Abstract
ABSTRACT τ 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 τ Sco’s properties with single-star models, an increase is necessary in the efficiency of rotational mixing by a factor of 3–10 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 τ Sco. However, the merger scenario also faces similar challenges as our magnetic single-star models to explain τ 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 τ Sco.
Highlights
Τ Scorpii (HD 149438) is a magnetic massive B0.2 V star in the Upper Sco association, which has been well-studied for almost an entire century (e.g. Struve & Dunham 1933; Unsold 1942; Traving 1955; Aller, Faulkner & Norton 1966; Lamers & Rogerson 1978; Wolff & Heasley 1985; Kilian 1992; Howk et al 2000)
The only solution far that predicts sufficiently slow rotation is a model with a constant surface magnetic dipole moment of μB = 1040 G cm3 corresponding to an approximately 270 kG field at the surface of a star with a radius of 5 R. Such a strong field is reached in the 3D MHD merger simulation (Schneider et al 2019), since the field strength remains quasi-constant on the main sequence, it is far too strong compared to any known OBA star and, in particular, to τ Sco’s measured surface field strength of a few hundred G (Donati et al 2006; Shultz et al 2018, and further details in Appendix B). These significant findings pose important questions, for example, i) are all fossil magnetic fields generated via a stellar merger event, and ii) can we confidently identify signs of a past stellar merger event and its evolutionary consequences when studying a star? This makes τ Sco a valuable laboratory to further our understanding of magnetic massive stars and for this reason the current discrepancies between models and observations need to be studied in more detail
Since in Geneva stellar evolution code (GENEC) only the outermost layers are ascribed to lose specific angular momentum, the use of angular momentum transport without a strong coupling means that significant shears can develop in the outer part of the stellar envelope, while meridional currents remain efficient to transport chemical elements close to the stellar core. (Note that in Modules for Experiments in Stellar Astrophysic (MESA), we model the opposite scenario: shears remain efficient close to the core but meridional circulation dominates the transport in the outer envelope.) The equatorial magnetic field strength is set to Beq = 300 G and is kept constant over time
Summary
Τ Scorpii (HD 149438) is a magnetic massive B0.2 V star in the Upper Sco association, which has been well-studied for almost an entire century (e.g. Struve & Dunham 1933; Unsold 1942; Traving 1955; Aller, Faulkner & Norton 1966; Lamers & Rogerson 1978; Wolff & Heasley 1985; Kilian 1992; Howk et al 2000). Walborn & Panek 1984; Peters & Polidan 1985; Rogerson & Ewell 1985; Cowley & Merritt 1987; Snow et al 1994), and infrared (e.g. Waters et al 1993; Zaal et al 1999; Repolust et al 2005). Spectropolarimetric observations of τ Sco by Donati et al (2006) led to the discovery of a surface magnetic field, which is unusually complex compared to other B-type stars whose field measurements can usually be reconciled with a dipolar configuration
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