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 z(1) = 1.47688, z(2) = 1.15628, and z(3) = 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 H-2. 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) = 10(2) cm(-3). If depletion of metals onto dust is similar to Galactic values or less than solar abundances are present, then n(H) (I) greater than or equal to 2 x 10(4) cm(-3) is probably ruled out in the immediate vicinity of the burst.