OH Maser Sources Research Articles

The discrete nature of circumstellar OH maser emission

The Arecibo radio telescope was used to obtain high-resolution, high-SNR 1612-MHz observations of seven circumstellar OH maser sources. Each spectrum displays the double-peaked emission line profile characteristics of an expanding shell. The jaggedness of the spectra, which varies from source to source, is consistent with statistical fluctuations in the number of discrete emitting elements contributing at a given velocity. In particular, it is found that the spectra of WX Psc and OH 53.6-0.2 contain narrow, weak, isolated emission features which may arise from individual emitting elements.

Submillimetre observations reveal that DR21 (OH) is a double source

The region around the OH maser source DR21 (OH) has been scanned with a 20-arcsec beam at a wavelength of 350 microns and a hitherto unknown double structure, with two peaks of roughly equal mass separated by 40 arcsec (0.1 pc) in declination, has been discovered. The northernmost peak is coincident with the OH maser position and the centroid of the far-infrared emission, while the southern source has not previously been found at any wavelength. Both sources are extremely dense and deeply embedded, with optical depths of order unity even in the far-infrared. The southern source may be one of the densest molecular cores yet found, and is a candidate protostar. These results confirm that OB star formation can occur deep within molecular clouds, and not just at their edges.

Circular Polarization at 1665 and 1667 MHz Towards OH Masers in NGC 6334N, NGC 7538N, NGC 7538S, and G45.07+0.13

VLBI OH maser observations simultaneously in right and left circular polarization at the 1665 and 1667 MHz transitions were conducted by taking advantage of the multichannel capability of the Mk III system. The OH maser maps of the right and left circular polarization at both transitions were obtained toward the OH maser sources NGC 6334N, NGC 7538N, NGC 7538S, and G45.07+0.13.

Survey of OH Masers at 1665 MHz. III. Galactic Longitudes 233° to 326°

We have searched the galactic plane from longitude 233� to 326� for OH emission on the 1665 MHz transition. Between longitudes 270� and 326�, where the survey zone was a uniformly sampled grid, we detected 35 OH maser sources, including 24 new ones; most are of the Type I variety that delineates sites of current star formation, and many of the sites are young, with no well-developed HII regions in the vicinity. By combining the present results with those of earlier surveys Oongitudes 326� ? 360� ? 2�: Parts I and II) we have improved on previous estimates of the galactic distribution of these masers.

Compact H II regions - Hydrogen recombination and OH maser lines

Hydrogen recombination lines at 15 and 22 GHz and hydroxyl (OH) maser emission at 1.6 GHz have been mapped toward nine compact H II regions using the VLA. The authors find that all OH masers have radial velocities which are redshifted from the velocities of the H II regions as deduced from their hydrogen recombination lines. The majority of the maser spots appear projected onto the most compact H II region in a complex. These observations are inconsistent with models in which the masers form in an expanding, shocked shell, and they are explained most simply if the OH maser sources are part of a remnant envelope which is still collapsing toward the newly formed star.

On the nature of OH/IR stars

OH/IR stars are the infrared counterparts of galactic OH maser sources which show a characteristic double-peaked emission-line profile. Their strong radio emission can be detected at large distances, making them excellent tracers of distribution and kinematics of evolved stars in the Milky Way. The OH maser profile is typical for line emission from an expanding circumstellar shell. The circumstellar shells of OH/IR stars absorb the optical emission of the central star nearly completely and reemit the energy in the infrared. Having luminosities ~ 105 L⊙ and energy distributions peaking around 10μm, they may make a major contribution to the interstellar radiation field beyond 5μm. With mass loss rates of 10-5 to 10-4 M⊙/yr they lose several solar masses in a few hundred thousand years. OH/IR stars are therefore important objects for recycling stellar matter into the interstellar medium.Progress has been made in understanding the nature of OH/IR stars. They are Mira-like large-amplitude variables with periods up to 5 years long. It is proposed that they are stars of intermediate mass (2–10 M⊙) on the asymptotic giant branch (AGB). They have not only larger masses than Mira variables proper, but also longer periods of pulsation and larger mass loss rates. As a result optically thick circumstellar dust shells are formed, which prevent the detection of these more massive Mira-like variables at optical wavelengths. Radial pulsation (Mira variability) is thus thought to occur for all intermediate-mass stars in the course of their evolution on the AGB. In view of their high mass-loss rates, these stars may be key objects in the study of the formation of planetary nebulae.

Merlin Observations of the OH Maser Emission from IRC+10420

MERLIN observations of the 1612 and 1665 Mhz OH masers from the supergiant IRC+10420 reveal several major departures from the usual morphology of OH/IR stars.IRC+10420 is one of the most unusual OH maser sources yet found. It is an F8 supergiant with strong OH maser emission at 1612, 1665 and 1667 Mhz. A north-south optical nebulosity with an angular size of 2 arcsec has been detected (Thompson and Boroson,1977) and its peculiar IR properties are reminiscent of Eta Carina. These characteristics make IRC+10420 unique among Type II OH/IR stars.

VLA observations of warm NH3 associated with mass outflows in W51

view Abstract Citations (74) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS VLA observations of warm NH3 associated with mass outflows in W 51. Ho, P. T. P. ; Genzel, R. ; Das, A. Abstract VLA observations of the (J, K) = (3,3) inversion line of NH3 toward the distant H II region complex in W51 are presented. Three small (about 0.1 pc), warm (about 100 K) NH3 condensations are found coincident with every one of the H20 and OH maser sources in the region. This positional emission arises from dense molecular condensations which are interacting with the mass outflows from young stars in these regions. These observations also represent the first direct detection of such high gas temperatures outside the Orion-KL region. Gas heating could be due to the radiation from the exciting sources of the compact H II regions and masers or from the input of mechanical energy due to the outflows. These warm condensations are embedded in a cooler and more extended envelope which dominates lower angular resolution studies. Publication: The Astrophysical Journal Pub Date: March 1983 DOI: 10.1086/160807 Bibcode: 1983ApJ...266..596H Keywords: Ammonia; Molecular Clouds; Radio Sources (Astronomy); Stellar Mass Ejection; Water Masers; Astronomical Maps; B Stars; Condensates; Early Stars; Gas Heating; H Ii Regions; High Temperature Gases; Molecular Spectra; O Stars; Orion Nebula; Astrophysics full text sources ADS | data products SIMBAD (4) NED (1)

Interferometric observations of four protostellar OH maser sources

Interferometric observations of four protostellar OH maser sources

Radio and infrared observations of the OH maser source OH 351.78-0.54

Observations of the region associated with the highly variable maser source OH 351.78--0.54 at several radio and infrared wavelengths are reported. Strong emission from water vapor and carbon monoxide was detected at nearly the same velocity as the hydroxyl maser emission. An H ii region was detected 12'' from the OH maser position, and another, weaker, radio continuum object, at the position of the OH masers. Strong, extended near-IR emission was also found. Synoptic monitoring of the OH maser flux density during the period 1980.6 to 1981.4 revealed little change in peak flux, which may indicate that the masers have become saturated. VLBI observations of the OH maser emission indicate a relatively large angular size, which is interpreted as evidence for interstellar scattering. The observations suggest a region of active star formation associated with a giant molecular cloud at a distance of approximately 6.5 kpc. The H/sub 2/O/OH maser site is likely associated with an OB star in an early evolutionary state.

Magnetic fields in dense interstellar clouds

Evidence is presented that shows that magnetic fields pervade the entire interstellar medium including interstellar gas clouds of both low and high density. The magnetic field in the ‘seed’ gas from which the denser clouds form is 0.2-0.3 nT (1T — 10 4 G). Zeeman effect measurements of neutral hydrogen show that stronger fields occur in denser clouds. These data, taken with the microtesla fields found in OH maser sources, indicate that magnetic flux is conserved during gravitational collapse of interstellar clouds from densities of ca. 5 to ca. 10 7 cm -3 . Magnetic fields appear to play a major role in the formation of dense interstellar clouds. Furthermore there is a strong indication that the magnetic field direction is preserved during cloud collapse.

OH emission in the direction of TV GEM and BI CYG

The NRAO-Green Bank 43-m telescope and 18-cm receiver have detected OH maser main-line emission at 1665 MHz, in the direction of the two late-type supergiants TV Gem and BI Cyg. The detection of the two new OH maser sources is noteworthy in view of the small number of late-type supergiants known to be molecular radio sources, or associated in close proximity with OH emission clouds. The IR, UV and radio properties of the two stars are discussed.

Positions of OH maser sources determined with the 100-m telescope

Positions of 65 OH maser sources obtained with the 100-m radio telescope are presented. The positional accuracy is typically 10'', a conservative estimate for sources stronger than approx.10 Jy. The emphasis of this work was on OH/IR sources and 51 of the objects are certain type II OH/IR sources, of which only one (GX Mon) is associated with an optically known star. Three of the sources show only one 1612-MHz line component and these are probably also OH/IR sources but with a large ratio between the intensities of the high- and low-velocity components. Six of the objects are certain type I OH sources, i.e., maser emission associated with recently formed massive stars, and five more are probably of this type, although in some of these cases the 1612-MHz line is stronger than one of the main lines. This work has incidentally revealed (i) that the 1665-MHz emission which previously was assumed to be positionally coincident with the 1720-MHz emission of the source ON-3 originates approx.2.'3 to the southwest of the interferometrically determined 1720-MHz position, and (ii) that to within the positional accuracy, the position of the 1612-MHz emission from the core of the Kleinmann--Low nebula (Orion) is the same as more » that of the 1665-MHz emission; (iii) that the position of the 1667-MHz emission from VX Sgr is the same as that of the 1612-MHz emission; and (iv) that the position of the 1612-MHz emission from W 3(OH) is the same as that of the 1665/1667-MHz emission. « less

VLBI aperture synthesis observations of the OH maser source W75 N

We conducted very long baseline interferometric (VLBI) observations of the OH maser source in W75 N at 1665 MHz. The OH emission was mapped with aperture synthesis techniques using a seven station array of telescopes across the United States which provided a synthesized beam of approx.0.''015 diameter. The maser emission consists of nine components in an elongated region that has a major axis of 1.''5 (0.15 pc) and a position angle of 20/sup 0/. The maser components systematically increase in velocity from 3.8 to 13.2 km s/sup -1/ along this axis. The absolute position of the OH maser source was measured to an accuracy of 0.''2, confirming the association of the OH maser with the ultracompact H II region W75 N. Very large array observations of the continuum emission from W75 N at 4885 MHz indicate that the OH maser components are aligned with an extension of the ultracompact H II region. Our observations suggest that a binary protostellar system may excite the H II region, and that the OH maser reside in a rotating disk of remnant material out of which the binary formed.

VLBI observations of the H2O masers in Sagittarius B2

view Abstract Citations (27) References (35) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS VLBI observations of the H2O masers in Sagittarius B2 Elmegreen, B. G. ; Genzel, R. ; Moran, J. M. ; Reid, M. J. ; Walker, R. C. Abstract A two-station VLBI experiment was used for the mapping of 33 velocity features of the H2O emission at 1.35 cm wavelength from Sagittarius B2 with an accuracy of 4 milliarcsec. The masers originate in three space groups spaced by 44 in. in the north-south direction; the middle and southern sources coincide with the principal OH maser sources. The 30 features in the northern and middle sources indicate that the middle source masers originate from an expanding or contracting envelope with a radial velocity gradient; the envelope may be the gravitationally collapsing core of the Sagittarius B2 cloud. The pattern of maser emission is symmetric; the middle source is at the midpoint of a line joining the northern and southern sources; the middle source features are almost exactly at the centroid of the emission. Publication: The Astrophysical Journal Pub Date: November 1980 DOI: 10.1086/158414 Bibcode: 1980ApJ...241.1007E Keywords: Galactic Nuclei; Interstellar Masers; Molecular Clouds; Very Long Base Interferometry; Water Masers; Astrometry; Interstellar Matter; Sagittarius Constellation; Astrophysics full text sources ADS |

Redox reactions and the optical properties of interstellar grains

view Abstract Citations (22) References (36) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Redox reactions and the optical properties of interstellar grains. Duley, W. W. Abstract The role of surface and photochemical reactions on the oxidation state of interstellar grain material is examined. The equilibrium in the Fe oxide series (Fe, FeO, Fe3O4, Fe2O3) as a function of ambient conditions is investigated showing primarily that pure Fe grains are unlikely to exist in H I regions because of low temperature oxidation to Fe oxides by O atoms. In addition, it is found that several optical effects usually ascribed to size changes in interstellar grains can be interpreted in terms of chemically induced modifications in the optical properties of the grains. The optical properties of a silicon oxide series SiO(x) is used to study a possible relationship between refractive index variations and variations in the wavelength of maximum linear polarization, suggesting that reduction of silicate or oxide grains in H II regions may be related to OH and H2O maser sources in these regions. Publication: The Astrophysical Journal Pub Date: September 1980 DOI: 10.1086/158308 Bibcode: 1980ApJ...240..950D Keywords: Granular Materials; Interstellar Matter; Optical Properties; Photochemical Reactions; Surface Reactions; H I Regions; H Ii Regions; Interstellar Masers; Iron Oxides; Refractivity; Silicon Oxides; Water Masers; Astrophysics; Interstellar Grains:Chemical Composition full text sources ADS |

Interpretation of type I OH maser sources

OH maser sources of type I, which are related to regions of star formation, have been known for more than a decade. However, despite great effort no satisfactory theoretical interpretation has yet been given. (For general reviews on cosmic masers see refs 1–5.) A model for the interpretation of type I OH maser sources is suggested here. This model combines the effect of velocity and magnetic field gradients with radiative pumping by means of the (2∏3/2, J= 5/2 →2∏1/2, J = 5/2) transition. Due to an overlap of particular hyperfine components, inversion of the 1,665 MHz line is favoured.

Accurate measurements of the positions of OH maser sources

Accurate measurements of the positions of OH maser sources

Absolute Positions of OH Masers Associated with HII Regions

We have measured the absolute positions of five OH maser sources associated with HII regions, using the Jodrell Bank phase compensated radio linked interferometer (Norris, Booth & Davis, 1980); these are reproduced in Table 1. In addition, we have mapped the relative positions of the features in the OH spectra at 1665 MHz for all of the sources and at 1667 MHz for 3 of them (W30H, NGC 7538 and W49). We are now able to compare the distribution of the OH masers in a given source with its radio continuum map and to investigate the proximity of masers in the 2 main line transitions and the H2O masers.

Interferometric Measurements of Flattend Circumstellar Envelopes

AbstractAn incoherent spatial interferometer has been used to measure the shapes of circumsteliar dust envelopes at infrared wavelengths. The objects IRC +10216, VY CMa, and NML Cyg exhibit a large variation of fringe visibility with position angle Their circumstellar envelopes are probably flattened disks. For IRC +10216, visibility measurements from 5.0 to 20µm are discussed in detail to illustrate the physical information available from interferometric work. In VY CMa and NML Cyg, disk orientation is strongly correlated with polarimetric position angles and with the distribution of OH maser sources. Interferometric measurements are shown to be relevant to studies of star/planet formation and to studies of mass loss in long-period variables