A Ca ii H&K survey of gamma Doradus candidates

July 27, 2017 | Autor: Anthony Kaye | Categoria: Oscillations, Chromosphere
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Descrição do Produto

Mon. Not. R. Astron. Soc. 294, L35–L39 (1998)

A Ca II H&K survey of g Doradus candidates Anthony B. Kaye1 * and Klaus G. Strassmeier2 * 1 2

Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30303, USA Institut fu¨r Astronomie, Universita¨t Wien, Tu¨rkenschanzstraße 17, A-1180 Wien, Austria

Accepted 1997 December 3. Received 1997 November 24; in original form 1997 October 10

A B S T R AC T

In an investigation of the starspot hypothesis as it applies to the ‘slowly variable’ F-type dwarfs, we spectroscopically observed eight promising g Doradus candidates to search for Ca ii H&K emission. We found that there are no significant emission reversals in the cores of these resonance lines. Based on the ceiling flux calculations of the Ca ii K line and on calculations of the Rossby number, we conclude that there is no support for the presence of strong magnetic activity and the starspot hypothesis in these objects. Key words: stars: activity – stars: chromospheres – stars: oscillations – stars: variables: other.

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INTRODUCTION

The ‘slowly variable’ F-type dwarfs, now known as g Doradus variables after the brightest member of the class (and the first to be discovered; see Cousins & Warren 1963), constitute a new type of variable star (Balona, Krisciunas & Cousins 1994; Mantegazza, Poretti & Zerbi 1994; Krisciunas & Handler 1995; Krisciunas et al. 1995; Kaye et al. 1997). In contrast to spectroscopically normal, single F-type dwarf stars, g Doradus variables show variation in both their light curves and their spectra. The photometric periods of these stars range between 0.5 and 3.5 d, periods which are an order of magnitude longer than the fundamental radial pulsation periods for these stars; an alternative physical mechanism for the observed variability must be found. The area of the Hertzsprung–Russell diagram occupied by these objects has long been thought to be a haven for photometrically ‘constant’ (i.e. stable) stars as most lie to the cool side of the Cepheid instability strip and so they should not pulsate. They are also hot enough so that they are thought to have radiative envelopes and therefore should not have large, magnetically induced starspots. Even so, recent investigations (Krisciunas et al. 1993, 1995; Zerbi et al. 1997; Kaye et al. 1997) have shown (much to the dismay of observers using these objects as comparison stars) that some g Doradus stars vary by more than
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