Molecular Clouds in Cepheus and Cassiopeia

Abstract

A large-scale 13CO(J = 1-0) survey for nearby molecular clouds was performed toward the Cepheus and Cassiopeia regions (100° < l < 130° and -10° < b < 20°) with the velocity coverage of -40 < VLSR < +20 km s-1 by using the two 4 m millimeter-wave telescopes at Nagoya University. An area of 866 square degrees was covered at an 8' grid spacing with a 27 beam, and 48,750 positions were observed. Significant 13CO emission (≥1.8 K km s-1 = 3 σ) is detected at 1015 positions. On the basis of the 13CO data, 188 distinct 13CO clouds are identified whose total mass is 1.0 × 105 M☉. Physical properties of the 13CO clouds such as molecular column densities, sizes, and masses are derived for each cloud. Astronomical objects associated with the 13CO clouds were searched for in the literature: 10 H II regions, eight reflection nebulae, 23 T Tauri type stars, 28 molecular outflows, and 125 IRAS point sources selected as candidates for protostars are likely to be associated with the 13CO clouds. Statistical studies on the 13CO clouds detected in this survey have been made. The mass spectra of the 13CO clouds in Cepheus and Cassiopeia are well represented by a power law, dNcloud/dMcloud ∝ Mcloud−α, with α = 1.7 ± 0.3 in a cloud mass range 101-105 M☉, which is similar to previously observed other regions like Cygnus. We also investigated the relation between the line width and the size of the 13CO clouds and found that there is no significant correlation among them in the cloud mass range 101-104 M☉. A virial analysis made for the 13CO clouds indicates that the relation between the virial mass Mvir and the mass measured in 13CO Mcloud is approximated well by (Mvir/M☉) = 2.8 × 101(Mcloud/M☉)0.62, suggesting that smaller clouds tend to be more weakly bound than larger clouds gravitationally or are dispersing if the possible external pressure is disregarded. It is found that 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 signs of star formation. There seems to be a threshold value in the peak H2 column density of N(H2) = 2.5 × 1021 cm-2 for stars to form in a13CO cloud. In order to study star formation activities in the 13CO clouds, we investigated the global luminosity distribution of the IRAS point sources and attempted to determine the distribution as a function of the parent molecular cloud mass. The IRAS luminosity function in a given cloud with mass Mcloud is found to be well approximated by a power law dnstar/dLstar = 6 × 10−3(Mcloud/M☉)0.8(Lstar/L☉)−1.5 L☉−1 for the IRAS luminosity range 1 < Lstar/L☉ < 105 and the cloud mass range 101 < Mcloud/M☉ < 105. The luminosity function does not depend significantly on the cloud mass, the regions sampled, and the existence of H II regions. The luminosity of the most luminous IRAS point source in a given molecular cloud Lstar,max increases systematically with the mass of the associated cloud, which is fitted by the power law (Lstar,max/L☉) = 0.098 × (Mcloud/M☉)1.43. These relations are found to be consistent with those derived in Cygnus by Dobashi, Bernard, & Fukui, suggesting that they may represent general relations between the maximum IRAS luminosity and the molecular clouds. We finally demonstrate that the slope of the initial mass function estimated from the IRAS luminosity function derived here is consistent with the stellar initial mass function for a stellar mass to luminosity relation of (Lstar/L☉) = (Mstar/M☉)3.45.