Estrellas de plomo/Stars of Lead

HE 2359-2844 artistic expression (C. S. Jeffery)

Las estrellas ricas en plomo, podrían representar una breve etapa de la evolución estelar sobre la que los científicos habían teorizado, pero que hasta ahora no habían observado. Los pequeños soles, HE 2359-2844 en la constelación del Escultor y HE 1256-2738 en Hydra, estaban entre los más ricos en helio según una búsqueda de estrellas anterior. Pero nuevos análisis han mostrado que estos dos soles son doblemente inusuales ya que poseen atmósferasr con concentraciones de plomo alrededor de 10.000 veces las observadas en la atmósfera de nuestro sol, según informan hoy, 1º de agosto, los investigadores online en el Monthly Notices de la Royal Astronomical Society. Las temperaturas superficiales de estas dos estrellas azuladas son de alrededor de 38.000 °C (mucho más calientes que la de la superficie de nuestro sol que es de alrededor de 5.500° C), tan caliente que los átomos de plomo presentes en sus atmósferas han sido despojados de tres electrones. Según los científicos, estas estrellas pueden estar pasando por una etapa de la evolución estelar que no dura más de unas pocas decenas de miles de años, una fase entre la de gigantes rojas (con 30 o 40 veces el tamaño de nuestro sol) y enanas azules (estrellas con un tamaño la quinta parte de nuestro sol, pero siete veces más calientes y 70 veces más brillantes). El plomo que rodea las estrellas, y que formaba parte de la nube inicial de gas y polvo de la que estas estrellas se formaron, y no se ha generado en las estrellas por medio de reacciones. Puede dispersarse dentro de una capa atmosférica de hasta 100 kilómetros de espesor (representado en rosa) y que en su conjunto pesa del orden de 100.000 millones de toneladas métricas.


The most lead-rich stars known to science may represent a brief stage in stellar evolution that scientists have theorized but previously haven’t seen. The small suns, known as HE 2359-2844 in the constellation Sculptor and HE 1256-2738 in Hydra, were among nine identified as being helium-rich in a previous survey of stars. But new analyses show that these two are doubly unusual because they also sport atmospheres with lead concentrations about 10,000 times those seen in the atmosphere of our sun, the researchers report online today, 1st August,  in Monthly Notices of the Royal Astronomical Society. The surface temperatures of these two bluish stars are estimated to be about 38,000°C (far hotter than our sun’s surface temperature of about 5500°C), so hot that lead atoms in their atmospheres have been stripped of three electrons. The stars may be passing through a stage of stellar evolution that lasts no more than a few tens of thousands of years, the scientists say—a phase between red giants (about 30 or 40 times the size of our sun) and blue subdwarfs (stars about one-fifth the size of our sun but seven times hotter and 70 times brighter). The lead surrounding the stars—which was part of the original cloud of gas and dust from which these stars formed, not generated by reactions in the evolving stars themselves—may be dispersed within an atmospheric layer as much as 100 kilometers thick (depicted patchily in pink) that altogether weighs up to 100 billion metric tons.

Tomado de/Taken from Science

Resumen de la publicación/Abstract of the paper
Discovery of extremely lead-rich subdwarfs: does heavy metal signal the formation of subdwarf B stars?
N. Naslim, C.S. Jeffery, A. Hibbert and N.T. Behara
Monthly Notices of the Royal Astronomical Society
Abstract
Hot subdwarfs represent a group of low-mass helium-burning stars formed through binary-star interactions and include some of the most chemically peculiar stars in the Galaxy. Stellar evolution theory suggests that they should have helium-rich atmospheres but, because radiation causes hydrogen to diffuse upwards, a majority are extremely helium poor. Questions posed include: when does the atmosphere become chemically stratified and at what rate?
The existence of several helium-rich subdwarfs suggests further questions: are there distinct subgroups of hot subdwarf, or do hot subdwarfs change their surface composition in the course of evolution? Recent analyses have revealed remarkable surface chemistries amongst the helium-rich subgroup. In this paper, we analyse high-resolution spectra of nine intermediate helium-rich hot subdwarfs. We report the discovery that two stars, HE 2359−2844 and HE 1256−2738, show an atmospheric abundance of lead which is nearly 10 000 times that seen in the Sun. This is measured from optical Pb iv absorption lines never previously seen in any star. The lead abundance is 10 to 100 times that measured in normal hot-subdwarf atmospheres from ultraviolet spectroscopy. HE 2359−2844 also shows zirconium and yttrium abundances similar to those in the zirconium star LS IV−14°116. The new discoveries are interpreted in terms of heavily stratified atmospheres and the general picture of a surface chemistry in transition from a new-born helium-rich subdwarf to a normal helium-poor subdwarf.