Showing papers by "Gary J. Melnick published in 1990"

A new submillimeter water maser transition at 325 GHz

Abstract: The first astronomical detection of the 325 GHz 5(15) - 4(22) transition of interstellar water vapor is reported. The sources toward which the line was detected, namely the luminous IR and radio continuum sources Orion-KL, W49N, and W51 Main and the low-luminosity cold source IRAS 1629A, are also strongly emitting in the 22 GHz water maser line. Both lines show high-velocity emission toward W49N. It is concluded that 325 GHz masers can arise in the same variety of sources as the 22 GHz masers. Emission at the frequency of the H2()-(18) line was tentatively detected toward Orion-KL. 21 refs.

Submillimeter water masers

Abstract: The excitation of maser emission in millimeter and submillimeter transitions of interstellar and circumstellar water is considered. Using an escape probability method, the equilibrium populations in 349 rotational states of both ortho- and para-water have been determined under varying conditions of gas temperature, density, water abundance and radiation field. Results indicate that under suitable conditions, collisional excitation of warm (400 K) interstellar gas may result in significant maser action in a total of 7 centimeter, millimeter, and submillimeter transitions of H2O, while at somewhat higher gas temperatures (1000 K) a total of 22 centimeter, millimeter, and submillimeter H2O transitions are found to mase. Under conditions of low atmospheric humidity, several of these transitions might be observed from mountaintop altitude, and all of them are potentially observable from airborne observatories. Under the assumption that water masers from different transitions originate in the same gas and that the emission is equally beamed, the ratio of maser luminosities from different transitions can be used to set limits on the temperature as well as the bundled quantity of hydrogen density, water abundance, and velocity gradient within the masing gas.

Interpretation of the newly discovered submillimeter water maser

Abstract: The 10(29)-9(36)321 GHz water maser discovered recently by Menten et al. (1990) may be pumped collisionally under a wide range of physical conditions. This range of conditions is similar but not identical to that required to excite the well-studied 22 GHz water maser. The ratio of the observed 22 GHz and 321 GHz intersteller maser luminosities can be explained by collisional excitation within the same warm, dense gas. Collisional pumping in a circumstellar envelope can account for the observed 321 GHz maser emission in the supergiant star VY CMa.

Further observations of rotationally excited far-infrared O-16H and O-18H emission in Orion-KL - Tighter constraints on the nature of the emitting region

Abstract: The Orion-KL region, within 1 arc minute, is observed. The rotational cross ladder (53.351 microns) and rotational ground state (120.1719 microns) transitions are studied. It is shown that these lines exhibit a P-Cygni profile and unambiguously show that the OH gas is expanding out from the central BN-KL infrared cluster. The OH-16 rotational ground state transition (119.234 microns) is velocity resolved and it is found that its intrinsic full width at half maximum is 75 km/s. The line fluxes and line profiles are modeled and it is shown that no single temperature and density component can reproduce the data. Rather, the best fit to the data requires emission from three main components of the gas: post shocked gas with the profiles of temperature, density, and OH abundance; a high density component to the cool post shocked region; and the plateau region.