G. A. Fuller

Dense cores in dark clouds. VIII: Velocity gradients

An analysis of motions consistent with uniform rotation in dense cores is presented. Twenty-nine of the 43 cores studied have a statistically significant gradient. Some gradients are spatially continuous and are consistent with uniform rotation, but other apparent gradients are caused by clump-clump motion, or sharp localized gradients, within a map. The motions in L1495, B217, L1251, L43, B361, and L1551 are discussed in detail. In L1551, the residuals of the fit to the NH3 velocity field indicate an outflow from IRS5 in the same direction as the CO outflow. Gradient orientation appears to be preserved over a range of density, as evidenced by comparing results of NH3 to fits of (C-18)O and CS maps. The specific angular momentum is found to scale roughly as F exp 3/2, where R represents the diameter of the FWHM intensity contour in a map.

The initial conditions of stellar protocluster formation - I. A catalogue of Spitzer dark clouds

Context. The majority of stars form in clusters. Therefore a comprehensive view of star formation requires understanding the initial conditions for cluster formation. Aims. The goal of our study is to shed light on the physical properties of infrared dark clouds (IRDCs) and the role they play in the formation of stellar clusters. This article, the first of a series dedicated to the study of IRDCs, describes techniques developed to establish a complete catalogue of Spitzer IRDCs in the Galaxy. Methods. We have analysed Spitzer GLIMPSE and MIPSGAL data to identify a complete sample of IRDCs in the region of Galactic longitude and latitude 10° < |l|< 65° and |b| < 1°. From the 8 m observations we have constructed opacity maps and used a newly developed extraction algorithm to identify structures above a column density of 1 1022 cm-2. The 24 m data are then used to characterize the star formation activity of each extracted cloud. Results. A total of 11 303 clouds have been extracted. A comparison with the existing MSX based catalogue of IRDCs shows that 80% of these Spitzer dark clouds were previously unknown. The algorithm also extracts ~20 000 to 50 000 fragments within these clouds, depending on detection threshold used. A first look at the MIPSGAL data indicates that between 20% and 68% of these IRDCs show 24 m  point-like association. This new database provides an important resource for future studies aiming to understand the initial conditions of star formation in the Galaxy.

Near-infrared and optical observations of IRAS sources in and near dense cores

This paper reports observations of 34 IRAS sources associated with dense cores in dark clouds at wavelengths 0.4-20 microns. The stars near cores tend to be visible T Tauri stars, while stars in cores tend to have circumstellar extinction 30-90 mag and luminosity about one solar, similar to that of T Tauri stars. The typical highly obscured star is probably accompanied by a luminous structure of substellar temperature, such as a circumstellar disk. In Taurus-Auriga, stars in cores probably become visible T Tauri stars less than 100,000 yr after they become detectable by IRAS; i.e., after they attain luminosity greater than about 0.1 solar. This implies that they are extremely young and may still be accreting. 54 references.

Dense cores in dark clouds. VI, Shapes

A comparison is presently conducted between 48 line-intensity maps of 16 dense cores in dark clouds, based on observations in the 13-mm line of NH3 and the 3-mm lines of CS and C(O-18). Core elongation in this sample does not differ significantly between cores with and without embedded stars; elongation appears to be a condition prior to star formation, rather than the consequence of formation. The characteristic elongation of dense cores implies that models of equilibrium between self-gravity and isotropic random motions are incomplete. The observed elongation of dense cores in projection is modeled as arising from a group of either prolate or oblate spheroids. 50 refs.

The 6-GHz methanol multibeam maser catalogue - I. Galactic Centre region, longitudes 345° to 6°

Original article can be found at: http://www3.interscience.wiley.com/journal/ Copyright Royal Astronomical Society

The connection between submillimeter continuum flux and binary separation in young binaries: evidence of interaction between stars and disks

We present 800 micron continuum photometry of pre-main-sequence binary stars with projected separations a_p < 150 AU in the Sco-Oph star-forming region. Combining our observations with published 1300 micron photometry, we find that binaries in Sco-Oph with 1 < a_p < 50--100 AU have lower submillimeter fluxes than wider binaries or single stars, as previously found for Taurus- Auriga binaries. The wide binaries and single stars have indistinguishable submillimeter flux distributions. Thus, binary companions with separations less than 50--100 AU significantly influence the nature of associated disks. We have explored the hypothesis that the reduction in submillimeter flux is the result of gaps cleared in disks by companions. Gap clearing produces the qualitative dependence of submillimeter flux on binary separation, and a simple model suggests that large gaps in disks with surface densities typical of wide-binary or single-star disks can reduce submillimeter fluxes to levels consistent with the observed limits. This model shows that the present submillimeter flux upper limits do not necessarily imply a large reduction in disk surface densities. Two-thirds of the young binaries were detected by IRAS, showing that most binaries have circumstellar disks. These fluxes place lower limits of 10^{-5} M_sun on circumstellar disk masses. The submillimeter fluxes place upper limits of 0.005 M_sun on circumbinary disk masses. Thus massive circumbinary disks are rare among binaries with separations between a few AU and 100 AU. Circumbinary disks are found around some close binaries.

Dense cores in dark clouds. VII : Line width-size relations

Observations of 14 dense cores in lines of CS, C 18 O and NH, are combined to examine the relations between the Doppler width of a line and the spatial extent of the map of line intensity. The line width increases with map size R as R p . The deduced width of the velocity distribution of the molecule of mean mass has p = 0.20 ± 0.04, and the deduced width of the nonthermal velocity distribution has p = 0.7 ± 0.1. The values of p for eight cores with embedded stars and for six cores without embedded stars are statistically indistinguishable, but p is better determined for cores without stars

The 6-GHz multibeam maser survey – I. Techniques

A new 7-beam 6 7 GHz receiver has been built to survey the Galaxy and the Magellanic Clouds for newly forming high-mass stars that are pinpointed by strong methanol maser emission at 6668 MHz. The receiver was jointly constructed by Jodrell Bank Observatory (JBO) and the Australia Telescope National Facility (ATNF) and allows simultaneous coverage at 6668 and 6035 MHz. It was successfully commissioned at Parkes in January 2006 and is now being used to conduct the Parkes-Jodrell multibeam maser survey of the Milky Way. This will be the first systematic survey of the entire Galactic plane for masers of not only 6668-MHz methanol, but also 6035-MHz excited-state hydroxyl. The survey is two orders of magnitude faster than most previous systematic surveys and has an rms noise level of �0.17Jy. This paper describes the observational strategy, techniques and reduction procedures of the Galactic and Magellanic Cloud surveys, together with deeper, pointed, follow-up observations and complementary observations with other instruments. It also includes an estimate of the survey detection efficiency. The 111 days of observationswith the Parkes telescope have so far yielded >800 methanol sources, of which �350 are new discoveries. The whole project will provide the first comprehensive Galaxy-wide catalogue of 6668-MHz and 6035-MHz masers.

The 6-GHz methanol multibeam maser catalogue - II. Galactic longitudes 6° to 20 °

The definitive version is available at: http://onlinelibrary.wiley.com Copyright Royal Astronomical Society.

Characterizing precursors to stellar clusters with Herschel

Context. Despite their profound effect on the universe, the formation of massive stars and stellar clusters remains elusive. Recent advances in observing facilities and computing power have brought us closer to understanding this formation process. In the past decade, compelling evidence has emerged that suggests infrared dark clouds (IRDCs) may be precursors to stellar clusters. However, the usual method for identifying IRDCs is biased by the requirement that they are seen in absorption against background mid-IR emission, whereas dust continuum observations allow cold, dense pre-stellar-clusters to be identified anywhere. Aims. We aim to understand what dust temperatures and column densities characterize and distinguish IRDCs, to explore the population of dust continuum sources that are not IRDCs, and to roughly characterize the level of star formation activity in these dust continuum sources. Methods. We use Hi-GAL 70 to 500 μm data to identify dust continuum sources in the l = 30° and l = 59° Hi-GAL science demonstration phase (SDP) fields, to characterize and subtract the Galactic cirrus emission, and perform pixel-by-pixel modified blackbody fits on cirrus-subtracted Hi-GAL sources. We utilize archival Spitzer data to indicate the level of star-forming activity in each pixel, from mid-IR-dark to mid-IR-bright. Results. We present temperature and column density maps in the Hi-GAL l = 30° and l = 59° SDP fields, as well as a robust algorithm for cirrus subtraction and source identification using Hi-GAL data. We report on the fraction of Hi-GAL source pixels which are mid-IR-dark, mid-IR-neutral, or mid-IR-bright in both fields. We find significant trends in column density and temperature between mid-IR-dark and mid-IR-bright pixels; mid-IR-dark pixels are about 10 K colder and have a factor of 2 higher column density on average than mid-IR-bright pixels. We find that Hi-GAL dust continuum sources span a range of evolutionary states from pre- to star-forming, and that warmer sources are associated with more star formation tracers. Additionally, there is a trend of increasing temperature with tracer type from mid-IR-dark at the coldest, to outflow/maser sources in the middle, and finally to 8 and 24 μm bright sources at the warmest. Finally, we identify five candidate IRDC-like sources on the far-side of the Galaxy. These are cold (~20 K), high column density (N(H2) > 1022 cm-2) clouds identified with Hi-GAL which, despite bright surrounding mid-IR emission, show little to no absorption at 8 μm. These are the first inner Galaxy far-side candidate IRDCs of which the authors are aware.