Going slitless:: Images of forbidden-line emission regions of classical T Tauri stars observed with the Hubble Space Telescope

We have observed five classical T Tauri stars known to have strong forbidden-line emission with STIS in slitless mode on the Hubble Space Telescope. This technique makes it possible to image jets within a few tens of AU of their exciting sources, a region of great interest for models of accretion disks and jets. Slitless images generate emission-line images at all wavelengths, including those where no narrowband filters exist. Images of the forbidden-line regions around each object, constructed by subtracting the stellar continuum and combining observations taken at different orientations, show [O I] jets from CW Tau, HN Tau, UZ Tau E, DF Tau, and the primary of DD Tau. Jets exist on both sides of the close binary DF Tau, either as a jet and its counterjet or as separate jets from the primary and secondary. Several emission lines not previously seen in jets close to the star exist in the HN Tau jet; the [ Fe II] lambda\7155/lambda8617 ratio is particularly useful because it measures the electron density in the densest regions of stellar jets, where log N(e)greater than or similar to6. Electron densities in the inner 30 AU of the HN Tau jet range from log N-e = 6.2 to 6.9. We construct diagnostic diagrams for the density, temperature, and ionization fraction in jets close to their stars, using various emission lines of O I and O II. The red auroral [ O II] lines are bright close to HN Tau, indicating that the emitting regions of the inner 35 AU of the jet have a substantial ionized component - 20% if the emission comes from a shock and 50% for an isothermal flow. We discuss mass-loss rates and filling factors for these two cases. The intensity of the HN Tau jet in [ O I] lambda6300 declines exponentially with distance beyond similar to 15 AU. The superior continuum subtraction with slitless data, as compared with narrowband images, makes it possible to resolve the widths of jets at distances as close as similar to 15 AU from the star. The two best examples, HN Tau and UZ Tau E, have jets that expand with distance. When projected back to the source, the width of the jet in HN Tau is a few AU at the 3 sigma level, while the jet in UZ Tau E is spatially unresolved. The new images of CW Tau reveal proper motions in this jet, which has ejected at least two knots since 1980. There is no indication that CW Tau brightened when it ejected the largest of these knots, but the photometric record of this star over the last two decades is fragmentary.