In
order to outline possible future directions in galaxy research, this book wants
to be a short stopover, a moment of self-reflection of the past century of achievements
in this area.
Since
the ...pioneering years of galaxy research in the early 20th century, the research
on galaxies has seen a relentless advance directly connected to the parallel
exponential growth of new technologies.
Through
a series of interviews with distinguished astronomers the editors provide a
snapshot of the achievements obtained in understanding galaxies. While many
initial questions about their nature have been addressed, many are still open
and require new efforts to achieve a solution. The discussions may reveal
paradigms worthwhile revisiting. With the help of some of those scientists who
have contributed to it, the editors sketch the history of this scientific
journey and ask them for inspirations for future directions of galaxy research.
ABSTRACT
Reliable models of the thermally pulsing asymptotic giant branch (TP-AGB) phase are of critical importance across astrophysics, including our interpretation of the spectral energy ...distribution of galaxies, cosmic dust production, and enrichment of the interstellar medium. With the aim of improving sets of stellar isochrones that include a detailed description of the TP-AGB phase, we extend our recent calibration of the AGB population in the Small Magellanic Cloud (SMC) to the more metal-rich Large Magellanic Cloud (LMC). We model the LMC stellar populations with the trilegal code, using the spatially resolved star formation history derived from the VISTA survey. We characterize the efficiency of the third dredge-up by matching the star counts and the Ks-band luminosity functions of the AGB stars identified in the LMC. In line with previous findings, we confirm that, compared to the SMC, the third dredge-up in AGB stars of the LMC is somewhat less efficient, as a consequence of the higher metallicity. The predicted range of initial mass of C-rich stars is between Mi ≈ 1.7 and 3 M⊙ at Zi = 0.008. We show how the inclusion of new opacity data in the carbon star spectra will improve the performance of our models. We discuss the predicted lifetimes, integrated luminosities, and mass-loss rate distributions of the calibrated models. The results of our calibration are included in updated stellar isochrones publicly available.
Dissecting the Gaia HR diagram within 200 pc Dal Tio, Piero; Mazzi, Alessandro; Girardi, Léo ...
Monthly notices of the Royal Astronomical Society,
10/2021, Letnik:
506, Številka:
4
Journal Article
Recenzirano
ABSTRACT
We analyse the high-quality Hertzsprung–Russell diagram (HRD) derived from Gaia data release 2 for the Solar Neighbourhood. We start building an almost complete sample within 200 pc and for ...|b| > 25○, so as to limit the impact of known errors and artefacts in the Gaia catalogue. Particular effort is then put into improving the modelling of population of binaries, which produce two marked features in the HRD: the sequence of near-equal mass binaries along the lower main sequence, and the isolated group of hot subdwarfs. We describe a new tool, BinaPSE, to follow the evolution of interacting binaries in a way that improves the consistency with PARSEC evolutionary tracks for single stars. BinaPSE is implemented into the TRILEGAL code for the generation of ‘partial models’ for both single and binary stellar populations, taking into account the presence of resolved and unresolved binaries. We then fit the Gaia HRD via Markov chain Monte Carlo methods that search for the star formation history and an initial binary fraction (by mass) that maximize the likelihood. The main results are (i) the binary fraction derived from the lower MS is close to 0.4, while twice larger values are favoured when the upper part of the HRD is fitted; (ii) present models predict the observed numbers of hot subdwarfs to within a factor of 2; and (iii) irrespective of the prescription for the binaries, the star formation rate peaks at values $\sim \!1.5\times 10^{-4}{\rm{M}_{\odot }}\mathrm{yr}^{-1}$ at ages slightly above 2 Gyr, and then decreases to $\sim \!0.8\times 10^{-4}{\rm{M}_{\odot }}\mathrm{yr}^{-1}$ at very old ages.
The early Universe had a chemical composition consisting of hydrogen, helium and traces of lithium; almost all other elements were subsequently created in stars and supernovae. The mass fraction of ...elements more massive than helium, Z, is known as 'metallicity'. A number of very metal-poor stars has been found, some of which have a low iron abundance but are rich in carbon, nitrogen and oxygen. For theoretical reasons and because of an observed absence of stars with Z < 1.5 × 10(-5), it has been suggested that low-mass stars cannot form from the primitive interstellar medium until it has been enriched above a critical value of Z, estimated to lie in the range 1.5 × 10(-8) to 1.5 × 10(-6) (ref. 8), although competing theories claiming the contrary do exist. (We use 'low-mass' here to mean a stellar mass of less than 0.8 solar masses, the stars that survive to the present day.) Here we report the chemical composition of a star in the Galactic halo with a very low Z (≤ 6.9 × 10(-7), which is 4.5 × 10(-5) times that of the Sun) and a chemical pattern typical of classical extremely metal-poor stars--that is, without enrichment of carbon, nitrogen and oxygen. This shows that low-mass stars can be formed at very low metallicity, that is, below the critical value of Z. Lithium is not detected, suggesting a low-metallicity extension of the previously observed trend in lithium depletion. Such lithium depletion implies that the stellar material must have experienced temperatures above two million kelvin in its history, given that this is necessary to destroy lithium.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
ABSTRACT
We recover the spatially resolved star formation history across the entire main body and Wing of the Small Magellanic Cloud (SMC), using 14 deep tile images from the VISTA survey of the ...Magellanic Clouds (VMC) in the YJKs filters. The analysis is performed on 168 subregions of size 0.143 deg2 covering a total contiguous area of 23.57 deg2. We apply a colour–magnitude diagram (CMD) reconstruction method that returns the best-fitting star formation rate SFR(t), age–metallicity relation, distance and mean reddening, together with their confidence intervals, for each subregion. With respect to previous analyses, we use a far larger set of the VMC data, updated stellar models, and fit the two available CMDs (Y − Ks versus Ks and J − Ks versus Ks) independently. The results allow us to derive a more complete and more reliable picture of how the mean distances, extinction values, star formation rate, and metallicities vary across the SMC, and provide a better description of the populations that form its Bar and Wing. We conclude that the SMC has formed a total mass of (5.31 ± 0.05) × 108 M⊙ in stars over its lifetime. About two-thirds of this mass is expected to be still locked in stars and stellar remnants. 50 per cent of the mass was formed prior to an age of 6.3 Gyr, and 80 per cent was formed between 8 and 3.5 Gyr ago. We also illustrate the likely distribution of stellar ages and metallicities in different parts of the CMD, to aid the interpretation of data from future astrometric and spectroscopic surveys of the SMC.
Context.The Sagittarius dwarf spheroidal galaxy is the nearest neighbor of the Milky Way. Moving along a short period quasi-polar orbit within the Halo, it is being destroyed by the tidal interaction ...with our Galaxy, losing its stellar content along a huge stellar stream. Aims.We study the detailed chemical composition of 12 giant stars in the Sagittarius dwarf Spheroidal main body, together with 5 more in the associated globular cluster Terzan 7, by means of high resolution VLT-UVES spectra. Methods.Abundances are derived for up to 21 elements from O to Nd, by fitting lines EW or line profiles against ATLAS 9 model atmospheres and SYNTHE spectral syntheses calculated ad-hoc. Temperatures are derived from $(V-I)_0$ or $(B-V)_0$ colors and gravities from $\ion{Fe}{i}$ – $\ion{Fe}{ii}$ ionization equilibrium. Results.The metallicity of the observed stars is between Fe/H = -0.9 and 0. We detected a highly peculiar “chemical signature”, with undersolar α elements, Na, Al, Sc, V, Co, Ni, Cu, and Zn, among others, and overabundant La, Ce, and Nd. Many of these abundance ratios (in particular light-odd elements and iron peak ones) are strongly at odds with what is observed within the Milky Way, so they may be a very useful tool for recognizing populations originating within the Sagittarius dwarf. This can be clearly seen in the case of the globular Palomar 12, which is believed to have been stripped from Sagittarius: the cluster shows precisely the same chemical “oddities”, thus finally confirming its extragalactic origin.
ABSTRACT
Starting from the Gaia DR3 HR diagram, we derive the star formation history (SFH) as a function of distance from the Galactic plane within a cylinder centred on the Sun with a 200 pc radius ...and spanning 1.3 kpc above and below the Galaxy’s midplane. We quantify both the concentration of the more recent star formation in the Galactic plane, and the age-related increase in the scale height of the Galactic disc stellar component, which is well-described by power laws with indices ranging from 1/2 to 2/3. The vertically-integrated star formation rate falls from $(1.147 \pm 0.039)\times 10^{-8}\, \text{M}_\odot \, \text{yr}^{-1} \, \text{pc}^{-2}$ at earlier times down to $(6.2 \pm 3.0) \times 10^{-9}\, \text{M}_\odot \, \text{yr}^{-1} \, \text{pc}^{-2}$ at present times, but we find a significant peak of star formation in the 2–3 Gyr age bin. The total mass of stars formed per unit area over time is $118.7 \pm 6.2\, \text{M}_\odot \, \text{pc}^{-2}$, which is nearly twice the present stellar mass derived from kinematics within 1 kpc from the Galactic plane, implying a high degree of matter recycling in successive generations of stars. The method is then modified by adopting an age-dependent correlation between the SFH across the different slices, which results in less noisy and more symmetrical results without significantly changing the previously mentioned quantities. This appears to be a promising way to improve SFH recovery in external galaxies.
Abstract
We analyze the oxygen abundances of a stellar sample representative of the two major Galactic populations: the thin and thick disks. The aim is to investigate the differences between members ...of the Galactic disks and contribute to the understanding of the origin of oxygen chemical enrichment in the Galaxy. The analysis is based on the O
i
= 6300.30 Å oxygen line in high-resolution spectra (
R
∼ 52,500) obtained from the Gaia-ESO public spectroscopic Survey (GES). By comparing the observed spectra with a theoretical data set computed in LTE with the SPECTRUM synthesis and ATLAS12 codes, we derive the oxygen abundances of 516 FGK dwarfs for which we have previously measured carbon abundances. Based on kinematic, chemical, and dynamical considerations, we identify 20 thin and 365 thick disk members. We study the potential trends of both subsamples in terms of their chemistry (O/H, O/Fe, O/Mg, and C/O versus Fe/H and Mg/H), age, and position in the Galaxy. The main results are that (a) O/H and O/Fe ratios versus Fe/H show systematic differences between thin and thick disk stars with an enhanced O abundance of thick disk stars with respect to thin disk members and a monotonic decrement of O/Fe with increasing metallicity, even at metal-rich regime; (b) there is a smooth correlation of O/Mg with age in both populations, suggesting that this abundance ratio can be a good proxy of stellar ages within the Milky Way; and (c) thin disk members with Fe/H ≃ 0 display a C/O ratio smaller than the solar value, suggesting a possibly outward migration of the Sun from lower Galactocentric radii.
Context. Elemental correlations and anti-correlations are known to be present in globular clusters (GCs) owing to pollution by CNO cycled gas. Because of its fragility Li is destroyed at the ...temperature at which the CNO cycling occurs, and this makes Li a crucial study for the nature of the contaminating stars. Aims. We observed 112 un-evolved stars at the Turnoff of the NGC 6752 cluster with FLAMES at the VLT to investigate the presence and the extent of a Li-O correlation. This correlation is expected if there is a simple pollution scenario. Methods. Li (670.8 nm) and O triplet (771 nm) abundances are derived in NLTE. All stars belong to a very narrow region of the color-magnitude diagram, so they have similar stellar parameters (Teff, log g). Results. We find that O and Li correlate, with a high statistical significance that confirms the early results for this cluster. At first glance this is what is expected if a simple pollution of pristine gas with CNO cycled gas (O-poor, Li-poor) occurred. The slope of the relationship, however, is about 0.4, and differs from unity by over 7σ. A slope of one is the value predicted for a pure contamination model. Conclusions. We confirm an extended Li-O correlation in non evolved stars of NGC 6752. At the same time the characteristic of the correlation shows that a simple pollution scenario is not sufficient to explain the observations. Within this scenario the contaminant gas must have been enriched in Li. This would rule out massive stars as main polluters, and favor the hypothesis that the polluting gas was enriched by intermediate or high-mass AGB stars, unless the former can be shown to be able to produce Li. According to our observations, the fraction of polluting gas contained in the stars observed is a considerable fraction of the stellar mass of the cluster.
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