SHON - Discussion

Dr. P. Molik of the Petrin Hill Observatory submitted for publication in the IBVS the following communication:

The phenomenon of "anti-flares" in eclipsing binaries of the W UMa-type

As reported in the literature (see e.g. Yang and Liu, 1985), there were observed short-term increases of brightness called "flares" in several binary stars of the W UMa-type. Huruhata (1952) during his photoelectric photometry of U Peg found an unusual brightening near maximum light which lasted about 30 minutes. The increase in brightness amounted to 0.3 mag in U passband. But there was no similar flare in V passband, so Binnendijk (1960) expressed its doubt about the reality of this phenomenon.

The most famous of these "flare" events was observed twelve years later in W UMa itself by Kuhi (1964). On April 26th, 1964 (JD 24 38 511.681) he registered a rapid increase in brightness lasting only 7 minutes and reaching about 1.5 mag at wavelength 330nm. It occurred in orbital phase 0.703, i.e. 25 minutes before secondary maximum. Later on, Egge and Petersen (1982) described a "flare" in VW Cep; Yang and Liu (1985) noted a "flare" in CN And. (Distortions in the light curve of 44 i Boo near maximum light which were observed by Eggen (1948) and which are cited in the literature as another example of "flares" in W UMa-stars are certainly not flares.)

However, the aim of this communication is to draw attention to just opposite phenomenon: In several W UMa-stars instead of short-term brightenings there were observed short-term decreases of brightness lasting a few minutes or even less than one minute. These decreases of brightness look like reverse flares or "anti-flares".

At least two events of this kind were observed photoelectrically: Castelaz (1979) during observation of BX And on the night of October 27/28, 1978, registered a very short-lasting (less than one minute) decrease of brightness in orbital phase 0.01. The depth of the decrease was 0.1 mag in V, 0.2 mag in B, and 0.3 mag in U passband. Bradstreet (1981) observed similar "sudden dip" in V523 Cas on the night of July 7/8, 1979. The decrease was 0.13 mag deep and occurred in phase 0.6. Neither Castelaz nor Bradstreet gave some explanation of this phenomenon.

The author of this communication observed very short-lasting decreases of brightness visually in V839 Oph. It has to be said that for a year he considered them to be observational errors and only later he get convinced of their reality due to the fact that they had occurred in certain orbital phases. Their duration was less than 10 minutes. Table 1 lists six such events observed in the years 1994 and 1995 that are with high probability real. Note that "anti-flares" No.1 and No. 6 as well as No. 2 and No. 3 differ exactly by half-period.

Table 1: Very short-lasting decreases of brighthess ("anti-flares") in V839 Oph

In contrast to "flares" that were recorded in several W UMa-stars in phases around maximum light all "anti-flares" were observed in phases around minimum light. The short-term increases of brightness can be (at least formally) compared to genuine flares but the phenomenon of "anti-flares" is much more puzzling. We do not know any intrinsic physical process that is able to cause such rapid and short-lasting decrease in energy output of stars. So the only plausible explanation of "anti-flares" seems to be eclipses of some parts of binary systems. Because of short duration of "anti-flares" these eclipsed areas must be relatively very small and much brighter than remaining parts of the surfaces of given stars. But even when these bright areas are very small the orbital motion of close binaries composed of main sequence stars is too slow to cause eclipses lasting only one or a few minutes.

Therefore it is necessary to take into consideration that these eclipses can be caused by an object that rotates only around one component star with a speed several times higher than is the speed of orbital motion of given binary system. Such circumstellar (not circumbinary) object can be a transient gaseous stream or permanent accretion disk. But this implies the semi-detached configuration instead of the contact one. The small and bright area which is sometimes eclipsed by a part of the circumstellar object could then be a hot spot or directly the surface of a white dwarf.

Thus the existence of "anti-flares" supports the author's hypothesis that at least some of the W UMa-stars are not contact binaries but semi-detached systems in which one component is a white dwarf surrounded by an accretion disk. For example, if we accept that the above mentioned "anti-flares" in V839 Oph have eclipsing nature (as suggests their occurrence in certain orbital phases) we can assume that they could be caused by additional eclipses of higher latitudes of the degenerated hot primary component (which are normally uneclipsed all over the orbital cycle due to relatively low inclination of orbital plane) with a clump of matter belonging to the accretion disk but orbiting temporarily well above the equatorial plane of the primary component. However, it is only a working hypothesis which can be questioned because it does not explain e.g. why the "anti-flares" occur only near orbital phases 0.0 and 0.5.

For understanding the true nature of "anti-flares" much more observations are needed. The author of this communication would greatly appreciate notifying about any published or unpublished observations of "anti-flares" in W UMa-stars or in other eclipsing binaries.

Petr MOLIK, Petrin Hill Observatory, Prague

References:

Binnendijk L., 1960: Astron. J., 65, 88

Bradstreet D.H., 1981: Astron. J., 86, 98

Castelaz M., 1979: Inform. Bull. Var. Stars, No. 1554

Egge K.E., Petersen B.R., 1982: In: Byrne P.B. and Rodono M. (eds.), IAU Colloquium 71, "Activity in Red-Dwarf Stars", p. 481

Eggen O.J., 1948: Astrophys. J., 108, 15

Huruhata M., 1952: Publ. Astron. Soc. Pacific, 64, 200

Kuhi L. V., 1964: PASP, 76, 430

Yang Yu-Lan, Liu Qing-Yao, 1985: Inform. Bull. Var. Stars, No. 2705

The co-editor of IBVS, Dr. L. Szabados, made a reply:

Dear Dr. Molik,

enclosed please find the referee report on your manuscript. It is clear that the referee who is an expert in this field of research, does not support acceptance of this note for the IBVS.

Yours sincerely,

Laszlo Szabados

co-editor

The author claims of having observed what he calls - with a unnecessarily confusing term - 'anti-flares'. These are actually dips of short duration in the light curve of a known contact binary, V839 Oph.

The only thing one learns from the text about his observations is that they are visual estimates and the dips are ~.1 mag and occur not far from the minima. He does not tell if these dips correspond to one or more observations, which comparison he used and so on.

V839 Oph has been observed photoelettrically by Niarchos, who published complete light curves and mentions that there the shape of the curve is variable at a level of 0.05m. I find therefore highly probable that the the dips are actually observational errors. The fact that they occur at phases when the system is dimmer certainly does not help. The 'similar effects observed by other authors' in other contact binaries, as reported in the text, were indeed treated by them as such.

Moreover, the explanation provided by the author for the dips is unacceptable, as it is on contrast with all the other observed properties of V839 Oph: according to him the system (that definitely has the light curve of a NORMAL CONTACT BINARY and a spectral type F8V!) should be a semidetached system with a WD primary surrounded by an accretion disc (a changing configuration of the disc would then be at the origin of the eclipse of some bright dots on the WD component). Without entering into the details of the model it is obvious that such a configuration will produce a completely different type of light curve.

So, I believe that this paper can only be rejected, though I always feel sorry in these cases for the time and the energy certainly wasted in the work.

and Dr. P. Molik replied:

Dear Dr. Szabados,

thank You very much for Your letter. I respect Your decission not to publish my contribution to IBVS. It is a great pleasure for me to read the oppinion of real expert on W UMa-stars. The only thing I disagree with is that my work represents wasting of time. I will continue searching for "anti-flares" in W UMa-stars.

Yours sincerely,

Petr Molik, Petrin Hill Observatory, Prague

We add to this correspondence that Dr. P. Molik presented a substantially improved (and more convincing) version of the communication at the above mentioned conference on variable star research in Brno.