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Kepler observations of δ Scuti stars
L. A. Balona
1
W. A. Dziembowski
0
0
Warsaw University Observatory
,
Al
. Ujazdowskie 4, 00-478 Warszawa,
Poland
1
South African Astronomical Observatory, PO Box 9, Observatory 7935, Cape Town,
South Africa
A B S T R A C T We analyse observations of 1568 Scuti stars in the public archive of the Kepler satellite. We compare the location of these stars in the Hertzsprung-Russell (HR) diagram with that calculated by linear, non-adiabatic pulsation models. There is generally a good agreement and it appears that many of the hotter Scuti stars are pulsating in overtones as high as radial order n = 8. Models predict unstable modes of intermediate and high spherical harmonic, l, which are trapped in the envelope. The instability strip for these modes extends well beyond the blue edge for l 4 and should be visible in the Kepler data. However, stars with these predicted properties are not seen. Moreover, we find that the observed frequency range does not agree with the models. Another challenge is to understand why more than half of the stars in the Sct instability strip are not pulsating. The distribution of amplitudes argues strongly against the notion that the pulsation amplitudes are below the Kepler detection limit. The mode density of Scuti stars is surprisingly low and suggests that modes of a high degree are not common. We do not find any stars with mode densities as high as that found in the CoRoT observations of HD 50844. The periodograms of stars in the same part of the HR diagram vary widely. However, we have identified a group of Scuti stars characterized by a single dominant frequency in which a period-luminosity law is present. In many cases the dominant frequency is accompanied by a close frequency of lower amplitude. The problem of low frequencies in hot Scuti stars remains unresolved. We confirm that these frequencies have long lifetimes and are not likely to be of stochastic origin. By carefully examining the periodograms of Sct stars in the low-frequency regime, we were able to identify a group of Sct stars with low-frequency peaks closely resembling those attributed to pure Dor stars. These stars lie in the region of the HR diagram where ground-based Dor stars are located. None is found hotter than the blue edge of Dor pulsation. We suspect that these are genuine Dor/ Sct hybrids.
1 I N T R O D U C T I O N
The Scuti class of variables are dwarfs or giants with spectral
types between A2 and F5. They lie on an extension of the Cepheid
instability strip with periods in the range of 0.020.3 d. Most of the
pulsational driving in these stars is by the mechanism in the He II
partial ionization zone with some contribution from H. What we
may call classical Scuti stars (pulsating main-sequence or giant
AF stars of solar composition) have been particularly well studied
because they promise to offer good prospects for asteroseismology.
In part this is because many independent pulsation modes have
been detected, but also because they are bright, numerous and easy
to observe. The high-amplitude Scuti (HADS) stars stand out
as transition objects between Scuti and classical Cepheids. The
HADS stars are mainly slow-rotating and pulsate radially with
peakto-peak amplitudes exceeding 0.3 mag in V. Most other Sct stars
are faster rotating with many low-amplitude radial and non-radial
modes.
The Kepler Mission is designed to detected earth-like planets
around solar-type stars by the transit method (Koch et al. 2010).
Kepler is continuously monitoring the brightness of over 100 000
stars for at least 3.5 yr in a 105 deg2 fixed field of view with
extremely accurate photometry (at the micromagnitude level). Kepler
observations of 750 AF stars have been recently discussed by
Uytterhoeven et al. (2011). They propose three main groups to
describe the observed variety in these stars: Dor, Sct and hybrid
stars. They find that about 64 per cent of the AF stars can be
classified into one of these three groups and identify the remaining
stars as rotationally modulated/active stars, red giants, binaries,
Btype stars or stars that show no clear periodic variability. They find
that 23 per cent of the AF stars have low-frequency variations in
addition to Sct pulsations, which is a much larger fraction than has
been observed from the ground. As already found by Grigahce`ne
et al. (2010), Uytterhoeven et al. (2011) find that these hybrid stars
are located over the entire Sct and Dor instability strips.
The low frequencies which are seen in Scuti stars are also
present in non- Sct A-type stars in the Kepler data (Balona 2011). It
turns out that the dominant frequency in the range below 5 cycle d1
is most probably the rotational frequency in AF stars, though it is
not clear why other low frequencies are also often present. In Sct
stars, however, the dominant low frequency appears to correspond
to twice the rotational frequency (Balona 2011), though why this
should be the case is a not understood.
In pre (...truncated)