Stars, metals and planets?

I. Neill ReidSTScI

The question

Over 100 extrasolar planets have been discovered since 1995 -this includes several multiplanet systems

Given the statistical properties of the parent stars coupled with our current knowledge of Galactic structure

How frequent are planetary systems likely to be in the Milky Way Galaxy?

Analysis requirements

1. An understanding of the selection effects inherent in identifying extrasolar planetary systems

2. A reference sample which provides an unbiased sampling of the local disk population(s)

3. A consistent set of empirical measurements for both datasets

Current statistics

As of October 31 2002, 90 nearby stellar systems are known to have planetary mass companions – A handful may be brown dwarfs in face-on orbits (e.g. HD 114762, [m/H]~-0.7) or misclassified chromospheric variations (e.g. HD 192263)

At least 25 of those systems have multiple planets

All discovered using the radial velocity technique

The ESP systems

Almost all known ESP systems have F, G, K primaries

reflects preference for solar-type stars in RV surveys

ESP detection biasesRadial velocity surveys are limited by:1. Velocity amplitude (i.e. mass/semi-major axis) RV = 1 to 4 m/sec [Jupiter/Sun V ~ 12 m/sec]2. Orbital Period ~150 stars have been monitored for >10 years >1500 stars have <5 years of observations3. Photospheric stability & binarity - active stars and close binaries excluded a priori

Current surveys are biased towards detecting planets with M > MJ and P < 5 years Majority of targets are late-F, G, K dwarfs, d < 50 pc. The ESP sample is not volume-limited

Metallicity and planet formation

Why is metallicity likely to be important for planet formation? - planets are made of metals A 1 MSun [m/H]=0 Solar Nebula has ~10-3 MSun in refractory elements – Si, Fe, Mg etc 1,000 Earth masses Jupiter’s central regions include 20-30 Earth masses of refractory material limited scope for reducing [m/H]

Metallicity and planet formation

Observationally:

1. Many ESP hosts appear to have super-solar [m/H] - Gonzalez (1997), Laughlin (1999), Santos et al (2000) indeed, some stars targeted based on high [m/H]

2. No planets detected as transits in 47 Tuc, [m/H]=-0.7 - low metallicity? or high density?

How significant is this effect?

Measuring [m/H]Wide variety of techniques: spectroscopic: high resolution – available for all ESP hosts but few field stars low resolution – photometric: broad band – UBV FG early-K medium band – uvby, DDO, Geneva Each has its advantages + range of application

Systematic differences can exist between different techniques

uvby is available for all ESP hosts and most nearby field stars

The reference sample

The most robust method of defining a reference sample is to select by proximity – volume-complete, distance-limited sample Now possible, thanks to Hipparcos:

1549 stars with > 40 mas

486 have 0.5 < (B-V) < 1.0 & 2.0 > MV > 7.0

419 of those stars have Stromgren photometry

Only 40% of the 25-pc. FGK sample are included in the Keck ESP survey

The disk metallicity distribution

1. Sun lies at mode of distribution2. only 5% of local stars have [m/H]<-0.5

Metallicity and planets1. Determine [m/H] distribution for ESP hosts - omit stars selected for observation based on [m/H]2. Determine scaling factor to match to 25-parsec sample - 24 of the 486 stars in the 25-pc. sample are known to have planetary-mass companions 4.9 %3. Scale ESP host star f([m/H]) and ratio with 25-pc sample f([m/H])

Metallicity and planetary orbitsNo significant evidencefor a significant correlation between [m/H] and a, e, & M

Metallicity and planets

Why the preference for high [m/H]?1. Nature – higher metallicity, more planet-building material2. Nurture – planet migration and destruction raises the metallicity in the outer stellar envelope: e.g. HD 82493

Metallicity and planetsPlanetary accretion occurs during stellar youth 108 to 109 years pollutes convection zone But the size of the convection zone varies with mass pollution has more effect on higher-mass stars

1. No evidence for increase in [m/H]max with mass2. No evidence for decrease on subgiant branch ESP hosts are born rich

Covering the Galaxy

Current results span only the immediate Solar neighbourhood

How do we extend coverage to the rest of the Galaxy?

Radial gradients1. Stellar density gradients (r) = 0 e-(R-R0)/h

where h ~ 2500 pc.2. Metallicity gradients Observations of HII regions i.e. young objects

Hy, Pr Pl

Radial abundance distributionsInferred orbital parameters for local disk dwarfs could be affected by orbital migration of stars and gas (Merrifield & Binney)

Cepheids – intermediate-age starsBroad plateau in <[Fe/H]> from 6 to 10 kpc.Andriessen et al, 2002

Planetary frequency

Define solar-like stars as G dwarfs with –0.15 < [m/H] < 0.15Based on the current sample of extrasolar planets: ~5% of solar-like stars have detectable companions Local space density of solar-like stars ~0.0044 pc-3

0.00022 pc-3 solar-like ESP hosts Assume similar abundance distribution for 5 < R < 10 kpc, -100 < Z < 100 pc ~11.4 million solar-like stars with currently-detectable planetary-mass companions

Note that this does not allow for jovian & sub-jovian mass planets and P > 5 years

Summary

1. Metal-rich stars are significantly more likely to have super-jovian mass planetary companions. Are lower-mass planets more common at lower [m/H]?2. Despite this trend, planetary companions to solar-like stars are not rare ~5% locally3. Expanding these calculations to the wider Galaxy requires better understanding of (amongst other issues) the abundance distribution as f(R)

r