This week we are looking at the young star LkHa154 in the Pelican Nebula. It is classified as an emission line star. This means, if you take a spectrum you can see strong emission at certain wavelengths. For young stars typically the most prominent and strongest is the Balmer Alpha line of atomic hydrogen. This typically comes from hot gas, which in young stars is created by material falling from the accretion disk onto the star at high speeds – or better, the material is channeled along magnetic field lines from the disk to the star. The kinetic energy of the accreting material is then converted into heat upon impact on the surface.
The long term light curve (feel free to look it up on the database server) looks quite messy. Essentially it looks like a very noisy set of data, scattering all the time by about 0.7-0.9mag. The average brightness of the star seems to have increased by about 0.1mag (10 percent) over the last seven years. Only when one zooms in, as in the bit shown above, one can see what is actually going on. We show about 2 months worth of data from some years ago.
One can see that the ‘noise’ is basically caused by short term variability on the timescales of a few days. There are phases where it looks almost regular with a period of about 3days, but then this changes and no variations are seen for a week. These short term, almost periodic features can be hot spots on the surface which appear and disappear if the mass accretion process changes. If this is the case, then any time the periodicity re-appears it would have the same period. There would of course be phase shifts, if the hot surface spot is created on a different part of the star (different longitude). Such sources are hard to find, as one will only find them as periodic if one searches for periods in small fractions of the data. This is something we are working on in an ongoing PhD project.
An alternative explanation is that the variations are caused by occultations by disk material. If the material moves out of periodic alignment, e.g. by being accreted onto the star, the periodicity can disappear. If then other material moves into alignment, the periodicity can come back, but not necessary with the same period, if the material is at a different separation to the star. There are plenty of things to look at in this star alone, never mind all the other light curves….