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(fwd) 2MASS J-Ks colours and spectral types (Forwarded message from John Greaves) Hi everybody peeps The "final" 2MASS All Sky dataset has been released. It goes quite faint and purportedly covers 99.998% of sky (I wonder where the .002% is?) It can be used to assess the spectral type of stars fainter than Tycho2 BT-VT values (or even double check the faintest Tycho2 values of said) using the table in section IV of Brian Skiff's downloadable file :- http://ftp.lowell.edu/pub/bas/starcats/uvby.calib This can give clues as to what type of variable a new object can be (I am not sure if J-Ks is relatively reddening free compared to B-V, which would be useful) and/or whether a faintish star is too red/blue to be used as a comparator on variable finder charts, etc, &c. Ostensibly the three passbands for each field were imaged simultaneously, though it should be noted that there is no guarantee that there are both J and Ks available for every star/object, especially at the faint end. The 2MASS all sky database can be interrogated via:- http://irsa.ipac.caltech.edu/applications/Gator/ and press the topmost 'select' button. Use a search radius of 3 to 5 arcsecs, dependent on how confident you are of your astrometry. The resultant file, when notified of and viewed, will give values such as j_m and k_m. subtract k_m from j_m to get the J-Ks colour and check it against the above noted table. Ideally the quality of these values should be assessed via the errors noted in the 2MASS data table, and especially by reference to the flags, which are explained in :- http://vsnet.ipac.caltech.edu/2mass/releases/allsky/doc/ancillary/pscformat.html (all one url) The blend, close companion and interference from bright star spikes etc flags are of most interest in that respect. Inclusion of the signal to noise ratio for the measures is also very nice. You'd better note the acknowledgement blurb too:- http://vsnet.ipac.caltech.edu/2mass/releases/allsky/doc/sec1_8b.html GATOR is a tad mulish for single object searches, but the full 2MASS catalogue will be available via VizieR soon, or so it is said. Finally, note that 2MASS All Sky PSC nomenclature is 2MASS Jhhmmssss+dd''""" (where + can be either + or -), which differs from the current 2MASS Jhhmmsss+dd''"" of the second incremental catalogue, which is now going to be referred to as 2MASSI Jhhmmsss+dd''"". This is a bit weird in terms of 'backwards compatibility' re 2MASS names already in the literature, but there you go. You should note though that 2MASS final release is a reprocessing and therefore the data for any object should be considered as not the same as for that object in the 2nd incremental or other releases. Cheers John
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