For the first time, an international group of scientists was able to demonstrate the existence of spots on the surface of the intermediate-mass star Vega. This unexpected result places important new constraints on the stellar evolution of intermediate mass stars and in particular on their magnetic field generation mechanisms, opening a window to formerly unreachable information. Two French institutes have contributed to this discovery: the institut de recherche en astrophysique et planétologie (IRAP – CNRS/Université Paul Sabatier Toulouse III), who conducted the work, and the institute de planétologie et d’astrophysique de Grenoble (IPAG – CNRS/Université Joseph Fourier). The results are published in the journal Astronomy & Astrophysics of 6 may 2015.
Vega (alpha, Lyrae) is a very bright northern hemisphere star, well
known by the astronomers. It is an intermediate-mass star (2 solar
masses) with a surface temperature of approximately 10000°C (spectral
class A). The star spins rapidly, with a period close to 0.68
terrestrial days. For more than 150 years it has been a stability
reference for light flux measurements (photometry). Despite the fact
that very small sporadic light variations were announced in the past, no
periodicity had been detected in its light curve.
In 2009 a very faint magnetic field was detected on Vega (see the “Nouvelle INSU” from the 18th of June 2009) and subsequently also on other stars of the same spectral class (A). While the solar magnetic field is generated by a dynamo mechanism in its convective envelope, the origin of magnetic field in stars lacking convective envelopes, such as Vega, remains mysterious. One of the characteristics of the solar dynamo is its temporal variability revealed by the appearance or disappearance of solar spots. Are there similar structures on the surface of Vega, and can they provide us with indications concerning the origin of its magnetic field? Recent research based on Kepler1 satellite data has suggested a rotational modulation of the photometric light curve (i.e. a variation in phase with the rotational period of the star) in 40 % of the observed A stars.
The group of scientists searched for direct signatures of such a modulation. To do this, they studied a set of 2500 high resolution and highly stabilized velocimetric spectra (visible domain; obtained 2012 with SOPHIE/OHP2). The precise acquisition moment of each of the 2500 acquired spectra was well known. For each spectrum, several hundreds of individual spectral lines contribute, as a function of their intensity, to an averaged spectroscopic line profile. Thereafter, the temporal evolution of this representative profile throughout the complete data set was studied.
When compared to an averaged equivalent line profile (of all 2500 spectra), tiny differences were detected in the individual profiles, enabling the scientists to extract the signatures of spots on the surface of the star (see the figure). The velocimetry is a key ingredient of this research and allows the determination, with extreme precision, the proper motions of stars, but also via the analysis of their spectral profile the motion and brightness inhomogeneities of structures on the stellar surface.
For the first time, and thanks to the exceptional quality of the data, the scientists have provided evidence of a structured surface showing bright or dark plages with a very tiny contrast on the prototype star Vega, representative of its spectral class A. These plages are dominantly located at lower latitudes, close to the stellar equator.
This discovery indicates that intermediate mass stars have a complex magnetic activity. Moreover, it opens a totally new range of observation potential. Indeed, some important additional observables of the physical conditions of A stars are now accessible: stellar rotation, surface magnetic activity and internal structure. Therefore Vega, despite numerous studies that have been devoted to it, still offers exciting new insights!
1 Kepler is a space telescope developed by the American space
agency NASA to detect exoplanets. It was launched in 2009. The
satellite uses the transit method. It observes stars using a 0.98 m
2 Spectrograph mainly used for the search for exoplanets by the radial velocity method, Observatoire de Haute Provence
- Discovery of starspots on Vega – First spectroscopic detection of surface structures on a normal A-type star, T. Böhm 1, 2, M. Holschneider 3, F. Lignières 1, 2, P. Petit 1, 2, M. Rainer 4, F. Paletou 1, 2, G. Wade 6, E. Alecian 5, H. Carfantan 1, 2, A. Blazère 1, 2, and G.M. Mirouh 1, 2, Astronomy & Astrophysics, 6 mai 2015.
- Nature Highlights
- Torsten Boehm, Torsten.Boehm@irap.omp.eu
- François Lignières, Francois.Lignieres@irap.omp.eu
- Pascal Petit, firstname.lastname@example.org
- Frédéric Paletou, email@example.com
- Hervé Carfantan, firstname.lastname@example.org