A 13CO survey of molecular clouds in Gemini and Auriga

A large-scale survey for molecular clouds in (13)CO(J = 1-0) was performed toward the Gemini and Auriga regions (170 degrees < l less than or equal to 196 degrees and -10 degrees less than or equal to b < 10 degrees) with velocity coverages of -30 < V(LSR) < + 30 km s(-1) and -20 < V(LSR) < +40 km s(-1) for 170 degrees < l less than or equal to 188 degrees and 188 degrees < l less than or equal to 196 degrees, respectively, by using the two 4 m millimeter-wave telescopes at Nagoya University. An area of 520 deg(2) was covered at an 8' grid spacing with a 2.'7 beam, and 29,250 positions were observed. Significant (13)CO emission (greater than or equal to 1.2 K km s(-1) = 3 sigma) is detected at 1032 positions, and 139 distinct (13)CO clouds are identified. Physical properties such as molecular column density, size, and mass are derived for each cloud. Comparison with known H II regions and other associated visible objects indicates that 98 of the 139 clouds are located at similar to 2 kpc, while the rest lie at less than or similar to 1 kpc. The candidates for protostars are selected from IRAS point sources and 141 sources are identified as protostellar candidates associated with the (13)CO clouds. A statistical study of the (13)CO clouds and of the candidates for protostars shows the following results: 1. The mass spectra of the (13)CO clouds are well represented by a power law, N(cloud)(greater than or equal to M(cloud)) = 1.4 x 10(4)(M(cloud)/M.)(-0.83) - 1.4 for the clouds at similar to 2 kpc and N(cloud)(greater than or equal to M(cloud)) = 3.5 x 10(2)(M(cloud)/M.)(-0.64) - 2.4 for the rest. 2. The line width, Delta V(comp), and the size, R, of the (13)CO clouds show a power-law relation with an index 0.24 +/- 0.06, where the dispersion in the fitting is fairly large. 3. A virial analysis made for the (13)CO clouds indicates that the relation between the virial mass, M(vir), and the mass measured in (13)CO, M(cloud), is approximated well by (M(vir)/M.) = 2.0 x 10(1) (M(cloud)/M.)(0.72), which suggests that smaller clouds tend to be more weakly bound gravitationally than larger clouds or are dispersing if the external pressure is negligible. This is probably the cause of a small index value of the line width-size relation. 4. The luminosity function of the IRAS point sources associated with (13)CO clouds are well represented by a power law with N(star)(greater than or equal to L(star)) = 4.0 x 10(2) L(star)(-0.29) - 1.9 x 10 for those at similar to 2 kpc and N(star)(greater than or equal to L(star)) = 2.3 x 10L(star)(-0.27) - 3.2 for the rest. 5. The luminosity of the most luminous IRAS point source in a given molecular cloud increases systematically with the mass of the associated cloud. 6. The (13)CO clouds associated with IRAS point sources, which are regarded as ongoing star-forming clouds, tend to be more massive and larger in size and to have higher column densities than those without any sign of star formation. These relations are found to be consistent with those derived in Cygnus by Dobashi, Bernard, & Fukui and in Cepheus-Cassiopeia by Yonekura et al. In order to study the star formation activities, we studied the ratio of virial mass and the mass measured in (13)CO, M(vir)/M(cloud), and its relation with star formation. It is clearly seen that the star-forming (13)CO clouds have low M(vir)/M(cloud), and all the clouds with high M(vir)/M(cloud) exhibit no sign of star formation. This suggests that star formation rarely occurs in clouds with overall kinetic energy higher than the gravitational energy.