(fwd) "TT ARI-94": CALL FOR OBSERVATIONS

>from root@astro.odessa.ua (Dr. Ivan L.Andronov) Sat Jul 16 23:27:50 1994

"TT ARI-94": CALL FOR OBSERVATIONS

Introduction

   TT Ari is one of the brightest cataclysmic variables and remains one
of the most interesting objects of this class due to a variety of
phenomena observed at different time scales from seconds to months.
Usually its brightness varies near B=10.7, but sometimes drops to ~16
mag for several weeks or even years (Wenzel et al., 1992). Spectra are
similar to those observed in dwarf novae during an outburst, thus
indicating that the usual "high" luminosity state of TT Ari may be
interpreted as a "continuous" outburst (Krautter et al., 1981) Cowley et
al. (1975) first mentioned a significant difference between the values
of the spectroscopic and photometric periods. Thorstensen et al. (1985)
obtained P(orb)=0.13755114 corresponding to their spectral observations.
Warner (1983) suspected that TT Ari belongs to the group of intermediate
polars, although rotational period of the white dwarf had not been
detected (it still isn't). Accretion geometry in this system undergoes
drastic changes - the "switchings" from the disk accretion in a "high"
luminosity state to a "cone" accretion in a "low" state (Vojkhanskaya
and Gnedin, 1983). Large-amplitude (>1mag) "dips" (Wenzel et al 1992)
and "flares" (Andronov et al 1992) are sometimes present.

"TT ARI-88"

   A variety of phenomena even in photometric behaviour argues for active
monitoring of the object at different observatories to obtain data with
smaller gaps. The first international campaign was organized in 1985, and
its results were summarized by Wenzel et al. (1986). A second campaign
covered the autumn months in 1988. Our recent re-analysis of these data
(Tremko, Andronov et al , Astronomy & Astrophysics, subm.) has lead to
the following conclusions:

1. Moments of 33 brightness maxima and 35 minima show large scatter
of phases within 0.6P-interval. The value of the primary photometric
period of P=0.132953(+-13)d differs from the previously published ones.
2. A secondary oscillation is present, the precise value of which may
not be determined from the present observations. Four secondary peaks
in the periodogram in the range 5.4-7.3h have nearly the same significance.
However, three of them are aliases of the fourth one.
3. Periodograms for detrended nightly runs show significant peaks
at different frequencies from 28 to 103 c/d. The most prominent of
which correspond to periods of 23.7, 15.2, 27.5 and 51.6 min.
This argues for transient quasi-periodic oscillations occuring at a
few preferred time scales rather than for a QPO with a time scale
decreasing from 27 min 1962 to 17 min in 1985 (Semeniuk et al., 1987).
Such behaviour may be explained by the "beat-frequency model"
(Hollander and van Paradijs, 1992). It seems that in TT Ari
we observe contributions of several instability mechanisms with
similar time scales. At higher frequencies 90-900 c/d the periodogram
obeys a power law with an index Gamma=1.600+-0.008.
4. The variations in UBVRI are highly correlated. A method for
determination of relative colours of "slow", "fast correlated" variations
and an "uncorrelated noise" is proposed. Corresponding parameters are
determined. Values U-B= -1.04, -1.24, -1.35 and B-V=0.19, 0.05, 0.12
for "mean brightness", "fast" (15min) and "slow" (3h) variations,
respectively.

"TT Ari-94" : how to observe

Super-long (~>30hours in each observatory) monitoring with time resolution
<3minutes, preferrably in B (V or R when high Moon background). One 8-hour
run is more informative than 2 4-hour ones in different nights. Observa-
tions at different continents starting in Asia, continued in Europe, America
and than again in Japan may allow to obtain at least few "super-long"
runs what is important to to "catch 5-7h period". Be careful to avoid false
"trends" when the star is not very high above the horizon. Multiwavelength
observations are welcome ONLY if they are simultaneous.
    Recommended comparison star:
"s" (BD+14o336), check "a" (BD+14o335). Your runs (>4hours) are welcome
at all dates, practically from August to January. They are VERY important
for precise determination of period(s). However, October 6-9 is selected
for "super-long" runs obtained in different observatories, which are achieved
to be critical for a secondary wave.
Amplitude of variations at each time scale ~0.1-0.2 mag, although dependent
on wavelength and date.
CCD observations at small telescopes with short focal length may be very
important.
   Not confirmed information is that in October the Hubble ST will be pointed
to TT Ari.

What questions may be answered:

1. best fit period of "20-min" variations. Does it really decreases from
  year to year?
2. What is a primary period? Is it changing or this is an illusion caused
  by distorsion of the light curve by other variations?
3. Is secondary wave periodic or elusive? If real, no theoretic explanation
  yet...
4. 4-d "beat period" between the orbital and primary photometric ones.
   Why these 3-hour periods are different?
5. "Flares" and "dips" are unpredictable, if occured, iss good, but for
   periodogram analysis they will be deleted.

Very preliminary list of observers:
N.Dorokhov (Ukraine), R.Luthardt (Germany), G.Oprescu (Romania), L.Patkos
(Hungary), I.Spychka (Ukraine), J.Tremko (Slovakia), S.Zola (Poland)

All time reduced series are planned to be analyzed together in December
and a joint article to be published in one of the international journals.
Your proposals, suggestions are highly appreciated.

Hope that Your contribution will be prominent!
Welcome to join us!

Dr. Ivan L.Andronov, Associate Professor,
Department of Astronomy, Odessa State University,
T.G.Shevchenko Park, Odessa 270014 Ukraine
e-mail: root@astro.odessa.ua (Relcom, Internet)