HT Cas - eclipsing dwarf nova during its superoutburst in 2010
K.Bakowska, A.Olech, A.Rutkowski, R.Koff, E.de Miguel, M.Otulakowska-Hypka
aa r X i v : . [ a s t r o - ph . S R ] D ec Contrib. Astron. Obs. Skalnaté Pleso , 1 – 5, (0) HT Cas - eclipsing dwarf nova during itssuperoutburst in 2010
K. Bąkowska , , A. Olech , A. Rutkowski , R. Koff ,E. de Miguel and M. Otulakowska-Hypka Nicolaus Copernicus Astronomical Center, Polish Academy of Sciencesul. Bartycka 18, 00-716 Warszawa, Poland Astronomical Observatory Institute, Faculty of Physics,A. Mickiewicz University, ul. Słoneczna 36, 60-286 Poznań, Poland Astronomical Observatory, Jagiellonian University, ul. Orła 171, 30-244Kraków, Poland Center for Backyard Astrophysics, Antelope Hills Observatory, 980 AntelopeDrive West, Bennett, CO 80102, USA Departamento de Fisica Aplicada, Facultad de Ciencias Experimentales,Unversidad de Huelva, 21071 Huelva, Spain
Received: ; Accepted:
Abstract.
We present results of a world-wide observing campaign of the eclips-ing dwarf nova - HT Cas during its superoutburst in November 2010. Usingcollected data we were able to conduct analysis of the light curves and wecalculated O − C diagrams.The CCD photometric observations enabled us to derive the superhumpperiod and with the timings of eclipses the orbital period was calculated. Basedon superhump and orbital period estimations the period excess and mass ratioof the system were obtained. Key words:
Stars: individual: HT Cas - binaries: close - novae, cataclysmicvariables
1. Introduction
Among close binary stars there are cataclysmic variables a containing whitedwarf (the primary) and a main-sequence star (the secondary or the donor).The primary accretes matter from the donor star through the inner Lagrangianpoint. In non-magnetic systems the material forms an accretion disk around theprimary.One of the subclasses of cataclysmic variables are dwarf novae and amongthem there are SU UMa type stars. They can be characterized by short orbitalperiod ( P orb < . h) and in their light curves we see two types of outbursts:normal and superoutbursts. Outbursts are about one magnitude fainter andlast shorter than superoutbursts. Tooth-shaped oscillations called superhumpsmanifest their presence during superoutburts (Hellier 2001). HT Cas was discovered 70 years ago and classified as a variable star (Hoffmeis-ter 1943). Unfortunately, for 35 years this object did not received any attention,until the eclipses of HT Cas were noticed (discovery made by Bond in 1978,private communication with Patterson) and this amazing dwarf nova became atop priority object for observing season organized in 1978. Patterson called HTCas "a Rosetta stone among dwarf novae" because of the variety of manifestedfeatures presented in light curves (Patterson 1981). Over 30 years since thisstatement literature concerning HT Cas is still growing, reaching several dozensof publications.
2. Observations
We present observations of superoutburst in HT Cas made during 21 nights.Data were collected between 2nd and 27th of November. During this world-widecampagin five observers were gathering observations in four countries: Poland,Turkey, Spain and USA, and eight telescopes with diameters ranging from 10to 100 cm were used simultaneously. Moreover, data collected by AAVSO or-ganization were used for this analysis.HT Cas was monitored in clear filter ("white" light). Bias, dark current andflat-field correction was made using IRAF package. Profile photometry wasobtained with DAOPHOTII (Stetson 1987). Relative manitudes were transformto the standard Johnson V manitudes using data published by Henden andHoneycutt (1997). In Fig. 1 there are presented resulting light curves from ourobserving campaign.The superoutburst lasted 11 nights (3rd-13th Nov) and we gathered datacovering almost whole this period except of the last night of the superoutburst.HT Cas reached maximum brightness of V ≈ mag and the amplitude of thissuperoutburst was about A ≈ mag.
3. Results
The O − C diagram is an excellent tool to check the stability of superhump ororbital period and to determine their values. We analyzed light curves from 10subsequent nights where superhumps clearly manifested their presents and weidentified 69 moments of maxima. The least squares linear fit to the gathereddata gave the following ephemeris for the maxima: HJD max = 2455504 . . · E (1) IRAF is distributed by the National Optical Astronomy Observatory, which is operated bythe Association of Universities for Research in Astronomy, Inc., under a cooperative agreementwith the National Science Foundation.
T Cas - eclipsing dwarf nova during its superoutburst in 2010 V i s . [ m a g ] Figure 1.
The photometric behavior of HT Cas in November 2010. During that timeone can observe an outburst precursor, developed later on into full superoutburst. Atthe end of November star came back to its quiescence level. O - C [ c y c l e s ] Figure 2.
The O − C diagram of the superhumps maxima. The solid line correspondsto the fit given by Eq. 2. One can observe the decreasing trend of superhump period presented inFig. 2. Due to this fact the second-order polynomial fit to the moments of max-ima was derived and the following ephemeris was obtained:
HJD max = 2455504 . . · E − . · − · E (2)Based on those calculations, we can confirm that the period of superhumpswas not stable and can be described by a decreasing trend with a rate of ˙P = − . . × − .To obtain the value of orbital period we constructed the O − C diagram forthe moments of minima. In total the timings of 70 eclipses from November 2010superoutburst were used to calculate the following ephemeris of the minima: HJD min = 2455504 . . × E (3) O - C [ c y c l e s ] Figure 3.
The O − C diagram for eclipses observed during superoutburst of HT Casin November 2010. Regular humps are the manifestation of beat period - compositionof the orbital and superhump period. There is a relation between superhump and orbital period (Osaki 1985): P sh = 1 P orb − P beat (4)and we used Eq. 4 to calculate the beat period of P beat = 2 . ± . days.This value is in full agreement with previous results presented by Zhang et al.(1986) for the superoutburst in HT Cas in 1985. In Fig. 3 one can observe regularhumps near cycles: 18, 47 and 80, we checked those features and it is beat periodmanifestation.We used values of superhump P sh and orbital P orb periods to calculate theperiod excess ε which can be obtained based on formula: ε = ∆ PP orb = P sh − P orb P orb (5)Based on Eq. 5 the value of period excess for HT Cas is ε = 3 . ± . and it is a typical value for SU UMa-type stars.There is an empirical formula with the relation between period excess andmass ratio of the binary q = M /M (Patterson 1998): ε = 0 . q . q (6)Based on Eq. 6 the mass ratio of HT Cas with the value of q = 0 . wasobtained.
4. Summary
To summarize the results of the autumn 2010 observations of the HT Cas wecan confirm:
T Cas - eclipsing dwarf nova during its superoutburst in 2010 – After 25 years of quiescence or normal outbursts in November 2010 the super-outburst in HT Cas was detected. This rare phenomenon had an amplitudeof about A = 4 mag, lasted 11 nights was triggered by an outburst precursor. – During superoutburst mesmerizing superhumps manifested their presenceand based on them the superhump period with a value of P sh = 0 . days and decreasing with an rate of ˙P = − . . × − was calculated. – Based on the timinings of eclipses observed during superoutburst an orbitalperiod with a value of P orb = 0 . days was obtained and its valueis in full accordance with results presented by other authors from earlierobservations (Horne et al. 1991, Feline et al. 1998, Ioannou et al. 1999,Borges et al. 2008,). No anomalies in orbital period were detected as it wasmentioned by Ioannou et al. (1999) or Borges et al. (2008). – From the November 2010 superoutburst the period excess with value of ε =3 . ± . was obtained and it has typical value for SU UMa type stars.The same value of period excess was derived from the superoutburst in 1985(Zhang et al. 1986). – Mass ratio with the value of q = 0 . was calculated and it confirms theresult obtained by Horne et al. (1991) where a different method and set ofobservations were used. Acknowledgements.
We acknowledge generous allocation of the Warsaw Observa-tory and Tzec Maun Foundation for telescope time. Data from AAVSO observers arealso appreciated. The project was supported by Polish National Science Center grantsawarded by decisions: DEC-2012/07/N/ST9/04172 to KB, DEC-2011/03/N/ST9/03289to MOH, and DEC-2012/04/S/ST9/00021 to AR.
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