equation, is demonstrated to be valid throughout the evolution. A semi-empirical M-L law for mass losing sequences during the main sequence phase is found and shown to be derivable from the above M-L law. A critical ZAMS mass loss rate above which a star evolved to fainter luminosities is derived analytically. The M-L law is also shown to be valid for the post main sequence phase. The internal structure of the stars as they evolve is also examined in detail. Explanations for the changes of physical variables are offered. It is shown analytically that M-L law makes it possible to understand many features of the evolution
[en] The interior structure and composition of Beta Lyrae are examined. Beta Lyrae is composed of a primary (B8 giant) and a secondary component; the primary component is less massive (2 solar masses) than the secondary component (12 solar masses) and is shedding its outer layer and transferring it to the secondary component. The development of binaries from main-sequence dwarfs to red giants is described. The chemical composition of Beta Lyrae and the CNO cycle are analyzed. The binary displays He poorness, He richness, C,N,O abundances characteristic of hydrogen burning, and a cosmic dose of other elements. The three episodes of mass transfer for Beta Lyrae, which will result in the Beta Lyrae system forming a single compact object, are discussed
[en] The rate of production of heavy elements is rediscussed using the models of Arnett (1978) for late stage nucleosynthesis in massive stars, the M(M,) relationship of Chiosi et al. (1978b) for losing mass models in the core H and He-Burning phases, and the stellar birth rate of Miller and Scalo (1978). Contrary to that found by Wheeler et al (1978), we do not encounter the difficulty of heavy element overproduction. The explosive nucleosynthesis from massive stars is still compatible with the observed abundance distribution of the solar system, and a stellar birth rate decreasing over the history of the galactic disk. (orig.)
[en] We report the discovery of TOI 694 b and TIC 220568520 b, two low-mass stellar companions in eccentric orbits around metal-rich Sun-like stars, first detected by the Transiting Exoplanet Survey Satellite (TESS). TOI 694 b has an orbital period of 48.05131 ± 0.00019 days and eccentricity of 0.51946 ± 0.00081, and we derive a mass of 89.0 ± 5.3 (0.0849 ± 0.0051 ) and radius of 1.111 ± 0.017 (0.1142 ± 0.0017 ). TIC 220568520 b has an orbital period of 18.55769 ± 0.00039 days and eccentricity of 0.0964 ± 0.0032, and we derive a mass of 107.2 ± 5.2 (0.1023 ± 0.0050 ) and radius of 1.248 ± 0.018 (0.1282 ± 0.0019 ). Both binary companions lie close to and above the hydrogen-burning mass threshold that separates brown dwarfs and the lowest-mass stars, with TOI 694 b being 2σ above the canonical mass threshold of 0.075 . The relatively long periods of the systems mean that the magnetic fields of the low-mass companions are not expected to inhibit convection and inflate the radius, which according to one leading theory is common in similar objects residing in short-period tidally synchronized binary systems. Indeed we do not find radius inflation for these two objects when compared to theoretical isochrones. These two new objects add to the short but growing list of low-mass stars with well-measured masses and radii, and highlight the potential of the TESS mission for detecting such rare objects orbiting bright stars.
[en] In this paper the properties of the stars close to the upper main sequence of OB associations are studied in order to provide constraints on the theory of evolution of massive stars. The spectral classification of northern single-lined O stars, most of them belonging to the associations Cyg OB1, Per OB1 and Cas OB6, is presented. The classification is performed according to Conti's classification scheme for O stars, amended as described in the first paper of this series; the new classification criteria introduced in that paper are rediscussed and refined. The behaviour of the C and N lines of the stars is studied, in order to determine whether CNO-processed material is visible at their surface. Three new ON stars have been discovered: BD + 360 4063 (09.71), HD 13268 (O8V), and HD 110360 (O7V). All in all, 9 ON stars and 25 other stars showing some morphological N enhancement are discussed