Detection of Wolf-Rayet stars in host galaxies of gamma-ray bursts (GRBs): are GRBs produced by runaway massive stars ejected from high stellar density regions?

Abstract

We have obtained deep spectroscopic observations of several nearby gamma-ray burst (GRB) host galaxies revealing for the first time the presence of Wolf-Rayet (WR) stars and numerous O stars located in rich and compact clusters or star forming regions. Surprisingly, high spatial resolution imaging shows that the GRBs and the associated supernovae did not occur in these regions, but several hundreds of parsec away. Considering various scenarios for GRB progenitors, we do not find any simple explanation of why they should be preferentially born in regions with low stellar densities. All the examined GRBs and associated SNe have occurred 400 to 800 pc from very high density stellar environments including large numbers of WR stars. Such distances can be travelled through at velocities of 100 km s −1 or larger, assuming the travel time to be the typical life time of WR stars. It leads us to suggest that GRB progenitors may be runaway massive stars ejected from compact massive star clusters. The ejection from such super star clusters may lead to a spin-up of these stars, producing the loss of the hydrogen and/or helium envelopes leading to the origin of the type Ibc supernovae associated with GRBs. If this scenario applies to all GRBs, it provides a natural explanation of the very small fraction of massive stars that emit a GRB at the end of their life. An alternative to this scenario could be a binary origin for GRBs, but this still requires an explanation of why it would preferentially occur in low stellar density regions.

Binary star progenitors of long gamma-ray bursts

“…Then the direction of motion of the runaway GRB progenitor must be orthogonal to the line of sight, allowing the progenitor, for the given space velocity, to obtain the maximum possible apparent separation from its formation region. The finding of Hammer et al (2006), that the nearest three long gammaray bursts may be due to runaway stars is in remarkable agreement with our scenario. While the collapsar progenitor in our binary model travels only 200 pc before it dies, compared to the 400...800 pc deduced by Hammer et al (2006), binary evolution resulting in higher runaway velocities are certainly possible (Petrovic et al 2005a).…”

“…The finding of Hammer et al (2006), that the nearest three long gammaray bursts may be due to runaway stars is in remarkable agreement with our scenario. While the collapsar progenitor in our binary model travels only 200 pc before it dies, compared to the 400...800 pc deduced by Hammer et al (2006), binary evolution resulting in higher runaway velocities are certainly possible (Petrovic et al 2005a). It remains to be analyzed whether the runaway scenario is compatible with the finding that long GRBs are more concentrated in the brightest regions of their host galaxies than core collapse supernovae (Fruchter et al 2006).…”

Two type Ic supernovae in low-metallicity, dwarf galaxies: diversity of explosions

“…Christensen et al (2008) have performed the most comprehensive study of the host galaxy of GRB 980425/SN 1998bw to date, using VIMOS integral field spectroscopy to produce a PP04 abundance map of the galaxy (ESO 184-G82). They find that of all regions in the galaxy the WR region located at a projected distance 0.8 Kpc North-West of the GRB/SN site (originally defined by Hammer et al 2006) has the lowest oxygen abundance (12 + log(O/H) PP04 = 8.16 ± 0.14) and that the GRB/SN region has the second lowest abundance (12 + log(O/H) PP04 = 8.30 ± 0.14). We use the abundance measured at the SN/GRB site to plot the host galaxy of GRB 980425/SN 1998bw in Fig.…”

Stellar Forensics with the Supernova‐GRB Connection

“…of the massive SN and GRB progenitor stars (≤ 10 million years for 20 M star, Woosley et al 2002), we do not expect them to move far from their birth HII region sites (but see Hammer et al 2006;Eldridge et al 2011 and below) and thus, we take the abundance of the HII region at the SN site to indicate the natal metallicty of the SN or GRB progenitor. In one GRB case, where there is an independent metallicity measurement from absorption-line ratios in the Xray spectra from the circumburst medium of SN 2006aj/XRF 060218 ) and the common nebular oxygenabundance measurement (e.g., Modjaz et al 2006), the two completely independently derived values are in broad agreement.…”