A Comparison of the X-Ray Properties of FU Ori-type Stars to Generic Young Stellar Objects

Abstract

Like other young stellar objects (YSOs), FU Ori-type stars have been detected as strong X-ray emitters. However, little is known about how the outbursts of these stars affect their X-ray properties. We assemble available X-ray data from XMM-Newton and Chandra observations of 16 FU Ori stars, including a new XMM-Newton observation of Gaia  17bpi during its optical rise phase. Of these stars, six were detected at least once, while 10 were non-detections, for which we calculate upper limits on intrinsic X-ray luminosity ( L X ) as a function of plasma temperature ( kT ) and column density ( N H ). The detected FU Ori stars tend to be more X-ray luminous than is typical for non-outbursting YSOs, based on comparison to a sample of low-mass stars in the Orion Nebula Cluster. FU Ori stars with high L X have been observed both at the onset of their outbursts and decades later. We use the Kaplan–Meier estimator to investigate whether the higher X-ray luminosities for FU Ori stars are characteristic or a result of selection effects, and we find the difference to be statistically significant ( p  < 0.01) even when non-detections are taken into account. The additional X-ray luminosity of FU Ori stars relative to non-outbursting YSOs cannot be explained by accretion shocks, given the high observed plasma temperatures. This suggests that, for many FU Ori stars, either (1) the outburst leads to a restructuring of the magnetosphere in a way that enhances X-ray emission, or (2) FU Ori outbursts are more likely to occur among YSOs with the highest quiescent X-ray luminosity.