Compact vs. Scattered: Suitability of Southern 6.7GHz Methanol Masers for VLBI astrometry

TL;DR

This study assesses the suitability of southern 6.7 GHz methanol masers for VLBI astrometry by surveying 187 masers with the LBA and identifying 69 sources that exhibit sufficiently compact, bright maser spots for parallax measurements. It develops a core–halo visibility model to extract compact core sizes from limited uv-coverage and uses fringe-rate spectra to detect compact emission, revealing a notable dependence of compactness on Galactic position likely driven by interstellar scattering. The results show a substantial fraction of southern masers are viable for VLBI, with compact sources clustering in nearby spiral-arm regions, while distant arms and the Galactic center display more scatter-broadened, less compact emission; this underscores the need for higher-frequency follow-up (12.2 GHz, 22 GHz) or next-generation arrays to expand the astrometric sample. The work provides a practical path for expanding southern VLBI parallax programs and improves our understanding of how intrinsic maser structure and ISM scattering affect astrometric prospects in the Milky Way.

Abstract

The 6.7~GHz methanol maser transition is exclusively associated with young, high-mass stars and represents a potential target for astrometric studies, including accurate determination of their distance through trigonometric parallax measurements. There are more than 1000 known 6.7~GHz methanol maser sources in the Milky Way; however, not all are suitable targets for astrometric measurements. We have used the Long Baseline Array to observe 187 southern 6.7~GHz methanol masers and identify 69 sources with one or more maser spots that are sufficiently compact and intense to be suitable targets for very long baseline interferometry astrometry with current instruments. Maser compactness appears to be a strong function of Galactic position, with masers that are likely in nearby spiral arms being more compact, while those associated with distant arms or the central Galactic region being less compact - a relationship we associate with scatter broadening. This has implications for astrophysical masers, especially distant ones employed for Galactic astrometry.

Figures (6)

• Figure 1: Example of the fringe-rate peak finding process for the maser 337.153-0.395. Visibility amplitude (blue squares) and phase (red dots) time series are on the left, while the fringe rate spectrum is on the right. Top: A clear detection in both the time and fringe rate domains on the CD-PA baseline. Bottom: A more tentative detection on the AT-WA baseline. The visibility time series does not have an obvious detection, but the fringe rate spectrum does. The green error bars in the fringe rate spectrum indicate the uncertainty in the peak measurement of the amplitude, while the purple dashed line represents the rice noise, which we are treating as the baseline noise threshold.
• Figure 2: Examples of the 5 "grades" of maser. Left to right, top to bottom: A, B, C, D and F. Blue points give the measured amplitude of the emission in the fringe-rate spectrum for the specified velocity channel, with the error bars giving the $3\sigma$ scatter in the fringe-rate spectrum, and the blue solid line is the fit to these data. Black dashed lines indicate the noise threshold in the fringe rate spectrum. Red points indicate detections below the acceptable threshold, which were not used in the fitting.
• Figure 3: Top: Distribution of maser grade and size vs. Galactic longitude (between 270 and 360 deg) and Galactic latitude. Bottom: Cumulative fraction of each maser grade vs. increasing Galactic longitude.
• Figure 4: Left: Distribution of maser grade and size on a longitude vs. velocity diagram with spiral arms from Reid2019. Maser grades are coloured as before. Right: The same arms on a plan view of the Milky Way for reference. Arms are coloured as: Carina (purple), Centaurus (blue), Norma (red), "3-kpc" (yellow), and Perseus (black). Dashed lines indicate arms past the tangent point. Galactic longitudes between $270\rightarrow360$ in increments of 30 deg are shown with thin black lines, the Sun is shown at (0,8.15 kpc) with the sun-symbol, and the Galactic centre is shown as "GC".
• Figure 5: Cumulative fraction of each maser grade against decreasing absolute Galactic latitude $|b|$. More compact masers are preferentially found at larger Galactic latitudes than less compact masers.