Resolving the Hα-emitting Region in the Wind of η Carinae

The massive evolved star η Carinae is the most luminous star in the Milky Way and has the highest steady wind mass-loss rate of any known star. Radiative transfer models of the spectrum by Hillier et al. predict that Hα is mostly emitted in regions of the wind at radii of 6–60 au from the star (2.5–25 mas at 2.35 kpc). We present diffraction-limited images (FWHM ~ 25 mas) with Magellan adaptive optics in two epochs, showing that η Carinae consistently appears ~2.5–3 mas wider in Hα emission compared to the adjacent 643 nm continuum. This implies that the Hα line-forming region may have a characteristic emitting radius of 12 mas or ~30 au, in very good agreement with the Hillier stellar-wind model. This provides direct confirmation that the physical wind parameters of that model are roughly correct, including the mass-loss rate of , plus the clumping factor, and the terminal velocity. Comparison of the Hα images (ellipticity and PA) to the continuum images reveals no significant asymmetries at Hα. Hence, any asymmetry induced by a companion or by the primary's rotation do not strongly influence the global Hα emission in the outer wind.

[1]  J. Vink,et al.  On the Hα behaviour of blue supergiants: rise and fall over the bi-stability jump , 2014, 1403.4097.

[2]  D. Morris,et al.  The 2014 X-Ray Minimum of η Carinae as Seen by Swift , 2017 .

[3]  Nathan Smith,et al.  The UV Scattering Halo of the Central Source Associated with η Carinae , 2006 .

[4]  S. Owocki,et al.  Modelling the Central Constant Emission X-ray component of η Carinae , 2016, 1603.01629.

[5]  N. Smith,et al.  The Structure of the Homunculus. I. Shape and Latitude Dependence from H2 and [Fe II] Velocity Maps of η Carinae , 2006, astro-ph/0602464.

[6]  S. Owocki,et al.  The extended interacting wind structure of Eta Carinae , 2009 .

[7]  N. Smith,et al.  A revised historical light curve of Eta Carinae and the timing of close periastron encounters , 2010, 1010.3719.

[8]  Tyson Hare,et al.  First closed-loop visible AO test results for the advanced adaptive secondary AO system for the Magellan Telescope: MagAO's performance and status , 2012, Other Conferences.

[9]  F. Millour,et al.  VLTI-AMBER velocity-resolved aperture-synthesis imaging of η Carinae with a spectral resolution of 12 000 - Studies of the primary star wind and innermost wind-wind collision zone , 2016, 1610.05438.

[10]  S. Owocki,et al.  On the Role of Continuum-driven Eruptions in the Evolution of Very Massive Stars and Population III Stars , 2006, astro-ph/0606174.

[11]  R. Humphreys,et al.  A SEA CHANGE IN ETA CARINAE, , 2010, 1004.3529.

[12]  J. Morse,et al.  Kinematics and Ultraviolet to Infrared Morphology of the Inner Homunculus of η Carinae , 2004 .

[13]  Kris Davidson,et al.  On the Nature of the Central Source in η Carinae , 2001 .

[14]  Robert D. Gehrz,et al.  Mass and Kinetic Energy of the Homunculus Nebula around η Carinae , 2003 .

[15]  Doug Tody,et al.  The Iraf Data Reduction And Analysis System , 1986, Astronomical Telescopes and Instrumentation.

[16]  E. R. Parkin,et al.  3D modelling of the colliding winds in η Carinae – evidence for radiative inhibition , 2009, 0901.0862.

[17]  Louis Antonelli,et al.  The 5.52 Year Cycle of Eta Carinae , 1996 .

[18]  Nathan Smith.  A blast wave from the 1843 eruption of η Carinae , 2008, Nature.

[19]  S. Owocki,et al.  The Spectral Temperature of Optically Thick Outflows with Application to Light Echo Spectra from η Carinae's Giant Eruption , 2016, 1606.03681.

[20]  S. Esposito,et al.  DIFFRACTION-LIMITED VISIBLE LIGHT IMAGES OF ORION TRAPEZIUM CLUSTER WITH THE MAGELLAN ADAPTIVE SECONDARY ADAPTIVE OPTICS SYSTEM (MagAO) , 2013, 1308.4155.

[21]  M. Schoeller,et al.  Direct measurement of the size and shape of the present-day stellar wind of $\eta$ Carinae , 2003, astro-ph/0310399.

[22]  D. Ebbets,et al.  Hubble Space Telescope Proper-Motion Measurements of the η Carinae Nebula , 2001 .

[23]  Tyson Hare,et al.  MagAO: Status and on-sky performance of the Magellan adaptive optics system , 2014, Astronomical Telescopes and Instrumentation.

[24]  Kazunori Ishibashi,et al.  Latitude-dependent Effects in the Stellar Wind of η Carinae , 2003, astro-ph/0301394.

[25]  G. Weigelt,et al.  IS ETA CARINAE A FAST ROTATOR, AND HOW MUCH DOES THE COMPANION INFLUENCE THE INNER WIND STRUCTURE? , 2010, 1006.4816.

[26]  N. Smith,et al.  Ancient eruptions of η Carinae: a tale written in proper motions , 2016, 1609.00362.

[27]  Stephen M. White,et al.  Radio evidence of recent mass ejection from η Carinae , 2003 .

[28]  S. Owocki,et al.  Constraining the absolute orientation of η Carinae’s binary orbit: a 3D dynamical model for the broad [Fe iii] emission , 2011, 1111.2226.

[29]  C. Moutou,et al.  Adaptive optics imaging of P Cygni in H , 2000 .

[30]  K. Gayley,et al.  A Porosity-Length Formalism for Photon-Tiring-limited Mass Loss from Stars above the Eddington Limit , 2004, astro-ph/0409573.

[31]  Armando Riccardi,et al.  MAGELLAN ADAPTIVE OPTICS FIRST-LIGHT OBSERVATIONS OF THE EXOPLANET β PIC b. I. DIRECT IMAGING IN THE FAR-RED OPTICAL WITH MagAO+VisAO AND IN THE NEAR-IR WITH NICI, , 2014, 1403.0560.