Results 1 - 10 of 12635
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[en] Stars are known to form mostly in group and in molecular clouds. The initial mass function seems to be definitely sensitive to the initial conditions of the gravitational collapse, i.e. the physical properties of the parental cloud. It appears that most of the massive stars preferentially form in the edges of the giant molecular clouds, which are relatively stable against collapse. This process therefore requires the presence of a triggering mechanism. The bulk of the low-mass stars form in colder molecular clouds, in sites apparently scattered throughout the clouds and the process directly results the spontaneous fragmentation of cold and pressurefree dense gas
[fr]S'il est admis que la plupart des etoiles se forment en groupe et dans les nuages moleculaires (NM), un point reste discute: c'est la sensibilite de la fonction de masse initiale aux proprietes des nuages geniteurs. A savoir, la grande majorite des etoiles massives semble se former dans les bords des nuages moleculaires geants (NMG), avec un taux de formation stimule superieur au taux de formation spontanee. Quant aux etoiles de faible masse, elles semblent se former de facon essentiellement spontanee, dans des nuages plus froids ou dans les coeurs des nuages moleculaires geants
[en] The increasing number of metal-poor stars observed to date has raised numerous questions concerning the elemental production in the early stages of our Galaxy. Theoretical studies have shown that extremely metal-poor (EMP) low- and intermediate-mass stars undergo extra mixing processes (proton ingestion episode-PIE) that leads to the production of large amounts of neutron in the early stages of the He-flash or the TP-AGB phase. In this work we explore the nucleosynthesis during the PIE of intermediate-mass stars, including the production of sprocess elements. We have performed models of stars with M<3.0Msun and Z<10-7. Our models show that a significant amount of s-process elements are produced by EMP stars during the PIE. Moreover, this production is strongly dependent on the duration of this phenomenon. (author)
[en] Theories that attempt to explain cosmic acceleration by modifying gravity typically introduces a long-range scalar force that needs to be screened on small scales. One common screening mechanism is the chameleon, where the scalar force is screened in environments with a sufficiently deep gravitational potential, but acts unimpeded in regions with a shallow gravitational potential. This leads to a variation in the overall gravitational G with environment. We show that such a variation can occur within a star itself, significantly affecting its evolution and structure, provided that the host galaxy is unscreened. The effect is most pronounced for red giants, which would be smaller by a factor of tens of percent and thus hotter by hundreds of Kelvin, depending on the parameters of the underlying scalar-tensor theory. Careful measurements of these stars in suitable environments (nearby dwarf galaxies not associated with groups or clusters) would provide constraints on the chameleon mechanism that are four orders of magnitude better than current large-scale structure limits and two orders of magnitude better than present solar system tests.
[en] A replot of period derivative against period is done for about 300 pulsars, and the main features of the plot are discussed. The significance of a 'gap' in this plot is reexamined and the existence of pulsars with nulling and subpulse drifting behaviour below this 'gap' is pointed out. The implications of this for pulsar evolution are also discussed. (orig.)
[en] We present an analysis of the star formation history (SFH) of the transition-type (dIrr/dSph) Local Group galaxy LGS-3 (Pisces) based on deep photometry obtained with the Advanced Camera for Surveys onboard the Hubble Space Telescope. Our observations reach the oldest main-sequence turnoffs at high signal to noise, allowing a time resolution at the oldest ages of σ ∼ 1.1 Gyr. Our analysis, based on three different SFH codes, shows that the SFH of LGS-3 is dominated by a main episode ∼11.7 Gyr ago with a duration of ∼1.4 Gyr. Subsequently, LGS-3 continued forming stars until the present, although at a much lower rate. Roughly 90% of the stars in LGS-3 were formed in the initial episode of star formation. Extensive tests of self-consistency, uniqueness, and stability of the solution have been performed together with the IAC-star/IAC-pop/MinnIAC codes, and these results are found to be independent of the photometric reduction package, the stellar evolution library, and the SFH recovery method. There is little evidence of chemical enrichment during the initial episode of star formation, after which the metallicity increased more steeply reaching a present-day value of Z ∼ 0.0025. This suggests a scenario in which LGS-3 first formed stars mainly from infalling fresh gas, and after about 9 Gyr ago, from a larger fraction of recycled gas. The lack of early chemical enrichment is in contrast to that observed in the isolated dSph galaxies of comparable luminosity, implying that the dSphs were more massive and subjected to more tidal stripping. We compare the SFH of LGS-3 with expectations from cosmological models. Most or all the star formation was produced in LGS-3 after the reionization epoch, assumed to be completed at z ∼ 6 or ∼12.7 Gyr ago. The total mass of the galaxy is estimated to be between 2 and 4 x 108 Msun corresponding to circular velocities between 28 km s-1 and 36 km s-1. These values are close to but somewhat above the limit of 30 km s-1 below which the UV background is expected to prevent any star formation after reionization. Feedback from supernovae (SNe) associated with the initial episode of star formation (mechanical luminosity from SNe Lw = 5.3 x 1038 erg s-1) is probably inadequate to completely blow away the gas. However, the combined effects of SN feedback and UV background heating might be expected to completely halt star formation at the reionization epoch for the low mass of LGS-3; this suggests that self-shielding is important to the early evolution of galaxies in this mass range.
[en] We report trigonometric parallaxes for three massive star-forming regions, corresponding to distances of 2.34+0.13-0.11 kpc for G12.89+0.49 (also known as IRAS 18089-1732), 1.98+0.14-0.12 kpc for G15.03-0.68 (in the M17 region), and 8.0+4.0-2.0 kpc for G27.36-0.16. Both G12.89+0.49 and G15.03-0.68 are located in the Carina-Sagittarius spiral arm.
[en] We present the discovery of a substellar companion on a wide orbit around the ∼ 2.5 Msun star HIP 78530, which is a member of the 5 Myr old Upper Scorpius association. We have obtained follow-up imaging over two years and show that the companion and primary share common proper motion. We have also obtained JHK spectroscopy of the companion and confirm its low surface gravity, in accordance with the young age of the system. A comparison with DRIFT-PHOENIX synthetic spectra indicates an effective temperature of 2800 ± 200 K and a comparison with template spectra of young and old dwarfs indicates a spectral type of M8 ± 1. The mass of the companion is estimated to be 19-26 MJup based on its bolometric luminosity and the predictions of evolutionary models. The angular separation of the companion is 4.''5, which at the distance of the primary star, 156.7 pc, corresponds to a projected separation of ∼710 AU. This companion features one of the lowest mass ratios (∼0.009) of any known companion at separations greater than 100 AU.