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Springer Theses Recognizing Outstanding Ph.D. Research Anna Y. Q. Ho The Landscape of Relativistic Stellar Explosions Springer Theses Recognizing Outstanding Ph.D. Research AimsandScope The series “Springer Theses” brings together a selection of the very best Ph.D. theses from around the world and across the physical sciences. Nominated and endorsed by two recognized specialists, each published volume has been selected for its scientific excellence and the high impact of its contents for the pertinent fieldofresearch.Forgreateraccessibilitytonon-specialists,thepublishedversions includeanextendedintroduction,aswellasaforewordbythestudent’ssupervisor explainingthespecialrelevanceoftheworkforthefield.Asawhole,theserieswill provideavaluableresourcebothfornewcomerstotheresearchfieldsdescribed,and for other scientists seeking detailed background information on special questions. Finally,itprovidesanaccrediteddocumentationofthevaluablecontributionsmade bytoday’syoungergenerationofscientists. Thesesmaybenominatedforpublicationinthisseriesbyheadsofdepartment at internationally leading universities or institutes and should fulfill all of the followingcriteria (cid:129) TheymustbewritteningoodEnglish. (cid:129) ThetopicshouldfallwithintheconfinesofChemistry,Physics,EarthSciences, EngineeringandrelatedinterdisciplinaryfieldssuchasMaterials,Nanoscience, ChemicalEngineering,ComplexSystemsandBiophysics. (cid:129) Theworkreportedinthethesismustrepresentasignificantscientificadvance. (cid:129) If the thesis includes previously published material, permission to reproduce this must be gained from the respective copyright holder (a maximum 30% of the thesis should be a verbatim reproduction from the author’s previous publications). (cid:129) They must have been examined and passed during the 12 months prior to nomination. (cid:129) Eachthesisshouldincludeaforewordbythesupervisoroutliningthesignificance ofitscontent. (cid:129) The theses should have a clearly defined structure including an introduction accessible to new PhD students and scientists not expert in the relevant field. IndexedbyzbMATH. Anna Y. Q. Ho The Landscape of Relativistic Stellar Explosions Doctoral Thesis accepted by California Institute of Technology, in USA AnnaY.Q.Ho CornellUniversity Ithaca,NY,USA ISSN2190-5053 ISSN2190-5061 (electronic) SpringerTheses ISBN978-3-031-15366-2 ISBN978-3-031-15367-9 (eBook) https://doi.org/10.1007/978-3-031-15367-9 ©TheEditor(s)(ifapplicable)andTheAuthor(s),underexclusivelicensetoSpringerNatureSwitzerland AG2022 Thisworkissubjecttocopyright.AllrightsaresolelyandexclusivelylicensedbythePublisher,whether thewholeorpartofthematerialisconcerned,specificallytherightsoftranslation,reprinting,reuse ofillustrations,recitation,broadcasting,reproductiononmicrofilmsorinanyotherphysicalway,and transmissionorinformationstorageandretrieval,electronicadaptation,computersoftware,orbysimilar ordissimilarmethodologynowknownorhereafterdeveloped. Theuseofgeneraldescriptivenames,registerednames,trademarks,servicemarks,etc.inthispublication doesnotimply,evenintheabsenceofaspecificstatement,thatsuchnamesareexemptfromtherelevant protectivelawsandregulationsandthereforefreeforgeneraluse. Thepublisher,theauthors,andtheeditorsaresafetoassumethattheadviceandinformationinthisbook arebelievedtobetrueandaccurateatthedateofpublication.Neitherthepublishernortheauthorsor theeditorsgiveawarranty,expressedorimplied,withrespecttothematerialcontainedhereinorforany errorsoromissionsthatmayhavebeenmade.Thepublisherremainsneutralwithregardtojurisdictional claimsinpublishedmapsandinstitutionalaffiliations. ThisSpringerimprintispublishedbytheregisteredcompanySpringerNatureSwitzerlandAG Theregisteredcompanyaddressis:Gewerbestrasse11,6330Cham,Switzerland Tomygrandparents, HoHengMeng,WongGeokKim,Robert EdwardLipsey,andSallyIreneLipsey,who haveinspiredmewiththeirlifelong curiosity, enthusiasm,andadventurousspirit. BeforetheireyesinsuddenviewappearThe secretsofthehoaryDeep—adarkIllimitable ocean,withoutbound,Withoutdimension... —ParadiseLost,II.890–91 Hetouchedthebrimofhishardhatand glancedbackatthedome,whiteandround likeanancienttemple,whileathought crossedhismind,bynomeansforthefirst time,thathewasonlypayinghisrespectsto atempleofscience. —FirstLightbyRichardPreston Supervisor’s Foreword Gamma-raybursts(GRBs)aredazzlinglighthousesmadebyNature.Unlikeman- made lighthouses, they last a short time, milliseconds to minutes but make up by their brilliance. The bursts, even if they occur at the edge of the Universe, can be readilydetectedbymeter-classtelescopes.Infact,oneburstwhichoccurredhalfway acrosstheUniversecouldbeseenwiththeaidofbinoculars!Theshortdurationof GRBs stymied progress in GRBs. In 1997, the discovery of afterglow emission— extendedradio-through-X-rayemission—revolutionizedthisfield. It is now believed that GRBs are the birth cries of newly formed spinning black holes which are accreting matter from stellar debris. In detail, the black holeconvertsaccretionpowerintooppositelydirectednarrow(conical)relativistic jets which plow through the stellar core. Collisions within the jet result in short burstsofgammaraysandafterglowemissionarisesinthecollisionofthejetwith circumstellarmatter.GRBswithdurationsofasecondorlongerarisefrommassive starswhereasthosewithshorterdurationfromthecoalescenceoftwoneutronstars. WhileouroverallmodelforGRBsmaybecorrect,threebasicobservationshave nosatisfyingexplanations.First,apparently,themassofthejetmatterisverysmall, comparable to that of Earth (which is just a bit over 3 millionths of the mass of the Sun). In contrast, in ordinary supernova explosions, the explosion energy is typicallycoupledtoasolarmassofmaterial.The“cleanliness”ofGRBexplosions has no fundamental explanation. Next, we lack a basic explanation for the small, a few degrees, inferred angular widths of the conical jets. Finally, there exist a class of “low-luminosity” GRBs which are only detectable when they occur close to us. Their volumetric rate seems to be very high compared to that of “classical” cosmological GRBs. These GRBs do not show strong evidence for powerful jets. WedonotunderstandifthereisadeepconnectionbetweenthecosmologicalGRBs andthelowluminosityGRBs. Adictuminastronomy,notedbyastronomersineverygeneration,isthat,given the large size of the Universe, the vast number of stars and stellar systems, “all possible ecological niches are occupied.” This dictum suggests that there should beGRBswithwideropeningangleswhichnaturallywouldcoupletomorematter. TheresultingjetwouldhaveasmallervelocityandproduceperhapsX-raysinstead ix x Supervisor’sForeword of gamma rays. However, such “dirty” fireballs would have afterglow emission not particularly distinguishable from those of classical GRBs. Thus, the resulting afterglowswouldbeasdetectableasthosefromGRBs. The quest for dirty fireballs was the goal of Anna Ho’s thesis. It was a timely project. The Zwicky Transient Facility with its large etendue (measured as the product of the collecting area provided by the telescope and the field of view) of about53m2deg2(whichisonlyatenththatofLSST)waswellsuitedtofindingrare events.Theprizewasevidentlyhigh:thediscoveryofanewsub-typeofGRBs(of which,thusfar,onlythreewereknown:short,long,andlow-luminosity).However, aswithmanyhigh-rewardprojectstherewereanumberofrisks.Itishardtostudy short-termphenomena,especiallyrare.Greatpreparationisneededtostudyshort- termphenomena.Finally,thebiggestriskisthatthehypothesizedsub-classmaynot exist. Therearemanyinterestingprojectsthatathesisstudentcanchoosefromandan advisor devise. The primary requirement and challenge to any thesis advisor is to understandtherisksofthethesisprojectandensurethattheskillsandtalentofthe studentarewellmatchedwiththeneedsoftheproject.Inmyassessment,thequest for dirty fireballs had a modest chance, say 10%, of succeeding. The worst-case outcome of such a thesis is that no new phenomenon is discovered and the upper limittotherateprovidesweakconstraints.Thestudenttakinguptheprojectshould have the ability to survive this worst-case scenario, that is, should have the agility and ability to make excellent lemonade when showered with lemons. I issued the appropriate warnings and Anna signed up for the thesis project. The rest, as they say,isthethesis. Anna was able to place a strong upper limit to the hypothesized class of dirty fireballs, namely the rate of these events is below that of GRBs. Thus, the GRB phenomenon has its roots in robust mechanisms. Anna did find a few candidates whicharenotassociatedwithknownburstsofgammarays.However,theabsence of evidence is not evidence of absence. The upcoming Chinese-French mission, SVOM,iswellsuitedtodefinitivelyresolvingthenatureofsuchobjects. Fortunately, along the way, Anna found a number of interesting short-term phenomena.Onephenomenonisanewclassofmildlyrelativisticexplosionstaking place in dense circumstellar material. The prototype of this group is the transient “AT2018cow,” discovered by the ATLAS survey. Anna found that AT2018cow is spectacularly luminous at millimeter wavelengths, far more so than ordinary supernovae. She devised search criteria that successfully identified two additional such events in the ZTF data and showed that luminous millimeter emission is a reliable characteristic. The time-domain sky is only beginning to be explored at millimeter wavelengths, so this class will be prime targets for future large-scale surveys.AT2018cowanditsilkwereasurprisediscoveryanddifferfromGRBsin several ways, including the presence of hydrogen in their spectra, the fact that the central engine is exposed to the observer, and the lack of a bright accompanying supernova. Another phenomenon may be related to the poorly understood phenomenon of low-luminosity GRBs. Nearby low-luminosity GRBs have accompanying super- Supervisor’sForeword xi novaewithaprominent“flash”ofopticallightwithinthefirstdayafterexplosion. Anna discovered a supernova with such a flash that turned out to have mildly relativistic ejecta, similar to low-luminosity GRBs. The promising implication is that we may be able to boost the rate of low-luminosity GRBs discovered in the localuniverseviaopticalsurveys. Upcoming wide-field millimeter surveys and time-domain astronomy surveys especially in the ultraviolet (such as UVEX) and X-ray bands (such as Einstein Probe)willkeepthisfieldsizzlingandAnnaiswellpreparedtoexploitthem. GeorgeElleryHaleProfessorofAstronomy ShriR.Kulkarni CaliforniaInstituteofTechnology