Constraining transport of angular momentum in stars

Combining asteroseismic observations of core helium burning stars and white dwarfs

¹ Konkoly Observatory, Konkoly Thege Miklós út 15-17, 1121 Budapest, Hungary
e-mail: jacqueline.den.hartogh@csfk.mta.hu
² Astrophysics Group, Lennard-Jones Laboratories, Keele University, ST5 5BG, UK
³ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
⁴ Kavli Institute for the Physics and Mathematics of the Universe (WPI), University of Tokyo, 5-1-5 Kashiwanoha, 277-8583 Kashiwa, Japan

Received: 27 September 2018
Accepted: 7 January 2019

Abstract

Context. Transport of angular momentum has been a challenging topic within the stellar evolution community, even more since the recent asteroseismic surveys. All published studies on rotation using asteroseismic observations show a discrepancy between the observed and calculated rotation rates, indicating there is an undetermined process of angular momentum transport active in these stars.

Aims. We aim to constrain the efficiency of this process by investigating rotation rates of 2.5 M_⊙ stars.

Methods. First, we investigated whether the Tayler-Spruit dynamo could be responsible for the extra transport of angular momentum for stars with an initial mass of 2.5 M_⊙. Then, by computing rotating models including a constant additional artificial viscosity, we determined the efficiency of the missing process of angular momentum transport by comparing the models to the asteroseismic observations of core helium burning stars. Parameter studies were performed to investigate the effect of the stellar evolution code used, initial mass, and evolutionary stage. We evolved our models into the white dwarf phase, and provide a comparison to white dwarf rotation rates.

Results. The Tayler-Spruit dynamo is unable to provide enough transport of angular momentum to reach the observed values of the core helium burning stars investigated in this paper. We find that a value for the additional artificial viscosity ν_add around 10⁷ cm² s⁻¹ provides enough transport of angular momentum. However, the rotational period of these models is too high in the white dwarf phase to match the white dwarf observations. From this comparison we infer that the efficiency of the missing process must decrease during the core helium burning phase. When excluding the ν_add during core helium burning phase, we can match the rotational periods of both the core helium burning stars and white dwarfs.