U. C. JoshiPRL, Ahmedabad

IDMC2014,

Cometary dust – Negative Branch of Polarization

Collaborators: KS Baliyan, S Ganesh

6840

Observations show:

-comets are very dark; albedo is very low (~4%) -> in (n-ik) k need to be large

- dust contains ~ 30% crystalline matterial (olivine, pyroxine)

-composition of amorphous silicate is similar as that of olivine and pyroxine

-in general colour of comets red even in near IR (require large grains)

-polarimetric colour also in general red

-negative polarization for phase angle < 20 deg

Comet C/2007 N3 (Lulin) : Polarization observations from Mt Abu Observatory

Joshi, Ganesh, Baliyan (under review)

IHW Filters: 3650/80A; 4845/65A; 6840/90A

Lulin observations through different size apertures

Comet C/2007 N3 (Lulin) : Polarization observations from Mt Abu Observatory

Joshi, Ganesh, Baliyan (under review)

IHW 4845 data

IHW 6840 data + 4845 curve overlaid

Present understanding:

Status of cometary dust based on remote sensing has been reviewed by Kolokolova et al (2004). They conclude: Aggregate models are best at explaining photometric, polarimetric and thermal properties of cometary dust. This model explains well not only the phase dependence of brightness and Polarization of comets but also explain the existence of two classes of Comets based on polarimetry (Levasseur-Regourd et al 96)

Lisse et al(2002) and Sitko et al(2004) argue – there are two classes of comets based on the strength of silicate feature in emission and the 10micron SED Temp excess over an equivalent BB temp at observed heliocentric distance of comet.

High %P -> strong Silicate features - > more fine grained dustlow %P -> weak Silicate emission -> relaively more course grained dust in the mixture

Negative polarization: - can be reproduced by ensemble of polydisperse spherical particle (Mukai et al 1987) - Kolokolova et al (2004) - > polarimetric, photometric and thermal IR characteristics: Require comet particle to be aggregates of submicron ensemble

- aggregate “structure” -> confirmed by Stardust in situ measurements (65% aggregate structure)

- reproduction of NBP -> can be modeled with aggregates with thousands monomers (kolokolova et al 2007) or Smaller aggregates with transparent silicates (Zubko et al 2009, Kimura et al 2001)

- comet Hale-Bopp: No -ve polarization in NIR (Jones & Gehrz 2000) Strong silicate features

Comet C/2007 N3 (Lulin): Typical NBP ; also exhibit NP at 1.65 micronNP -> demand large aggregate particlesThermal emission -> gives a

p ~ 0.9 micron; D=2.727

(relatively compact grains) Monomer size 0.1 micron + large aggregate -> is consistent with NPweak silicate (woodward et al 2011)

Dust in Lulin is dominated by large and compact aggregate particles of thousand monomers

NIC & EC comets: Sitko et al (2004) -> Based on 10 micron emission find - old comets (EC) in general dominated by compact grains & NIC with fluffy grains

Lulin-> Polarization similar to Oort cloud comets but thermal properties similar to EC -> indicate that grain structure may account for this observation

Composite dust particles (consisting of submicron monomers -size ~0.1micron) With composition Halley type or Stardust sample which include – silicates, amorphous carbon, organic refractory

Above prescription Explian well -> P(pahse); NPB;low geometric albedo (~5%)Red photometric and polarimetric color; brightness increase at low phase angle

For proper characterization of observed low albedos and red photometric color: The value of (n-ik) is crucial If k <0.4 the color of comet -> blue (Kimura et al 2006)

Lasue et al (2009) : mixed BAs (BPCA, BCCA) with speroidal silicate grains and got good fit with P(phase); but not provided photometric characteristics (n-ik) -> organics (1.88 + i 0.1) & silicates (1.62 + i 0.003)

Similarly Shen et al (2009) : aggregate model + (n+ ik) -> visible (1.72+ i 0.029) & IR (1.71 + 0.031) P(phase) OK but lead to high albedo + blue color

AK Sen & Himadri Das (Assam Univ) :

So far the most consistent model -> Kolokolova & Kimura (2010) Stardust sample type mixture + large aggregate(1024monomers) + k>0.4 Explain all observed parameters well except NBP

Kolokolova & Kimura (2010)

Modeling results: Shen et al (2009), Lause et al (2009), Zubko et al (2011) : Red polarimetric colour in NBP

Kimura (2003) : blue polarimetric colour

Red

Blue

Summary● Comet C/2007 N3 (Lulin) shows negative P in the phase angle range 1.7 – 11 deg in the filter bands IHW BC & RC

● NBP of comets in BC & RC bands is not neutral. The absolute value of P in red wave band is larger than in blue wave band. Though there seems to be some overlap of error bars, the trend shows P in red wave band is systematically deeper than in the blue wave band i.e. blue polarimetric colour in NBP.

Thanks