From instrumental design
to scientific exploitation
Home > Scientific Pole > Stellar and Galactic physics > News > The Galactic evolution of phosphorus

The Galactic evolution of phosphorus

» Tuesday 9 August 2011

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 cold 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 PI, 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 the figure.
If we use [Fe/H] (log10(NFe/NH)-log10(NFe/NH))
as a measure for the stellar metallicity,
[P/Fe], defined as
is the logarithm of the ratio phosphorus to iron abundances.
The figure plots the relative abundance of phosphorus, [P/Fe], as a function of the metallicity, [Fe/H], for stars known to have planets (red squares) and stars without known planet (black hexagons). A model of Galactic evolution of P (Kobayashi et al. 2006) is superimposed.
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 phosphorus 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

The paper The Galactic evolution of phosphorus (Caffau, Bonifacio, Faraggiana & Steffen) will be published in A&A.