The cluster origin of the solar system

S.Pfalzner Max-Planck-Institut für Radioastronomie

Today the solar system is located in a relatively sparse area of the Milky Way

local stellar density:0.122 stars/pc3

Did the solar system form in such an environment?

Perseus arm

Sagitarius arm

• http://idw-online.de/pages/de/newsimage?id=69365&size=screenhttp://idw-online.de/pages/de/newsimage?id=69365&size=screen

Dr. Udo Barckhausen, Bundesanstalt für Geowissenschaften und Rohstoffe des Geozentrum Hannover

MeteoritesChondrites - unmolten

meteorites

86.2% of meteorites found on

Originate from primitive astroids

not been modified due to melting

contain chondrules• mm-cm sized silicate droplets

• originated as freely floating, molten or partially molten droplets in space

• Abundances differ little from those of the sun

• formed very early in the history of the solar system

• representative of entire solar system

Birth environment of solar system: Isotopes

Ejected material caught by protoplanetary disc aroundyoung Sun

Supernova exploded at a distance of 0.2-0.3 pc from Sun (F. Adams ARAA 48, 2010)

The onion-like layers of a massive, evolved star just prior to core collapse.

Decay products in chondrites presence of 26Al and 60Fe

26 Al and 60Fe isotopes must have formed by supernova explosion (Williams & Gaidos 2007)

Meteorite abundances determine type of progenitor star for supernova

Massive star with ~25 Msun

Birth environment of solar system: Initial mass function

Sun formed close to massive star with ~25 Msun

Massive stars do not form in isolation but as part of clusters of stars

IMF: distribution in mass of a newlyformed stellar population

massive star with ~25 Msun

Sun must have formed in a cluster of several 1000 stars

Infos from Meteorits

Solar system formed:

• In a star cluster

Nstars > 1000 stars

• In vicinity of supernova

• Distance to supernova

0.2 - 0.3 pc

Solar System properties

Mass distribution in Solar System:

Sun

Planets

Kuiper belt

Oort cloud

Planets mostly oncircular orbits

System was not perturbed inside 30 AU

Relicts of the Solar System history:Mass distribution

Mass distribution in Solar System:

30 AU drop in mass distribution (Bernstein et al. 2004)

What can cause such a drop?

• Encounter during disc phase

• Photoevaporation during disc phase

• Binary star

Relicts of the Solar System history:Mass distribution

Mass distribution in Solar System:

30 AU drop in mass distribution (Bernstein et al. 2004)

What can cause such a drop?

• Encounter during disc phase

• Photoevaporation during disc phase (Mann & Williams 2009)

• Binary star

UV radiation

Relicts of the Solar System history:Sedna

trans-Neptunian object discovered in 2003

Perhelion: 76 AUApelion: 937 AUPeriod: 11400 yrEccentricity: 0.8527

High eccentricity of Sedna NOT caused by planets (Gaidos et al. 2005)

Possible explanation: Encounter(Moribelli & Levison 2004)

But circular orbits of planets

No encounter after solar system formed (30 Myr) (Malhorta 2008)

Relicts of the Solar System history:Mass distribution and Sedna orbit

Mass and dynamics in Solar System:

30 AU drop in Sedna orbitmass distribution

What can cause drop and Sedna orbit?

• Encounter during disc phase

•What kind of encounter is necessary to obtain these two features?

100 -1000 AU encounter

(for summary see Adams 2010)

Encounter assumptions:

• solar-type star • coplanar, prograde• cut-off at 1/3 periastron

Limits on Solar System birth environment:

Meteorit composition

30AU cut-off in mass distribution

Sedna orbit

Circular orbits of planets

Supernova within 0.2pc 25 Msun progenitor

Encounter or photo-evaporation Both require high stellar density If encounter with solar type star, then rmin =100-1000AU

Stellar density at solar system location

System undisturbed for solar system age > 30 Myr

The Solar System birth cluster:

Sevaral 103Msun < Mcluster < several 104 Msun

103 Msun/ pc3 < ρcentral < 104 Msun/pc3

Nstar > 1000 Radio-isotopes Adams (2010)

Nstar > 4000 Chemical composition

Lee et al. (2008)

Nstar< 105 Radiation field Adams (2010)

ρcentral<103 Msunpc-3 Sedna orbit Brasser (2008)

ρcentral<104Msunpc-3 Sedna orbit Schwamb (2011)

Estimates or simulations of encounter probabilities in clusters of different size or density:

Mean stellar mass in cluster: 0.5 Msun

Sun close to massive star

Sun close to cluster

center

Mass segregationMany young cluster are

mass segregated

Massive stars are predominantly found in

central regions of cluster

ONC – all O stars are within 0.5pc

Density distribution in young clusters

Requirement for solar birth clustercentral stellar density: 103 – 104 stars/ pc 3

Inside cluster:High stellar density at center, but steepsteep gradient towardsoutskirts

Central density of 103 – 104 stars/ pc 3

translates into average density 10 – 1000 stars/ pc 3

The Solar System birth cluster:

Sevaral 103Msun < Mcluster < several 104 Msun

10 Msun/ pc3 < ρmean < 1000 Msun/pc3

Nstar > 1000 Radio-isotopes Adams (2010)

Nstar > 4000 Chemical composition

Lee et al. (2008)

Nstar< 105 Radiation field Adams (2010)

ρmean<10 Msunpc-3 Sedna orbit

ρmean<1000Msunpc-3 Sedna orbit

Estimates or simulations of encounter probabilities in clusters of different size or density:

Mean stellar mass in cluster: 0.5 Msun

Trapezium in ONC

Quintuplett

High densitymany O stars Gravitational interaction

Photoevaporation Hernandez et al, ApJ 662(2007)

σ Ori cluster

Different young cluster environments

HST image

Most stars form in clusters Lada & Lada (2003)

Many more clusters with ages < 10 Myr than at older ages for same time span

Clusters dissolve early on in development

Young clusters with M > 103 Msun

Cluster with same mass as solar birth cluster mass exist today in Milky Way

Note: relatively large error bars for cluster age

Temporal evolution of young clustersCluster mass

No mass lossConsiderablemass loss

Pfalzner 2011

Temporal evolution of young clustersCluster mass

No mass lossConsiderablemass loss

2 cluster types: Snapshots in cluster developmentrather than multitude of cluster types

„Starburst cluster“ -Sequenz

Trumpler14

1 Myr 20 Myr

Densities of young clusters with M > 103 Msun

What about the densities of these young massive clusters?

The mass density of such young clusters spans 7 orders of magnitude:

From ~0.01 to 105 Msun pc-3

Pfalzner A&A 498, L37,2009

Expansion velocity

Relation between cluster radius and age givesexpansion velocity in both types of clusters

Star burst clusters:

Rc ~ tc

vexp = 0.1 - 0.2 pc/Myr

Leaky clusters:Rc ~ tc

0.6-0.7

vexp ~ 2pc/Myr

Cluster size gives directly its age

Pfalzner A&A 498, L37,2009

The Solar System birth cluster:

Sevaral 103Msun < Mcluster < several 104 Msun

10 Msun/ pc3 < ρmean < 1000 Msun/pc3

Nstar > 1000 Radio-isotopes Adams (2010)

Nstar > 4000 Chemical composition

Lee et al. (2008)

Nstar< 105 Radiation field Adams (2010)

ρmean<10 Msunpc-3 Sedna orbit

ρmean<1000Msunpc-3 Sedna orbit

Estimates or simulations of encounter probabilities in clusters of different size or density:

Mean stellar mass in cluster: 0.5 Msun

?

Temporal evolution of young clustersCluster density

Pfalzner A&A 498, L37,2009

2 types of clusters in solar birth cluster mass range

Star burst clusters ρc ∼ Rc

-3 Diffusion

Leaky clusters OB associations

ρc ~ Rc-4

Diffusion + Ejection

Radius-age transformation1 Myr 20 Myr 1 Myr 20 Myr

Radial development translates into age development Age

Solar birth cluster a Starburst-Cluster?

average density of 10 – 103 stars/ pc 3

Overlap with starburst

clusters after 5Myr

But ...

Starburst cluster

Leaky-Cluster

During first 5 Myr density in starburst clusters extremely high Many close encounters

Discs would be destroyed No planetary system

Starburst cluster unlikely solar birth environment

Solar system has likely developed in leaky cluster environment

Solar birth cluster a leaky cluster?

Density development ρc ~ C t-3.7

Interaction with other stars unlikely after solar system gives naturally circular orbits

average density of 10 – 103 stars/ pc 3

Overlap in early stages of development

Starburst cluster

Leaky-Cluster

Solar system formed in the central regions of a leaky cluster

Initially high stellar density:What does that means for the solar system?

Modelling of solar birth cluster

Average encounterAverage encountereffect on protoplanetare effect on protoplanetare disc in clusterdisc in cluster

Only coplanar, prograde encounters

Cluster simulation

Encounter simulation

Dynamical model of clusters single stars no gas component Code: NBODY6++ List of encounter parameters for all

Modelling of the solar birth cluster development

Gas expulsion at end of star formationprobably resposible for cluster expansion

Uncertainities in gas expulsion process

Instead: Model clusters at different densities

ONC-like cluster profile

Sun formed close to massive star

Solar-type stars close to cluster center

Probability of solar system forming encounter

Single encounterwith 100 AU <rperi< 1000 AU

Encounter probabilty functionof cluster density

Higher density=higher likelihood of encounter

But very high densities

Multiple or close encounters No solar system

Resulting encounter history

Probabilty of encounter as function of solar system age

Probability of encounter decreases with cluster age

During 1st Myr after gas expulsion30% chance of solar system forming encounter

Such an encounter likely event for solar-type star close leaky cluster center

After 3-4 Myr significantly reduced encounter probability

Leaky cluster: ρc ~ C t-3.7

Encounter partner history

Solar-type stars mainlyhave encounter with

• Low-mass stars mstar< 0.5 Msun

• High-mass stars mstar< 10 Msun

With a preference for low mass stars

This preference for encounters with low-mass stars decreases the older the cluster becomes

Encounter eccentricity historyEccentricity of encounterfunction of cluster density

Dense clustersStrongly hyperbolic encounters

Less dense clustersnearly parabolic encounters

If encounter was early on in cluster developemnt (< 2Myr) then

Most likely strongly hyperbolic encounter

Why are we not still within this birth cluster?

1. Spreading and mass loss during first 20Myr

Why are we not still within this birth cluster?

1. Spreading and mass loss during first 20Myr

2.Solar system circles around Galactic Center

v = 220 km/sec r = 8kpc tsun= 4.57 109 yr

About 22 orbits since its formation

Tidal disruption of cluster

Portegies Zwart (2010)

Today the solar system is located in a relatively sparse area of the Milky Way

local stellar density:0.122 stars/pc3

Perseus arm

Sagitararius arm

Artist‘s impression of the nightsky in a cluster

Scientific American, Ron Miller

Impression of thenight sky when the sun was born

Sun formed in a massive cluster

Such clusters exist in two forms Starburst and leaky cluster

Sun most likely formed in leaky cluster

Density development ρc ~ C t-3.7

Solar system forming encounter:• low mass star• highly eccentric orbit