ON THE UNUSUAL GAS COMPOSITION IN THE beta PICTORIS DEBRIS DISK
2013 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 762, no 2, 114- p.Article in journal (Refereed) Published
Themetallic gas associated with the beta Pic debris disk is not believed to be primordial, but arises from the destruction of dust grains. Recent observations have shown that carbon and oxygen in this gas are exceptionally overabundant compared to other elements, by some 400 times. We study the origin of this enrichment under two opposing hypotheses: preferential production, where the gas is produced with the observed unusual abundance (as may happen if gas is produced by photodesorption from C/O-rich icy grains), and preferential depletion, where the gas evolves to the observed state from an original solar abundance (if outgassing occurs under high-speed collisions) under a number of dynamical processes. We include in our study the following processes: radiative blowout of metallic elements, dynamical coupling between different species, and viscous accretion onto the star. We find that, if gas viscosity is sufficiently low (the conventional a parameter less than or similar to 10(-3)), differential blowout dominates. While gas accumulates gradually in the disks, metallic elements subject to strong radiation forces, such as Na and Fe, deplete more quickly than C and O, naturally leading to the observed overabundance of C and O. On the other hand, if gas viscosity is high (alpha greater than or similar to 10(-1), as expected for this largely ionized disk), gas is continuously produced and viscously accreted toward the star. This removal process does not discriminate between elements so the observed overabundance of C and O has to be explained by a preferential production that strongly favors C and O to other metallic elements. One such candidate is photodesorption off the grains. We compare our calculation against all observed elements (similar to 10) in the gas disk and find a mild preference for the second scenario, based on the abundance of Si alone. If true, beta Pic should still be accreting at an observable rate, well after its primordial disk has disappeared.
Place, publisher, year, edition, pages
2013. Vol. 762, no 2, 114- p.
circumstellar matter, planetary systems, planets and satellites: formation, protoplanetary disks, stars: individual (Beta Pictoris)
Astronomy, Astrophysics and Cosmology
IdentifiersURN: urn:nbn:se:su:diva-87694DOI: 10.1088/0004-637X/762/2/114ISI: 000313008900048OAI: oai:DiVA.org:su-87694DiVA: diva2:605853