Interstellar detection of the intersystem line Si II lambda 2335 toward zeta Ophiuchi

Details Interstellar detection of the intersystem line Si II lambda 2335 toward zeta Ophiuchi Interstellar detection of the intersystem line Si II lambda 2335 toward zeta Ophiuchi

Jan 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...420l..29c&link_type=abstract

Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X), vol. 420, no. 1, p. L29-L32

Astronomy and Astrophysics

Astronomy

30

Abundance, Cosmic Dust, Hydrogen, Iron, Magnesium, Silicon, Transition, Ultraviolet Astronomy, Ultraviolet Spectra, Carbon, Echelle Gratings, Hubble Space Telescope, Interstellar Gas, Spectrographs

Scientific paper

We report on the detection of the weak intersystem transistion of Si II lambda 2335 A in the sight line toward zeta Oph using the Ech-B mode (3.5 km/s resolution) of the Goddard High Resolution Spectrograph. The high-quality spectrum is characterized by an empirically measured signal-to-noise of 450, in excellent agreement with that expected from photon-statistics. The measured equivalent width of the Si II line is Wlambda = 0.48 +/- 0.12 mA. Using the new experimental f-value of Calamai, Smith, and Bergeson, we find a Si II column density of 2.34 (+/- 0.58) x 1015 atoms/sq cm and (Si/H)zeta Oph = 1.78 (+/- 0.44) x 10-6 for the principal absorbing component(s) at vsun approx. = -15 km/s. Analysis of the Si II lambda 1808 absorption over the same velocity range using the new experimental f-value of Bergeson & Lawler yields a column density (corrected for saturation) that is consistent within the weak line errors and confirms the relative accuracies of these new f-values. Furthermore, these results indicate that accurate abundances can now be derived for Si II, particularly from the weak Si II lambda 2335 A since it is free of saturation effects. For the zeta Oph vsun approx. = -15 km/s component(s), we find that greater than 95% of the available cosmic abundance (i.e. the 1989 meteoritic abundances of Anders & Grevesse) of Mg, Fe, and Si is 'missing' from the gas phase and is presumably locked up in the dust. These elements are present in the dust grains in ratios of Fe/Si approximately equals 0.9 and Mg/Si approximately equals 1.1, consistent with the ratio of their cosmic abundances. These ratios are in sharp contrast to more diffuse clouds like those seen toward the high-latitude halo star HD 93521 where in the dust Fe/Si approximately equals 1.8 and Mg/Si approximately equals 2.1.

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