bluuebunny/crossref_metadata_2025_split huggingface dataset:
Magnetic fields in the non-masing ISM
Authors: Crutcher Richard M. 
Published: January 2008

AbstractObservations of the Zeeman effect in OH and H2O masers provide valuable information about magnetic field strength and direction, but only for the very high density gas in which such masers are found. In order to understand the role of magnetic fields in the evolution of the interstellar medium and in the star formation process, it is essential to consider the maser results in the broader context of magnetic fields in lower density gas. This contribution will (very briefly) summarize the state of observational knowledge of magnetic fields in the non-masing gas. Magnetic fields in H I and molecular clouds may be observed via the Zeeman effect, linear polarization of dust emission, and linear polarization of spectral-line emission. Useful parameters that can be inferred from observations are the mass-to-flux ratio and the scaling of field strength with density. The former tells us whether magnetic fields exert sufficient pressure to provide support against gravitational contraction; the latter tells whether or not magnetic fields are sufficiently strong to determine the nature (spherical or disk geometry) of the contraction. Existing observations will be reviewed. Results are that the strength of interstellar magnetic fields remains roughly invariant at 5-10 microgauss between densities of 0.1 cm−3 < n(H) < 1,000 cm−3 but increases proportional to approximately the square root of density at higher densities. Moreover, the mass-to-flux ratio is significantly subcritical (strong magnetic support with respect to gravity) in diffuse H I clouds that are not self-gravitating, but becomes approximately critical in high-density molecular cloud cores. This suggests that MCs and GMCs form primarily by accumulation of matter along magnetic field lines, a process that will increase density but not magnetic field strength. How clumps in GMCs evolve will then depend crucially on the mass-to-flux ratio in each clump. Present data suggest that magnetic fields play a very significant role in the evolution of molecular clouds and in the star formation process.
VLBI OH maser polarimetry with the Australian Long Baseline Array: the star-forming region G340.054–0.244
Authors: Bains I. , Caswell J. , Richards A. M. S. , Phillips C. , Tingay S. , Kramer B. Hutawarakorn , Cohen R. J. , Cunningham M. 
Published: January 2008

AbstractUntil recently, high spatial resolution full Stokes maser polarimetry was the sole domain of northern interferometers and a wealth of sources in the far south remained unexplored due to a lack of suitable instrumentation having both high spatial and high velocity resolution. The Australia Telescope Long Baseline Array (LBA) has now switched to disk-based software correlation, permitting full Stokes observing in spectral line mode with velocity channels which are sufficiently narrow to sample usefully the polarization structure. To illustrate the utility of this valuable addition to radio astronomy, we present preliminary results of the first such polarimetric observation, the subject of which are the OH masers in the star-forming region G340.054–0.244.
Possible CO clouds interacting with the SNRs G21.8-0.6 and G32.8-0.1
Authors: Zhou Jianjun , Esimbek Jarken , Zhang Xizheng , Zhang Hongbo 
Published: January 2008

AbstractWe have obtained the first CO(1-0) maps towards the SNRs G21.8-0.6 and G32.8-0.1, which are associated with OH 1720 MHz masers. Based on the morphological correspondence between the radio remnant and the CO cloud, and the velocity and position agreement between the OH maser and CO clouds, we tentatively suggest the clouds may be interacting with G21.8-0.6 and G32.8-0.1. However, the shock-excited line broadening was not obvious for these two SNRs. We discuss the possible reasons for this.
Maser science at Tidbinbilla
Authors: Horiuchi Shinji , Lovell Jim 
Published: January 2008

AbstractThe 70 m antenna at Canberra Deep Space Communication Complex (Tidbinbilla) is the largest and most sensitive dish in the southern hemisphere, working at 1, 3, 13 and 18 cm, and as such is in high demand from astronomers both in Australia and overseas. In this paper we present the current status of the single dish spectroscopy system and highlight some recent results in maser science.
How do methanol masers manage to appear in the youngest star vicinities and isolated molecular clumps?
Authors: Sobolev A. M. , Cragg D. M. , Ellingsen S. P. , Gaylard M. J. , Goedhart S. , Henkel C. , Kirsanova M. S. , Ostrovskii A. B. , Pankratova N. V. , Shelemei O. V. , van der Walt D. J. , Vasyunina T. S. , Voronkov M. A. 
Published: January 2008

AbstractGeneral 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.
Methanol masers as tools to study high-mass star formation
Authors: Pestalozzi Michele R. 
Published: January 2008

AbstractIn this contribution I will attempt to show that the study of Galactic 6.7 and 12.2 GHz methanol masers themselves, as opposed to the use of methanol masers as signposts, can yield important conclusions contributing to the understanding of high-mass star formation. Due to their exclusive association with star formation, methanol masers are the best tools to do this, and their large number allows us to probe the entire Galaxy. In particular I will focus on the determination of the luminosity function of methanol masers and on the determination of an unambiguous signature for a circumstellar masing disc seen edge-on. Finally I will try to point out some future fields of research in the study of methanol masers.
A documentary of high-mass star formation: Probing the dynamical evolution of Orion Source I on 10–100 AU scales using SiO masers
Authors: Matthews L. D. , Goddi C. , Greenhill L. J. , Chandler C. J. , Reid M. J. , Humphreys E. M. L. 
Published: January 2008

AbstractA comprehensive picture of high-mass star formation has remained elusive, in part because examples of high-mass young stellar objects (YSOs) tend to be relatively distant, deeply embedded, and confused with other emission sources. These factors have impeded dynamical investigations within tens of AU of high-mass YSOs—scales that are critical for probing the interfaces where outflows from accretion disks are launched and collimated. Using observations of SiO masers obtained with the Very Large Array (VLA) and the Very Long Baseline Array (VLBA), the KaLYPSO project is overcoming these limitations by mapping the structure and dynamical/temporal evolution of the material 10-1000 AU from the nearest high-mass YSO: Radio Source I in the Orion BN/KL region. Our data include ~40 epochs of VLBA observations over a several-year period, allowing us to track the proper motions of individual SiO maser spots and to monitor changes in the physical conditions of the emitting material with time. Ultimately these data will provide 3-D maps of the outflow structure over approximately 30% of the outflow crossing time. Here we summarize recent results from the KaLYPSO project, including evidence that high-mass star formation occurs via disk-mediated accretion.
Massive star-formation in G24.78+0.08 studied by means of maser VLBI and thermal interferometric observations
Authors: Moscadelli L. , Goddi C. , Cesaroni R. , Beltràn M. T. 
Published: January 2008

AbstractThis work presents the results of VLBI observations of 6.7 GHz methanol and 22.2 GHz water masers towards the mm core A in the massive star-forming region G24.78+0.08. Comparing the maser with previous millimeter interferometric and recent continuum VLA observations, the physical properties and the gas kinematics of the G24 A core on linear scales from ~100 AU to ~0.1 pc are determined.
A variability study of the H2CO 6cm maser in IRAS 18566+0408
Authors: Araya E. , Hofner P. , Kurtz S. , Linz H. , Sewilo M. , Olmi L. , Watson C. 
Published: January 2008

AbstractWe report the first detection of a short-duration (<3 months) outburst of an H2CO 6cm maser based on multi-epoch observations of IRAS18566+0408 obtained with Arecibo, the Green Bank Telescope, and the Very Large Array. The H2CO maser was observed nine times between 2002 and 2005. In May 2006 we began a two-year program of monthly monitoring with the Arecibo Telescope. The H2CO maser in IRAS18566+0408 is coincident with a young massive stellar object and the line profile of the maser suggests that the H2CO emission originates from two different regions, perhaps associated with the kinematics of a circumstellar disk.
Polarization of 6.0-GHz OH masers in W3(OH)
Authors: Harvey-Smith L. , Soria-Ruiz R. , Richards A. M. S. , Cohen R. J. 
Published: January 2008

AbstractWe present preliminary results of a MERLIN polarization study of 6.0-GHz OH masers in the W3(OH) region of massive star-formation. We have detected a small but significant amount of linear polarization in the 6031-MHz masers, as well as a much larger fraction of circular polarization. We have found many Zeeman pairs, the analysis of which (along with the results at 6035-MHz) will be presented elsewhere. MERLIN is well-suited to detecting all the single-dish maser flux in these regions, owing to its range of short baselines (tens of kilometers) and resulting sensitivity to extended emission. We have found core-halo structure in the 6031-MHz OH maser emission in the region of the highly extended 4.7-GHz OH and 6.7-GHz methanol maser filament found by Harvey-Smith & Cohen (2005, 2006).
LBA observations of OH masers in the star-forming region OH 330.953–0.182
Authors: Kramer B. Hutawarakorn , Caswell J. L. , Sukom A. , Reynolds J. E. 
Published: January 2008

AbstractOH masers are sensitive probes of the kinematics, physical conditions, and magnetic fields in star-forming regions. The maser site OH 330.953-0.182 has been studied using the Long Baseline Array of the Australia Telescope National Facility. Simultaneous observations of the 1665- and 1667-MHz hydroxyl ground-state transitions yield a series of maps at velocity spacing 0.09kms−1, in both right- and left-hand circular polarization, with tenth-arcsec spatial resolution. Several clusters of maser spots have been detected within a five-arcsec region. Eight Zeeman pairs were found, and in one case, at 1665 MHz, there is a nearby 1667-MHz pair indicating a similar value of magnetic field and velocity. Over the whole site, all magnetic field estimates are toward us (negative), and range from -3.7 to -5.8 mG. We also compared the morphology and kinematics of the 1665- and 1667-MHz maser spots with those from the excited state of OH at 6035 MHz and from methanol at 6668 MHz.
Low-mass star formation in bright rimmed clouds
Authors: Migenes V. , Trinidad M. A. , Valdettaro R. , Palla F. , Brand J. 
Published: January 2008

AbstractBright Rimmed Clouds (BRCs) are clouds that have been compressed by an external ionization-shock front which focuses the neutral gas into compact globules. The boundary layer between the neutral gas and the gas ionized by the incident photons is often called “bright rim” but the clumps are sometimes classified also as speck globules or cometary globules depending on their appearance. Small globules with bright rims have been considered to be potential sites of star formation and have been studied in several individual regions. We present the first high resolution VLA observations of 20 of these BRCs, but only three detections were obtained. The low detection rate seems to support the idea that BRCs produce mostly low-luminosity objects, for which maser emission is weak and episodic, and that the embedded sources are in a more advanced evolutionary phase than class 0 objects.
An OH maser flare with a strong magnetic field in W75N
Authors: Slysh V. I. , Migenes V. 
Published: January 2008

AbstractA flare of OH maser emission was discovered in W75N in 2000. Its location was determined with the VLBA to be within 110 AU from one of the ultra-compact HII regions, VLA2. The flare consisted of several maser spots. Four of the spots were found to form Zeeman pairs, all of them with a magnetic field strength of about 40 mG. This is the highest ever magnetic field strength found in OH masers, an order of magnitude higher than in typical OH masers. We discuss the possible source for the enhanced magnetic field and its relation to the flare event.
Groundstate OH masers associated with Herbig-Haro objects
Authors: de Witt A. , Smits D. P. 
Published: January 2008

AbstractJets from newly-formed stars plow into the surrounding ISM, creating nebulous regions known as Herbig-Haro (HH) objects. Signatures of C-type shocks have been found in many HH objects. Models of shock-excited 1720-MHz OH masers associated with SNRs suggest that the masers could be signposts of C-type shocks. We have conducted a 1720MHz survey towards a number of HH objects to see if any shock-excited masers occur in association with these objects. Some results of our search are reported. When present, the 1720-MHz OH line can occur as emission, absorption or as P Cygni profiles. OH mainline emission is found in many of our sources. We have identified several potential 1720-, 1665- and 1667-MHz OH masers.
The Australia Telescope campaign to study southern class I methanol masers
Authors: Voronkov M. A. , Brooks K. J. , Sobolev A. M. , Ellingsen S. P. , Ostrovskii A. B. , Caswell J. L. 
Published: January 2008

AbstractThe Australia Telescope Compact Array (ATCA) and the Mopra facility have been used to search for new southern class I methanol masers at 9.9, 25(J=5) and 104 GHz, which are thought to trace more energetic conditions in the interface regions of molecular outflows, than the widespread class I masers at 44 and 95 GHz. One source shows a clear outflow association.
A MERLIN study of 6 GHz excited OH & 6.7 GHz methanol masers in ON1
Authors: Green James A. , Richards A. M. S. , Flood H. , Vlemmings W. H. T. , Cohen R. J. 
Published: January 2008

AbstractMERLIN observations of 6.668-GHz Methanol and 6.035-GHz OH emission from the known massive star-formation region ON1 are presented. Maser components are found to lie at the southern edge of the UCHII with consistent polarization angles across the strongest features. Zeeman splitting of OH shows magnetic field strengths between +0.4 to −5.3 mG and from cross-correlation a tentative methanol magnetic field of −18mG is detected.
G23.657−0.127, what can we learn from a perfect methanol maser source?
Authors: Bartkiewicz Anna , van Langevelde Huib Jan , Szymczak Marian , Brunthaler Andreas 
Published: January 2008

AbstractIn the course of following up compact methanol masers at 6.7 GHz which were found in the Torun blind survey (Szymczak et al. 2002), we discovered a ring structure in the source G23.657−00.127. This source provides interesting insights into whether methanol masers arise in rotating disks around massive stars, in their outflows, or behind shocks. By monitoring the 12.2 GHz masers, which fortunately follow the same structure, we hope to resolve the kinematics of the ring. Moreover, the symmetry of the source points to the existence of a central source. Early results on the nature of this source indicate the existence of a hyper–compact H II region.
Towards constraining the environments of methanol masers
Authors: Szymczak M. , Bartkiewicz A. , Richards A. M. S. 
Published: January 2008

AbstractObservations of 10 transitions of HCO+, CO and CS isotopomers were taken with the IRAM 30 m telescope for a sample of 28 methanol maser sources detected in the Torun unbiased survey of the galactic plane. About 64% of the sources show line wings in one or more transitions, indicating the presence of molecular outflows. Two sources show evidence for infall motion. Comparison of the widths of line wings and methanol maser emission suggests that the 6.7 GHz maser line traces the environment of massive young stellar objects of various kinematic regimes. Calculations based on the CO and HCO+ lines and the CS line intensity ratios refine the physical parameters. Specifically, a gas density of <107 cm−3 is sufficient for strong maser emission and a high methanol fractional abundance (>5×10−7) is required.
Recent southern maser surveys
Authors: Caswell J. L. 
Published: January 2008

AbstractThis discussion of recent maser surveys in the southern sky will allow us to see the context of the new methanol multibeam survey, which is currently in progress and described elsewhere at this symposium. My emphasis will be on masers that arise in star-forming regions in the disc of our Galaxy. I discuss outcomes from existing surveys, and comparison of different maser species, with special attention to the recognition of blue-shifted outflows.
Investigating high-mass star formation through maser surveys
Authors: Ellingsen S. P. , Voronkov M. A. , Cragg D. M. , Sobolev A. M. , Breen S. L. , Godfrey P. D. 
Published: January 2008

AbstractInterstellar masers are unique probes of the environments in which they arise. In studies of high-mass star formation their primary function has been as signposts of these regions and they have been used as probes of the kinematics and physical conditions in only a few sources. With a few notable exceptions, we know relatively little about the evolutionary phase the different maser species trace, nor their location with respect to other star formation tracers. While detailed studies of a small number of maser regions can reveal much about them, other information can only be obtained through large, systematic searches. In particular, such surveys are vital in efforts to determine an evolutionary sequence for the common maser species, and there is growing evidence that methanol masers may trace an earlier phase than the other common maser species of OH and water.
A 20-year H2O maser monitoring program with the Medicina 32-m telescope
Authors: Brand J. , Felli M. , Cesaroni R. , Codella C. , Comoretto G. , Di Franco S. , Massi F. , Moscadelli L. , Nesti R. , Olmi L. , Palagi F. , Palla F. , Panella D. , Valdettaro R. 
Published: January 2008

AbstractThe Arcetri/Bologna H2O maser group has been monitoring the 1.3-cm water maser emission from a sample of 43 star-forming regions (SFRs) and 22 late-type stars for about 20 years at a sampling rate of 4-5 observations each year, using the 32-m Medicina Radio Telescope (HPBW 1.′9 at 22 GHz). For the late-type stars we observe representative samples of OH/IR-stars, Mira's, semi-regular variables, and supergiants. The SFR-sample spans a large interval in FIR luminosity of the associated Young Stellar Object (YSO), from 20 L⊙ to 1.5 × 106 L⊙, and offers a unique data base for the study of the long-term (years) variability of the maser emission in regions of star formation.This presentation concerns only the masers in SFRs. The information obtained from single-dish monitoring is complementary to what is extracted from higher-resolution (VLA and VLBI) observations, and can better explore the velocity domain and the long-term variability therein.We characterize the variability of the sources in various ways and we study how it depends on the luminosity and other properties of the associated YSO and its environment.
The large-scale distribution and physical properties of massive star forming regions in the Milky Way
Authors: Esimbek Jarken , Jianjun Zhou , Xingwu Zheng 
Published: January 2008

AbstractWe are conducting a large survey with the Urumqi 25 m radio telescope to study the distribution and physical properties of massive star forming regions in the Milky Way. We will accomplish this by observing the H110α hydrogen recombination line at 4.874 GHz and the H2CO absorption line at 4830 GHz. These lines are associated with compact HII regions and dense molecular clouds. As a test of the 6 cm spectral line receiving system, we observed the two spectral lines toward massive star forming regions. We plan to study the large scale distribution and physical properties of massive star forming regions in the Milky way.
Wide field imaging of Giant Molecular Clouds with the ATA
Authors: Forster James R. 
Published: January 2008

AbstractGalactic Giant Molecular Cloud (GMC) complexes are among the largest objects in the sky. In order to get the full picture of Galactic star formation a large field of view is important. The Allen Telescope Array (ATA) is the first of a new generation of large-N, small-D radio telescopes designed to provide high quality images on large angular scales. We present here the first wide field images of GMCs made with the ATA.
A search for OH 6 GHz maser emission towards southern supernova remnants
Authors: McDonnell Korinne E. , Vaughan Alan E. , Wardle Mark 
Published: January 2008

AbstractOH masers at 1720 MHz have proven to be excellent indicators of interactions between supernova remnants and molecular clouds. Recent calculations suggest that the 6049 MHz OH maser line is excited at higher column densities than for the 1720 MHz line. It is therefore a potentially valuable indicator of remnant-cloud interaction.We present preliminary results of a survey using the Parkes Methanol Multibeam receiver for 6049 MHz and 6035/6030 MHz OH masers towards 36 supernova remnants and four fields in the Large and Small Magellanic Clouds. While no 6049 MHz masers have been found, three new sites of 6035 and 6030 MHz OH maser emission have been discovered in star-forming regions.
Millimetre wavelength methanol masers survey towards massive star forming regions
Authors: Umemoto T. , Mochizuki N. , Shibata K. M. , Roh D.-G. , Chung H.-S. 
Published: January 2008

AbstractWe present the results of a mm wavelength methanol maser survey towards massive star forming regions. We have carried out Class II methanol maser observations at 86.6 GHz, 86.9 GHz and 107.0 GHz, simultaneously, using the Nobeyama 45 m telescope. We selected 108 6.7 GHz methanol maser sources with declinations above −25 degrees and fluxes above 20 Jy. The detection limit of maser observations was ~3 Jy. Of the 93 sources surveyed so far, we detected methanol emission in 25 sources (27%) and “maser” emission in nine sources (10%), of which thre “maser” sources are new detections. The detection rate for maser emission is about half that of a survey of the southern sky (Caswell et al. 2000). There is a correlation between the maser flux of 107 GHz and 6.7 GHz/12 GHz emission, but no correlation with the “thermal” (non maser) emission. From results of other molecular line observations, we found that the sources with methanol emission show higher gas temperatures and twice the detection rate of SiO emission. This may suggest that dust evaporation and destruction by shock are responsible for the high abundance of methanol molecules, one of the required physical conditions for maser emission.