V. Migenes

The Subparsec-Scale Structure and Evolution of Centaurus A: The Nearest Active Radio Galaxy

?????The subparsec-scale structure of Cen A is complex, consisting of a bright jet and a fainter counterjet. The bright jet contains components that have subluminal speeds of approximately 0.1c and undergo irregular episodes of rapid internal evolution. The rapid evolution sometimes observed could be interpreted as evidence for an underlying jet flow much faster (>0.45c) than observed from the proper motion of components (~0.1c). Considering the large-scale morphology of the source, the motions and temporal variations in the jet, and the detection of a counterjet, we conclude that the axis of the Cen A jet lies between ~50? and ~80? to our line of sight. We find that the estimated times of component ejection from the compact core are reasonably coincident with enhancements in hard X-ray intensity and 22 GHz flux density. In the context of the radio galaxy population, Cen A is a low-luminosity FR I?type source and in general has the properties observed in other FR I radio galaxies. Overall, the observations of Cen A presented here, and from other investigations, are consistent with the idea that sources with an FR I appearance are not aligned with our line of sight and have relativistic flow on the subparsec scale. The apparently subluminal subparsec-scale jet components are interpreted as being slow patterns on the relativistic flow.

The VSOP Prelaunch H2O Maser Survey. I. VLBA Observations

We present the first results from an H2O maser survey conducted with the VLBA for determining candidate sources for space VLBI missions. We observed 60 sources in total: four late-type stars, 29 star-forming regions, and 27 H II regions. These are the first interferometric observations of any kind for 50% of the sources. Approximately 30% of the sources we observed exhibit highly compact structure and very strong emission, which make them ideal targets for space VLBI missions.

Total Linear Polarization in the OH Maser W75 N: VLBA Polarization Structure

W75 N is a star-forming region containing various ultracompact H II regions and OH, H2O, and CH3OH maser emission. Our VLBA map shows that the OH masers are located in a thin disk rotating around an O star, the exciting star of the ultracompact H II region VLA 1. A separate set of maser spots is associated with the ultracompact H II region VLA 2. The radial velocity of OH maser spots varies across the disk from 3.7 to 10.9 km s-1. The diameter of the disk is 4000 AU. All maser spots are strongly polarized. These are the first OH masers showing nearly 100% linear polarization in several spots. Two maser spots seem to be Zeeman pairs, corresponding to a magnetic field of 5.2 and 7.7 mG, and in one case we tentatively find a Zeeman pair consisting of two linearly polarized components. The linearly polarized maser spots are shown to be σ components, which is the case when the magnetic field is perpendicular to the line of sight. The direction of the magnetic field as determined from linearly polarized spots is perpendicular to the plane of the disk, although the Galactic Faraday rotation may significantly affect this conclusion.

The three-dimensional kinematics of water masers around the semiregular variable RT Virginis.

We report observations of water masers around the semiregular variable RT Virginis, which have been made with the Very Long Baseline Array of the National Radio Astronomy Observatory at five epochs, each separated by 3 weeks of time. We detected about 60 maser features at each epoch. Overall, 61 features, detected at least twice, were tracked by their radial velocities and proper motions. The three-dimensional maser kinematics exhibited a circumstellar envelope that is expanding roughly spherically with a velocity of 8 km s-1. Asymmetries in both the spatial and velocity distributions of the maser features were found in the envelope, but they were less significant than those found in other semiregular variables. Systematic radial velocity drifts of individual maser features were found with amplitudes of ≤2 km s-1 yr-1. For one maser feature, we found a quadratic position shift with time along a straight line on the sky. This apparent motion indicates an acceleration with an amplitude of 33 km s-1 yr-1, implying the passage of a shock wave driven by the stellar pulsation of RT Vir. The acceleration motion is likely seen only on the sky plane because of a large velocity gradient formed in the accelerating maser region. We estimated the distance to RT Vir to be about 220 pc on the basis of both the statistical parallax and model-fitting methods for the maser kinematics.

The Extreme Scattering Event toward PKS 1741–038: VLBI Images

We report multiepoch VLBI observations of the source PKS 1741-038 (OT 068) as it underwent an extreme scattering event (ESE). Observations at four epochs were obtained, and images were produced at three of these. One of these three epochs was when the source was near the minimum flux density of the ESE, the other two were as the flux density of the source was returning to its nominal value. The fourth epoch was at the maximum flux density during the egress from the ESE, but the VLBI observations had too few stations to produce an image. During the event the source consisted of a dominant, compact component, essentially identical to the structure seen outside the event. However, the sources diameter increased slightly at 13 cm, from near 0.6 mas outside the ESE to near 1 mas during the ESE. An increase in the sources diameter is inconsistent with a simple refractive model in which a smooth refractive lens drifted across the line of sight to PKS 1741-038. We also see no evidence for ESE-induced substructure within the source or the formation of multiple images, as would occur in a strongly refractive lens. However, a model in which the decrease in flux density during the ESE occurs solely because of stochastic broadening within the lens requires a larger broadening diameter during the event than is observed. Thus, the ESE toward PKS 1741-038 involved both stochastic broadening and refractive defocusing within the lens. If the structure responsible for the ESE has a size of order 1 AU, the level of scattering within an ESE lens may be a factor of 107 larger than that in the ambient medium. A filamentary structure could reduce the difference between the strength of scattering in the lens and ambient medium, but there is no evidence for a refractively induced elongation of the source. We conclude that, if ESEs arise from filamentary structures, they occur when the filamentary structures are seen lengthwise. We are able to predict the amount of pulse broadening that would result from a comparable lens passing in front of a pulsar. The pulse broadening would be no more than 1.1 μs, consistent with the lack of pulse broadening detected during ESEs toward the pulsars PSR B1937+21 and PSR J1643-1224. The line of sight toward PKS 1741-038 is consistent with a turbulent origin for the structures responsible for ESEs. The source PKS 1741-038 lies near the radio Loop I and is seen through a local minimum in 100 μm emission.

Protoplanetary Disk and/or Bipolar Outflow Traced by H2O Masers in IC 1396N

The H2O maser connected with the young stellar object in the globule IC 1396N has been mapped with the VLBA during its highest state of activity in 1996 June. The spectrum of the H2O maser consisted of a dense group of strong low-velocity features near the LSR velocity of the globule, and two high-velocity features: one redshifted to 9.3 km s-1 and the other blueshifted to -14.1 km s-1. The map of low-velocity features displays a remarkable chain of at least eight maser spots located very close to a straight line about 15 AU in extent, with LSR velocities varying linearly along the line. The two high-velocity features are offset from the low-velocity group by 410 and 10,000 AU for the blue and red features, respectively. We discuss three models that can describe the observed distribution of maser spots: a Keplerian disk, a shock front, and a molecular outflow. The final model that we propose incorporates all three of these models: the low-velocity features arise in the Keplerian disk with maser emission excited by shock waves traveling in the disk, while the high-velocity features arise at the root of the molecular outflow originating from the central 4 M☉ young star or a protostar. The mass of the disk and its angular momentum are similar to those of the solar system planets. It is suggested that it is a circumstellar accretion disk accumulating the excess angular momentum of the collapsing molecular core, which may give rise to the formation of a planetary system. This model can be tested by measurements of the proper motion and radial velocity variations of the maser spots.

Very Large Array Simultaneous 1.3?cm Continuum and H2O Maser Observations toward IRAS?20126+4104

We have observed simultaneously the 1.3 cm continuum and H2O maser emission toward the high-mass protostar IRAS 20126+4104 with the Very Large Array (VLA) in its A configuration, providing an angular resolution of 01. We have detected an unresolved continuum source [I20126N(1.3 cm)] nearly coinciding with the strongest peak of the northern double continuum source previously detected at 3.6 cm in the field and suggested to be a radio jet. In addition, we have detected 29 water maser spots, 21 of which are tightly grouped in a cluster of 01 size displaced 007 (120 AU) northwest of I20126N(1.3 cm). The relative positions of the masers with respect to I20126N(1.3 cm) have been established with 10 mas of accuracy. The overall spatial distribution and line-of-sight velocity components of the water maser spots associated with I20126N(1.3 cm) are fully consistent with those previously found by Moscadelli and collaborators through VLBI measurements and interpreted as tracing a conical outflow, with the powering source (suggested to be traced by a one-sided radio continuum jet) located at the cone vertex. Within this very reasonable scenario, our observations would indicate that the source I20126N(1.3 cm) is showing the position of the high-mass protostar. Analyzing the spatiokinematic distribution of the VLA water masers, we propose that their motions also show, in addition to proper motions on the order of 100 km s-1 seen in the plane of the sky, a component of rotation with velocities on the order of 20 km s-1. The water masers seem then to be both rotating (as evidenced from their radial velocities) and changing their position in the plane of the sky (from the proper motions). Within this scenario, some of the water maser spots could be within a rotating circumstellar disk of 170 AU size around a protostar of 20 M⊙ if it is located at the center of the water maser cluster rather than located at the position of I20126N(1.3 cm). Then, I20126N(1.3 cm) would trace one of the two peaks of a two-sided thermal jet, with the high-mass protostar located in between. We think that simultaneous high angular resolution and sensitive multifrequency (from centimeter to submillimeter wavelengths) observations are the key studies to knowing both the nature of the continuum emission (one- or two-sided jet?) and the location of the high-mass protostar, which is very relevant for modeling this important object.

Parsec-Scale Morphology of PKS 2152-699 and the Radio/Optical Misalignment

We present the first very-long-baseline interferometry (VLBI) image of the parsec-scale structure in the nearby (z=0.0282) Fanaroff-Riley type II radio galaxy PKS 2152-699, revealing the core-jet morphology typical of compact radio sources. Tadhunter et al. [MNRAS, 235, 403 (1988)] have identified this object as one in which the radio axis and optical emission line features on the kiloparsec-scale appear to have an obvious connection but are somewhat misaligned. They interpret this relationship as most likely being due to an interaction between the radio jet and an extra-nuclear cloud of gas, the misalignment possibly caused by the deflection of the radio jet in the interaction. The VLBI observations presented here further strengthen the evidence for the interaction interpretation. The parsec-scale radio jet aligns strongly with optical emission line features, and not with the kiloparsec-scale radio structure. We explore a mechanism which could produce the misalignment and which is consistent with the interaction concept, namely oblique shocks in a relativistic jet. With a minimal model we find that reasonable values of the model parameters can provide the required misalignment over a wide range of initial conditions.

High-Resolution H2O Maser Observations toward IRAS Sources in Bright-Rimmed Clouds

Bright-rimmed clouds (BRCs) are clouds that have been compressed by an external ionization shock front. We present the first high-resolution VLA observations of 20 of these BRCs in the northern hemisphere. We detected water maser emission from three objects: IRAS 21346+5714 (BRC 36), IRAS 21388+5622 (BRC 37), and IRAS 21445+5712 (BRC 39). The low detection rate supports the evidence that BRCs produce mostly low-luminosity objects, for which maser emission is weak and episodic, and suggests that the embedded sources are in a more advanced evolutionary phase than Class 0 objects.

INTERFEROMETRIC OBSERVATIONS TOWARD THE HIGH-MASS YOUNG STELLAR OBJECT IRAS 23139+5939: RADIO CONTINUUM AND WATER MASER EMISSION

We present simultaneous interferometric observations of continuum (3.5 and 1.3 cm) and water maser line emission (1.3 cm) carried out with the A configuration of the Very Large Array (VLA) toward the high-mass object IRAS 23139+5939. We detected two radio continuum sources at 3.5 cm separated by ~05 (~2400 AU), I23139 and I23139S, the latter being a new detection. I23139 is also detected at 1.3 cm. We also detected a cluster of water masers toward these two VLA sources, obtaining an accuracy of the order of ~10 mas in the relative positions between the radio continuum emission and the H2O masers. In addition, based on the observed continuum flux density and the spectral index, we suggest that I23139 is a thermal radio jet associated with a high-mass young stellar object. This jet appears to be the best candidate for the driving source of the more spatially extended molecular outflow observed in the region. On the other hand, based on the spatiokinematical distribution of the water masers, together with the continuum emission information, we speculate that I23139S is also a jet source powering some of the masers detected in the region.