Observational Characteristics of the SW Sextantis Stars
In the early 1990s, three cataclysmic variables (CVs; SW Sex, DW UMa, and V1315 Aql) were proposed as the founding members of a new CV sub-class, the SW Sex stars (see 1986ApJ...302..388H, 1990ApJ...361..235S, and 1991AJ....102..272T). Additional CVs matching the characteristics of the first three were subsequently identified. The original defining properties of the SW Sex stars are as follows (also see 1991AJ....102..272T and the review in 1995cvs..book.....W):
- They are novalike CVs. Unlike the more well-known dwarf nova class of CV, members of the novalike class do not undergo quasiperiodic outbursts. Novalikes are instead characterized by an approximately steady, high rate of mass transfer (and correspondingly prominent accretion disk) that quenches the disk instability mechanism responsible for dwarf nova outbursts.
- Their optical light curves show deep eclipses of the WD+disk by the secondary star, requiring the system inclination to be high (i > 70°–80°).
- They have orbital periods of 3–4 hr, just above the 2–3 hr “period gap” (in which few non-magnetic CVs are found) that is thought to arise during the secular evolution of CVs due to a change in angular momentum loss mechanism, likely triggered by mass-loss-induced evolution of the donor star. This range of orbital periods is also associated with the largest range and highest extreme of predicted rates of mass transfer in CVs, above expectations from the standard theory for the secular evolution of CVs.
- They display high levels of spectral excitation, including He II λ4686 emission that is often comparable in strength to Hβ.
- Their spectra exhibit single-peaked emission lines rather than the double-peaked lines expected from near-edge-on disks.
- The Balmer and He I emission lines are only shallowly (or not at all) eclipsed compared to the continuum (implying emission originating above the orbital plane).
- The zero-crossings of their emission line radial velocities exhibit pronounced phase offsets relative to their eclipse ephemerides (implying a non-uniform distribution of emitting regions in the disk).
- Transient absorption features appear in their Balmer and He I emission line cores, typically around photometric phase φ ≈ 0.5 (i.e., the superior conjunction of the secondary star, opposite the eclipse).
The number of confirmed and probable SW Sex stars has now swelled (e.g., see The Big List of SW Sextantis Stars). In part, this is due to accepting that the SW Sex classification does not have a rigid definition linked to a specific CV morphology. Rather, the original SW Sex stars represent extreme cases of a pathology that is present at some level in many high mass transfer rate CVs. Consequently, SW Sex behavior has been recognized in CVs that do not eclipse (e.g., V442 Ophiuchi – 2000ApJ...537..936H), have orbital periods outside the range 3–4 hr (e.g., BT Monocerotis – 1998MNRAS.296..465S), or in which the transient absorption occurs outside the orbital phase range φ ≈ 0.4–0.6 (e.g., UU Aquarii – 1998MNRAS.294..689H).
Also see Hoard et al. 2003, Astronomical Journal, v.126, p. 2473 – 2003AJ....126.2473H.