Transcript Next Gen VLA Observations of Protoplanetary Disks

Next Gen VLA Observations of
Protoplanetary Disks
ALMA (NRAO/ESO/NAOJ); C. Brogan, B. Saxton (NRAO/AUI/NSF)
A. Meredith Hughes
Wesleyan University
What can a Next-Gen VLA give us?
2. Low optical
depth across
disk radii
3. Access to
terrestrial planetforming regions
1. Pebbles and rocks
throughout the disk
Why Pebbles and Rocks?
1cm – 1m is VERY interesting grain size for theory of planet formation.
C. Dullemond
Brauer et al. (2007)
Radial Drift
Meter size barrier
Modified from Fu et al. (2014)
Time evolution:
- Planet formation is quick after meter-size barrier
crossed, but when/how does this happen?
Seeing Pebbles and Rocks
Turnover at 2πa
At a given wavelength,
large grains (a>λ) are the most
efficient emitters
1
Log λ
Net effect: Smallest grain that can emit
efficiently will dominate flux at a given
wavelength.
Grain size ≈ Wavelength of observation
Need long wavelengths to see pebbles
Log [Number of Grains (N)]
Log [Emission Efficiency (Q)]
a
Many more small grains than large.
Small grains dominate surf area
Log [Grain Size (a)]
Seeing Pebbles and Rocks
One more piece of the puzzle: κν ∞ λ-1
(opacity)
Millimeter flux (optically thin): Fν ∞ Σ * κν * Bν(T)
(Surface density)
∞ 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 Depth
Optical 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 = 3cm
(NGVLA)
τ = 1 at λ = 1mm
(ALMA)
We can only trace underlying mass distribution of solids where τ < 1
Want to know when, where, how much mass in pebbles exists
Terrestrial Planet-Forming Regions
ALMA Band 9, 15km baselines -> 6mas resolution
0.9 AU at distance of Taurus, 2.5 AU in Orion
But disk is optically thick at this
radius/wavelength!
ALMA (ESO/NAOJ/NRAO), T. Sawada
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
Chemistry
Lots of exciting chemistry: volatiles in planet-forming regions, complex organics, etc.
Most exciting to me: Ammonia!
Nitrogen chemistry & TEMPERATURE
One example: turbulence in protoplanetary disks
Degeneracy between temperature and turbulence
disk wind
FUV-ionized
active layer
ambipolar
damping zone
Hall dominated
inner disk
CO freeze-out
near midplane
CO isotopologue emission
throughout molecular layer
10 AU
Simon, Hughes et al. (submitted)
CO emission from
near surface of warm
molecular layer
CO photodissociated
NGVLA will provide:
Views of pebbles and rocks in protoplanetary disks
Radial drift, meter-size barrier
Low optical depth, which is necessary to trace dust
mass distribution within 10 AU
Access to terrestrial planet-forming regions: mass distribution,
changes on ~1yr timescales
Chemistry, particularly ammonia for accurate
temperature determination