[vsnet-alert 11252] re New flare star candidates

[kstars at Safe-mail.net]

A fascinating little bunch of contradictions this stars is.

First of all, could you kindly contextualise this ? :-

"Hideo Nishimura (Shizuoka, Japan) found brightning of GSC 02170-01762  
on 21 Mar. 2009 during his photographic nova patrol.
Yukio Sakurai (Ibaraki, Japan) and Koichi Itagaki (yamagata, Japan) confirmed it."

I take it the two confirmations are observations and not only independent to the discovery, but also to each other?  What are the times of all these?  If two others confirmed it, what time and magnitudes did their confirmations have, relative to the discovery?  A flare of seconds, minutes, hours, a coupla days?  Or did they confirm by just viewing the discovery image?

"Spectroscopic observation is encouraged to confirm nature of the star."

Anyway, the spectral type.  You've already discovered the spectral type in your email.  Possibly...

You correctly cross identify it as CSS 1237, ie that's star number 1237 in Stephenson's Catalogue of S stars (I sometimes wonder where would we all be without Stephenson and/or Sanduleak).

Spectral type M2S.

Which makes no sense whatsoever for as you rightly point out this thing has quite some proper motion looking more like a Halo dwarf star then a red giant.  mu_alpha and mu_delta 150 and -38 mas/y according to USNO B1.0, which can be a bit awry sometimes, but comparison of positions between USNO A2.0 and CMC14 seem to roughly support that, and further the PPMX gives 144 and -28 for the same proper motions, which is within the UNSO B1.0 errors.

Your B-V of 1.6 doesn't really tally with the CMC14 r' versus J-Ks colours.  They fit a K dwarf.  J-Ks is around where it could be a K or M dwarf, but r'-J isn't enough, it's too blue, it's K spectrum, probably late K.  There are things like metal poor K giants in the Halo, and Halo objects are randomly distributed (on the whole) and can as much appear near the Galactic Plane as at high Galactic Latitudes.

But M2S.  ie ZrO detected (Zirconium Oxide bands), diagnostic of Disc red giants.  Checking Stephenson's 2nd Catalogue for CSS 1237 the B1900 position _and_ magnitude fit this star well and aren't likely to be another adjacent star due to lower precision astrometry.

So the thing's too "blue" to be a red star, and the "bluer than usual" red stars that can be found in the Halo are mostly bluer than usual coz they're metal poor to very metal poor which leads to 'em burning hotter at lower masses and stuff like that, etc, etc.

Maybe Stephenson mistook another line in the relatively low dispersion objective prism plates?  Well, it looks unlikely.  It appears he used a "band head" near 647 nm, that's going to be clear enough.  Maybe Stephenson mistood some coincident nearby C or CH line or something?  Not very likely considering his familiarity with all such things.  After all, Stephenson's S star catalogue is only one of his many works, which include also his C star catalogues, so he knows all about spectral line diagnostics.

Maybe the thing's some sort of combined stars, say a symbiotic or such, and that's why it's photometry's a bit blue?  Checking near 647 nm will show 656 nm easily enough, any sharp Halpha emission would've been clear.

Nice little bits of contradictions, I'm not familiar with M dwarfs having S subtypes, but maybe they do exist and I've just missed the facts.  S, or ZrO, means giants.  Even if Barium star, which can be mistaken for S stars apparently (although I'm not sure if that applies to the 647 nm line too) that still means giants.

Yet there's the relatively large proper motion.

And there's the likely association with a ROSAT bright xray source, of middling hardness.  Hints of a chromospherically active star.  The sort liable to give flares, the sort that have amongst their number nearby red dwarfs of at times relatively high proper motion.

So :-

i) the encouraged spectroscopic follow up needs to be at as high a classificatory dispersion as possible given the vagaries of the object.  It looks like low dispersion will likely confirm M star easily enough, but what will it say about the mild ZrO lines hinted at in Stephenson's red objective prism plates?  And something with better dispersion than the latter is needed to confirm, or modify, that.  The colours from 2MASS and CMC14 don't quite fit an M dwarf, though J-Ks on it's own is not too far off.  (Although this is possibly a somewhat reddened field given its location, this star is bound to be in front of the vast majority of interstellar dust within the line of sight).

ii) the details re discovery and confirmation are needed.  Red dwarf flares usually barely last an hour, and are of low amplitude in the optical, I don't think they frequently are known to reach 3 magnitudes in the optical, and if it's red sensitive photography that's usually minutes for a flare.  So the details of time between discovery, confirmation and what equipment was used for these confirmations and their level of independence, and guesstimates of magnitudes will help there.

iii) I ain't no spectroscopy genius.  There may well be a rare but known subset of red dwarfs with zirconium oxide lines enough to be picked up and included in literatures somewhere.  To me MS stars are red giants, and in the coupla books I've looked in, that's the usual.  But there might simply be some other line that on rare occasions can get confused with ZrO and I wouldn't be familiar with said.


Given a "long" flare event, this is an interesting star.  It's actually an interesting star for going around having red dwarf characteristics (relativley high proper motion, potential chromospheric activity signatured by likely xray source association, possible flaring activity) but a red giant spectrum (M2S).  Photometric colours from 2MASS and CMC14 however preclude a giant star.  So the spectrum's the only contradictory bit.  However, a "long" flaring event's a bit weird too.  Normally red dwarf flares are short and sharp, don't linger at maximum, and are done with by the time anyone's had chance to tell anyone else about them.

Stuff like that.

All errors of science interpretation in the above are of course mine own.  There ain't any targetted books on stuff like this, you've to look up diverse info and try and synthesize a whole.

Finally, the thing looks constant enough on a quick glance of ASAS3 and NSVS data, though any less than < 0.2 V amplitude sinusoid of several day's period of the sort not unknown in some chromospherically active variables wouldn't necessarily be detectable in said data.  I ain't done a period analysis to look though, someone else can do that this time.  The data're there, the apps exist.

Cheers

John

John Greaves