Publication: Detección de planetas en estrellas activas y estudio de la interacción estrella-planeta: el caso de BD+20 1790
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2016-02-10
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Universidad Complutense de Madrid
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Abstract
Desde que en 1995 se descubriera el primer exoplaneta, la búsqueda de planetas alrededor de otras estrellas se ha convertido en uno de los campos con mayor productividad en Astrofísica, una auténtica cacería que ha dado como fruto cerca de 2000 nuevos mundos. La fauna planetaria descubierta presenta una diversidad nunca antes imaginada, tan amplia que ha llevado a replantear las teorías de formación de planetas, para poder explicar su existencia. La búsqueda de exoplanetas se ha centrado tradicionalmente en estrellas con niveles muy bajos de actividad. Los surveys de búsqueda de planetas descartaban de manera sistemática las estrellas activas y jóvenes, en favor de estrellas más viejas y menos activas. Esto introduce un importante sesgo observacional en los resultados, dejando sin explorar un importante régimen en el espacio de parámetros, imprescindible para ayudar a reconstruir los escenarios de formación y describir las etapas más tempranas de la evolución de los sistemas planetarios. Detectar planetas en estrellas jóvenes puede proporcionar una información muy valiosa de los mecanismos de formación y su escala temporal. Sin embargo, hay muy pocos planetas detectados en estrellas con edades menores de 100 millones de años. Esta Tesis doctoral se centra en el estudio de la estrella BD+ 20 1790, de tipo espectral K5Ve, excepcionalmente activa (log R′ HK=-3.7) y joven, con una edad estimada de 35-80 millones de años. En comparación con objetos del mismo tipo espectral, velocidad de rotación y rango de edad, BD+20 1790 presenta un nivel de actividad muy elevado, que se manifiesta en una plétora de fenómenos en todas las capas de la atmósfera. Mediante espectroscopía multilongitud de onda con alta resolución espectral y datos fotométricos de varias épocas, se ha podido determinar qué fenómenos ocurren en las distintas capas de la atmósfera de la estrella, su escala temporal y su intensidad. En la cromosfera se detectaron estructuras tipo protuberancia co-rotando con la estrella, localizadas en zonas de inestabilidad lejos del plano ecuatorial, y emisión debida a playas. A partir del análisis fotométrico se encontraron manchas fotosféricas con un factor de cobertura entre 9% y 15%, y que parecen regenerarse en un rango de latitudes. Son numerosas las fulguraciones muy intensas observadas, con una fase de decaimiento de varias horas. La tasa de ocurrencia de fulguraciones es altísima, del orden del 60%. La luminosidad en rayos X es mucho mayor que la promedio para una estrella K de similar edad. Observaciones espectropolarimétricas han permitidomedir por primera vez el campomagnético de la estrella. Las variaciones en la velocidad radial tienen una semiamplitud en torno a 1 km/s. Este hecho junto con el elevado nivel de actividad de la estrella, que no se justifica por su juventud, motivó el estudio de la naturaleza de la variación de las velocidades radiales...
Since the first exoplanet was discovered in 1995, the search of planets around other stars has become one of the more productive fields in Astrophysics, a real "hunt" that has provided about 2000 new worlds. The planetary "zoo" found until now presents a diversity that could not be imagined before and that has led to reconsider planetary formation theories in order to explain their existence. The exoplanet searches have been centered traditionally in stars with low activity levels. Planet search surveys avoid systematically active and young stars, favoring older and less active ones. This introduces an important observational bias in the results, leaving unexplored an important parameter space, essential to aid in the formation scenario reconstruction and to describe the earliest stages of planetary systems evolution. Detecting planets in young stars can provide a valuable information about the migration processes or their atmosphere evolution. There is an important dearth of planets around stars with ages under 100 Million years. This PhDwork is focused in the study of BD+ 20 1790, a KV5e young star, exceptionally active (log R′ HK=-3.7), with an estimated age of 35-80 million years. In comparison with stars of similar spectral type, rotational velocity and age, BD+ 20 1790 presents a high activity level, that manifests in a plethora of phenomena in all atmospheric layers. Through an spectroscopic multiwavelength coverture of high resolution echelle spectra, and the analysis of photometric data obtained in several epochs, the different phenomena that occur in different layers of the star atmosphere could be determined, as well as their temporal scale and intensity. In the chromosphere, protuberance-like structures that co-rotate with the star have been detected, located in unstable latitudes far from the equatorial plane. Also plage emissions are found. From the photometric analysis, photospheric spots are detected with a spot filling factor of the stellar disc ranging between 9% and 15%. The spots seem to regenerate in a determined range of latitudes. Flare events are numerous and very energetic, with several hours of decay phase. The flare rate for BD+20 1790 is very high, about 60%. While the average luminosity of a K type star of similar age is logLX ∼ 28.8, BD+20 1790 is a markedly active star, with a logLX ∼ 29.2. Spectropolarimetric observations have allowed to measure the magnetic field of the star for the first time, being 67 G in average. Radial velocity variations found have a semiamplitude of 1 km/s. This fact, joint to the high activity level of the star, that cannot be explained by its youth, was the motivation for the study of the radial velocity variation sources. Based in a careful activity phenomena study, and in a deep analysis and discussion of the activity effects on radial velocity, the presence of a planetary companion to the star 3 4 Summary is the best explanation for the radial velocity variations...
Since the first exoplanet was discovered in 1995, the search of planets around other stars has become one of the more productive fields in Astrophysics, a real "hunt" that has provided about 2000 new worlds. The planetary "zoo" found until now presents a diversity that could not be imagined before and that has led to reconsider planetary formation theories in order to explain their existence. The exoplanet searches have been centered traditionally in stars with low activity levels. Planet search surveys avoid systematically active and young stars, favoring older and less active ones. This introduces an important observational bias in the results, leaving unexplored an important parameter space, essential to aid in the formation scenario reconstruction and to describe the earliest stages of planetary systems evolution. Detecting planets in young stars can provide a valuable information about the migration processes or their atmosphere evolution. There is an important dearth of planets around stars with ages under 100 Million years. This PhDwork is focused in the study of BD+ 20 1790, a KV5e young star, exceptionally active (log R′ HK=-3.7), with an estimated age of 35-80 million years. In comparison with stars of similar spectral type, rotational velocity and age, BD+ 20 1790 presents a high activity level, that manifests in a plethora of phenomena in all atmospheric layers. Through an spectroscopic multiwavelength coverture of high resolution echelle spectra, and the analysis of photometric data obtained in several epochs, the different phenomena that occur in different layers of the star atmosphere could be determined, as well as their temporal scale and intensity. In the chromosphere, protuberance-like structures that co-rotate with the star have been detected, located in unstable latitudes far from the equatorial plane. Also plage emissions are found. From the photometric analysis, photospheric spots are detected with a spot filling factor of the stellar disc ranging between 9% and 15%. The spots seem to regenerate in a determined range of latitudes. Flare events are numerous and very energetic, with several hours of decay phase. The flare rate for BD+20 1790 is very high, about 60%. While the average luminosity of a K type star of similar age is logLX ∼ 28.8, BD+20 1790 is a markedly active star, with a logLX ∼ 29.2. Spectropolarimetric observations have allowed to measure the magnetic field of the star for the first time, being 67 G in average. Radial velocity variations found have a semiamplitude of 1 km/s. This fact, joint to the high activity level of the star, that cannot be explained by its youth, was the motivation for the study of the radial velocity variation sources. Based in a careful activity phenomena study, and in a deep analysis and discussion of the activity effects on radial velocity, the presence of a planetary companion to the star 3 4 Summary is the best explanation for the radial velocity variations...
Description
Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Astrofísica y Ciencias de la Atmósfera, leída el 15-12-2015.