Harvard Astronomy 201b

Observing Disks: The SED


The SED, or Spectral Energy Distribution, is a description of how the flux of light from an object (in this case a disk around a star), depends on the frequency of light that is observed. This applies to broad band color characterization, as well as to more detailed spectral measurements. By analyzing an SED, one can obtain information about a disk’s geometry, inclination, and constituents.

Why Study Disks?

Disks are important astronomical objects for a variety of reasons. The most obvious reason to study disk is that disks are the hotbed (or coldbed, depending on your perspective) of planet formation. Studying systems that may be in the process of forming planets is important in order to bridge the gap in knowledge from the initial disk configuration to the final statistical distribution of planets being studied by Kepler.

Aside from planet formation and dynamics, a wealth of physical phenomena are taking place in the disk, from complicated disk chemistry, viscous and turbulent flows, and accretion onto the young (or old in the case of debris disks) star.

A Disk Model

(Dullemond et al)

Broadband Trends as Evidence for Disk Flaring

Initial disk modeling utilized simple assumptions – that of a flat disk of material being heated by the (spherical) star. By setting blackbody cooling of the disk equal to the irradiated flux from the star, one obtains a temperature profile that goes as T_{eff} \propto r^{-3/4}. This leads to an SED with \nu F_\nu \propto \nu^{4/3}. However, SEDs typically have larger slopes of ~0.6-1.0 (Kenyon and Hartmann 1995). This issue can be resolved if a disk if flared at larger radii – it provides a larger surface area to absorb stellar flux, which it then reprocess and emits in the far infrared:

(Dullemond et al)

Spectral Information

More detailed spectral information of a disk hosting star can shed further light (pun intended) onto the structure of the disk geometry and even the nature of the disk itself. Accretion dominated disks (those shining as a blackbody, and whose primary source of heat is the gravitational energy from accreting towards the center) can be easily differentiated from irradiation-dominated disks (disks in a quasi-steady state with the host star, whose primary heating mechanisms come from the light from the host star):

(Dullemond et al)

Furthermore, the degree of flaring, and the size of the gap between the disk and the host star can be inferred from the spectrum of the disk hosting star.

Spectral Line Information

The shape of individual spectral lines can provide information not just about the disk’s composition, but also its dynamics. In some cases, depending on where a  line is formed in the disk, it is possible to produce a double-horned profile indicative of rotation:

Jonkheid et al. (2004)

Multiple lines can be found in the mid to far-infrared that are useful for characterizing a disk. However, it is not always clear at what temperature (and what location in the disk) these lines have formed.

Gorti and Hollenbock (2004)

Further Reading

(Dullemond et al) (good review from U. Washington)

Jonkheid et al. (2004)

Kenyon and Hartmann 1995

Gorti and Hollenbock (2004)

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