Abstract
According to the magnetospheric accretion scenario, young low-mass stars are surrounded by circumstellar disks which they interact with through accretion of mass. The accretion builds up the star to its nal mass and is also believed to power the mass out ows, which may in turn have a signicant role in removing the excess angular momentum from the star-disk system. Although the process of mass accretion is a critical aspect of star formation, some of its mechanisms are still to be fully understood. On the other hand, strong aring activity is a common feature of young stellar objects (YSOs). In the Sun, such events give rise to perturbations of the interplanetary medium. Similar but more energetic phenomena occur in YSOs and may in uence the circumstellar environment. In fact, a recent study has shown that an intense flaring activity close to the disk may strongly perturb the stability of circumstellar disks, thus inducing mass accretion episodes (Orlando et al. 2011). Here we review the main results obtained in the eld and the future perspectives.
Highlights
Observations in the X-ray band reveal that low-mass pre-main-sequence stars are strong sources with X-ray luminosities 3−4 orders of magnitude greater than that of the present-day Sun
Strong magnetic fields are believed to connect the star with a Keplerian circumstellar disk, funneling accretion onto limited portions of the stellar surface (e.g. Hartmann 1998) where shocks are produced by the impact of the accretion streams (e.g. Orlando et al 2010)
As a follow-up of the previous study, we explored in more details the possibility that significant mass accretion in young stars can be triggered by a storm of small-to-medium flares occurring on the accretion disk (Orlando et al 2014)
Summary
Observations in the X-ray band reveal that low-mass pre-main-sequence stars are strong sources with X-ray luminosities 3−4 orders of magnitude greater than that of the present-day Sun. The source of this high-energy radiation is plasma with temperatures of 1 − 100 MK in the stellar outer atmospheres (coronae), heated by magnetic activity analogous to the solar one but higher by factors up to 106 Such a magnetic activity manifests through very different phenomena that may occur in several places of the stellar atmosphere and circumstellar environment. X-ray observations in the last decades have shown that flares in young stellar objects (YSOs) have amplitudes much larger than solar analogues and occur much more frequently. Examples of these flares are those collected by the Chandra satellite in the Orion star-formation region (COUP enterprise; Favata et al 2005). Since the central star is surrounded by a circumstellar disk accreting material onto the star, it is natural to ask whether strong flaring activity involves the disk and even perturbs its stability, possibly affecting the mass accretion to the star
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