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Die transmissiespektrum van die Aarde-atmosfeer is so (Kaltenegger & Traub 2009):
Soos u kan sien, kan u baie absorberingslyne vind wat verband hou met sommige komponente van die Aarde se atmosfeer: H$_2$O, O$_2$, CO$_2$, O$_3$, CH$_4$... Maar die stikstof, N$_2$, word nêrens in die spektrum opgespoor nie. Waarom N$_2$ word nie bespeur nie, maar O$_2$is byvoorbeeld?
Aangesien u vraag gebaseer is op die plot wat u geplaas het, stel ek voor dat u na 'n laer golflengte van die atmosferiese elektromagnetiese absorpsie soek. 'N Vinnige soektog op google het my hierdie vraestel gegee, wat sê:
Die belangrikheid van molekulêre stikstof is die meeste in die atmosfeer van die aarde. Die sterk absorpsiebande in die omgewing van 80-100 nm beskerm die aarde se oppervlak teen die ekstreme ultraviolet (XUV) deel van die sonstraling.
Soos u kan sien, is die absorpsiebande van diatomiese stikstof in die <1000 nm, wat links van u plot is, in die skadu van die skaal (1 mikrometer = 1000 nanometer).
Spectra in die laboratorium
Elke chemiese element het 'n unieke 'handtekening' wat openbaar kan word deur die lig wat dit afgee, te ontleed. Dit word gedoen deur die lig in 'n spektrum te versprei - basies 'n reënboog.
Dit kan opmerklik lyk of ons die samestelling van verre sterre kan leer deur die lig wat hulle uitstraal, te bestudeer. In werklikheid kan ons baie leer, nie net oor die chemiese elemente wat daar is nie, maar ook oor fisiese toestande. Die sleutel is om die lig volgens kleur uit te sprei en 'n spektrum soos in Fig. 1 getoon. Hierdie laboratorium ondersoek 'n paar basiese idees wat gebruik word om spektra te analiseer.
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| Fig. 1. 'n Spektrum. Die lig - in hierdie geval van 'n gewone gloeilamp - is in verskillende kleure versprei. Die skale bo en onder die spektrum word hieronder uiteengesit. |
ATOME EN FOTONE
Die aard van die saak is duisende jare lank bespreek. Gestel jy het byvoorbeeld 'n stuk goud en jy begin dit in kleiner en kleiner stukke sny. Kan jy altyd sny enige stuk, selfs a baie klein een, in twee kleiner stukke goud? Of is daar 'n minimum grootte wat 'n stuk goud kan hê? Ons weet wat die antwoord is: die kleinste moontlike stuk bevat net een atoom van goud. Atome is die boustene van materie. Daar is ongeveer honderd verskillende soorte atome in die heelal - dit staan bekend as die chemikalie elemente.
Die aard van lig het 'n baie soortgelyke vraag gestel: bestaan lig uit golwe of uit deeltjies? As lig golwe is, kan 'n mens altyd die hoeveelheid lig verminder deur die golwe swakker te maak. As lig deeltjies is, is daar 'n minimum hoeveelheid lig wat jy kan hê - 'n enkele 'deeltjie' lig. In 1905 vind Einstein die antwoord: lig is albei! In sommige situasies gedra dit soos golwe, terwyl dit in ander soos deeltjies optree. Dit lyk miskien vreemd en misties, maar dit beskryf die aard van die lig baie goed.
'N Golf van lig het 'n golflengte, gedefinieer as die afstand van die een kruin van die golf na die volgende, en geskryf met behulp van die simbool. Die golflengtes van sigbare lig is redelik klein: tussen 400 nm en 650 nm, waar 1 nm = 10-9 m 'n nanometer 'is - een miljardste van 'n meter. In figuur 1 toon die skaal aan die onderkant golflengtes in nanometers soos u kan sien, rooi lig het lang golflengtes, terwyl blou lig kort golflengtes het.
'N Deeltjie lig, bekend as 'n foton, het 'n energie E. Die energie van 'n enkele foton sigbare lig is klein, skaars genoeg om een atoom te versteur. Ons gebruik eenhede van 'elektronvolts', afgekort as eV, om die energie van fotone te meet. In figuur 1 toon die skaal aan die bokant energie in elektronvolt-fotone met rooi lig het lae energie, terwyl fotone van blou lig hoë energie het.
Die verband tussen energie E en golflengte is een van die mees basiese vergelykings van kwantumfisika:
Hier c is die snelheid van die lig, en h staan bekend as die konstante van Planck. Albei c en h is konstantes van die natuur is hulle nooit nie verander. Vanuit ons oogpunt is die betekenis van hierdie vergelyking daardie energie E en golflengte is omgekeerd eweredig aan mekaar, en die verhouding tussen hulle is die dieselfde in 'n laboratorium op aarde en in die verste sterre en sterrestelsels.
HANDTEKENINGE VAN DIE ELEMENTE
Namate die kwantumfisika ontwikkel het, het fisici 'n ander legkaart begin verstaan. Die lig wat deur atome in 'n warm verdunde gas afgegee word, vorm nie 'n spektrum van alle kleure soos in Fig. 1 nie, maar slegs sommige kleure is teenwoordig, en elke element lewer 'n unieke patroon, soos getoon in Fig. 2. Waarom is dit warm? atome so optree? Die antwoord behels twee sleutelidees: eerstens bevat elke atoom een of meer elektrone wentel om 'n sentrale kern tweedens, in atome van 'n gegewe element, is slegs sekere wentelbane toegelaat, en 'n baie spesifieke hoeveelheid energie is betrokke wanneer 'n elektron van een baan na 'n ander spring.
Figuur 3 illustreer dit vir waterstof, wat slegs een elektron het. Die toegelate wentelbane van 'n elektron in 'n waterstofatoom kan met behulp van die simbool genommer word n, met n = 1 vir die baan die naaste aan die kern, n = 2 vir die volgende een uit, ensovoorts. Vir wentelbaan n, is die hoeveelheid energie benodig om die elektron volledig van die kern te skei
Hierdie hoeveelheid En is die energievlak van die baan n. Byvoorbeeld, 'n elektron in 'n baan n = 2 benodig energie E2 = 3.4 eV om van die kern te skei, terwyl 'n elektron in 'n baan is n = 3 benodig slegs E3 = 1,51 eV dus, wentelbaan n = 3 is minder styf aan die kern gebonde as die baan n = 2. Wanneer 'n elektron van die baan af spring n = 3 om te wentel n = 2, dit gee energie af E = E2 - E3 = 1,89 eV. Dit is presies die energie van die fotone waaruit die rooi lyn van waterstof bestaan in Fig. 2. Net so, elektrone wat van die baan spring n = 4 om te wentel n = 2 lewer die blou-groen lyn en elektrone wat van 'n baan spring n = 5 om te wentel n = 2 lewer die diepblou lyn. As 'n elektron van 'n baan met 'n hoë nommer na 'n lae nommerbaan spring, is die atoom uitstraal 'n foton.
| Fig. 3. Energievlakke (horisontale lyne) en afwaartse spring (pyle) van waterstof. Die wankelrige pyle in kleur stel die fotone voor wat geproduseer word wanneer 'n elektron van die een baan na die ander spring. Om plek te bespaar, is die laagste vlak (n = 1) word nie getoon nie. |
Wat gebeur as 'n elektron in 'n waterstofatoom spring op na 'n hoër baan? Dit verg energie wat êrens vandaan moet kom. Een manier om die energie te voorsien is met 'n foton, maar die foton moet hê presies die regte hoeveelheid energie - nie meer nie, en nie minder nie. Byvoorbeeld, 'n elektron in 'n baan n = 2 kan opspring na 'n wentelbaan n = 3 as dit absorbeer 'n foton met energie E = E2 - E3 = 1,89 eV.
Soortgelyke prosesse van emissie en absorpsie vind plaas in atome van ander elemente. Vir atome met meer as een elektron word die fisika baie meer kompleks, maar die basiese idee dat elektrone net sekere toegelate wentelbane het, bly steeds geld. Elke element het 'n ander stel toegelate wentelbane, dus elke element stuur of absorbeer fotone met verskillende energieë - en dus verskillende golflengtes. Dit is net wat ons in Fig. 2 sien!
Molekules produseer ook spektrale lyne, maar hul spektra is veel meer kompleks as die spektra van enkele atome, en toon gewoonlik breed bands in plaas van smal lyne, soos in Fig. 4.
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| Fig. 4. 'n Spektrum lug. Die helder bande is as gevolg van molekulêre suurstof (O2), molekulêre stikstof (N2), en ander molekules. |
TIPES SPECTRA
As u verskillende soorte lig met 'n spektroskoop ondersoek, kan u 'n wye verskeidenheid spektra sien. Die voorkoms van 'n spektrum vertel ons iets oor die fisiese toestande wat die lig produseer.
Byvoorbeeld, a deurlopende spektrum, soos die een aan die bokant van Fig. 5, is 'n kenmerkende reënboog van kleur. Hierdie soort spektrum is die kenmerk van 'swart liggaam'-bestraling (sogenaamd omdat 'n swart voorwerp, verhit totdat dit gloei, hierdie soort lig uitstraal). 'N Warm vaste, vloeibare of baie digte gas produseer 'n deurlopende spektrum, terwyl 'n wye verskeidenheid golflengtes altyd teenwoordig is. Die algehele kleur van die lig hang af van die temperatuur. Byvoorbeeld, 'n ysterstaaf wat in 'n vuur verhit word, dof rooi as dit meer verhit word, gloei dit oranje en as dit verby sy smeltpunt verhit word, skyn dit met 'n blink blou-wit lig.
Daarteenoor is 'n emissiespektrum, soos die een in die middel van Fig. 5, bestaan uit helder lyne of bande op 'n donker agtergrond. Emissiespektra word geproduseer wanneer atome van 'n verdunde gas 'opgewek' word (in werklikheid verhit word) deur 'n elektriese stroom, ultravioletstraling of 'n ander bron van energie. Opgewonde atome het elektrone in hoë wentelbane, en hulle straal fotone met spesifieke golflengtes uit wanneer hulle terugspring na laer wentelbane (soos hierbo uiteengesit). Neontekens produseer emissiespektra, so ook voorwerpe soos die Lagoon Nebula (M8) en die Ring Nebula (M57).
Laastens, 'n absorpsiespektrum, soos die spektrum sonlig wat onder in Fig. 5 getoon word, bestaan uit donker lyne of bande bo-op 'n deurlopende spektrum. Absorpsiespektra word geproduseer wanneer lig van 'n warm voorwerp deur 'n koeler, verdunde gas beweeg. Wanneer 'n foton met presies die regte golflengte tref 'n atoom van die koel gas, dit word geabsorbeer en sy energie word gebruik om 'n elektron in 'n hoër baan te skop as daar genoeg atome gas is, al die fotone van die golflengtes word geabsorbeer, terwyl fotone met ander golflengtes deurkom. . Die atmosfeer van sterre lewer absorpsiespektra op.
'N Element lewer helder en donker lyne met die dieselfde golflengtes. Waterstof het byvoorbeeld drie prominente lyne met golflengtes van 434 nm, 486 nm en 656 nm. Dit lyk donker as die waterstof lig absorbeer, en helder as dit lig uitstraal, maar dieselfde drie golflengtes word in beide gevalle gesien.
In sommige situasies vind ons spektra wat verskillende soorte kenmerke meng: 'n deurlopende spektrum met helder emissielyne bo-op mekaar. Sommige sterre produseer, terwyl hulle ouer word, deurlopende spektra met donker absorpsielyne en helder emissielyne is dit gewoonlik 'n teken dat die ster gas in 'n sterwind uitstoot.
EKSPERIMENTE MET SPECTRA
In die laboratorium sal ons verduidelik hoe u die spektroskoop kan gebruik en hoe u dit kan verstel sodat u golflengtes akkuraat kan meet. U het dan die kans om verskillende soorte spektra te sien.
Ons sal verskillende ontladingsbuise opstel waarin verskillende elemente elektries opgewek word. U sal gevra word om hierdie elemente te identifiseer deur na u lig te kyk met behulp van u spektroskoop. Die betrokke elemente is onder die wat in Fig. 2 verskyn.
Ons sal ook 'n ligbron opstel wat 'n helder spektraallyn produseer en u vra om die golflengte van hierdie lyn te meet. Nadat u dit gedoen het, kan u die betrokke element identifiseer deur na die tabel op u spektrometer te kyk.
Uiteindelik moet u die spektroskoop vir 'n week huis toe neem om na verskillende ligbronne te kyk en hul spektra te skets. Klassifiseer in elk geval die tipe spektrum (deurlopend, emissie, absorpsie of gemeng) en meet die golflengtes van enige helder of donker lyne wat u kan sien. U moet kyk na:
- 'n fluoresserende lig
- sonlig weerkaats van die wolke middag (moenie wys die spektroskoop direk op die son!)
- sonlig weerkaats deur wolke teen sononder (vergelyk met sonlig teen die middaguur)
- 'n 'neon' teken (wenk: soek na emissie lyne, en probeer 'n ander teken as u dit nie sien nie)
- 'n straatlig
- een (of meer) ligbronne van u eie keuse.
WEBHULPBRONNE
Gebruik hierdie grafiek om spektra van verskillende ligbronne te skets. As u lyne of ander kenmerke sien, plaas dit op die toepaslike golflengte met behulp van die skaal en lys die golflengtes wat u meet.
Tabel van taksonomiese eienskappe
| Klas | Mg II | HI | C IV | Fe II |
| DLA / HI-Rich | Die versadigingsnelheid van die swart bodem versprei soortgelyk aan Classics - smal bereik van 40-60 km / s. | Baie groot W (Lya), sommige as gevolg van dempende vlerke, ander op stompgedeelte van die groeikurwe, nie almal het N (HI)> 2e20 nie. | Hulle het gemiddeld nie 'n tekort aan CIV nie, maar minder as Klassieke in 'n aparte hoër ioniseringsfase, wat klein is in vergelyking met Klassieke (miskien as gevolg van kinematika). | Swartbodemversadiging W (FeII) en W (MgII) ongeveer gelyk en groot. |
| Dubbel | W (MgII) groot, maar minder as DLA's, groot kinematiese verspreiding met 'n hoë vlak van kompleksiteit en wissel in wolkgroottes. | Gemiddeld W (Lya) groter as vir Klassieke as gevolg van kinematiese verbreding, wat dikwels in 'n hoër ioniseringsfase ontstaan. | Baie groot in vergelyking met alle ander klasse wat verband hou met MgII kinematiese verspreiding ontstaan in 'n aparte, hoër ioniseringsfase. | Gemiddeld sterker as vir Classics ontstaan in die sterkste MgII-wolke. |
| Klassiek | Gemiddelde W (MgII) en kinematika het dikwels klein wolke met groot snelhede. | Gemiddelde W (Lya) is byna altyd 'n Lyman-limietstelsel | Gemiddelde W (CIV) ontstaan dikwels in 'n aparte hoë-ioniseringsfase (nie foto-ionisasie-balans in MgII-wolke nie). | Gemiddelde W (FeII) korreleer met W (MgII) met groot verspreiding. |
| C IV - tekortkomend | Soortgelyk aan Klassieke wat nie deur klein wolke teen 'n groot snelheid gemerk is nie. | Soortgelyk aan Klassieke. Sommige is Lyman-limietstelsels, ander nie. | Gemiddeld ontbreek blykbaar 2-sigma onder gemiddeld van Classics a kinematies verbreed, afsonderlike hoë-ioniseringsfase. | Soortgelyk aan Classics. |
| Enkellopend / swak | Enkele, smal, onopgeloste wolke. (TUSSEN: blykbaar nie geassosieer met normale, helder sterrestelsels nie.) | Gemiddeld is W (Lya) kleiner as Classics en CIV-tekort, maar groot variasie as gevolg van die teenwoordigheid van 'n aparte, hoër ioniseringsfase met C IV in sommige Geen Lyman-limietstelsels nie. | Gemiddeld onder klassieke, maar toon 'n volledige reikwydte in W (CIV), maar geen so groot soos vir die dubbele teenwoordigheid van CIV en FeII in dieselfde wolk as gevolg van afsonderlike ioniseringsfases. | Ongeveer die helfte van die bevolking het N (FeII) ongeveer gelyk aan N (MgII), hoewel klein Dit beteken lae ionisasie (hoë digtheid), klein (10s pc!) Wolke. |
Ons waarsku dat die klassifikasieskema hierbo nie geneem moet word nie, wat daarop dui dat Mg II-absorbeerders groepeer gediskretiseerde klasse. Diskretisering is 'n neweproduk van groeperingsanalise. In werklikheid word die verspreidingsfunksies van die ekwivalente breedtes gekenmerk deur enkele modusse en dalende sterte. Die uitsonderings is die ekwivalente breedte Mg I, Fe II en Lyman-alpha, wat as gevolg van die DLA / HI-Rich-klas bimodaal is. As sodanig word enige absorpsie-eienskap, wat op hierdie eenveranderlike manier beskou word, deurlopend versprei. Vanuit die perspektief van 'n multivariate analise is dit egter duidelik dat die algehele eienskappe van Mg II absorbeerders groepeer in goed gedefinieerde streke van 'n 'multi-dimensionele ruimte'. Interpretasie van die groeperingsresultate moet uiteindelik gekoppel word aan die vraag of daar 'n stelselmatige verband is met die omgewings waarin die verskillende klasse ontstaan.
Die sterk korrelasie tussen die C IV ekwivalente breedte, W (CIV), en die kinematiese verspreiding en kompleksiteit van Mg II absorpsie word geïllustreer in die onderstaande figuur, wat die vlak W (CIV) teenoor W (MgII) is, maar met die Mg II 2796 profiele (HIRES / Keck) is op hul onderskeie plekke W (CIV) - W (MgII) geteken. Daarbenewens is die groot verskeidenheid W (CIV) vir die enkel / swak stelsels en die kleiner W (CIV) vir die DLA / HI-ryk stelsels te sien. Hierdie syfer moet vergelyk word met ( paneel c ) van die onderste figuur in die afdeling getiteld "Hoe korreleer C IV-absorpsie met Mg II-kinematika?".
Hier sien ons dat W (CIV) nie soveel verband hou met W (MgII) as wat dit verband hou met die kinematiese "morfologie" van die Mg II-opname nie. 'N Belangrike bestanddeel wat die aanwesigheid van groot W (CIV) dryf, blyk die voorkoms van klein wolke met groot snelhede. Let byvoorbeeld op hoe die DLA / HI-ryke stelsels groot W (MgII) het, maar eenvoudige kinematika vertoon (versadigde onderkant en geen materiaal met hoër snelheid nie). As die W (CIV) korrelasie met Mg II kinematika verband hou met die teenwoordigheid van klein W (MgII) wolke met groter snelhede, kan afgelei word dat hierdie 'buiteligter' wolke hoogs geïoniseerd is [dit is die klein W (MgII) en groot W (CIV)]. Met ander woorde, die sterk C IV is te wyte aan ionisasiebalans in wolke wat versprei is in snelheid. Wanneer CLOUDY-fotoionisasiemodelle van hierdie stelsels ondersoek word, kan die groot W (CIV) egter nie in aanmerking geneem word nie, selfs nie onder die aanname dat die gas so hoog geïoniseer is as wat toegelaat word deur die beperkings van die lae ioniseringsdata (hoofsaaklik Fe II en Mg II). Die ekwivalente breedte van hierdie klein wolke is te klein om in hoë ionisasiegas te ontstaan onder die aanname van son- of subsolarmetale in die wolke. Sien Churchill et al. (1999, ApJS, 120, 51), Churchill & Charlton (1999, AJ, 118, 59), en Churchill et al. (1999b, ApJ, ingedien) vir besonderhede.
Daarom word die gevolgtrekking gemaak dat die grootste deel van die C IV gas ontstaan in 'n hoër ioniseringsfase wat fisies verskil van die aanleiding tot die Mg II absorpsie. Dit dui op 'n fisiese verband tussen die aanwesigheid van hierdie klein wolke in die finaalheid van 'n sterrestelsel en 'n hoë ionisasie-struktuur, miskien 'n koronastruktuur wat nie anders is as die wat rondom die Melkweg gesien word nie.
| Byskrif: Die "W (CIV) versus W (MgII) -vlak" met die Mg II 2796-profiel (HIRES / Keck) van elke stelsel word op hierdie plek in hierdie vlak geteken. Die vyf taksonomiese klasse (sien hierbo) skei op hierdie vlak uit en word geïdentifiseer met die kleurkode-omlystkassies. Verwerk uit Churchill et al. (1999b, ApJ, ingedien). Die stelsel regs bo in geel omring is 'n hoër rooiverskuiwingstelsel (slegs z = 0,4 - 1,4 word hier voorgestel). Verskeie baie groot W (MgII) stelsels by hoër rooiverskuiwings het hierdie profielmorfologie. Aangesien hierdie stelsels van z = 2 tot z = 1 ontwikkel, dui dit daarop dat daar evolusie op die W (CIV) versus W (MgII) vlak is en dat dit ook gekoppel is aan die Mg II kinematika. |
Om dit alles saam te stel (in aanbou)
- watter tipe absorpsieseleksie 'n volledige, eenvormige en onbeperkte monster sterrestelsels sal kies (dws 'n wye verskeidenheid sterrestelselmassas, soorte en omgewings dek)
- watter tipe direkte waarnemingsbeperkings sal bied oor so 'n wye rooiverskuiwingsreeks as moontlik en oor die reeks (s) waar die verwagting is dat evolusie die meeste sal uitkom (dws kinematika, ionisasie en chemiese evolusie).
- bekend is dat diegene met ekwivalente breedtes groter as 0,3 Ang direk met sterrestelsels verband hou (Bergeron & Boisse '1991, A&A, 243, 344 Steidel, Dickinson, & Persson 1994, ApJ, 437, L75 Churchill, Steidel, & Vogt 1996, ApJ , 471, 164) en / of sub-galaktiese metaalverrykte omgewings (Yanny 1992, PASP, 104, 840 Yanny & York, 1992, ApJ, 391, 569) en aangesien magnesium 'n alfa-proseselement is wat deur Type II supernovas opgelewer word. , word verwag dat die vereniging die hoogste rooiverskuiwings sal hou
- dit kom voor in strukture met 'n reeks HI-kolomdigthede van vyf dekades, insluitend sub-Lyman-limietstelsels (Churchill et al. 1999 ApJS, 120, 51), Lyman-limietstelsels (bv. Steidel & Sargent 1992, ApJS, 80, 1), en gedempte Lyman-alfa-stelsels (bv. Le Brun et al. 1997 A&A, 321, 733 Rao & Turnshek 1998, ApJ, 500, L115 Boisse 'et al. 1998, A&A, 333, 841), wat beteken dat 'n groot verskeidenheid galaktiese omgewings sal gebruik word
- by z 2 (bv. Lilly et al. 1996, ApJ, 460, L1 Connolly et al. 1997, ApJ, 486, L11 Steidel et al. 1999, ApJ, 519, 1).
- Vir 0 0,5 sterrestelsels is blouer as 'n hedendaagse Sbc-sterrestelsel sterk ontwikkel (Guillemin & Bergeron 1997, A&A, 328, 499). Hierdie resultaat stem ooreen met die resultate van die Redshift Survey van Kanada - Frankryk (CFRS Lilly et al. 1995, ApJ, 455, 108), wat 'n I-band-geselekteerde sterrestelsels is.
Daar is verskeie modelle van wêreldwye sterrestelsels en elkeen bied 'n soortgelyke scenario vir die evolusie van sterrestelsels vanaf die hoogste rooiverskuiwings (nou tot en met z.
4). 'N Mooi, heuristiese model is aangebied deur Pei, Fall en Hauser (1999, ApJ, 522, 604), en ons gebruik hierdie model om die volgende gedeelte te motiveer om die evolusie van sterrestelsels vanuit 'n absorpsielynperspektief te verstaan.
Soos gesien in die onderstaande figuur, is Pei et. al. drie fases van die sterrestelsel-evolusie te identifiseer: a groeiperiode van gasvormige herfs by z> 3, a werksperiode gekenmerk deur hoër vlakke van stervorming vanaf 1 aftree-periode wanneer stervorming afneem omdat gas nie meer in die sterrestelsels by z aangevul word nie aftree-tydperk , vanaf z = 1, beslaan meer as 50% van die tydperk van sterrestelsel evolusie. Dit is belangrik om daarop te let dat die scenario nie goed beperk word deur die data vir z> 3. Dit kan wees dat die groeiperiode eindig met 'n hoër rooiverskuiwing.
| Byskrif: NIE VOLTOOI NIE. Ons huidige kennisbasis van Mg II-absorpsie stop by z = 2.2 en dit is op 'n relatief lae sensitiwiteitsvlak in lae resolusie spektra. Die kinematika en die verdeling van wolksnelheid is slegs vanaf 0,4 waargeneem |
Aangesien Mg II-absorpsie-geselekteerde stelsels gemotiveer is as die beste oplossing vir die verskaffing van 'n absorpsielyndatabasis vir die beperking van modelle van sterrestelsels (sien hierbo), kan ons nou herkou oor wat die absorpsie-lynhandtekeninge sal wees in die konteks van modelle soos aangebied deur Pei, Fall en Hauser (1999, ApJ, 522, 604).
Die groeiperiode sou die tydvak wees wanneer sterrestelsels werklike intergalaktiese gas en / of proto-galaktiese polle (PGC's) versamel [bv. Rauch, Haehnelt en Steinmetz (1997, ApJ, 481, 601)]. Let daarop dat hierdie rooiverskuiwingsregime en die tydperk van sterrestelsel-evolusie onontgin bly in Mg II-opname!
Die werksperiode sou die tydvak wees wanneer sterrestelsels van die gloeidraad ontkoppel is en PGC's nie meer volop was as fundamentele boustene nie, maar sterrestelsels bly plaaslike materiaal teen hoë massa-oordragtempo's [d.w.s. gebonde "satelliet" -voorwerpe (York et al. 1986, ApJ, 311, 610) ergo die toevoer van materiaal vir voortgesette stervorming]. Hierdie periode sou dan onderskei word deur 'n groot fraksie van Mg II absorbeerders met profiele soortgelyk aan die wat in geel skadu gebied buite die regter boonste vlak van die W (CIV) versus W (MgII) vlak is (hierbo getoon). Let daarop dat die boonste helfte van hierdie rooiverskuiwingsregime en die tydperk van sterrestelsel-evolusie onontgin bly in Mg II-opname en feitlik onontgin word met 'n hoë resolusie!
Die aftree-periode , wat ongeveer ooreenstem met die laer bereik van die intermediêre rooiverskuiwingsregime wat met hierdie werk bestudeer is, sou die tydvak wees as die aantal samevoegings van sterrestelsels en satellietaanwas verminder het tot die punt dat die kosmiese stervormingstempo verminder. Die komplekse Mg II-absorpsieprofiele met veelvuldige snelheidsverdelings en laer totale optiese dieptes sou dan 'n weerspieëling van die verminderde gasmassa wees. Dit sou impliseer dat die meerderheid sterrestelsels deur self in regulerende stelsels verander
1 en op 'n meer geïsoleerde manier begin ontwikkel, in 'n rigting wat afhanklik is van hul vermoë om aan te hou om sterre te vorm. Sterrestelsels wat ryk aan gas was en in staat was om 'n groot aantal molekulêre wolke in hul interstellêre media te vorm (dws laat-sterrestelsels), sou voortgaan om sterre te vorm en evolusie te vertoon, terwyl diegene wat minder bekwaam is, geen waarneembare evolusie sou toon nie. So 'n scenario stem ooreen met die evolusie van differensiële helderheid wat deur Lilly et al. (1995, ApJ, 455, 108).
Ons gaan natuurlik Mg II-absorpsie oor die volle herontwikkelingsomvang van evolusie waar, natuurlik! ONDER KONSTRUKSIE.
1. Inleiding
Interstellêre DC3N is die eerste keer ontdek in die donker wolk TMC-1 (Taurus Molecular Cloud 1) deur die waarneming in emissie van sy J & # x0003d 5 & # x02212 4 rotasie-oorgang wat byna 42,2 & # x000a0GHz lê (Langer et al., 1980) . Vervolgens is gederutateerde sioanoasetileen baie waargeneem in koue molekulêre wolke, soos L1498, L1544, L1521B, L1400K en L1400G (Howe et al., 1994), sowel as in die warm kern van die sterrevormende streke met groot massa. Orion KL en Boogskutter B2(N) (Esplugues et al., 2013 Belloche et al., 2016). Onlangs het DC3N is aangewend om die evolusiestadium van massiewe stervormende streke te ondersoek: Rivilla et al. (2020) het die emissie van GS opgespoor3N (J & # x0003d 11 & # x02212 10 oorgang) in 'n steekproef van 15 bronne wat beide koue en warm ster-vormende kerne bevat, en verkry die eerste emissiekaart van DC3N in die hoë-massa proto-cluster IRAS 05358 & # x0002b3543.
Die astrofisiese relevansie van DC3N hou verband met die alomteenwoordige teenwoordigheid van die ouer spesies (HC3N) in die ruimte en tot sy deuterasie. Die studie van interstellêre D-bevattende molekules verskaf inderdaad belangrike inligting oor die eienskappe en die evolusie van stervormende streke (sien bv. Ceccarelli et al., 2014 en verwysings daarin), en is 'n belangrike instrument om die chemikalie te volg. geskiedenis van die materiale wat uiteindelik in die samestelling van planetêre liggame ingaan.
Op algemene grond word die identifikasie van 'n interstellêre molekule verkry deur die opsporing van die rotasie- en / of vibrasie-spektrale eienskappe daarvan (McGuire, 2018). Die afgelope jaar het die waarneming van molekules in die interstellêre medium (ISM) vooruitgegaan danksy die ontwikkeling van nuwe teleskope met 'n hoë sensitiwiteit wat werk met 'n golflengte wat strek van sentimeter tot mikrometer. Die Atacama Large Millimeter / Sub-Millimeter Array (ALMA) is een van die beste fasiliteite vir die waarneming van molekulêre handtekeninge in 'n verskeidenheid astrofisiese voorwerpe, waaronder afgeleë sterrestelsels en planete. Met tien verskillende bande (van 0,32 tot 3,6 & # x000a0mm) bedek ALMA 'n spektrale venster waarin die meeste rotasie-oorgange val, waardeur 'n aantal chemiese (ligte en mediumgrootte molekules) en opwekkingstoestande (gastemperatuur) ondersoek kan word.
Aan die ander kant is die waarneming van vibrasie-handtekeninge van interstellêre molekules relatief moeiliker as gevolg van die beperkte deursigtigheid van die Aarde-atmosfeer in die Mid- en Ver-infrarooi gebiede. Ondanks die feit dat 'n aantal kragtige teleskope toegerus is met moderne moderne resolusie (R & # x0003e 50.000) infrarooi spektrograwe (bv. CRIRES by ESO Very Large Telescope, TEKSTE by NASA IR Telescope-fasiliteit), steeds die identifisering van vibrasie-eienskappe van spesies wat verskil van die hoofspore (H2O, CH4, HCN, NH3, ens.) bly uitdagend as gevolg van 'n kombinasie van seinswakheid, lynoorvleueling en kontaminasie deur stratosferiese OH-lyne. In hierdie konteks word 'n toekomstige belangrike bydrae verwag van die James Webb-ruimteteleskoop (JWST) wat in Oktober 2021 gelanseer sal word as 'n opvolgmissie van die Hubble-ruimteteleskoop. JWST, wat buite die Aarde-atmosfeer geleë is, maak voorsiening vir die waarneming van hoë-sensitiewe breëband-infrarooi-spektra met sy ingeboude infrarooi-spektrograwe. Die Mid-Infrared Instrument (MIRI) dek die 5 & # x0201328 & # x000a0 & # x003bcm-streek met middeloplossing (R & # x0223c 3.000) deurlopend met vier bande (Banks et al., 2008), terwyl NIRSPEC ontwerp is om die korter golflengte aan te dui. reeks (1.8 & # x020135.2 & # x000a0 & # x003bcm). Sulke fasiliteite sal 'n volledige spektrale oorsig van die bronne bied, en sodoende die volledige vibrasie-manifold van die molekulêre teikens gelyktydig kan monster: dws die buigstreek onder 12 & # x000a0 & # x003bcm wat reeds opgewek is in die warm gas (T & # x0003c 500 & # x02009 K), en die CH-strekgebied rondom 3 & # x000a0 & # x003bcm, wat gekenmerk word deur hoë opwindingsenergieë (E / k & # x0223c 4.000 & # x02009 K) en nuttig om die binneste (AU-grootte) dele van die proto- te ondersoek. planetêre skywe en die atmosfeer van warm eksoplanete.
Die laboratorium spektroskopiese kennis van DC3N oor die volle spektrum van belangstelling vir sterrekunde is egter nie homogeen nie. Terwyl die rotasiespektrum van GS3N is noukeurig bestudeer en dit is goed geskik om astronomiese waarnemings op millimeter golflengtes te rig, 'n gedetailleerde kennis van sy infrarooi spektrum is beperk tot die spektrale bereik tussen 200 en 1100 & # x000a0cm & # x022121 (Melosso et al., 2020). By hoër frekwensies is slegs spektroskopiese data met 'n lae akkuraatheid in die literatuur beskikbaar (Mallinson en Fayt, 1976). Dieselfde geld vir die MIR-spektrum van HC3N bo 1.000 & # x000a0cm & # x022121, waarvoor die ro-vibrasiebande opgeteken is by lae resolusie van 0,025 en 0,050 & # x000a0cm & # x022121 (Mallinson en Fayt, 1976) of in 'n noue spektrale interval (Yamada et al. ., 1980 Yamada en Winnewisser, 1981 Yamada et al., 1983). Sulke ongemaklike resultate was hoofsaaklik te wyte aan die tipe instrumentasie wat gebruik is, dit wil sê die Ebert-tipe en Diode-laser-spektrometers. Tans word hierdie beperkings maklik oorkom deur Fourier-transform infrarooi (FTIR) spektrometers, wat die moontlikheid bied om hoë resolusie ro-vibrasie spektra in 'n wye frekwensie op te neem.
Om die tekort aan bostaande inligting te vul, rapporteer ons in hierdie werk 'n omvattende ondersoek na die hoë-resolusie MIR-spektra van DC3N en HC3N, verkry met behulp van FTIR-spektroskopie. Daarbenewens het die suiwer rotasiespektrum in sommige vibrerende opgewekte toestande van DC3N is aangeteken om hoogs akkurate spektroskopiese parameters te bepaal. Die nuwe toewysings van die IR- en millimetergolfspektra is saamgevoeg in 'n enkele globale pasvorm waaruit 'n konstante stel spektroskopiese konstantes bepaal is. Die analise van DC3N bied 'n baie akkurate rusfrekwensie-katalogus wat nuttig is vir astronomiese waarnemings.
Bespreking
Die onafhanklike seleksiemodusse van genoomvolgorde is nou verwant aan die G + C-inhoud van genoomvolgorde. Daarom is die G + C-inhoud 'n uitdrukking van reëls vir die evolusie van die volgorde. Dit is bekend dat G + C-inhoud nie eenvormig is nie en dat dit altyd in DNA-rye saamgevoeg word. Deur die verspreiding van die G + C-inhoud in DNA-reekse te ontleed, kan ons die evolusietoestande van plaaslike DNA-segmente of die evolusietoestande van verskillende reekse, soos proteïenkoderingsreekse, introne en CpG-eilandreekse, ens. Onthul. Dit is nuttig vir ons die oorsprong en evolusieverhoudinge van verskillende rye diep te verstaan.
Genoom evolusie is 'n deurlopende proses. Alhoewel die verskynsel van TA-onafhanklike seleksie basies in primaat- en knaagdiergenome verdwyn het, bestaan die spoor van TA-onafhanklike seleksie steeds en word die ooreenstemmende funksionele elemente steeds voorbehou. As gevolg van die algemene koderingsreëls in proteïenkoderingsreekse, moet die spoor van TA-onafhanklike seleksie geërf word in proteïenkoderingsreekse van menslike en knaagdiere. Om dit te verifieer, is die spektra van drie TA- en drie CG-motiewe-onderstelle van proteïenkoderingsreekse in menslike en muisgenome aangebied in Addisionele lêer 5: Figuur S2. Alhoewel die CG-onafhanklike seleksie voor die hand liggend is, kan gesien word dat die TA-onafhanklike seleksie ook duidelik is in proteïenkoderingsvolgordes van menslike en knaagdiergenome.
Op grond van die evolusiemeganisme van genome, is dit moontlik om die raaisels op te los wat tydens die bestudering van die evolusionêre verwantskappe van genome ondervind word. K-mer-frekwensies van genoomvolgorde bevat die inligting van die hele genoomvolgorde op die vlak van die ryksamestelling. Wanneer ons die totale k-mer-frekwensies gebruik om die genoom-evolusie-verhoudings te bestudeer, word die nadeel vermy om gedeeltelike ry-inligting in plaas van hele genoom-volgorde-inligting te gebruik. Because some researchers did not know what kinds of k-mers are sensitive to genome evolution, they had to filter out some k-mers in total k-mer set to obtain the acceptable phylogenetic trees, such as filtering the k-mers with the highest or lowest frequencies. Filtering out some k-mers destroys the integrity of genomic information. Since the selected k-mer number has no theoretical support and has a certain degree of arbitrariness, it cannot obtain a consistent evolutionary relationship of species, and it cannot be used as a standard for species identification. Independent selection laws show that there are three types of independent k-mers and the spectra of the k-mers containing CG or TA dinucleotide are sensitive to genome evolution and the spectra of CG0/TA0 k-mers reflect the basic structures of a genome sequence. Thus, the three types of k-mers contribute differently to genome evolution. If we can consider the weighting factors of the three types of k-mers and do not filter any k-mer, we thought it is the most reasonable method to construct the evolutionary relationship among species genomes.
Our results have important guiding significance for biological information mining of nucleotide sequences. The independent selection laws reveal the composition rule of nucleotide sequences. It shows that the three kinds of CG motifs and the three kinds of TA motifs have evolutionary independence, and the k-mers containing CG and TA dinucleotides are functional motifs. That is to say, any nucleotide sequence is composed by the six kinds of motifs. The proportion of these motifs and their distribution forms in a nucleotide sequence determine its biological functions. If biological information mining in nucleotide sequences is considered in this way, the problem will become clear and simple. Our proposal may provide us with a new idea from theory to sequence.
Our results showed that the living habits of species are closely related to the independent selection mode adopted by species genomes. We can study further the interaction relationships between different species from the perspective of the independent selections of genome sequences. Such as, why some bacteria infect plants and why some others only infect animals.
The CG and TA independent selection laws and their mutual inhibition relationships in genome sequences have been revealed by studying the intrinsic laws of k-mer spectra of genome sequences. But the relations between the sequence structure of each k-mer and its occurrence frequency in genome sequence are not clear. Just as the atomic structure was revealed by studying the laws of atomic spectra, we believe that the mechanism of the composition and the evolution of genome sequences will be improved further by studying the structures and usage of all k-mers in genome sequences.
Thermal Ramping of the Target Ice and Observation of the Long-Lived N ( 2 D)
The target ice was then subjected to a series of temperature variations and concurrently recorded emission spectra labeled “Em-1” to “Em-3,” of which temporal and spectral profiles of the α-line from N ( 2 D) are shown in Figs. 2B and 4, respectively. Sequentially, we raised the temperature of the icy sample from 3.5 to 15 K over a period of 8.5 min and maintained it at 15 K for 45 min (Em-1), raised from 15 to 20 K over a period of 5 min and maintained it at 20 K over a period of 15 min (Em-2), cooled to 3.5 K again for 20 min, and raised from 3.5 to 20 K over a period of 16.5 min and maintained it at 20 K for 20 min (Em-3).
From spectra Em-1 to Em-3, the observation of N α-line emission indicated that some portion of N was still in state 2 D and not all in ground state as previously supposed. After elimination of l-N3, we waited until no emission of the α-line was measurable as “iEm-2” in Fig. 2 this result indicates that no N was in state 2 D. But, the temporal profiles of Em-1 to “Em-3” in Fig. 2 recorded the α line, the results indicate that the target ice still contained N in an excited state 2 D which might be different from N ( 2 D) in emissions of “iEm-1” and iEm-2. To distinguish the N ( 2 D) with a lifetime of 25 s, we thus label this long-lived state as N ( 2 Dlank).
After the thermal ramping and recording visible spectra Em-1 to Em-3, we then recorded the IR spectrum “IR-4” in Fig. 5, in which the absorption of l-N3 appeared again at 1,657.8 cm −1 , but notably corresponding to only 1 site, whereas, the other site of feature absorption at 1,652.6 cm −1 is absent here. Considering the nascent threshold wavelengths of l-N3 and N to be 145.6 nm, we reasonably suppose that l-N3 was synthesized from N2 combining with atomic N that might be in an excited state. First considering N2, no trace of N2 (A) was found as the A-X emission was not observed it is hence reasonable to exclude the effect of N2 (A) in further formation of l-N3 and thermal warming. For atomic N, emissions of series of N α-lines were observed. N might hence be present in configuration 2s 2 2p 3 with 4 S, 2 D, or other terms. According to a calculation of Galvão (29), a synthesis of l-N3 has a greater probability from N ( 2 D) than from N ( 4 S). We thus suggest that the formation of l-N3 at 1,657.8 cm −1 in the warming process follows Eq. 4. N ( D long 2 ) + N 2 ( X ) → l - N 3 ( X 2 Π g 1,657.8 ) . [4]
4 Proposed Mechanisms Based on Ice-Sheet Feedbacks
This section discusses different feedback mechanisms that have been proposed as (contributing) causes of the MPT that mainly focus on ice sheets (ice-climate feedbacks, basal conditions, ice-dynamical instabilities, etc.). Mechanisms involving sea ice, ocean circulation, and the carbon cycle, will be discussed in Section 5.
4.1 A Framework of Ice-Sheet Stability Thresholds
Several different mechanisms that have been proposed to explain the so-called 100-kyr Late Pleistocene glacial cycles involve nonlinear feedback mechanisms between ice sheets and the global climate, the solid Earth, and other Earth system components (Abe-Ouchi et al., 2013 Bintanja & van de Wal, 2008 Clark & Pollard, 1998 Oerlemans, 1980 Pollard, 1983 Raymo, 1997 ). Here, we propose a framework, where the sensitivity of an ice sheet to changes in insolation and climate is separated into three size regimes, separated by two thresholds (Figure 4). In the “small” regime, the ice sheet is too small to survive an insolation maximum, leading to the nearly linear response of ice volume to the 41-kyr variations of ISI visible in reconstructions for the Early Pleistocene (Bintanja & van de Wal, 2008 Köhler & van de Wal, 2020 ). The “small” regime is separated from the “medium” regime by a threshold ice-sheet size. Above this threshold, certain positive feedbacks in the ice-sheet – climate system, such as the ice-albedo and elevation-temperature feedbacks, create enough self-sustained ice-sheet growth to allow the ice sheet to survive an insolation maximum. The “medium” regime is separated from the “large” regime by a second threshold. Above this second threshold, different physical mechanisms, such as the bedrock-mass-balance feedback and calving, lead to the ice sheet becoming unstable or more vulnerable. An insolation maximum will then trigger a self-sustained retreat, leading to the rapid disintegration of the ice sheet. We will show that the majority of studies that propose different ice-dynamical processes as explanations for the MPT, fit within this two-threshold framework. All of these studies explain the MPT as the result of a slow, global cooling trend throughout the Pleistocene. Before the MPT, they suggest, global temperatures were warm enough that, during insolation minima, the ice sheet size never reached the first threshold. This implied that ice sheets were too small to survive an insolation maximum, consequently leading to a near-linear response of land ice volume to insolation, and a periodicity in ice-sheet size that is similar to that of the relevant incoming insolation (here 41 kyr). The MPT marks the point in time when temperatures became cold enough for the ice sheets to reach the size of the first threshold, thus surviving the next insolation maximum and continuing their growth through the next cycle of insolation or obliquity. This is repeated until the growing ice sheet crosses the second size threshold and becomes unstable. The next insolation maximum then triggers a deglaciation (or termination), moving the world to an interglacial state. The right-hand panel of Figure 4 shows how an extremely simple zero-dimensional model (see Appendix A) of such a two-threshold system, forced with a simple sinusoid insolation plus a linear cooling term, can reproduce the basic features of the MPT. Paillard ( 1998 , 2001 ) showed that a similar three-regime model can, after some careful tuning, produce an ice volume history that closely matches the observed benthic δ 18 O record in terms of the timing, duration, and relative magnitude of glacial cycles.
(a) The two-threshold framework. In the “small” regime, ice sheets are sensitive to changes in insolation, and react linearly. In the “medium” regime, self-sustained growth reduces melt during insolation maxima, resulting in a lower sensitivity. In the “large” regime, ice sheets become highly sensitive again, so that an insolation maximum can trigger a termination. (b) Ice-sheet size over time, from an extremely simple zero-dimensional model of this two-threshold system. The model is forced with a 41-kyr sinusoid insolation, plus a small linear cooling term.
4.2 Positive Feedbacks for Ice-Sheet Growth
The existence of the lower threshold in ice sheet size in our framework has been linked to both the ice-albedo and elevation-temperature feedbacks. As an ice sheet advances, the bare soil and rock of the adjacent tundra become snow-covered for increasing parts of the year, until it gets covered by the ice sheet itself. Since the albedo of snow and ice is much higher than that of rock and soil, this locally reinforces the cooling that caused the initial ice-sheet advance, and also reduces the sensitivity of the local climate to changes in insolation. This effect has been known for a long time in the scientific literature (e.g., Budyko, 1969 Milankovic, 1941 Sellers, 1969 ), and several studies have been able to capture it in ice-sheet models without requiring full dynamic coupling to General Circulation Models (GCMs) (Abe-Ouchi et al., 2013 Berends et al., 2018 ). The elevation-temperature feedback (Weertman, 1961 ) is a basic atmospheric property based on adiabatic cooling as a parcel of air moves up, the pressure drops, causing the air to both expand and cool. Since the mass balance of an ice sheet, and in particular the melt, strongly depends on temperature, this leads to a positive feedback, where ice-sheet growth leads to a surface cooling, reducing melt and enhancing the growth. Using a relatively simple one-dimensional (1-D) ice-sheet model, Clark and Pollard ( 1998 ) show how the elevation-temperature feedback can allow a large enough ice sheet to remain cold enough to prevent a complete retreat, even during an insolation maximum. At the same time, the presence of a large ice sheet affects atmospheric circulation on the local, regional, and global level. Orographic forcing of precipitation, where air masses moving up the flank of an ice sheet cool down and precipitate their moisture content, results in increased precipitation near the ice-sheet margin, while the interior of the ice sheet becomes a plateau desert (Roe, 2002 Roe & Lindzen, 2001 ). This does not greatly affect the net mass balance over the entire ice sheet, only the spatial distribution of accumulation. Relatively simple models describing these effects have been shown to produce ice sheets that agree reasonably well with geomorphological evidence of ice-sheet extent (Berends et al., 2018 van den Berg et al., 2008 ).
Another proposed mechanism for enhanced ice-sheet growth above the lower size threshold involves the topography of North America. Geomorphological evidence indicates that, during the last deglaciation, the North American ice sheet complex separated into the Cordilleran and Laurentide ice sheets (Dyke, 2004 ). The Laurentide subsequently separated into the Labrador, Keewatin, and Baffin ice sheets, all of which disappeared within a few millennia. Although geomorphological evidence for locations of inception is scarce, model studies (Bahadory et al., 2021 Berends et al., 2018 Calov et al., 2005 Choudhury et al., 2020 de Boer et al., 2013 ) indicate that glacial inception also occurred separately at all four locations. These models show that, while the Labrador, Keewatin and Baffin ice sheets quickly merged into the Laurentide, the Laurentide and Cordilleran likely did not merge until much later. Bintanja and van de Wal ( 2008 ) and Gregoire et al. ( 2012 ) suggest that the merging of these two ice sheets presents a strong positive feedback, or even an instability Bintanja and van de Wal ( 2008 ) propose that their merging led to a rapid increase in ice volume with no substantial change in climatic forcing, while Gregoire et al. ( 2012 ) show that their separation during the deglaciation led to a rapid retreat. Bintanja and van de Wal ( 2008 ) propose that this feedback, and the resulting rapid increase in ice volume, was triggered for the first time during the MPT, when a long-term Pleistocene cooling trend finally resulted in Laurentide and Cordilleran ice sheets that grew large enough to touch, and subsequently merge.
The ice-albedo and elevation-temperature feedbacks are gradual in nature for these mechanisms, the first size threshold marks the point where the effects of these positive feedbacks become strong enough to negate the melt from an insolation maximum. The merging ice-dome feedback is more abrupt in nature in this case, the first size threshold directly corresponds to the geographic size required for the separate domes to touch, which is around 45 m of sea level equivalent in North America. Whereas the ice-albedo and elevation-temperature feedbacks are essentially universal, the merging ice-dome feedback is rather a result of the particular topography of the North American continent, making schematic experiments for studying this effect less straightforward.
4.3 Regolith: Basal Sliding and Dust
Clark and Pollard ( 1998 ) proposed that the disappearance of the regolith cover beneath the North American and Eurasian ice sheets can explain the MPT. In their theory, before the onset of the Pleistocene glacial cycles, these continents were entirely covered by the thick (10–50 m) layer of regolith that is still found today in the lower-latitude regions that have never been covered by ice sheets. They propose that a regolith substrate easily deforms under the driving stress of an ice sheet, leading to much higher basal velocities than an ice sheet lying on top of hard bedrock. This would lead to the ice sheet being thinner and wider than would be the case under present-day circumstances, in agreement with sparse geological evidence that the pre-MPT Laurentide ice sheet reached a substantially larger extent than during the Last Glacial Maximum (LGM) (Balco & Rovey, 2010 Balco et al., 2005 Boellstorf, 1978 ). The more gentle surface slopes of this thinner, wider ice sheet would have resulted in a larger ablation zone, which is more sensitive to changes in surface climate. This means that the first size threshold, above which an ice sheet can survive an insolation maximum, lies much higher. They propose that, during the Early Pleistocene, this threshold was never reached, and every insolation maximum led to a deglaciation. However, as the regolith deforms and erodes, it is advected along the direction of ice flow, moving away from the central parts of the glaciated areas. In their theory, the MPT marks the moment when all the regolith was eroded away, and the bare bedrock underneath became exposed. The decrease in basal sliding resulted in ice sheets that were both thicker (providing more stability through the altitude-temperature feedback) and narrower (making them less sensitive to changes in insolation), essentially lowering the first threshold. The ice sheet volume that could be formed during a single insolation cycle was now large enough to survive an insolation maximum, thus creating the 82/123 kyr glacial cycles of the Late Pleistocene.
Using a coupled ice-sheet – climate – carbon cycle model, Willeit et al. ( 2019 ) showed that 100-kyr cycles can occur both with and without a prescribed regolith cover, but that the combination of both a prescribed global cooling trend, and a prescribed gradual removal of regolith during the Pleistocene, gave them the best fit to the observed δ 18 O record. However, they still find a maximum ice-sheet extent that is substantially smaller pre-MPT than post-MPT, at odds with the (sparse) geomorphological evidence (Balco & Rovey, 2010 Balco et al., 2005 Boellstorf, 1978 ).
Regolith erosion has been suggested to influence glacial dynamics not only through reduced basal sliding, but also through its impact on dust fluxes. During the LGM, regolith erosion by the North American ice-sheet complex resulted in increased amounts of atmospheric dust (Kohfeld & Harrison, 2001 McGee et al., 2010 ). Different studies have suggested that, as some of this dust precipitated onto the ice sheet, the resulting decrease in albedo could have accelerated the retreat of the ice sheet (Peltier & Marshall, 1995 Willeit et al., 2019 ). In this theory, the MPT is the result of the disappearance of regolith over an increasingly large area of the North American continent, resulting in an increasingly larger ice-sheet size that is required to produce the necessary glaciogenic dust. Proxy data from the North Atlantic support a largely increased glacial dust source from the North American continent starting at 2.7 Myr ago (Naafs et al., 2012 ). This would be in line with the idea that ice sheets have an impact on dust fluxes. Although the viability of including sediment transport (or tracer tracking in general) in an ice-sheet model has long been demonstrated (Lhomme et al., 2005 Melanson et al., 2013 ), this has not yet become common practice. Willeit et al. ( 2019 ) included glaciogenic dust (using a prescribed regolith mask rather than a transport model), and its effect on both ice-sheet albedo and oceanic productivity, in their coupled ice-sheet – climate – carbon cycle model, but did not explicitly investigate the magnitude of these effects.
4.4 Mechanisms for Increased Sensitivity of Very Large Ice Sheets
The existence of an ice-sheet size threshold required for triggering a termination was already suggested by Raymo ( 1997 ). Different mechanisms have been proposed to explain why ice sheets could become more sensitive to changes in insolation when their size exceeds this threshold. Model studies by Abe-Ouchi et al. ( 2013 ), Clark and Pollard ( 1998 ), and Pollard ( 1983 ) suggest that calving might be an important factor. The massive Late Pleistocene ice sheets in North America and Eurasia created deep depressions in the Earth's crust, which, either through oceanic incursion or through the accumulation of meltwater in proglacial lakes, could have resulted in a large portion of the ice-sheet margin becoming marine-based. Clark and Pollard ( 1998 ) find that prescribing a sizable calving flux for these marine margins is crucial for achieving a complete deglaciation in their 1-D ice-sheet model. Abe-Ouchi et al. ( 2013 ) included, but did not investigate the effect of, a prescribed calving flux in their three-dimensional ice-sheet model. Instead, they suggested that the effect of bedrock depression on the surface mass balance, through the elevation-temperature feedback, is what causes a larger ice sheet to retreat more rapidly. All of these studies prescribed a fixed calving rate for ice margins lying below (local) sea level, implying that calving only occurs when their modeled ice sheet grows large enough to depress the bedrock below sea level or grows off the continental shelf during inception. Furthermore, all of these studies used shallow-ice models, which do not simulate floating ice shelves that might serve as an amplifying factor for ice sheet retreat.
Another proposed mechanism for the reduced stability of large ice sheets involves the thermodynamics of the ice sheet. Bintanja and van de Wal ( 2008 ) and Marshall and Clark ( 2002 ) propose that the build-up of geothermal heat at the base of an ice-sheet, combined with the insulative properties of the ice, lead to basal temperatures that increase with ice-sheet size. While during the inception phase, the ice sheets are thin enough to remain frozen to the bedrock, they might get thick enough during the glacial highstand to reach pressure melting point at the base, yielding increased basal sliding and rapid thinning of the ice sheet. Clark and Pollard ( 1998 ) and Pollard ( 1983 ) do not include thermodynamics in their ice-sheet modes, while Abe-Ouchi et al. ( 2013 ) include thermodynamics but do not use the resulting basal temperatures to adjust basal friction, such that this feedback process is not present in their models.
Bespreking
First encouraging results were obtained after 7 hours of irradiation at 89.2 nm with a flux estimated at about 2 × 10 10 photons s −1 cm −2 . In spite of the precautions taken, the mass spectrum analysis highlights the presence of oxygen species coming from the slow but inevitable micro-leaks with the closed cell reactor, which prevents us from being too affirmative in the results put forward in this paper. Despite this, complex species comprising several carbon and nitrogen atoms were found through extensive statistical analysis without any insertion of oxygen atoms 64 . Their formation can be explained simply by some reactions resulting from the photochemistry initiated by the photons with the initial gas mixture (N2 and CH4). These species will now be placed in the context of Titan’s atmospheric chemistry in order to highlight the interest of these first promising results.
The detection of heavy hydrocarbons with several carbon atoms demonstrates the feasibility of the reactor to initiate a molecular growth similar to that of Titan’s upper atmosphere despite the wavelength selectivity of this experiment 65 . Small molecules such as acetylene (C2H2) and ethylene (C2H4) support the experiment’s ability to trigger Titan’s complex chemical network from the photolysis of methane and nitrogen because they are abundant products in Titan’s chemistry 66 . C2H4 is formed in the upper atmosphere and diffuses downwards while being photolysed and is the main source of C2H2 in the bulk of the atmosphere. In Titan’s upper atmosphere, C2H4 is responsible for the growth of hydrocarbons with, in particular, the formation of propyne (CH3C2H) and allene (CH2CCH2) isomers. However, in our reactor the statistical analysis found only one of the two isomers although both FPs are present in the database. This absence comes from the fact that CH2CCH2 tends to be easily isomerized to CH3C2H. The abundance of CH3CCH2H is therefore self-sustaining while CH2CCH2 tends to disappear but both isomers are likely to be formed in the reactor. The fact that our statistical analysis finds only one of the two isomers, shows us its sensitivity to be able to discriminate the presence of molecule with the same raw formula once the respective FPs are known.
On the opposite, butane (C4H10) is formed via a different formation pathway than in Titan’s atmosphere. In Titan’s atmosphere, C4H10 is formed from propylene (C3H6) which itself is derived from C2H6 15 . However, the formation of C2H6 is not optimum at 89.2 nm, due to the fact that the main reaction involves two CH3 radicals. At 89.2 nm the photolysis branching ratios of CH4 favour the formation of (<<
m
Thus, despite wavelength selectivity, this work demonstrates that it is possible to form complex hydrocarbons similar to those found on Titan but via different formation pathways. The analysis also identified two even higher hydrocarbons methyl vinylacetylene (C5H6) and ethyl vinylacetylene (C6H8) which have not yet been officially detected in Titan’s atmosphere but have been found in thermal degradation studies of Titan aerosol analogues 67 . However, the detection of these species is to be taken with caution because their dominant peaks, which is above mass 60, does not appear. These molecules are formed via the dissociative recombination of heavy ions ( (<<
m
As far as the formation of nitrogen species is concerned, it is initiated by the dissociation of molecular nitrogen above 600 km in Titan and for wavelengths below 100 nm, like the one used in this work, which produce both atoms in their ground and excited state. A few nitrogenous species were detected in this work, including hydrogen cyanide (HCN) which is the smallest molecule observed and a very stable molecule well known from Titan’s atmospheric chemistry due to its C ≡ N triple bond that is difficult to break 68 . Its main route of formation in Titan and the reactor is via the reaction between nitrogen atoms and methylene-amidogen (H2CN) radicals and constitutes building blocks to build up more complex nitrile species through proton-transfer reactions with ions, whose associated neutrals have a lower proton affinity. Similarly, acetonitrile (CH3CN) once formed will protonate in Titan’s atmosphere and comes mainly from the reaction between N( 2 D) and C2H4. However in our reactor C2H4 is mostly photodissociated or ionized and it is likely that a larger source of CH3CN comes from the reaction between CH and methanimine (CH2NH), a reaction that is not mentioned in the photochemical models of Titan although it would be an efficient source of CH3CN. Although this product was not found in the reactor because its fragmentation pattern is not known, there is every reason to believe that it is present because of the detection of the two isomers: cyanamide (HNCNH) and diazomethane (H2CNN) which are the products of the reaction of CH2NH with nitrogen atoms. The detection of these two isomers is a very interesting result because current photochemical models, although taking into account nitriles to model nitrogen chemistry, only contain species with a single nitrogen atom. There are no diazo and triazo species taken into account although their involvement is necessary to explain the formation of observed nitrogen-rich aerosols in Titan’s atmosphere. Laboratory analysis of aerosol analogues identified nitrogen-rich aromatic species for which cyanamide was presented as a possible heteroaromatic structure to explain the observed N = N patterns along with other CN2H2 isomers 69,70,71,72 . Referring to the importance of anion chemistry on the growth of aerosols in the atmosphere of Titan 8 , recent experimental results have identified new negatively charged di- and tri-nitrogen species ( (<<
m
Finally the detection of dimethyldiazene (CH3NNCH3) and 2-propanamine ((CH3)2CHNH2) is consistent with the high nitrogen incorporation observed by the Huygens probe aerosol collector pyrolysis instrument, which identified NH3 and HCN as fingerprints of the chemical structure of the complex nitrogenated organic compounds that make up the aerosol nucleus. 76 . These are the largest masses produced in the present experiment after 7 hours of irradiation at 89.2 nm and are a very encouraging result on the path of formation of photochemical aerosol analogues in the laboratory. Once again the detection of these molecules demonstrates with a single wavelength, the feasibility of producing complex molecules with several nitrogen atoms that are absent from Titan’s photochemical models. In conclusion, our results highlight a photochemistry at 89.2 nm close to the atmospheric chemistry of Titan with large hydrocarbons especially nitrogen-rich organic compounds, up to 2 nitrogen atoms. Among these nitrogen compounds, new species that have not been observed in situ corroborate previous experimental measurements during laboratory simulations on similar gas mixtures and on the chemical composition of aerosol analogues. This work represents an important step in the use of a closed cell chamber for the generation of Titan-type photochemical aerosol analogues to better constrain the nitrogen fixation processes in Titan’s atmosphere and its relevance to the evolution of primitive life. This project is the first step in a long-term strategy to exploit various VUV sources. This paper highlights the potential of the HHG as a source of VUV for planetary atmospheric studies. Current limitations due to contamination from slow but unavoidable micro-leaks and residual water that introduce oxygen into the chemical system will require improvements such as the use of a higher EUV flux to reduce the irradiation time. This new type of high repetition rate EUV sources (above 200 kHz) has recently been optimised 41 , paving the way for shorter irradiation time and therefore oxygen-free experiments. The cold trap could also be used before performing the experiment in order to minimize the water signal in the chamber. Developments are also under way to house the reactor in a secondary containment with the possibility of heating the entire facility. This secondary containment would be filled with dry N2, to avoid micro-leaks that lead to the introduction of water and oxygen into the reactor, even over long irradiation periods. I have an unknown gas in a glass container. I shine a bright white light through one side of the container and measure the spectrum of transmitted light. I notice that there is a black line (absorption line) in the middle of the visible red band at ( ext<642>) ( ext We have an absorption line at ( ext<642>) ( ext
The absorbed photons had an energy of ( ext <3,1> imes ext<10>^<- ext<19>>) ( ext Figure 12.7 shows various energy level transitions. The transitions related to visible wavelengths are marked as the transitions beginning or ending on Energy Level 2. Let us find the energy of those transitions and compare with the energy of the absorbed photons we have just calculated. Energy of transition (absorption) from Energy Level 2 to Energy Level 3: Therefore the energy of the photon that an electron must absorb to jump from Energy Level 2 to Energy Level 3 is ( ext <3,1> imes ext<10>^<- ext<19>>) ( ext
This is the same energy as the photons which were absorbed by the gas in the container! Therefore, since the transitions of all elements are unique, we can say that the gas in the container is hydrogen. The transition is absorption of a photon between Energy Level 2 and Energy Level 3. The energy of the photon does not correspond to the energy of an energy level, it corresponds to the difference in energy between two energy levels. The study of spectra from stars and galaxies in astronomy is i called spektroskopie. Spectroscopy is a tool widely used in astronomy to learn different things about astronomical objects. Measuring the spectrum of light from a star can tell astronomers what the star is made of. Since each element emits or absorbs light only at particular wavelengths, astronomers can identify what elements are in the stars from the lines in their spectra. From studying the spectra of many stars we know that there are many different types of stars which contain different elements and in different amounts. You have already learnt in Chapter 9 about the Doppler effect and how the frequency (and wavelength) of sound waves changes depending on whether the object emitting the sound is moving in die rigting van of weg from you. The same thing happens to electromagnetic radiation (light). If the object emitting the light is moving in die rigting van us, then the wavelength of the light appears shorter (called blueshifted). As die voorwerp beweeg weg from us, then the wavelength of its light appears stretched out (called redshifted). The Doppler effect affects the spectra of objects in space depending on their motion relative to us on the earth. For example, the light from a distant galaxy that is moving away from us at some velocity will appear redshifted. This means that the emission and absorption lines in the galaxy's spectrum will be shifted to a longer wavelength (lower frequency). Knowing where each line in the spectrum would normally be if the galaxy was not moving and comparing it to the redshifted position, allows astronomers to precisely measure the velocity of the galaxy relative to the Earth. The sun emits radiation (light) over a range of wavelengths that are mainly in the visible part of the spectrum. Radiation at these wavelengths passes through the gases of the atmosphere to warm the land and the oceans below. The warm earth then radiates this heat at longer infrared wavelengths. Carbon dioxide (one of the main greenhouse gases) in the atmosphere has energy levels that correspond to the infrared wavelengths that allow it to absorb the infrared radiation. It then also emits at infrared wavelengths in all directions. This effect stops a large amount of the infrared radiation from getting out of the atmosphere, which causes the atmosphere and the earth to heat up. More radiation is coming in than is getting back out. So increasing the amount of greenhouse gases in the atmosphere increases the amount of trapped infrared radiation and therefore the overall temperature of the earth. The earth is a very sensitive and complicated system upon which life depends and changing the delicate balances of temperature and atmospheric gas content may have disastrous consequences if we are not careful. [45] This work has demonstrated that the latest photochemistry can accurately account for the N2 + , NO + , O2 + , and N + density measurements on the AE-C satellite. In particular, the earlier problem with the models overestimating the N2 + density by a factor of 2 appears to have been resolved by the latest laboratory reaction rate measurements, solar EUV irradiances, and photoelectron fluxes. [46] The agreement between the photochemical model and measurement is remarkable when it is considered that the calculations involve multiple quantities such as densities, reaction rates, and photoionization rates that may have uncertainties as large as 30%. Nevertheless, the agreement within the statistical errors for so many ion densities provides confidence in our ability to successfully model the ionospheric photochemical system.
Worked example 4: Absorption
What is given and what needs to be done?
Calculate the energy of the absorbed photons
Find the energy of the transitions resulting in radiation at visible wavelengths
Applications of emission and absorption spectra (ESCQV)
Identifying elements in astronomical objects using their spectra
Determining velocities of galaxies using spectroscopy
Global warming and greenhouse gases
4. Conclusions
