Literature #7: Analyzing [OI] lines

The structure of protoplanetary disks surrounding three young intermediate mass stars

I. Resolving the disk rotation in the [OI] 6300 A line

G. van der Plas, M.E. van den Ancker, D. Fedele, B. Acke, C. Dominik, L. B. F. M. Waters, and J. Bouwman 

HD 172555: detection of 6300 A [OI] emission in a debris disc

P. Riviere-Marichalar, D. Barrado, J.-C. Augereau, W.F. Thi, A. Roberge, C. Eiroa, B. Montesinos, G. Meeus, C. Howard,G . Sandell, G. Duchene, W.R.F. Dent, J. Lebreton, I. Medigutia, N. Huelamo, F. Menard, and C. Pinte.

These two papers caught my eye because the both focus on analysis of [OI] line in circumstellar disks. The van der Plas et al. paper focuses on three Herbig AeBe stars while the Riviere-Marichalar et al. paper focuses on a single star that is a member of the Beta Pictoris moving group. Both papers address the importance of studying the gaseous materials in disk, van der Plas et al. noting that 99% of disk mass is gas, but they diverge in their interests in the [OI] line. It’s interesting to read these two papers together because both teams are essentially studying the same thing, the very beginnings of the formation of planets, but the stars they’re choosing to study are at different evolutionary points. This difference is reflected in their different foci. van der Plas et al. use the [OI] to refine the determination of the disk structures of their three stars while Riviere-Marichalar et al. are focused on determining the composition of the gas in the debris disk.

(I’m going to take an aside and wonder about the disk in the the Riviere-Marichalar paper. The star, HD 17255, is a part of the BMPG and as such as an age around 12-20 Myr so I’m curious to why it has a disk at all. A previous paper I read said that these types of disks dissipate long before that. So why does this star have a disk at all?)

Riviere-Marichalar et al. derived the [OI] mass in the disk by first creating an SED by compiling the observations from numerous sources. By fitting the SED with a blackbody spectrum the authors were able to determine the radius of dust distribution. Then from computing the infrared excess and the mass of the dust disk were able to determine the final dust mass (I really don’t understand how these calculations were done). From there the authors compute the [OI] mass (the paper has an appendix where they go over this computation that I haven’t gone over enough to understand).

The authors of the other paper followed a method created by Acke et al. (2005) which allows for determining disk rotation and distribution of gas in the disk. I haven’t read that paper (it is on my reading list) and van der Plas et al. leave the details of that method out of this paper.  They essentially have expected disk shapes from the SEDs for each star and then they refine the structure of disks using the [OI] line by finding the distance from the star that the [OI] line was emitted.

Both papers discuss the possible origins of the [OI] line – the thing that I’m most interested for my project. van der Plas et al. split the origin of [OI] between broad and narrow lines. According to them, broad lines in T Tauri stars are, “due to a combination of a dense stellar jets and a disk wind or magnetic accretion columns.”  Then they go onto to say the narrow lines in HAeBe stars (I’m not sure if they are saying, in general, T Tauri stars have broad lines and, in general, HAeBe stars have narrow lines or not) are from the photodissociation of OH in the upper layers of the disk.

Both of these papers used the [OI] line in interesting ways. And they both made mention of the difficulty in detecting the line so I’m eager to reduce the data I’ve gotten so far and see if I’ve picked up on [OI]. I will definitely have to think about how each of these terms used the [OI] line and if and how I can apply either of these methods.

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