Atomic Processes and the Interstellar Medium Some Typical Condi{Ons

22/04/2016 Astrophysics seminars Usually Monday aernoons, Atomic Processes and the 2pm: Interstellar Medium Today: Dr. S.-J. Paardekooper, Queen Mary Extrac<ng quan<tave measurements University of London Migraon of Rocks, Planets from astronomical observaons and Hurricanes in Patrick Roche Protoplanetary Discs But note that the Hintze Lecture is on Tuesday May 10 and the Halley Lecture on Wednesday June 8 at 5pm Synopsis • Astronomical spectroscopy, lines in different spectral regions, recap of Some typical condi<ons atomic physics and selec<on rules, forbidden and allowed transi<ons, cosmic abundances • The two level atom, A, B and C coefficients and their useful regimes, • 90% H atoms, ~9% He, ~1% everything else (by number) thermal populaons, IR fine structure lines, cri<cal density, mass • Stellar surface temperatures 2000 <T< 40000 K es<mates Densi<es ~ few gram/m3 for main sequence stars • Recombinaon and ionizaon processes, the Stromgren sphere, • Ionized nebulae e.g. HII regions, planetary nebulae ionizaon balance, effec<ve temperature es<mates. – T(electron) ~ 10000K, • The 3 level atom: diagnos<cs of electron temperature and electron 6 12 -3 density. – n(electron)~ N(proton) 10 - 10 m • Absorp<on lines, equivalent width and the curve of growth. Column – T(dust) ~ 50K densi<es and abundances • Cold and denser molecular clouds (T<< 100 K) • The interstellar medium. Atomic and ionic absorp<on lines, abundance • Hot and lower-density plasmas - e.g. shock heated gas, of gas, molecules and dust. Hyperfine transi<ons: 21cm line of H, T~106K, n<100 Galac<c structure • 4 • Interstellar ex<nc<on, dust components, thermal emission, equilibrium Veloci<es: cold ISM 1km/s - SN ouhlows 10 km/s and stochas<c processes • Overall density of the Universe is ~ 10 orders of magnitude lower • The sun. Ionizaon and sources of opacity, radiave transfer, the Gray than the best lab vacuum atmosphere limb darkening, absorp<on line formaon 1 22/04/2016 Astronomical Spectroscopy Imaging provides informaon about structure and morphology, Whilst photometry permits es<mates of luminosity and variability. We have to analyse spectra to understand the composi<on and Observaons across the electromagne<c spectrum probe: e.g. physical condi<ons (temperature, density, excitaon) of High energy processes e.g. accre<on onto compact objects; K-shell X- galaxies, stars and nebulae and the intervening material ray transi<ons between the Earth and the object Photo-ionised gas, recombinaon and forbidden lines, stellar Spectra provide informaon on the structure and dynamics of atmospheres in UV, op<cal, Infrared stars, planets and galaxies – and have provided evidence for the Rotaonal-vibraonal molecular transi<ons, fine-structure line Big Bang, expansion of the Universe, dark maer, exosolar transi<ons , dust emission in the IR planets Molecular rotaonal lines, synchrotron and free-free emission in the microwave and radio, 21cm line tracing atomic H In fact almost everything interes<ng in astrophysics! To first order, astronomical objects have very similar composi<ons, Different techniques using a range of ground-based and space facili<es but their appearances vary dramacally Here I will concentrate on op<cal/infrared transi<ons, but the same e.g. the surface of the sun and a nebula principles apply to other wavelength regimes Quan<tave analysis allows us to probe and understand this A brief history of astronomical spectroscopy 1672 Newton’s prism – sunlight split into cons<tuent colours 1800 Herschel noted that infrared light is present beyond the visible red bands 1804 Wollaston noted dark lines in the solar spectrum 1814 Fraunhofer rediscovered them and iden<fied 475 dark lines including one coincident with Fraunhofer Spectrum of the Sun that produced by salt in flame A,B – telluric atmospheric absorp<on bands by ozone C – Hydrogen (Balmer alpha) 1870 Kirchoff and Bunsen iden<fied 70 lines with iron vapour D – Sodium Doublet 1864 Nebulium was proposed by Huggins to explain a green line seen in nebula E – Iron 1869 Coronium invoked to explain a green line seen in solar prominences F,G – Hydrogen (Balmer beta, gamma) 1870 Lockyer and Janssen proposed a new element Helium from solar spectra H,K - Ca+ Helium was confirmed in 1895 by Ramsay. The explanaon for Nebulium did not emerge un<l 1928 when Bowen demonstrated that the Note that although Hydrogen is by far the most abundant element, the strongest lines lines at 4959 and 5007A arise from the 1D -3P forbidden transi<on in OIII are due to ‘trace’ elements – we need to use Atomic Physics and stas<cal mechanics 2 2 to understand this. The presence of dark lines, suggests a temperature gradient in Coronium was iden<fied by Edlen and Grotrian in 1939 as a transi<on from Fe XIV, arising from the surface layers an ion with an ionizaon poten<al of 361eV. Since then, many other unexpected phenomena have been discovered – masers, transi<ons from Note also that for accurate analysis, we need to calibrate the spectra – compensate for short-lived species, highly relavis<c mo<ons etc, there are s<ll many as-yet uniden<fied the effects of transmission through the atmosphere and instrument (and in more lines from ions and molecules. distant objects, the effects of the interstellar medium). 2 22/04/2016 Stellar Detailed spectral characteris<cs Spectra depend on pressure (surface gravity), element abundances etc. Examples of • E.g. White dwarf : the end product of stellar spectra intermediate mass star evolu<on aer going Sirius A and B with Teff through planetary nebula phase. ranging from • Dense. High surface gravity leads to 30000 - 2800 K pressure-broadening of lines (Y-P Chen et al 2014) • Strong quasi-blackbody con<nuum emission but with marked spectral structure and absorp<on lines: the break at ~350nm (the ‘Balmer Jump’ due to an excitaon edge in H), narrow atomic lines and broader molecular bands • Note the increasingly prominent absorp<on as temperature decreases Effects of Metallicity Stars with reduced heavy element Interpretaon of Spectra abundances show fewer and narrower absorp<on bands. • Con<nuum with emission and/or absorp<on lines (and Spectroscopic surveys have molecular bands and solid state dust features in the infrared) iden<fied stars in the halo of our • Con<nuum due to a range of processes, galaxy with very low abundances – thermal emission (characteris<c of the temperature of a star, of heavy elements. nebula or dust) : λmaxT = 2898 [ T in K, λ in μm ] – Element abundance paerns in Bremsstrahlung from electron-electron interac<ons in plasma – Non-thermal (synchrotron) emission at long wavelengths the most metal-poor stars reflect – pollu<on from first generaon of Non-thermal (compton) emission at short wavelengths stars formed: SMSS 0313-6708 • In the simplest case, a cold layer in front of a hot star will has [Fe/H] < -7.1 and [C/H] = -2.6 produce absorp<on lines, while and may show the imprint of a • A hot gas will produce emission lines from atoms and ions (see single early-<me Supernova arc lamps) ; a cool gas may produce emission from molecules and neutral atoms. (Keller et al 2014) Figure from – In astronomical objects, condi<ons are oGen very different from Frebel & Norris 2015. those in terrestrial laboratories. 3 22/04/2016 The search for the most distant objects Informaon from observaons of lines z=5.80 • Radial veloci<es and velocity components and For some purposes, distribu<ons and hence dynamics, rotaon, expansion and/or contrac<on, bulk or turbulent mo<ons, iden<ficaon of lines and ouhlow and/or accre<on, pulsaons, flares, z=5.82 wavelengths may be astroseismology sufficient, e.g. redshiG • The physical condi<ons of astronomical bodies: determinaon. But even – density, temperature, ionizaon state etc z=5.99 here, need to know which • Ionic and element abundances, isotopic raos in lines have been detected some cases (e.g. Lyman Alpha in these • Magne<c field strengths (Zeeman effect) z=6.28 high-z QSOs) • Molecule (gas and condensed phases) and dust species, astrochemistry Interstellar Absorp<on lines towards Planetary nebulae and nearby stars Photoionized gas Observaons of absorp<on • laboratories for understanding lines produced by ions and atomic processes and atoms in the interstellar photoionizaon by hot stars medium between the Earth and nearby stars are used to – Isolated systems es<mate the amounts of – Single (or at least a small number of stars) different species in the gas – Central star excites gas ejected by the precursor phase of the ISM. • Clues to stellar evolu<on – End point of red giants High spectral resolu<on – Enrichment of the ISM observaons isolate different velocity components (along – Produc<on of white dwarfs the line of sight), and the • Central stars have 30,000 K < T < 250,000 K iden<ficaon of different – Ionizaon state of gas reflects stellar temperature clumps of material – Chemical abundances reflect stellar evolu<on: • nuclear processing and dredge-up With narrow lines, and high – Density structure reflects ejec<on mechanism and quality spectra, may pick up protoplanetary structure hyperfine structure due to different isotopes – Formaon and processing of molecules and dust 4 22/04/2016 Planetary Nebula NGC 7009 • Photo-ionised gas emission spectrum – Bright emission lines from hydrogen, helium, oxygen, nitrogen etc. • Recombinaon lines of Hydrogen (and Helium): protons capture electrons in excited states which then cascade down • Collisionally excited forbidden states in heavy elements, which have low transi<on probabili<es, but can decay radiavely at low densi<es, cooling and ac<ng as a thermostat for the gas Visible spectrum of Planetary Nebula NGC 7027 flanked by the spectra of Hydrogen (top) and H + He (bolom) – with logarithmic scaling [from Kevin Volk] Zoom in to the op<cal spectrum Spectra of Planetary Nebulae showing line iden<ficaons Planetary nebulae spectra: Hubble 12 (top), Jonckhere 900.

Load more