科研之友
微信
新浪微博
Facebook
分享链接
摘要
Accretion in young stellar objects (YSOs) is at least partially episodic, i.e. periods with high accretion rates (%26apos;bursts%26apos;) are interspersed by quiescent phases. These bursts manifest themselves as eruptive variability. Here we present a systematic survey for eruptive YSOs aiming to constrain the frequency of accretion bursts. We compare mid-infrared photometry from Spitzer and WISE separated by similar to 5 yr for two samples of YSOs, in nearby star-forming regions and in the Galactic plane, each comprising about 4000 young sources. All objects for which the brightness at 3.6 and 4.5 mu m is increased by at least 1 mag between the two epochs may be eruptive variables and burst candidates. For these objects, we carry out follow-up observations in the near-infrared. We discover two new eruptive variables in the Galactic plane which could be FU Ori-type objects, with K-band amplitudes of more than 1.5 mag. One object known to undergo an accretion burst, V2492 Cyg, is recovered by our search as well. In addition, the young star ISO-Oph-50, previously suspected to be an eruptive object, is found to be better explained by a disc with varying circumstellar obscuration. In total, the number of burst events in a sample of 4000 YSOs is 1-4. Assuming that all YSOs undergo episodic accretion, this constraint can be used to show that phases of strong accretion (%26gt; 10(-6) M-circle dot yr(-1)) occur in intervals of about 10(4) yr, most likely between 5000 and 50 000 yr. This is consistent with the dynamical time-scales for outflows, but not with the separations of emission knots in outflows, indicating that episodic accretion could either trigger or stop collimated large-scale outflows.
- 出版日期2013-4
