AN EMPIRICAL PLANETESIMAL BELT RADIUS … EMPIRICAL PLANETESIMAL BELT RADIUS STELLAR LUMINOSITY RELATION LUCA MATRÀ SUBMILLIMETER ARRAY (SMA) FELLOW, HARVARD-SMITHSONIAN CENTER FOR

A N E M P I R I C A L

P L A N E T E S I M A L B E LT R A D I U S

S T E L L A R L U M I N O S I T Y

R E L AT I O N

L U C A M AT R À S U B M I L L I M E T E R A R R AY ( S M A ) F E L L O W, H A R VA R D - S M I T H S O N I A N C E N T E R F O R A S T R O P H Y S I C S

S . M A R I N O , M . W YAT T ( U N I . C A M B R I D G E , U K ) , G . K E N N E D Y ( U N I . W A R W I C K , U K ) , D . W I L N E R , K . Ö B E R G ( H A R VA R D - S M I T H S O N I A N C FA )

D U S T A N D G A S F R O M P L A N E T E S I M A L B E LT S

Wyatt & Dent 2002

Wyatt 2008

• Dust is second-generation, produced by asteroids and/or comets in a collisional cascade

200 AUe.g. Matthews+ 2014, Backman+ 1993, Wyatt+ 2002, Matra+ 2017b

• CO (and other?) gas is also being released in the process, indicating cometary compositions consistent with the Solar System.

CO+CO2

CO+CO2 CO+CO2

U B I Q U I T Y O F P L A N E T E S I M A L B E LT S A N D P L A N E T S

Wyatt 2008

Wang+ 2016, Wilner+ 2018, Marois+ 2008, Matrà+ 2018c in prep

• Both planets AND planetesimal belts are everywhere.

• And yes, they go together, including potential correlations between the two.

Video credit: J. Wang

20 AU

β Pictoris

HR 8799

• At least ~20% of stars have belts of dust emitting at IR wavelengths

W H Y S H O U L D Y O U C A R E ?

Wyatt 2008

• Terrestrial planet formation is still ongoing interior to most planetesimal belts, and can be detected through the dust it produces from impacts

• Observe gas and probe composition of cometary belts when volatile delivery events through inward cometary scattering are most likely.

20 AU

• Infer the presence of otherwise undetectable planets at large separations through planet-belt dynamical interactions

• Learn about planet formation from its outcome: why did planetesimal belts form?

W H Y S H O U L D Y O U C A R E ?

Wyatt 2008

• Terrestrial planet formation is still ongoing interior to most planetesimal belts, and can be detected through the dust it produces from impacts

• Observe gas and probe composition of cometary belts when volatile delivery events through inward cometary scattering are most likely.

20 AU

• Infer the presence of otherwise undetectable planets at large separations through planet-belt dynamical interactions

• Learn about planet formation from its outcome: why did planetesimal belts form?

T H E E M P I R I C A L R - L ★ R E L AT I O N O F P L A N E T E S I M A L B E LT S

Wyatt & Dent 2002

Wyatt 2008

• Location of the first 26 belts resolved with SMA and ALMA (black) strongly correlates with luminosity of host stars, with R ~ 73 L0.19, and ~17% scatter

• Kuiper belt (red) and CO snow lines observed in protoplanetary disks (orange) lie close to the relation

Matrà+ 2018b, submitted

• Radii defined as ‘centre’ of the belt, uncertainties conservatively taken as half widths.

T H E E F F E C T O F O B S E R VAT I O N A L B I A S

Wyatt & Dent 2002

Wyatt 2008

Belt selection effects:1) Detectability in the IR2) Single-dish detectability in the mm 3) Resolvability by mm interferometry

JCMT

ALMA, SMA

Spitzer,Herschel

Use Monte Carlo analysis: What is the probability of finding a relation if none was present?

Very low! <10-5

But… we live in a BIASphere!

Wyatt 2008

Scatter

B I A S A L O N E C A N ’ T E X P L A I N S M A L L S C AT T E R

• Belt locations R not as scattered as would be expected from observing an underlying uncorrelated population

• In other words, observed locations don’t fill all detectable [R, L] space.

T H E E F F E C T O F O B S E R VAT I O N A L B I A S

Wyatt & Dent 2002

Wyatt 2008

JCMT

ALMA, SMA

Spitzer,HerschelBut - belt mass decreases with age

through collisional evolution

tcoll ∝ M R 1) Closer-in belts evolve and become undetectable faster2) At the same distance, belts around more massive stars evolve faster

CO+CO2

CO+CO2 CO+CO2

★- 4 / 3 1 3 / 3

Age

Age

e.g. Wyatt+ 2007

T H E E F F E C T O F O B S E R VAT I O N A L B I A S

Wyatt & Dent 2002

Wyatt 2008

Belt selection effects:1) Detectability in the IR2) Single-dish detectability in the mm 3) Resolvability by mm interferometry

JCMT

ALMA, SMA

Spitzer,Herschel

Use Monte Carlo analysis: What is the probability of finding a relation if none was present?

Very low! <10-5

But… we live in a BIASphere!

Scatter

C O L L I S I O N A L E V O L U T I O N S T I L L S T R U G G L E S

• Belt locations R are still not quite as scattered as would be expected from observing an underlying uncorrelated population

• Probability of finding a relation if none was present is still low: ~10-4

This could mean belts form at specific preferential locations within young protoplanetary disks

P R E F E R E N T I A L F O R M AT I O N L O C AT I O N

Wyatt & Dent 2002

Wyatt 2008

This suggests a preferential formation location of planetesimal belts in protoplanetary disks, raising the fundamental question:

Efficient planet formation?

• Enhanced planetesimal formation - or - inhibited growth to planets?

Does the (CO?) snow line set the location of planetesimal belt formation?

Wyatt 2008E.g. Stevenson&Lunine 1988, Drazkowska+ 2017, Pinilla+2017, Schoonenberg+2017

Problem:CO snow line has been shown to both favor, but also disfavor, planetesimal

formation

If favored, planets need to also migrate inwards to leave a planetesimal belt behind

P R E F E R E N T I A L F O R M AT I O N L O C AT I O N

Wyatt 2008

This suggests a preferential formation location of planetesimal belts in protoplanetary disks, raising the fundamental question:

• Enhanced planetesimal formation - or - inhibited growth to planets?

Is planet formation - from planetesimals up - simply too

slow at large separations?

Wyatt 2008

Efficient planet

formation?

E.g. Kenyon&Bromley 2008

Upward trend can be understood by simple core

accretion for a fixed timescale, though slope and intercept are

dependent on assumptions

S U M M A R Y

Planetesimal belts, as well as planets, are everywhere.But why do planetesimal belts form?Discovery of strong R-L* correlation, which although subject to observational bias and collisional evolution, still has a significantly smaller scatter than expected if no correlation was present.

This indicates that planetesimal belts form at preferential locations in protoplanetary disks.

This means planetesimal belt formation does take place at large distances from the star, and may be linked to either the CO snow line or planet formation timescales.

L A R G E R P O P U L AT I O N S T U D Y N E E D E D

Wyatt & Dent 2002

Wyatt 2008

Efficient planet formation?

• REASONS: REsolved ALMA and SMA Observations of Nearby Stars

• Double the sample of resolved belts from 26 to 52 to confirm the relation at higher significance

• Reduce uncertainty on derived parameters for comparison with planet formation models.

The first resolved legacy survey of planetesimal belts at mm wavelengths

Simulation of typical REASONS belt observations with ALMA, Cycle 5

T H E K E Y: R E S O LV E D M M O B S E R VAT I O N S

• Where are the planetesimals? Need to observe planetesimal-tracing dust

Wyatt 2008

• Need accurate radius measurement: high resolution and little to no stellar contamination

• Need a population study: sufficient sensitivity