Constraints on gamma-ray burst and supernova progenitors through circumstellar absorption lines

¹ Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA, Utrecht, The Netherlands e-mail: [A.vanMarle; N.Langer]@astro.uu.nl
² Instituto de Astronomía-UNAM, APDO Postal 877, Ensenada, 22800 Baja California, Mexico e-mail: ggs@astrosen.unam.mx

Received: 26 May 2005
Accepted: 27 July 2005

Abstract

Long gamma-ray bursts are thought to be caused by a subset of exploding Wolf-Rayet stars. We argue that the circumstellar absorption lines in early supernova and in gamma-ray burst afterglow spectra may allow us to determine the main properties of the Wolf-Rayet star progenitors which can produce those two events. To demonstrate this, we first simulate the hydrodynamic evolution of the circumstellar medium around a 40 star from the creation and evolution of a wind-blown, photo-ionized bubble around the star up to the time of the supernova explosion. Knowledge of density, temperature, and radial velocity of the circumstellar matter as function of space and time allows us to compute the column density in the line of sight to the centre of the nebula, as a function of radial velocity, angle, and time. While without radiative transfer modeling and without detailed knowledge of the spatial distribution of chemical elements we cannot produce spectra, our column density profiles indicate the possible number, strengths, widths, and velocities of absorption line components in supernova and gamma-ray burst afterglow spectra. Our example calculation shows four distinct line features during the Wolf-Rayet stage, at about 0, 50, 150–700, and 2200, with only those of the lowest and highest velocity present at all times. The 150–700 feature decays rapidly as a function of time after the onset of the Wolf-Rayet stage. It consists of a variable number of components, and, especially in its evolved stage, depends strongly on the particular line of sight. A comparison with absorption lines detected in the afterglow of GRB 021004 suggests that the high velocity absorption component in GRB 021004 may be attributed to the free streaming Wolf-Rayet wind, which is consistent with the steep density drop indicated by the afterglow light curve. The presence of the intermediate velocity components implies that the duration of the Wolf-Rayet phase of the progenitor of GRB 021004 was much smaller than the average Wolf-Rayet life time, which strongly constrains its progenitor evolution.