NOTES ON SOME ECLIPSING AND VISUAL BINARIES
by T. HERCZEG, Budapest
The subsequent paper deals with three different topics, presenting:
1. observations of the well-known eclipsing binary VV Cephei ;
2. notes on the duplicity of Nova DQ Herculis (1934) and
3. discussions of two interesting and problematic visual pairs.*
1. Photoelectric observations of eclipsing binaries
First I shall give a short account about current observations of eclipsing
binaries at the Budapest Observatory. The following stars are in the
observational program of various observers:
lambda Tauri, VW Cephei (Dr. Detre)
GO Cygni (Ozsváth)
SX, TT and BF Aurigae (Herczeg).
The photoelectric light curve of lambda Tauri and VW Cephei are practically
complete and a discussion will be soon published. Measurements made till
now of the four other above mentioned stars are giving a very fragmentary
light curve only.
Besides these stars we are observing regularly the eclipses of Zeta
Aurigae-type double stars. Observations of the 1950 and 1955-56 eclipses of
Zeta Aurigae are reported elsewhere. The session of to-day give an opportunity
of publishing my two-colour photoelectric observations of VV Cephei.
I observed the present eclipse of VV Cephei with the 1P21 tube attached
to the 24-inch reflecting telescope in blue and ultraviolet light in order to get
a lightcurve of eclipse which is as free as possible from the erratic changes
of the M-type component. These measurements will be included into widespread
co-operative program organised by Dr. F. B. Wood, but for the sake
of completeness I give here the results of 19 observations made between
July 5 and September 22, 1956.
The Budapest observations fall on the descending branch and seem
to indicate the beginning of constant phase.** Measurements before
July 5 were, unfortunately, prevented by the delayed silvering of the 24-inch
mirror and (primarily) by unfavourable weather.
The filters used are Schott BG 12 and Schott UG 1; the isophotic
wavelength is 4340 A in blue light, in ultraviolet it is not yet determined.
It would be prematured to discuss these measurements without taking
into considerations the observations made elsewhere. Nevertheless, let me
* This last section appeared as Nr. 35, of our Mitteilungen [CoKon No. 35]
** Note added in proof: Later observations have shown that this constant
phase is only a hump on the descending branch.
Fig. 1.
Photoelectric observations of VV Cep.
Circles: blue magnitudes, dots:
ultraviolet magnitudes.
Table 1.
Photoelectric observations of VV Cephei
Obs. J. D. Number Delta mm -m_vv Atmospheric
number (heliocentric) of cdmp conditions
obs. in blue in UV
1. 2435 638.515 1 +0.437 moderate
2. 660.530 1 +0.088 poor
3. 664.468 2 -0.068 fair
4. 664.486 1 +0.522 "
5. 665.470 3 -0.035 poor
6. 665.488 1 +0.540 "
7. 671.511 2 -0.208 good
8. 671.524 2 +0.438 "
9. 673.477 3 -0.306 fair
10. 673.502 2 +0.435 fair
11. 680.510 2 +0.327 poor, Moon
12. 680.525 1 -0.520 "
13. 681.516 2 -0.524 good; Moon
14. 681.532 2 +0.308 "
15. 684.449 3 -0.647 good
16. 684.489 2 +0.236 "
17. 690.399 3 +0.259 "
18. 690.431 3 -0.656 "
19. 694.527 2 -0.645 fair
20. 694.545 2 +0.260 "
21. 726.494 2 -0.775 "
22. 726.518 2 +0.252 "
23. 728.481 1 -0.636: poor^1
24. 732.508 2 -0.754 poor; Moon
25. 732.524 1 +0.240 "
26. 739.444 2 -0.809 moderate; Moon
27. 2435 739.461 2 +0.175 "
^1 Observations interrupted by clouds; magnitudes very uncertain !
allowed to notice that the provisory elements due to S. Gaposchkin give very
good prediction for the "second contact". This is perhaps already indicated
by the end of the loss of light after J. D. 2435684, the O-C value being only
of the order of -20d. But there is a surprisingly great difference between
predicted and observed durations of the partial phase.
The rate of gradual dimming during the ingress is according to Gaposchkin
0.038m per day, in photographic light, whereas our observations indicate
a change 0.018m per day in blue and even in the ultraviolet only 0.033m per
day.
2. Note on Nova DQ Herculis (1934)
The all-important discovery by Walker [1] of the close binary nature of
Nova Herculis was put in the right order by Kukarkin [2] saying that Walker's
discovery is of greater importance than all the photometric and spectroscopic
observations made before. It raises the natural question whether or not
the present binary system existed before the outburst. Detection of duplicity
in the preoutburst stadium must be based exclusively on patrol plates and is
therefore very difficult though not entirely hopeless. I hope to return to this
point later. In the present article I propose an other approach to the problem.
It is well known that Nova Herculis like other slow novae exhibited
a deep minimum after the outburst, dropping almost to its prenova magnitude in
the first days of May, 1935. Is it perhaps possible to detect duplicity during
this short interlude of comparatively low brightness? Obviously one cannot
expect any indication of an eclipse during the period of the overwhelming
luminosity of the nova, the secondary being imbedded in a highly luminous
layer of gas. But a discussion of all the available photographic observations
gave the somewhat unexpected result that there are some slight indications
of binary nature as early as during this short period of minimum brightness,
suggesting the existence of duplicity before the outburst.
The question is closely connected with theories about the physical
interpretation of nova phenomenon. Accepting e. g. the "opaque dust cloud"
explanation put forward by Chandrasekhar and Stratton, we may conclude that
the suspected eclipse I shall discuss subsequently is clearly spurious. On the
other hand my suggestion is well compatible with the simple picture of a gas
layer either contracting or gradually getting transparent. The best explanation
so far proposed for the following increase of nova brightness is due to
Grotrian. [3].
The discussion is based on a closer scrutiny of this minimum. I collected
all the photographic observations available in our library published for the
interval of about 12 days, between April 28 and May 10, 1935. Visual
observations are hardly useful for our purposes and can define a general trend
of light variation only. (This we call as the "ground level of magnitudes".)
The photographic observations I used were the following.
1. Best data are the observations of Schneller [4] and those of Lavdovsky [5]
made at Babelsberg and Pulkova, respectively. These plates were measured
photoelectrically.
2. Fischer (Innsbruck) made some observations [6], but with different
telescopes and measured only the diameters of the photographic images. Fresa's
observations were made on panchromatic plates and he probably made very
long exposures, reducing the usefulness of his data [7].
3. Brunner published [8] two and Gaposchkin three observations [9]
for this critical time-interval; they are actually only estimates of the
photographic brightness.*
Table 2 contains the photometric data I used, both the original and the
somewhat modified magnitudes. These modified values are the results of an
attempt of reducing the published data to a homogeneous scale, undoubtedly
a rather problematic procedure.
The magnitude reductions were made on the following grounds.
Fig. 2.
Lavdovsky and Schneller quote international photographic magnitudes
and their data - adopted without any correction - were considered as the
basis of the whole investigation. The observations of Brunner and Gaposchkin
(based similarly on international standards) were, faute de mieux, also adopted
without changing, because their small number is not suited to a discussion of
possible systematic differences.
Further, the "1922 correction" [10] was applied to the results of Fischer.
Fresa's observations had a different effective wave length; to them
an empirical correction of +0.6 was applied, derived by the aid of the known
change of colour-index [11] during the deep minimum.
Then I tried to determine a "groundlevel" of brightness, fixed by the
general trend of light variation.
This "ground level" or "zero level" of light variaton was then a
symmetrical interpolating curve**, the elevation of which at the end of the
interval considered is very strictly determined by numerous observations made
in the adjacent period of fast perfectly linear increase of brightness
(between 15th May and 5th June). Near the minimum this construction seems to
be in some extent arbitrary, but, practically, a small vertical shift of the
"ground level" do not alter the remarkably distribution of the magnitude
differences we are just going to discuss.
* I received Gaposchkin's paper just after the end of the colloquium and
therefore his data were incorporated only afterwards into figures and text.
** S. Figure 2.
Table 2.
Photographic observations of the deep minimum 1935
Date of obs. Observed Reduced Deflection from
No Observer Phase
J. D. 2427000+ magnitude magnitude "zero-level"
1 920.56 Lavdovsky 13.58 13.58 -0.05 0.087
2 921.562 Fresa 13.0 13.6 +0.08 .262
3 921.80 Gaposchkin 14.22 14.22 -0.49 .49(1)
4 923.550 Fresa 13.6 14.2 -0.33 .529
5 924.504 Lavdovsky 14.09 14.09 -0.17 .456
6 924.69 Gaposchkin 14.22 14.22 -0.29 .41(6)
7 925.388 Fischer 13.55 13.80 +0.13 .021
8 925.401 Fischer 13.76 14.01 -0.07 .088
9 925.466 Lavdovsky 13.88 13.88 +0.05 .424
10 925.501 Fresa 13.7 14.3 -0.37 .605
11 925.516 Lavdovsky 13.79 13.79 +0.14 .682
12 925.546 Brunner 13.8 13.8 +0.13 .837
13 926.408 Fischer 14.06 14.31 -0.39 .289
14 926.500 Brunner 14.0 14.0 -0.09 .764
15 927.372 Fischer 13.65 13.90 -0.04 .268
16 927.431 Schneller 13.88 13.88 -0.02 .572
17 927.70 Gaposchkin 14.13 14.13 -0.29 .96(2)
18 928.519 Schneller 13.72 13.72 +0.04 .191
19 929.484 Schneller 13.53 13.53 +0.11 .175
20 903.509 Fresa 12.9 13.5 -0.01 .469
21 931.504 Lavdovsky 13.36 13.36 -0.05 .068
The next step was namely the fixing of the differences in brightness
from the "zero level" (Column 6 in Table 2).
These resulting magnitude differences I reduced by the aid of the usual
P^-1 (t-t_0) phase formula, using as initial epoch quite arbitrarily J. D.
2427923.254 corresponding to the computed phase = 0.5. (Column 7, Table 2.)
The period was taken P = 0.193627d, i. e. Walker's first, only approximate
value. The time interval considered is not longer than 11 days, therefore even
an error in the 4th decimal of the period is not at all of influence.
The resulting "light curve" is shown in Fig. 3. It is unmistakable that
these observations show a clear tendency toward being "depressed" in a
rather narrow interval about phase = 0.5 suggesting perhaps an eclipse of
the proper duration and of a reduced amplitude. (This is to be expected because
of the higher luminosity of the system.) In fact, all but two or three observations
are in rough agreement with a light curve of a hypothetical eclipsing system.
Let me notice, that quite apart from errors of photographic photometry,
longexposure plates are especially unfavourable in detecting a possibly light
variation of no more than 1 hour duration. Supposed now that this - very
doubtful - minimum is real, we can, of course, determine its date. Taking
JD 2427923.448 as the time of this minimum, we get the following residuals:
1. with Rosino's period [12] O - C =+0.04d (that is 20 per cent of the period);
2. with Walker's improved period O - C = +0.01d, a remarkably small
value. But it is necessary to underline that these residuals cannot be regarded
as any convincing evidence at all. It is, in the contrary, easily possible,
that this seemingly very good agreement is merely fortuitous, for a change of
one unit in the 6th decimal of P, will shift - after 20 years - the
time of minimum by about 0.04d i. e. 20 per cent of the period!*
Fig. 3.
Adopting, however, this hypothetical eclipse as real, we obtain for the
"improved" period the alternative values P = 0.1936251d or P = 0.1936198d,
the number of epochs being n = 36314 or n = 36315, respectively.
*
This discussion can perhaps underline the importance of the problem
of a possible pre-outburst duplicity of Nova Herculis. In this respect the
patrol plates taken at the Harvard Observatory and at the Sternwarte Sonneberg
are extremely interesting and it is highly desirable to rediscuss this
valuable material.
Literature
[1] M. F. Walker PASP 66, 230, 1954 and Ap. J. 123, 68, 1956.
[2] B. V. Kukarkin IAU Draft report. Dublin meeting.
[3] W. Grotrian Zs. f. Ap. 13, 215, 1937.
[4] H. Schneller A. N. 256 108, 1935.
[51 V. Lavdovsky A. N. 256 251, 1935.
[6] H. Fischer A. N. 256, 106, 1935.
[7] A. Fresa A. N. 255, 430 and 256, 68, 1935.
[8] W. Brunner jr. A. N. 258, 129, 1936.
[9] S. Gaposchkin A. J. 61, 36, 1956.
[10] H. Shapley and M. Walker, Harv. Bull. 781, 1922.
[11] C. Payne-Gaposchkin-F. L. Whipple Harv. Circ. 433, 1939.
[12] L. Rosino Asiago Contrib. No. 63, 1955.
* At this point I am very indebted to Prof. L. Rosino and
Prof. A. van Hoof for their valuable criticism.