We present results of observational programs with the Navy Prototype Optical Interferometer (NPOI) in astrometry, the study the angular diameters of individual stars, and the measurement of binary star systems. These observations include measurements of the closure phase, which allow us to make images of simple systems.

We used the Mark III Optical Interferometer on Mt. Wilson in 1991 and 1992 to measure the diameters of 42 stars with 10 nm wide filters centered at 712 nm and at 754 nm. These filters probe the stellar atmosphere in a strong TiO band (712 nm) and in a “continuum” band relatively free of TiO absorption (754 nm). The data were taken on a North-South baseline that could be configured to lengths between 3.0 m and 31.5 m. Observations of the target stars were interleaved with frequent measurements of calibrator stars. The square of the calibrated visibility amplitude V2 was determined as a function of baseline length, and a uniform disk model fitted to these data (for details see Mozurkewich et al. 1991). The average formal error of the diameter measurements - derived from the χ2 of the model fits - is 2.5%. Data for a subset of 12 stars have already been published (Quirrenbach et al. 1993).

Symbiotic stars are a heterogenous group of interacting binaries, where the combination of a mass-losing giant and a hot compact companion create conditions ideal for studying interacting winds, dust formation, nebular shaping and accretion phenomena. We present HST and VLA observations of the symbiotic nova HM Sge as an example of our work. In this case, we characterise hot and cool parts of the extended nebula, determine the angular binary separation to be 40±9 milli-arcsec, and the interstellar reddening to be no more than E(B-V) = 0.35. Comparison with a suggested binary separation of 50 au gives a distance of 1250±280 pc, consistent with the sparsely-sampled distance-extinction mapping in the direction of HM Sge. A number of other observations of additional high spatial-resolution observations of symbiotic stars have been completed or are being planned.

Deep optical images of the circumstellar envelope of IRC+10216 have been obtained with CFHT and HST, with angular resolutions of 1 and 0.1, respectively. In the outer regions, the envelope is seen in dust-scattered galactic light, and is detected up to ∼200 from the star, corresponding to 8000 years of expansion. The envelope is composed of discrete, nearly circular, thin, limb-brightened shells. These shells are spaced at irregular intervals, and the shell/intershell density contrast is typically 3. Although the HST observations show a distinct bipolar geometry very near the star, the global shell pattern suggests a roughly isotropic, episodic mass loss mechanism.

An analysis of the spatially resolved (0.1) main-sequence stars of four compact multiple systems located in the Large Magellanic Cloud (LMC) is given. For this purpose we compared theoretical synthetic spectra with the observations obtained by means of the Faint Object Spectrograph (FOS) aboard the Hubble Space Telescope (HST).

We summarize multifold evidence for the presence of a hot spot region in contact binary VW Cephei. We interpret this feature as a photospheric burn and the preferential site of flares in EUV and X rays. With its trigonometric parallax of 0.041 arcsec VW Cep is a promising target for a milliarcsecond resolution investigation of the nature of surface features both in optical region and in IR.

Interferometry of red supergiants before 2000: The M2 supergiant Betelgeuse (α Ori) is the first star whose angular diameter was measured by the Michelson stellar interferometer (Michelson & Pease 1921). Since then, however, we had to wait half a century before we witnessed the renaissance of interferometric observations in optical astronomy with novel methods such as speckle interferometry (Laberie 1970). Michelson's classical method was also extended to the infrared regime (e.g. McCarthy et al. 1977). The initial results served not only to compare stellar temperature scale with the measured angular diameters but also to probe scattering and thermal emission in the dust envelope around red supergiant stars (e.g. Tsuji 1978,1979). Interferometry has been more active in the radio domain: In connection with our subject, VLBI observations revealed many masering water clouds around red supergiants such as VY CMa (Imai et al. 1997) and S Per (Richards et al. 1999), but their origin in unknown. Also, radio observation of Betelgeuse with the VLA (Lim et al. 1998) revealed the presence of a new component of modest temperatures (Tex ≲ 3500 K) over the same height range as the classical chromosphere (Tex ≳ 5000 K), but again the nature of the new component remains obscure.

We have made 3.6 cm VLBI observations of the RS CVn binary star IM Pegasi (HR 8703) approximately four times per year since 1997 in support of the NASA/Stanford Relativity Gyroscope Experiment (Gravity Probe B). Phase-referenced maps reveal structural changes in the radio emission of the star on hour time scales during several of the sessions. Analyses of the VLBI phase delays with a Kalman-filter estimator reveal submilliarcsecond motions of the radio centroid of the star on hour and even subhour time scales. The observed structural changes and centroid motions often coincide with rapid changes in the star's flux density, as measured with the VLA. We report on our latest results and summarize our findings to date.

We present preliminary results of our observations of the 7 mm (v=1 and v=2 J=1-0) and 3 mm (v=1 J=2-1) SiO masers in the circumstellar envelopes of evolved stars. The 7mm masers were observed with the Very Large Baseline Array (VLBA) and the European VLBI Network (EVN), and the 3mm ones with the Coordinated Millimeter VLBI Array (CMVA). We find similarities between the maps, despite the non-coincidence in the location of most spots in the different 7 mm lines. The visibilities found for most compact spots are compatible with maser sources as small as 0.13 mas (or 0.023 AU at the distance of R Cas).

The variety of molecular megamasers trace different classes of galaxies and provide information on their various nuclear components. H2O megamaser regions may trace the inner parsec-scale regions with keplerian disks of the LINER and Seyfert 2 nuclei, which are relatively naked and not dominated by dust or starforming regions. The OH and H2CO megamaser emissions exhibit the physics and dynamics of the inner few hundred parsecs of ultra-luminous FIR galaxies, which are dusty, compact, and whose perturbed nuclei are powered by compact starbursts.

The first VLBI images of H2O maser emission in the Circinus Galaxy AGN show both a warped, edge-on accretion disk and an outflow 0.1 to 1 pc from the central engine. The inferred central mass is 1.3 × 106 M⊙, while the disk mass may be on the order of 105 M⊙, based on a nearly Keplerian rotation curve. The bipolar, wide-angle outflow appears to contain “bullets” ejected from within < 0.1 pc of the central mass. The positions of filaments and bullets observed in the AGN ionization cone on kpc-scales suggest that the disk channels the flow to a radius of ∼ 0.4 pc, at which the flow appears to disrupt the disk.

We present results from fitting of water-vapor megamaser emission from NGC 4258 and NGC 1068. Using the radiative transfer, velocity, and coordinate equations, we derive parameters of the maser disc-rings.

We present the first sub-kpc (∼ 0.7 ≈ 0.8 kpc) resolution 12CO(1–0) observations of the ISM in the host galaxy of the QSO I Zw 1 which were obtained with the BIMA mm-interferometer in its A configuration. The measurements, which are part of a multi-wavelength study of I Zw 1, will allow comparison of the ISM properties of a QSO host with those of nearby galaxies and place constraints on galaxy formation/evolution models. Our maps of the 12CO(1–0) line emission from the host galaxy of a QSO show a ring-like structure in the circumnuclear molecular gas distribution with a radius of about 900 pc. The presence of such a molecular gas ring was already predicted from earlier lower angular resolution PdBI observations (Schinnerer, Eckart, & Tacconi 1998). A first comparison of the BIMA data with new PdBI 12CO(2–1) observations with 0.9 angular resolution shows variations in the excitation conditions of the molecular gas in the innermost 3 comprising the nuclear region of I Zw 1.

We present results from the most recent Plateau de Bure observation of the 12CO(2-1) line towards the nearby QSO I Zw 1.

Molecular gas in the host galaxy of the lensed quasar QSO 0957+561 has been detected at the redshift of 1.414 using the IRAM Plateau de Bure interferometer. This detection shows the extended nature of the molecular gas distribution. The molecular gas mass is 2-4 109 M⊙ and it is distributed in a disk that is several kiloparsecs in diameter. A second, weaker component of CO is interpreted as arising from a close companion galaxy moving at a velocity 440 km s−1 higher than the host galaxy.

Using a sample of 7 barred spirals from the BIMA Survey of Nearby Galaxies (SONG), we compare the molecular gas distribution in the bar, to recent massive star formation activity. In all 7 galaxies, Hα is offset azimuthally from the CO on the downstream side. The maximum offset, at the bar ends, ranges from 170-570 pc, with an average of 320±120 pc. We discuss whether the observed offsets are consistent with the description of gas flows in bars provided by the two main classes of models: n-body models and hydrodynamic models.

We present the first results of the OTF CO (J=1-0) mapping of spiral galaxies in the Ursa Major cluster, obtained with the NRAO 12m radio telescope during June 20-27, 1999 and March 23-30, 2000.

I present a unique data set for the study of molecular gas in galaxies: a complete, high-resolution survey of the CO in M 31 and additional local studies. The fully sampled survey has an angular resolution of 23 FWHM and the interferometric data attain the pc-scale with sub-arcsecond resolution. For the first time it is now possible to study large and small scales in conjunction. Thus we are able to derive the global structure and study the links down to the individual cloud complexes and star formation regions.

We have observed three molecular clumps in the Galactic center region in the CS J=2-1 line with the Nobeyama Millimeter Array. We have determined the mass spectrum and other statistical relations for the Galactic center cloud cores. The derived mass spectrum is somewhat steeper than for galactic cloud cores based on molecular line observations. In addition, the spectrum is steeper than that of the large scale Galactic center clumps.

With sub-arcsecond angular resolution accompanied by fast time resolution and spatially resolved spectral capabilities, the Chandra X-ray Observatory provides a unique capability for the study of supernova remnants (SNRs) and pulsars. Though in its relative infancy, Chandra has already returned stunning images of SNRs which reveal the distribution of ejecta synthesized in the stellar explosions, the distinct properties of the forward and reverse shocks, and the presence of faint shells surrounding compact remnants. Pulsar observations have uncovered jet features as well as small-scaled structures in synchrotron nebulae. In this brief review we discuss results from early Chandra studies of pulsars and SNRs.