Kinematic properties and ages of extended fast, neutral gas around η Carinae: tracing the pre-eruption bipolar wind

We present proper-motion measurements and long-slit spectroscopy of the Mg II nebula around η Carinae obtained with the Wide Field Camera 3 and Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope. Detailed kinematics of the Mg II-emitting material constrain the geometry and history of mass-loss from η Car, and provide estimated ejection dates, assuming linear, ballistic motions. These measurements show that the neutral gas immediately outside the Homunculus – i.e. material into which the Homunculus is now expanding – was expelled over several decades prior to the Great Eruption, thus representing unshocked pre-eruption stellar wind. Material outside the Homunculus is therefore not part of a Hubble-like flow from the Great Eruption itself. This result discriminates between versions of merger-in-a-triple models for η Car. The STIS spectrum of Mg II-emitting gas along the projected outflow axis displays radial velocities consistent with bipolar expansion, redshifted several hundred km s⁻¹ towards the northwest, similarly blueshifted towards the southeast, and with low internal velocity dispersion. The η Car system was therefore losing mass in a relatively fast, low-density polar wind for several decades that probably traces the critical inspiral phase preceding a merger event.