Radioactive 26Al from the Scorpius-Centaurus association

Abstract

Context. The Scorpius-Centaurus association is the most-nearby group of massive and young stars. As nuclear-fusion products are ejected by massive stars and supernovae into the surrounding interstellar medium, the search for characteristic γ-rays from radioactivity is one way to probe the history of activity of such nearby massive stars on a My time scale through their nucleosynthesis. 26Al decays with a radioactivity lifetime t∼1 My, 1809 keV γ-rays from its decay can be measured with current γ-ray telescopes. Aims.We aim to identify nucleosynthesis ejecta from the youngest subgroup of Sco-Cen stars, and interpret their location and bulk motion from 26Al observations with INTEGRAL's γ-ray spectrometer SPI. Methods. Following earlier 26Al γ-ray mapping with NASA's Compton observatory, we test spatial emission skymaps of 26Al for a component which could be attributed to ejecta from massive stars in the Scorpius-Centaurus group of stars. Such a model fit of spatial distributions for large-scale and local components is able to discriminate 26Al emission associated with Scorpius-Centaurus, in spite of the strong underlying nucleosynthesis signal from the Galaxy at large. Results. We find an 26Al γ-ray signal above 5s significance, which we associate with the locations of stars of the Sco-Cen group. The observed flux of 6 × 10-5 ph cm-2 s -1 corresponds to ∼1.1 × 10-4 M⊙ of 26Al. This traces the nucleosynthesis ejecta of several massive stars within the past several million years. Conclusions.We confirm through direct detection of radioactive 26Al the recent ejection of massive-star nucleosynthesis products from the Sco-Cen association. Its youngest subgroup in Upper Scorpius appears to dominate 26Al contributions from this association. Our 26Al signal can be interpreted as a measure of the age and richness of this youngest subgroup. We also estimate a kinematic imprint of these nearby massive-star ejecta from the bulk motion of 26Al and compare this to other indications of Scorpius-Centaurus massive-star activity. © 2010 ESO.