A. Brunthaler

The distance to the Orion Nebula

We have used the Very Long Baseline Array to measure the trigonometric parallax of several member stars of the Orion Nebula Cluster showing non-thermal radio emission. We have determined the distance to the cluster to be 414 ± 7 pc. Our distance determination allows for an improved calibration of luminosities and ages of young stars. We have also measured the proper motions of four cluster stars which, when accurate radial velocities are measured, will put strong constraints on the origin of the cluster.

The Proper Motion of Sagittarius A*. II. The Mass of Sagittarius A*

We report measurements with the Very Long Baseline Array (VLBA) of the position of Sgr A* with respect to two extragalactic radio sources over a period of 8 yr. The apparent proper motion of Sgr A* relative to J1745� 283 is 6:379 � 0:024 mas yr � 1 along a position angle of 209N60 � 0N18, almost entirely in the plane of the Galaxy. The effects of the orbit of the Sun around the Galactic center can account for this motion, and the residual proper motion of Sgr A* perpendicular to the plane of the Galaxy is � 0:4 � 0: 9k m s � 1 . A maximum likelihood analysis of the motion expected for a massive object within the observed Galactic center stellar cluster indicates that Sgr A*

A TRIGONOMETRIC PARALLAX OF Sgr B2

We have measured the positions of H(2)O masers in Sgr B2, a massive star-forming region in the Galactic center, relative to an extragalactic radio source with the Very Long Baseline Array. The positions measured at 12 epochs over a time span of one year yield the trigonometric parallax of Sgr B2 and hence a distance to the Galactic center of R(0) = 7.9(-0.7)(+0.8) kpc. The proper motion of Sgr B2 relative to Sgr A* suggests that Sgr B2 is approximate to 0.13 kpc nearer than the Galactic center, assuming a low-eccentricity Galactic orbit.

A strong magnetic field around the supermassive black hole at the centre of the Galaxy

Earth’s nearest candidate supermassive black hole lies at the centre of the Milky Way. Its electromagnetic emission is thought to be powered by radiatively inefficient accretion of gas from its environment, which is a standard mode of energy supply for most galactic nuclei. X-ray measurements have already resolved a tenuous hot gas component from which the black hole can be fed. The magnetization of the gas, however, which is a crucial parameter determining the structure of the accretion flow, remains unknown. Strong magnetic fields can influence the dynamics of accretion, remove angular momentum from the infalling gas, expel matter through relativistic jets and lead to synchrotron emission such as that previously observed. Here we report multi-frequency radio measurements of a newly discovered pulsar close to the Galactic Centre and show that the pulsar’s unusually large Faraday rotation (the rotation of the plane of polarization of the emission in the presence of an external magnetic field) indicates that there is a dynamically important magnetic field near the black hole. If this field is accreted down to the event horizon it provides enough magnetic flux to explain the observed emission—from radio to X-ray wavelengths—from the black hole.

Parallaxes and proper motions of interstellar masers toward the Cygnus X star-forming complex - I. Membership of the Cygnus X region

Context. Whether the Cygnus X complex consists of one physically connected region of star formation or of multiple independent regions projected close together on the sky has been debated for decades. The main reason for this puzzling scenario is the lack of trustworthy distance measurements. Aims. We aim to understand the structure and dynamics of the star-forming regions toward Cygnus X by accurate distance and proper motion measurements. Methods. To measure trigonometric parallaxes, we observed 6.7 GHz methanol and 22 GHz water masers with the European VLBI Network and the Very Long Baseline Array. Results. We measured the trigonometric parallaxes and proper motions of five massive star-forming regions toward the Cygnus X complex and report the following distances within a 10% accuracy: 1.30 +0.07 ―0.07 kpc for W 75N, 1.46 +0.09 ―0.08 kpc for DR 20, 1.50 +0.08 ―0.07 kpc for DR 21, 1.36 +0.12 ―0.11 kpc for IRAS 20290+4052, and 3.33 +0.11 ―0.11 kpc for AFGL 2591. While the distances of W 75N, DR 20, DR 21, and IRAS 20290+4052 are consistent with a single distance of 1.40 ± 0.08 kpc for the Cygnus X complex, AFGL 2591 is located at a much greater distance than previously assumed. The space velocities of the four star-forming regions in the Cygnus X complex do not suggest an expanding Stromgren sphere.

TRIGONOMETRIC PARALLAX OF W51 MAIN/SOUTH

We report measurement of the trigonometric parallax of W51 Main/South using the Very Long Baseline Array. We measure a value of 0.185 ± 0.010 mas corresponding to a distance of 5.41+0.31 –0.28 kpc. W51 Main/South is a well-known massive star-forming region near the tangent point of the Sagittarius spiral arm of the Milky Way. Our distance to W51 yields an estimate of the distance to the Galactic center of R 0 = 8.3 ± 0.46 (statistical) ±1.0 (systematic) kpc by simple geometry. Combining the parallax and proper motion measurements for W51, we obtained the full-space motion of this massive star-forming region. We find W51 is in a nearly circular orbit about the Galactic center. The H2O masers used for our parallax measurements trace four powerful bipolar outflows within a 0.4 pc size region, some of which are associated with dusty molecular hot cores and/or hyper- or ultra-compact H II regions.

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 Geometric Distance and Proper Motion of the Triangulum Galaxy (M33)

We measured the angular rotation and proper motion of the Triangulum Galaxy (M33) with the Very Long Baseline Array by observing two H2O masers on opposite sides of the galaxy. By comparing the angular rotation rate with the inclination and rotation speed, we obtained a distance of 730 ± 168 kiloparsecs. This distance is consistent with the most recent Cepheid distance measurement. M33 is moving with a velocity of 190 ± 59 kilometers per second relative to the Milky Way. These measurements promise a method to determine dynamical models for the Local Group and the mass and dark-matter halos of M31, M33, and the Milky Way.

Trigonometric parallaxes of 6.7 GHz methanol masers

Aims. Emission from the 6.7 GHz methanol maser transition is very strong, is relatively stable, has small internal motions, and is observed toward numerous massive star-forming regions in the Galaxy. Our goal is to perform high-precision astrometry using this maser transition to obtain accurate distances to their host regions.