Revising the kinematics of 12GHz CH3OH masers in W3(OH)

We derive accurate proper motions of the CH3OH 12 GHz masers towards the W3(OH) UCHII region, employing seven epochs of VLBA observations spanning a time interval of about 10 yr. The achieved velocity accuracy is of the order of 0.1 km/s, adequate to precisely measure the relative velocities of most of the 12 GHz masers in W3(OH), with amplitude varying in the range 0.3 - 3 km/s. Towards W3(OH), the most intense 12 GHz masers concentrate in a small area towards the north (the northern clump) of the UCHII region. We have compared the proper motions of the CH3OH 12 GHz masers with those (derived from literature data) of the OH 6035 MHz masers, emitting from the same region of the methanol masers. In the northern clump, the two maser emissions emerge from nearby (but likely distinct) cloudlets of masing gas with, in general, a rather smooth variation of line-of-sight and sky-projected velocities, which suggests some connection of the environments and kinematics traced by both maser types. The conical outflow model, previously proposed to account for the 12 GHz maser kinematics in the northern clump, does not reproduce the new, accurate measurements of 12 GHz maser proper motions and has to be rejected. We focus on the subset of 12 GHz masers of the northern clump belonging to the "linear structure at P.A. = 130-140 degree", whose regular variation of LSR velocities with position presents evidence for some ordered motion. We show that the 3-dimensional velocities of this "linear distribution" of 12GHz masers can be well fitted considering a flat, rotating disk, seen almost edge-on.Comment: 32 pages, 10 figures; accepted in ApJ (Main Journal

Observations of extragalactic masers in bright IRAS sources

We report the first results of an ongoing survey at 22 GHz with the 100-m Effelsberg telescope to search for water maser emission in bright IRAS sources. We have detected water vapor emission in IC 342. The maser, associated with a star forming region ~10-15 arcsec west of the nucleus, consists of a single 0.5 km/s wide feature and reaches an isotropic luminosity of 0.01 L(sun) (D = 1.8 Mpc). Our detection raises the detection rate among northern galaxies with IRAS point source fluxes S(100micron) > 50 Jy to 16%.Comment: 6 pages, 3 figures. Proceeding of the conference "SRT: the impact of large antennas on Radio Astronomy and Space Science", Cagliari, Italy, 7-10 Nov. 2001, Eds. Porceddu et al. Needs srt_style.st

Tracing the base of protostellar wind(s) towards the high-mass star forming region AFGL 5142: VLA continuum and VLBA water maser observations

We have conducted phase-reference multi-epoch observations of the 22.2 GHz water masers using the VLBA and multi-frequency study of the continuum emission using the VLA towards the high-mass SFR AFGL 5142. The water maser emission comes from two elongated structures (indicated as Group I and Group II), with the measured proper motions aligned along the structures' elongation axes. Each group consists of two (blue- and red-shifted) clusters of features separated by a few hundreds and thousands of AU respectively for Group I and Group II. The maser features of Group II have both positions and velocities aligned along a direction close to the axis of the outflow traced by HCO+ and SiO emission on angular scales of tens of arcsec. We predict that the maser emission arises from dense, shocked molecular clumps displaced along the axis of the molecular outflow. The two maser clusters of Group I are oriented on the sky along a direction forming a large angle (> 60 degrees) with the axis of the jet/outflow traced by Group II maser features. We have detected a compact (8.4 and 22 GHz) continuum source that falls close to the centroid of Group I masers, indicating that the source ionizing the gas is also responsible for the excitation of the water masers. The kinematic analysis indicates that the Group I masers trace outflowing rather than rotating gas, discarding the Keplerian disk scenario proposed in a previous paper for Group I. Since the axis joining the two maser clusters of Group II does not cross the position of the continuum source, Group II masers might be excited by an (undetected) massive YSO, distinct from the one (pinpointed by the VLA continuum emission) responsible for the excitation of the Group I masers.Comment: 12 pages, 3 figures, accepted for publication in A&

Infall and outflow within 400 AU from a high-mass protostar. 3-D 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 of 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 and methanol masers. Remarkably, our Very Long Baseline Interferometry (VLBI) observations of molecular masers over six years provided us with the 3-D velocity field of circumstellar molecular gas with a resolution of 0.001-0.005 arcseconds and at radii <0.23 arcseconds (or 400 AU) from the protostar. In particular, our measurements of methanol maser emission enabled, for the first time, a direct measurement of infall of a molecular envelope (radius of 300 AU and velocity of 5 km/s) onto an intermediate- to high-mass protostar. We estimate an infall rate of 0.0006 n_8 Msun/year, where n_8 is the ambient volume density in units of 10^8 cm-3 (required for maser excitation). In addition, our measurements of water maser (and radio continuum) emission identify a collimated bipolar molecular outflow (and ionized jet) from MM-1. The evidence of simultaneous accretion and outflow at small spatial scales, makes AFGL 5142 an extremely compelling target for high-angular resolution studies of high-mass star formation.Comment: 6 pages, 4 figures, 1 table, accepted for publication in A&A Letter

Infrared variability, maser activity, and accretion of massive young stellar objects

Methanol and water masers indicate young stellar objects. They often exhibit flares, and a fraction shows periodic activity. Several mechanisms might explain this behavior but the lack of concurrent infrared (IR) data complicates to identify the cause. Recently, 6.7 GHz methanol maser flares were observed, triggered by accretion bursts of high-mass YSOs which confirmed the IR-pumping of these masers. This suggests that regular IR changes might lead to maser periodicity. Hence, we scrutinized space-based IR imaging of YSOs associated with periodic methanol masers. We succeeded to extract the IR light curve from NEOWISE data for the intermediate mass YSO G107.298+5.639. Thus, for the first time a relationship between the maser and IR variability could be established. While the IR light curve shows the same period of ~34.6 days as the masers, its shape is distinct from that of the maser flares. Possible reasons for the IR periodicity are discussed.Comment: 4 pages, 3 figures, to be published in: Proceedings IAU Symposium 336 "Astrophysical Masers: Unlocking the Mysteries of the Universe", Editors: A. Tarchi, M.J. Reid & P. Castangia, updated version with hyperlinks adde

Unveiling the gas kinematics at 10 AU scales in high-mass star-forming regions (Milliarcsecond structure of 6.7 GHz methanol masers)

This work presents a study of the milliarcsecond structure of the 6.7 GHz methanol masers at high-velocity resolution (0.09 km s^(-1)) in four high-mass star-forming regions: G16.59-0.05, G23.01-0.41, IRAS20126+4104, and AFGL5142. We studied these sources by means of multi-epoch VLBI observations in the 22 GHz water and 6.7 GHz methanol masers, to determine the 3-D gas kinematics within a few thousand AU from the (proto)star. The present work exploits the 6.7 GHz maser data collected so far to investigate the milliarcsecond structure of this maser emission at high-velocity resolution. Most of the detected 6.7 GHz maser features present an ordered (linear, or arc-like) distribution of maser spots on the plane of the sky, together with a regular variation in the spot LSR velocity (V_LSR) with position. Typical values for the amplitude of the V_LSR gradients (defined in terms of the derivative of the spot V_LSR with position) are found to be 0.1-0.2 km s^(-1) mas^(-1). In each of the four target sources, the orientation and the amplitude of most of the feature V_LSR gradients remain remarkably stable in time, on timescales of (at least) several years. We also find that the data are consistent with having the V_LSR gradients and proper motion vectors in the same direction on the sky, considered the measurement uncertainties. The time persistency, the ordered angular and spatial distribution, and the orientation generally similar to the proper motions, altogether suggest a kinematical interpretation for the origin of the 6.7 GHz maser V_LSR gradients. This work shows that the organized motions (outflow, infall, and rotation) revealed by the (22 GHz water and 6.7 GHz methanol) masers on large scales (~100-1000 AU) also persist to very small (~10 AU) scales.Comment: 14 pages, 7 figures, accepted for publication in Astronomy and Astrophysics Journa

Water masers in the massive protostar IRAS 20126+4104: ejection and deceleration

We report on the first multi-epoch, phase referenced VLBI observations of the water maser emission in a high-mass protostar associated with a disk-jet system. The source under study, IRAS 20126+4104, has been extensively investigated in a large variety of tracers, including water maser VLBA data acquired by us three years before the present observations. The new findings fully confirm the interpretation proposed in our previous study, namely that the maser spots are expanding from a common origin coincident with the protostar. We also demonstrate that the observed 3-D velocities of the maser spots can be fitted with a model assuming that the spots are moving along the surface of a conical jet, with speed increasing for increasing distance from the cone vertex. We also present the results of single-dish monitoring of the water maser spectra in IRAS 20126+4104. These reveal that the peak velocity of some maser lines decreases linearly with time. We speculate that such a deceleration could be due to braking of the shocks from which the maser emission originates, due to mass loading at the shock front or dissipation of the shock energy.Comment: 11 pages, 8 figures. Accepted for publication in A&

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 $2.34^{+0.13}_{-0.11}$ kpc for G12.89+0.49 (also known as IRAS 18089-1732),$1.98^{+0.14}_{-0.12}$ kpc for G15.03-0.68 (in the M17 region), and $8.0^{+4.0}_{-2.0}$ kpc for G27.36-0.16. Both G12.89+0.49 and G15.03-0.68 are located in the Carina-Sagittarius spiral arm

A study on subarcsecond scales of the ammonia and continuum emission toward the G16.59-0.05 high-mass star-forming region

We wish to investigate the structure, velocity field, and stellar content of the G16.59-0.05 high-mass star-forming region, where previous studies have established the presence of two almost perpendicular (NE-SW and SE-NW), massive outflows, and a rotating disk traced by methanol maser emission. We performed Very Large Array observations of the radio continuum and ammonia line emission, complemented by COMICS/Subaru and Hi-GAL/Herschel images in the mid- and far-infrared (IR). Our centimeter continuum maps reveal a collimated radio jet that is oriented E-W and centered on the methanol maser disk, placed at the SE border of a compact molecular core. The spectral index of the jet is negative, indicating non-thermal emission over most of the jet, except the peak close to the maser disk, where thermal free-free emission is observed. We find that the ammonia emission presents a bipolar structure consistent (on a smaller scale) in direction and velocity with that of the NE-SW bipolar outflow detected in previous CO observations. After analyzing our previous N2H+(1-0) observations again, we conclude that two scenarios are possible. In one case both the radio jet and the ammonia emission would trace the root of the large-scale CO bipolar outflow. The different orientation of the jet and the ammonia flow could be explained by precession and/or a non-isotropic density distribution around the star. In the other case, the N2H+(1-0) and ammonia bipolarity is interpreted as two overlapping clumps moving with different velocities along the line of sight. The ammonia gas also seems to undergo rotation consistent with the maser disk. Our IR images complemented by archival data allow us to derive a bolometric luminosity of about 10^4 L_sun and to conclude that most of the luminosity is due to the young stellar object associated with the maser disk.Comment: 11 pages, 12 figures, published in Astronomy and Astrophysic

Trigonometric Parallaxes of Massive Star Forming Regions: II. Cep A & NGC 7538

We report trigonometric parallaxes for the sources NGC 7538 and Cep A, corresponding to distances of 2.65 [+0.12/-0.11] kpc and 0.70 [+0.04/-0.04] kpc, respectively. The distance to NGC 7538 is considerably smaller than its kinematic distance and places it in the Perseus spiral arm. The distance to Cep A is also smaller than its kinematic distance and places it in the Local arm or spur. Combining the distance and proper motions with observed radial velocities gives the location and full space motion of the star forming regions. We find significant deviations from circular Galactic orbits for these sources: both sources show large peculiar motions (> 10 km/s) counter to Galactic rotation and NGC 7538 has a comparable peculiar motion toward the Galactic center.Comment: 21 pages, 8 figures; to appear in the Astrophysical Journa