On the Absence of Wolf-rayet Signatures in Grb Afterglow Spectra: Constraints on the Grb Progenitor Environment

We present a detailed study of the circumstellar medium (CSM) around the progenitor stars of longduration γ-ray bursts (GRBs) at redshift z > 1.5. We establish a statistical sample of five GRB spectra for which the data were collected and analyzed with no prior knowledge of the line-of-sight properties, and carry out a systematic search of Wolf-Rayet wind signatures, as represented by blue-shifted, highvelocity (|∆ v| = 1000− 5000 km s) C IV λλ 1548, 1550 absorption doublets (designated as C IV15). We report the detection of a C IV15 absorber at ∆ v ≈ −1500 km s −1 for GRB050730 and none in the rest of the sample. Our search yields an estimate of ≈ 20% for the incidence of C IV15 absorbers with rest-frame absorption equivalent width of EW(C IV 1548) > 0.2 Å from the GRB host galaxies. This is consistent with the incidence of C IV15 around classical damped Lyα absorbers toward quasar sightlines, and suggests that the majority of C IV15 absorbers originate in foreground galaxies. To understand the nature of the detected C IV15 absorbers, we first demonstrate that the presence of associated H I λ 1215, C II λ 1334, or Si II λ 1260 absorption argues against a CSM origin. The absence of fine-structure absorption such as C II* or Si II* further rules out the progenitor-CSM hypothesis, because it indicates negligible UV pumping. The lack of Wolf-Rayet features is consistent with the expectation that the circumburst medium is fully ionized by the afterglow radiation field. The null result therefore constrains the extent of free expanding winds, which we estimate ≤ 30 pc for a homogeneous CSM based on early-time afterglow light-curve observations. We also demonstrate that the survival of the C ions requires (1) the presence of abundant He ions and/or (2) the presence of a clumpy wind for shielding the ionizing photons at ∼ 1 pc. For example, a clumpy wind of single-clump mass Mc ∼ 10 M⊙ and a total number of ∼ 10 clumps is sufficient. Our observations can, therefore, constrain models for the clumpiness of Wolf-Rayet winds surrouding GRB progenitors. Subject headings: gamma rays: bursts—ISM: abundances—ISM: kinematics—intergalactic medium