arXiv subjects

Paola Marigo

Publications and source records attributed to Paola Marigo.

At least 55 records · Page 3Linked to original sources

arXiv2018

New PARSEC database of alpha-enhanced stellar evolutionary tracks and isochrones I. Calibration with 47 Tuc (NGC104) and the improvement on RGB bump

Precise studies on the Galactic bulge, globular cluster, Galactic halo and Galactic thick disk require stellar models with alpha enhancement and various values of helium content. These models are also important for extra-Galactic population synthesis studies. For this purpose, we complement the existing PARSEC models, which are based on the solar partition of heavy elements, with alpha-enhanced partitions. We collect detailed measurements on the metal mixture and helium abundance for the two populations of 47 Tuc (NGC 104) from the literature, and calculate stellar tracks and isochrones with these alpha-enhanced compositions. By fitting the precise color-magnitude diagram with HST ACS/WFC data, from low main sequence till horizontal branch, we calibrate some free parameters that are important for the evolution of low mass stars like the mixing at the bottom of the convective envelope. This new calibration significantly improves the prediction of the RGB bump brightness. Comparison with the observed RGB and HB luminosity functions also shows that the evolutionary lifetimes are correctly predicted. As a further result of this calibration process, we derive the age, distance modulus, reddening, and the red giant branch mass loss for 47 Tuc. We apply the new calibration and alpha-enhanced mixtures of the two 47 Tuc populations ( [alpha/Fe] ~0.4 and 0.2) to other metallicities. The new models reproduce the RGB bump observations much better than previous models. This new PARSEC database, with the newly updated alpha-enhanced stellar evolutionary tracks and isochrones, will also be part of the new stellar products for Gaia.

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arXiv2017

Estimating the dust production rate of carbon stars in the Small Magellanic Cloud

We employ newly computed grids of spectra reprocessed by dust for estimating the total dust production rate (DPR) of carbon stars in the Small Magellanic Cloud (SMC). For the first time, the grids of spectra are computed as a function of the main stellar parameters, i.e. mass-loss rate, luminosity, effective temperature, current stellar mass and element abundances at the photosphere, following a consistent, physically grounded scheme of dust growth coupled with stationary wind outflow. The model accounts for the dust growth of various dust species formed in the circumstellar envelopes of carbon stars, such as carbon dust, silicon carbide and metallic iron. In particular, we employ some selected combinations of optical constants and grain sizes for carbon dust which have been shown to reproduce simultaneously the most relevant color-color diagrams in the SMC. By employing our grids of models, we fit the spectral energy distributions of $\approx$3100 carbon stars in the SMC, consistently deriving some important dust and stellar properties, i.e. luminosities, mass-loss rates, gas-to-dust ratios, expansion velocities and dust chemistry. We discuss these properties and we compare some of them with observations in the Galaxy and LMC. We compute the DPR of carbon stars in the SMC, finding that the estimates provided by our method can be significantly different, between a factor $\approx2-5$, than the ones available in the literature. Our grids of models, including the spectra and other relevant dust and stellar quantities, are publicly available at http://starkey.astro.unipd.it/web/guest/dustymodels

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arXiv2017

A Remarkable Oxygen-rich Asymptotic Giant Branch Variable in the Sagittarius Dwarf Irregular Galaxy

We report and discuss JHKs photometry for Sgr dIG, a very metal-deficient galaxy in the Local Group, obtained over 3.5 years with the Infrared Survey Facility in South Africa. Three large amplitude asymptotic giant branch variables are identified. One is an oxygen-rich star that has a pulsation period of 950 days, that was until recently undergoing hot bottom burning, with Mbol~-6.7. It is surprising to find a variable of this sort in Sgr dIG, given their rarity in other dwarf irregulars. Despite its long period the star is relatively blue and is fainter, at all wavelengths shorter than 4.5microns, than anticipated from period-luminosity relations that describe hot bottom burning stars. A comparison with models suggests it had a main sequence mass Mi~5 times solar and that it is now near the end of its AGB evolution. The other two periodic variables are carbon stars with periods of 670 and 503 days (Mbol~-5.7 and -5.3). They are very similar to other such stars found on the AGB of metal deficient Local Group Galaxies and a comparison with models suggests Mi~3 times solar. We compare the number of AGB variables in Sgr dIG to those in NGC6822 and IC1613, and suggest that the differences may be due to the high specific star formation rate and low metallicity of Sgr dIG.

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arXiv2017

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.

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arXiv2017

A new generation of PARSEC-COLIBRI stellar isochrones including the TP-AGB phase

We introduce a new generation of PARSEC-COLIBRI stellar isochrones that include a detailed treatment of the thermally-pulsing asymptotic giant branch (TP-AGB) phase, and covering a wide range of initial metallicities (0.0001<Zi<0.06). Compared to previous releases, the main novelties and improvements are: use of new TP-AGB tracks and related atmosphere models and spectra for M and C-type stars; inclusion of the surface H+He+CNO abundances in the isochrone tables, accounting for the effects of diffusion, dredge-up episodes and hot-bottom burning; inclusion of complete thermal pulse cycles, with a complete description of the in-cycle changes in the stellar parameters; new pulsation models to describe the long-period variability in the fundamental and first overtone modes; new dust models that follow the growth of the grains during the AGB evolution, in combination with radiative transfer calculations for the reprocessing of the photospheric emission. Overall, these improvements are expected to lead to a more consistent and detailed description of properties of TP-AGB stars expected in resolved stellar populations, especially in regard to their mean photometric properties from optical to mid-infrared wavelengths. We illustrate the expected numbers of TP-AGB stars of different types in stellar populations covering a wide range of ages and initial metallicities, providing further details on the C-star island that appears at intermediate values of age and metallicity, and about the AGB-boosting effect that occurs at ages close to 1.6 Gyr for populations of all metallicities. The isochrones are available through a new dedicated web server.

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arXiv2017

Determining stellar parameters of asteroseismic targets: going beyond the use of scaling relations

Asteroseismic parameters allow us to measure the basic stellar properties of field giants observed far across the Galaxy. Most of such determinations are, up to now, based on simple scaling relations involving the large frequency separation, Δν, and the frequency of maximum power, ν$_{max}$. In this work, we implement Δν and the period spacing, ΔP, computed along detailed grids of stellar evolutionary tracks, into stellar isochrones and hence in a Bayesian method of parameter estimation. Tests with synthetic data reveal that masses and ages can be determined with typical precision of 5 and 19 per cent, respectively, provided precise seismic parameters are available. Adding independent information on the stellar luminosity, these values can decrease down to 3 and 10 per cent respectively. The application of these methods to NGC 6819 giants produces a mean age in agreement with those derived from isochrone fitting, and no evidence of systematic differences between RGB and RC stars. The age dispersion of NGC 6819 stars, however, is larger than expected, with at least part of the spread ascribable to stars that underwent mass-transfer events.

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arXiv2017

The adventure of carbon stars - Observations and modelling of a set of C-rich AGB stars

Modeling stellar atmospheres is a complex, intriguing task in modern astronomy. A systematic comparison of models with multi-technique observations is the only efficient way to constrain them. Aims: We performed a self-consistent modeling of the atmospheres of six C-rich AGB stars: R Lep, R Vol, Y Pav, AQ Sgr, U Hya and X TrA, with the aim of enlarging the knowledge of the dynamic processes occurring in their atmospheres. Methods. We used VLTI/MIDI interferometric observations, in combination with spectro-photometric data, and compared them with self-consistent dynamic models atmospheres. Results: The models can reproduce SED data well at wavelengths longwards of 1μm, and the interferometric observations between 8μm and 10μm. Discrepancies could be due to a combination of data- and model-related effects. The models best fitting the Miras are significantly extended, with a prominent shell-like structure, while the models best fitting the non-Miras are more compact, showing lower average mass-loss. The mass loss is of episodic or multi-periodic nature, but causes the visual amplitudes to be notably larger than the observed ones. Stellar parameters were derived from the model fitting: T_eff, L_bol, M, C/O, $\dot{\text{M}}$. Our findings agree well with literature values within the uncertainties. T_eff and L_bol are also in good agreement with the T derived from the angular diameter θ_{(V-K)} and L_bol from the SED fitting, except for AQ Sgr. Finally, θ_Ross and θ_(V-K) agree with each other better for the Miras than for the non-Miras,which is probably connected to the episodic nature of the latter models. We also located the stars in the H-R diagram, comparing them with evolutionary tracks. We found that the main derived properties from the model fitting are in good agreement with TP-AGB evolutionary calculations for carbon stars (COLIBRI code).

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arXiv2016

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 TP-AGB, for which we compute spectra and colors. Then we compare our modeled colors 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 reproduces several colors 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 colors greater than two magnitudes in some cases. We conclude that the complete set of observed NIR and MIR colors are best reproduced by small grains, with sizes between $\sim$0.035 and $\sim$0.12~$μ$m, rather than by large grains between $\sim0.2$ and $0.7$~$μ$m. The inability of large grains to reproduce NIR and MIR colors 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.

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arXiv2016

Evolution of Thermally Pulsing Asymptotic Giant Branch Stars V: Constraining the Mass Loss and Lifetimes of Intermediate Mass, Low Metallicity AGB Stars

Thermally-Pulsing Asymptotic Giant Branch (TP-AGB) stars are relatively short lived (less than a few Myr), yet their cool effective temperatures, high luminosities, efficient mass-loss and dust production can dramatically effect the chemical enrichment histories and the spectral energy distributions of their host galaxies. The ability to accurately model TP-AGB stars is critical to the interpretation of the integrated light of distant galaxies, especially in redder wavelengths. We continue previous efforts to constrain the evolution and lifetimes of TP-AGB stars by modeling their underlying stellar populations. Using Hubble Space Telescope (HST) optical and near-infrared photometry taken of 12 fields of 10 nearby galaxies imaged via the ACS Nearby Galaxy Survey Treasury and the near-infrared HST/SNAP follow-up campaign, we compare the model and observed TP-AGB luminosity functions as well as the number ratio of TP-AGB to red giant branch stars. We confirm the best-fitting mass-loss prescription, introduced by Rosenfield et al. 2014, in which two different wind regimes are active during the TP-AGB, significantly improves models of many galaxies that show evidence of recent star formation. This study extends previous efforts to constrain TP-AGB lifetimes to metallicities ranging -1.59 < [Fe/H] < -0.56 and initial TP-AGB masses up to ~ 4 Msun, which include TP-AGB stars that undergo hot-bottom burning.

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arXiv2016

Synthetic photometry for M and K giants and stellar evolution: hydrostatic dust-free model atmospheres and chemical abundances

Based on a grid of hydrostatic spherical COMARCS models for cool stars we have calculated observable properties of these objects, which will be mainly used in combination with stellar evolution tracks and population synthesis tools. The high resolution opacity sampling and low resolution convolved spectra as well as bolometric corrections for a large number of filter systems are made electronically available. We exploit those data to study the effect of mass, C/O ratio and nitrogen abundance on the photometry of K and M giants. Depending on effective temperature, surface gravity and the chosen wavelength ranges variations of the investigated parameters cause very weak to moderate and, in the case of C/O values close to one, even strong shifts of the colours. For the usage with stellar evolution calculations they will be treated as correction factors applied to the results of an interpolation in the main quantities. When we compare the synthetic photometry to observed relations and to data from the Galactic Bulge, we find in general a good agreement. Deviations appear for the coolest giants showing pulsations, mass loss and dust shells, which cannot be described by hydrostatic models.

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arXiv2015

Envelope Overshooting in Low Metallicity Intermediate- and High-mass Stars: a test with the Sagittarius Dwarf Irregular Galaxy

We check the performance of the {\sl\,PARSEC} tracks in reproducing the blue loops of intermediate age and young stellar populations at very low metallicity. We compute new evolutionary {\sl\,PARSEC} tracks of intermediate- and high-mass stars from 2\Msun to 350\Msun with enhanced envelope overshooting (EO), EO=2\HP and 4\HP, for very low metallicity, Z=0.0005. The input physics, including the mass-loss rate, has been described in {\sl\,PARSEC}~V1.2 version. By comparing the synthetic color-magnitude diagrams (CMDs) obtained from the different sets of models with envelope overshooting EO=0.7\HP (the standard {\sl\,PARSEC} tracks), 2\HP and 4\HP, with deep observations of the Sagittarius dwarf irregular galaxy (SagDIG), we find an overshooting scale EO=2\HP to best reproduce the observed loops. This result is consistent with that obtained by \citet{Tang_etal14} for Z in the range 0.001-0.004. We also discuss the dependence of the blue loop extension on the adopted instability criterion and find that, contrary to what stated in literature, the Schwarzschild criterion, instead of the Ledoux criterion, favours the development of blue loops. Other factors that could affect the CMD comparisons such as differential internal extinction or the presence of binary systems are found to have negligible effects on the results. We thus confirm that, in presence of core overshooting during the H-burning phase, a large envelope overshooting is needed to reproduce the main features of the central He-burning phase of intermediate- and high-mass stars.

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arXiv2015

Connecting the evolution of thermally pulsing asymptotic giant branch stars to the chemistry in their circumstellar envelopes -- I. The case of hydrogen cyanide

We investigate the formation of hydrogen cyanide (HCN) in the inner circumstellar envelopes of thermally pulsing asymptotic giant branch (TP-AGB) stars. A dynamic model for periodically shocked atmospheres, which includes an extended chemo-kinetic network, is for the first time coupled to detailed evolutionary tracks for the TP-AGB phase computed with the COLIBRI code. We carried out a calibration of the main shock parameters (the shock formation radius and the effective adiabatic index) using the circumstellar HCN abundances recently measured for a populous sample of pulsating TP-AGB stars. Our models recover the range of the observed HCN concentrations as a function of the mass-loss rates, and successfully reproduce the systematic increase of HCN moving along the M-S-C chemical sequence of TP-AGB stars, that traces the increase of the surface C/O ratio. The chemical calibration brings along two important implications: i) the first shock should emerge very close to the photosphere, and ii) shocks are expected to have a dominant isothermal character in the denser region close to the star (within ~ 3-4 R), implying that radiative processes should be quite efficient. Our analysis also suggests that the HCN concentrations in the inner circumstellar envelopes are critically affected by the H-H2 chemistry during the post-shock relaxation stages.

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arXiv2015

PARSEC evolutionary tracks of massive stars up to $350 M_\odot$ at metallicities 0.0001$\leq Z \leq$0.04

We complement the PARSEC data base of stellar evolutionary tracks with new models of massive stars, from the pre-main sequence phase to the central carbon ignition. We consider a broad range of metallicities, 0.0001$\leq Z \leq$0.04 and initial masses up to $M_{\rm ini}=350\,M_\odot$. The main difference with respect to our previous models of massive stars is the adoption of a recent formalism accounting for the mass-loss enhancement when the ratio of the stellar to the Eddington luminosity, $Γ_e$, approaches unity. With this new formalism, the models are able to reproduce the Humphreys-Davidson limit observed in the Galactic and Large Magellanic Cloud colour-magnitude diagrams, without an ad hoc mass-loss enhancement. We also follow the predictions of recent wind models indicating that the metallicity dependence of the mass-loss rates becomes shallower when $Γ_e$ approaches unity. We thus find that the more massive stars may suffer from substantial mass-loss even at low metallicity. We also predict that the Humphreys-Davidson limit should become brighter at decreasing metallicity. We supplement the evolutionary tracks with new tables of theoretical bolometric corrections, useful to compare tracks and isochrones with the observations. For this purpose, we homogenize existing stellar atmosphere libraries of hot and cool stars (PoWR, ATLAS9 and Phoenix) and we add, where needed, new atmosphere models computed with WM-basic. The mass, age and metallicity grids are fully adequate to perform detailed investigations of the properties of very young stellar systems, both in local and distant galaxies. The new tracks supersede the previous old Padova models of massive stars.

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arXiv2015

When do stars in 47 Tucanae lose their mass?

By examining the diffusion of young white dwarfs through the core of the globular cluster 47 Tucanae, we estimate the time when the progenitor star lost the bulk of its mass to become a white dwarf. According to stellar evolution models of the white-dwarf progenitors in 47 Tucanae, we find this epoch to coincide approximately with the star ascending the asymptotic giant branch ($3.0 \pm 8.1$ Myr before the tip of the AGB) and more than ninety million years after the helium flash (with ninety-percent confidence). From the diffusion of the young white dwarfs we can exclude the hypothesis that the bulk of the mass loss occurs on the red-giant branch at the four-sigma level. Furthermore, we find that the radial distribution of horizontal branch stars is consistent with that of the red-giant stars and upper-main-sequence stars and inconsistent with the loss of more than 0.2 solar masses on the red-giant branch at the six-sigma level.

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arXiv2015

Uncertainties on near-core mixing in red-clump stars: effects on the period spacing and on the luminosity of the AGB bump

Low-mass stars in the He-core-burning phase (HeCB) play a major role in stellar, galactic, and extragalactic astrophysics. The ability to predict accurately the properties of these stars, however, depends on our understanding of convection, which remains one of the key open questions in stellar modelling. We argue that the combination of the luminosity of the AGB bump (AGBb) and the period spacing of gravity modes (DP) during the HeCB phase, provides us with a decisive test to discriminate between competing models of these stars. We use the MESA, BaSTI, and PARSEC stellar evolution codes to model a typical giant star observed by Kepler. We explore how various near-core-mixing scenarios affect the predictions of the above-mentioned constraints, and we find that DP depends strongly on the prescription adopted. Moreover we show that the detailed behaviour of DP shows the signature of sharp variations in the Brunt-Vaisala frequency, which could potentially give additional information about near-core features. We find evidence for the AGBb among Kepler targets, and a first comparison with observations shows that, even if standard models are able to reproduce the luminosity distribution, no standard model can account for satisfactorily the period spacing of HeCB stars. Our analysis allows us to outline a candidate model to describe simultaneously the two observed distributions: a model with a moderate overshooting region characterized by an adiabatic thermal stratification. This prescription will be tested in the future on cluster stars, to limit possible observational biases.

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arXiv2015

Lithium evolution in metal-poor stars: from Pre-Main Sequence to the Spite plateau

Lithium abundance derived in metal-poor main sequence stars is about three times lower than the value of primordial Li predicted by the standard Big Bang nucleosynthesis when the baryon density is taken from the CMB or the deuterium measurements. This disagreement is generally referred as the lithium problem. We here reconsider the stellar Li evolution from the pre-main sequence to the end of the main sequence phase by introducing the effects of convective overshooting and residual mass accretion. We show that $^7$Li could be significantly depleted by convective overshooting in the pre-main sequence phase and then partially restored in the stellar atmosphere by a tail of matter accretion which follows the Li depletion phase and that could be regulated by EUV photo-evaporation. By considering the conventional nuclear burning and microscopic diffusion along the main sequence we can reproduce the Spite plateau for stars with initial mass $m_0=0.62 - 0.80 M_{\odot}$, and the Li declining branch for lower mass dwarfs, e.g, $m_0=0.57 - 0.60 M_{\odot}$, for a wide range of metallicities (Z=0.00001 to Z=0.0005), starting from an initial Li abundance $A({\rm Li}) =2.72$. This environmental Li evolution model also offers the possibility to interpret the decrease of Li abundance in extremely metal-poor stars, the Li disparities in spectroscopic binaries and the low Li abundance in planet hosting stars.

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arXiv2015

On the Interpretation of Sub-Giant Branch Morphologies of Intermediate-Age Star Clusters with Extended Main Sequence Turnoffs

Recent high-quality photometry of many star clusters in the Magellanic Clouds with ages of 1$\,-\,$2 Gyr revealed main sequence turnoffs (MSTOs) that are significantly wider than can be accounted for by a simple stellar population (SSP). Such extended MSTOs (eMSTOs) are often interpreted in terms of an age spread of several $10^8$ yr, challenging the traditional view of star clusters as being formed in a single star formation episode. Li et al. and Bastian & Niederhofer recently investigated the sub-giant branches (SGBs) of NGC 1651, NGC 1806, and NGC 1846, three star clusters in the Large Magellanic Cloud (LMC) that exhibit an eMSTO. They argued that the SGB of these star clusters can be explained only by a SSP. We study these and two other similar star clusters in the LMC, using extensive simulations of SSPs including unresolved binaries. We find that the shapes of the cross-SGB profiles of all star clusters in our sample are in fact consistent with their cross-MSTO profiles when the latter are interpreted as age distributions. Conversely, SGB morphologies of star clusters with eMSTOs are found to be inconsistent with those of simulated SSPs. Finally, we create PARSEC isochrones from tracks featuring a grid of convective overshoot levels and a very fine grid of stellar masses. A comparison of the observed photometry with these isochrones shows that the morphology of the red clump (RC) of such star clusters is also consistent with that implied by their MSTO in the age spread scenario. We conclude that the SGB and RC morphologies of star clusters featuring eMSTOs are consistent with the scenario in which the eMSTOs are caused by a distribution of stellar ages.

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arXiv2015

The AGB bump: a calibrator for the core mixing

The efficiency of convection in stars affects many aspects of their evolution and remains one of the key-open questions in stellar modelling. In particular, the size of the mixed core in core-He-burning low-mass stars is still uncertain and impacts the lifetime of this evolutionary phase and, e.g., the C/O profile in white dwarfs. One of the known observables related to the Horizontal Branch (HB) and Asymptotic Giant Branch (AGB) evolution is the AGB bump. Its luminosity depends on the position in mass of the helium-burning shell at its first ignition, that is affected by the extension of the central mixed region. In this preliminary work we show how various assumptions on near-core mixing and on the thermal stratification in the overshooting region affect the luminosity of the AGB bump, as well as the period spacing of gravity modes in core-He-burning models.

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