INTEGRAL Picture Of the Month
January 2017

INTEGRAL POM
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The SPI 26Al spectrum from the Orion region

The Orion OB1 association contains the largest groups of young massive stars in the vicinity of the Solar system. Within a distance of 400 pc in the direction of Orion, the stellar populations are well known, which allows precise estimates of their energy content and mass flows into interstellar space. Massive stars are producing heavy elements, eject them during their wind phases and supernovae, and thereby enrich the interstellar medium. A special tracer of the history of massive star actions in such stellar groups is the radioactive isotope 26Al which is decaying after a characteristic lifetime of about one million years. The decay gamma-rays from 26Al at 1809 keV can be measured with the gamma-ray spectrometer SPI aboard INTEGRAL (see INTEGRAL POM March 2015, January 2014, April 2013, March, October & December 2010, June 2009, January 2006, May 2004, July 2003 ).

The predecessor gamma-ray telescope COMPTEL aboard the NASA satellite CGRO could reveal the large scale structure of 1809 keV emission in the Milky Way (top left panel), showing that the 26Al ejecta appear clustered at groups of massive stars and spiral arms. Many massive star groups could be identified, but discriminating them from the dominant Galactic plane emission is difficult. This is because the morphology is following the interstellar conditions, the age of the massive star (sub)-groups and the decay time of 26Al, and the number and distribution of stars and how they eject their nucleosynthesis products. The Orion OB1 association, however, can easily be distinguished from Galactic background as its position in the sky is off the Galactic plane and its distance to the Sun makes it possible to follow the path of 26Al ejecta.

Million years ago, winds and supernovae in the Orion OB1a subgroup blew the Eridanus cavity, whose boundary is a dense shell of atomic hydrogen (HI, blue band in the top right panel; also shown is the constellation of Orion as white dots) and in which hot X-ray gas (red) and 26Al (green) accumulates. The bulk of 26Al is found at a distance away from the star groups (lower right panel) as expected from the ejecta velocities and the decreasing age of the a, b, and c subgroups. Testing for emission with the high spectral resolution of SPI exactly at the position of the stellar associations (black circles in the top right panel) reveals no significant signals, but 5°-15° further away, corresponding to 100-300 pc in the direction of the Sun, an excess at photon energies of 1809.16 keV is seen in the gamma-ray spectrum (red hatched area indicating the Doppler-shift in the lower left panel with respect to the lab energy as dashed green line). The flux of the line as determined by a Gaussian line fit (1, 2, 3 sigma confidence bands indicated by blue and grey areas) is 3.7 × 10-5 ph cm-2 s-1 which corresponds to a mass in 26Al nuclei of about 4 × 10-4 M⊙, moving at a speed of 100 km/s towards the Sun. The Doppler-shifted 26Al line from the Orion region supports the scenario for the Galactic-wide 1809 keV emission, being dominated by transport within superbubbles around massive-star groups which extend asymmetrically around ejection sites.

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