COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 2113 Konkoly Observatory Budapest 1982 March 19 HU ISSN 0374-0676 CENTRAL STAR OF PLANETARY NEBULA NGC 2346: NEW ECLIPSING BINARY NGC 2346 (215+3d1; AR_1950 = 7h06m49.7s, D_1950 =-0d43'29") is a bipolar planetary nebula having an A-type central star. This spectral type is too late for planetaries and the most plausible explanation seems to be a binary model of the nucleus, already discussed by Kohoutek and Senkbeil (1973), and Mendez (1978). Mendez and Niemela (1978), and Mendez (1980) observed variable radial velocity of this star with a period of 16 days. We have searched for variability of this object since 1974 at the European Southern Observatory, La Silla, Chile. In January and February 1982 we have found drastic changes of the nuclear brightness (reported already in IAU Circ. No. 3667). Our photoelectric UBV and BV measurements of NGC 2346 are summarized in Table I They were carried out using (A) the 50 cm telescope and a pulse counting photometer (EMI 6256A photomultiplier, diaphragm 21 arcsec), internal accuracy about +-0.01 mag; (B) the 1 m telescope and a pulse counting photometer (EMI 9658R, dia. 22.9 arcsec), accuracy about +-0.02 mag. In (C) we present supplementary BV measurements kindly provided by R. Kiehling with the Bochum 61 cm telescope (dia. 18.2 arcsec). Table I contains stellar magnitudes only; the contribution of the nebular radiation was eliminated by observing through different diaphragms (15, 21, 30 and 40 arcsec). Stars in the E-regions Nos. 2-7 (Cousins, 1973) served as photometric standards. In case of observations given in (B) and (C) the brightness of NGC 2346 was referred to the local comparison star "b". The UBV magnitudes of five comparison stars listed in Table II were measured at ESO, La Silla (50 cm and 1 m telescopes) during 1978-82; their internal accuracy is +-0.01 mag or better. The following conclusions can be made from the V and B light curve of NGC 2346: 1) The deep, very broad but sharp minimum at Min.I = JD 2445010.85 +-.15 can be interpreted as a partial eclipse (probably close to totality) of the main A-type component of the close-binary system, The amplitudes are: A_V = 2.2 mag, A_B = 2.6 mag. 2) In January a part of the ascending branch of the previous minimum was observed, probably at Min.I= JD 2444993.65 +-.35, so that the orbital period P = 17.2 +-0.4 day can be estimated. Table I Photoelectric observations of the central star of NGC 2346 in January - February 1982 (A) Measurements with 50 cm telescope JD V B-V U-B n 2440000+ ---------------------------------------------- 4995.620 12.660 +0.425 +0.487 8 4997.631 11.650 0.322 0.341 9 4998.647 11.396 0.305 0.315 6 4999.686 11.338 0.285 0.290 9 5000.575 11.337 +0.277 +0.308 9 (B) Measurements with 1 m (C) Measurements with 61 cm telescope telescope made by R. Kiehling JD V B-V n JD V B-V n 2445000+ 2445000+ ----------------------------- --------------------------------- 2.657 11.29 - 2 08.571 12.31 +0.55 1 4.656 11.34 - 2 09.561 12.92 0.67 1 5.666 11.44 - 2 10.573 13.43 0.58 2 6.577 11.69 +0.28 1 11.568 13.04 0.62 2 7.573 11.97 0.36 1 12.538 12.57 0.48 2 8.572 12.40 +0.42 1 13.538 12.20 0.50 2 14.539 11.88 0.42 1 n - number of measurements 15.535 11.54 +0.38 1 Table II UBV magnitudes of the comparison stars --------------------------------------- Star V B - V U - B n --------------------------------------- a 10.24 +0.14 +0.17 4 b 11.02 0.36 0.13 6 c 12.01 0.11 0.12 2 d 12.80 0.25 +0.21 7 e 13.21 +0.09 -0.15 8 [FIGURE 1] Fig. 1. Finding chart for NGC 2346 and for comparison stars a,b,c,d,e. [FIGURE 2] Fig. 2. V light curve of the central star of NGC 2346 in 1982. 3) The extremely broad minimum and the shape of the light curve outside the eclipse could be explained by the non-spherical components and the possible gaseous stream between them. 4) The secondary component is of later spectral type, V = 13.50, B - V = +0.70 can be found assuming the total eclipse. 5) The secondary minimum has not been detected. We tentatively classify the system as a semi-detached close binary. Adopting K = 18 km.s^-1 for the half amplitude of the radial-velocity curve (Mendez, 1980), we obtain a = 0.03 AU for the semi-major axis of the relative orbit, and f(M)= 0.01 for the mass function, There is no evidence for large light changes of NGC 2346 in the past, although small light variations were possible according to our observations. We expect that the conditions for the geometrical eclipse of the system have changed due to fast rotation of the line of apsides caused by (a) non-spherical components, (b) strongly elliptical orbit, and (c)presence of a third body (the actual planetary nucleus). Further systematic observations are planned. Acknowledgements: I wish to thank especially R. Kiehling for his very valuable additional observations at the Bochum 61 cm telescope, and H.M. Maitzen who left to me some of his observing time at the 1 m telescope, The observations have been collected at the European Southern Observatory, La Silla, Chile. L. KOHOUTEK Hamburger Sternwarte D-2050 Hamburg 80 Federal Republic of Germany References: Cousins, A.W.J., 1973, Mem.Roy.Astr.Soc. 77, 223 [BIBCODE 1973MmRAS..77..223C ] Kohoutek, L., 1982, IAU Circ, No. 3667 [BIBCODE 1982IAUC.3667....2K ] Kohoutek, L., Senkbeil, G., 1973, Mem.Soc.Roy.Sci. Liege, Ser. VI, 5, 485 [BIBCODE 1973MSRSL...5..485K ] Mendez, R.H., 1978, Mon.Not.Roy.Astr.Soc, 185, 647 [BIBCODE 1978MNRAS.185..647M ] Mendez, R.H., 1980, IAU Symp. No. 88, 567 [BIBCODE 1980IAUS...88..567M ] Mendez, R.H., Niemela. V.S., 1978, Mon.Not.Roy.Astr.Soc. 184, 351 [BIBCODE 1978MNRAS.184..351M ]