An extremely slowly rotating young star
Super-WASP lightcurve (see link) of the candidate young star SST042200.9+235430 (hereafter SST0422). The lightcurve shows a very obvious period of 60-65 days in two seasons, which are plotted with different colours. The dashed lines mark the zero level for the two datasets. The phase offset between 2004-5 and 2006-7 is 0.6. This finding was a result of a summer project by Donna Rodgers-Lee at the Dublin Institute for Advanced Studies.
SST0422 is a possible member of the young association in Taurus (age 2 Myr), first listed by Rebull et al. (2010). According to their analysis, it has a spectral type of M3, but requires further follow-up to confirm its youth. If it is indeed a young object, a rotation period of 60 days would be highly unusual and extremely long; the typical period range at this age is 1-20d, with a maximum around 30d (Herbst et al. 2007). However, most previous period searches are not sensitive to significantly longer periods - we might see an example of the 'tail' of slow rotators among young stars.
In this context it is interesting to note that SST0422 is very bright for an M3 star at this age - the absolute J-magnitude after extinction correction is 2.5-2.8mag, a young M3 star is expected to be 0.7-1mag fainter. This could indicate that SST0422 is in fact a binary, i.e. two gravitationally bound stars with similar spectral type. We know little about the rotation periods of young binaries, but it may not be surprising to find that they are slow rotators, if they are not close enough to be tidally locked.
Two more (unlikely) options remain and require further verification:
1. When interpreting the period as a rotation period, we assume that it is caused by spots on the surface of the star. Alternatively, the period could arise in a disk around the star, for example as the consequence of a dust cloud which periodically obscures the central object. In this case and assuming Keplerian rotation in the disk, the obscuring object would be less than 1AU away from the star. This, however, seems unlikely, because the mid-infrared colours from 3 to 8mu give no evidence for the presence of circumstellar dust. The [8]-[24]mu colour, on the other hand, is slightly enhanced (which is the reason why the object appears as a candidate member of the Taurus association in Rebull et al. (2010)). Further analysis is needed to definitely rule out this scenario.
2. Since the evidence for youth is weak, SST0422 could still be a star in the foreground of the Taurus association. It is located in the outskirts of the associations and does not have mid-infrared excess or Halpha emission, the typical indicators of youth. The 60 days period would fit quite well with an old M3 star in the field, and the J-band magnitude would imply a distance of 10-30pc (very rough estimate). The problem for this scenario is the amplitude of the lightcurve: The difference between minimum and maximum is at least 0.1mag in 2004-5 and 0.3mag in 2006-7, whereas typical amplitudes for old field M dwarfs are only 0.01-0.05mag (Irwin et al. 2011). SST0422 would have to be an unusually active old field dwarf, which is at odds with the lack of Halpha emission.
