Royal Astronomical Society of New Zealand, Variable Star Section (RASNZ-VSS)
Thu, 11/07/2013 - 05:12

 TZ Cen : variability confirmed

  TZ Cen has been an enigma since first announced as HV 1259 by Leavitt (HC 120, 1906). The reported range is 11.5-13.0p, but Hoffmeister (1943) could find no star brighter than 14mag (GCVS, 3rd ed. 1969). In 1994 I checked the spot in the Guide Star Catalogue (v.1) and found a 12th mag. star, and this finding was reported in IBVS 4037. There appeared to be no problem in identifying the star.      Further, Bidelman and MacConnell reported that TZ Cen was of spectral type M7 in 1998 (IBVS 4612). This is corroborated by 2MASS data, J-K=1.66, indicating a very red star, with B-V >2.0. This casts severe doubt on the photographic range given by Leavitt, and immediately explains Hoffmeister's failure to find the star. On blue-sensitive plates TZ Cen would rarely be seen brighter than B=14.0. The ASAS-3 lightcurve clearly shows the variability of TZ Cen. The star, known as ASAS 130407-6046.0, has an extreme range of 11.75 - 13.0V, made up of SR variations of P=120d, superimposed on a longer term fluctuation, P=1200d. The individual SR cycles usually have a range of no more than 0.5mag, and one would need to follow the star for about 10 cycles to observe the star over its extreme range. Curiously, the published range is fairly close to the mark, but in V, rather than photographic or B. Its position is quite secure; the published range is the misleading parameter, rather than the position.