Dwarf novae in the Hamburg quasar survey: rarer than expected

¹ Department of Physics, University of Warwick, Coventry CV4 7AL, UK e-mail: A.Aungwerojwit@warwick.ac.uk
² Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
³ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁴ Universitäts-Sternwarte, Scheinerstr. 1, 81679 München, Germany
⁵ Braeside Observatory, PO Box 906, Flagstaff AZ 86002, USA
⁶ Institute of Space Applications and Remote Sensing, National Observatory of Athens, PO Box 20048, Athens 11810, Greece
⁷ Department of Physics, Section of Astrophysics, Astronomy & Mechanics, University of Thessaloniki, 541 24 Thessaloniki, Greece
⁸ Hida Observatory, Kyoto University, Kamitakara, Gifu 506-1314, Japan
⁹ Bischmisheim, Am Probstbaum 10, 66132 Saarbrücken, Germany
¹⁰ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
¹¹ Harvard-Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, MA 02138, USA

Received: 20 February 2006
Accepted: 11 May 2006

Abstract

Aims.We report the discovery of five new dwarf novae that were spectroscopically identified in the Hamburg Quasar Survey (HQS), and discuss the properties of the sample of new dwarf novae from the HQS.

Methods.Follow-up time-resolved spectroscopy and photometry have been obtained to characterise the new systems.

Results.The orbital periods determined from analyses of the radial velocity variations and/or orbital photometric variability are P_orb min or P_orb min for HS 0417+7445, P_orb min for HS 1016+3412, P_orb min for HS 1340+1524, P_orb min for HS 1857+7127, and P_orb min for HS 2214+2845. HS 1857+7127 is found to be partially eclipsing. In HS 2214+2845 the secondary star of spectral type is clearly detected, and we estimate the distance to the system to be  pc. We recorded one superoutburst of HS 0417+7445, identifying the system as a SU UMa-type dwarf nova. HS 1016+3412 and HS 1340+1524 have rare outbursts, and their subtype is yet undetermined. HS 1857+7127 frequently varies in brightness and may be a Z Cam-type dwarf nova. HS 2214+2845 is a U Gem-type dwarf nova with a most likely cycle length of 71 d.

Conclusions.To date, 14 new dwarf novae have been identified in the HQS. The ratio of short-period (<3 h) to long-period (>3 h) systems of this sample is 1.3, much smaller compared to the ratio of 2.7 found for all known dwarf novae. The HQS dwarf novae display typically infrequent or low-amplitude outburst activity, underlining the strength of spectroscopic selection in identifying new CVs independently of their variability. The spectroscopic properties of short-period CVs in the HQS, newly identified and previously known, suggest that most, or possibly all of them are still evolving towards the minimum period. Their total number agrees with the predictions of population models within an order of magnitude. However, the bulk of all CVs is predicted to have evolved past the minimum period, and those systems remain unidentified. This suggests that those post-bounce systems have markedly weaker Hβ emission lines compared to the average known short-period CVs, and undergo no or extremely rare outbursts.