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[en] Stothers and Chin(1972) examined numerically thermal instability of H burning shells in very massive stars. They reported the discovery of the thermal pulses in the shell on a stage of the contracting core just after the core exhaustion. Their results, however, are inconsequential because the pulses are very feeble and the mechanism of such feeble pulses is not well known at the shell burning phases. In this note, the authors summarize the analytical method and the results on the possibility of such feeble pulses in H burning shells. (Auth.)
[en] The mass function and statistics of binaries provide important diagnostics of the star formation process. Despite this importance, the mass function at low masses remains poorly known due to observational difficulties caused by the faintness of the objects. Here we report the microlensing discovery and characterization of a binary lens composed of very low mass stars just above the hydrogen-burning limit. From the combined measurements of the Einstein radius and microlens parallax, we measure the masses of the binary components of 0.10 ± 0.01 Msun and 0.09 ± 0.01 Msun. This discovery demonstrates that microlensing will provide a method to measure the mass function of all Galactic populations of very low mass binaries that is independent of the biases caused by the luminosity of the population.
[en] Stellar multiplicity properties have been studied for the lowest and the highest stellar masses, but intermediate-mass stars from F-type to late A-type have received relatively little attention. Here, we report on a Gemini/NICI snapshot imaging survey of 138 such stars in the young Scorpius-Centaurus (Sco-Cen) region, for the purpose of studying multiplicity with sensitivity down to planetary masses at wide separations. In addition to two brown dwarfs and a companion straddling the hydrogen-burning limit which we reported previously, here we present 26 new stellar companions and determine a multiplicity fraction within 0.''1-5.''0 of 21% ± 4%. Depending on the adopted semimajor axis distribution, our results imply a total multiplicity in the range of ∼60%-80%, which further supports the known trend of a smooth continuous increase in the multiplicity fraction as a function of primary stellar mass. A surprising feature in the sample is a distinct lack of nearly equal-mass binaries, for which we discuss possible reasons. The survey yielded no additional companions below or near the deuterium-burning limit, implying that their frequency at >200 AU separations is not quite as high as might be inferred from previous detections of such objects within the Sco-Cen region
[en] A re-investigation of the secular problem is presented taking into account perturbations of chemical abundances. In particular the role of the equation of state is exposed. It is shown, in the frame of an analytical one-zone model, that an evolutionary instability results from the coupling of thermal and nuclear-chemical variables. Strong arguments indicate that the secular behaviour of a star (in the hydrogen burning phase) is represented by the thermal spectrum joined with the Continuous Nuclear Spectrum . The existence of an additional discrete (unstable) nuclear spectrum is called in question. (orig.)
[en] Nuclear astrophysics combines astronomy/astrophysics with nuclear physics and aims at unveiling the origin of the chemical elements and the astrophysical sites where they are formed. Recent years have witnessed tremendous advances in powerful observatories, laboratory reaction measurements, radioactive ion-beam facilities providing highly unstable nuclei, and progress in astrophysical and nuclear modeling. The authors describe the stellar evolution, the two types of supernova explosion mechanism and present the 2 great mysteries in nuclear astrophysics: the r-process which is responsible for the production of the heaviest elements up to Th, U and Pu and the synthesis of stable proton-rich isotopes. The r-process runs through nuclei with such an extreme neutron excess that most of them have never been produced in the laboratory yet. The synthesis of stable proton-rich isotopes can happen in a hot bath of photons during supernova explosions (where photo-disintegrations combined with beta decays can populate stable proton-rich isotopes) or during explosive hydrogen burning occurring in binary stellar systems on the surface of white dwarfs or neutron stars, this explosive burning causes a rapid proton-capture process. (A.C.)
[en] We present new Spitzer IRAC [3.6], [4.5], [5.8] and [8.0] photometry of nine very late-type T dwarfs. Combining this with previously published photometry, we investigate trends with type and color that are useful for both the planning and interpretation of infrared surveys designed to discover the coldest T or Y dwarfs. Brown dwarfs with effective temperature (Teff) below 700 K emit more than half their flux at wavelengths longer than 3 (micro)m, and the ratio of the mid-infrared flux to the near-infrared flux becomes very sensitive to Teff at these low temperatures. We confirm that the color H (1.6 (micro)m) - [4.5] is a good indicator of Teff with a relatively weak dependence on metallicity and gravity. Conversely, the colors H - K (2.2 (micro)m) and [4.5] - [5.8] are sensitive to metallicity and gravity. Thus near- and mid-infrared photometry provide useful indicators of the fundamental properties of brown dwarfs, and if temperature and gravity are known, then mass and age can be reliably determined from evolutionary models. There are twelve dwarfs currently known with H - [4.5] > 3.0, and ∼ 500 < Teff K ∼< 800, which we examine in detail. The ages of the dwarfs in the sample range from very young (0.1 - 1.0 Gyr) to relatively old (3 - 12 Gyr). The mass range is possibly as low as 5 Jupiter masses to up to 70 Jupiter masses, i.e. near the hydrogen burning limit. The metallicities also span a large range, from [m/H]= -0.3 to [m/H]= +0.2. The small number of T8 - T9 dwarfs found in the UKIRT Infrared Deep Sky Survey to date appear to be predominantly young low-mass dwarfs. Accurate mid-infrared photometry of cold brown dwarfs is essentially impossible from the ground, and extensions to the mid-infrared space missions warm-Spitzer and WISE are desirable in order to obtain the vital mid-infrared data for cold brown dwarfs, and to discover more of these rare objects.
[en] In a recent investigation Klapp et al. 1987 obtained a critical mass of 440 Msun for the overstability of very massive extreme population I stars at the main sequence. In this work we investigate the dependence of Klapp et al. 1987 results upon the program input physics. We find that stars in the 100 - 500 Msun range are marginally stable (or unstable) and that his mass range should be considered as a transition region from stability to overstability of very massive stars. (Author)
[en] Helium core white dwarfs (WDs) with mass M ∼< 0.20 M sun undergo several Gyr of stable hydrogen burning as they evolve. We show that in a certain range of WD and hydrogen envelope masses, these WDs may exhibit g-mode pulsations similar to their passively cooling, more massive carbon/oxygen core counterparts, the ZZ Cetis. Our models with stably burning hydrogen envelopes on helium cores yield g-mode periods and period spacings longer than the canonical ZZ Cetis by nearly a factor of 2. We show that core composition and structure can be probed using seismology since the g-mode eigenfunctions predominantly reside in the helium core. Though we have not carried out a fully nonadiabatic stability analysis, the scaling of the thermal time in the convective zone with surface gravity highlights several low-mass helium WDs that should be observed in search of pulsations: NLTT 11748, SDSS J0822+2753, and the companion to PSR J1012+5307. Seismological studies of these He core WDs may prove especially fruitful, as their luminosity is related (via stable hydrogen burning) to the hydrogen envelope mass, which eliminates one model parameter.
[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.)