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

which provide mass reservoirs from which the stars can grow through circumstellar
accretion. Typical mass accretion rates for T Tauri stars are around 10 9 –10 8 M
yr 1 (Calvet et al., 2004), though this value can vary widely. Accretion of circumstel-
lar material onto the central star results in flux increases in the X-ray through infrared
wavelength regimes and also causes the star to appear variable as the accretion rate
fluctuates or the accretion stream geometry changes. In addition, infalling material
absorbs some of the bluer flux from the emission line, reducing the overall amount of
the blue-wing emission of some emission features. In cases where the absorption is
strong enough, the emission line blue wings go into absorption while the remainder
of the feature stays in emission, producing what is known as an inverse P-Cygni pro-
file.As material impacts the photosphere, the energy generated also ejects material
from the star, generating a wind. The signature of this wind is observed in cTTS as
aP-Cygniprofileintheprominentemissionlines. Thoughawindwouldbeexpected
to be produced in any direction (such that a redshifted and blueshifted component
should be observed), the blueshifted wind component is observed most due to the
optically thick circumstellar disk blocking our view of the redshifted component.
2.2 FUors and EXors
There are two prominent classes of cTTSs that are known to erupt because of
sudden, large-scale accretion events: FUors and EXors. In general, it is thought
that the main di↵erence between these types of stars is the level of their “quiescent”
accretion rates and the duration and magnitude of the large accretion episodes that
they experience. Figure 4 illustrates they physical setup of these types stars and how
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