This week we have a look at the star LkHa137A, situated in the Pelican Nebula. It is classified as a Hydrogen Alpha emission line star (hence the name 🙂 ) and a long term variable. Thus, it is most likely a young star with strong mass accretion, and thus a disk around it.

In the V, R, and I-band light curve of the object shown above, one can see where the long-term variable classification comes from. The star seems to undergo various dimming events of different duration and depth. The first two, which are about a year apart, are about half a magnitude deep and of quite short duration. After that, the star underwent a very long, almost 4yr!!!!, dimming event. In part that was more than one magnitude deep (roughly for one year in the middle), and the star has now basically recovered to the baseline brightness it had about 6-9yr ago.


Since People where asking: There are two likely scenarios for the dips: A larger structure in the outer disk, moving slowly across the line of sight, or dust forming close to the star triggered by some instability and it then gets accreted, or sublimated over time. One can distinguish these by checking how the object varies in the mid-infrared (this is available from the NEO-WISE survey). Formation of dust will cause the mid-IR brightness to increase, while optical dimming due to outer disk material should slightly dimm the mid-IR as well. There is of course also the option of a decrease in the accretion rate, but that is less likely given the shape of the optical light curve. Accretion rates usually spike for short bursts and brightness increase and then return to the faint state.