Discovery of starspots on Vega^⋆

First spectroscopic detection of surface structures on a normal A-type star

¹ Université de Toulouse, UPS-OMP, IRAP, Toulouse, France
e-mail: torsten.boehm@irap.omp.eu
² CNRS; IRAP ; 14 avenue Édouard Belin, 31400 Toulouse, France
³ Institut für Mathematik, Universität Potsdam, DYCOS, 14469 Potsdam, Germany
⁴ INAF–Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
⁵ Observatoire de Grenoble, Université Joseph Fourier, IPAG, 38041 Grenoble, France
⁶ Dept. of Physics, Royal Military College of Canada, PO Box 17000, Stn Forces, Kingston, Canada K7KK 7B4

Received: 27 November 2014
Accepted: 20 March 2015

Abstract

Context. The theoretically studied impact of rapid rotation on stellar evolution needs to be compared with these results of high-resolution spectroscopy-velocimetry observations. Early-type stars present a perfect laboratory for these studies. The prototype A0 star Vega has been extensively monitored in recent years in spectropolarimetry. A weak surface magnetic field was detected, implying that there might be a (still undetected) structured surface. First indications of the presence of small amplitude stellar radial velocity variations have been reported recently, but the confirmation and in-depth study with the highly stabilized spectrograph SOPHIE/OHP was required.

Aims. The goal of this article is to present a thorough analysis of the line profile variations and associated estimators in the early-type standard star Vega (A0) in order to reveal potential activity tracers, exoplanet companions, and stellar oscillations.

Methods. Vega was monitored in quasi-continuous high-resolution echelle spectroscopy with the highly stabilized velocimeter SOPHIE/OHP. A total of 2588 high signal-to-noise spectra was obtained during 34.7 h on five nights (2 to 6 of August 2012) in high-resolution mode at R = 75 000 and covering the visible domain from 3895−6270 Å. For each reduced spectrum, least square deconvolved equivalent photospheric profiles were calculated with a T_eff = 9500 and log g = 4.0 spectral line mask. Several methods were applied to study the dynamic behaviour of the profile variations (evolution of radial velocity, bisectors, vspan, 2D profiles, amongst others).

Results. We present the discovery of a spotted stellar surface on an A-type standard star (Vega) with very faint spot amplitudes ΔF/Fc ~ 5 × 10^-4. A rotational modulation of spectral lines with a period of rotation P = 0.68 d has clearly been exhibited, unambiguously confirming the results of previous spectropolarimetric studies. Most of these brightness inhomogeneities seem to be located in lower equatorial latitudes. Either a very thin convective layer can be responsible for magnetic field generation at small amplitudes, or a new mechanism has to be invoked to explain the existence of activity tracing starspots. At this stage it is difficult to disentangle a rotational from a stellar pulsational origin for the existing higher frequency periodic variations.

Conclusions. This first strong evidence that standard A-type stars can show surface structures opens a new field of research and ask about a potential link with the recently discovered weak magnetic field discoveries in this category of stars.