L2: The building-up of the chemical elements

Transcription

1 credit: NASA L2: The building-up of the chemical elements UCL Certificate of astronomy Dr. Ingo Waldmann

2 What ordinary stuff is made of What ordinary stuff is made of Build up of metallicity 2

3 What are we made of? Composition of man What are we made of? Composition of homo sapiens 96.2% of body weight comes from "organic elements : Oxygen 65.0% Carbon 18.5% Hydrogen 9.5% Nitrogen 3.2% 3.9% of body weight comes from elements present in the form of salts: Calcium 1.5% Phosphorus 1.0% Potassium 0.4% Sulfur 0.3% Sodium 0.2% Chlorine 0.2% Magnesium 0.1% Iodine 0.1% Iron 0.1% Build up of metallicity 3

4 What are we made of? Composition of the Earth What are we made of? Composition of the Earth Atmosphere : 78% nitrogen 21% oxygen 1% other stuff (CO 2, H 2 O, Ar, ) Oceans : Water Solid crust : 47 % oxygen 28 % silicon 8 % aluminum 2-5 % of iron, calcium, potassium, sodium, etc. Intermediate mantle : mostly oxygen and silicon some iron, magnesium, etc. Central core : mainly iron smaller amounts of nickel and cobalt Build up of metallicity 4

5 Of all objects, the planets are those which appear to us under the least varied aspect. We see how we may determine their forms, their distances, their bulk, and their motions, but we can never known anything of their chemical or mineralogical structure; and, much less, that of organized beings living on their surface... Auguste Comte, The Positive Philosophy, Book II, Chapter 1 (1842) Build up of metallicity 5

6 Spectroscopy proves Comte wrong: Decomposing light into its colours allows us to probe the composition of a light source Sky &Telescope (Longer) (Shorter) Wavelength (i.e. colour) Build up of metallicity 6

7 Spectroscopy of the sun Spectroscopy An example... the Sun Solar Spectrum N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF Build up of metallicity 7 Joseph von Frauenhofer

8 Composition of the sun Composition of the Sun Element Abundance Abundance (% Number) (% Mass) Hydrogen Helium Oxygen Carbon Nitrogen Silicon Magnesium Neon Iron Sulfur Build up of metallicity 8

9 Composition of other stars: Stellar metallicities Composition of other stars: Stellar metallicities X = Mass Fraction in Hydrogen (X Ꙩ =0.70) Y = Mass Fraction in Helium (Y Ꙩ =0.28) Z = Mass Fraction in Metals (Z Ꙩ =0.02): Metallicity Arcturus (α Bootis) N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF Build up of metallicity 9

10 Beware! Metallicity is NOT really a measure of the amount of metals in a star. It is best to define metallicity as the MASS fraction of a star in elements other than hydrogen and helium. Remember the Earth is mostly made up of Fe, Mg, Si, O, C. Stars with a very small metallicity cannot generate a Solar System like ours no life possible! For instance, stars in the outskirts of our Galaxy have very low metallicities, whereas the central region of the Galaxy (Bulge) is dominated by stars with high metallicity. Build up of metallicity 10

11 The connection between stellar age and metallicity The connection between stellar age and metallicity Old stars have lower metallicities: in the past, the gas feeding star formation did not have much of C, N, O, Fe, ωcen stars Metallicity Hilker et al Time Build up of metallicity 11

12 Metals in Galaxies Metals in Galaxies Unresolved stellar populations (>100 million stars). Similar values of metallicity as individual stars. Build up of metallicity 12

13 The mass-metallicity relationship The mass-metallicity relationship The metallicity of the stars in a galaxy correlate with the total galaxy mass: More massive galaxies retain more metals from previous star formation, so that the next generations of stars achieve higher metallicities. Low mass galaxies are too weak to retain this gas (and metals), so that subsequent generations of stars are still formed at low metallicities. CONFUSING BIT: Massive galaxies have old stars, but they have formed very quickly, and were very efficient at recycling the gas, achieving high metallicities. Tremonti et al SDSS Galaxies Build up of metallicity 13

14 Where does everything come from? The big bang

15 Structure Growth and Galaxy Formation Structure Growth and Galaxy Formation You are here 300,000yr Now (13.5Gyr) Time V. Springel (MPA) The photon gas decoupled from matter 300,000 years after the Big Bang (about 13 Gyr ago), after which ordinary material (protons, neutrons and electrons) condensed to form structures, eventually leading to stars, planets and us! Build up of metallicity 15

16 Primordial Abundances Primordial Abundances: Were the elements formed in the Big Bang? were elements formed in the Big Bang? NO! 4 He The initial stages of the Universe were very hot. However, the gas was not dense enough and it kept on cooling down (because of the expansion of the Universe). As a result, only small amounts of elements were synthesized: mostly Helium, Deuterium (an isotope of Hydrogen) and traces of Lithium. No C, N, O whatsoever!! Build up of metallicity 16

17 The Big Bang Periodic Table of Elements The Big Bang Periodic Table of the Elements Build up of metallicity 17

18 Where else can elements be formed??? Where else can elements be formed? We need systems that keep high enough temperatures for nuclear reactions to take place, and at high enough densities so that the process generates a significant amount of "new elements". STARS!! Build up of metallicity 18

19 Stars as furnaces (old days) Stars as furnaces (Old days) The energy from the Sun and the stars was originally thought to come from the cooling of a hot cinder However: L SUN =2x10 26 J/s The Earth is ~ 4.5 billion years old The Sun must have a process to generate 3x10 43 J Mass of the Sun is M SUN =2x10 30 kg Should generate ~1.5x10 13 J per kg!!! Build up of metallicity 19

20 Chemistry (molecular and atomic binding energies) Chemistry (Molecular and atomic binding energies) Chemistry cannot generate those energies!!! Hydrogen ionization: E~13.6 ev T CHEM ~10 4 K 10 9 J per kg To fuse Hydrogen nuclei, we need much higher temperatures: T~10 7 K!!! Nuclear physics!! Build up of metallicity 20

21 This a snapshot of the outer layers of the Sun (its atmosphere), at a temperature ~6,000K. The heat and turbulence you see is caused by the energy input from the inner regions ~700,000 km below, where nuclear reactions take place. Build up of metallicity 21

22 PP chain products (Main sequence) PP chain products (Main Sequence) Hydrogen Helium Build up of metallicity 22

23 Second phase: Helium Carbon In the Sun a second channel for H He synthesis operates (CNO cycle) which transforms most of the C present in the star into N 4 He + 4 He 8 Be But 8 Be is unstable!! Lucky Coincidence! 8 Be can hold itself for s Enough to allow the next stage: 4 He + 8 Be 12 C + γ T ~ 100 million K Build up of metallicity 23

24 How about Massive Elements: Ne, Mg, S, Si, Fe??? Massive stars (~10 M Ꙩ, ~1,000R Ꙩ ) Build up of metallicity 24

25 Hydrostatic nucleosynthesis in massive stars Hydrostatic nucleosynthesis in massive stars Onion-like structure: the inner shells are hotter and will synthesize more massive elements Build up of metallicity 25

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27 Iron is the end of the alchemist s fusion road! Iron is the end of the alchemist s fusion road! Build up of metallicity 26

28 The subsequent burning phases are hotter, with shorter duration. Take a 20M Ꙩ star: H (Main Sequence) 10 Myr 10 7 K He(HB) 1 Myr 10 8 K C 1,000 yr 10 9 K Ne/O 1 yr K S/Si 12 days K When the core is left with iron: Next step in reaction chain ABSORBS energy Free electrons (which support core) are absorbed by nuclear material Suddenly the core behaves like dust Build up of metallicity 27

29 Supernova collapse! In a small fraction of a star s life, it outshines the whole galaxy!! ~6 billion times brighter than Sun! Build up of metallicity 28

30 Elements more massive than Iron are generated by the irradiation of the lighter nuclei with neutrons, in two possible ways: r-process (rapid) during the SN explosion s-process (slow) during AGB phase Build up of metallicity 29

31 in a nutshell... in a nutshell Hydrogen is the most simple chemical element (proton+electron): no nucleosynthesis necessary: truly primordial. About 25% in mass is transformed from 1 H to 4 He right ~100s after the Big Bang (Big Bang Nucleosynthesis). D (= 2 H), 3 He and 7 Li generated as well. Stars burn H into He in their cores. This process sustains the energetic balance of the star during most of its lifetime (~10 Gyr for the Sun). At later stages, He is burned into C although the total yield depends sensitively on the mass of the star. Stars like the Sun mainly generate He and N Stars with masses ~3-4M Ꙩ generate He, C and N Only massive stars (>10M Ꙩ ) generate heavier elements: O, Mg, Si, Fe Build up of metallicity 30

32 Metallicity and planet formation Metallicity and planet formation Metals are needed to form planetary systems, not only rocky planets: planets grow from small seeds (rocky or ices of water, ammonia, methane) that merge forming larger seeds that can eventually accrete gas. So even for Jupiter-like planets, those seeds are needed. Marcy et al Metallicity Build up of metallicity 31

33 However, its is also found that most exoplanetary systems feature hot Jupiters : massive gas giants very close to the parent star. This could also be related to metallicity (higher growth rate of massive planets with increasing metallicity), causing a problem for the presence of Earth-like planets in the inner regions, where water could exist in liquid form Z Ꙩ Z Ꙩ Build up of metallicity 32

34 An optimal metallicity? An optimal metallicity? There is a balance between these two competing forces so that a characteristic metallicity favours the formation of Earths Could that be solar metallicity?? Lineweaver 2000 Build up of metallicity 33

35 Aims and Objectives Aims and Objectives 1. Put the Periodic Table of the elements in context with cosmic evolution 2. Relate the formation of elements to stellar evolution 3. Establish a connection between stellar mass - main characteristic of a star - with the elements synthesized. A star like our Sun could not have generated the amount of C, N, O, Mg, Fe needed to form a planet like the Earth. 4. Present the concept of metallicity as a property that roughly increases with cosmic time 5. and the connection of metallicity with the presence of Earth-like planets and life. Build up of metallicity 34