Winds - Australian National Universitygeoff/AGD/Winds.pdf · Astrophysical Gas Dynamics: Stellar...

Stellar Winds

vwStar

Stellar Winds © Geoffrey V. Bicknell

1 Characteristics of stellar winds

Solar wind

Velocity at earth’s orbit:

Density:

Temperature:

Speed of sound:

v 400 km/s

n 107 m 3–

T 105 K

cs 50 km/s=

Astrophysical Gas Dynamics: Stellar Winds 2/66

Mass flux (spherically symmetric wind):

Other stars

Red giants:

O&B type stars:

Protostars:

M· 4nmvrr2 314–10 solar masses /yr= =

M· 10 11– Msun/yr

M· 10 7– 10 6–– Msun/yr

M· 10 4– Msun/yr

2 Why winds?

Yohkoh Soft X-ray Tele-scope (SXT) full-field imag-es from the Hiraiso Solar Terrestrial Research Center / CRL

White-light Mk. 4coronameter images

http://umbra.nas-com.nasa.gov/images/

The Sun's outer atmosphere as it ap-pears in ultraviolet light emitted by ionized oxygen flowing away from the Sun to form the solar wind (region outside black circle), and the disk of the Sun in light emitted by ionized iron at temperatures near two million degrees Celsius (region inside circle). This composite image taken by two in-struments (UVCS, outer region and EIT, inner region) aboard the SOHO spacecraft shows dark areas called coronal holes at the poles and across the disk of the Sun where the highest speed solar wind originates. UVCS has discovered that the oxygen atoms

flowing out of these regions have extremely high energies corresponding to temperatures of over 200 million degrees Celsius and accelerate to supersonic outflow velocities within 1.5 solar radii of the solar surface. The structure of the corona is controlled by the Sun's magnetic field which forms the bright active regions and the ray-like structures originating in the coro-nal holes. The composite image allows one to trace these structures from the base of the co-rona to millions of kilometers above the solar surface. (http://sohowww.nascom.nasa.gov/gallery/UVCS/)

2.1 Hydrostatic atmospheresAnalysis of the X-ray and radio emission from the solar corona

indicates a . Gas at this temperature cannotbe held in by the gravitational field of the Sun.To show this, wefirst consider hydrostatic atmospheres and show that an hydro-static atmosphere is not feasible.

The momentum equations are:

temperature 106K

vi vj xj

vi+1---

Gravitational potential=

Hydrostatic solutions

Spherical symmetry

dpdr------

ddr------–=

Suppose we consider isothermal solutions with the pressure de-fined by the following equation:

pkTm----------=

Molecular weight 0.62Atomic mass unit

1.6627–10 Kg

Temperature 106KBoltzmanns constant 1.38

23–10 J K 1–

The potential exterior to a spherical star is

---------–=

Newtons gravitational constant

6.6711–10 SI units=

Mass of sun

2.03010 kg=

For an isothermal atmosphere, the hydrostatic equation be-comes:

kTm--------

ddr------

ddr------–=

Integrating ln mkT--------– Constant+=

Suppose the density at a point within the corona is . Then

the constant of integration is

Constant 0lnmkT--------0+=

0------ln m

kT-------- 0– –

------------------1r---

1r0-----–

0------ GMm

kT------------------

1r0-----–

Since the pressure is proportional to the density, the pressure inthis atmosphere is given by:

The pressure at – the pressure cooker model

The pressure described by the above equation decreases as. However, it does not decrease to zero. The asymptotic

value is

pp0-----

------------------1r---

1r0-----–

expGMm

kTr0------------------– 1

r0r-----–

exp= =

PP0-------

GMmkTr0

------------------–exp=

The interpretation of this pressure is that this is what is requiredin addition to the gravitational field to confine the atmosphere.In the case of the sun, this confining pressure has to be providedby the interstellar medium.

2.2 Some numbers

Excerpt from the Handbookof Astronomy and Astrophys-ics, available from:

http://adsabs.harvard.edu/books/hsaa/idx.html

For indicative calculations let us insert some numbers into theabove equations, with the view in mind of determining whatpressure we would need to hold the sun’s corona in. The follow-ing parameters relate to the corona at a solar radius.

0.62= G 6.6711–10 SI=

m 1.6627–10 kg=

T 1.5610 K= Solar radius 6.96

810 m=

Electron density 1.55810 cm 3– 1.55

1410 m 3–= =

For order of magnitude purposes, it is not necessary to distin-guish between electron density and total number density. How-ever, it is useful to go though the exercise, linking electrondensity to total number density and mass density.

Number density, electron density and mass density in a fullyionised plasma

Suppose that the plasma consists of ionised Hydrogen, and fullyionised Helium and the associated electrons. Then,

Electron density ne nH 2nHe+= =

nH 1 2nHenH---------+

The total number density is:

The mass density is given by:

n nH nHe ne+ +=

2nH 3nHe+ =

nH 2 3nHenH---------+

nHm 4nHem+=

nHm 1 4nHenH---------+

If we adopt a solar abundance of Helium, then

Usually, based on more accurate composition and allowing formetals, i.e. elements heavier than Helium, we adopt and

Also from the above numbers:

nHenH--------- 0.085 ne 1.17nH n 2.26nH

1.34nHm 0.59m

0.62nm=

n 1.9ne

Required pressure for the pressure-cooker model

At a solar radius , we have

The pressure at the base of the corona, is

pp0-------

GMmkTr0

------------------–exp=

r0 6.96810 m=

GMmkTr0

------------------ 7.2=pp0------- 7.5

4–10

p0 nkT 1.9nekT 8.13–10 N m 2–= = =

Therefore, the ISM pressure required by the pressure cookermodel is:

The ISM is a multi-phase medium, with all phases in pressureequilibrium. Take the warm phase, for example:

The ISM pressure fails by a factor of about to containthe solar atmosphere! Hence, the solar atmosphere flows out ina wind.

p 7.54–10 8.1

3–10 N m 2– 6.16–10 N m 2–= =

n 1 cm 3– 106 m 3– T 104K

pISM 1.413–10 N m 2–

2.3 Preliminary estimate of mass fluxIn order to gain some idea of the mass flux we might expect, letus suppose that the solar wind flows out spherically from a solarradius at the escape velocity from the Sun.

(28)Escape velocity

------------- 1 2/

6.2510 m s 1–= =

620 km s 1–=

This estimate is out by about three orders of magnitude mainlybecause our estimate of the density of the outflowing wind is inerror. It is interesting to see how the correct theory makes allow-ance for this.

Mass flux 4 r02 0 Vesc=

4 6.96810 2 1.2nem 6.2

1.21210 kg s 1–=

2.011–10 solar masses per year=

3 Analysis of spherically symmetric winds

3.1 Fundamental equations

Euler equations

See Landau & Lifshitz Fluid Mechanics for expressions for thegas dynamics equations in cylindrical and spehrical coordinates.

vi vj xj

vi+1---

Spherical symmetry vr r

vr 1---

For a spherical star of mass

Equation of state

In the latter, is not necessarily . (in

an expanding flow).

---------–=

r2---------–=

Adiabatic: p K s =

Polytropic: p C=

cp cv 5 3 Heating

Isothermal equation of state

Source of heat for the solar wind: Dissipation by waves in solarwind generated in photosphere/chromosphere.

Speed of sound

where the derivative is no longer at constant entropy. We alsodefine the

pkTm--------= T constant=

ddp C 1–= =

Isothermal sound speedkTm--------=

Radial momentum equation

Pressure gradient:

so that

dvr as2

------

rdd GM

r2---------––=

Mass flux

From now on .

xi vi +

r2-----

r2vr 0= =

r2vr ConstantM·

4------= =

M· vinidS

Sphere 4r2vr= =

Also note that this equation gives us a way of estimating the den-sity given the velocity. Solving for :

4vr2---------------=

3.2 Sonic pointAn important feature of all winds is the existence of a sonic pointwhere the flow makes a transition from subsonic to supersonicflow. To show the existence of this we consider both the massflux equation and the momentum equation.

Mass flux

(41)4vR2 M·=

Taking logs and differentiating:

4ln vln+ln 2 Rln+ + M·ln=

rddv 2

r---+ + 0=

rddv 2

Substitute into momentum equation:

1–rd

dv as2

v------

r------

r2---------–+=

v2 as2–

r------

r2---------–

r------

r2---------–

v2 as2–

----------------------------------=

The equation for v has a critical point where the numerator anddenominator of the right hand side of this equation are both zero.That is, where:

v askTcm

-----------= =

r------

r2---------= rc GM

---------GMm2kTc

------------------= =

e.g. the Sun ( ):

Solar radius: .

vc 170 km/sT

106--------- 1 2/

rc 4.8910

106--------- 1–

metres 6.9RsunT

106--------- 1–

Rsun 6.96810= m

The actual measured coronal temperature is approximately

. This implies that:

3.3 A better estimate of mass fluxLet us suppose that the corona of the Sun is approximately deter-mined by the hydrostatic solution out to the sonic point. Then

2610 K

rc 2.0910 m 3.5 solar radii=

vc 340 km s 1–

0------

GMmkTr0

------------------– 1r0r-----–

We have

Hence our mass flux estimate becomes:

We are now only about an order of magnitude out!

GMmkTr0

------------------ 7.2 rr0----- 3.5

0------ 2.1

3–10

M· 4 3.5 6.96810 2 2.1

3–10

1.2 1.551410 m 3.4

2.613–10 solar masses per year=

4 Critical point analysis

4.1 Splitting equationsIf we want a wind solution which accelerates to supersonic thenwe need to negotiate the critical point:

(50)rd

r---------

r2---------–

v2 as2–

----------------------------------=

Split this equation into 2 by introducing a parameter and writethe equations as:

These equations have no potential infinities anywhere and aremuch better behaved numerically and easier to analyse mathe-matically.

drdu------ r2 v2 as

2– f1 r v = =

dvdu------ v 2as

2r GM– f2 r v = =

4.2 Critical pointThis is now defined by:

Expansion in neighbourhood of critical point:

drdu------

dvdu------ 0= =

r rc r'+= v vc v'+=

udd r'

f1 r2 v2 as2– = f2 v 2as

2r GM– =

The functions and involve the function and in order to

evaluate the partial derivatives of these functions, we need to

evaluate the partial derivatives of . This involves evaluating

Bernoulli’s equation.

4.3 Bernoulli’s equation:Since

f1 f2 as2

dpdr------ C 2– d

dr------=

(58)as2 dp

d------ C 1–= =

dv C 2–rd

r2---------–=

2---v2 C 2–

r2---------–=

12---v2 C

1–----------- 1––

--------- constant+ +=

12---v2 C

1–----------- 1– GM

r---------–+ constant=

as2 1– constant

---------12---v2–+

The last equation implies:

then, at the critical point:

as2 1– GM

r2---------–=

as2 1– v–=

f1 r2 v2 as2– = f2 v 2as

2r GM– =

f1 2r v2

as2– r2

as2– 1– GM= =

f1 r2 2vv

as2–

r2 1+ v= =

f2 v 2as2 2r

r--------- 2 1– GM

r---------–

f2 3 2– vGMr

---------=

f2 2as2r GM– 2vr

as2+ 2 1– as

2r–= =

f2 1– GM–=

1– GM 1+ vcrc2

---------vc 3 2– 1– GM–=

udd r'

1– GM 1+ vcr2

---------vc 3 2– 1– GM–

The solution of these equations requires the eigenvalues and ei-genvectors of the matrix. Denoting the eigenvalues by :

This simplifies to:

1– – GM 1+ – vcrc2

---------– vc 3 2– 1– GM+0=

2 GM 2 5 3– 2

-------------------=

GM5 3–

2-------------- 1 2/

Eigenvectors:

1– GM– 1+ vrc2–

1– GM– u1 1+ vcrc2u2– 0=

u2 1= u1 1+

GM-----------

5 3–2

-------------- 1 2/

1– –------------------------------------------------------=

The general solution in the neighbourhood of the critical point is:

where and are the two independent eigenvectors.

Slopes of lines through critical point and topology of the criticalpoint:

v'A1u1 1uexp A2u2 2uexp+=

Critical Point

dvdr------

u2u1-----

--------

5 3–2

-------------- 1 2/

1– –

1+----------------------------------------------------= =

5 Scaling of wind equations for numerical solutions

When we have analytical solutons of differential (or other) equa-tions, the dependence on physical parameters is evident. Howev-er, when we construct numerical solutions, it is best to scale theequations in order to derive solutions depending upon the small-est number of physical parameters. Numerical solutions of thewind equations were given in section 5. The physical equationsshould be scaled before numerical solutions can be determined.One reason for this scaling is that the numerical solutions canthen be presented using the smallest number of parameters. Var-ious parameters (specifically the velocity, radius and density atthe critical point) then enter as scaling parameters.

The wind equations in physical units are:

For numerical purposes we scale these by the values of velocityand radius at the critical point.

NB The primed variables here are not the same as the perturba-tions that were used to study solutions in the neighbourhood ofthe critical point.

drdu------ r2 v2 as

2– =

dvdu------ v 2as

2r GM– =

r rcr= v vcv= as vcas= c=

5.1 Scaling of the sound speedThe solutions that were shown in this lecture are all constrainedto have the same mass flux. Hence,

4------ vr2 cvcrc

2= =c----- v

vc----- 1– r

rc---- 2–

v 1– r 2–=

as2 C 1–= ac

2 Cc 1–=

asac----- 2

c----- 1–

(79)as2 1– =

5.2 Normalisation of the differential equations

rcdrdu------ rc

2vc r2 v2 as2– =

----------- dr'du------ r2 v2 as

2– =

vcdv'du------- rcvc

3v 2as2r GM

-----------–

2rcvc3v as

2r 1– =

-----------dvdu------- 2 as

2r 1– =

The last equation follows from the condition at the critical point

We now simply make the transformation of the parameter :

and the normalised equations become:

----------- 2=

du rcvc2du=

drdu-------- r2 v2 as

2– =

dvdu-------- 2 as

2r 1– =

These can be easily solved with a numerical differential equationsolver (I simply use the NAG library). To start solutions near thecritical point, the perturbation equations, developed in this lec-ture, need to be used.

v 1– r 2–= as2 1–=

6 Numerical solution of equations

0 1 2 3 4 50

Velocity vs radius for γ = 1.2

This plot shows the numerical integration of the two curves through the critical point and the numeri-cal solution of other curves not passing through the critical point.

Blue curve: wind

Red curve: accretion

1e-01 1e+00 1e+011e-02

ρ/ρ c

Density vs radius for γ=1.2

Density for the two critical curves

Blue: wind

Red: accretion

7 Mass flux

We can estimate the mass flux from the sun with this simplemodel.

Estimate parameters at the critical point assuming a temperature

of there.

The mass flux is:

where the velocity and critical radius are given by

Tc 106K=

M· 4cvcrc2=

vckTm--------- 1.27 102 km/s= =

Using our numerical solution, we estimate that the density at the

solar radius, i.e. at is .

Hence, we can write the mass flux as

---------GMm2kTc

------------------ 4.14 109 m 5.96 Rsun= = = =

5.96----------= 230 c

M· 4cvcrc2 M· 40

c0------

vcrc2= = =

Earlier we used an electron number density at the base of the co-

rona of so that the density

ne 1.55810 cm 3–

0 1.9ne m= 3.016–10 gm cm 3–

M· 4 3.016–10

230------------------------- 1.27

710 4.11110 2

M· 3.61310 gm s 1– 5.7

13–10 Msun yr 1–

This is about a factor of 20 larger than the observed value. In or-der to obtain better agreement with observation, theory needs totake into account the effect of magnetic fields and coronal holes.

8 Asymptotic wind solutions

8.1 Deductions from Bernoulli’s equation

As the density in the wind goes to zero and thereforesince

12---v2 C

1–----------- 1– GM

r---------–+ constant=

12---v2

1–-----------

---------–+ constant=

We can use Bernoulli’s equation to estimate the asymptotic ve-locity as follows. The constant on the right hand side of theabove equation can be estimated from the conditions at the crit-ical point.

As , and so that

constant12---vc

1–-----------

---------–+ vc2 1

1 1–----------- 2–+= =

5 3–2 1– -------------------= vc

r as2 0 GM

r--------- 0

v2 5 3–

1– ----------------vc

2= v 5 3– 1–

----------------vc=

We have been using , in which case

Thhis desn’t compare too badly with the observed value of

measured at the Earth.

Reasons for the discrepancy between theory and observation

• Magnetic effects are important in the initial phase of the solar wind

• The effective outside the solar region is greater than 1.2

8.2 Density in the asymptotic regionSince

v 2.65vc 2.65 127 km s 1– 320 km s 1–= = =

400 km s 1–

M· 4vr2=

then asymptotically

This expression for the density is frequently used for stellarwinds well outside the sonic point where the wind can be consid-ered to have achieved its terminal velocity, . As we have sen

with the sun, the sonic point is reasonably close to the star.

4vr2-------------------=

Winds - Australian National Universitygeoff/AGD/Winds.pdf · Astrophysical Gas Dynamics: Stellar...

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