The origin of gas in debris disks Alexis Brandeker University of Toronto Rodrigo Fernández Yanqin Wu

Why gas? Inner dust disk Inner gas disk Haisch, Lada, & Lada (2001) Jayawardhana et al. (2006) Inner dust disk Inner gas disk

Favourable case: b Pictoris

Velocity at ionisation

Braking ions Magnetic fields Ion self-braking Ion-atom interaction Charged dust

Ion fluid

Gas friction 0.1 MEarth [km/s] If we assume that the disk is of cosmic composition and just scale the detected metals in gas form, we get too little gas to effectively brake the atoms – for the terminal velocities to be within a few km/s, we need a hydrogen column density two orders of magnitude higher than the observed upper limits. One can speculate that perhaps some invisible gas other than hydrogen, like oxygen, is responsible for the gas drag. Why this gas should be enhanced over hydrogen to that high degree is not clear, however, though we note that this is the case in e.g. Earth’s atmosphere, meaning this state is not impossible. [km/s]

Dust braking

Key clues to the origin of gas Metals approximately of solar abundance (Lagrange et al. 1998) Strong upper limits on H2 (Lecavelier des Etangs et al. 2001) Gas spatial distribution similar to dust (Olofsson et al. 2001, Brandeker et al. 2004) Gas almost at rest relative to star (Lagrange et al. 1998, Brandeker et al. 2004) NEW C is ~15× overabundant compared to metals (Roberge et al. 2006)

Roberge et al. 2006

Possible sources Primordial gas Comet evaporation Grain-grain collisions

Other systems β Pic AU Mic

Work published in tomorrow’s issue of ApJ: Results - predictions Gas most likely generated in grain-grain collisions (linked to dust kinematics?) Little Be and P in b Pic-disk, relative abundance of metals similar to grains, more volatiles relative to metals. Very high Fe/Na ratio in AU Mic disk Work published in tomorrow’s issue of ApJ: Fernández, Brandeker, & Wu 2006, ApJ 643:509

Extra Material

References Brandeker, et al. 2004, A&A, 413:681 Fernández, Brandeker, Wu 2006, ApJ, 643:509 Jayawardhana et al. 2006, ApJ, submitted Lagrange et al. 1998, A&A, 330, 1091 Lecavelier des Etangs et al. 2001, Nature, 412:706 Haisch, Lada, Lada 2001, ApJ, 553, L153 Olofsson, Liseau, Brandeker 2001, ApJ, 563, L80 Roberge et al. 2006, Nature, in press