L. Moscadelli

TRIGONOMETRIC PARALLAXES OF MASSIVE STAR-FORMING REGIONS. I. S 252 & G232.6+1.0

We are conducting a large program with the NRAO Very Long Baseline Array (VLBA) to measure trigonometric parallaxes of massive star-forming regions across the Milky Way. Here we report measurement of the parallax and proper motion of methanol masers in S 252 and G232.6+1.0. The parallax of S 252 is 0.476 +/- 0.006 mas (2.10+(0.027)(-0.026) kpc), placing it in the Perseus spiral arm. The parallax of G232.6+1.0 is 0.596 +/- 0.035 mas (1.68+-(0.11)(0.09) kpc), placing it between the Carina-Sagittarius and Perseus arms, possibly in a Local ( Orion) spur of the Carina-Sagittarius arm. For both sources, kinematic distances are significantly greater than their parallax distances. Our parallaxes and proper motions yield full space motions accurate to approximate to 1 km s(-1). Both sources orbit the Galaxy similar to 13 km s(-1) slower than circular rotation.

ON THE NATURE OF THE LOCAL SPIRAL ARM OF THE MILKY WAY

Trigonometric parallax measurements of nine water masers associated with the Local Arm of the Milky Way were carried out as part of the BeSSeL Survey using the Very Long Baseline Array. When combined with 21 other parallax measurements from the literature, the data allow us to study the distribution and three-dimensional motions of star forming regions in the spiral arm over the entire northern sky. Our results suggest that the Local Arm does not have the large pitch angle characteristic of a short spur. Instead its active star formation, overall length (> 5 kpc), and shallow pitch angle (similar to 10 degrees) suggest that it is more like the adjacent Perseus and Sagittarius Arms; perhaps it is a branch of one of these arms. Contrary to previous results, we find the Local Arm to be closer to the Perseus than to the Sagittarius Arm, suggesting that a branching from the former may be more likely. An average peculiar motion of near zero toward both the Galactic center and north Galactic pole, and counter rotation of similar to 5 km s(-1) were observed, indicating that the Local Arm has similar kinematic properties as found for other major spiral arms.

The Bar and Spiral Structure Legacy (BeSSeL) survey: Mapping the Milky Way with VLBI astrometry†

Astrometric Very Long Baseline Interferometry (VLBI) observations of maser sources in the Milky Way are used to map the spiral structure of our galaxy and to determine fundamental parameters such as the rotation velocity (Theta(0)) and curve and the distance to the Galactic center (R-0). Here, we present an update on our first results, implementing a recent change in the knowledge about the Solar motion. It seems unavoidable that the IAU recommended values for R0 and Theta(0) need a substantial revision. In particular the combination of 8.5 kpc and 220 km s(-1) can be ruled out with high confidence. Combining the maser data with the distance to the Galactic center from stellar orbits and the proper motion of Sgr A* gives best values of R-0 = 8.3 +/- 0.23 kpc and Theta(0) = 239 or 246 +/- 7 km s(-1), for Solar motions of V-circle dot = 12.23 and 5.25 km s(-1), respectively. Finally, we give an outlook to future observations in the Bar and Spiral Structure Legacy (BeSSeL) survey

Trigonometric Parallaxes of Massive Star Forming Regions: III. G59.7+0.1 and W 51 IRS2

We report trigonometric parallaxes for G59.7+0.1 and W 51 IRS2, corresponding to distances of 2.16+(0.10)(-0.09) kpc and 5.1+(2.9)(-1.4) kpc, respectively. The distance to G59.7+0.1 is smaller than its near kinematic distance and places it between the Carina-Sagittarius and Perseus spiral arms, probably in the Local (Orion) spur. The distance to W 51 IRS2, while subject to significant uncertainty, is close to its kinematic distance and places it near the tangent point of the Carina-Sagittarius arm. It also agrees well with a recent estimate based on O-type star spectro/photometry. Combining the distances and proper motions with observed radial velocities gives the full space motions of the star-forming regions. We find modest deviations of 5-10 km s(-1) from circular Galactic orbits for these sources, both counter to Galactic rotation and toward the Galactic center.

TRIGONOMETRIC PARALLAXES OF MASSIVE STAR-FORMING REGIONS. V. G23.01–0.41 AND G23.44–0.18

We report trigonometric parallaxes for the massive star-forming regions G23.01-0.41 and G23.44-0.18, corresponding to distances of 4.59+(0.38)(-0.33) kpc and 5.88+(1.37)(-0.93) kpc, respectively. The distance to G23.01-0.41 is smaller than its near kinematic distance assuming a standard model of the Milky Way and less than half of its far kinematic distance, which has usually been assumed. This places it in the Crux-Scutum spiral arm. The distance to G23.44-0.18 is close to its near kinematic distance and most likely places it in the Norma spiral arm near the end of the Galactic bar. Combining the distance and proper motions with observed radial velocities gives the location and full space motion of the star-forming regions. We find large deviations from circular Galactic orbits for these sources: both sources show peculiar motions of 20-30 km s(-1) counter to Galactic rotation and toward the Galactic center. These large peculiar motions might be the result of gravitational perturbations from the Galactic bar.

TRIGONOMETRIC PARALLAXES OF MASSIVE STAR-FORMING REGIONS. VIII. G12.89+0.49, G15.03–0.68 (M17), AND G27.36–0.16

We report trigonometric parallaxes for three massive star-forming regions, corresponding to distances of 2.34(-0.11)(+0.13) kpc for G12.89+0.49 (also known as IRAS 18089-1732), 1.98(-0.12)(+0.14) kpc for G15.03-0.68 (in the M17 region), and 8.0(-2.0)(+4.0) kpc for G27.36-0.16. Both G12.89+0.49 and G15.03-0.68 are located in the Carina-Sagittarius spiral arm.

TRIGONOMETRIC PARALLAXES OF MASSIVE STAR-FORMING REGIONS. IV. G35.20-0.74 AND G35.20-1.74

We report trigonometric parallaxes for the high-mass star-forming regions G35.20-0.74 and G35.20-1.74, corresponding to distances of 2.19+(0.24)(-0.20) kpc and 3.27+(0.56)(-0.42) kpc, respectively. The distances to both sources are close to their near kinematic distances and place them in the Carina-Sagittarius spiral arm. Combining the distances and proper motions with observed radial velocities gives the locations and full space motions of the star-forming regions. Assuming a standard model of the Galaxy, G35.20-0.74 and G35.20-1.74 have peculiar motions of approximate to 13 km s(-1) and approximate to 16 km s(-1) counter to Galactic rotation and approximate to 9 km s(-1) toward the North Galactic Pole.

Methanol and water masers in IRAS 20126+4104: the distance, the disk, and the jet

Context. Knowledge of the distance to high-mass star forming regions is crucial to obtain accurate luminosity and mass estimates of young OB-type (proto)stars and thus better constrain their nature and age. IRAS20126+4104 is a special case, being the best candidate of a high-mass (proto)star surrounded by an accretion disk. Such a fact may be used to set constraints on theories of highmass star formation, but requires confirmation that the mass and luminosity of IRAS20126+4104 are indeed typical of a B0.5 star, which in turn requires an accurate estimate of the distance. Aims. The goal of our study is twofold: to determine the distance to IRAS20126+4104, using the parallax of H2O masers associated with the source, and unveil the 3D velocity field of the disk, through proper motion measurements of the 6.7 GHz CH3OH masers. At the same time, we can also obtain an estimate of the systemic velocity in the plane of the sky of the disk+star system. Methods. We used the Very Long Baseline Array and the European VLBI Network to observe the 22.2 GHz H2O and 6.7 GHz CH3OH masers in IRAS20126+4104 at a number of epochs suitably distributed in time. The absolute positions of the maser features were established with respect to reference quasars, which allowed us to derive absolute proper motions. Results. From the parallax of the H2O masers we obtain a distance of 1.64 ± 0.05 kpc, which is very similar to the value adopted so far in the literature (1.7 kpc) and confirms that IRAS20126+4104 is a high-mass (proto)star. From the CH3OH masers we derive the component in the plane of the sky of the systemic velocity of the disk+star system (−16 km s −1 in right-ascension and +7.6 km s −1 in declination). Accurate knowledge of the distance and systemic velocity allows us to improve on the model fit to the H2O maser jet presented in a previous study. Finally, we identify two groups of CH3OH maser features, one undergoing rotation in the disk and possibly distributed along a narrow ring centered on the star, the other characterised by relative proper motions indicating that the features are moving away from the disk, perpendicular to it. We speculate that the latter group might be tracing the disk material marginally entrained by the jet. Conclusions. VLBImulti-epoch observations with phase referencing are confirmed to be an excellent tool for distance determinations and for the investigation of the structure and 3D velocity field within a few 100 AU from newly born high-mass stars.

THE PARALLAX OF W43: A MASSIVE STAR-FORMING COMPLEX NEAR THE GALACTIC BAR

We report trigonometric parallax measurements of masers in the massive star-forming complex W43 from Very Long Baseline Array observations as part of the Bar and Spiral Structure Legacy Survey. Based on measurements of three 12 GHz methanol maser sources (G029.86-00.04, G029.95-00.01, and G031.28+00.06) and one 22 GHz water maser source (G031.58+ 00.07) toward W43, we derived a distance of 5.49(-0.34)(+0.39) kpc to W43. By associating the masers with CO molecular clouds, and associating the clouds kinematically with CO longitude-velocity spiral features, we assign W43 to the Scutum spiral arm, close to the near end of the Galactic bar. The peculiar motion of W43 is about 20 km s(-1) toward the Galactic Center and is very likely induced by the gravitational attraction of the bar.

TRIGONOMETRIC PARALLAXES OF MASSIVE STAR-FORMING REGIONS. VII. G9.62+0.20 AND THE EXPANDING 3 kpc ARM

We report a trigonometric parallax of 12 GHz methanol masers associated with the massive star-forming region G9.62+0.20, corresponding to a distance of 5.2(-0.6)(+0.6) kpc. With an LSR velocity of about 2 km s(-1), the regions kinematic distances of 0.5 and 16 kpc differ greatly from the distance derived here. Our measurement of the peculiar motion of the star-forming region shows a very large deviation from a circular Galactic orbit: 41 km s(-1) radially outward from the Galactic center and 60 km s(-1) counter to Galactic rotation. The combination of its radial velocity and distance places G9.62+0.20 in the inner region of the Galaxy close to the expanding near 3 kpc arm, where the bulge/bar potential has strong gravitational influence. We also map the distribution of 12 GHz methanol masers, locate them with respect to a hypercompact H II region, and compare our data with the periodic flare phenomenon reported previously for this source.