Lecture 11: Ages and Metalicities from Observations A Quick Review

Transcription

1 Lecture 11: Ages and Metalicities from Observations A Quick Review

2 Ages from main-sequence turn-off stars Main sequence lifetime: lifetime = fuel / burning rate $ M " MS = 7 #10 9 % & M $ L " MS = 7 #10 9 % & L. ' ( ) ' $ ( ) L % & L * 3 4. yr ' ( ) *1 (since L + M 4, M + L 1/ 4 ). yr M V M V (TO) HR diagram B-V Luminosity at the top of the main sequence (turn-off stars) gives the age t.

3 Ages from main-sequence turn-off stars M V (TO) = 2.70 log ( t / Gyr ) [Fe/H] Globular Cluster in Halo Open Clusters in Disk 47 Tuc: 12.5 Gyr M67: 4 Gyr NGC188: 6 Gyr

4 Multiple Ages of stars in Omega Cen

5 Star Formation Rates!"#$%&'$(#)'*+$#",+!!"#$%&'$(#)'*+-#*+.,+(,#/0$,1+2*+(#*3+4#3/5+!!!! :+ C,$,+4,+=''P+#"+'*,5+QR,+C!+$,-'(.2*#)'*+=2*,>+! 678+$#1+&$'(+$,-,*"+/"#$%&'$(+2'*2/,/+-='01+4R2-R+ $,-'(.2*,1+#*1+-#/-#1,/+! ;*=3+/"#$/+42"R+STKUS'+42"R+=2&,)(,/+VWUS3$+<$'10-, SFR(M " yr #1 ) = 7.9 $10#42 L(H% )(ergs#1 ) = 1.08 $10#53 Q(H o )(s#1 ) Q(H o ) is the ionising photon luminosity constants are derived from evol. synthesis models (e.g., Kennicutt 1982)!

6 Cosmic Star Formation History!"#$%&'#()*+,"*$)-".'/%#("*0' 1()*+,"*$)-".'/)#'23&4%.32'56+,"42'#%.&3' )'*32#/%7'",'5'89/)4,'):3'",';.%<3*#3=>'?"#('#(344)*'$)##',"*$32'3)*40'

7 Abundance Measurements Star spectra: absorption lines Gas spectra: emission lines Galaxy spectra: both Metal-rich/poor stars: stronger/weaker metal lines relative to H. HII region spectra Stellar spectra

8 Lab measurements: Unique signature (pattern of wavelengths and strengths of lines) for each element.

9 High-Resolution Spectra Measure line strengths (equivalent widths) for individual elements. Equivalent Width measures the strength (not the width) of a line. EW is width of a 100% deep line with same area.

10 Abundance Measurements! Spectra Line strengths (equivalent widths)! Astrophysics Stellar atmosphere models! Physics Laboratory calibrations Abundances: (Temperature, surface gravity, and metal abundances in the stellar atmosphere models are adjusted until they fit the observed equivalent widths of lines in the observed spectrum. Full details of this are part of other courses) " # $ + + " Fe H % & ', etc.

11 Bracket Notation Bracket notation for Fe abundance of a star relative to the Sun: " # $ Fe H % & ' ( log ) 10+ * n(fe) n(h),. - / ) n(fe), 0 log * n(h) - ) ( n(fe) n(h) ), = log 10 / + *( n(fe) n(h) ).. - And similarly for other metals, e.g. relative to Fe:. atoms of Fe atoms of H Star with solar Fe abundance: Twice solar abundance: Half solar abundance: " # $ O Fe % & ', " Fe # $ H " Fe # $ H " # $ " # $ C Fe Fe H % & ',... % & ' = 0.0 % & ' = log 2 10 % & ' = log 1/2 10 ( ) = +0.3 ( ) = (0.3

12 Z = X = Metallicity Metalicity (by mass): " metals A i n i n(h) + 4 n(he) + n(h) n(h) + 4 n(he) + Z X = f n(mg) n(h) " metals " metals A i n i A i n i To infer Z from a single line: f " # metals A i n i n(mg) " # $ Z " vs Abundance Abundance (by number): Mg H % & ' ( log ) 10+ * Z = X f " * X. f.. n(mg) n(h) ) Z, = log * f X - ) Z = log * Z.,. - f * f 0 * ) n(mg), / log * n(h) - ) Z, / log * f X -. [ 10 Mg H ] X X 0.,. -.. [ #10 Mg H ] Primordial: X p = 0.75, Y p = 0.25, Z p = 0.00 Solar: X = 0.70, Y = 0.28, Z = 0.02

13 Solar Abundances!"#$%&'&(&)$*%+,%-&%&./&0$1%!"#$%&$'()*+)(,(-($./)0$).1()/*,&2)/3/.(-4)35&60/),*7&20.1-0')

14 Solar Abundances

15 Primordial He/H measurement Emission lines from H II regions in low-metalicity galaxies. Measure abundance ratios: He/ H, O/H, N/H, Stellar nucleosynthesis increases He along with metal abundances. Find Y p by extrapolating to zero metal abundance.

16 Most metals enrich at approx same rate as Fe (e.g. to a factor of 2-3 over a factor of 30 enrichment). Some elements (Mg,O,Si,Ca,Ti,Al) formed early, reaching 2-3 x Fe abundance in metal-poor stars Lowest metal abundance seen in stars: [Fe/H] ~ -4 [X i /Fe] vs [Fe/H] O Mg Si Ca Ti Ni Na Al Y Zr Ba Nd

17 Enhancement of α Elements α-elements = multiples of He, more stable, produced by Type II Supernovae (high-mass stars, M > 8M ) 3!"#"$"%&'4'")56'$7#89#"5',:' ;"<'=69>,?7*&5',:'%,>$)#'@A'.B69"'CC2' D)#)E-"5'F-&('(-G('! $75&' ;)H"':,>$"?':)5&'=":,>"' I)%6'@%C)J5'-%K"*&"?'&("' %,%!!'"#"$"%&5'-%&,'&("'' C@I' Stars with high α elements must have formed early, e.g. before a less α-enhanced mix added to ISM by Type Ia SNe (WD collapse due to accretion from binary companion). I6'=7#G"':,>$"?':)5&')%?' ")>#6<'=":,>"'&("'?-5*L'!!"#"$"%&'"%()%*"$"%&' [O/Fe] [O/Fe] +,**)#-'"&')#'./0012' Most MW bulge stars are α- enhanced => Bulge must have formed early. [Fe/H]

18 Some Key Observational Results Z =!ylog(µ) for Z < y Gas consumption: More gas used --> higher metallicity. Radius: more metals near galaxy centre µ 1 Near centre of galaxy: Shorter orbit period--> More passes thru spiral shocks --> More star generations --> µ lower --> Z higher. (Also, more infall of IGM on outskirts.) Galaxy Mass : Low-mass galaxies have lower metallicity. Dwarf irregulars: form late (young galaxies), have low Z because µ is still high. Dwarf ellipticals: SN ejecta expel gas from the galaxy, making µ low without increasing Z. y Z

19 M31: Andromeda in Ultraviolet Light UV light traces hot young stars, current star formation. Gas depleted, hence no current star formation in the inner disk.

20 More metals near Galaxy Centres Ellipticals (NGC 3115) Spirals (M100)

21 !"#$%%&'&#()*+$,&"-#)&-)!.)!/)0"#$%%&'&#()*+$,&"-#) &01%&"2)$-)&-2&,"345#) 64+0$74-)1+4'"22)64+)#8")!.)

22 Mass-Metalicity relation Why are low-mass galaxies are metal poor?!1/2,%"##,3"("4)&#,6"7&, (/2&-,%&'"()*)'+8,26+9, -!:/;03&-, Some are young (not much -!/-&,3"#,(/##, gas used yet, -!5)36&-,)0<"((, so ISM not yet enriched).!"##$%&'"(()*)'+,-&("./0, 1/2,%"##,, 3"("4)&#, Supernovae eject the enriched gas from small galaxies. 5)36,%"##, 3"("4)&#,

23 Less Metals in Small Galaxies faint ----> bright

24 SFR Stellar Mass Two fundamental parameters seem to determine observed metallicity: mass and SFR. This forms a fundamental metallicity relation (FMR). Despite extremely complex underlying physics, the relation seems to hold out to z = 2.5 and in a huge range of galaxies / environments. Stellar Mass

25 More Metals => More Planets Doppler wobble surveys find Jupiters orbiting 5% of stars with solar metalicity. This rises to 25% for stars with 3x solar abundance [Fe/H]=+0.5 Fischer & Valenti 2005

26 A Quick Review Main events in the evolution of the Universe: The Big Bang (inflation of a bubble of false vacuum) Symmetry breaking matter/anti-matter ratio Quark + antiquark annihilation photon/baryon ratio The quark soup heavy quark decay Quark-Hadron phase transition and neutron decay n/p ratio Big Bang nucleosynthesis primordial abundances X p = 0.75 Y p = 0.25 Z p = 0.0 Matter-Radiation equality R ~ t 1/2 R ~ t 2/3 Recombination/decoupling the Cosmic Microwave Background CMB ripples (ΔT/T~10-5 at z=1100) seed galaxy formation Galaxy formation and chemical evolution of galaxies

27 Main events in the chemical evolution of galaxies: Galaxy formation Jeans Mass ( ~10 6 M ) Ellipticals Initial mass and angular momentum, plus mergers. Spirals Star formation history S( t ), gas fraction µ( t ) Irregulars Star formation α = efficiency of star formation The IMF ( e.g., Salpeter IMF power-law with slope -7/3 ) First stars (Population III) from gas with no metals (none seen) Stellar nucleosynthesis metals up to Fe Supernovae (e.g. SN 1987A) metals beyond Fe p, s, and r processes white dwarfs (M < 8 M ) or black holes, neutron stars (M > 8 M ). Galaxy enrichment models: (e.g. Z = - y ln(µ), yield y ) Metal abundances rise X = 0.70 Y = 0.28 Z = 0.02 (solar abundances) Gas with metals Stars with Planets Life!