lunes, 5 de septiembre de 2011

Una estrella que no debiera existir/The Star That Should Not Exist

Un equipo de astrónomos europeos utilizó el Very Large Telescope (VLT) de ESO en Cerro Paranal, en Chile, para localizar a una estrella en la Vía Láctea que para muchos no debiera existir. Los científicos descubrieron que esta estrella se compone casi totalmente de hidrógeno y helio, con cantidades muy pequeñas de otros elementos químicos. Esta inusual composición la coloca en la "zona prohibida" de una teoría de formación estelar ampliamente aceptada, lo que implica que esta estrella es prácticamente imposible. Los resultados aparecerán en la edición del 1 de septiembre de 2011 de la revista Nature. 
La estrella SDSS J102915+172927, situada en la constelación de Leo, ha resultado ser la que posee la menor cantidad de elementos más pesados ​​que el helio (lo que los astrónomos llaman "metales") de todas las estrellas estudiadas hasta ahora. Tiene una masa más pequeña que la del Sol y probablemente tiene más de 13 mil millones de años. Su proporción de metales es más de 20.000 veces más pequeña que la del Sol, lo que significa que es muy primitiva
Según Elisabetta Caffau (Zentrum für Astronomie der Universität Heidelberg, Alemania y el Observatoire de Paris, Francia), autora principal del estudio: "Una teoría ampliamente aceptada predice que las estrellas de este tipo, con poca masa y cantidades extremadamente bajas de metales, no debiera existir, porque las nubes de material en donde se formaron nunca podrían haberse condensado .../... Fue sorprendente encontrar por primera vez una estrella en esta ‘zona prohibida’, y esto significa que tendrán que revisarse algunos de los modelos de formación estelar". 
Los cosmólogos creen que los elementos químicos más ligeros -como hidrógeno y helio- se crearon poco después del Big Bang, junto con algo de litio, mientras que casi todos los demás elementos se formaron posteriormente al interior de las estrellas. Las explosiones de supernova fueron las responsables de esparcir este material estelar hacia el medio interestelar, volviéndolo más rico en metales. Nuevas estrellas se formaron a partir de este medio enriquecido, las que posee una mayor cantidad de metales en su composición que las estrellas más viejas. Por lo tanto, la proporción de metales en una estrella nos indica cuántos años tiene
La estrella es tan antigua que su composición debería ser similar la del Universo poco después del Big Bang, con un poco más de metales en su interior. Sin embargo el equipo encontró que la proporción de litio en la estrella es al menos cincuenta veces menor del esperado en el material producido por el Big Bang. Sin embargo, esta inusual estrella probablemente no es única. "Hemos identificado varias estrellas candidatas que podrían tener niveles de metales similares o incluso inferiores a los de SDSS J102915+172927. Ahora estamos planeando observarlas con el VLT para ver si se confirman", concluye Caffau.


A team of European astronomers has used ESO’s Very Large Telescope (VLT) to track down a star in the Milky Way that many thought was impossible. They discovered that this star is composed almost entirely of hydrogen and helium, with only remarkably small amounts of other chemical elements in it. This intriguing composition places it in the “forbidden zone” of a widely accepted theory of star formation, meaning that it should never have come into existence in the first place. The results will appear in the 1 September 2011 issue of the journal Nature. 
A faint star in the constellation of Leo, called SDSS J102915+172927, has been found to have the lowest amount of elements heavier than helium (what astronomers call “metals”) of all stars yet studied. It has a mass smaller than that of the Sun and is probably more than 13 billion years old. The proportion of metals in the star is more than 20 000 times smaller than that of the Sun, meaning it is very primitive.
According Elisabetta Caffau (Zentrum für Astronomie der Universität Heidelberg, Germany and Observatoire de Paris, France), lead author of the paper: “A widely accepted theory predicts that stars like this, with low mass and extremely low quantities of metals, shouldn’t exist because the clouds of material from which they formed could never have condensed ... / ... It was surprising to find, for the first time, a star in this ‘forbidden zone’, and it means we may have to revisit some of the star formation models.”
Cosmologists believe that the lightest chemical elements — hydrogen and helium — were created shortly after the Big Bang, together with some lithium , while almost all other elements were formed later in stars. Supernova explosions spread the stellar material into the interstellar medium, making it richer in metals. New stars form from this enriched medium so they have higher amounts of metals in their composition than the older stars. Therefore, the proportion of metals in a star tells us how old it is
The star is so old, that its composition should be very similar to wthat of the Universe shortly after the Big Bang, with a few more metals in it, but the researcheres have found that the proportion of lithium in the star was at least fifty times less than expected in the material produced by the Big Bang. However, this freakish star is probably not unique. “We have identified several more candidate stars that might have metal levels similar to, or even lower than, those in SDSS J102915+172927. We are now planning to observe them with the VLT to see if this is the case,” concludes Caffau.

Tomado de ESO/ Taken from ESO 

Abstract of the paper
An extremely primitive star in the Galactic halo 
Elisabetta Caffau, Piercarlo Bonifacio, Patrick François, Luca Sbordone, Lorenzo Monaco, Monique Spite, François Spite, Hans-G. Ludwig, Roger Cayrel, Simone Zaggia, François Hammer, Sofia Randich, Paolo Molaro & Vanessa Hill 
Nature 477,67–69 (01 September 2011) doi:10.1038/nature10377R 

The early Universe had a chemical composition consisting of hydrogen, helium and traces of lithium; almost all other elements were subsequently created in stars and supernovae. The mass fraction of elements more massive than helium, Z, is known as ‘metallicity’. A number of very metal-poor stars has been found, some of which have a low iron abundance but are rich in carbon, nitrogen and oxygen. For theoretical reasons and because of an observed absence of stars with Z < 1.5 × 10−5, it has been suggested that low-mass stars cannot form from the primitive interstellar medium until it has been enriched above a critical value of Z, estimated to lie in the range 1.5 × 10−8 to 1.5 × 10−6, although competing theories claiming the contrary do exist. (We use ‘low-mass’ here to mean a stellar mass of less than 0.8 solar masses, the stars that survive to the present day.) Here we report the chemical composition of a star in the Galactic halo with a very low Z (≤ 6.9 × 10−7, which is 4.5 × 10−5 times that of the Sun) and a chemical pattern typical of classical extremely metal-poor stars —that is, without enrichment of carbon, nitrogen and oxygen. This shows that low-mass stars can be formed at very low metallicity, that is, below the critical value of Z. Lithium is not detected, suggesting a low-metallicity extension of the previously observed trend in lithium depletion11. Such lithium depletion implies that the stellar material must have experienced temperatures above two million kelvin in its history, given that this is necessary to destroy lithium.