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 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 is 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.

Measuring Optical Extinction Towards Young Stellar Objects Using Diffuse Interstellar Bands

“…However, FU Ori is not completely obscured, and the X-ray N (H) value is clearly over-estimating the extinction to the optical source, even if it does accurately describe the obscuration to the X-ray continuum. For the few targets with Xray derived A V s from Kuhn & Hillenbrand (2019), there is, however, some agreement with our DIBs-inferred A V s for the two other sources in common (V960 Mon and Z CMa).…”

“…Extinctions derived from X-rays seem less influenced by outburst variability (Kuhn & Hillenbrand 2019). However, they cannot always be simply converted into V band extinctions.…”

“…l Work by Kuhn & Hillenbrand (2019) using Gaia DR2 parallaxes places V1057 Cyg at 795 pc, rather than 550 pc cited in Connelley & Reipurth (2018). The updated luminosity would be 209L .…”

Diagnosing FU Ori-like Sources: The Parameter Space of Viscously Heated Disks in the Optical and Near-infrared

“…For example, the analytical formula for a viscously heated disk assumes that the disk is geometrically thin, even though the innermost disk is likely thick. The star-disk interaction region is also complicated; the attenuation of X-rays from FUor objects may even indicate that the inner disk envelops the star (Kuhn & Hillenbrand 2019). We do not consider wind absorption, although P Cygni absorption profiles are commonly detected in strong lines (e.g., Calvet et al 1993;Lee et al 2015;Milliner et al 2019).…”

Measuring Optical Extinction toward Young Stellar Objects Using Diffuse Interstellar Bands

“…a We derive a spectral type closer to A0 or F8 for RNO 1A; see Appendix D for more details. b A V s from Kuhn & Hillenbrand (2019), derived from XMM-Newton X-Ray spectra or adopted for their analysis. c According to characteristics of a blue spectrum.…”