  We present optical and near-infrared (NIR) photometry and spectroscopy as
well as modelling of the lightcurves of the Type IIb supernova (SN) 2011dh. Our
extensive dataset spans two years, and complemented with Spitzer mid-infrared
(MIR) data, we use it to build an optical-to-MIR bolometric lightcurve between
days 3 and 732. To model the bolometric lightcurve before day 400 we use a grid
of hydrodynamical SN models and a bolometric correction determined with
steady-state NLTE modelling. Using this method we find a helium core mass of
3.1 (+0.7-0.4) solar masses for SN 2011dh, consistent within error bars with
previous results. We present bolometric and broad-band lightcurves between days
100 and 500 for the Jerkstrand et al. (2014) steady-state NLTE models, and the
preferred 12 solar masses (initial mass) model shows a good overall agreement
with the observed lightcurves. We find an excess in the K and the MIR bands
developing between days 100 and 250, during which an increase in the optical
decline rate is also observed. A local origin of the excess is suggested by the
depth of the HeI 2.058 micron absorption. Steady-state NLTE models with a
modest dust opacity in the core, turned on during this period, reproduce the
observed behaviour, but an additional excess in the Spitzer 4.5 micron band
remains. CO first-overtone band emission is detected at day 206, and possibly
at day 89, and assuming the additional excess to be dominated by CO fundamental
band emission, we find fundamental to first-overtone band ratios considerably
higher than observed in SN 1987A. This paper concludes our extensive
observational and modelling work on SN 2011dh. The results from hydrodynamical
modelling, steady-state NLTE modelling, and stellar evolutionary progenitor
analysis are all consistent, and suggest an initial mass of 12 solar masses for
the progenitor.
