A Statistical Study of Accretion Disk Model Spectra for Cataclysmic Variables

Abstract

We have performed a statistical test of the currently used accretion disk models for cataclysmic variables (CVs) using a set of 33 CVs with steady disks (10 old novae and 23 nova-like systems). The mass transfer rate (Ṁ) for each system was also calculated. Ultraviolet (UV) data were fitted by model spectra using a multiparametric optimization method, aiming to constrain theṀ values. It was verified that these accretion disk models fail to fit both color and flux simultaneously, as previously noted when composite stellar atmosphere models were fitted to the UV spectra of CVs by Wade. By applying such models to a sample of novae and nova-like CVs, we confirm that the limb-darkening effect must be taken into account when estimating mass transfer rates, especially for high-inclination systems. Important fitting degeneracies of the basic disk parameters are analyzed. Our simulations suggest that to reproduce the observations a revision of the temperature profile, at least in the innermost parts of the disk, seems to be required, and possibly the vertical distribution of the viscosity should be revised. In addition, an optically thin layer or an extended disk component should be considered. This component may be physically represented by a disk wind and/or a chromosphere. A physical description of the emission-line profiles may help to break the degeneracies that appear when only the continuum is analyzed. The average value ofṀ found for nova-like systems is $9:3 ; 10 À9 M yr À1 , while $1:3 ; 10 À8 M yr À1 is found for old classical novae. No clear evidence is found for either the presence or absence of a correlation betweenṀ and the orbital period. Such correlation analysis was performed for high accretion rate systems (15 nova-like systems and 10 old novae), but we were not able to find a well-defined correlation as found by Patterson. By measuring the equivalent width of the emission lines (C iv k1550 and He ii k1640) we found a lack of systems with lowṀ and strong UV emission lines. A correlation between the equivalent width of such lines and the orbital inclination (i) was also confirmed.

An archive study of 18 old novae

“…Our E B−V values are in fair agreement with those of Duerbeck (1981) (with the exception of V603 Aql and DK Lac), those of Downes & Duerbeck (2001) (with the exception of T Aur and DK Lac), and, for the ten objects in common, with those by Puebla et al (2007) (with the only exception of DI Lac). However, there are significant differences from the values given by Shafter (1997) and from those, generally higher, found by Weight (1994) and by Ringwald et al (1996) in studies of the optical and IR spectra of cataclysmic variables.…”

“…We note that, with the exception of Puebla et al (2007), most of the E B−V values listed in Cols. 3-12 are not uniform, because they were obtained by various methods from the literature (e.g., Balmer decrement, IR maps, optical spectra, UV colors).…”

“…In this respect it is worth pointing out that, in contrast to the large number of UV (mostly IUE) and far-UV (mostly FUSE) studies for other classes of CVs (see, for example, Verbunt & Wade 1984;La Dous 1991;Mauche et al 1997;Araujo-Betancor et al 2005;Long 2006; Gänsicke et al 2006;Hamilton et al 2007;Sion et al 2008;Froning et al 2012), in the case of old novae, A&A 560, A49 (2013) the studies have been fairly preliminary (Gilmozzi et al 1994;Selvelli 2004) or limited to a subset of objects (Szkody et al 1991;Puebla et al 2007Puebla et al , 2011.…”

Spectroscopy From the Hubble Space Telescope Cosmic Origins Spectrograph of the Southern Nova-Like Bb Doradus in an Intermediate State

“…While some NLs have been successfully modeled with standard disk spectra (e.g., IX Vel, V3885 Sgr, V794 Aql, RZ Gru; Godon et al 2007;Linnell et al 2007Linnell et al , 2009Bisol et al 2012) others appear impossible to model with standard disk spectra (e.g., RW Sex, V751 Cyg, V380 Oph, UX UMa, QU Car; Linnell et al 2008aLinnell et al , 2008bLinnell et al , 2010Zellem et al 2009). The results of the statistical study of Puebla et al (2007) suggest that the temperature profile of the standard disk model be revised at least in the innermost parts of the disk in order to properly model some of the disk-dominated CV systems. With, so far, no existing alternative disk model to the standard one, the current results for BB Dor are at least consistent with typical WD temperatures in VY Scl NLs, and improved disk models, as well as accurate distances obtained by Gaia in the near future, will provide better constraints on the physical parameters of this system.…”

Influence of the velocity gradient on the line formation in discs of cataclysmic variables

“…On the other hand, it provides a good possibility for testing our knowledge about the accretion disc structure. However, the statistical studies based on observations (Warner 1986(Warner , 1987Morales-Rueda & Marsh 2002;or Puebla et al 2007) are not at sufficient accuracy. Above all, this comes from the uncertainty in determining the inclination angle and the fact that only objects with similar properties can be compared.…”