Fine-Structure Fe II and Si II Absorption in the Spectrum of GRB 051111: Implications for the Burst Environment

We present an analysis of fine-structure transitions of Fe II and Si II detected in a high-resolution optical spectrum of the afterglow of GRB051111 (z = 1.54948). The fine-structure absorption features arising from Fe II* to Fe II****, as well as Si II*, are confined to a narrow velocity structure extending over ±30 km s −1 , which we interpret as the burst local environment, most likely a star forming region. We investigate two scenarios for the excitation of the fine-structure levels by collisions with electrons and radiative pumping by an infra-red or ultra-violet radiation field produced by intense star formation in the GRB environment, or by the GRB afterglow itself. We find that the conditions required for collisional excitation of Fe II fine-structure states cannot be easily reconciled with the relatively weak Si II* absorption. Radiative pumping by either IR or UV emission requires > 10 3 massive hot OB stars within a compact star-forming region a few pc in size, and in the case of IR pumping a large dust content. On the other hand, it is possible that the GRB itself provides the source of IR and/or UV radiation, in which case we estimate that the excitation takes place at a distance of � 10 20 pc from the burst. Detailed radiative transfer calculations are required in order to verify that excitation of the low-ionization fine-structure states is possible given the intense UV flux from the burst. Still, it is clear that GRB absorption spectroscopy can provide direct information on the mode and conditions of star formation at high redshift. Subject headings: gamma-rays:bursts — ISM:abundances — ISM:kinematics