  Type IIn SNe show spectral evidence for strong interaction between their
blast wave and dense circumstellar material (CSM) around the progenitor star.
SN2010jl was the brightest core-collapse SN in 2010, and it was a Type IIn
explosion with strong CSM interaction. Andrews et al. recently reported
evidence for an IR excess in SN2010jl, indicating either new dust formation or
the heating of CSM dust in an IR echo. Here we report multi-epoch spectra of
SN2010jl that reveal the tell-tale signature of new dust formation:
emission-line profiles becoming systematically more blueshifted as the red side
of the line is blocked by increasing extinction. The effect is seen clearly in
the intermediate-width (400--4000 km/s) component of H$\alpha$ beginning
roughly 30d after explosion. Moreover, we present near-IR spectra demonstrating
that the asymmetry in the hydrogen-line profiles is wavelength dependent,
appearing more pronounced at shorter wavelengths. This evidence suggests that
new dust grains had formed quickly in the post-shock shell of SN 2010jl arising
from CSM interaction. Since the observed dust temperature has been attributed
to an IR echo and not to new dust, either (1) IR excess emission at $\lambda <
5 \mu$m is not a particularly sensitive tracer of new dust formation in SNe, or
(2) some assumptions about expected dust temperatures might require further
study. Lastly, we discuss one possible mechanism other than dust that might
lead to increasingly blueshifted line profiles in SNeIIn, although the
wavelength dependence of the asymmetry argues against this hypothesis in the
case of SN2010jl.
