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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] 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] 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] 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] 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] 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] We report the result of the analysis of the light curve of the microlensing event MOA-2009-BLG-016. The light curve is characterized by a short-duration anomaly near the peak and an overall asymmetry. We find that the peak anomaly is due to a binary companion to the primary lens and the asymmetry of the light curve is explained by the parallax effect caused by the acceleration of the observer over the course of the event due to the orbital motion of the Earth around the Sun. In addition, we detect evidence for the effect of the finite size of the source near the peak of the event, which allows us to measure the angular Einstein radius of the lens system. The Einstein radius combined with the microlens parallax allows us to determine the total mass of the lens and the distance to the lens. We identify three distinct classes of degenerate solutions for the binary lens parameters, where two are manifestations of the previously identified degeneracies of close/wide binaries and positive/negative impact parameters, while the third class is caused by the symmetric cycloid shape of the caustic. We find that, for the best-fit solution, the estimated mass of the lower-mass component of the binary is (0.04 ± 0.01) Msun, implying a brown-dwarf companion. However, there exists a solution that is worse only by Δχ2 ∼ 3 for which the mass of the secondary is above the hydrogen-burning limit. Unfortunately, resolving these two degenerate solutions will be difficult as the relative lens-source proper motions for both are similar and small (∼1 mas yr-1) and thus the lens will remain blended with the source for the next several decades.
[en] We present the results of deep and high-resolution (FWHM ∼ 0.''35) JHK near-infrared (NIR) observations with the Subaru telescope, to search for very low mass young stellar objects in the W3 Main star-forming region. The NIR survey covers an area of ∼ 2.6 arcmin2 with 10σ limiting magnitude exceeding 20 mag in the JHK bands. The survey is sensitive enough to provide unprecedented details in W3 IRS 5 and IRS 3a regions and reveals a census of the stellar population down to objects below the hydrogen-burning limit. We construct JHK color-color and J - H/J and H - K/K color-magnitude diagrams to identify very low luminosity young stellar objects and to estimate their masses. Based on these color-color and color-magnitude diagrams, we identified a rich population of embedded YSO candidates with infrared excesses (Class I and Class II), associated with the W3 Main region. A large number of red sources (H - K > 2) have also been detected around W3 Main, which are arranged from the northwest toward the southeast regions. Most of these are concentrated around W3 IRS 5. We argue that these red stars are most probably pre-main-sequence (PMS) stars with intrinsic color excesses. We find that the slope of the K-band luminosity function of W3 Main is lower than the typical values reported for young embedded clusters. Based on the comparison between theoretical evolutionary models of very low mass PMS objects with the observed color-magnitude diagram, we find there exists a substantial substellar population in the observed region. The mass function does not show the presence of cutoff and sharp turnover around the substellar limit, at least at the hydrogen-burning limit. Furthermore, the mass function slope indicates that the number ratio of young brown dwarfs and hydrogen-burning stars in the W3 Main is probably higher than those in Trapezium and IC 348. The presence of mass segregation, in the sense that relatively massive YSOs lie near the cluster center, is seen. The estimated dynamical evolution time indicates that the observed mass segregation in the W3 Main may be the imprint of the star formation process.
[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.)