Nucleosynthesis and stellar lifecycles

Outline:

2. What nucleosynthesis is, and where it occurs

3. Molecular clouds

4. YSO & protoplanetary disk phase

5. Main Sequence phase

6. Old age & death of low mass stars

7. Old age & death of high mass stars

8. Nucleosynthesis & pre-solar grains

Ste

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lifec

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What nucleosynthesis is,and where it occurs

Nucleosynthesis

formation of elements

Except for H, He(created in Big Bang),all other elements createdby fusion processes instars

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lati

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ab

un

da

nce

StellarNucleosynthesis

Some H destroyed;all elements withZ > 2 produced

Various processes,depend on

(1) star mass (determines T)

(2) age (determinesstarting composition)

Z = no. protons, determines element

Beta Stability Valley.

Nucleons with rightmix of neutrons (n) toprotons (p) are stable.

Those that lie outsideof this mix are radioactive.

n >

p >

Beta Stability Valley.

Too many n:beta particle (electron)emitted, n convertedto p. (Beta Decay)

e.g. 26Al -> 26Mg + betae.g. 53Mn -> 53Cr + beta

Some stellarnucleosynthesisresulted inn-rich nucleonsthat are short-livednuclides.

n >

p > too

many n

Beta Stability Valley.

Too many p:electron captured bynucleus, p convertedto n.

e.g.,41Ca + electron -> 41K

Other stellarnucleosynthesisproduced short-livedp-rich nucleons.

n >

p >

toomany p

Stellar lifecycles: from birth to death

low massstar (< 5 Msun)

high massstar (> 5 Msun)

Stellar lifecycles: low mass stars

1 & 5.molecularcloud

low massstar (< 5 Msun)

3. Red Giant2. Main Seq.

4. Planetary nebula

4. White dwarf

Stellar nucleosynthesis

Nucleosynthesis possibleif white dwarf in binary system(during nova or supernova)

Stellar lifecycles: high mass stars

1 & 6.molecularcloud

high massstar (>5 Msun)

3. Red Giant/ Supergiant

2. Main Seq.(luminous)

4. Supernova

5. Black hole

5. Neutron star

Stellar nucleosynthesis

Track stellar evolution on H-R diagram of T vs luminosity

Luminosity: energy / time

Distribution ofstars onH-R diagram.

When corrected forintrinsic brightness,there are MANY morecool Main Sequencestars than hot.

On main sequence, luminosity depends on mass

L ~ M3.5

Molecular clouds:

Where it begins & ends

molecularcloud

Molecular cloudscold, dense areas ininterstellar medium (ISM)

Horsehead Nebula

Mainly molecular H2,also dust, T ~ 10s of K

Famous EagleNebula image.

Cool dark cloudsare close to hotstars that arecausing them toevaporate.

Dust in ISM consists of:

-- ices, organic molecules, silicates, metal, graphite, etc.-- some of these preserved as pre-solar grains & organic components in meteorites

A largerInterplanetary DustParticle (IDP)

2 atoms

3 atoms

4 atoms

5 atoms

6 atoms

7 atoms

H2 C3* c-C3H C5* C5H C6H

AlF C2H l-C3H C4H l-H2C4 CH2CHCN

AlCl C2O C3N C4Si C2H4* CH3C2H

C2** C2S C3O l-C3H2 CH3CN HC5N

CH CH2 C3S c-C3H2 CH3NC CH3CHO

CH+ HCN C2H2* CH2CN CH3OH CH3NH2

CN HCO NH3 CH4* CH3SH c-C2H4O

CO HCO+ HCCN HC3N HC3NH+ H2CCHOHCO+ HCS+ HCNH+ HC2NC HC2CHO

CP HOC+ HNCO HCOOH NH2CHO

SiC H2O HNCS H2CNH C5N

HCl H2S HOCO+ H2C2O l-HC4H* (?)

KCl HNC H2CO H2NCN l-HC4NNH HNO H2CN HNC3

NO MgCN H2CS SiH4*

NS MgNC H3O+ H2COH+

NaCl N2H+ c-SiC3

OH N2O CH3*

2 atoms

3 atoms

4 atoms

5 atoms

6 atoms

7 atoms

PN NaCN

SO OCS

SO+ SO2

SiN c-SiC2

SiO CO2*

SiS NH2

CS H3+*

SH* SiCN

HD AlNC

FeO? SiNC

O2 ?

8 atoms

9 atoms

10 atoms 11 atoms12 atoms

13 atoms

CH3C3N CH3C4H CH3C5N (?) HC9N C6H6* (?) HC11N

HCOOCH3 CH3CH2CN (CH3)2CO

CH3COOH (CH3)2O (CH2OH)2 (?)

C7H CH3CH2OHH2NCH2COOHGlycine ?

H2C6 HC7N CH3CH2CHO

CH2OHCHO C8H

l-HC6H* (?)

CH2CHCHO (?)

All molecules have been detected (also) by rotational spectroscopy in the radiofrequency to far-infrared regions unless indicated otherwise. * indicates molecules that have been detected by their rotation-vibration spectrum,** those detected by electronic spectroscopy only.

http://www.ph1.uni-koeln.de/vorhersagen/molecules/main_molecules.html

Molecules inISM as of12 / 2004

Note manyC-compounds

HF H2D+, HD2+

Photochemistry can occur in icy mantles to createcomplex hydrocarbons from simple molecules

Gravity in molecularclouds helps promotecollapse of cloud

…and sometimes isassisted by a trigger

Young stellar objects (YSOs)& protoplanetary disks (proplyds)

YSOs

YSOs & Proplyds:Molecular cloud fragments that have collapsed– no fusion yet

< Protoplanetary disk around glowing YSO in Orion

Solar nebula:the Protoplanetary diskout of which our solar

system formed

Herbig-HaroObjects--

• YSOs withdisks & bipolaroutflows

Magnetic fields aroundYSOs can create polarjets and X winds

Collapse of molecular cloud fragments occurs rapidly

~105 to 107 yrs,depending on mass

Protostellar diskphase lasts ~106 yrs

Single collapsing molecular cloud produces manyfragments, each of which can produce a star

Main Sequence phase:Middle age

Main sequence

Star “turns on” when nuclear fusion occurs

main sequence star – either proton-proton chain or CNO cycle nucleosynthesis

P-P chain net: 4 H to 1 He

CNO cycle – more efficient method, but requires higher internal temperature, so only for stars with mass higher than 1.1 solar masses

12C + p -> 13N 13N -> 13C

13C + p -> 14N

14N + p -> 15O 15O -> 15N

15N + p -> 12C + 4He

CNO cycle net reaction : 4 H to 1 He

Star stays on main sequence in stable condition– so long as H remains in the core

A more massive star must produce more energy to support its own weight – reason there is a correlation of mass and luminosity on main sequence

But– eventually the H runs out

Lifetime on main sequence = fuel / rate of consumption~ M / L ~ M / M3.5

lifetime ~ 1/M2.5

So a 4 solar mass star will have a main sequence lifetime 1/32 as long as our sun

So, what happens when the core runs out of hydrogen?

• Star begins to collapse, heats up

• Core contains He, continues to collapse

• But H fuses to He in shell– greatly inflating star

RED GIANT (low mass)or SUPERGIANT (high mass)

What happens next depends on stellar mass

Old age and death of low mass stars

Planetary nebula

White dwarf

Red Giant

There are different types of Red Giant Stars

1) RGB (Red Giant Branch)2) Horizontal branch3) AGB (Asymptotic Giant Branch)

These differ in position on H-R diagram and ininterior structure

Red Giant (RGB) star: H burning in shell

Red Giant (Horizontal branch) star: He fusion in coreRed Giant (AGB) star: He burning in shell

AGB star

Convective dredge-ups bring productsof fusion to surface

Red Giant includes: s-process nucleosynthesis

s-processnucleosynthesis:

slow neutronaddition

beta decaykeeps pacewith n addition

No.

pro

tons

(Z

)

An AGB can lose its outer layers—Ultimately a planetary nebula forms,leaving a white dwarf in the center

Planetary nebula

White dwarf

Note: planetary nebula have nothing to do with planets!

Planetary nebulas

Nuclear fusionstops whenthe star becomesa white dwarf—

It gradually cools down

Old age & death of high mass stars

SupernovaBlack hole

Super Giant

Neutron star

High-mass stars: Progressive core fusionof elements heavier than C

Includes: s-process nucleosynthesis as Supergiant,r-process nucleosynthesis during core collapse

r-processnucleosynthesis:

rapid neutron addition

beta decay does notkeep pace withn addition

No.

pro

tons

(Z

)

End for high mass star comes as it tries to fuse core Fe into heavier elements– andfinds this absorbs energy

STAR COLLAPSES & EXPLODES AS SUPERNOVA

--Fe core turns into dense neutrons--Supernova forms because overlying star falls onto dense core & bounces off of it

Supernova remnants

Crab Nebulasupernovaremnant.

A spinningneutron star(pulsar) occursin the centralregion.

There are different types of Supernovae

1) Type 2 (kept upper H-rich portion)2) Type 1b (lost H, but kept He-rich portions)3) Type 1c (lost both H & He portions)4) Type 1a (explosion on white dwarf in binary system)

Type 2 supernovae had intact upper layers

Type 1b & c supernovae had lost upper layers

Type 1a supernovae occur in binary systemswhen material from companion falls onto whitedwarf

Nucleosynthesis &pre-solar grains

process main commentproducts

H-burning 4He main seq.

He-burning 12C, 16O Red Giant

C-O-Ne-Si 20Ne, 28Si, 32Si, Supergiantsburning up to 56Fe

s-process many elements Red Giants, Supergiants

r-process many heavy supernovaelements

Summary of nucleosynthesis processes

material suggested astrophysical site

Ne-E exploding novaS-Xe Red Giant or SupergiantXe-HL supernovaeMacromolecular C low-T ISM

SiC C-rich AGB stars, supernovaeCorundum Red Giant & AGB starsNanodiamond supernovaeGraphite, Si3N4 supernovae

Pre-solar material in meteorites

Solar system formed out of diverse materials.