# Presentation: 368922

## IAC-04-IAA-1.1.1.06SEARCHING FOR DYSON SPHERES WITH PLANCK SPECTRUM FITS TO IRAS

**Dick Carrigan**
**Fermilab**

- Infrared Processing and Analysis Center, Caltech/JPL.
- IPAC is NASA's Infrared Astrophysics Data Center.

**Notes:**
Good morning. I am Dick Carrigan and today I’m going to be talking about searching for so-called Dyson spheres using the database from the IRAS satellite shown on the left. Dyson spheres were suggested by Freeman Dyson many years ago. The idea is that an advanced civilization could break up the planets around a star to form a spherical cloud or “shell” that would absorb most of the light coming from the sun and thereby harvest more of the stars energy. 

## The radio SETI paradigm

- Credit: Allen Telescope

**Notes:**
Interest in searching for signs of life beyond the earth is quickening. The principal scientific thrust is to look for biological signatures. A second approach is to search for signs of extraterrestrial intelligence. This program is called SETI. Note that there is a distinction here, life can include some very simple forms while intelligence might just possibly transcend life.

The principal SETI programs now underway search for radio or optical signals. The new Allen telescope at the SETI Institute is a very powerful device. Even without that radio SETI has monitored many sun-like stars out to several hundred light years.

The current SETI paradigm is heavily dependent on the presumption that the originating civilization will provide a beacon, much in the spirit of lighthouses indicating coast lines. The problem here is understanding just why a civilization would bother. My own view is that most of the justifications would be unfavorable to the receiving civilizations.

A different approach is to look for material and electromagnetic artifacts of advanced civilizations like Dyson Spheres. These correspond to objects like the pottery shards, garbage dumps, or pyramids that are the messages of archaeology. Most of these archaeological signals were not intended as messages to the future. They did not require a conscious decision to try to communicate. 

## Cosmic archaeology

- ... and Dyson Spheres

- G. Lemarchand**, **_**[SETIQuest](http://www.coseti.org/sq_v1_n1.htm)**__**[, Volume 1, Number 1, p. 3.](http://www.coseti.org/sq_v1_n1.htm)**_** **On the web at http://www.coseti.org/lemarch1.htm

- Kardashev civilizations: I-planet, II-star,
-     III-galaxy
-    Annis-JBIS **52**, 33 (1999)-elliptical gal
-        log(v dispersion)+sur bright (mag) vs
-            log radius
-        outlier line 1.5 mag or 75% of energy

**Notes:**
There are many possibilities for cosmic archaeological signals. One of the more interesting perspectives was a classification scheme suggested by Kardashev. He divides civilizations into those harvesting all the energy of a planet, of a star, and of a galaxy. A Dyson Sphere would be a type II Kardashev civilization. Jim Annis, an astronomer at my laboratory, has looked for type III Kardashev civilizations by plotting a surrogate for the galaxy brightness versus the size of the galaxy for 106 elliptical galaxies. This follows an astronomical paradigm called the Tully-Fisher relation. Most of the galaxies lie rather close to the line. The outlier line is 1.5 magnitudes higher (representing a 75% dimunition of the light output. No galaxies reach the outlier suggesting that there are few 75% efficient type III Kardashev civilizations.

Another signal suggested by Drake and Shklovski is the appearance of unusual spectral lines such as Technetium in a star due to purposeful salting of the star. They estimate that it would require 105 tons to produce a recognizable signature. Nuclear waste disposal to the surface of a star might produce a similar signal. 

Other signatures might arise from trying to manage the life cycle of a star. Candidates have included so-called star lifting proposed by Criswell and the possibility of  managing the H-R curve. The latter possibility may require too much material.

And finally, there is the possibility of Dyson Spheres.

## Dyson spheres

-     instead swarm of smaller pieces

- Searches-problem fixing distance

- Signature
-     infrared
-     stellar luminosity (distance problem)
-     pure Planck
-     no star for pure DS

- Types of Dyson Spheres - ...pure, partial, rings,

- Based on www.daviddarling.info/encyclopedia/D/Dysonsp.html

- Types
-     pure – star completely obscured
-     partial

- Energy to assemble
-     800 solar years to take Jupiter apart

- Sagan and Walker, Astrophysical Journal **144(3)**, 1216 (1966)
-     search feasible even with sixties technology but that the possible confusion
-     with natural signatures could require searches for other artifacts of intelligence

**Notes:**
If we take earth and break it in to small pieces we could establish a cloud around the sun with a material thickness corresponding to 4 mm. This is thick enough to adsorb all  the visible light. The spherical swarm would radiate the energy away at a temperature of about 300 degrees Kelvin. Building the sphere comes at a cost. Dyson estimates that breaking Jupiter into a sphere would require 800 years of the sun’s energy output.

Dyson spheres could come in several varieties. A pure Dyson Sphere would contain all of the visible energy of the star so that no visible star would be present. A partial Dyson Sphere would only partly cover the star. This could occur for something like a ring distribution or because the overall density was such that there was not total obscuration.

The signature of a Dyson sphere is a strong infrared signal with the luminosity of a normal star. For a pure Dyson sphere there will be no visible star. 

This presents a problem because it will be difficult to determine the distance and thereby the absolute luminosity unless there is some sort of companion object as in a binary or a cluster.

As far back as 1966 Sagan and Walker showed that that Dyson Spheres could be detected out to 1000 parsecs. They did note the problem of confusion with natural signals and suggested that SETI searches could be used to sort out the problem.

## Dyson Sphere surrogates

- Brown dwarfs but temperature is typically higher
-     absolute luminosity is lower

**Notes:**
A number of objects can be confused with a Dyson Sphere. Planetary nebula might be confused with Dyson Spheres with temperatures in the 200 oK region. Globus et al. note that dust clouds from planetary formation will contain spectroscopic features of silicates, aerosols, and hydrocarbons. Further, the age of a star can help to distinguish a cloud from a partial Dyson Sphere since a dust cloud will be young while a Dyson Sphere is expected to be near an old star.

Other possibilities include late type stars with shells such as C stars or “Miras” variables.  Very thick dust clouds occur when there is an extreme phase of mass loss. Since this is a fast process these are apparently rare. In addition for a thick cloud of dust one sees an agglomeration of blackbody spectra for many different temperatures.

Very young stars or protostars are typically in situations where there is a lot of dust and a lot of star formation. An example is Orion.

Brown dwarf temperatures are generally higher than those expected for Dyson Spheres. The luminosities are also expected to be lower.

## Earlier searches

- Jugaku and Nishimura, *Bioastronomy 2002: Life Among the Stars*,
-     Norris and Stootman, eds, IAU Symposium, **213**, 437 (2002) and earlier
-     use the **2.2 μm K band as an indicator** of the photospheric radiation
-     of a star hosting a partial Dyson Sphere and then look for an
-     infrared **excess in the IRAS infrared satellite 12 μm** band
-     1 mag difference for 1% sphere. See less than 0.3 mag for 384 stars inside 25 pc.

- Slysh, in *The Search for Extraterrestrial Life: Recent Developments*,  Papagiannis (Ed) 1985. Timofeev, Kardashev, and Promyslov, AAJournal, **46**, 655 (2000) [TKP]
-     **Four band IRAS** Planck fit. Several candidates, limited sky. 100, 300 °K. 98 stars

- Conroy and Werthimer, preprint (2003)
-     Jugaku technique to **older stars**. 1000 nearby older stars from Wright and Marcy
-     Older stars eliminate thick dust clouds around young stars
-     Correlate with the K band near-infrared ground based data from 2MASS
-     33 candidates in the 12 μm IRAS band with 3 σ excesses from mean.

- Globus, Backman, and Witteborn, preprint (2003)
-     look for a **temperature/luminosity anomaly** due to the fact that the luminosity of a star
-     surrounded by a partial DS would be lowered compared to naked star with same T

**Notes:**
A number of Dyson Sphere searches have been carried out. Jugaku and his colleagues have a continuing program to use the 2.2 micron K band as an indicator of photospheric radiation and then look for an excess in the 12 micron IRAS band. This presupposes a partial Dyson sphere. A 1 magnitude difference could arise for a sphere absorbing 1% of the stellar light. The measured differences are less than 0.3 magnitudes. Looking at 378 of the 458 stars with infrared excesses within 25 pc of the sun they have found no cases of reasonable partial Dyson Sphere candidates. They do note a few interesting sources.

Slysh and Timofeev, Kardashev, and Promyslov (TKP) use IRAS data in a different way. They concentrate on sources near the poles, possibly to avoid Zodiacal light confusion, and then look at 98 sources with IR temperatures in the 100 and 300 degree K range. They then fit a Planck spectrum to the the four IRAS filter bands. Outside of the fact that the sample is limited by the Temperature and polar cuts there is another problem with this approach. The IRAS 100 micron filter is very sensitive to galactic cirrus so the 100 micron filter often rides high. One of two possible candidates have been identified in their search but most could be confused with something else. 

Recently Conroy and Werthimer have argued that older stars may be more fertile possibilities for Dyson Sphere locations. They have applied the Jugaku technique to 1000 nearby older stars. They have found 33 candidates with a 3 sigma excess from the mean in the IRAS 12 micron band.

Globus, Backman, and Witteborn have looked for a temperature/luminosity anomaly in the same spirit as the Annis search for type III Kardashev civilizations. Up to this point they have reported a data set that requires more investigation.

## IRAS

- Picture from Infrared Processing and Analysis Center, Caltech/JPL. IPAC is NASA's Infrared Astrophysics Data Center.

- Only available all-sky survey at 12 μm
-     12, 25, 60, 100 μm micron filters
-     A main purpose – dust, mirror only 0.6 m
-         cosmic cirrus problems in 100, 60 μm

- Performance
-     sensitivity – 0.5 Jy 12 – 60 μm,
-         1 Jy for 100 μm
-     250 K point sources
-     angular resolution – O(1')
-     positional – 2 to 6" in-scan, 8 – 16" cross

- Requirement for Dyson Sphere search
-     all sky – useful
-     100 < T < 600 °K

- 2MASS
-     much more sensitive,
-         500 M point sources
-     IRAS 12 μm must be at least 10 Jy to
-         register in the 2MASS 2.17 μm filter

**Notes:**
An ideal instrument for a Dyson sphere study is an all sky survey, like the ground based 2MASS project. The detectors should be sensitive to infrared over a wide wavelength band centered in the 10 micron regime. This corresponds to the 300 degree Kelvin temperature range. These two requirements narrow the choice to the IRAS satellite that flew in the mid-eighties. IRAS had filters at 12, 25, 60, and 100 microns. A central purpose  was to study cosmic dust. As a result there was no premium on resolution and the mirror had a diameter of 0.6 m.  Still, the performance was quite satisfactory for an initial Dyson Sphere all sky survey. The sensitivity was in the 0.5 to 1 Jansky range while the angular resolution was on the order of 1 minute. Two hundred fifty thousand point sources were identified. The data base is widely available and quite comprehensive. The ground based 2MASS survey contains several thousand times more sources with much better resolution. It is of limited use for a Dyson Sphere survey because a 300 degree Kelvin source must be twenty times the IRAS minimum in the 12 micron filter to register in the 2MASS 2.17 micron filter.

## Analysis

- Infrared cirrus
-      significant presence of emission in the 100 μm band
-         on a wide range of angular scales from so-called
-         infrared cirrus due to interstellar dust
-     often well above Planck
-     do not use 100 μm for fit
-         (Slysh and TKP used all four filters)

- Flux quality factors, FQUAL(i)
-     do not use if only an upper limit
-         leaves 19572 sources
-     limits upper temperature range
-         e. g. missing 60 μm looks like high temperature DS

- Temperature range
-     limited to 150 to 500 °K

**Notes:**
In the spirit of TKP I have used the IRAS database to search for Dyson Spheres by fitting Planck spectra to the filters values. I have used the full IRAS sample rather than applying their tight temperature and geometrical constraints. As noted one of the findings illuminated by IRAS was the significant presence of emission in the 100 μm band on a wide range of angular scales from so-called infrared cirrus due to interstellar dust. This is frequently noticed when comparing Planck distributions fits to the 12, 25, and 60 μm bands. For a large fraction of the cases the 100 μm band is riding well above the blackbody distribution. For this reason, the 100 μm band was not used for blackbody fitting.
The IRAS database includes flux quality factors, FQUAL(i), for each of the bands with values ranging from 3 for a high quality case to 1 for an upper limit. For this analysis sources were not used if they had only an upper limit in any one of the 12, 25, or 60 μm bands.  This cut left a sample with 19572 sources. Note that this choice limits the range of temperatures that can be observed. For example, a high temperature source visible in the 2MASS bands and the 
IRAS 12 μm band might have a small value in the IRAS 25 and 60 μm bands. Based on that and some consideration of the most likely prospects for a Dyson Sphere the temperature range investigated was constrained to lie between 150 and 500 degree K.

## Planck fitting (IRAS 06176-1036 “Red Rectangle”)

- F is IRAS flux, K is Planck color correction,
- P is Planck dist., sigma is weight, and a is fit para

**Notes:**
Two approaches were used to determine the putative temperature and temperature deviation for the fit to the three filters. The first formed a weighted least squares for  an array of temperatures from 100 to 600 degrees Kelvin in 50 degree increments for the roughly twenty thousand sources. In the formula F is an IRAS flux, K is an IRAS Planck color correction, P is the Planck distribution at the filter “center”, sigma is a weight, and a is the fitting parameter (related to the luminosity). The minimum of this distribution was identified from the array and this was used to refit the Planck distribution. The distribution shown here is the case for a bright planetary nebula called the “Red Rectangle.” This is a well studied object and is not a Dyson Sphere. The diamonds show the IRAS filter values. Note that the 100 micron filter is not riding high here. The position of the 2MASS filter is shown in green. Note that the Planck contribution at 2.2 microns is very small.

## Color color fitting

- Left is 12/25, rt 25/60

- Fit with arbitrary polynomial

- Diamonds are extragalactic
- Dots - no catalog entry, pure DS
- Dyson Spheres along BB line

- Follow Pottasch et al. AA **205**, 248 (88), Fig. 1

**Notes:**
A second approach is to use the so-called color-color technique. The ratio of a pair of IRAS filters can be used to deduce a Planck temperature that would fit the ratio. The IRAS Explanatory Supplement includes tables to facilitate this determination as shown in the left panel. In practice an arbitrary curve was fitted to the 12/25 and 25/60 pairs and this was used to obtain the appropriate color temperatures for all the sources. In general the 12/25 ratio is expected to be better determined because the 60 micron filter often has cirrus contamination.  Following Pottasch et al. the panel on the right shows plots of these ratios for IRAS extragalactic sources (diamonds) and IRAS sources with no identifiable counterpart in other data bases (dots). These are candidates for pure Dyson spheres. The pure Planck cases lie on the solid line. Note that there is no obvious peaking along the line and one could even imagine a depletion in the 250 to 400 degree regime.

## Relation of color-color fits to Planck fits: no star

- ↓blackbody

- The lowest LSQ rises quickly as dT/T moves away from 0
-     By dT/T = 0.2 LSQ almost half of the maximum LSQ at dT/T = 0
-     may be possible to rule out LSQ values greater than 1-2E-7
-  There is a direct relation between dT/T and LSQ

**Notes:**
An estimate of the uncertainty and dispersion away from the single Planck spectrum fit can be found by forming the quantity dT/T as defined by the formula. The Planck and color-color dispersions are then plotted in the figure. This particular case is done for IRAS  IDTYPE = 0, that is no positional association. This choice rules out partial Dyson Spheres and leaves a sample of 2356 IRAS sources. Note that that the least squares is plotted on a logarithmic scale. Pure Dyson Spheres should populate the lower left hand corner of the graph. The least squares rises quickly away from dT/T = 0. For a value of 0.2 it has doubled. Obviously there is a direct relation between the two measures. The least squares parameter is probably more sensitive. The banding or aliasing has to do with binning used for the initial Planck temperature arrays.

## dT/T distribution from color color fitting

- No obvious peaking at dT/T = 0 where pure Dyson Sphere should be
- 370 sources with no catalog entry between -0.1 < dT/T < 0.1
-     so that 1 out of every 600 IRAS sources in interval
-     However distribution statistically flat in the region of dT/T = 0
- A 3 σ peak in one bin might require about 25 sources
-     or one in 10,000 of the IRAS sources.

- ↓blackbody line

**Notes:**
We can look directly at the distribution of dT/T to see if there is any peaking at  0 where Dyson spheres should be. There is no peaking. There are 370 pure Dyson Sphere candidates in the region between plus and minus 0.1. This corresponds to one out of every 600 IRAS point sources. However the distribution is flat. Twenty five sources above the flat distribution would be needed to see a 3 sigma signal in one bin. This would correspond to one Dyson Sphere for every ten thousand IRAS point sources.There is no obvious peaking at dT/T = 0 where pure Dyson Spheres should be located. 

## Future plans

- Determining the absolute luminosity
-     need distance to source
-     possible ways to determine distance
-         clusters like Pleiades
-         multiple star system
-         center of galaxy at 8 kpc
-         partial of Jugaku  or Dyson Sphere – use source distance of companion

- The Pleadies
-     at 125 pc contains about 1200 objects
-     subtends a field of roughly four square degrees or approximately 10-4 of the sky
-     contain about 25 members of a randomly distributed sample of 250K sources
- Thus association with a nearby cluster will be a useful tool for less than 0.1%
-     of the objects in IRAS sample

- Small dT/T Dyson Sphere candidates
-     individual cases need to be matched with other information available on the source
-     Following TKP one can look for further information from mm wave measurements,
-          2MASS, or even additional measurements using SIRTF

**Notes:**
This analysis has not yet considered the important need to measure the absolute luminosity of the IRAS sources by determining the distance to the source. Once a candidate is identified as a source with a single blackbody spectrum it is either necessary to associate it with a known cluster such as the Pleiades or a multiple star system. A problem with the Pleadies is that they cover only 10-4 of the sky so that only about 25 IRAS point sources should be within the cluster area on the sky. Another possibility is to look in the direction of the galactic center and assume sources are 8 kpc away. In the case of a Jugaku Dyson Sphere establishing the source distance is straightforward because the signature is an excess in the infrared tail of the star.
In the future the candidates with small dT/T or LSQ will be looked at individually to examine their proximity to other sources and to determine the impact of cirrus. These can be checked against 2MASS and mm wave sources as suggested by TKP. It is hoped that this will either rule out possible candidates or identify likely objects requiring further investigations.

## Summary

- For pure Dyson Spheres do need distance
- to get luminosity. Clusters, galactic center, Virgo?

**Notes:**
In summary, artifacts like Dyson Spheres offer a different paradigm for searching for ETI. Like pyramids and other artifacts from archaeology, Dyson Spheres require no purposeful effort to originate a signal.
The IRAS infrared satellite offers a useful tool for whole sky Dyson Sphere searches. Even with the problem of angular resolution it offers the best available possibility. Even now it appears to be able to rule out pure Dyson Spheres at a level of about 1 in ten thousand. One does find good Planck fits but many of these are surrogates like the planetary nebula “Red Rectangle.” 
Determining the distance and thereby the absolute luminosity remains a problem for pure Dyson Spheres.

## Questions?