On the use of the main sequence knee (saddle) to measure globular
  cluster ages
  In this paper we review the operational definition of the so-called main
sequence knee (MS-knee), a feature in the color-magnitude diagram (CMD)
occurring at the low-mass end of the MS. The magnitude of this feature is
predicted to be independent of age at fixed chemical composition. For this
reason, its difference in magnitude with respect to the MS turn-off (MS-TO)
point has been suggested as a possible diagnostic to estimate absolute globular
cluster (GC) ages. We first demonstrate that the operational definition of the
MS-knee currently adopted in the literature refers to the inflection point of
the MS (that we here more appropriately named MS-saddle), a feature that is
well distinct from the knee and that cannot be used as its proxy. The MS-knee
is only visible in near-infrared CMDs, while the MS-saddle can be also detected
in optical-NIR CMDs. By using different sets of isochrones we then demonstrate
that the absolute magni- tude of the MS-knee varies by a few tenths of a dex
from one model to another, thus showing that at the moment stellar models may
not capture the full systematic error in the method. We also demonstrate that
while the absolute magnitude of the MS-saddle is almost coincident in different
models, it has a systematic dependence on the adopted color combinations which
is not predicted by stellar models. Hence, it cannot be used as a reliable
reference for absolute age determination. Moreover, when statistical and
systematic uncertainties are properly taken into ac- count, the difference in
magnitude between the MS-TO and the MS-saddle does not provide absolute ages
with better accuracy than other methods like the MS-fitting.
