Regularities in frequency spacings of Delta Scuti stars: The Kepler star KIC 9700322

TL;DR

This study analyzes the frequency spectrum of the Delta Scuti star KIC 9700322 observed by Kepler, revealing dominant radial modes, rotation effects, and the star's stability against nonradial pulsations, with implications for interpreting low-frequency signals.

Contribution

It provides detailed frequency analysis of KIC 9700322, confirming slow rotation, identifying mode regularities, and highlighting the star's stability against nonradial pulsations, which refines understanding of Delta Scuti star oscillations.

Findings

Dominant radial modes at 9.79 and 12.57 c/d identified.

Star exhibits slow rotation with v sin i = 19 km/s.

Absence of additional independent frequencies suggests stability against nonradial pulsations.

Abstract

In the faint star KIC 9700322 observed by the Kepler satellite, 76 frequencies with amplitudes from 14 to 29000 ppm were detected. The two dominant frequencies at 9.79 and 12.57 c/d (113.3 and 145.5 \mu Hz), interpreted to be radial modes, are accompanied by a large number of combination frequencies. A small additional modulation with a 0.16 c/d frequency is also seen; this is interpreted to be the rotation frequency of the star. The corresponding prediction of slow rotation is confirmed by a spectrum from which v sin i = 19 \pm 1 km/s is obtained. The analysis of the spectrum shows that the star is one of the coolest {\delta} Sct variables. We also determine Teff = 6700 \pm 100 K and log g = 3.7 \pm 0.1, compatible with the observed frequencies of the radial modes. Normal solar abundances are found. An \ell = 2 frequency quintuplet is also detected with a frequency separation consistent with predictions from the measured rotation rate. A remarkable result is the absence of additional independent frequencies down to an amplitude limit near 14 ppm, suggesting that the star is stable against most forms of nonradial pulsation. The frequency spectrum of this star emphasizes the need for caution in interpreting low frequencies in {\delta} Sct stars as independent gravity modes. A low frequency peak at 2.7763 c/d in KIC 9700322 is, in fact, the frequency difference between the two dominant modes and is repeated over and over in various frequency combinations involving the two dominant modes. The relative phases of the combination frequencies show a strong correlation with frequency, but the physical significance of this result is not clear.