We present the results of spectral energy distribution analysis for 162 of the 405 sources reported in the SIMBA survey of Hill et al. (2005). The fits reveal source specific parameters including: the luminosity, mass, temperature, H2 number density, the surface density and the luminosity-to-mass ratio. Each of these parameters are examined with respect to the four classes of source present in the sample. Obvious luminosity and temperature distinctions exist between the mm-only cores and those cores with methanol maser and/or radio continuum emission, with the former cooler and less luminous than the latter. The evidence suggests that the mm-only cores are a precursor to the methanol maser in the formation of massive stars. The mm-only cores comprise two distinct populations distinguished by temperature. Analysis and conclusions about the nature of the cool-mm and warm-mm cores comprising the mm-only population are drawn.

We present VLBI and single dish observations of a strong (1000 Jy) OH maser flare at 1665 MHz in the star-forming region W75N. This new flare appeared in 2003, after a 100 Jy flare in 2000. Three major spectral features N1, N2, and N3 were present at all epochs, with N1 declining and N2 increasing in flux density during observations taken in 2003–2006. All three features have strong linear polarization approaching 90%. VLBI maps show that along with the strong features N1-3, many weaker features have appeared within 100 mas of the ultra compact HII region VLA 2. The proper motion of the flare feature N1 was measured: Δα=1.36±0.3; Δδ=-0.3±0.3 mas in 1.75 years, corresponding to a tangential velocity of about 8 km s−1 relative to the rest of the features, and is comparable to the radial velocity range in the VLA 2 disk.

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.

Maser emission from the SiO molecule has been widely observed in the near-circumstellar envelopes of late-type, evolved stars. VLBI images can resolve individual SiO maser spots, providing information about the kinematics and magnetic field in the extended atmospheres of these stars. This poster presents full polarization images of several SiO maser lines towards the supergiant star VY CMa. VY CMa is a particularly strong SiO maser source and allows observations of a wide range of maser transitions. We discuss implications of these observations for VY CMa morphology, polarization, and pumping models.

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.

We present a review of the field of formaldehyde (H2CO) 6cm masers in the Galaxy. Previous to our ongoing work, H2CO 6cm masers had been detected in the Galaxy only toward three regions: NGC7538 IRS1, Sgr B2, and G29.96–0.02. Current efforts by our group using the Very Large Array, Arecibo, and the Green Bank Telescope have resulted in the detection of four new H2CO 6cm maser regions. We discuss the characteristics of the known H2CO masers and the association of H2CO 6cm masers with very young regions of massive star formation. We also review the current ideas on the pumping mechanism for H2CO 6cm masers.

We are using a Japanese VLBI network (JVN) for VLBI observations of 6.7-GHz methanol masers associated with massive star-forming regions. Here we present results for Cepheus A (Cep A) from observations taken on September 9, 2006. The distribution of the maser spots indicates either a spherical bubble or a disk having an inclination of several tens degree. We construct a disk model with an inclination of ~70° and a radius of ~700 AU. From a luminosity of 1.7 × 104L⊙ for a source observed in this region, the excitation of the maser is probably radiative.

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.

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.

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.

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.

We present the results of recent surveys in the 44GHz methanol maser line toward regions of massive star formation using the Haystack 37m telescope and the VLA. We discuss a possible shock origin of this maser line and present evidence for variability from multi-epoch observations of selected sources.

We present H2O masers associated with the massive-star forming region G192 observed with the Japan VLBI network since the year 2005, The spatio-kinematical structure of the maser feature clusters has well persisted since previous observations, in which the masers are associated with two young stellar objects (YSOs) separated by ~1200 AU and expected to be associated with a highly-collimated bipolar jet and an infalling-rotating disk in the northern and southern YSOs, respectively. We estimated a jet speed of ~100 km s−1 and re-estimated a dynamical age of the whole jet to be 5.6×104 years. The spatial distribution of maser Doppler velocities found during the previous and present observations, relative proper motions of H2O maser features in the southern cluster found in the present observations, a relative bulk motion between the two maser clusters are well explained by a model of an infalling-rotating disk with a radius of ~1000 AU and a central stellar mass of ~8 M⊙.

We use second epoch images taken with WFPC2 on the HST and imaging polarimetry taken with the HST/ACS/HRC to explore the three dimensional structure of the circumstellar dust distribution around the red supergiant VY Canis Majoris. Transverse motions, combined with radial velocities, provide a picture of the kinematics of the ejecta, including the total space motions. The fractional polarization and photometric colors provide an independent method of locating the physical position of the dust along the line-of-sight. Most of the individual arc-like features and clumps seen in the intensity image are also features in the fractional polarization map, and must be distinct geometric objects. The location of these features in the ejecta of VY CMa using kinematics and polarimetry agree well with each other, and strongly suggest they are the result of relatively massive ejections, probably associated with magnetic fields.

ISO mid-infrared absorption data helps to distinguish between radiative and collisional pumping mechanisms in 1612 MHz OH masers in various environments. Archive data of OH absorption at 34.6 and 53.3μm shows different behavior and different pumping rates for different types of maser sources.

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.

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.

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.

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.

We have re-analyzed observations of the water maser emission in IRAS 17347-3139 carried out previously and compared them with new higher angular resolution and more sensitive radio-continuum observations to explain the shift between the position of the peak of the radio-continuum with respect to the center of the ring-like distribution. From our analysis, we found that the water maser emission is not distributed in a closed ring-like structure, but in what appears to be a segment of an ellipse centered in the position of the peak of the radio-continuum. These results can explain the shift between the radio-continuum and the water maser emission. We also present interferometric observations of the OH maser emission towards IRAS 17347-3139 from which we have found that the OH maser emission detected previously is not associated with this source. Furthermore, since our observations are more sensitive, we have detected for the first time weak OH maser emission associated with IRAS 17347-3139.