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Register a new userA dust disk surrounding the young A star HR4796A
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Ray Jayawardhana
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We report the codiscovery of the spatially-resolved dust disk of the Vega-like star HR 4796A. Images of the thermal dust emission at $lambda = 18 mu$m show an elongated structure approximately 200 AU in diameter surrounding the central A0V star. The position angle of the disk, $30^{circ} pm 10^{circ}$, is consistent to the position angle of the M companion star, $225^{circ}$, suggesting that the disk-binary system is being seen nearly along its orbital plane. The surface brightness distribution of the disk is consistent with the presence of an inner disk hole of approximately 50 AU radius, as was originally suggested by Jura et al. on the basis of the infrared spectrum. HR 4796 is a unique system among the Vega-like or $beta$ Pictoris stars in that the M star companion (a weak-emission T Tauri star) shows that the system is relatively young, $sim 8 pm 3$ Myr. The inner disk hole may provide evidence for coagulation of dust into larger bodies on a timescale similar to that suggested for planet formation in the solar system.
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[Abridged] We present the first direct comparison of the distribution of the gas, as traced by the [OI] 6300 AA emission, and the dust, as traced by the 10 micron emission, in the protoplanetary disk around three intermediate-mass stars: HD 101412, HD 135344 B and HD 179218. N-band visibilities were obtained with VLTI/MIDI. Simple geometrical models are used to compare the dust emission to high-resolution optical spectra in the 6300 AA [OI] line of the same targets. The disks around HD 101412 and HD 135344 B appear strongly flared in the gas, but self-shadowed in the dust beyond ~ 2 AU. In both systems, the 10 micron emission is rather compact (< 2 AU) while the [OI] brightness profile shows a double peaked structure. The inner peak is strongest and is consistent with the location of the dust, the outer peak is fainter and is located at 5-10 AU. Spatially extended PAH emission is found in both disks. The disk around HD 179218 is flared in the dust. The 10 micron emission emerges from a double ring-like structure with the first ring peaking at ~ 1 AU and the second at ~ 20 AU. No dust emission is detected between ~ 3 -- 15 AU. The oxygen emission seems also to come from a flared structure, however, the bulk of this emission is produced between ~ 1 -- 10 AU. This could indicate a lack of gas in the outer disk or could be due to chemical effects which reduce the abundance of OH -- the parent molecule of the observed [OI] emission -- further away from the star. The three systems, HD 179218, HD 135344 B and HD 101412, may form an evolutionary sequence: the disk initially flared becomes flat under the combined action of gas-dust decoupling, grain growth and dust settling.
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