ABSTRACT We study the M-type asymptotic giant branch (AGB) population of the Large Magellanic Cloud (LMC) by characterizing the individual sources in terms of the main properties of the progenitors ...and of the dust present in the circumstellar envelope. To this aim we compare the combination of the spectroscopic and photometric data collected by Spitzer, complemented by additional photometric results available in the literature, with results from AGB modelling that include the description of dust formation in the wind. To allow the interpretation of a paucity stars likely evolving through the post-AGB phase, we extended the available evolutionary sequences to reach the PN phase. The main motivation of the present analysis is to prepare the future observations of the evolved stellar populations of Local Group galaxies that will be done by the James Webb Space Telescope (JWST), by identifying the combination of filters that will maximize the possibilities of characterizing the observed sources. The present results show that for the M-star case the best planes to be used for this purpose are the colour magnitude (F770W–F2550W, F770W) and (KS–F770W, F770W) planes. In these observational diagrams the sequences of low-mass stars evolving in the AGB phases before the achievement of the C-star stage and of massive AGBs experiencing hot bottom burning are clearly separated and peculiar sources, such as post-AGB, dual-dust chemistry, and iron-dust stars can be easily identified.
We use images acquired with the Hubble Space Telescope Wide Field Camera 3 and new models to probe the horizontal branch (HB) population of the Galactic globular cluster (GC) NGC 2419. A detailed ...analysis of the composite HB highlights three populations: (1) the blue luminous HB, hosting standard helium stars (Y = 0.25) with a very small spread of mass; (2) a small population of stars with intermediate helium content (0.26 < Y ... 0.29); and (3) the well-populated extreme HB. We can fit the last group with models having high helium abundance (Y ~ 0.36), half of which (the hottest part, 'blue hook' stars) are identified as possible 'late flash mixed stars'. The initial helium abundance of this extreme population is in nice agreement with the predicted helium abundance in the ejecta of massive asymptotic giant branch (AGB) stars of the same metallicity as NGC 2419. This result further supports the hypothesis that second-generation stars in GCs formed from the ashes of intermediate-mass AGB stars. We find that the distribution in magnitude of the blue hook stars is larger than that predicted by theoretical models. We discuss the possible uncertainties in the magnitude scales and different attempts to model this group of stars. Finally, we suggest that consistency can be better achieved if we assume core masses larger than predicted by our models. This may be possible if the progenitors were fast rotators on the main sequence. If further study confirms this interpretation, a fast initial rotation would be a strong signature of the peculiarity of extreme second-generation stars in GCs. (ProQuest: ... denotes formulae/symbols omitted.)
All models for the formation of multiple populations in globular clusters (GCs) imply an initial mass of the systems several times greater than the present mass. A recent study of the dwarf ...spheroidal galaxy Fornax, where the low-metallicity (Fe/H −2) stars contained in GCs appear to account for ∼20 per cent of the total number, seems to constrain the initial mass of the four low-metallicity GCs in Fornax to be at most a factor of 5-6 greater than their present mass. We examine the photometric data for Fornax clusters, focusing our attention on their horizontal branch (HB) colour distribution and, when available, on the fraction and period distribution of RR Lyrae variables. Based on our understanding of the HB morphology in terms of varying helium content (and red giant mass-loss rate) in the context of multiple stellar generations, we show that the clusters F2, F3 and F5 must contain substantial fractions of second-generation stars (∼54-65 per cent). On the basis of a simple chemical evolution model we show that the helium distribution in these clusters can be reproduced by models with cluster initial masses ranging from values equal to ∼4 to ∼10 times greater than the current masses. Models with a very short second-generation star formation episode can also reproduce the observed helium distribution but require greater initial masses up to about 20 times the current mass. While the lower limit of this range of possible initial GC masses is consistent with those suggested by observations of the low-metallicity field stars, we also discuss the possibility that the metallicity scale of field stars (based on Ca ii triplet spectroscopy) and the metallicities derived for the clusters in Fornax may not be consistent with each other. In this case, observational constraints would allow greater initial cluster masses. Two interesting hypotheses are needed in order to reproduce the HB morphology of the clusters F2, F3 and F5. (i) The first-generation HB stars all lie at 'red' colours; that is, they populate only the RR Lyraes and the red HB region. According to this interpretation, the low-metallicity stars in the field of Fornax, populating the HB at colours bluer than the blue side (V − I)0 0.3 or (B − V)0 0.2 of the RR Lyraes, should be second-generation stars born in the clusters. A preliminary analysis of available colour surveys of Fornax field provides a fraction ∼20 per cent of blue HB stars, in the low-metallicity range. (ii) The mass loss from individual second-generation red giants is a few per cent of a solar mass larger than the mass loss from first-generation stars.
Erratum: DQWD Milone, A P; Vesperini, E; Marino, A F ...
Monthly Notices of the Royal Astronomical Society,
03/2020, Letnik:
492, Številka:
3
Journal Article
ABSTRACT The origin of extended main-sequence turn-offs (eMSTO) in star clusters younger than 2 Gyr still challenges our current understanding of stellar evolution. Exploiting data from Gaia Data ...Release 3 (DR3), we investigate eMSTOs in a large sample of 32 Galactic open clusters younger than 2.4 Gyr. We first validate Gaia rotational velocities from radial velocity spectrometer spectra by comparing them with literature values and assessing their correlation with magnetic activity measurements from LAMOST spectra. We detect a general positive correlation between turn-off colour and projected stellar rotation, with slow-rotating stars predominantly found on the bluer side of the turn-off. Comparing our observations with theoretical models, we find that the eMSTO morphology is well reproduced by a single population formed with a high rotation rate, and observed with rotation axis inclination ranging between 0° (pole-on) and 90° (edge-on). This contrasts with observations of Magellanic Cloud clusters, where a population of non-rotating stars appears to be ubiquitous in clusters younger than 700 Myr. However, we note that our interpretation, while successfully explaining the overall eMSTO morphology, cannot fully explain the observed projected rotational velocities. Additionally, two young clusters, NGC 3532 and NGC 2287, exhibit moderate evidence of a split-main sequence in colour and rotation, suggesting a possible small spread in the initial rotation rate. Finally, we advise caution in determining the ages of young clusters from non-rotating isochrones, as neglecting the effects of stellar rotation can impact the isochrone dating by up to factors of 5–20 per cent.
We present a new asteroseismic analysis of the stars in the globular cluster (GC) M4 based on the data collected by the K2 mission. We report the detection of solar-like oscillation in 37 stars, 32 ...red giant branch (RGB) and six red horizontal branch (rHB) stars, which is the largest sample for this kind of study in GCs up to date. Combining information from asteroseismology and multi-band photometry, we estimate both the masses and the radii of our targets. Our estimates are in agreement with independent sources, serving as a crucial verification of asteroseismology in the low metallicity regime. As M4 is an old GC, it hosts multiple stellar populations differing in light element abundances and in helium mass fraction. This generates a mass difference between the populations along the RGB, which in the case of M4 is estimated to be 0.017
M
⊙
. With this wealth of information, we can assign population membership and estimate the average mass of the stellar populations; however, the current uncertainties do not allow us to resolve this mass difference. The population membership and the seismic data of RGB and HB stars allow us, however, to assess the integrated mass loss along the RGB of the first generation stars in the cluster. We obtain Δ
M
= 0.227 ± 0.028
M
⊙
, which is in good agreement with independent estimates. Finally, we observe the presence of a statistically significant mass-temperature gradient in the rHB stars. This represents the first direct, model-independent observation of the colour-temperature-mass correlation predicted by the theory.
Context.
Knowledge of the
Gaia
, DR3 parallaxes of Galactic post-asymptotic giant branch (AGB) stars makes it possible to exploit these objects as tracers of AGB evolution, nucleosynthesis, and dust ...production as well as to use them to shed new light on still poorly known physical processes experienced by AGB stars.
Aims.
The goal of this study is to reconstruct the evolution and the dust formation processes during the final AGB phases of a sample of carbon-rich, post-AGB Galactic stars, with particular attention to the determination of the past mass-loss history.
Methods.
We study the IR excess of Galactic sources classified as post-AGB single stars by means of dust formation modelling where dust grains form and grow in a static wind and expand from the surface of the star. The method is applied to various evolutionary stages of the final AGB phase of stars with different masses and metallicities. The results from a spectral energy distribution (SED) fitting are used to infer information on mass loss, efficiency of dust formation, and wind dynamics.
Results.
The detailed analysis of the SED of the sources investigated, which included the derivation of the luminosities and the dust properties, allows us to confirm previous results, mostly based on the surface chemical composition, that most of the investigated sources descend from low-mass (
M
< 1.5
M
⊙
) progenitors that reached the C-star stage. Metal-poor carbon stars are characterised by higher IR excesses with respect to their more metal-rich counterparts of similar luminosity due to a higher surface carbon-to-oxygen excess. This work confirms previous conclusions based on a limited sample of carbon-rich post-AGB objects in the Magellanic Clouds, namely that more luminous stars descending from higher-mass progenitors are generally more opaque due to shorter evolutionary timescales that place the dust shell closer to the central object. Through the study of the dynamics of the outflow and results from stellar evolution modelling, we find that the mass-loss rate at the tip of the AGB phase of metal-rich low-mass carbon stars is approximately 1#x2212;1.5 × 10
−5
M
⊙
yr
−1
, whereas in the metal-poor domain
Ṁ
∼ 4 − 5 × 10
−5
M
⊙
yr
−1
is required. These results indicate the need for an upwards revision of the theoretical mass-loss rates of low-mass carbon stars in the available literature, which in turn require a revised determination of carbon dust yields by AGB stars.
Context.
The study of stars that evolve through the asymptotic giant branch (AGB) proves crucial in several astrophysical contexts because these objects provide important feedback to the host system ...in terms of the gas that is poured into the interstellar medium after being exposed to contamination from nucleosynthesis processes, and in terms of the dust that forms in their wind. Most of the studies conducted so far have been focused on AGB stars with solar and sub-solar chemical composition, whereas the extremely metal-poor domain has been poorly explored.
Aims.
We study the evolution of extremely metal-poor AGB stars with metallicities down to Fe/H = −5 to understand the main evolutionary properties and the efficiency of the processes able to alter their surface chemical composition, and to determine the gas and dust yields.
Methods.
We calculated two sets of evolutionary sequences of stars in the 1−7.5
M
⊙
mass range that evolved from the pre-main sequence to the end of the AGB phase. To explore the extremely metal-poor chemistries, we adopted the metallicities
Z
= 3 × 10
−5
and
Z
= 3 × 10
−7
, which correspond to Fe/H = −3 and Fe/H = −5, respectively. The results from stellar evolution modelling were used to calculate the yields of the individual chemical species. We also modelled dust formation in the wind to determine the dust produced by these objects.
Results.
The evolution of AGB stars in the extremely metal-poor domain we explored proves highly sensitive to the initial mass of the star.
M
≤ 2
M
⊙
stars experience several third-dredge-up events, which favour the gradual surface enrichment of
12
C and the formation of significant quantities of carbonaceous dust, ∼0.01
M
⊙
. The
13
C and nitrogen yields are found to be significantly smaller than in previous explorations of low-mass metal-poor AGB stars because the proton ingestion episodes experienced during the initial AGB phases are weaker.
M
≥ 5
M
⊙
stars experience hot bottom burning, and their surface chemistry reflects the equilibria of a very advanced proton-capture nucleosynthesis; little dust production takes place in their wind. Intermediate-mass stars experience both third dredge-up and hot bottom burning: they prove efficient producers of nitrogen, which is formed by proton captures on
12
C nuclei of primary origin dredged up from the internal regions.
Context.
Abundances of slow neutron-capture process (
s
-process) elements in stars with exquisite asteroseismic, spectroscopic, and astrometric constraints offer a novel opportunity to study stellar ...evolution, nucleosynthesis, and Galactic chemical evolution.
Aims.
We investigate one of the least studied
s
-process elements in the literature, cerium (Ce), using stars with asteroseismic constraints from the
Kepler
, K2, and TESS missions.
Methods.
We combined the global asteroseismic parameters derived from precise light curves obtained by the
Kepler
, K2, and TESS missions with stellar parameters and chemical abundances from the latest data release of the large spectroscopic survey APOGEE and astrometric data from the
Gaia
mission. Finally, we computed stellar ages using the code PARAM with a Bayesian estimation method.
Results.
We investigated the different trends of Ce/Fe as a function of metallicity,
α
/Fe, and age taking into account the dependence on the radial position, especially in the case of K2 targets, which cover a wide galactocentric range. We finally explored the Ce/
α
ratios as a function of age in different galactocentric intervals.
Conclusions
The studied trends display a strong dependence of the Ce abundances on the metallicity and star formation history. The Ce/Fe ratio shows a non-monotonic dependence on Fe/H with a peak around −0.2 dex. Moreover, younger stars have higher Ce/Fe and Ce/
α
ratios than older stars, confirming the latest contribution of low- and intermediate-mass asymptotic giant branch stars to the Galactic chemical enrichment. In addition, the trends of Ce/Fe and Ce/
α
with age become steeper moving towards the outer regions of the Galactic disc, demonstrating more intense star formation in the inner regions than in the outer regions. Cerium is thus a potentially interesting element to help constrain stellar yields and the inside-out formation of the Milky Way disc. However, the large scatter in all the relations studied here suggests that spectroscopic uncertainties for this element are still too large.
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