Extinction and nebular line properties of a Herschel-selected lensed dusty starburst AT z=1.027

We present Hubble Space Telescope Wide Field Camera 3 (WFC3) imaging and grism spectroscopy observations of the 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 0.8-1.7 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 +/- 4.4, when corrected for [NII], results in an extinction for the starburst galaxy of E(B-V)= 0.8 +/- 0.5. The H alpha-based star formation rate (SFR), when corrected for extinction, is 60 +/- 50 M-circle dot yr-1, lower than the instantaneous SFR of 390 +/- 90 M-circle dot yr-1 from the total IR luminosity. We also compare the nebular line ratios of HATLASJ1429-0028 with other star-forming and submillimeter 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 +/- 0.16. Such a low value is below the average relation for stellar mass versus metallicity of galaxies at z similar to 1 for a galaxy with a stellar mass of similar to 2x10(11) M-circle dot. The combination of high stellar mass, the lack of AGN indicators, low metallicity, and the high SFR of HATLASJ1429-0028 suggest that this galaxy is currently undergoing a rapid formation.