arXiv subjects

Aki Roberge

Publications and source records attributed to Aki Roberge.

75 records · Page 5Linked to original sources

arXiv2006

The Carbon-Rich Gas in the Beta Pictoris Circumstellar Disk

The edge-on disk surrounding the nearby young star Beta Pictoris is the archetype of the "debris disks", which are composed of dust and gas produced by collisions and evaporation of planetesimals, analogues of Solar System comets and asteroids. These disks provide a window on the formation and early evolution of terrestrial planets. Previous observations of Beta Pic concluded that the disk gas has roughly solar abundances of elements [1], but this poses a problem because such gas should be rapidly blown away from the star, contrary to observations of a stable gas disk in Keplerian rotation [1, 2]. Here we report the detection of singly and doubly ionized carbon (CII, CIII) and neutral atomic oxygen (OI) gas in the Beta Pic disk; measurement of these abundant volatile species permits a much more complete gas inventory. Carbon is extremely overabundant relative to every other measured element. This appears to solve the problem of the stable gas disk, since the carbon overabundance should keep the gas disk in Keplerian rotation [3]. New questions arise, however, since the overabundance may indicate the gas is produced from material more carbon-rich than the expected Solar System analogues.

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arXiv2005

Rapid Dissipation of Primordial Gas from the AU Microscopii Debris Disk

The disk around AU Microscopii, an M1 star in the Beta Pictoris Moving Group, is extraordinarily well-suited for comparison to the Beta Pic debris disk (type A5V). We use far-UV absorption spectroscopy of AU Mic to probe its edge-on disk for small amounts of molecular hydrogen, the primary constituent of gas giant planets. Our conservative upper limit on the line-of-sight H_2 column density is 1.7 x 10^{19} cm^{-2}, which is 18.5 times lower than the limit obtained from non-detection of sub-mm CO emission (Liu et al. 2004). In addition, there is a hint of H_2 absorption at a column density an order of magnitude or more below our upper limit. The H_2-to-dust ratio in the AU Mic disk is < 6:1, similar to that in the Beta Pic disk. This shows that the primordial gas has largely been dissipated in less than about 12 Myr for both disks, despite their very different stellar masses and luminosities. It is extremely difficult to form a giant planet around AU Mic with current core-accretion scenarios in such a short time.

astro-ph↗

arXiv2004

HST-STIS Spatially Resolved Spectroscopy and Coronagraphic Imaging of the TW Hydrae Circumstellar Disk

We present the first spatially resolved spectrum of scattered light from the TW Hydrae protoplanetary disk. This nearly face-on disk is optically thick, surrounding a classical T Tauri star in the nearby 10 Myr old TW Hya association. The spectrum was taken with the HST-STIS CCD, providing resolution R ~ 360 over the wavelength range 5250 - 10300 A. Spatially resolved spectroscopy of circumstellar disks is difficult due to the high contrast ratio between the bright star and faint disk. Our novel observations provide optical spectra of scattered light from the disk between 40 AU and 155 AU from the star. The scattered light has the same color as the star (gray scattering) at all radii, except the innermost region. This likely indicates that the scattering dust grains are larger than about 1 micron all the way out to large radii. From the spectroscopic data, we also obtained radial profiles of the integrated disk brightness at two position angles, over almost the same region as previously observed in HST-WFPC2 and NICMOS coronagraphic images (35 AU to 173 AU from the star). The profiles have the same shape as the earlier ones, but show a small azimuthal asymmetry in the disk not previously noted. Our STIS broad-band coronagraphic images of TW Hya confirm the reality of this asymmetry, and show that the disk surface brightness interior to 140 AU has a sinusoidal dependence on azimuthal angle. The maximum brightness occurs at a position angle of 233.6 +/- 5.7 degrees East of North. This might be caused by the combination of forward-scattering and an increase in inclination in the inner region of the disk, suggesting that the TW Hya disk has a warp like that seen in the Beta Pictoris debris disk.

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