T. Natan | Searcharxiv

arXiv2026

Results from the first spectropolarimetric survey of $\gamma$ Dor pulsators

Context. Magnetic fields can have an important influence on stellar structure and evolution. In intermediate-mass (A- and F-type) stars, there are many known stars with directly measured strong, globally organized magnetic fields, as well as indications of weak and/or small-scale variable fields. However, among the intermediate-mass $\gamma$ Dor pulsators, there are no known stars with strong surface magnetic fields. Aims. The broad goal of this work is to search for evidence of strong, globally organized fields at the surface of $\gamma$ Dor pulsators. Methods. We identified objects consistent with being $\gamma$ Dor pulsators based on an analysis of space photometry from the Transiting Exoplanet Survey Satellite (TESS) mission. A spectropolarimetric survey was then conducted on a subset of 47 of these objects, with a precision sufficient to detect dipolar surface magnetic fields down to a threshold of about 10 -- 100 G. Results. We detected strong magnetism in three targets. However, upon closer inspection, none of these appear to be genuine $\gamma$ Dor pulsators. We found no evidence of surface magnetism in any of the remaining 44 objects. Conclusions. We conclude that either strong, globally organized magnetic fields and $\gamma$ Dor pulsation are mutually exclusive, or that such stars are exceedingly rare. A possible explanation is that strong global fields inhibit the excitation mechanism, which prevents $\gamma$ Dor pulsations from being driven in strongly magnetic intermediate-mass stars. The dipolar surface magnetic field strength upper limits we derive for this sample ($\lesssim$ 50 -- 100 G) provide valuable constraints for surface boundary conditions for asteroseismic models that include magnetism for $\gamma$ Dor stars.

astro-ph.SR↗

arXiv2017

The evolutionary roads leading to low effective spins, high black hole masses, and O1/O2 rates of LIGO/Virgo binary black holes

All ten LIGO/Virgo binary black hole (BH-BH) coalescences reported from the O1/O2 runs have near zero effective spins. There are only three potential explanations of this fact. If the BH spin magnitudes are large then (i) either both BH spin vectors must be nearly in the orbital plane or (ii) the spin angular momenta of the BHs must be oppositely directed and similar in magnitude. Or, (iii) the BH spin magnitudes are small. We test the third hypothesis within the framework of the classical isolated binary evolution scenario of the BH-BH merger formation. We test three models of angular momentum transport in massive stars: a mildly efficient transport by meridional currents (as employed in the Geneva code), an efficient transport by the Tayler-Spruit magnetic dynamo (as implemented in the MESA code), and a very-efficient transport (as proposed by Fuller et al.) to calculate natal BH spins. We allow for binary evolution to increase the BH spins through accretion and account for the potential spin-up of stars through tidal interactions. Additionally, we update the calculations of the stellar-origin BH masses, include revisions to the history of star formation and to the chemical evolution across cosmic time. We find that we can match simultaneously the observed BH-BH merger rate density, BH masses, and effective spins. Models with efficient angular momentum transport are favored. The updated stellar-mass weighted gas-phase metallicity evolution now used in our models appears to be a key in better reproducing the LIGO/Virgo merger rate estimate. Mass losses during the pair-instability pulsation supernova phase are likely overestimated if the merger GW170729 hosts a BH more massive than 50 Msun. We also estimate rate of BH-NS mergers from recent LIGO/Virgo observations. Our updated models of BH-BH, BH-NS and NS-NS mergers are now publicly available at www.syntheticuniverse.org.

astro-ph.HE↗