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Astron. Astrophys. 323, L9-L12 (1997)

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1. Introduction

Since the pioneering work of Geiss & Reeves (1972), a number of papers have been devoted to the fundamental question of the estimate of the protosolar abundance of deuterium as reviewed by Geiss (1993), hereafter referred as G93. The current value of [FORMULA] [FORMULA], is that derived by G93 from a reanalysis of the measurements of [FORMULA] in the Solar Wind (SW).

At the end of the pre-main sequence the young Sun is still fully convective with a temperature, at center, of a few million K; the mixing makes that all the deuterium is converted to [FORMULA] via the reaction [FORMULA]. As pointed out by Geiss & Reeves (1972), if [FORMULA] [FORMULA] is no longer changed at the surface then, [FORMULA] is equal (G93) to [FORMULA] times the difference between [FORMULA] presently observed at the surface and the protosolar [FORMULA]:

[EQUATION]

[FORMULA] is measured only in the solar wind, from the data available G93 concluded to [FORMULA] [FORMULA]. However G93 analyzing the various processes which may fractionate the helium isotopes in the interior and in the atmosphere of the Sun, has estimated that the actual ratio to be considered for inferring [FORMULA] is the solar wind value divided by a factor [FORMULA], resulting in [FORMULA] [FORMULA]. [FORMULA] has been estimated by G93 from meteoritic data to be equal to [FORMULA] [FORMULA].

An other way to determine the protosolar deuterium abundance is to evaluate [FORMULA] in Jupiter. This approach is based on the fact that Jupiter is mainly made of hydrogen which originates from the primordial solar nebula. According to current models of formation of Jupiter, some amount of ices, presumably enriched in deuterium with respect to the protosolar value, may have been mixed to hydrogen during the planetary formation, but their mass is currently considered as too small to have significantly increased [FORMULA] (Hubbard & MacFarlane 1980). Although the various estimates of [FORMULA] from remote sensing observations of HD and of [FORMULA] are uncertain, it was generally considered, prior to the arrival of the Galileo mission to Jupiter, that the Jovian deuterium abundance is consistent with the G93 value (Lecluse et al. 1996). The preliminary result of the mass spectrometer aboard the Galileo atmospheric probe - [FORMULA] [FORMULA] (Niemann et al. 1996) - is however substantially higher than the G93 value. This result, if confirmed, would have profound cosmological implications.

Compared to the present day InterStellar Medium (ISM) value of [FORMULA] [FORMULA] (Linsky et al. 1993), it suggests a deuterium destruction in 4.55 Gyr, the solar age, inconsistent with current models of chemical evolution of galaxies. Alternatively, it might be that [FORMULA] varies upon the line of sight so that, the concept of a well defined ISM deuterium abundance is not reliable, as advocated by some authors e.g., Ferley et al. (1995). A third possibility is that the current protosolar value [FORMULA], is underestimated.

Two circumstances prompted us to reanalyze the determination of G93: - First, Niemann et al. (1996) have measured in Jupiter [FORMULA] [FORMULA] which must be the exact ratio in the primitive solar nebula. As pointed out by these authors, this results in an increase of the estimation of [FORMULA]. - Second, since 1993 the introduction of microscopic diffusion in solar modeling have significantly increased the accuracy of models (e.g., Basu et al. 1996). A consequence of the microscopic diffusion is the change, with respect to time, of surface values of [FORMULA] and [FORMULA]. A result directly relevant to this Letter is the inferred helium mass fraction at the surface namely [FORMULA] (Pérez Hernández & Christensen-Dalsgaard 1994) substantially less than the protosolar value [FORMULA] (Bahcall & Pinsonneault 1995). Therefore, we propose here to complete the approach of G93 by using a solar evolutionary model with microscopic diffusion. The modeling of the solar evolution and the data employed are described in Section 2; Section 3 is devoted to results and discussions. We conclude in Section 4.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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