Stellar Activity or a Planet? Revisiting dubious Radial Velocity signals in the M-dwarf system GJ581
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Date
2024
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Universidad de Concepción
Abstract
Las estrellas enanas M, las más abundantes en el universo, son candidatas ideales para albergar planetas potencialmente habitables. GJ581, una estrella M3V, alberga un sistema multiplanetario que ha sido ampliamente estudiado. Con un periodo de rotación estelar reportado de 132 días, se han confirmado tres planetas: GJ581b, GJ581c y GJ581e. Otras señales periódicas atribuidas a GJ581d y GJ581g, con periodos orbitales reportados de 66.8 y 36.5 días, respectivamente, han generado debate ya que sus periodos están cerca de los armónicos del periodo de rotación estelar, lo que genera dudas sobre el origen de las señales de velocidad radial (RV). Algunos investigadores argumentan que estas señales son resultado de la actividad estelar, mientras que otros sugieren orígenes planetarios, con uno posiblemente en la zona habitable. Esta tesis tiene como objetivo investigar las señales periódicas atribuidas a GJ581d y GJ581g para determinar su verdadero origen. Utilizando datos de los instrumentos HARPS, HIRES y CARMENES, analizamos las RV junto con los índices de actividad estelar (Hα, Na I D y S-index) mediante modelos simultáneos con órbitas Keplerianas para las RV y regresión de Procesos Gaussianos (GP) para los índices de actividad. Derivamos el periodo de rotación modelando Hα con un GP, obteniendo 132.79+3.21 −2.60 días. Nuestro mejor modelo es un 3-Kepleriana + GP con un valor de ∆ BIC de 174.95 sobre el modelo 4-Kepleriana + GP, indicando tres planetas: GJ581b (Porb : 5.368 [d]), GJ581c (Porb : 12.918 [d], and GJ581e (Porb : 3.148 [d]). La semiamplitud de la tercera Kepleriana (1.78 m/s) es significativamente menor que las otras, lo que complica el modelado de señales más allá de tres Keplerianas debido a las limitaciones de precisión de las RV. Para la señal atribuida a GJ581g, identificamos una señal periódica de 33.08 días, cercana al valor de la literatura pero mejor explicada como un armónico del periodo de rotación estelar. Nuestros hallazgos destacan la importancia de tener en cuenta la actividad estelar al buscar señales planetarias, además de demostrar la efectividad del modelado simultáneo de RV y trazadores de actividad en la diferenciación de señales planetarias del ruido estelar. Observaciones futuras con espectrógrafos más precisos, como ESPRESSO, podrían proporcionar mayor claridad, pero actualmente, nuestros modelos favorecen la presencia de tres planetas en GJ581.
M-dwarf stars, the most abundant in the universe, are ideal candidates for hosting potentially habitable planets. GJ581, an M3V star, hosts a multi-planetary system and has been extensively studied. However, controversy remains regarding the number of planets it harbors. With a reported stellar rotation period of 132 days, three planets—GJ581b, GJ581c, and GJ581e—have been confirmed. Additional periodic signals attributed to GJ581d and GJ581g, with reported orbital periods of 66.8 and 36.5 days, respectively, have sparked debate. These periods are close to the harmonics of the stellar rotation period, raising doubts about the origin of radial velocity (RV) signals. Some researchers argue these signals result from stellar activity, while others suggest planetary origins, with one possibly in the habitable zone. This thesis aims to investigate the dubious periodic signals attributed to GJ581d and GJ581g to determine their true origin. Using data from the HARPS, HIRES, and CARMENES instruments, we analyzed RV alongside spectroscopic activity indices (Hα, Na I D, and S-index) using simultaneous modeling with Keplerian orbits for RVs and Gaussian Process (GP) regression for activity indices. We derived the stellar rotation period using GP regression with a Quasi-Periodic kernel on the Hα index, yielding 132.79+3.21 −2.60 days. Our best-fit model is a 3 Keplerian + GP with a ∆ BIC value of 174.95 over the 4-Keplerian + GP model, indicating three planets: GJ581b (Porb : 5.368 [d]), GJ581c (Porb : 12.918 [d], and GJ581e (Porb : 3.148 [d]). The semi-amplitude of the third Keplerian (1.78 m/s) is significantly lower than the others, complicating the modeling of signals beyond three Keplerians due to RV precision limitations. For the signal attributed to GJ581g, we identified a 33.08-day periodic signal, close to the literature value but better explained as a harmonic of the stellar rotation period. Our findings highlight the importance of accounting for stellar activity when searching for planetary signals, as it can induce apparent RV variations mimicking planetary signals. This work demonstrates the effectiveness of simultaneous RV and activity tracer modeling in distinguishing planetary signals from stellar noise. Future observations with more precise spectrographs, such as ESPRESSO, may provide further clarity, but currently, our models favor the presence of three planets in the GJ581 system.
M-dwarf stars, the most abundant in the universe, are ideal candidates for hosting potentially habitable planets. GJ581, an M3V star, hosts a multi-planetary system and has been extensively studied. However, controversy remains regarding the number of planets it harbors. With a reported stellar rotation period of 132 days, three planets—GJ581b, GJ581c, and GJ581e—have been confirmed. Additional periodic signals attributed to GJ581d and GJ581g, with reported orbital periods of 66.8 and 36.5 days, respectively, have sparked debate. These periods are close to the harmonics of the stellar rotation period, raising doubts about the origin of radial velocity (RV) signals. Some researchers argue these signals result from stellar activity, while others suggest planetary origins, with one possibly in the habitable zone. This thesis aims to investigate the dubious periodic signals attributed to GJ581d and GJ581g to determine their true origin. Using data from the HARPS, HIRES, and CARMENES instruments, we analyzed RV alongside spectroscopic activity indices (Hα, Na I D, and S-index) using simultaneous modeling with Keplerian orbits for RVs and Gaussian Process (GP) regression for activity indices. We derived the stellar rotation period using GP regression with a Quasi-Periodic kernel on the Hα index, yielding 132.79+3.21 −2.60 days. Our best-fit model is a 3 Keplerian + GP with a ∆ BIC value of 174.95 over the 4-Keplerian + GP model, indicating three planets: GJ581b (Porb : 5.368 [d]), GJ581c (Porb : 12.918 [d], and GJ581e (Porb : 3.148 [d]). The semi-amplitude of the third Keplerian (1.78 m/s) is significantly lower than the others, complicating the modeling of signals beyond three Keplerians due to RV precision limitations. For the signal attributed to GJ581g, we identified a 33.08-day periodic signal, close to the literature value but better explained as a harmonic of the stellar rotation period. Our findings highlight the importance of accounting for stellar activity when searching for planetary signals, as it can induce apparent RV variations mimicking planetary signals. This work demonstrates the effectiveness of simultaneous RV and activity tracer modeling in distinguishing planetary signals from stellar noise. Future observations with more precise spectrographs, such as ESPRESSO, may provide further clarity, but currently, our models favor the presence of three planets in the GJ581 system.
Description
Tesis presentada para optar al grado académico de Magíster en Astronomía
Keywords
Astrometría, Astrofísica, Análisis espectral