Maser Pumping Research Articles

Using VLBI to Probe the Orion KL Outflow on AU Scales

We present the first contemporaneous 43GHz and 86GHz VLBI images of the v=1 J=2-1 and J=1-0 SiO masers in the Orion-KL nebula. Both maser species exhibit the same general morphology of earlier J=1-0 maser images which appear to trace the edges of a bi-polar conical outflow. Surprisingly, the J=2-1 masers form further from the central protostar than the J=1-0 masers, a fact not readily explained by current SiO maser pumping models. The average magnitude of offsets between corresponding regions of the two masing transitions is approximately 14% of the total radial extent of the SiO maser emission. This offset indicates that each transition must trace different physical conditions.

OH 1720-MHz masers in southern star-forming regions

A search for maser emission at the 1720-MHz transition of OH has been made with the Australia Telescope Compact Array (ATCA) towards more than 200 southern sites of 1665- and 6035-MHz OH masers. 28 of the 36 detected 1720-MHz masers precisely coincide with the targets (15 previously known; 13 new discoveries) and are thus almost certainly of the star formation region (SFR) variety. Our observations revealed highly variable intensities for some previously known 1720-MHz masers, in extreme cases now preventing their detection. We interpret six new masers serendipitously discovered offset from any target position as an SFR subclass. Two other masers were chance detections of a quite different maser class associated with supernova remnants. Several instances of prominent 1720-MHz OH absorption occur. The 1720-MHz masers mostly coincide, to arcsecond accuracy, with a stronger 1665-MHz OH maser counterpart; in a few instances, 1665-MHz counterparts are much weaker, and the six 1720-MHz masers discovered offset from any target position (with no detectable 1665-MHz emission) are the ultimate examples, qualifying them as a distinct SFR subclass. At the sensitivity of the current survey, we find that SFR 1720-MHz masers are approximately one-sixth as common as 1665-MHz masers. The 1720-MHz masers preferentially occur at the sites hosting 6035-MHz masers – explicable in the context of current maser pumping models if 1720-MHz and 6035-MHz masers trace regions where the OH column density is higher, and the kinetic temperature lower, than at sites displaying only 1665-MHz masers. For each 1720-MHz maser, spectra of the two circular polarizations at high-velocity resolution were taken with the Parkes telescope. Similar observations, to monitor several previously known northern 1720-MHz masers, augment the southern sample. The Parkes spectra commonly reveal Zeeman splitting in magnetic fields of a few milligauss, and the largest field is a ‘record-setting’ 16 mG. Compared with fields derived from other OH transitions at the same site, the 1720-MHz values generally imply the same direction of field, but the field strength tends to be higher, by as much as a factor of 2. This may be understood if the 1720-MHz masers preferentially occur at locations compressed to higher density, with enhanced frozen-in magnetic field. Such higher fields support a pumping scheme requiring high densities for the 1720-MHz masers. Towards most of the 1720-MHz maser sites lies an associated 6668-MHz methanol maser, consistent with pumping expectations that the methanol masers are also favoured by high densities. We also explore some sites in more detail, with new results obtained in the radio continuum and recombination lines, and at other masing transitions – a new position for 328.809+0.633 at the 1612-MHz OH transition, for example, confirms a rare spatial near-coincidence with a 1720-MHz maser (although offset in velocity).

Masers: High Resolution Probes of Massive Star Formation

Astrophysical masers are one of the most readily detected signposts of high-mass star formation. Their presence indicates special conditions, probably indicative of a specific evolutionary phase. Masers also represent the ultimate high-resolution probe of star formation with the potential to reveal information on the kinematics and physical conditions within the region at milli-arcsecond resolution. To date this potential has largely remained unfulfilled, however, recent advances suggest that this will soon change.The key to unlocking the potential of masers lies in identifying where they fit within the star formation jigsaw puzzle. I will review recent high resolution observations of OH, water and methanol maser transitions and what they reveal. I also briefly discuss how multi-transition observations of OH and methanol masers are being used to constrain maser pumping models and through this estimate the physical conditions in the masing region.

OH inversions in star-forming regions and ALI radiation transfer

I introduce a new model of OH maser pumping in star-forming regions, based on exact radiation transfer models of the interface between an expanding ucHII region and undisturbed molecular material. The model incorporates the most recent set of rate-coefficients for collisions of OH with molecular hydrogen, and an improved model of continuum emission from dust. FIR line overlap is incorporated into the model in a manner which avoids the inconsistencies associated with the Sobolev approximation.

Detection of new sources of 4765-MHz OH masing

We have used the Australia Telescope Compact Array (ATCA) to make a sensitive search for maser emission from the 4765-MHz transition of OH towards a sample of 55 star formation regions. Maser emission with peak flux densities in excess of 100 mJy were detected in 14 sites, with 10 of these being new discoveries. Unlike the ground-state OH transitions the 4765-MHz transition is not predicted to be circularly polarised and none of the masers observed have detectable levels of linear, or circular polarisation. Combining our results with those of previous high resolution observations of other OH transitions we are able to investigate various theoretical models for the pumping of OH masers.

VLBA observations of the 7mm SiO masers in TX Cam and R Cas

Simultaneous VLBI observations of the SiO masers of the J = 1 → 0 rotational line in the v = 1 and v = 2 vibrational levels toward Mira variables are presented. Because SiO maser lines are formed deep in circumstellar envelopes they serve as a unique tool to study the innermost envelopes of evolved stars. Although the first interpretation of SiO maser emission was made in 1974, observational features are only partially explained by models which have been suggested since then. Positional coincidence of the J = 1 → 0 masers of v = 1 and v = 2 has been argued as a way to distinguish among the maser pumping models, but it requires simultaneous observations of the two lines using high resolution. We have developed the technique for such observations during the last few years and here we report successful results of our simultaneous observations of the SiO v = 1 and v = 2 J = 1 → 0 masers using the Very Long Baseline Array (VLBA). We discuss the pumping mechanism in terms of our observational results.

A new pumping mechanism for a series of class II methanol masers in the J 0 →J -1 E transitions.

A new pumping mechanism – methanol masers without population inversion is presented in this paper. It can be used to explain the formation of a series of J0 → J-1E methanol masers, while the 21 → 30A+ methanol masers are regarded as a driving coherent micrwave field. In the new mechanism, the intensities of J0 - J-1E methanol masers are increased with the decreasing transition frequencies (or with rotational number J, approximately). These results agree with Slysh et al. (1995) and Slysh et al. (1999) J ≤ 5 observations for G3345.01+1.79 and W48, in which both J0 → J-1E and 21 → 30A+ methanol masers are detected coincidentally. Other astronomical conditions, such as magnetic field, 21 → 30A+ coherent radition, incoherent pumping rate by thermal radition and so on are also discussed. The new mechanism can operate as a complement to other ordinary maser pumping mechanisms for some class II methonal maser sources.

Maser emission from OH at the 6035-MHz transition

The Australia Telescope Compact Array (ATCA) has been used to measure positions with subarcsec accuracy for more than 50 masers at the 6035-MHz transition of excited OH. Eleven of the masers are new discoveries; the remainder complete the detailed study of masers from an earlier single-dish survey of the southern Galactic plane using the Parkes telescope. All previously known masers in the region were strong enough to be studied in the present observations, despite the common occurrence of variability. Typically, the intensity varies by less than a factor of two over several years, but one of the new masers is, remarkably, 100 times stronger than the upper limit measured six years earlier. The precise positions confirm that most of the masers coincide with a 1665-MHz ground-state OH maser counterpart that is usually stronger. However, there are a few instances where any 1665-MHz counterpart is either much weaker or not detected. When surveyed with comparable sensitivity, we show that the 6035-MHz masers are approximately one-third as common as 1665-MHz masers. We also add to earlier evidence that 1720-MHz masers preferentially occur at the sites hosting 6035-MHz masers. These facts are discussed in the context of current maser pumping models. Towards most of the 6035-MHz maser sites we find a coincident 6668-MHz methanol maser, and towards some of the maser sites we report new detections of ultracompact H ii regions. Clusters of two or three sites, each showing one, or several, of the principal molecular masing transitions, are found to be common; future comparative studies of these sites within small clusters will elucidate the precise conditions favouring different maser species. Several instances of absorption occur at 6035 MHz, one of them especially striking since it extends over a broad velocity range adjacent to maser emission related to the ultracompact H ii region 5.885−0.392. The absorption provides new clues to understanding the geometry and excitation of 5.885−0.392. Three sources were observed at 6030 MHz, an OH transition from the same excited state (2Π3/2, J=5/2) as the 6035-MHz line; in one case, the matching velocity patterns at 6030 and 6035 MHz, their similar circular polarization characteristics, and their spatial coincidence, reveal Zeeman splitting in a magnetic field of −9 mG, a field strength close to the upper limit found for any such masers.

Dust radiation fields and pumping of excited state OH masers in W3(OH)

I show that it is quite possible to obtain saturating amplification in the 13.44-GHz maser which is so far unique to W3(OH), using a combined radiative and collisional pumping scheme. The dominant radiative part of the pump involves far-infrared line overlap, and the far-infrared continuum is provided by dust, modelled as either a two-component mixture or composite grains. Transport of the far-infrared radiation is carried out via the accelerated lambda iteration method. The observational link between 13.44-GHz and 6035-MHz masers is reproduced by the model. Inadequate amplification in Sobolev models probably results from the optical depth limitations imposed by this approximation. I review the dust models used in far-infrared pumping models of OH masers, and conclude that the main consequence of moving from skewed blackbody functions to more sophisticated models is a selective pumping enhancement when ice mantles are included.

Variations of the H2O maser emission of W51M in 1981–1998

The results of spectral monitoring of the maser source W51M carried out in the water-vapor line at 1.35 cm (22GHz) on the 22-m telescope of the Pushchino Radio Astronomy Observatory in 1981–1998 are reported and interpreted. Long-term variations of the maser flux with a period of 12–13 years are found. W51M may be a rotating and simultaneously expanding toroidal cloud of gas and dust around a young star with a mass of the order of ∼15M⊙, with numerous high-velocity jets of maser condensations flowing out in two broad cones along the polar axis of the torus. A stellar wind with a velocity of about 2000–3000 km/s is responsible for the maser pumping.

Variability of the H2O maser associated with U Orionis

H2O line observations at =1 :35 cm of the Mira Ceti-type variable star U Ori are reported. The ob- servations cover the time interval from March, 1980, to September, 1999. Variations of the integral flux and ve- locity centroid of the H2O line are analysed. The flux in general correlates with the visual light curve, following it with some phase delay ' 0:2 0:4P (P is the period of the star). The maser emission is generated in a quasi- stationary layer of gas and dust at a distance of about 10 14 cm from the stellar centre. The maser variability is explained by the action of periodic shocks, driven by stel- lar pulsation and arriving to the maser in each stellar cy- cle. The shocks provide the maser pumping, whereas the sink of the waste energy is controlled by the dust, period- ically heated by the stellar radiation near the light maxi- mum; this accounts for the correlation of the maser radi- ation maximum with the descending branch of the light curve. Temporary weakness of the maser emission may be due to decay of the quasi-stationary layer, which is then rebuilt by a powerful shock, carrying away from the star a portion of the lost mass, once per a few stellar periods { the \superperiod. In its turn, the superperiod may re- flect multiperiodic pulsation of the star or the presence of a long-term activity cycle, connected with restructuring of the stellar magnetic eld, which is known to be strong in U Ori.

No Water Masers Associated with Supernova Remnants

We have used the Very Large Array to search for water masers in the three supernova remnants W28, W44, and IC 443. Only one candidate maser was detected, in W44. Follow-up observations using the VLA at 1612 MHz demonstrate that the water maser in W44 is likely associated with a late-type star and not a shock in the supernova remnant. The nondetection of water masers toward shocked regions in these supernova remnants places constraints on the postshock density and is consistent with the density determined by molecular observations and the limits from 1720 MHz OH maser pumping.

A Survey of [TSUP]30[/TSUP]S[CLC]i[/CLC]O Emission from Evolved Stars

We present the first detection of 30SiO v = 1, J = 1–0 maser emission toward the Mira variable TX Cam among 18 surveyed evolved stars. The line width of this maser is much narrower than that of 30SiO v = 0, J = 1–0 masers and the intensity is also weaker, indicating that it is unsaturated. The detection of the 30SiO maser emission in the lowest rotational transition of the first vibrationally excited state, together with nearly simultaneous observations of the large number of 29SiO and 28SiO masers, provides good constraints for establishing the SiO maser pumping model of TX Cam. The global features of 30SiO and 29SiO v = 1, J = 1–0 lines are similar, but detailed characteristics, such as their half-widths, their peak intensity ratios of the v = 1, J = 1–0 line to the v = 0, J = 1–0 line, and their intensity variation with time, are different. New detection of the 30SiO v = 0, J = 1–0 and J = 2–1 lines is also reported in six evolved stars: TX Cam, R Cas, χ Cyg, W Hya, U Lyn, and WX Psc. They show a different masing characteristics of the 30SiO that depends on both sources and transitions. This is probably related to a critical condition for masing.

Detection by ISO of the far-infrared OH maser pumping lines in IRC+10420

The population inversions which lead to circumstellar hydroxyl (OH) maser emission from cool stars have long been thought to be radiatively pumped. However, the proposed pumping mechanism has not been directly observed before now, because the spectral lines that correspond to the rotational transitions of the pumping cycle lie in the far-infrared, and are unobservable from the ground. We present ISO observations of IRC+10420, obtained with its Short and Long Wavelength Spectrometers (SWS and LWS, respectively). This object is an F-type hypergiant and a strong OH maser source. The SWS spectrum clearly shows the 34.6-μm doublet in absorption, and the ensuing rotational cascade lines at 98.7,163 and 79 μm are clearly detected in emission in the LWS spectrum. These spectra provide the first direct confirmation of the radiative pumping cycle for circumstellar OH masers. © 1997 RAS.

A starburst origin of the OH-megamaser emission from the galaxy Arp220

Ultraluminous infrared galaxies have been known for more than a decade, but the source of their very large far-infrared luminosities remains controversial. It may reflect a quasar-like active nucleus surrounded by a torus of dense gas and dust, the latter absorbing the energetic photons from the nuclear region and re-emitting at infrared wavelengths1, or a huge burst of massive-star formation in dense dusty clouds of molecular gas close to the nucleus2, which heats the surrounding dust. A number of ultraluminous galaxies are also a source of OH-megamaser emissions (intense laser-like spectral lines at microwave frequencies), an observation that may hold important clues as to the main power source in these galaxies. A general feature of many models3,4 is that the masers are pumped radiatively by the absorption of infrared photons. Identifying the source of the maser pump may therefore indicate whether the ultimate energy source is a burst of star formation, or an active nucleus. Here we report the detection of a strong mid-infrared OH absorption line in the prototypical megamaser and ultraluminous galaxy5, Arp220. We find that the power absorbed in this line alone is sufficient to pump the megamaser emission seen at radio wavelengths. Moreover, the warm, extended nature of the pumping region is suggestive of a starburst origin for the ultraluminous infrared emissions.

Determination of Λ-doublet resolved cross-sections for inelastic scattering of OH by para- and normal-H2

In this paper we report the measurement of Λ-doublet resolved state-to-state cross sections for inelastic collisions of OH by H2 at a translational energy of 595 cm−1. Experimental values are obtained for transitions from both the upper (f) and the lower (e) Λ-doublet substates of the lowest rotational state (J=3/2) of OH(2Π3/2) to almost all other states within the range of the collision energy. Cross sections for scattering by both para- and normal-H2 have been determined. The main difference between para- and normal-H2 scattering is seen in the Λ-doublet cross section. The results are compared to He-scattering which shows that H2 (J=0) scattering behaves similar to He scattering. When averaged over the Λ-doublet states, the cross sections are in good agreement with the measurements of Andresen et al. [J. Chem. Phys. 81, 571 (1984)], although the conclusions with regard to collisional pumping of interstellar OH masers are different. The quantum calculations of Offer et al. [J. Chem. Phys. 100, 362 (1994)] show a surprisingly good correspondence with the measured values. In this comparison the full initial state distribution of the OH and H2 beams has been taken into account.

The Warp in the Subparsec Molecular Disk in NGC 4258 as an Explanation for Persistent Asymmetries in the Maser Spectrum

Neufeld & Maloney have proposed a model in which the warp in the subparsec-scale molecular disk in NGC 4258 allows direct illumination of the disk by a central X-ray source, heating the gas to temperatures appropriate for collisional pumping of water vapor masers. One element of this model is the creation of a layer of warm atomic gas above the cooler molecular gas where the masing occurs. Here, we examine the effects of this warm atomic gas on the maser emission. We find that for a range of plausible warps, thermal free-free extinction in the atomic gas can generate opacities of 2-3 at a wavelength of 1.3 cm. Furthermore, because of the orientation of the warp with respect to the line of sight, we argue that this extinction will only occur on the blueshifted side of the disk. Thus, we demonstrate that such thermal absorption is a likely explanation for the persistent weakness of the blueshifted maser features relative to the redshifted ones.

OH Masers as Diagnostics of Physical Conditions in Star-forming Regions. II. Effects of Large Velocity Gradients and Infrared Radiation

We have studied the pumping of OH masers in star-forming regions by introducing four pumping mechanisms one at a time. The four mechanisms are collisions, local line overlap, nonlocal line overlap, and external infrared radiation field. The first two have been investigated in a previous paper (Paper I). Here we study the effects of nonlocal line overlap and of an external infrared radiation field. As in Paper I, we have used the most accurate collision rates of OH with ortho- and para-H<SUB>2</SUB>, which became available recently, and we have taken the effects of line overlap into account as accurately as possible using a recently developed formalism. We have found that, despite the significant change in the collision rates, the effects of nonlocal line overlap and of an external infrared radiation field are necessary for the explanation of the main lines in the ground state of OH. The same is true for the explanation of the general qualitative behavior of H II/OH masers. Our calculations have revealed a number of diagnostics of the physical conditions in star-forming regions. These diagnostics are enumerated in our conclusions.

A water-vapour giga-maser in the active galaxy TXFS2226-184.

Active galactic nuclei are thought to be powered by gas falling into a massive black hole; the different types of active galaxy may arise because we view them through a thick torus of molecular gas at varying angles of inclination. One way to determine whether the black hole is surrounded by a torus, which would obscure the accretion disk around the black hole along certain lines of sight, is to search for water masers, as these exist only in regions with plentiful molecular gas. Since the first detection of an extra-galactic water maser in 1979, they have come to be associated primarily with active galaxies, and have even been used to probe the mass of the central engine. Here we report the detection of a water giga-maser in the radio galaxy TXFS2226-184. The strength of the emission supports a recently proposed theory of maser pumping that allows for even more powerful masers, which might be detectable at cosmological distances. Water masers may accordingly provide a way to determine distances to galaxies outside the usual distance ladder, providing an independent calibration of the Hubble constant.

New handle on a maser pump

New handle on a maser pump