We report the discovery of H 2 O maser emission at 1.35 cm wavelength in seven active galactic nuclei (at distances up to < 80 Mpc) during a survey conducted at the 70-m diameter antenna of the NASA Deep Space Network near Canberra, Australia. The detection rate was ∼ 4%. Two of the maser sources are particularly interesting because they display satellite high-velocity emission lines, which are a signature of emission from the accretion disks of supermassive black holes when seen edge on. Three of the masers are coincident, to within uncertainties of 0. ′′ 2, with continuum emission sources we observed at about λ1.3 cm. We also report the discovery of new spectral features in the Circinus galaxy H 2 O maser that broaden the known velocity range of emission therein by a factor of ∼ 1.7. If the new spectral features originate in the Circinus accretion disk, then molecular material must survive at radii ∼ 3 times smaller than had been believed previously (∼ 0.03 pc or ∼ 2 × 10 5 Schwarzschild radii).

L1498 is a classic example of a dense cold pre-protostellar core. To study the evolutionary status, the structure, dynamics, and chemical properties of this core we have obtained high spatial and high spectral resolution observations of molecules tracing densities of 10(3)-10(5) cm-3. We observed CCS, NH3, C3H2, and HC7N with NASA's DSN 70 m antennas. We also present large-scale maps of C18O and 13CO observed with the AT&T 7 m antenna. For the high spatial resolution maps of selected regions within the core we used the VLA for CCS at 22 GHz, and the Owens Valley Radio Observatory (OVRO) MMA for CCS at 94 GHz and CS (2-1). The 22 GHz CCS emission marks a high-density [n(H2) > 10(4) cm -3] core, which is elongated with a major axis along the SE-NW direction. NH3 and C3H2 emissions are located inside the boundary of the CCS emission. C18O emission traces a lower density gas extending beyond the CCS boundary. Along the major axis of the dense core, CCS, NH3 and C3H2 emission show evidence of limb brightening. The observations are consistent with a chemically differentiated onion-shell structure for the L1498 core, with NH3 in the inner and CCS in the outer parts of the core. The high angular resolution (9"-12") spectral line maps obtained by combining NASA Goldstone 70 m and VLA data resolve the CCS 22 GHz emission in the southeast and northwest boundaries into arclike enhancements, supporting the picture that CCS emission originates in a shell outside the NH3 emitting region. Interferometric maps of CCS at 94 GHz and CS at 98 GHz show that their emitting regions contain several small-scale dense condensations. We suggest that the differences between the CCS, CS, C3H2, and NH3 emission are caused by a time-dependent effect as the core evolves slowly. We interpret the chemical and physical properties of L1498 in terms of a quasi-static (or slowly contracting) dense core in which the outer envelope is still growing. The growth rate of the core is determined by the density increase in the CCS shell resulting from the accretion of the outer low-density gas traced by C18O. We conclude that L1498 could become unstable to rapid collapse to form a protostar in less than 5 x 10(6) yr.

We report the discovery of water maser emission in eight active galactic nuclei (AGN) with the 70-m NASA Deep Space Network (DSN) antennas at Tidbinbilla, Australia and Robledo, Spain. The positions of the newly discovered masers, measured with the VLA, are consistent with the optical positions of the host nuclei to within 1σ (0. ′′ 3 radio and 1. ′′ 3 optical) and most likely mark the locations of the embedded central engines. The spectra of two sources, NGC 3393 and NGC 5495, display the characteristic spectral signature of emission from an edge-on accretion disk, with orbital velocities of ∼ 600 and ∼ 400 km s −1 , respectively. In a survey with DSN facilities of 630 AGN selected from the NASA Extragalactic Database, we have discovered a total of 15 water maser sources. The resulting incidence rate of maser emission among nearby (v sys < 7000 km s −1) Seyfert 1.8 − 2.0 and LINER systems is ∼ 10% for a typical rms noise level of ∼ 14 mJy over 1.3 km s −1 spectral channels. As a result of this work, the number of nearby AGN (v sys < 7000 km s −1) observed with < 20 mJy rms noise has increased from 130 to 449.

A B S T R A C TPositions with subarcsecond accuracy have been measured for seven 22-GHz H 2 O masers associated with H II regions in the Large Magellanic Cloud (LMC); two of the masers are new detections. Initial position measurements were obtained with the 70-m antenna of the Canberra NASA Deep Space Network during a period of more than two years in which the antenna was used to monitor the maser emission. The positions were further improved using 22-GHz observations involving three antennas of the Australia Telescope Compact Array.The positions have been compared with those of 1.6-GHz continuum emission and other LMC masers (of OH and CH 3 OH). The H 2 O maser positions range from within 1 arcsec (270 mpc) of the centre of a compact H II component to beyond the boundary of significant continuum emission. Three of the four masers located near continuum peaks are close to OH masers. In two cases the positional agreement is better than 0.2 arcsec (53 mpc); in the third case the agreement is worse (0.9 arcsec) but the positions of the individual H 2 O features appear to be spread over more than 1 arcsec. The velocities of the OH masers are within the spread of the H 2 O velocities. The three H 2 O masers offset from continuum centres are located 3 -7 arcsec from optical or infrared phenomena probably associated with very early stages of star formation; no other molecular masers are known in these directions.

We have investigated the physical and chemical status of the pre-protostellar core B68. A previous extinction study suggested that the density profile of B68 is remarkably consistent with a Bonnor-Ebert sphere with 2.1 M at 16 K. We mapped B68 in C 3 H 2 , CCS, and NH 3 with the Deep Space Network (DSN) 70 m telescope at Goldstone. Our results show that the NH 3 peak coincides with the dust continuum peak, whereas CCS and C 3 H 2 are offset from the NH 3 and dust peaks. The B68 chemical structure is consistent with that seen in other such pre-protostellar cores (L1498 and L1544) and is explained by time-dependent chemical models that include depletion. We measured the kinetic temperature of B68 with NH 3 (1, 1) and (2, 2) spectra obtained with a DSN 34 m telescope. We find that the kinetic temperature of B68 is only 11 K, which is significantly lower than that previously assumed. We also derive the nonthermal line width in B68 and show that B68 is thermally dominated with little contribution from turbulence support (less than 10%). We consider a modified Bonnor-Ebert sphere to include effects of turbulence and magnetic fields and use it to constrain the uncertainties in its distance determination. We conclude that the distance to B68 is $95 pc with a corresponding mass of $1.0 M . If some magnetic field is present, it can be further away (beyond $100 pc) and still satisfy the density structure of a Bonnor-Ebert sphere. The sulfur (CS and CCS) and carbon chain (C 3 H 2 ) molecules are heavily depleted in B68 and do not trace the dense interior region. We see some evidence for depletion of NH 3 at the core center roughly on a scale similar to that of N 2 H + . Our observations do not preclude any instability such as the onset of collapse, or slow contraction, occurring in the center of the core, which cannot be resolved with our beam size (45 00 ).