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Register a new userStar Formation in a Strongly Magnetized Cloud
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We study star formation in the Center Ridge 1 (CR1) clump in the Vela C giant molecular cloud, selected as a high column density region that shows the lowest level of dust continuum polarization angle dispersion, likely indicating that the magnetic field is relatively strong. We observe the source with the ALMA 7m-array at 1.05~mm and 1.3~mm wavelengths, which enable measurements of dust temperature, core mass and astrochemical deuteration. A relatively modest number of eleven dense cores are identified via their dust continuum emission, with masses spanning from 0.17 to 6.7 Msun. Overall CR1 has a relatively low compact dense gas fraction compared with other typical clouds with similar column densities, which may be a result of the strong magnetic field and/or the very early evolutionary stage of this region. The deuteration ratios, Dfrac, of the cores, measured with N2H+(3-2) and N2D+(3-2) lines, span from 0.011 to 0.85, with the latter being one of the highest values yet detected. The level of deuteration appears to decrease with evolution from prestellar to protostellar phase. A linear filament, running approximately parallel with the large scale magnetic field orientation, is seen connecting the two most massive cores, each having CO bipolar outflows aligned orthogonally to the filament. The filament contains the most deuterated core, likely to be prestellar and located midway between the protostars. The observations permit measurement of the full deuteration structure of the filament along its length, which we present. We also discuss the kinematics and dynamics of this structure, as well as of the dense core population.
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Recent galaxy observations show that star formation activity changes depending on galactic environments. In order to understand the diversity of galactic-scale star formation, it is crucial to understand the formation and evolution of giant molecular clouds in an extreme environment. We focus on observational evidence that bars in strongly barred galaxies lack massive stars even though quantities of molecular gas are sufficient to form stars. In this paper, we present a hydrodynamical simulation of a strongly barred galaxy, using a stellar potential which is taken from observational results of NGC1300, and we compare cloud properties between different galactic environments: bar, bar-end and spiral arms. We find that the mean of clouds virial parameter is ~1 and that there is no environmental dependence, indicating that the gravitationally-bound state of a cloud is not behind the observational evidence of the lack of massive stars in strong bars. Instead, we focus on cloud-cloud collisions, which have been proposed as a triggering mechanism for massive star formation. We find that the collision speed in the bar is faster than those in the other regions. We examine the collision frequency using clouds kinematics and conclude that the fast collisions in the bar could originate from random-like motion of clouds due to elliptical gas orbits shifted by the bar potential. These results suggest that the observed regions of lack of active star-formation in the strong bar originate from the fast cloud-cloud collisions, which are inefficient in forming massive stars, due to the galactic-scale violent gas motion.
A recent discovery of two stellar clusters associated with the diffuse high-latitude cloud HRK 81.4-77.8 has important implications for star formation in the Galactic halo. We derive a plausible distance estimate to HRK 81.4-77.8 primarily from its gaseous properties. We spatially correlate state-of-the-art HI, far-infrared and soft X-ray data to analyze the diffuse gas in the cloud. The absorption of the soft X-ray emission from the Galactic halo by HRK 81.4-77.8 is used to constrain the distance to the cloud. HRK 81.4-77.8 is most likely located at an altitude of about 400 pc within the disk-halo interface of the Milky Way Galaxy. The HI data discloses a disbalance in density and pressure between the warm and cold gaseous phases. Apparently, the cold gas is compressed by the warm medium. This disbalance might trigger the formation of molecular gas high above the Galactic plane on pc to sub-pc scales.
Star formation is a fundamental process for galactic evolution. One issue over the last several decades has been determining whether star formation is induced by external triggers or is self-regulated in a closed system. The role of an external trigger, which can effectively collect mass in a small volume, has attracted particular attention in connection with the formation of massive stellar clusters, which in the extreme may lead to starbursts. Recent observations have revealed massive cluster formation triggered by cloud-cloud collisions in nearby interacting galaxies, including the Magellanic system and the Antennae Galaxies as well as almost all well-known high-mass star-forming regions such as RCW 120, M20, M42, NGC 6334, etc., in the Milky Way. Theoretical efforts are laying the foundation for the mass compression that causes massive cluster/star formation. Here, we review the recent progress on cloud-cloud collisions and triggered star-cluster formation and discuss the future prospects for this area of research.
Motivated by the idea that a subset of HVCs trace dark matter substructure in the Local Group, we search for signs of star formation in the Smith Cloud, a nearby ~2x10^6 Msun HVC currently falling into the Milky Way. Using GALEX NUV and WISE/2MASS NIR photometry, we apply a series of color and apparent magnitude cuts to isolate candidate O and B stars that are plausibly associated with the Smith Cloud. We find an excess of stars along the line of sight to the cloud, but not at a statistically significant level relative to a control region. The number of stars found in projection on the cloud after removing an estimate of the contamination by the Milky Way implies an average star formation rate surface density of 10^(-4.8 +/- 0.3) Msun yr^(-1) kpc^(-2), assuming the cloud has been forming stars at a constant rate since its first passage through the Milky Way ~70 Myr ago. This value is consistent with the star formation rate expected based on the average gas density of the cloud. We also discuss how the newly discovered star forming galaxy Leo P has very similar properties to the Smith Cloud, but its young stellar population would not have been detected at a statistically significant level using our method. Thus, we cannot yet rule out the idea that the Smith Cloud is really a dwarf galaxy.
W51A is one of the most active star-forming region in our Galaxy, which contains giant molecular clouds with a total mass of 10^6 Msun. The molecular clouds have multiple velocity components over ~20 km/s, and interactions between these components have been discussed as the mechanism which triggered the massive star formation in W51A. In this paper, we report an observational study of the molecular clouds in W51A using the new 12CO, 13CO, and C18O (J=1-0) data covering a 1.4x1.0 degree region of W51A obtained with the Nobeyama 45-m telescope at 20 resolution. Our CO data resolved the four discrete velocity clouds at 50, 56, 60, and 68 km/s with sizes and masses of ~30 pc and 1.0-1.9x10^5 Msun. Toward the central part of the HII region complex G49.5-0.4, we identified four C18O clumps having sizes of ~1 pc and column densities of higher than 10^23 cm^-3, which are each embedded within the four velocity clouds. These four clumps are distributed close to each others within a small distance of 5 pc, showing a complementary distribution on the sky. In the position-velocity diagram, these clumps are connected with each others by bridge features with intermediate intensities. The high intensity ratios of 13CO (J=3-2/J=1-0) also indicates that these four clouds are associated with the HII regions. We also found these features in other HII regions in W51A. The timescales of the collisions are estimated to be several 0.1 Myrs as a crossing time of the clouds, which are consistent with the ages of the HII regions measured from the size of the HII regions in the 21 cm continuum emissions. We discuss the cloud-cloud collision scenario and massive star formation in W51A by comparing with the recent observational and theoretical studies of cloud-cloud collision.
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