Hydrogen line and continuum emission in young stellar objects. III - Line ratios and physical conditions

A detailed analysis of the dependence of Br-gamma/Br-alpha and other hydrogen line ratios on nucleon density (over the range 10 to the 10th - 10 to the 12th/cu cm), column density (about 10 to the 18th - 10 to the 24th/sq cm), young stellar object (YSO) luminosity (about 10-10,000 solar luminosities), and distance of the gas cloud from the YSO, r (about 10 to the 12th - 10 to the 14th cm). For a given continuum model, the value of Br-gamma/Br-alpha can provide a constraint on r. The ionization and thermal structures of the emission region are described. The electron fraction is fairly constant and is small (less than 10 percent) in the region where most of the hydrogen line fluxes are produced. The temperature in this region is also quite constant, with a value of 5000-7000 K.