ESA Science & Technology - Massive stars in the Cygnus region

The ²⁶Al and ⁶⁰Fe radio-isotopes are thought to be predominantly produced by massive stars at various stages of their evolution. Through stellar winds and supernova explosions, they are released in the interstellar medium (ISM) where they decay over timescales of 10⁶ years. The study of the corresponding nuclear gamma-ray line emission at 1809 keV (²⁶Al) and 1173/1332 keV (⁶⁰Fe) can be a useful probe of massive star evolution and nucleosynthesis.

The Cygnus region harbours a strong concentration of massive stars: about 170 O stars distributed in half a dozen OB associations and a dozen open clusters, all at a distance of just 1-2 kpc from us and collectively referred to as the Cygnus complex. This region was observed by CGRO/COMPTEL to shine at 1809 keV. The flux measured by COMPTEL was found to be 2-3 times higher than predicted from the stellar models available in the late 1990s-early 2000s. Now, the Cygnus complex has been re-evaluated using INTEGRAL/SPI observations and improved stellar models.

About 5 years of INTEGRAL observations were used to derive the spectra of the ²⁶Al (see also related image "Galactic Nucleosynthesis" in the right-hand menu) and ⁶⁰Fe decay lines from Cygnus. The 1809 keV (²⁶Al) flux from Cygnus is ~ 6.0 × 10^-5 ph cm^-2 s^-1, while an upper-limit was derived on the ⁶⁰Fe line flux at 1173/1332 keV of 1.6 × 10^-5 ph cm^-2 s^-1. Yet, when taking into account the Galactic foreground and background contribution in the direction of Cygnus, it turns out that only ~ 60% of the 1809 keV flux, (3.9 ±1.1) × 10^-5 ph cm^-2 s^-1, is attributable to the Cygnus complex itself.

The results have been compared with population synthesis predictions based on recent stellar models including some effects of stellar rotation and a coherent estimate of the contribution of SNIb/c. As shown in the above figure, the observations now agree with the theoretical predictions (the vertical shaded bar indicates the estimated current position along the time axis).

References:
"New estimates of the gamma-ray line emission of the Cygnus region"
P. Martin et al., 2009, A&A, Vol. 506, pp. 703-710

"Predicted gamma-ray line emission from the Cygnus complex"
P. Martin et al., 2009, accepted by A&A, http://arxiv.org/abs/1001.1522