Multicolour photometry of unusual dwarf nova star HS 0218+3229

1. Golysheva P., Shugarov, S., Katysheva N., Chochol D. Multicolour photometry of unusual dwarf nova star HS 0218+3229 Sternberg Astronomical Institute, Moscow and Astronomical Institute of Slovak Academy of Sciences, TatranskaLomnica

2. Dwarf Novae Cataclysmic variables (CVs) are highly evolved close binaries consisting of a red dwarf transferring a matter to his companion – a white dwarf. This matter creates an accretion disc around the white dwarf. Dwarf novae (dNe) are a subclass of cataclysmic variables (CVs). dNe usually live in quiescence but sometimes flare up. Their outbursts is caused by accumulation a matter in accretion disc and possibly a sudden increasing of mass rate. Outbursts of U Gem-, or SS Cyg-type of dNe last a few days. HS 0218+3229 (RA = 2h21m34s, Del = +32o 43'.4, J2000) was suspected as a CV by Gänsicke et al (ASP Conf. Ser., 261, 190, 2002).

3. Later the system was studied by Rodrigues-Gil et al. (Astron. Ap. 496, 805, 2009).They didn’t observe any outbursts, but their spectral observations showed the emission lines of hydrogen and neutral helium of weak accretion disc and the absorption lines of a cold component.

4. Rodrigues-Gil et al. determined an orbital period of 0.297 days, during which a double wave is seen, and valued some parameters of HS 0218+3229: They described second component as a K5V star which contributes of 80-85% to the R-brightness of the system. i = 59o± 3o 0.44< M1/MSun<0.65 d= 0.9 - 1 kpc 0.52<q<0.650.23< M2/MSun<0.44 HS 0218+3229 was also identified as X-ray source RXS J022133.6+324343 and as IR-source 2MASS J02213348+3243239.

5. Independently HS 0218+3229 was discovered by S. Antipin in 2006 yr on the plates of the photo collections of SAI. He found one outburst with amplitude 4m.5 in Bpg– band among the 200 photoplates in the interval 1963-1997 yrs. Then we carried out more than 2000 UBVRI CCD frames from 2006 to 2010 yrs. The observations were performed at the 50-cm telescope in StaraLesna (Slovakia) and 50-and 125-cm telescopes of Crimean laboratory of SAI.

6. We began our observations in Sept., 2007. And in October we caught the outburst of HS 0221. The outburst light curve is present below. The nightly LCs are in the separate windows. Left LC shows the orbital wave with the ellipsoidal effect, this effect is not visible on the other nightly LCs because of the outburst disc.

7. The UBVRcIc-band light curves from 2006 to 2010 yrs. The observations in RJ IJ -bands are plotted by grey points. During outburst (JD 2454384-385): Vmax ≈ 12m.34(05) U≈ 11m.3±0m.2 B ≈ V≈ 12m.3(2) RJ ≈ IJ ≈ 12m.2(2) At the quiescence there is a small variability with an amplitude of 0m.2 and with a characteristic time of hundreds of days.

8. Let’s go back to outburst of the October. We can see that the form of the LC is more symmetrical than the other dNe. Symmetry coefficient (the ratio of the duration of the ascending branch to the descending branch) is ~0.22. For famous dN SS Cyg this value is 0.10-0.13. The length of outburst was valued in 15-16 d.

9. Such form of the LC HS 0218+3229 can be explained by the low rate of mass transfer in the system. In this case it is “inside-out” outburst in accordance with Smak(PASP, 96, 575, 1984). Outburst starts from the inner parts of the disk and extends outward. The nightly light curves are present in the next slides.

10. The nightly light curves of HS 0218+3229 in quiescence (Mag vs JD). Some of the curves clearly show a double wave per a period.

11. The nightly light curves of HS 0218+3229 in quiescence (Mag vs JD). We can see the fast variability in U, B-bands with a characteristic time of 40-50 min. Maximum amplitude in U-band is of 0m.7, in B-band - 0m.4.

12. Phased light curves We got next ephemeris which satisfies both our observations and data by Rodriguez-Gil et al. (2009) : The initial epoch of the minimum is the same as in the paper by Rodriguez-Gil et al. (2009). This epoch corresponds to the inferior conjunction (the RD before the WD)

13. V B U V The phased LCs are plotted above. Double wave per a period is clear seen,especially in V, Rc-bands. In the U & B-bands it is not so obvious but there is some fading at phase 0 . U-B Rc B-V Ij V-Rc Ic (R-I)c

14. We can see that the form of LC changed: before outburst the depths of the main and the secondary minima were different and the LC amplitude was larger. After outburst the depths and amplitude of both waves were the same. There are summed LCs before (grey) and after (black) outburst.

15. This can be explained by the fact that, immediately before an outburst, the saturated accretion disk makes a larger contribution to the luminosity of the system than afterward, when it is exhausted. The heights of the maxima in the pre-outburst state are different because we view the region where the stream and the outer parts of the disk interact from different angles. On the other hand, the depths of the minima are different because the “nose” of the red star, which is turned toward the white dwarf, is heated by radiation from the disk. After an outburst the luminosity of the disk and the spot on it decrease, and these effects are less marked. Rodrigues-Gil et al., marked that during January-February 2001, the heights of both maxima and the depths of the minima were essentially the same, but in October 2005, differences between them could be seen. These curves are very similar to our curves constructed using data obtained prior to the outburst of 2007. It can be assumed that at the end of October 2005 or just after that, the star underwent an outburst that was missed.

16. U-B, B-V U-B The positions of HS 0218 at max, min and at the decline note by “ * ”. Comparison stars are marked as the “diamonds”. The position of the star at maximum is quite uncertain. B-V

17. V-Rj, (R-I)j V-Rj At minimum position of our system is close to K5V – K6V stars. In the U-B, B-V diagram the UV-excess caused by the radiation of hot outburst disc is seen. (R-I)j

18. Summary 1.We detected two outbursts of HS 0218+3229 in 1980 and 2007 yrs. Once more outburst (in 2002) was found by Р. Willsin the database NEAT. An duration of 2007 outburst was 15-16 days and asymmetry coefficient ~0.22. The amplitudes were maximal in U,B-bands - 5m, and 4m, 3m.5 and 3m in V, R, I, accordingly. 2. In quiescence we can see a double wave per a period. And we give:JD(Min I) = 2453653.0286+(0.d2973559±0.0000010)·E (Porb=7h 8m 11s. 5) .

19. 3. Two-color diagrams show that the star was bluer than during in quiescence. It is connected with a dotation of bright accretion disc into total luminosity. Outburst color temperature was of 15000 К. Color-indices in quiescence correspond to the star К5V.4. As a result, we classified this cataclysmic variable as a very rare subtype of UGSS of dNe with low accretion rate and sparse and more symmetric (“inside-out”) outbursts than usual U Gem- and SS Cyg-type dNe have. 5. We also valued some parameters of this binary: disc luminosity LV= 1.6∙1034erg/s, Lbol~4∙1034erg/s, the orbital size 1.5-1.9 Rsun, the distance 0.66 kpc

20. Thanks!