Evolutionary Prescriptions Research Articles

Anomalous Cepheids: Updated Theoretical Period–Luminosity–Color and Period–Wesenheit Relations

AbstractIn the context of a project aimed to provide an updated theoretical scenario for various classes of radially pulsating stars, we present the first results obtained for anomalous Cepheids. By adopting reliable and updated evolutionary prescriptions concerning the luminosity levels for various core He-burning stellar models with masses suitable for entering the instability strip, we have computed nonlinear convective pulsation models for both fundamental and first-overtone mode anomalous Cepheids by exploring the impact of varying the metal abundance as well as the efficiency of super-adiabatic convection. These numerical simulations have allowed us to retrieve the boundaries of the instability strip and all relevant pulsation properties, namely period, amplitude, bolometric light and radial velocity curves. This theoretical scenario has been transformed into the Gaia photometric system to derive the first theoretical Period–Luminosity–Colour and Period–Wesenheit relations in the Gaia bands.

Stellar Population Astrophysics (SPA) with TNG

Context.Open clusters are excellent tracers of the chemical evolution of the Galactic disc. The spatial distribution of their elemental abundances, through the analysis of high-quality and high-resolution spectra, provides insight into the chemical evolution and mechanisms of element nucleosynthesis in regions characterised by different conditions (e.g. star formation efficiency and metallicity).Aims.In the framework of the Stellar Population Astrophysics (SPA) project, we present new observations and spectral analysis of four sparsely studied open clusters located in the solar neighbourhood, namelyCollinder 350,Gulliver 51,NGC 7044,andRuprecht 171.Methods.We exploit the HARPS-N spectrograph at the TNG telescope to acquire high-resolution optical spectra for 15 member stars of four clusters. We derive stellar parameters (Teff, logg, [Fe/H] andξ) using both the equivalent width (EW) analysis and the spectral fitting technique. We compute elemental abundances for light,α-, iron-peak, andn-capture elements using the EW measurement approach. We investigate the origin of the correlation between metallicity and stellar parameters derived with the EW method for the coolest stars of the sample (Teff < 4300 K). The correlation is likely due to the challenging continuum setting and to a general inaccuracy of model atmospheres used to reproduce the conditions of very cool giant stars.Results.We locate the properties of our clusters in the radial distributions of metallicity and abundance ratios, comparing our results with clusters from theGaia-ESO and APOGEE surveys. We present the [X/Fe]−[Fe/H] and [X/Fe]−RGCtrends for elements in common between the two surveys. Finally, we derive the C and Li abundances as a function of the evolutionary phase and compare them with theoretical models.Conclusions.The SPA survey, with its high-resolution spectra, allows us to fully characterise the chemistry of nearby clusters. With a single set of spectra, we provide chemical abundances for a variety of chemical elements, which are comparable to those obtained in two of the largest surveys combined. The metallicities and abundance ratios of our clusters fit very well in the radial distributions defined by the recent literature, reinforcing the importance of star clusters to outline the spatial distribution of abundances in our Galaxy. Moreover, the abundances of C and Li, modified by stellar evolution during the giant phase, agree with evolutionary prescriptions (rotation-induced mixing) for their masses and metallicities.

THE CARINA PROJECT. VI. THE HELIUM-BURNING VARIABLE STARS

We present new optical (BVI) time-series data for the evolved variable stars in the Carina dwarf spheroidal galaxy. The quality of the data and the observing strategy allowed us to identify 14 new variable stars. Eight out of the 14 are RR Lyrae (RRL) stars, four are Anomalous Cepheids (ACs) and two are geometrical variables. Comparison of the period distribution for the entire sample of RRLs with similar distributions in nearby dSphs and in the Large Magellanic Cloud indicates that the old stellar populations in these systems share similar properties. This finding is also supported by the RRL distribution in the Bailey diagram. On the other hand, the period distribution and the Bailey diagram of ACs display significant differences among the above stellar systems. This evidence suggests that the properties of intermediate-age stellar populations might be affected both by environmental effects and structural parameters. We use the BV Period--Wesenheit (PW) relation of RRLs together with evolutionary prescriptions and find a true distance modulus of 20.09+/-0.07(intrinsic)+/-0.1(statistical) mag that agrees quite well with similar estimates available in the literature. We identified four peculiar variables. Taking into account their position in the Bailey diagram and in the BV PW relation, two of them (V14, V149) appear to be candidate ACs, while two (V158, V182) might be peculiar RRLs. In particular, the variable V158 has a period and a V-band amplitude very similar to the low-mass RRL ---RRLR-02792---recently identified by Pietrzynski at al. (2012) in the Galactic bulge.

ON THE CENTRAL HELIUM-BURNING VARIABLE STARS OF THE LeoI DWARF SPHEROIDAL GALAXY

We present a study of short period, central helium-burning variable stars in the Local Group dwarf spheroidal galaxy LeoI, including 106 RR Lyrae stars and 51 Cepheids. So far, this is the largest sample of Cepheids and the largest Cepheids to RR Lyrae ratio found in such a kind of galaxy. The comparison with other Local Group dwarf spheroidals, Carina and Fornax, shows that the period distribution of RR Lyrae stars is quite similar, suggesting similar properties of the parent populations, whereas the Cepheid period distribution in LeoI peaks at longer periods (P \sim 1.26d instead of ~0.5d) and spans over a broader range, from 0.5 to 1.78d. Evolutionary and pulsation predictions indicate, assuming a mean metallicity peaked within -1.5<= [Fe/H]<=-1.3, that the current sample of LeoI Cepheids traces a unique mix of Anomalous Cepheids (blue extent of the red--clump, partially electron degenerate central helium-burning stars) and short-period classical Cepheids (blue-loop, quiescent central helium-burning stars). Current evolutionary prescriptions also indicate that the transition mass between the two different groups of stars is MHeF \sim 2.1 Mo, and it is constant for stars metal-poorer than [Fe/H]\sim-0.7. Finally, we briefly outline the different implications of the current findings on the star formation history of LeoI.

Updated pulsation models for BL Herculis stars

Population II pulsatingvariables play a relevant role both as distance indicators and as tracers of the properties of old stellar populations. In this paper we present an update and homogeneous pulsational scenario for a wide range of stellar parameters typical of BL Her stars i.e., Population II Cepheids with periods shorter than 8 days. To this purpose, we adopt a nonlinear convective hydrodinamical code evaluate the stability and full amplitude behaviour of an extensive set of BL Her pulsation models. Various assumptions of mass, luminosity and metallicity consistent with the most recent evolutionary prescriptions, are adopted. We obtain the theoretical instability strip for both fundamental and first overtone pulsators and present a detailed atlas of light/radial-velocity curves. Some relations for the boundaries of the instability strip and for the dependence of the absolute magnitude on period, mass and color, as well as the fundamental period-amplitude relations are derived.Finally, we provide the theoretical period-radius relation for BL Her and find that it is in excellent agreement with the empirical relation by Burki & Meylan and consistent with the one holding for shorter periods for RR Lyrae stars.

The Distance of the Large Magellanic Cloud Cluster NGC 1866

Hubble Space Telescope V,I photometry of stars in the Large Magellanic Cloud Cluster NGC 1866 shows a well defined cluster main sequence down to V=25 mag, with little contamination from field or foreground stars. We use the main sequence fitting procedure to link the distance of NGC 1866 to the Hipparcos determination of the distance for the Hyades MS stars, making use of evolutionary prescriptions to allow for differences in the chemical composition. On this basis we find a true distance modulus for NGC 1866 of 18.35 +/- 0.05 mag. If the cluster is assumed to lie in the LMC plane then the LMC modulus is 0.02 mag less.

A pulsational approach to the luminosity of horizontal branch stellar structures

We discuss an alternative approach to constrain the absolute bolometric luminosity of zero-age horizontal branch (ZAHB) structures by using the observational pulsational properties of ab-type RR Lyrae stars, theoretical relations connecting these properties to the evolutionary ones, i.e. luminosity, mass and effective temperature, and also the location in the Hertzsprung–Russell diagram of the instability strip boundaries for fundamental pulsators. As the main goal of this work is to obtain an evaluation of the ZAHB bolometric luminosity as independent of stellar evolution theory as possible, we have minimized the use of evolutionary prescriptions, the only adopted evolutionary input being the allowed mass range for fundamental pulsators. Nevertheless, the effects on our final results related to these evolutionary prescriptions have been carefully checked. In order to test the accuracy of the current framework, we have carefully investigated the effective temperature scale provided by De Santis and adopted in the present work. It has been found that the pulsational colour–effective temperature scale fixed by this temperature scale and the relation between intrinsic (B−V)0 colour, blue amplitude and metallicity given by Caputo & De Santis appear fully consistent with both theoretical colour transformations, given by Castelli, Gratton & Kurucz and Buser & Kurucz, and a semi-empirical one, provided by Green. In contrast, there is a large discrepancy between this pulsational colour–temperature scale and the one based on the Kurucz model atmospheres. The effects of adopting the temperature scale of Catelan, Sweigart & Borissova have been investigated. The reliability of the method is shown by applying it to a selected sample of globular clusters (GCs), the heavy element abundances of which cover almost all the complete GC metallicity range. In order to verify the accuracy of the results obtained by using ab-type RR Lyrae stars, a similar analysis has also been performed by using both observational and theoretical evidence for first-overtone variables. The results obtained for the ZAHB bolometric luminosities have been critically analysed and a comparison with recent evolutionary prescriptions has also been performed. The existence of evident mismatches between current results and some evolutionary models has been verified and discussed. Finally, our investigation has been extended to field variables to check for a luminosity difference between cluster RR Lyrae stars and field ones as suggested by Gratton. The comparison between the pulsational properties of field and cluster variables does not show the existence of any significant difference in their intrinsic luminosity, thus providing further support to the results obtained by Catelan.

Metal‐rich RR Lyrae Variables. II. The Pulsational Scenario

We present a theoretical investigation on the pulsational behavior of metal-rich RR Lyrae variables over the range of evolutionary parameters suitable for stars with metallicities Z = 0.006, 0.01, and 0.02. With the addition of similar results for metal-poor pulsators, we discuss the theoretical pulsational scenario covering the metallicity range Z = 0.0001-0.02. By connecting pulsational constraints to evolutionary prescriptions for He-burning stars, we discuss the observed behavior of the RR Lyrae population in the Galactic field. We find that the distribution of field ab-type RR Lyrae stars in the period-metallicity plane can be easily understood within the framework of the present theoretical scenario, suggesting that the Oosterhoff dichotomy also affects field variables. Theoretical predictions concerning the amplitude-period diagram are discussed and compared with observational data. We find a general agreement for metal-poor ([Fe/H] < -1.4) RR Lyrae stars, whereas more metal-rich variables show amplitudes smaller than those predicted for pulsators originated from old, low-mass evolving stars. Alternatively, the agreement between theory and observations would require that a substantial fraction of metal-rich RR Lyrae variables in the Galactic field were younger than ≈ 2 Gyr. The comparison between the pulsational behavior of RR Lyrae either in the Galactic field or in the Galactic bulge discloses the evidence that, at least as far as RR Lyrae variables are concerned, the metal-rich components of the bulge and of the field population appear quite similar. We finally suggest that the peak at log P -0.55 in the period frequency distribution of first-overtone RR Lyrae stars in the Large Magellanic Cloud, considered as possible evidence for second-overtone pulsators, could be more simply taken as evidence of a metal-rich stellar population.