Latitudal Dependence of Density and Outflow Velocity in the Stellar Wind of Eta Carinae

By Nathan Smith

Under the supervision of Dr. Kris Davidson and Dr. Roberta M. Humphreys

ABSTRACT

The Homunculus reflection nebula around the massive star eta Carinae provides the rare opportunity to observe the spectrum of a star from multiple directions. Doppler velocities of emission lines can be used to estimate the nebula's geometry accurately enough to infer how reflected stellar-wind profiles vary with latitude. HST/STIS spectra of hydrogen and helium lines seen in the Homunculus reveal a non-spherical stellar wind, with bipolar symmetry. P Cygni absorption in Balmer lines depends on latitude, with relatively high velocities and strong absorption near the poles. Higher velocities there are expected due to higher escape velocity if the star is rotating. However, the stronger hydrogen P Cygni absorption at high latitudes is surprising. It suggests higher mass flux toward the poles, perhaps resulting from equatorial gravity darkening on a rotating star. Reflected profiles of He I lines are more puzzling, and offer clues to eta Car's wind geometry and circumstellar ionization structure. In March 2000, during eta Car's normal state between periodic `spectroscopic events' that repeat every 5.5 years, the wind appears to have had a fast, high-density polar wind, with higher ionization at low/mid latitudes. In spite of the lower densities at low/mid latitudes, a thin equatorial disk-wind may also be present. The bipolar wind geometry may imply that intrinsically asymmetric ejection helped form the Homunculus, rather than an externally constrained outflow, and it has interesting implications for the long-term variability of eta Carinae and excitation of its ejecta.

Older STIS data obtained since 1998 reveal that this global stellar-wind geometry changes during eta Car's 5.5 year cycle, suggesting that the star's periodic spectroscopic events are shell ejections. Whether or not a companion star triggers these outbursts remains ambiguous. Dramatic changes in the wind occur at low latitudes, while the dense polar wind remains relatively stable during an event. The observed wind geometry and its variability have critical implications for the 5.5 year cycle, but do not provide a clear alternative to an eccentric binary system for generating eta Car's variable X-ray emission.