PDF model based on Langevin equation for polydispersed two-phase flows applied to a bluff-body gas-solid flow,

The aim of the paper is to discuss the main characteristics of a complete theoretical and numerical model for turbulent polydispersed two-phase flows, pointing out some specific issues. The theoretical details of the model have already been presented [Minier and Peirano, Physics Reports, Vol. 352/1-3, 2001 ]. Consequently, the present work is mainly focused on complementary aspects, that are often overlooked and that require particular attention. In particular, the following points are analysed : the necessity to add an extra term in the equation for the velocity of the fluid seen in the case of twoway coupling, the theoretical and numerical evaluations of particle averages and the fulfilment of the particle mass-continuity constraint. The theoretical model is developed within the PDF formalism. The important-physical choice of the state vector variables is first discussed and the model is then expressed as a stochastic differential equation (SDE) written in continuous time (Langevin equations) for the velocity of the fluid seen. The interests and limitations of Langevin equations, compared to the single-phase case, are reviewed. From the numerical point of view, the model corresponds to an hybrid Eulerian/Lagrangian approach where the fluid and particle phases are simulated by different methods. Important aspects of the Monte Carlo particle/mesh numerical method are emphasised. Finally, the complete model is validated and its performance is assessed by simulating a bluff-body case with an important recirculation zone and in which two-way coupling is noticeable.Comment: 23 pages, 10 figure

Spectral Energy Distributions of 6.7 GHz methanol masers

The 6.7 GHz maser transition of methanol has been found exclusively towards massive star forming regions. A majority of the masers have been found to lack the presence of any associated radio continuum. This could be due to the maser emission originating prior to the formation of an HII region around the central star, or from the central object being too cool to produce a HII region. One way to distinguish between the two scenarios is to determine and model the spectral energy distributions (SEDs) of the masers. We observed a sample of 20 6.7 GHz methanol masers selected from the blind Arecibo survey, from centimeter to submillimeter wavelengths. We combined our observations with existing data from various Galactic plane surveys to determine SEDs from centimeter to near-infrared wavelengths. We find that 70% of the masers do not have any associated radio continuum, with the rest of the sources being associated with hypercompact and ultracompact HII regions. Modeling the SEDs shows them to be consistent with rapidly accreting massive stars, with accretion rates well above 10^{-3} M_sun/yr. The upper limits on the radio continuum are also consistent with any ionized region being confined close to the stellar surface. This confirms the paradigm of 6.7 GHz methanol masers being signposts of early phases of massive star formation, mostly prior to the formation of a hypercompact HII region.Comment: 15 pages, 4 figures; Accepted by A&

New class I methanol masers

We review properties of all known collisionally pumped (class I) methanol maser series based on observations with the Australia Telescope Compact Array (ATCA) and the Mopra radio telescope. Masers at 36, 84, 44 and 95 GHz are most widespread, while 9.9, 25, 23.4 and 104 GHz masers are much rarer, tracing the most energetic shocks. A survey of many southern masers at 36 and 44 GHz suggests that these two transitions are highly complementary. The 23.4 GHz maser is a new type of rare class I methanol maser, detected only in two high-mass star-forming regions, G357.97-0.16 and G343.12-0.06, and showing a behaviour similar to 9.9, 25 and 104 GHz masers. Interferometric positions suggest that shocks responsible for class I masers could arise from a range of phenomena, not merely an outflow scenario. For example, some masers might be caused by interaction of an expanding HII region with its surrounding molecular cloud. This has implications for evolutionary sequences incorporating class I methanol masers if they appear more than once during the evolution of the star-forming region. We also make predictions for candidate maser transitions at the ALMA frequency range.Comment: 8 pages, 2 figures, to appear in proceedings for IAUS 287: Cosmic Masers - from OH to H

How do methanol masers manage to appear in the youngest star vicinities and isolated molecular clumps?

General characteristics of methanol (CH3OH) maser emission are summarized. It is shown that methanol maser sources are concentrated in the spiral arms. Most of the methanol maser sources from the Perseus arm are associated with embedded stellar clusters and a considerable portion is situated close to compact HII regions. Almost 1/3 of the Perseus Arm sources lie at the edges of optically identified HII regions which means that massive star formation in the Perseus Arm is to a great extent triggered by local phenomena. A multiline analysis of the methanol masers allows us to determine the physical parameters in the regions of maser formation. Maser modelling shows that class II methanol masers can be pumped by the radiation of the warm dust as well as by free-free emission of a hypercompact region hcHII with a turnover frequency exceeding 100 GHz. Methanol masers of both classes can reside in the vicinity of hcHIIs. Modelling shows that periodic changes of maser fluxes can be reproduced by variations of the dust temperature by a few percent which may be caused by variations in the brightness of the central young stellar object reflecting the character of the accretion process. Sensitive observations have shown that the masers with low flux densities can still have considerable amplification factors. The analysis of class I maser surveys allows us to identify four distinct regimes that differ by the series of their brightest lines.Comment: 8 pages, 4 figures, invited presentation at IAU242 "Astrophysical Masers and their environments

The Formaldehyde Masers in NGC 7538 and G29.96-0.02: VLBA, MERLIN, and VLA Observations

The 6 cm formaldehyde (H2CO) maser sources in the compact HII regions NGC 7538-IRS1 and G29.96-0.02 have been imaged at high resolution (beams < 50 mas). Using the VLBA and MERLIN, we find the angular sizes of the NGC 7538 masers to be ~10 mas (30 AU) corresponding to brightness temperatures ~10^8 K. The angular sizes of the G29.96-0.02 masers are ~20 mas (130 AU) corresponding to brightness temperatures ~10^7 K. Using the VLA, we detect 2 cm formaldehyde absorption from the maser regions. We detect no emission in the 2 cm line, indicating the lack of a 2 cm maser and placing limits on the 6 cm excitation process. We find that both NGC 7538 maser components show an increase in intensity on 5-10 year timescales while the G29.96-0.02 masers show no variability over 2 years. A search for polarization provides 3-sigma upper limits of 1% circularly polarized and 10% linearly polarized emission in NGC 7538 and of 15% circularly polarized emission in G29.96-0.02. A pronounced velocity gradient of 28 km/s/arcsecond (1900 km/s/pc) is detected in the NGC 7538 maser gas.Comment: accepted to ApJ, 15 figures, 11 table

A Search for Propylene Oxide and Glycine in Sagittarius B2 (LMH) and Orion

We have used the Mopra Telescope to search for glycine and the simple chiral molecule propylene oxide in the Sgr B2 (LMH) and Orion KL, in the 3-mm band. We have not detected either species, but have been able to put sensitive upper limits on the abundances of both molecules. The 3-sigma upper limits derived for glycine conformer I are 3.7 x 10^{14} cm^{-2} in both Orion-KL and Sgr B2 (LMH), comparable to the reported detections of conformer I by Kuan et al. However, as our values are 3-sigma upper limits rather than detections we conclude that this weighs against confirming the detection of Kuan et al. We find upper limits for the glycine II column density of 7.7 x 10^{12} cm^{-2} in both Orion-KL and Sgr B2 (LMH), in agreement with the results of Combes et al. The results presented here show that glycine conformer II is not present in the extended gas at the levels detected by Kuan et al. for conformer I. Our ATCA results (Jones et al.) have ruled out the detection of glycine (both conformers I and II) in the compact hot core of the LMH at the levels reported, so we conclude that it is unlikely that Kuan et al. have detected glycine in either Sgr B2 or Orion-KL. We find upper limits for propylene oxide abundance of 3.0 x 10^{14} cm^{-2} in Orion-KL and 6.7 x 10^{14} cm^{-2} in Sgr B2 (LMH). We have detected fourteen features in Sgr B2 and four features in Orion-KL which have not previously been reported in the ISM, but have not be able to plausibly assign these transitions to any carrier.Comment: 12 pages, 3 figures. Accepted by MNRAS 12th January 200

Evidence of triggered star formation in G327.3-0.6. Dust-continuum mapping of an infrared dark cloud with P-ArT\'eMiS

Aims. Expanding HII regions and propagating shocks are common in the environment of young high-mass star-forming complexes. They can compress a pre-existing molecular cloud and trigger the formation of dense cores. We investigate whether these phenomena can explain the formation of high-mass protostars within an infrared dark cloud located at the position of G327.3-0.6 in the Galactic plane, in between two large infrared bubbles and two HII regions. Methods: The region of G327.3-0.6 was imaged at 450 ? m with the CEA P-ArT\'eMiS bolometer array on the Atacama Pathfinder EXperiment telescope in Chile. APEX/LABOCA and APEX-2A, and Spitzer/IRAC and MIPS archives data were used in this study. Results: Ten massive cores were detected in the P-ArT\'eMiS image, embedded within the infrared dark cloud seen in absorption at both 8 and 24 ?m. Their luminosities and masses indicate that they form high-mass stars. The kinematical study of the region suggests that the infrared bubbles expand toward the infrared dark cloud. Conclusions: Under the influence of expanding bubbles, star formation occurs in the infrared dark areas at the border of HII regions and infrared bubbles.Comment: 4 page

The Herschel view of the on-going star formation in the Vela-C molecular cloud

As part of the Herschel guaranteed time key program 'HOBYS', we present the photometric survey of the star forming region Vela-C, one of the nearest sites of low-to-high-mass star formation in the Galactic plane. Vela-C has been observed with PACS and SPIRE in parallel mode between 70 um and 500 um over an area of about 3 square degrees. A photometric catalogue has been extracted from the detections in each band, using a threshold of 5 sigma over the local background. Out of this catalogue we have selected a robust sub-sample of 268 sources, of which 75% are cloud clumps and 25% are cores. Their Spectral Energy Distributions (SEDs) have been fitted with a modified black body function. We classify 48 sources as protostellar and 218 as starless. For two further sources, we do not provide a secure classification, but suggest they are Class 0 protostars. From SED fitting we have derived key physical parameters. Protostellar sources are in general warmer and more compact than starless sources. Both these evidences can be ascribed to the presence of an internal source(s) of moderate heating, which also causes a temperature gradient and hence a more peaked intensity distribution. Moreover, the reduced dimensions of protostellar sources may indicate that they will not fragment further. A virial analysis of the starless sources gives an upper limit of 90% for the sources gravitationally bound and therefore prestellar. We fit a power law N(logM) prop M^-1.1 to the linear portion of the mass distribution of prestellar sources. This is in between that typical of CO clumps and those of cores in nearby star-forming regions. We interpret this as a result of the inhomogeneity of our sample, which is composed of comparable fractions of clumps and cores.Comment: 9 pages, 7 figures, accepted by A&

The Aquila prestellar core population revealed by Herschel

The origin and possible universality of the stellar initial mass function (IMF) is a major issue in astrophysics. One of the main objectives of the Herschel Gould Belt Survey is to clarify the link between the prestellar core mass function (CMF) and the IMF. We present and discuss the core mass function derived from Herschel data for the large population of prestellar cores discovered with SPIRE and PACS in the Aquila Rift cloud complex at d ~ 260 pc. We detect a total of 541 starless cores in the entire ~11 deg^2 area of the field imaged at 70-500 micron with SPIRE/PACS. Most of these cores appear to be gravitationally bound, and thus prestellar in nature. Our Herschel results confirm that the shape of the prestellar CMF resembles the stellar IMF, with much higher quality statistics than earlier submillimeter continuum ground-based surveys