COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 1367 Konkoly Observatory Budapest 1977 December 5 CPD -55d 5216: A COOL Ap STAR SHOWING EXTREMELY LARGE VARIATIONS AT 4100 Angstrom In 1971-72 a large programme of uvby photometry of bright southern stars was carried out by Gronbech and Olsen (1976). Many visual companions brighter than about 9m were also observed for the purpose of a later investigation of visual double stars. CPD -55d 5216 (=CoD -55d 4778 = SAO 240236 = GC 17353 = = IDS 12406S5556B) is a companion to HR 4848 (about 53" distant) for which two very discordant measures were obtained. Our immediate reaction was that two different companions had been measured, and since the magnitude and indices of the last measure fitted very well to the information in the Henry Draper catalogue concerning HD 110910 (8.9, A5), which is about 150" from HR 4848 this measure was ascribed to HD 110910 and so published in the catalogue. To settle the matter beyond doubt two more uvby observations and two beta observations were made of each star in 1976. Again, the two uvbry measures of CPD -55d 5216 disagreed violently, and it was concluded that the two measures from 1972 did indeed both refer to CPD -55d 5216 (cf. Olsen, 1977). In March-April 1977 twelve uvby observations were made and in the Table the 16 available measures are listed. The magnitudes u, v and b on the standard system have been computed by combining the y (= V) magnitude with the indices. The photometric variations are very remarkable, but not unique. In the u(3500Angstrom) and b(4700Angstrom) bands the range of variation is relatively small, 0.08m and 0.05m, respectively, and the variations are approximately in the same sense, as shown by a graph or by the scatter in the (u-b) index, which is only 0.019m. In sharp contrast to this behaviour the v(4100Angstrom) band, situated between u and b, shows a photometric range of 0.25m, while the y (5500Angstrom) band has a more moderate range of 0.12m. However, the variations in v and y are in the opposite sense, as shown by the scatter in y + v/2, which is only 0.019m. As a consequence of the described variations, the indices (b-y) and m_1 vary together, with ranges 0.12m and 0.16m respectively, while the Balmer discontinuity index c_1 varies in the opposite sense with the range 0.45m. When the star is brightest, it is also reddest, has the largest m_1 value, and the smallest c_1 value. In the ([m_1] ,[c_1]) diagram the star jumps back and forth in a narrow band between the two extremes: the point (0.20m, 0.90m) corresponding approximately to an A7 type star near the ZAMS, and the point (0.39m, 0.43m), which lies in a region populated by composite stars and cepheids, and on the red border of the region of cool Ap stars (cf. Figs. 3 and 5 in Cameron 1967). Thackeray (1966) has obtained four spectra of CPD -55d 5216 (=HD 110956f) in order to determine the radial velocity, which is -4 km/s. Thackeray classifies the spectrum A3p and comments: "CaII weak. Some signs of spectral variation with mild features of a Cr star". Later, Andrews and Thackeray (1973) have published UBV photometry for CPD -55d 5216, but they do not refer to any photometric variability. Andersen (1977, private communication) has obtained a 20Angstrom/mm coude spectrogram (ESO 1.5 m telescope, HJD = 2443271.695). He gives the following description: "The spectrogram shows a late A type spectrum, but with the CaII K line far too weak. Several CrII lines are considerably strengthened, as are possibly also MnI lambdalambda 4030-33 and EuII lambdalambda 4205, 4129. The SiII and SrII lines do not appear enhanced, and the ScII lines not abnormally weak. There is a noticeable broadening of the lines, with vsin i about 30 km/s. A rough measurement yields a radial velocity of -3 km/s, in good agreement with the value by Thackeray (1966). There is thus no indication of appreciable variation of the radial velocity". There is no doubt that CPD -55d 5216 is a cool Ap star with very large photometric variations. It is very reminiscent of Osawa's star HD 221568, which has a smaller range in m_1 and c_1, but a similar range in V and (b-y) (cf. Osawa 1967). Somewhat smaller variations in the v band relative to neighbouring bands have also been found in HD 71866 and 125248, and, to an even lesser degree, in several other stars (cf. Adelman et al. 1976 and references therein). Adelman et al. explain this as partly due to two EuII bound-free discontinuities at 4200Angstrom. The periods of the three stars just mentioned range from 7 to 159 days, while the period for CPD -55degree 5216 (if it has a period) is considerably shorter, probably about 2 days. The star thus seems to be the most extreme in this group of Ap stars. HR 7575 has also been found to show variations in the v band. In Cameron's (1967) investigation it was the Ap star with the largest m_1 value, and his photometry was confirmed by Gronbech and Olsen (1976). However, a single measurement in August 1976 has yielded (V, (b-y), m_1, c_1) - (5.66m, 0.051m, 0.370m, 0.665m).The u, b, and y magnitude thus show no changes while v has changed by 0.06m. In this connection attention is drawn to Gronbech and Olsen's (1976) Table 4 and their notes to Table 2, which contain many variable Ap stars. Since no further observations of CPD -55d 5216 are being planned at this observatory, we strongly recommend this noteworthy object to other observers. Simultaneous spectrographic and photometric observations should prove particularly valuable in investigations of the sources of the observed variations. ERIK HEYN OLSEN Copenhagen University Observatory Brorfelde, Denmark References: Adelman, S.J., Shore, S.N.,and Wolken, P.R. 1976, Physics of Ap Stars, IAU Colloquium No. 32. eds. Weiss, Jenkner and Wood, Vienna Univ.Obs., p. 189 [BIBCODE 1976paps.coll..189A ] Andrews, P.J. and Thackeray, A.D. 1973, MNRAS, 165, 1 [BIBCODE 1973MNRAS.165....1A ] Cameron, R.C. 1967, The Magnetic and Related Stars, ed. R.C. Cameron, Mono Book Corporation, Baltimore, p. 471 [BIBCODE 1967mrs..conf..471C ] Gronbech, B. and Olsen, E.H. 1976, Astron.Astrophys.Suppl. 25, 213 [BIBCODE 1976A&AS...25..213G ] Olsen, E.H. 1977, Astron.Astrophys.Suppl. 29, 313 [BIBCODE 1977A&AS...29..313O ] Osawa, K. 1967, The Magnetic and Related Stars, ed. R.C. Cameron, Mono Book Corporation, Baltimore, p. 363 [BIBCODE 1967mrs..conf..363O ] Thackeray, A.D. 1966, Memoirs Roy.Astron.Soc. 70, 33 [BIBCODE 1966MmRAS..70...33T ] HJD 2440000+ V b v u b-y m_1 c_1 1367.79664 8.716m 8.895m 9.408m 10.387m 0.179m 0.334m 0.466 1368.80087 8.778 8.871 9.163 10.328 0.093 0.199 0.873 2831.85748 8.792 8.889 9.171 10.331 0.097 0.185 0.878 2832.80867 8.704 8.889 9.369 10.373 0.185 0.295 0.524 3220.77962 8.712 8.894 9.406 10.400 0.182 0.330 0.482 3222.81860 8.794 8.890 9.204 10.355 0.096 0.218 0.837 3226.71967 8.704 8.901 9.398 10.383 0.197 0.300 0.488 3228.81928 8.773 8.906 9.267 10.376 0.133 0.228 0.748 3230.76144 8.806 8.903 9.174 10.365 0.097 0.174 0.920 3232.70323 8.732 8.914 9.358 10.385 0.182 0.262 0.583 3233.72544 8.821 8.920 9.201 10.389 0.099 0.182 0.907 3234.79941 8.755 8.914 9.354 10.411 0.159 0.281 0.617 3236.64818 8.802 8.884 9.176 10.347 0.082 0.210 0.879 3237.71561 8.720 8.894 9.339 10.391 0.174 0.271 0.607 3238.56246 8.757 8.922 9.320 10.372 0.165 0.233 0.654 3238.64723 8.767 8.925 9.300 10.377 0.158 0.217 0.702