This thesis focuses on the quantification of the impact of stellar activity on observations of transiting planets and on the identification of a methodology to correct these effects. In particular, the role of starspots, both un-occulted and occulted is considered. The presence of spots on the visible stellar disk may distort the primary transit light curve in a wavelength-dependent way, mimicking the presence of an atmosphere. To take into account this bias, the thesis presents an innovative method to estimate the spots properties and their distribution from the out-of-transit observations and, on this basis, correct the planetary transit light curves, avoiding possible degeneracy between the presence of the planetary atmosphere and of the spots. The method is developed for low-resolution transit spectroscopy and is tested on realistic simulations of future observations of planetary transits with Ariel, a space mission dedicated to the observation of about 1000 transiting exoplanets whose launch is expected in 2029. Then, the method is applied to real observations of HST of a transiting planet. The first chapter of the thesis presents the state of the art of efforts done so far to mitigate the effect of the spots in planetary observations (chapter 1). Chapter 2 presents a method for correcting the effect of spots not-crossed by the transiting planet, starting from a basic model of the stellar activity, where the spotted star is simulated as a linear combination of stellar spectra at different temperatures and the star has a uniform emission. The method is tested on 3 simulated targets of transiting systems that will be observed by the Ariel mission. In Chapter 3, I introduced another component in the stellar model, making it more realistic. In fact, I simulate again the same targets analyzed in Chapter 2 but including the limb darkening effect to the stellar model. In this chapter, I show the importance of taking into account this effect if a good estimate of the spots’ parameters and a good correction of the planet's atmosphere have to be obtained. In Chapter 4 the approach presented in the previous chapters, and tested on simulations, is applied to observations of planetary transits of the planet LHS 1140 b, acquired with the Hubble Space Telescope. The analysis leads to two possible scenarios: in the first one the star is very active and almost the 65% of its surface is covered by spots (against other indicators in the literature suggesting a quiet star); in the second one, the star is quiet but is about 300 K cooler than in the first scenario. In both cases, the observed chromatic modulation derived from the planetary transit light curves is not due to the presence of a planetary atmosphere. In Chapter 5, I analyze the TESS light curves of the active star V1298 Tau, by modeling the star with a model dominated by 4 spots, co-rotating with the stellar surface. Such a study allows not only to derive the distribution of spots on V1298 Tau but also to correct the transit light curves of the 4 planets orbiting around the star for the effect of non-occulted spots. The results show that the spots’ effect may produce different transit depths for the 4 planets in the TESS and the K2 band, thus justifying the discrepancy between the planetary radii in the two bands reported in the scientific literature.
Il tema centrale della tesi è l'impatto dell'attività stellare sulle osservazioni dei pianeti in transito e sull'identificazione di una metodologia per correggere questi effetti. In particolare, viene approfondito l'effetto delle macchie stellari. La presenza di macchie sul disco stellare visibile può distorcere la curva della luce di transito primaria in modo dipendente dalla lunghezza d'onda, mimando la presenza di un'atmosfera planetaria. Per tener conto di questo effetto, la tesi presenta un metodo innovativo per stimare le proprietà delle macchie e la loro distribuzione dalle osservazioni fuori transito e correggere le curve di luce del transito planetario, evitando possibili degenerazioni tra la presenza delle atmosfera planetaria e delle macchie. Il metodo è sviluppato per la spettroscopia di transito a bassa risoluzione ed è testato su simulazioni realistiche di future osservazioni di transiti planetari con Ariel, una missione spaziale dedicata all'osservazione di circa 1000 esopianeti in transito il cui lancio è previsto nel 2029. Successivamente, il metodo viene applicato alle osservazioni reali di HST di un pianeta in transito. Il capitolo 1 della tesi presenta la letteratura relativa al fenomeno. Il capitolo 2 presenta un metodo per correggere l'effetto delle macchie non attraversate dal pianeta in transito, partendo da un modello base dell'attività stellare, dove la stella macchiata è simulata come una combinazione lineare di spettri stellari a diverse temperature e la stella ha un'emissione uniforme. Il metodo è testato su 3 sistemi transitanti che saranno osservati dalla missione Ariel. Nel capitolo 3 è introdotto un altro componente nel modello di attività stellare, rendendolo più realistico. Infatti, sono simulati nuovamente gli stessi target analizzati nel Capitolo 2 ma includendo l'effetto di oscuramento al bordo. In questo capitolo è discussa l'importanza di tener conto di questo effetto se si vuole ottenere una buona stima dei parametri delle macchie e una buona correzione dell'atmosfera del pianeta. Nel Capitolo 4 l'approccio presentato nei capitoli precedenti, e testato su simulazioni, è applicato a osservazioni di transiti planetari del pianeta LHS 1140 b con il Telescopio Spaziale Hubble. L'analisi porta a due possibili scenari: nel primo la stella è molto attiva e quasi il 65% della sua superficie è ricoperta da macchie (contro altri indicatori in letteratura che suggeriscono una stella tranquilla); nel secondo, la stella è silenziosa ma è circa 300 K più fredda rispetto al primo scenario. In entrambi i casi, la modulazione cromatica osservata derivata dalle curve di luce del transito planetario non è dovuta alla presenza di un'atmosfera planetaria. Nel Capitolo 5, vengono analizzate le curve di luce TESS della stella attiva V1298 Tau, modellando la stella con un modello a 4 macchie rotanti sulla superficie stellare. Tale studio permette non solo di ricavare la distribuzione delle macchie su V1298 Tau ma anche di correggere le curve di luce di transito dei 4 pianeti orbitanti intorno alla stella per effetto delle macchie non occultate. I risultati mostrano che l'effetto delle macchie può produrre differenti profondità di transito per i 4 pianeti in banda TESS e K2, giustificando così la discrepanza tra i raggi planetari nelle due bande riportata nella letteratura scientifica.
(2023). Mitigation of the impact of stellar activity on observations of transiting planets.
Mitigation of the impact of stellar activity on observations of transiting planets
Cracchiolo, Gianluca
2023-01-01
Abstract
This thesis focuses on the quantification of the impact of stellar activity on observations of transiting planets and on the identification of a methodology to correct these effects. In particular, the role of starspots, both un-occulted and occulted is considered. The presence of spots on the visible stellar disk may distort the primary transit light curve in a wavelength-dependent way, mimicking the presence of an atmosphere. To take into account this bias, the thesis presents an innovative method to estimate the spots properties and their distribution from the out-of-transit observations and, on this basis, correct the planetary transit light curves, avoiding possible degeneracy between the presence of the planetary atmosphere and of the spots. The method is developed for low-resolution transit spectroscopy and is tested on realistic simulations of future observations of planetary transits with Ariel, a space mission dedicated to the observation of about 1000 transiting exoplanets whose launch is expected in 2029. Then, the method is applied to real observations of HST of a transiting planet. The first chapter of the thesis presents the state of the art of efforts done so far to mitigate the effect of the spots in planetary observations (chapter 1). Chapter 2 presents a method for correcting the effect of spots not-crossed by the transiting planet, starting from a basic model of the stellar activity, where the spotted star is simulated as a linear combination of stellar spectra at different temperatures and the star has a uniform emission. The method is tested on 3 simulated targets of transiting systems that will be observed by the Ariel mission. In Chapter 3, I introduced another component in the stellar model, making it more realistic. In fact, I simulate again the same targets analyzed in Chapter 2 but including the limb darkening effect to the stellar model. In this chapter, I show the importance of taking into account this effect if a good estimate of the spots’ parameters and a good correction of the planet's atmosphere have to be obtained. In Chapter 4 the approach presented in the previous chapters, and tested on simulations, is applied to observations of planetary transits of the planet LHS 1140 b, acquired with the Hubble Space Telescope. The analysis leads to two possible scenarios: in the first one the star is very active and almost the 65% of its surface is covered by spots (against other indicators in the literature suggesting a quiet star); in the second one, the star is quiet but is about 300 K cooler than in the first scenario. In both cases, the observed chromatic modulation derived from the planetary transit light curves is not due to the presence of a planetary atmosphere. In Chapter 5, I analyze the TESS light curves of the active star V1298 Tau, by modeling the star with a model dominated by 4 spots, co-rotating with the stellar surface. Such a study allows not only to derive the distribution of spots on V1298 Tau but also to correct the transit light curves of the 4 planets orbiting around the star for the effect of non-occulted spots. The results show that the spots’ effect may produce different transit depths for the 4 planets in the TESS and the K2 band, thus justifying the discrepancy between the planetary radii in the two bands reported in the scientific literature.File | Dimensione | Formato | |
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