Context. The Kepler spacecraft is providing time series of photometric data with micromagnitude precision for hundreds of A-F type stars. Aims. We present a first general characterization of the pulsational behaviour of A-F type stars as observed in the Kepler light curves of a sample of 750 candidate A-F type stars, and observationally investigate the relation between γ Doradus (γ Dor), δ Scuti (δ Sct), and hybrid stars. Methods. We compile a database of physical parameters for the sample stars from the literature and new ground-based observations. We analyse the Kepler light curve of each star and extract the pulsational frequencies using different frequency analysis methods. We construct two new observables, “energy ”a nd “efficiency”, related to the driving energy of the pulsation mode and the convective efficiency of the outer convective zone, respectively. Results. We propose three main groups to describe the observed variety in pulsating A-F type stars: γ Dor, δ Sct, and hybrid stars. We assign 63% of our sample to one of the three groups, and identify the remaining part as rotationally modulated/active stars, binaries, stars of different spectral type, or stars that show no clear periodic variability. 23% of the stars (171 stars) are hybrid stars, which is a much higher fraction than what has been observed before. We characterize for the first time a large number of A-F type stars (475 stars) in terms of number of detected frequencies, frequency range, and typical pulsation amplitudes. The majority of hybrid stars show frequencies with all kinds of periodicities within the γ Dor and δ Sct range, also between 5 and 10 d −1 , which is a challenge for the current models. We find indications for the existence of δ Sct and γ Dor stars beyond the edges of the current observational instability strips. The hybrid stars occupy the entire region within the δ Sct and γ Dor instability strips and beyond. Non-variable stars seem to exist within the instability strips. The location of γ Dor and δ Sct classes in the (Teff ,l ogg)-diagram has been extended. We investigate two newly constructed variables, “efficiency ”a nd “energy”, as a means to explore the relation between γ Dor and δ Sct stars. Conclusions. Our results suggest a revision of the current observational instability strips of δ Sct and γ Dor stars and imply an investigation of pulsation mechanisms to supplement the κ mechanism and convective blocking effect to drive hybrid pulsations. Accurate physical parameters for all stars are needed to confirm these findings.

/pdf/the-kepler-characterization-of-the-variability-among-a-and-f-y9gptafczn.pdf

The Kepler characterization of the variability among A- and F-type stars I. General overview

The Kepler spacecraft is providing time series of photometric data with micromagnitude precision for hundreds of A-F type stars. We present a first general characterization of the pulsational behaviour of A-F type stars as observed in the Kepler light curves of a sample of 750 candidate A-F type stars. We propose three main groups to describe the observed variety in pulsating A-F type stars: gamma Dor, delta Sct, and hybrid stars. We assign 63% of our sample to one of the three groups, and identify the remaining part as rotationally modulated/active stars, binaries, stars of different spectral type, or stars that show no clear periodic variability. 23% of the stars (171 stars) are hybrid stars, which is a much larger fraction than what has been observed before. We characterize for the first time a large number of A-F type stars (475 stars) in terms of number of detected frequencies, frequency range, and typical pulsation amplitudes. The majority of hybrid stars show frequencies with all kinds of periodicities within the gamma Dor and delta Sct range, also between 5 and 10 c/d, which is a challenge for the current models. We find indications for the existence of delta Sct and gamma Dor stars beyond the edges of the current observational instability strips. The hybrid stars occupy the entire region within the delta Sct and gamma Dor instability strips, and beyond. Non-variable stars seem to exist within the instability strips. The location of gamma Dor and delta Sct classes in the (Teff,logg)-diagram has been extended. We investigate two newly constructed variables 'efficiency' and 'energy' as a means to explore the relation between gamma Dor and delta Sct stars. Our results suggest a revision of the current observational instability strips, and imply an investigation of other pulsation mechanisms to supplement the kappa mechanism and convective blocking effect to drive hybrid pulsations.

/pdf/the-kepler-characterization-of-the-variability-among-a-and-f-2au724c8rd.pdf

The Kepler characterization of the variability among A- and F-type stars. I. General overview

In this paper we describe a new class of pulsating stars, the prototype of which is the bright, early, F‐type dwarf γ Doradus These stars typically have between 1 and 5 periods ranging from 04 to 3 days with photometric amplitudes up to 01 mag in Johnson V The mechanism for these observed variations is high‐order, low‐degree, nonradial, gravity‐mode pulsation

https://iopscience.iop.org/article/10.1086/316399/pdf

γ Doradus Stars: Defining a New Class of Pulsating Variables

By studying the oscillations of a star's surface, it is possible to extract information about stellar strata, age, and dynamics. Though one might guess that observation of surface oscillatory motions would be limited to our Sun, high-precision brightness measurements of distant stars, performed over many years, have enabled the field of asteroseismology. This comprehensive review covers the recent development of this field, the necessary blending of numerical simulation and data, and the way in which this new information enhances our understanding of stellar evolution.

/pdf/probing-the-interior-physics-of-stars-through-2dqd8qjata.pdf

Probing the interior physics of stars through asteroseismology

In this paper we describe a new class of pulsating stars, the prototype of which is the bright, early, F-type dwarf, Gamma Doradus. These stars typically have between 1 and 5 periods ranging from 0.4 to 3 days with photometric amplitudes up to 0.1 in Johnson V. The mechanism for these observed variations is high-order, low-degree, non-radial, gravity-mode pulsation. Gamma Doradus stars exhibit variability on a time scale that is an order of magnitude slower than Delta Scuti stars. They may offer additional insight into stellar physics when they are better understood (e.g., they may represent the cool portion of an "iron opacity instability strip" currently formed by the Beta Cephei stars, the SPB stars, and the subdwarf B stars; they may also offer insight into the presence of g-modes in solar-like stars).

Gamma Doradus Stars: Defining a New Class of Pulsating Variables

The RS CVn-type eclipsing binary AR Lac has been observed in two colours,B andV, in 1978 and 1979. Several times of minima have been obtained during the observations and the new light elements have been calculated. The orbital period of the system was found to be decreasing with an amount of 14.6 s century−1. The light curves of the system obtained inB andV show a significant wave-like distortion outside eclipses. The amplitudes of the wave-like distortion inB andV are about 0
 .
 09 and 0
 .
 10 in 1978, and, 0
 .
 12 and 0
 .
 13 in 1979, respectively. The minima of the wave-like distortion have shifted with an amount of 0
 .
 40 in one year, thus indicating the migration period of the wave-like distortion is about 2.5 yr. A noticeable variation of light during the totality of primary minimum has also been observed. This variation can be attributed to the distortion wave which has been moving towards decreasing phases.

Two-colour photoelectric investigation of AR Lacertae

The eclipsing variable RT Lac, which is classified as an RS CVn-type binary by Hall, was observed in two colours,B andV, during the summer seasons of 1978 and 1979. The observations made during two successive seasons indicate that, outside eclipses, the system has brightened by about 0.12 and 0.15 magnitudes on average inB andV, respectively. Moreover, RT Lac was even fainter when observed by Milone in 1965. While the total brightness of the system remained unchanged at mid-secondary, it increased at mid-primary and outside eclipses. This may be interpreted as indicating that the cooler component is responsible for the peculiar light variations.

Light curve variations of RT lacertae

In this paper we present the observations of the peculiar eclipsing binary RT Lac, which were obtained in 1980 and 1981. Examining the light curves obtained in successive four years we estimate the period of the migrating light curve distortion to be about four years. The orbital period of the system was also found to be decreasing with an amount 34s century−1. The brightnesses both at primary and secondary are changing with the time, which is clearly seen from the last light curves. The variations occurred at primary is three times greater than secondary eclipse. Therefore, we suggest that the star, in front at primary eclipse, is more responsible for the distorted light curves and it should be the earlier spectral type and also the smaller one.

Extensive photometric study of RT lacertae

The RS CVn-type eclipsing binary SZ Psc was observed in two coloursB andV, in 1981. The rate of change of the period was calculated using two different least-squares procedures. The amplitudes of the light variations at outside eclipses are 0 . m 181 and 0 . m 213 inB andV colours, respectively. The light curves of the system have been solved using a new approximation. The radius of the cooler component is only 82% of its Roche-radius and the system is detached one.

Two-colour photoelectric investigation of SZ Pisces

The observations of the peculiar eclipsing binary RT Lac were continued during the observing seasons of 1982 and 1983. The salient properties of the light curves were presented. The shallowest primary eclipse was observed in 1983. When the light curves, obtained in six years sucessively, are interpreted together it is seen that the variations of the brightness at mid-primary and second maximum are too similar to each other. The colour variations at mid-primary also follow those variations in the brightness. These circumstances indicate that the starspot hypothesis is insufficient alone to explain all the phenomena observed. The earlier spectral type G9 companion may be intrinsically variable and the unusual distortion on the light curves following the secondary eclipse may arise from the gas-stream from larger K star to the G9 star.

Photoelectric photometry of the peculiar system RT Lacertae

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