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

• Published in: Monthly notices of the Royal Astronomical Society 2024-01, Vol.527 (3), p.9176-9184
• Main Authors: Morse, Jon A, Smith, Nathan
• Format: Article
• Language: English
• Subjects: Astronomical models; Field cameras; Kinematics; Nebulae; Neutral gases; Space telescopes; Stellar winds
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stad3790

ABSTRACT 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.