First evidence of pulsations in Vega?^⋆

Results of today’s most extensive spectroscopic search

¹ Université de Toulouse; UPS-OMP; IRAP;, Toulouse, France
e-mail: boehm@obs-mip.fr
² CNRS; IRAP ; 14 avenue Edouard Belin, 31400 Toulouse, France
³ Instituut voor Sterrenkunde, Celestijnenlaan 200D BUS 2401, Belgium
⁴ Dept. of Physics, Royal Military College of Canada, PO Box 17000, Stn Forces, Kingston K7KK 7B4, Canada
⁵ Observatoire de Paris-Meudon, LESIA, 92190 Meudon, France

Received: 27 September 2011
Accepted: 7 December 2011

Abstract

Context. The impact of rapid rotation on stellar evolution theory remains poorly understood as of today. Vega is a special object in this context as spectroscopic and interferometric studies have shown that it is a rapid rotator seen nearly pole one, a rare orientation particularly interesting for seismic studies. In this paper, we present a first systematic search for pulsations in Vega.

Aims. The goal of the present work is to detect for the first time pulsations in a rapidly rotating star seen nearly pole-on.

Methods. Vega was monitored in quasi-continuous high-resolution echelle spectroscopy. A total of 4478 spectra were obtained. More precisely in 2008 we obtained 1213 spectra during 19.9 h on 3 nights (26th, 27th and 29th of July 2008) with NARVAL/TBL (at R = 65   000 and R = 75   000), in 2009 we obtained 1293 spectra during 13.7 h on 3 nights (9th–11th of September 2009) with ESPaDOnS/CFHT (at R = 68   000) and in 2010 we gathered again 1972 with NARVAL/TBL during 28.8 h on five nights (July 15th–19th). This data set should represent the most extensive high S/N, high resolution quasi-continuous survey obtained on Vega as of today. Least square deconvolved (LSD) profiles were obtained for each spectrum representing the photospheric absorption profile potentially deformed by the presence of pulsations. In addition, we calculated for each spectrum a telluric line LSD profile subsequently used as radial velocity reference. LSD profile centroids were adjusted and velocity differences (stellar-telluric) determined. These residual velocities were analysed and periodic low amplitude variations, potentially indicative of stellar pulsations, detected. In a subsequent step, the temporal line profile variations during the longest (2010) data set was calculated for each individual velocity bin of 1.8 km s^-1  by computing a 2D (velocity-frequency) Lomb-Scargle periodogram.

Results. Based on high resolution echelle spectroscopy, we have obtained indications of periodic variations of very small amplitudes within the residual radial velocity curves of Vega. All three data sets revealed the presence of residual periodic variations: 5.32 and 9.19 d^-1  (A ≈ 6 m s^-1) in 2008, 12.71 and 13.25 d^-1  (A ≈ 8 m s^-1) in 2009 and 5.42 and 10.82 d^-1  (A ≈ 3–4 m s^-1) in 2010. However, it is too early to conclude that the variations are due to stellar pulsations, and a confirmation of the detection with a highly stable spectrograph is a necessary next step.

Conclusions. If pulsations are confirmed, their very small amplitudes show that the star would belong to a category of very “quiet” pulsators.