478 research outputs found

    An Extinction Threshold for Protostellar Cores in Ophiuchus

    Full text link
    We have observed continuum emission at lambda = 850 microns over ~4 square degrees of the Ophiuchus star-forming cloud using SCUBA on the JCMT, producing a submillimetre continuum map twenty times larger than previous Ophiuchus surveys. Our sensitivity is 40 mJ/beam, a factor of ~2 less sensitive than earlier maps. Using an automated identification algorithm, we detect 100 candidate objects. Only two new objects are detected outside the boundary of previous maps, despite the much wider area surveyed. We compare the submillimetre continuum map with a map of visual extinction across the Ophiuchus cloud derived using a combination of 2MASS and R-band data. The total mass in submillimetre objects is ~ 50 Msun compared with ~ 2000 Msun in observed cloud mass estimated from the extinction. The submillimetre objects represent only 2.5% of the cloud mass. A clear association is seen between the locations of detected submillimetre objects and high visual extinction, with no objects detected at A_V<7 magnitudes. Using the extinction map, we estimate pressures within the cloud from P/k ~2x10^5 cm^-3 K in the less-extincted regions to P/k ~2x10^6 cm^-3 K at the cloud centre. Given our sensitivities, cold (T_d ~15K) clumps supported by thermal pressure, had they existed, should have been detected throughout the majority of the map. Such objects may not be present at low A_V because they may form only where A_V > 15, by some mechanism (e.g., loss of non-thermal support).Comment: 12 pages, 1 figure. Accepted by Astrophysical Journal Letter

    The Role of Turbulence and Magnetic Fields in Simulated Filamentary Structure

    Get PDF
    We use numerical simulations of turbulent cluster-forming regions to study the nature of dense filamentary structures in star formation. Using four hydrodynamic and magnetohydrodynamic simulations chosen to match observations, we identify filaments in the resulting column density maps and analyze their properties. We calculate the radial column density profiles of the filaments every 0.05 Myr and fit the profiles with the modified isothermal and pressure confined isothermal cylinder models, finding reasonable fits for either model. The filaments formed in the simulations have similar radial column density profiles to those observed. Magnetic fields provide additional pressure support to the filaments, making `puffier' filaments less prone to fragmentation than in the pure hydrodynamic case, which continue to condense at a slower rate. In the higher density simulations, the filaments grow faster through the increased importance of gravity. Not all of the filaments identified in the simulations will evolve to form stars: some expand and disperse. Given these different filament evolutionary paths, the trends in bulk filament width as a function of time, magnetic field strength, or density, are weak, and all cases are reasonably consistent with the finding of a constant filament width in different star-forming regions. In the simulations, the mean FWHM lies between 0.06 and 0.26 pc for all times and initial conditions, with most lying between 0.1 to 0.15 pc; the range in FWHMs are, however, larger than seen in typical Herschel analyses. Finally, the filaments display a wealth of substructure similar to the recent discovery of filament bundles in Taurus.Comment: 18 pages, 8 figures, 7 tables. Accepted for publication in Ap

    Dynamics of Dense Cores in the Perseus Molecular Cloud

    Full text link
    We survey the kinematics of over one hundred and fifty candidate (and potentially star-forming) dense cores in the Perseus molecular cloud with pointed N2H+(1-0) and simultaneous C18O(2-1) observations. Our detection rate of N2H+ is 62%, rising to 84% for JCMT SCUBA-selected targets. In agreement with previous observations, we find that the dense N2H+ targets tend to display nearly thermal linewidths, particularly those which appear to be starless (using Spitzer data), indicating turbulent support on the small scales of molecular clouds is minimal. For those N2H+ targets which have an associated SCUBA dense core, we find their internal motions are more than sufficient to provide support against the gravitational force on the cores. Comparison of the N2H+ integrated intensity and SCUBA flux reveals fractional N2H+ abundances between 10^-10 and 10^-9. We demonstrate that the relative motion of the dense N2H+ gas and the surrounding C18O gas is less than the sound speed in the vast majority of cases (~90%). The point-to-point motions we observe within larger extinction regions appear to be insufficient to provide support against gravity, although we sparsely sample these regions.Comment: 49 pages, 20 figures. Accepted for publication in the Astrophysical Journa

    Leaders’ transitions : experiences of four directors of education during school division restructuring in rural Saskatchewan

    Get PDF
    The purpose of this study was to build an understanding of the transitional experiences of four Directors of Education who led school division restructuring initiatives in rural Saskatchewan. The data regarding the participants’ experiences were collected through participant observation, a survey, and a series of three interviews in which the participants were asked to describe their transitional experiences. Additionally, an interview with a member of the Restructuring Coordinating Committee and a process of document analysis provided further information about the restructuring initiative in which the Directors were involved. The conceptual framework for this study included the work of several researchers who had previously focused on transitions. It included the concept that transitions took place in three non-linear stages: the ending, the neutral zone, and the beginning. Additionally, the conceptual framework reflected the belief that transitions are influenced by the individual’s personal circumstances. This study added to the collective knowledge of change leadership as it chronicled the lived experiences of leaders who went through personal transitions while in the process of leading change and managing the transitions of employees within their organization. This study utilized a multiple case study design to explore the transitional experiences of educational leaders, charged with the task of leading organizational restructuring. Participants included four Directors of Education from rural Saskatchewan school divisions that were involved in the January 2006 provincial restructuring initiative. The data analysis resulted in three understandings: a) Variations in personal circumstances have a significant impact on the way that individuals experience transitions. b) Relationship building is a key component of the transition process. c) The leader must first negotiate interpersonal relationships and then focus on repatterning processes and building relationships within the organization

    The Dynamics of Dense Cores in the Perseus Molecular Cloud II: The Relationship Between Dense Cores and the Cloud

    Get PDF
    We utilize the extensive datasets available for the Perseus molecular cloud to analyze the relationship between the kinematics of small-scale dense cores and the larger structures in which they are embedded. The kinematic measures presented here can be used in conjunction with those discussed in our previous work as strong observational constraints that numerical simulations (or analytic models) of star formation should match. We find that dense cores have small motions with respect to the 13CO gas, about one third of the 13CO velocity dispersion along the same line of sight. Within each extinction region, the core-to-core velocity dispersion is about half of the total (13CO) velocity dispersion seen in the region. Large-scale velocity gradients account for roughly half of the total velocity dispersion in each region, similar to what is predicted from large-scale turbulent modes following a power spectrum of P(k) ~ k^{-4}.Comment: Accepted for publication in ApJ. 47 pages (preprint format), 20 figures, 5 table
    • …
    corecore