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YSO Bulletin
- 1 August 2020 -

- Getting ready for Summer -

Flares are in!

No, not that sort of flares! I am talking here about Hubble Flares. In the 1930s he noticed that certain areas of the sky next to, but not on, the plane of the Galaxy, had high concentrations of the ISM (from which stars are born) and called these areas flares. Probably the best studied of these is the Cepheus Flare. If you think of Cepheus as a steep-roofed house then the Cepheus Flare is basically the loft. It contains not only several YSOs but also a plethora of nebulae and other areas of the ISM that exhibit bow shock fronts, suggesting multiple supernova outbursts in the distant past. Several degrees to the east of the main flare area is an arced wall of OB associations (Cep OB2 to OB4) which further bear this hypothesis out. Along the arc we also find large concentrations of nebulae such as the well-known IC1396 which decorates the heading of our modest publication every month.
To be honest, this area is so full of interest that I could bring out a monthly 'Cepheus Flare' newsletter as well, but instead I will direct you to a publication edited by our very own mentor Bo Reipurth. Once on the page you can download the 104-page booklet as a pdf file.

YY Orionis?

Never heard of them? Partly this is because of the haphazard way that YSOs were categorised. The GCVS still lists subgroups such as IA, INS, INAS, IB and so on. Other systems use the terms T Tau, CTTS, WTTS. It seems some of the creatures are in danger of escaping from the YSO zoo! Things might seem easier to relate to when you realise that a well-known YY star is our old friend RW Aurigae. These stars are T tauri objects that display inverse P Cygni characteristics. 'Normal' P Cygni stars show strong stellar winds streaming away from the star - so inverse P Cyg behaviour in the spectrum means that material is falling onto the star - in this case from its parent disc or cloud. It is possible that this infalling material produces star-spotting, since the prototype YY Ori was shown to be more heavily spotted than most classical T Tau stars (CTTS). Their lightcurves show (outside of the more violent 'infalling' episodes) gentler changes over an average of about a week or less, implying rapid rotation which is another signature of youth.
More about these amazing stars as winter approaches!

Triggered Star Formation

As stars form and the material builds up on their surfaces their temperatures rise and gravity becomes balanced by radiation from the star's core, with the result being that the still-infalling material now has to go somewhere. That somewhere is the point of least resistance which are the star's poles. The material is expelled from there producing a bipolar outflow which streams out at sometimes very high velocities and often becoming visible as a Herbig-Haro (HH) Object. The material may then plough into other areas of the adjoining starforming regions to initiate more star formation. The picture is of the HH46, 47 area in Vela. Distance between the two edges is about 3 parsecs.
HH 46 and 47In a paper accepted by the Monthly Notices of the RAS, Areal et al demonstrate evidence for multi-generational star formation by just this process. They say: "...We present observational evidence for triggered star formation relating three generations of stars in the neighbourhood of the star LS II +26 8. We carried out new spectroscopic observations of LS II +26 8, revealing that it is a B0 III-type star. We note that LS II +26 8 is located exactly at the geometric centre of a semi-shell-like HII region complex. The most conspicuous component of this complex is the HII region Sh2-90, which is probably triggering a new generation of stars. The distances to LS II +26 8 and to Sh2-90 are in agreement (between 2.6 and 3 kpc). Analysis of the interstellar medium on a larger spatial scale shows that HII region complex lies on the northwestern border of an extended H2 shell. The radius of this molecular shell is about 13 pc, which is in agreement with what an O9V star(the probable initial spectral type of LS II +26 8 as inferred from evolutive tracks) can generate through its winds in the molecular environment. In conclusion, the spatial and temporal correspondences derived in our analysis enable us to propose a probable triggered star formation scenario initiated by the evolved massive star LS II +26 8 during its main sequence stage, followed by stars exciting the HII region complex formed in the molecular shell, and culminating in the birth of YSOs around Sh2-90."

A new Dipper?

A team of Italian researchers have found a new dipper, in an active starforming region. They report on the discovery of periodic dips in the multiband lightcurve of ISO-ChaI 52, a young stellar object in the Chamaeleon I dark cloud. This is one among the peculiar objects that display very low or negligible accretion both in their UV continuum and spectral lines, although they present a remarkable infrared excess emission characteristic of optically-thick circumstellar disks. From the rotational period of the star/disk system of 3.45 days they estimate a disk inclination of 36°. The depth of the dips in different bands has been used to gain information about the occulting material. A two-component model of a disk warp composed of a dense region with a gray extinction and an upper layer with an ISM-type extinction provides a good fit of the data.
Dippers are YSOs that show a regularity in their fades, usually on a fairly short timescale.