Infrared Spectroscopic Observations of the Secondary Stars of Short-Period Sub-Gap Cataclysmic Variables

Abstract

We present K-band spectroscopy of short period, "sub-gap" cataclysmic variable (CV) systems obtained using ISAAC on the VLT. We show the infrared spectra (IR) for nine systems below the 2-3 hour period gap: V2051 Oph, V436Cen, EX Hya, VW Hyi, Z Cha, WX Hyi, V893 Sco, RZ Leo, and TY PsA. We are able to clearly detect the secondary star in all but WX Hyi, V893 Sco, and TY PsA. We present the first direct detection of the secondary stars of V2051 Oph, V436 Cen, and determine new spectral classifications for EX Hya, VW Hyi, Z Cha, and RZ Leo. We find that the CO band strengths of all but Z Cha appear normal for their spectral types, in contrast to their longer period cousins above the period gap. This brings the total number of CVs and pre-CVs with moderate resolution (R 1500) IR spectroscopy to sixty-one systems: nineteen pre-CVs, thirty-one non-magnetic systems, and eleven magnetic or partially magnetic systems. We discuss the trends seen in the IR abundance patterns thus far, and highlight a potential link between anomalous abundances seen in the IR with the C IV/N V anomaly seen in the ultraviolet. We present a compilation of all systems with sufficient resolution IR observations to assess the CO band strengths, and, by proxy, obtain an estimate on the C abundance on the secondary star.

Infrared photometry of the dwarf nova V2051 Ophiuchi – I. The mass-donor star and the distance★

“…The derived colours matches those of a typical lower mainsequence star of spectral type M(8.0 ± 1.5), with temperatures in the T eff = 2700 K range, in good agreement with the spectral type inferred by Hamilton et al (2011) and that expected for a dwarf nova with similar orbital period (Knigge 2006).…”

Evolution of Cataclysmic Variables and Related Binaries Containing a White Dwarf

“…In contrast to the central chemical composition, the surface composition might be inferred either via spectral features of the donor star, or from an accretion disk around the accreting WD (see, e.g., Hamilton et al 2011). In Figures 7-9 we show the median values of the surface composition ratios of C/O, N/O, and C/N, respectively.…”

“…An order of magnitude more systems have measured orbital periods covering a wide range, but the donor masses have not been determined. A number of groups have attempted to measure the chemical composition in CV donor stars via direct spectroscopic observations in the K-band region (Harrison et al 2004(Harrison et al , 2005Howell et al 2010;Hamilton et al 2011). They collected spectroscopic information on 19 pre-CVs, 31 non-magnetic CVs, and 11 magnetic CVs.…”

“…We make robust predictions as to how the ratio of observable isotopes such as N/O would have to change in order for us to be able to conclude that a particular PDCV descended from a high-mass primordial donor. Observations of the abundance ratios for C/O and N/ O inferred from the chemical properties of the donor star and the accretion disk in CVs have been carried out by Harrison et al (2004), Howell et al (2010, and Hamilton et al (2011). They found evidence for carbon deficiencies in CVs that would seem to imply that some CVs for which they measured the abundances could have been derived from much more massive systems.…”

Population types of cataclysmic variables in the solar neighbourhood

“…However, the faintness of CVs restricts most of their photometric and spectroscopic studies to the Solar neighbourhood. Although some abundance anomalies in UV and IR were found for CVs (Hamilton et al, 2011;Gänsicke et al, 2005), there are no reliable metallicity measurements for CVs. In the absence of metallicity measurements, only kinematical and the dynamical methods are expected to provide reliable results in the recognition of the thick disc and halo CVs in the Solar neighbourhood.…”