Structure and star forming activities of the cold, massive molecular cloud G216-2.5.

We have studied the gas and dust properties and the star forming activity in G216-2.5 (also called Maddalena\u27s cloud), a cold, massive GMC that has no evidence for star formation. We have mapped 11 square degrees in the J = 1-0 transitions of \sp{12}CO and \sp{13}CO using the QUARRY fifteen-beam array receiver on the 14 m telescope at FCRAO. We have confirmed the results of Maddalena and Thaddeus (1985) that the cloud is unusually cold and has very broad linewidths $(\Delta V\sim8$ km s\sp{-1}). The visual extinction to \sp{13}CO column density ratio is found to be similar to that in local molecular clouds, suggesting that if the gas to dust ratio is normal, the \sp{13}CO abundance relative to molecular hydrogen in this cloud is similar to that found for clouds near the sun. The low gas and dust temperatures found for this cloud are largely due to the absence of heating by massive stars. The exceptionally low gas temperature can be explained if the cosmic-ray heating rate is reduced by a factor of two in the outer Galaxy. Low luminosity, and presumably low mass, young stars have been identified in this cloud. These young stars are found preferentially on the edge of the cloud. Clusters of T Tauri stars are found in two locations within G216-2.5. The mass of the cloud has been established by several techniques, and masses between 1 $\times$ 10\sp5 and 6 $\times$ 10\sp5 M\sb\odot were obtained. It is suggested that G216-2.5 is a remnant cloud from a past episode of massive star formation for the following reasons. First, the cloud has a relatively large velocity dispersion for a non-star-forming GMC. Second, there is clear evidence for shells and rings within the cloud, which may be the fossil remains of its earlier star formation activity. Third, the kinematics of the cloud is dominated by a global velocity gradient suggesting that the cloud is part of a very large expanding shell. Lastly, the discrepancy between the LTE and virial masses may be explained if the cloud has been severely perturbed and is currently expanding. G216-2.5 may be part of a larger star forming complex that includes the adjacent H scII region S287 and its molecular cloud

The Scaleheight of Giant Molecular Clouds is Less than that of Smaller Clouds

We have used an antenna temperature thresholding algorithm on the Bell Laboratories 13CO Milky Way Survey to create a catalog of 1,400 molecular clouds. Of these, 281 clouds were selected for having well-determined kinematic distances. The scaleheight, luminosity, internal velocity dispersion, and size of the cloud sample are analyzed to show that clouds smaller than ~200,000 solar masses have a scaleheight which is about 35 pc, roughly independent of cloud mass, while larger clouds, the Giant Molecular Clouds, have a reduced scaleheight which declines with increasing cloud mass.Comment: 12 pages including 4 color figures and 1 table. Submitted to ApJ

The Bell Laboratories (13)CO Survey: Longitude-Velocity Maps

A survey is presented of the Galactic plane in the J=1-0 transition of (13)CO. About 73,000 spectra were obtained with the 7 m telescope at Bell Laboratories over a ten-year period. The coverage of survey is (l, b) = (-5 to 117, -1 to +1), or 244 square degrees, with a grid spacing of 3' for |b| < 0.5, and a grid spacing of 6' for |b| > 0.5. The data presented here have been resampled onto a 3' grid. For 0.68 km/s channels, the rms noise level of the survey is 0.1 K on the $T_R^*$ scale. The raw data have been transformed into FITS format, and all the reduction processes, such as correcting for emission in the reference positions, baseline removal and interpolation were conducted within IRAF using the FCRAO task package and additional programs. The reduced data are presented here in the form of longitude-velocity color maps at each latitude. These data allow identification and classification of molecular clouds with masses in excess of ~ 1,000 solar masses throughout the first quadrant of the Galaxy. Spiral structure is manifested by the locations of the largest and brightest molecular clouds.Comment: 23 pages, 7 figures, ApJS submitted (out of 41 frames of Figure4, only one is included becaue of size limit

The Spitzer c2d Survey of Nearby Dense Cores. V. Discovery of a VeLLO in the "Starless" Dense Core L328

This paper reports the discovery of a Very Low Luminosity Object (VeLLO) in the "starless" dense core L328, using the Spitzer Space Telescope and ground based observations from near-infrared to millimeter wavelengths. The Spitzer 8 micron image indicates that L328 consists of three subcores of which the smallest one may harbor a source, L328-IRS while two other subcores remain starless. L328-IRS is a Class 0 protostar according to its bolometric temperature (44 K) and the high fraction ~72 % of its luminosity emitted at sub-millimeter wavelengths. Its inferred "internal luminosity" (0.04 - 0.06 Lsun) using a radiative transfer model under the most plausible assumption of its distance as 200 pc is much fainter than for a typical protostar, and even fainter than other VeLLOs studied previously. Note, however, that its inferred luminosity may be uncertain by a factor of 2-3 if we consider two extreme values of the distance of L328-IRS (125 or 310 pc). Low angular resolution observations of CO do not show any clear evidence of a molecular outflow activity. But broad line widths toward L328, and Spitzer and near-infrared images showing nebulosity possibly tracing an outflow cavity, strongly suggest the existence of outflow activity. Provided that an envelope of at most ~0.1 Msunis the only mass accretion reservoir for L328-IRS, and the star formation efficiency is close to the canonical value ~30%, L328-IRS has not yet accreted more than 0.05 Msun. At the assumed distance of 200 pc, L328-IRS is destined to be a brown dwarf.Comment: 29 pages, 8 figures, 1 table, to be published in Astrophysical Journa

Giant Molecular Clouds are More Concentrated to Spiral Arms than Smaller Clouds

From our catalog of Milky Way molecular clouds, created using a temperature thresholding algorithm on the Bell Laboratories 13CO Survey, we have extracted two subsets:(1) Giant Molecular Clouds (GMCs), clouds that are definitely larger than 10^5 solar masses, even if they are at their `near distance', and (2) clouds that are definitely smaller than 10^5 solar masses, even if they are at their `far distance'. The positions and velocities of these clouds are compared to the loci of spiral arms in (l, v) space. The velocity separation of each cloud from the nearest spiral arm is introduced as a `concentration statistic'. Almost all of the GMCs are found near spiral arms. The density of smaller clouds is enhanced near spiral arms, but some clouds (~10%) are unassociated with any spiral arm. The median velocity separation between a GMC and the nearest spiral arm is 3.4+-0.6 km/s, whereas the median separation between smaller clouds and the nearest spiral arm is 5.5+-0.2 km/s.Comment: 11 pages, 3 figure

Precessing Jet and Large Dust Grains in the V380 Ori NE Star-forming Region

The V380 Ori NE bipolar outflow was imaged in the SiO and CO J = 1 - 0 lines, and dense cores in L1641 were observed in the 2.0-0.89 mm continuum. The highly collimated SiO jet shows point-symmetric oscillation patterns in both position and velocity, which suggests that the jet axis is precessing and the driving source may belong to a non-coplanar binary system. By considering the position and velocity variabilities together, accurate jet parameters were derived. The protostellar system is viewed nearly edge-on, and the jet has a flow speed of 35 km/s and a precession period of 1600 years. The CO outflow length gives a dynamical timescale of 6300 years, and the protostar must be extremely young. The inferred binary separation of 6-70 au implies that this protobinary system may have been formed through the disk instability process. The continuum spectra of L1641 dense cores indicate that the emission comes from dust, and the fits with modified blackbody functions give emissivity power indices of beta = 0.3-2.2. The emissivity index shows a positive correlation with the molecular line width, but no strong correlation with bolometric luminosity or temperature. V380 Ori NE has a particularly low value of beta = 0.3, which tentatively suggests the presence of millimeter-sized dust grains. Because the dust growth takes millions of years, much longer than the protostellar age, this core may have produced large grains in the starless core stage. HH 34 MMS and HH 147 MMS also have low emissivity indices.Comment: To appear in the Astrophysical Journal Supplement Serie

A CO Line and Infrared Continuum Study of the Active Star-Forming Complex W51

We present the results of an extensive observational study of the active star-forming complex W51 that was observed in the J=2-1 transition of the 12CO and 13CO molecules over a 1.25 deg x 1.00 deg region with the University of Arizona Heinrich Hertz Submillimeter Telescope. We use a statistical equilibrium code to estimate physical properties of the molecular gas. We compare the molecular cloud morphology with the distribution of infrared (IR) and radio continuum sources, and find associations between molecular clouds and young stellar objects (YSOs) listed in Spitzer IR catalogs. The ratios of CO lines associated with HII regions are different from the ratios outside the active star-forming regions. We present evidence of star formation triggered by the expansion of the HII regions and by cloud-cloud collisions. We estimate that about 1% of the cloud mass is currently in YSOs.Comment: 18 pages, 29 figures; accepted for publication in ApJ

Triggered Star Formation in a Double Shell near W51A

We present Heinrich Hertz Telescope CO observations of the shell structure near the active star-forming complex W51A to investigate the process of star formation triggered by the expansion of an HII region. The CO observations confirm that dense molecular material has been collected along the shell detected in Spitzer IRAC images. The CO distribution shows that the shell is blown out toward a lower density region to the northwest. Total hydrogen column density around the shell is high enough to form new stars. We find two CO condensations with the same central velocity of 59 km/s to the east and north along the edge of the IRAC shell. We identify two YSOs in early evolutionary stages (Stage 0/I) within the densest molecular condensation. From the CO kinematics, we find that the HII region is currently expanding with a velocity of 3.4 km/s, implying that the shell's expansion age is ~1 Myr. This timescale is in good agreement with numerical simulations of the expansion of the HII region (Hosokawa et al. 2006). We conclude that the star formation on the border of the shell is triggered by the expansion of the HII region.Comment: 9 pages, 10 figures, accepted for publication in Ap