Tipos de Digestores Anaerobios

flux and the flux of the star (with lightning flux density of ∼ 10 Jy, Fig. 6.2, bottom panel) is

∼102. If every single lightning flash on HAT-P-11b would emit_∼1012W, the optical emission of lightning that would produce the radio emission, would outshine the host star by two orders of magnitude.

6.4

Lightning Chemistry

Lightning produces non-equilibrium species affecting the composition of the local atmosphere (Chapter3, Sect.3.2). It is important, therefore, to consider what the chemical effects of lightning activity on HAT-P-11b would be on the local atmosphere, and on the spectrum of the planet. Here, the effect of large, Saturn-like lightning storms on HCN chemistry is considered. Lewis

(1980) estimated that lightning produces HCN at a rate of 2_×10−10 kg/J. Their model was set up for Jupiter, and is applicable for any hydrogen-dominated atmosphere with roughly solar composition. Farrell et al.(2007) estimated the dissipative energy of a single lightning flash on Saturn, with peak frequency, f₀=10 kHz, and spectral roll-off,n=3.5, to be:

Ed ≈260 J ‚ fSat f0 Œn , (6.6)

where f_Sat=10 MHz is the frequency at which the lightning on Saturn was observed. One can multiply the dissipative energy by the flash density of 1 flash km−2h−1. Multiplying the lightning energy density by the production rate of HCN, it is estimated that 5_×10−7kg m−2 s−1 of HCN is produced, of the order of 109greater than the estimate ofLewis(1980) for Jupiter. Accepting the energetics arguments fromLewis(1980), the resulting HCN will achieve a volume mixing ratio of

∼10−6within the mbar regime of the atmosphere.Moses et al.(2013) found that similar mixing ratios (their fig. 11) should have significant observational consequences in theL(3.0−4.0µm) andN(7.5₋14.5µm) IR bands, which they show in their fig. 16 comparing their model spectra both with and without HCN.

In order to estimate the chemical timescale3for HCN on HAT-P-11b, a semi-analytical pressure₋ temperature profile is developed, appropriate for the object using the method ofHansen(2008). The parameters for HAT-P-11b (mass, radius, distance from host star) given byBakos et al.(2010) andLopez & Fortney(2014), and the stellar temperature fromBakos et al.(2010) are used for this

Figure 6.4: The chemical lifetime of HCN[s]plotted vs. pressure [bar], along with various dynamical timescales with eddy diffusion coefficients rangingKzz=108. . . 1012cm2s−1. If HCN is produced in the

10−1_{bar pressure level in the atmosphere, than it will live long enough (days) to be transported to the} higher atmosphere, where it will be present for a couple of years after the storm have occurred.

profile. To determine the XUV flux impinging on the atmosphere, a spectrum appropriate for a K4-type star was taken, the X-Exospheres synthetic spectrum for HD 111232 (Sanz-Forcada et al.,

2011). It is assumed that the atmosphere is hydrogen rich, and the atmospheric gas of HAT-P-11b is at roughly solar metallicity with respect to C, N and O, i.e. that the primordial concentrations of these elements in HAT-P-11b is solar and that there is no elemental depletion into clouds. The at- mospheric chemistry is calculated using the semi-analytic temperature profile and synthetic XUV flux, with the STAND2015 chemical network and the ARGO diffusion-photochemistry model from

Rimmer & Helling(2016). Then a variety of locations in the atmosphere is examined, injecting HCN at a mixing ratio of 10−6, and evolving the atmospheric chemistry in time to determine the chemical timescale for HCN as a function of pressure, shown in Fig.6.4. The dynamical timescale for vertical mixing is overlaid on the top of this plot, assuming a range of constant eddy diffusion coefficients.

The chemical timescale for HCN ranges from 100 milliseconds at the bottom of the model at- mosphere (10 bar), to 2.5 years at 5 mbar (Fig. 6.4). At pressures less than 5 mbar, the timescale for HCN slowly drops down to about 4 months at 10µbar, and then drops precipitously at lower pressures, until at 1µbar it achieves a timescale of about 30 minutes (Fig. 6.4). These results can be compared to the dynamical timescale of the atmosphere, represented approximately by

6.5. Summary

the eddy diffusion coefficient (see Lee et al., 2015, for details). If the chemical timescale for HCN is smaller than the dynamical timescale, then the HCN would be destroyed before it is trans- ported higher into the atmosphere. If the chemical timescale for HCN is larger than the dynamical timescale, then the HCN will survive long enough to reach other parts of the atmosphere, where it will survive longer. Assuming that lightning takes places on HAT-P-11b at pressures of®0.1 bar,

the produced HCN will survive long enough to be transported into the mbar regime, where it will survive for 2−3 years before being chemically destroyed. If, on the other hand, HCN is formed much below the 0.1 bar level, at pressures of¦1 bar, the chemical timescale is too short for the

HCN to escape, and it will be rapidly destroyed before it could be observed. This suggests that to confirm the presence of lightning in an atmosphere, future radio observations can be followed up by IR observations looking for molecules that are the result of lightning chemistry.

6.5

Summary

In this chapter, I presented an interpretation of the radio observations of HAT-P-11b made byL13

under the assumption that these transient radio signals are real and were caused by lightning on HAT-P-11b. I estimated that the flash density necessary to explain the average radio signal ranges between 29 and 2_×109 flashes km−2 h−1, depending on the parameters I use. These values range between the same order of magnitude as terrestrial volcanic eruptions show, and thunderstorms never seen in the Solar System before. I also examined the optical emission such a storm would generate, as well as the impact of this storm on the atmospheric chemistry, assuming a hydrogen-rich atmosphere.

In summary, I found that

1. the radio emission of a few mJy at 150 MHz, at the distance of HAT-P-11b, requires unreal- istically high flash densities if this lightning is like in the Solar System. However, if we let the parameters of spectral roll-off and lightning duration vary, the flash density can be as low as values seen during volcano eruptions. Nevertheless, a large part of the parameter space requires extremely large lightning activity, therefore lightning produced radio emis- sion most probably cannot be observed by current radio telescopes at frequencies of 150 MHz or higher, from distances of several tens of pc.

2. The optical counterpart of the enormous lightning storm would be as bright as the host star itself.

mosphere of an irradiated exoplanet with strong winds, may in some cases yield detectable quantities that linger in the atmosphere for 2−3 years after the advent of the lightning storm. If the lightning occurs much deeper in the atmosphere, the HCN will react away before it can diffuse into the upper atmosphere, and will probably not be observable.

The results show that the radio emission on HAT-P-11b is unlikely to be caused by lightning, if lightning properties similar to the Solar System ones are assumed. However, intermittent, pow- erful thunderstorms are not unprecedented in the Solar System: In 2010/11 a huge storm was observed in Saturn, producing the largest flash densities ever observed, with a total power com- parable to Saturn’s total emitted power (Fischer et al.,2011b). Such large, or even more powerful storms may occur on exoplanets. The current study also shows a new interpretation that could be applied to high frequency (up to_∼30₋50 MHz) radio observations, where it is more probable to observe lightning, because of its radiating properties (Eq. 6.1). The calculations explained in this chapter can be also applied to determine the minimum storm size detectable within an exoplanetary atmosphere using current or future radio instruments. The recommendation to ob- servers who detect radio emission in the frequency range of a few tens of MHz, especially if it is unpolarized, from an exoplanet would be to follow up these observations with infrared observa- tions made in theLandNbands when possible, in order to look for HCN emission, which should be observable for 2−3 years if lightning occurs around the 0.1 bar level of an atmosphere with reasonably large vertical convective velocities. If HCN is detected at that time, and if both the radio emission and the HCN turn out to be transient, this would be strong evidence for lightning on an exoplanet.

7

Looking for lightning on the closest brown dwarfs

7.1

Introduction

In the previous chapter, I introduced a method to estimate lightning activity on extrasolar objects from observed radio emission. Based on the results I estimated the optical emission a thunder- storm on the exoplanet HAT-P-11b could produce if its radio emission was the observed one in

Lecavelier des Etangs et al.(2013). In this chapter, I turn the method the other way around and ask the question: what is the optical flux of lightning from a target object, if we assume that lightning on that body has the same statistical and energetic properties that is known from the Solar System. The purpose of this short project is to estimate whether lightning optical emission can be observed with the 1.54-m Danish Telescope in La Silla, Chile1. The estimates were used as part of an observing proposal for the telescope. The telescope is equipped with a CCD and an EMCCD camera, and standard Johnson-Cousins System filters, of which I estimated observability in I, V and U bands. The sensitivity of the CCD camera of the Danish Telescope is∼90% in I, 80%

Brown Dwarf Distance[pc] Spectral Type Apparent magnitude Right Ascension Declination Luhman 16AB

(Luhman,2013)

2 L7.5 and T0.5(1) _{I: 14.95}(1) ₁₀h₄₉m_18.9s(1) ₋₅₃◦₁₉0_10.100(1)

εIndi (Scholz et al.,2003) (Volk et al.,2003) 3.6 T1 and T6(2) _{V: 24.12}(2) I: 15.60 22h₀₄m_10.5s(2) ₋₅₆◦₄₆0_57.700(2) SCR 1845-6357 (Biller et al.,2006) 3.85 T6(3) _{J: 13.29}(3) ₁₈h₄₅m_05.5s(3) ₋₆₃◦₅₇0_46.300(3) 1_{http://simbad.u-strasbg.fr/simbad/sim-id?Ident=2MASS%20J10491891-5319100}

2_{http://simbad.u-strasbg.fr/simbad/sim-id?Ident=*+eps+Ind+B} 3_{http://simbad.u-strasbg.fr/simbad/sim-id?Ident=SCR+J1845-6357B}

in V and 20% in U bands, while the EMCCD is_∼90% sensitive in all bands (Uffe G. Jorgensen, private communication).

Electromagnetic radiation propagates as r−2, therefore, the closer the object to us the larger the chance to receive electromagnetic flux from the same source on the object. The targets of this study are the three closest brown dwarf systems observable from Chile: Luhman 16,εIndi, and SCR 1845-6357. Luhman 16 is a binary brown dwarf system (Luhman,2013),εIndi is composed of a K5 star and a brown dwarf binary separated by_∼1450 AU from the primary (Scholz et al.,

2003;Volk et al.,2003). The third target, SCR 1845-6357, is a brown dwarf orbiting an M8.5 red dwarf (Biller et al.,2006). Table7.1lists the intrinsic properties of the brown dwarfs, which are important for lightning flux estimates, and for planning observations.

In Sect. 7.2, I shortly summarize the method used to calculate optical fluxes. The equations first appear in Chapter6, however for better understanding, I repeat them here as well. In the same section, I introduce the parameters I used for my calculations. In Sect. 7.3, I present the results and discuss them. I conclude the chapter in Sect. 7.4.