0.8-2.5 micron spectroscopy of Nova Ophiuchi 1998

We report 0.8-2.5 mu m spectroscopy of the very fast Nova Ophiuchi 1998 fro m June 19 and 22 to October 2.15 1998 UT. On the first night the Paschen an d Brackett emission lines dominated the spectrum, although He I 1.0830 mu m 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 wit h 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 mu m also appeared the second night, a line that we have heretofore seen only in Wolf-Rayet stars and which almost cer tainly indicates an overabundance of carbon. The broad emission wings were also present. About 110 days later on October 2.15 UT, the emission lines w ere 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 fro m dust. The continuum magnitudes on the dates of observation at J (1.25 mic rons) 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 broa d lines profiles suggest that they originate in an optically thin, spherica lly expanding shell. The line ratios from the narrow components deviate sig nificantly from case B values and seem to come from an optically thick (tau = 10-100 at line center in Pa alpha), high-density (n(e) similar to 10(11) cm(-3)) gas at around 10,000 K.