Meeting the neighbours: NStars and 2MASS

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Photometric searches (II): 2MASS near-infrared data and optical counterparts

Main investigators: Cruz, Reid, Liebert
Late-type M, L and T dwarfs span a wide range of photometric properties . This variety means that no single set of search criteria is suitable for finding all nearby ultracool dwarfs. We therefore use a combination of techniques to identify candidate nearby stars and brown dwarfs, each technique targeting a somewhat different regime in (absolute magnitude, colour/spectral type) space.

1. Ultracool Dwarfs: Near-infrared Photometric Selection

1.1 Defining the sample

The (MJ, (J-K)) colour-magnitude diagram for nearby stars shows that (J-K) is essentially degenerate for early- and mid-type M dwarfs, but can provide a means of identifying cooler dwarfs, with distinctively redder (J-K) colours. The second incremental release of 2MASS data provides accurate JHKS photometry for hundreds of millions of point sources covering 47% of the sky. That cxatalogue provides a homogeneous dataset to search for candidate ultracool dwarfs.

Figure A1.1: The (J, (J-K)) selection criteria

Figure A1.1 illustrates the search criteria adopted for this phase of our NStars survey. As in Figure 1.11, we plot photometric data for nearby stars and L dwarfs with known parallaxes, but in this case the magnitudes are scaled to a distance of 20 parsecs. The lines superimposed on the figure delineate our selection criteria: (J-K) > 1.0, and J < Jlim, where

Jlim = 3.0 (J-K) + 10.5
This limit lies sufficiently below the observed sequence that we would expect to include all r < 20 parsecs ultracool dwarfs, with additional cool dwarfs scattered in from larger distances.

Figure A1.2: Colour selection criteria for ultracool dwarfs in the (J-H)/(H-K) two-colour diagram: blue triangles are nearby main-sequence stars; green points ultracool M and L dwarfs; red stars are T dwarfs; deep blue circles are M subdwarfs; and purple crosses are giants. The box outlines the (J-H)/(H-K) selection limits given in the text.

The simple colour-magnitude limits outlined in Figure A1.2 encompass a wide range of astronomical sources: red giants, carbon stars, reddened Galactic stars, T Tauris and dusty QSOs, besides the targeted ultracool dwarfs. Those contaminants can be eliminated to some extent by imposing selection limits in the (J-H)/(H-K) plane. Our chosen limits are

(J-H) > 1.75 (H-K) - 0.475, for (H-K) > 0.5
(J-H) < 1.75 (H-K) + 0.1875, for (H-K) > 0.35
As Figure A1.2 shows, these limits include the reddest ultracool dwarfs, while excluding the bulk of the red giant sequence and reddened main-sequence stars.

As an indication of the stellar sample segregated by these criteria, consider the two archetypical ultracool dwarfs, VB8 and VB 10
VB8, spectral type M7, has MJ=10.71, or J=12.21 at 20 parsecs, and (J-K)=0.95
VB10, spectral type M8, has MJ=11.05, or J=12.55 at 20 parsecs, and (J-K)=1.10
The (J-H)/(H-K) colours of the two stars are
VB8: (J-H)=0.58, (H-K)=0.47; VB 10: (J-H)=0.66, (H-K)=0.44
VB8 fails to meet our criteria; VB10 is well within the sample limits. Thus, our sample should isolate ultracool dwarfs with spectral types later than ~M8.

Figure A1.3: A first cut at ultracool dwarf selection for part of the northern sky

Figure A1.3 illustrates the (J, (J-K)) distribution for point-source selection based on these colour criteria. The data are drawn from approximately one-third of the sky (although the 2MASS second incremental covers only half of that area, so effectively ~17% sky coverage). We also exclude all sources within 10 degrees of the Galactic Plane.

Figure A1.4: The ((J-H), (H-K)) two-colour diagram for the sources plotted in Figure A1.3

The colour-coding of the sources in the (J-H)/(H-K) colour-magnitude diagram provides a hint as to the likely make-up of the sample. The uncoloured open squares predominantly represent the brightest sources in J. It is clear that most of those sources lie near the upper bound of the JHK selection box, close to the locus of red giants and AGB stars. Indeed, follow-up observations have generally confirmed most of those objects as evolved stars. The fainter `good quality' sources (blue points) lie mainly in the vicinity of the dwarf sequence in the JHK plane, with some extending upwards along the L dwarf sequence. Many of these have indeed proven to be ultracool dwarfs. Note, however, the increased number density in a band across the L dwarf region at (J-H)~ 1.4: most of these stars prove to be reddened sources.

1.2 Refining the sample

Section 1.1 outlined the basic technmiques employed in our survey. This section describes in more detail the application of those techniques to analysis of data from the 2MASS Second Incremental Release.

Figure A1.5: Photometry and spatial distribution of the 1794 surviving targets in the 2nd IncR ultracool sample.

There are a total of 2224 sources in the Second Incremental Release which meet the colour/magnitude criteria of the ultracool sample, and which have Galactic latitudes |b| > 10o. Visual inspection of those sources allows the immediate elimination of 430 sources as confused, near bright stars, parts of large galaxies, etc. The celestial distribution and near-infrared properties of the remaining 1794 targets are shown in Figure A1.5. Clear concentrations remain at low latitudes, particularly south of the equator around 6 hours (Orion, Monoceros), 16 hours (Ophiuchus, Sco-Cen, Lupus) and 20 hours (Capricorn, Aquarius)

Figure A1.6: The three ultracool subsets: faint (green), intermediate (magenta) and bright. Red crosses at J > 9 are sources eliminated from the sample based on optical photometry.

We have divided the ultracool sample into three subsets based on their apparent magnitudes, as illustrated in Figure A1.6. These are defined as follows, with each sub-sample described in more detail on the linked page:

Figure A1.7: Ultracool candidates which still require observations; February, 2002

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page by Neill Reid, last updated 28/02/2002