arXiv subjects

E. M. L. Humphreys

Publications and source records attributed to E. M. L. Humphreys.

40 records · Page 3Linked to original sources

arXiv2003

Recent Progress on a New Geometric Distance to NGC 4258

We report on our ongoing, high-resolution study of H2O masers in the innermost parsec of NGC 4258. Over thirty epochs of VLBA and VLA data, taken over six years, are being used to monitor the velocities, accelerations, positions and proper motions of water masers rotating in a warped, Keplerian disk about a supermassive central object. Our extensive monitoring results in an improved accuracy in the distance determination to this galaxy (i) via a reduction in experimental random errors due to a longer time-baseline than in previous work; and (ii) via a better modeling of sources of systematic error, such as disk eccentricity. These data can therefore yield an extremely accurate geometric distance to NGC 4258.

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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

First SIMBA observations toward CH3OH masers

We report SIMBA 1.2 mm dust continuum observations of the environments of eight methanol maser sources, all discovered during spatially fully-sampled, untargeted surveys of the galactic plane. We summarise our search for possible associations of the masers with IR sources (IRAS and MSX) and find that it is not always possible to make definite associations. A preliminary characterisation of the IR sources found in the maser neighbourhood is given according their position in the [60-25] -- [25-12] colour-colour diagram.

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arXiv1997

Testing the clump model of SiO maser emission

Building on the detection of J=7-6 SiO maser emission in both the v=1 and v=2 vibrational states towards the symbiotic Mira, R Aquarii, we have used the James Clerk Maxwell Telescope to study the changes in the SiO maser features from R Aqr over a stellar pulsational period. The observations, complemented by contemporaneous data taken at 86 GHz, represent a test of the popular thermal-instability clump models of SiO masers. The `clump' model of SiO maser emission considers the SiO masers to be discrete emitting regions which differ from their surroundings in the values of one or more physical variables (SiO abundance, for example). We find that our observational data are consistent with a clump model in which the appearance of maser emission in the J=7-6 transitions coincides with an outward-moving shock impinging on the inner edge of the maser zone.

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