The Ultracool sample: III. The bright sources

Cross-referencing the sources

Figure c1.1 plots the spatial distribution and colour-magnitude diagrams for the 650 sources with J < 9. Our expectation is that a substantial fraction of this sample is contributed by red giants, AGB stars and highly-reddened young stars; the concentration towards the Galactic Plane is clearly in line with those expectations.

As bright sources, there is a reasonable chance that many of these sources are already known. We have cross-referenced the sample against the SIMBAD, using a search radius of 2.0 arcminutes centred on the 2MASS position. The results are as follows:

Figure c1.2: Characteristics of the ultracool sources identified with IRAS sources.

1. 147 are within 10 to 15 arcseconds of a source from the IRAS catalogue; given the positional uncertainties of the IRAS astrometry, plus the expectation that IRAS sources should be red in (J-K), these are very likely to be the correct identification for the 2MASS source. Figure c1.2 shows that most lie close to the Plane, as would be expected for either protostars/T Tauris or AGB stars.

   Figure c1.3: Stellar ultracool sources; the blue points mark the proper motion stars.

   Figure c1.4: Stellar ultracool sources - optical/near-infrared distributions.

2. 138 lie within 2-3 arcseconds of stars in one of either the Henry Draper, Bonner Durchmusterung, Cape Durchmusterung, Cape Photographic Durchmusterung, Guide Star Catalogue, or PPM catalogue. Most of the sample lie in the vicinity to the giant sequence in JHK, with approximately 15 stars overlapping with the L dwarf regime. As discussed further below, these are likely to be carbon stars. Figure c1.4 shows the optical/near-IR distributions, using the optical photometry listed in SIMBAD (which may be of dubious origins in some cases).
3. Three sources match with known proper motion stars: G180-11, G139-3 and BPM 65125. The two Giclas stars are known to be within 15 parsecs of the Sun; the BPM star is a new nearby candidate. The JHK colours of G 180-11 and BPM 65215 are somewhat surprising - significantly redder than one would expect given their spectral types of M3/M4. On the other hand, with photometryc of > 10⁸ sources in 2MASS, some scatter is not unexpected.

   Figure c1.5: The known caarbon stars

4. Sixteen stars in the sample are classified in SIMBAD as carbon stars. Figure c1.5 shows their distribution on the sky and in the near-infrared plane - there's an obvious overlap with the L dwarfs in the JHK plane.

   Figure c1.6: The mira variables

5. 98 stars are identified as Miras - M-type long-period variables. Most of these stars lie close to the Galactic Plane, with a particular concentration in the ScoCen region (towards the Bulge).

   Figure c1.7: The semi-regular variables (solid squares) and `others' (open triangles).

6. Eighteen stars are identified as semi-regular (AGB/RGB) variables. As might be expected, the spatial and colour-magnitude distributions are similar to those of the miras (Figure c1.7).
7. a hodge-podge of 30 red stars: WOH S 11, listed as an ``M-type star'' (no cross-references) in SIMBAD - an OH maser?; StM 78, 161 and 218, from Stephenson's (ApJ 301, 927, 1985) sample of high-latitude red giants (spectra in Sharples et al, 1995, MNRAS 272, 139); BR B0954-0947 from the APM QSO survey; FBS 1518+626 and 1724+644 from the Byurakan survey; the symbiotic star, TX CVn, which presumably gets in through the red giant/supergiant component; the T Tauri, HT Lupi; UZ Sco, and `Orion-type' variable (pre-MS?); a DENIS source from Phan-Bao {\sl et al.} (A\&A 380, 590, 2001: a known red giant variable); and an assortment of M-type variables, some of which may well be Miras or semi-regulars, but including EZ Aqr (Gliese 866).

   Figure c1.8: The spatial distribution and JHK distributions of the 203 unidentified sources in the bright ultracool sample.

8. 203 sources have no obvious counterpart in the SIMBAD database. Based on the distribution on the sky, most, if not all, of these sources are likely to be pre-main-sequence stars or AGB stars.

Discussion

A: Hidden L dwarfs?

Of the 650 sources, the overwhelming majority are clearly red giants. There are 34 sources of the 203 unidentified sources and 11 stellar sources with (H-K)$>$0.45: optical photometry (or spectra) of those sources can discriminate between AGB star and T Tauris (likely) and the L dwarf on our doorstep (not really likely at all, but who knows). Considering those in turn

• The stellar sources - the optical/2MASS positions all agree to better than a few arcseconds, so the cross-identification is highly likely. (Put another way, the absence of a bright 2MASS source at that position would be extremely unlikely.) A star-by-star check indicates that all eleven are probably giants - three (V355 Gem, CD CVn and BD-08 2741) are definitely giants.
• The unidentified sources - 16 can be ruled out as possible L dwarfs, based either on the magnitude of the optical counterpart or our spectroscopy; eleevn of the remaining eighteen are possible, but unlikely, late-type dwarfs; seven are at high latitude and are worth checking (although they may simply be carbon stars

B: Confirmed nearby dwarfs

There are five confirmed dwarfs in the sample:

• G180-11 = 1RXS J155532.2+351207, M_V=13.11, d=13+/-3.9 pc (spec. parallax), Sp=M4.5;
• G139-3, M_V=12.43, d=13.8+/-3.9 pc (spec. parallax), Sp=M4;
• Gl 866 = EZ Aqr. triple system, M_V=15.5, d=3.4+/-0.03 pc (spec. parallax), Sp=M5.5;
• BPM 65125, from Luyten's Bruce Proper Motion survey;
• BD-01 3925D, listed as a K0 dwarf.

There is no ambiguity about the identification of the two Giclas stars: the positional agreement is within 3 arcseconds, and there is no other bright 2MASS source within 30 arcseconds. Both are listed as flare stars in Gershberg et al. (1999), but only G180-11 is detected at X-ray wavelengths. Figure c1.9 plots the location of these two stars in the (M$_V$, (V-K)) colour-magnitude diagram; G139-3 lies towards the upper edge of the main-sequence, while G180-11 sits above the main-sequence, suggestive of an equal-mass binary (or enhanced IR flux, since G180-11 has unusually red JHK colours for its spectral type). McCarthy, Zuckerman & Becklin (2001) identify a candidate binary companion to the latter star, with I=12.6 and a separation of 1.5 arcseconds at PA=266^o. There is little evidence for that object on the 2MASS scans - perhaps a slight enhancement at the proposed location. Certainly there's nothing as bright as J=10, the expected magnitude of a red dwarf companion, but even a white dwarf should have J~12 and be easily visible. In any case, this potential companion doesn't explain the near-IR colours.

Gl 866ABC is a well-known nearby star.

BPM 65125 also has unusually red near-IR colours for its estimated spectral type. There is only one bright source in the 2MASS catalogue (and on the 2MASS images), but SIMBAD lists three sources within 2 arcminutes of the 2MASS position: BPM 65125 (also identified as [LM84] 4-20 from Lu (1984 - Contrib. VVO 2, 33), [LM84] 4-18 (also from Lu), and IRAS F19495-3817. The position listed for the IRAS faint source is closest to the 2MASS source, lying 10 seconds East and 4 seconds North; that offset might reflect proper motion between the early 80s and the 2MASS imaging in 1998. [LM84] 4-20 is ~ 100 arcseconds west (RA=19 52.7, Dec = -38 10), and there's no bright 2MASS source within about 60 arcseconds; the nominal position of BPM 65125 is (RA=19 52.9, Dec = -38 11), or 1 arcminute South of the 2MASS source - but there isn't any other 13th magnitude star within 3 or 4 arcminutes, so the offset is probably due to a bad posiion for the BPM star.

BD-01 3925D is listed as a companion of the K dwarf, BD -1:3925A (HD 192263), which has an Hipparcos parallax measurement of 50.27+/-1.13 milliarcseconds. HD 192263 (M_V=6.30, (B-V)=0.94, K2V), is an extrasolar planet host (Msin(i) = 0.73 M_J, Santos et al, 2000). BD-01 3925D has M_K=5.61 for d=19.9 parsecs, consistent with a spectral type of M0/M1. The V magnitude quoted in SIMBAD places the star at the lower edge of the main sequence (Figure c1.9), but is unlikely to be very accurate. As with G180-11 and BPM 65125, the near-IR colours are redder than expected. no indication of problems on the 2MASS images.

Net results

Five nearby dwarfs, three already included in the pCNS3 (Gl 866 and the two Giclas stars).

References

• Gershberg et al 1999 AAS 139, 555
• McCarthy, Zuckerman & Becklin 2001 AJ 121, 3259
• Santos et al, 2000, A&A 356, 599

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