Detection of high-degree prograde sectoral mode sequences in the A-star KIC 8054146?

TL;DR

This study identifies and analyzes a sequence of high-frequency prograde sectoral modes in the Delta Scuti star KIC 8054146, revealing specific frequency patterns and their relation to stellar rotation and mode stability.

Contribution

It provides the first detailed identification of high-degree prograde sectoral modes in KIC 8054146, linking observed frequency sequences to stellar rotation and mode visibility.

Findings

Frequency sequence follows a precise linear pattern

Prograde sectoral modes can explain observed frequency and amplitude patterns

Star rotates at about 70% of break-up velocity, viewed near equator

Abstract

This paper examines the 46 frequencies found in the Delta Sct star KIC 8054146 involving a frequency spacing of exactly 2.814 c/d (32.57 microHz), which is also a dominant low-frequency peak near or equal to the rotational frequency. These 46 frequencies range up to 146 c/d. Three years of Kepler data reveal distinct sequences of these equidistantly spaced frequencies, including the basic sequence and side lobes associated with other dominant modes (i.e., small amplitude modulations). The amplitudes of the basic sequence show a high-low pattern. The basic sequence follows the equation fm = 2.8519 + m * 2.81421 c/d with m ranging from 25 to 35. The zero-point offset and the lack of low-order harmonics eliminate an interpretation in terms of a Fourier series of a non-sinusoidal light curve. The exactness of the spacing eliminates high-order asymptotic pulsation. The frequency pattern is not compatible with simple hypotheses involving single or multiple spots, even with differential rotation. The basic high-frequency sequence is interpreted in terms of prograde sectoral modes. These can be marginally unstable, while their corresponding low-degree counterparts are stable due to stronger damping. The measured projected rotation velocity (300 km/s) indicates that the star rotates with app. 70% of the Keplerian break-up velocity. This suggests a near equator-on view. We qualitatively examine the visibility of prograde sectoral high-degree g-modes in integrated photometric light in such a geometrical configuration and find that prograde sectoral modes can reproduce the frequencies and the odd-even amplitude pattern of the high-frequency sequence.