Radioactive 26Al from the Scorpius-Centaurus Association
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, and the 1809 keV γ-rays from its decay can be
measured with current γ-ray telescopes.
Following earlier 26Al γ-ray mapping with NASA’s
Compton observatory, the spatial emission sky-maps of 26Al for
a component, which could be attributed to ejecta from massive stars in the
Scorpius-Centaurus group of stars, have been tested. 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.
Using data from the SPI spectrometer onboard INTEGRAL, a significant
26Al γ-ray signal has been detected, which can be
associated with the locations of massive stars of the Sco-Cen group.
The observed flux of 6×10−5ph cm−2s−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. The Figure shows the all-sky image (top)
of 26Al γ-ray (1809 keV) as derived from CGRO-Comptel
observations. The γ-ray spectrum of the Sco-Cen region (10°
diameter, lower left panel) shows the 26Al line emission at a
statistical significance of 6σ. The 26Al flux increases
with radius of the region of interest (lower right panel), centered on
l = -10°, b=20°, up to a 10° radius, then remains constant
for larger radii until the emission from the galactic plane is contributing
to the observed signal for r ~ 20°.
The observed flux is plausibly associated with nucleosynthesis from mainly
the youngest subgroup of this stellar association. Indications for bulk
streaming of the hot ISM from this region towards the Sun have been found.
This result demonstrates that the interstellar medium in the Solar vicinity
has been shaped by massive stars very nearby and attributed to this stellar
association.
