arXiv subjects

Monique Spite

Publications and source records attributed to Monique Spite.

22 records · Page 2Linked to original sources

arXiv2010

Non-LTE abundances of Mg and K in extremely metal-poor stars and the evolution of [O/Mg], [Na/Mg], [Al/Mg] and [K/Mg] in the Milky Way

LTE abundances of light elements in extremely metal-poor (EMP) stars have been previously derived from high quality spectra. New derivations, free from the NLTE effects, will better constrain the models of the Galactic chemical evolution and the yields of the very first supernovae. The NLTE profiles of the magnesium and potassium lines have been computed in a sample of 53 extremely metal-poor stars with a modified version of the program MULTI and adjusted to the observed lines in order to derive the abundances of these elements. The NLTE corrections for magnesium and potassium are in good agreement with the works found in the literature. The abundances are slightly changed, reaching a better precision: the scatter around the mean of the abundance ratios has decreased. Magnesium may be used with confidence as reference element. Together with previously determined NLTE abundances of sodium and aluminum, the new ratios are displayed, for comparison, along the theoretical trends proposed by some models of the chemical evolution of the Galaxy, using different models of supernovae.

astro-ph.GA↗

arXiv2007

Line shift, line asymmetry, and the 6Li/7Li isotopic ratio determination

Context: Line asymmetries are generated by convective Doppler shifts in stellar atmospheres, especially in metal-poor stars, where convective motions penetrate to higher atmospheric levels. Such asymmetries are usually neglected in abundance analyses. The determination of the 6Li/7Li isotopic ratio is prone to suffering from such asymmetries, as the contribution of 6Li is a slight blending reinforcement of the red wing of each component of the corresponding 7Li line, with respect to its blue wing. Aims: The present paper studies the halo star HD 74000 and estimates the impact of convection-related asymmetries on the Li isotopic ratio determination. Method: Two methods are used to meet this aim. The first, which is purely empirical, consists in deriving a template profile from another element that can be assumed to originate in the same stellar atmospheric layers as Li I, producing absorption lines of approximately the same equivalent width as individual components of the 7Li I resonance line. The second method consists in conducting the abundance analysis based on NLTE line formation in a 3D hydrodynamical model atmosphere, taking into account the effects of photospheric convection. Results: The results of the first method show that the convective asymmetry generates an excess absorption in the red wing of the 7Li absorption feature that mimics the presence of 6Li at a level comparable to the hitherto published values. This opens the possibility that only an upper limit on 6Li/7Li has thus far been derived. The second method confirms these findings. Conclusions: From this work, it appears that a systematic reappraisal of former determinations of 6Li abundances in halo stars is warranted.

astro-ph↗

arXiv2004

The evolution of the Milky Way from its earliest phases : constraints on stellar nucleosynthesis

We computed the evolution of the abundances of O, Mg, Si, Ca, K, Ti, Sc, Ni, Mn, Co, Fe and Zn in the Milky Way. We made use of the most widely adopted nucleosynthesis calculations and compared the model results with observational data with the aim of imposing constraints upon stellar yields. In order to fit at best the data in the solar neighborhood, when adopting the Woosley and Weaver (1995) yields for massive stars and the Iwamoto et al. (1999) ones for type Ia SNe, it is required that: i) the Mg yields should be increased in stars with masses from 11 to 20 M_sun and decreased in masses larger than 20 M_sun. The Mg yield should be also increased in SNe Ia. ii) The Si yields should be slightly increased in stars above 40 M_sun, whereas those of Ti should be increased between 11 and 20 M_sun and above 30 M_sun. iii) The Cr and Mn yields should be increased in stars with masses in the range 11-20 M_sun, iv) the Co yields in SNe Ia should be larger and smaller in stars in the range 11-20 M_sun, v) the Ni yield from type Ia SNe should be decreased, vi) the Zn yield from type Ia SNe should be increased. vii) The yields of O (metallicity dependent SN models), Ca, Fe, Ni, and Zn (the solar abundance case) in massive stars from Woosley and Weaver (1995) are the best to fit the abundance patterns of these elements since they do not need any change. We adopted also the yields by Nomoto et al. (1997) and Limongi and Chieffi (2003) for massive stars and discussed the corrections required in these yields in order to fit the observations. Finally, the small spread in the [el/Fe] ratios in the metallicity range from [Fe/H]=-4.0 up to -3.0 dex (Cayrel et al. 2003) is a clear sign that the halo of the Milky Way was well mixed even in the earliest phases of its evolution.

astro-ph↗

arXiv1998

Lithium-6 : Evolution from Big Bang to Present

The primordial abundances of Deuterium, he4, and li7 are crucial to determination of the baryon density of the Universe in the framework of standard Big Bang nucleosynthesis (BBN). li6 which is only produced in tiny quantities and it is generally not considered to be a cosmological probe. However, recent major observational advances have produced an estimate of the li6/li7 ratio in a few very old stars in the galactic halo which impacts the question whether or not the lithium isotopes are depleted in the outer layers of halo stars, through proton induced reactions at the base of (or below) the convective zone. li6 is a pure product of spallation through the major production reactions, fast oxygen and alphas interacting on interstellar H, He (especially in the early Galaxy). The rapid nuclei are both synthesized and accelerated by SN II. In this context, the \li6 evolution should go in step with that of beryllium and boron, recently observed by the Keck and HST telescopes. Li6 adds a new constraint on the early spallation in the Galaxy. In particular, if confirmed, the Li6/Be9 ratio observed in two halo stars (HD 84937, BD +263578) gives strong boundary conditions on the composition and the spectrum of the rapid particles involved. We show that Li6 is essentially intact in halo stars, and a fortiori \li7. We can define a range of the Li6 abundance in the very early Galaxy consistent with Big Bang nucleosynthesis (5.6 10(-14) to 3. 10(-13) . Following the evolution at increasing metallicity, we explain the abundance in the solar system within a factor of about 2.

astro-ph↗