  Aims: Stars twinkle because their light propagates through the atmosphere.
The same phenomenon is expected at a longer time scale when the light of remote
stars crosses an interstellar molecular cloud, but it has never been observed
at optical wavelength. In a favorable case, the light of a background star can
be subject to stochastic fluctuations on the order of a few percent at a
characteristic time scale of a few minutes. Our ultimate aim is to discover or
exclude these scintillation effects to estimate the contribution of molecular
hydrogen to the Galactic baryonic hidden mass. This feasibility study is a
pathfinder toward an observational strategy to search for scintillation,
probing the sensitivity of future surveys and estimating the background level.
Methods: We searched for scintillation induced by molecular gas in visible dark
nebulae as well as by hypothetical halo clumpuscules of cool molecular hydrogen
($\mathrm{H_2-He}$) during two nights. We took long series of 10s infrared
exposures with the ESO-NTT telescope toward stellar populations located behind
visible nebulae and toward the Small Magellanic Cloud (SMC). We therefore
searched for stars exhibiting stochastic flux variations similar to what is
expected from the scintillation effect. According to our simulations of the
scintillation process, this search should allow one to detect (stochastic)
transverse gradients of column density in cool Galactic molecular clouds of
order of $\sim 3\times 10^{-5}\,\mathrm{g/cm^2/10\,000\,km}$. Results: We found
one light-curve that is compatible with a strong scintillation effect through a
turbulent structure characterized by a diffusion radius $R_{diff}<100\, km$ in
the B68 nebula. Complementary observations are needed to clarify the status of
this candidate, and no firm conclusion can be established from this single
observation. We can also infer limits on the existence of turbulent dense cores
(of number density $n>10^9\, cm^{-3}$) within the dark nebulae. Because no
candidate is found toward the SMC, we are also able to establish upper limits
on the contribution of gas clumpuscules to the Galactic halo mass. Conclusions:
The limits set by this test do not seriously constrain the known models, but we
show that the short time-scale monitoring for a few $10^6 star\times hour$ in
the visible band with a $>4$ meter telescope and a fast readout camera should
allow one to quantify the contribution of turbulent molecular gas to the
Galactic halo. The LSST (Large Synoptic Survey Telescope) is perfectly suited
for this search.
