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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. Figure c1.1: The (RA, Dec), (J, (J-K)) and (J-H)/(H-K)
distributions of the bright sources. The dwarf (yellow) and giant (blue) sequences are plotted in the JHK plane,
while the open circles mark data for known L dwarfs. The dotted lines mark the selection criteria,
Note the concentration at low Galactioc latitudes.
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: 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)$>$0.45: optical
photometry (or spectra) of those sources can discriminate between AGB star and
T Tauris (likely) and the L dwarf on our doorstep (not really likely at all, but who knows).
Considering those in turn
The Ultracool sample: III. The bright sources
Cross-referencing the sources
Discussion
A: Hidden L dwarfs?
There are five confirmed dwarfs in the sample:
Figure c1.9: The (MV, (V-K)) diagram for the three proper motion stars - the main sequence is defined by nearby stars with accurate parallaxes. The solid squares mark G 180-11 and G139-3; the green triangle is BD-01 3925D. |
There is no ambiguity about the identification of the two Giclas stars: the positional agreement is within 3 arcseconds, and there is no other bright 2MASS source within 30 arcseconds. Both are listed as flare stars in Gershberg et al. (1999), but only G180-11 is detected at X-ray wavelengths. Figure c1.9 plots the location of these two stars in the (M$_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=266o. 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 (MV=6.30,
(B-V)=0.94, K2V), is an extrasolar planet host (Msin(i) = 0.73 MJ, Santos et al, 2000).
BD-01 3925D has MK=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.
Five nearby dwarfs, three already included in the pCNS3 (Gl 866 and the two Giclas stars).
Net results
References
NStars Photometric
surveys index
page by Neill Reid, last updated 1/03/2002