Multicomponent stellar wind and chemical peculiarity in A stars cka - - PowerPoint PPT Presentation

Multicomponent stellar wind and chemical peculiarity in A stars

Jiˇ r´ ı Krtiˇ cka1 Jiˇ r´ ı Kub´ at2

1Masaryk University Brno, Czech Republic 2Astronomical Institute, Ondˇ

rejov, Czech Republic

IAU Symposium No. 224 “The A-Star Puzzle”, Poprad, Slovakia, July 8-13, 2004 – p. 1

Acceleration of hot star winds

Hot star winds are accelerated mainly by absorption of radiation in resonance lines:

• 1. absorption of radiation by C, N, O, Fe, etc. (minor

wind component), light scattering by free electrons

• 2. transfer of obtained momentum to the major wind

component (H, He) high density winds – no multicomponent effects (e.g. stellar winds of galactic O stars) low density winds – frictional heating, possible decoupling of wind components, etc.

IAU Symposium No. 224 “The A-Star Puzzle”, Poprad, Slovakia, July 8-13, 2004 – p. 2

Multicomponent model equations

Stationary spherically symmetric hydrodynamic equations for multicomponent radiatively driven stellar wind (Krtiˇ cka & Kubát 2001): continuity equation for each component a d dr

• r2ρavra
• = 0,

ρa is the density of component a vra is the radial velocity of component a

IAU Symposium No. 224 “The A-Star Puzzle”, Poprad, Slovakia, July 8-13, 2004 – p. 3

Multicomponent model equations

momentum equation for each component a vra dvra dr = grad

a

−g − 1 ρa d dr

• a2

aρa

• + qa

ma E +

• b=a

gfric

ab

grad

a

is the radiative acceleration either due to the lines in the CAK approximation (Castor, Abbott & Klein 1975) or due to free electrons g is the gravity acceleration aa is the isothermal sound speed E is the electric polarization field gfric

ab is the frictional acceleration

IAU Symposium No. 224 “The A-Star Puzzle”, Poprad, Slovakia, July 8-13, 2004 – p. 4

Multicomponent model equations

energy equation for each component a 3 2vraρa da2

a

dr +a2

aρa

r2 d dr

• r2vra
• = Qrad

a +

• b=a
• Qex

ab+Qfric ab

• Qrad

a

is the radiative heating/cooling calculated using the thermal balance of electrons method (Kubát et al. 1999) Qex

ab is the heat exchange

Qfric

ab is the frictional heating

we neglect Gayley-Owocki heating (Gayley & Owocki 1994)

IAU Symposium No. 224 “The A-Star Puzzle”, Poprad, Slovakia, July 8-13, 2004 – p. 5

Important simplifications

radiative force in the CAK approximation with force parameters after Abbott (1982) ⇒ possibly incorrect wind parameters ionization equilibrium approximated using “nebular approximation” (Mihalas 1978) ⇒ significantly influences the frictional force neglected wind instabilities (Owocki & Puls 1999) neglected magnetic fields (ud-Doula & Owocki 2002)

• nly Coulomb collisions accounted for the

calculation of the frictional force

IAU Symposium No. 224 “The A-Star Puzzle”, Poprad, Slovakia, July 8-13, 2004 – p. 6

The frictional force

The frictional force depends on the velocity difference via the Chandrasekhar function G(x) = Φ(x) − xdΦ(x) dx 2x2 , x = |vrb − vra| αab .

0.05 0.1 0.15 0.2 0.25 1 2 3 4 5 G(xab) xab Chandrasekhar function G(xab)

for x 1 is G(x) decreasing function, this behaviour enables decoupling of wind components

IAU Symposium No. 224 “The A-Star Puzzle”, Poprad, Slovakia, July 8-13, 2004 – p. 7

Types of hot star winds

winds with negligible multicomponent effects (e.g. winds of OB supergiants)

IAU Symposium No. 224 “The A-Star Puzzle”, Poprad, Slovakia, July 8-13, 2004 – p. 8

Types of hot star winds

winds with negligible multicomponent effects (e.g. winds of OB supergiants) wind temperature influenced by frictional heating (e.g. winds of some main-sequence B stars)

IAU Symposium No. 224 “The A-Star Puzzle”, Poprad, Slovakia, July 8-13, 2004 – p. 8

Types of hot star winds

winds with negligible multicomponent effects (e.g. winds of OB supergiants) wind temperature influenced by frictional heating (e.g. winds of some main-sequence B stars) decoupling of wind components occurs in the wind helium decoupling hydrogen and helium decoupling

IAU Symposium No. 224 “The A-Star Puzzle”, Poprad, Slovakia, July 8-13, 2004 – p. 8

Types of hot star winds

winds with negligible multicomponent effects (e.g. winds of OB supergiants) wind temperature influenced by frictional heating (e.g. winds of some main-sequence B stars) decoupling of wind components occurs in the wind helium decoupling hydrogen and helium decoupling decoupling of wind components in the atmosphere helium decoupling ⇒ helium-free wind, possible helium overabundance in the atmosphere hydrogen and helium decoupling ⇒ metallic stellar wind only

IAU Symposium No. 224 “The A-Star Puzzle”, Poprad, Slovakia, July 8-13, 2004 – p. 8

Helium decoupling

proposed by Hunger & Groote (1999) as the explanation of chemical peculiarity of Bp stars Helium decoupling in the atmosphere – the frictional acceleration lower than the gravity acceleration: gfric

αp < g,

⇒ for solar metallicity stars helium decouples when the mass-loss rate is lower than ˙ M 2 · 10−16 M⊙ year−1 M M⊙ Teff 104 K 3/2 z−2

α

IAU Symposium No. 224 “The A-Star Puzzle”, Poprad, Slovakia, July 8-13, 2004 – p. 9

Helium decoupling in HR diagram

1 101 102 103 104 105 106 F0 A7 A5 A3 A2 A1 A0 B9 B8 B7 B6 L* /LS Ia Ib II III V helium-free wind

due to its low charge zα helium is not present in the stellar wind of A stars (Kubát & Krtiˇ cka 2003) helium may be overabundant in A star atmospheres

IAU Symposium No. 224 “The A-Star Puzzle”, Poprad, Slovakia, July 8-13, 2004 – p. 10

Frictional heating

The multicomponent effects are found to be important when the velocity difference is comparable with the thermal speed (Krtiˇ cka et al. 2003), vri − vrp

• 2kT

mp

0.1. For solar metallicity stars the multicomponent effects become important for mass-loss rates lower than ˙ M 10−10 M⊙ year−1 v∞ 108 cm s−1 3 R∗ R⊙ Teff 104 K 1 z2

Hz2 i

.

IAU Symposium No. 224 “The A-Star Puzzle”, Poprad, Slovakia, July 8-13, 2004 – p. 11

Frictional heating in the stellar wind

Example of the frictionally heated wind of A5II star (Teff = 8 300 K, M∗ = 5.5 M⊙, R∗ = 15.1 R⊙)

50 100 150 200 250 300 350 400 450 1 2 5 10 20 50 100 vr [km s-1] r/R* 5000 10000 15000 20000 25000 30000 100 50 20 10 5 2 1 T [K] r/R* absorbing ions hydrogen electrons

IAU Symposium No. 224 “The A-Star Puzzle”, Poprad, Slovakia, July 8-13, 2004 – p. 12

Frictional heating in HR diagram

1 101 102 103 104 105 106 F0 A7 A5 A3 A2 A1 A0 B9 B8 B7 B6 L* /LS Ia Ib II III V frictionally heated wind

stellar wind of bright giants is heated by friction

IAU Symposium No. 224 “The A-Star Puzzle”, Poprad, Slovakia, July 8-13, 2004 – p. 13

Hydrogen decoupling

Hydrogen decoupling occurs when the velocity difference is equal to the thermal speed, vri − vrp

• 2kT

mp

≈ 1. After decoupling hydrogen leaves the star if vH > vesc, hydrogen falls back onto the stellar surface or forms clouds above the surface (Porter & Skouza 1999), hydrogen decouples in the atmosphere, only pure metallic wind exist (Babel 1995, Babel 1996).

IAU Symposium No. 224 “The A-Star Puzzle”, Poprad, Slovakia, July 8-13, 2004 – p. 14

Hydrogen decoupling

Hydrogen decoupling in the wind of A0III star (Teff = 9 600 K, M∗ = 2.7 M⊙, R∗ = 3.63 R⊙)

10 20 30 40 50 60 70 1 1.001 1.002 1.003 1.004 1.005 1.006 vr [km s-1] r/R* 20000 40000 60000 80000 100000 120000 1 1.001 1.002 1.003 1.004 1.005 1.006 T [K] r/R* absorbing ions hydrogen electrons

IAU Symposium No. 224 “The A-Star Puzzle”, Poprad, Slovakia, July 8-13, 2004 – p. 15

Hydrogen decoupling in HR diagram

1 101 102 103 104 105 106 F0 A7 A5 A3 A2 A1 A0 B9 B8 B7 B6 L* /LS Ia Ib II III V hydrogen decoupling, pure metallic wind or no wind

hydrogen decouples in the stellar wind of A giants

• r A giants may have pure metallic wind

IAU Symposium No. 224 “The A-Star Puzzle”, Poprad, Slovakia, July 8-13, 2004 – p. 16

Conclusions

1 101 102 103 104 105 106 F0 A7 A5 A3 A2 A1 A0 B9 B8 B7 B6 L* /LS Ia Ib II III V helium-free wind frictionally heated wind hydrogen decoupling, pure metallic wind or no wind

multicomponent effects may be important for those A stars which have stellar wind more advanced (NLTE) models are necessary to study these effects in detail

IAU Symposium No. 224 “The A-Star Puzzle”, Poprad, Slovakia, July 8-13, 2004 – p. 17