A tunable integrated system to simulate colder stellar radiation Marco S. Erculiani*a, b, Riccardo Claudi b, Diego Barbisan d, Enrico Giro b, Matteo Bonato b,e , Lorenzo Cocola c, Giancarlo Farisato b, Matteo Meneghini d , Luca Poletto c, Bernardo Salasnichb and Nicola Trivellin d a CISAS-"G.Colombo" Centre of Studies and Activities for Space Via Venezia 15, 35131, Padua Italy ; b INAF - Astronomical Observatory of Padova (Italy), Vicolo dell'osservatorio, 5, 35100 Padova; c LUXOR - Photonics and Nanotechnology institute - CNR, Via Trasea 7 Padova, 35131, Italy; d Department of Information Engineering-DEI, via Gradendo 6B, Padova ; e Department of Astronomy, University of Padova, Vicolo dell'osservatorio, 5, 35100 Padova ABSTRACT In the last years, a lot of extrasolar planets have been discovered in any direction of the Galaxy. More interesting, some of them have been found in the habitable zone of their host stars. A large diversity of spectral type, from early types (A) to colder ones (M), is covered by the planetary system host stars. A lot of efforts are done in order to find habitable planets around M stars and indeed some habitable super earths were found. In this framework, “Atmosphere in a Test Tube”, a project started at Astronomical observatory of Padua, simulates planetary environmental condition in order to understand how and how much the behavior of photosynthetic bacteria in different planetary/star scenarios can modify the planet atmosphere. The particular case of an habitable planet orbiting a M dwarf star is under study for the time being. The irradiation of an M star, due to its lower surface temperature is very different in quality and quantity by the irradiation of a star like our Sun. We would like to describe the study of feasibility of a new kind of tunable led stellarlight simulator capable to recreate the radiation spectrum of M type stars (but with the potential to be expanded even to F, G, K star spectra types) incident on the planet. The radiation source is a multiple LED matrix cooled by means of air fan technology. In order to endow it with modularity this device will be composed by a mosaic of circuit boards arranged in a pie-chart shape, on the surface of which will be welded the LEDs. This concept is a smart way in order to replace blown out pieces instead of changing the entire platform as well as implement the device with new modules suitable to reproduce other type of stars. The device can be driven by a PC to raise or lower the intensity of both each LED and the lamp, in order to simulate as close as possible a portion of the star spectrum. The wavelength intervals overlap the limits of photosynthetic pigment absorption range (280-850 nm), while the range of the radiation source will be between 365 nm and 940 nm. The reason why we chose a higher outer limit is that M stars have the emission peak at about 1000 nm and we want to study the effects of low-light radiation on bacterial vitality. The innovative concept behind this radiative source is the use of the LED components to simulate the main stellar absorption lines and to make this a dynamic-light. Last but not least the use of LED is crucial to keep the device compact and handy. This device could help us to better understand the link between radiation and NIR-photosynthesis and could find applications in the field of photobioreactors as a test bench for the choice of the wavelength to be used in order to maximize the production rate. Other fields of application are the microscopy light sources field and the yeasts growth sector. Keywords: LED, stellar simulator, photosynthesis, extrasolar planets.
1. INTRODUCTION The discovery of light emitting diodes has opened a window on the way to enlighten and their versatility has set them as the broadest-spectrum light devices that can be used in an infinite variety of scientific and not-scientific applications. LED's adjustability can led to the construction of lights that can help humans to regulate their circadian life cycles and the simulation of the solar radiation can be useful to enlighten environments that real sunlight doesn't reach. Sunlight simulation is useful in order to test photovoltaic panels as well as light-exposed devices. But what about the light of other stars? How their radiation spectrum influences the surface of planets it floods on?
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Optical Systems Design 2015: Optical Design and Engineering VI, edited by Laurent Mazuray, Rolf Wartmann, Andrew P. Wood, Proc. of SPIE Vol. 9626, 96262D · © 2015 SPIE · CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2189053 Proc. of SPIE Vol. 9626 96262D-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 09/28/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx
In what follows, we describe in detail our study and the first steps towards the construction of a colour tunable LED radiation source capable to reproduce the spectrum of F,G, K and M type stars. In chapter 2 will be described the scientific rationale, the "WHY" we want to realize this device. Then we'll describe thoroughly the instrument design and its components as well as a glimpse on the first mechanical parts. In this chapter will be described the cooling device too. In chapter 4 we'll talk about the preliminary tests done in order to understand the LED palette to install and its behaviour with respect to the datasheet provided. In the same chapter will be described the working principle and the flow chart of the routine developed in order to produce the best fit of the given spectra. In chapter 5 will be described the development of a software in order to drive the lamp from a personal computer. Then chapter 6 will give a brief overview on the other research fields in which the device could operate. Finally in chapter 7 will be summarized the conclusions.
2. SCIENTIFIC RATIONALE In the last years, a lot of extrasolar planets have been discovered in any direction of the Galaxy. More interesting, some of them have been found in the habitable zone of their host stars. A lot of efforts are done in order to find habitable planets. Our research will focus on F, G, K, and M stars. In fact these stellar types are long-lived enough to support an habitable zone (HZ) capable of evolving life. By habitable zone, we mean the range of orbital distances from a star that will allow 4,16 for the existence of liquid water on a planet . Moreover, we'll focus on terrestrial-type planets, since the existence of life forms on gas giants requires speculation beyond analogous examples on Earth. In particular planets orbiting around M stars are very interesting because of their high statistic density and because of 1,2 their capability of hosting earth-like planets. Nowadays, some habitable super-earths orbiting these type of stars have already been found. In this framework, “Atmosphere in a Test Tube”, a project started at Astronomical observatory of 3 Padua , simulates planetary environmental condition in order to understand how and how much the behavior of photosynthetic bacteria in different planetary/star scenarios can modify the planet atmosphere. The particular case of an habitable planet orbiting an M dwarf star is under study for the time being. The irradiation of an M star, due to its lower photospheric temperature is very different in quality and quantity by the irradiation of a star like our Sun. Because of 4 their dimness, the habitable zone of M stars is very close to the star , such that planets may become tidally locked (one 5 side constantly facing the star ). The different radiation regimes of F, K, and M stars lead to different atmospheric 6 photochemistry as well as a much different photon density. This feature can strongly impact with the metabolic cycle of photosynthetic organisms. 7 Kopparapu et al., (2013) developed an HZ approach evaluating the stellar flux ( ) reaching the top of the atmosphere directly from a climatic model in a dependent way by the spectral type of the star considered. The relationships between and the stellar effective temperature ( ): +d where
=
-5780 and
is the effective temperature of the star.
provides a better metric for habitability than In fact,
(1)
for 2600 K
