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(fwd) colour - colour and exotic variables? The following message is from John Greaves. The files are placed under http://vsnet.kusastro.kyoto-u.ac.jp/pub/vsnet/others/JG/ (This method is apparently an extension of the well-received photometric method for searching for Herbig Ae/Be stars and related objects, when no spectroscopy is available. Anyway, please enjoy the beutiful result!) === I've been playing again. Two colour photometry is nice. Two lots of two coloured photometry cubicly so. I scythed off some TASS data, out of interest, with Welch-Stetson indices twixt 50 and 500. The upper cut off was because above that value it tends to be very small number of observations with one outlier, which affects the mean magnitudes in the two passbands, and not necessarily evenly, thus shifting the colours sometimes. The lower limit is to force the chance of variability, though a somewhat lower limit could have been chosen, this was a first attempt, and I wanted some things at least to be fairly certain. Then, just about five minutes before I noticed that the _full_ allsky 2MASS dataset was now far more conveniently available via vizier (typical bad timing), I figured out the formatting style GATOR requires and formatted up a request list which I parsed through it to get 2MASS J-Ks for these objects. One arcsec matches, both datasets being good to said. (Indeed, problems arose more through photometry problems than cross identification problems for a the couple of objects that gave trouble). The attached graph shows the 1200 common objects (well, 1199 to be exact). It shows TASS V-Ic for the subset against 2MASS J-Ks. (I can't remember the convention directionwise re y axis in colour-colour diagrams, so I hope I've got it right). A nice doglegged main sequence, which was no great surprise as I've seen similar before in far red colour-colour diagrams. With some objects well off the beaten track. Now, these are mostly variables we are talking about, so obviously their magnitudes vary, and probably differently in different passbands, so they will vary in colour during variation too. But most variables do not drift all that much in colour during variation, and if they do, like in some EA stars, then often that event is relatively short lived upon the lightcurve. This is meant with respect to none catastrophic variables. So, the mean colour (TASS is multi-epoch data averaged; 2MASS simultaneous measures) should be fairly representative. You need good photometry though. The slightest problem and your colour is totally fallacious. There are a few stars identified on the graph. A couple of YSOs, a probable ZAND and a couple more 'more mundane' objects. Even these latter might be more interesting than expected, as there will certainly be other carbon stars and LPVs within the main sequence, so why are these objects special? I note also that some 'slow irregular variables' have turned out to be YSOs rather than LPVs on further examination. The stars have notes below. The actual point was to try and find a way of digging out more exotic variables from the mass in an easy manner. Moderately successful, although numbers are few, and all are known (though not necessarily well known). I was actually hoping to find a few Be stars with near infrared excess, but that didn't come off. The WS index criteria could be widened though. Anyway, the point is, with these new more expansive two colour photometry datasets available, that this may be a quick way of assessing for certain less common types of variability. Even a single star could have it's V-Ic assessed against it's J-Ks to see if it was highly distant from the main. Some TASS V-Ic is now available, full 2MASS is now available, for the south a combination of ASAS3 (carefully assessed) and DENIS gunn i could be used for the V-I (though not for DENIS data of gunn i > 10, ie brighter than 10, which has problems [I'm so embarrassed about alluding blame to the wrong dataset on that one, that I'm not even going to give the reference!]). Although you still have to make sure you are happy with the quality of some of this data before you use it. ASAS3 and TASS do allow plotting of the epoch photomtery for assessment. So, if you find a new variable star, and it seems a bit different, you could always try that route for a bit more circumstantial (at least) evidence. But as to its true applicability and frequency of utility, I dunno. Comments/extensions/caveats welcome, half bricks not. (Especially in the context of other colour matches (but only where good data is readily available, which isn't even the case for B-V!) and 'exotic' variables: would J-H or H-Ks be better or lead to different objects? MSX_5C carries photometry on bands close to the [Cousins?/Kron?] M band, and K-L vs L-M has uses, so what about J-Ks versus Ks - MSX_B1 or similar?) stars on t'attached graph UX Ori YYOri or even proposed UXOR subclass HD 53542 A0. binary Argelander 1742, but the 13th mag comes at 3" should not intefere in terms of either being resolved or inherent brightness (??) GT Ori semiregular - but is it... GSC 0168 1700 HBH emission star and LS luminous OB star: B star for once! DR Tau YYOri StHA55 A possible ZAND, according to newvar.cat, more I know not (GSC 0128 0816) EI Ori Lb - CGCS 1190 many CGCS 1697, TmzV296, 1FASTT 336, others? plenty caught it, none identified it (of course, all these would just have to be past season so further variabity checks and double checks end up on hold!) Cheers John ACKNOWLEDGEMENTS TASS (www.tass-survey.org) data is courtesy of Tom Droege's dataset, also see here for the same data (and newer): http://sallman.tass-survey.org/servlet/markiv/template/Download.vm 2MASS is an IRSA/IPAC - University of Massachusetts Project http://vsnet.ipac.caltech.edu/2mass/releases/allsky/doc/sec1_8b.html VizieR is part of the CDS Strasbourg, http://vizier.u-strasbg.fr/viz-bin/VizieR Welch-Stetson index: AJ 105 1813 (1993) http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993AJ....105.1813W&db_ key=AST&link_type=ABSTRACT&high=3ef9bb138c08349 DENIS 2nd data release is available via vizier ASAS3 epoch photmetry is available via http://vsnet.astrouw.edu.pl/~gp/asas//asas_menu.html
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