Multiwavelength Photometry and Hubble Space Telescope Spectroscopy of the Old Nova V842 Centaurus

TL;DR

This study combines optical, infrared, and HST spectroscopy to analyze the old nova V842 Cen, revealing a low accretion rate and challenging previous assumptions about its white dwarf's magnetic properties and post-nova behavior.

Contribution

It provides the first comprehensive multiwavelength analysis of V842 Cen, showing a non-magnetic accretion disk and unexpectedly low accretion rate shortly after nova eruption.

Findings

Optical light curves show a 56.5s periodicity linked to white dwarf rotation.

HST spectra favor a non-magnetic, full accretion disk model over truncated disks.

Accretion rate is surprisingly low for a nova only 24 years post-eruption.

Abstract

We present ground-based optical and near infrared photometric observations and Hubble Space Telescope COS spectroscopic observations of the old nova V842 Cen (Nova Cen 1986). Analysis of the optical light curves reveals a peak at 56.5 +/- 0.3s with an amplitude of 8.9 +/- 4.2 mma, which is consistent with the rotation of a magnetic white dwarf primary in V842 Cen that was detected earlier by Woudt et al., and led to its classification as an intermediate polar.However, our UV lightcurve created from the COS time-tag spectra does not show this periodicity. Our synthetic spectral analysis of an HST COS spectrum rules out a hot white dwarf photosphere as the source of the FUV flux. The best-fitting model to the COS spectrum is a full optically thick accretion disk with no magnetic truncation, a low disk inclination angle, low accretion rate and a distance less than half the published distance that was determined on the basis of interstellar sodium D line strengths.Truncated accretion disks with truncation radii of 3Rwd and 5Rwd yielded unsatisfactory agreement with the COS data. The accretion rate is unexpectedly low for a classical nova only 24 years after the explosion when the accretion rate is expected to be high and the white dwarf should still be very hot, especially if irradiation of the donor star took place. Our low accretion rate is consistent with low accretion rates derived from X-ray and ground-based optical data.