Evidences of extragalactic origin and planet engulfment in the metal-poor twin pair HD 134439/HD 134440

Recent studies of chemical abundances in metal-poor Halo stars show the existence of different populations, which is important for studies of Galaxy formation and evolution. Here we revisit the twin pair of chemically anomalous stars HD 134439 and HD 134440, using high resolution (R $\sim 72 000$) and high S/N ratio (S/N $\sim 250$) HDS/Subaru spectra. We compare them to the well-studied Halo star HD 103095, using the line-by-line differential technique to estimate precise stellar parameters and LTE chemical abundances. We present the abundances of C, O, Na, Mg, Si, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Sr, Y, Ba, La, Ce, Nd, and Sm. We compare our results to the precise abundance patterns of Nissen & Schuster (2010) and data from dwarf Spheroidal Galaxies (dSphs). We show that the abundance pattern of these stars appears to be closely linked to that of dSphs with [$\alpha$/Fe] knee below [Fe/H] $<-1.5$. We also find a systematic difference of $0.06\pm0.01$ dex between the abundances of these twin binary stars, which could be explained by the engulfment of a planet, thus suggesting that planet formation is possible at low metallicities ([Fe/H]=$-1.4$).Comment: Published in MNRA

The Dependence of Iron-rich Metal-poor Star Occurrence on Galactic Environment Supports an Origin in Thermonuclear Supernova Nucleosynthesis

It has been suggested that a class of chemically peculiar metal-poor stars called iron-rich metal-poor (IRMP) stars formed from molecular cores with metal contents dominated by thermonuclear supernova nucleosynthesis. If this interpretation is accurate, then IRMP stars should be more common in environments where thermonuclear supernovae were important contributors to chemical evolution. Conversely, IRMP stars should be less common in environments where thermonuclear supernovae were not important contributors to chemical evolution. At constant $[\text{Fe/H}] \lesssim -1$, the Milky Way's satellite classical dwarf spheroidal (dSph) galaxies and the Magellanic Clouds have lower [\text{\alpha/Fe}] than the Milky Way field and globular cluster populations. This difference is thought to demonstrate the importance of thermonuclear supernova nucleosynthesis for the chemical evolution of the Milky Way's satellite classical dSph galaxies and the Magellanic Clouds. We use data from the Sloan Digital Sky Survey (SDSS) Apache Point Observatory Galactic Evolution Experiment (APOGEE) and Gaia to infer the occurrence of IRMP stars in the Milky Way's satellite classical dSph galaxies $\eta_{\text{dSph}}$ and the Magellanic Clouds $\eta_{\text{Mag}}$ as well as in the Milky Way field $\eta_{\text{MWF}}$ and globular cluster populations $\eta_{\text{MWGC}}$. In order of decreasing occurrence, we find $\eta_{\text{dSph}}=0.07_{-0.02}^{+0.02}$, $\eta_{\text{Mag}}=0.037_{-0.006}^{+0.007}$, $\eta_{\text{MWF}}=0.0013_{-0.0005}^{+0.0006}$, and a 1-$\sigma$ upper limit $\eta_{\text{MWGC}}<0.00057$. These occurrences support the inference that IRMP stars formed in environments dominated by thermonuclear supernova nucleosynthesis and that the time lag between the formation of the first and second stellar generations in globular clusters was longer than the thermonuclear supernova delay time.Comment: 13 pages, 1 figure, and 3 tables in aastex631 format; accepted for publication in A

Iron-rich Metal-poor Stars and the Astrophysics of Thermonuclear Events Observationally Classified as Type Ia Supernovae. I. Establishing the Connection

The progenitor systems and explosion mechanisms responsible for the thermonuclear events observationally classified as Type Ia supernovae are uncertain and difficult to uniquely constrain using traditional observations of Type Ia supernova host galaxies, progenitors, light curves, and remnants. For the subset of thermonuclear events that are prolific producers of iron, we use published theoretical nucleosynthetic yields to identify a set of elemental abundance ratios infrequently observed in metal-poor stars but shared across a range of progenitor systems and explosion mechanisms: [Na,Mg,Co/Fe]<0. We label stars with this abundance signature ``iron-rich metal-poor'' or IRMP stars. We suggest that IRMP stars formed in environments dominated by thermonuclear nucleosynthesis and consequently that their elemental abundances can be used to constrain both the progenitor systems and explosion mechanisms responsible for thermonuclear explosions. We identify three IRMP in the literature and homogeneously infer their elemental abundances. We find that the elemental abundances of BD+80 245, HE 0533--5340, and SMSS J034249.53--284216.0 are best explained by the (double) detonations of sub-Chandrasekhar mass CO white dwarfs. If our interpretation of IRMP stars is accurate, then they should be very rare in globular clusters and more common in the Magellanic Clouds and dwarf spheroidal galaxies than in the Milky Way's halo. We propose that future studies of IRMP stars will quantify the relative occurrences of different thermonuclear event progenitor systems and explosion mechanisms.Comment: Accepted for publication at A

First high-precision differential abundance analysis of extremely metal-poor stars

Context. Studies of extremely metal-poor stars indicate that chemical abundance ratios [X/Fe] have a root mean square scatter as low as 0.05 dex (12%). It remains unclear whether this reflects observational uncertainties or intrinsic astrophysical scatter arising from physical conditions in the interstellar medium at early times. Aims. We measure differential chemical abundance ratios in extremely metal-poor stars to investigate the limits of precision and to understand whether cosmic scatter or observational errors are dominant. Methods. We used high-resolution (R ~ 95 000) and high signal-to-noise (S/N = 700 at 5000 Å) HIRES/Keck spectra to determine high-precision differential abundances between two extremely metal-poor stars through a line-by-line differential approach. We determined stellar parameters for the star G64-37 with respect to the standard star G64-12. We performed EW measurements for the two stars for the lines recognized in both stars and performed spectral synthesis to study the carbon abundances. Results. The differential approach allowed us to obtain errors of σ(Teff) = 27 K, σ(log g) = 0.06 dex, σ( [Fe/H] ) = 0.02 dex and σ(vt) = 0.06 km s-1. We estimated relative chemical abundances with a precision as low as σ([X/Fe]) ≈ 0.01 dex. The small uncertainties demonstrate that there are genuine abundance differences larger than the measurement errors. The observed Li difference cannot be explained by the difference in mass because the less massive star has more Li. Conclusions. It is possible to achieve an abundance precision around ≈ 0.01−0.05 dex for extremely metal-poor stars, which opens new windows on the study of the early chemical evolution of the Galaxy

The relationship between photometric and spectroscopic oscillation amplitudes from 3D stellar atmosphere simulations

We establish a quantitative relationship between photometric and spectroscopic detections of solar-like oscillations using ab initio, three-dimensional (3D), hydrodynamical numerical simulations of stellar atmospheres. We present a theoretical derivation as proof of concept for our method. We perform realistic spectral line formation calculations to quantify the ratio between luminosity and radial velocity amplitude for two case studies: the Sun and the red giant $\epsilon$ Tau. Luminosity amplitudes are computed based on the bolometric flux predicted by 3D simulations with granulation background modelled the same way as asteroseismic observations. Radial velocity amplitudes are determined from the wavelength shift of synthesized spectral lines with methods closely resembling those used in BiSON and SONG observations. Consequently, the theoretical luminosity to radial velocity amplitude ratios are directly comparable with corresponding observations. For the Sun, we predict theoretical ratios of 21.0 and 23.7 ppm/[m/s] from BiSON and SONG respectively, in good agreement with observations 19.1 and 21.6 ppm/[m/s]. For $\epsilon$ Tau, we predict K2 and SONG ratios of 48.4 ppm/[m/s], again in good agreement with observations 42.2 ppm/[m/s], and much improved over the result from conventional empirical scaling relations which gives 23.2 ppm/[m/s]. This study thus opens the path towards a quantitative understanding of solar-like oscillations, via detailed modelling of 3D stellar atmospheres.Comment: 16 pages, 7 figures, accepted for publication in MNRA

Constraining cosmic scatter in the Galactic halo through a differential analysis of metal-poor stars

© ESO 2017.Context. The chemical abundances of metal-poor halo stars are important to understanding key aspects of Galactic formation and evolution. Aims. We aim to constrain Galactic chemical evolution with precise chemical abundances of metal-poor stars (−2.8 ≤ [Fe/H] ≤ −1.5). Methods. Using high resolution and high S/N UVES spectra of 23 stars and employing the differential analysis technique we estimated stellar parameters and obtained precise LTE chemical abundances. Results. We present the abundances of Li, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Zn, Sr, Y, Zr, and Ba. The differential technique allowed us to obtain an unprecedented low level of scatter in our analysis, with standard deviations as low as 0.05 dex, and mean errors as low as 0.05 dex for [X/Fe]. Conclusions. By expanding our metallicity range with precise abundances from other works, we were able to precisely constrain Galactic chemical evolution models in a wide metallicity range (−3.6 ≤ [Fe/H] ≤ −0.4). The agreements and discrepancies found are key for further improvement of both models and observations. We also show that the LTE analysis of Cr II is a much more reliable source of abundance for chromium, as Cr I has important NLTE effects. These effects can be clearly seen when we compare the observed abundances of Cr I and Cr II with GCE models. While Cr I has a clear disagreement between model and observations, Cr II is very well modeled. We confirm tight increasing trends of Co and Zn toward lower metallicities, and a tight flat evolution of Ni relative to Fe. Our results strongly suggest inhomogeneous enrichment from hypernovae. Our precise stellar parameters results in a low star-to-star scatter (0.04 dex) in the Li abundances of our sample, with a mean value about 0.4 dex lower than the prediction from standard Big Bang nucleosynthesis; we also study the relation between lithium depletion and stellar mass, but it is difficult to assess a correlation due to the limited mass range. We find two blue straggler stars, based on their very depleted Li abundances. One of them shows intriguing abundance anomalies, including a possible zinc enhancement, suggesting that zinc may have been also produced by a former AGB companion.Peer reviewedFinal Published versio

Spectacular Nucleosynthesis from Early Massive Stars

Stars that formed with an initial mass of over 50 M ⊙ are very rare today, but they are thought to be more common in the early Universe. The fates of those early, metal-poor, massive stars are highly uncertain. Most are expected to directly collapse to black holes, while some may explode as a result of rotationally powered engines or the pair-creation instability. We present the chemical abundances of J0931+0038, a nearby low-mass star identified in early follow-up of the SDSS-V Milky Way Mapper, which preserves the signature of unusual nucleosynthesis from a massive star in the early Universe. J0931+0038 has a relatively high metallicity ([Fe/H] = −1.76 ± 0.13) but an extreme odd–even abundance pattern, with some of the lowest known abundance ratios of [N/Fe], [Na/Fe], [K/Fe], [Sc/Fe], and [Ba/Fe]. The implication is that a majority of its metals originated in a single extremely metal-poor nucleosynthetic source. An extensive search through nucleosynthesis predictions finds a clear preference for progenitors with initial mass >50 M ⊙, making J0931+0038 one of the first observational constraints on nucleosynthesis in this mass range. However, the full abundance pattern is not matched by any models in the literature. J0931+0038 thus presents a challenge for the next generation of nucleosynthesis models and motivates the study of high-mass progenitor stars impacted by convection, rotation, jets, and/or binary companions. Though rare, more examples of unusual early nucleosynthesis in metal-poor stars should be found in upcoming large spectroscopic surveys

Abundâncias diferenciais em estrelas pobres em metais: impactos no estudo da Galáxia, evolução estelar e nucleossíntese primordial.

This thesis presents the main results of the studies developed during the PhD at the Instituto de Astronomia, Geofísica e Ciências Atmosféricas, of the Universidade de São Paulo, that resulted in four papers as the first author and one peer-reviewed proceeding. All papers are attached in Appendix A. The main interest throughout the PhD is to study the Formation and early Chemical Evolution of the Galaxy via precise chemical abundances of the metal-poor component of the Galactic inner halo. We explored the viability of a chemical analysis via line-by-line differential abundance analysis in low metallicity stars, and showed that the increased precision might reveal small differences and inhomogeneities that can not be seen in a regular spectroscopic analysis. For this, we employed extremely high-resolution, and high S/N spectra, and we also showed how the quality of our data influences the final abundance precision. The line-by-line differential abundance technique was employed in a larger sample of high-resolution, high S/N, spectra, to study the formation of the inner halo and put constraints on chemical evolution models and the nucleosynthesis processes therein. As byproduct of our analysis, we studied binary stellar evolution through Blue Straggler Stars, which were used to constrain both nucleosynthesis in AGB stars and Blue Straggler formation. We also present the results of a study focused on a pair of binary stars with implications for the formation of the inner halo and the use of chemical tagging to constrain the birth environment of the stars. As a byproduct of this analysis we found implications of possible planetary formation in metal-poor stars. At last, we show the development of an atomic model for a non-LTE analysis of potassium line formation, and its application for the study of the chemical evolution of potassium through cosmic history, with implications for the nucleosynthesis of potassium and Galactic chemical evolution models with yields from massive rotating stars.Nesta tese apresentamos os resultados do trabalho de doutoramento desenvolvido no Instituto de Astronomia, Geofísica e Ciências Atmosféricas, da Universidade de São Paulo, publicados em quatro artigos científicos como primeiro autor, e um artigo arbitrado em proceedings. Todos os artigos estão anexados no Apêndice A. O interesse principal deste trabalho de doutorado é estudar a formação e evolução química da Galáxia através de análises precisas de abundâncias químicas de estrelas pobres em metais do componente interno do halo galáctico. Exploramos a viabilidade do uso da técnica de análise diferencial e mostramos que o aumento em precisão fornecido pelo uso da técnica é capaz de revelar inomogeneidades não observáveis em uma análise clássica. Também é discutido como a resolução e o sinal/ruído dos espectros, pode influenciar a precisão final das abundâncias obtidas, analisando espectros de diferentes qualidades de forma sistemática. Em seguida, é apresentado um trabalho de análise química através da técnica diferencial, onde são discutidas a formação do componente interno do halo galático e a evolução química de diversos elementos, com implicações para processos de nucleossíntese e modelos de evolução química da Galáxia. Como subproduto deste estudo apresentamos uma análise da evolução de estrelas binárias através da análise química de duas estrelas Blue Stragglers, com implicações para a nucleossíntese em estrelas AGB e para a evoluçaõ de sistemas binários. Também mostramos resultados de um estudo de um par de estrelas binárias, focado na formação do halo e no uso da técnica de chemical tagging para a identificação do local de nascimento de duas estrelas extra-galáticas. Uma diferença entre as abundâncias do par de estrelas é evidência de formação planetária em estrelas de baixa metalicidade. Finalizando a tese, mostramos o desenvolvimento de um modelo atômico para estudar a formação das linhas espectroscópicas de potássio, e a aplicação deste modelo no estudo da evolução química do elemento, mostrando que, possivelmente, estrelas massivas em alta rotação podem ser uma importante fonte nucleossintética de potássio