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\title{Quasi-Dyson Spheres}
\author{Al Globus\\ CSC \\ NASA Ames Research Center
\and Dana Backman\\  Franklin and Marshall College
\and Fred Witteborn\\ SETI \\ NASA Ames Research Center
}

\begin{document}
\maketitle

\section{Abstract}

Suppose
\begin{enumerate}
\item there is no technological short-cut to the laws of thermodynamics, as we know them
\item our civilization continues to expand for perhaps a million years, a small fraction of the lifetime of a star
\item we start building orbital space colonies in the next thousand years
\end{enumerate}

In this case, we might be observable to a civilization orbiting a nearby star possessing instruments similar to what we have today. Turning this around, if civilizations similar to ours a million years hence are common in our stellar neighborhood, we may be able to observe them with current or soon-to-be deployed instruments. Specifically, a star orbited by very large numbers of space colonies may have excess emissions in the infrared, be dimmed, or perhaps even exhibit a detectable ring of infrared emissions. Such a star may be described as a Quai-Dyson Sphere (QDS). Freeman Dyson [Dyson 1960] proposed artificial structures completely enclosing a star, but such structures are gravitationally unstable [reference]. However, sufficiently large numbers of orbital colonies around a star would partially block and convert a star's energy. If n\% of a star's energy is blocked, we may say that this star is a n\% QDS.  Given instruments that can detect an n\% QDS, the existence of such civilizations, at least in our stellar neighborhood, becomes a testable, falsafyable hypothesis.

One approach to detecting a QDS is to look for an infrared excess produced by colony thermal radiators. Colonies must absorb a star's energy (or produce new energy), use it and reject the heat to deep space (assuming there is no work-around to thermodynamics as we know it).  Assuming water based life, the temperature of the heat rejection systems can be assumed to be in the range 273-373K (the temperature of liquid water). The higher the temperature, the more efficient the heat rejection but rejection must be below the temperature the energy was used. Thus, a QDS star's energy output curve should be somewhat different than a purely natural solar system, with a small artificial peak in the 10-20 micron range. The hight of the peak is related the completeness of the QDS. Once a star with such an infrared excess is found, it is necessary to distinguish the infrared excess of space colonies and dust. A space colony generated excess might be expected to exhibit a smaller range of temperatures (assuming some optimal heat rejection temperature given their technology) than dust.  This temperature difference may be detectable by examining the shape of the 10-20 micron spectrum. A QDS should be closer to black body radiation at a single temperature whereas dust  emissions, at a greater variety of temperatures, should be flatter. Since dust is usually associated with younger stars, a reliable means for determining the age of stars would help distinguish QDS from dust since a QDS would be expected around older starts since it has taken ~5 billion years for sol to be accompanied by our civilization.

Jugaku, Noguchi, and Nishimura have searched 53 nearby stars with a 1.26 m infrared telescope in Japan and examined IRAS data for 135 more looking for a 10-20 micron infrared excess [Jugaku, Noguchi, and Nishimura 1995]. The sensitivity of the instruments should be sufficient to detect a 1\% QDS. No candidates have yet been found. 

Slyesh [Slyesh 1995] examined the IRAS data for 100\% DS candidates, specifically 0507+528 P05, 0453+444 P03, 0536+467 P05, and 0259+601 P02 without finding a candidates, however noted that G 357 .3-1.3 [Gautier et al. 1984] is strong source with a 220K blackbody spectrum and claims this is a good 100\% QDS candidate, however the temperature is suspiciously low. Slyesh notes that it is difficult to distinguish a 100\% QDS from circumstellar dust shells around an evolved red giant star.

Another approach to finding an n\% QDS with high n is to search for main sequence stars with low luminosity relative to other main sequence stars of the same temperature. A main sequence star's mass determines it's temperature [reference?] and therefor it's absolute luminosity.  If the distance to a star is accurately know, it's absolute luminosity can be calculated from it's apparent brightness. The European Hipparchos satellite [reference] calculated very accurate distance to nearby stars using parallax. We have extracted 299 stars from the Hipparchos catalogue the exhibit low luminosity relative to other stars with similar temperatures. The stars were selected to have:
 
\begin{eqnarray}
p & \ge & 20 \\
T_c & = & \frac{7300}{B-V + 0.73} \\
T_c & \le & 8000 \\
m_h & \le & 27.31 - 0.002149 T_c \\
m_h & \ge & 17.805 - 0.001681 T_c
\end{eqnarray}

where $p$ is the Hipparchos parallax, $B-V$ is from the Hipparchos catalogue, and $m_h$ is the Hipparchos (absolute) magnitude.

infrared torus

Dana approach

\section{References}
\noindent
[Dyson 1960] Freeman Dyson, Science, 131, 1967.

\noindent
[Gautier et al. 1984] T. Gautier et al., Ap J. (Letters), volume 278, page L57

\noindent
[Jugaku, Noguchi, and Nishimura 1995] Jun Jugaku, Kunio Noguchi, and Shiro Nishimura, "A Search for Dyson Spheres Around Late-Type Stars in the Solar Neighborhood," Progress in the Search for Extraterrestrial Life, ASP Conference Series, Volume 74, 1995, G. Seth Shostak editor.

Hipparchos reference

Russian reference

\end{document}
