Eta Carinae's 2014.6 spectroscopic event: Clues to the long-term recovery from its Great Eruption

Abstract

Aims. Every 5.5 years, eta Car's light curve and spectrum change remarkably across all observed wavelength bands. These so-called spectroscopic events are most likely caused by the close approach of a companion. We compare the recent spectroscopic event in mid-2014 to the events in 2003 and 2009 and investigate long-term trends.

Methods. Eta Car was observed with HST STIS, VLT UVES, and CTIO 1.5 m CHIRON for a period of more than two years in 2012-2015. Archival observations with these instruments cover three orbital cycles and the events of 2003.5, 2009.1, and 2014.6. The STIS spectra provide high spatial resolution and include epochs during the 2014 event when observations from most ground-based observatories were not feasible. The strategy for UVES observations allows for a multidimensional analysis, because each location in the reflection nebula is correlated with a different stellar latitude.

Results. Important spectroscopic diagnostics during eta Car's events show significant changes in 2014 compared to previous events. While the timing of the first He II lambda 4686 flash was remarkably similar to previous events, the He II equivalent widths were slightly larger, and the line flux increased by a factor of similar to 7 compared to 2003. The second He II peak occurred at about the same phase as in 2009, but was stronger. The He I line flux grew by a factor of similar to 8 in 2009-2014 compared to 1998-2003. The N II emission lines also increased in strength. On the other hand, Ha and Fe II lines show the smallest emission strengths ever observed in eta Car. The optical continuum brightened by a factor of similar to 4 in the past 10-15 years. The polar spectrum shows fewer changes in the broad wind emission lines: the Fe II emission strength decreased by a factor of similar to 2 (compared to a factor of similar to 4 in our direct line of sight). The He II equivalent widths at FOS4 were larger in 2009 and 2014 than during the 2003 event.

Conclusions. The basic character of eta Car's spectroscopic events has changed in the past two to three cycles. The ionizing UV radiation dramatically weakened during each pre-2014 event but not in 2014. The strengthening of He I and N II emission and the weakening of the lower-excitation H alpha and Fe II wind features in our direct line of sight implies a substantial change in the physical parameters of the emitting regions. The polar spectrum at FOS4 shows fewer changes in the broad wind emission lines, which may be explained by the latitude-dependent wind structure of eta Car. The quick and strong recovery of the He II emission in 2014 supports a scenario, in which the wind-wind shock may not have completely collapsed as was proposed for previous events. As a result, the companion did not accrete as much material as in previous events. All this may be the consequence of just one elementary change, namely a strong decrease in the primary's mass-loss rate. This would mark the beginning of a new phase, in which the spectroscopic events can be described as an occultation by the primary's wind.