An archive study of 18 old novae

I. The UV spectra ^{⋆,⋆⋆,⋆⋆⋆}

¹ INAF – Ossservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
e-mail: selvelli@oats.inaf.it
² European Southern Observatory, Karl Schwarzschild-Str. 2, 85748 Garching, Germany
e-mail: rgilmozz@eso.org

Received: 25 October 2012
Accepted: 14 September 2013

Abstract

Aims. We present an overview of the UV spectral properties of old novae as a class. The data and results of this paper, together with data from the outburst phases, will be utilized in a follow-up study to determine statistical properties and to investigate correlations among the physical parameters of the quiescent and eruptive phases.

Methods. All the available IUE, HST, and FUSE archive data for 18 old novae were used to derive accurate and homogeneous estimates of the reddening from the 2175 Å extinction bump and to determine the intrinsic spectral energy distribution corresponding to the utilization of both existing extinction curves. We also measured all the emission and absorption lines.

Results. We have found good agreement between spectra taken at different epochs and by different instruments, a clear indication of the near constancy of the SED on timescales of several years. With the possible exception of GK Per, the dereddened UV continua are represented well by a single-curve power-law distribution F_λ ∝ λ^− α, with α in the range 0.32–2.55 for one curve and 0.35–2.88 for the other one. The extrapolation of the various UV power laws to the optical range yields values that are in good agreement with the V magnitudes. We interpret this result as evidence that the SED is dominated by the accretion disk in the UV and optical spectral regions. A detailed study of the emission spectrum has led to measuring and identifying more than one hundred features and to detecting several lines that are rather uncommon in other CVs and whose identification is uncertain.