A. Sanna

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.

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

VLBI study of maser kinematics in high-mass SFRs. I. G16.59-0.05

Aims. To study the high-mass star-forming process, we have started a large project to unveil the gas kinematics close to young stellar objects (YSOs) through the Very Long Baseline Interferometry (VLBI) of maser associations. By comparing the high spatial resolution maser data, tracing the inner kinematics of the (proto)stellar cocoon, with interferometric thermal data, tracing the largescale environment of the hot molecular core (HMC) harbouring the (proto)stars, we can investigate the nature and identify the sources of large-scale motions. The present paper focuses on the high-mass star-forming region G16.59‐0.05. Methods. Using the VLBA and the EVN arrays, we conducted phase-referenced observations of the three most powerful maser species in G16.59‐0.05: H2O at 22.2 GHz (4 epochs), CH3OH at 6.7 GHz (3 epochs), and OH at 1.665 GHz (1 epoch). In addition, we performed high-resolution ( 0: 00 1), high-sensitivity (< 0:1 mJy) VLA observations of the radio continuum emission from the star-forming region at 1.3 and 3.6 cm. Results. This is the first work to report accurate measurements of the relative proper motions of the 6.7 GHz CH3OH masers. The di erent spatial and 3-D velocity distribution clearly indicate that the 22 GHz water and 6.7 GHz methanol masers are tracing di erent kinematic environments. The bipolar distribution of 6.7 GHz maser l.o.s. velocities and the regular pattern of observed proper motions suggest that these masers are tracing rotation around a central mass of about 35 M . The flattened spatial distribution of the 6.7 GHz masers, oriented NW‐SE, suggests that they can originate in a disk/toroid rotating around the massive YSO which drives the 12 CO (2‐ 1) outflow, oriented NE‐SW, observed on arcsec scale. The extended, radio continuum source observed close to the 6.7 GHz masers could be excited by a wide-angle wind emitted from the YSO associated with the methanol masers, and such a wind is proven to be su ciently energetic to drive the NE‐SW 12 CO (2‐1) outflow. The H2O masers distribute across a region o set about 0: 00 5 to the NW of the CH3OH masers, in the same area where emission of high-density molecular tracers, typical of HMCs, was detected. We postulate that a distinct YSO, possibly in an earlier evolutionary phase than that exciting the methanol masers, is responsible for the excitation of the water masers and the HMC molecular lines.

Infall and outflow within 400 AU from a high-mass protostar 3D velocity fields from methanol and water masers in AFLG 5142

Observational signatures of infalling envelopes and outflowing material in early stages of protostellar evolution and at small radii from the protostar are essential to progress in the understanding of the mass-accretion process in star formation. In this Letter, we report a detailed study on the accretion and outflow structure around a protostar in the well-known high-mass star-forming region AFGL 5142. We focus on the mm source MM–1, which exhibits hot-core chemistry, radio continuum emission, and strong water (H2O) and methanol (CH3OH) masers. Remarkably, our Very Long Baseline Interferometry (VLBI) observations of molecular masers over six years provided us with the 3D velocity field of circumstellar molecular gas with a resolution of 0.001–0.005 �� and at radii <0. �� 23 (or 400 AU) from the protostar. In particular, our measurements of CH3OH maser emission for the first time provided a direct measurement of the infall of a molecular envelope (radius of 300 AU and velocity of 5 km s −1 ) onto an intermediate- to high-mass

Disk-mediated accretion burst in a high-mass young stellar object

Observations show that, like light solar-mass stars, heavy stars also form through episodic disk-accretion; but faster, more energetic and emitting more light. Solar-mass stars form via disk-mediated accretion. Recent findings indicate that this process is probably episodic in the form of accretion bursts1, possibly caused by disk fragmentation2,3,4. Although it cannot be ruled out that high-mass young stellar objects arise from the coalescence of their low-mass brethren5, the latest results suggest that they more likely form via disks6,7,8,9. It follows that disk-mediated accretion bursts should occur10,11. Here we report on the discovery of the first disk-mediated accretion burst from a roughly twenty-solar-mass high-mass young stellar object12. Our near-infrared images show the brightening of the central source and its outflow cavities. Near-infrared spectroscopy reveals emission lines typical for accretion bursts in low-mass protostars, but orders of magnitude more luminous. Moreover, the released energy and the inferred mass-accretion rate are also orders of magnitude larger. Our results identify disk-accretion as the common mechanism of star formation across the entire stellar mass spectrum.

PARALLAXES OF STAR-FORMING REGIONS IN THE OUTER SPIRAL ARM OF THE MILKY WAY

We report parallaxes and proper motions of three water maser sources in high-mass star-forming regions in the Outer Spiral Arm of the Milky Way. The observations were conducted with the Very Long Baseline Array as part of Bar and Spiral Structure Legacy Survey and double the number of such measurements in the literature. The Outer Arm has a pitch angle of 14 degrees.9 +/- 2 degrees.7 and a Galactocentric distance of 14.1 +/- 0.6 kpc toward the Galactic anticenter. The average motion of these sources toward the Galactic center is 10.7 +/- 2.1 kms(-1) and we see no sign of a significant fall in the rotation curve out to 15 kpc from the Galactic center. The three-dimensional locations of these star-forming regions are consistent with a Galactic warp of several hundred parsecs from the plane.

CLUSTERED STAR FORMATION AND OUTFLOWS IN AFGL 2591

We report on a detailed study of the water maser kinematics and radio continuum emission toward the most massive and young object in the star-forming region AFGL 2591. Our analysis shows at least two spatial scales of multiple star formation, one projected across 0.1 pc on the sky and another one at about 2000 AU from a ZAMS star of about 38 M ☉. This young stellar object drives a powerful jet- and wind-driven outflow system with the water masers associated to the outflow walls, previously detected as a limb-brightened cavity in the NIR band. At about 1300 AU to the north of this object a younger protostar drives two bow shocks, outlined by arc-like water maser emission, at 200 AU either side of the source. We have traced the velocity profile of the gas that expands along these arc-like maser structures and compared it with the jet-driven outflow model. This analysis suggests that the ambient medium around the northern protostar is swept up by a jet-driven shock (>66 km s–1) and perhaps a lower-velocity (~10 km s–1) wind with an opening angle of about 20° from the jet axis.

TRIGONOMETRIC PARALLAXES OF MASSIVE STAR-FORMING REGIONS. IX. THE OUTER ARM IN THE FIRST QUADRANT

We report a trigonometric parallax measurement with the Very Long Baseline Array for the water maser in the distant high-mass star-forming region G75.30+1.32. This source has a heliocentric distance of 9.25(-0.40)(+0.45) kpc, which places it in the Outer arm in the first Galactic quadrant. It lies 200 pc above the Galactic plane and is associated with a substantial H i enhancement at the border of a large molecular cloud. At a Galactocentric radius of 10.7 kpc, G75.30+1.32 is in a region of the Galaxy where the disk is significantly warped toward the North Galactic Pole. While the star-forming region has an instantaneous Galactic orbit that is nearly circular, it displays a significant motion of 18 km s(-1) toward the Galactic plane. The present results, when combined with two previous maser studies in the Outer arm, yield a pitch angle of about 12 degrees for a large section of the arm extending from the first quadrant to the third.

THE VLBA CALIBRATOR SEARCH FOR THE BeSSeL SURVEY

We present the results of a survey of radio continuum sources near the Galactic plane using the Very Long Baseline Array (VLBA). Our observations are designed to identify compact extragalactic sources of milliarcsecond size that can be used for parallax measurements in the Bar and Spiral Structure Legacy Survey. We selected point sources from the NVSS and CORNISH catalogs with flux densities above 30 mJy and within 1 degrees.5 of known maser targets. Of the 1529 sources observed, 199 were detected. For sources detected on three or more baselines, we determined accurate positions and evaluated their quality as potential calibrators. Most of the 1330 sources that were not detected with the VLBA are probably of extragalactic origin.

A near-infrared spectroscopic survey of massive jets towards extended green objects

We aim at deriving the main physical properties of massive jets from near-IR observations, comparing them to those of a large sample of jets from low-mass YSOs, and relating them to the main features of their driving sources. We present a NIR imaging (H2 and Ks) and low-resolution spectroscopic (0.95-2.50 um) survey of 18 massive jets towards GLIMPSE extended green objects, driven by intermediate- and high-mass YSOs, which have Lbol between 4x10^2 and 10^5 Lsun. As in low-mass jets, H2 is the primary NIR coolant, detected in all the analysed flows, whereas the most important ionic tracer is [FeII], detected in half of the sampled jets. Our analysis indicates that the emission lines originate from shocks at high temperatures and densities. No fluorescent emission is detected along the flows, regardless of the source Lbol. On average, the physical parameters of these massive jets (i.e. Av, temperature, column density, mass, and luminosity) have higher values than those measured in their low-mass counterparts. The morphology of the H2 flows is varied, mostly depending on the complex, dynamic, and inhomogeneous environment in which these massive jets form and propagate. All flows and jets in our sample are collimated, showing large precession angles. Additionally, the presence of both knots and jets suggests that the ejection process is continuous with burst episodes, as in low-mass YSOs. We compare the flow H2 luminosity with the source Lbol confirming the tight correlation between these two quantities. Five sources, however, display a lower L(H2)/Lbol efficiency, which might be related to YSO evolution. Most important, the inferred L(H2) vs Lbol relationship agrees well with the correlation between the momentum flux of the CO outflows and the bolometric luminosities of high-mass YSOs indicating that outflows from high-mass YSOs are momentum driven, as are their low-mass counterparts.