Explaining UX Orionis Star Variability with Self-Shadowed Disks

C.P. Dullemond, M.E. Van Den Ancker, B. Acke, and R. van Boekel

2003

The purpose of this paper is to delineate the problems with the current models of where the dust filaments responsible for the UXOR characteristics originate and propose a better model. The models that the authors are critiquing are that the obscuration events are caused by protocometary clouds or comets. Which they reject noting that the model is not consistent with observed SEDs of UXORs. They then go into discussing the idea of protoplanetary disks around the stars and what geometry could explain the observed phenomena. The main proposal for the disk geometry around these stars was a flared disk. However, if that was the case then the obscuring dust would be in the outer regions of the disk and thus would not be able to account for the variations seen in UXORs. Even adding a puffed up inner rim like Natta et al. (2001) then flared disk would still obscure the star totally and thus would not be seen as HAeBe star. So it’s a logical impossibility for UXORs to be the product of a flared disk.

What the authors propose is a self-shadowed disk with a puffed up inner rim.

[Aside: I really need to get some sort of drawing program on my computer so I can make diagrams.]

The puffed inner rim would be the source of obscuring clouds and the outer, self-shadowed part would not obscure the cloud and inner rim.

What the authors do to provide evidence for this model is try to correlate SED shape with UXOR stars. Our of 86 HAeBe stars they determined that 47 belong to Group I and 39 belong to Group II. Then by defining UXORs as stars spectral type B9 or later and experience variations of ~1 magnitude on time scales of days or weeks they determined that 10 UXORs are in in Group II and 1 in Group I. The authors associate self-shadowed disks with modest FIR excess. So they conclude that “UXOR-type phenomena should only occur in in self-shadowed disk,”

I think that overall the self-shadowed disk is an good way of looking at the system. And I have now sorted out most of my confusion from my  previous post. It’s not disk geometry vs. obscuration by dust clouds, it’s about understanding where in the disk geometry the dust clouds originate. And I think I’m understanding that these “dust clouds” or “obscuring clouds” are the planetesismals that other papers refer to.

There are a couple things that bother me about this paper. First, the sample in this paper did not include any T Tauri stars and T Tauri stars produce UXORs as well. So T Tauri stars would have to have a self-shadowed disk. The authors touch on this point at the very end of the paper by qualitatively saying that there would be a lot less T Tauri UXORs than HAeBe UXORs. Which is true but it is unsatisfying that they did not go into understanding how a T Tauri star would produce the same disk structure of a HAeBe star. Also, I just want to double check there use of Group I and Group II.

And the talk about the different time scales from the different dust cloud origins interested me. I wonder where dust clouds would have to originate to explain the ~100 day variation we observed at MMO.