We report accurate positions in the International Celestial Reference Frame (ICRF) for 11 radio stars. Observations were made using the Multi-Element Radio Linked Interferometer Network at a radio frequency of 5 GHz. The positions are estimated to be accurate at the 5 mas level. Positions were obtained directly in the ICRF by phase referencing the radio stars to ICRF quasars whose positions are estimated to be accurate at the 0.25 mas level. We use our results together with results of previous observations to obtain proper-motion estimates for these stars. The average proper-motion uncertainties are 1.1 mas yr-1 in μα cos δ and 1.2 mas yr-1 in μδ, comparable to the Hipparcos values.

We present observational evidence that the OH/IR star OH 12.8-0.9 is the fourth in a class of objects previously dubbed "water- fountain" sources. Using the Very Long Baseline Array, we produced the first images of the H2O maser emission associated with OH 12.8-0.9. We find that the masers are located in two compact regions with an angular separation of similar to 109 mas on the sky. The axis of separation between the two maser regions is at a position angle of 1 (.) over circle5 east of north, with the blueshifted (-80.5 to -85.5 km s(-1)) masers located to the north and the redshifted (-32.0 to -35.5 km s) masers to the south. In addition, we find that the blue- and redshifted masers are distributed along arclike structures -10 -12 mas across, oriented roughly perpendicular to the separation axis. The morphology exhibited by the H2O masers is suggestive of an axisymmetric wind with the masers tracing bow shocks formed as the wind impacts the ambient medium. This bipolar jet-like structure is typical of the three other confirmed water- fountain sources. When combined with the previously observed spectral characteristics of OH 12.8-0.9, the observed spatiokinematic structure of the H2O masers provides strong evidence that OH 12.8-0.9 is indeed a member of the water-fountain class.

We report on recent observations of AGB stars obtained with the VLT Interferometer (VLTI). We illustrate in general the potential of interferometric measurements to study stellar atmospheres and circumstellar envelopes, and demonstrate in particular the advantages of a coordinated multi-wavelength approach including near/mid-infrared as well as radio interferometry. We report on studies of the atmospheric structure of non-Mira and Mira variable giants. We have used VLTI observations of the near- and mid-infrared stellar sizes and concurrent VLBA observations of the SiO maser emission. So far, this project includes studies of the Mira stars S Ori and RR Aql as well as of the supergiant AH Sco. The results from our first epochs of S Ori measurements have recently been published and the main results are reviewed here. The S Ori maser ring is found to lie at a mean distance of approximately 2 stellar radii, a result that is virtually free of the usual uncertainty inherent in comparing observations of variable stars widely separated in time and stellar phase. We discuss the status of our more recent S Ori, RR Aql, and AH Sco observations, and present an outlook on the continuation of our project.

We observed v = 1, J = 1-0 43 GHz SiO maser emission toward the Mira variable IK Tauri (IK Tau) using the Very Long Baseline Array (VLBA). The images resulting from these observations show that SiO masers form a highly elliptical ring of emission approximately 58 × 32 mas with an axial ratio of 1.8 : 1. The major axis of this elliptical distribution is oriented at a position angle of ~59°. The line-of-sight velocity structure of the SiO masers has an apparent axis of symmetry consistent with the elongation axis of the maser distribution. Relative to the assumed stellar velocity of 35 km s-1, the blue- and redshifted masers were found to lie to the northwest and southeast of this symmetry axis, respectively. This velocity structure suggests a northwest-southeast (NW-SE) rotation of the SiO maser shell with an equatorial velocity, which we determine to be ~3.6 km s-1. Such a NW-SE rotation is in agreement with a circumstellar envelope geometry invoked to explain previous H2O and OH maser observations. In this geometry, H2O and OH masers are preferentially created in a region of enhanced density along the NE-SW equator orthogonal to the rotation/polar axis suggested by the SiO maser velocities.

We present first imaging results and preliminary source structure analysis of 108 extragalactic objects observed using the Very Long Baseline Array (VLBA) at 24 GHz and 43 GHz as part of a joint NASA, USNO, NRAO and Bordeaux Observatory program to extend the ICRF to higher radio frequencies.

We present the first coordinated Very Long Baseline Array (VLBA) / Very Large Telescope Interferometer (VLTI) measurements of the stellar diameter and circumstellar atmosphere of a Mira variable star. Observations of the v = 1, J = 1-0 (43.1 GHz) and v = 2, J = 1-0 (42.8 GHz) SiO maser emission toward the Mira variable S Ori were conducted using the VLBA. Coordinated near-infrared K-band measurements of the stellar diameter were performed using VLTI-VINCI closely spaced in time to the VLBA observations. Analysis of the SiO maser data recorded at a visual variability phase 0.73 shows the average distance of the masers from the center of the distribution to be 9.4 mas for the v = 1 masers and 8.8 mas for the v = 2 masers. The velocity structure of the SiO masers appears to be random, with no significant indication of global expansion/infall or rotation. The determined near-infrared, K-band, uniform disk (UD) diameters decreased from ~10.5 mas at phase 0.80 to ~10.2 mas at phase 0.95. For the epoch of our VLBA measurements, an extrapolated UD diameter of Θ = 10.8 ± 0.3 mas was obtained, corresponding to a linear radius of R = 2.3 ± 0.5 AU or R = 490 ± 115 R☉. Our coordinated VLBA/VLTI measurements show that the masers lie relatively close to the stellar photosphere at a distance of ~2 photospheric radii, consistent with model estimates. This result is virtually free of the usual uncertainty inherent in comparing observations of variable stars widely separated in time and stellar phase.

We have made the first unambiguous detection of vibrational ground-state maser emission from 28SiO toward six evolved stars. Using the Very Large Array (VLA), we simultaneously observed the v = 0, J = 1-0, 43.4 GHz ground-state transitions and the v = 1, J = 1-0, 43.1 GHz first excited-state transitions of 28SiO toward the oxygen-rich evolved stars IRC +10011, o Ceti, W Hya, RX Boo, NML Cyg, and R Cas and the S-type star χ Cyg. We detected at least one v = 0 SiO maser feature from six of the seven stars observed, with peak maser brightness temperatures ranging from 10,000 to 108,800 K. In fact, four of the seven v = 0 spectra show multiple maser peaks, a phenomenon that has not been previously observed. Ground-state thermal emission was detected for one of the stars, RX Boo, with a peak brightness temperature of 200 K. Comparing the v = 0 and v = 1 transitions, we find that the ground-state masers are much weaker, with spectral characteristics different from those of the first excited-state masers. For four of the seven stars, the velocity dispersion is smaller for the v = 0 emission than for the v = 1 emission; for one star, the dispersions are roughly equivalent; and for two stars (one of which is RX Boo), the velocity spread of the v = 0 emission is larger. In most cases, the peak flux density in the v = 0 emission spectrum does not coincide with the v = 1 maser peak. Although the angular resolution of these VLA observations was insufficient to completely resolve the spatial structure of the SiO emission, the SiO spot maps produced from the interferometric image cubes suggest that the v = 0 masers are more extended than their v = 1 counterparts.

Ethyl cyanide (CH3CH2CN) emission and absorption have been imaged with the Very Large Array toward Sagittarius B2(N-LMH) by means of the 5(15)-4(14) rotational transition at 43.5 GHz (lambda similar to mm). The 1."5 x 1."4 VLA beam shows two principal sources of ethyl cyanide emission: an unresolved source similar to5" north of the LMH that is kinematically consistent with simple expansion, contraction, or small-scale turbulence, and the resolved LMH core source itself that shows kinematics indicating an edge-on rotating disk that extends greater than or equal to3" (similar to0.1 pc) in the approximate east-west direction. A search for the 7(07) - 6(06) rotational transition of the amino acid glycine (NH2CH2COOH) at 43.7 GHz toward Sgr B2(N-LMH) gave negative results.

High-resolution images of the hypercompact H II (HC H II) regions in W3 IRS 5 taken with the Very Large Array (VLA) at 1.3 and 0.7 cm are presented. Four HC H II regions were detected with sufficient signal-to-noise ratios to allow the determination of relevant parameters, such as source position, size, and flux density. The sources are slightly extended in our similar to0."2 beams; the deconvolved radii are less than 240 AU. A comparison of our data with VLA images taken at epoch 1989.1 shows proper motions for sources IRS 5a and IRS 5f. Between 1989.1 and 2002.5, we find a proper motion of 210 mas at a position angle of 12degrees for IRS 5f and a proper motion of 190 mas at a position angle of 50degrees for IRS 5a. At the assumed distance to W3 IRS 5, 1.83 +/- 0.14 kpc, these offsets translate to proper motions of similar to135 and similar to122 km s(-1), respectively. These sources are either shock-ionized gas in an outflow or ionized gas ejected from high-mass stars. We find no change in the positions of IRS 5d1/d2 and IRS 5b, and we show through a comparison with archival NICMOS 2.2 mum images that these two radio sources coincide with the infrared double constituting W3 IRS 5. These sources contain B or perhaps O stars. The flux densities of the four sources have changed compared to the epoch 1989.1 results. In our epoch 2002.5 data, none of the spectral indices obtained from flux densities at 1.3 and 0.7 cm are consistent with optically thin free-free emission; IRS 5d1/d2 shows the largest increase in flux density from 1.3 to 0.7 cm. This may be an indication of free-free optical depth within an ionized wind, a photoevaporating disk, or an accretion flow. It is less likely that this increase is caused by dust emission at 0.7 cm.

We have used the Very Large Array, linked with the Pie Town Very Long Baseline Array antenna, to determine the astrometric positions of 19 radio stars in the International Celestial Reference Frame (ICRF). The positions of these stars were directly linked to the positions of distant quasars through phase-referencing observations. The positions of the ICRF quasars are known to 0.25 mas, thus providing an absolute reference at the angular resolution of our radio observations. Average values for the errors in our derived positions for all sources were 13 and 16 mas in α cos δ and δ, respectively, with accuracies approaching 1–2 mas for some of the stars observed. Differences between the ICRF positions of the 38 quasars and those measured from our observations showed no systematic offsets, with mean values of -0.3 mas in α cos δ and -1.0 mas in δ. Standard deviations of the quasar position differences of 17 and 11 mas in α cos δ and δ, respectively, are consistent with the mean position errors determined for the stars. Our measured positions were combined with previous Very Large Array measurements taken from 1978 to 1995 to determine the proper motions of 15 of the stars in our list. With mean errors of ≈1.6 mas yr-1, the accuracies of our proper motions approach those derived from Hipparcos and, for a few of the stars in our program, are better than the Hipparcos values. Comparing the positions of our radio stars with the Hipparcos Catalogue, we find that at the epoch of our observations, the two frames are aligned to within formal errors of approximately 3 mas. This result confirms that the Hipparcos frame is inertial at the expected level.

We previously reported Very Large Array and Berkeley-Illinois-Maryland Association (BIMA) array observations that suggested rotation of the SiO maser shell surrounding the long-period variable (LPV) in the R Aquarii binary system. In the present Very Long Baseline Array (VLBA) work, we report high spatial and spectral resolution observations of the v = 1, J = 1-0, SiO maser line that confirm our previous result and further suggest that the LPV maser shell is undergoing differential rotation. The 8-34 yr range of rotational periods resulting from differential rotation of the maser shell contains the ~18 yr period reported previously. The velocity structure of the VLBA data suggests a rotation symmetry axis oriented at a position angle of ~150°. The differential rotation model can be envisioned as a series of nested thin spherical shells that have a common rotation axis; each thin shell is characterized by its radius, r, with the innermost shell rotating fastest and the outermost shell slowest, in accordance with an equatorial plane velocity law of the form v ∝ 1/2. We find that q ≈ 1.09 is necessary to approximate the VLBA data, suggesting that the differential rotation is approximately Keplerian.

Future space-based optical astrometric satellite missions present the possibility of directly linking the radio and optical reference frames at the microarcsecond level. We have evaluated the current database of radio observations of the extragalactic objects that make up the International Celestial Reference Frame (ICRF) to determine the optimum candidates in terms of their radio properties for use as radiooptical frame tie sources. We use up-to-date radio astrometric and ancillary data to evaluate the sources in terms of their suitability for use by the Space Interferometry Mission (SIM), but the results apply equally well to any optical astrometric satellite or optical ground-based observations with sufficient brightness sensitivity to measure precise astrometric positions of extragalactic objects directly. Currently, the ICRF limits the accuracy of any radio-optical frame tie based on future optical astrometric satellite observations if the projected accuracies for these missions are realized. We have attempted to construct a suitable set of frame tie sources but found a significant deficit of candidate sources in the southern hemisphere. If the SIM optical frame is to be successfully tied to the ICRF, additional radio observations are necessary to overcome the limitations of the radio frame.

We have discovered a new peak of emission in the v = 1, J = 1-0, SiO maser spectrum of the supergiant NML Cygni. This new peak of emission is blueshifted by ~18 km s-1 from the previously detected SiO masers around VLSR ≈ 0 km s-1. A double-peaked SiO maser profile makes NML Cyg unique among evolved stars. We present the first images of the SiO masers toward NML Cyg observed using the Very Long Baseline Array. These images span the entire spectrum of NML Cyg, including the new blueshifted peak. We find that the masers form an elliptical ring of emission approximately 33 × 27 mas with the major axis oriented at a position angle of ~141°. The orientation of the ring is consistent with the northwest-southeast elongation previously observed in both the OH and H2O maser shells. The elongation axis of the SiO maser shell also provides an axis of symmetry for the velocity structure of the masers. The observed velocity structure is interpreted as rotation of the SiO maser shell with a velocity Vm sin i ≈ 11 km s-1 relative to a systemic velocity of -6.6 km s-1.

We report Very Large Array (VLA) observations taken in 1996 November and 1998 May of the v = 1, J = 1-0, SiO maser line and BIMA array observations taken in 1999 December and 2000 February of the v = 1, J = 2-1, SiO maser line associated with the long-period variable (LPV) in the R Aquarii binary system that suggest rotation of the maser shell. From these interferometric data cubes, we determine that the maser shell rotation axis is approximately northeast-southwest, thus aligning approximately with the direction of the R Aqr jet; the sense of the maser shell rotation is such that northwest is approaching and southeast is receding; the period of rotation is ~17 yr. Alternatively, co-adding 72 time series spectra of the v = 1, J = 1-0, SiO maser line obtained during the period 1984 July-1990 May with a single-dish antenna, we constructed a composite spectral emission envelope that shows the LSR velocity limits of maser emission over this epoch. From this composite spectral emission envelope and Very Long Baseline Array observations in 1996 February of the v = 1, J = 1-0, SiO maser line, which show the maximal spatial extent of the maser shell, we obtain a shell rotation period of ~18 yr, which is in excellent agreement with the VLA and BIMA array results and represents the maximum rotation period of the LPV if corotating with the maser shell. On the other hand, we obtain a minimum rotation period for the LPV of ~5 yr if the LPV supplies material to the maser shell under the constraint of conservation of angular momentum. The ~5-18 yr range for the rotational period of the LPV determined here and the ~18 yr rotational period for the hot companion determined by previous investigators suggest that tidal effects at successive periastron passages in the R Aqr binary system are tending to synchronize these stellar rotational periods to the orbital period of ~44 yr.

We observed the v = 1, J = 2 --> 1, 86 GHz SiO maser transition from Mira (o Ceti) near stellar pulsation phase phi = 0.3 with the Coordinated Millimeter VLBI Array. Maser emission was detected on two of the baselines, and general size and brightness parameters were extracted. For this epoch (1998.26), we find that the maser features extend over roughly 1.5 to 2 stellar radii R-* in the north-south direction. The limited image fidelity was insufficient to assess whether a ringlike morphology typical of other evolved stars with SiO masers appears in Mira also. The brightness temperatures (less than or equal to 10(10.5) K) and spot sizes (greater than or equal to 0." 001) generally resemble those of other maser sources measured by VLBI at 86 GHz, such as VX Sgr and R Gas. We find no coherent velocity structure suggested by the 86 GHz SiO masers toward Mira.

We present the first very long baseline interferometric observations of the 616–523, 22 GHz H2O maser emission associated with a proto–planetary nebula candidate. We obtained three epochs of observation of the water masers associated with IRAS 19296+2227, using the Very Long Baseline Array, operated by the National Radio Astronomy Observatory. These observations show that the water masers are distributed over a 50 mas arclike structure covering a limited velocity range of ∼9 km s-1. The masers most likely reside in an asymptotic giant branch–like circumstellar shell, although we cannot rule out the possibility that they lie in a shocked region. Over the 40 days spanning the epochs of observation, we have measured an error-weighted average proper motion of 6.16 ± 1.32 km s-1, indicating that the masers are not in a high-velocity outflow.

SIM anticipates using an all-sky stellar grid for astrometric, instrumental calibration, and to perform wide-angle astrometry A small number of quasars will be included in the SIM astrometric grid star observational program to provide both an anchor for the astrometric grid as well as to tie the grid to the international Celestial Reference Frame (ICRF). We report preliminary results of a study of the astrometric properties of the radio sources comprising the ICRF with a focus on suitability for use in the SIM astrometric grid.

We have observed v = 1, J = 1 - 0, 43-GHz SiO maser emission towards the symbiotic Mira variable RAqr at four different epochs in the stellar pulsation cycle using the VLBA.

We monitored the 22 GHz H2O maser emission toward W49N from December 1989 through May 1990. During this period we observed an outburst in a component at -66.25 km s-1. The flux density of the flaring component increased by a factor of ≳10 to a maximum of 4020 Jy over a period of 24 days and decreased over the following 34 days to 1400 Jy on the last day of monitoring. During the flux increase the line narrowed from about 1.1 to 0.8 km s-1; it subsequently rebroadened to 1.0 km s-1. Most interestingly, during the flaring behavior the line center shifted by approximately 0.5 km s-1 over the 58 day period. To explain the flare, particularly its shifting line center, we present a model of two interacting maser clouds. In the model, saturated maser radiation produced in a background cloud is amplified by an unsaturated, masing foreground cloud. Motion of the foreground cloud across the line of sight produces a flaring line, accompanied by line narrowing. We demonstrate that the observed flare in W49N may be explained by such a model where a rotating foreground cloud passes in front of a nonrotating background cloud. The differential amplification of the background cloud's radiation produces the observed increase in flux density, line narrowing, and systematic shift in center velocity.

We have made the first detection of circumstellar SiO maser proper motions in the envelope of a late-type star. Using the Very Long Baseline Array (VLBA), we have obtained observations at four epochs of the 43 GHz, v = 1, J = 1-0 SiO maser emission toward the Mira variable in the symbiotic binary R Aqr. The maser emission has a ringlike structure approximately 31 mas across with a slight elongation in the north-south direction. We find that the emission changes significantly over a timescale of about 1-2 months with almost no similarity in structure for timescales ≳6 months. Our observations show that over a 98 day period the masers have an average inward proper motion of about 1 mas. This contraction of the ring implies an infall velocity of about 4 km s-1 for the SiO masers in the circumstellar envelope.