Diagnostic Diagram for [O I] Emission Lines

Temperature, Density, and Ionization Fraction diagnostic diagrams for forbidden lines in jets. (a) The [O I] line ratio of 5577A / 6300A depends on both temperature and density. The heavy curve is the typical temperature of [O I] emission in postshock gas. The horizontal lines mark the observed, and +/- 1 sigma values for the ratio observed close to HN Tau, which implies a density of ~ 10^7 cm^-3. (b) The ratio of the red [O II] quartet to [O I] 5577 is insensitive to density for a fixed ionization fraction. The point, discussed in the paper, assumes both O I and O II emit at 10^4K. (c) As in (b), but curves are shown for a variety of ionization fractions with T(O II) = 10^4 K and T(O I) = 6500 K, as appropriate for low velocity radiative shocks. As in (b), the point is for the HN Tau jet about 5 AU from the star. (d) The [O II] 7325 / [O I] 6300 ratio is shown as a function of temperature and density for an ionization fraction of 0.5. (e) Same as (d) but for different ionization fractions at 10^4 K. The triangle, circle, and square are for positions in the jet 30 AU, 16 AU, and 5 AU from the star, respectively. The densities for the triangle and circle were determined from [Fe II] line ratios, and the square from the [O I] 5577 / [O I] 6300 ratio, as described in the text. (f) Same as (e) but for temperatures appropriate for shocked gas. These plots show that the HN Tau jet has a substantial ionization fraction within 50 AU of the star.