Extinction and nebular line properties of a Herschel-selected lensed
  dusty starburst AT z=1.027
  We present Hubble Space Telescope (HST) WFC3 imaging and grism spectroscopy
observations of the {\it Herschel}-selected gravitationally-lensed starburst
galaxy HATLASJ1429-0028. The lensing system consists of an edge-on foreground
disk galaxy at $z=0.218$ with a nearly complete Einstein ring of the infrared
luminous galaxy at $z=1.027$. The WFC3 spectroscopy with G102 and G141 grisms,
covering the wavelength range of 0.8 to 1.7 $\mu$m, resulted in detections of
H$\alpha$+[NII], H$\beta$, [SII], and [OIII] for the background galaxy from
which we measure line fluxes and ratios. The Balmer line ratio
H$\alpha$/H$\beta$ of $7.5 \pm 4.4$, when corrected for [NII], results in an
extinction for the starburst galaxy of $E(B-V)=0.8 \pm 0.5$. The $H\alpha$
based star-formation rate, when corrected for extinction, is $60 \pm 50$
M$_{\odot}$ yr$^{-1}$, lower than the instantaneous star-formation rate of 390
$\pm$ 90 M$_{\odot}$ yr$^{-1}$ from the total IR luminosity. We also compare
the nebular line ratios of HATLASJ1429-0028 with other star-forming and sub-mm
bright galaxies. The nebular line ratios are consistent with an intrinsic
ultra-luminous infrared galaxy with no evidence for excitation by an active
galactic nucleus (AGN). We estimate the metallicity, $12 + log(O/H)$, of
HATLASJ1429-0028 to be 8.49 $\pm$ 0.16. Such a low value is below the average
relations for stellar mass vs. metallicity of galaxies at $z \sim 1$ for a
galaxy with stellar mass of $\sim 2 \times 10^{11}$ M$_{\odot}$. The
combination of high stellar mass, lack of AGN indicators, low metallicity, and
the high star-formation rate of HATLASJ1429-0028 suggest that this galaxy is
currently undergoing a rapid formation.
