Theta Tuc (HR 139) was first discovered to be a variable star by Cousins \& Largerwey (MNRASSA 30, 12, 1971). The period of light variation was in the range 70-80 min., and the amplitude was found to be variable with a maximum in V light of 0.06 mag typical of Delta Scuti type variability. Later, two independent data sets (556 integrations on six nights in 1972 and 954 observations obtained during 70 hours on 21 nights in 1979) were published and analysed by Stobie & Shobbrook (MNRAS 174, 401, 1976) and Kurtz (MNRAS 193, 61, 1980), respectively. The two independent analyses led to two alternative hypotheses: the first is that the frequencies present in the light curve of Theta Tuc are unstable and may change in a time span as short as 24 hours, the second is that all frequencies in Theta Tuc are completely stable, but that beating among the various frequencies makes them appear to come and go. No definite frequency solution, but only a possible set of frequencies were given.
To make a decision between the two alternate hypotheses, a long time-base multisite campaign of Theta Tuc was undertaken at three observatories on three continents. From mid-September to end of October, 1993 we obtained:
As the duration of observations in Table 1 shows two compact sets of data, between HJD 2449254.51 - 59.26 (coverage 65 %) and 70.26 - 74.77 (coverage 64 %), were achieved resulting an excellent spectral window which makes us to avoid the serious one-cycle per day aliasing present in single-observatory data. The coverage is of 37% from the beginning to 14/15 Oct and 24% for the whole time span. (To extend a time base line Theta Tuc was only followed at ESO over one cycle per night during the last two weeks of the campaign.)
The figure on the cover of the Newsletter shows the light curves of Theta Tuc in y during the second compact set of data.
In Table 1 the overlaps are not counted although there are overlaps between the observations of Perth Observatory and SAAO and the runs of SAAO and ESO. These overlaps can help to check the extinction correction and the transformation of the different instrumental systems into a homogeneous data set. Such overlap could be seen on the middle three panels of the figure on cover page between 0.52 - 0.62. The two different Stromgren instrumental system could be matched perfectly.
A total of 246 hours and of 2316 integrations (five times 20 sec at ESO and SAAO) were obtained in y and Johnson V. The duration and number of integrations in b, v and u are slightly less.
The data analysis and interpretation are still underway but some preliminary results have already been obtained.
We used mostly the traditional three-star technique where the two comparison stars (HR 83 and HR 169) were observed alternately between two variable star observations. This approach gives the possibility to check the behaviour of the two comparison stars precisely.
Our data seems to be enough for a detailed investigation of the problem of the mean magnitude of Theta Tuc which was reported by the previous authors.
A preliminary inspection of the power spectrum shows the presence of the major frequency determined by Stobie & Shobbrook and Kurtz, however, at the present stage of the analysis it is much too early to give any definite numerical value.
On the basis of the extended colour observations a mode identification of the frequencies will be carried out.
The paper with detailed results on the stellar pulsational characteristics, mode identification and physical parameters will be submitted to Astronomy & Astrophysics while the data transformed into standard Stromgren system will be published elsewhere.
We have high hopes to make decision between the two hypotheses mentioned above.
Date 1993 JD t n Observatory 2449200.+ hr
18/19 Sep 49 6.96 28 ESO 21/22 Sep 52 7.44 77 ESO 22/23 Sep 53 7.44 73 ESO 23/24 Sep 54 8.66 107 ESO 24/25 Sep 55 20.66 271 Perth+SAAO+ESO 25/26 Sep 56 17.04 224 Perth+ESO 26/27 Sep 57 8.16 115 ESO 27/28 Sep 58 12.24 128 Perth+SAAO 28/29 Sep 59 6.96 58 Perth 1/2 Oct 62 3.6 6 SAAO 2/3 Oct 63 8.4 52 SAAO 3/4 Oct 64 1.2 8 SAAO 4/5 Oct 65 6.72 53 ESO 5/6 Oct 66 5.28 60 ESO 6/7 Oct 67 14.88 101 SAAO+ESO 7/8 Oct 68 12.96 104 SAAO+ESO 8/9 Oct 69 8.16 84 ESO 9/10 Oct 70 14.64 109 SAAO+ESO 10/11 Oct 71 14.88 140 SAAO+ESO 11/12 Oct 72 15.12 133 SAAO+ESO 12/13 Oct 73 12.48 107 SAAO+ESO 13/14 Oct 74 12.48 102 SAAO+ESO 14/15 Oct 75 7.44 49 SAAO 17/18 Oct 78 1.44 19 ESO 18/19 Oct 79 1.92 18 ESO 19/20 Oct 80 1.44 13 ESO 20/21 Oct 81 0.96 9 ESO 21/22 Oct 82 1.68 14 ESO 28/29 Oct 89 1.20 16 ESO 29/30 Oct 90 2.16 18 ESO 30/31 Oct 91 1.68 20 ESO
Total 246.28 2316
Table 1: The list of the nights on which observations were obtained, the duration of those observations in hours, the number of magnitudes measurements for Theta Tuc and the observatories where the observation was taken on the date given.