Dust emission. D.Maino. Radio Astronomy II. Physics Dept., University of Milano. D.Maino Dust emission 1/24

Transcription

1 Dust emission D.Maino Physics Dept., University of Milano Radio Astronomy II D.Maino Dust emission 1/24

2 New insight on Dust Emission Before WMAP and Planck with only COBE-DMR, dust is well described by amorphous silicates: ν 1.7 and T d = 9.5K carbonaceous components: ν 2.7 and T d = 16K In 1997 Leitch et al. with OVRO (Owens Valley Radio Observatory) at 14.5 and 32 GHz found an excess of microwave emission strongly correlated with dust as traced by IRAS@100µm spectral index 2 resambling free-free emission high dust temperature 10 6 K but not visible in X-Ray This emission is dubbed AME - Anomalous Microwave Emission D.Maino Dust emission 2/24

3 New insight on Dust Emission D.Maino Dust emission 3/24

4 New insight on Dust emission Draine & Lazarian (1998) proposed a new mechanism of emission from dust grains Main idea is related to the electrical dipole of dust grains In order to compute such emission one needs: total number of small grains grain electrical dipole moment rotational velocities D.Maino Dust emission 4/24

5 New insight on Dust emission Number of grains derived from IR observations and 25µm Dipole moments: for non-spherical dust grains consider displacement between mass and charge centroids Grains acquire charge from photoelectric emission and collisions Rotational ω: both thermal and from damping/excitation from plasma drag i.e. grain interaction with passing ions DL98 consider different phases of ISM: Cold (Warm) Neutral Medium - C(W)NM, Warm Ionized Medium (WIM), Molecular Cloud (MC) and Dark Cloud (DC) described by n H, T, χ where χ is starlight intensity wrt average D.Maino Dust emission 5/24

6 Physics of Rotation Dust Grains D.Maino Dust emission 6/24

7 Physics of Rotating Dust Grains Angular Momentum I is in general quantized If rotational kinetic energy is 3/2kT rot the quantum number classical rotation J = I ω ( ) 1/2 ħ 20 Trot 1 100K Radiation power from rotating grain: P = 2 ω 4 µ 2 sin 2 θ 3 c 3 where ω angular velocity and µ grain dipole moment D.Maino Dust emission 7/24

8 Physics of Rotating Dust Grains Grain dipole moment µ µ = µ i + ɛzea x where µ i is intrinsic dipole moment of uncharged grain, Ze is total charge and ɛa x is displacement between mass and charge centroids a x is the equivalent excitation radius i.e. for a non-spherical grain the radius of the sphere with the same rate of collisions displaced from center rotational energy Displacement is usually assumed to be 10% of a x thus ( a ) ( ) x ɛ Zea x ɛ = Z debye cm 0.1 D.Maino Dust emission 8/24

9 Physics of Rotating Dust Grains Grain intrinsic dipole moment µ i βn 1/2 As for β: hydrocarbon molecules has µ i 1debye while perfectly symmetric molecules like C 24 H 12 (PAH) has µ i = 0 From consideration on interstellar environment (i.e. stellar radiation) β = 0.4debye Last hypothesis is that µ orientation is un-correlated with ω sin 2 θ = 2/3 P = 4 µ 2 ω 4 9 c 3 D.Maino Dust emission 9/24

10 Physics of Rotation Dust Grains D.Maino Dust emission 10/24

11 Physics of Rotating Dust Grains Gas-grain and plasma-grain interactions as well as infrared and radio emission will damp rotation i.e. lower ω Thermal collision, excitation from field of passing-by ions/electrons instead increase ω Dipole moment in general depends on grain size (through the a x quantity): the model assumes 50% of grains with nominal β while 25% will have half of this and 25% double of this For each a x and β there will be a range on ω assumed to follow Boltzmann distribution Emissivity per H j ν n H = ( ) 8 1/2 1 3 n H c 3 da dn µ 2 ω 6 ) da ω 2 exp ( 3ω2 2/3 2 ω 2 D.Maino Dust emission 11/24

12 Physics of Rotating Dust Grains D.Maino Dust emission 12/24

13 Physics of Rotating Dust Grains D.Maino Dust emission 13/24

14 Observational Evidence of AME OVRO: Leicth et al COBE-DMR: Kogut et al. 1996, Banday et al Tenerife: de Oliveira-Costa 2002,2004 Python V: Mukherjee et al Green Bank: Finkbeiner 2002, Finkbeiner et al Cosmosomas: Watson et al. 2005, Battistelli et al VSA: Scaife et al. 2007, Tibbs et al CBI: Casassus et al. 2004,2006 WMAP: Bennett et al 2003, Dickinson et al Planck: Planck collaboration 2011 D.Maino Dust emission 14/24

15 Observational Evidence of AME: Planck D.Maino Dust emission 15/24

16 Observational Evidence of AME: Planck D.Maino Dust emission 16/24

17 Perseus AME region Take aperture photometry of the region Inner circle: signal + background. Outer anulus: background estimation D.Maino Dust emission 17/24

18 Perseus AME region D.Maino Dust emission 18/24

19 Perseus AME region Model: Free-free (orange), thermal dust (cyan), high ρ molecular gas (magenta) and low ρ atomic gas (green) D.Maino Dust emission 19/24

20 Perseus AME region Model: Free-free (orange), thermal dust (cyan), high ρ molecular gas (magenta) and low ρ atomic gas (green) D.Maino Dust emission 20/24

21 ρ Ophiuchi AME region Model: Free-free (orange), thermal dust (cyan), high ρ molecular gas (magenta), low ρ atomic gas (green) and CMB (black - dotted) D.Maino Dust emission 21/24

22 ρ Ophiuchi AME region Model: Free-free (orange), thermal dust (cyan), high ρ molecular gas (magenta) low ρ atomic gas (green) and CMB (black - dotted) D.Maino Dust emission 22/24

23 Diffuse AME emission Most of the AME evidence in specific astrophysical objects e.g. molecular clouds Large scale correlation between low-frequency and high-frequency D.Maino Dust emission 23/24

24 Diffuse AME emission Most of the AME evidence in specific astrophysical objects e.g. molecular clouds Large scale correlation between low-frequency and high-frequency Other observations are required to better understand spinning dust grain model D.Maino Dust emission 24/24