Meeting the neighbours: NStars and 2MASS

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Candidate nearby dwarfs in the NLTT: photometric follow-up

1. Southern stars: SAAO photometry

We are obtaining Cousins (B)VRI photometry for the brighter southern stars. Initial results from observations of stars from NLTT sample 1 were obtained from the South African Astronomical Observatory, and are presented by Reid, Kilkenny & Cruz (AJ 2002). The main results are summarised below; full details are given here.

Figure C2.1: The colour-magnitude diagram and distribution on the sky of the NLTT stars in Paper 2.

The initial NLTT sub-sample includes 209 targets with mr < 14.0 and Right Ascension between 2 hours and 18 hours (Figure C2.1), broken down into the following subsets:

Figure C2.2: Optical and near-infrared colour-colour diagrams for the NLTT stars observed from SAAO. The magenta points plot data for neraby stars with accurate parallaxes; the green lines plot the dwarf (dotted) and giant (dashed) JHK sequences.

Figure C2.2 compares the 2MASS near-infrared and SAAO optical photometry of the NLTT dwarfs against data for well-studied nearby stars. The agreement is excellent.

Figure C2.3: Optical/near-infrared 2-colour diagrams for the NLTT sample. As in C2.2, the magenta points mark standard stars.

There are more outliers in Figure C2.3, which matched optical and optical/near-infrared colours for the NLTT stars against standard sequences. In most cases those discrepancies can be traced to the presence of additional stars (either physical companions or background systems) in the aperture during the SAAO photometry. The faint apparent magnitudes of these stars at blue wavelengths makes the B-band photometry particularly susceptible to this problem.

Figure C2.4: Photometric residuals from the SAAO observations: solid squares are matched against observations by Weis; open squares against Eggen (1986); triangles versus Patterson et al (1998); five-point stars against Leggett's (1992) compilations; and open circles against Bessell's (199) data.

We have cross-referenced the SAAO sample against the literature using SIMBAD, and have identified 97 photometric observations of 75 stars by Weis (1991a,b; 1996; 1999), Eggen (1987), Leggett (1992), Patterson et al (1998) and Bessell (1990). Figure C2.4 compares the photometry - we have transformed the Kron-system RI data from Weis and Eggen onto the Cousins system using the transformations given by Bessell & Weis (1987). The breakdown is as follows:

Reference N1 V (V-R) (V-I) N2 (B-V)
Weis 33 -0.011 -0.006 -0.005 23 -0.010
0.033 0.018 0.005 0.019
Eggen 15 -0.039 -0.037 -0.067 1 0.006
0.106 0.082 0.067
Leggett 10 0.002 0.004 0.013 10 -0.024
0.012 0.008 0.013 0.00
Patterson et al 3 0.019 -0.016 0.014
0.025 0.013 0.014
Bessell 25 -0.012 -0.003 -0.001 25 -0.014
0.022 0.009 0.014 0.017

The agreement is excellent, save for the comparison with Eggen's photometry. Most of the V magnitudes in the last sample are given to an accuracy of only 0.1 magnitude, and may well be estimates, rather than measurements.

Figure C2.5: Distance estimates for the SAAO stars the upper panel plots distance modulus, derived from the average photometric parallax, against (V-I), with the dotted line marking the 20-parsec distance limit; the lower compares photometric and trigonometric parallaxes for stars with Hipparcos data.

We have used the optical and near-infrared photometry to derive photometric parallaxes, combining the individual (V-K), (V-I) and (I-J)-based estimates using the same prescription as adopted in Paper I.. Figure C2.5 summarises the results. We have made explicit allowance for known binaries where the SAAO photometry is contaminated by the companion, using the magnitudes listed in the NLTT to estimate the relative contribution of both components. Of the 190 targets, 108 have formal distance moduli less than or within 1-sigma of 1.5 magnitudes, i.e. potential distances of less than 20 parsecs.

2. Northern stars: APO photometry

Just as the brighter southern stars have been targetted for optical photometry at SAAO, the brighter northern targets are being observed with the 20-inch Photometric Telescope at Apache Point Observatory . This telescope has a 1024-square CCD covering a 0.8x0.8 degree field, and is equipped with filters matching those used in the Sloan Digital Sky Survey, During dark time, the telescope is used to provide secondary standards for photometric calibration of SDSS imaging; during bright time, when the main survey is not operating, the telescope is used for a number of other projects, including a photometric survey of stars known to be within 10 parsecs (Golimowski, Henry et al, in prep.).

The initial target list for APO includes 558 northern sources from NLTT sample 1 with magnitudes in the range 10 < mr < 14.5.


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page by Neill Reid, last updated 25/02/2002