Tombaugh 2: The first open cluster with a significant abundance spread or embedded in a cold stellar stream?

Abstract

We present new high-resolution spectroscopy from which we derive abundances and radial velocities for stars in the field of the open cluster Tombaugh 2 (To2), which has been suggested to be one of a group of clusters previously identified with the Galactic Anticenter Stellar Structure (GASS) (also known as the Monoceros stream). Using Very Large Telescope/Fibre Large Array Multi-Element Spectrograph (VLT/FLAMES) with the Ultra-Violet and Visible Echelle Spectrograph (UVES) and GIRAFFE spectrographs, we find a radial velocity (RV) of 〈Vr〉 = 121 ± 0.4 km s-1 using 18 To2 cluster stars; this is in agreement with previous studies, but at higher precision. We also make the first measurement of To2's velocity dispersion, which is σint = 1.8 ± 0.3 km s-1. Our abundance analysis of RV-selected members finds that To2 is more metal rich than previous studies have found; moreover, unlike the previous work, our larger sample also reveals that stars with the velocity of the cluster show a relatively large spread in chemical properties (e.g. Δ[Fe/H] > 0.2). This is the first time a possible abundance spread has been observed in an open cluster, though this is one of several possible explanations for our observations. While there is an apparent trend of [α/Fe] with [Fe/H], the distribution of abundances of these 'RV cluster members' also may hint at a possible division into two primary groups with different mean chemical characteristics - namely (〈[Fe/H]〉, 〈[Ti/Fe]〉) ∼ (-0.06, +0.02) and (-0.28, +0.36). Isochrone fitting to the colour-magnitude distribution of apparent To2 members yields an age of 2.0 Gyr, E(B - V) = 0.3, and (m - M)0 = 14.5 or d = 7.9 kpc for both populations - parameters that are within the range of previous findings. Based on position and kinematics To2 is a likely member of the GASS/Monoceros stream, which makes To2 the second star cluster within the originally proposed GASS/Monoceros family after NGC 2808 to show some evidence for internal population dispersions. However, we explore other possible explanations for the observed spread in abundances and two possible subpopulations, with the most likely explanation being that the metal poor ([Fe/H] = -0.28), more centrally concentrated population being the true To2 clusters stars and the metal-rich ([Fe/H] = -0.06) population being an overlapping, and kinematically associated, but 'cold' (σV < 2 km s-1) stellar stream at Rgc ≥ 15 kpc.