COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 2689 Konkoly Observatory Budapest 22 March 1985 HU ISSN 0374 - 0676 PHOTOMETRIC VARIATIONS OF THE SUPERGIANT B3 Ia : HD 178129 * The B and A supergiants are now known to exhibit semi-periodic photometric variations with semi-amplitudes up to 0.07 magnitude (Sterken, 1977). The semiperiods tend to increase with the absolute visual magnitudes (Sterken, 1977). Until now, to our knowledge, only 32 stars have known semi-periods and only 3 of them have spectral types comprised between B2 I and B8 I (Burki, 1978). So it is useful to complete this sample by more observations and to check whether the new observed supergiants share the properties already derived for this class of objects. In that context it is interesting to report the observations of the B3 Ia star HD 178129, though more data would be useful to better determine its semi-period. Actually the detection of the photometric variations of HD 178129 is a byproduct of observations obtained for another purpose and it is why its publication was delayed. Incidentally the B3 Ia star HD 178129 was used as one of the two comparison stars in a search for photometric variations of a mild Ap star HD 179761. At the time of our observations (1970), photometric variations were detected only for two B supergiant stars and no general trend was reported (Sterken, 1977), It turned out that the variations of the differential magnitude between the second comparison HD 178484 and HD 179761 is less than 0.02 m whereas it is as large as 0.1 m (in the u filter) between the supergiant and the second comparison. The photometric observations were performed in Stromgren's uvby photometric system during nine nights from the 18th to the 26th of August 1970 at the ESO. We used the standard equipment at the 50 cm telescope. The sequence of the observations already described in Megessier-Garnier (1972) was C_1-C_2-A_p- C_1-C_2-A_p-C_1, the recording time being 20 s in each filter. It was followed by a measurement of the sky in the four filters. The main characteristics of the three stars are given in Table I and the journal of observations is given in Table II as well as the instrumental differential magnitudes in the four filters. The variation curves are represented in Fig. 1 where a free-hand curve is drawn to show the likely semi-period of the supergiant. The * The observations have been performed at ESO, Chile. Table I star spectral type m_v M_v --------------------------------------------- HD 178129 B3 Ia 7.41 -6.1 HD 178484 K0 6.56 HD 179761 B8 III, Ap 5.15 --------------------------------------------- instrumental differential magnitudes between HD 178484 and HD 179761 (Fig. 1, bottom) indicates the precision of the differential measurements. Their small variations (Delta m < 0.02 m) during the observational period should be due to the Ap star variations. Table II uvby instrumental differential magnitudes between HD 178129 and HD 178484. -------------------------------------------------------------------------- date JD Delta u Delta v Delta b Delta y August 1970 2440000+ -------------------------------------------------------------------------- 18 817.565 1.479 0.583 - 0.372 - 0.811 817.587 1.189 0.586 - 0.376 - 0.816 19 818.533 1.494 0.598 - 0.317 - 0.795 818.655 1.483 0.605 - 0.356 - 0.796 20 819.533 1.485 0.591 - 0.365 - 0.798 819.555 1.189 0.593 - 0.370 - 0.803 21 820.522 1.450 0.564 - 0.400 - 0.832 820.511 1.413 0.543 - 0.402 - 0.834 1.467 0.519 - 0.402 - 0.837 22 821.527 1.422 0.531 - 0.421 - 0.857 821.550 1.422 0.531 - 0.427 - 0.855 23 822.526 1.470 0.590 - 0.367 - 0.814 822.515 1.477 0.584 - 0.381 - 0.811 26 825.531 1.526 0.611 - 0.339 - 0.776 825.551 1.513 0.604 - 0.315 - 0.783 1.517 0.627 - 0.332 - 0.773 -------------------------------------------------------------------------- [FIGURE 1] Figure 1. Top: Instrumental differential magnitudes between the supergiant HD 178129 and HD 178484; Bottom: Instrumental differential magnitudes between HD 178484 and HD 179761 The amplitudes of the variations of the supergiant are of the same order of magnitude in the four filters (Delta y_max~=0.08, Delta v_max~=0.08, Delta b_max~=0.09, Delta u_max~=0.10 magnitude). More data are needed to determine the semi-period with some accuracy. Nevertheless these observations show that variations with same periodicity are certainly present and an order of the semi-period T may be eye- estimated. The free-hand curve in Fig. 1 corresponds to T = 7 days. Since its absolute visual magnitude was determined by Hutchings (1971) (see Table I) we can place the representative point of HD 178129 in the M_v vs log T diagram given by Sterken (1977). It fits very well the general trend and it is the lowest point in the diagram. In the same way HD 178129 takes a logical place in both diagrams M_v/spectral type and M_v/log T given by Burki (1978). The half amplitude of the y variations A(y) is about 0.04 which is in agreement with the general feature suggested by Sterken (1977), i.e. an increase of A(y) with increasing luminosity. We reported evident large photometric variations for the supergiant B3 Ia HD 178129. Although we have not a large number of measurements we can suggest a likely semi-period of the order of 7 days. If one adopts that semi-period the star shares the already derived properties for B supergiants. So, even if more observations are needed to confirm the semi-period, these facts support the estimated value. C. MEGESSIER Observatoire de Paris-Meudon F-92195 Meudon Principal Cedex FRANCE References: Burki, G., 1978, Astron. Astrophys. 65, 357. [BIBCODE 1978A&A....65..357B ] Hutchings, J.B., 1971, Publ. Dom. Astrophys. Obs. 14, 355. [BIBCODE 1976PDAO...14..355H ] Megessier, C., Garnier, R., 1972, Astrophys. Letters 11, 113. [BIBCODE 1972ApL....11..113M ] Sterken, C., 1977, Astron. Astrophys. 57, 361. [BIBCODE 1977A&A....57..361S ]