COMMISSION 27 OF THE I. A. U. INFORMATION BULLETIN ON VARIABLE STARS Number 3146 Konkoly Observatory Budapest 9 February 1988 HU ISSN 0374-0676 TW HYA: A T TAURI STAR FAR FROM ANY DARK CLOUD UBVRI OBSERVATIONS IN 1986 AND 1987 TW Hya is one of the most interesting objects among young late-type stars. Herbig (1978) pointed it out as a possible candidate for being one of - then very rare - post-T Tauri stars. However, a multi-angled investigation of Rucinski and Krautter (1983; hereinafter RK) definitely showed it to be an absolutely normal T Tauri star. The main point here is that TW Hya is quite far (~/= 13deg) from any dark cloud, star formation complex, or other concentration of young proto-stellar material. Yet, it is definitely not "naked" - as defined by Walter (1987) for stars which exhausted supplies of the circumstellar accretable matter - and shows clear signatures of a classical T Tauri stage. TW Hya may have formed in an isolated small cloud, as de la Reza et al., (1986) suggest, but it must have retained enough circumstellar matter to insure the continuing accretion. The variability of TW Hya was observed in 1982 to be quite erratic (RK). Short time-scale trends of a few hours in duration were seen to be superimposed on, or combined with longer night-to-night variations. The total range was 10.9 < V < 11.3. The colours were redder for lower brightness, but parallel trends in colour-magnitude plots on individual nights suggested a few sources simultaneously contributing to the variability. Dr. F. Vrba (private communication) noticed a possible periodicity of about 2 days in the RK data which could have been the spot modulation, as observed in other T Tauri and naked T Tauri stars (Bouvier et al., 1986a, 1986b; Herbst 1986; Rydgren and Vrba 1983; Rydgren et al., 1984). An independent analysis of the RK data has shown that such a periodicity is definitely present. Unfortunately, its reality may be questioned because it is practically identical with the quasi-Nyquist frequency for the data sampled in close groups at one-day intervals. The present observations were obtained in June 1986 and in March 1987 at the Las Campanas observatory using the 61 cm telescope of the University of Toronto and the single-channel photometer. The E regions were observed to place the observations in the Cousins UBVRcIc system. The weather conditions were quite poor during both runs. The observations were made differentially relative to the same comparison stars as used in 1982. The new determinations of the UBVRI data for the comparison stars were made on 7 nights in 1986 and 5 nights in 1987 (Table I). When compared with the 1982 data which were obtained during perfect weather conditions at ESO, the new data reveal a generally good agreement of colours but some shifts in the V magnitudes. Since the magnitude difference between the comparison stars also changed, we may suspect some small long-term variability (by about 0.04 - 0.08 in V ) in one or both stars. This would not entirely unexpectable in view of both comparison stars most probably being late-type giants. The new data for TW Hya are listed in Table II and are shown in graphical form in the figure. The picture is exactly the same as in 1982. The range of variability Table I THE COMPARISON STARS FOR TW HYA U-B B-V V V-Rc V-Ic Reference C1 (SAO 202005) 1.095 1.172 9.116 0.599 1.139 Rucinski and Krautter (1983) 1.071 1.152 9.072 0.586 1.163 Las Campanas 1986 1.080 1.156 9.090 0.570 1.158 Las Campanas 1987 C2 (HD 95470, SAO 202001, K 2/3III) 1.412 1.307 8.744 0.678 1.273 Rucinski and Krautter (1983) 1.406 1.275 8.652 0.662 1.300 Las Campanas 1986 1.418 1.278 8.664 0.647 1.288 Las Campanas 1987 [FIGURE 1] Table II UBVRcIc OBSERVATIONS OF TW HYA JD(hel) U-B B-V V V-Rc V-Ic 2446000+ 582.523 -0.44: 0.930 10.884 0.850 1.586 582.527 -0.36: 0.891 10.896 0.850 1.575 583.506 -0.27 0.983 10.890 0.845 1.575 583.514 -0.29 0.987 10.877 0.845 1.572 584.507 -0.28 1.085 11.005 0.905 1.678 584.518 -0.26 1.091 11.022 0.907 1.679 585.494 -0.27 0.867 10.871 0.837 1.544 585.498 -0.33 0.885 10.869 0.819 1.522 587.512 -0.30 1.020 10.950 0.878 1.630 587.516 -0.30 1.056 10.957 0.886 1.637 588.507 -0.20 1.022 10.983 0.885 1.635 588.518 -0.14 1.016 10.982 0.877 1.617 589.493 -0.20: 1.122 11.062 0.933 1.696 589.498 -0.31: 1.087 11.045 0.926 1.674 593.501 -0.31: 1.003 10.917 0.875 1.590 593.510 -0.40: 0.982 10.913 0.866 1.621 595.493 -0.58 0.921 10.826 0.893 1.556 596.512 -0.22: 0.993 10.987 0.895 1.642 860.637 0.06 1.186 11.130 0.945 1.712 860.657 -0.09 1.150 11.122 0.935 1.705 860.853 -0.05 0.999 11.017 0.885 1.628 861.616 -0.31 0.932 10.952 0.867 1.564 861.624 -0.24 0.974 10.900 0.845 1.577 861.668 -0.31 0.916 10.887 0.827 1.545 861.676 -0.26 0.937 10.880 0.816 1.545 861.823 -0.21 0.941 10.914 0.851 1.555 861.831 -0.28 0.957 10.931 0.865 1.565 862.682 0.12 1.197 11.124 0.932 1.707 862.689 0.06 1.164 11.117 0.947 1.735 862.698 0.18 1.131 11.168 0.937 1.710 864.631 -0.24 1.074 11.007 0.900 1.623 864.642 -0.30 1.093 10.979 0.893 1.626 864.696 -0.38 1.037 10.994 0.889 1.618 864.704 -0.21 0.988 10.981 0.895 1.619 864.716 -0.26 1.010 10.974 0.892 1.611 865.667 -0.35 1.053 10.909 0.845 1.576 865.675 -0.40 0.937 10.905 0.841 1.561 865.688 -0.30 0.935 10.880 0.825 1.532 865.813 -0.28 0.954 10.885 0.825 1.545 866.736 -0.34 0.971 10.898 0.908 1.568 866.748 -0.47 0.991 10.961 0.941 1.612 867.708 0.09 1.175 11.144 0.900 1.717 867.716 0.21 1.148 11.128 0.914 1.693 867.731 0.39 1.281 11.156 0.928 1.706 868.689 -0.04 1.166 11.138 0.921 1.705 868.706 0.40 1.211 11.128 0.904 1.659 868.714 0.27 1.221 11.090 0.900 1.685 869.722 0.15 1.015 11.070 0.874 1.654 869.730 0.23 1.241 11.038 0.879 1.632 869.738 -0.19 1.120 11.079 0.910 1.658 869.746 0.17 1.226 11.089 0.907 1.695 870.707 1.131 11.055 0.917 1.658 870.714 1.039 11.052 0.909 1.663 870.722 1.046 11.054 0.916 1.646 is similar, as is a general relationship between brightness and colours. The data were analysed for periodicity but, contrary to the 1982 observations, they do not reveal any clear signal of a few days in duration. This does not prove that the 2-day periodicity observed before was entirely spurious because this time scale is actually the hardest to prove or disprove anyway. A visual inspection of nightly data again seems to suggest existence of trends with a characteristic scale of about 0.2 day. This work has been supported by an operating grant of the Natural Sciences and Engineering Research Council of Canada. S. M. RUCINSKI David Dunlap Observatory University of Toronto P. O. Box 360 Richmond Hill, Ontario L4C 4Y6 Canada References: Bouvier, J., Bertout, C., and Bouchet, P. 1986a, Astron. Astrophys. 158, 149. [BIBCODE 1986A&A...158..149B ] Bouvier, J., Bertout, C., Benz, W., and Mayor, M. 1986b, Astron. Astrophys. 165, 110. [BIBCODE 1986A&A...165..110B ] de la Reza, R., Quast, G., Torres, C. A. O., Mayor, M., Meylan, G., Llorente de Andres, F. 1986, in New insights in Astrophysics, ESA SP-263, p. 107. [BIBCODE 1986ESASP.263..107D ] Herbig, G. H. 1978, in Problems of Physics and Evolution of the Universe, ed. L.V. Mirzoyan, Publ.Armenian Acad.Sci., Yerevan, p. 171. [BIBCODE 1978ppeu.book..171H ] Herbst, W. 1986, Publ. Astron. Soc. Pacific, 98, 1088. [BIBCODE 1986PASP...98.1088H ] Rucinski, S. M., and Krautter, J. 1983, Astron. Astrophys. 121, 217. [BIBCODE 1983A&A...121..217R ] Rydgren, A. E., and Vrba, F. J. 1983, Astrophys.J. 267, 191. [BIBCODE 1983ApJ...267..191R ] Rydgren, A. E., Zak, D. S., Vrba, F. J., Chugainov, P. F., Zajtseva, G. W. 1984, Astron. J. 89, 1015. [BIBCODE 1984AJ.....89.1015R ] Walter, F. M. 1987, Publ. Astron. Soc. Pacific, 99, 31. [BIBCODE 1987PASP...99...31W ]