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 , which has been suggested to be one of a group of clusters previously identified with the Galactic Anticenter Stellar Structure ( also known as the Monoceros stream ) .
Using VLT/FLAMES with the UVES and GIRAFFE spectrographs , we find a radial velocity ( RV ) of \langle V _ { r } \rangle = 121 \pm 0.4 km s ^ { -1 } using eighteen Tombaugh 2 cluster stars ; this is in agreement with previous studies , but at higher precision .
We also make the first measurement of Tombaugh 2 ’ s velocity dispersion , which is \sigma _ { int } = 1.8 \pm 0.3 km s ^ { -1 } .
Our abundance analysis of RV-selected members finds that Tombaugh 2 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. , \Delta [ 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 [ \alpha /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 ( \langle [ Fe/H ] \rangle , \langle [ Ti/Fe ] \rangle ) \sim ( - 0.06 , + 0.02 ) and ( - 0.28 , + 0.36 ) .
Isochrone fitting to the colour-magnitude distribution of apparent Tombaugh 2 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 Tombaugh 2 is a likely member of the GASS/Monoceros stream , which makes Tombaugh 2 the second star cluster within the originally proposed GASS/Monoceros family after NGC2808 to show some evidence for internal population dispersions .
However , we explore other possible explanations for the observed spread in abundances and two possible sub-populations , with the most likely explanation being that the metal-poor ( [ Fe/H ] = - 0.28 ) , more centrally-concentrated population being the true Tombaugh 2 clusters stars and the metal-rich ( [ Fe/H ] = - 0.06 ) population being an overlapping , and kinematically associated , but “ cold ” ( \sigma _ { V } < 2 km s ^ { -1 } ) stellar stream at R _ { gc } \geq 15 kpc .