Be and O in the ultra metal-poor dwarf 2MASS J18082002-5104378: the Be–O correlation

Abstract

Context. Measurable amounts of Be could have been synthesised primordially if the Universe were non-homogeneous or in the presence of late decaying relic particles. Aims. We investigate the Be abundance in the extremely metal-poor star 2MASS J1808-5104 ([Fe/H]=-3.84) with the aim of constraining inhomogeneities or the presence of late decaying particles. Methods. High resolution, high signal-to-noise ratio UV spectra were acquired at ESO with the Kueyen 8.2 m telescope and the UVES spectrograph. Abundances were derived using several model atmospheres and spectral synthesis code. Results. We measured log(Be/H) = −14.3 from a spectrum synthesis of the region of the Be line. Using a conservative approach, however we adopted an upper limit two times higher, i.e. log(Be/H) < −14.0. We measured the O abundance from UV OH lines and find [O/H]=-3.46 after a 3D correction. Conclusions. Our observation reinforces the existing upper limit on primordial Be. There is no observational indication for a primordial production of 9 Be. This places strong constraints on the properties of putative relic particles. This result also supports the hypothesis of a homogeneous Universe, at the time of nucleosynthesis. Surprisingly, our upper limit of the Be abundance is well below the Be measurements in stars of similar [O/H]. This may be evidence that the Be-O relation breaks down in the early Galaxy, perhaps due to the escape of spallation products from the gas clouds in which stars such as 2MASS J1808-5104 have formed.

Inhomogeneity in the early Galactic chemical enrichment exposed by beryllium abundances in extremely metal-poor stars

“…A similar possibility of increased scatter at low metallicities was discussed by Spite et al (2019), but mainly relied on the relation between log(Be/H) and [O/H]. Unfortunately, we cannot derive reliable oxygen abundances here, as they require the use of three-dimensional hydrodynamical models.…”

“…The evidence supporting the flattening of the relation was made weaker by the upper limit of the Be abundance in BD+ 44 493, a star with [Fe/H] = −3.7, derived by Ito et al (2009Ito et al ( , 2013 and later refined by Placco et al (2014). Similar evidence against the flattening is given by the more recent Be upper limit for star 2MASS J1808-5104 (Spite et al 2019). In both cases, the inferred limits are below the level of the possible plateau.…”

The 6Li/7Li isotopic ratio in the metal-poor binary CS22876–032

“…It could simply be that this system is one that significantly deviates from the Galactic Be-O relation. Spite et al (2019) found that 2MASS J1808-5104, which has a similar [Fe/H] but lower [O/H] than CS 22876-32, has no detectable Be and its upper limit lies well below the Galactic Be-O line. Another similar case is the CEMP-no star BD +44 • 493 (Ito et al 2013), again a similar [Fe/H] and no detectable Be.…”

“…8 at the lowest T eff and are Li depleted, but with measurable Li. The explanation suggested by Spite et al (2019) for these stars was that their age is old enough that cosmic rays did not have enough time to build significant quantities of Be. New higher-quality observations of Be in CS 22876-32 would be very valuable, since they would allow us to establish if this system lies on the Galactic Be-O line or if it is below the line, like 2MASS J1808-5104 and BD +44 • 493.…”

High-speed stars: Galactic hitchhikers

“…Quite unexpectedly, the most metal-poor star known, SDSS J1029+1729, is on a thick-disc orbit (Caffau 2018;Sestito et al 2019;Di Matteo et al 2020). Another extremely metal-poor star 2MASS J18082002-5104378 (Meléndez et al 2016) with an iron abundance [Fe/H] = −3.84 (Spite et al 2019) is on a thin-disc orbit (Schlaufman et al 2018;Sestito et al 2019;Di Matteo et al 2020). More generally it now appears that when a sample of stars is chemically selected as metal poor there is always a significant fraction that is on disc orbits (see e.g.…”