Picture of the Month

March 2010

Massive Stars in the Cygnus Region

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 ISM where they decay over Myr timescales. 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 nearby Cygnus region harbours a strong concentration of massive stars: about 170 O stars distributed in half a dozen associations, thereafter called the Cygnus complex. It 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 POM June 2009) and 60Fe decay lines from Cygnus. The 1809 keV (26Al) flux from Cygnus is ~ 6.0 x 10-5 ph/cm2/s, while we derived an upper-limit on the 60Fe line flux at 1173/1332 keV of 1.6 x 10-5 ph/cm2/s. 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) x 10-5 ph/cm2/s, 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 Figure, the observations now agree with the theoretical predictions (the vertical shaded bar indicates our estimated current position along the time axis).

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