We present yields from stars of mass in the range M ⊙ M 8M ⊙ of metallicities Z = 3 × 10 −4 and Z = 8 × 10 −3 , thus encompassing the chemistry of low-and high-Z Globular Clusters. The yields are based on full evolutionary computations, following the evolution of the stars from the pre-Main Sequence through the Asymptotic Giant Branch phase, until the external envelope is lost.Independently of metallicity, stars with M < 3M ⊙ are dominated by Third Dredge-Up, thus ejecting into their surroundings gas enriched in carbon and nitrogen. Conversely, Hot Bottom Burning is the main responsible for the modification of the surface chemistry of more massive stars, whose mass exceeds 3M ⊙ : their gas shows traces of proton-capture nucleosynthesis.The extent of Hot Bottom Burning turns out to be strongly dependent on metallicity. Models with Z = 8×10 −3 achieve a modest depletion of oxygen, barely reaching −0.3 dex, and do not activate the Mg-Al chain. Low-Z models with Z = 3 × 10 −4 achieve a strong nucleosynthesis at the bottom of the envelope, with a strong destruction of the surface oxygen and magnesium; the most extreme chemistry is reached for models of mass ∼ 6M ⊙ , where δ[O/Fe]∼ −1.2 and δ[Mg/Fe]∼ −0.6. Sodium is found to be produced in modest quantities at these low Z's, because the initial increase due to the combined effect of the second dredge-up and of 22 Ne burning is compensated by the later destruction via proton capture. A great increase by a factor ∼ 10 in the aluminium content of the envelope is also expected. These results can be used to understand the role played by intermediate mass stars in the self-enrichment scenario of globular clusters: the results from spectroscopic investigations of stars belonging to the second generation of clusters with different metallicity will be used as an indirect test of the reliability of the present yields.The treatment of mass loss and convection are confirmed as the main uncertainties affecting the results obtained in the context of the modeling of the thermal pulses phase. An indirect proof of this comes from the comparison with other investigations in the literature, based on a different prescription for the efficiency of convection in transporting energy and using a different recipe to determine the mass loss rate.
We use Hubble Space Telescope (HST) and ground-based imaging to study the multiple populations of 47 Tuc, combining high-precision photometry with
We investigate the relation between star formation rate (SFR) and stellar mass (M), i.e. the Main Sequence (MS) relation of star-forming galaxies, at 1.3 ≤ z < 6 in the first four HST Frontier Fields, based on rest-frame UV observations. Gravitational lensing combined with deep HST observations allows us to extend the analysis of the MS down to log M/M ⊙ ∼ 7.5 at z 4 and log M/M ⊙ ∼ 8 at higher redshifts, a factor of ∼10 below most previous results. We perform an accurate simulation to take into account the effect of observational uncertainties and correct for the Eddington bias. This step allows us to reliably measure the MS and in particular its slope. While the normalization increases with redshift, we fit an unevolving and approximately linear slope. We nicely extend to lower masses the results of brighter surveys. Thanks to the large dynamic range in mass and by making use of the simulation, we analyzed any possible mass dependence of the dispersion around the MS. We find tentative evidence that the scatter decreases with increasing mass, suggesting larger variety of star formation histories in low mass galaxies. This trend agrees with theoretical predictions, and is explained as either a consequence of the smaller number of progenitors of low mass galaxies in a hierarchical scenario and/or of the efficient but intermittent stellar feedback processes in low mass halos. Finally, we observe an increase in the SFR per unit stellar mass with redshift milder than predicted by theoretical models, implying a still incomplete understanding of the processes responsible for galaxy growth.
Observations of the pulsations of stars can be used to infer their interior structure and test theoretical models. The main-sequence γ Doradus (Dor) and δ Scuti (Sct) stars with masses 1.2-2.5 M are particularly useful for these studies. The γ Dor stars pulsate in high-order g-modes with periods of order 1 day, driven by convective blocking at the base of their envelope convection zone. The δ Sct stars pulsate in low-order g-and p-modes with periods of order 2 hr, driven by the κ mechanism operating in the He ii ionization zone. Theory predicts an overlap region in the Hertzsprung-Russell diagram between instability regions, where "hybrid" stars pulsating in both types of modes should exist. The two types of modes with properties governed by different portions of the stellar interior provide complementary model constraints. Among the known γ Dor and δ Sct stars, only four have been confirmed as hybrids. Now, analysis of combined Quarter 0 and Quarter 1 Kepler data for hundreds of variable stars shows that the frequency spectra are so rich that there are practically no pure δ Sct or γ Dor pulsators, i.e., essentially all of the stars show frequencies in both the δ Sct and the γ Dor frequency range. A new observational classification scheme is proposed that takes into account the amplitude as well as the frequency and is applied to categorize 234 stars as δ Sct, γ Dor, δ Sct/γ Dor or γ Dor/δ Sct hybrids.
The first detection of the period doubling phenomenon is reported in the Kepler RR Lyrae stars RR Lyr, V808 Cyg and V355 Lyr. Interestingly, all these pulsating stars show Blazhko modulation. The period doubling manifests itself as alternating maxima and minima of the pulsational cycles in the light curve, as well as through the appearance of half‐integer frequencies located halfway between the main pulsation period and its harmonics in the frequency spectrum. The effect was found to be stronger during certain phases of the modulation cycle. We were able to reproduce the period‐doubling bifurcation in our non‐linear RR Lyrae models computed by the Florida–Budapest hydrocode. This enabled us to trace the origin of this instability in RR Lyrae stars to a resonance, namely a 9:2 resonance between the fundamental mode and a high‐order (ninth) radial overtone showing strange‐mode characteristics. We discuss the connection of this new type of variation to the mysterious Blazhko effect and argue that it may give us fresh insights into solving this century‐old enigma.
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