Francesco Trotta

YERAC, Manchester - 2011

Using mm observations to constrain variations of dust

propertiesin circumstellar disks

Advised by:

Leonardo Testi

Planet Formation in circumstellar disks

Observational evidence of dust grain growth

Constrain the radial variation of dust properties with

high-resolution mm-observation

Future prospective with ALMA

Outline

1

2

3

4

What are the circumstellar disks and why do we study them?

• Disks are presumed to be the birthplace of planetary system

Forming star

Rotating disk Massaccretion

• Central forming star accretes most of its mass throught the disk

dominates the mass

the dynamics of the disk

dominates the opacity

thermal and gemetrical structure of the disk

the emission properties of the disk

GAS99%

DUST1%

Angular Momentumtransported outward

Circumstellar disks play a fundamental role in the process of star and planet formation

TurbulenceTransport angular

momentum

Mass accretion onto the star

with time: (1) Mstar↗ Mdisk↘ (2) Disk spreads out

1mm 1m 1km 103km Log a1mm

ISMdust

grains

coupled to the gas gravitycoupled to the gas + gravity

Early growth Mid-life growth Late growth

Gas sweeping

Gravitational interactionAereodynamic interaction

Core accretion

model

weakwell

Growth of 12 order of magn. in size in a few Myr

From dust to planet

1mm 1m 1km 103km Log a1mm

ISMdust

grains

coupled to the gas gravitycoupled to the gas + gravity

Early growth Mid-life growth Late growth

Gas sweeping

Gravitational interactionAereodynamic interaction

weakwell

From dust to planet

Directly observable

Exo-Planets

Growth of 12 order of magn. in size in a few Myr

Core accretion

model

Which observations do we need?

)1))((()( )(ReRTBRI

)()()( RRR

The dust thermal emission (at each radius)

withOpticaldepth

Optical depth is very high (at least at short l) LIMITATION:

Which observations do we need?

)1))((()( )(ReRTBRI

)()()( RRR

The dust thermal emission (at each radius)

withOpticaldepth

Optical depth is very high (at least at short l) LIMITATION:

Probe the bulk of the dust mass in the disk mid-plane

Information only on grain located in the surface layer (tiny fraction of dust mass)

Limited to the outer regions of the disks

V

IR

R

Longer ,l bigger fraction of the dust optically

thin

How to observe dust grain size at mm-l?

Small (compact) grain (a<<l)(es. ISM dust b ~ 1.7)

For a optically thin disk and in RJ regime the dust thermal emission at mm-l

b=2

Solid bodies with a>> /2l p(es. Rocks)

b=0

mm-size particles

0< <2b

Optically thick inner region

Deviation from RJ regime

+ b > a - 2

(diagnostic of dust size,shape,composition)

We are observing b

Fn ~n4

)2(F

Fn ~n2

Fn ~ n2:4

Draine & Lee (1984)

Gray opacity

)1( e2)( TB

Log

F

Log l

Grain growth evidence from mm spectral index

If b < bISM ~ 1.7DUST GRAIN

GROWTH

Resolved (large) disks make (2) improbable

Need of Spatially Resolved Disk at mm l

(hi-res interferometry)

Twopossibility

Testi & al. (2003)

CQ Tau

VLA 7mm res~0.8’’(~100 AU)

Observational

evidence

Shallow SED(a mesured are small

)

(1) Optically thin disk & low b(2) Optically thick disk & any b

Disk models(with radial variation

of the grain size distrib)

High-resolution observ (at more l)

How to constrain the radial variation of dust properties?

Dust evolution models predict grain growth different dust properties in function the position on the

disk

kdust(x)

We are trying tho constrain the radial

opacity profile

Surface layer

Interior

Surface DensitySimilari

tySolution

qaRan ),()(005.0 max Raam

Grain size distribution

PowerLawapproximati

on

max)/()( 0max0max

bRRaRa

starsurface

interior

total

SURFACE LAYERDominate the flux ~ 60 mm (mid-IR)

STARBB emission picco a l - 1 mm (near-IR)

INTERIOR LAYERDominate the flux a l > 100 mm (sub-

mm/mm)

Disk models

How to constrain the radial variation of dust properties?

Isella & al. (2010)

1.3mm 2.8mm

6.92mm

res~0.5’’(~70 AU)

res~0.3’’(~40 AU)

res~0.15’’(~20 AU)

CARMA

VLA

High angular resolution observations at 3 different mm-l of RY Tau

new data

High-res observation

CARMA

VLA

How to constrain the radial variation of dust properties?

Disk around RYTau

We use c2 fitting procedure (directly on visibility)

Choose the grid models (n free-param with a wide range of value)

• Produce disk images

• Fourier trasform it and sampled at the (u,v) points corrisponding to the observed samples

• Computed the c2 value

Calculate the best fitting model (minimum of the c2 hypercube_sum)

2/2)Im(Im2ReRe2 toto

Costruct the c2 hypercube(for each l)

1

2

3

Free parameter

Str Rtr g a0max bmax inc P.A.

How to constrain the disk parameters?

First resultsRtr = 30 [AU]Str = 3.4 [g/cm^2]g = -0.53

Rtr = 34 [AU]Str = 2.3 [g/cm^2]g = -0.35

Rtr = 36 [AU]Str = 1.8 [g/cm^2]g = -0.9

The best fit values we found are ~ in agreement with the Isella result

a0max=0.03cm bmax= 0P.A. = 24°Inclination = 66°

Compare with Isella & al. 2010

1.3mm

2.8mm

6.92mm

But large error-bars

Evidence of radial variation of dust

properties

BEST FIT VALUE

Future prospects

ALMA

To place more stringent constrains on the radial variation of the dust opacity we need of observations with:

higher angular resolution

higher sensitivity

At least 50X12m Antennasmax resolution <0.01’’

at 870 mm

Will be able to resolve structure of

few AU

(at near star forming region)

Should be possible detect spiral structure

of few AU

Simulated observations of massive self-graviting circumstellar disk with ALMA

Should be possible detect spiral

structure of few AU

Cossin,Lodato,Testi (2010)

Intensity maps at sub-mm l from SPH simulation of disk

Image maps at that sub-mm l with various

array conf.

CASA ALMA simulator

(Taurus-Auriga star-forming

region)

Conclusions

• mm spectral slopes indicate presence of mm-size dust grains in the disk (dust grain growth)

• To study the radial variation of the dust properties we need of observations with higher angular resolution and sensitivity

• High angular resolution observation show us radial variation of dust property in circumstellar disk

ALMA will play a crucial role in the next future

However

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