EVIDENCE OF ACCRETION-GENERATED X-RAYS IN THE YOUNG ...

exhibiting this plasma) was much smaller than what is observed for normal coronal
X-ray sources. They concluded that the plasma generating these spectral features
likely arises from accretion hotspot(s) on the stellar photosphere. With the derived
electron density and using an assumed filling factor and accretion stream velocity, the
accretion rate was calculated for this object.
The X-ray spectrum of YSOs can also yield information about the circumstellar
environment. The most notable example is the fluorescence of “cold” iron (iron that
is weakly ionized) in the cirumstellar disk. Iron emission lines are often observed
in the X-ray spectra of YSOs in the 6.7–7.0 keV range. This feature is the He-like
Fe K↵-line triplet at 6.64, 6.67, and 6.70 keV. More recently, a neutral iron K-↵
doublet at 6.4 keV has been observed in the spectra of some YSOs, including those
that are heavily embedded. These emission lines are the result of higher-energy X-ray
photons (energy 7.11 keV) weakly ionizing iron atoms or ions through the removal
of a 1s electron. Imanishi et al. (2001) provided the first detection of the neutral iron
doublet in the X-ray spectrum of a deeply embedded YSO when observing YLW 16A
in ⇢ Ophichus (Fig. 8), and this feature was also detected in the X-ray spectrum of
objects, such as V1647 Ori (Grosso et al., 2005; Teets et al., 2011) and seven sources
in the ONC (Tsujimoto et al., 2005).
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