Cairo University

MTPR Journal

 

PLASMA OPACITY of THE SUN and EXOPLANETARY HOST STARS

2016-12-17
Sultana N. Nahar
Department of Astronomy, The Ohio State University, Columbus, Ohio, USA
Vol./Issue: 16 , id: 185

Thousands of extra-solar planets have been discovered thus far, mainly by the space based observatory KEPLER. Host stars of exoplanets generally fall under a category of "cool dwarfs" like our Sun, specially G, K, F, and M-class stars. Most stars belong to this category, with stellar fluxes that are not too intense for planetary atmospheres to exist. Of particular interest are Earth-like planets and solar-type stars, forming a star-planet system in the so-called "habitable or Goldilocks" zone where liquid water may also be found. The primary technique for detecting exoplanets is planetary transit across the host star, and measurement of resultant variation in stellar flux. However, the variations are extremely slight, and interpretation of observation requires precise knowledge of stellar atmospheres and detailed knowledge of emitted spectrum. In particular, the predicted near-UV flux is important since Earth-like life forms are highly sensitive to it. However, current model spectra of cool stars do not accurately reproduce observed fluxes even for the Sun. The problem lies in the attenuation of transmitted flux due to the opacity of plasma comprising stellar atmospheres. The computations for opacity are highly complex, requiring non-localthermodynamic- equilibrium (NLTE) models using a vast amount of atomic data for a plethora of UV lines and continuous opacity, especially for relatively low ionization stages of abundant elements such as carbon, sililcon and iron, and ions such as nuetral and singly ionized Fe I-II. Radiative atomic processes responsible for photon absorption and opacity can now be computed with high accuracy using state-of-the-art atomic physics and computer codes. The most powerful theoretical methodology is the R-Matrix method that is capable of producing highly detailed photoionization or bound-free cross sections and millions of transition probabilities among hundreds to thousands of atomic levels.