This week we have another look at the fascinating case of the young star KH15D, or V582Mon. It is situated near the Cone Nebula, just south of the NGC2264 cluster. We have looked at this source in week 83, and also included an image of it in week 90. This image shows, that using the pipeline HOYS photometry for the object is problematic usually, as it is near a bright star, and sometimes it gets very faint.
The source is a binary which is surrounded by a dust torus. The dust torus and binary orbit are not aligned, indeed they are almost perpendicular to each other, and the torus is precessing. Thus, in the past the system dimmed with the period of the binary, when both stars we behind the dust torus. Very roughly at that time the bright state of the system was 14th magnitude in the I-band and about 17th magnitude in the faint state.
The I-band light curve from the 2022/23 observing season from HOYS shown above, shows that this has changed due to the precession of the dust torus. The bright state is still roughly the same brightness and the periodic dimming events every 48.4d are still there. However, the dimming events are less deep, and in the middle of the dimming a 3-5d brightening can be seen. This is because now the other star appears briefly outside the dust torus during it’s orbital motion. In a few years the system will return to ‘normal’ dimming events, which will get ever shorter and eventually they will stop when the torus has precessed out of alignment.
However for now, the short term variations in the system brightness are caused by the small scale structure in the dust torus, which we can probe due to the orbital motion of the two stars. This is hence a very interesting source to look at very small scale structures in disks or tori surrounding young stars.