0.8–2.5 Micron Spectroscopy of Nova Ophiuchi 1998

Abstract

We report 0.8-2.5 μm spectroscopy of the very fast Nova Ophiuchi 1998 from June 19 and 22 to October 2.15 1998 UT. On the first night the Paschen and Brackett emission lines dominated the spectrum, although He I 1.0830 μm was the strongest single line present and He II was very weak. There were also broad, symmetric emission wings underlying the H I and He I lines with FWHM of about 10,000 km s-1. Three nights later the rapid evolution of the nova to higher excitation conditions was evident from the much stronger He II lines which had increased by a factor of 4 relative to the hydrogen lines. The C III line at 0.9710 μm also appeared the second night, a line that we have heretofore seen only in Wolf-Rayet stars and which almost certainly indicates an overabundance of carbon. The broad emission wings were also present. About 110 days later on October 2.15 UT, the emission lines were very weak (5%-15% of the continuum) and only a few Paschen features and lines of He II were present. At no time during our observations was there any evidence of a long-wavelength upturn indicative of thermal emission from dust. The continuum magnitudes on the dates of observation at J (1.25 microns) were 9.4, 10.9 to 15.0, respectively. The optical decline of 0.37 mag day-1 makes this nova one of the fastest ever seen. Model fits of the broad lines profiles suggest that they originate in an optically thin, spherically expanding shell. The line ratios from the narrow components deviate significantly from case B values and seem to come from an optically thick (τ=10-100 at line center in Paα), high-density (ne~1011 cm-3) gas at around 10,000 K. This work supported by the Aerospace Independent Research and Development Program.