Literature Review #4: Flaring and Shadowing

On the interplay between flaring and shadowing in diss around Herbig Ae/Be stars
B. Acke, M. Min, M.E van den Ancker, J. Bouwman, B. Ochsendorf, A. Juhasz, and L. B. F. M Waters

Meeus et al. (2001) used SEDS of a sample of Herbig stars to determine that members of the class can be split up into two groups; group I have much larger far-infrared excess than those of group II. This paper focuses on the proposed physical reason for this discrepancy which is that stars in group II have an outer disk that is protected from stellar radiation from the “puffed up” inner disk. However there has been evidence that if the disks of group II have a large flaring angle (meaning not a steep wall) then the outer disk is not as protected and will have IR excess comparable to the stars of group I.

This paper is actually a letter to the editor with a follow up paper planned. But it presents the preliminary results of 33 Herbig Ae/Be sources with the goal of better understanding the disk geometry and the nature of that geometry.

This was a fun paper to read after the last because it compliments the previous paper in focus but is much more digestible. The last paper was about the changes in dust size and structure as the disks around these stars age and this paper is about how the dust size affects the structure of the disk. The authors did this by comparing their spectra to self-consistent passive-disk models (I don’t know what that means). They found evidence that in the inner disks (~1 AU from the star) and the outer disks (~10s of AU) of the sources in their sample are related to each other. In an attempt to understand the physical cause they made disk models using the radiative transfer code MCMax (Min et al. 2009). Here is a list of their input parameters:
1.) Central star is a main sequence of spectra type A0 (T_eff = 10,000 K, R=2R_sun, M_*=2.5 M_sun)
2.) Assumed hydrostatic equilibrium and thermal coupling between dust and gas
3.) gas-to-dust ratio = 100
4.) Dust is made of astronomical silicates with a grain size of 0.12 microns

They used these parameters to compute models at different dust masses (m). They also varied the surface density power law and the inclination of the disk system (i).

They found that different dust masses gives the most change and spread in the models. The surface density and inclination give either inconclusive pro negligible results.

The authors conclude that the geometry of the outer disk is dependent on the inner disk and that the degree of flaring is determined by the mass of small dust grains and the degree of shadowing is dependent on inner disk scale height.

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