Some considerations on the non-detection of interstellar Li and B towards the SN1987a

Abstract

A very low upper limit of 0.15 mÅ for the interstellar λ6707 Å Lii line has been recently derived towards the SN1987a by Baade and Magain (1988). This value corresponds toN(Li)<1.4×10¹¹ cm⁻² and gives [Li/H]<5.4×10⁻¹¹ assumingN(Hi)=2.6×10²¹ cm⁻² for the hydrogen column density in the LMC towards SN1987a. This value is lower than the Li abundance found in the Population II stars and lower than the minimum abundance allowed in the framework of the standard Big-Bang theory. We indirectly estimate the Li depletion usingKi observations and show that a depletion of 1.2 dex is plausible. Therefore, an interstellar abundance [Li/H] as high as 0.8×10⁻⁹ cannot be excluded. Any improvement in the above-mentioned upper limit will place important constraints on current theories for lithium nucleosynthesis.

High-resolution IUE spectra of the SN1987a have been analysed in search for IS 1362 ÅBii resonance lines. A minimum detectable equivalent width of 22 mÅ has been found, that impliesN(B)<1.2×10⁻¹² cm⁻² and [B/H]<4.7×10⁻¹⁰ cm⁻², i.e., comparable to the solar value of [B/H]=4×10⁻¹⁰. This limit is the most stringent derived so far for an external galaxy, and suggests that the rate of spallation processes in the LMC has not been higher than in our own Galaxy.