Low Mass Star Forma-on: The T Tauri Stars

Transcription

1 Low Mass Star Forma-on: The T Tauri Stars

2 Cloud Collapse Thermally- supported non- rota-ng cloud Inside- out collapse R=c s t (c s : sound speed) m acc =m dot t Gm acc m H /R = m H v 2 (by VT); v=c 2 s G(m dot t)/c s t = c 2 s m dot = c s3 /G = 2x10-6 M yr - 1 (c s /0.2 km s - 1 ) 3

3 How Bright is a Protostar? L = E/T Let T = τ ff ~ (Gρ) - ½ For ρ = 10 3 m H cm - 3, τ ff ~ sec = 3x10 6 yrs Let M = 1 M E = 1/2 E grav ~GM 2 /R ~ 2x10 48 erg L ~ 2x10 48 erg/3x10 6 yrs ~ 5L T =GM 2 /RL is the Kelvin- Helmholtz -mescale

4 Birthline Protostar forms by quasi- sta-c contrac-on Temperature profile set by hydrosta-c equilibrium When T core reaches about 10 6 K, D+p + - > 3 He D burning acts as a thermostat Star becomes isothermal and convec-ve Strong winds disperse infalling outer cloud [D/H]=2.5x10-5 For m dot =10-5 M yr - 1, L D = 15L Since T~M/R and T constant; M~R

5 Deuterium Burning In a convec-ve star (γ = 5/3, polytropic index n = 3/2) T C = 0.54 GM * µm H /k B R * T C 7.5 x 10 6 (M * /M )(R /R * ) 2 D burns at T C ~ 106 K Birthline: R birth ~ 7.5 M * (solar units)

6

7 PMS evolu-on

8 Evolu-onary Track Caveats Ages are arbitrary Models do not account for Rota-on Magne-c fields Con-nuing accre-on

9 Expected Protostar SED Assume free fall at rate m dot onto core of mass m core v r -½ ; ρ r -3/2 ρ(r) = m dot /4πr 2 v = m dot r -3/2 /4π(2Gm core ) - ½ Integrate inwards to get τ(r,λ) Assume τ dust independent of r for T<1500K

10 Dust Opacity. Absorp-on efficiency for silicate grains Suh, K.- W.,1999, MNRAS, 304, 389

11 Expected Protostar SED τ λ = κ λ m dot r - ½ / 2π (2GM core ) 1/2 κ λ is the opacity [cm 2 g - 1 ] assumed due to dust Note: τ λ ρ(r) r Define photosphere r λ where τ λ = 2/3 r λ = 9 κ λ m dot 2 / 32π 2 GM core Use Rosseland mean opacity κ m ~ 5 cm 2 g - 1 valid K; µm Assume a black body: L = 4πr m2 σt m 4 For m=0.6 M ; m dot = 3x10-6 M yr - 1 r m ~20au and T m = 177K

12 The Herbig- Haro Objects PMS ouqlows Central object obscured Bowshock + knots Length: au Densi-es: cm - 3 Knot masses: planetary Veloci-es ~ few x100 km/s Timescale: centuries

13 The Herbig- Haro (HH) Objects Pat Har-gan s movies at hsp://sparky.rice.edu/movies.html

14 FU Orionis Objects (FUOrs) Brightening event in PMS star Extreme accre-on event? Disk instability Spectral type depends on λ

15 Is accre-on steady or discrete? 10 3 FUOri events over 10 6 years accumulates as much mass as a steady accre-on over the same -me. EXor events

16 ctts: Basic Characteris-cs Spectra type G- M Co- spa-al with dark clouds Hα emission Li I absorp-on IR/UV excesses Blue veiling Spectral evidence for inflows and ouqlows

17 Aside: Stars Equilibrium supported by steady H- burning Hydrosta-c equilibrium g(r) = GM(r)/r 2 ; F G =GM(r)/r 2 ρdr dp/dr =- GM(r)ρ/r 2 Con-nuity of mass: dm(r)/dr = 4πr 2 ρ dt/dr depends on energy transport Radia-ve convec-ve dl/dr = 4πr 2 ε(r)

18 Aside: Stars Photosphere: temperature minimum Observe absorp-on lines T eff from Absorp-on lines Spectral shape (op-cally photosphere radiates like a black body to first order) Emission lines require temperature increase with height

19 Classes Classes I III defined by near- IR (2-10 µm) spectral slope; νf ν ν α O - protostars I imbedded/extreme T Tauri stars α > 0.3; -0.3 < α < 0.3 (flat) II classical T Tauri stars -1.6 < α < -0.3 III naked or weak- lined T Tauri stars α < -1.6

20 Environs

21 T Tau

22 T Tau F140LP

23 T Tau F140L

24 T Tau 2µm Roddier & Roddier, IFA

25 YSO SEDs

26 Class I ctts. hsp://

27 A Normal Star: ε Eridani Spectral type: K2V

28

29

30

31

32 Class I

33

34 class I- II

35 class I- II

36 class I- II

37 class I- II

38 class I- II

39 class I- II

40 class I- II

41 class I- II

42 TW Hya Source: Thi, W.F. et al. 2010, A&A 518, L125

43 Class II

44 Class II

45 Class II

46 Class II

47 Class II

48 Class II

49 Class III

50 Lithium Solar System abundance ~ 3x10-9 (n Li =3.5) Primordial abundance: n~2.5 (Pop II stars) Solar photospheric abundance: n~1 Li is created In the Big Bang By CR spalla-on reac-ons In novae Li is destroyed at T~10 6 K 6 Li (p + e - ) 7 Li 7 Li (p + γ) 2 4 He

51 IR Excess Dusty disk T dust < 1500K

52 UV excess boundary layer Accre-on luminosity L acc = ½m dot v 2 /R * v ~ v esc = (2GM/R) ½ Re- radiated as black body from frac-on of surface area T acc = (L/(4π) 2 fr 2 σ) ¼ = (GMm dot / (4π) 2 fr 3 σ) ¼ For R=2R and f=0.02, T acc = 11,000 K Explains blue veiling

53 XZ Tau Jets and ouqlows

54 XZ Tau Jets and ouqlows

55 X- rays Class I: shock emission from jet (DG Tau) Class II: some-mes detected Class III: ac-ve stellar corona, L x /L bol 10-3

56 X- rays Class I: shock emission from jet Class II: some-mes detected O}en highly absorbed Class III: ac-ve stellar corona, L x /L bol 10-3

57 Rota-on and magne-c braking Class II rota-on periods typically 7-10 days Periods regulated by magne-c braking? Class III rota-on periods typically 1-5 days Spinup due to contrac-on?

58 . Edwards et al., 1993, AJ, 106, 372

59 A Few Words on Classifica-on Classes: based on SED slope in near- IR Classical T Tauri: based on EW(Hα) > 10Α Weak- lined T Tauri: based on EW(Hα) < 10Α Equivalent Width is a ra-o of fluxes Con-nuum flux depends on photospheric temperature Naked T Tauri: essen-ally Class III

60 Herbig Ae/Be Stars Higher- mass analogs of T Tauri stars Spectra class A or B Hα in emission Near- IR excesses