aa r X i v : . [ a s t r o - ph . S R ] N ov Mem. S.A.It. Vol. 75, 282 c (cid:13) SAIt 2008
Memorie della
The Search for SW Sex Type Stars
Linda Schmidtobreick , Pablo Rodr´ıguez-Gil , , , and Boris T. G¨ansicke European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile Instituto de Astrof´ısica de Canarias, V´ıa L´actea, Santa Cruz de Tenrife, Spain Departmento de Astrof´ısica, Universidad de La Laguna, Santa Cruz de Tenerife, Spain Isaac Newton Group of Telescopes, Apartado de correos 321, 38700 Santa Cruz de laPalma, Spain Department of Physics, University of Warwick, Coventry CV4 7AL, UK
Abstract.
All eclipsing nova-likes in the 2.8–4 h orbital period range belong to the groupof SW Sex stars, and as such experience very high mass transfer rates. Since the physicalproperties of a star should be independent of the inclination it is observed at, this suggeststhat all or at least a large fraction of the non- or weakly-magnetic cataclysmic variables inthis period range are physically SW Sex stars.We here present preliminary results of a large campaign to search for SW Sex characteristicfeatures in the spectra of such stars. We find that 14 out of the 18 observed non-eclipsingcataclysmic variables belong to the group of SW Sex stars the classification of the otherfour is uncertain from our data. This confirms the domination of SW Sex stars in the periodrange of 2.8–4 h just above the period gap. Since all long-period systems need to cross thisrange before entering the gap, the SW Sex phenomenon is likely to be an evolutionary stagein the life of a cataclysmic variable.
1. Introduction
SW Sextantis stars are a sub-group of cata-clysmic variables (CVs) that were originallycharacterised by Thorstensen et al. (1991).They were defined as eclipsing nova–like starswith high velocity, emission line wings extend-ing up to 4000 km / s, inconsistent with an originin a standard accretion disc. They show nar-row absorption features in the Balmer and He i lines near the inferior conjunction of the whitedwarf, and large orbital phase o ff sets ( ∼ . Send o ff print requests to : L. Schmidtobreick tremely high accretion rates exceeding the ex-pected rates based on standard magnetic brak-ing as angular momentum loss mechanism(Townsley & G¨ansicke 2009).While SW Sex stars were considered rareobjects at the beginning, later surveys (seeG¨ansicke (2005) for an overview) haveshown that a surprisingly large number of thenewly identified systems are deeply eclips-ing SW Sextantis stars with orbital periods be-tween 2.8 h and 4 h. Populating the upper edgeof the orbital period gap makes them very in-teresting objects in the context of CV evolu-tion. In detail, 13 out of 48 nova-like systemsin this period range – these are all the eclipsingones – belong to the sub-class of SW Sex stars. chmidtobreick et al.: The Search for SW Sex Type Stars 283 Since being an eclipsing system is not anintrinsic physical property of the star but ratherdepends on the angle under which the binary isobserved, it appears entirely plausible that allnon- or weakly-magnetic CVs just above theperiod gap are physically SW Sex stars, i.e. ex-perience a very high mass transfer rate. Thiswould be of major significance for the evolu-tionary theory of CVs, as all long-period CVshave to pass through this range before enteringthe period gap.During the last years, we have conducted aproject to test the hypothesis that all nova–likestars in the 2.8–4 h period regime are physicalSW Sex type stars, even if they are not eclips-ing. Examples for such stars are e.g. RR Pic(Schmidtobreick et al. 2003) or V533 Her(Rodr´ıguez-Gil & Mart´ınez-Pais 2002). Weselected a sample of candidates: non-eclipsingnova-likes and old novae with an orbital periodof 2.8–4 h that are su ffi ciently bright to performtime-series spectroscopy. We aim to analysethe emission lines of these stars searching forthe presence of SW Sex characteristics such asbroad line wings with large–amplitude radialvelocity variations, single–peaked line profileswith phase-dependent central absorption, andphase lags between the radial velocity modula-tion in the line cores and wings. We also checkfor line flaring, an additional feature that is of-ten observed in SW Sex stars but also in inter-mediate polars and that is manifested in fast os-cillations of the emission line flux and velocitywith periods around 10–20 min. First results ofthis campaign are discussed in Rodr´ıguez-Gilet al. (2007b).
2. Data
In total, we have observed 18 non-eclipsingCVs with periods between 2.8 h and 4 h. Timeseries spectroscopy was done covering at leastone orbit but most of the stars were followedover longer time ranges covering up to two or-bits. The data of the first set are described inRodr´ıguez-Gil et al. (2007b).The new set of candidate SW Sex starswere observed in January and December2009 with EFOSC2 (Buzzoni et al. 1984)mounted at the NTT of the European Southern Observatory on La Silla, Chile. Grism α was used to perform time-seriesspectroscopy covering at least one orbit foreach system. The data were reduced using stan-dard procedures in PAMELA and MOLLY . Thewavelength calibration yielded a final FWHMresolution of 3.8 Å. All further analysis wasdone using MIDAS .
3. Preliminary Results
In Figure 1 the average spectra of the newcandidates are plotted. All of them are dom-inated by H α in emission and also show theHe I λ ff er-ent components in the emission line is neededto determine the correct zero-phase. Also theflaring has not yet been checked for in any ofthe new stars. We leave these results for a laterpaper. Still, with the presence of the S–waveas well as the central absorption we are pretty Tom Marsh’s packages PAMELA and MOLLYare available at http: // deneb.astro.warwick.ac.uk / phsaap / software / MIDAS is distributed by ESO at http: // / sci / software / esomidas /
84 Schmidtobreick et al.: The Search for SW Sex Type Stars
Fig. 1.
Averaged normalised spectra of the new setof SW Sex candidate stars. confident that all our new candidates are in factSW Sex stars.Thus, from the 18 observed nova-likes inthe orbital period range of 2.8–4 h, 14 belongto the group of SW Sex stars, see Table 1 fordetails. The data for V849 Her and V393 Hyaare rather poor and do not allow a conclusiveclassification. LQ Peg is of very low inclinationand much higher spectral resolution is neededto find any features in the emission line. Thespectrum of V992 Sco is still dominated by thenova outburst and does not yet show any signa-ture of the binary.
Fig. 2.
Trailed spectra of the H α line of BB Dor(left) and KUV03580 + ffi cient ac-curacy to be extrapolated to our observations. Theorbital period P = .
143 d for KUV03580 +
4. Conclusions
With the results so far, we have shown thatthe mayority of the nova-like stars in the 2.8–4 h period range are of SW Sex nature. Theremight be the one or other oddball among theCVs in this range, but the population in clearlydominated by SW Sex stars. On the other hand,due to angular momentum loss, a CV thatstarted with a long orbital period will eventu-ally cross the 2.8–4 h period range. Our find-ings imply that these CVs will turn into anSW Sex star during this phase of their lifetime.Therefore, the SW Sex phenomenon can be nolonger considered a feature of individual starsbut rather an evolutionary stage of the CV pop-ulation.
Acknowledgements.
We acknowledge the use ofTom Marsh’s
PAMELA and
MOLLY packages. We alsomade thorough use of the
SIMBAD database oper-ated at CDS, Starssbourg, France. The data werecollected at ESO, La Silla, Chile under program IDs082.D-0138 and 084.D-0646.
References
Buzzoni B., Delabre B., Dekker H., Dodorico chmidtobreick et al.: The Search for SW Sex Type Stars 285
Table 1.
The observed SW Sex characteristics and the final classification for all candidates
System single peak S–wave 0.5–abs. phases flaring SW Sex?HL Aqr ∗ √ a √ √ √ x yesBO Cet ∗ √ √ √ √ √ yesBB Dor ∗∗ √ √ √ √ ? yesIM Eri ∗∗ √ √ √ √ ? yesV849 Her ∗ √ x x x x no c V393 Hya ∗ x x x x x no c AQ Men ∗∗ x √ √ ? ? yesAH Men ∗ √ √ √ √ x yesKQ Mon ∗∗ √ √ √ √ √ yesV380 Oph ∗ √ √ √ √ √ yesV1193 Ori ∗∗ √ √ √ √ ? yesLQ Peg ∗ √ x x x x no c AH Pic ∗ √ a √ x √ x yesV992 Sco ∗ - - - - - - b LN UMa ∗ √ √ √ √ x yesV382 Vel ∗∗ x b √ √ √ ? yesSDSS J0920 + ∗∗ √ √ √ √ ? yesKUV 03580 + ∗∗ x √ √ √ ? yes a in absorption.; b spectrum still dominated by shell emission from nova explosion; c need higher quality data ∗ Rodr´ıguez-Gil et al. (2007b); ∗∗ Schmidtobreick et al. (2011)
S., Enard D., Focardi P., Gustafsson B.,Nees W., Paureau J., Reiss R., 1984, ESOMessenger 38, 9G¨ansicke, B.T., 2005, In: