COMMISSIONS 27 AND 42 OF THE IAU INFORMATION BULLETIN ON VARIABLE STARS Number 3856 Konkoly Observatory Budapest 9 March 1993 HU ISSN 0374 - 0676 NGC 2169-12, a photometric and spectroscopic Silicon variable * In the course of a Durchmusterung of open clusters for the presence of chemically peculiar ('CP') stars of the upper main sequence using delta a photometry, the object No. 12 (numbering according to Hoag et al., 1961) in the young open cluster NGC 2169 has been identified as peculiar (Maitzen, 1993). Moreover, its Delta a-index turned out to have a large scatter. Table 1 gives the log of 7 observations obtained at ESO-La Silla both at the 1m-ESO and the 61cm-Bochum telescopes (description of the equipment: Maitzen, 1993). Since such a low number prevents us from applying the usual period search codes we have relied on visual inspection of phase diagrams in order to single out possible periods. An important starting point for period search is the uvby-photometry of Delgado et al. (1992) who discovered photometric variability of NGC 2169-12 through two extended night series (duration: about 7 hours each) separated by 3 days. Since they observed in both nights a decline in brightness in all colours (with decreasing range from blue to red) at roughly the same level, they conclude that the period should be larger than two times the duration of the observing run, hence about 14 hours. Delgado et al., however, retreated to the suggestion of Perry et al. (1978) who concluded from their 2 spectrograms (taken with a 0.9m telescope) that the star is a spectroscopic binary and ascribed the light variation to a binary system with components of rather different type. Had they taken into account that Young and Martin (1973) already classified it as Silicon star both the possible period length and the amplitude increase from y to u would have appeared to them as typical features of a hot CP2 star. Fig. 1 shows the variation of the Delta a-index with phase for P = 1.56 days yielding the smoothest picture. While P = 0.77 has still to be retained as possible period, our photometry renders a period near 3 days improbable although it has shown up prima facie from the uvby-photometry of Delgado et al. [FIGURE 1] Fig. 1: Delta a in mmags versus phase (elements of the variation: Tab. 1) [FIGURE 2] Fig. 2: Density tracings of NGC 2169-12 spectrograms between Hdelta (left) and Hgamma (right) obtained at the observing nights indicated. The location of the Si II doublet 4128-31 is marked by an arrow. Gaps in the tracings are due to emulsion defects. TABLE 1. Delta a-photometry of NGC 2169-12 HJD phi Delta a n m.e. telesc. 2447524.70 0.08 52 4 2.7 1m ESO 25.62 0.67 38 9 1.6 1m ESO 27.67 0.99 53 4 3.2 1m ESO 30.69 0.92 55 7 5.0 Bochum 31.70 0.57 37 16 2.8 Bochum 32.67 0.19 40 5 3.9 Bochum 33.65 0.82 41 5 1.3 Bochum Notes: the phases phi were calculated with delta a(max) = 2447524.57 +1.56E. The photometric quantities are expressed in mmags. TABLE 2. Spectroscopic observations at Mt. Schopfl Plate No. HJD night exposure S811-3 2446769.51 86-12-04 81 min. S878-3 2447862.47 89-12-01 60 min. S974-3 2449021.35 93-02-02 184 min. S978-1 2449031.38 93-02-12 175 min. Although there is triple evidence (one spectroscopic identification, our delta a-values and uvby-variability) for the CP2-nature of NGC 2169-12, we were worrying about the fact that 3 other spectroscopic classifications indicated normal main sequence types: Hoag and Applequist (1965): A0V, van Rensbergen et al. (1978) and Perry et al. (1978): B9V. Maitzen (1993) pointed out that one reason for missing peculiarity could have been the insufficient spectrum widening forced by the fact that for the telescopes and photographic technique used the star is practically at the magnitude limit. Since the star is at delta = 13deg, hence observable also from the northern hemisphere we decided to resort to our home observatory, the Leopold-Figl-Observatorium fur Astrophysik on Mt. Schopfl and to obtain spectrum with its Boller & Chivens Spectrograph at dispersion 125 angstrom/mm on IIaO emulsion. The widening of the spectrograms on the plate was 500 microns. Table 2 contains the log of the spectrographic observations and Fig. 2 the density tracings obtained at the PDS 1000 microdensitometer of the Vienna Institute for Astronomy (step size: 2 microns, slit width: 4 microns). From the behaviour of the Si II 4128-31 doublet it is clear that NGC 2169-12 is a spectrum variable. Our spectrograms 5811-3 and 5974-3 do not show any prominence of the Si feature and we would have classified the star as normal like the other sources except Young and Martin. On the other hand, 5978-1 and to a lesser extent also 5878-3 exhibit the Silicon doublet as outstanding. Hence, the problem of controversial classifications is resolved by the existence of intrinsic changes of peculiar spectral features. We conclude therefore that all existing observational evidence consistently identifies No. 12 as CP2-Silicon star with strong variability in Silicon line strength, delta a and uvby, and rather fast rotation. A final note is due to the appearance of an object 'No. 12' with Geneva photometry (reported e.g. in the SIMBAD database): This number refers to the designation of Cuffey and McCuskey (1956) and represents Hoag No. 15 (North, 1993). Acknowledgments: The treatment of our spectrograms was done with the program SPE kindly provided by S.G. Sergeev from the Crimean Astrophysical Observatory. One of us (H.M.M.) expresses his special gratitude to Mr. H. Kapun from the Institut fur Astronomie for immediate rescue action after the observing night on Feb 12, 1993 when the observatory car got stuck in a snow cornice at Mt. Schopfl. The support of the other observer on duty Dr. A. Schnell on this occasion is also gratefully acknowledged. H.M. MAITZEN Th. LEBZELTER Institut fur Astronomie Universitat Wien Turkenschanzstrasse 17 A-1180 Wien, Austria MAITZEN @ AVIA.UNA.AC.AT References Cuffey J., McCuskey S.W., 1956, ApJ 123, 59 [BIBCODE 1956ApJ...123...59C ] Delgado A.J., Alfaro E.J., Garcia-Pelayo J.M., Garrido R., 1992, AJ 103, 891 [BIBCODE 1992AJ....103..891D ] Hoag A.A., Applequist N.L., 1965, ApJS 12, 215 [BIBCODE 1965ApJS...12..215H ] Hoag A.A., Johnson H.L., Iriarte B., Michel R.I., Hallam K.L., Sharpless S., 1961, Publ. U.S. Naval Obs. 2nd Ser. 17, 349 [BIBCODE 1961PUSNO..17....1H ] Maitzen H.M., 1993, A&AS, submitted. North P., 1993, private communication Perry C.L., Lee P.D., Barnes J.V., 1978, PASP 90, 73 [BIBCODE 1978PASP...90...73P ] van Rensbergen W., Hammerschlag-Hensberge G., van den Heuvel E.P.J., 1978, A&A 64, 131 [BIBCODE 1978A&A....64..131V ] Young A., Martin A.E., 1973, ApJ 181, 805 [BIBCODE 1973ApJ...181..805Y ] * based on observations collected at the European Southern Observatory (ESO) on La Silla, Chile, and at the Leopold-Figl-Observatorium fur Astrophysik on Mount Schopfl, Austria. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France.