Radioactive²⁶Al 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. 26 Al 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 26 Al observations with INTEGRAL's γ-ray spectrometer SPI. Methods. Following earlier 26 Al γ-ray mapping with NASA's Compton observatory, we test spatial emission skymaps of 26 Al 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 26 Al emission associated with Scorpius-Centaurus, in spite of the strong underlying nucleosynthesis signal from the Galaxy at large. Results. We find an 26 Al 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 26 Al. This traces the nucleosynthesis ejecta of several massive stars within the past several million years. Conclusions. We confirm through direct detection of radioactive 26 Al the recent ejection of massive-star nucleosynthesis products from the Sco-Cen association. Its youngest subgroup in Upper Scorpius appears to dominate 26 Al contributions from this association. Our 26 Al 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 26 Al and compare this to other indications of Scorpius-Centaurus massive-star activity.

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.…”

“…11, and in specific, localized regions (see below). In the inner Galactic ridge, a flux at 1808.63 keV of (2.63 ±0.2) 10 −4 ph cm −2 s −1 rad −1 has been derived [28]. The total mass of 26 Al in the Galaxy had been inferred from earlier COMP-TEL measurements to be 2-3 M [101].…”

“…18), and provide a more specific test for massive-star models than can be obtained from Galaxy-wide analysis as discussed above. With deeper exposure, this became possible in the late INTEGRAL mission for the Cygnus [85], Carina [140], and Scorpius-Centaurus [28] regions; for other candidate locations of massivestar groups INTEGRAL's sensitivity is insufficient, due to their fainter 26 Al emission.…”

“…Similar to the Orion region, this promises that even modest spatial telescope resolution such as from SPI (2.7 • ) could reveal 26 Al emission displaced from its sources, and thus teach us about ejecta flows. With SPI, 26 Al emission from the direction overlapping the current location of the Upper-Sco group of stars could be discriminated against the large-scale Galactic 26 Al emission, due to its favorable location about 20 • above the plane of the Galaxy [28]. The 26 Al ejected from the Scorpius-Centaurus stars should in fact be distributed over a large region on the sky.…”