We are making good progress on the writing of the next few papers with HOYS data, so this week we have a look back at a plot from an earlier paper “A survey for variable young stars with small telescopes – V. Analysis of TX Ori, V505 Ori, and V510 Ori, the HST ULLYSES targets in the σ Ori cluster” to discuss briefly how we can identify young stars in our clusters/star forming regions and then analyse their light curves.

Originally young stars were recognised as variable and due to the disks around them they were typically redder than ‘normal’ stars. Thus, if one wanted to select young stars in a cluster, one identifies all the variable objects with certain colours. One of the main things one could learn from that was that young stars are variable and had disks. But what if there were young stars that were not variable and did not have any disks? Where they there? Where there many? Are they important and what could they teach us about star formation?

One workaround would be to measure the young star distance via their parallax to determine if they are in the cluster or not. From the ground this is very hard, especially for distant and faint objects – i.e. most of them. What was possible however (well, with a few decades of effort), was to determine the proper (apparent) motion of the stars on the sky. Most cluster members would move with roughly the same direction and speed across the sky because they are formed out of the same cloud of gas and dust. This helped, but Gaia was a gamechanger. We now have parallax (and thus distance, which is d=1/p) and proper motion for most stars we can detect in the HOYS images and can ‘play’ with them to see what we find.

In the plot on the top there are four panels. They are all for the σ Ori target region. They are colour coded for different groups of stars. For example all the blue dots are the same stars in each panel.

Top Left: This is a histogram of the parallax values. One can see that there are two peaks, each of them indicating a larger number of objects at that parallax/distance, i.e. a cluster. There is a main group near p=2.5mas, which is a distance of about 400pc, and a second group at p=2.7mas, i.e. slightly in the foreground at 370pc. Thus, as a first surprise, what looks like a cluster of stars in the sky actually seems to be two groups of stars projected into the same direction.

Top Right: This shows the proper motion on the sky for all objects. One can see that the majority of the main ‘red’ group have very similar motions. But so do the ‘green’ objects. Hence, without the distance information it would not be possible to distinguish these two groups, despite the fact that one of them is about 10% more distant than the other. And there is a second surprise. A subgroup of the ‘red’ objects, the ‘blue’ sources, has a completely different motion on the sky, despite being at the same distance. Thus, dynamically there are actually three different clusters in this field, despite that it looks like one. And we can even argue if the main ‘red’ group is split into two groups – there seems to be a gap at about PM_RA=+2mas.

Bottom Left: This shows the position of all the stars on the sky. We can see that some of the ‘red’ group stars clearly clusters in the centre, around the bright star σ Ori. But a large fraction of this group and both of the others are very wide spread out. Indeed, if one would zoom out, one would find that the ‘red’ and ‘blue’ group are about limited to the 1×1 deg field shown. However, the foreground ‘green’ group is spread out over a much larger area. Indeed this is a foreground layer of young stars that covers much of the northern part of the Orion star forming region.

Bottom Right: This shows the colours (x-axis, red things are on the right, blue things on the left) and the absolute brightness/intrinsic luminosity (y-axis, bright things are on the top, faint things are on the bottom) of all the stars. This is basically a Hertzsprung-Russel diagram. The majority of the ‘black’ group (these are the normal stars) line up on a ‘main sequence’, which is diagonal in this plot. The young stars are towards the top and right of that sequence. The reason for this is that they are typically larger than normal stars and slightly colder (redder). The gap between the main sequence and the young stars is a rough measure of the age of the young stars. The older they get, the closer to the main sequence they will appear. We can see that the ‘green’ and ‘red’ groups are basically the same age, while the ‘blue’ group might be slightly older.

What have we learned from this – despite that there are multiple groups of young stars? Well, the objects that people have looked at the most to investigate how stars form in the region are the brightest (fair enough, they are easier to see) but also by far the most variable. The latter is interesting and shows that your typical young star might actually be much less variable than we thought. We are now starting to do this exercise of identifying all the young stars with Gaia data for all of our fields. We can then look at all their light curves to see: a) How many are there?; b) How many are variable and by wich amount? c) How do they vary? Are they bursting, dipping or chaotic? Are they periodic?

This will take a while, but should uncover a few interesting objects along the way – we will show them here of course …… 🙂