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Download e-book for kindle: Astrobiology: Future Perspectives (Astrophysics and Space by P. Ehrenfreund (Editor), W.M. Irvine (Editor), T. Owen

By P. Ehrenfreund (Editor), W.M. Irvine (Editor), T. Owen (Editor), Luann Becker (Editor), Jen Blank (E

ISBN-10: 1402023049

ISBN-13: 9781402023040

ISBN-10: 1402023057

ISBN-13: 9781402023057

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AGB stars are also objects in which carbonaceous dust particles are formed and then ejected into the interstellar medium. A detailed review 24 Chapter 2 of the formation of molecules and grains in late-type stars has been given by Millar (2003); here we merely sketch the salient points. Close to the stellar surface of an AGB star, the gas temperature is on the order of 2000 K and LTE holds. The result, in the case of C-rich objects, is the formation of CO, which essentially uses up all of the oxygen atoms.

These measurements indicate the presence of scale free density distributions with central condensations of the order of 106 cmÀ3 . The abundance of HCOþ yields a lower limit on the fractional ionization in these envelopes of $ 10À8 . 3 Disks and Hot Molecular Cores Circumstellar disks regulate the accretion of material onto the central star through a boundary layer, and are believed to be 10–100 AU on theoretical grounds. It is important to constrain this radial extent better, and to determine the surface density and temperature proWle of these disks (see also Markwick & Charnley 2004, this volume).

For a DIB with equivalent width W at wavelength ˚ ), the column density of an absorber having oscillator l (both in A strength f needed is: N ¼ 1:13 Â 1020 W =l2 f In the z Oph diVuse cloud the total hydrogen column density is about ˚, 1:3 Â 1021 cmÀ2 . Thus, to account for a strong narrow DIB at 6000 A ˚ having W ¼ 0:1 A and f ¼ 0:001, typical for a small carbon chain, a fractional abundance of about 3 Â 10À7 is required, much larger than Organic Molecules in the Interstellar Medium 21 that of C3 in z Oph, 2 Â 10À9 (Maier et al.

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Astrobiology: Future Perspectives (Astrophysics and Space Science Library) by P. Ehrenfreund (Editor), W.M. Irvine (Editor), T. Owen (Editor), Luann Becker (Editor), Jen Blank (E


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