Abundances in Presolar Grains, Stars and the Galaxy

Larry R. NittlerDepartment of Terrestrial Magnetism

Carnegie Institution of Washington

or

Larry R. NittlerDepartment of Terrestrial Magnetism

Carnegie Institution of Washington

What can presolar grains tell us about the history of the Galaxy?

Larry R. NittlerDepartment of Terrestrial Magnetism

Carnegie Institution of Washington

Galactic Chemical Evolution• As succeeding generations of stars are born, die and

eject newly-synthesized nuclei into the ISM, chemical composition of the Galaxy evolves.

• ISM composition at any time or place depends on entire history of stellar birth, death, and nucleosynthesis, as well as galactic dynamics

• In particular, average metallicity increases with time

– Z=mass fraction of elements heavier than He (Z=1.5-2%)

– [Fe/H]= log (Fe/H) – log (Fe/H)

GCE: Astronomical Clues

• Many (but not all) studies indicate general decline in metallicity with stellar age.

• Very large scatter in metallicity for stars of given age and galactocentric radius.

GCE: Astronomical Clues

• “alpha”-elements overabundant relative to Fe at low metallicity– Type Ia versus Type II

supernovae

Reddy et al (2003)

• Presolar grain compositions reflect interplay of galactic chemical evolution, nucleosynthesis, physicochemical conditions of stellar dust formation

• Represent sample of stars that was present shortly before formation of solar System but had formed over long history of galactic disk.– Provide complementary data set to

astronomical observations on Galactic Chemical Evolution

GCE: Presolar Grains

GCE and Presolar Stardust

• Can we use presolar grain data to constrain nucleosynthesis and GCE models?

• Can we constrain homogeneity of ISM with presolar grain data?

• Are there metallicity effects on the abundances and types of dust formed by stars?

GCE of Isotopic Ratios• “Primary” isotopes can be made in star of H, He;

“Secondary” require pre-existing CNO.– e.g., 28Si, 16O, 48Ti primary, rare isotopes secondary

• D. D. Clayton (1988):– (S/P)/(S/P) Zp/Zp

– e.g.: (29Si/28Si)/ (29Si/28Si) Z/Z

• Actual evolution depends on balance of yields from different types of stars and many details of Galactic evolution

Presolar Silicon Carbide (SiC)

• Best-studied presolar phase

• 0.1 – 20 m, ~20ppm• Anomalous in C, Si, N,

Ca, Mg, Ti, Ne, Xe, Kr, Ba, Nd, Sr, Sm, Dy, Mo, Zr

• At least six isotopically distinct groups

• Mainstream (90%), Y (2%) and Z (2%) grains from AGB stars– C, N, Mo, Zr … reflect

nucleosynthesis in parent stars (see talk by M. Lugaro)

1m

• Minimum of 30-100 sources (Alexander 1993)

GCE and presolar SiC

• Si and Ti reflect GCE

GCE and rare presolar SiC types

Nittler and Alexander (2003)

GCE and rare presolar SiC types• 30Si, C isotopes in Z

grains point to low-mass, low-metallicity AGB origin– Supported also by Ti

data for a few Z grains – Lie on extension of

mainstream correlations; depleted in secondary isotopes as expected for GCE

Zinner et al, 2005

GCE and rare presolar SiC types

• Use large grid of AGB nucleosynthesis models (R. Gallino) to estimate initial metallicity of Z grains

• Early rise of 29Si– Role of SN Ia?

Nittler et al. (2004) Nuclei in the Cosmos; Zinner et al (2006), ApJ, in press

GCE of N isotopes• N nucleosynthesis poorly understood:

– 14N mostly secondary (CNO-cycle in massive stars), partly primary from low-Z SNe, red giants

– 15N??• ~Secondary in novae; primary in SNe, but both

sources fail to make enough

• Does 14N/15N increase or decrease with Z?

N isotopic evolution• Dahmen et al. (1995)

found N isotopic gradient:– 14N/15N decreases with Z

(secondary 15N)

• Chin et al. (1999) found 14N/15N ~ 100 in starburst galaxies and LMC SFR:– 14N/15N increases with Z

(primary 15N)

N isotopic evolution• Owen et al (2001):

– 14N/15N of Jupiter (Solar?) ~ 430

• Presolar grains:– SNe-derived SiC imply 15N production higher than

predicted (e.g., Nittler et al. 1995, Amari talk)– Consistent with Chin results– Mainstream SiC 15N/14N ranges from ~200 to 20,000,

implies that presolar parent stars started with lower ratios than solar

– Data point towards primary 15N production, but inconsistent with galactic gradient.

Presolar O-rich Grains

• More difficult to locate in meteorites than C-rich grains

• Several hundred oxide and silicate grains identified

Data from Nittler et al. 1994-2005; Choi et al 1997, 1998; Zinner et al. 2003; Messenger et al 2003, Nguyen and Zinner 2004; Messenger et al 2005

Most grains formed in red giants/AGB stars

Most grains formed in red giants/AGB stars

• Group 1 and 3 grains well-explained by 1st dredge-up models; require range of masses and metallicities

• Simulate oxide distribution– Assume parent stars 1.1-2.3 Solar Masses– No age-metallicity relationship (Nordstrom et

al 2004)

• Simulate oxide distribution– Assume parent stars 1.1-2.3 Solar Masses– No age-metallicity relationship (Nordstrom et

al 2004)– Poor Match to Data

• Simulate oxide distribution– Assume parent stars 1.1-2.3 Solar Masses– Assume age-metallicity relationship of Reddy

et al (2003)– Much better match!

Data point to AMR with scatter

GCE of Mg Isotopes

• Stellar obs. and high-precision Mg measurements of presolar spinel support shallow GCE model of Fenner et al (role of low-Z AGB stars)

• Scatter large in stars– Mass transfer from companion AGB stars in low-Z binary systems??

SiC-Oxide Consistency?

• Use Si-Ti correlation in SiC and Ti-Z (determined from 18O/16O) correlation in oxides to predict Si-Z relationship for Galaxy

SiC-Oxide Consistency?

• Predicted trend is somewhat consistent with limited silicate dataset (Nguyen et al 2006; Mostefaoui & Hoppe 2004)– Instrumental

effects can strongly affect plot

– Need better data

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end

Homogeneity of GCE• Scatter in age-metallicity relation suggests ISM

is heterogeneous(1) Inhomogeneous mixing of stellar ejecta?(2) Orbital diffusion of stars?(3) Inhomogeneous infall of low-Z matter from halo?

• Low scatter in element/Fe ratios in nearby stars argues against (1), but uncertainty ~10-20% (Edvardsson et al. 1993, Reddy et al. 2003)

– Can high-precision presolar grain data help?

Heterogeneous GCE and SIC• Lugaro et al.

(1999) used Monte Carlo methods to model heterogeneous mixing of SN ejecta into ISM

• Can explain range and scatter of SiC Si isotopes very well

Heterogeneous GCE and SIC

• As found by Lugaro et al. (1999), Si isotopes can be well explained

• Correlation between Si and Ti not reproduced, though.– Grain correlation indicates Si and Ti isotopes locally well-mixed in the

ISM!

Nittler, ApJ, 2005

• Maximum heterogeneity allowed by Si-Ti correlation provides constraints on elemental variations in ISM– Few % for el/Fe

Nittler, ApJ, 2005

Mineralogy vs metallicity?

• “High-density” presolar graphite probably from AGB stars (e.g. s-process enrichments, Croat et al. 2005)

• Higher 12C/13C ratios than SiC point to either higher-mass or lower-metallicity parents

Mineralogy vs metallicity?• Recent Spitzer

observations indicate that low-metallicity (SMC) C stars produce similar amounts of dust as Galactic ones, but much less SiC (Sloan et al, ApJ, 2006)– Low-Z AGB stars have

much higher C/Si ratios, preferentially produce graphite?

– Presolar grains might provide information on how galactic inventory of dust changes with Z

Conclusions

• Presolar grains can be used as sensitive probes of Galactic Chemical Evolution– Indicate enhanced low- metallicity production of 29,30Si

and 25Mg (role of SN-Ia and AGB stars, respectively)– Grain data indicate 15N is primary isotope and 15N/14N

ratio increases with metallicity– Oxide/Silicate data indicate that age-metallicity

relation (AMR) with large scatter does exist– Isotopic correlations indicate ISM is well-mixed;

heterogeneous mixing of SN ejecta does not explain scatter in AMR

– C isotopes in SiC and graphite indicate former from high metallicity, latter from low metallicity stars, consistent with recent Spitzer observations

Presolar Galactic Merger?(D. D. Clayton 2003)

• Explains correlation lines (Si, Ti)

• Might explain 18O/17O of Sun

• Requires supporting evidence

Presolar galactic merger?

• Max 12C/13C decreases with increasing 29Si/28Si

• Larger 12C/13C requires higher mass AGB stars (>3 M for Y grains)– Shorter lived

• Implies temporal evolution down mainstream line?– Increasing mixing

fraction from merging galaxy?

More than 3700 grains plotted; Nittler & Alexander 2003

GCE of Si isotopes

• 29Si/28Si, 30Si/28Si increase with Z ([Fe/H])

• 30Si/29Si misses Solar!– (“renormalize” calc.)

(Timmes & Clayton 1996)

GCE and presolar SiC• Slope of Si trend (1.3)

steeper than GCE model predictions (1.0)– Indicates low metallicity

source of 30Si not included in GCE models

– Novae?– AGB stars?

• Most grains isotopically heavy, from stars with [Fe/H]>0, despite being older than Sun– Sun isotopically

anomalous?– Large observed scatter in

metallicity of stars in galaxy