[vsnet-chat 7645] TT Ari: Double-Humped Positive Superhumps and QPOs on Return to Usual Bright State

[Ivan Andronov]

TT Ari: Double-Humped Positive Superhumps and QPOs on Return to Usual Bright State 

TT Ari has undergone an excursion from usual ~11mag state to ~17 mag since September, 2009. Such event is a rare one, as the previous year-scale fading of large depth occurred in 1980 (Bortle et al., 1980IAUC.3541....1B; Krautter et al, 1981, 1981A%26A....98...27K). Long-term photographic photometry was discussed by Hudec et al. (1984Obs...104....1H) who found no periodicity in excursions from usullay bright state to a lower one. Bianchini (1990AJ.....99.1941B) suggested a 12.6 year cyclicity in such switches, noting a long "superlow" state (B~16.5) in 1979-1985. Yoon et al. (2006ATel..698....1Y) reported on a transit between the intermediate and bright luminosity states.
Kim et al. (2009A%26A...496..765K) studied dependence of the periods and amplitudes of QPOs with time and brightness.

The observations were obtained in the Crimean Astrophysical Observatory at the 1.25m telescope AZT-11 equipped with an UBVRI photometer-polarimeter constructed by V.Piirola. The calibration of the comparison star "c" (Goetz, 1985IBVS.2823....1G) is U=12.04, B=11.72, V=11.01, R=10.41, I=9.97 (Efimov et al. 1998BCrAO..94..215E).
Alltogether, 2048 10-sec exposures were obtained from HJD 2455501.38826 to 2455501.51121 with a mean time 2455501.421. The mean brightness and colors with error estimate are
Filter value    sigma
U     11.8587  0.0033 
B     12.6964  0.0030 
V     12.6745  0.0029 
R     12.4160  0.0024 
I      12.2434  0.0021 
U-B  -0.8377  0.0010 
B-V   0.0218  0.0015 
V-R   0.2585  0.0012 
R-I    0.1727  0.0011 


Table of best fit periods and amplitudes obtained using the "Lambda" scalegram (Andronov, 2003ASPC..292..391A) is below:
FIlter;Period1,days;semi-amplitude1, mmag; Period2,days;semi-amplitude2, mmag;
U    0.0103   76     0.0752  93
B    0.0105   68     0.0751  90
V    0.0105   61     0.0746  84
R    0.0104   51     0.0703  70
I     0.0104   43     0.0754  52

The first "period" value corresponds to the quasi-periodic oscillations (QPOs) and is of 15 minutes. ALthough this value is close to that obtained by Virnina and Andronov (http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/12275) 
on October 16, the semi-amplitude of QPOs in V (61mmag) is slightly larger than that obtained by Virnina and Andronov (46+-15mmag) at the state whith a mean brightness of V=13.101+-0.012, i.e. by 0.43mag fainter than during the present observations.

The second period is strange and is close to one half of the positive superhump period. However, for both periods there is an increase of amplitudes with decreasing wavelength (from I to R) by a factor of 1.77 and 1.79, respectively. This agrees with finding of Tremko et al (1996A&A...312..121T) that QPOs have a "bluer" spectral energy distribution than the negative superhumps and even more than the mean brightness.

The periodogram analysis was made using one- and two- harmonic approximations
(Andronov, 1994OAP.....7...49A) realized in the program MCV (Andronov I.L.,
Baklanov A.V., 2004, Astronomy School Reports, 5, 264, http://uavso.pochta.ru/mcv).
The two-harmonic fit to the observations in U (largest amplitude) corresponds to a best fit period of 0.1472days)
T_Max=2455501.37116   +- 0.00099  Ampl=0.08305 +- 0.00364 S/N=22.8 (main period)
T_Max=2455501.45179 +- 0.00036  Ampl=0.11302 +- 0.00354 S/N=31.9 (harmonic)
So the half-period variations are larger than that at the main period.

For V,
Ampl=0.07050 +- 0.00325 S/N=21.7
Ampl=0.10057 +- 0.00314 S/N=32.0
The value of 0.071mag is remarkably similar to that 0.077+-0.015 obtained for a fainter state by Virnina and Andronov.

The light and color index curve with this second-degree trigonometric polynomial (TP2) fit for U is shown at
http://uavso.pochta.ru/TT101031.gif
The dominating amplitude of the variations at "half-period" is clearly seen. However, the length of the run is comparable with the determined photometric period. Thus new observations of 6-8 hors are needed.


Ivan L.Andronov, Kirill A.Antoniuk
Odessa National Maritime University, Ukraine
Crimean Astrophysical Observatory, Ukraine