AActa Prima Aprilia • April 2017 • Vol. III
Detecting the Ultimate Power in theUniverse with LSST M ichael B. L und
Vanderbilt [email protected]
Abstract
Large time-domain surveys, when of sufficient scale, provide a greatly increased probability of detectingrare and, in many cases, unexpected events. Indeed, it is these unpredicted and previously unobservedobjects that can lead to some of the greatest leaps in our understanding of the cosmos. The events that maybe monitored include not only those that help contribute to our understanding of sources astrophysicalvariability, but may also extend to the discovery and characterization of civilizations comprised of othersentient lifeforms in the universe. In this paper we examine if the Large Synoptic Survey Telescope (LSST)will have the ability to detect the immediate and short-term effects of a concave dish composite beamsuperlaser being fired at an Earth analog from an alien megastructure.
I. I ntroduction S o me of the most significant discoveriesin astronomy have been those that haveoccurred incidental to observations thathad other objectives. Quasars were initially dis-covered as radio sources without optical com-ponents during all-sky radio surveys (Schmidt,1963). During tests of a radio antenna, unex-pected static was discovered that would laterbe explained as the leftover signal from theBig Bang (Penzias and Wilson, 1965). Unusualradio pulses on the order of one per secondwere tongue-in-cheek coined ’LGM1’, referringto ’Little Green Men’, before being better ex-plained as pulsars (Hewish et al., 1968). De-tectors that were intended to monitor nuclearlaunches from Earth discovered other signalsthat came from sources outside the Earth, nowknown as gamma ray bursts (Klebesadel et al.,1973).Other observations continue to hold on toan aspect of mystery. The famed ’WOW!’ sig-nal observed by SETI in 1977 possessed manyof the characteristics that had been expectedof an intentional radio message. Forty yearsafter it was initially received, and despite fur-ther research, the true nature of this signal isstill unknown (Gray, 1994). It still remains acandidate for extraterrestrial communication or some astrophysical event that we are not yetaware of. Much more recently, the observationsof KIC 8462852 by Kepler have provided an ex-tremely unusual variable signal (Boyajian et al.,2016). Some of the suggested interpretationsof this phenomenon have been astrophysicalin nature, however there has also been spec-ulation and follow-up observations that haveframed this in the context of alien civilizations(Harp et al., 2016; Wright et al., 2015).In this paper, we look directly at the abilityof telescopes to observe a photometric eventthat would be indicative of potential extrater-restrial activity. In doing this, we focus on avery specific test case, a concave dish compositebeam superlaser as part of an alien megastruc-ture being used to destroy an Earth-like planet.We further limit the scope of our considera-tion here by primarily addressing this questionin the context of what will be possible withthe Large Synoptic Survey Telescope (LSST),but this could easily be broadened to includeadditional ground- and space-based facilities.
II. L arge S ynoptic S urvey T elescope (LSST) The Large Scale Synoptic Survey Telescope(LSST) is an 8.4-meter telescope currently being1 a r X i v : . [ phy s i c s . pop - ph ] M a r cta Prima Aprilia • April 2017 • Vol. III
Figure 1:
The throughputs (including atmospheric effects) for the six bands that LSST will observe in. constructed in Chile (Ivezic et al., 2008). Withfirst light anticipated in 2020, LSST will spendten years observing the entire southern sky insix photometric bands ugrizy , with sensitivityfrom 16th to 24th magnitude. While a largenumber of potential science results have beenexamined previously, the matter we discuss inthis paper seems to have not received any con-sideration as of yet (LSST Science Collaborationet al., 2009). Many of the observational param-eters for LSST have not been finalized, but forpurposes of this paper we use the LSST filterthroughputs that have been defined thus far .The throughputs of all six bands are shown inFigure 1. III. B last M odeling We investigate the activity and immediate af-termath of a planet-destroying laser blast witha series of approximations. We consider ourtarget to be an Earth-analog, and so we use theproperties in Table 1 for this planet, with valuesfrom Kite et al. (2009). We additionally use ap-proximations of the average temperature of thecore and mantle as 6000K and 1270K, respec-tively. Previous work has already examinedthe question of the energy needed to destroya planet in this way, and we use their valueof 2 ∗ J in order to destroy an Earth-likeplanet (Boulderstone et al., 2011). However,it would not be realistic to treat the super-weapon as fully efficient, and so we use valuesbased off of nuclear explosions, where 50% of https://github.com/lsst/throughputs/tree/master/baseline • April 2017 • Vol. IIIthe energy goes into the kinetic energy of theplanet, 35% into thermal radiation that raisesthe temperature of the planetary material, and15% into an immediate, short-duration flashof electromagnetic radiation . The observableenergy from the explosion then comes fromtwo components, the immediate release of en-ergy during the explosion (what we refer to asthe ’flash’) and the long-term thermal radiationfrom the debris of the planet (what we refer toas the ’remnant’). For the flash, we treat thisas a blackbody with a surface of the Earth thatwill release all of the energy of this componentin 2 seconds, or the equivalent of blackbodyradiation for a surface at 10 K. We considerthe debris of the planet to be well-mixed andbe of a single temperature, and when this iscalculated for the total energy, we find it tobe a blackbody with a temperature of 29,000K.As this is occurring while the planet is beingdestroyed, the radius will be increasing, how-ever as the escape velocity is 11 km/s we treatthis object as consistent with earth-sized for theimmediate aftermath. A more time-dependentexamination would require accounting for thedebris cloud growing in size, as well as thecooling of the debris (a time scale on orderof 100 days if approximated as linear cooling)and changes to the optical depth of the debriscloud.
Table 1:
Earth Properties
Parameter Value UnitsEarth mass 5.97 ∗ kgEarth radius 6.37 ∗ mCore mass fraction 0.325Specific heatcapacity, mantle 914 J K − kg − Specific heatcapacity, core 800 J K − kg − We show the blackbody curves for the flashand the remnant in Figure 2. We also include ablackbody curve for a Sun-like star at 5800K forcomparison. We then convolve each of theseblackbody curves with the filter throughputs for LSST. Unsurprisingly considering the hightemperatures involved, we see that the mostsignificant contributions from both the flashand the remnant will occur in the bluer bands.We treat the solar-mass star as our refer-ence for calibrating the absolute magnitudesby using the method for determining the abso-lute magnitude in each band using the methodthat was outlined in Lund et al. (2015). Wethen compare the total flux in each bandpassfor the Sun and for the flash and remnant inorder to get relative magnitudes, followed byabsolute magnitudes. An important considera-tion here is that the radius of the planet mustbe included in these calculations, and so theremnant is a close analogue of a white dwarf inradius and temperature. The absolute magni-tudes that we determine are listed in Table 2. Itbecomes readily apparent that the remnant isgenerally no more than 1% of the brightness ofa solar-mass star, and the flash is only brighterthan a solar-mass star in the u band. Table 2:
Absolute Magnitudes
Band Sun Flash Remnant u g r i z y • April 2017 • Vol. III
Figure 2:
Blackbody curves for the initial flash of the explosion as well as the debris remaining afterward. The blackbodycurve for a solar mass star is included for comparison. light from the flash and remnant would haveto be of considerable brightness with respectto the host star to be observed, and it does notappear that this is the case.There are, however, three scenarios thatmay result in the destruction event still beingdetectable. The first is if the star and planetare close enough to our Solar System that theplanet’s destruction can be angularly resolved.Given that LSST will saturate at 16th magni-tude, however, it seems extremely unlikely thatany geometry exists where this would be pos-sible. The second is if the planet is orbiting asmaller star. A red dwarf, for example, will beseveral magnitudes fainter, particularly on thebluer end of the LSST filter set. In this case,the flash, and possibly the remnant, will bebrighter than the host star. While red dwarfs have not been the typical stars searched forplanets in the past, there is no reason to thinkthat an inhabited planet could not orbit arounda red dwarf. Finally, the flash in the u band isstill brighter than a solar mass star if it is ob-served instantaneously. In the case of LSST orother survey, this could also be accomplishedby having a shorter exposure time, and so anexposure of 2-3 seconds would mean that anyflash from a planetary explosion will be signif-icantly brighter than the host star. In the caseof LSST, however, the costs of this change tothe observing schedule greatly outweigh thisbenefit as it would significantly curtail the ob-servations that LSST will be able to make offainter objects.4cta Prima Aprilia • April 2017 • Vol. III
Figure 3:
The flash and remnant blackbody curves when convolved with the LSST filter throughputs. We include aSun-like star as well for comparison.
IV. S ummary
Astronomy has a relatively unique pattern ofdiscovery when compared to other sciences inthat much of astronomy is simply collectinglarge amounts of data with the hope that inter-esting and novel objects are discovered this way.Many major astronomical discoveries were notnecessarily the results of pointed searches, butrather the luck of observing in the right placeat the right time. This has certainly been thecase for discoveries of astrophysical events, butmay well be the case for observations of eventslinked to alien civilizations also. In this paperwe have briefly explored the ability of the LargeSynoptic Survey Telescope to observe an alienmegastructure destroying a terrestrial planet.While it does not appear that LSST (or indeed,most telescopes) would be able to detect thisfor a terrestrial planet around a solar-mass star,there is a new hope that such an event would be easily observable in the case of the destruc-tion of a terrestrial planet that orbits around ared dwarf. R eferences Boulderstone, D., Meredith, C., and Clapton,C. (2011). That’s no moon.
Journal of PhysicsSpecial Topics , 9.Boyajian, T. S., LaCourse, D. M., Rappaport,S. A., Fabrycky, D., Fischer, D. A., Gandolfi,D., Kennedy, G. M., Korhonen, H., Liu, M. C.,Moor, A., Olah, K., Vida, K., Wyatt, M. C.,Best, W. M. J., Brewer, J., Ciesla, F., Csák, B.,Deeg, H. J., Dupuy, T. J., Handler, G., Heng,K., Howell, S. B., Ishikawa, S. T., Kovács, J.,Kozakis, T., Kriskovics, L., Lehtinen, J., Lin-tott, C., Lynn, S., Nespral, D., Nikbakhsh, S.,Schawinski, K., Schmitt, J. R., Smith, A. M.,Szabo, G., Szabo, R., Viuho, J., Wang, J., Weik-5cta Prima Aprilia • April 2017 • Vol. IIIsnar, A., Bosch, M., Connors, J. L., Good-man, S., Green, G., Hoekstra, A. J., Jebson,T., Jek, K. J., Omohundro, M. R., Schwen-geler, H. M., and Szewczyk, A. (2016). PlanetHunters IX. KIC 8462852 â ˘A¸S where’s theflux?
Monthly Notices of the Royal Astronomi-cal Society , 457(4):3988–4004.Gray, R. H. (1994). A search of the ’Wow’ lo-cale for intermittent radio signals.
Icarus ,112:485–489.Harp, G. R., Richards, J., Shostak, S., Tarter,J. C., Vakoch, D. A., and Munson, C. (2016).Radio seti observations of the anomalousstar kic 8462852.
The Astrophysical Journal ,825(2):155.Hewish, A., Bell, S. J., Pilkington, J. D. H., Scott,P. F., and Collins, R. A. (1968). Observationof a Rapidly Pulsating Radio Source.
Nature ,217:709–713.Ivezic, Z., Tyson, J. A., Acosta, E., Allsman,R., Anderson, S. F., Andrew, J., Angel, R.,Axelrod, T., Barr, J. D., Becker, A. C., Be-cla, J., Beldica, C., Blandford, R. D., Bloom,J. S., Borne, K., Brandt, W. N., Brown, M. E.,Bullock, J. S., Burke, D. L., Chandrasekha-ran, S., Chesley, S., Claver, C. F., Connolly,A., Cook, K. H., Cooray, A., Covey, K. R.,Cribbs, C., Cutri, R., Daues, G., Delgado, F.,Ferguson, H., Gawiser, E., Geary, J. C., Gee,P., Geha, M., Gibson, R. R., Gilmore, D. K.,Gressler, W. J., Hogan, C., Huffer, M. E., Ja-coby, S. H., Jain, B., Jernigan, J. G., Jones,R. L., Juric, M., Kahn, S. M., Kalirai, J. S.,Kantor, J. P., Kessler, R., Kirkby, D., Knox, L.,Krabbendam, V. L., Krughoff, S., Kulkarni,S., Lambert, R., Levine, D., Liang, M., Lim,K.-T., Lupton, R. H., Marshall, P., Marshall,S., May, M., Miller, M., Mills, D. J., Monet,D. G., Neill, D. R., Nordby, M., O’Connor,P., Oliver, J., Olivier, S. S., Olsen, K., Owen,R. E., Peterson, J. R., Petry, C. E., Pierfederici,F., Pietrowicz, S., Pike, R., Pinto, P. A., Plante,R., Radeka, V., Rasmussen, A., Ridgway, S. T., Rosing, W., Saha, A., Schalk, T. L., Schindler,R. H., Schneider, D. P., Schumacher, G., Se-bag, J., Seppala, L. G., Shipsey, I., Silvestri,N., Smith, J. A., Smith, R. C., Strauss, M. A.,Stubbs, C. W., Sweeney, D., Szalay, A., Thaler,J. J., Berk, D. V., Walkowicz, L., Warner, M.,Willman, B., Wittman, D., Wolff, S. C., Wood-Vasey, W. M., Yoachim, P., Zhan, H., and Col-laboration, f. t. L. (2008). LSST: from ScienceDrivers to Reference Design and AnticipatedData Products. page 34.Kite, E. S., Manga, M., and Gaidos, E. (2009).Geodynamics and Rate of Volcanism on Mas-sive Earth-like Planets.
The Astrophysical Jour-nal , 700:1732–1749.Klebesadel, R. W., Strong, I. B., and Olson, R. A.(1973). Observations of Gamma-Ray Burstsof Cosmic Origin.
The Astrophysical JournalLetters , 182:L85.LSST Science Collaboration, Abell, P., Allison,J., Anderson, S., Andrew, J., Angel, J., Ar-mus, L., Arnett, D., Asztalos, S., Axelrod, T.,and al., E. (2009).
LSST Science Book . LSSTScience Collaboration, Tucson, AZ, version2. edition.Lund, M. B., Pepper, J., and Stassun, K. G.(2015). Transiting Planets With LSST. I. Po-tential for LSST Exoplanet Detection.
TheAstronomical Journal , 149(1):16.Penzias, A. A. and Wilson, R. W. (1965). AMeasurement of Excess Antenna Tempera-ture at 4080 Mc/s.
The Astrophysical Journal ,142:419–421.Schmidt, M. (1963). 3C 273 : A Star-Like Objectwith Large Red-Shift.
Nature , 197:1040.Wright, J. T., Cartier, K. M. S., Zhao, M., Jontof-Hutter, D., and Ford, E. B. (2015). The ˆGsearch for extraterrestrial civilizations withlarge energy supplies. iv. the signatures andinformation content of transiting megastruc-tures.