It has been suggested that high C/O ratios ( > 0.8 ) in circumstellar disks lead to the formation of carbon dominated planets .
Based on the expectation that elemental abundances in the stellar photospheres give the initial abundances in the circumstellar disks , the frequency distributions of C/O ratios of solar-type stars have been obtained by several groups .
The results of these investigations are mixed .
Some find C/O > 0.8 in more than 20 % of stars , and C/O > 1.0 in more than 6 % .
Others find C/O > 0.8 in non of the sample stars .
These works on solar-type stars are all differential abundance analysis with respect to the Sun and depend on the adopted C/O ratio in the Sun .
Recently a method of molecular line spectroscopy of M dwarfs with which carbon and oxygen abundances are derived respectively from CO and H _ { 2 } O lines in the K band , has been developed .
The resolution of the K band spectra is 20,000 .
Carbon and oxygen abundances of 46 M dwarfs have been obtained by this non-differential abundance analysis .
Carbon-to-oxygen ratios in M dwarfs derived by this method are more robust than those in solar-type stars derived from neutral carbon and oxygen lines in the visible spectra due to the difficulty in the treatment of oxygen lines .
We have compared the frequency distribution of C/O distributions in M dwarfs with those of solar-type stars and have found that low frequency of high C/O ratios is preferred .