We analyse high-resolution near-UV and optical spectra of the afterglow of GRB 080310 , obtained with the Very Large Telescope Ultraviolet and Visual Echelle Spectrograph ( VLT/UVES ) , to investigate the circumburst environment and the interstellar medium of the gamma-ray burst ( GRB ) host galaxy .
The VLT rapid-response mode ( RRM ) enabled the observations to start only 13 minutes after the Swift trigger and a series of four exposures to be collected before dawn .
A low neutral-hydrogen column-density ( log N ( H i ) = 18.7 ) is measured at the host-galaxy redshift of z = 2.42743 .
At this redshift , we also detect a large number of resonance ground-state absorption lines ( e.g. , C ii , Mg ii , Al ii , Si ii , Cr ii , C iv , Si iv ) , as well as time-varying absorption from the fine-structure levels of Fe ii .
Time-varying absorption from a highly excited Fe iii energy level ( ^ { 7 } S _ { 3 } ) , giving rise to the so-called UV 34 line triplet , is also detected , for the first time in a GRB afterglow .
The Cr ii ground-state and all observed Fe ii energy levels are found to depopulate with time , whilst the Fe iii ^ { 7 } S _ { 3 } level is increasingly populated .
This absorption-line variability is clear evidence of ionization by the GRB , which is for the first time conclusively observed in a GRB afterglow spectrum .
We derive ionic column densities at each epoch of observations by fitting absorption lines with a four-component Voigt-profile model .
We perform CLOUDY photo-ionization modelling 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 the integrated 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 .
These peculiar chemical abundances can not easily be explained by current models of supernova yields .
They are indicative of a low dust content , whilst dust destruction may also contribute to the marked Fe and Cr overabundances .
The overall high iron enhancement along the line-of-sight suggests that there has been negligible recent star formation in the host galaxy .
Thus , the occurrence of a GRB indicates that there has been episodes of massive star formation in the GRB region .