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(fwd) Re: YSOs and J-Ks The following message is from John Greaves: Brian Skiff wrote: > the apparently very large 2MASS J-K of 1.6, suggesting instead a > very red star, it looks to me as though this must be some sort of > pre-main-sequence object with a large IR excess. Yes, a bit of serendipity from plotting stuff in the V-Ic and J-Ks plane seems to find these objects nicely, though the sample is very, very small at present. It was a happenstance discovery whilst trying to find Be stars with infrared excess etc. Some points on things I've been noting: I plotted 33000+ selected as non-variable objects in the same plane and normal stars do not appear to have any candidates with IR excess. This is from TASS stuff with large numbers of measures and small standard deviations thereon, and mostly from a 5 degree circumsky declination strip, so it should be a fairly representative mix of populations and types and colours. The 'control' experiment. Incidentally, my original plot of around 1200 likely variable objects that first hinted at this YSO clustering was not dereddened. It looks likely that reddening is still a bit of a worry at J-Ks, about three tenths relative to V at J and about one tenth relative to V at K, which will likely be more or less the same at Ks in this instance. That pulls GT Ori down to about J-Ks +1.4 The YSOs at present, (miniscule sample of roughly half a dozen so far) are landing between about 1.1 and 1.4 J-Ks. It is giving an early impression that the surrounding matter is physically very similar between objects. Ic is probably in excess too. The stars tend to be a fair bit 'earlier' in colour than any known spectral type suggests. UX Ori with a dereddened V-Ic of -0.27 and spectrum A2 for instance. B-V versus J-Ks might be a better test, but it has to exist to be used. Dereddened MisV1147 and dereddened UX Ori live very close together in V-Ic vs J-Ks space, which spreads the sample out to at least one more OB Assn. ;) I'm using a distance modulus plus apparent magnitude plus extinction based absolute magnitude, and, a spectral type plus spectral class based absolute magnitude as a check for association membership. Objects are coming out at perhaps just a bit too bright, but I think that's okay for stars that haven't quite settled onto the main sequence yet (Ayashi tracks and all that stuff). You would actually have to fairly markedly _reduce_ the extinction values to make things much better, which is unlikely as the distances are reasonable enough. Certainly they aren't too bright to be within the Associations. There might well be an island/zone for YSOs in this colour-colour space, or more than one depending on whether the object is an Herbig Ae/Be or a TTau object. J-Ks looks likely to be the the more set value, which seems sensible (ie generic warm gas/dust clouds of similar temperatures and nature). (I haven't checked the literature yet. 2MASS did some astrophysical stuff on things like 'L' and 'T' dwarfs, but I don't know if they touched on PMS/YSO objects yet. Also, this all may already be noted in other passbands). There's a project for someone here, measuring B-V for known YSOs in a slew of the better known OB associations (so that information re colour excess correction and distance modulus can be used), though as V seems intractable enough as is, I dunno how many people are going to be able to manage succesful B! Which is atmospherically, and detectorly, more demanding. And I personally lump amateurs and professionals together in that statement, else Brian Skiff's loneos.phot file wouldn't have to be a _critical_ compilation (critical as in assessed, not as in derogatory): he'd just include everything published otherwise. 2MASS J-Ks comes as a special free offer for VizieR users. The objects won't be too faint on the whole for measurement with relatively modest kit, though. Maybe it'd be lucky enough to find B-V measures in the literature, but then there'd have to have some way of ensuring the things were measured near maximum, because these things often change colour during fade events (caused by circumstellar material). V-Ic and/or V-Rc would still suffice, but some values for nonexcess reference objects would be required. Heck, enough effort and time it could end up being big paper stuff. Cheers John PS NSV 16694, probably better known to Japanese observers as HadV02, is a nice one that popped out of looking at the TASS colour data against 2MASS colour. There's something of an ASAS3 V lightcurve online too. It dereddens to V-Ic +0.1 and J-Ks about +1.1 (interesingly B-V from NPM2 of +0.6 gives +0.3 dereddened for average Orion OB1 exitinction, making V-Ic seem too blue again. Tycho2 BT appears quite wrong, probably due to background level problems as there is an associated reflection nebula, as VT is not suitable for for fade events). Nebulosity is involved here, so there may actually be further extinction to allow for. It appears to be associated with a reflection nebula, an Herbig-Haro Object (haven't looked that particular catalogue up yet to see how they defined their search and criteria), and the IRAS sources, both faint and point, get brighter in the 12 -> 100 microns direction. It's probably also the adjacent faint and soft xray source. Likely there's already a lot in the literature on this one. Sideways, Ondrej Pejcha was good enough to inform me that StHA 55, a possible ZAnd, is probably one of the very few known Galactic carbon star symbiotics (symbiotic carbon stars, whichever way round it should be), a type of object relatively more common in the LMC it seems. This too stood out due to J-Ks excess.
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