Flares in the young stars of NGC 2264 observed in optical, infrared and X-rays

Most of the stars, mainly the low mass stars, produce their own magnetic field in their interior. The interaction between the magnetic field and stellar plasma is the cause of phenomena such as photosperic spots, faculae, protuberances, coronal mass ejections, and flares. The study of this activity is important both because it reveals how intense magnetic fields interact with high-temperature plasma, and because it can probe the stellar internal structure and its evolution.

 

Given its proximity, the Sun is the only star where we can spatially resolve flares and the other phenomena associated with magnetic activity. In all the other stars, we can only study their integrated emission in different bands of the electromagnetic spectrum and in the time domain, e.g. studying stellar variability and their light curves. Stellar flares require a multi-band approach since they are complex phenomena, characterized by interconnected phenomena occurring in different parts of stellar atmosphere and emitting different kind of radiation. For instance, the pre-flare, during which the plasma in the photosphere is heated up to million degrees and it starts evaporating in the upper atmosphere, is characterized by hard X-ray emission and it is typically followed by a white-light flare with emission in infrared, optical, and UV light. This is then followed by the coronal flare, during which the evaporating plasma fills the coronal loops formed by the magnetic fields, where the plasma is heated up to several millions degrees and it starts emitting soft X-rays.

 

Stellar flares are observed in almost all kind of stars, but they are preferentially studied in young stars. Magnetic activity, in fact, scales with stellar rotation, which scales with stellar age. For this reason, young pre-main sequence stars are order of magnitudes more active than old stars such as the Sun. Besides, young stars are often part of stellar clusters, which offer a large number of targets in a small area, being easy to be observed with telescopes with a small field of view.

 

The study: “A multi-wavelength view of magnetic flaring from PMS stars” of E. Flaccomio (Inaf – OAPA), which is part of the project Coordinated Synoptic Investigation of NGC 2264”  and it recently appeared in Astronomy & Astrophysics, is one of the rare papers based on multi-wavelength observations of flares occurring in young stars (the members of NGC 2264, a 1-2 Myrs old cluster). The authors have analyzed 68 flares occurred in 65 stars, observed in X-rays (with Chandra/ACIS-I) and simultaneously in optical (CoRoT) and/or infrared (Spitzer/IRAC). This has allowed to analize and quantify the correlation between the energy emitted in optical during the flares and that emitted in X-rays. Besides, evidence has been found that in stars with protoplanetary disks (the disks typically observed around young stars and that may evolve in planetary systems) infrared flares are brighter than in stars without disks. This is likely due to the  of flare emission absorbed and reprocessed by the dust in the disk, an evidence for the importance of stellar flares in the early planets-forming environments.

 

The figure (link) shows some of the flares analyzed in this paper.

 

by Mario Giuseppe Guarcello    ( follow mguarce)