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P. J. Diamond

Publications and source records attributed to P. J. Diamond.

49 records · Page 3Linked to original sources
arXiv2002

Numerical simulations of stellar SiO maser variability. Investigation of the effect of shocks

A stellar hydrodynamic pulsation model has been combined with a SiO maser model in an attempt to calculate the temporal variability of SiO maser emission in the circumstellar envelope (CE) of a model AGB star. This study investigates whether the variations in local physical conditions brought about by shocks are the predominant contributing factor to SiO maser variability because, in this work, the radiative part of the pump is constant. We find that some aspects of the variability are not consistent with a pump provided by shock-enhanced collisions alone. In these simulations, gas parcels of relatively enhanced SiO abundance are distributed in a model CE by a Monte Carlo method, at a single epoch of the stellar cycle. From this epoch on, Lagrangian motions of individual parcels are calculated according to the velocity fields encountered in the model CE during the stellar pulsation cycle. The potentially masing gas parcels therefore experience different densities and temperatures, and have varying line-of-sight velocity gradients throughout the stellar cycle, which may or may not be suitable to produce maser emission. At each epoch (separated by 16.6 days), emission lines from the parcels are combined to produce synthetic spectra and VLBI-type images. We report here the results for v=1, J=1-0 (43-GHz) and J=2-1 (86-GHz) masers.

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arXiv2002

Strongly decelerated expansion of SN1979C

We observed SN1979C in M100 on 4 June 1999, about twenty years after explosion, with a very sensitive four-antenna VLBI array at the wavelength of 18cm. The distance to M100 and the expansion velocities are such that the supernova cannot be fully resolved by our Earth-wide array. Model-dependent sizes for the source have been determined and compared with previous results. We conclude that the supernova shock was initially in free expansion for 6 +/- 2 yrs and then experienced a very strong deceleration. The onset of deceleration took place a few years before the abrupt trend change in the integrated radio flux density curves. We estimate the shocked swept-up mass to be about 1.6 solar masses, assuming a standard density profile for the CSM. Such a swept-up mass for SN1979C suggests a mass of the hydrogen-rich envelope ejected at explosion no larger than $M_{\rm env} \sim$ 0.9 solar masses. If SN1979C originated in a binary star the low value of M_env suggests that the companion of the progenitor star stripped off most of the hydrogen-rich envelope mass of the presupernova star prior to the explosion.

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arXiv2001

A search for extragalactic H2O maser emission towards IRAS galaxies - Detection of a maser from an infrared-luminous merger, NGC6240

We report the result of an on-going survey for 22 GHz H2O maser emission towards infrared luminous galaxies. The observed galaxies were selected primarily from the IRAS bright galaxy sample. The survey has resulted in the detection of one new maser. The new maser was discovered towards the [U]LIRG/merger galaxy NGC6240, which contains a LINER nucleus. This is the first detection of an H2O maser towards this class of galaxy, they are traditionally associated with OH megamaser sources. The detected maser emission is highly redshifted (~ 260-300 km/s) with respect to the adopted systemic velocity of the galaxy, and we identified no other significant emission at velocities < +- 500 km/s relative to the systemic velocity. The presence of high-velocity maser emission implies the possible existence of a rotating maser disk formed in the merging process. The large maser luminosity (~ 40 Lo) suggests that an active galactic nucleus could be the energy source that gives rise to the water emission. Alternatively, the maser emission could be associated with the previously observed double radio source in the centre of the galaxy. Interferometric observations with high angular resolution will be able to clarify the origin of the new maser.

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arXiv2001

Circular Polarization of Circumstellar Water Masers around S Per

We present the first circular polarization measurements of circumstellar H2O masers. Previously the magnetic field in circumstellar envelopes has been estimated using polarization observations of SiO and OH masers. SiO masers are probes of the high temperature and density regime close to the central star. OH masers are found at much lower densities and temperatures, generally much further out in the circumstellar envelope. The detection of the circular polarization of the (6_16 - 5_23) rotational transition of the H2O maser could be attributed to Zeeman splitting due to the magnetic field in the intermediate temperature and density regime. The fields inferred here agree well with predicted values for a combination of the r^{-2} dependence of a solar-type magnetic field, and the coupling of the field to the high density masing regions. We also discuss the unexpected narrowing of the circular polarization spectrum.

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arXiv2000

Global VLBI Observations of Compact Radio Sources in M82

Observations of the starburst galaxy, M82, have been made with a 20-station global VLBI array at $λ$18cm. Maps are presented of the brightest young supernova remnants (SNR) in M82 and the wide-field mapping techniques used in making images over a field of view of $\sim$1 arcminute with 3 milliarcsecond resolution are discussed. A limit has been placed on the power law deceleration of the young SNR, 43.31+592 with an index greater than 0.73 $\pm$ 0.11 from observations with the European VLBI Network. Using the global array we have resolved compact knots of radio emission in the source which, with future global observations, will enable better constraints to be placed on the expansion parameters of this SNR. The latest global observations have also provided high resolution images of the most compact radio source in M82, 41.95+575. We determine an upper limit to the radial expansion rate along the major axis of 2000 km s$^{-1}$. However, the new images also show structure resembling that of collimated ejection which brings into question the previous explanation of the source as being due to the confinement of a supernova by a high density circumstellar medium. It is apparent that we are now able to image the brightest supernova remnants in M82 with a linear scale which allows direct comparison with galactic SNR such as Cassiopeia A.

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arXiv2000

VLBI Astrometry of the Stellar Image of U Herculis, Amplified by the 1667 OH Maser

The OH 1667 MHz maser in the circumstellar shell around the Mira variable U Herculis has been observed with the NRAO Very Long Baseline Array (VLBA) at 6 epochs, spread over 4 years. Using phase referencing techniques the position of the most blue-shifted maser spot was monitored with respect to two extra-galactic radio sources. The absolute radio positions of the maser can be compared with the stellar optical position measured by the Hipparcos satellite to 15 mas accuracy. This confirms the model in which one of the maser spots corresponds to the stellar continuum, amplified by the maser. The stellar proper motion and the annual parallax (5.3 +/- 2.1 mas) were measured.

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arXiv1999

A 4% Geometric Distance to the Galaxy NGC4258 from Orbital Motions in a Nuclear Gas Disk

The water maser in the mildly active nucleus in the nearby galaxy NGC4258 traces a thin, nearly edge-on, subparsec-scale Keplerian disk. Using the technique of very long baseline interferometry, we have detected the proper motions of these masers as they sweep in front of the central black hole at an orbital velocity of about 1100 km/s. The average maser proper motion of 31.5 microarcseconds per year is used in conjunction with the observed acceleration of the masers to derive a purely geometric distance to the galaxy of 7.2 +- 0.3 Mpc. This is the most precise extragalactic distance measured to date, and, being independent of all other distance indicators, is likely to play an important role in calibrating the extragalactic distance scale.

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arXiv1998

Coexisting conical bipolar and equatorial outflows from a high-mass protostar

The BN/KL region in the Orion molecular cloud is an archetype in the study of the formation of stars much more massive than the Sun. This region contains luminous young stars and protostars, but it is difficult to study because of overlying dust and gas. Our basic expectations are shaped to some extent by the present theoretical picture of star formation, the cornerstone of which is that protostars acrete gas from rotating equatorial disks, and shed angular momentum by ejecting gas in bipolar outflows. The main source of the outflow in the BN/KL region may be an object known as radio source I, which is commonly believed to be surrounded by a rotating disk of molecular material. Here we report high-resolution observations of silicon monoxide (SiO) and water maser emission from the gas surrounding source I; we show that within 60 AU (about the size of the Solar System), the region is dominated by a conical bipolar outflow, rather than the expected disk. A slower outflow, close to the equatorial plane of the protostellar system, extends to radii of 1,000 AU.

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arXiv1998

Methanol Masers as Tracers of Circumstellar Disks

We show that in many methanol maser sources the masers are located in lines, with a velocity gradient along them which suggests that the masers are situated in edge-on circumstellar, or protoplanetary, disks. We present VLBI observations of the methanol maser source G309.92+0.48, in the 12.2 GHz transition, which confirm previous observations that the masers in this source lie along a line. We show that such sources are not only linear in space but, in many cases, also have a linear velocity gradient. We then model these and other data in both the 6.7 GHz and the 12.2 GHz transition from a number of star formation regions, and show that the observed spatial and velocity distribution of methanol masers, and the derived Keplerian masses, are consistent with a circumstellar disk rotating around an OB star. We consider this and other hypotheses, and conclude that about half of these methanol masers are probably located in edge-on circumstellar disks around young stars. This is of particular significance for studies of circumstellar disks because of the detailed velocity information available from the masers.

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arXiv1998

Polarimetric Observations of the Masers in NGC 4258: An Upper Limit on the Large-Scale Magnetic Field 0.2 pc from the Central Engine

We report VLA 1 sigma upper limits of 1.5% and 3% on the intrinsic circular and linear fractional polarizations, respectively, of the water vapor maser emission 0.2 pc from the central engine of NGC 4258. A corresponding 0.5% upper limit on any Zeeman-splitting-induced circular polarization translates to a 1 sigma upper limit on the parallel, or toroidal, component of the magnetic field of 300 mG. Assuming magnetic and thermal pressure balance in the disk, this magnetic field upper limit corresponds to a model-dependent estimate of the accretion rate through the molecular disk of 10^-1.9 alpha solar masses per year for the case where the magnetic field lies along the line of sight.

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arXiv1996

VLBI observations of the 6.7 and 12.2 GHz methanol masers associated with NGC 6334F

We present milliarcsecond resolution images of the 6.7 and 12.2 GHz methanol maser emission associated with the well-known star formation region NGC 6334F. The images agree well with previous lower resolution observations, but detect approximately double the number of spots seen in the earlier work. Comparison of the relative positions of the 6.7 and 12.2 GHz maser spots shows that five of them are coincident to within the positional accuracy of these observations (approximately 4 milli-arcsec). Menten et al. (1992) observed similar positional coincidence for W3(OH) and in each case the flux density of the 6.7 GHz maser spot was greater than that of the 12.2 GHz methanol maser spot. However, for NGC 6334F several of the coincident maser spots have a larger flux density at 12.2 GHz than at 6.7 GHz. We also detected several 12.2 GHz methanol maser spots with no coincident 6.7 GHz emission. This implies that, although the 6.7 GHz methanol masers usually have a greater flux density than their 12.2 GHz counterparts, regions within the gas cloud exist where the conditions are more favourable for 12.2 than for 6.7 GHz methanol maser emission. If the conditions which give rise to the various observed ratios can be determined, then VLBI images of class II methanol masers will allow the physical conditions of the star formation region to be probed with unprecedented resolution.

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arXiv1994

Anisotropic Scattering of OH/IR Stars Towards the Galactic Center

We have made angular broadening measurements in the satellite line of the OH molecule at 1612 MHz of 8 OH/IR stars located near the Galactic Center with the Very Large Array. Early observations with the Very Long Baseline Array had revealed a region of pronounced scattering approximately centered on SgrA*. These new observations show that the compact line emission from these stars is scattered by an amount that is two to three times larger than had been previously seen. Additionally, we measure non-circular scattering disks for these objects, evidence that the electron density variations are distributed anisotropically. The present data suggest that the enhanced turbulence is located near the Galactic Center. However, we cannot completely rule out that the scattering is caused by an unrelated region along the line of sight.

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