We present yields from stars of mass in the range M ≤ M ≤ 8 M of metallicities Z = 3 × 10−4 and 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.
Independent of metallicity, stars with M < 3 M are dominated by third dredge-up, thus ejecting into their surroundings gas enriched in carbon and nitrogen. Conversely, hot bottom burning is mainly responsible for the modification of the surface chemistry of more massive stars, whose mass exceeds 3 M: 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 ∼6 M, where δO/Fe ∼ −1.2 and δMg/Fe ∼ −0.6. Sodium is found to be produced in modest quantities at these low Zs, because the initial increase due to the combined effect of the second dredge-up and of 22Ne burning is compensated by the later destruction via proton capture. A great increase by a factor of ∼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 is confirmed as the main uncertainties affecting the results obtained in the context of the modelling 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 present deep spectroscopic observations of a Lyman break galaxy (LBG) candidate (hereafter MACS1149-JD) at z ∼ 9.5 with the Hubble Space Telescope (HST) WFC3/IR grisms. The grism observations were ...taken at four distinct position angles, totaling 34 orbits with the G141 grism, although only 19 of the orbits are relatively uncontaminated along the trace of MACS1149-JD. We fit a three-parameter (z, F160W mag, and Ly equivalent width EW) LBG template to the three least contaminated grism position angles using a Markov chain Monte Carlo approach. The grism data alone are best fit with a redshift of (68% confidence), in good agreement with our photometric estimate of (68% confidence). Our analysis rules out Ly emission from MACS1149-JD above a 3 EW of 21 , consistent with a highly neutral IGM. We explore a scenario where the red Spitzer/IRAC 3.6-4.5 color of the galaxy previously pointed out in the literature is due to strong rest-frame optical emission lines from a very young stellar population rather than a 4000 break. We find that while this can provide an explanation for the observed IRAC color, it requires a lower redshift (z 9.1), which is less preferred by the HST imaging data. The grism data are consistent with both scenarios, indicating that the red IRAC color can still be explained by a 4000 break, characteristic of a relatively evolved stellar population. In this interpretation, the photometry indicates that a Myr stellar population is already present in this galaxy only ∼500 Myr after the big bang.
The color-magnitude diagrams (CMDs) of Galactic open clusters are widely considered to be the prototypes of single stellar populations. By using photometry in ultraviolet and optical bands we ...discovered that the nearby young cluster NGC 6705 (M11) exhibits an extended main-sequence turnoff (eMSTO) and a broadened main sequence (MS). This is the first evidence of multiple stellar populations in a Galactic open cluster. By using high-resolution Very Large Telescope (VLT) spectra we provide direct evidence that the multiple sequences along the CMD correspond to stellar populations with different rotation rates. Specifically, the blue MS (bMS) is formed of slow-rotating stars, while red-MS (rMS) stars are fast rotators. Moreover, we exploit photometry from Gaia data release 2 (DR2) to show that three Galactic open clusters, namely NGC 2099, NGC 2360, and NGC 2818, exhibit the eMSTO, thus suggesting that it is a common feature among these objects. Our previous work on the Large Magellanic Cloud star cluster NGC 1818 shows that slowly and rapidly rotating stars populate the bMS and rMS observed in its CMD. The similarities between M11 and the young clusters of the Magellanic Clouds (MCs) suggest that rotation is responsible for the appearance of multiple populations in the CMDs of both Milky Way open clusters and MCs young clusters.
The stars in the Magellanic Clouds with the largest degree of obscuration are used to probe the highly uncertain physics of stars in the asymptotic giant branch (AGB) phase of evolution. Carbon stars ...in particular provide key information on the amount of third dredge-up and mass-loss. We use two independent stellar evolution codes to test how a different treatment of the physics affects the evolution on the AGB. The output from the two codes is used to determine the rates of dust formation in the circumstellar envelope, where the method used to determine the dust is the same for each case. The stars with the largest degree of obscuration in the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) are identified as the progeny of objects of initial mass 2.5–3 M⊙ and ∼1.5 M⊙, respectively. This difference in mass is motivated by the difference in the star formation histories of the two galaxies, and offers a simple explanation of the redder infrared colours of C-stars in the LMC compared to their counterparts in the SMC. The comparison with the Spitzer colours of C-rich AGB stars in the SMC shows that a minimum surface carbon mass fraction X(C) ∼ 5 × 10−3 must have been reached by stars of initial mass around 1.5 M⊙. Our results confirm the necessity of adopting low-temperature opacities in stellar evolutionary models of AGB stars. These opacities allow the stars to obtain mass-loss rates high enough (≳10−4 M⊙ yr−1) to produce the amount of dust needed to reproduce the Spitzer colours.
Ultra-faint galaxies are hosted by small dark matter halos with shallow gravitational potential wells, hence their star formation activity is more sensitive to feedback effects. The shape of the ...faint end of the high-z galaxy luminosity function (LF) contains important information on star formation and its interaction with the reionization process during the Epoch of Reionization. High-z galaxies with have only recently become accessible thanks to the Frontier Fields (FFs) survey combining deep HST imaging and the gravitational lensing effect. In this paper we investigate the faint end of the LF at redshift >5 using the data of FFs clusters Abell 2744 (A2744), MACSJ0416.1-2403 (M0416), MACSJ0717.5+3745 (M0717), and MACSJ1149.5+2223 (M1149). We analyze both an empirical and a physically motivated LF model to obtain constraints on a possible turnover of LF at faint magnitudes. In the empirical model the LF drops fast when the absolute UV magnitude is much larger than a turnover absolute UV magnitude . We obtain (15.2) at the 1 (2) confidence level (C.L.) for z ∼ 6. In the physically motivated analytical model, star formation in halos with circular velocity below is fully quenched if these halos are located in ionized regions. Using updated lensing models and new additional FFs data, we re-analyze previous constraints on and fesc presented by Castellano et al. using a smaller data set. We obtain new constraints on km s−1 and fesc 56% (both at 2 C.L.) and conclude that there is no turnover detected so far from the analyzed FFs data. Forthcoming JWST observations will be key to tightening these constraints further.
We present nucleosynthesis predictions (HeCNOCl) from asymptotic giant branch (AGB) models, with diffusive overshooting from all the convective borders, in the metallicity range Z.../4 < Z < 2 Z... ...They are compared to recent precise nebular abundances in a sample of Galactic planetary nebulae (PNe) that is divided among double-dust chemistry (DC) and oxygen-dust chemistry (OC) according to the infrared dust features. Unlike the similar subsample of Galactic carbon-dust chemistry PNe recently analysed by us, here the individual abundance errors, the higher metallicity spread, and the uncertain dust types/subtypes in some PNe do not allow a clear determination of the AGB progenitor masses (and formation epochs) for both PNe samples; the comparison is thus more focused on a object-by-object basis. The lowest metallicity OC PNe evolve from low-mass (~1 M...) O-rich AGBs, while the higher metallicity ones (all with uncertain dust classifications) display a chemical pattern similar to the DC PNe. In agreement with recent literature, the DC PNe mostly descend from high-mass (M greater than or equal to 3.5 M...) solar/supersolar metallicity AGBs that experience hot bottom burning (HBB), but other formation channels in low-mass AGBs like extra mixing, stellar rotation, binary interaction, or He pre-enrichment cannot be disregarded until more accurate C/O ratios would be obtained. Two objects among the DC PNe show the imprint of advanced CNO processing and deep second dredge-up, suggesting progenitors masses close to the limit to evolve as core collapse supernovae (above 6 M...). Their actual C/O ratio, if confirmed, indicate contamination from the third dredge-up, rejecting the hypothesis that the chemical composition of such high-metallicity massive AGBs is modified exclusively by HBB. (ProQuest: ... denotes formulae/symbols omitted.)
We interpret the stellar population of ω Centauri by means of a population synthesis analysis, following the most recent observational guidelines for input metallicities, helium and (C+N+O)/Fe ...contents. We deal at the same time with the main sequences, sub-giant and horizontal branch (HB) data. The reproduction of the observed colour–magnitude features is very satisfying and bears interesting hints concerning the evolutionary history of this peculiar stellar ensemble. Our main results are: (1) no significant spread in age is required to fit the colour–magnitude diagram. Indeed, we can use coeval isochrones for the synthetic populations, and we estimate that the ages fall within a ∼0.5 Gyr time interval; in particular the most metal-rich population can be coeval (in the above meaning) with the others, if its stars are very helium-rich (Y ∼ 0.37) and with the observed CNO enhancement ((C+N+O)/Fe = +0.7); (2) a satisfactory fit of the whole HB is obtained, consistent with the choice of the populations providing a good reproduction of the main sequence and sub-giant data; (3) the split in magnitude observed in the red HB is well reproduced assuming the presence of two stellar populations in the two different sequences observed: a metal-poor population made of stars evolving from the blue side (luminous branch) and a metal richer one whose stars are in a stage closer to the zero age HB (dimmer branch). This modelization also fits satisfactorily the period and the Fe/H distribution of the RR Lyrae stars.
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.
PDF
