Next Gen VLA Observations of

Protoplanetary Disks

A. Meredith HughesWesleyan University

ALMA (N

RAO/ESO

/NAO

J); C. Brogan, B. Saxton (NRAO

/AUI/N

SF)

What can a Next-Gen VLA give us?

3. Access to terrestrial planet-forming regions

1. Pebbles and rocks throughout the disk

2. Low optical depth across disk radii

Why Pebbles and Rocks?1cm – 1m is VERY interesting grain size for theory of planet formation.

Radial Drift

Meter size barrier

Time evolution: - Planet formation is quick after meter-size barrier

crossed, but when/how does this happen?

Brauer et al. (2007)C. Dullem

ond

Modified from Fu et al. (2014)

Seeing Pebbles and RocksLo

g [E

mis

sion

Effi

cien

cy (Q

)]

Log λ

1

Turnover at 2πa

a

At a given wavelength, large grains (a>λ) are the most efficient emitters

Log [Grain Size (a)]Log

[Num

ber o

f Gra

ins

(N)]

dN/da ∞ a -3.5

Many more small grains than large.Small grains dominate surf area

Net effect: Smallest grain that can emit efficiently will dominate flux at a given

wavelength.

Grain size ≈ Wavelength of observationNeed long wavelengths to see pebbles

Seeing Pebbles and Rocks

One more piece of the puzzle: κν ∞ λ-1

(opacity)

(Surface density)

Millimeter flux (optically thin): Fν ∞ Σ * κν * Bν(T) ∞ λ-3

(Planck function ∞ λ-2)

The bottom line: Flux drops off like crazy with wavelength. Need LOTS of sensitivity

to image pebbles.

Low Optical DepthOptical Depth: τ = Σ κν

Longer wavelengths have lower optical depth, but only until surface density gets high.

Andrews et al. (2009)

Surface density profile in outer disk similar to MMSN: Σ ∞ R-1

At λ = 1mm, τ = 1 at 10 AU

Radius at which τ=1 is inversely proportional to wavelength of observation!

Why Low Optical Depth?

τ = 1 at λ = 1mm (ALMA)

τ = 1 at λ = 3cm(NGVLA)

We can only trace underlying mass distribution of solids where τ < 1Want to know when, where, how much mass in pebbles exists

Terrestrial Planet-Forming Regions

ALMA (ESO/NAOJ/NRAO), T. Sawada

ALMA Band 9, 15km baselines -> 6mas resolution0.9 AU at distance of Taurus, 2.5 AU in Orion

But disk is optically thick at this radius/wavelength!

NGVLA will allow us to see inside terrestrial planet-forming regions

Time domain: changes on ~1 year!

Don’t need to improve over ALMA resolution; need to make sensitivity/resolution of VLA comparable

ChemistryLots of exciting chemistry: volatiles in planet-forming regions, complex organics, etc.

Most exciting to me: Ammonia! Nitrogen chemistry & TEMPERATURE

One example: turbulence in protoplanetary disksDegeneracy between temperature and turbulence

Simon, Hughes et al. (submitted)

Low optical depth, which is necessary to trace dust mass distribution within 10 AU

NGVLA will provide:

Views of pebbles and rocks in protoplanetary disksRadial drift, meter-size barrier

Access to terrestrial planet-forming regions: mass distribution, changes on ~1yr timescales

Chemistry, particularly ammonia for accurate temperature determination