Radioactive ²⁶Al from the Scorpius-Centaurus association

¹ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching Germany
e-mail: rod@mpe.mpg.de
² Universitätssternwarte, Scheinerstr. 1, 81679 München, Germany
³ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China
⁴ Excellence-Cluster “Origin and Structure of the Universe”, 85748 Garching, Germany
⁵ Centre d’Étude Spatiale des Rayonnements, BP 4346, 31028 Toulouse Cedex 4, France

Received: 22 February 2010
Accepted: 18 July 2010

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 τ ~ 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 5σ 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.