Astrometry of Galactic Star-Forming Region G48.61+0.02 with VERA

We performed the astrometry of H2O masers in the Galactic star-forming region G48.61 + 0.02 with the VLBI Exploration of Radio Astrometry (VERA). We derived a trigonometric parallax of 199 ± 7 μas, which corresponds to a distance of 5.03 ± 0.19 kpc. The distance to G48.61 + 0.02 is about a half of its far kinematic distance, which was often assumed previously. This distance places G48.61 + 0.02 in the Sagittarius-Carina arm and near the active star forming region and the supernova remnant W51. The obtained distance is very close to the parallactic distance of W51 Main/South, which is measured to be 5.41+0.31 kpc by the H2O maser astrometric observations with VLBA [1]. We also obtained the three dimensional motion of G48.61 + 0.02, and found that it has a large peculiar motion of 40 ± 5 km s−1. The kinetic energy of the peculiar motion is estimated to be (2 ± 1) × 1051 erg. What is the origin of this peculiar motion? This peculiar motion would be originated with the multiple supernovae explosions in W51, or the streaming motion across the Sagittarius-Carina arm. W51 C is the nearest SN remnant from G48.61 + 0.02. Figure 1(a) shows the position of G48.61 + 0.02 superimposed on Fermi LAT counts map in 2–10 GeV around W51 C [2]. The separation between G48.61 + 0.02 and the center of W51 C is 0.70 deg (62 pc). Figure 1(b) and (c) show the integrated intensity and the position velocity maps in the 13CO J = 1–0 line around W51 C [3]. The explosion kinetic energy of W51 C is estimated to be ~ 5 × 1051 erg [2]. Considering the energetics, the SN explosion is the possible origin. G48.61 + 0.02 appears to be located on the SagittariusCarina arm. Therefore, the streaming motion across the spiral arm is one possible origin for the large peculiar motion of G48.61 + 0.02. This study is published in Nagayama et al. (2011), PASJ, 63, 719. Astrometry of Galactic Star-Forming Region G48.61+0.02 with VERA