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Interstellar isotope ratios from mm-wave molecular absorption spectra

R. Lucas tex2html_wrap_inline1462 , H. Liszt tex2html_wrap_inline1482
tex2html_wrap_inline1462 Institut de Radioastronomie Millimétrique, 300 Rue de la Piscine, F-38406 Saint Martin d'Hères, France
tex2html_wrap_inline1482 National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA, USA 22903-2475


Figure: H12,C16,O tex2html_wrap_inline1490 , H13,CO tex2html_wrap_inline1490 ( tex2html_wrap_inline1528 ), H12,C14,N, H13,C14,N ( tex2html_wrap_inline1530 ), H14,N12,C, and 12,C32,S (J=2-1) absorption spectra toward B0415+379=3C111 (solid lines) with H12,C18,O tex2html_wrap_inline1490 ( tex2html_wrap_inline1536 ), H12,C15,N ( tex2html_wrap_inline1538 ), H14,N13,C ( tex2html_wrap_inline1540 ) and C34,S overlaid (shaded).

We have measured galactic tex2html_wrap_inline1488 mm absorption spectra of HCO tex2html_wrap_inline1490 , HCN, HNC, and CS toward compact extragalactic continuum sources in order to derive the isotopic abundance ratios tex2html_wrap_inline1358 C/ tex2html_wrap_inline1494 C, tex2html_wrap_inline1496 N/ tex2html_wrap_inline1498 N, tex2html_wrap_inline1500 O/ tex2html_wrap_inline1502 O, and tex2html_wrap_inline1504 S/ tex2html_wrap_inline1506 S in local diffuse clouds. For carbon, our data confirm recent results for the local ISM: we find tex2html_wrap_inline1358 C/ tex2html_wrap_inline1494 C = tex2html_wrap_inline1512 . For nitrogen, we find tex2html_wrap_inline1514 consistent with the Solar value of 270, but substantially smaller than the values inferred from HCN emission in dense clouds. For sulfur we find tex2html_wrap_inline1504 S/ tex2html_wrap_inline1506 S\ tex2html_wrap_inline1520 consistent with the Solar value of 23. We also find one striking individual anomaly: toward 3C111 (B0415+379), H12,CN/H13,CN = 170 tex2html_wrap_inline1522 in one kinematic component (Fig. 3). We attribute this to the fractionation of 13,C into 13,CO which may be so great that HCN is starved for 13,C.
Preprints: lucas/papers/