An infrared transient from a star engulfing a planet
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Planets with short orbital periods (roughly under 10 days) are common around stars like the Sun1,2. Stars expand as they evolve and thus we expect their close planetary companions to be engulfed, possibly powering luminous mass ejections from the host star3–5. However, this phase has never been directly observed. Here we report observations of ZTF SLRN-2020, a short-lived optical outburst in the Galactic disk accompanied by bright and long-lived infrared emission. The resulting light curve and spectra share striking similarities with those of red novae6,7—a class of eruptions now confirmed8 to arise from mergers of binary stars. Its exceptionally low optical luminosity (approximately 1035 erg s−1) and radiated energy (approximately 6.5 × 1041 erg) point to the engulfment of a planet of fewer than roughly ten Jupiter masses by its Sun-like host star. We estimate the Galactic rate of such subluminous red novae to be roughly between 0.1 and several per year. Future Galactic plane surveys should routinely identify these, showing the demographics of planetary engulfment and the ultimate fate of planets in the inner Solar System.
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An infrared transient from a star engulfing a planet
- 1. Nature | Vol 617 | 4 May 2023 | 55 Article Aninfraredtransientfromastarengulfinga planet Kishalay De1✉, Morgan MacLeod2 , Viraj Karambelkar3 , Jacob E. Jencson4 , Deepto Chakrabarty1 , Charlie Conroy2 , Richard Dekany5 , Anna-Christina Eilers1 , Matthew J. Graham3 , Lynne A. Hillenbrand3 , Erin Kara1 , Mansi M. Kasliwal3 , S. R. Kulkarni3 , Ryan M. Lau6 , Abraham Loeb2,7 , Frank Masci8 , Michael S. Medford9,10 , Aaron M. Meisner6 , Nimesh Patel2 , Luis Henry Quiroga-Nuñez11 , Reed L. Riddle5 , Ben Rusholme8 , Robert Simcoe1 , Loránt O. Sjouwerman12 , Richard Teague2,13 & Andrew Vanderburg1 Planetswithshortorbitalperiods(roughlyunder10 days)arecommonaroundstars liketheSun1,2 .Starsexpandastheyevolveandthusweexpecttheircloseplanetary companionstobeengulfed,possiblypoweringluminousmassejectionsfromthe hoststar3–5 .However,thisphasehasneverbeendirectlyobserved.Herewereport observationsofZTFSLRN-2020,ashort-livedopticaloutburstintheGalacticdisk accompaniedbybrightandlong-livedinfraredemission.Theresultinglightcurve andspectrasharestrikingsimilaritieswiththoseofrednovae6,7 —aclassoferuptions nowconfirmed8 toarisefrommergersofbinarystars.Itsexceptionallylowoptical luminosity (approximately 1035 erg s−1 ) and radiated energy (approximately 6.5 × 1041 erg)pointtotheengulfmentofaplanetoffewerthanroughlytenJupiter massesbyitsSun-likehoststar.WeestimatetheGalacticrateofsuchsubluminousred novaetoberoughlybetween 0.1andseveralperyear.FutureGalacticplanesurveys shouldroutinelyidentifythese,showingthedemographicsofplanetaryengulfment andtheultimatefateofplanetsintheinnerSolarSystem. Using data from the Zwicky Transient Facility (ZTF) time domain survey9 , we searched for slowly evolving outbursts near the Galactic plane (Supplementary Information 1). We identified a transient opti- cal source, named ZTF 20aazusyv, at celestial J2000 coordinates α = 19:09:39.783, δ = +05:35:04.269 (Supplementary Information 5; hereafter,ZTFSLRN-2020),thatexhibitedarapidrisefromquiescence to peak outburst flux in approximately 10 days, subsequently fading byabouttenfoldover6 months(Fig.1andExtendedDataFigs.1and2). The long optical outburst duration, together with its faint peak flux, distinguishesitfromcommonGalacticplanetransientsresultingfrom whitedwarfswithclosebinarycompanions(the‘dwarf’and‘classical’ novae10 ). The transient also exhibits a mid-infrared (IR) brightening starting at around 7 months before the optical outburst, together with bright mid-IR emission (over 50-fold brighter than the optical r-bandat around 4 monthsafteropticalpeak),thatlastedforroughly atleast 15 months.NoX-rayemissionwasdetectedinfollow-upobser- vations during the outburst using the Swift telescope11 , ruling out an unstable disk accretion episode around a neutron star or black hole12 (Supplementary Information 4). The bright mid-IR emission during the outburst is suggestive of emission from a warm dust shell surrounding the stellar photo- sphere. We model the optical to mid-IR spectral energy distribution (SED; Supplementary Information 12) at around 120 days after the optical peak. The analysis shows a relatively hot inner photosphere (approximately 9,000 K)surroundedbyawarmdustshellofapproxi- mately 1,000 K, located behind a dust visual extinction column of AV (approximately 3.6 mag; Fig. 2, Extended Data Figs. 5 and 6 and Extended Data Table 3). Using the 90% confidence interval on the foregroundextinction,togetherwiththree-dimensionalGalacticdust distributionmaps(SupplementaryInformation13andExtendedData Fig.7),weinferthesourcetobelocatedatadistanceof2–7 kpc.Ajoint analysis of the overlap between the different dust maps suggests a probable distance of roughly 4 kpc. Performing the same analysis at around 320 days after peak, we find the SED to have predominantly shifted into the IR bands caused by an increase in dust optical depth that can be attributed to the formation of about 10−6 M⊙ of dust (for a distance of 4 kpc). Using the best estimate for foreground extinction, we construct a bolometric luminosity light curve for the outburst (Fig. 2 and Sup- plementary Information 13). The light curve is characterized by an initialplateauataluminosityofaround 1035 × (d/4 kpc)2 erg s−1 lasting approximately 25days(SupplementaryInformation13)beforefading by a factor of 5 over the next (roughly) 100 days. The effective tem- peratureofthephotosphereremainsconstantataround (6−7) × 103 K duringtheplateauphase,presumablyregulatedbytherecombination temperatureofhydrogen13 ,beforefadingandcoolingtoabout 5 × 103 K. https://doi.org/10.1038/s41586-023-05842-x Received: 29 September 2022 Accepted: 14 February 2023 Published online: 3 May 2023 Check for updates 1 Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA, USA. 2 Center for Astrophysics/Harvard & Smithsonian, Cambridge, MA, USA. 3 Cahill Center for Astrophysics, California Institute of Technology, Pasadena, CA, USA. 4 Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD, USA. 5 Caltech Optical Observatories, California Institute of Technology, Pasadena, CA, USA. 6 NSF’s National Optical-Infrared Astronomy Research Laboratory, Tucson, AZ, USA. 7 Black Hole Initiative, Harvard University, Cambridge, MA, USA. 8 IPAC, California Institute of Technology, Pasadena, CA, USA. 9 Department of Astronomy, University of California, Berkeley, Berkeley, CA, USA. 10 Lawrence Berkeley National Laboratory, Berkeley, CA, USA. 11 Department of Aerospace, Physics and Space Sciences, Florida Institute of Technology, Melbourne, FL, USA. 12 National Radio Astronomy Observatory, Array Operations Center, Socorro, NM, USA. 13 Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA. ✉e-mail: kde1@mit.edu
- 2. 56 | Nature | Vol 617 | 4 May 2023 Article Thetotalradiatedenergyisapproximately 6.5 × 1041 (d/4 kpc)2 ergover the first (roughly) 150 days. On UT 2020 November 20, we obtained an optical spectrum of the transientusingtheKeck-Itelescope(ExtendedDataTable2).Thespec- trum(Fig.3)exhibitsanalmostfeaturelessredcontinuumcontaining onlyatomic(Na,Ba,HandMg)andmolecularabsorptionfeatures(VO andTiO).Theabsenceofatomicemissionlinesduringtheoutburstis inconsistentwithcommonGalacticplanetransientssuchasaccretion/ thermonuclearoutburstsindwarfandclassicalnovae10 ,wherehotgas produces emission lines due to atomic recombination and ions (Sup- plementaryInformation11andExtendedDataFigs.3and4).Whereas ZTF SLRN-2020 shows some similarities to outbursting young stars, itexhibitsstrikingdifferencesinitsrapidcoolingtoaSEDdominated by IR emission (suggestive of a cooling photosphere), complete lack ofemissionlines(thatis,ubiquitousinhotgasaccretion)aswellasits sky location away from known star-forming regions (Supplementary Information 11). Instead, the molecular features are suggestive of a cool outer photosphere consistent with an M4-III-type giant star and effective temperature of around 3,600 K (Supplementary Informa- tion11).Contemporaneousnear-IR(NIR)spectra(Fig.3)obtainedwith the Palomar 200 inch telescope show only broad molecular absorp- tion bands from H2O, and probably from TiO, VO and CO, consist- ent with an M7-III-type giant possessing an extended, cool envelope (SupplementaryInformation11).Late-timeNIRspectra(Fig.3)obtained around 700 days after the optical peak with the Magellan Baade and Keck-II telescopes show only a relatively featureless continuum with broad H2O absorption. The distinctively cool molecular spectroscopic features, together withsubstantialreddeningoftheSEDduringtheoutburst,arereminis- centoftheclassofrednovae8,14–17 .Directphotometricobservationsof thebinaryorbitaldecaybeforetherednovaV1309Sco8 providestrong evidence for the association of these events with catastrophic merg- ers of binary stars. Because the primary star engulfs its companion, a powerful outflow is launched from the binary system that gradually cools and powers a ‘plateau’ in the light curve of the optical transient via recombination of hydrogen6 . Subsequently, the expanding enve- lopecoolsandformsdustleadingtotheemergenceofaphotosphere dominated by molecular absorption and a luminous, long-lived IR transient18,19 . The photometric and spectroscopic properties of ZTF SLRN-2020 share striking similarities with both Galactic and extragalactic red novae(SupplementaryInformation10andExtendedDataFigs.1and2). Nevertheless, its remarkably low luminosity, even if placed on the far side of the Galactic disk (no nore than around a few × 1036 erg s−1 ), makesitexceptionalamongthepopulationofrednovaethatreachthe Eddingtonluminosityoftheprimarystars(around 1038 erg s−1 fora1 M⊙ 20′′ N E 2.0′′ –400 0 200 400 Time from i-band peak (MJD 58993; days) 10–3 10–2 10–1 100 101 Difference in flux (mJy) I O W W SED outburst SED late ZTF-r ZTF-g ZTF-i ATLAS-o ATLAS-c WISE-W1 WISE-W2 –40 0 10–2 10–1 UKIRT JHK composite 2007–2009 a unWISE W1+ W2 2020 October b c WIRC JHK composite 2021 March d Gemini-S/GSAOI J 2022 April e –20 –200 Fig.1|DiscoverylocationandmulticolourlightcurvesofZTFSLRN-2020. a,AfaintprogenitoridentifiedinarchivalNIRimagesfromtheUKIRTGalactic PlaneSurvey.b,Mid-IRtransientsourcedetectedinNEOWISEimagesfrom 2020.c,NIRcompositefollow-upimage(whitesquaremasksoutanearby regionwithadetectorartefact)ofthetransientduring2021.Thespatialscales inpanelsa–careidentical.d,Zoomed-in,high-spatial-resolutionimageofthe IRremnantin2022.e,MulticolourlightcurvesoftheoutburstfromtheZTF9 , ATLAS48 andNEOWISE49 surveys(asindicatedinthelegend).Errorbarsareshown at1σconfidence,and5σupperlimitsareshownassymbolswithdownward arrows.IandOindicatethetimesofNIRandopticalspectroscopyofthe transient,respectively,withWshowingtheepochofP200NIRimaging. VerticaldashedlinesrepresentthetimerangesusedtoperformSEDmodelling duringtheoutburstin2020(SEDoutburst)andaftertheopticaltransienthad fadedawayin2021(SEDlate).Insetshowsaclose-upoftheearly-timelight curve(shadedingreyinthemainpanel),showingfainti-bandprecursor emissionanddetectionofmid-IRemissionbeforetheonsetoftheoptical outburst.
- 3. Nature | Vol 617 | 4 May 2023 | 57 star). We show ZTF SLRN-2020 in the phase space of luminosity and timescales (Fig. 4 and Supplementary Information 13) for red novae, togetherwithanalyticalcontoursforthemassandvelocityofejecta,in modelsofstellarmergers7 .Theextremelylowluminosityisreasonably explained by around only 10−5 −10−4 M⊙ of hydrogen launched in the outflow7 . The small ejected mass is consistent with the nondetection of radio molecular line emission in follow-up observations with the SubmillimeterArray(SMA)andVeryLargeArray(VLA;Supplementary Information 8 and 9). Usingahigh-spatial-resolutionimageobtainedwiththeGemini-South telescope about 2 years after the outburst peak, we identified a faint progenitor source (Supplementary Information 14) in archival NIR imagesfromtheUnitedKingdomInfraredTelescope(UKIRT)Galactic planesurvey20 .Althoughlimitedbyphotometricerrors,itsbrightness andcoloursareconsistentwithastarofaround 0.8−1.5 M⊙ onthemain sequenceorearlyinthesub-giantbranch(radius,1−4 R⊙;ExtendedData Fig. 8 and Extended Data Table 1). The IR progenitor is thus similar to theSun,andtotheprimarystarofV1309Sco,whichwasamergerevent involving a low-mass q = 0.1 companion, where q is the companion to primary mass ratio8,21–24 . We draw constraints on the mass of the merging object from both the light curve and pre-outburst detections. ZTF SLRN-2020’s ejecta massandradiatedenergyarebothapproximately 103 -foldlowerthan thoseofV1309Sco(Fig.4).Linearlyscalingthecompanionmasswith thesepropertiesimpliesagiantplanetcompanionofaround 0.1 MJ.The presenceofpre-outburstdustandgassuggestsanongoinginteraction (Extended Data Fig. 9) that escalates to produce the outburst22,24,25 , rather than a direct collision. The pre-outburst dust model that best matchesthedatahasq = 10−2 oracompanionmassofabout 10 MJ (Fig.4 and Supplementary Information 18). Therefore, all our estimates 100 Wavelength (μm) 10–13 10–12 Flux OF λ (erg cm –2 s –1 ) Phase +125 days AV = 3.6 mag L = 5.0 L W = 1.8 Td = 1,014 K T* = 8,970 K rin = 1.0 AU a 100 10–13 10–12 Phase +320 days L = 5.5 L W = 13.0 Td = 415 K T* = 4,300 K rin = 3.7 AU 1034 1035 1036 L (erg s −1 ) L ∝ t−4/5 V838 Mon ×10–4 V1309 Sco ×10–3 V4332 Sgr ×10–2 M31-OT 2015 ×10–4 AT2019zhd ×10–4 AT2018bwo ×10–5 2 4 6 8 T (10 3 K) ZTFSLRN-2020 101 102 Time since peak (days) 1011 1012 R (cm) V838Mon ×10–3 V1309Sco ×10–2 V4332Sgr ×10–2 M31-OT2015 ×10–2 AT2019zhd ×10–2 AT2018bwo ×10–3 c b Flux OF λ (erg cm –2 s –1 ) Wavelength (μm) Fig.2|Temporalevolutionofthespectralenergydistributionand integratedluminosityofZTFSLRN-2020.a,Best-fitmodel(parametersare shown)fortheSEDofZTFSLRN-2020(shownasblackcircles)ataround 125 days afteroutburstpeak.Blacksolidlinesdenotetotalflux,browndot-dashedlines denotedustemissionandgreendashedandbluedottedlinesdenotescattered andattenuatedstellaremission,respectively.b,Asinabutroughly 320 days afteroutburstpeak.c,Bolometricluminosity(top),temperature(middle)and radius(bottom)evolutionofZTFSLRN-2020foranestimateddistanceof4 kpc andaforegroundinterstellarextinctionofAV = 3.6 mag.Redsquaresinthe toppanelrepresentluminosityestimatedfromthetwoepochsofDUSTY modelling.Forcomparison,wealsoshowtheevolutionoftheseparametersfor previousrednovae,thebest-fitlightcurveplateaumodel(blackdottedline)and theL ∝ t−4/5 luminositydecayexpectedforamergerremnant(blackdashedline; SupplementaryInformation13).Theradiusandluminosityofarchivalevents havebeenscaledasindicated.Errorbarsareshownat1σconfidence.
- 4. 58 | Nature | Vol 617 | 4 May 2023 Article squarely point to a close planetary companion to the primary star, plausibly a Neptune- or Jupiter-like planet strikingly similar to known systems26 . Theoveralldurationofthelightcurvesuggestsamassejectionveloc- ity of roughly 30 km s−1 (Fig. 4), which is substantially lower than the stellarescapevelocity.Wecanunifytheseestimateswithourtheoretical understandingofhowplanetaryengulfmentmightaffectahoststar.In particular, the smaller the engulfed companion the less dramatic the disturbance to the primary star and the smaller fraction of material thatisexpectedtobeejectedatsufficientlyhighvelocitiestobecome unbound27–29 . The short-lived (around 25 days) plateau phase in ZTF SLRN-2020 may be powered by the ejection and unbinding of a small amount(lessthanabout10−4 M⊙)ofmassatvelocitiesapproachingthe stellarescapevelocity(about100 km s−1 ;Fig.4).TheradiusofZTFSLRN- 2020(Fig.2)remainsroughlyconstantatabout 3 × 1011 × (d/4 kpc) cm duringtheplateauandrecedesduringthefadingphase,similartothat expectedforthegravitationalcontractionofamergerremnant19 .These features suggest that the late-time decay over the next 100 days or so is powered by hydrodynamic and thermal readjustment of the star following the ingestion of its planetary companion. Our interpretation of ZTF SLRN-2020 as the engulfment event of a planetarymassobjectbyaSun-likestarprovidesevidenceforamissing link in our understanding of the evolution and final fates of planetary systems.Ithaslongbeenknownthatthepopulationofplanetsinshort orbital periods1,2,30,31 has sufficiently low orbital angular momentum suchthattheyareunstabletotidaldissipationandareboundtomerge withtheirhoststars32–35 .Thisisconsistentwiththelackofoldplanetary systems with short orbital periods36,37 , as well as the dearth of close planets around sub-giant stars38–40 . Therefore, to our knowledge, the observationsreportedhereofferthefirstdirectinsightintotheeffectof planetaryengulfmentontheirhoststarstointerpretcommonindirect techniques used to infer past planetary engulfment via its effects on long-term stellar luminosity5,41 , chemical enrichment42–44 and stellar rotation45–47 . With empirical and theoretical rate predictions ranging from0.1toseveralperyear(SupplementaryInformation16and20)for similar ‘subluminous red novae’, upcoming combined optical and IR surveys of the Galactic plane may show many similar events via their distinctive, short-lived optical outbursts accompanied by long-lived IR emission. Onlinecontent Anymethods,additionalreferences,NaturePortfolioreportingsum- maries, source data, extended data, supplementary information, 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Wavelength (μm) Scaled F λ + constant TiO 8859 TiO 9209 Hα TiO 9728 Na ID Ba II Mg II TiO 7589 VO 7912 VO 8624 ZTFSLRN-2020 Keck/LRIS +180 days HV2255 M4-III V838Mon +61 days AT2018bwo +51 days a 0.58 0.60 0.62 0.64 0.66 Ba II Hα Na D 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 P200/TSpec P200/TSpec +160 days +160 days AT2018bwo +103 days HD108849 M7-III Magellan/Keck Magellan/Keck +690 days +690 days H2O TiO VO TiO VO 12CO b Scaled F λ + constant Wavelength (μm) H2O Fig.3|OpticalandIRspectraofZTFSLRN-2020.a,Opticalspectrumofthe outburst(black)obtainedabout 180 daysafterpeak.Thespectrumshows clearatomicandmolecularabsorptionfeatures,similartotheM4-III-type giantHV 2255.Forcomparison,late-timeopticalspectraofapreviousGalactic rednova(V838 Mon,inorange)andanextragalacticluminousrednova (AT 2018bwo,inmagenta)areshownafterapplyingtheinferredforeground extinction50 .Theinsetshowsaclose-upofthespectraaroundtheregionof atomiclinesNa D,HαandBa II.b,NIRspectrumofZTFSLRN-2020(black)at around +160 daysandaround +690 daysafteropticalpeak,showingclear broadmolecularabsorptionfeaturesofH2O,TiO,VOandprobablyCO,similar totheM7-IIIgiantHD 108849(showninred).Similarfeaturesarealsoseenin NIRspectraofthepreviouslyknownextragalacticrednovaAT 2018bwo(shown inmagenta).
- 5. Nature | Vol 617 | 4 May 2023 | 59 acknowledgements,peerreviewinformation;detailsofauthorcontri- butionsandcompetinginterests;andstatementsofdataandcodeavail- ability are available at https://doi.org/10.1038/s41586-023-05842-x. 1. Winn, J. N. & Fabrycky, D. C. The occurrence and architecture of exoplanetary systems. Ann. Rev. Astron. Astrophys. 53, 409–447 (2015). 2. Dawson, R. I. & Johnson, J. A. Origins of hot Jupiters. Ann. Rev. Astron. Astrophys. 56, 175–221 (2018). 3. Metzger, B. D., Giannios, D. & Spiegel, D. S. Optical and X-ray transients from planet-star mergers. Mon. Not. R. Astron. Soc. 425, 2778–2798 (2012). 4. MacLeod, M., Cantiello, M. & Soares-Furtado, M. Planetary engulfment in the Hertzsprung- Russell Diagram. Astrophys. J. 853, L1 (2018). 5. Stephan, A. P., Naoz, S., Gaudi, B. S. & Salas, J. M. Eating planets for lunch and dinner: signatures of planet consumption by evolving stars. Astrophys. J. 889, 45 (2020). 6. Ivanova, N., Justham, S., Avendano Nandez, J. 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Bound outflows, unbound ejecta, and the shaping of bipolar remnants during stellar coalescence. Astrophys. J. 868, 136 (2018). 101 102 Outburst duration (days) 1035 1036 1037 1038 1039 1040 1041 Luminosity L 90 (erg s −1 ) V4332 Sgr V838 Mon V1309 Sco NGC 4490-OT2011 M31-OT2015 M101-OT2015 SNHunt 248 AT 2014ej AT 2017jfs AT 2018bwo AT2018hso AT 2019zhd AT 2020kog AT2020hat ZTFSLRN-2020 ZTFSLRN-2020 10−5 M( 10−4 M( 10−1 MJ 10–3 M( 100 MJ 10−2 M( 101 MJ 10−1 M( 102 MJ 100 M( 103 MJ 101 M( 104 MJ 102 M( 1,000 km s−1 a −2.0 −1.5 −1.0 −0.5 0 Time from merger (years) 10−8 10−7 10−6 Dust mass (M ( ) q = 10−1 q = 10−2 q = 10−3 100 km s−1 10 km s−1 b 10–2 MJ Fig.4|ComparisonofZTFSLRN-2020outburstwiththephasespaceof luminosity(L90)andtimescales(t90)forpreviouslyknownrednovae. a, Contoursshowinferredejectaphysicalparameters7 —ejectamassinunitsof solarandJupitermassontheleft-handside,andoutflowvelocityinunitsof km s−1 (inmagenta)ontheright-handside.SolidandhollowstarsdenoteZTF SLRN-2020viaitslightcurveplateaudurationandtimetakentorelease90%of thetotalradiatedenergy,respectively(SupplementaryInformation13).The inferredejectamassofZTFSLRN-2020is100-foldlowerthananyotherknown rednova,andthecharacteristicvelocitiesarealsonearlyanorderofmagnitude lower.b,Comparisonofpre-outburstdustmassestimatedfromprecursor mid-IR emission of ZTF SLRN-2020 (denoted by stars) with models of precoalescencemasslossfora1 M⊙ starevolvingoffthemainsequencewith binarymassratioq = 10−3 −10−1 .Solidlinescorrespondtoaninitialradiusof2 R⊙, andtheshadedregionshowsvariationsforradiiof1−4 R⊙.Errorbarsinpanelsa andbareshownat1σconfidence.
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HST-HF2-51477.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA under contract no. NAS5-26555. M.M.’s contributions were supported by the National Science Foundation under grant no. 1909203. A.-C.E. acknowledges support by NASA through NASA Hubble Fellowship grant no. HF2-51434 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA under contract no. NAS5-26555. S.R.K. thanks the Heising-Simons Foundation for supporting his research. We thank B. Metzger, T. Matsumoto, M. Soares-Furtado and J. van Roestel for discussions. The discovery of the optical transient was based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility (ZTF) project. ZTF is supported by the National Science Foundation under grant nos. AST-1440341 and AST-2034437, and by a collaboration including Caltech, IPAC, the Weizmann Institute of Science, the Oskar Klein Center at Stockholm University, the University of Maryland, the University of Washington, Deutsches Elektronen- Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortium of Taiwan, Trinity College Dublin, the University of Wisconsin at Milwaukee, IN2P3 France, Lawrence Livermore National Laboratories and Lawrence Berkeley National Laboratories. Operations are conducted by COO, IPAC and UW. The ZTF forced-photometry service was funded under Heising-Simons Foundation grant no. 12540303 (PI: Graham). Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership including the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics, and is funded by the Smithsonian Institution and Academia Sinica. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community; we are most fortunate to have had the opportunity to conduct observations from this mountain. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. Author contributions K.D. identified the object, initiated follow-up observations, carried out the analysis and wrote the manuscript. M.M. and A.L. led the theoretical interpretation of the transient, created the models presented in this work and wrote the manuscript. V.K., J.E.J., A.-C.E., L.A.H., M.M.K. and R.M.L. assisted with optical/IR follow-up observations, data interpretation and analysis. D.C., C.C., E.K., S.R.K., R.S. and A.V. assisted with interpretation of the data. R.D., M.J.G., F.M., M.S.M., R.L.R. and B.R. are builders of the ZTF observing system and contributed to survey operations during the observations presented here. A.M.M. assisted with analysis of NEOWISE data. N.P. and R.T. assisted with acquisition of SMA data and carried out SMA data analysis. L.H.Q.-N. and L.O.S. assisted with acquistion of VLA data and carried out VLA data analysis. All authors contributed to scientific interpretation. Competing interests The authors declare no competing interests. Additional information Supplementary information The online version contains supplementary material available at https://doi.org/10.1038/s41586-023-05842-x. Correspondence and requests for materials should be addressed to Kishalay De. Peer review information Nature thanks Smadar Naoz and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Reprints and permissions information is available at http://www.nature.com/reprints.
- 8. Article ExtendedDataFig.1|ComparisonoftherandibandlightcurveofZTF SLRN-2020(shownasredandyellowcirclesrespectively,asindicated)to visual light curves of Galactic and extragalactic red novae from the literature.Thelightcurveshavebeennormalizedtopeakmagnitude.Foreach comparisonobject,weindicatethephotometricbandofthearchivallight curveinthelegend.ThecomparisonobjectsincludeV1309Sco8 , OGLE- BLG-36017 ,V838Mon51 ,M31-LRN-201552,53 ,AT2019zhd54 ,NGC3437-OT55 , AT2017jfs56 andAT2018hso57 .Errorbarsareshownat1σconfidenceand5σupper limitsareshownassymbolswithdownwardarrows.
- 9. ExtendedDataFig.2|Comparisonoftheg−r(panel(a))andr−i(panel(b)) observedcolorevolutionofZTFSLRN-2020(shownassquaresandcircles respectively) to a sample of red novae from the literature that have contemporaneousmulti-bandcoverageduringtheoutburst.Theliterature objectsincludesomeoftheobjectsshowninExtendedDataFig.1inadditionto AT2020hat,AT 2020kog58 andAT2018bwo59 .Ineachpanel,weshowablack arrowindicatingtheestimatedshiftinthecolorevolutionofZTFSLRN-2020 accountingforthebestestimatedline-of-sightextinction.Errorbarsareshown at1σconfidence.
- 11. ExtendedDataFig.4|IdentificationofspectroscopicfeaturesintheNIR spectraofZTFSLRN-2020duringandaftertheoutburst.Ineachpanel, thegray/blacklinesrepresenttheraw/binnedspectrumduringtheoutburst ( ≈ 160 dafterpeak),whilethelightbrown/brownlinesshowtheraw/binned spectrumobtainedafterthefadingoftheinfraredtransient( ≈ 690dafterpeak). WealsoshowacomparisonwiththeM7-IIItypestarHD108849(inred). Prominentatomicandmolecularabsorptionfeaturesaremarked.
- 16. Article ExtendedDataFig.9|EvolutionoftheSEDofZTFSLRN-2020progenitor priortotheonsetoftheopticaltransient.Theorangepointsshowthe optical/IRphotometryfrom ≈ 6−12yearsbeforetheoutburst.Theblackpoints showtheevolutionoftheprogenitorinthemid-IRstarting ≈ 1.7yearsbefore thetransient.Solidpointsindicatedetectionswhilehollowpointsdenote3σ upperlimits.Forthe−30dand−244depochs,weshowthebestfitSilicatedust emissionmodelfitusingthemid-IRphotometry(seetext),whileweshow estimatedupperlimitstothedustemissionusingtheW2photometry. Forthelastepoch,wealsoshowthepre-outburstopticalbrighteningofthe progenitor,coincidentwiththemid-IRsourcedetectedinNEOWISE.Errorbars areshownat1σconfidenceand5σupperlimitsareshownassymbolswith downwardarrows.
- 17. Extended Data Table 1 | Archival photometry of the progenitor of ZTF SLRN-2020 along with the time of observation Upper limits are reported at 5σ confidence.
- 18. Article Extended Data Table 2 | Spectroscopic follow-up of ZTF SLRN-2020 The spectra denoted by † were stacked together to obtain the final binned late-time NIR spectrum.
- 19. Extended Data Table 3 | Derived dust parameters from the multi-epoch DUSTY modeling of ZTF SLRN-2020 The inner shell radius and ejecta mass are for an estimated distance of 4 kpc.