New estimates of the gamma-ray line emission of the Cygnus region from INTEGRAL/SPI observations

Abstract

Context. The Cygnus region harbours a huge complex of massive stars at a distance of 1.0-2.0 kpc from us. About 170 O stars are distributed over several OB associations, among which the Cyg OB2 cluster is by far the largest with about 100-120 O stars. As a consequence of their successive nuclear-burning episodes, these massive stars inject large quantities of radioactive nuclei into the interstellar medium such as 26 Al and 60 Fe. The gamma-ray line signal from the latter is a solid tracer of ongoing nucleosynthesis. Aims. We want to compare the decay emission from the Cygnus region with predictions of recently improved stellar models. As a first step, we establish observational constraints upon the gamma-ray line emission from 26 Al and 60 Fe, with particular emphasis placed on separating the emission due to the Cygnus complex from the foreground and background mean Galactic contributions. Methods. We used the high-resolution gamma-ray spectrometer INTEGRAL/SPI to analyse the 26 Al and 60 Fe decay signal from the Cygnus region. The weak gamma-ray line emissions at 1809 and 1173/1332 keV have been characterised in terms of photometry, spectrometry, and source morphology. Results. The 1809 keV emission from Cygnus is centred on the position of the Cyg OB2 cluster and has a typical size of 9 • or more. The total 1809 keV flux from the Cygnus region is (6.0 ± 1.0) × 10 −5 ph cm −2 s −1 , but the flux really attributable to the Cygnus complex reduces to (3.9 ± 1.1) × 10 −5 ph cm −2 s −1 . The 1809 keV line centroid agrees with expectations considering the direction and distance of the Cygnus complex, and the line width is consistent with typical motions of the interstellar medium. No decay emission from 60 Fe has been observed from the Cygnus region and an upper limit of 1.6 × 10 −5 ph cm −2 s −1 was derived. Conclusions. The determination in a coherent way of the gamma-ray line emission of the clustered OB population of the nearby Cygnus complex will allow a more accurate comparison of observations with theoretical expectations based on the most recent stellar models.

26Al kinematics: superbubbles following the spiral arms?

“…At such galactocentric distances, individual 26 Al-emission regions are only a few, they are faint, and they are not associated with spiral arms. Thus, we do not expect large 26 Al velocity asymmetries, in good agreement with the measurements in Cygnus (Martin et al 2009) and Scorpius-Centaurus (Diehl et al 2010). Bagetakos et al (2011).…”

Cosmic Gamma-Ray Spectroscopy

“…Yet, the irregularity of the large-scale emission as seen by COMPTEL from Galactic 26 Al already suggested that the massive-star population in the Galaxy may be more clumpy than such large-scale models describe. The localized 26 Al emission seen from the Cygnus and Sco-Cen regions [84,28] supports this view. Accounting for such localized enhancements, with more-advanced INTEGRAL measurements the Galactic mass of 26 Al has been re-determined as ranging from 1.5 to 3.6 M , considering all uncertainties.…”

“…no bulk motion exceeding tens of km s −1 ) and with instrumental line width (i.e. no excessive Doppler broadening beyond ∼200 km s −1 [84]. Note that 26 Al ejection from Wolf Rayet winds would be with ∼1500 km s −1 , decelerating as circum-stellar gas is swept up.…”

“…Instead, Wolf-Rayet-wind ejected 26 Al from hydrostatic nucleosynthesis may be assumed to dominate, currently originating from Cyg OB2 stars. [71,84]. Accounting for a large-scale galactic background, the contribution from the Cygnus complex is ∼3.9 10 −5 ph cm −2 s −1 [84].…”

“…[71,84]. Accounting for a large-scale galactic background, the contribution from the Cygnus complex is ∼3.9 10 −5 ph cm −2 s −1 [84]. Accumulation of the expected 26 Al production from the stellar census has always shown that 26 Al gamma-rays seemed 2-3 times brighter than predicted.…”