We report the first detection of submillimeter water maser emission toward water-fountain nebulae, which are post-AGB stars that exhibit high-velocity water masers. Using APEX we found emission in the ortho-H2O (10_29-9_36) transition at 321.226 GHz
toward three sources: IRAS 15445-5449, IRAS 18043-2116 and IRAS 18286-0959. Similarly to the 22 GHz masers, the submillimeter water masers are expanding with a velocity larger than that of the OH masers, suggesting that these masers also originate in fast bipolar outflows. In IRAS 18043-2116 and IRAS 18286-0959, which figure among the sources with the fastest water masers, the velocity range of the 321 GHz masers coincides with that of the 22 GHz masers, indicating that they likely coexist. Towards IRAS 15445-5449 the submillimeter masers appear in a different velocity range, indicating that they are tracing different regions. The intensity of the submillimeter masers is comparable to that of the 22 GHz masers, implying that the kinetic temperature of the region where the masers originate should be Tk > 1000 K. We propose that the passage of two shocks through the same gas can create the conditions necessary to explain the presence of strong high-velocity 321 GHz masers coexisting with the 22 GHz masers in the same region.
CRL 618 is an object that exhibits characteristics of both AGB and post-AGB star. It also displays a spectacular array of bipolar lobes with a dense equatorial region, which makes it an excellent object to study the development of asymmetries in evol
ved stars. In the recent decades, an elliptical compact HII region located in the center of the nebula has been seen to be increasing in size and flux. This seems to be due to the ionization of the circumstellar envelope by the central star, and it would be indicating the beginning of the planetary nebula phase for CRL 618. We analyzed interferometric radio continuum data at ~5 and 22 GHz from observations carried out at seven epochs with the VLA. We traced the increase of the flux of the ionized region over a period of ~26 years. We measured the dimensions of the HII region directly from the brightness distribution images to determine the increase of its size over time. For one of the epochs we analyzed observations at six frequencies from which we estimated the electron density distribution. We carried out model calculations of the spectral energy distribution at two different epochs to corroborate our observational results. We found that the radio continuum flux and the size of the ionized region have been increasing monotonically in the last three decades. The size of the major axis of the HII region shows a dependance with frequency, which has been interpreted as a result of a gradient of the electron density in this direction. The growth of the HII region is due to the expansion of an ionized wind whose mass-loss rate increased continuously for a period of ~100 years until a few decades ago, when the mass-loss rate experienced a sudden decline. Our results indicate that the beginning of the ionization of the circumstellar envelope began around 1971, which marks the start of the planetary nebula phase of CRL 618.
K 3-35 is a planetary nebula (PN) where H2O maser emission has been detected, suggesting that it departed from the proto-PNe phase only some decades ago. Interferometric VLA observations of the OH 18 cm transitions in K~3-35 are presented.OH maser em
ission is detected in all four ground state lines (1612, 1665, 1667, and 1720 MHz). All the masers appear blueshifted with respect to the systemic velocity of the nebula and they have different spatial and kinematic distributions.The OH 1665 and 1720 MHz masers appear spatially coincident with the core of the nebula, while the OH 1612 and 1667 MHz ones exhibit a more extended distribution. We suggest that the 1665 and 1720 masers arise from a region close to the central star, possibly in a torus, while the 1612 and 1667 lines originate mainly from the extended northern lobe of the outflow. It is worth noting that the location and velocity of the OH 1720 MHz maser emission are very similar to those of the H2O masers (coinciding within 0.1 and ~2 km/s, respectively). We suggest that the pumping mechanism in the H2O masers could be produced by the same shock that is exciting the OH 1720 MHz transition. A high degree of circular polarization (>50%) was found to be present in some features of the 1612, 1665, and 1720 MHz emission.For the 1665 MHz transition at ~ +18 km/s the emission with left and right circular polarizations (LCP and RCP) coincide spatially within a region of ~0.03 in diameter.Assuming that these RCP and LCP 1665 features come from a Zeeman pair, we estimate a magnetic field of ~0.9 mG within 150 AU from the 1.3 cm continuum peak. This value is in agreement with a solar-type magnetic field associated with evolved stars.
We present observations of continuum (lambda = 0.7, 1.3, 3.6 and 18 cm) and OH maser (lambda = 18 cm) emission toward the young planetary nebula IRAS 17347-3139, which is one of the three planetary nebulae that are known to harbor water maser emissio
n. From the continuum observations we show that the ionized shell of IRAS 17347-3139 consists of two main structures: one extended (size ~1. 5) with bipolar morphology along PA=-30 degrees, elongated in the same direction as the lobes observed in the near-infrared images, and a central compact structure (size ~0. 25) elongated in the direction perpendicular to the bipolar axis, coinciding with the equatorial dark lane observed in the near-infrared images. Our image at 1.3 cm suggests the presence of dense walls in the ionized bipolar lobes. We estimate for the central compact structure a value of the electron density at least ~5 times higher than in the lobes. A high resolution image of this structure at 0.7 cm shows two peaks separated by about 0. 13 (corresponding to 100-780 AU, using a distance range of 0.8-6 kpc). This emission is interpreted as originating in an ionized equatorial torus-like structure, from whose edges the water maser emission might be arising. We have detected weak OH 1612 MHz maser emission at VLSR ~ -70 km/s associated with IRAS 17347-3139. We derive a 3 sigma upper limit of < 35% for the percentage of circularly polarized emission. Within our primary beam, we detected additional OH 1612 MHz maser emission in the LSR velocity ranges -5 to -24 and -90 to -123 km/s, associated with the sources 2MASS J17380406-3138387 and OH 356.65-0.15, respectively.
