new RCB variable candidate in LMi: MASTER OT J095310.04+335352.8

This star appears to be unusual among R CrB variables, and may be one of the most interesting and important MASTER discoveries. After all, surveys are looking for flaring stars, and this one was the first fading star for 101 confirmed MASTER CVs (plus 26 possible CVs) and 101 SNe and PSNe!

The most important thing about TYC 2505-672-1 is its unusual location in the Milky Way. The generally adopted view is that RCB stars are concentrated to the galactic bulge and have absolute magnitudes M_V=-4..-5. If you look at Fig. 1 in Tisserand et al., arXiv:1211.2475 (http://arxiv.org/abs/1211.2475), you will see that most RCBs are at low galactic latitudes. The only exception is R CrB itself which is at b=51. But R CrB has galactic longitude l=45 which makes it reside in the direction of galactic bulge. Yet, at V_max=5.9 it is likely just a kiloparsec away (for the distance modulus m-M=10), and several hundred parsecs above the galactic plane. But if we take M_V=-4.3 for TYC 2505-672-1 with its V=10.7, distance modulus m-M=15 will put it at 10 kpc from the Sun in almost opposite direction from the galactic center! As stated in our discovery ATel, galactic coordinates (l, b) for this new variable are (191.7, +51.3). 10 kpc*sin(51.3) makes it 7.8 kpc above the galactic plane, which is not just beyond the bulge - it's far in the outer halo of Milky Way! The galactic extinction is negligeable in this direction, which is also confirmed by rather moderate (for the red giant) J-K color index of 1.04 in 2MASS.

So, either this is not a true RCB star (which one then? something even more interesting?), or it will put an end to the long-time speculations about the bulge concentration of R CrB variables. Or we will have to reconsider the intrinsic magnitude scale for this type of (super)giants.

Good web pages say (and they don't lie) that RCB is a short phase in the evolution of a star. If there are ~100 stars of this type known in our own Galaxy, and RCB phase lasts for ~1000 years, there should be a new star every ~10 years entering this phase for the first time. It would be very interesting to know if the first fading episode (when there's no carbon dust around) is different from the next ones. For instance, the typical RCB variables which have already "been there, done that" are fading from the max to the min within 1 week to 1 month. TYC 2505-672-1, however, was likely constant at least during the last 60 years, and took more than 3 months to fade by ~4m. This also doesn't look like a typical RCB behavior. But, again, we may just have not enough statistics on the RCB fading episodes and the intervals between them.

We are still missing the spectrum of this variable. Actually, even the color was not measured! So, if you have RZ LMi in your CCD monitoring program, please consider changing the coordinates of your telescope's pointing center to someting like 09 52 30 +34 00 10! Of course, if your field of view is larger than 15'x15' - there's 21.5' between the two variables. The star is added to AAVSO VSX with AUID 000-BKW-029, see http://www.aavso.org/vsx/index.php?view=detail.top&oid=297394.

I think it is very unlikely that the interpretation in,

 http://www.astronomerstelegram.org/?read=4784

is correct. As far as we know, hydrogen-rich red giants do not evolve to become hydrogen-deficient RCB stars. 

Geoff Clayton

Hi Robert,

There is a spectrum here: http://www.astronomerstelegram.org/?read=4834.

I suspect it is a rather unusual eclipsing binary.

Patrick

The spectrum illustrated is consistent with the published M2III, with the addition of weak H-alpha emission. The JHK colours are also consistent with this. This is rather an early type for a semiregular variable, especially as it is well below maximum light - much of the light drop in red variables of spectral type M results from the increasing strength of the TiO bands, which define the spectral subtype, as the star approaches minimum. It could however be a metal poor star with intrinsically weak bands. An interesting alternative is that it is an M star which has produced a dust cloud. This is well established for carbon stars (not just RCB but also regular SR and Mira types).  This has been seen in an SC star, UY Cen, so maybe it can also occur in an M star of less or no C enrichment.

The most useful observation which could be made now, in addition to continued monitoring, is to re-observe the JHK colours. If it has ejected dust in the line of sight, it should have become substantially redder than appropriate for the spectral type, possibly with an IR excess. The latter would be best seen at L, but might show at K.

Tom.