Period spacings of γ Doradus pulsators in the Kepler field: Rossby and gravity modes in 82 stars

Abstract

Rossby modes are the oscillations in a rotating fluid, whose restoring force is the Coriolis force. They provide an additional diagnostic to understand the rotation of stars, which complicates asteroseismic modelling. We report 82 γ Doradus stars for which clear period spacing patterns of both gravity and Rossby modes have been detected. The period spacings of both show a quasi-linear relation with the pulsation period but the slope is negative for the gravity modes and positive for the Rossby modes. Most Rossby modes have k = −2, m = −1. For only one star a series of k = −1, m = −1 modes is seen. For each pattern, the mean pulsation period, the mean period spacing, and the slope are measured. We find that the slope correlates with the mean period for Rossby mode patterns. The traditional approximation of rotation is used to measure the near-core rotation rate, assuming the star rotates rigidly. We report the near-core rotation rates, the asymptotic period spacings, and the radial orders of excited modes of these 82 main-sequence stars. The near-core rotation rates lie between 0.6 d −1 and 2.3 d −1 . Six stars show surface rotation modulations, among which only KIC 3341457 shows differential rotation while the other five stars have uniform rotations. The radial orders of excited modes show different distributions for the dipole and quadrupole gravity modes versus the Rossby modes.

Gravity-mode period spacings and near-core rotation rates of 611 γ Doradus stars with Kepler

“…We examined 2085 stars with effective temperature between 6000 K to 10000 K, where we used the input temperatures from the Kepler DR25 data release (Mathur et al 2017). We found 960 clear period-spacing patterns in 611 stars, including 50 stars by Van Reeth et al (2015), 22 stars with splittings by Li et al (2019a), 82 stars with r-mode patterns by Li et al (2019b), 30 stars by Chowdhury et al (2018), 44 stars by Murphy et al (2018), 344 stars found by Barbara et al (in prep), and the rest we found by visually inspecting light curves and their Fourier transforms (the samples overlap). Barbara et al (in prep) applied a Gaussian mixture model in a reduced 5D space to classify 12066 stars in the Kepler field.…”

“…Hence the near-core rotation rate, the asymptotic spacing, and the radial orders can be obtained by fitting these pulsation periods to the observed pattern using a Markov chain Monte Carlo (MCMC) optimising code described by Li et al (2019b). Fig.…”

“…Kaye et al 1999;Van Reeth et al 2016). The pulsations of γ Dor stars are gravity modes with high radial order (20 n 100) low degree (l 4) with typical pulsation period from 0.3 to 3 d (Balona et al 1994;Kaye et al 1999;Saio et al 2018b;Van Reeth et al 2018;Li et al 2019b). Gravity modes have their highest mode energy in the near-core regions (e.g.…”

“…Now, splittings or period spacings of g modes from tens of γ Dor stars were found, both in single stars and binaries, whose rotation profiles are almost uniform (e.g. Triana et al 2015;Saio et al 2015;Keen et al 2015;Van Reeth et al 2015;Li et al 2019a;Guo et al 2017;Kallinger et al 2017;Li et al 2019b).…”

“…Using the period-spacing patterns from g and r modes, the near-core rotation of tens of γ Dor stars were measured to be around 1 d −1 (e.g. Saio et al 2018a;Van Reeth et al 2018;Li et al 2019b). Many of the stars in our sample also show r modes.…”