Ref Arxiv: 2010.14311
DOI: 10.1093/mnras/staa3396
Ref. & Cit.: NASA ADS
Comments: 7 pages, 5 figures, accepted to MNRAS

Ref Arxiv: 2101.09269
DOI: 10.1051/0004-6361/202039890
Ref. & Cit.: NASA ADS
Abstract:

We study the evolutionary and physical properties of evolved O stars in the Small Magellanic Cloud (SMC), with a special focus on their surface abundances to investigate the efficiency of rotational mixing as a function of age, rotation and global metallicity. We analyse the UV + optical spectra of thirteen SMC O-type giants and supergiants, using the stellar atmosphere code CMFGEN to derive photospheric and wind properties. We compare the inferred properties to theoretical predictions from evolution models. For a more comprehensive analysis, we interpret the results together with those we obtained for O-type dwarfs in a former study. Most dwarfs lie in the early phases of the main-sequence. For a given initial mass, giants are farther along the evolutionary tracks, confirming that they are more evolved than dwarfs. Supergiants have higher initial masses and are located past the terminal age main-sequence. We find no clear trend of a mass discrepancy, regardless of the diagram that was used to estimate the evolutionary mass. CNO abundances are consistent with nucleosynthesis from the CNO cycle. Comparisons to theoretical predictions reveal that the initial mixture is important when the observed trends in the N/C versus N/O diagram are to be reproduced. A trend for stronger chemical evolution for more evolved objects is observed. More massive stars, are on average, more chemically enriched at a given evolutionary phase, qualitatively consistent with evolutionary models. Abundance ratios supports the theoretical prediction that massive stars at low metallicity are more chemically processed than their Galactic counterparts. Finally, models including rotation generally reproduce the surface abundances and rotation rates when different initial rotational velocities are considered. Nevertheless, there are objects for which a stronger braking and/or more efficient mixing is required.

Comments: 30 pages. 28 figures, 6 tables. Accepted in A&A. English edited version. Réf Journal: A&A 647, A134 (2021)

Ref Arxiv: 2103.13465
DOI: 10.1093/mnras/stab893
Ref. & Cit.: NASA ADS
Abstract:

$\tau$ Sco, a well-studied magnetic B-type star in the Upper Sco association, has a number of surprising characteristics. It rotates very slowly and shows nitrogen excess. Its surface magnetic field is much more complex than a purely dipolar configuration which is unusual for a magnetic massive star. We employ the CMFGEN radiative transfer code to determine the fundamental parameters and surface CNO and helium abundances. Then, we employ MESA and GENEC stellar evolution models accounting for the effects of surface magnetic fields. To reconcile $\tau$ Sco's properties with single-star models, an increase is necessary in the efficiency of rotational mixing by a factor of 3 to 10 and in the efficiency of magnetic braking by a factor of 10. The spin down could be explained by assuming a magnetic field decay scenario. However, the simultaneous chemical enrichment challenges the single-star scenario. Previous works indeed suggested a stellar merger origin for $\tau$ Sco. However, the merger scenario also faces similar challenges as our magnetic single-star models to explain $\tau$ Sco's simultaneous slow rotation and nitrogen excess. In conclusion, the single-star channel seems less likely and versatile to explain these discrepancies, while the merger scenario and other potential binary-evolution channels still require further assessment as to whether they may self-consistently explain the observables of $\tau$ Sco.

Comments: Accepted for publication in MNRAS. A full reproduction package is shared on zenodo in accordance with the Research Data Management plan of the Anton Pannekoek Institute for Astronomy at the University of Amsterdam: https://doi.org/10.5281/zenodo.4633408

Ref Arxiv: 2104.13988
Ref. & Cit.: NASA ADS
Abstract:

Multiple populations in globular clusters are usually explained by the formation of stars out of material with a chemical composition that is polluted to different degrees by the ejecta of short-lived, massive stars of various type. Among other things, these polluters differ by the amount of helium they spread in the surrounding medium. In this study we investigate whether the present-day photometric method used to infer the helium content of multiple populations indeed gives the true value or underestimates it by missing very He-rich, but rare stars. We focus on the specific case of NGC6752. We compute atmosphere models and synthetic spectra along isochrones produced for this cluster for a very broad range of He abundances covering the predictions of different pollution scenarios, including the extreme case of the fast-rotating massive star (FRMS) scenario. We calculate synthetic photometry in HST filters best suited to study the helium content. We subsequently build synthetic clusters with various distributions of stars. We finally determine the maximum helium mass fraction of these synthetic clusters using a method similar to that applied to observational data. We build toy models of clusters with various distributions of multiple populations and ensure that we are able to recover the input maximum Y. We then build synthetic clusters with the populations predicted by the FRMS scenario and find that while we slightly underestimate the maximum Y value, we are still able to detect stars much more He-rich than the current observed maximum Y. It is easier to determine the maximum Y on main sequence stars than on red giant branch stars, but qualitatively the results are unaffected by the sample choice. We show that in NGC6752 it is unlikely that stars more He-rich than the current observational limit of about 0.3 are present.

Comments: 17 pages, 21 figures + appendix. Accepted in Astronomy & Astrophysics

DOI: 10.1016/j.cplett.2021.139241
Abstract:

In this work, we present the efficient combination of Smolyak representations with time indepen- dent quantum mechanical approach using absorbing boundary conditions for the cumulative reac- tion probability calculations of a multidimensional reactive scattering problem. Our approach uses both kinds of Smolyak representations (finite basis and grid) which drastically reduces the size of the basis representation for the cumulative reaction operator. The cumulative reaction probability is thus obtained by solving the eigenvalue problem within the context of reaction path Hamiltonian using the compact Smolyak basis combined with an iterative Lanczos algorithm. Benchmark cal- culations are presented for reactive scattering models with a linear reaction coordinate and applied to a 25D model highlighting the efficiency of the present approach for multidimensional reactive processes.

DOI: 10.1051/0004-6361/201832698e

Ref Arxiv: 2012.05407
DOI: 10.1093/mnras/staa3878
Ref. & Cit.: NASA ADS
Abstract:

Studying cool star magnetic activity gives an important insight into the stellar dynamo and its relationship with stellar properties, as well as allowing us to place the Sun's magnetism in the context of other stars. Only 61 Cyg A (K5V) and $\tau$ Boo (F8V) are currently known to have magnetic cycles like the Sun's, where the large-scale magnetic field polarity reverses in phase with the star's chromospheric activity cycles. ${\tau}$ Boo has a rapid $\sim$240 d magnetic cycle, and it is not yet clear whether this is related to the star's thin convection zone or if the dynamo is accelerated by interactions between ${\tau}$ Boo and its hot Jupiter. To shed light on this, we studied the magnetic activity of HD75332 (F7V) which has similar physical properties to ${\tau}$ Boo and does not appear to host a hot Jupiter. We characterized its long term chromospheric activity variability over 53 yrs and used Zeeman Doppler Imaging to reconstruct the large-scale surface magnetic field for 12 epochs between 2007 and 2019. Although we observe only one reversal of the large-scale magnetic dipole, our results suggest that HD75332 has a rapid $\sim$1.06 yr solar-like magnetic cycle where the magnetic field evolves in phase with its chromospheric activity. If a solar-like cycle is present, reversals of the large-scale radial field polarity are expected to occur at around activity cycle maxima. This would be similar to the rapid magnetic cycle observed for ${\tau}$ Boo, suggesting that rapid magnetic cycles may be intrinsic to late-F stars and related to their shallow convection zones.

Comments: 23 pages, 14 figures, accepted for publication in MNRAS