ASTROMETRICALLY REGISTERED SIMULTANEOUS OBSERVATIONS OF THE 22 GHz H₂O AND 43 GHz SiO MASERS TOWARD R LEONIS MINORIS USING KVN AND SOURCE/FREQUENCY PHASE REFERENCING

Abstract

Oxygen-rich Asymptotic Giant Branch (AGB) stars can be intense emitters of SiO (v=1 and 2, J=1→0) and H 2 O maser lines at 43 and 22 GHz, respectively. VLBI observations of the maser emission provide a unique tool to probe the innermost layers of the circumstellar envelopes in AGB stars. Nevertheless, the difficulties in achieving astrometrically aligned H 2 O and v=1 and v=2 SiO maser maps have traditionally limited the physical constraints that can be placed on the SiO maser pumping mechanism. We present phase referenced simultaneous spectral-line VLBI images for the SiO v=1 and v=2, J=1→0, and H 2 O maser emission around the AGB star R LMi, obtained from the Korean VLBI Network (KVN). The simultaneous multi-channel receivers of the KVN offer great possibilities for astrometry in the frequency domain. With this facility we have produced images with bona-fide absolute astrometric registration between high frequency maser transitions of different species to provide the positions of the H 2 O maser emission, and the centre of the SiO maser emission, and hence reducing the uncertainty in the proper motion for R LMi by an order of magnitude over that from Hipparcos. This is the first successful demonstration of source frequency phase referencing for mm-VLBI spectral-line observations and also where the ratio between the frequencies is not an integer.

High-Precision Astrometric Millimeter Very Long Baseline Interferometry Using a New Method for Multi-Frequency Calibration

“…The Source Frequency Phase Referencing method, which has been described in detail elsewhere Rioja et al , 2014Rioja et al , 2015Dodson et al 2014), consists of two calibration steps. In a first step, the observations at the higher frequency bands are calibrated using near-simultaneous Rioja et al 2014) or simultaneous (Rioja et al , 2015Dodson et al 2014) observations at a lower frequency band, for each source.…”

“…In a first step, the observations at the higher frequency bands are calibrated using near-simultaneous Rioja et al 2014) or simultaneous (Rioja et al , 2015Dodson et al 2014) observations at a lower frequency band, for each source. This is done for all frequency pairs which have an integer 2 frequency ratio, by which the low frequency calibration phase solutions are scaled.…”

FIRST PARALLAX MEASUREMENTS TOWARD A 6.7 GHz METHANOL MASER WITH THE AUSTRALIAN LONG BASELINE ARRAY—DISTANCE TO G 339.884−1.259.

“…This crucial step was required in order to prevent the introduction of spurious R-L polarization phase differences into the dataset, and the efficacy of this procedure (see Appendix B) is demonstrated by the similarity in the phase solutions between the different polarizations in Figure 2. Similar phase transfer issues between datasets with differing frequency properties have previously been resolved using comparable methods (Rioja et al 2008;Dodson et al 2014). Figure 1 shows that there are multiple strong (>100 Jy) 6.7 GHz methanol maser components in G 339.884-1.259 that offer potential spectral channels for astrometry.…”

The Power of (Near) Simultaneous Multi-Frequency Observations for mm-VLBI and Astrometry

“…Fast switching limits the maximum frequency for the method, as the rates become unmanageable. It is highly recommended that the frequencies are at an integer ratio ( [31], but it is possible, with care, for non-integer ratios). SFPR requires two sources, which can both be detected at the lower frequency and at least one of which can be detected at the higher frequency (although the longer coherence time from the FPT helps with this step).…”