Magnetic activity and convection must be accounted for studying young stars . The paper: “Gaia-ESO Survey: Role of magnetic activity and starspots on pre-main-sequence lithium evolution” of E. Franciosini (INAF – OA Arcetri) recently appeared on A&A
Star clusters contain a rich population of stars covering a wide range of stellar masses (sometimes from the brown dwarfs to massive stars) with a well defined age. For this reason, star clusters represent fundamental targets to test the theoretical models describing stellar physics and evolution. However, this requires precise measurement of stellar clusters age. The method which is commonly used to measure clusters age consists in comparing the measured star colors and magnitudes with those predicted by models describing stars with fixed age and a large range of masses (isochrones). A method which has been introduced more recently, instead, consists in measuring the abundance of lithium in stars over a wide range of masses. This chemical element, in fact, is rapidly consumed in stellar interior at a temperature of 2.5-3 million degrees. Thus, the superficial abundance of lithium decreases as a function of the efficiency of the convection in stars outer layers, which in turns depends on stellar mass and evolutionary phase. For this reason, in clusters the boundary in mass between lithium-rich and lithium-depleted stars depends on the cluster age (Lithium Depletion Boundary).
Recent studies have shown that in some young clusters isochrones obtained from theoretical models can not fit simultaneously the observed distributions in the magnitude vs. colors and the lithium abundance vs. stellar temperature diagrams. For instance, in the star cluster Gamma Velorum, isochrones with an age of 18-21 million of years can reproduce the observed distributions only assuming that stars are inflated by about 10% with respect to previous models. Similar results have been obtained in other star clusters, stars in binary systems, and magnetically active field stars. In the Iades and Pleiades star clusters (620 and 100 million years old, respectively) inflated stars are those with rapid rotation and thus a high level of magnetic activity.
From the analysis of spectroscopic data, obtained in the framework of the Gaia-ESO Survey, and both photometric and kinematic data obtained with the Gaia satellite of the European Space Agency, the team of researchers led by the astronomer E. Franciosini (INAF – Osservatorio Astrofisico di Arcetri) verified that in order to reproduce the colors, magnitudes and lithium abundances observed in five star clusters with the age ranging from 10 to 100 million of years, it is necessary to adopt models that account for the effects of the magnetic field on the convection and the filling factor on stellar surface of photospheric spots (which are regions of stellar surface colder than the stellar effective temperature and characterized by an intense magnetic field). In particular, stars younger than 20 million of years require isochrones obtained from models that account for a large coverage by dark spots and the inhibition of convection by the intense magnetic field. In older cluster, instead, stellar colors, magnitudes, and lithium abundances can be reproduced by standard models. Only for low-mass stars it is necessary to adopt isochrones that assume a spot filling factor of about 20%. The study is described in the paper: “Gaia-ESO Survey: Role of magnetic activity and starspots on pre-main-sequence lithium evolution“, recently published by Astronomy & Astrophysics, and it shed some light on the importance of getting a deeper insight on star physics in order to understand their early evolution. Among the collaborators, there are astronomers Sara Bonito, Loredana Prisinzano and Francesco Damiani dell’INAF – Astronomical Observatory of Palermo.
The figure (click here to visualize the entire image) shows the comparison between stellar measured quantities (magnitude vs. colors obtained with Gaia in the left panel; lithium abundances vs. effective temperatures in the right panel) and those predicted by models for the case of the cluster 25 Ori. The age of the isochrones is set to 19 million years, the interstellar reddening to E(B-V)=0.065. The blue line is an isochrone obtained from a model which assumes 20% spots filling factor, while the green line does not account for spots coverage. Both models account for a reduced convection efficiency.
Mario Giuseppe Guarcello ( follow mguarce) ( youtube)
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