Scattered OH Maser sources in the direction of W49N

Abstract

Abstract. OH masers spots are observed with the VLBA at 1612, 1665 and 1667 MHz. The orientation of the ellipses resulting from scattering are all nearly aligned perpendicular to the galactic plane. These ordered orientation could be due to the galactic magnetic field. Keywords. OH masers -W49N -interstellar scattering Observations and resultsUsing VLBA observations of 2005 October 6 towards W49N at the 1612 MHz, 1665 MHz and 1667 MHz OH frequencies, 215 maser spots were detected (Fig.1). LCP and RCP are observed simultaneously in 240 spectral channels with a resolution of 0.1 km/sec in a range of about 22 km/sec. The beam size is about 20 mas by 15 mas at a position angle of 84 degrees. The galactic plane is at a position angle of 30 degrees. The magnetic field at the maser emitting region obtained based on Zeeman pairs gives values from -4 to 6 mGauss. All for a region of about one arcsecond square near the center of the field. The deconvolved major axis (mjax) of the spots are between 30 to 70 mas and the ratio of mjax to minor axis (mnax) range from about 1.5:1 to 3:1.The center of expansion (CE) of Gwinn et al. (1992) for H 2 O maser spots at W49N is denoted by + in Fig. 1. The velocities of the nearest spots are slightly higher or smaller than 9 km/s, a value near the middle of the entire velocity range that we use as reference in Fig. 1a). Red shifted spots predominantly appear at the right (as also observed by Kent & Mutel (1982)) and blue shifted spots are located at the left, slightly to the north respect the red shifted spots, as occurs for H 2 O spots (Gwinn et al. 1992). However, the left hand side H 2 O spots are red shifted while the OH of the same side are blue shifted and vice versa.The principal axis (PA) takes values from about 80 to 130 degrees, i.e., even though the spots are spread more than 7×10 4 AU, their PAs are all nearly aligned. Most of the ellipses of the right group are oriented as if they would be traced from the CE but the ellipses of the left hand side group do not follow any given orientation respect the CE.The larger PA values take place at the larger mnax (Fig. 2) and a similar correlation is observed between PA and mjax. However, there is no correlation between PA and the majax/minax ratio. This suggests that it is the size of the spots and not the ratio of their axes that correlates with PA. 95available at https://www.cambridge.org/core/terms. https://doi