Pulsation spectrum of δ Scuti stars: the binary HD 50870 as seen with CoRoT and HARPS ^⋆

¹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy
e-mail: luciano.mantegazza@brera.inaf.it
² LESIA, UMR 8109, Observatoire de Paris, 5 place Jules Janssen, 92195 Meudon Cedex, France
³ Institut d’Astrophysique Spatiale, CNRS, Université Paris XI UMR 8617, 91405 Orsay, France
⁴ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía S/N, 18008 Granada, Spain
⁵ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92195 Meudon, France
⁶ Observatorio Astronómico Nacional, Instituto de Astronomia, UNAM, Apto Postal 877, Ensenada, BC 22860 México, Mexico
⁷ Institut de Recherche en Astrophysique et Planétologie, UMR 5277, 57 avenue d’Azereix, 65000 Tarbes, France
⁸ Departamento de Astrofísica, Centro de Astrobiología (INTA-CSIC), PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain

Received: 19 December 2011
Accepted: 15 February 2012

Abstract

Aims. We present the results obtained with the CoRoT satellite for HD 50870, a δ Sct star which was observed for 114.4 d. The aim of these observations was to evaluate the results obtained for HD 50844, the first δ Sct star monitored with CoRoT, on a longer time baseline.

Methods. The 307,570 CoRoT datapoints were analysed with different techniques. The photometric observations were complemented over 15 nights of high-resolution spectroscopy with HARPS on a baseline of 25 d. These spectra were analysed to study the line profile variations and to derive the stellar physical parameters. Some uvby photometric observations were also obtained to better characterize the pulsation modes.

Results. HD 50870 proved to be a low-amplitude, long-period spectroscopic binary system seen almost pole-on (i ≃ 21°). The brighter component, which also has the higher rotational velocity (vsini = 37.5 km s^-1), is a δ Sct-type variable with a full light amplitude variation of about 0.04 mag. There is a dominant axisymmetric mode (17.16 d^-1). Moreover, there are two groups of frequencies (about 19) in the intervals 6 − 9 and 13 − 18 d^-1, with amplitudes ranging from a few mmag to 0.3 mmag. After the detection of about 250 terms (corresponding to an amplitude of about 0.045 mmag) a flat plateau appears in the power spectrum in the low-frequency region up to about 35 d^-1. We were able to detect this plateau only thanks to the short cadence sampling of the CoRoT measurements (32 s). The density distribution vs. frequency of the detected frequencies seems to rule out the possibility that this plateau is the result of a process with a continuum power spectrum. The spacings of the strongest modes suggest a quasi-periodic pattern. We failed to find a satisfactory seismic model that simultaneously matches the frequency range, the position in the HR diagram, and the quasi-periodic pattern interpreted as a large separation. Nineteen modes were detected spectroscopically from the line profile variations and associated to the photometric ones. Tentative ℓ,m values have been attributed to the modes detected spectroscopically. Prograde as well as retrograde modes are present with ℓ values up to 9. There are no traces of variability induced by solar-like oscillations.