Significant Dust-Obscured Star Formation in Luminous Lyman-break
  Galaxies at $z$$\sim$$7$$-$$8$
  We make use of ALMA continuum observations of $15$ luminous Lyman-break
galaxies at $z$$\sim$$7$$-$$8$ to probe their dust-obscured star-formation.
These observations are sensitive enough to probe to obscured SFRs of $20$
$M_{\odot}$$/$$yr$ ($3\sigma$). Six of the targeted galaxies show significant
($\geq$$3$$\sigma$) dust continuum detections, more than doubling the number of
known dust-detected galaxies at $z$$>$$6.5$. Their IR luminosities range from
$2.7$$\times$$10^{11}$ $L_{\odot}$ to $1.1$$\times$$10^{12}$ $L_{\odot}$,
equivalent to obscured SFRs of $20$ to $105$ $M_{\odot}$$/$$yr$. We use our
results to quantify the correlation of the infrared excess IRX on the
UV-continuum slope $\beta_{UV}$ and stellar mass. Our results are most
consistent with an SMC attenuation curve for intrinsic $UV$-slopes
$\beta_{UV,intr}$ of $-2.63$ and most consistent with an attenuation curve
in-between SMC and Calzetti for $\beta_{UV,intr}$ slopes of $-2.23$, assuming a
dust temperature $T_d$ of $50$ K. Our fiducial IRX-stellar mass results at
$z$$\sim$$7$$-$$8$ are consistent with marginal evolution from $z$$\sim$$0$. We
then show how both results depend on $T_d$. For our six dust-detected sources,
we estimate their dust masses and find that they are consistent with dust
production from SNe if the dust destruction is low ($<$$90$%). Finally we
determine the contribution of dust-obscured star formation to the star
formation rate density for $UV$ luminous ($<$$-$$21.5$ mag:
$\gtrsim$$1.7$$L_{UV} ^*$) $z$$\sim$$7$$-$$8$ galaxies, finding that the total
SFR density at $z$$\sim$$7$ and $z$$\sim$$8$ from bright galaxies is
$0.18_{-0.10}^{+0.08}$ dex and $0.20_{-0.09}^{+0.05}$ dex higher, respectively,
i.e. $\sim$$\frac{1}{3}$ of the star formation in $\gtrsim$$1.7$$L_{UV} ^*$
galaxies at $z$$\sim$$7$$-$$8$ is obscured by dust.
