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Radioactive nuclei are the key to understanding the circumstances of the birth of our Sun because meteoritic analysis has proven that many of them were present at that time. Their origin, however, has been so far elusive. The ERC-CoG-2016 RADIOSTAR project is dedicated to investigating the production of radioactive nuclei by nuclear reactions inside stars, their evolution in the Milky Way Galaxy, and their presence in molecular clouds. So far, we have discovered that: (i) radioactive nuclei produced by slow (107Pd and 182Hf) and rapid (129I and 247Cm) neutron captures originated from stellar sources —asymptotic giant branch (AGB) stars and compact binary mergers, respectively—within the galactic environment that predated the formation of the molecular cloud where the Sun was born; (ii) the time that elapsed from the birth of the cloud to the birth of the Sun was of the order of 107 years, and (iii) the abundances of the very short-lived nuclei 26Al, 36Cl, and 41Ca can be explained by massive star winds in single or binary systems, if these winds directly polluted the early Solar System. Our current and future work, as required to finalise the picture of the origin of radioactive nuclei in the Solar System, involves studying the possible origin of radioactive nuclei in the early Solar System from core-collapse supernovae, investigating the production of 107Pd in massive star winds, modelling the transport and mixing of radioactive nuclei in the galactic and molecular cloud medium, and calculating the galactic chemical evolution of 53Mn and 60Fe and of the p-process isotopes 92Nb and 146Sm.
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Phosphorus-bearing molecules PO and PN at the edge of the Galaxy
Abstract Despite its importance in planet formation and biology1, phosphorus has been identified only in the inner 12 kpc of the Galaxy2–19. The study of this element has been hindered in part by unfavourable atomic transitions2,4,20. Phosphorus is thought to be created by neutron capture on29Si and30Si in massive stars20,21, and released into the interstellar medium by Type II supernova explosions2,22. However, models of galactic chemical evolution must arbitrarily increase the supernovae production23to match observed abundances. Here we present the detection of gas-phase phosphorus in the Outer Galaxy through millimetre spectra of PO and PN. Rotational lines of these molecules were observed in the dense cloud WB89-621, located 22.6 kpc from the Galactic Centre24. The abundances of PO and PN in WB89-621 are comparable to values near the Solar System25. Supernovae are not present in the Outer Galaxy26, suggesting another source of phosphorus, such as ‘Galactic Fountains’, where supernova material is redistributed through the halo and circumgalactic medium27. However, fountain-enriched clouds are not found at such large distances. Any extragalactic source, such as the Magellanic Clouds, is unlikely to be metal rich28. Phosphorus instead may be produced by neutron-capture processes in lower mass asymptotic giant branch stars29which are present in the Outer Galaxy. Asymptotic giant branch stars also produce carbon21, flattening the extrapolated metallicity gradient and accounting for the high abundances of C-containing molecules in WB89-621.
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- Award ID(s):
- 1907910
- PAR ID:
- 10484977
- Publisher / Repository:
- Springer
- Date Published:
- Journal Name:
- Nature
- Volume:
- 623
- Issue:
- 7986
- ISSN:
- 0028-0836
- Page Range / eLocation ID:
- 292 to 295
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Presolar grains are stardust particles that condensed in the ejecta or in the outflows of dying stars and can today be extracted from meteorites. They recorded the nucleosynthetic fingerprint of their parent stars and thus serve as valuable probes of these astrophysical sites. The most common types of presolar silicon carbide grains (called mainstream SiC grains) condensed in the outflows of asymptotic giant branch stars. Their measured silicon isotopic abundances are not significantly influenced by nucleosynthesis within the parent star but rather represent the pristine stellar composition. Silicon isotopes can thus be used as a proxy for galactic chemical evolution (GCE). However, the measured correlation of29Si/28Si versus30Si/28Si does not agree with any current chemical evolution model. Here, we use a Monte Carlo model to vary nuclear reaction rates within their theoretical or experimental uncertainties and process them through stellar nucleosynthesis and GCE models to study the variation of silicon isotope abundances based on these nuclear reaction rate uncertainties. We find that these uncertainties can indeed be responsible for the discrepancy between measurements and models and that the slope of the silicon isotope correlation line measured in mainstream SiC grains agrees with chemical evolution models within the nuclear reaction rate uncertainties. Our result highlights the importance of future precision reaction rate measurements for resolving the apparent data–model discrepancy.more » « less
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Abstract Phosphorus is a key element that plays an essential role in biological processes important for living organisms on Earth. The origin and connection of phosphorus-bearing molecules to early solar system objects and star-forming molecular clouds is therefore of great interest, yet there are limited observations throughout different stages of low-mass (M < a few solar masses) star formation. Observations from the Yebes 40 m and IRAM 30 m telescopes detect for the first time in the 7 mm, 3 mm, and 2 mm bands multiple transitions of PN and PO, as well as a single transition of PO+, toward a low-mass starless core. The presence of PN, PO, and PO+is kinematically correlated with bright SiO(1–0) emission. Our results reveal not only that shocks are the main driver of releasing phosphorus from dust grains and into the gas phase but that the emission originates from gas not affiliated with the shock itself but quiescent gas that has been shocked in the recent past. From radiative transfer calculations, the PO/PN abundance ratio is found to be , consistent with other high-mass and low-mass star-forming regions. This first detection of PO+toward any low-mass star-forming region reveals a PO+/PO ratio of , a factor of 10 lower than previously determined from observations of a Galactic center molecular cloud, suggesting its formation can occur under more standard Galactic cosmic-ray ionization rates. These results motivate the need for additional observations that can better disentangle the physical mechanisms and chemical drivers of this precursor of prebiotic chemistry.more » « less
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Context. The 26 Al short-lived radioactive nuclide is the source of the observed galactic diffuse γ -ray emission at 1.8 MeV. While different sources of 26 Al have been explored, such as asymptotic giant branch stars, massive stellar winds, and supernovae, the contribution of very massive stars has not been studied so far. Aims. We study the contribution of the stellar wind of very massive stars, here, stars with initial masses between 150 and 300 M ⊙ , to the enrichment in 26 Al of the galactic interstellar medium. Methods. We studied the production of 26 Al by studying rotating and non-rotating very massive stellar models with initial masses between 150 and 300 M ⊙ for metallicities Z = 0.006, 0.014, and 0.020. We compared this result to a simple Milky Way model and took the metallicity and the star formation rate gradients into account. Results. We obtain that very massive stars in the Z = 0.006 − 0.020 metallicity range might be very significant contributors to the 26 Al enrichment of the interstellar medium. Typically, the contribution of the winds of massive stars to the total quantity of 26 Al in the Galaxy increases by 150% when very massive stars are considered. Conclusions. Despite their rarity, very massive stars might be important contributors to 26 Al and might overall be very important actors for nucleosynthesis in the Galaxy.more » « less
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Abstract The APOGEE Open Cluster Chemical Abundances and Mapping survey is used to probe the chemical evolution of the s-process element cerium in the Galactic disk. Cerium abundances were derived from measurements of Ce ii lines in the APOGEE spectra using the Brussels Automatic Code for Characterizing High Accuracy Spectra in 218 stars belonging to 42 open clusters. Our results indicate that, in general, for ages < 4 Gyr, younger open clusters have higher [Ce/Fe] and [Ce/ α -element] ratios than older clusters. In addition, metallicity segregates open clusters in the [Ce/X]–age plane (where X can be H, Fe, or the α -elements O, Mg, Si, or Ca). These metallicity-dependent relations result in [Ce/Fe] and [Ce/ α ] ratios with ages that are not universal clocks. Radial gradients of [Ce/H] and [Ce/Fe] ratios in open clusters, binned by age, were derived for the first time, with d [Ce/H]/ d R GC being negative, while d [Ce/Fe]/ d R GC is positive. [Ce/H] and [Ce/Fe] gradients are approximately constant over time, with the [Ce/Fe] gradient becoming slightly steeper, changing by ∼+0.009 dex kpc −1 Gyr −1 . Both the [Ce/H] and [Ce/Fe] gradients are shifted to lower values of [Ce/H] and [Ce/Fe] for older open clusters. The chemical pattern of Ce in open clusters across the Galactic disk is discussed within the context of s-process yields from asymptotic giant branch (AGB) stars, gigayear time delays in Ce enrichment of the interstellar medium, and the strong dependence of Ce nucleosynthesis on the metallicity of its AGB stellar sources.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1038/s41586-023-06616-1
