Structured He II 4686A emission in eta Car near periastron

ATel #6334; Frederick M Walter (Stony Brook University)
on 21 Jul 2014; 22:38 UT
Credential Certification: Fred Walter (fwalter@mail.astro.sunysb.edu)

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We are obtaining frequent high-SN, high resolution optical spectra of eta Car to monitor changes in the high excitation He II 4686 line as it approaches its 2014 periastron. The Chiron spectrograph (Tokovinin et al., PASP, 125, 1336, 2013) is a fiber-fed spectrograph on the SMARTS/CTIO 1.5m telescope optimized for radial velocities, working from 460 through 880 nm. We observe with the image slicer, which gives a resolution R approx 78,000. We have obtained 28 spectra since 16 May 2013, with 19 taken between 9 May and 17 July 2014.

The transient He II 4686 emission that appears in eta Car prior to periastron is likely diagnostic of the colliding winds. The emission line has been measurable in spectra since 9 May 2014, but the line profile varies. It often resembles a single Gaussian with a velocity that has generally tracked that seen in 2009 (Mehner et al. ApJ, 740, 80, 2011). But on 8 occasions it has separated into two or three components, with significant changes within 24 hours. This may indicate a highly structured velocity- and time-variable shock. The Gaussian width of the stronger emission component has systematically increased from about 3 to 6 Angstroms over the past 2 months. The equivalent width of the He II emission tripled between 22 June and 17 July 2014.

We thank Emily MacPherson of the SMARTS consortium for scheduling the Chiron observations, and the SMARTS service observers, including Rodrigo Hernandez, for taking the data.