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Register a new userThe Initial Conditions of Stellar Protocluster Formation: III. The Herschel counterparts of the Spitzer Dark Cloud catalogue
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Galactic plane surveys of pristine molecular clouds are key for establishing a Galactic-scale view of the earliest stages of star formation. For this reason Peretto & Fuller (2009) built an unbiased sample of IRDCs in the 10 deg < |l| < 65 deg, |b|<1 deg region of the Galactic plane using Spitzer 8micron extinction. However, in absorption studies, intrinsic fluctuations in the mid-infrared background can be mis-interpreted as foreground clouds. The main goal of the study presented here is to disentangle real clouds in the Spitzer Dark Cloud (SDC) catalogue from artefacts due to fluctuations in the mid-infrared background. We constructed H_2 column density maps at ~18 resolution using the 160micron and 250micron data from the Herschel Galactic plane survey Hi-GAL. We also developed an automated detection scheme that confirms the existence of a SDC through its association with a peak on these Herschel column density maps. Detection simulations, along with visual inspection of a small sub-sample of SDCs, have been performed to get better insight into the limitations of our automated identification scheme. Our analysis shows that 76(+/-19)% of the catalogued SDCs are real. This fraction drops to 55(+/-12)% for clouds with angular diameters larger than ~1 arcminute. The contamination of the PF09 catalogue by large spurious sources reflect the large uncertainties associated to the construction of the 8micron background emission, a key stage towards the identification of SDCs. A comparison of the Herschel confirmed SDC sample with the BGPS and ATLASGAL samples shows that SDCs probe a unique range of cloud properties, reaching down to more compact and lower column density clouds than any of these two (sub-)millimetre Galactic plane surveys.
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The earliest phases of clustered star formation and the origin of the stellar initial mass function (IMF) are currently much debated. In order to constrain the origin of the IMF, we investigated the internal and relative motions of starless condensations and protostars previously detected by us in the dust continuum at 1.2mm in the L1688 protocluster of the Ophiuchus molecular cloud complex. The starless condensations have a mass spectrum resembling the IMF and are therefore likely representative of the initial stages of star formation in the protocluster. We carried out detailed molecular line observations, including some N2H+(1-0) mapping, of the Ophiuchus protocluster condensations using the IRAM 30m telescope. We measured subsonic or at most transonic levels of internal turbulence within the condensations, implying virial masses which generally agree within a factor of ~ 2 with the masses derived from the 1.2mm dust continuum. This supports the notion that most of the L1688 starless condensations are gravitationally bound and prestellar in nature. We measured a global one-dimensional velocity dispersion of less than 0.4 km/s between condensations. This small relative velocity dispersion implies that, in general, the condensations do not have time to interact with one another before evolving into pre-main sequence objects. Our observations support the view that the IMF is partly determined by cloud fragmentation at the prestellar stage. Competitive accretion is unlikely to be the dominant mechanism at the protostellar stage in the Ophiuchus protocluster, but it may possibly govern the growth of starless, self-gravitating condensations initially produced by gravoturbulent fragmentation toward an IMF, Salpeter-like mass spectrum.
We present the largest catalogue to date of optical counterparts for HI radio-selected galaxies, Hopcat. Of the 4315 HI radio-detected sources from the HI Parkes All Sky Survey (Hipass) catalogue, we find optical counterparts for 3618 (84%) galaxies. Of these, 1798 (42%) have confirmed optical velocities and 848 (20%) are single matches without confirmed velocities. Some galaxy matches are members of galaxy groups. From these multiple galaxy matches, 714 (16%) have confirmed optical velocities and a further 258 (6%) galaxies are without confirmed velocities. For 481 (11%), multiple galaxies are present but no single optical counterpart can be chosen and 216 (5%) have no obvious optical galaxy present. Most of these blank fields are in crowded fields along the Galactic plane or have high extinctions. Isolated Dark galaxy candidates are investigated using an extinction cut of ABj < 1 mag and the blank fields category. Of the 3692 galaxies with an ABj extinction < 1 mag, only 13 are also blank fields. Of these, 12 are eliminated either with follow-up Parkes observations or are in crowded fields. The remaining one has a low surface brightness optical counterpart. Hence, no isolated optically dark galaxies have been found within the limits of the Hipass survey.
We present N2D+ 3-2 (IRAM) and H2D+ 1_11 - 1_10 and N2H+ 4-3 (JCMT) maps of the small cluster-forming Ophiuchus B2 core in the nearby Ophiuchus molecular cloud. In conjunction with previously published N2H+ 1-0 observations, the N2D+ data reveal the deuterium fractionation in the high density gas across Oph B2. The average deuterium fractionation R_D = N(N2D+)/N(N2H+) ~ 0.03 over Oph B2, with several small scale R_D peaks and a maximum R_D = 0.1. The mean R_D is consistent with previous results in isolated starless and protostellar cores. The column density distributions of both H2D+ and N2D+ show no correlation with total H2 column density. We find, however, an anticorrelation in deuterium fractionation with proximity to the embedded protostars in Oph B2 to distances >= 0.04 pc. Destruction mechanisms for deuterated molecules require gas temperatures greater than those previously determined through NH3 observations of Oph B2 to proceed. We present temperatures calculated for the dense core gas through the equating of non-thermal line widths for molecules (i.e., N2D+ and H2D+) expected to trace the same core regions, but the observed complex line structures in B2 preclude finding a reasonable result in many locations. This method may, however, work well in isolated cores with less complicated velocity structures. Finally, we use R_D and the H2D+ column density across Oph B2 to set a lower limit on the ionization fraction across the core, finding a mean x_e, lim >= few x 10^{-8}. Our results show that care must be taken when using deuterated species as a probe of the physical conditions of dense gas in star-forming regions.
Massive clumps tend to fragment into clusters of cores and condensations, some of which form high-mass stars. In this work, we study the structure of massive clumps at different scales, analyze the fragmentation process, and investigate the possibility that star formation is triggered by nearby HII regions. We present a high angular resolution study of a sample of 8 massive proto-cluster clumps. Combining infrared data, we use few-arcsecond resolution radio- and millimeter interferometric data to study their fragmentation and evolution. Our sample is unique in the sense that all the clumps have neighboring HII regions. Taking advantage of that, we test triggered star formation using a novel method where we study the alignment of the centres of mass traced by dust emission at multiple scales. The eight massive clumps have masses ranging from 228 to 2279 $M_odot$. The brightest compact structures within infrared bright clumps are typically associated with embedded compact radio continuum sources. The smaller scale structures of $R_{rm eff}$ $sim$ 0.02 pc observed within each clump are mostly gravitationally bound and massive enough to form at least a B3-B0 type star. Many condensations have masses larger than 8 $M_odot$ at small scale of $R_{rm eff}$ $sim$ 0.02 pc. Although the clumps are mostly infrared quiet, the dynamical movements are active at clump scale ($sim$ 1 pc). We studied the spatial distribution of the gas conditions detected at different scales. For some sources we find hints of external triggering, whereas for others we find no significant pattern that indicates triggering is dynamically unimportant. This probably indicates that the different clumps go through different evolutionary paths. In this respect, studies with larger samples are highly desired.
Understanding the mechanism of O star formation is one of the most important issues in current astrophysics. It is also an issue of keen interest how O stars affect their surroundings and trigger secondary star formation. An H,emissiontype{II} region RCW79 is one of the typical Spitzer bubbles alongside of RCW120. New observations of CO $J=$ 1--0 emission with Mopra and NANTEN2 revealed that molecular clouds are associated with RCW79 in four velocity components over a velocity range of 20 km s$^{-1}$. We hypothesize that two of the clouds collided with each other and the collision triggered the formation of 12 O stars inside of the bubble and the formation of 54 low mass young stellar objects along the bubble wall. The collision is supported by observational signatures of bridges connecting different velocity components in the colliding clouds. The whole collision process happened in a timescale of $sim$1 Myr. RCW79 has a larger size by a factor of 30 in the projected area than RCW120 with a single O star, and the large size favored formation of the 12 O stars due to the larger accumulated gas in the collisional shock compression.
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