Picture of the Month

March 2010

Massive Stars in the Cygnus Region

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).

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