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Register a new userConvective core sizes in rotating massive stars: I. Constraints from solar metallicity OB field stars
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S\\'ebastien Martinet
Publication date
2021
fields
Physics
and research's language is
English
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Spectroscopic studies of Galactic O and B stars show that many stars with masses above 8 M$_{odot}$ are observed in the HR diagram just beyond the Main-Sequence (MS) band predicted by stellar models computed with a moderate overshooting. This may be an indication that the convective core sizes in stars in the upper part of the HR diagram are larger than predicted by these models. Combining stellar evolution models and spectroscopic parameters derived for a large sample of Galactic O and B stars, including brand new information about their projected rotational velocities, we reexamine the question of the convective core size in MS massive stars. We confirm that for stars more massive than about 8 M$_{odot}$, the convective core size at the end of the MS phase increases more rapidly with the mass than in models computed with a constant step overshoot chosen to reproduce the main sequence width in the low mass range (around 2 M$_{odot}$). This conclusion is valid for both the cases of non-rotating models and rotating models either with a moderate or a strong angular momentum transport. The increase of the convective core mass with the mass obtained from the TAMS position is, however, larger than the one deduced from the surface velocity drop for masses above about 15 M$_{odot}$. Although observations available at the moment cannot decide what is the best choice between the core sizes given by the TAMS and the velocity drop, we discuss different methods to get out of this dilemma. At the moment, comparisons with eclipsing binaries seem to favor the solution given by the velocity drop. While we confirm the need for larger convective cores at higher masses, we find tensions in-between different methods for stars more massive than 15 M$_{odot}$. The use of single-aged stellar populations (non-interacting binaries or stellar clusters) would be a great asset to resolve this tension.
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Aluminium-26 is a short-lived radionuclide with a half-life of 0.72Myr, which is observed today in the Galaxy via gamma-ray spectroscopy and is inferred to have been present in the early Solar System via analysis of meteorites. Massive stars are considered the main contributors of Al26. Although most massive stars are found in binary systems, the effect, however, of binary interactions on the Al26 yields have not been investigated since Braun & Langer (1995). Here we aim to fill this gap. We have used the MESA stellar evolution code to compute massive (10Msun<=M<=80Msun), non-rotating, single and binary stars of solar metallicity (Z=0.014). We computed the wind yields for the single stars and for the binary systems where mass transfer plays a major role. Depending on the initial mass of the primary star and orbital period, the Al26 yield can either increase or decrease in a binary system. For binary systems with primary masses up to ~35-40Msun, the yield can increase significantly, especially at the lower mass-end, while above ~45Msun the yield becomes similar to the single star yield or even decreases. Our preliminary results show that compared to supernova explosions, the contribution of mass-loss in binary systems to the total Al26 abundance produced by a stellar population is minor. On the other hand, if massive star mass-loss is the origin of Al26 in the early Solar System, our results will have significant implications for the identification of the potential stellar, or stellar population, source.
Context. Rotation is known to affect the nucleosynthesis of light elements in massive stars, mainly by rotation-induced mixing. In particular, rotation boosts the primary nitrogen production. Models of rotating stars are able to reproduce the nitrogen observed in low-Z halo stars. Aims. Here we present the first grid of stellar models for rotating massive stars at low Z, where a full s-process network is used to study the impact of rotation-induced mixing on the nucleosynthesis of heavy elements. Methods. We used the Geneva stellar evolution code that includes an enlarged reaction network with nuclear species up to bismuth to calculate 25 M$_odot$ models at three different Z and with different initial rotation rates. Results. First, we confirm that rotation-induced mixing leads to a production of primary $^{22}$Ne, which is the main neutron source for the s process in massive stars. Therefore rotation boosts the s process in massive stars at all Z. Second, the neutron-to-seed ratio increases with decreasing Z in models including rotation, which leads to the complete consumption of all iron seeds at Z < 1e-3 by the end of core He-burning. Thus at low Z, the iron seeds are the main limitation for this boosted s process. Third, as Z decreases, the production of elements up to the Ba peak increases at the expense of the elements of the Sr peak. We studied the impact of the initial rotation rate and of the uncertain $^{17}$O$(alpha,gamma)$ rate (which strongly affects the neutron poison strength of $^{16}$O) on our results. This study shows that rotating models can produce significant amounts of elements up to Ba over a wide range of Z. Fourth, compared to the He-core, the primary $^{22}$Ne production in the He-shell is even higher (> 1% in mass fraction at all Z), which could open the door for an explosive neutron capture nucleosynthesis in the He-shell, with a primary neutron source.
In this work we make use of DR14 APOGEE spectroscopic data to study a sample of 92 known OB stars. We developed a near-infrared semi-empirical spectral classification method that was successfully used in case of four new exemplars, previously classified as later B-type stars. Our results agree well with those determined independently from ECHELLE optical spectra, being in line with the spectral types derived from the canonical MK blue optical system. This confirms that the APOGEE spectrograph can also be used as a powerful tool in surveys aiming to unveil and study large number of moderately and highly obscured OB stars still hidden in the Galaxy.
Radioactive nuclei were present in the early Solar System, as inferred from analysis of meteorites. Many are produced in massive stars, either during their lives or their final explosions. In the first paper in this series (Brinkman et al. 2019), we focused on the production of $^{26}$Al in massive binaries. Here, we focus on the production of another two short-lived radioactive nuclei, $^{36}$Cl and $^{41}$Ca, and the comparison to the early Solar System data. We used the MESA stellar evolution code with an extended nuclear network and computed massive (10-80 M$ _{odot} $), rotating (with initial velocities of 150 and 300 km/s) and non-rotating single stars at solar metallicity (Z=0.014) up to the onset of core collapse. We present the wind yields for the radioactive isotopes $^{26}$Al, $^{36}$Cl, and $^{41}$Ca, and the stable isotopes $^{19}$F and $^{20}$Ne. In relation to the stable isotopes, we find that only the most massive models, $geq$ 60M$_{odot}$ and $geq$ 40M$_{odot}$ give positive $^{19}$F and $^{20}$Ne yields, respectively, depending on the initial rotation rate. In relation to the radioactive isotopes, we find that the early Solar System abundances of $^{26}$Al and $^{41}$Ca can be matched with by models with initial masses $geq$40M$_{odot}$, while $^{36}$Cl is matched only by our most massive models, $geq$60M$_{odot}$. $^{60}$Fe is not significantly produced by any wind model, as required by the observations. Therefore, massive star winds are a favoured candidate for the origin of the very short-lived $^{26}$Al, $^{36}$Cl, and $^{41}$Ca in the early Solar System.
We present a chemical abundance analysis of a metal-poor star, ROA 276, in the stellar system omega Centauri. We confirm that this star has an unusually high [Sr/Ba] abundance ratio. Additionally, ROA 276 exhibits remarkably high abundance ratios, [X/Fe], for all elements from Cu to Mo along with normal abundance ratios for the elements from Ba to Pb. The chemical abundance pattern of ROA 276, relative to a primordial omega Cen star ROA 46, is best fit by a fast-rotating low-metallicity massive stellar model of 20 Msun, [Fe/H] = -1.8, and an initial rotation 0.4 times the critical value; no other nucleosynthetic source can match the neutron-capture element distribution. ROA 276 arguably offers the most definitive proof to date that fast-rotating massive stars contributed to the production of heavy elements in the early Universe.
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