TRIGONOMETRIC PARALLAXES OF STAR FORMING REGIONS IN THE SCUTUM SPIRAL ARM

Abstract

We report measurements of trigonometric parallaxes for six high-mass\nstar-forming regions in the Scutum spiral arm of the Milky Way as part of the\nBeSSeL Survey. Combining our measurements with 10 previous measurements from\nthe BeSSeL Survey yields a total sample of 16 sources in the Scutum arm with\ntrigonometric parallaxes in the Galactic longitude range from 5 deg to 32 deg.\nAssuming a logarithmic spiral model, we estimate a pitch angle of 19.8 (+/-3.1)\ndeg for the Scutum arm, which is larger than pitch angles reported for other\nspiral arms. The high pitch angle of the arm may be due to the arm's proximity\nto the Galactic bar. The Scutum arm sources show an average peculiar motion of\n4 km/s slower than the Galactic rotation and 8 km/s toward the Galactic center.\nWhile the direction of this non-circular motion has the same sign as determined\nfor sources in other spiral arms, the motion toward the Galactic center is\ngreater for the Scutum arm sources.\n