Exoplanets and stellar activity. The study: “Gliese 49: activity evolution and detection of a super-Earth. A HADES and CARMENES collaboration” of M. Perger (Institut de Ciències de l’Espai) recently appeared on A&A
There is no static star in the Universe. Stars move around the center of their galaxy, or they move together with other stars born from the same parental cloud, or they orbit around other stars in binary or multiple systems. Besides, stars with planets can oscillate in our sky while they orbit around the center of mass in common with their planetary system. These periodic oscillations can be observed by detecting periodic Doppler shifts of spectral lines in their spectra observed with moderate to high spectral resolution. This is the basic concept of the radial velocity method used to discover and characterize exoplanets.
The radial velocity method thus consists in detecting periodic Doppler shifts in spectral lines from the analysis of long time series of spectroscopic observations with moderate to high spectral resolution. However, periodic signals similar to those due to the presence of exoplanets can be also produced by the stellar surface magnetic activity, such as starspots and faculae. The signal from these phenomena, in fact, can be modulated by stellar rotation inducing periodic signals in stellar spectra. For this reason, in order to select stars with planets, it is mandatory to analyze in details the surface magnetic activity of the studied stars and identify their signals.
The international team led by the astronomer M. Perger (Institut de Ciències de l’Espai), also counting the astronomers from INAF – Astronomical Observatory of Palermo L. Affer, G. Micela e J. Maldonado, has analyzed a long time series of spectroscopic observations of the M1.5V star Gliese 49, in order to search for the presence of exoplanets. The spectra were acquired from the High Accuracy Radial velocity Planet Searcher in the South (HARPS) and North (HARPS-N), the Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical echelle Spectrographs (CARMENES) and the Iodine-cell HIgh Resolution echelle Spectrograph (HIRES). The study, described in the paper: “Gliese 49: activity evolution and detection of a super-Earth. A HADES and CARMENES collaboration” recently appeared on Astronomy & Astrophysics, confirms the presence of a super-Earth of 5.6 Earth masses orbiting the star every 13.85 days, in an eccentric orbit with a semi-major axis of 0.09 AU (1 AU is the average Eart-Sun distance, equal to 150 million of km). The planet as an average temperature of 350 K. The signal due to the planet was however well hidden in signals due to the stellar surface magnetic activity. The authors have identified two main signals: A long period signal (with a period longer than 1500 days) due to the motion and change in size of starspots, and a short period (40-80 days) signal due to the evolution of starspots.