Literature Review #5: Emission Lines and Accretion Disks

P Manoj, H.C. Bhatt, G. Maheswar, S. Muneer

This paper is centered around the question: How long does emission line activity persist in young stars? This is a relevant question because emission line activity is associated with the presence of an active accretion disk around the star in both T Tauri and HAeBe stars. Since the matter that feeds accretion disks is taken from the the gas rich disk that surrounds the star we can characterize how long it takes Circumstellar disks to get to the planet forming phase by looking at when their emission line activity drops off. As the circumstellar disk loses material to the beginning stages of planet formation and other processes the rate of accretion goes down. This paper cites various other papers to say that the most stars lose their inner disks by 5 Myr. They don’t discuss what happens to to the outer disk and they also don’t explicitly say that once the inner disk is gone so is the accretion disk.

The authors compare low-mass T Tauri stars to intermediate mass HAeBe stars. Apparently, the origin of emission lines in pre-main sequence T Tauri stars is understood to be from magnetospheric accretion (Uchida & Shibata 1985, and several other papers). There is no such general understanding for emission lines in HAeBe stars but it is thought to be similar. In Muzerolle et al. (2004) the authors were able to apply the magnetospheric accretion model to HAeBe stars.

The Manoj et al. sample includes 91 HAeBe stars including RR Tau and a couple other UXors such as KK Oph. They found the equivalent width of H-Alpha for each of these stars and compared those values to the stellar age of each star. They found that H-Alpha line strength decreases with increasing stellar age and thus accretion gradually declines during the pre main sequence phase. They also find that inner disks dissipate on a similar timescale which they say suggests that inner disks dissipate after the accretion activity has fallen below a certain level. This seems like a backwards interpretation to me if it’s the inner disk is what feeds the accretion disk.

I’m also curious about the validity of only including one equivalent width value for H-Alpha per star. For UXors like RR Tau the equivalent width of the H-Alpha line varies so it’s curious to me at what point in RR Tau’s variation did the authors get the equivalent width.

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