Chapter 13: Debris disks - uni-jena.dekrivov/lecturing/planetensysteme/...Now we know that debris...

Chapter 13: Debris disks Comets, asteroids, and dust around stars

Outline

What are “debris disks”?

What do we see around other stars?

How to interpret what we see?

What can we learn about extrasolar comets?

Are there also extrasolar asteroids?

How about extrasolar planets?

Is Kuiper belt similar to other debris disks?

What are „debris disks“?

KB dust KB AB Zodi

Giant planets Terrestrial planets

• Debris disks are belts of comets, asteroids, and their dust • Debris disks are descendants of protoplanetary disks • Debris disks, like planets, are natural outcome of planet formation

Outline








6

The story begins in the 1980s…

There is dust around mature stars

A serendipitous IRAS discovery of an infrared excess around a main-sequence star, Vega (Aumann et al. 1984)

“Vega phenomenon” was promptly found for three other stars (IRAS team 1984)

The dust is arranged in a disk

The first image of a debris disk in scattered light with a 2.5 m telescope at Las Campanas: b Pictoris (Smith & Terrile 1984)

For a decade, only these „fabulous four“ known

b Pic

e Eri

a Lyr a PsA

The same four imaged at

submillimeter wavelengths

(Holland et al. 1998, Greaves et al. 1998)

~1000 of debris disks discovered by IR excesses

over the photosphere and many got densely sampled SEDs from optical through mm wavelengths

Now we know that debris disks are ubiquitous…

Example: HD 207129 (Marshall et al. 2010)

Herschel/PACS resolved

image at 70mm

~100 of debris disks spatially resolved

in thermal emission or scattered light. For some, even

polarimetry is available

…and that they persist at all stellar ages

Picture credit: Guy Ottewell / Universal Workshop

Incidence rates around main-sequence stars: ~22±7% for FGKs (Montesinos et al. 2016)

~ 24±5% for A stars (Thureau et al. 2014)

remain disputable for Ms (Lestrade et al. 2012)

debris

disk rate

1-14%

debris

disk rate

~11%

debris

disk rate

<10%

debris

disk rate

~22%

debris

disk rate

~24%

Herschel discovered debris disks around subgiants at a rate of 11±2% (Bonsor et al. 2013, 2014)

Debris is also known to exist around 1-14% of white dwarfs (“polluted” and “dusty” WDs; e.g.

Kilic & Redfield 2007, Barber et al.

2012, Dufour et al. 2012)

Outline








The modeling paradigm

Infe

r

Co

mp

ute

Dust

Dust emission

Planets

Formation and evolution of a planetary system

Planetesimals

Architecture of a planetary system

Physical processes: collisional cascade

14

planetesimals... boulders ... dust

This is usually the case in

debris disks. Collisional cascade

grinds planetesimals to ever-

smaller fragments, down to

dust sizes

Benz & Asphaug 1999

If impact energy is larger than

critical fragmentation energy,

the bodies are disrupted

Physical processes: stellar “photogravity”

15

Planetesimals in nearly-circular orbits, dust grains in elliptic ones, fine dust blown out in hyperbolas

Size-spatial distributions and disk appearance

Animation credit: Torsten Löhne

An application to Vega disk

Spitzer/MIPS 70 mm (Su et al. 2005) and Herschel/PACS 100 mm (Sibthorpe et al. 2010)

Spitzer found a huge disk extent of ~1000 AU, which was interpreted as a halo of unbound grains. Would imply an unfeasible mass loss of ~3000 Earth masses / Gyr (Su et al. 2005)

Later, we showed the observations to be fully consistent with a halo of bound grains formed in a steady-state disk. Reduces the mass loss to ~10 Earth masses / Gyr (Müller, Löhne, & Krivov 2010)

Outline








We can measure radii of extrasolar Kuiper belts

Tbb = 30K 50K 100K

Large scatter from 40 to 300 AU, no significant trend Disk dimensions not set by ice lines or other T-dependent processes

(Pawellek, Krivov, Marshall et al. 2014)

Sun

Size and radial distribution of dust reflect stirring, or excitation, level: lower eccentricities imply larger dust grain sizes and less pronounced halos

(Thebault & Wu 2008, Krivov et al. 2013, Matthews et al. 2014)

We can deduce <e> and <I> of cometary orbits

We can deduce <e> and <I> of cometary orbits

We infer eccentricities and inclinations of ~0.03…0.05 for disks of late-type stars and ~0.10 for A-stars (cf. the Kuiper belt level of ~0.08)

Disks of luminous are stirred more strongly – contain more massive bodies?

Pawellek & Krivov (2015)

We can infer the presence of large bodies

Kenyon & Bromley (2008), Mustill & Wyatt (2009)

Secular stirring by a planet

with a=5 AU, e=0.1

Self-stirring (by “Plutos”) :

time to form Pluto-sized bodies

in an ~xm x MMSN disk

To excite disks to the observed level, disks must either contain embedded Pluto-sized bodies or giant planets in their cavities

We start to probe chemical composition

Dent et al. (2014)

ALMA has detected CO, C, and O gas in 12 debris disks around young (10-40 Myr) early-type (A and F) stars. Gas (10-6 to 10-1 Earth mass) is typically colocated with dust (10-3 to 1 Earth mass) Like dust, this gas must be a destruction product of volatile-rich comets

Kral et al. (2017)

Mass of the

known EKB

0.007 M

Let’s look at our Kuiper belt

Vitense, Krivov, & Löhne (2010)

Mass of the

“true” EKB

0.12 M

We can even test planetesimal formation models

Statistics of TNOs together

with in-situ dust data can be

used to test initial

size distribution of Kuiper belt

objects






objects

Vitense, Krivov, Löhne, & Kobayashi (2012)


Inconsistent with formation by slow accretion (e.g., Kenyon et al. 2008), consistent with rapid graviturbulent formation (e.g. Johansen et al. 2012)





objects


Outline


What’s seen around other stars?






Two-component debris disks

About 2/3 of the debris disks appear to have an additional warm

component (Morales et al. 2011, 2013; Ballering et al. 2013, Chen at al. 2014,

Pawellek et al. 2014, Kennedy & Wyatt 2014), whose origin is as yet unclear

Duchene et al. (2015)

Crv

A natural explanation would be “asteroid belts”

Ricci et al. (2015)

HD 107146 It is indeed a viable

explanation for some of the

systems, such as z Lep (Moerchen, Telesco, &

Packham 2010), HD 107146 (Ricci, Carpenter, Fu et al.

2015) or q1 Eridani (Schüppler, Krivov, Löhne et

al. 2016). This would also

imply giant planets in

between – akin to the

Solar system

Instead, warm dust can be dragged-in

Reidemeister, Krivov, Stark, et al. (2011)

e Eri Alternatively, in disks of later-

type stars strong stellar winds

may effectienly transport dust

inward from the Kupier belts

to where it is observed.

Examples are e Eri (Reidemeister, Krivov, Stark et al.

2011), HIP 17439 (Schüppler,

Löhne, Krivov et al. 2014), and

AU Microscopii (Schüppler,

Löhne, Krivov et al. 2015)

However, recent SOFIA

observations of e Eri favor

asteroid belt (Su et al. 2017)

We don‘t yet know which scenario is true

Synthetic images for proposed ALMA observations of HIP 17439 Schüppler, Krivov, Löhne, et al. (2014)

„Asteroid belts“ can actually be common

220 out of 224 two-temperature disks (98%) in a Spitzer sample are consistent with two distinct belts. Such systems could stem from

protoplanetary disks of 0.001-0.01 solar mass

Geiler & Krivov (2017)

„Hot exozodis“

NIR/MIR interferometry reveals “hot exozodiacal clouds” around ~20% of stars Dust grains are tiny (20-500 nm) and close to the stars (0.01-1AU)

Origin of this dust is unknown

Kirchschlager, Wolf, Krivov et al. (2017) Kral, Krivov, Defrere et al. (2017)

Outline








~4000 stars with

planets

~1000 stars with disks

~40 stars with planets

and disks

Debris disks and planets should coexist, but…

…we look at different regions of parameter space

Samples differ, parameter regions do not overlap spatially Thus only ~40 systems known to have both planets and disks

All resolved disks are structured

A lot of structure is seen in the resolved images Structure does not mean planets, but planets always mean structure

Disk cavities: are they opened by planets?

Faber & Quillen 2007, Shannon et al. 2016

Likely, but not necessarily. Extremely difficult to prove: Dynamical calculations show a few Earth- to Jupiter-mass planets would suffice Direct imaging only sensitive to planets more massive than Jupiter

Disk offsets: eccentric planets?

Kalas et al. 2005, Acke et al. 2012, Boley et al. 2012

Eccentric planets impose forced complex eccentricity on the disk particles This results in the disk offset and “pericenter glow” However, Fomalhaut example is controversial…

a PsA

Disk warps: inclined planets?

Lagrange et al. (2009, 2010) Chauvin et al. (2012)

In a similar way, inclined planets create forced complex inclination This leads to disk warping, visible in some edge-on disks b Pic is the best evidence of planet predicted and found by disk structure

b Pic

Disk clumps: resonances with planets?

ε Eri

Lestrade & Thilliez (2015)

Outward-migrating planet traps planetesimals in resonances, making their distribution clumpy (Wyatt 2003)

Also, inward-transported dust can be captured by planet in resonances, creating clumps (Krivov et al. 2007)

However, clumps may also be signatures of recent collisions or just background contamination of sub-mm images

Outline








Kuiper belt may seem to look like the others…

Liou & Zook (1999) Vitense, Krivov, & Löhne (2014)

Models agree that: • main structure is a ring outside Neptune • Jupiter and Saturn prevent the KB dust from entering the inner region • resonances with planets may cause asymmetric structure

… but it is much more tenuous. Why?


The solar system’s Kuiper belt from 10 pc A typical extrasolar debris disk

The Late Heavy Bombardment 3.8 Gyr ago

Formation

of giants

0 Gyr 4.6 Gyr

Formation

of terrestrial

planets

LHB

0.8 Gyr

Present-day

Solar

System

The Late Heavy Bombardment 3.8 Gyr ago

Simulation and animation: Alessandro Morbidelli

Debris disks as mirrors of planetary shakedowns

Booth et al. (2009)

The history of solar

system’s debris disk:

• A pre-LHB disk

would be among the

brightest sources

• A post-LHB disk is

below the detection

limits

So, how typical could be Kuiper belt analogs?

Montesinos, Eiroa, Krivov et al. (2016)

Extrapolating Herschel

detection statistics

down to low dustiness

level of our Kuiper belt,

we expect than 0-75%

of solar-type stars may

harbor Kuiper belts as

tenuous as ours…

Future facilities like Origins (FIR) may detect them

Figure courtesy: Grant Kennedy

Summary








Summary


Belts of comets, asteroids, and their dust around stars


Emission of dust from comets and asteroids


By collisional and dynamical modeling


Mass, location, excitation, sizes, … even formation


There must be some


There are some, and we expect many more


It was similar, it isn’t anymore

Chapter 13: Debris disks - uni-jena.dekrivov/lecturing/planetensysteme/...Now we know that debris...

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