Distances of Stars by mean of the Phase-lag Method

Variable OH/IR stars are Asymptotic Giant Branch (AGB) stars with an optically thick circumstellar envelope that emit strong OH 1612 MHz emission. They are commonly observed throughout the Galaxy but also in the LMC and SMC. Hence, the precise inference of the distances of these stars will ultimately result in better constraints on their mass range in different metallicity environments. Through a multi-year long-term monitoring program at the Nancay Radio telescope (NRT) and a complementary high-sensitivity mapping campaign at the eMERLIN and JVLA to measure precisely the angular diameter of the envelopes, we have been re-exploring distance determination through the phase-lag method for a sample of stars, in order to refine the poorly-constrained distances of some and infer the currently unknown distances of others. We present here an update of this project.Comment: 4 pages, 1 figure, 2 tables, to appear in the Proceedings of the IAU Symposium No. 336: Astrophysical Masers: Unlocking the Mysteries of the Univers

The Masers Towards IRAS 20126+4104

We present MERLIN observations of OH, water and methanol masers towards the young high mass stellar object IRAS 20126+4104. Emission from the 1665-MHz OH, 22-GHz H_2O and 6.7-GHz CH_3OH masers is detected and all originates very close to the central source. The OH and methanol masers appear to trace part of the circumstellar disk around the central source. The positions and velocities of the OH and methanol masers are consistent with Keplerian rotation around a central mass of ~ 5 Msun. The water masers are offset from the OH and methanol masers and have significantly changed since they were last observed, but still appear to be associated to the outflow from the source. All the OH masers components are circularly polarised, in some cases reaching 100 percent while some OH components also have linear polarisation. We identify one Zeeman pair of OH masers and the splitting of this pair indicates a magnetic field of strength ~ 11 mG within ~ 0.5" (850 AU) of the central source. The OH and methanol maser emission suggest that the disk material is dense, n > 10^6 cm^-3, and warm, T > 125 K and the high abundance of methanol required by the maser emission is consistent with the evaporation of the mantles on dust grains in the disk as a result of heating or shocking of the disk materialComment: 9 pages, 7 figures and 6 table

Maser Flare Simulations from Oblate and Prolate Clouds

We investigated, through numerical models, the flaring variability that may arise from the rotation of maser clouds of approximately spheroidal geometry, ranging from strongly oblate to strongly prolate examples. Inversion solutions were obtained for each of these examples over a range of saturation levels from unsaturated to highly saturated. Formal solutions were computed for rotating clouds with many randomly chosen rotation axes, and corresponding averaged maser light curves plotted with statistical information. The dependence of results on the level of saturation and on the degree of deformation from the spherical case were investigated in terms of a variability index and duty cycle. It may be possible to distinguish observationally between flares from oblate and prolate objects. Maser flares from rotation are limited to long timescales (at least a few years) and modest values of the variability index ($\lesssim 100$), and can be aperiodic or quasi-periodic. Rotation is therefore not a good model for H$_2$O variability on timescales of weeks to months, or of truly periodic flares.Comment: 11 pages, 12 figures, accepted for publication in MNRA

Insight into the OH polarimetric structure of OH 26.5+0.6

We present the first view of the magnetic field structure in the OH shell of the extreme OH/IR star OH 26.5+0.6. MERLIN interferometric observations of this object were obtained in December 1993 in full polarisation, at 1612, 1665 and 1667 MHz. The maser spots show a spheroidal distribution both at 1612 and 1667 MHz, while at 1665 MHz emission from the blue-shifted maser peak is concentrated on the stellar position, and the red-shifted peak emission exhibits a filamentary structure oriented on a SE-NW axis. The linear polarisation in both main lines is rather faint, ranging from 9 to 20% at 1665 MHz and from 0 to 30% at 1667 MHz. At 1612 MHz most maser spots exhibit a similar range of linear polarisation although those in the outermost parts of the envelope reach values as high as 66%. This is particularly apparent in the southern part of the shell. The detailed distribution of the polarisation vectors could only be obtained at 1612 MHz. The polarisation vectors show a highly structured distribution indicative of a poloidal magnetic field inclined by 40-60$^\circ$ to the line of sight. The velocity distribution of the maser spots with respect to the radial distance is well explained by an isotropic outflow at constant velocity in the case of a prolate shaped spheroid envelope, also tilted about 45-65$^\circ$ to the line of sight.Comment: 20 pages, 16 figures, accepted for publication in MNRA

A New 3D Maser Code Applied to Flaring Events

We set out the theory and discretization scheme for a new finite-element computer code, written specifically for the simulation of maser sources. The code was used to compute fractional inversions at each node of a 3-D domain for a range of optical thicknesses. Saturation behaviour of the nodes with regard to location and optical depth were broadly as expected. We have demonstrated via formal solutions of the radiative transfer equation that the apparent size of the model maser cloud decreases as expected with optical depth as viewed by a distant observer. Simulations of rotation of the cloud allowed the construction of light-curves for a number of observable quantities. Rotation of the model cloud may be a reasonable model for quasi-periodic variability, but cannot explain periodic flaring.Comment: Accepted for publication in MNRAS, 2018 February 23, 12 pages, 6 figure

OH spectral evolution of oxygen-rich late-type stars

We investigated the main-line spectral evolution with shell thickness of oxygen rich AGB stars. The study is based on a sample of 30 sources distributed along the IRAS colour-colour diagram. The sources were chosen to trace the Miras with thick shells and the whole range of OH/IR stars. The Miras exhibit a 1665 MHz emission strength comparable to that at 1667 MHz. Even though the Miras of the study have quite thick shells, their spectral characteristics in both main lines attest to a strong heterogeneity in their OH shell with, in particular, the presence of significant turbulence and acceleration. The expansion velocity has been found to be about the same at 1665 and 1667 MHz, taking into account a possible velocity turbulence of 1-2km/s at the location of the main-line maser emission. An increase in the intensity ratio 1667/1665 with shell thickness has been found. A plausible explanation for such a phenomenon is that competitive gain in favour of the 1667 MHz line increases when the shell is getting thicker. There is an evolution in the spectral profile shape with the appearance of a substantial inter-peak signal when the shell is getting thicker. Also, inter-peak components are found and can be as strong as the external standard peaks when the shell is very thick. This trend for an increase of the signal in between the two main peaks is thought to be the result of an increase of the saturation with shell thickness. All sources but two - a Mira and an OH/IR star from the lower part of the colour-colour diagram - are weakly polarized. The strong polarization observed for those two particular objects is thought to be the result of perturbations in their shells.Comment: 19 pages, 12 figures, accepted for publication in A&

Astrometry of OH/IR stars using 1612 MHz hydroxyl masers. I. Annual parallaxes of WX Psc and OH138.0+7.2

We report on the measurement of the trigonometric parallaxes of 1612 MHz hydroxyl masers around two asymptotic giant branch stars, WX Psc and OH138.0+7.2, using the NRAO Very Long Baseline Array with in-beam phase referencing calibration. We obtained a 3-sigma upper limit of <=5.3 mas on the parallax of WX Psc, corresponding to a lower limit distance estimate of >~190 pc. The obtained parallax of OH138.0+7.2 is 0.52+/-0.09 mas (+/-18%), corresponding to a distance of 1.9(+0.4,-0.3) kpc, making this the first hydroxyl maser parallax below one milliarcsecond. We also introduce a new method of error analysis for detecting systematic errors in the astrometry. Finally, we compare our trigonometric distances to published phase-lag distances toward these stars and find a good agreement between the two methods.Comment: Preprint, accepted for publication in The Astronomical Journal (January 17, 2017