Cyclical period changes in HT Cassiopeiae: a difference between systems above and below the period gap

Abstract

Aims. We report identification of cyclical changes in the orbital period of the eclipsing cataclysmic variable HT Cas. Methods. We measured new white-dwarf mid-eclipse timings and combined them with published measurements to construct an observed-minus-calculated diagram covering 29 years of observations. Results. The data present a 36 yr period modulation of semi-amplitude ∼40 s, with a statistical significance greater than 99.9 percent with respect to a constant period. Conclusions. We combine our results with those in the literature to revisit the issue of cyclical period changes in cataclysmic variables and their interpretation in terms of a solar-type magnetic activity cycle in the secondary star. A diagram of fractional period change (∆P/P) versus the angular velocity of the active star (Ω) for cataclysmic variables, RS CVn, W UMa, and Algols, reveal that close binaries with periods above the gap (secondaries with convective envelopes) satisfy a relationship ∆P/P ∝ Ω −0.7±0.1 . Cataclysmic variables below the period gap (with fully convective secondaries) deviate from this relationship by more than 3-σ, with average fractional period changes 6 times smaller than those of the systems above the gap.

The X-ray eclipse of the dwarf nova HT Cassiopeiae observed by theXMM-Newton satellite: spectral and timing analysis

“…As for many other CVs, HT Cas is a short period binary system (P orb 1.77 h, Borges et al 2008), close to the lower edge of the period gap at ∼2.5 h. The system is characterized by a visual magnitude of ∼16.4 during its quiescent state, relatively rare outbursts (Horne et al 1991), and white dwarf eclipses well observed in several optical campaigns. A detailed analysis of the light curves can be found in Patterson (1981) and Wood et al (1985).…”

“…There has also been a report of a cyclical change in the HT Cas orbital period (ΔP/P ∼ 4 × 10 −7 , see Borges et al 2008, for details). The authors attributed this effect to a loss of angular momentum caused by the coupling of the stellar wind from the companion star to the surface of the compact object.…”

“…In this respect, we use this ephemeris and note that the first eclipse appearing in the EPIC light curve occurs at the time MJD = 52 506.479 21 ± 0.000 03 days (as expected using the linear ephemeris). Figure 3 presents the folded OM light curve using the Borges et al (2008) linear ephemeris period of P 0 = 0.073 647 202 9(±3) days (the error refers to last digit), which is equal to 1.77 h (Borges et al 2008). The left panel shows the two-period folded light curve, and the right shows a zoom centered on the eclipse.…”

Long-term photometry of the eclipsing dwarf nova V893 Scorpii

“…On the contrary, they appear rather to be the rule. Borges et al (2008) compiled a list of 14 systems exhibiting this phenomenon which contains most of the brighter eclipsing CVs, well observed over a long time base. An updated version was published by Pilarčík et al (2012), who also included some post-common envelope binaries (PCEBs) with similar period variations.…”

Stellar magnetic cycles

“…The same phenomenon is observed in semi-detached binaries, such as Algols and cataclysmic variables. The latter are particularly interesting because they show the phenomenon also when their secondary components are fully convective (Ak et al 2001;Borges et al 2008), thus providing a circumstantial evidence of magnetic cycles in fully convective dwarf stars with a sufficiently fast rotation. For more information on the connection between orbital period modulation and dynamo action see, e.g., Lanza et al (1998), Lanza & Rodonò (1999), Lanza & Rodonò (2004), Lanza (2005), and Lanza (2006a).…”