The upper mass limit for the formation of TP-SAGB stars and the dredge-out phenomenon

We have computed the evolution of Super-AGB stars from the main sequence and up to a few hundred thermal pulses, with special attention to the low metallicity cases (Z = 1010; 105; 104 and 103). Our computations have been performed using time– dependent mixing and new opacity tables that admit variations in the abundances of carbon and oxygen. By following the evolution along the main central burning stages and the early TP-SAGB, we resolve the upper mass limits for the formation of TP-SAGB stars and determine the mass range at which the dredge-out phenomenon occurs. This phenomenon involves the merger of a convective shell sustained by helium burning at the top of the degenerate core with the hydrogen–rich convective envelope and the occurrence of a hydrogen flash. The dredge–out allows elements synthesised through helium burning to be transported to the stellar surfaces and therefore it can a ect the initial composition of the TP-SAGB stars.Peer ReviewedPostprint (published version

Super-AGB Stars and their role as Electron Capture Supernova progenitors

We review the lives, deaths and nucleosynthetic signatures of intermediate mass stars in the range approximately 6.5-12 Msun, which form super-AGB stars near the end of their lives. We examine the critical mass boundaries both between different types of massive white dwarfs (CO, CO-Ne, ONe) and between white dwarfs and supernovae and discuss the relative fraction of super-AGB stars that end life as either an ONe white dwarf or as a neutron star (or an ONeFe white dwarf), after undergoing an electron capture supernova. We also discuss the contribution of the other potential single-star channels to electron-capture supernovae, that of the failed massive stars. We describe the factors that influence these different final fates and mass limits, such as composition, the efficiency of convection, rotation, nuclear reaction rates, mass loss rates, and third dredge-up efficiency. We stress the importance of the binary evolution channels for producing electron-capture supernovae. We discuss recent nucleosynthesis calculations and elemental yield results and present a new set of s-process heavy element yield predictions. We assess the contribution from super-AGB star nucleosynthesis in a Galactic perspective, and consider the (super-)AGB scenario in the context of the multiple stellar populations seen in globular clusters. A brief summary of recent works on dust production is included. Lastly we conclude with a discussion of the observational constraints and potential future advances for study into these stars on the low mass/high mass star boundary.Comment: 28 pages, 11 figures. Invited review for Publications of the Astronomical Society of Australia, to be published in special issue on "Electron Capture Supernovae". Submitte

Is there a chance for SNeI1/2?

The evolution of primordial stars of initial masses between 5 and 10 M⊙ has been computed and analyzed in order to determine the nature of the remnants of massive intermediate–mass primordial stars and to check the influence of overshooting in their evolution. We have obtained the values for the limiting masses of Population III progenitor stars leading to carbon–oxygen and oxygen–neon compact cores. We have also obtained the limiting mass for which isolated primordial stars would lead to core–collapse supernovae after the end of the main central burning phases. Considering a moderate amount of overshooting, the mass thresholds at the ZAMS for the formation of carbon–oxygen and oxygen–neon degenerate cores shift to smaller values by about 2 M⊙. As a by–product of our calculations, we have also obtained the structure and composition profiles of the resulting compact remnants. We find that the final fate of the considered stars could not be to become white dwarfs, as it is the case of objects of larger metallicity of analogous initial masses. Instead, as we show by means of a synthetic code, they might end their lives as SNI1/2.Peer ReviewedPostprint (published version

The upper-mass limit for the formation of super-agb stars and the dredge-out phenomenon

We have computed the evolution of Super-AGB stars from the main sequence and up to a few hundred thermal pulses, with special attention to the low metallicity cases (Z = 1010; 105; 104 and 103). Our computations have been performed using time– dependent mixing and new opacity tables that admit variations in the abundances of carbon and oxygen. By following the evolution along the main central burning stages and the early TP-SAGB, we resolve the upper mass limits for the formation of TP-SAGB stars and determine the mass range at which the dredge-out phenomenon occurs. This phenomenon involves the merger of a convective shell sustained by helium burning at the top of the degenerate core with the hydrogen–rich convective envelope and the occurrence of a hydrogen flash. The dredge–out allows elements synthesised through helium burning to be transported to the stellar surfaces and therefore it can a ect the initial composition of the TP-SAGB stars.Peer ReviewedPostprint (published version

Further evidence of the long-term thermospheric density variation using 1U CubeSats

Faculty members, undergraduate and graduate students of the School of Communication and Aerospace Engineering (Polytechnical University of Catalonia) are participating in a series of studies to determine the thermospheric density. These studies involve planning a space mission, designing and constructing small satellites, and performing related data analysis. This article presents a method for determining the thermospheric density and summarises the academic context in which we develop our work. Several studies have reported the existence of a downtrend in thermospheric density, with relative values ranging from –2% to –7% per decade. Although it is well known that solar and geomagnetic activity are the main drivers of the variations of the thermospheric density, this downtrend was reported to be caused by the rise of greenhouse gases. We present an update of this progression, considering the last solar cycle (2009-2021) and using Two-Line Elements sets (TLE) of 1U CubeSats and the spherical satellites ANDE-2. TLEs were used to propagate the orbits numerically using SGP4 (Simplified General Perturbations), and then compute the average density between two consecutive TLEs by integrating the appropriate differential equation. Then, using the NRLMSISE-00 (Picone 2002) and JB2008 (Bowman 2008) atmospheric models, we calculated an average density deviation per year. We built a comprehensive time series of the thermospheric density values, ranging from 1967 to the present. We merged Emmert (2015) thermospheric density data and our results computed both with NRLMSISE-00 and with JB2008. A linear regression on the combined dataset yields a decreasing trend of –5.1% per decade. We also studied the geomagnetic and solar activity to isolate the possible greenhouse gasses effect during the considered period. Our results show a strong correlation between geomagnetic activity and density deviation near the solar minima, and we propose that the cause of the previously reported long-term density deviation could be a poor adjustment of the effects of geomagnetic activity. Finally, we proved that orbital information from small satellites could be efficiently used to assess the evolution of thermospheric density variations. Additional data obtained from future missions (as the one proposed by our group) will eventually allow a better characterisation of the atmospheric density and help disentangle the possible greenhouse gasses effects on its variation

Hiding in plain sight - red supergiant imposters? Super-AGB stars

Super Asymptotic Giant Branch (Super-AGB) stars reside in the mass range ˜ 6.5-10 M¿ and bridge the divide between low/intermediate-mass and massive stars. They are characterised by off-centre carbon ignition prior to a thermally pulsing phase which can consist of many tens to even thousands of thermal pulses. With their high luminosities and very large, cool, red stellar envelopes, these stars appear seemingly identical to their slightly more massive red supergiant counterparts. Due to their similarities, super-AGB stars may therefore act as stellar imposters and contaminate red supergiant surveys. The final fate of super-AGB stars is also quite uncertain and depends primarily on the competition between the core growth and mass-loss rates. If the stellar envelope is removed prior to the core reaching ˜ 1.375 M¿, an O-Ne white dwarf will remain, otherwise the star will undergo an electron-capture supernova (EC-SN) leaving behind a neutron star. We determine the relative fraction of super-AGB stars that end life as either an O-Ne white dwarf or as a neutron star, and provide a mass limit for the lowest mass supernova over a broad range of metallicities from the Z=0.02 to 0.0001.Peer ReviewedPostprint (published version

Primordial black holes capture by stars and induced collapse to low-mass stellar black holes

Primordial black holes in the asteroid-mass window, which might constitute all the dark matter, can be captured by stars when they traverse them at low enough velocity. After being placed on a bound orbit during star formation, they can repeatedly cross the star if the orbit happens to be highly eccentric, slow down by dynamical friction and end up in the stellar core. The rate of these captures is highest in halos of high dark matter density and low velocity dispersion, when the first stars form at redshift ∼ 20. We compute this capture rate for low-metallicity stars of 0.3 to 1 M, and find that a high fraction of these stars formed in the first dwarf galaxies would capture a primordial black hole, which would then grow by accretion up to a mass that may be close to the total star mass. We show the capture rate of primordial black holes does not depend on their mass over this asteroid-mass window, and should not be much affected by external tidal perturbations. These low-mass stellar black holes could be discovered today in low-metallicity, old binary systems in the Milky Way containing a surviving low-mass mainsequence star or a white dwarf, or via gravitational waves emitted in a merger with another compact object. No mechanisms in standard stellar evolution theory are known to form black holes below the Chandrasekhar mass, so detecting a low-mass black hole would fundamentally impact our understanding of stellar evolution, dark matter and the early Universe.We would like to acknowledge helpful discussions and advice fromN. Bellomo, J. L. Bernal, A. Escrivà, C. Germani, and J. Sal-vadó. This work was supported in part by Spanish grants CEX-2019-000918-M funded by MCIN/AEI/10.13039/501100011033,AYA2015-71091-P, and PID2019-108122GB-C32.Peer ReviewedPostprint (author's final draft

Super and massive AGB stars - IV. Final fates - Initial to final mass relation

We explore the final fates of massive intermediate-mass stars by computing detailed stellar models from the zero age main sequence until near the end of the thermally pulsing phase. These super-AGB and massive AGB star models are in the mass range between 5.0 and 10.0 Msun for metallicities spanning the range Z=0.02-0.0001. We probe the mass limits M_up, M_n and M_mass, the minimum masses for the onset of carbon burning, the formation of a neutron star, and the iron core-collapse supernovae respectively, to constrain the white dwarf/electron-capture supernova boundary. We provide a theoretical initial to final mass relation for the massive and ultra-massive white dwarfs and specify the mass range for the occurrence of hybrid CO(Ne) white dwarfs. We predict electron-capture supernova (EC-SN) rates for lower metallicities which are significantly lower than existing values from parametric studies in the literature. We conclude the EC-SN channel (for single stars and with the critical assumption being the choice of mass-loss rate) is very narrow in initial mass, at most approximately 0.2 Msun. This implies that between ~ 2-5 per cent of all gravitational collapse supernova are EC-SNe in the metallicity range Z=0.02 to 0.0001. With our choice for mass-loss prescription and computed core growth rates we find, within our metallicity range, that CO cores cannot grow sufficiently massive to undergo a Type 1.5 SN explosion.Comment: 15 pages, 7 figures, accepted for publication in MNRA