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Register a new userSpectroscopy of hot Gamma Doradus and A-F hybrid Kepler candidates close to the hot border of the Delta Scuti instability strip
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Added by
Filiz Kahraman Alicavus
Publication date
2020
fields
Physics
and research's language is
English
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If Gamma Dor-type pulsations are driven by the convective blocking mechanism, a convective envelope at a sufficient depth is essential. There are several hot Gamma Dor and hybrid star candidates in which there should not be an adequate convective envelope to excite the{gamma}Dor-type oscillations. The existence of these hot objects needs an explanation. Therefore, we selected, observed and studied 24 hot{gamma}Dor and hybrid candidates to investigate their properties. The atmospheric parameters, chemical abundances and vsini values of the candidates were obtained using medium-resolution (R= 46 000)spectra taken with the FIES instrument mounted at the NordicOptical Telescope. We also carried out frequency analyses of theKeplerlong- and short-cadence data to determine the exact pulsation contents. We found only five bona-fide hot{gamma}Dor and three bona-fide hot hybrid stars in our sample. The other 16 stars were found to benormal{gamma}Dor,{delta}Sct, or hybrid variables. No chemical peculiarity was detected in the spectra of the bona-fide hot{gamma}Dor and hybrid stars. We investigated the interplay between rotation and pulsational modes. We also found that the hot{gamma}Dor stars havehigherGaialuminosities and larger radii compared to main-sequence A-F stars.
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Context: Several hundred candidate hybrid pulsators of type A-F have been identified from space-based observations. Their large number allows both statistical analyses and detailed investigations of individual stars. This offers the opportunity to study the full interior of the genuine hybrids, in which both low-radial-order p- and high-order g-modes are self-excited at the same time. However, a few other physical processes can also be responsible for the observed hybrid nature, related to binarity or to surface inhomogeneities. The finding that most delta Scuti stars also show long-period light variations represents a real challenge for theory. Methods: Fourier analysis of all the available Kepler light curves. Investigation of the frequency and period spacings. Determination of the stellar physical parameters from spectroscopic observations. Modelling of the transit events. Results: The Fourier analysis of the Kepler light curves revealed 55 significant frequencies clustered into two groups, which are separated by a gap between 15 and 27 c/d. The light variations are dominated by the beating of two dominant frequencies located at around 4 c/d. The amplitudes of these two frequencies show a monotonic long-term trend. The frequency spacing analysis revealed two possibilities: the pulsator is either a highly inclined moderate rotator (v~70 km/s, i > 70 deg) or a fast rotator (v~200 km/s) with i~20 deg. The transit analysis disclosed that the transit events which occur with a ~197 c/d period may be caused by a 1.6 R_Jup body orbiting a fainter star, which would be spatially coincident with KIC 9533489.
We have obtained CCD photometry and medium-resolution spectroscopy of a number of $delta$ Scuti and $gamma$ Doradus stars in the Kepler field-of-view as part of the ground-based observational efforts to support the textit{Kepler} space mission. In this work we present the preliminary results of these observations.
Observations of the pulsations of stars can be used to infer their interior structure and test theoretical models. The main sequence $gamma$ Doradus (Dor) and $delta$ Scuti (Sct) stars with masses 1.2-2.5 $M_{sun}$ are particularly useful for these studies. The $gamma$ Dor stars pulsate in high-order $g$ modes with periods of order 1 day, driven by convective blocking at the base of their envelope convection zone. The $delta$ Sct stars pulsate in low-order $g$ and $p$ modes with periods of order 2 hours, driven by the $kappa$ mechanism operating in the Heii ionization zone. Theory predicts an overlap region in the Hertzsprung-Russell diagram between instability regions, where hybrid stars pulsating in both types of modes should exist. The two types of modes with properties governed by different portions of the stellar interior provide complementary model constraints. Among the known $gamma$ Dor and $delta$ Sct stars, only four have been confirmed as hybrids. Now, analysis of combined Quarter 0 and Quarter 1 Kepler data for hundreds of variable stars shows that the frequency spectra are so rich that there are practically no pure $delta$ Sct or $gamma$ Dor pulsators, i.e. essentially all of the stars show frequencies in both the $delta$ Sct and $gamma$ Dor frequency range. A new observational classification scheme is proposed that takes into account the amplitude as well as the frequency, and is applied to categorize 234 stars as $delta$ Sct, $gamma$ Dor, $delta$ Sct/$gamma$ Dor or $gamma$ Dor/$delta$ Sct hybrids.
By using a non-local and time-dependent convection theory, we have calculated radial and low-degree non-radial oscillations for stellar evolutionary models with $M=1.4$--3.0,$mathrm{M}_odot$. The results of our study predict theoretical instability strips for $delta$ Scuti and $gamma$ Doradus stars, which overlap with each other. The strip of $gamma$ Doradus is slightly redder in colour than that of $delta$ Scuti. We have paid great attention to the excitation and stabilization mechanisms for these two types of oscillations, and we conclude that radiative $kappa$ mechanism plays a major role in the excitation of warm $delta$ Scuti and $gamma$ Doradus stars, while the coupling between convection and oscillations is responsible for excitation and stabilization in cool stars. Generally speaking, turbulent pressure is an excitation of oscillations, especially in cool $delta$ Scuti and $gamma$ Doradus stars and all cool Cepheid- and Mira-like stars. Turbulent thermal convection, on the other hand, is a damping mechanism against oscillations that actually plays the major role in giving rise to the red edge of the instability strip. Our study shows that oscillations of $delta$ Scuti and $gamma$ Doradus stars are both due to the combination of $kappa$ mechanism and the coupling between convection and oscillations, and they belong to the same class of variables at the low-luminosity part of the Cepheid instability strip. Within the $delta$ Scuti--$gamma$ Doradus instability strip, most of the pulsating variables are very likely hybrids that are excited in both p and g modes.
Gamma Doradus are F-type stars pulsating with high order g-modes. Their instability strip (IS) overlaps the red edge of the delta Scuti one. This observation has led to search for objects in this region of the HR diagram showing p and g-modes simultaneously. The existence of such hybrid pulsators has recently been confirmed (Handler 2009) and the number of candidates is increasing (Matthews 2007). From a theoretical point of view, non-adiabatic computations including a time-dependent treatment of convection (TDC) predict the existence of gamma Dor/delta Sct hybrid pulsators (Dupret et al. 2004; Grigahcene et al. 2006). Our aim is to confront the properties of the observed hybrid candidates with the theoretical predictions from non-adiabatic computations of non-radial pulsations including the convection-pulsation interaction.
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