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We analyse high-resolution near-UV and optical spectra of the afterglow ofGRB 080310, obtained with the Very Large Telescope Ultraviolet and VisualEchelle Spectrograph (VLT/UVES), to investigate the circumburst environment andthe interstellar medium of the gamma-ray burst (GRB) host galaxy. The VLTrapid-response mode (RRM) enabled the observations to start only 13 minutesafter the Swift trigger and a series of four exposures to be collected beforedawn. A low neutral-hydrogen column-density (log N (HI) = 18.7) is measured atthe host-galaxy redshift of z = 2.42743. At this redshift, we also detect alarge number of resonance ground-state absorption lines (e.g., CII, MgII, AlII,SiII, CrII, CIV, SiIV), as well as time-varying absorption from thefine-structure levels of FeII. Time-varying absorption from a highly excitedFeIII energy level (7S3), giving rise to the so-called UV34 line triplet, isalso detected, for the first time in a GRB afterglow. The CrII ground-state andall observed FeII energy levels are found to depopulate with time, whilst theFeIII 7S3 level is increasingly populated. This absorption-line variability isclear evidence of ionization by the GRB, which is for the first timeconclusively observed in a GRB afterglow spectrum. We derive ionic columndensities at each epoch of observations by fitting absorption lines with afour-component Voigt-profile model. We perform CLOUDY photo-ionizationmodelling of the expected pre-burst ionic column densities, to estimate that,before the onset of the burst, [C/H] = -1.3 \pm 0.2, [O/H] < -0.8, [Si/H] =-1.2 \pm 0.2, [Cr/H] = +0.7 \pm 0.2, and [Fe/H] = +0.2 \pm 0.2 for theintegrated line profile, indicating strong overabundances of iron and chromium.For one of the components, we observe even more extreme ratios of [Si/Fe] \leq-1.47 and [C/Fe] \leq -1.74. [abridged]

astronomy and astrophysics

Rapid-response mode VLT/UVES spectroscopy of super iron-rich gas exposed  to GRB 080310. Evidence of ionization in action and episodic star formation  in the host