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YSO Bulletin
- 3 January 2020 -

Cloudlet Capture Events

Star Formation Newsletter (SFN) No.324 carries the abstract of an article by Dullemond et al that may be of interest to observers of, among others, RW Aur. It concerns the capture of minor gas clouds by a major one during the formation process: "Some of this cloudlet material may accrete onto the star+disk system, while other material may fly by in a hyperbolic orbit. In contrast to the original cloud collapse event, this process will be a 'cloudlet flyby' and/or 'cloudlet capture' event..." say the authors. This is widely believed to relate to the recent deep fading events on RW Aur.
Such an event would lead to arc-shaped or tail-shaped reflection nebulosity near the star. Those shapes of reflection nebulosity can be seen around several transitional disks and FU Orionis stars. Although the masses in the those arcs appears to be much less than the disk masses in these sources, it may be that higher-mass cloudlet capture events happen occasionally. If so, they may lead to the tilting of the outer disk, because the newly infalling matter will have an angular momentum orientation entirely unrelated to that of the disk. This may be one possible explanation for the highly warped/tilted disk geometries found in several transitional disks. It may also be that such events, if massive enough, may lead to FU Orionis outbursts.

More on H-H 212

Last month's newsletter carried a brief article on the outflow from H-H 212, and further work on this object has emerged. In the previous issue of the SFN, using Spitzer and VLT sources it is seen that this outflow has a strong symmetry, with jet/counterjet knot pairs with ∆x <1" position offsets. These knots are ejected with 6-yearly intervals and velocity differences of about 2km / second. Looking at the deviations of the knot positions perpendicular to the outflow axis suggests a binary orbital motion of the outflow source, whose mass is deduced at about 0.7 solar, with a separation of ~80AU between the components of the binary (assuming equal masses for the two components).

NGC 7129

This photogenic nebula in Cepheus is a dynamic region of star formation and a recent long-term study from 1998 to 2016 has found extensive variability in several young stars, across all wavelengths. Long-term light curves of the five stars studied indicate typical classical T Tauri variations in brightness with large amplitudes. No reliable periodicity in the brightness variations of any of the stars followed were found. Results from spectral observations showed that all studied stars can be classified as classical T Tauri stars with rich emission line spectra and strong variability in the profiles and intensity of emission lines.
In the NASA photo here, the star at the centre of the central arc is V361 Cep, on which the field in the study was centred, and the star at 11 o'clock (apologies - I am unsure as to the orientation of the NASA picture!) is the more visually active V373 Cep. Even purely visually, this area is clearly full of nebulosity; note the reddish jets and bowshocks in the neighbourhood also.

Binocular YSOs!

To close on matters purely observational, did you know that there are a small number of YSOs that you don't even need a telescope for
The most obvious is AB Aur which varies between magnitudes 7 and (occasionally) 8ish. Fades are infrequent but do happen! Owners of larger binoculars may also be able to follow its neighbour SU Aur, which has shown a lot of activity recently. Not far away, owners of those more powerful glasses should be able to catch CQ Tau when bright. This is an extremely rapidly-varying object which should be looked at on every possible occasion!