Abundance Derivations for the Secondary Stars in Cataclysmic Variables From Near-Infrared Spectroscopy

Abstract

We derive metallicities for 41 cataclysmic variables (CVs) from near-infrared spectroscopy. We use synthetic spectra that cover the 0.8 µm ≤ λ ≤ 2.5 µm bandpass to ascertain the value of [Fe/H] for CVs with K-type donors, while also deriving abundances for other elements. Using calibrations for determining [Fe/H] from the K-band spectra of M-dwarfs, we derive more precise values for T eff for the secondaries in the shortest period CVs, and examine whether they have carbon deficits. In general, the donor stars in CVs have sub-solar metallicities. We confirm carbon deficits for a large number of systems. CVs with orbital periods > 5 hr are most likely to have unusual abundances. We identify four CVs with CO emission. We use phase-resolved spectra to ascertain the mass and radius of the donor in U Gem. The secondary star in U Gem appears to have a lower apparent gravity than a main sequence star of its spectral type.Applying this result to other CVs, we find that the later-than-expected spectral types observed for many CV donors is mostly an affect of inclination. All of the magnetic CVs, except the low accretion rate polar MQ Dra, have donors with subsolar metallicities. We find that two systems with unusual spectra, EI Psc and QZ Ser, that have large excesses of sodium, and extreme deficits of carbon.Synthetic spectra that have a reduced abundance of hydrogen are best able to explain the spectra of these two objects. The observed dearth of CVs with 2 hr ≤ P orb ≤ 3 hr (see Kolb et al. 1998).unpublished data is presented in Table 1, and it lists the UT date, start and stop times, the length of the exposure times used for each object, the instrument used, the orbital period, the orbital phase at the mid-point of the observational sequence (except for U Gem where full orbits were covered), and the outburst state for each of the CVs.We also present previously unpublished spectra obtained with NIRSPEC 2 on Keck.We used NIRSPEC in low-resolution, single order mode with a 0.38" slit. The grating tilt was set so as to cover the wavelength region 2.04 µm ≤ λ ≤ 2.46 µm, with a dispersion of 4.27Å/pixel. We employed the two-nod script, and used 4 minute exposure times for all of the program CVs. To correct for telluric absorption, we observed bright A0V stars located close to the program objects so as to minimize their relative differences in air mass. These data were reduced using the IDL routine REDSPEC 3 , specially developed for NIRSPEC.Finally, we used TripleSpec 4 (Wilson et al. 2004) on the Apache Point Observatory (APO) 3.5 m telescope to obtain phase-resolved near-IR spectra of U Gem over two orbital periods, and for 50% orbital period coverage of QZ Ser. TripleSpec is similar to SPEX in that it provides cross-dispersed spectra covering the near-IR, from 0.95 µm to 2.46 µm, at a resolution of R ∼ 3500. The 1.1" slit was used, and all exposures were four minutes in length. These data were reduced using "Triplespectool," a version of SPEXTOOL modified by Michael Cushing for use at APO. Nearby A0V stars were used to pr...

V1460 Her: a fast spinning white dwarf accreting from an evolved donor star

“…Thorstensen et al (2002b,a) argued that the over-luminous donors in EI PSc and QZ Ser were the stripped cores of companions that were initially more massive, > ∼ 1 M , bearing the signatures of CNO burning in the form of enhanced sodium abundances. Anomalous carbon and nitrogen abundances were confirmed in EI PSc in the far-ultraviolet (Gänsicke et al 2003) and infrared (Harrison 2016). In the ultraviolet, these anomalies are reflected in an inversion of the flux ratio of the N v and C iv resonance lines.…”

“…In CVs with (quasi) main-sequence donors, N v/C iv 0.5, whereas the C iv line is undetected in the Hubble Space Telescope (HST ) ultraviolet spectrum of EI Psc, with a lower limit of N v/C iv > ∼ 8 (Gänsicke et al 2003). Harrison (2016) report that the infrared spectra of both QZ Ser and EI Psc suggest an H deficiency of about 30 per cent, and a depletion of C based on the weak or absent CO absorption bands -corroborating the abundance anomalies detected in the ultraviolet spectroscopy of EI Psc. Sufficiently high initial donor star masses, i.e.…”

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“…Individually they are GW Lib (Szkody et al 2002), BW Scl (Gänsicke et al 2005), LL And (Howell et al 2002), WZ Sge , AL Com (Szkody et al 2003), SW UMa (Gänsicke et al 2005), HV Vir (Szkody et al 2002), WX Cet (Sion et al 2003), EG Cnc (Szkody et al 2002), BC UMa (Gänsicke et al 2005), VY Aqr (Sion et al 2003), EK Tra (Gänsicke et al 2001;Godon et al 2008), VW Hyi , EF Peg (Howell et al 2002), MV Lyr (Hoard et al 2004), DW UMa (Knigge et al 2000), WW Cet (Godon et al 2012), U Gem (Long et al 2006;Godon et al 2017), SS Aur (Sion et al 2004), RX And (Sion et al 2001), RU Peg (Sion et al 2004;Godon et al 2008), BY Cam (Mouchet et al 2003), BV Cen (Godon et al 2012), CH UMa (Godon et al 2012), EY Cyg (Godon et al 2008), EM Cyg (Godon et al 2012), SS Cyg (Godon et al 2012), UU Aql (Godon et al 2012), SS Aur (Godon & Sion 2021), TU Men (Godon & Sion 2021), and BZ UMa (Godon et al 2011). Infrared synthetic spectra of K-type secondaries have been compared to 41 CVs in a massive study by Harrison (2016); Harrison & Marra (2017). Harrison (2018) has also generated synthetic spectra with H-deficient stellar atmospheres and finds four systems that have significant hydrogen deficits.…”

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“…* It is noted that the study of Knigge et al (2011) was stemed from dwarf novae with solar metallicities. magnitude of the secondary, and the absolute magnitude of the super/long outburst peak can be derived from the orbital period (Beuermann et al 1998;Mennickent et al 1999;Knigge et al 2011;Harrison 2016;Otulakowska-Hypka et al 2016). To derive the absolute magnitude of the super/long outburst and the other parameters, we use the equation (4) of Patterson (2011) and Tables of Knigge et al (2011).…”

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