Calibrating the thermally-pulsing asymptotic giant branch phase through resolved stellar populations in nearby galaxies

Most of the physical processes driving the Thermally-Pulsing Asymptotic Giant Branch (TP-AGB) evolution are not yet fully understood and they need to be modelled with parametrised descriptions. The uncertainties of the models affect the interpretation of the spectrophotometric properties of galaxies up to high-redshift. In the framework of the ERC - STARKEY project, the aim of this Thesis is to constrain the uncertain parameters, i.e. third dredge-up and mass-loss, that still affect the TP-AGB models. To this purpose, I perform detailed simulations of AGB star populations in the Small Magellanic Cloud (SMC) based on robust measurements of the space-resolved star formation history as derived from the deep near-infrared photometry of the VISTA survey of the Magellanic Clouds. I compare the resulting synthetic catalogues with high-quality observations of resolved stellar populations in the infrared passbands of 2MASS and Spitzer. A large grid of TP-AGB evolutionary tracks is computed with several combinations of third dredge-up and mass-loss prescriptions. By requiring the models to reproduce the star counts and the luminosity functions of the observed Oxygen-, Carbon-rich and extreme-AGB stars, I put quantitative constraints on the efficiencies of the third dredge-up and mass-loss. The observed luminosity functions in all the available infrared photometric filters are successfully reproduced by two set of models, one with a relatively high mass-loss efficiency for Oxygen-rich stars and the second with a lower mass-loss efficiency and a lower efficiency of the third dredge-up for the more massive TP-AGB stars, i.e. initial masses larger than three solar masses. On the basis of the best-fitting model I present a complete characterisation of the AGB population in terms of stellar parameters, including the predicted mass-loss rates, initial masses, and Carbon-to-Oxygen ratio. I use the TP-AGB models calibrated in the SMC to model the population of Long Period Variables (LPVs) in the Large Magellanic Cloud as observed by Gaia. The remarkable agreement between models and observations allows us to guide the interpretation of a new observational diagram that is able to photometrically distinguish the evolutionary stages, the initial masses and the chemical type of these stars. In the context of the Large Synoptic Survey Telescope (LSST) science collaboration, I produce catalogues containing the synthetic photometry of the Magellanic Clouds in the Gaia and LSST filters. These catalogues, together with the all-sky simulations of the Milky Way will be made available to the community through the NOAO Data Lab to help defining the observing strategy of the LSST mini-surveys. In addition, I simulate samples of AGB stars in Local Group dwarf galaxies and find a general agreement with the data. However, to properly consider these objects in the TP-AGB models calibration, the simulations should be improved to take into account the crowding effects and the different areas used for the star formation histories derivation and the AGB stars identification. Finally, the products of this work, namely calibrated stellar isochrones and pulsation periods of LPVs, will be publicly available and ready to use for the interpretation of the data coming from present and future observing facilities. The calibrated TP-AGB models may be included in population synthesis models used to probe the integrated light of galaxies in the extragalactic Universe

A new interpretation of the period-luminosity sequences of long-period variables

Period-luminosity (PL) sequences of long period variables (LPVs) are commonly interpreted as different pulsation modes, but there is disagreement on the modal assignment. Here, we re-examine the observed PL sequences in the Large Magellanic Cloud, including the sequence of long secondary periods (LSPs), and their associated pulsation modes. Firstly, we theoretically model the sequences using linear, radial, non-adiabatic pulsation models and a population synthesis model of the LMC red giants. Then, we use a semi-empirical approach to assign modes to the pulsation sequences by exploiting observed multi-mode pulsators. As a result of the combined approaches, we consistently find that sequences B and C$^{\prime}$ both correspond to first overtone pulsation, although there are some fundamental mode pulsators at low luminosities on both sequences. The masses of these fundamental mode pulsators are larger at a given luminosity than the mass of the first overtone pulsators. These two sequences B and C$^{\prime}$ are separated by a small period interval in which large amplitude pulsation in a long secondary period (sequence D variability) occurs, meaning that the first overtone pulsation is not seen as the primary mode of pulsation. Observationally, this leads to the splitting of the first overtone pulsation sequence into the two observed sequences B and C$^{\prime}$. Our two independent examinations also show that sequences A$^{\prime}$, A and C correspond to third overtone, second overtone and fundamental mode pulsation, respectively.Comment: 10 pages, 7 figures, accepted for publication in Ap

Modelling Long-Period Variables -- II. Fundamental mode pulsation in the nonlinear regime

Long-period variability in luminous red giants has several promising applications, all of which require models able to accurately predict pulsation periods. Linear pulsation models have proven successful in reproducing the observed periods of overtone modes in evolved red giants, but they fail to accurately predict their fundamental mode periods. Here, we use a 1D hydrodynamic code to investigate the long-period variability of M-type asymptotic giant branch stars in the nonlinear regime. We examine the period and stability of low-order radial pulsation modes as a function of mass and radius, and find overtone mode periods in complete agreement with predictions from linear pulsation models. In contrast, nonlinear models predict an earlier onset of dominant fundamental mode pulsation, and shorter periods at large radii. Both features lead to a substantially better agreement with observations, that we verify against OGLE and Gaia data for the Magellanic Clouds. We provide simple analytic relations describing the nonlinear fundamental mode period-mass-radius relation. Differences with respect to linear predictions originate from the readjustment of the envelope structure induced by large-amplitude pulsation. We investigate the impact of turbulent viscosity on linear and nonlinear pulsation, and probe possible effects of varying metallicity and carbon abundance.Comment: 18 pages, 17 figures; accepted for publication in MNRA

Constraining dust properties in circumstellar envelopes of C-stars in the Small Magellanic Cloud: optical constants and grain size of carbon dust

We present a new approach aimed at constraining the typical size and optical properties of carbon dust grains in circumstellar envelopes (CSEs) of carbon-rich stars (C-stars) in the Small Magellanic Cloud (SMC). To achieve this goal, we apply our recent dust growth description, coupled with a radiative transfer code to the CSEs of C-stars evolving along the thermally pulsing asymptotic giant branch, for which we compute spectra and colours. Then, we compare our modelled colours in the near- and mid-infrared (NIR and MIR) bands with the observed ones, testing different assumptions in our dust scheme and employing several data sets of optical constants for carbon dust available in the literature. Different assumptions adopted in our dust scheme change the typical size of the carbon grains produced. We constrain carbon dust properties by selecting the combination of grain size and optical constants which best reproduce several colours in the NIR and MIR at the same time. The different choices of optical properties and grain size lead to differences in the NIR and MIR colours greater than 2 mag in some cases. We conclude that the complete set of observed NIR and MIR colours are best reproduced by small grains, with sizes between ~0.035 and ~0.12 μm, rather than by large grains between ~0.2 and 0.7 μm. The inability of large grains to reproduce NIR and MIR colours seems independent of the adopted optical data set. We also find a possible trend of the grain size with mass-loss and/or carbon excess in the CSEs of these stars. © 2016 The Authors

YBC: a stellar bolometric corrections database with variable extinction coefficients: application to PARSEC isochrones

We present the YBC database of stellar bolometric corrections, in which we homogenise widely used theoretical stellar spectral libraries and provide BCs for many popular photometric systems, including Gaia filters. The database can easily be extended to additional photometric systems and stellar spectral libraries. The web interface allows users to transform their catalogue of theoretical stellar parameters into magnitudes and colours of selected filter sets. The BC tables can be downloaded or implemented into large simulation projects using the interpolation code provided with the database. We computed extinction coefficients on a star-by-star basis, hence taking into account the effects of spectral type and non-linearity dependency on the total extinction. We illustrate the use of these BCs in PARSEC isochrones. We show that using spectral-type dependent extinction coefficients is necessary for Gaia filters whenever A(V)greater than or similar to 0.5 mag. Bolometric correction tables for rotating stars and tables of limb-darkening coefficients are also provided

The first comprehensive Milky Way stellar mock catalogue for the Chinese Space Station Telescope Survey Camera

The Chinese Space Station Telescope (CSST) is a cutting-edge two-meter astronomical space telescope currently under construction. Its primary Survey Camera (SC) is designed to conduct large-area imaging sky surveys using a sophisticated seven-band photometric system. The resulting data will provide unprecedented data for studying the structure and stellar populations of the Milky Way. To support the CSST development and scientific projects related to its survey data, we generate the first comprehensive Milky Way stellar mock catalogue for the CSST SC photometric system using the TRILEGAL stellar population synthesis tool. The catalogue includes approximately 12.6 billion stars, covering a wide range of stellar parameters, photometry, astrometry, and kinematics, with magnitude reaching down to g=27.5 mag in the AB magnitude system. The catalogue represents our benchmark understanding of the stellar populations in the Milky Way, enabling a direct comparison with the future CSST survey data. Particularly, it sheds light on faint stars that are hidden from current sky surveys. Our crowding limit analysis based on this catalogue provides compelling evidence for the extension of the CSST Optical Survey (OS) to cover low Galactic latitude regions. The strategic extension of the CSST-OS coverage, combined with this comprehensive mock catalogue, will enable transformative science with the CSST

Dissecting the Gaia HR diagram II. The vertical structure of the star formation history across the Solar Cylinder

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 ± 0.039) × 10-8 M⊙yr-1pc-2 at earlier times down to (6.2 ± 3.0) × 10-9 M⊙yr-1pc-2 at present times, but we find a significant peak of star formation in the 2 to 3 Gyr age bin. The total mass of stars formed per unit area over time is 118.7 ± 6.2 M⊙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

Dissecting the Gaia HR diagram II. The vertical structure of the star formation history across the Solar Cylinder

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 $(1.147 \pm 0.039)\times10^{-8}\, \text{M}_{\odot} \text{yr}^{-1} \text{pc}^{-2}$ at earlier times down to $(6.2 \pm 3.0) \times10^{-9}\, \text{M}_{\odot} \text{yr}^{-1} \text{pc}^{-2}$ at present times, but we find a significant peak of star formation in the 2 to 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.Comment: Accepted for publication in MNRA

Constraining the thermally pulsing asymptotic giant branch phase with resolved stellar populations in the Small Magellanic Cloud

The thermally pulsing asymptotic giant branch (TP-AGB) experienced by low- and intermediate-mass stars is one of the most uncertain phases of stellar evolution and the models need to be calibrated with the aid of observations. To this purpose, we couple high-quality observations of resolved stars in the Small Magellanic Cloud (SMC) with detailed stellar population synthesis simulations computed with the TRILEGAL code. The strength of our approach relies on the detailed spatially resolved star formation history of the SMC, derived from the deep near-infrared photometry of the VISTA survey of the Magellanic Clouds, as well as on the capability to quickly and accurately explore a wide variety of parameters and effects with the COLIBRI code for the TP-AGB evolution. Adopting a well-characterized set of observations - star counts and luminosity functions - we set up a calibration cycle along which we iteratively change a few key parameters of the TP-AGB models until we eventually reach a good fit to the observations. Our work leads to identify two best-fitting models that mainly differ in the efficiencies of the third dredge-up and mass-loss in TP-AGB stars with initial masses larger than about 3 M⊙. On the basis of these calibrated models, we provide a full characterization of the TP-AGB stellar population in the SMC in terms of stellar parameters (initial masses, C/O ratios, carbon excess, mass-loss rates). Extensive tables of isochrones including these improved models are publicly available.This work is supported by the European Research Council (ERC) Consolidator Grant funding scheme (project STARKEY, G.A. n. 615604). We thank the entire VMC team for producing the spaceresolved SFH used in this work and J. Cummings and J. Kalirai for kindly providing us with their IFMR data. MRLC acknowledges funding from the ERC under the European Union’s Horizon 2020 research and innovation programme (G.A. n. 682115). Many thanks go to C. Maraston, S. Charlot, and G. Bruzual for providing us with their stellar population synthesis models