2004A&A...421..991P

We present CCD photometry in the Washington system C and T₁ passbands down to T₁∼18.5mag in the field of Tombaugh 1, a little studied open cluster located in the third Galactic quadrant. We measured T₁ magnitudes and C-T₁ colours for a total of 1351 stars distributed throughout an area of 13 .6'x13.6'. A cluster radius of 4.3'±0.3' was estimated from star counts in 100-pixel a side boxes distributed throughout the entire observed field. Based on the best fits of isochrones computed by the Geneva group for Z=0.008 to the T₁ vs. C-T₁ colour-magnitude diagram, we derive a colour excess E(C-T₁)=0.55±0.10, equivalent to E(B-V)=0.30±0.05, a distance of (2.2±0.5)kpc from the Sun and an age of 1.3^+0.1_–0.2Gyr. The latter value is in good agreement with that derived from the independent metallicity δT₁ index defined in Geisler et al. (1997AJ....114.1920G). An independent metallicity estimation using the [M_T1, (C-T₁)₀] plane with the standard giant branches of Geisler & Sarajedini (1999AJ....117..308G) yields [Fe/H]=-0.30±0.25dex, a value which lends support to the one obtained from the isochrone fit. Tombaugh 1 is then found to be a relatively metal-poor intermediate-age open cluster.