1. Interstellar dust distribution in the astrospheres formation of the filamentary structures
Olga Katushkina1, Dmitry Alexashov1,3,
Vladislav Izmodenov1,2,3, Vasiliy Gvaramadze1,4
Space Research Institute, Moscow, Russia
Lomonosov Moscow State University , Moscow, Russia
Institute for Problem in mechanics , Moscow, Russia
Sternberg Astronomical Institute , Moscow, Russia

2. Content Introduction – motivation Our numerical model: plasma and dust
Results of the modelling:
- Physical effect of gyrorotation – formation of the filamentary structure
- Qualitative comparison of the model results with two observed astrospheres (around stars kCas and Qcar )

3. Motivation and goals Our goals:
Most astrospheres are observed by measurements of the dust emission => we should know how the dust is distributed;
Recent high angular resolution mid-infrared observations show that many astrospheres have a filamentary structure.
Our goals:
Apply our numerical 3D MHD model of the astrosphere to calculate the interstellar dust distribution;
Propose a new possible mechanism of formation of the filamentary structures;

4. Numerical model
Problem of interaction of the supersonic stellar wind with the surrounded interstellar medium
plasma (previous talk):
ideal MHD equations (stationary, 3D);
no radiation cooling and thermal conduction;
Interstellar dust – test particles:
charged interstellar dust grains interact with the interstellar magnetic field through the Lorentz force;
stellar gravitation, radiation pressure and drag force can be included to the model optionally;
Monte-Carlo method for calculations of dust number density and bulk velocity.

5. Parameters of calculation:

6. Motion of a charge dust grain
Gyrorotation + drifts of the guiding center
E x B drift: Vplasma
Drifts due to spatial gradients of the magnetic and electric field
Example of a trajectory and grain’s velocity:

7. Dust density distribution
Dust motion equation in dimensionless form:
- parameter characterizing the dust grain, it is dimensionless q/m.
ad ~ 1/b2, b – radius of dust grain, i.e. ad is smaller for larger grains.
dimensionless quantities:
- gyroradius;
Dgyr – is average distance along Z axis passing during one gyrorotation;

10. Physical explanation
Bulk velocity Vz of dust, ad=0.01

11. What do we see depends on the line-of-sight
What do we see depends on the line-of-sight! If we observe an astrosphere at different angles we will see different numbers of filaments.

12. Qualitative comparison with two astrospheres
kCas:
BISM ┴ VISM
ad=0.01
(~ md≈ g)
qCar:
BISM ║ VISM
ad=0.02
(~ md≈ g)

13. Summary
We suggest a new physical mechanism for formation of the filamentary structures observed for some astrospheres;
It is shown that the alternating minima and maxima of the dust density occur between the astrospheric bow shock and the astropause due to periodical gyromotion of the dust grains;
These filamentary structures are obtained by the model in the case of particles for which the gyrorotational period is comparable with the character time of the dust motion between the bow shock and the astropause.

14. Thank you for your attention!
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