Intermediate‐Mass Star Models with Different Helium and Metal Contents

Abstract

We present a comprehensive theoretical investigation of the evolutionary properties of intermediate-mass stars. The evolutionary sequences were computed from the Zero Age Main Sequence up to the central He exhaustion and often up to the phases which precede the carbon ignition or to the reignition of the H-shell which marks the beginning of the thermal pulse phase. The evolutionary tracks were constructed by adopting a wide range of stellar masses (3 ≤M/M ⊙ ≤ 15) and chemical compositions. In order to account for current uncertainties on the He to heavy elements enrichment ratio (∆Y /∆Z ), the stellar models were computed by adopting at Z=0.02 two different He contents (Y=0.27, 0.289) and at Z=0.04 three different He contents (Y=0.29, 0.34, and 0.37). Moreover, to supply a homogeneous evolutionary scenario which accounts for young Magellanic stellar systems the calculations were also extended toward lower metallicities (Z=0.004, Z=0.01), by adopting different initial He abundances.We evaluated for both solar (Z=0.02) and super-metal-rich (SMR, Z=0.04) models the transition mass M up between the stellar structures igniting carbon and those which develop a full electron degeneracy inside the carbon-oxygen core. We found that M up is of the order of 7.7 ± 0.5M ⊙ for solar composition, while for SMR structures an increase in the He content causes a decrease in M up , and indeed it changes from 9.5 ± 0.5M ⊙ at Y=0.29, to 8.7 ± 0.2M ⊙ at Y=0.34, and to 7.7 ± 0.2M ⊙ at Y=0.37. We also show that M up presents a nonlinear behavior with metallicity, and indeed it decreases when moving from Z=0.04 to Z ≈ 0.001 and increases at lower metal contents. This finding confirms the predictions by Cassisi & Castellani (1993) and more recently by Umeda et al. (1999) and suggests that the rate of SNe type Ia depends on the 3 chemical composition of the parent stellar population.This approach allows us to investigate in detail the evolutionary properties of classical Cepheids. In particular, we find that the range of stellar masses which perform the blue loop during the central He-burning phase narrows when moving toward metal-rich and SMR structures. This evidence and the substantial decrease in the evolutionary time spent by these structures inside the instability strip bring out that the probability to detect long-period Cepheids in SMR stellar systems is substantially smaller than in more metal-poor systems.Moreover and even more importantly, we find that the time spent by Cepheids along the subsequent crossings of the instability strip also depends on the stellar mass. In fact, our models suggest that low-mass, metal-poor Cepheids spend a substantial portion of their lifetime along the blueward excursion of the blue loop, while at higher masses (M/M ⊙ ≥ 8) the time spent along the redward excursion becomes longer. Models at solar chemical composition present an opposite behavior i.e. the time spent along the redward excursion is longer than the blueward excursion among low-mass Cepheids and vice versa for high-massCepheids. Oddly ...

Uncertainties on the theoretical predictions for classical Cepheid pulsational quantities

“…Iben 1972;Brunish et al 1990;Bono et al 2000). This point is discussed in the following subsections in the light of updated nuclear cross sections measurements.…”

“…Bono et al 2000) we take into account the three crossings of the instability strip: the first crossing during the H burning and the second and third crossings during the He burning phases. The mean luminosity is obtained by averaging the luminosity behaviour during the three subsequent crossings with the corresponding evolutionary times.…”

“…(3), with the one by Bono et al (2000), which explicitly takes into account the dependence on the chemical composition, computed for the values of Y and Z corresponding to our standard case: Y = 0.26, Z = 0.005. We found that the ML relation derived in this paper is flatter than Bono et al (2000) with the disagreement reaching about 0.1 dex at masses around 10 M . However for M ≤ ∼ 8 M the two relations are consistent within the uncertainties.…”

A comparison of evolutionary tracks for single Galactic massive stars

“…Padova stellar evolutionary code (Bertelli et al 2009) The evolutionary models for massive stars computed with the Padova code are described in Bertelli et al (2009). Additional information regarding the code can be found in Bono et al (2000) and Pietrinferni et al (2004Pietrinferni et al ( , 2006. Rotation is not included in the grid of Bertelli et al (2009).…”

Evolutionary Properties of Intermediate-Mass Stars

“…The recent literature for intermediate-mass stars is quite rich (see Bono et al 2000 and references therein), and a detailed discussion of the main evolutionary and structural properties of these stars can be easily found elsewhere. Here we wish to shed light on the questions one has to face when trying to model this class of stars.…”