Probing the possibility of a /sup 12/C/sup 13/C abundance gradient from observations of interstellar CH/sup +/ Page: 3 of 15
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be locally homogeneous and well mixed. The most direct way to assess the degree of homo-
geneity and the amount of chemical evolution of the ISM, is by an accurate determination
of element abundances toward different lines of sight. The measured abundances can then
be compared with solar system values, A hich are representative of the ISM abundances of
4.6 billion years ago. Recent very accurate results have shown that the local ISM 1C/11C
ratio within 1 kpc of the Sun is homogeneous within 12%, and has decreased by a factor
of two since the formation of the Sun (Hawkins, Meyer, and Jura 1985; Hawkins and Jura
1987). These results stem from optical observations of the interstellar CH+ molecule in
nearby diffuse clouds. We believe that the measured 12CH+/"CH+ ratios are represen-
tative of 12C/i3C in the gas because of the close similarity among the values toward five
stars and four lines of sight. This finding coupled with theories of CH+ formation provide
evidence that, unlike CO and other species, CH+ does not suffer from chemical fraction-
ation or other isotope-selective effects in these regions. Because this molecule seems to
be a sensitive and unambiguous probe of 120/13C in diffuse clouds, observations toward
more distant, reddened stars up to 2 kpc from the Sun can assess the existence of a carbon
isotope abundance gradient on a larger scale.
Oxygen and nitrogen negative radial abundance gradients have been observed in the
plane of our Galaxy (Shaver et at. 1983). In addition, similar trends are seen in the
N/O abundance ratio (Lester et z(. 1986) and possibly in 12C/15C (Henkel, Wilson and
Bieging 1982; Henkel, Gusten, and Gardner 1985). Gradients in the abundance ratio of
closely related isotopic species are easier to assess because the isotopes are most likely not
affected by "selective" depletion onto dust grains or molecules. Models of Galactic chemical
evolution which relax the instantaneous recycling approximation and include bimodal star
formation (variable element yields) and infall of halo gas have successfully explained the
observed gradients (Gusten and Mezger 1982). However, the evidence for a L2C/DC
abundance gradient comes from measurements of isotopic formaldehyde absorption toward
strong continuum sources. These determinations suffer from uncertainties due to saturation
and fractionation effects which are very difficult to take into account. Despite the smaller
distances which can be sampled with optical observations of CH+, this molecule is a more
sensitive probe of the true 12C and '2C abundances in the observed regions since, unlike
H2CO, the line formation process is not strongly coupled to the physical conditions of
individual clouds.
OBSERVATIONS
The advent of sensitive solid state detectors with a very stable, linear response, has
made possible the acquisition of high resolution (< 0.1 A), high signal-to-noise (> 500)
data toward stars as faint as B=: 8, and the detection of features as weak as 0.3 mA.
Observations with an 1872-element Reticon detector, coupled with the coudd spectrograph
of the 3.0 m telescope at Lick Observatory, have yielded 5 a detections of the weak 31CH+
feature toward five bright (B < 5) stars located within a few hundred parsecs of the Sun.
These spectra, with S/N ratios of 1000 - 2500, have allowed us to make a very accurate
determination of the local ISM 12C/13C isotope ratio. The measured values toward five
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Hawkins, I. Probing the possibility of a /sup 12/C/sup 13/C abundance gradient from observations of interstellar CH/sup +/, article, September 16, 1987; California. (https://digital.library.unt.edu/ark:/67531/metadc1053474/m1/3/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.