Modelling the polarization from dust in protoplanetary disks

Transcription

1 Modelling the polarization from dust in protoplanetary disks rançois Ménard Laboratoire ranco-chilien d Astronomie CNRS-rance & U de Chile With very special thanks to Bruce Lites, Stefano Bagnulo & Jos de Boer

2 Outline Observations of Protoplanetary disks A brief history Polarimetry and Protoplanetary Disks Geometry Dust Content Summary. Menard IAUS 305 Costa Rica, 5dec2014 2

3 Brief History irst direct images of disks: HH 30 discovered by HST in 1996 Beta Pic 1984 GG Tau discovered by IRAM in 1994 or memory, 51 Peg b was discovered in 1995! Gledhill & Scarrott 1989 Image credit: HST Heritage. Menard IAUS 305 Costa Rica, 5dec2014 3

4 Brief history Polarimetry was used before 1990 to search for disks Look for aligned vectors Tracing multiple scattering Traces of disks, really?. Menard IAUS 305 Costa Rica, 5dec2014 4

5 Polarimetry of disks Gledhill & Scarrott 1989 Polarimetry was used to trace geometry as early as 1986 Mike Scarrott s group Aligned grains Pierre Bastien s group Multiple scattering Multiple scattering codes applied to disk geometries were developed. Continuum (dust) only. Some of them were the ancestors of the very potent codes still in use today. Main geometric parameters were estimated ~correctly. Size, position angle, inclination Bastien & Menard 1990, PhD work.. Menard IAUS 305 Costa Rica, 5dec2014 5

6 Polarimetry of Disks ALMA science verification Band 6 15km baselines Nov Menard IAUS 305 Costa Rica, 5dec2014 6

7 Polarimetry of Disks Models of linear polar map gave us sizes, PA, and inclinations... High polarisation sources are good targets to find disks and reflection nebulae Because scattering medium is highly asymmetric Message No1: New polarimetric surveys are needed. Menard IAUS 305 Costa Rica, 5dec2014 7

8 Polarimetry of Disks Previous images were scattered light Continuum (dust) radiative transfer allowed to study geometry mm-wave polarimetry will soon be sensitive enough to look for disks Dichroic emission Aligned grains But WHAT alignment mechanism??? Is polarisation tracing the magnetic field? Other alignment mechanisms exist RAT Mechanical alignment Stephens et al CARMA Message No2: Get deeper radio data and understand alignment mechanism. Menard IAUS 305 Costa Rica, 5dec2014 8

9 Polarimetry of disks: The dust scattering matrix α=180 deg Θ Θ Intensity Lin. Pol. Circular Pol. I Q U V sc sc sc sc Stokes vector Phase function Scattering Matrix - 21 / 11 Deg. Lin. Pol I Q U V in in in in Stokes Vector Depends on: shape size refractive index orientation wavelength. Menard IAUS 305 Costa Rica, 5dec2014 9

10 Polarimetry of disks: main mechanism Multiple scattering on dust particles MIE particles Centrosymmetric pattern No alignment needed ISM size distribution extended to a few microns 60% polar requires small particles (Rayleigh) SED requires a ~1mm Grain growth! Silber et al Menard IAUS 305 Costa Rica, 5dec

11 HR 4796: better data, finer models Schneider et al. 1999, Menard IAUS 305 Costa Rica, 5dec

12 HR 4796: understand geometry first Milli et al Milli et al Menard IAUS 305 Costa Rica, 5dec

13 HR 4796: so west side is closer... Perrin et al. 2014, GEMINI / GPI data. Menard IAUS 305 Costa Rica, 5dec

14 HR 4796:...then understand dust Message No 3: calibrate!!! get P and I whenever possible Perrin et al Menard IAUS 305 Costa Rica, 5dec

15 Towards complex dust shapes GMT 150 units a= 0.1µm K-band. Menard IAUS 305 Costa Rica, 5dec

16 Summary Modelling disks is complex but useful!!! Needed to understand early stellar evolution Needed to understand planet formation Message No1: More surveys: need more disks Message No2: Need radio data, polar in emission in disks Message No3: Measure P and I, not just PI Message No4: As data get better, models must follow Complex dust properties, couple dust and gas, etc. Menard IAUS 305 Costa Rica, 5dec

17 . Menard IAUS 305 Costa Rica, 5dec

18 Polarimetry of disks: MIE particles only? Circular polarisation images Multiple scattering on dust particles is sufficient MIE particles don t always work! some alignment needed modest aspect ratio needed -> : 1 Message No3: Circular polarisation is rare but useful! Menard 1998; Kwon et al Menard IAUS 305 Costa Rica, 5dec