Time-dependent Optical Spectroscopy of GRB 010222: Clues to the Gamma-Ray Burst Environment^*

We present sequential optical spectra of the afterglow of GRB 010222 obtained 1 day apart using the Low-Resolution Imaging Spectrometer (LRIS) and the Echellette Spectrograph and Imager (ESI) on the Keck Telescopes. Three low-ionization absorption systems are spectroscopically identified at z₁ = 1.47688, z₂ = 1.15628, and z₃ = 0.92747. The higher resolution ESI spectrum reveals two distinct components in the highest redshift system at z_1a = 1.47590 and z_1b = 1.47688. We interpret the z_1b = 1.47688 system as an absorption feature of the disk of the host galaxy of GRB 010222. The best-fitted power-law optical continuum and [Zn/Cr] ratio imply low dust content or a local gray dust component near the burst site. In addition, we do not detect strong signatures of vibrationally excited states of H₂. If the gamma-ray burst took place in a superbubble or young stellar cluster, there are no outstanding signatures of an ionized absorber either. Analysis of the spectral time dependence at low resolution shows no significant evidence for absorption-line variability. This lack of variability is confronted with time-dependent photoionization simulations designed to apply the observed flux from GRB 010222 to a variety of assumed atomic gas densities and cloud radii. The absence of time dependence in the absorption lines implies that high-density environments are disfavored. In particular, if the GRB environment was dust free, its density was unlikely to exceed n_{H I} = 10² cm^-3. If depletion of metals onto dust is similar to Galactic values or less than solar abundances are present, then n_{H I} ≥ 2 × 10⁴ cm^-3 is probably ruled out in the immediate vicinity of the burst.