MHz OH Masers Research Articles

Errata: VLA Observations of 1612 and 1665 MHz OH Masers Associated with IRC +10420

Errata: VLA Observations of 1612 and 1665 MHz OH Masers Associated with IRC +10420

Kinematics of W3(OH) - First proper motions of OH masers from VLBI measurements

view Abstract Citations (86) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Kinematics of W3(OH): First Proper Motions of OH Masers from VLBI Measurements Bloemhof, E. E. ; Reid, M. J. ; Moran, J. M. Abstract The relative proper motions of the 1665 OH maser spots in W3(OH) were measured by comparing data from VLBI experiments at epochs 1978 and 1986. A significant ordered motion with apparent transverse velocities of a few km/s is found which provides important direct information about the kinematics in the immediate vicinity of a recently formed massive star. The possibility that the OH maser spots are in a collapsing circumstellar envelope is ruled out. The observed pattern of motions suggests expansion from the ultracompact H II region seen in radio continuum emission. Alternatively, a velocity field resembling that expected for a cometary bow shock is consistent with the data. Significant temporal variations of the strengths of maser spots were measured. A map of the magnetic field in the OH maser clusters of W3(OH) is also obtained. Publication: The Astrophysical Journal Pub Date: October 1992 DOI: 10.1086/171807 Bibcode: 1992ApJ...397..500B Keywords: Hydroxyl Emission; Interstellar Masers; Very Long Base Interferometry; Astronomical Maps; Bow Waves; Field Strength; H Ii Regions; Interstellar Magnetic Fields; Zeeman Effect; Astrophysics; ISM: INDIVIDUAL ALPHANUMERIC: W3; ISM: KINEMATICS AND DYNAMICS; MAGNETIC FIELDS; MASERS; TECHNIQUES: INTERFEROMETRIC full text sources ADS | data products SIMBAD (4)

VLA observations of 1612 and 1665 MHz OH masers associated with IRC + 10420

view Abstract Citations (49) References (65) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS VLA Observations of 1612 and 1665 MHz OH Masers Associated with IRC +10420 Nedoluha, G. E. ; Bowers, P. F. Abstract Results are presented of simultaneous spectral line VLA observations at 1612 and 1665 MHz in right and left circular polarizations obtained for the OH masers associated with the highly luminous supergiant IRC + 10420. Comparison with previous data indicates that the 1612 MHz emission has increased by at least a factor of 3 at all velocities over the past 14 yr and by a factor of 60 at 55 km/s. Meanwhile, the 1665 MHz emission has varied erratically, and the 1667 MHz emission has declined by a factor of 3. The unusual temporal behavior is similar to that observed for the Mira variable U Orionis. Most of the 1665 MHz maser components are distributed in a shell with a radius of 1.3 arcsec, marginally smaller than the value of 1.5 arcsec for the main shell at 1612 MHz. There are indications of clumps or portions of shells at much smaller and larger radii. At the radius of the maser shells (about 8000 AU), the magnetic field is estimated to be 15-1/6 mG. Publication: The Astrophysical Journal Pub Date: June 1992 DOI: 10.1086/171423 Bibcode: 1992ApJ...392..249N Keywords: Hydroxyl Radicals; Interstellar Masers; Very Large Array (Vla); Circular Polarization; Data Reduction; Interstellar Gas; Mira Variables; Oblate Spheroids; Supergiant Stars; Astrophysics; MASERS; POLARIZATION; STARS: INDIVIDUAL ALPHANUMERIC: IRC; 10420 full text sources ADS | data products SIMBAD (2) Related Materials (1) Erratum: 1992ApJ...399..743N

Circular polarization in 1612 MHz OH maser emission from OH/IR stars

In order to study the incidence of circular polarization in the 1612 MHz OH emission from envelopes of OH/IR stars, the Arecibo radio telescope was used to obtain high-resolution, high signal-to-noise 1612 MHz spectra in both circular polarizations for several OH/IR stars. The method of Troland and Heiles (1982) was used to estimate the intensities of magnetic fields in the envelopes. The fields appear to have complex structures, and intensities on the order of 1-100 microG. The spectra are weakly polarized, probably as a result of weakly polarized emission from individual emitting elements. Integration over the Stokes parameter V yields nonzero results for three of the stars, suggesting that another polarizing mechanism (in addition to simple Zeeman splittings) is at work.

Large proper motions in the remarkable triple radio source in Serpens

view Abstract Citations (106) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Large Proper Motions in the Remarkable Triple Radio Source in Serpens Rodriguez, L. F. ; Curiel, S. ; Moran, J. M. ; Mirabel, I. F. ; Roth, M. ; Garay, G. Abstract The triple radio source in the Serpens star formation region consists of a central object and two lobes with spectral indices characteristic of nonthermal emission. Measurements made at the Arecibo Observatory and the VLA indicate that this triple source is probably associated with the Serpens star-forming region. A weak, circularly polarized 1667 MHz OH maser detected at Arecibo has been observed with the VLA and found to coincide in position to within 1 arcsec with the central source. The position of this central source also falls within the position error ellipsoid of IRAS 18273+0113. VLA observations at 6 cm clearly show that the outer components are moving away from the central source. The angular motion of each component with respect to the central one is 1.3 + or - 0.2 arcsec, implying velocities of 300 + or - 100 km/s at a distance of 500 + or - 200 pc. Publication: The Astrophysical Journal Pub Date: November 1989 DOI: 10.1086/185585 Bibcode: 1989ApJ...346L..85R Keywords: Molecular Clouds; Radio Sources (Astronomy); Star Formation; Emission Spectra; Herbig-Haro Objects; Hydroxyl Emission; Pre-Main Sequence Stars; Very Large Array (Vla); Water Masers; Astrophysics; INFRARED: SOURCES; NEBULAE: GENERAL; RADIO SOURCES: GENERAL; STARS: PRE--MAIN-SEQUENCE full text sources ADS | data products SIMBAD (2)

OH Maser Emission from Young Planetary Nebulae

OH/IR stars are now generally accepted to be progenitors of planetary nebulae. We have carried out a project to try to find objects in the transition phase between OH/IR stars and PN. Transition objects should be characterized by having both continuum radio emission and a 1612 MHz OH maser line. The continuuum emission would indicate that the central star has become hot enough to ionize part of the envelope, while the OH maser indicates that a large part of the envelope is still neutral. Only one such object was known, namely Vy 2-2.

Envelope structure of the Mira variable U Orionis

The authors have obtained simultaneous VLA measurements of the distributions of 1612 and 1655 MHz OH masers associated with U Orionis. At 1665 MHz they find an incomplete ring distribution near the stellar velocity of -39 km s<SUP>-1</SUP>. The observations indicate that radial expansion is the dominant kinematic component at the radius of the OH masers (≡50 AU). From consideration of all available OH, H<SUB>2</SUB>O, and SiO data, it is found that a model of axisymmetric, biconical OH density concentrations embedded in a spherical outflow at a constant velocity provides a qualitative basis for understanding the overall properties of the profile structures and the angular distributions.

Kinematic structure of OH/IR stars

The discovery of OH maser emission in late-type stars has created the controversy of the kinematic nature of the sources and for several years it was not clear whether the characteristic doublepeak profile of OH/IR stars is due to rotation, shock waves, expansion or collapse. This problem was finally resolved when quantitative calculations taking into account the physics of masers show that no model other than a uniformly expanding model can produce the double-peak profile. With the development of VLA/VLBI observations for OH/IR stars, the availability of OH intensity distribution from interferometric data in addition to the total flux from single-dish measurements allows a more rigorous testing of the kinematic models of OH/IR stars. It is also possible that the physical parameters responsible for the creation of the maser can be determined with higher accuracy than before. In this paper, a kinematic model is constructed for 1612 MHz OH maser emission in OH/IR stars. The spatial distributions for OH maser intensity are calculated from a model of a sphericallysymmetric uniformly-expanding circumstellar shell. By comparing VLA/VLBI maps of OH/IR stars with model results, the acceptable of combinations of physical parameters M/Ve, range (nH2)ma x , (nH2)min' fOH (=[nOH]/[nH2]) are derived. The theoretical relations between OH shell radius Ro and mass loss rate H and between OH maser luminosity LOH and M are also obtained. These relations are in good agreement with empirical relations established by Bowers et al. (1983) and Baud and Habing (1983). The physical conditions

Detection of the first extra-galactic OH/IR star

A search has been made for 1612 MHz OH maser emission from OH/IR stars in the Magellanic Clouds. Candidate objects were selected mainly on the basis of their 25μm flux densities and the 25 to 12μm flux ratio as given in the IRAS point source catalog; two known long-period variables and two HII regions (30 Doradus and N159) were also examined. One OH source (IRAS 04553-6825) was detected, this being the first OH/IR star found in the Magellanic Clouds. Upper limits were placed on the flux for 17 other sources. The expansion velocity of the circumstellar material surrounding IRAS 04553-6825, as indicated by the OH peak separation of 11 km s−1, is surprisingly small compared to Galactic sources of similar bolometric and OH luminosity. The OH intensity of IRAS 04553-6825, and the upper flux limits placed on many of the other objects examined, indicate that Magellanic Cloud OH/IR stars do not emit OH as strongly as their Galactic counterparts of similar 25μm/12μm flux ratio. Both the low expansion velocity of IRAS 04553-6825 and the low OH intensity of the Magellanic Cloud infrared sources may be explained by the low metal abundance in the Clouds.

Saturation and beaming in astrophysical masers. III - Asymmetrically pumped masers

There is reason to believe that the physical conditions in many astrophysical masers are far from homogeneous. In particular, if the maser pump involves the absorption of infrared photons, the pump rate may vary rapidly (perhaps exponentially) across the cloud. The consequences of such variation of the pump rate for the maser radiation are calculated, and it is shown that there are substantial asymmetries in the output of the maser. A general result is that the maser radiation is preferentially emitted back toward the source of pump radiation. The implication of this result for the interpretation of the circumstellar 1612 MHz OH masers is briefly discussed.

Radiative transfer in circumstellar 1612 MHz OH masers

A detailed investigation of radiative transfer in the 1612 MHz OH maser in the expanding envelopes around evolved stars is made in the context of the standard model in which the mass outflow is windlike. The most sensitive diagnostic of the models is the maser emission at the velocity of the star for which VLA and MERLIN data exist. Fully saturated models are analytically eliminated. Detailed numerical modeling also eliminates partially saturated models. The need to examine alternative models for the mass outflow is suggested by this conclusion. A model for the mass outflow from these stars is advocated in which all mass is lost by the ejection of blobs of material. These blobs, acted on by radiation pressure on grains, evolve into platelike structures in the OH maser region. It is shown that the peculiar difficulties of the standard model for the OH masers do not occur in this picture.

VLA observations of 1612 and 1667 MHz OH masers associated with IRC +10420

view Abstract Citations (41) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS VLA observations of 1612 and 1667 MHz OH masers associated with IRC +10420. Bowers, P. F. Abstract The star IRC +10420 is a high-luminosity (F8 Ia) star for which evidence of a substantial circumstellar dust shell has been obtained. This star is the only known OH/IR star with a spectral type earlier than M. It is also a member of the class of supergiant OH/IR stars. The present investigation is concerned with simultaneous Very Large Array (VLA) observations of the ground-state rotational transitions of OH maser emission at 1612 and 1667 MHz. An attempt is made to determine the distribution of the circumstellar material as well as the relative locations of emission from the two OH transitions. Attention is given to the velocity structure of the microwave emission, the spatial structure of the OH emission, the position of the star, a model of the circumstellar envelope, and the relative location of emission at 1612 and 1667 MHz. Publication: The Astrophysical Journal Pub Date: April 1984 DOI: 10.1086/161895 Bibcode: 1984ApJ...279..350B Keywords: Hydroxyl Emission; Infrared Spectra; Interstellar Masers; Peculiar Stars; Radio Astronomy; Supergiant Stars; Astrometry; Distance; Line Spectra; Optical Pumping; Spectral Energy Distribution; Stellar Envelopes; Very Large Array (Vla); Astrophysics full text sources ADS | data products SIMBAD (1)

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.

The three-dimensional structure of a circumstellar maser

A 900-day observation of the line profile of the unusual 1667 MHz OH maser in the bipolar nebula OH 231.8+4.2 has indicated that it varies sinusoidally by a factor of about 3, with a period of 684 + or - 40 days. This is in agreement with recent IR observations. The phase of the variations varies strongly with velocity. By interpreting phase differences in terms of the line-of-sight light travel time, the emission at each of 15 velocity intervals was assigned a relative line-of-sight distance. The overall, 70-day phase lag across the profile implies that this is one of the largest circumstellar masers, with a radius of at least 6000 AU, and that the source is expanding, since the redshifted emission has the greatest phase lag and is thus on the far side of the source. This data, together with maps previously made of OH 231.8+4.2 with the VLA at a number of velocities within the 1667 MHz line profile, reveals the three-dimensional structure of the maser. This structure agrees with the qualitative model previously suggested on the basis of VLA data alone.

H2O Masers in the Galactic Plane. I. Longitude 340º to the Galactic Centre

Twenty-five new H20 masers in the southern galactic plane have been discovered using a 22 GHz maser receiver on the Parkes telescope. The search was conducted at the locations of all the known 1665 MHz OH masers in the longitude range 340� through the galactic centre to 2�; of the 46 type I OH masers 38 are now found to have an H20 maser nearby; three other main-line OH emitters, possibly unusual late-type stars, also show H20 emission. We give an up-to-date tabulation of all 46 currently known H20 masers in the region which includes for completeness several H20 masers with no readily detectable associated type I OH emission.

VLBI measurements of the relative position of the 1665 MHz and 1667 MHz OH masers toward W49N and NGC 6334N

VLBI measurements of the relative position of the 1665 MHz and 1667 MHz OH masers toward W49N and NGC 6334N

OBSERVATIONS OF COOL STARS AT 20,25 AND 33 MUM.

ABSTRACT Broad-band 20-33 micron photometry is presented for a sample of cool stars. The long-wavelength flux is interpreted in terms of dust-shell models. It is found that the amount of silicate dust necessary to produce the observed 10 micron features can also produce the long-wavelength flux. The dust observations are used to derive lower limits for mass-loss rates. Sufficient 35 micron flux exists to pump the 1612 MHz OH masers for all stars observed in this sample, with the exception of R Aql. The data do not rule out the possibility of infrared pumping of the main-line masers.

VLA observations of a highly symmetric OH maser in a bipolar nebula

view Abstract Citations (65) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS VLA observations of a highly symmetric OH maser in a bipolar nebula. Morris, M. ; Bowers, P. F. ; Turner, B. E. Abstract The Very Large Array was used to map 1667 MHz OH maser emission from the bipolar nebula OH 231.8+4.2 at 23 distinct velocities within the unusual, 100 km/s wide profile. The source is large and well resolved, and displays ordered large-scale velocity gradients. At most velocities, the maser maps display an unmistakable symmetry about the bipolar axis defined by the optical and infrared reflection nebulae. Most of the data can be accounted for by an axisymmetric model in which the masering OH is concentrated toward the system's equatorial plane and is expanding radially away from the central star. The observation of complete rings of maser emission at some velocities, however, shows that the maser is also present at high latitudes above the equatorial plane. A model which incorporates these features plus other known aspects of bipolar nebulae is presented and discussed. Publication: The Astrophysical Journal Pub Date: August 1982 DOI: 10.1086/160198 Bibcode: 1982ApJ...259..625M Keywords: Hydroxyl Emission; Interstellar Masers; Nebulae; Radio Interferometers; Bipolarity; Emission Spectra; Very Large Array (Vla); Astrophysics full text sources ADS | data products SIMBAD (4) NED (1)

Extended OH maser emission from circumstellar shells and the envelope around OH 26.5 + 0.6

view Abstract Citations (50) References (11) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Extended OH maser emission from circumstellar shells and the envelopearound OH 26.5+0.6. Baud, B. Abstract The extended diffuse component of the 1612 MHz OH maser emission from circumstellar envelopes around OH/IR stars has been studied with the VLA spectral line system. The first results show extended emission, in some cases distributed over several arc seconds, with maser line brightness temperatures of 10,000-1,000,000 K. The emission from OH 26.5 + 0.6 shows an overall velocity structure consistent with an expanding circumstellar shell model. The inner and outer radii of the masing shell are 0.4-1.1 and 4.6 x 10 to the 16th cm, respectively. Large-scale spatial asymmetries appear in the shell, with evidence of a ringlike structure seen edge-on. This structure appears to be a reflection of actual density variations within the envelope, rather than the effect of velocity crowding or of variations in the infrared pump rate. New, independent determinations give distances for IRC + 10011 and OH 26.5 + 0.6 of 570 and 800 pc, respectively. Publication: The Astrophysical Journal Pub Date: November 1981 DOI: 10.1086/183678 Bibcode: 1981ApJ...250L..79B Keywords: Hydroxyl Emission; Infrared Stars; Interstellar Masers; M Stars; Stellar Envelopes; Stellar Mass Ejection; Brightness Temperature; Spectral Energy Distribution; Stellar Models; Supergiant Stars; Astronomy full text sources ADS | data products SIMBAD (21)

Observations of a circumstellar shell around the OH/IR star OH127.8–0.0

The 1,612 MHz OH maser emission associated with cool long-period variable stars is generally characterized by two main regions of emission separated in velocity by ∼10–50 km s−1. This has been taken to indicate that the OH lies in an expanding shell surrounding the star, the main peaks originating at the near or far sides of the shell along the line of sight where the maser column lengths may be greatest. Here we present the first high-angular resolution measurements of the OH emission from such a source which demonstrates dramatically that this picture is correct.