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Simon Eriksson Simon.C.Eriksson@Gmail.Com Department of Astronomy { Stockholm University A Photometric Variability Study Using Brown Dwarfs As Giant Planet Analogues Investigating rotation periods and cloud structure Master of Science Thesis 3 June 2016 Author: Simon Eriksson [email protected] Supervisor: Markus Janson Abstract Recent discoveries of large numbers of low-mass field brown dwarfs below the deuterium-burning limit, offers up a substantial sample of giant planet analogues which can be directly imaged without the severe contrast difficulties that regularly bound planets suffer from. By detecting significant periodic variability in such objects, enhanced by their lower surface-gravities, over several hours we can obtain estimates of their rotation periods. As this periodicity is possibly the result of heterogeneous cloud features in the upper atmosphere of a brown dwarf, atmospheric cloud models can then be used to discern some properties of its cloud structure. In this work we independently investigate 19 L0-L7 and 2 T2.5-T3.5 brown dwarfs with a mass-range of ∼ 6 − 22 MJup, spanning the deuterium-burning limit. We detect significant large amplitude (> 2%) sinusoidal variability of < 9:3 ± 2:0% through near-infrared observations of PSO J318.5-22. This unusually red, 6:5 MJup, giant planet analogue was observed in JS and KS at the NTT/SOFI through the ESO observing program 194.C-0827(A) by PI: Biller, B. We are able to constrain the rotation period of PSO 318, estimating a likely period of at least 7 hours. We further detect entirely new and significant (> 99% confidence) variability in 3 targets, with tentative detections in another 10, out of which 9 of are new discoveries. Our results indicate a minimum variability fraction for these targets, primarily outside the L/T transition, as high +8 as fmin = 70−12%. We conclude that very low-mass brown dwarfs, many of which show unusually red colours and signs of low-gravity, might be more likely to exhibit both greater amplitudes and frequencies of rotationally modulated variability. ii Contents 1 Introduction 1 2 Background and Theory 3 2.1 Brown Dwarfs . .3 2.1.1 The Definition of BD vs. GP . .3 2.1.2 Observational properties . .4 2.1.3 Formation . .7 2.2 Giant Planets . .8 2.2.1 Observational properties . .8 2.2.2 Formation . .9 2.3 Previous Works . 10 2.3.1 Observational . 10 2.3.2 Modelling . 13 2.4 Summary . 13 3 Observations 14 3.1 NTT/SOFI . 17 3.1.1 Data reduction . 17 3.2 VLT/HAWK-I . 19 3.2.1 Data reduction . 20 4 Analysis 22 4.1 Aperture Photometry . 22 4.2 Light Curve Calibration . 26 4.3 Light Curve Analysis . 28 4.3.1 Polynomial subtraction . 28 4.3.2 Lomb-Scargle periodogram . 29 5 Results 30 5.1 Significantly Variable Targets . 31 5.2 Tentatively Variable Targets . 38 5.3 Non-Variable Targets . 45 6 Discussion 48 6.1 Implications of Binarity in the Sample . 48 6.2 PSO 318 . 49 6.3 HN Peg B . 51 6.4 Photometric Precision and the Reduction Process . 52 6.5 Frequency of Variability . 53 6.6 Comparisons with Previous Works . 53 6.7 Future Prospects . 55 7 Conclusions 55 References 57 A Binned light curves & finding charts of non-variable targets 61 iii B Polynomial subtraction results for tentative & non-variable targets 65 C L-S periodograms for tentative & non-variable targets 68 iv List of Figures 1 Optical spectra of SpT M7-T8 from 6800 to 8700 A................˚ 5 2 Radigan et al. (2014) SpT vs. 2MASS J − KS colour diagram . .6 3 Planet-Metallicity Correlation . .9 4 Biller et al. (2015) light curve for PSO 318 . 10 5 Radigan et al. (2012) light curves for 2M2139 . 11 6 Reduced light curves from Metchev et al. (2015) . 12 7 Colour-Magnitude diagram of our sample . 14 8 Single raw 60 s exposure from NTT/SOFI. 18 9 Typical master dark frame from NTT/SOFI . 18 10 Typical master flat field from NTT/SOFI . 19 11 Illumination correction . 19 12 HD 106906 b close-up . 20 13 HD 106906 b median box filtering . 21 14 HD 106906 b identification . 22 15 Raw light curve examples . 23 16 HD 106906 b Standard deviation vs. Median flux . 24 17 PSO 318 Ks frame combination effects . 25 18 PSO 318 Js Nov. raw light curves . 25 19 Calibrated light curve of CB reference star . 26 20 Binned light curve of CB reference star . 27 21 Polynomial subtraction of the CB reference star . 29 22 Simulated sinusoidal data . 29 23 Lomb-Scargle periodogram of simulated data . 29 24 Lomb-Scargle periodogram for the CB reference star . 30 25 PSO 318 JS Oct. binned light curves & finding chart . 31 26 PSO 318 JS Nov. binned light curves & finding chart . 32 27 Polynomial subtraction results for PSO 318 JS Oct, Nov. 33 28 PSO 318 KS Nov. binned light curves & finding chart . 33 29 Polynomial subtraction result for PSO 318 KS Nov. 34 30 Lomb-Scargle periodograms for PSO 318 . 34 31 2M0045 binned light curves & finding chart . 35 32 Polynomial subtraction result for 2M0045. 35 33 2M0117 binned light curves & finding chart . 36 34 2M0501 binned light curves & finding chart . 37 35 Polynomial subtraction results for 2M0117 and 2M0501. 37 36 Lomb-Scargle periodograms for 2M0045, 2M0117 and 2M0501 . 38 37 2M0303 binned light curves & finding chart . 40 38 2M0326 binned light curves & finding chart . 41 39 2M0342 binned light curves & finding chart . 41 40 2M0355 binned light curves & finding chart . 42 41 2M0421 binned light curves & finding chart . 42 42 2M0536 binned light curves & finding chart . 43 43 2M2224 binned light curves & finding chart . 43 44 GU Psc b binned light curves & finding chart . 44 45 HN Peg B binned light curves & finding chart . 44 46 SIMP2154 binned light curves & finding chart . 45 47 HD 106906 b and control star comparison . 46 v 48 Model fitting for PSO 318 by B15 . 50 49 November 2014 light curve for PSO 318 by B15 . 52 50 SpT vs. J-KS diagram . 54 51 2M0103 binned light curves & finding chart . 61 52 2M0234 binned light curves & finding chart . 61 53 2M0323 binned light curves & finding chart . 62 54 2M0357 binned light curves & finding chart . 62 55 2M0518 binned light curves & finding chart . 63 56 2M2322 binned light curves & finding chart . 63 57 HD 106906 b binned light curves & finding chart . 64 58 2M0045 - Example with all RS . 64 59 Polynomial subtraction plots I . 65 60 Polynomial subtraction plots II . 66 61 Polynomial subtraction plots III . 67 62 Lomb-Scargle periodograms I . 68 63 Lomb-Scargle periodograms II . 69 List of Tables 1 SOFI observations . 15 2 HAWK-I observations . 15 3 Field object properties . 16 4 Companion object properties . 16 5 Detections of Variability . 47 vi 1 Introduction - 1 - Section 1.0 1 Introduction technology allows. The detection method used for the Kepler mis- In this thesis project we investigate 21 directly im- sion focuses on photometric observations of plane- aged low-mass brown dwarfs (BD's) for periodical tary transits in front of their host stars (e.g. Char- photometric variability. With a mass-range of 6-22 bonneau et al. 2007), producing light curves with Jupiter-masses (MJup) and with the majority at a clearly distinguishable drop in intensity which or just below the deuterium burning (DB) mass- can be analysed.
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