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Дата изменения: Fri Jun 8 23:32:41 2007 Дата индексирования: Mon Oct 1 22:09:32 2012 Кодировка: Поисковые слова: iceland |
The 1051 sources with K > 9.5 have been cross-referenced against the GSC 2.2 catalogue by Rick
White at STScI. That catalogue includes about 435,000,000 sources from the
ST scans of POSS (both I and II) plates in the northern hemisphere and
from the ESO and UK Schmidt plates in the south. The catalogue lists F, BJ
and V magnitudes, and includes some
sources as faint as 19th magnitude in R, although it is not complete at those
magnitudes. There are a number of 2MASS sources which have
(faint) counterparts on POSS II F (R) or N (I), but
don't have an entry in the GSC 2.2. However, brighter sources should
be reasonably complete, so cross-referencing the ultracool list againt
the GSC2.2 will identify most of the bluer stars scattered into
the sample due to either photomteric uncertainties in the 2MASS data or
intrinsic scatter near the (J-K)=1 cutoff. It is possible that some images might
have no GSC counterpart due to proper motion taking the source outside the 6
arcsecond search radius between the palte-taking and the 2MASS observation.
In the north, the mean epoch of GCS2.2 is around
1991, so that would imply
a significant annual motion, so the source is probably worth checking anyway.
In the south, the ESO R plates were taken in the late 70s and early 80s, and the
second epoch UKST F-survey isn't complete - but even so, the annual motion would
have to be around 0.3 arcsec yr-1.
Bottom line is that we're interested in sources with no GSC counterpart or
(F-J) (effectively (R-J)) colours which match the late-M/L sequence in the RJK plane.
Figure a1.1: The (F-J)/(J-K) two-colour diagram for faint
sources with GSC2.2 counterparts. The magenta points mark data for nearby M dwarfs; the yellow points
plot known L dwarfs; and the cyan points plot data for the faint sample. The green line is the
selection boundary.
80 of the 1051 sources gave GCS2.2 images have GSC counterparts. Figure a1.1
plots the 2-colour RJK diagram for those sources. Data for nearby M and L dwarfs
are superimposed for reference. Clearly there are a lot of sources with
implausible colours for ultracool dwarfs. There are also a large number of sources right where
the M dwarf sequence meets the (J-K)=1 boundary, and a few sources along the
L dwarf sequence. We have applied the folowing cutoff to remove the obvious
interlopers
The Ultracool sample: I. The faint sources
That reduces the sample of 1051 sources to 809 targets
Figure a1.2: The (RA, Dec) and JHK distributions for the 809 targets which survive the R-magnitude cut. |
Figure a1.2 plots the (RA, Dec) and near-infrared colour-colour and colour-magnitude diatributions for the reduced sample.There are 326 sources at dec > 0o and 482 south of the equator. The larger number of southern sources probably reflects both the brighter limiting magnitude of the southern ESO R plates and crowding in the Galactic Bulge.
Figure a1.3: The (RA, Dec) and JHK distributions for the 584 viable candidates amongst the faint sample: 307 in the North, 277 in the South. |
As a further check, we have gone through the sample and checked each of the surviving objects. A number of targets are obviously in star forming regions, with counterparts not in the GSC2.2; others are associated with the Fornax dwarf, LMC or SMC (probably AGB/carbon stars). Cutting those stars reduces the sample to a total of 584 targets.
Cross referencing against the literature (via SIMBAD) plus our own observations gives classifications for over 340 of the surviving sample. That sample includes over 45 L dwarfs, 30 of which are new identifications. Figure a1.4 plots the targets still requiring observation. Relatively few remain with (J-K) colours ufficiently red to be L dwarf.
NStars Photometric surveys index