Astrophysics 2 stellar and circumstellar physics
Download
1 / 29

Astrophysics 2: Stellar and Circumstellar Physics - PowerPoint PPT Presentation


Astrophysics 2: Stellar and Circumstellar Physics. 4. Stellar Winds (1). http://www.arc.hokkai-s-u.ac.jp/ ~okazaki/astrophys-2/. 4.1.1 Solar wind. 4.1 Observations of stellar winds. Radiative core Convective envelope, where dynamo process is going on

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha

Download Presentation

Astrophysics 2: Stellar and Circumstellar Physics

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


Astrophysics 2:Stellar and Circumstellar Physics

4. Stellar Winds (1)

http://www.arc.hokkai-s-u.ac.jp/ ~okazaki/astrophys-2/


4.1.1 Solar wind

4.1 Observations of stellar winds

  • Radiative core

  • Convective envelope, where dynamo process is going on

  • Corona, where the solar wind begins to blow

Structure of the Sun


Interaction of the Earth’s magnetosphere with the solar wind


Northern lights


Boundary between the solar system and the interstellar space


Why the solar wind blows? (Parker 1958)

Suppose the solar corona is static, then the equation of motion is given by

If we assume the corona to be isothermal, i.e., with being the isothermal sound speed, we have


where

Therefore, the solar corona can’t be static.


4.1.2 Winds from massive stars: P Cygni profiles

P Cygni profile: Profile characterized by strong emission lines with corresponding blueshifted absorption lines.


P Cygni profiles: lines from an expanding atmosphere/stellar wind

Emission

Absorption

E

E

A

Total

observer

wavelength


http://casswww.ucsd.edu/public/tutorial/Stars.html


Formation of a P Cygni Line- Profile

By S. Owocki


4.2 General equations and formalism for stellar winds

4.2.1 What is a stellar wind?

  • A stellar wind is:

  • a sustained outflow in the outer layers of a star, through which the star loses its mass continuously.

  • a source of mass, angular momentum, and energy to the interstellar matter.


4.2.2 Hydrostatic equilibrium in the base of a wind

Eq of motion:

Eq of state:

T varies gradually


In the base of a wind, the atmosphere is exponentially stratified with a scale height much smaller than the stellar radius.

e.g., Solar photosphere


4.2.3. General dynamical equations

Mass

Momentum

Internal energy

EOS


Steady, spherical expansion

Mass loss rate

Momentum

Total energy

work

heating

conduction


kinetic energy

potential energy

Energy requirement

work

heating

conduction


Driving mechanism of coronal winds = gas pressure gradient

4.2.4 A simple model of coronal wind: an isothermal wind

Assumptions

  • Steady & spherically symmetric.

  • Forces taken into account are only gravity and pressure gradient force.


Coronal wind

Corona

heating

Convective envelope

Coronal winds are driven by gas pressure due to a high T in the corona.


Wind eq:

Basic equations

Eq of continuity:

Eq of motion:

Eq of state:


Wind eq has a singularity at

  • The critical point is at

  • The critical point is of saddle type (x-type), which is stable for perturbations

  • At the critical point,

(sonic point),


Solution curves for an isothermal coronal wind

(transonic solution)


4.2.5 Temperature sensitivity of mass loss rate

At the bottom of a subsonic wind with

we have


The density distribution is


Mass loss rate is very sensitive to the temperature!

Mass loss rate


Mass loss rate vs. temperature

(Owocki 2000)


4.3 Analogy of De laval nozzles

Critical solutions have an analogy with flows in rocket nozzles.


Basic equations

Eq of continuity:

Eq of motion:

Eq of state:

Flow eq:


Wind eq:

Flow eq:

Both equations would be identical if


ad
  • Login