Stellar population templates in the near-infrared

• Main Author: Brasseur, Crystal
• Other Authors: Stetson, Peter Brailey
• Language: English; en
• Published: 2009
• Subjects: Stellar populations; infrared; UVic Subject Index::Sciences and Engineering::Physics::Astronomy
• Online Access: http://hdl.handle.net/1828/1671

We have obtained broad-band NIR-photometry for six Galactic star clusters, M92,M15,M13, NGC1851, M71 and NGC6791, as observed with the WIRCam wide-field imager on the Canada-France-Hawaii Telescope; supplemented by images taken with HAWK-I on VLT. From the resultant (V − J)-V and (V − K)-V colour-magnitude diagrams, fiducial sequences spanning the range in metallicity, −2.4 ≤ [Fe/H] ≤+0.3, have been defined which extend from the tip of the red-giant branch to ∼ 2.5 magnitudes below the main-sequence turnoff. These fiducials provide a valuable set of empirical isochrones for the interpretation of stellar population data in the 2MASS system. From the NIR data, the reddenings of M15, M71 and NGC6791 — which have been subject to considerable controversy — were found to be E(B−V)=0.075, 0.22 and 0.155 mag respectively. Comparisons of our CMDs to Victoria isochrones that have been transformed using the MARCS model colour-Teff relations reveal that the models reproduce the giant branches of clusters more metal-rich than [Fe/H] ≈ −1.3, but they become systematically redder than the observed RGBs as the cluster metallicity decreases. These discrepancies are seen consistently in the two colours and therefore may indicate that the temperature scale of the stellar evolutionary models for giant stars at low metallicity is too cool.MARCS colour transformations were also tested using the classic Population II subdwarfs. The MARCS colours show redward offsets of ∼ 0.03 mag when compared with the observed (V − K) and (J − K) colours (assuming best estimates of Teff, log g, and [Fe/H]), and a systematic blue offset relative to the isochrone temperatures. Together with the indications from the cluster (V − K) and (V − J) CMDs, these results suggest that there is a problem with the MARCS colour transformations involving J.