Near-Infrared H and K band studies of the 2006 outburst of the recurrent nova RS Ophiuchi

TL;DR

This study presents near-infrared spectroscopic observations of the 2006 outburst of recurrent nova RS Ophiuchi, revealing bipolar ejecta flow, excitation mechanisms of Fe II lines, and the evolution of coronal lines over time.

Contribution

It provides detailed near-IR spectral analysis of RS Oph's outburst, identifying bipolar flow, excitation processes, and shockfront evolution, which were not previously characterized in this detail.

Findings

Bipolar velocity flow observed in ejecta.

Fe II lines excited by fluorescence and collisional processes.

Shockfront had not yet broken out by the end of observations.

Abstract

We present near-IR photo-spectroscopy in the H and K bands of the 2006 outburst of the recurrent nova RS Ophiuchi (RS Oph). The observations cover the period between 1 to 94 days after the eruption. The near IR light curve and an extensive set of spectra are presented, lines identified and the general characteristics of the spectra discussed. Analysis of the HI line profiles show the presence of broad wings on both flanks of a strong central component indicating the presence of a bipolar velocity flow in the ejecta. Such a flow is kinematically consistent with the bipolar structure that the object displays in high-resolution spatial images. We discuss the behaviour and origin of the Fe II lines at 1.6872 and 1.7414 micron and show that Lyman alpha and Lyman continuum fluorescence are viable mechanisms to excite these lines. We draw upon the result, that collisional excitation can also contribute in exciting and significantly enhancing the strength of these Fe II lines, to propose that these lines originate from a site of high particle density. Such a likely site could be the high-density, low temperature contact surface that should exist in the shockfront in between the shocked ejecta and red giant wind. Recombination analysis of the HI lines indicate deviations from Case B conditions during most of the span of our observations indicating optical depth effects. It appears likely that the breakout of the shockfront had not yet occured till the end of our observations. An analysis is made of the temporal evolution of the [Si VI] 1.9641 micron coronal line and another coronal line at 2.0894 micron which is attributed to [Mn XIV]. Assuming collisional effects to dominate in the hot coronal gas, estimates are made of the ion temperature in the gas.