Return to Home Page
# From John Greaves. It seems to me quite a few people has taken his words # too seriously. I must comment before forwarding that this kind of analysis # can easily be too speculative, and should better regarded as a simple # number trick before more substantial evidence becomes available. # Please don't attempt to write a flash news entitled: # "Astronomers discovered an unreasonably luminous object potentially # associated with transient X-ray emission of unknown origin" ;-) === On 22 Sep 2003, Taichi Kato forwarded from Berto Monard: > I think the bright red star at 17 54 25.3 -26 19 52 might be corona > active, sending all those X rays to us, what do you think Taichi? and later replied: > I don't think that the bright star at 17 54 25.3 -26 19 52 > is identified with the ROSAT source. Okay, well using the dereddening 'Q' parameter of Q = (U-B) - 0.645(B-V) for B1V-A1V [Heintze 1973, IAU Symp 54] and a public domain look up table in a file sourced via Brian Skiff's public ftp, and the Henden photometry for this star based on 3 observations, where U-B is +0.552 and B-V +1.762 with quoted errors of 0.03 and 0.05 respectively, we get: Q = 0.552-(0.645*1.762) = -0.584, or roughly spectral type B3 to B4 for this star Not that a B3 star necessarily has to be a bright xray source, but rather than coronal activity, think more exotic. Some xray binaries have B primaries, for instance, with compact secondaries. Or maybe just total coincidence. Both 2MASS and DENIS J-Ks are about +0.8, which would normally be an M0 star, whereas about -0.2 should be the value for a B3 star, so this object is reddened in the near infrared too. Now, I haven't quite gotten my brain around J-Ks yet, but I believe it is relatively little reddened by _interstellar extinction_. Extinction at the K band being one tenth of extinction at the V band (0.11 actually) and extinction at the J band about three tenths at V (0.28 actually), so I'm going to crudely assume E(J-Ks) ~ 0.2 times E(B-V), though in fact it is probably less (as extinction at B is ~ 1.4 times extinction at V). Even though this part of the sky has large interstellar extinction, the E(B-V) of about +2.0 then becomes equivalent to an E(J-Ks) of +0.4, whereas E(J-Ks) is actually about +1.0 (-0.2 - +0.8 from above stuff on B3 stars versus the actual measures). So, there's a lot of circumstellar extinction local to the star, probably in the form of small dust grains or large molecules (Ks has wavelength 2.2 microns, so that's the sort of sizes we're looking at, at least). Not much in the way of large warm dust clouds though, because we have no IRAS or MSX5C source at this position. Probably too much dust for a normal Be star, probably not too much for a noneruptive gammaCas star, or maybe just something else. B star types are a bit muddled at present, I think. Well, I get confused on them, at least jargon wise, as different groups seem not to quite use the same jargon. Going back to the B-V, Av, absorption at V due to interstellar extinction, is given in the books as nearly always being ~ 3.3(E(B-V)), or 6.6 or so in this case. In other words, the _unreddened_ apparent V magnitude of this star is ~6!!! B3 dwarfs have absolute magnitude about -2, superigiants about -5 to -6. The resulting distance modulus of 8 for a dwarf star is 400 pc, not at all distant, and not likely to have that much interstellar absorption. A resulting distance modulus of 12 for a supergiant star is 2500 pc, which will allow more absorption (there'll be a more absorption towards Galactic Centre anyway than on average), and more likely to be in the next spiral arm across. Possibly. Anyway, at the end of all that juggling numbers (hopefully correctly) and general blathering, the star you mention has more to it than first sight suggests. Whether it is the same as the bright xray source, and whether either are to do with the recent xray transient/object, is still another matter. But I can agree that this star can't just be instantly dismissed as irrelevant. Cheers John John Greaves
Return to the Powerful Daisaku Nogami
vsnet-adm@kusastro.kyoto-u.ac.jp
