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A Method to Derive the Absolute Composition of the Sun, the Solar System, and the Stars

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A Method to Derive the Absolute Composition of the Sun, the Solar System, and the Stars A&A 462, 1051–1062 (2007) Astronomy DOI: 10.1051/0004-6361:20054505 & c ESO 2007 Astrophysics A method to derive the absolute composition of the Sun, the solar system, and the stars L. Piersanti1, O. Straniero1, and S. Cristallo1 INAF-Osservatorio Astronomico di Collurania Teramo, via Mentore Maggini, snc, 64100 Teramo, Italy e-mail: [cristallo;piersanti;straniero]@oa-teramo.inaf.it Received 10 November 2005 / Accepted 12 October 2006 ABSTRACT Context. The knowledge of isotopic and elemental abundances of the pristine solar system material provides a fundamental test of galactic chemical evolution models, while the composition of the solar photosphere is a reference pattern to understand stellar abun- dances. However, spectroscopic or meteoritic abundance determinations are only possible for an incomplete sample of the 83 elements detected in the solar system. Therefore, only relative abundances are experimentally determined, with respect to H or to Si for spec- troscopic or meteoritic measurements, respectively. For this reason, the available compilations of solar abundances are obtained by combining spectroscopic and meteoritic determinations, a procedure requiring the knowledge of the chemical modification occurring in the solar photosphere. Aims. We provide a method to derive the mass fractions of the 83 elements (and their most abundant isotopes) in the early solar system material and in the present-day solar surface. Methods. By computing a solar model, we investigate physical processes responsible for the variation of the solar surface composi- tion in the last 4.57 Gyr. An extended network, from H to U, is coupled to our stellar evolutionary code. The effects of microscopic diffusion, rotational-induced mixing in the tachocline and radiative acceleration are discussed. Results. The abundances of the 83 elements are given for both the pristine solar system and the solar photosphere. Calculations are repeated by adopting the most widely adopted compilations of solar abundances. Since for a given [Fe/H], the total metallicity depends on (Z/X), a 30% reduction of Z is found when passing from the classical Anders & Grevesse to the most recent Lodders compilation. Some implications are discussed, such as the increase of about 700 Myr in the estimated age of Globular Clusters. Conclusions. Within the experimental errors, the complete set of relative solar abundances, as obtained by combining meteorite and photosphere measurements, are consistent with the variations implied by the quoted physical processes. The few deviations can be easily attributed to the decay of long-lived radioactive isotopes. The large lithium depletion is only partially explained by introducing a rotation-induced mixing in the tachocline. Key words. Sun: abundance – Sun: evolution – solar system: general – stars: abundances 1. Introduction overabundance, also shown by the r-process elements, like eu- ropium, is interpreted as evidence of the fact that the early The Sun is the only star for which a full set of elemen- Galaxy, unlike pre-solar nebulae, was mainly polluted by mas- tal abundances is available. For this reason, its composition sive stars. Notwithstanding these limitations, the solar composi- represents the reference framework for abundance analyses in tion remains an important test for models of galactic chemical Astrophysics. In standard spectroscopic notation, abundances evolution. Indeed, it represents a typical pattern for the evolved / = are given with respect to the solar composition (e.g. [Fe H] galactic composition, like the one shown by disk stars. log (XFe/XH) − log (XFe/XH)). Then, according to a widely accepted procedure, the abundances of the various elements are Abundances of the 83 elements found in nature have been obtained by re-scaling the solar composition: a trace element obtained from various components of the solar system, like (usually iron) is used to determine the scaling factor, which is the Earth, Moon, planets, comets, meteorites, solar wind, so- applied to all the other elements, except H and He. Thus, the lar corona and, obviously, the solar photosphere. For some el- distribution of heavy elements in the Sun is assumed as a sort ements, abundances in several solar system environments are of cosmic composition. Such a scenario obviously contrasts available, whilst others can be measured just in one component. with much theoretical and observational evidence. We know that For example, heavy elements abundances are generally best the distribution of heavy elements in the interstellar medium measured in meteorites. On the contrary, H and other volatile changes with time, as a consequence of the pollution caused elements (like C, N, O) are strongly depleted in meteorites and by different generations of stars. The case of the enhancement their abundances are typically derived from solar spectra. In of the α elements (O, Ne, Mg, Si, S, Ca, Ti) in the galactic some cases, as for the noble gases Ne and Ar, poor information halo, where [α/Fe] ∼ 0.3−0.6, represents a well known exam- can be found both in meteorites and in the solar photosphere, ple of a deviation from the scaled solar composition. A similar so that their abundances are derived from high energy spectra of the hot solar corona or from the solar wind. The abundances Tables 3 and 4 are only available in electronic form at of Kr and Xe cannot be directly measured and they are usually http://www.aanda.org estimated by nucleosynthesis models. Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20054505 1052 L. Piersanti et al.: Chemical abundances of stars Thus, the data set obtained from a single solar-system com- errors. The most clear deviation from this rule is represented by ponent is incomplete, i.e. it only includes a subsample of the lithium, whose abundance relative to Si is about 2 orders of mag- 83 elements existing in the solar system. For this reason, only nitude larger in meteorites than at the solar photosphere. Such a relative abundances1 can be obtained (see Sect. 2). Moreover, strong depletion of the photospheric Li is considered evidence great caution should be used when these relative abundances, for nuclear burning (mainly via p-captures) occurring at the bot- as collected from different solar system environments, are com- tom of the solar convective envelope or immediately below it bined to obtain a complete compilation. Changes with respect to (D’Antona & Mazzitelli 1984; Pinsonneault et al. 1989). Note the pristine composition have been produced by different phys- that since p-captures rates are significantly different from one ical processes (Anders & Grevesse 1989; Grevesse & Noels isotope to another, they represent a typical example of physical 1993; Grevesse & Sauval 1998; Lodders 2003; Palme & Jones process not preserving the relative abundances. 2004). For example, melting or crystallization could have pro- In this paper, on the base of up-to-date Standard Solar duced a certain differentiation in planets and minor bodies. Models (SSMs), we discuss the modifications that have occurred Among the various meteoritic phases, CI carbonaceous chon- in the solar photosphere, since the pre-Main Sequence phase. We drites represent the best primitive sample of the solar system will show, in particular, that in SSMs where the modification of composition. On the contrary, the composition of the solar pho- the surface composition is only caused by convective mixing and tosphere has been substantially modified since the epoch of solar microscopic diffusion, relative abundances between heavy ele- system formation. Indeed, the solar surface is at the top of a deep ments are preserved, with only a few exceptions, and, therefore, convective region, which penetrates about 30% of the stellar ra- they fulfill the constraints imposed by the comparison between dius. At the base of this region, where the temperature attains meteorite and photosphere relative abundances of the best mea- ∼2 × 106 K, light isotopes (e.g. D or Li) are burned via pro- sured refractory elements. ton captures. The temperatures experienced within the convec- However, in many astrophysical applications and, in partic- tive layer of the Sun were even larger in the past 4.5 Gyr (up ular, in the calculation of stellar models, absolute, rather than to 3 ÷ 4 × 106 K during the pre-Main Sequence). In addition, relative, abundances are required, but there is no way to de- other phenomena, such as mass loss, coupled with the external rive the solar absolute composition on the basis of the available convection, may have contributed to the variation of the surface measurements only. Forgetting, for a while, the differences in composition of the Sun. Indeed, the He abundance derived from the chemical composition of the various solar system compo- the inversion of helioseismic data (Y = 0.2485 ± 0.034 – Basu nents and adopting a compilation obtained by combining dif- & Antia 2004) can be only explained by invoking gravitational ferent data sources, one may obtain a partial solution of this settling or, more in general, microscopic diffusion. Note that dif- problem. The mass fraction of He (Y) of the present-day-solar fusion affects the whole chemical pattern. In particular, the abun- photosphere can be independently estimated according to he- dances of all the isotopes more massive than A = 12 are depleted lioseismic measurements. Then, the detailed absolute compo- at the solar surface (∼–10% with respect to the original compo- sition of the surface layers of the present Sun can be obtained sition), whilst the mass fraction of H increases (about +5%). by deriving the hydrogen mass fraction (X) from the relation For this reason, photospheric abundances, usually expressed rel- X + Y + Z = Y + (1 + Z/X) · X = 1, where Z/X is known from the ative to H, cannot represent the original composition of the solar compilation of relative abundances. However, also in this case, system. This occurrence should be carefully taken into account, the composition of the pristine solar system would remain unde- especially for those elements whose abundances are almost ex- termined.
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