Preparing the COROT space mission: Incidence and characterisation of pulsation in the lower instability strip^*

¹ INAF-Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate, Italy
² Instituto de Astrofisica de Andalucia, C.S.I.C., Apdo. 3004, 18080 Granada, Spain
³ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200 B, 3001 Leuven, Belgium
⁴ South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa
⁵ Institut für Astronomie, Universität Wien, Türkenschanzstrasse 17, 1180 Wien, Austria
⁶ Instituto de Astrofisica de Canarias, C/ via Lactea s/n, 38200 La Laguna, Tenerife, Spain
⁷ Observatoire de Paris, LESIA, FRE 2461, 92195 Meudon, France
⁸ Observatoire de la Côte d'Azur, UMR 6528, BP 4229, 06304 Nice Cedex, France
⁹ Dept. of Astronomy, University of Cape Town, Rondebosch 7700, South Africa
¹⁰ Dept. of Physics, University of the North-West, Private Bag X2046, Mmabatho 2735, South Africa
¹¹ Observatoire de Paris, GEPI, 92195 Meudon, France
¹² Laboratoire d'Astrophysique de Marseille, Traverse du Siphon, BP 8, 13376 Marseille Cedex 12, France
¹³ Laboratoire d'Astrophysique, Observatoire Midi-Pyrénées, 14 avenue E. Belin, 31400 Toulouse, France

Corresponding author: E. Poretti, poretti@merate.mi.astro.it

Received: 24 February 2003
Accepted: 18 April 2003

Abstract

By pursuing the goal to find new variables in the COROT field–of–view we characterised a sample of stars located in the lower part of the instability strip. Our sample is composed of stars belonging to the disk population in the solar neighbourhood. We found that 23% of the stars display multiperiodic light variability up to a few mmag in amplitude, i.e., easily detectable on a single night of photometry. photometry fixed most of the variables in the middle of the instability strip and high–resolution spectroscopy established that they have km s^-1. An analysis of the Rodriguez & Breger ([CITE]) sample (δ Sct stars in the whole Galaxy) shows slightly different features, i.e., most δ Sct stars have a 0.05–mag redder index and lower values. Additional investigation in the open cluster NGC 6633 confirms the same incidence of variability, i.e., around 20%. The wide variety of pulsational behaviours of δ Sct stars (including unusual objects such as a variable beyond the blue edge or a rapidly rotating high–amplitude pulsator) makes them very powerful asteroseismic tools to be used by COROT. Being quite common among bright stars, δ Sct stars are suitable targets for optical observations from space.