Vreeswijk, P. M.P. M.VreeswijkFruchter, A.A.FruchterKaper, L.L.KaperRol, E.E.RolGalama, T. J.T. J.Galamavan Paradijs, J.J.van ParadijsKouveliotou, C.C.KouveliotouWijers, RAMJRAMJWijersPian, E.E.PianPalazzi, E.E.PalazziMasetti, N.N.MasettiFrontera, F.F.FronteraSavaglio, S.S.SavaglioReinsch, K.K.ReinschHessman, Frederic V.Frederic V.HessmanBeuermann, KlausKlausBeuermannNicklas, H.H.Nicklasvan den Heuvel, E.E.van den Heuvel2018-11-072018-11-072001https://resolver.sub.uni-goettingen.de/purl?gro-2/30489We present time-resolved optical spectroscopy of the afterglows of the gamma-ray bursts GRB 990510 and GRB 990712. Through the identification of several absorption lines in the first-epoch GRB 990510 spectrum, we determine the redshift for this burst at z greater than or equal to 1.619. No clear emission lines are detected. The strength of the Mg I feature is indicative of a dense environment, most likely the host galaxy of GRB 990510. Although the host is extremely faint the GRB afterglow allows us to probe its inter-stellar medium and-in principle-to measure its metallicity. The optical spectrum of GRB 990712 (whose host galaxy is the brightest of the known GRB hosts at cosmological redshifts) shows clear features both in emission and absorption, at a redshift of z = 0.4331 +/- 0.0004. On the basis of several line emission diagnostic diagrams, we conclude that the host galaxy of GRB 990712 is most likely an H II galaxy. We derive an unreddened [O II] star formation rate of 2.7 +/- 0.8 M-. yr(-1). Correcting for the measured extinction intrinsic to the host galaxy (A(v) = 3.4(-1.7)(+2.4)), this value increases to 35(-25)(+178) M-. yr(-1). The [O II] equivalent width, compared to that of field galaxies at z less than or equal to1, also suggests that the host of GRB 990712 is vigorously forming stars. We employ the oxygen and H beta emission-line intensities to estimate the global oxygen abundance for the host of GRB 990712: log (O/H) = 3.7 +/- 0.4, which is slightly below the lowest metallicity one finds in nearby spiral galaxies. For both GRBs we study the time evolution of the adsorption lines, whose equivalent width might be expected to change with time if the burst resides in a dense compact medium. We find no evidence for a significant change in the Mg II width.journal_article10.1086/318308000166460500006