Stellar variability at the main-sequence turnoff of the intermediate-age
  LMC cluster NGC 1846
  Intermediate-age star clusters in the LMC present extended main sequence
turnoffs (MSTO) that have been attributed to either multiple stellar
populations or an effect of stellar rotation. Recently it has been proposed
that these extended main sequences can also be produced by ill-characterized
stellar variability. Here we present Gemini-S/GMOS time series observations of
the intermediate-age cluster NGC 1846. Using differential image analysis, we
identified 73 new variable stars, with 55 of those being of the Delta Scuti
type, that is, pulsating variables close the MSTO for the cluster age.
Considering completeness and background contamination effects we estimate the
number of Delta Scuti belonging to the cluster between 40 and 60 members,
although this number is based on the detection of a single Delta Scuti within
the cluster half-light radius. This amount of variable stars at the MSTO level
will not produce significant broadening of the MSTO, albeit higher resolution
imaging will be needed to rule out variable stars as a major contributor to the
extended MSTO phenomenon. Though modest, this amount of Delta Scuti makes NGC
1846 the star cluster with the highest number of these variables ever
discovered. Lastly, our results are a cautionary tale about the adequacy of
shallow variability surveys in the LMC (like OGLE) to derive properties of its
Delta Scuti population.
