Near-infra-red imaging observations of the two highly reddened globular clusters M4 and M71 are presented. These data represent the deepest K observations yet published for any cluster.
The photometric accuracy and repeatability of the infra-red camera IRCAM is investigated. Observations made at wavelengths of 1.25/¿m (J) and 2.2/¿m (K) show clearly that the camera-array combination is capable of a photometric accuracy of better than 1% on bright stars, with no evidence of residual non-linearity of > 2% over 6 magnitudes..
The reduction procedure is discussed in detail. The profile fitting package DAOPHOT produces accurate and repeatable results from these small frames with large pixels. Zero- point calibrations are examined, and the data are combined with optical data.
M71: Fitting new (V-K)-V and (V-K)-K isoclxrones finds for the four free parameters: E(B-V)=0.26 ± 0.03; (m-M)0=12.85 ± 0.15; age=13 ± 1 Gyr and [Fe/H]=-0.78 ± 0.3, which agree very well with the adopted values from the literature. Combining these with the unified literature results gives the best yet estimates of the cluster parameters: E(B- V)=0.266 ± 0.015; (m-M)c=12.87 ± 0.07; age=14.5 ± 2 Gyr and [Fe/H]=-0.7 ± 0.4. No significant binary fraction is detected on the main sequence.
M4: An extensive re-assessment of the literature concludes that a value of R of 4.0 ± 0.2 is appropriate and that the ‘best’ literature parameters are: E(B-V)=0.37 ± 0.01; (m-M)0=11.22 ± 0.11 ; age=16 ± 2 Gyr and [Fe/H ]=-l.l ± 0.25. The Alcaino and Liller (1984) V magnitudes are found to contain a zero-point error. Allowing for this, isochrone fitting finds an age of 16 ± 3 Gyr and [Fe/H]=-1.0 ± 0.4.
wo-colour (V-K)-(B-V) models are constructed and agree well with the M4 and M71 data for metallicities of -1 and -0.5 respectively. The relative offsets between the two clusters in the two-colour diagram are in excellent agreement with the predictions of the models, based on the adopted metallicities.
The K-magnitude luminosity function is constructed for M4, although no allowance can be made for field star contamination. Once converted into a synthetic V-magnitude LF, it compares well with previous work and supports the proposition that the more metal-rich clusters have flatter mass functions, with x=0 for M4.