bluuebunny/crossref_metadata_2025 huggingface dataset:
Low-mass star formation in bright rimmed clouds
Authors: Migenes V. , Trinidad M. A. , Valdettaro R. , Palla F. , Brand J.
Published: January 2008

Bright 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

A 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

Jets 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

The 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

MERLIN 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

In 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

Observations 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

This 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

Interstellar 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

The 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

We 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

Galactic 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

OH 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

We 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.
Stellar masers, circumstellar envelopes and supernova remnants
Authors: Kemball Athol J.
Published: January 2008

This paper reviews recent advances in the study or circumstellar masers and masers found toward supernova remnants. The review is organized by science focus area, including the astrophysics of extended stellar atmospheres, stellar mass-loss processes and outflows, late-type evolved stellar evolution, stellar maser excitation and chemistry, and the use of stellar masers as independent distance estimators. Masers toward supernova remnants are covered separately. Recent advances and open future questions in this field are explored.
The Mira star S Ori: SiO maser shells related to the stellar photosphere, the molecular layers, and the dust shell at three epochs
Authors: Wittkowski M. , Boboltz D. A. , Ohnaka K. , Driebe T. , Scholz M.
Published: January 2008

We present recent results from our project of concurrent radio and infrared interferometry of oxygen-rich Mira stars. Our pilot study on the Mira variable S Ori included coordinated VLBA mapping of the v=1, J=1-0 43.1 GHz and v=2, J=1-0 42.8 GHz SiO maser radiation at one epoch and concurrent near-infrared K-band interferometry to constrain the stellar photospheric diameter. We recently obtained new maps of these SiO maser transitions toward S Ori at three additional epochs with the VLBA and concurrent mid-infrared interferometric data with VLTI/MIDI. The MIDI data are analyzed using recent self-excited dynamic model atmospheres including molecular shells close to continuum-forming layers, which we complement by a radiative transfer model of the dust shell. The modelling of our MIDI data results in phase-dependent continuum photospheric angular diameters. The dust shell can best be modelled with Al2O3 grains alone. We find that the mean SiO maser ring radii lie between about 1.9 and 2.4 stellar continuum radii. The maser spots mark the region of the molecular atmospheric layers shortly outward of the steepest decrease of the mid-infrared model intensity profile. Our results suggest that the SiO maser shells are co-located with the Al2O3 dust shell near minimum visual phase. Their velocity structure indicates a radial gas expansion. Preliminary results from a study of the Mira star GX Mon suggest a similar picture.
Turbulent, steamy red supergiant winds
Authors: Richards A. M. S. , Bains I. , Bartkiewicz A. , Cohen R. J. , Diamond P. J. , Etoka S. , Gray M. D. , Lekht E. E. , Masheder M. R. W. , Mendoza-Torres E. , Murakawa K. , Szymczak M. , van Langevelde H. J. , Vlemmings W. , Yates J. A.
Published: January 2008

Rapidly-evolving red supergiants (RSG) lose half or more of their mass before ending their lives as supernovae. Masers allow us to study the mass loss from 4 nearby RSG in AU-scale detail using MERLIN and EVN/global VLBI. The water maser clouds are over-dense and over-magnetised with respect to the surrounding wind. In most cases, the brighter an individual maser component is the smaller its apparent (beamed) FWHM appears, as predicted for approximately spherical clouds. Individual water maser features have a typical half-life of 5-10 yr, but comparison with single dish monitoring suggests that the water vapour clouds themselves survive many decades (the water maser shell crossing time), within which the local masers wink on and off. OH mainline masers are found in the tenuous surrounding gas, overlapping the water maser shell, surrounded by OH 1612-MHz masers at a greater distance from the star.
SiO maser observations of a wide dust-temperature range sample
Authors: Nakashima Jun-ichi , Deguchi Shuji
Published: January 2008

We present the results of SiO line observations of a sample of known SiO maser sources covering a wide dust-temperature range. The aim of the present research is to investigate the causes of the correlation between infrared colors and SiO maser intensity ratios among different transition lines. We observed in total 75 SiO maser sources with the Nobeyama 45m telescope quasi-simultaneously in the SiO J = 1-0 v = 0, 1, 2, 3, 4 and J = 2-1 v = 1, 2 lines. We also observed the sample in the 29SiO J = 1-0 v = 0 and J = 2-1 v = 0, and 30SiO J = 1-0 v = 0 lines, and the H2O 61,6-52,3 line. As reported in previous papers, we confirmed that the intensity ratios of the SiO J = 1-0 v = 2 to v = 1 lines clearly correlate with infrared colors. In addition, we found possible correlation between infrared colors and the intensity ratios of the SiO J = 1-0 v = 3 to v = 1&2 lines.
A study of the close environments of evolved stars from SiO masers
Authors: Bujarrabal Valentín , Alcolea J. , Colomer F. , Desmurs J.-F. , Contreras C. Sánchez , Soria-Ruiz R.
Published: January 2008

SiO maser emission allows us to study the innermost circumstellar layers around AGB and post-AGB stars, at a few AU from the stellar photosphere, what is crucial to understand the mass loss process in AGB stars and the jet launching mechanisms in post-AGB stars. Observations of SiO masers are also useful to address the question of the pumping mechanism itself, still under debate. In particular, VLBI observations of such emissions enables the study of the innermost shells with extremely high spatial resolution, equivalent (in nearby stars) to a few 1012 cm, about one tenth of the stellar radius. In this contribution, recent results on this topic obtained by our group are summarized.
Stellar molecular jets traced by maser emission
Authors: Imai Hiroshi
Published: January 2008

Highly collimated jets found in AGB and post-AGB stars are expected to play an important role for shaping planetary nebulae. Recent VLBI observations of H2o maser sources have revealed that some of the spatio-kinematical structures of H2o maser sources exhibit stellar jets with extremely spatially and kinematically high collimation. Such stellar H2o maser jets, so-called “water fountains”, have been identified in about 10 sources to date. Here we review recent VLBI observations and relevant observational results of the water fountain sources. They have revealed a typical dynamical age and the detailed kinematical structures of the water fountains, the possibility of the existence of “equatorial flows”, and the evolutionary status of the host stars. The location and kinematics of one of the water fountain sources in the Galaxy is also revealed and shown here.
Maser emission in planetary nebulae
Authors: Gómez Yolanda
Published: January 2008

Stars at the top of the asymptotic giant branch (AGB) can exhibit maser emission from molecules like SiO, H2O and OH. These masers appear in general stratified in the envelope, with the SiO masers close to the central star and the OH masers farther out in the envelope. As the star evolves to the planetary nebula (PN) phase, mass-loss stops and ionization of the envelope begins, making the masers disappear progressively. The OH masers in PNe can be present in the envelope for periods of ~1000 years but the H2O masers can survive only hundreds of years. Then, H2O maser emission is not expected in PNe and its detection suggests that these objects are in a very particular moment of its evolution in the transition from AGB to PNe. We discuss the unambiguous detection of H2O maser emission in two planetary nebulae: K 3-35 and IRAS 17347-3139. The water-vapor masers in these PNe are tracing disk-like structures around the core and in the case of K3-35 the masers were also found at the tip of its bipolar lobes. Kinematic modeling of the H2O masers in both PNe suggest the existence of a rotating and expanding disk. Both PNe exhibit a bipolar morphology and in the particular case of K 3-35 the OH masers are highly polarized close to the core in a disk-like structure. All these observational results are consistent with the models where rotation and magnetic fields have been proposed to explain the asymmetries observed in planetary nebulae.
OH (1720 MHz) masers: signposts of SNR/molecular cloud interactions
Authors: Brogan Crystal L.
Published: January 2008

Over the last decade it has been demonstrated that supernova remnant (SNR) OH (1720 MHz) masers are unique tracers of SNR/molecular cloud interactions. Here I briefly review the current state of our observational understanding of these masers including results from recent MERLIN and VLBA full polarization studies of the masers in W28, W44, and W51C. Some of our findings include that (1) in accordance with theory, the linear polarization position angles are either parallel or perpendicular to the plane-of-sky magnetic field determined through other observations; (2) the maser spot sizes are fairly large (~ 1014 cm) and exhibit a core/halo morphology; and (3) while the magnetic field strengths do increase slightly with higher resolution, this effect can be completely explained by spectral/spatial blending.
Extended OH(1720 MHz) maser emission from supernova remnants
Authors: Hewitt J. W. , Yusef-Zadeh F. , Wardle M. , Roberts D. A.
Published: January 2008

Compact OH(1720 MHz) masers have proven to be excellent signposts for the interaction of supernova remnants with adjacent molecular clouds. Less appreciated has been the weak, extended OH(1720 MHz) emission which accompanies strong compact maser sources. Recent single-dish and interferometric observations reveal the majority of maser-emitting supernova remnants(SNRs) have accompanying regions of extended maser emission. Enhanced OH abundance created by the passing shock is observed both as maser emission and absorption against the strong background of the remnant. Modeling the observed OH profiles gives an estimate of the physical conditions in which weak, extended maser emission arises. I will discuss how we can realize the utility of this extended maser emission, particularly the potential to measure the strength of the post-shock magnetic field via Zeeman splitting over these large-scales.
VLBI studies of SiO masers around VX Sagittarii
Authors: Chen X. , Shen Z.-Q. , Jiang D.-R.
Published: January 2008

Observations of 43 GHz 28SiO (v = 1, J = 1-0) masers in the circumstellar envelope of the M-type semi-regular variable star VX Sgr were performed using the VLBA at 3 epochs during 1999 April-May. By tracing 42 matched maser features appearing in all the three epochs, we determined the contraction of an SiO maser shell toward VX Sgr at a velocity of about 4 km s−1 at a distance of 1.7 kpc to VX Sgr. We also report on some preliminary results from our first epoch of simultaneous VLBA observations of three 7 mm SiO masers toward VX Sgr.
Observations of SiO J=2–1 and J=3–2 masers towards evolved stars with the TRAO 14 m telescope
Authors: Cho Se-Hyung
Published: January 2008

We present the results of survey and time-monitoring observations of SiO J=2–1 and J=3–2 masers towards evolved stars with the 14 m radio telescope at Taeduk Radio Astronomy Observatory (TRAO) from 1995 February to 2001 February. The first detection of SiO v=3, J=2–1 maser emission toward S-type Mira variable χ Cyg is also presented.
Full polarization VLBA maps of a proto-planetary nebula
Authors: McAlpine Kim , Kemball A.J. , Jonas J. L.
Published: January 2008

The morphological evolution of stars from the asymptotic giant branch (AGB) to the planetary nebula (PN) phases presents an intriguing problem in stellar research. Planetary nebulae show bright-rims, well-defined shell-like structures and a large proportion of them are axisymmetric. In contrast their progenitor AGB stars are largely spherically symmetric (Sahai, 2004). Studies of proto-planetary nebulae, which are objects in transition between these two phases of stellar evolution, offer insight into the mechanisms which are responsible for the onset of axisymmetry. This poster presents the first polarization VLBI observations of the 1612 MHz OH maser emission from the proto-planetary nebula candidate OH O.9+1.3. The morphology, kinematics and polarization properties of these masers are discussed.
SiO maser emission in oxygen AGB stars
Authors: Soria-Ruiz R. , Colomer F. , Alcolea J. , Bujarrabal V. , Desmurs J.-F.
Published: January 2008

We present high resolution observations of SiO masers in the late-type star R Cas as part of a multi-line study of SiO circumstellar masers in oxygen-rich AGB stars. We used the NRAO Very Long Baseline Array (VLBA) to map the 43 GHz maser emission of 28SiO. We study the relative spatial distribution of the different maser lines and discuss the relevance of line overlaps in the SiO pumping theory.
Collisional excitation of OH(6049 MHz) masers in supernova remnant – molecular cloud interactions
Authors: Wardle Mark
Published: January 2008

OH (1720 MHz) masers serve as indicators of SNR – molecular cloud interaction sites. These masers are collisionally excited in warm (50–100 K) shocked gas with densities of order 105cm−3 when the OH column density is in the range 1016−1017cm−2. Here I present excitation calculations which show that when the OH column density exceeds 1017cm−2 at similar densities and temperatures, the inversion of the 1720 MHz line switches off and instead the 6049 MHz transition in the first excited rotational state of OH becomes inverted. This line may serve as a complementary signal of warm, shocked gas when the OH column density is large.
Outburst in OH17.7-2.0
Authors: Szymczak M. , Gérard E. , Wolak P.
Published: January 2008

We present OH 1612 MHz maser observations towards the pre-planetary nebula OH17.7–2.0 taken with the Nançay radio telescope in 2002–2007. A bursting feature at 73 kms−1 grew by a factor of 11 in 740 days in left-hand circular polarization intensity, reaching a maximum in mid-2005. The OH flux density of features at neighbouring velocities exhibited a rise by a factor of 5–7. During the burst, the degree of circular polarization increased from 7 to 83%. The blue-shifted emission showed weak variations of 7–10% in amplitude with the exception of the bluest feature for which the OH flux decreased with the rate of ~4.4 Jy year−1. Very unusual OH outbursts of individual features and remarkable changes in the shape of the maser profile appear to be linked with recent evolutionary changes in the detached shell.
Masers and Galactic structure: Micro-arcsecond astrometry with the VLBA
Authors: Reid M. J. , Brunthaler A. , Menten K. M. , Ye Xu , Xing-Wu Zheng , Moscadelli L.
Published: January 2008

Astrometric observations with the VLBA with accuracies approaching ~ 10 μas are being conducted in order to better understand the Galaxy. The location of Sgr A* on infrared images can be determined with an accuracy of a few mas, using stars with SiO maser emission as a calibration grid for infrared images. The apparent proper motion of Sgr A*, which is dominated by the effects of the orbit of the Sun around the Galactic center, has been measured with high accuracy. This measurement strongly constrains Θ0R0 and offers a dynamical definition of the Galactic plane with Sgr A* at its origin. The intrinsic motion of Sgr A* is very small and comparable to that expected for a supermassive black hole. When combined with infrared results, this provides overwhelming evidence that Sgr A* is a supermassive black hole. Finally, we are engaged in a large project to map the spiral structure and kinematics of the Galaxy. Preliminary trigonometric parallaxes, obtained with the VLBA, to eight massive star forming regions are presented.
Do maser stars trace a rotating Galactic bar?
Authors: Habing H. J.
Published: January 2008

I have analyzed the radial velocities in two large samples of maser stars that cover the Galactic plane from l = +40deg to l = −40deg through the Galactic Centre. I compare a diagram of longitude versus radial velocity as observed with a diagram obtained from the calculation of orbits in a simple, but realistic potential containing a weak, rotating bar. Almost all stars belong to the Galactic disk but there is a small sample with high velocities close to the Galactic Centre and in the so-called forbidden quadrants. These velocities cannot be explained by a combination of pure Galactic rotation and a modest dispersion in velocity. An acceptable explanation is one in which the stars of this small sample move up and down a rotating Galactic bar.
Galactic rotation measurements based on H2O maser astrometry with VERA
Authors: Honma Mareki , Bushimata Takeshi , Choi Yoon Kyung , Hirota Tomoya , Imai Hiroshi , Iwadate Kenzaburo , Jike Takaaki , Kameya Osamu , Kamohara Ryuichi , Kan-ya Yukitoshi , Kawaguchi Noriyuki , Kijima Masachika , Kobayashi Hideyuki , Kuji Seisuke , Kurayama Tomoharu , Manabe Seiji , Miyaji Takeshi , Nagayama Takumi , Nakagawa Akiharu , Oh Chung Sik , Omodaka Toshihiro , Oyama Tomoaki , Sakai Satoshi , Sato Katsuhisa , Sasao Tetsuo , Shibata Katsunori M. , Shintani Motonobu , Suda Hiroshi , Tamura Yoshiaki , Tsushima Miyuki , Yamashita Kazuyoshi
Published: January 2008

We present results of astrometric observations of S269 H2O maser performed with VERA (VLBI Exploration of Radio Astrometry). We have monitored the positions of S269 H2O masers for 1 year and successfully detected its parallax to be 189±8 micro-arcsecond. This corresponds to a source distance of 5.28+0.24−0.22kpc, and is the smallest parallax (and thus the largest distance) that has ever been measured by means of annual parallax. Proper motions of S269 H2O maser were also measured and used to determine the Galactic rotation velocity at the position of S269. Our measurements show that the Galactic rotation velocity at S269 is the same to that at the Sun within 3%, indicating that the Galactic rotation curve is flat out to R~13 kpc.
TCS-CAIN: NIR survey of the Galactic plane
Authors: González-Fernández C. , Cabrera-Lavers A. , Garzón F. , Hammersley P. L. , López-Corredoira M. , Vicente B.
Published: January 2008

We present TCS-CAIN, a NIR survey of the Galactic plane, recently made public at the Instituto de Astrofísica de Canarias, and some results derived from it: with star counts derived directly the structure of the inner the Milky Way can be dissected, obtaining also estimations for the extinction toward the inner MW.
The structure of the accretion disk in NGC 4258
Authors: Moran James M. , Humphreys Elizabeth , Greenhill Lincoln , Reid Mark , Argon Alice
Published: January 2008

A wealth of new information about the structure of the maser disk in NGC 4258 has been obtained from a series of 18 VLBA observations spanning three years, as well as from 32 additional epochs of spectral monitoring data from 1994 to the present, acquired with the VLA, Effelsberg, and GBT. The warp of the disk has been defined precisely. The thickness of the maser disk has been measured to be 12 micro-arcseconds (FWHM), which is slightly smaller than previously quoted upper limits. Under the assumption that the masers trace the true vertical distribution of material in the disk, from the condition of hydrostatic equilibrium the sound speed is 1.5 km s−1, corresponding to a thermal temperature of 600K. The accelerations of the high velocity maser components have been accurately measured for many features on both the blue and red side of the spectrum. The azimuthal offsets of these masers from the midline (the line through the disk in the plane of the sky) and derived projected offsets from the midline based on the warp model correspond well with the measured offsets. This result suggests that the masers are well described as discrete clumps of masing gas, which accurately trace the Keplerian motion of the disk. However, we have continued to search for evidence of apparent motions caused by “phase effects.” This work provides the foundation for refining the estimate of the distance to NGC 4258 through measurements of feature acceleration and proper motion. The refined estimate of this distance is expected to be announced in the near future.
Precision cosmology with H2O megamasers: progress in measuring distances to galaxies in the Hubble flow
Authors: Braatz J. , Greenhill L. , Reid M. , Condon J. , Henkel C. , Lo K.-Y.
Published: January 2008

A determination of the Hubble Constant (H0) to better than 3% would be the best complement to cosmic microwave background (CMB) data to constrain the equation of state of Dark Energy. Water vapor megamasers provide perhaps the best opportunity for measuring direct distances to galaxies out to about 200 Mpc. We have formed a team of astronomers in the Megamaser Cosmology Project to pursue the ambitious goal of making a precise measurement of H0 by measuring such distances using the techniques pioneered on the disk maser in NGC 4258 by Herrnstein et al (1999). In recent surveys we have made significant progress identifying new maser systems analogous to that in NGC 4258, but more distant. Once the appropriate candidates are identified, two types of observations are necessary to ultimately measure a distance: single-dish monitoring to measure the acceleration of gas in the disk, and sensitive VLBI imaging in order to measure the angular size of the disk, measure the rotation curve, and model radial displacement of the systemic maser features. We have recently obtained preliminary VLBI maps of the masers in two systems, NGC 6323 and UGC 3789. The maser disks in both galaxies were discovered and monitored with the Green Bank Telescope (GBT) and subsequently imaged with the High Sensitivity Array (VLBA + GBT + Effelsberg). In this contribution we present a map of the maser distribution in one of those systems, NGC 6323. The map demonstrates that pc-scale maser disks as distant as ~ 100 Mpc can be imaged with existing telescopes. Results on UGC 3789 will be presented in a later publication.
Investigating the nature of low-luminosity extragalactic H2O masers
Authors: Castangia Paola , Tarchi A. , Henkel C. , Menten K. M.
Published: January 2008

We have searched for 22 GHz water maser emission in a sample of FIR bright galaxies and detected two new kilomaser sources. The newly detected masers have been promptly followed-up using interferometric observations to derive positions and constraints on the size and brightness temperature of the emitting spots. Here we report results related to the newly detected kilomasers, also including the well-known kilomaser source in in NGC 253. These are discussed within the framework of the kilomaser/megamaser dichotomy.
PV diagrams for the maser emission from a Keplerian ring
Authors: Uscanga L. , Cantó J. , Raga A. C.
Published: January 2008

We study the maser emission arising from a thin, planar, gaseous ring in Keplerian rotation around a central mass when it is observed edge-on. Assuming that the absorption coefficient within the amplifying ring is a decreasing function of distance from the central mass (i.e., κ = κ0r−q), we calculated position-velocity (PV) diagrams for the most intense maser features using different values of the exponent q. We found that, depending on the value of q, these diagrams can be qualitatively different.
Water-vapor maser disk at the nucleus of the Seyfert 2 IC 2560
Authors: Yamauchi Aya , Nakai Naomasa , Ishihara Yuko , Diamond Philip , Sato Naoko
Published: January 2008

We observed the H2O maser at the nucleus of the Seyfert 2, IC 2560, using the VLBA and the phased VLA. The systemic, red-shifted and blue-shifted maser features and a continuum component have been detected. We propose a maser disk in the nuclear region. The systemic and red-shifted features are emitted from a nearly edge-on disk with the position angle of PA = −47°. The thickness is 2H < 0.025 pc. The binding mass is 3.5 × 106M⊙. Assuming the Keplerian rotation, the radii at the disk are r = 0.087-0.335 pc and the rotation velocities are 213–418 km s−1. The mean density within the inner radius is 1.3 × 109M⊙ pc−3, suggesting a massive black hole at the center. A continuum component is considered as a jet ejected from the nucleus, with an angle of 70° from the disk. The blue-shifted maser feature is located on the continuum component, being interpreted to be a ‘jet maser’.
Statistical properties of extragalactic H2O maser sources
Authors: Jiang-Shui Zhang , Christian Henkel
Published: January 2008

Recently, with the development of sensitive maser receiver and wide-band spectrometer, maser emission were detected toward more and more galaxies. 69 galaxies beyond the Magellanic Clouds has been reported with H2O maser emission (to 2006.12). The statistical properties of the sample of extragalactic H2O masers are not well explored. Here we collect observation data of all these detected extragalactic H2O maser sources and give a statistical analysis to investigate the statistical properties of the sample.
OH megamasers as extragalactic diagnostics
Authors: Pihlström Ylva M.
Published: January 2008

Luminous, extragalactic OH masers are excellent tracers of merger triggered, intense star formation in Ultra Luminous Infrared Galaxies (ULIRGs). As such, they are tracers of high density regions in the centers of active galaxies. From high resolution imaging of the 1667 MHz emission in a small sample of OH megamaser galaxies, the general consensus is that most of the maser emission arises in thick, circumnuclear structures. Here we summarize the current work on OH megamasers, and present the preliminary results on VLA observations of the 1720 MHz satellite line in III Zw 35, Mrk 231 and Arp 220. We also briefly mention ongoing work on investigating the nature of the high molecular gas density environment in a sample of ULIRGs, using CO (J=3–2) observations. A subset of the ULIRG sample harbors OH megamaser emission, and is therefore suited for comparing gas properties between ULIRGs with and without OH megamaser emission.
A compact starburst ring traced by clumpy OH megamaser emission
Authors: Parra Rodgrigo , Conway John E. , Elitzur Moshe , Pihlström Ylva M.
Published: January 2008

We model the OH megamaser emission from the luminous infrared galaxy IIIZw35 as arising from a narrow rotating starburst ring of radius 22 pc enclosing a mass of 7×106M⊙. We show how both the compact and apparently diffuse maser emission from this ring can arise from a single phase of unsaturated maser clouds amplifying background radio continuum. The masering clouds are estimated to have a diameter of <0.7 pc and internal velocity dispersion of ∽20 kms−1. We find that the clouds are neither self-gravitating nor pressure confined, and they could be magnetically confined or freely expanding. Their dispersal lifetimes may set the vertical thickness of the ring. For an estimated internal density of 3×103cm−3, cloud masses are of order 24M⊙. The observed spectral features and velocity gradients indicate that the clouds must be outflowing and escaping the nucleus. The cloud mass outflow rate is estimated to be 0.8M⊙yr−1, while the star formation rate is ∽19M⊙yr−1. Associated ionised gas, possibly generated from dissipated clouds, provides free-free absorption along the source axis, explaining the observed East-West asymmetries. We show that the clumpiness of a maser medium can have a dramatic effect on what is observed even in a relatively low gain OH megamaser. Specifically, in IIIZw35 our clumpy maser model naturally explains the large line to continuum ratios, the large 1667MHz:1665MHz line ratios and the wide velocity dispersions seen in the compact maser spots. Other astrophysical masers showing both compact and apparently diffuse emission might be explained by similar clumpy structures.
Molecular properties of (U)LIRGs: CO, HCN, HNC and HCO+
Authors: Loenen A. F. , Baan W. A. , Spaans M.
Published: January 2008

The observed molecular properties of a sample of FIR-luminous and OH megamaser (OH-MM) galaxies have been investigated. The ratio of high and low-density tracer lines is found to be determined by the progression of the star formation in the system. The HCO+/HCN and HCO+/HNC line ratios are good proxies for the density of the gas, and PDR and XDR sources can be distinguished using the HNC/HCN line ratio. The properties of the OH-MM sources in the sample can be explained by PDR chemistry in gas with densities higher than 105.5cm−3, confirming the classical OH-MM model of IR pumped amplification with (variable) low gains.
Zeeman splitting in OH megamasers
Authors: Robishaw Timothy , Heiles Carl , Quataert Eliot
Published: January 2008

We detected significant Zeeman splitting in the 1667 MHz OH megamaser emission from four ultraluminous galaxies. These detections, in addition to being the first extragalactic detection of the Zeeman effect in an emission line, suggest that OH megamasers are excellent extragalactic magnetometers.
Submillimeter and millimeter masers
Authors: Humphreys E. M. L.
Published: January 2008

Despite theoretical predictions of the existence of many submillimeter masers, and some pioneering observational discoveries over the past few decades, these lines have remained relatively unstudied due to (i) challenges associated with observing at shorter wavelength; and, (ii) the lack of possibility of high (< 14″ at 345 GHz) angular resolution observations. With the advent of the SMA, the first submillimeter imaging array capable of sub-arcsecond resolution, APEX, and the promise of ALMA, opportunities are opening for performing new science with millimeter/submillmeter masers. In this talk, I will review recent work in the field - including extragalactic H2O millimeter masers, hydrogen recombination masers, submillimeter masers in star-forming regions, and in the envelopes of evolved stars - and discuss prospects for the future.
Submillimeter Array observations of 321 GHz water maser emission in Cepheus A
Authors: Patel Nimesh A. , Curiel Salvador , Zhang Qizhou , Sridharan T. K. , Ho Paul T. P. , Torrelles José M.
Published: January 2008

Using the Submillimeter Array (SMA) we have imaged for the first time the 321.226 GHz, 1029 − 936 ortho-H2O maser emission. This is also the first detection of this line in the Cepheus A high-mass star-forming region. The 22.235 GHz, 616 – 523 water masers were also observed with the Very Large Array 43 days following the SMA observations. Three of the nine detected submillimeter maser spots are associated with the centimeter masers spatially as well as kinematically, while there are 36 22 GHz maser spots without corresponding submillimeter masers. In the HW2 source, both the 321 GHz and 22 GHz masers occur within the region of ~1″ which includes the disk-jet system, but the position angles of the roughly linear structures traced by the masers indicate that the 321 GHz masers are along the jet while the 22 GHz masers are perpendicular to it. We interpret the submillimeter masers in Cepheus A to be tracing significantly hotter regions (600~2000 K) than the centimeter masers.
SMA imaging of the masers in MWC349A
Authors: Weintroub Jonathan , Moran James , Rao Ramprasad , Shinnaga Hiroko , Wilner David , Young Ken
Published: January 2008

In September 2004 and September 2005 we used the Submillimeter Array (SMA) to map the distribution of hydrogen recombination line (HRL) maser spots that originate in the envelope of the peculiar star MWC349A. We measured positions in the line over a 100 km/s velocity range with 1 km/s spectral resolution. Relative angular position was estimated to an accuracy of about 0.01 arcseconds. The H30α maser components can be tracked continuously in frequency between the red- and blue-shifted peaks of the spectrum. The masers appear to trace a nearly edge-on disk-like structure. The position-velocity relationship is quite remarkable, and reminiscent of that of NCG4258. However, the “wings” in the PV relation are too steep for a simple Keplerian prescription to be correct. We present our data, and suggest a simple model to account for the steep wings.
Opportunities for maser studies with the Square Kilometre Array
Authors: Green Anne J. , Baan Willem A.
Published: January 2008

The Square Kilometre Array (SKA) is the radio telescope of the next generation, providing an increase in sensitivity and angular resolution of two orders of magnitude over existing telescopes. Currently, the SKA is expected to span the frequency range 0.1-25 GHz with capabilities including a wide field-of-view and measurement of polarised emission. Such a telescope has enormous potential for testing fundamental physical laws and producing transformational discoveries. Important science goals include using H2O megamasers to make precise estimates of H0, which will anchor the extragalactic distance scale, and to probe the central structures of accretion disks around supermassive black holes in AGNs, to study OH megamasers associated with extreme starburst activity in distant galaxies and to study with unprecedented precision molecular gas and star formation in our Galaxy.
IAU 242 closing summary
Authors: Reid M. J.
Published: January 2008

This paper focuses on highlights of the symposium and attempts to synthesize recent results in terms of key advances and unsolved problems related to maser research.
On the aerosol number concentration–wind speed relationship during a severe cyclonic storm over south Indian Ocean
Authors: Pant Vimlesh , Deshpande C. G. , Kamra A. K.
Published: January 2008

Measurements of the number concentration and size distribution of aerosols in the range of 0.5–20 μm diameter were made during a severe cyclonic storm in the Southern Hemisphere at 53.3°S, 52.5°E on 19 February 2004. Data were analyzed to study the aerosol number concentration–wind speed relationship. It was found that in conformity with past observations, total aerosol number concentration increases with increase in wind speed from 4 to 11 m s−1, measured at 10 m above sea level. However, contrary to most of the earlier studies, total aerosol number concentration decreases with increase in the wind speed from 16 to 22 m s−1 and then maintains an almost constant value up to the maximum wind speed of 33.1 m s−1. The total aerosol number concentration varies a little with wind speed in the range of 11–16 m s−1. While increase in the total aerosol number concentration corresponding to the increase in wind speed from 4 to 11 m s−1 is spread over the whole range of particle sizes, decrease in the aerosol number concentration associated with increase in wind speed from 16 to 22 m s−1 is mainly due to decrease in the concentration of particles of <1 μm. The increase in aerosol number concentration is well recognized as being due to the enhanced bubble‐breaking activity at the sea surface and increased entrainment of sea‐salt particles, once produced. We propose that decrease in the aerosol number concentration may possibly occur because of the scavenging of aerosols by larger seawater drops injected into the atmosphere at high wind speeds. This scavenging process may act as a built‐in sink, which becomes operative at high wind speeds and restricts the enhancement in concentration of marine aerosols.
Effect of three‐dimensional slab geometry on deformation in the mantle wedge: Implications for shear wave anisotropy
Authors: Kneller Erik A. , van Keken Peter E.
Published: January 2008

Shear‐wave splitting observations from many subduction zones show complex patterns of seismic anisotropy that commonly have trench‐parallel fast directions. Three‐dimensional flow may give rise to trench‐parallel stretching and provide an explanation for these patterns of seismic anisotropy. Along‐strike variations in slab geometry produce trench‐parallel pressure gradients and are therefore a possible mechanism for three‐dimensional flow. In this study we quantify the effects of variable slab dip, curved slabs, oblique subduction, and slab edges on flow geometry and finite strain in the mantle wedge of subduction zones. Temperature, dynamic pressure, velocity, and strain are calculated with high‐resolution three‐dimensional finite element models. These models include temperature‐ and stress‐dependent rheology and parameterized slab and trench geometry. Thick layers (20–60 km) with strong trench‐parallel stretching are observed in the mantle wedge when slab geometry involves a transition to slab dip less than 15° or strong curvature in the slab. In these cases, strong trench‐parallel stretching develops when flow lines have an oblique to trench‐normal orientation. This suggests that trench‐parallel seismically fast directions may not indicate trench‐parallel flow lines in systems with large along‐strike variations. An oblique component of stretching is confined to a 20–30 km layer above the slab in systems with oblique subduction. The effects of slab edges include strong toroidal flow and focusing in the mantle near slab edges and trench‐parallel flow that extends 50–100 km into the core of the mantle wedge.