MODELING OF THE HYDROGEN MASER DISK IN MWC-349

Maser amplification in a Keplerian circumstellar disk seen edge on-the idea put forward by Gordon (1992), Martin-Pintado, & Serabyn (1992), and Thum, Martin-Pintado, & Bachiller (1992) to explain the millimeter hydrogen recombination lines in MWC 349-is further justified and deve loped here. The double-peaked (vs. possible triple-peaked) form of the observed spectra is explained by the reduced emission from the inner portion of the disk, the portion responsible for the central (''zero v elocity'') component of a triple-peaked spectrum. Radial gradient of e lectron density and/or free-free absorption within the disk are identi fied as the probable causes of this central ''hole'' in the disk and o f its opacity. We calculate a set of synthetic maser spectra radiated by a homogeneous Keplerian ring seen edge-on and compare them to the H 30alpha observations of Thum et al., averaged over about 1000 days. We used a simple graphical procedure to solve an inverse problem and ded uced the probable values of some basic disk and maser parameters. We f ind that the maser is essentially unsaturated, and that the most proba ble values of electron temperature, Doppler width of the microturbulen ce, and electron density, all averaged along the amplification path ar e, correspondingly, T(e) less-than-or-equal-to 11,000 K, v(micro) less -than-or-equal-to 14 km s-1, n(e) almost-equal-to (3 +/- 2) x 10(7) cm -3. The model shows that radiation at every frequency within the spect rum arises in a monochromatic ''hot spot.'' The maximum optical depth within the 'hot spot '' producing radiation at the spectral peak maxim um is tau(max) almost-equal-to 6 +/- 1; the effective width of the mas ing ring is almost-equal-to 0.4-0.7 times its outer diameter; the size of the ''hot spot'' responsible for the radiation at the spectral pea k frequency is almost-equal-to 0.2-0.3 times the distance between the two ''hot spots'' corresponding to two peaks. An important derivation of our model is the dynamical mass of the central star, M almost-equa l-to 26 M.(D/1.2 kpc), D being the distance to the star. Prospects for improving the model are discussed.