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

in the spectra of star-disk systems. Modeling of stellar spectra allows astronomers
to determine many properties of stars. To nail down stellar parameters to within a
high confidence level, the observed spectrum of the star has to match a synthesized
spectrum very well. In the case of an “ordinary” star, the observed spectrum is usually
modeled fairly easily, especially if the star is not extremely faint and if there are no
peculiarities associated with the star’s spectrum. The observed spectrum of a star
surrounded by a circumstellar disk can be dramatically di↵erent, however, especially
when one observes the system at infrared and longer wavelengths. Figure 3 shows
the spectral energy distribution (SED) of the star GM Auriga, a star that is known
to have a circumstellar disk. The SED of a system such as GM Auriga exhibits a
flux contribution from the disk as well as from the star. The shorter wavelength
flux of the SED, primarily those of the optical, are contributed mostly by the star
itself. Longer wavelength data provide evidence of the disk. Instead of observing
the expected decline in flux past the optical wavelength regime, the flux actually
decreases much less due to the circumstellar material, which is radiating at infrared
wavelengths after being warmed by its star or by accretionary growth of the disk
itself. This “infrared excess” is the contribution of the disk to the SED. The bluer
wavelengths, which are contributed by the star, provide the spectral information that
allow one to model the stellar spectrum. This is not always an easy task as the more
heavily-embedded the object is, the redder it appears due to scattering of the bluer
wavelengths by intervening dust. If enough visible light emerges from the star-disk
system, then a stellar spectrum can be derived. With that, then by carefully studying
and modeling the shape of the infrared excess, one can determine many aspects of
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