Nucleosynthesis

Nucleosynthesis is the process that creates new atomic nuclei from pre-existing nucleons (protons and neutrons) and nuclei. According to current theories, the first nuclei were formed a few minutes after the Big Bang, through nuclear reactions in a process called Big Bang nucleosynthesis. After about 20 minutes, the universe had expanded and cooled to a point at which these high-energy collisions among nucleons ended, so only the fastest and simplest reactions occurred, leaving our universe containing hydrogen and helium. The rest is traces of other elements such as lithium and the hydrogen isotope deuterium. Nucleosynthesis in stars and their explosions later produced the variety of elements and isotopes that we have today, in a process called cosmic chemical evolution. The amounts of total mass in elements heavier than hydrogen and helium (called 'metals' by astrophysicists) remains small (few percent), so that the universe still has approximately the same composition. Stars fuse li

The last stages of star formation in the dwarf spheroidal galaxy Sextans

Romaine Theler

This thesis is anchored in the research of better understanding of the chemical evolution of galaxies. Many dwarf spheroidal galaxies (dSph) are orbiting the Milky Way displaying various star formation histories. The formation of these dwarf galaxies and the drivers of their evolution are still not well understood. We studied in detail one of them, Sextans, a low mass dSph composed of an old population. 90% of Sextans stars were formed 11-14 Gyr ago (Lee et al. 2009). Battaglia et al. (2011) show the presence of a radial metallicity gradient in Sextans considering observation based on intermediate resolution spectroscopy until the tidal radius. For a projected radius R < 0.8 deg stars cover the whole range of [Fe/H], while for R > 0.8 deg all stars are metal-poor [Fe/H] < -2.2 dex. Due to its distance and its low surface brightness high resolution spectra are required to obtain accurate abundances. Up to recently only 14 Sextans stars have been observed in high resolution spectroscopy (Shetrone et al. 2001; Aoki et al. 2009; Tafelmeyer et al. 2010; Honda et al. 2011) but in majority they were focused only on extremely metal-poor stars. In order to understand the chemical evolution of Sextans we needed to study a statistically significant sample of stars covering a larger metallicity range. This has been done in this work based on the largest high resolution spectroscopic sample of 87 red giant branch stars ever obtained in such low mass dwarf spheroidal galaxies. Our sample of stars located in the center of Sextans displays a large metallicity distribution ranging from -1.0 dex to -3.5 dex. We derived abundances of 10 elements : 3 alpha-elements (Mg, Ca and Ti), 5 iron-peak elements (Sc, Cr,Mn, Co and Ni) and 2 neutron-capture elements (Ba and Eu). Combining the results we had the first picture of the chemical evolution of Sextans. From [alpha/Fe] distribution reflecting the ratio between supernovae [SNe II/SNe Ia], we found that the domination of SNeIa in chemical evolution occurred around [Fe/H] = -2.0 dex. A lower knee than for Fornax and Sculptor suggests that star formation was less efficient in Sextans in agreement with their respective estimated mass. The absence of intrinsic scatter in [alpha/Fe] showed that in the center of Sextans the interstellar medium has been homogeneously enriched through star formation. Some hypothesis like tidal interactions with the Milky Way could explain this abundance homogeneity in the center of the galaxy and the metallicity gradient toward the outskirts. Abundances in iron-peak elements and neutron-capture elements give new observational constraints for a better understanding of the various nucleosynthesis sources of these elements.

EPFL2015

The chemical evolution of the dwarf spheroidal galaxy Sextans

Pascale Jablonka, Carmela Lardo, Romain Ely Roland Lucchesi, Pierre North, Romaine Theler

We present our analysis of the FLAMES dataset targeting the central 25 ' region of the Sextans dwarf spheroidal galaxy (dSph). This dataset is the third major part of the high-resolution spectroscopic section of the ESO large program 171.B-0588(A) obtained by the Dwarf galaxy Abundances and Radial-velocities Team. Our sample is composed of red giant branch stars down to V similar to 20.5 mag, the level of the horizontal branch in Sextans, and allows users to address questions related to both stellar nucleosynthesis and galaxy evolution. We provide metallicities for 81 stars, which cover the wide [Fe/H] = -3.2 to -1.5 dex range. The abundances of ten other elements are derived: Mg, Ca, Ti, Sc, Cr, Mn, Co, Ni, Ba, and Eu. Despite its small mass, Sextans is a chemically evolved system, showing evidence of a contribution from core-collapse and Type Ia supernovae as well as low-metallicity asymptotic giant branch stars (AGBs). This new FLAMES sample offers a sufficiently large number of stars with chemical abundances derived with high accuracy to firmly establish the existence of a plateau in [alpha/Fe] at similar to 0.4 dex followed by a decrease above [Fe/H] similar to -2 dex. These features reveal a close similarity with the Fornax and Sculptor dSphs despite their very different masses and star formation histories, suggesting that these three galaxies had very similar star formation efficiencies in their early formation phases, probably driven by the early accretion of smaller galactic fragments, until the UV-background heating impacted them in different ways. The parallel between the Sculptor and Sextans dSph is also striking when considering Ba and Eu. The same chemical trends can be seen in the metallicity region common to both galaxies, implying similar fractions of SNeIa and low-metallicity AGBs. Finally, as to the iron-peak elements, the decline of [Co/Fe] and [Ni/Fe] above [Fe/H] similar to -2 implies that the production yields of Ni and Co in SNeIa are lower than that of Fe. The decrease in [Ni/Fe] favours models of SNeIa based on the explosion of double-degenerate sub-Chandrasekhar mass white dwarfs.

EDP SCIENCES S A2020

The chemical evolution of the dwarf spheroidal galaxy Sextans

Pascale Jablonka, Carmela Lardo, Romain Ely Roland Lucchesi, Pierre North, Romaine Theler

EDP SCIENCES S A2020

The last stages of star formation in the dwarf spheroidal galaxy Sextans

Romaine Theler

EPFL2015

The last stages of star formation in the dwarf spheroidal galaxy Sextans

The chemical evolution of the dwarf spheroidal galaxy Sextans

Primordial magnetic fields, anomalous isocurvature fluctuations and big bang nucleosynthesis

The chemical evolution of the dwarf spheroidal galaxy Sextans

Primordial magnetic fields, anomalous isocurvature fluctuations and big bang nucleosynthesis

The last stages of star formation in the dwarf spheroidal galaxy Sextans