arXiv subjects

G. A. Fuller

Publications and source records attributed to G. A. Fuller.

At least 91 records · Page 5Linked to original sources

arXiv2014

A new HCN maser in IRAS 15082-4808

We have identified a new vibrational HCN maser at 89.087 GHz in the asymptotic giant branch (AGB) star IRAS 15082-4808, a maser which is thought to trace the innermost region of an AGB envelope. The observations of this maser at three epochs are presented: two positive detections and one null detection. The line profile has varied between the positive detections, as has the intensity of the maser. The major component of the maser is found to be offset by -2.0+/-0.9 km/s with respect to the systemic velocity of the envelope, as derived from the 88.631 GHz transition of HCN. Similar blueshifts are measured in the other 9 sources where this maser has been detected. Maser variability with pulsation phase has been investigated for the first time using the 10 stars now available. Comparisons with AGB model atmospheres constrain the position of the formation region of the maser to the region between the pulsation shocks and the onset of dust acceleration, between 2 and 4 stellar radii.

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arXiv2013

SDC13 infrared dark clouds: Longitudinally collapsing filaments?

Formation of stars is now believed to be tightly linked to the dynamical evolution of interstellar filaments in which they form. In this paper we analyze the density structure and kinematics of a small network of infrared dark filaments, SDC13, observed in both dust continuum and molecular line emission with the IRAM 30m telescope. These observations reveal the presence of 18 compact sources amongst which the two most massive, MM1 and MM2, are located at the intersection point of the parsec-long filaments. The dense gas velocity and velocity dispersion observed along these filaments show smooth, strongly correlated, gradients. We discuss the origin of the SDC13 velocity field in the context of filament longitudinal collapse. We show that the collapse timescale of the SDC13 filaments (from 1 Myr to 4 Myr depending on the model parameters) is consistent with the presence of Class I sources in them, and argue that, on top of bringing more material to the centre of the system, collapse could generate additional kinematic support against local fragmentation, helping the formation of starless super-Jeans cores.

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arXiv2013

MALT90: The Millimetre Astronomy Legacy Team 90 GHz Survey

The Millimetre Astronomy Legacy Team 90 GHz (MALT90) survey aims to characterise the physical and chemical evolution of high-mass star-forming clumps. Exploiting the unique broad frequency range and on-the-fly mapping capabilities of the Australia Telescope National Facility Mopra 22 m single-dish telescope, MALT90 has obtained 3' x 3' maps toward ~2000 dense molecular clumps identified in the ATLASGAL 870 um Galactic plane survey. The clumps were selected to host the early stages of high-mass star formation and to span the complete range in their evolutionary states (from prestellar, to protostellar, and on to HII regions and photodissociation regions). Because MALT90 mapped 16 lines simultaneously with excellent spatial (38") and spectral (0.11 km/s) resolution, the data reveal a wealth of information about the clump's morphologies, chemistry, and kinematics. In this paper we outline the survey strategy, observing mode, data reduction procedure, and highlight some early science results. All MALT90 raw and processed data products are available to the community. With its unprecedented large sample of clumps, MALT90 is the largest survey of its type ever conducted and an excellent resource for identifying interesting candidates for high resolution studies with ALMA.

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arXiv2013

Global collapse of molecular clouds as a formation mechanism for the most massive stars

The relative importance of primordial molecular cloud fragmentation versus large-scale accretion still remains to be assessed in the context of massive core/star formation. Studying the kinematics of the dense gas surrounding massive-star progenitors can tell us the extent to which large-scale flow of material impacts the growth in mass of star-forming cores. Here we present a comprehensive dataset of the 5500(+/-800) Msun infrared dark cloud SDC335.579-0.272 (hereafter SDC335) which exhibits a network of cold, dense, parsec-long filaments. Atacama Large Millimeter Array (ALMA) Cycle 0 observations reveal two massive star-forming cores, MM1 and MM2, sitting at the centre of SDC335 where the filaments intersect. With a gas mass of 545(+770,-385) Msun contained within a source diameter of 0.05pc, MM1 is one of the most massive, compact protostellar cores ever observed in the Galaxy. As a whole, SDC335 could potentially form an OB cluster similar to the Trapezium cluster in Orion. ALMA and Mopra single-dish observations of the SDC335 dense gas furthermore reveal that the kinematics of this hub-filament system are consistent with a global collapse of the cloud. These molecular-line data point towards an infall velocity V_{inf} =0.7(+/-0.2) km/s, and a total mass infall rate \dot{M}_{inf} = 2.5(+/-1.0) x 10^{-3} Msun/yr towards the central pc-size region of SDC335. This infall rate brings 750(+/-300) Msun of gas to the centre of the cloud per free-fall time (t_{ff}=3x10^5 yr). This is enough to double the mass already present in the central pc-size region in 3.5(+2.2,-1.0) x t_{ff}. These values suggest that the global collapse of SDC335 over the past million year resulted in the formation of an early O-type star progenitor at the centre of the cloud's gravitational potential well.

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arXiv2012

The mid-infrared environments of 6.7 GHz Methanol Masers from the Methanol Multi-Beam Survey

We present a study of the mid-infrared environments and association with star formation tracers of 6.7 GHz methanol masers taken from the Methanol Multi-Beam (MMB) Survey. Our ultimate goal is to establish the mass of the host star and its evolutionary stage for each maser site. As a first step, the GLIMPSE survey of the Galactic Plane is utilised to investigate the environment of 776 methanol masers and we find that while the majority of the masers are associated with mid-infrared counterparts, a significant fraction (17%) are not associated with any detectable mid-infrared emission. A number of the maser counterparts are clearly extended with respect to the GLIMPSE point spread function and we implement an adaptive non-circular aperture photometry (ANCAP) technique to determine the fluxes of the maser counterparts. The ANCAP technique doubles the number of masers with flux information at all four wavelengths compared to the number of the corresponding counterparts obtained from the GLIMPSE Point Source Catalogue. The colours of the maser counterparts are found to be very similar to the smaller study carried out by Ellingsen (2006). The MMB masers are weakly associated with Extended Green Objects (EGOs) and Red MSX Survey (RMS) embedded sources (YSO and HII classifications) with 18% and 12% of masers associated with these objects respectively. The majority of MMB masers (60%) have detectable GLIMPSE infrared counterparts but have not been identified with previously recognised tracers of massive star formation; this confirms that the MMB survey has the potential to identify massive star forming regions independent of infrared selection.

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arXiv2012

The Co-ordinated Radio and Infrared Survey for High Mass Star Formation (The CORNISH Survey) - I. Survey Design

We describe the motivation, design and implementation of the CORNISH survey, an arcsecond resolution radio continuum survey of the inner Galactic plane at 5 GHz using the Karl G. Jansky Very Large Array (VLA). It is a blind survey co-ordinated with the northern Spitzer GLIMPSE I region covering 10 deg < l < 65 deg and |b| < 1 deg at similar resolution. We discuss in detail the strategy that we employed to control the shape of the synthesised beam across this survey that covers a wide range of fairly low declinations. Two snapshots separated by 4 hours in hour angle kept the beam elongation to less that 1.5 over 75% of the survey area and less than 2 over 98% of the survey. The prime scientific motivation is to provide an unbiased survey for ultra-compact HII regions to study this key phase in massive star formation. A sensitivity around 2 mJy will allow the automatic distinction between radio loud and quiet mid-IR sources found in the Spitzer surveys. This survey has many legacy applications beyond star formation including evolved stars, active stars and binaries, and extragalactic sources. The CORNISH survey for compact ionized sources complements other Galactic plane surveys that target diffuse and non-thermal sources as well as atomic and molecular phases to build up a complete picture of the ISM in the Galaxy.

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arXiv2012

The molecular gas content of the Pipe Nebula I. Direct evidence of outflow-generated turbulence in B59?

The Pipe Nebula is a molecular cloud hosting the B59 region as its only active star-forming clump. While the particular importance of outflows in active star forming regions is subject of debate, the quiet nature of the gas in B59 makes it a good site to directly see the impact of protostellar feedback on the quiescent dense gas. Using HARP at the JCMT, we mapped the B59 region with the J=3-2 transition of 12CO to study the kinematics and energetics of the outflows, and 13CO and C18O to study the overall dynamics of the ambient cloud, the physical properties of the gas, and the hierarchical structure of the region. The B59 region has a total of 30Msun of cold and quiescent material, mostly gravitationally bound, with narrow line widths throughout. Such low levels of turbulence in non-star-forming sites of B59 are indicative of the intrinsic initial conditions of the cloud. On the other hand, close to the forming protostars the impact of the outflows is observed as a localised increase of both line widths from 0.3 to 1 km/s, and 13CO excitation temperatures by 2-3K. The impact of the outflows is also evident in the low column density material which shows signs of being pushed, shaped and carved by the outflow bow shocks as they pierce their way out of the cloud. Much of this structure is readily apparent in a dendrogram analysis of the cloud. The low mass of B59 together with its intrinsically quiescent gas and small number of protostars, allows the identification of specific regions where the outflows from the embedded sources interact the dense gas. Our study suggests that outflows are an important mechanism for injecting and sustaining supersonic turbulence at sub-parsec size scales. We find that less than half of the outflow energy is deposited as turbulent energy of the gas, however this turbulent energy is sufficient to slow down the collapse of the region.

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arXiv2012

Isolated starless cores in IRDCs in the Hi-GAL survey

In a previous paper we identified cores within infrared dark clouds (IRDCs). We regarded those without embedded sources as the least evolved, and labelled them starless. Here we identify the most isolated starless cores and model them using a three-dimensional, multi-wavelength, Monte Carlo, radiative transfer code. We derive the cores' physical parameters and discuss the relation between the mass, temperature, density, size and the surrounding interstellar radiation field (ISRF) for the cores. The masses of the cores were found not to correlate with their radial size or central density. The temperature at the surface of a core was seen to depend almost entirely on the level of the ISRF surrounding the core. No correlation was found between the temperature at the centre of a core and its local ISRF. This was seen to depend, instead, on the density and mass of the core.

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arXiv2012

Extended warm and dense gas towards W49A: starburst conditions in our Galaxy?

The star formation rates in starburst galaxies are orders of magnitude higher than in local star-forming regions, and the origin of this difference is not well understood. We use sub-mm spectral line maps to characterize the physical conditions of the molecular gas in the luminous Galactic star-forming region W49A and compare them with the conditions in starburst galaxies. We probe the temperature and density structure of W49A using H_2CO and HCN line ratios over a 2'x2' (6.6x6.6 pc) field with an angular resolution of 15" (~0.8 pc) provided by the JCMT Spectral Legacy Survey. We analyze the rotation diagrams of lines with multiple transitions with corrections for optical depth and beam dilution, and estimate excitation temperatures and column densities. Comparing the observed line intensity ratios with non-LTE radiative transfer models, our results reveal an extended region (about 1'x1', equivalent to ~3x3 pc at the distance of W49A) of warm (> 100 K) and dense (>10^5 cm^-3) molecular gas, with a mass of 2x10^4 - 2x10^5 M_Sun (by applying abundances derived for other regions of massive star-formation). These temperatures and densities in W49A are comparable to those found in clouds near the center of the Milky Way and in starburst galaxies. The highly excited gas is likely to be heated via shocks from the stellar winds of embedded, O-type stars or alternatively due to UV irradiation, or possibly a combination of these two processes. Cosmic rays, X-ray irradiation and gas-grain collisional heating are less likely to be the source of the heating in the case of W49A.

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arXiv2012

The JCMT Legacy Survey of the Gould Belt: mapping 13CO and C18O in Orion A

The Gould Belt Legacy Survey will map star-forming regions within 500 pc, using HARP (Heterodyne Array Receiver Programme), SCUBA-2 (Submillimetre Common-User Bolometer Array 2) and POL-2 (Polarimeter 2) on the James Clerk Maxwell Telescope (JCMT). This paper describes HARP observations of the J = 3-2 transitions of 13CO and C18O towards Orion A. The 1500-resolution observations cover 5 pc of the Orion filament, including OMC1 (inc. BN-KL and Orion Bar), OMC 2/3 and OMC 4, and allow a comparative study of the molecular gas properties throughout the star-forming cloud. The filament shows a velocity gradient of ~1 km/s /pc between OMC 1, 2 and 3, and high velocity emission is detected in both isotopologues. The Orion Nebula and Bar have the largest masses and line widths, and dominate the mass and energetics of the high velocity material. Compact, spatially resolved emission from CH3CN, 13CH3OH, SO, HCOOCH3, C2H5OH, CH3CHO and CH3OCHO is detected towards the Orion Hot Core. The cloud is warm, with a median excitation temperature of ~24 K; the Orion Bar has the highest excitation temperature gas, at >80 K. The C18O excitation temperature correlates well with the dust temperature (to within 40%). The C18O emission is optically thin, and the 13CO emission is marginally optically thick; despite its high mass, OMC 1 shows the lowest opacities. A virial analysis indicates that Orion A is too massive for thermal or turbulent support, but is consistent with a model of a filamentary cloud that is threaded by helical magnetic fields. The variation of physical conditions across the cloud is reflected in the physical characteristics of the dust cores....continued

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arXiv2012

Cores in Infra-Red Dark Clouds (IRDCs) seen in the Hi-GAL survey between l = 300° and l = 330°

We have used data taken as part of the Herschel infrared Galactic Plane survey (Hi-GAL) to study 3171 infrared-dark cloud (IRDC) candidates that were identified in the mid-infrared (8 μm) by Spitzer (we refer to these as 'Spitzer-dark' regions). They all lie in the range l=300 - 330 \circ and |b| 6 1 \circ. Of these, only 1205 were seen in emission in the far-infrared (250-500 μm) by Herschel (we call these 'Herschel-bright' clouds). It is predicted that a dense cloud will not only be seen in absorption in the mid-infrared, but will also be seen in emission in the far-infrared at the longest Herschel wavebands (250-500 μm). If a region is dark at all wavelengths throughout the mid-infrared and far-infrared, then it is most likely to be simply a region of lower background infrared emission (a 'hole in the sky'). Hence, it appears that previous surveys, based on Spitzer and other mid-infrared data alone, may have over-estimated the total IRDC population by a factor of 2. This has implications for estimates of the star formation rate in IRDCs in the Galaxy.We studied the 1205 Herschel-bright IRDCs at 250 μm, and found that 972 of them had at least one clearly defined 250-μm peak, indicating that they contained one or more dense cores. Of these, 653 (67 per cent) contained an 8-μm point source somewhere within the cloud, 149 (15 per cent) contained a 24-μm point source but no 8-μm source, and 170 (18 per cent) contained no 24-μm or 8-μm point sources. We use these statistics to make inferences about the lifetimes of the various evolutionary stages of IRDCs.

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arXiv2012

The 6-GHz methanol multibeam maser catalogue IV: Galactic longitudes 186 to 330 including the Orion-Monoceros region

We present the fourth portion of a Galactic Plane survey of methanol masers at 6668 MHz, spanning the longitude range 186 degrees to 330 degrees. We report 207 maser detections, 89 new to the survey. This completes the southern sky part of the Methanol Multibeam survey and includes a large proportion of new sources, 43%. We also include results from blind observations of the Orion-Monoceros star forming region, formally outside the latitude range of the Methanol Multibeam survey; only the four previously known methanol emitting sites were detected, of which we present new positions and spectra for masers at Orion-A (south) and Orion-B, obtained with the MERLIN array.

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arXiv2011

The Masers Towards IRAS 20126+4104

We present MERLIN observations of OH, water and methanol masers towards the young high mass stellar object IRAS 20126+4104. Emission from the 1665-MHz OH, 22-GHz H_2O and 6.7-GHz CH_3OH masers is detected and all originates very close to the central source. The OH and methanol masers appear to trace part of the circumstellar disk around the central source. The positions and velocities of the OH and methanol masers are consistent with Keplerian rotation around a central mass of ~ 5 Msun. The water masers are offset from the OH and methanol masers and have significantly changed since they were last observed, but still appear to be associated to the outflow from the source. All the OH masers components are circularly polarised, in some cases reaching 100 percent while some OH components also have linear polarisation. We identify one Zeeman pair of OH masers and the splitting of this pair indicates a magnetic field of strength ~ 11 mG within ~ 0.5" (850 AU) of the central source. The OH and methanol maser emission suggest that the disk material is dense, n > 10^6 cm^-3, and warm, T > 125 K and the high abundance of methanol required by the maser emission is consistent with the evaporation of the mantles on dust grains in the disk as a result of heating or shocking of the disk material

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arXiv2011

Major structures of the inner Galaxy delineated by 6.7-GHz methanol masers

We explore the longitude-velocity distribution of 6.7-GHz methanol masers in the context of the inner structure of our Galaxy. We analyse the correlation in velocities within this distribution and identify density enhancements indicating large-scale regions of enhanced star formation. These are interpreted as the starting points of the spiral arms and the interaction of the Galactic bar with the 3-kpc arms. The methanol masers support the presence of a long thin bar with a 45 degree orientation. Signatures of the full 3-kpc arm structure are seen, including a prominent tangent at approximately -22 degrees Galactic longitude. We compare this distribution with existing models of the gas dynamics of our Galaxy. The 3-kpc arm structure appears likely to correspond to the radius of corotation resonance of the bar, with the bar on its inner surface and the starting points of the spiral arms on its outer surface.

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arXiv2011

The JCMT Spectral Legacy Survey: physical structure of the molecular envelope of the high-mass protostar AFGL2591

The understanding of the formation process of massive stars (>8 Msun) is limited, due to theoretical complications and observational challenges. We investigate the physical structure of the large-scale (~10^4-10^5 AU) molecular envelope of the high-mass protostar AFGL2591 using spectral imaging in the 330-373 GHz regime from the JCMT Spectral Legacy Survey. Out of ~160 spectral features, this paper uses the 35 that are spatially resolved. The observed spatial distributions of a selection of six species are compared with radiative transfer models based on a static spherically symmetric structure, a dynamic spherical structure, and a static flattened structure. The maps of CO and its isotopic variations exhibit elongated geometries on scales of ~100", and smaller scale substructure is found in maps of N2H+, o-H2CO, CS, SO2, CCH, and methanol lines. A velocity gradient is apparent in maps of all molecular lines presented here, except SO, SO2, and H2CO. We find two emission peaks in warm (Eup~200K) methanol separated by 12", indicative of a secondary heating source in the envelope. The spherical models are able to explain the distribution of emission for the optically thin H13CO+ and C34S, but not for the optically thick HCN, HCO+, and CS, nor for the optically thin C17O. The introduction of velocity structure mitigates the optical depth effects, but does not fully explain the observations, especially in the spectral dimension. A static flattened envelope viewed at a small inclination angle does slightly better. We conclude that a geometry of the envelope other than an isotropic static sphere is needed to circumvent line optical depth effects. We propose that this could be achieved in envelope models with an outflow cavity and/or inhomogeneous structure at scales smaller than ~10^4 AU. The picture of inhomogeneity is supported by observed substructure in at least six species.

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arXiv2011

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. 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 bright mid-IR emission, whereas dust continuum observations allow cold, dense pre-stellar-clusters to be identified anywhere. Aims: We aim to understand what physical properties characterize IRDCs, to explore the population of dust continuum sources that are not IRDCs, and to roughly characterize the star formation activity in dust continuum sources. 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. We report on the fraction of Hi-GAL sources which are mid-IR-dark, -neutral, or -bright in both fields. We find significant trends in column density and temperature between mid-IR-dark and -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. 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 micron 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) > 10^22 cm^-2) clouds identified with Hi-GAL which, despite bright surrounding mid-IR emission, show little to no absorption at 8 micron. (abridged)

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arXiv2011

The initial conditions of high-mass star formation: radiative transfer models of IRDCs seen in the Herschel Hi-GAL survey

The densest infrared dark clouds (IRDCs) may represent the earliest observable stage of high-mass star formation. These clouds are very cold, hence they emit mainly at far-infrared and sub-mm wavelengths. For the first time, Herschel has provided multi-wavelength, spatially resolved observations of cores within IRDCs, which, when combined with radiative transfer modelling, can constrain their properties, such as mass, density profile and dust temperature. We use a 3D, multi-wavelength Monte Carlo radiative transfer code to model in detail the emission from six cores in three typical IRDCs seen in the Hi-GAL survey (G030.50+00.95, G031.03+00.26 and G031.03+00.76), and thereby to determine the properties of these cores and compare them with their low-mass equivalents. We found masses ranging from 90 to 290 solar masses with temperatures from 8 to 11K at the centre of each core and 18 to 28K at the surface. The maximum luminosity of an embedded star within each core was calculated, and we rule out the possibility of significant high mass star formation having yet occurred in three of our cores.

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arXiv2011

Was a cloud-cloud collision the trigger of the recent star formation in Serpens?

The complexity of the ISM is such that it is unlikely that star formation is initiated in the same way in all molecular clouds. While some clouds seem to collapse on their own, others may be triggered by an external event such as a cloud/flow collision forming a gravitationally unstable enhanced density layer. This work tests cloud-cloud collisions as the triggering mechanism for star formation in the Serpens Main Cluster as has been suggested by previous work. A set of smoothed particle hydrodynamics (SPH) simulations of the collision between two cylindrical clouds are performed and compared to (sub)millimetre observations of the Serpens Main Cluster. A configuration has been found which reproduces many of the observed characteristics of Serpens, including some of the main features of the peculiar velocity field. The evolution of the velocity with position throughout the model is similar to that observed and the column density and masses within the modeled cloud agree with those measured for the SE sub-cluster. Furthermore, our results also show that an asymmetric collision provides the ingredients to reproduce lower density filaments perpendicular to the main structure, similar to those observed. In this scenario, the formation of the NW sub-cluster of Serpens can be reproduced only if there is a pre-existing marginally gravitationally unstable region at the time the collision occurs. This work supports the interpretation that a collision between two clouds may have been the trigger of the most recent burst of star formation in Serpens. It not only explains the complicated velocity structure seen in the region, but also the temperature differences between the north (in "isolated" collapse) and the south (resulting from the shock between the clouds). In addition it provides an explanation for the sources in the south having a larger spread in age than those in the north.

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