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Register a new userHerschel detects oxygen in the beta Pictoris debris disk
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The young star beta Pictoris is well known for its dusty debris disk, produced through the grinding down by collisions of planetesimals, kilometre-sized bodies in orbit around the star. In addition to dust, small amounts of gas are also known to orbit the star, likely the result from vaporisation of violently colliding dust grains. The disk is seen edge on and from previous absorption spectroscopy we know that the gas is very rich in carbon relative to other elements. The oxygen content has been more difficult to assess, however, with early estimates finding very little oxygen in the gas at a C/O ratio 20x higher than the cosmic value. A C/O ratio that high is difficult to explain and would have far-reaching consequences for planet formation. Here we report on observations by the far-infrared space telescope Herschel, using PACS, of emission lines from ionised carbon and neutral oxygen. The detected emission from C+ is consistent with that previously reported being observed by the HIFI instrument on Herschel, while the emission from O is hard to explain without assuming a higher-density region in the disk, perhaps in the shape of a clump or a dense torus, required to sufficiently excite the O atoms. A possible scenario is that the C/O gas is produced by the same process responsible for the CO clump recently observed by ALMA in the disk, and that the re-distribution of the gas takes longer than previously assumed. A more detailed estimate of the C/O ratio and the mass of O will have to await better constraints on the C/O gas spatial distribution.
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Context: The dusty debris disk around the $sim$20 Myr old main-sequence A-star {beta} Pictoris is known to contain gas. Evidence points towards a secondary origin of the gas as opposed to being a direct remnant form the initial protoplanetary disk, although the dominant gas production mechanism is so far not identified. The origin of the observed overabundance of C and O compared to solar abundances of metallic elements, e.g. Na and Fe, is also unclear. Aims: Our goal is to constrain the spatial distribution of C in the disk, and thereby the gas origin and its abundance pattern. Methods: We used the HIFI instrument onboard Herschel to observe and spectrally resolve CII emission at 158 $mu$m from the {beta} Pic debris disk. Assuming Keplerian rotation, we use the spectrally resolved line profile to constrain the spatial distribution of the gas. Results: We show that most of the gas is located around $sim$100 AU or beyond. We estimate a total C gas mass of $1.3times10^{-2}$ M$_oplus$. The data suggest that more gas is located on the southwest side of the disk than on the northeast side. The data are consistent with the hypothesis of a well-mixed gas (constant C/Fe ratio throughout the disk). Assuming instead a spatial profile expected from a simplified accretion disk model, we found it to give a significantly worse fit to the observations. Conclusions: Since the bulk of the gas is found outside 30 AU, we argue that the cometary objects known as falling evaporating bodies are unlikely to be the dominant source of gas; production from grain-grain collisions or photodesorption seems more likely. The incompatibility of the observations with a simplified accretion disk model could favour a preferential depletion explanation for the overabundance of C and O. More stringent constraints on the spatial distribution will be available from ALMA observations of CI at 609 $mu$m.
We obtained Herschel PACS and SPIRE images of the thermal emission of the debris disk around the A5V star {beta} Pic. The disk is well resolved in the PACS filters at 70, 100, and 160 {mu}m. The surface brightness profiles between 70 and 160 {mu}m show no significant asymmetries along the disk, and are compatible with 90% of the emission between 70 and 160 {mu}m originating in a region closer than 200 AU to the star. Although only marginally resolving the debris disk, the maps obtained in the SPIRE 250 - 500 {mu}m filters provide full-disk photometry, completing the SED over a few octaves in wavelength that had been previously inaccessible. The small far-infrared spectral index ({beta} = 0.34) indicates that the grain size distribution in the inner disk (<200AU) is inconsistent with a local collisional equilibrium. The size distribution is either modified by non-equilibrium effects, or exhibits a wavy pattern, caused by an under-abundance of impactors which have been removed by radiation pressure.
We present observations at 1.3 millimeters wavelength of the beta Pictoris debris disk with beam size 4.3 x 2.6 arcsec (83 x 50 AU) from the Submillimeter Array. The emission shows two peaks separated by ~7 arsec along the disk plane, which we interpret as a highly inclined dust ring or belt. A simple model constrains the belt center to 94+/-8 AU, close to the prominent break in slope of the optical scattered light. We identify this region as the location as the main reservoir of dust producing planetesimals in the disk.
Many stars are surrounded by disks of dusty debris formed in the collisions of asteroids, comets and dwarf planets. But is gas also released in such events? Observations at submm wavelengths of the archetypal debris disk around $beta$ Pictoris show that 0.3% of a Moon mass of carbon monoxide orbits in its debris belt. The gas distribution is highly asymmetric, with 30% found in a single clump 85AU from the star, in a plane closely aligned with the orbit of the inner planet, $beta$ Pic b. This gas clump delineates a region of enhanced collisions, either from a mean motion resonance with an unseen giant planet, or from the remnants of a collision of Mars-mass planets.
The debris disk surrounding $beta$ Pictoris has a gas composition rich in carbon and oxygen, relative to solar abundances. Two possible scenarios have been proposed to explain this enrichment. The preferential production scenario suggests that the gas produced may be naturally rich in C and O, while the alternative preferential depletion scenario states that the enrichment has evolved to the current state from a gas with solar-like abundances. In the latter case, the radiation pressure from the star expels the gas outwards, leaving behind species less sensitive to stellar radiation such as C and O. Nitrogen is also not sensitive to radiation pressure due to its low oscillator strength, which would make it also overabundant under the preferential depletion scenario. As such, the abundance of N in the disk may provide clues to why C and O are overabundant. We aim to measure the N column density in the direction of $beta$ Pic, and use this information to disentangle these different scenarios explaining the C and O overabundance. Using far-UV spectroscopic data collected by the HSTs Cosmic Origins Spectrograph (COS) instrument, we analyse the spectrum and characterise the NI triplet by modelling the absorption lines. We measure the N column density in the direction of $beta$ Pic for the first time, and find it to be $log(N_{mathrm{NI}}/1,mathrm{cm}^2) = 14.9pm0.7$. The N gas is found to be consistent with solar abundances and Halley dust. The solar N abundance supports the preferential production hypothesis, in which the composition of gas in $beta$,Pic is the result of photodesorption from icy grains rich in C and O or collisional vaporisation of C and O rich dust in the disk. It does not support the hypothesis that C and O are overabundant due to the insensitivity of C and O to radiation pressure thereby leaving them to accumulate in the disk.
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