Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004phdt.......183f&link_type=abstract
Thesis (PhD). THE JOHNS HOPKINS UNIVERSITY, Source DAI-B 64/10, p. 4984, Apr 2004, 127 pages.
Astronomy and Astrophysics
Astronomy
Scientific paper
We present here the detection and initial chemical characterization of an extrasolar cometary system around IRC+10216, a carbon-rich Asymptotic Giant Branch (AGB) star. Observations by the Submillimeter Wavelength Astronomy Satellite (SWAS) indicated the presence of large quantities of water vapor (x(H2O) = 1.2 × 10-6) around the carbon-rich star; the presence of this water vapor was used to infer the existence of vaporizing comet-like icy bodies. We also address and reject possible alternative explanations for the presence of water vapor, including shocks and photochemistry. We have also developed a detailed model for the vaporization of cometary systems by an evolving central star and demonstrate that such a model can easily produce water vapor abundances relative to molecular hydrogen of x(H2O) ˜10-6 assuming the cometary system contains only 10 M⊕ of water ice. We show that a vaporizing cometary system around an AGB star should produce OH, through photodissociation of water vapor, and discuss our detection of OH around IRC+10216. We find a beam averaged fractional abundance x(OH) ˜4 × 10-8. The OH lines we detected had an unexpectedly narrow shape and were slightly blueshifted with respect to the systemic velocity of IRC+10216. The most likely explanations for the unexpected OH line shapes are either masing or an asymmetric distribution of OH molecules around IRC+10216. We have also detected formaldehyde (H2CO) around IRC+10216, a molecule found in comets but not in carbon-rich stars. We find a formaldehyde abundance, relative to water vapor, of x(H2CO)/ x(H2O) = 1.1%, consistent with abundances found in Solar System comets. We failed to detect methanol at abundances comparable to those found in normal Solar System comets (x(CH3OH)/ x(H2O) < 0.077%) and suggest possible causes for the discrepancy. Finally, we discuss how to detect and characterize more extrasolar cometary systems. Specifically, we make predictions for observations by the Herschel Space Observatory (HSO, formerly known as FIRST), the Space Infrared Telescope Facility (SIRTF) and the Atacama Large Millimeter Array (ALMA). We discuss plans for further observational and theoretical studies of the cometary system around IRC+10216 and other, yet-to-be-discovered systems.
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