Nuevos vecinos en la vecindad solar
David Barrado y Navascués
Una zona de moda para vivir… La vecindad solar se está llenando de nuevos inquilinos: estrellas, enanas marrones y planetas orbitando entorno a aquéllas.
Hace diez años los astrónomos creíamos que el censo de las estrellas más próximas al Sol estaba prácticamente completo. En un radio de 25 parsecs (81.5 años luz) se habían catalogado unas 3800 estrellas de distintos tipos espectrales: desde A0 hasta M9, con temperaturas superficiales (más precisamente, temperaturas efectivas) en el rango 10,000-2,400 grados kelvin (K).
Sin embargo, desde el advenimientos de las cámaras de gran campo, nuevos detectores ópticos e infrarrojos, y de los barridos sistemáticos del cielo realizados desde los años 90 (2MASS, DENIS, Sloan, UKIDSS), una nueva población de objetos de baja masa ha sido revelada. Son objetos que por su debilidad intrínseca (entre 100 y 10,000 veces menor que la solar) son muy difíciles de detectar. Más aún, sus rojos colores (esto es, el hecho de que emiten primordialmente en longitudes de onda infrarroja, no detectables directamente por el ojo humano) complicó su detección inicial. Sus propiedades son tan peculiares que ha sido necesario la introducción de dos nuevos tipos espectrales, L y T, caracterizados entre otras particularidades por la presencia de intensas líneas correspondientes a elementos alcalinos (potasio y sodio), y bandas moleculares debidas al metano, respectivamente.
Entre las nuevas sorpresas destaca el sistema múltiple de epsilon indi, la vigesimo cuarta estrella más cercana al Sol, a unos 11.8 años luz, y formado por una primaria de tipo K5V (estrella de la secuencia principal con temperatura de unos 4500 grados kelvin) y una compañera secundaria de tipo T, cuyo espectro infrarrojo está dominado por intensas bandas de absorción debidas al metano. Técnicas de óptica adaptativa han mostrado que esta secundaria está compuesta a su vez por dos miembros: dos enanas marrones (objetos con propiedades a medio camino entre las características de los planetas y la de las estrellas) con temperaturas de 1100 y 800 grados kelvin.
El último descubrimiento corresponde a un grupo liderado por Beth Biller, de la Universidad de Arizona. Para ello han utilizado uno de los telescopios de 8 metros del Observatorio Austral Europeo (ESO) y NACO/SDI, el sistema de óptica adaptativa combinado con imágenes diferenciales. Así, han sido capaces de detectar una enana marrón que orbita en torno a la estrella SCR 1845-6357. Este objeto subestelar tendría una masa entre 9 y 65 masas de Júpiter y una temperatura superficial de unos 1000 grados kelvin.
Diagrama tridimensional de las estrellas más cercanas al sistema solar (adaptación de ESO de uno similar creado por R Powell)
Imagen compuesta a tres colores del sistema SCR1845-6357
Secuencia de tres imagenes tomada por B. Biller y colaboradores con NACO/SDI en diferentes filtros en el infrarrojo. Este sistema cancela la emisión de la estrella central y resalta la emisión a 1.575 micras y la debilidad a 1.625 micras, provocada por una banda de molecular de metano, de la enana marrón.
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