The Sun as a star. The study: “Temporal evolution and correlations of optical activity indicators measured in Sun-as-a-star observations” of J. Maldonado (INAF-OAPA) recently appeared on A&A
Stellar magnetic activity results from the interaction between stellar magnetic field and the plasma in photosphere, chromosphere, and corona. This activity produces well known phenomena, such as flares, spots, faculae, and prominences, observed in stars of almost every type and mass. This activity is typically studied by analyzing spectroscopic activity indicators produced by these phenomena, such as the H and K lines of singly ionized Calcium (the Ca II H&K lines at 3969 e 3934 Å), produced by chromospheric activity; the Balmer series lines produced by electronic transitions in the Hydrogen atoms, such as the Hα (6563 Å) e Hβ (4861 Å) lines; and emission lines of elements such as Helium and Sodium.
The study of stellar magnetic activity is important for two reasons. First, it probes the stellar magnetic fields, their topology and their interaction with the stellar plasma at temperatures of thousands to million degrees. This interaction can not be reproduced in laboratories on Earth, and thus stars are the only places where we can observe it. Besides, when exoplanets are searched with the radial velocity technique (e.g. the detection of the oscillation of stars with planets due to the mutual gravitational attraction between star and planets, which can be observed with medium to high resolution spectroscopy thanks the to Doppler effect), stellar magnetic activity produces signals that can mimic the presence of exoplanets or dominate the signals due to the planets. For instance, the radial velocity signal due to a super-Earth orbiting around a low mass star is about 10 cm/sec, more than one order of magnitude fainter than the signals due to stellar magnetic activity.
Magnetic phenomena are spatially resolved only in the Sun. Due to their large distance from us, we observe the magnetic activity of the other stars integrated over their full disk, with no information on where the observed phenomena are actually occurring. In order to take advantage of the spatially resolved observations on the Sun to understand the magnetic phenomena occurring in the other stars, the HARPS-N instrument mounted on the Telescopio Nazionale Galileo monitors the solar magnetic activity with the more common activity indicators or the indicators used to search for exoplanets with the technique of the radial velocity. These data have been analyzed by the international team of astronomers led by J. Maldonado (INAF – Astronomical Observatory of Palermo). This team demonstrated that it is possible to retrieve the Sun rotation period using all the activity indicators, with the only exception of the Hσ line. The measured rotation periods span over the 26.3-31.2 days range. These differences are likely due to migration and evolution of photospheric spots. Evident correlation have been also found between several activity indicators and both the integrated intensity of the magnetic field and the radial velocity signal produced by stellar activity. The results of this study are published in the paper “Temporal evolution and correlations of optical activity indicators measured in Sun-as-a-star observations“, recently appeared on Astronomy & Astrophysics.
The figure (link) shows the time series of the activity indicators and the periodograms obtained from these observations.
by Mario Giuseppe Guarcello ( follow mguarce)