# Steady-state nucleosynthesis throughout the Galaxy

Roland Diehl111Email<EMAIL_ADDRESS>Martin G.H. Krause222current affiliation:
Centre for Astrophysics Research, University of Hertfordshire, Hertfordshire,
UK Karsten Kretschmer333current affiliation: DLR Oberpfaffenhofen, Germany
Michael Lang444current affiliation: Airbus Defense and Space, Taufkirchen,
Germany Moritz M.M. Pleintinger  Thomas Siegert555current affiliation:
University of California, San Diego, CA, USA Wei Wang666current affiliation:
School of Physics and Technology, Wuhan University, Wuhan, China Max Planck
Institut für extraterrestrische Physik, D-85748 Garching, Germany  Laurent
Bouchet  Pierrick Martin  Institut de Recherche en Astrophysique et
Planétologie, 31028 Toulouse, France

###### Abstract

The measurement and astrophysical interpretation of characteristic
$\gamma$-ray lines from nucleosynthesis was one of the prominent science goals
of the INTEGRAL mission and in particular its spectrometer SPI. Emission from
${}^{26\\!}$Al and from ${}^{60\\!}$Fe decay lines, due to their My decay
times, originates from accumulated ejecta of nucleosynthesis sources, and
appears diffuse in nature. ${}^{26\\!}$Al and ${}^{60\\!}$Fe are believed to
originate mostly from massive star clusters. The radioactive decay
$\gamma$-ray observations open an interesting window to trace the fate and
flow of nucleosynthesis ejecta, after they have left the immediate sources and
their birth sites, and on their path to mix with ambient interstellar gas. The
${}^{26\\!}$Al emission image obtained with INTEGRAL confirms earlier findings
of clumpiness and an extent along the entire plane of the Galaxy, supporting
its origin from massive-star groups. INTEGRAL spectroscopy resolved the line
and found Doppler broadenings and systematic shifts with longitude,
originating from large-scale galactic rotation. But an excess velocity of
$\sim$200 km s-1 suggests that ${}^{26\\!}$Al decays preferentially within
large superbubbles that extend in forward directions between spiral arms. The
detection of ${}^{26\\!}$Al line emission from the nearby Orion clusters in
the Eridanus superbubble supports this interpretation. Positrons from
$\beta^{+}$decays of ${}^{26\\!}$Al and other nucleosynthesis ejecta have been
found to not explain the morphology of positron annihilation $\gamma$-rays at
511 keV that have been measured by INTEGRAL. The ${}^{60\\!}$Fe signal
measured by INTEGRAL is diffuse but too weak for an imaging interpretation, an
origin from point-like/concentrated sources is excluded. The ${}^{26\\!}$Al
/${}^{60\\!}$Fe ratio is constrained to a range 0.2 – 0.4. Beyond improving
precision of these results, diffuse nucleosynthesis contributions from novae
(through 22Na radioactivity) and from past neutron star mergers in our Galaxy
(from r-process radioactivity) are exciting new prospects for the remaining
mission extensions.

###### keywords:

nucleosynthesis – interstellar medium – massive stars – supernovae – novae –
spectroscopy – telescopes: gamma rays

††journal: New Astronomy Reviews

##  1 The 26Al isotope

Radioactive decay of the ${}^{26\\!}$Al nucleus (see Fig. 1) provides a
special opportunity to astrophysics, as this unstable isotope is unusually
long-lived ($\tau$=1.04 My) among the abundantly-produced lighter nuclei up to
the iron group. This long radioactive lifetime of ${}^{26\\!}$Al is due to
special nuclear levels arising from its odd-odd composition of 13 protons and
13 neutrons, leading to a high threshold for $\beta$ transitions from this
particular shell configuration of neutrons and protons. The ground state of
${}^{26\\!}$Al is an (unusually-high) $5+$ spin state, while lower-lying
states of the neighbouring isotope 26Mg have states $2+$ and $0+$, so that a
rather large change of angular momentum must be carried by radioactive-decay
secondaries. This causes the large $\beta$-decay lifetime of ${}^{26\\!}$Al .
Excited states of ${}^{26\\!}$Al at energies 228, 417, and 1058 keV with $0+$
and $3+$ spins are more favourable for decay due to their smaller angular
momentum differences to the 26Mg states. This leads to a lifetime of 9.15 s
for the $0+$ _metastable_ state 26mAl, which could be thermally excited in
nucleosynthesis sites exceeding a few 108K. ${}^{26\\!}$Al thus may decay via
this state without any $\gamma$-ray emission.

${}^{26\\!}$Al is an excellent and promising tracer for stellar
nucleosynthesis, as it survives long enough after production inside a star or
explosion to have a good chance to decay only after being ejected into the
interstellar medium. Additionally, it serves as a thermometer for its
production environment, hence also provides astrophysical diagnostics for its
production site. Moreover, due to the long radioactive lifetime, decay and
$\gamma$-ray emission occur millions of years after nucleosynthesis, thus
tracing the fate and flow of ejecta after supernova explosions. Note that
individual supernova remnants can only be observed in radio or X rays up to
ages of several 10 ky after a supernova explosion.

Figure 1: Radioactive-decay schemes for ${}^{26\\!}$Al (top) and
${}^{60\\!}$Fe (bottom). The ${}^{26\\!}$Al nucleus ground state has a long
radioactive lifetime, due to the large spin difference of its state to lower-
lying states of the daughter nucleus 26Mg. An isomeric excited state of
${}^{26\\!}$Al exists at 228 keV excitation energy. If thermally excited,
${}^{26\\!}$Al may decay through this state. Secondary products, lifetime, and
radioactive energy available for deposits and observation depend on the
environment. The ${}^{60\\!}$Fe isotope decays with a radioactive lifetime of
3.8 My through 60Co to 60Ni. Note that per decay, two $\gamma$-ray photons are
obtained.

Candidate cosmic sources of ${}^{26\\!}$Al are all environments of H burning,
as pre-existing 25Mg converts to ${}^{26\\!}$Al through the
25Mg(p,$\gamma$)26Al reaction. Nova explosions first come to mind, being
explosive hydrogen burning sites on the surface of a white dwarf resulting
from stellar evolution and hence likely including 25Mg (or else, producing it
from more-abundant 24Mg through hydrogen burning, 24Mg(p,$\gamma$)25Mg ).
Novae thus were considered the prime candidate for ${}^{26\\!}$Al production
[1]. In principle, hydrogen burning occurs in all stars during the main
sequence. Stars of high masses above 10–25 $M_{\odot}$ evolve rather rapidly,
within $\sim$10 My through the entire main sequence [2]. Thus, a significant
part of the ${}^{26\\!}$Al produced in this phase will still be alive while
the star enters the giant phase with core helium burning and shell hydrogen
burning. This is accompanied with substantial structural changes of the star,
leading to significant outward mixing of hydrogen-burning ashes into the
envelope, and also to mass loss with strong stellar winds. Thus,
${}^{26\\!}$Al may be carried away with the wind, in particular when a star
enters the Wolf-Rayet phase [3, 4], which is thought to occur for stars with
masses above $\sim$25 $M_{\odot}$ at solar metallicity. Such massive stars
then end their evolution soon after the giant transition as core-collapses,
likely producing supernova explosions, at least for masses below $\sim$40
$M_{\odot}$. The supernova now will eject the envelope, including all
${}^{26\\!}$Al that may still reside therein, and enhanced by some additional
${}^{26\\!}$Al nucleosynthesis from explosive burning as the supernova shock
rushed through the star. Also massive stars that are below the minimum mass to
evolve towards a core-collapse supernova, i.e. AGB stars of the mass range 3–8
$M_{\odot}$ have been discussed as candidate ${}^{26\\!}$Al sources [5, 6, 7].
Here, in particular, nuclear burning at the hot inner edge of the hydrogen-
burning shell provides a suitable environment for ${}^{26\\!}$Al synthesis.
Massive stars in general, as well as their supernovae, are thus plausible
sources of ${}^{26\\!}$Al , from an extended phase during their evolution, and
eventually from the terminal core-collapse supernova. Predictions were derived
from theoretical modeling for the entire ${}^{26\\!}$Al amount present in the
Galaxy from each of these source types. Supernovae from core collapse of
massive stars were thought to contribute 2.1$\pm$1.1 $M_{\odot}$ [8]. The
earlier Wolf-Rayet phase was estimated to contribute a galactic total of 0.5
$M_{\odot}$ with a factor $\sim$3 uncertainty [see 9, and references therein].
Models for these two candidate sources were best established, while models for
novae and AGB stars are more uncertain, by comparison. Estimates for
${}^{26\\!}$Al from novae range from 0.1 to 5 $M_{\odot}$, with large
uncertainties, mainly from a lack of progenitor knowledge, and from lacking
still an also quantitatively-realistic nova model. From more-recent models,
maybe 0.2 $M_{\odot}$ of Galactic ${}^{26\\!}$Al may be due to classical
novae, with again a factor $\sim$3 uncertainty [see 10, for a description of
nova nucleosynthesis]. The AGB star contribution is now estimated [11] to be
below 10% of that of massive stars. Note that both AGB stars and novae are
clearly identified as ${}^{26\\!}$Al producers from interstellar-grain
inclusions in meteorites [see 12, for a review]. Both these two types of
sources are copious producers of dust grains, more so than supernovae or WR
stars; interstellar-grain samples are biased towards measuring ${}^{26\\!}$Al
from such dust-producing sources.

###  1.1 Pre-INTEGRAL measurements of ${}^{26\\!}$Al

The first report of a $\gamma$-ray line from a radioactive nucleus of cosmic
origin arose from the HEAO-C mission 1979/1980 measurements. This satellite
carried a Ge detector with high spectral resolution, so that a line at 1809
keV could plausibly be attributed it to decay of live ${}^{26\\!}$Al in the
Galaxy’s interstellar medium [13]. ${}^{26\\!}$Al radioactive decay within 1
My requires a source within the past few million years, which is a rather
brief recent time span in the Galaxy’s history, which spans 12 Gy or more.
Therefore this is proof of currently-ongoing nucleosynthesis, observed most-
directly through the characteristic $\gamma$-ray emission that is a
consequence of the decay of this radioactive nucleus777Cosmic-ray activation
of this nuclear line in 26Mg would also produce other, brighter, lines from
more abundant species, which are not observed..

Figure 2: _Top:_ The ${}^{26\\!}$Al sky as deconvolved from the COMPTEL data,
using a maximum-entropy method [14]. _Bottom:_ The ${}^{26\\!}$Al sky as
deconvolved from INTEGRAL’s sky survey from ten years of data with the SPI
imaging spectrometer, also using the maximum-entropy imaging method [15].

The COMPTEL sky survey, accumulated over nine years, then provided a sky image
in the ${}^{26\\!}$Al $\gamma$-ray line [16, 17, 14] (Fig. 2, top). This
effectively is a map of nucleosynthesis activity in our current Galaxy, with
an effective exposure of the recent few million years due to ${}^{26\\!}$Al
decay. The ${}^{26\\!}$Al maps that were obtained from COMPTEL event
measurements through different varieties of iterative deconvolution methods
all showed structured ${}^{26\\!}$Al emission, though in detail at different
degrees depending on methods and sky exposure. All images presented extended
along the plane of the Galaxy [17, 14, 18, 19, 20], in broad agreement with
earlier expectations of ${}^{26\\!}$Al being produced throughout the Galaxy
and mostly from massive stars and their supernovae [21, 22].

The irregular distribution of ${}^{26\\!}$Al emission all along the plane of
the Galaxy provided a main argument for the consensus that massive stars
dominate the production of ${}^{26\\!}$Al [9]. Massive stars have been
recognised to preferentially occur in clusters [23, 24]. The star formation
regions also would be expected to be located near spiral arms, where
hierarchical gravitational collapse and turbulence are efficient in creating
new stars, and feedback from massive stars shapes surrounding gas [25, 26].
This plausibly explains structured emission along the plane of the Galaxy.
Prominent more-nearby massive-star regions appear particularly bright in
${}^{26\\!}$Al emission, such as the Cygnus region (see below), which appears
in all image variants. Compared to this, ${}^{26\\!}$Al ejections from novae
or intermediate-mass AGB stars would appear with a much smoother spatial
distribution [1, 5], due to the much larger number of individually-
contributing sources with their individually-lower ${}^{26\\!}$Al yields.

26Al emission from prominent and particularly-bright regions had been
discussed in theoretical work. The science goal for such individual source
regions was to make use of the knowledge about the stellar content, in
particular the age estimates that can be obtained from the census of the
remaining population of (high mass) stars that can still be observed, i.e.
that had not already ended their evolution in a gravitational collapse.
Population synthesis studies and their astrophysical potential in the context
of ${}^{26\\!}$Al measurements had been first discussed in detail by Cervino
et al. (2001), and later by Voss et al. (2009). The first convincing
association of a ${}^{26\\!}$Al $\gamma$-ray signal with a specific massive-
star region was presented for the Cygnus region, based on COMPTEL data [27,
28, 29]. The Cygnus OB2 association in particular was recognised as one of the
most-massive star groups in our Galaxy (Knoedlseder et al. 1999).
Interpretations of ${}^{26\\!}$Al emission in the region along the entire line
of sight towards Cygnus include about 9 stellar associations along the line of
sight and from cavities of different sizes [30, 31, 32]. Hints of
${}^{26\\!}$Al emission for other nearby massive-star regions also had been
derived from COMPTEL measurements: Most prominently, emissions were discussed
from the Orion region at $\sim$ 400 pc distance, and the Vela region with
candidate sources at distances from $\sim$ 200 out to 1500 pc [33].

A Galaxy-wide interpretation of the ${}^{26\\!}$Al $\gamma$-ray measurements
is attractive, as the total mass of ${}^{26\\!}$Al seen in observations can be
confronted with theoretical estimates of the amounts due to the entire
Galactic population of sources of a particular type of candidate source. The
population of sources can be estimated from their link to the Galaxy’s star
formation rate, and a spatial distribution that reflects the evolution in its
simplest form. Such analysis needs to resolve the distance uncertainty, when
assigning a measured flux along a line of sight to the brightness of
particular source regions. Localised regions along the line of sight which may
efficiently produce ${}^{26\\!}$Al need to be accounted for properly, using
other astronomical knowledge. The massive star census in the Galaxy is well
known from astronomical measurements of their thermal as well as maser
emission, the former only out to distances of a few kpc. Beyond, many regions
of the Galaxy are occulted for direct measurements. Therefore, one is left
with some uncertainty about their Galaxy-wide distribution. Probably, the
_molecular ring_ around the center of our Galaxy at a radial distance of 3–4
kpc from the center is a prominent birth site for massive stars, as are
molecular clouds swept up along the Galaxy’s spiral arms [34, 26]. The total
amount of ${}^{26\\!}$Al in the Galaxy can be estimated from the measured
$\gamma$-ray flux, using as a plausible assumption for the distances to the
emission regions a galaxy-wide distribution. Based on COMPTEL data, an
${}^{26\\!}$Al amount around 3 $M_{\odot}$ had been advertised [16, 35]. Under
plausible standard assumptions about the stellar population and their
evolution, this converts into a Galactic star formation rate of $\sim$5
$M_{\odot}$ y-1, or a core-collapse supernova rate of 3–4 per century [36].
[For a detailed discussion of such conversions, see 37].

The GRIS balloon experiment, carrying high-resolution Ge detectors, had
provided an indication that the ${}^{26\\!}$Al line was significantly
broadened beyond instrumental resolution to 6.4 keV (FWHM) [38]. This implied
kinematic Doppler broadening of astrophysical origin, with velocities of 540
km s-1. Considering the $1.04$ My decay time of 26Al, such a large line width
would naively translate into kpc-sized cavities around ${}^{26\\!}$Al sources,
or major fractions of ${}^{26\\!}$Al to be condensed on grains that could
retain their momentum in a ballistic flow through tenuous interstellar gas
[39, 40]. COMPTEL’s scintillation $\gamma$-ray detectors lacked the spectral
resolution required for line identification and spectroscopic studies: It had
$\sim$ 200 keV instrumental resolution (FWHM), compared to $\sim$ 3 keV for Ge
detectors, at the energy of the ${}^{26\\!}$Al line. Therefore, another
measurement of the ${}^{26\\!}$Al line at high spectral resolution was also
important to place these tantalizing hints on firmer footing.

###  1.2 INTEGRAL measurements and lessons

The INTEGRAL space observatory with its Ge-detector based spectrometer SPI,
launched in 2002, provided a wealth of high-quality spectroscopic data
accumulated over its long mission lifetime. This allowed a first measurement
of kinematic data about ${}^{26\\!}$Al [41] and a higher-precision study of
the Galaxy-wide ${}^{26\\!}$Al aspects [42, 37] and their role to understand
stellar feedback [43].

An all-sky image had been constructed from 10 years of data [15], and is shown
in Fig. 2 (lower graph) together with the COMPTEL image. The sky exposure
during the INTEGRAL mission was largely concentrated towards the inner Galaxy
and their X-ray binaries, so that any signals and features in the outer Galaxy
would be seen with less significance. Any such image is potentially
susceptible to systematics and uncertainties from the instrument, due to
statistical noise (Poisson) and a response that cannot be directly inverted,
so that forward convolution methods are applied. But due to different imaging
methods and detectors, as well as backgrounds, systematics effects will be
different among instruments, making comparisons a valuable assessment of the
astronomical imaging results. The total background count rate of a coded-mask
instrument is much higher than for the Compton telescopes. On the other hand,
the imaging principle is more straightforward, with a single interaction and
detector type. The ${}^{26\\!}$Al images from INTEGRAL and COMPTEL show
striking similarities, and several features of the COMPTEL-based
${}^{26\\!}$Al image re-appear in the INTEGRAL result (see Fig. 2). In view of
the different ways those images were obtained, we consider as confirmed that
there is diffuse emission all along the plane of the Galaxy, with emission
peaks or hot spots reminiscent of known massive star groups.

####  1.2.1 The mass of ${}^{26\\!}$Al in the Galaxy

Figure 3: The ${}^{26\\!}$Al line as seen with INTEGRAL’s high-resolution
spectrometer SPI and 13 years of measurements integrated [44] (left).
${}^{26\\!}$Al line variation with Galactic longitude[37] (right). This shift
of the line centroid reflects the kinematics of ${}^{26\\!}$Al towards the
inner Galaxy in INTEGRAL/SPI measurements [41].

Figure 4: (Top:) Kinematics of ${}^{26\\!}$Al towards the inner Galaxy, from
INTEGRAL/SPI measurements. This longitude-velocity diagram for hot ISM as
traced through ${}^{26\\!}$Al in the inner Galaxy shows the trend from the
Galaxy’s large-scale rotation. The underlying color plots show the
corresponding kinematics from molecular gas as traced through CO data. The
${}^{26\\!}$Al traced hot gas shows systematically higher velocities by about
200 km s-1 in the direction of Galactic rotation. (Bottom): Scenario for
asymmetric surroundings of ${}^{26\\!}$Al sources. At time of nucleosynthesis
product ejection, massive star groups could be located at the leading edges of
spiral arms, thus presenting more material moving away from spiral arms at
higher velocities than moving ’backwards’ into higher density regions [43].
[41]

The integrated flux of ${}^{26\\!}$Al $\gamma$-rays provides a measurement of
the total mass of ${}^{26\\!}$Al in our Galaxy. INTEGRAL/SPI data were used to
obtain a value of (2.8 $\pm$0.8) $M_{\odot}$ of ${}^{26\\!}$Al [37], in a
study comparing many alternative views of massive star activity in the Galaxy.
A systematic limitation of any galaxy-wide parameter determination occurs from
potential bias of the specific location of the Sun within the Galaxy and with
respect to spiral arms. Gamma rays are at an advantage here due to their
penetrating nature, reaching us also from sources of the distant regions of
our Galaxy. However, there may be sources in the vicinity of our vantage point
in the solar system, 8 kpc from the Galaxy’s center. An account for the more-
nearby sources (discussed in the next subsection) leads to a reduction of the
Galaxy-wide ${}^{26\\!}$Al content. Successively accounting for foreground
sources as they could be discriminated, improved values were determined.
Therefore, the ${}^{26\\!}$Al mass determined as such from the INTEGRAL data
directly range from 1.7$\pm$0.2 $M_{\odot}$[45] to 2.0$\pm$0.3 $M_{\odot}$[46,
47]; the quoted uncertainty includes estimates of residual systematics from
such foreground. Alternatively, a bottom-up modelling of the INTEGRAL-observed
${}^{26\\!}$Al sky from massive-star clusters [48] has shown that this quoted
uncertainty estimate appears realistic. A large-scale simulation had indicated
that the ${}^{26\\!}$Al mass in the Galaxy could be even lower [49]. On the
other hand, other Galactic-scale hydrodynamic simulations with ${}^{26\\!}$Al
sources distributed along spiral arms reproduce the INTEGRAL observations with
a steady-state mass of $\sim$4 $M_{\odot}$, values between 2.5 and 6
$M_{\odot}$ being consistent with these simulations [51]. It remains to be
seen how morphology improvements from imaging as well as simulation work will
refine the role of nearby and intermediate-distance clusters.

With a mass estimate from $gamma$-ray data, we can use the theoretical yield
estimates per mass and integrate over the mass distribution function to obtain
the normalization factor of the Galactic star formation or core-collapse
supernova rate. This is an important independent way to derive this key
quantity for our Galaxy, because it derives from a measured star formation
tracer that is unaffected by occultation and thus galaxy-wide, in contrast to,
e.g.,OB or WR star counts[see 37, 52, for a discussion of measurement
alternatives]. From INTEGRAL/SPI ${}^{26\\!}$Al measurements, a value of
1.9$\pm$1.1 SN century-1 has been estimated [37]. Correspondingly scaled down
due to the above-discussed foreground sources, this obtains 1.3$\pm$0.4 SN
century-1 [47] .

####  1.2.2 ${}^{26\\!}$Al decay within superbubbles

One of the main achievements in INTEGRAL/SPI ${}^{26\\!}$Al measurements is
the high-resolution spectroscopy of the ${}^{26\\!}$Al line, which resolves
the line and allows to analyze its shape (see Fig. 3). An astrophysical
broadening of the all-sky integrated ${}^{26\\!}$Al line was discovered, with
a value of 1.4 keV ($\pm$0.3 keV). This is much smaller than what had been
reported from the balloon instrument data with a Ge detector [38], and
corresponds to 175 km s-1 ($\pm$45 km s-1) in velocity space. The line width
of the ${}^{26\\!}$Al line as seen from the Galaxy in Fig. 3 (left) reflects
Doppler broadening from the large-scale rotation within the Galaxy, and from
peculiar ${}^{26\\!}$Al motion. As shown in Fig. 3 (right), there is a clear
and systematic trend with longitude, that shows the kinematic signature from
large-scale Galactic rotation. This could be clearly detected [41] with
sufficient INTEGRAL exposure [confirming earlier indications [37]; see
discussion in 53, for how sufficient detail had been accumulated over the
years of the INTEGRAL mission to enable such a measurement]. As shown in the
upper graph of Fig. 4, the bulk velocities seen in the Doppler shifts of the
${}^{26\\!}$Al line as viewed towards different Galactic longitudes turn out
to be on the order of several 100 km s-1. This is a much higher velocity than
commonly found in cold and dense gas, such as the bulk velocities of molecular
clouds traced by carbon monoxide [34], which characterises the large-scale
rotation of objects in the Galaxy in general. Evidently, the velocities
measured from ${}^{26\\!}$Al kinematics exceed typical Galactic rotation
significantly, by up to $\approx$200 km s-1. Clearly, the bulk of massive-star
ejecta as traced by ${}^{26\\!}$Al is not mixed with cold interstellar gas on
a time scale $\approx$106y after their ejection, as otherwise smaller line-of-
sight velocities would be expected.

When we consider that ${}^{26\\!}$Al will stream into the medium surrounding
its sources while decaying, it is clear that the velocity measurement from
${}^{26\\!}$Al will be affected by the conditions in the vicinity of the
source. Ejection from the sources plausibly should be isotropic. Typical
ejection velocities are on the order of 1000 km s-1, both for core-collapse
supernovae [54] and for Wolf-Rayet stellar winds [55]. If ejecta could travel
freely for about a kpc before ${}^{26\\!}$Al decay, such high initial
velocities would still be reflected in the ${}^{26\\!}$Al $\gamma$-ray line.
Structures in the interstellar medium with their characteristic sizes at or
below this scale can therefore constrain ejecta expansion in different
directions, thus making expansion deviate from isotropic.

Based on such ideas, an asymmetry scenario was proposed (see sketch in lower
graph of Fig. 4): Massive stars inside the Galaxy’s co-rotation radius would
be formed within spiral arms, but travel towards the leading edges of spiral
arms while evolving into their Wolf Rayet phases, and in any case before they
explode as supernovae. Then, the ejection of nucleosynthesis ejecta would
occur in a region that is characterised by a density gradient, higher density
surroundings from the spiral arm being located preferentially behind the
massive stars, as seen from an observer in the direction of Galactic
rotation.Massive-star ejecta are quickly thermalised at shocks, and fill the
superbubbles created from earlier massive-star winds and radiation. It is well
known that such superbubbles are asymmetric, being shaped by shear, local
density gradients, and embedded sources [56, 57, 58]. So, ejecta would be
decelerated if streaming towards the spiral arms, while streaming at higher
velocities would be allowed into the lower-density inter-arm regions. Such a
scenario is illustrated in Fig. 4 [43], placing the candidate ${}^{26\\!}$Al
sources along inner spiral arms and preferentially onto their inner ends
approaching the Galaxy’s bar. This results in a longitude-velocity trend as
shown by the green-dashed line in Fig. 4. In these calculations, the pitch
angle of a logarithmic spiral structure model has been fitted (and obtains
values in agreement with common results inferred otherwise), as well as an
ejecta velocity of 200 km s-1, which corresponds to the typical expected sound
velocity in superbubbles. The significant enhancement of apparent velocities
as observed from ${}^{26\\!}$Al therefore suggests that sources of
${}^{26\\!}$Al may be typically surrounded by large interstellar cavities
(i.e., superbubbles). These large cavities could have been left over from
previous stellar generations and their massive-star activity. Or,
alternatively, the winds of the most-massive stars of a massive-star group,
which evolve on shortest time scales, enter their Wolf-Rayet phase with strong
winds only about 3 My after their formation and thus create such a cavity.
Superbubbles of order $\sim$kpc as typical surroundings of massive stars with
ages of few to tens of My then present a new scenario for how new nuclei may
be recycled into the general flow of cosmic gas as part of cosmic chemical
evolution [43]. The sizes of such cavities plausibly extend up to one kpc.
This exceeds the scale height of the molecular and atomic interstellar medium.
Massive star ejecta as traced by ${}^{26\\!}$Al therefore may recycle to the
next generation of stars on a long way over the halo of the Milky Way on
timescales likely exceeding 10 Myrs. Recent galaxy-scale hydrodynamics
simulations with ${}^{26\\!}$Al ejection show that the ${}^{26\\!}$Al scale
height is sensitive to the assumed galaxy structure, clustering of star
formation as well the gas density in the lower halo [49, 50, 51]. With
realistic assumptions, scale heights of about 2 kpc near the solar radius are
found [51], similar to values derived from the INTEGRAL data [50]. The
simulations thus support the interpretation of the INTEGRAL data as tracing
massive-star ejecta through superbubbles into the Galactic halo.

Ejecta may thus be returned into the ambient interstellar medium on time
scales beyond 107 years. INTEGRAL observations of ${}^{26\\!}$Al have shed new
light onto the evolution of enriched gas on such time scales, which are
difficult to constrain through observations otherwise.

Figure 5: (Top:) The ${}^{26\\!}$Al signal measured from the Cygnus region
[59]. (Bottom:) The time history of ${}^{26\\!}$Al production in the Cygnus
complex, as predicted from population synthesis, is compared to INTEGRAL’s
early $\gamma$-ray flux measurements[60]. Expectations from such populations
synthesis are on the low side of observed ${}^{26\\!}$Al $\gamma$-rays, in
particular as the lower metallicity compared to solar for the Cygnus region is
adopted. The horizontal shaded area presents the range given by the
${}^{26\\!}$Al $\gamma$-ray data, the dashed lines bracket the uncertainty
range of predictions from recent massive-star models through population
synthesis, and the vertical shaded area indicates the current time (from
adopted cluster ages; the impact of stellar rotation on age estimates
determines the width of the shaded area). [Figure adapted from 60].

####  1.2.3 Specific regions and massive-star groups

A detailed test of our understanding of massive-star activity can be made in
specific, nearby, and well-constrained massive-star groups, i.e., for which
the formation of stars likely is coeval (i.e., due to one single star
formation epoch), and the stellar census as well as other astronomical
constraints are available.

The Gould Belt has been recognised as a region of local and nearby stellar
associations arranged in an elliptical-belt-like structure[61, 62, 63, 64,
65]. Several associations believed to be Gould Belt members also appear to be
aligned with ${}^{26\\!}$Al emission peaks. Interesting results on
${}^{26\\!}$Al from these regions were obtained with INTEGRAL for the Cygnus
[66, 45], Orion [59], Vela [67], Carina [68], and Scorpius-Centaurus [46]
groups of massive stars.

The Cygnus region [69] has been prominent as an ${}^{26\\!}$Al source even in
first images of ${}^{26\\!}$Al gamma rays [70]. A detailed investigation of
the Cygnus region and its ${}^{26\\!}$Al sources was provided from a PhD
thesis [31] based on COMPTEL observations, following earlier work [27].
INTEGRAL observations confirmed the prominent ${}^{26\\!}$Al emission from the
Cygnus region [66, 60], with indications of a slightly-broadened
${}^{26\\!}$Al line pointing to ${}^{26\\!}$Al decay from a hot medium
(Doppler broadening). Fig. 5 (left) shows the ${}^{26\\!}$Al spectrum after 13
years of cumulative exposure [44].

Figure 6: (Top) A sketch of a projection onto the Galactic plane of the region
between us and the molecular clouds in Orion at about 450 pc distance, with
the OB1 stellar association and its subgroups on the near side of the clouds,
and the Eridanus cavity extending from the clouds towards the Sun [from 71]. A
scenario of ${}^{26\\!}$Al distribution from ejecta of the Orion OB1
association, blown into the Eridanus cavity, is indicated. (Bottom) The
predicted time dependence of ejections of kinetic energy (above) and
${}^{26\\!}$Al (below), from population synthesis of the Orion OB1 association
and its subgroups [72].

Several star clusters at distances between 800 and 1500 pc were identified as
candidate sources [32, 73, 28, 74]. A population synthesis approach made use
of the astronomical knowledge about these clusters, and derived predictions
for the expected ${}^{26\\!}$Al emission, as well as the expected ionising
luminosity from starlight, and kinetic energy injection from massive stars and
supernovae that would lead to swept-up shells and cavities [75, 76]. Applying
this to the Cygnus region [74, 77, 45, 60], it became clear that age and
metallicity of the major sources would drive the observed ${}^{26\\!}$Al
emission (see Fig. 5, right), and that Cyg OB2 was revealed to be a more-
massive star cluster than had been thought before, as it is hidden behind
molecular gas occulting starlight from its massive-star members.

Figure 7: The ${}^{26\\!}$Al signal disentangled from the Orion region (above;
[44]), and from the Scorpius-Centaurus region (below; [46, 44]). In the Orion-
region spectrum, the hatched region indicates the uncertainty of the measured
line centroid; it is slightly offset (1.1 $\sigma$) from the ${}^{26\\!}$Al
laboratory energy of 1808.63 keV (vertical green-dashed line).

Figure 8: An illustration of the scenario of successive erosion of star-
forming molecular clouds by stellar feedback [58].

The Orion region is located towards the outer Galaxy, and holds one of the
most-nearby massive star clusters, at a distance of $\approx$450 pc (see Fig.
6, top). The Orion OB1 cluster subgroups have some spread in ages, between 1.5
and 15 My [78], which may indicate sequential, or even triggered, star
formation within the former giant molecular cloud that is now visible as Orion
A and Orion B. Interestingly, atomic HI data as well as X-ray data have
identified a rather large cavity in the foreground of these sources [79],
extending towards the location of the Sun (see sketch in upper part of Fig.
6). From the different subgroup ages, population synthesis here predicts a
rather flat time profile of ${}^{26\\!}$Al emission (see Fig. 6, bottom), even
rising in future [72]. ${}^{26\\!}$Al emission from Orion had first been
reported from COMPTEL data [80]. Cumulative INTEGRAL observations confirmed
Orion as a source of ${}^{26\\!}$Al emission [59], see Fig. 7(right).

The Orion region with its foreground cavity provides an interesting nearby
setting reminiscent to the superbubble scenario discussed above as an
explanation of the large-scale Galactic Doppler shifts of the ${}^{26\\!}$Al
line (see Fig. 4). In this case, a single cavity and cluster source set a
unique geometry, as shown in Fig. 6. Simulations of the interstellar medium as
it faces massive-star activity had demonstrated how one could expect to
observe erosion of the parental molecular clouds and production of an outflow
of hot gas [81]. It is interesting that the INTEGRAL measurement at high
spectral resolution provides an indication for a blue shift of the
${}^{26\\!}$Al (see Fig. 7, upper graph), as it is expected from the above
scenario and this particular geometry.

The Scorpius-Centaurus massive-star groups are even closer, at distances of
120–160 pc only [82]. Due to its proximity, the stars as well as the swept-up
cavity interiors and walls are spread over rather large regions of the sky.
This makes astronomical observations difficult for telescopes with modest
fields of view, and thus led to debates about memberships of stars [83, 84,
85]. ${}^{26\\!}$Al emission from this region appeared only weakly indicated
in the COMPTEL image. INTEGRAL measurements, however, allowed to discriminate
${}^{26\\!}$Al $\gamma$-ray emission from the Sco-Cen star clusters against
the bright ${}^{26\\!}$Al emission from the Galactic plane, see Fig. 7(lower
graph) [46]. We conducted a multi-wavelength effort involving latest radio
data from atomic hydrogen [86], as well as interpretations of Planck and
Herschel data associated with star forming activity of the Scorpius-Centaurus
region [87], and simulations of the interstellar medium in 3D hydro. From
this, we developed an understanding of how star formation may proceed
successively in and around a parental molecular cloud [58]. The scenario is
illustrated in Fig. 8, and shows that hot gas flowing out of a molecular cloud
due to massive-star activity (as seen in Orion) would surround the cloud
remains, and squash these through converging flows and shocks, thus triggering
new star formation at different locations within the same cloud. INTEGRAL
measurements of ${}^{26\\!}$Al gamma rays have contributed significantly to
establish this picture, which revises and modifies what was often discussed as
_triggered star formation_ [88, 89, 90].

Figure 9: The ${}^{26\\!}$Al image with contours overlaid from a best-fitting
model reflecting the spatial morphology of the $\gamma$-ray emission from
positron annihilation.

###  1.3 Connections to INTEGRAL e+ annihilation results

With INTEGRAL’s energy range also including emission from positron
annihilation near 500 keV and down to 20 keV, the positrons emitted from
$\beta^{+}$ decays of radioactive by-products of nucleosynthesis (see Fig. 1,
left) can be associated with the diffuse $\gamma$-ray results of
${}^{26\\!}$Al [44]. Note that other isotopes such as 56Ni, 44Ti, and 22Na
also contribute to such positrons from nucleosynthesis. As the radioactivity
in this case would be more short-lived, such contributions may be less of a
diffuse nature and could be localized to their source regions more than from
${}^{26\\!}$Al positrons. Therefore we focus our discussion on ${}^{26\\!}$Al
connections. (See also the special article on positron annihilation $\gamma$
rays, also included in this Volume [91].)

From the ${}^{26\\!}$Al observations, one may use the decay scheme physics
combined with the measured ${}^{26\\!}$Al $\gamma$-ray flux and an assumption
about positron annihilation efficiency to estimate the sky brightness in 511
keV $\gamma$-rays from ${}^{26\\!}$Al positrons alone. This led to a
contribution estimate of order 10% of ${}^{26\\!}$Al to the total observed
Galactic $\gamma$-ray emission from positron annihilation[44, 92]. Adding
contributions from other nucleosynthesis of $\beta^{+}$-decaying isotopes, it
was shown that the combined nucleosynthesis contribution to positron
annihilation $\gamma$-ray emission would be at odds with the Galactic
longitude and latitude profiles of the positron annihilation $\gamma$-ray
emission as observed by INTEGRAL. Fig. 9 illustrates a comparison of the
$\gamma$-ray emission morphologies for ${}^{26\\!}$Al emission and for
positron annihilation emission as seen by COMPTEL and INTEGRAL, respectively
[from 44] [see also 93, 94].

##  2 The 60Fe isotope

60Fe is a prominent neutron-rich isotope of Fe, which is radioactive with a
decay time of 3.61 My [95, 96]. The characteristic production reactions are
neutron captures on abundant stable 54Fe to 56Fe nuclei; thus ${}^{60\\!}$Fe
is produced by an s process reaction flow, allowing for (only some!)
intermediate $\beta$ decay as successive neutrons are captured by the nucleus.
This keeps the reaction path close to the region of stable isotopes, as is
characteristic for the s process. But 60Fe is two mass units away from this
region, hence requires a suitably-long $\beta$-decay lifetime of intermediate
nucleus 57Fe, and/or conditions that may involve higher neutron irradiation
intensities, such as characteristic for an $i$\- or r process. Favourable
production environments of ${}^{60\\!}$Fe are found in the helium-burning
zones inside the more-massive stars above 6–8 $M_{\odot}$, as here the
22Ne($\alpha$,n)25Mg reaction liberates neutrons [97, 98]. Also in rare
thermonuclear explosions, ${}^{60\\!}$Fe could be produced [99], although
massive-stars have been considered more plausible [8]. Being sensitive to
mixing and temperature structure in a massive star, the delicate production
process of ${}^{60\\!}$Fe may tell us about the interiors of massive stars,
through the yield in 60Fe. Different models have predicted yields that vary by
as much as two orders of magnitude for stars at the higher mass end, i.e.
beyond about 20-30 $M_{\odot}$.

As discussed above, ${}^{26\\!}$Al ejection is attributed to massive stars
mainly; therefore, INTEGRAL observations of ${}^{26\\!}$Al and of
${}^{60\\!}$Fe decay $\gamma$-rays may be unique probes of such massive-star
physics, and the emission ratio as a diagnostic quantity would even eliminate
uncertainties about source locations and distances in steady-state Galactic
nucleosynthesis. As such a steady state between current galactic production
and decay of both these isotopes may be plausibly assumed, the determination
of the ratio of their yields $r={Y_{60Fe}}/Y_{26Al}$ should be a very
sensitive global diagnostic of the validity of massive-star nucleosynthesis
models [e.g. 100, and references therein]. According to models, ${}^{60\\!}$Fe
is produced rather deep inside a massive star, in the Ne-O rich regions where
He burning had been effective. This implies that ${}^{60\\!}$Fe is expected to
be ejected _only_ by the SN explosion and not by the stellar wind, contrary to
the above case of ${}^{26\\!}$Al . Current models predict a $\gamma$-ray flux
ratio around 16% [8, 100] or $\sim$18 ($\pm$4)% [98].

Radioactive decay of ${}^{60\\!}$Fe (see Fig. 1, bottom) emits $\gamma$-rays
at energies of 1173 and 1332 keV, from cascade transitions towards the ground
state within the final 60Ni daughter nucleus of the
60Fe$\rightarrow^{60}$Co$\rightarrow^{60}$Ni decay chain; a low-energy photon
at 59 keV energy from a transition within the primary daughter 60Co occurs in
2% of 60Fe-decays only, hence its brightness is below current instrument
sensitivities.

${}^{60\\!}$Fe $\gamma$-rays have been hard to detect with current telescopes.
The team of the Ramaty solar spectrometer mission RHESSI first reported a
marginal signal (2.6 $\sigma$ for the combined ${}^{60\\!}$Fe 1.173 and 1.332
MeV lines) [101]. RHESSI also features Ge detectors, however with more modest
spectral resolution as cosmic rays eroded charge collection, and no annealings
were performed. Their Earth occultation, as the only available imaging
information, suggested that the observed ${}^{60\\!}$Fe emission may arise
from the inner Galaxy. Important was that the ${}^{60\\!}$Fe emission appeared
only at the 10%-level of ${}^{26\\!}$Al brightness.

###  2.1 INTEGRAL measurements and lessons

Figure 10:  (Top:) The ${}^{60\\!}$Fe signal (both $\gamma$-ray lines
superimposed at their laboratory energies of 1173 and 1332 keV) as observed
with INTEGRAL/SPI from the Galaxy [102] (Bottom:) The intensity of background
line emission from 60Co, here scaled with the varying continuum intensity of
background. This radioactivity is built up from cosmic-ray bombardment of
INTEGRAL in orbit, hence grows in relation to continuum background. The
${}^{60\\!}$Fe result shown on the left derives from data in orbits 17-359,
i.e. the early part of the mission. Note that instrumental background varies
somewhat, in anti-correlation with solar activity, which is shown as the grey
line.

INTEGRAL observations in the early mission years could reveal a weak signal
from ${}^{60\\!}$Fe in the characteristic $\gamma$-ray lines at 1173 and 1332
keV: Fig. 10 (top graph) shows the signal in both lines superimposed [102],
which were derived after about 3 mission years of data.

${}^{60\\!}$Fe detectability with INTEGRAL suffers from a specific problem:
The isotope 60Co apparently is built up within the INTEGRAL spacecraft due to
irradiation of the spacecraft materials with cosmic rays. With a radioactive
lifetime of 7.6 years, build-up and decay should reach a steady state within a
few lifetimes, and a quasi-linear growth is seen during the first $\approx$10
mission years in Fig. 10 (lower graph) [103]. From the data of the first
mission years, a ${}^{60\\!}$Fe to ${}^{26\\!}$Al brightness ratio of 14
($\pm$6)% was reported [102].

Although we expect the sources of ${}^{26\\!}$Al and ${}^{60\\!}$Fe to be the
same category of massive stars, there are, however, some important
differences: First, the radioactive lifetimes are different, ${}^{60\\!}$Fe
being 3.46 times longer-lived; this could make ${}^{60\\!}$Fe emission more
diffuse, from the same source, if propagation is the same. Second, ejection of
${}^{60\\!}$Fe only occurs at the time of the core-collapse supernova, while
${}^{26\\!}$Al ejection may start already in the strong-wind phase of a
massive star’s evolution. This implies that the interstellar medium has been
pre-shaped by winds before ${}^{60\\!}$Fe ejection, and thus the channeling of
${}^{26\\!}$Al ejecta by surrounding gas would be more restrictive than for
60Fe. These plausible cases can best be investigated in 3D-hydrodynamic
simulations. Such have been made recently at large-enough (galactic) scale
[49, 51]. In principle, those simulations confirm the above simple arguments.
However, from the limitations of required and different assumptions about the
basic structure within a galaxy, e.g. spiral arms and gravitational fields
from stars, gas, and dark matter, no quantitative conclusions can be derived
yet from the ${}^{60\\!}$Fe measurements. Hence, we must address the unknown
morphology of ${}^{60\\!}$Fe $\gamma$-ray line emission through different
ways. However, being weak compared to ${}^{26\\!}$Al emission, construction of
an image of ${}^{60\\!}$Fe emission is not feasible from INTEGRAL data. In
particular, a challenge is the important test whether the spatial distribution
of ${}^{60\\!}$Fe emission is identical or not to that of ${}^{26\\!}$Al .

Figure 11:  (Left:) The spectrum of diffuse $\gamma$-ray emission in the
0.8–2.0 MeV range, with continuum bands and lines from ${}^{60\\!}$Fe at 1173
and 1332 keV and from ${}^{26\\!}$Al at 1809 keV. Modelling the sky as a
diffuse exponential disk (left)) or by the COMPTEL ${}^{26\\!}$Al image
(right) leads to different values in detail, illustrating uncertainties from
the emission morphology. [From 104]

Figure 12:  The ${}^{60\\!}$Fe /${}^{26\\!}$Al ratio derived from SPI
observations depends on the adopted emission morphology (see Fig. 11). From a
range of exponential-disk models with different parameters (dark-blue
histogram) through models from specific source tracers (color bars), a
plausibly-constrained range of 0.2–0.4 emerges. [From 104]

A new attempt has been made, using all available accumulated INTEGRAL data,
and exploring results for a wide range of potential morphologies of
${}^{60\\!}$Fe emission [104]. Here, within a broad band of data from 0.8 to 2
MeV, several continuum bands and the lines from ${}^{60\\!}$Fe and from
${}^{26\\!}$Al were fitted consistently to the same adopted morphology.
Exponential-disk morphologies could be varied through their scale radius and
scale height parameters within a broad plausible range. For comparison,
astronomical tracers such as the ${}^{26\\!}$Al image from COMPTEL or SPI,
dust emission, and a range of other, more-or-less far-fetched tracers were
used. Fig. 11 shows an example of spectra obtained for a typical exponential
disk model that rather closely resembles ${}^{26\\!}$Al (but maybe not 60Fe)
emission (left), and for the COMPTEL ${}^{26\\!}$Al image within the inner
Galaxy (right). Fig. 12 shows the resulting $\gamma$-ray line intensity ratios
${}^{60\\!}$Fe /${}^{26\\!}$Al for such a broad range of potential
morphologies. This analysis found that for ${}^{60\\!}$Fe data alone,
morphologies akin a concentrated point source do not fit the data well, being
inferior in quality by a significant amount compared to diffuse and extended
morphologies. This alone confirms the essentially-diffuse nature of
${}^{60\\!}$Fe emission. But no further imaging constraints are feasible from
INTEGRAL data so far. On the other hand, the ${}^{60\\!}$Fe /${}^{26\\!}$Al
ratio varies ($\pm$4%) around 18.4% for all exponential-disk variants. Other
plausible tracers apparently obtain significantly higher ratio values such as
0.28 for the dust emission map. From this analysis, a range for the
${}^{60\\!}$Fe /${}^{26\\!}$Al ratio of 0.2 – 0.4 has been derived, in account
of these systematic uncertainties on ${}^{60\\!}$Fe emission morphology, for
the Galactic diffuse line emission [104].

## Summary

INTEGRAL with its $\gamma$-ray spectrometer SPI has measured diffuse
nucleosynthesis line emission from the ${}^{26\\!}$Al and ${}^{60\\!}$Fe
isotopes, and (less directly) from positron annihilation emission with its 511
keV line. ${}^{26\\!}$Al measurements have established the current production
within the extended entire Galaxy, most-plausibly ejected from massive stars
and their supernovae. The investigations of specific massive star clusters
which are within distances of order one kpc have allowed specific tests of the
massive-star origin hypothesis. Population synthesis model comparisons and
multi-frequency studies have been carried through, analysing other
implications of massive-star activity such as large cavities recognised
through emission of hot gas in their interiors and 21cm H1 emission in their
swept-up shells. On the galactic scale, Doppler shifts and broadenings of the
${}^{26\\!}$Al $\gamma$-ray line have been compared with kinematic data from
other objects. This has led to the insight that production of large cavities
and superbubbles are a typical characteristic of interstellar medium into
which ${}^{26\\!}$Al is injected from its sources. ${}^{60\\!}$Fe $\gamma$-ray
emission has been clearly detected, and is found significantly weaker in
intensity than ${}^{26\\!}$Al emission. This, and the radioactive build-up of
a particular background in 60Co emitting the same lines, makes astrophysical
analysis for ${}^{60\\!}$Fe much more challenging. ${}^{60\\!}$Fe most
probably also is diffuse in nature within our Galaxy, rather than from a few
bright individual sources. The intensity ratio found from INTEGRAL of
60Fe/${}^{26\\!}$Al is constrained to lie in the range 0.2–0.4. This appears
consistent with constraints derived from solar-system probes such as cosmic
rays [105] and deposits on Earth and Moon [106].

In summary, INTEGRAL results from 15 years of data have reached a precision
that enables detailed astrophysical studies for ${}^{26\\!}$Al , and
${}^{60\\!}$Fe data are sufficient for coarse tests of astrophysical models.
Future mission years will enhance the statistical precision, but even more-
importantly the recognition of systematics from the large instrumental
background. This should increase the overall sensitivity for diffuse
nucleosynthesis approximately linearly with time, stimulating hope for more
astrophysical results in this field before its mission end in 2029. Other
diffuse emission in our Galaxy may, e.g., be expected from 22Na radioactivity
as produced by novae, or from long-lived r-process radioactivity produced in
neutron star mergers.

## Acknowledgements

We are grateful for constructive comments of the referee, which helped the
quality of this review paper. The INTEGRAL/SPI project has been completed
under the responsibility and leadership of CNES; we are grateful to ASI, CEA,
CNES, DLR (No. 50 OG 1101, 1601, and 1901, and No. 50 OR 0901, 1206, 1611, and
1316), ESA, INTA, NASA and OSTC for support of this ESA space science mission.
This research was also supported by the DFG cluster of excellence _Origin and
Structure of the Universe_ , and by the ChETEC Action (CA16117) of COST
(European Cooperation in Science and Technology) .

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