A¹³CO Survey of Molecular Clouds in Gemini and Auriga

A large-scale survey for molecular clouds in ¹³CO(J = 1-0) was performed toward the Gemini and Auriga regions (170° < l ≤ 196° and -10° ≤ b < 10°) with velocity coverages of -30 < V_LSR < +30 km s^-1 and -20 < V_LSR < +40 km s^-1 for 170° < l ≤ 188° and 188° < l ≤ 196°, respectively, by using the two 4 m millimeter-wave telescopes at Nagoya University. An area of 520 deg² was covered at an 8' grid spacing with a 27 beam, and 29,250 positions were observed. Significant ¹³CO emission (≥1.2 K km s^-1 = 3 σ) is detected at 1032 positions, and 139 distinct ¹³CO 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 ~2 kpc, while the rest lie at ≲ 1 kpc. The candidates for protostars are selected from IRAS point sources and 141 sources are identified as protostellar candidates associated with the ¹³CO clouds.

A statistical study of the ¹³CO clouds and of the candidates for protostars shows the following results:

1. The mass spectra of the ¹³CO clouds are well represented by a power law, N_cloud(≥M_cloud) = 1.4 × 10⁴(M_cloud/M_☉)^-0.83 - 1.4 for the clouds at ~2 kpc and N_cloud(≥M_cloud) = 3.5 × 10²(M_cloud/M_☉)^-0.64 - 2.4 for the rest.

2. The line width, ΔV_comp, and the size, R, of the ¹³CO 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 ¹³CO clouds indicates that the relation between the virial mass, M_vir, and the mass measured in ¹³CO, M_cloud, is aproximated well by (M_vir/M_☉) = 2.0 × 10¹(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 cuase of a small index value of the line width-size relation.

4. The luminosity function of the IRAS point sources associated with ¹³CO clouds are well represented by a power law with N_star(≥L_star) = 4.0 × 10²L-1.9×10 for those at ~2 kpc and N_star(≥L_star) = 2.3 × 10L-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 ¹³CO 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 ¹³CO, M_vir/M_cloud, and its relation with star formation. It is clearly seen that the star-forming ¹³CO 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.