arXiv subjects

D. Nandal

Publications and source records attributed to D. Nandal.

4 recordsLinked to original sources

arXiv2026

The Ashes of Supermassive Stars: Globular Cluster-like Aluminum Enhancement in Little Red Dots

The relative abundances of elements in galaxies serve as fossil records of the physical conditions and processes by which they were forged. While the Big Bang produced only the lightest elements, subsequent stellar nucleosynthesis imprinted characteristic abundance patterns onto the surrounding gas, set initially by the temperatures reached inside stars and subsequently shaped by how the processed material was mixed and released. Globular clusters - dense, ancient groups of stars - provide a striking unique example. Some contain stars depleted in magnesium and enriched in aluminum, showing that they formed from gas exposed to exceptionally hot hydrogen burning. The stars responsible remain unknown. Little Red Dots may provide this missing engine. These compact, luminous objects formed at cosmic epochs similar to those associated with globular-cluster formation and are enshrouded by dense gas whose chemical composition can be measured with the James Webb Space Telescope. Here, using deep spectroscopy from the SPURS program, we show that this abundance pattern characterizes the LRD central engine: magnesium-depleted and aluminum-enhanced gas with a metallicity only 1% that of the Sun. This pattern is not produced by ordinary massive stars at these redshifts and cannot be mimicked by ionization, gas geometry or dust. Instead, it is reproduced by hot hydrogen burning in fully convective supermassive stars, with the measured abundances implying masses of at least 10,000 solar masses - approximately 100 times larger than any star observed in the present-day Universe. Little Red Dots may therefore reveal supermassive stars during their brief lives or in the immediate aftermath of their direct collapse, simultaneously identifying the long-sought source of globular cluster abundance anomalies and a formation pathway for massive black hole seeds.

astro-ph.GA↗

arXiv2025

Constraints on the properties of macroscopic transport in the Sun from combined lithium and beryllium depletion

Context. The Sun is a privileged laboratory of stellar evolution, thanks to the quality and complementary nature of available constraints. Using these observations, we are able to draw a detailed picture of its internal structure and dynamics which form the basis of the successes of solar modelling. Amongst such constraints, the depletion of lithium and beryllium are key tracers of the required efficiency and extent of macroscopic mixing just below the solar convective envelope. Thanks to revised determinations of these abundances, we may use them in conjunction with other existing spectroscopic and helioseismic constraints to study in detail the properties of macroscopic transport. Aims. We aim at constraining the efficiency of macroscopic transport at the base of the convective envelope and determining the compatibility of the observations with a suggested candidate linked with the transport of angular momentum in the solar radiative interior. Methods. We use recent spectroscopic observations of lithium and beryllium abundance and include them in solar evolutionary model calibrations. We test the agreement of such models in terms of position of the convective envelope, helium mass fraction in convective zone, sound speed profile inversions and neutrino fluxes. Results. We constrain the required efficiency and extent of the macroscopic mixing at the base of the solar convective envelope, finding that a power law of density with an index n between 3 and 6 would reproduce the data, with efficiencies at the base of the envelope of about 6000 cm2 /s, depending on the value of n. We also confirm that macroscopic mixing worsens the agreement with neutrino fluxes and that the current implementations of the magnetic Tayler instability are unable to explain the observations.

astro-ph.SR↗

arXiv2023

The evolution and impact of 3000 M$_\odot$ stars in the early Universe

We present evolutionary models of massive, accreting population III stars with constant and variable accretion rates until the end of silicon burning, with final masses of 1000 - 3000 Msol. In all our models, after the core-hydrogen-burning phase, the star expands towards the red side of the Hertzsprung-Russell diagram is where it spends the rest of its evolution. During core helium burning, the models exhibit an outer convective envelope as well as many large intermediate convective zones.These intermediate zones allow for strong internal mixing to occur which enriches the surface in helium. The effect of increasing metallicity at a constant accretion rate of 10^{-3} Msol/yr shows an increase in the lifetime, final mass and distribution of helium in the envelope. Our fiducial model with mass of 3000 Msol has a final surface helium abundance of 0.74 and 9% of its total mass or 50% of the core mass, has a value of Gamma1 < 4/3 at the end of core silicon burning. If the collapse of the core is accompanied by the ejection of the envelope above the carbon-oxygen core, this could have a significant impact on the chemical evolution of the surroundings and subsequent stellar generations. The model has a final log(N/O) ~ 0.45, above the lower limit in the recently detected high-redshift galaxy GN-z11. We discuss the impact of a single 3000 Msol star on chemical, mechanical and radiative feedback, and present directions for future work.

astro-ph.SR↗

arXiv2022

Very massive star winds as sources of the short-lived radioactive isotope $^{26}$Al

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 AGB stars, massive stars winds, and supernovae, the contribution of very massive stars has never been studied. We study the stellar wind contribution of very massive stars, i.e stars with initial masses between 150 and 300 M$_\odot$, to the enrichment in $^{26}$Al of the galactic interstellar medium. We discuss the production of $^{26}$Al by studying rotating and non-rotating very massive stellar models with initial masses between 150 and 300 M$_\odot$ for metallicities Z=0.006, 0.014, and 0.020. We confront this result to a simple Milky Way model taking into account both the metallicity and the star formation rate gradients. 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. Very massive stars, despite their rarity, might be important contributors to $^{26}$Al and overall very important actors for nucleosynthesis in the Galaxy.

astro-ph.SR↗