VIS3COS: III. environmental effects on the star formation histories of
  galaxies at z~0.8 seen in [OII], H$\delta$, and Dn4000
  [ABRIDGED] We present spectroscopic observations of 466 galaxies in and
around a superstructure at $z\sim0.84$ targeted by the VIMOS Spectroscopic
Survey of a Supercluster in the COSMOS field (VIS$^{3}$COS). We use
[OII]$\lambda$3727, H$\delta$, and $D_n4000$ to trace the recent, mid-, and
long-term star formation histories and investigate how stellar mass and the
local environment impacts those. By studying trends both in individual and
composite galaxy spectra, we find that both stellar mass and environment play a
role in the observed galactic properties. We find that the median [OII]
equivalent width (|EW$_\mathrm{[OII]}|$) decreases from $27\pm2$ {\AA} to
$2.0_{-0.4}^{+0.5}$ {\AA} and $D_n4000$ increases from $1.09\pm0.01$ to
$1.56\pm0.03$ with increasing stellar mass (from $\sim10^{9.25}$ to
$\sim10^{11.35}\ \mathrm{M_\odot}$). Concerning the dependence on the
environment, we find that at fixed stellar mass |EW$_\mathrm{[OII]}|$ is
tentatively lower in higher density environments. Regarding $D_n4000$, we find
that the increase with stellar mass is sharper in denser environments, hinting
that such environments may accelerate galaxy evolution. Moreover, we find
larger $D_n4000$ values in denser environments at fixed stellar mass,
suggesting that galaxies are on average older and/or more metal-rich in such
dense environments. This set of tracers depicts a scenario where the most
massive galaxies have, on average, the lowest sSFRs and the oldest stellar
populations (age $\gtrsim1$ Gyr, showing a mass-downsizing effect). We also
hypothesize that the observed increase in star formation (higher
EW$_\mathrm{[OII]|}$, higher sSFR) at intermediate densities may lead to
quenching since we find the quenched fraction to increase sharply from the
filament to cluster-like regions at similar stellar masses.
