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YSO Bulletin
- 1 March 2020 -

Two challenging YSOs!

For northern-hemisphere observers at least the YSO season is starting to wind down somewhat so I thought I would present a couple of stars which are, for different reasons, not too straightforward but good objects to stretch yourself on. One is a rather close double and the other is near a very bright star (and is a close double into the bargain!)
In each case, clicking on the pictures below will bring up a larger view.

The variable on the left is BE Cas, and the really bright star is Caph (β Cas)! When I first went to observe this object I thought it would be doomed to failure because of the closeness of the 2nd-magnitude star but no... using a fairly high power not only makes it easier to put Caph out of the field but also reveals BE Cas to be a close double as well. BE Cas normally hovers around magnitude 12½ and the companion is of mag. 13.3 so one star does not dominate the other. It was discovered by the great VS observer Hoffmeister who was carrying out a survey of the field around beta Cas. Well, he didn't have far to look! Its spectrum of type F suggests that it may be a star of the UX Orionis type.
On the right we have at least three variables in the same field. At the top and shrouded in its nebula is the bright YSO AB Aurigae with the rather more interesting SU Aur to the left. However at the bottom right is a fainter star, GM Aur. Again this looks like a difficult one on first view because of a close companion of approximately the same brightness (GM fluctuates from about mag 12-and-a-bit to 13-and-a-bit, cementing its status as a classical T tauri star (technically CTTS/ROT indicating that a component of the light variations are starspots caused by its fast rotation, a characteristic of young stars). Its spectrum is K7 which means that eventually it will end up as a star really very like the Sun, though at the moment its rotation period is six days - four times faster than the Sun!
From the Star Formation Newsletter
The February SFN includes a long and interesting article about distances to Starforming regions. An abstract by a group of British astronomers (Cadman et al) about the effects of mass on disc fragmentation is worth quoting as well: "We investigate how a protoplanetary disc’s susceptibility to gravitational instabilities and fragmentation depends on the mass of its host star. We use 1D disc models in conjunction with 3D SPH simulations to determine the critical disc-to-star mass ratios at which discs become unstable against fragmentation, finding that discs become increasingly prone to the effects of self-gravity as we increase the host star mass. The actual limit for stability is sensitive to the disc temperature, so if the disc is optically thin stellar irradiation can dramatically stabilise discs against gravitational instability. However, even when this is the case we find that discs around 2M☉ stars are prone to fragmentation, which will act to produce wide-orbit giant planets and brown dwarfs. The consequences of this work are two-fold:that low mass stars could in principle support high disc-to-star mass ratios, and that higher mass stars have discs that are more prone to fragmentation, which is qualitatively consistent with observations that favour high-mass wide-orbit planets around higher mass stars. We also find that the initial masses of these planets depends on the temperature in the disc at large radii, which itself depends on the level of stellar irradiation."
I think many of us thought this might be the case, but it's good to have it demonstrated.

The same issue also features a followup on the HOYS-CAPS project which some of you might be thinking of becoming involved with.