Calibrated stellar models for metal-poor populations (original) (raw)
Related papers
Stellar evolution models at the Magellanic Cloud metallicities
Proceedings of the …, 2008
The Magellanic Clouds are great laboratories to study the evolution of stars at two metallicities lower than solar. They provide excellent testbeds for stellar evolution theory and in particular for the impact of metallicity on stellar evolution. It is important to test stellar evolution models at metallicities lower than solar in order to use the models to predict the evolution and properties of the first stars. In these proceedings, after recalling the effects of metallicity, we present stellar evolution models including the effects of rotation at the Magellanic Clouds metallicities. We then compare the models to various observations (ratios of subgroups of massive stars and supernovae, nitrogen surface enrichment and gamma-ray bursts) and show that the models including the effects of rotation reproduce most of the observational constraints.
The Astrophysical …, 2011
We present new analysis of 11 intermediate-age (1-2 Gyr) star clusters in the Large Magellanic Cloud based on Hubble Space Telescope imaging data. Seven of the clusters feature main sequence turnoff (MSTO) regions that are wider than can be accounted for by a simple stellar population, whereas their red giant branches indicate a single value of [Fe/H]. The star clusters cover a range in present-day mass from about 1 × 10 4 M ⊙ to 2 × 10 5 M ⊙ . We compare radial distributions of stars in the upper and lower parts of the MSTO region, and calculate cluster masses and escape velocities from the present time back to a cluster age of 10 Myr. Our main result is that for all clusters in our sample with estimated escape velocities v esc 15 km s −1 at an age of 10 Myr, the stars in the brightest half of the MSTO region are significantly more centrally concentrated than the stars in the faintest half and more massive red giant branch and asymptotic giant branch stars. This is not the case for clusters with v esc 10 km s −1 at an age of 10 Myr. We argue that the wide MSTO region of such clusters is mainly caused by to a ∼ 200 − 500 Myr range in the ages of cluster stars due to extended star formation within the cluster from material shed by first-generation stars featuring slow stellar winds. Dilution of this enriched material by accretion of ambient interstellar matter is deemed plausible if the spread of [Fe/H] in this ambient gas was very small when the second-generation stars were formed in the cluster.
2013
In the recent controversy about the role of TP-AGB stars in evolutionary population synthesis (EPS) models of galaxies, one particular aspect is puzzling: TP-AGB models aimed at reproducing the lifetimes and integrated fluxes of the TP-AGB phase in Magellanic Cloud (MC) clusters, when incorporated into EPS models, are found to overestimate, to various extents, the TP-AGB contribution in resolved star counts and integrated spectra of galaxies. In this paper, we call attention to a particular evolutionary aspect, linked to the physics of stellar interiors, that in all probability is the main cause of this conundrum. As soon as stellar populations intercept the ages at which RGB stars first appear, a sudden and abrupt change in the lifetime of the core He-burning phase causes a temporary "boost" in the production rate of subsequent evolutionary phases, including the TP-AGB. For a timespan of about 0.1 Gyr, triple TP-AGB branches develop at slightly different initial masses, causing their frequency and contribution to the integrated luminosity of the stellar population to increase by a factor of ∼ 2. The boost occurs for turn-off masses of ∼ 1.75M , just in the proximity of the expected peak in the TP-AGB lifetimes (for MC metallicities), and for ages of ∼ 1.6 Gyr. Coincidently, this relatively narrow age interval happens to contain the few very massive MC clusters that host most of the TP-AGB stars used to constrain stellar evolution and EPS models. This concomitance makes the AGB-boosting particularly insidious in the context of present EPS models. As we discuss in this paper, the identification of this evolutionary effect brings about three main consequences. First, we claim that present estimates of the TP-AGB contribution to the integrated light of galaxies derived from MC clusters, are biased towards too large values. Second, the relative TP-AGB contribution of single-burst populations falling in this critical age range cannot be accurately derived by approximations such as the fuel consumption theorem, that ignore, by construction, the above evolutionary effect. Third, a careful revision of AGB star populations in intermediate-age MC clusters is urgently demanded, promisingly with the aid of detailed sets of stellar isochrones.
Astronomy and Astrophysics, 2007
We have studied the optical spectra of a sample of 28 O-and early B-type stars in the Large Magellanic Cloud, 22 of which are associated with the young star forming region N11. Our observations sample the central associations of LH9 and LH10, and the surrounding regions. Stellar parameters are determined using an automated fitting method , which combines the stellar atmosphere code fastwind with the genetic algorithm based optimisation routine pikaia (Charbonneau 1995). We derive an age of 7.0 ± 1.0 and 3.0 ± 1.0 Myr for LH9 and LH10, respectively. The age difference and relative distance of the associations are consistent with a sequential star formation scenario in which stellar activity in LH9 triggered the formation of LH10. Our sample contains four stars of spectral type O2. From helium and hydrogen line fitting we find the hottest three of these stars to be ∼49−54 kK (compared to ∼45−46 kK for O3 stars). Detailed determination of the helium mass fraction reveals that the masses of helium enriched dwarfs and giants derived in our spectroscopic analysis are systematically lower than those implied by non-rotating evolutionary tracks. We interpret this as evidence for efficient rotationally enhanced mixing leading to the surfacing of primary helium and to an increase of the stellar luminosity. This result is consistent with findings for SMC stars by . For bright giants and supergiants no such mass discrepancy is found; these stars therefore appear to follow tracks of modestly or non-rotating objects. The set of programme stars was sufficiently large to establish the mass loss rates of OB stars in this Z ∼ 1/2 Z environment sufficiently accurate to allow for a quantitative comparison with similar objects in the Galaxy and the SMC. The mass loss properties are found to be intermediate to massive stars in the Galaxy and SMC. Comparing the derived modified wind momenta D mom as a function of luminosity with predictions for LMC metallicities by yields good agreement in the entire luminosity range that was investigated, i.e. 5.0 < log L/L < 6.1.
Calibrating Stellar Population Models with Magellanic Cloud Star Clusters
The Astrophysical Journal, 2013
Stellar population models are commonly calculated using star clusters as calibrators for those evolutionary stages that depend on free parameters. However, discrepancies exist among different models, even if similar sets of calibration clusters are used. With the aim of understanding these discrepancies, and of improving the calibration procedure, we consider a set of 43 Magellanic Cloud (MC) clusters taking age and photometric information from the literature. We carefully assign ages to each cluster based on up-to-date determinations ensuring that these are as homogeneous as possible. To cope with statistical fluctuations, we stack the clusters in five age bins deriving for each of them integrated luminosities and colors. We find that clusters become abruptly red in optical and optical-IR colors as they age from ∼ 0.6 to ∼1 Gyr, which we interpret as due to the development of a well-populated thermally pulsing asymptotic giant branch (TP-AGB). We argue that other studies missed this detection because of coarser age binnings. Maraston (2005) and Girardi et al. (2010) models predict the presence of a populated TP-AGB at ∼0.6 Gyr, with a correspondingly very red integrated color, at variance with the data; Bruzual & Charlot (2003) and Conroy et al. (2009) models run within the error bars at all ages. The discrepancy between the synthetic colors of Maraston (2005) models and the average colors of MC clusters results from the now obsolete age scale adopted. Finally, our finding that the TP-AGB phase appears to develop between ∼ 0.6 − 1 Gyr is dependent on the adopted age scale for the clusters and may have important implications for stellar evolution.
The Pisa Stellar Evolution Data Base for low-mass stars
Astronomy & Astrophysics, 2012
Context. The last decade showed an impressive observational effort from the photometric and spectroscopic point of view for ancient stellar clusters in our Galaxy and beyond, leading to important and sometimes surprising results. Aims. The theoretical interpretation of these new observational results requires updated evolutionary models and isochrones spanning a wide range of chemical composition so that the possibility of multipopulations inside a stellar cluster is also taken also into account. Methods. With this aim we built the new "Pisa Stellar Evolution Database" of stellar models and isochrones by adopting a welltested evolutionary code (FRANEC) implemented with updated physical and chemical inputs. In particular, our code adopts realistic atmosphere models and an updated equation of state, nuclear reaction rates and opacities calculated with recent solar elements mixture. Results. A total of 32 646 models have been computed in the range of initial masses 0.30 ÷ 1.10 M for a grid of 216 chemical compositions with the fractional metal abundance in mass, Z, ranging from 0.0001 to 0.01, and the original helium content, Y, from 0.25 to 0.42. Models were computed for both solar-scaled and α-enhanced abundances with different external convection efficiencies. Correspondingly, 9720 isochrones were computed in the age range 8÷15 Gyr, in time steps of 0.5 Gyr. The whole database is available to the scientific community on the web. Models and isochrones were compared with recent calculations available in the literature and with the color-magnitude diagram of selected Galactic globular clusters. The dependence of relevant evolutionary quantities, namely turn-off and horizontal branch luminosities, on the chemical composition and convection efficiency were analyzed in a quantitative statistical way and analytical formulations were made available for reader's convenience. These relations can be useful in several fields of stellar evolution, e.g. evolutionary properties of binary systems, synthetic models for simple stellar populations and for star counts in galaxies, and chemical evolution models of galaxies.
Intermediate‐Mass Stars: Updated Models
The Astrophysical Journal, 1999
A new set of stellar models in the mass range 1.2 to 9 M ⊙ is presented. The adopted chemical compositions cover the typical galactic values, namely 0.0001 ≤ Z ≤ 0.02 and 0.23 ≤ Y ≤ 0.28. A comparison among the most recent compilations of similar stellar models is also discussed. The main conclusion is that the differencies among the various evolutionary results are still rather large.
The Astrophysical …, 2011
We present new analysis of 11 intermediate-age (1-2 Gyr) star clusters in the Large Magellanic Cloud based on Hubble Space Telescope imaging data. Seven of the clusters feature main sequence turnoff (MSTO) regions that are wider than can be accounted for by a simple stellar population, whereas their red giant branches indicate a single value of [Fe/H]. The star clusters cover a range in present-day mass from about 1 × 10 4 M ⊙ to 2 × 10 5 M ⊙ . We compare radial distributions of stars in the upper and lower parts of the MSTO region, and calculate cluster masses and escape velocities from the present time back to a cluster age of 10 Myr. Our main result is that for all clusters in our sample with estimated escape velocities v esc 15 km s −1 at an age of 10 Myr, the stars in the brightest half of the MSTO region are significantly more centrally concentrated than the stars in the faintest half and more massive red giant branch and asymptotic giant branch stars. This is not the case for clusters with v esc 10 km s −1 at an age of 10 Myr. We argue that the wide MSTO region of such clusters is mainly caused by to a ∼ 200 − 500 Myr range in the ages of cluster stars due to extended star formation within the cluster from material shed by first-generation stars featuring slow stellar winds. Dilution of this enriched material by accretion of ambient interstellar matter is deemed plausible if the spread of [Fe/H] in this ambient gas was very small when the second-generation stars were formed in the cluster.
Astronomy & Astrophysics, 2015
We present new models for low-mass stars down to the hydrogen-burning limit that consistently couple atmosphere and interior structures, thereby superseding the widely used BCAH98 models. The new models include updated molecular linelists and solar abundances, as well as atmospheric convection parameters calibrated on 2D/3D radiative hydrodynamics simulations. Comparison of these models with observations in various colour-magnitude diagrams for various ages shows significant improvement over previous generations of models. The new models can solve flaws that are present in the previous ones, such as the prediction of optical colours that are too blue compared to M dwarf observations. They can also reproduce the four components of the young quadruple system LkCa 3 in a colour-magnitude diagram with one single isochrone, in contrast to any presently existing model. In this paper we also highlight the need for consistency when comparing models and observations, with the necessity of using evolutionary models and colours based on the same atmospheric structures.
2010
We present a new method for the evaluation of the age and age-spread among pre-main-sequence (PMS) stars in star-forming regions in the Magellanic Clouds, accounting simultaneously for photometric errors, unresolved binarity, differential extinction, stellar variability, accretion and crowding. The application of the method is performed with the statistical construction of synthetic color-magnitude diagrams using isochrones from two families of PMS evolutionary models. We convert each isochrone into 2D probability distributions of artificial PMS stars in the CMD by applying the aforementioned biases that dislocate these stars from their original CMD positions. A maximum-likelihood technique is then applied to derive the probability for each observed star to have a certain age, as well as the best age for the entire cluster. We apply our method to the photometric catalog of ∼ 2 000 PMS stars in the young association LH 95 in the Large Magellanic Cloud, based on the deepest HST/ACS imaging ever performed toward this galaxy, with a detection limit of V ∼ 28, corresponding to M ∼ 0.2 M . We assume the Initial Mass Function and reddening distribution for the system, as they are previously derived by us. Our treatment shows that the age determination is very sensitive to the considered grid of evolutionary models and the assumed binary fraction. The age of LH 95 is found to vary from 2.8 Myr to 4.4 Myr, depending on these factors. We evaluate the accuracy of our age estimation and we find that the method is fairly accurate in the PMS regime, while the precision of the measurement of the age is lower at higher luminosities. Our analysis allows us to disentangle a real age-spread from the apparent CMD-broadening caused by the physical and observational biases. We find that LH 95 hosts an age-spread well represented by a gaussian distribution with a FWHM of the order of 2.8 Myr to 4.2 Myr depending on the model and binary fraction. We detect a dependence of the average age of the system with stellar mass. This dependence does not appear to have any physical meaning, being rather due to imperfections of the PMS evolutionary models, which tend to predict lower ages for the intermediate masses, and higher ages for low-mass stars. Subject headings: stars: formation -stars: pre-main sequence -Hertzsprung-Russell and C-M diagrams -open clusters and associations: individual (LH 95) -methods: statistical