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Register a new userWater Maser Kinematics in the Jet of OH 12.8-0.9
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We present Very Long Baseline Array observations of the kinematics of the water masers associated with OH 12.8-0.9, the fourth member of the so-called water fountain class of sources. We find that the masers occupy two distinct regions at the ends of a bipolar jet-like structure oriented north-south, with the blue-shifted masers located to the north and the red-shifted masers to the south. The masers are distributed along arc-like structures 12-20 mas across oriented perpendicular to the separation axis with an angular separation of ~110 mas on the sky. Our multi-epoch observations, show the two maser arcs to be expanding away from each other along the axis of separation. The relative proper motions of the two maser regions is 2.7 mas/yr (~105 km/s at the assumed distance of 8 kpc). The measured radial velocity difference between the northern, blue-shifted masers and the southern, red-shifted masers is 48.4 km/s. The radial velocity, when combined with the proper motion, yields a three-dimensional expansion velocity of 58 km/s and an inclination angle of 24 deg. for the jet. By combining our radial velocities with historical values, we estimate the three dimensional acceleration of the masers to be ~0.63 km/s/yr and a dynamical age for the collimated outflow of ~90 yr.
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We report on annual parallax and proper motion observations of H2O masers in S235AB-MIR, which is a massive young stellar object in the Perseus Arm. Using multi-epoch VLBI astrometry we measured a parallax of pi = 0.63 +- 0.03 mas, corresponding to a trigonometric distance of D = 1.56+-0.09 kpc, and source proper motion of ( u alpha cos d , u d) = (0.79 +- 0.12, -2.41 +- 0.14) mas/yr. Water masers trace a jet of diameter 15 au which exhibits a definite radial velocity gradient perpendicular to its axis. 3D maser kinematics were well modelled by a rotating cylinder with physical parameters: v_out = 45+-2 km/s, v_rot = 22+-3 km/s, i = 12+-2 degrees, which are the outflow velocity, tangential rotation velocity and line-of-sight inclination, respectively. One maser feature exhibited steady acceleration which may be related to the jet rotation. During our 15 month VLBI programme there were three `maser burst events caught `in the act which were caused by the overlapping of masers along the line of sight.
We report phase-referencing VLBA observations of H2O masers near the star-forming region W3(OH) to measure their parallax and absolute proper motions. The measured annual parallax is 0.489 +/- 0.017 milli-arcseconds (2.04 +/- 0.07 kpc), where the error is dominated by a systematic atmospheric contribution. This distance is consistent with photometric distances from previous observations and with the distance determined from CH3OH maser astrometry presented in a related paper. We also find that the source driving the H2O outflow, the ``TW-object, moves with a 3-dimensional velocity of > 7 km/s relative to the ultracompact HII region W3(OH).
We have conducted VLBI observations of water maser and radio continuum emission in the nucleus of NGC 3079. The 22 GHz maser emission arises in compact clumps, distributed along an axis that is aligned with the major axis of the galactic disk. The velocities of the masers are consistent with their lying in the inner parsec of a molecular disk rotating in the same sense as the rest of the galaxy. However, the velocity field has a significant non-rotational component, which may indicate supersonic turbulence. The bright maser emission is not coincident with any detected compact 22 GHz continuum source, suggesting the high apparent luminosity of the maser may not due to beamed amplification of continuum emission. We observed two compact continuum sources that have inverted spectra between 5 and 8 GHz, and steep spectra between 8 and 22 GHz. NGC 3079 may be a nearby, low-luminosity example of the class of compact symmetric gigahertz-peaked spectrum (GPS) radio sources. We detected a third continuum component that lies along the same axis as the other two, strongly suggesting that this galaxy possesses a nuclear jet. Faint maser emission was detected near this axis, which may indicate a second population of masers associated with the jet.
Observations of H$_2$O masers towards the post-AGB star and water fountain source OH 009.1--0.4 were made as part of HOPS (The H$_2$O southern galactic Plane Survey), with the Mopra radiotelescope. Together with followup observations using the Australia Telescope Compact Array (ATCA), we have identified H$_2$O maser emission over a velocity spread of nearly 400km/s (--109 to +289km/s). This velocity spread appears to be the largest of any known maser source in our Galaxy. High resolution observations with the ATCA indicate the maser emission is confined to a region $0farcs3 times 0farcs3$ and shows weak evidence for a separation of the red- and blueshifted maser spots. We are unable to determine if the water fountain is projected along the line of sight, or is inclined, but either way OH 009.1--0.4 is an interesting source, worthy of followup observations.
VLBI multi-epoch water maser observations are a powerful tool to study the dense, warm shocked gas very close to massive protostars. The very high-angular resolution of these observations allow us to measure the proper motions of the masers in a few weeks, and together with the radial velocity, to determine their full kinematics. In this paper we present a summary of the main observational results obtained toward the massive star-forming regions of Cepheus A and W75N, among them: (i) the identification of different centers of high-mass star formation activity at scales of 100 AU; (ii) the discovery of new phenomena associated with the early stages of high-mass protostellar evolution (e.g., isotropic gas ejections); and (iii) the identification of the simultaneous presence of a wide-angle outflow and a highly collimated jet in the massive object Cep A HW2, similar to what is observed in some low-mass protostars. Some of the implications of these results in the study of high-mass star formation are discussed.
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