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The Galactic evolution of phosphorus

1er août 2011

August 2011 — The chemical composition of the various stars in the Galaxy is an interesting tracer of its evolution. Up to now, phosphorous has remained mysterious, because it is difficult to detect in cool stars. An international team of astronomers, including researchers from Paris Observatory, has measured with the VLT/CRIRES the phosphorus abundance of twenty cool stars in the Galactic disk. They show that the ratio of phosphorus to iron (P/Fe) behaves similarly to sulphur, increasing towards lower metallicity stars. The observed rise in the P/Fe abundance ratio is steeper than predicted by Galactic chemical evolution models, and the different processes to produce phosphorus in stars have to be revised. In addition, there is no difference in (P/Fe) for stars with planets and stars without any detected planet.

Phosphorus, with an atomic number of Z=15, is in the nitrogen group in the periodic table of elements. It lies between silicon (Z=14) and sulphur (Z=16). At variance with phosphorus, both silicon and sulphur are well-studied, and their abundance is measured in the photospheres of "cool" stars. Phosphorus is an abundant element in the Universe, and is essential for the life, as we know it on Earth. In fact, phosphorus is a component of DNA, RNA, ATP (phosphate) and of the cell membranes (phospholipids). In the solar photosphere phosphorus is among the top twenty most abundant elements. In spite of being a common element, abundant in the Universe, and important for our life, up to now phosphorus has never been systematically analysed in the Galaxy. The reason for this lack of investigation is due to the absence of P signature in the stellar spectra ; no line of phosphorus is detected in the wavelength range usually observed in cool stars. Since few years CRIRES, an infra-red high resolution spectrograph is available, at the 8 m class telescopes, ESO-Chile. The weak lines of P I, in the range 1050-1082 nm, are detectable with CRIRES. It is then possible to derive the abundance of phosphorus in stars at different stages of the Galaxy’s life, to trace the evolution of phosphorus in the Milky Way. A team of astronomers led by Elisabetta Caffau (ZAH,GEPI) investigated a sample of twenty Galactic stars, with a metallicity in the range from 1/10 to twice the Solar metallicity, to derive the first picture of the Galactic evolution of phosphorus. The sample contained stars both with and without detected planets. The result is shown in Fig.1.

Figure 1 : L’abondance relative du phosphore, [P/Fe], en fonction de la metallicité, [Fe/H]. Les étoiles connues pour avoir des planètes (carrés rouges) ont des symboles différents des étoiles sans planète connue (hexagones noirs). Un modèle d’évolution galactique du phosphore (Kobayashi et al. 2006) est ajouté (ligne continue) pour la comparaison aux données observées. Cliquer sur l’image pour l’agrandir

If we use [Fe/H] = (log10(NFe/NH)-log10(NFe/NH)Sun) as a measure for the stellar metallicity, [P/Fe], defined as log10(NP/NH)-log10(NP/NH)Sun-[Fe/H], is the logarithm of the ratio phosphorus to iron abundances. We can see that [P/Fe] is close to zero for solar metallicity stars, and increases as metallicity decreases, i.e. more metal poor stars have a higher phosphorous to iron ratio than the Sun. There is no evident difference in behaviour between stars with and without planets. This behaviour was unexpected, and prompts for a revision of our models.

E. Caffau (1,2), P. Bonifacio (2), R. Faraggiana (3), M. Steffen (4,2) ((1) ZAH, LSW, (2) GEPI-Obs. Paris, (3) Univ. Trieste, (4), Leibniz Institut fuer Astrophysik, Potsdam) A&A in press

Contact

  • Piercarlo Bonifacio
    Observatoire de Paris-GEPI, CNRS
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