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X-rays from Magnetically Channeled Winds of OB Stars

Explore the use of X-rays in identifying embedded, active young OB stars and discriminating magnetic sources from non-magnetic ones. Discover diagnostics of the hot plasma in the extended atmospheres of magnetic OB stars and the passive probe of cooler circumstellar material.

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X-rays from Magnetically Channeled Winds of OB Stars

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  1. X-rays from Magnetically Channeled Winds of OB Stars David Cohen Swarthmore College with M. Gagné, S. St. Vincent, A. ud-Doula, S. Owocki, R. Townsend

  2. What can X-rays do for us? Identify embedded, active young OB stars Can they discriminate magnetic sources from non-magnetic ones? Diagnostics of the properties of the hot (>106 K) plasma in the extended atmospheres of magnetic OB stars (Somewhat) passive probe of cooler circumstellar material

  3. Outline Context q1 Ori C and the MCWS mechanism Other applications of MCWS and X-rays

  4. M17: ~0.5Myr soft medium hard courtesy M. Gagné 4’

  5. Orion Nebula Cluster: ~1Myr soft medium hard dashed arrows point to very early B stars courtesy M. Gagné 4’

  6. Tr 14: ~0.5 - 2 Myr soft medium hard courtesy M. Gagné 4’

  7. NGC 6611: ~5Myr No hard sources soft medium hard courtesy M. Gagné 7’

  8. What’s happened to the hard, variable O stars by 5 Myr?

  9. Let’s focus on one well-understood magnetic hot star: q1 Ori C

  10. Dipole magnetic field (> 1 kG) measured on 1 Ori C Wade et al. (2006) Magnetic field obliquity,  ~ 45o, inclination, i ~ 45o

  11. Babel and Montmerle (1997a,b) Channeling, confinement, shock-heating Steady state? Cooling disk? Insights such as centrifugal acceleration

  12. Fortuitous access to all viewing angles of the magnetic field Cartoon showing viewing angles of θ1 Ori C for Chandra observations. Phase 0 is when the disk is viewed face-on (α=4 deg), while phase 0.5 occurs when the disk is viewed edge-on (α=87 deg) Note: slow rotation (centrifugal force negligible); field consistent with large-scale dipole

  13. Rotational modulation of the X-ray emission simply from variation in the occultation of the x-ray emitting magnetosphere by the star To 1st order: depth of eclipse depends on how close the shock-heated plasma is to the star

  14. 0.4 1.5 0.3 1.0 Simulation EM (1056 cm-3) 0.2 θ1 Ori C ACIS-I count rate (s-1) 0.5 0.1 0.0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 Rotational phase (P=15.422 days) Chandra broadband count rate vs. rotational phase Model from MHD simulation

  15. Subsequent numerical MHD simulations by ud-Doula & Owocki (2002, etc.) Interplay between magnetic tension and wind kinetic energy – self-consistent field configuration Dynamical treatment (what happens as material accumulates at the tops of closed magnetic loops?) Aside: enhanced UV wind absorption at disk-on viewing angles rather than pole-on viewing angles Complementary analyses: RRM, RFHD

  16. 2-D MHD simulation ofq1Ori C: density courtesy A. ud-Doula

  17. 2-D MHD simulation ofq1Ori C: temperature courtesy A. ud-Doula

  18. 2-D MHD simulation ofq1 Ori C: speed courtesy A. ud-Doula

  19. Predictions from MHD simulations (and original analysis of Babel and Montmerle): Strong shocks – plasma very hot (few 107 K) Post-shock plasma moving quite slowly (Doppler broadening of X-ray emission lines should be quite modest – will there be a dependence on viewing angle?) Bulk of hot plasma is in the closed field region (< Alfven radius; h(r) < 1)

  20. Differential emission measure (temperature distribution) MHD simulation of 1 Ori C reproduces the observed differential emission measure Wojdowski & Schulz (2005)

  21. z Pup(O4 If) Line profiles: resolved, but narrow q1 Ori C: Ne X Ly-alpha

  22. Distribution of X-ray line widths in q1 Ori C cooler lines: broad (LDI wind shocks) hotter lines: narrow, but resolved Gagné et al. (2005)

  23. EM per unit volume (1110 ks) 5 z-axis (stellar radii) 0 -5 -5 0 5 x-axis (stellar radii) Line profile (1110 ks) – tilt: 0 deg Line profile (1110 ks) – tilt: 90 deg 1x1055 1x1055 8x1054 8x1054 6x1054 6x1054 EM (cm-3) EM (cm-3) 4x1054 4x1054 2x1054 2x1054 0 0 -500 0 500 -500 0 500 Line-of-Sight Velocity (km/s) Line-of-Sight Velocity (km/s) MHD sims: HWHM ~ 200 km/s No viewing angle dependence

  24. There’s one more powerful x-ray spectral diagnostic that can provide useful information to test the wind-shock scenario: Certain x-ray line ratios provide information about the location of the x-ray emitting plasma Distance from the star via the line ratio’s sensitivity of helium-like f/i ratios to the local UV radiation field

  25. Helium-like ions (e.g. O+6, Ne+8, Mg+10, Si+12, S+14) – schematic energy level diagram 1s2p 1P 10-20 eV 1s2p 3P 1s2s 3S resonance (r) forbidden (f) 1-2 keV intercombination (i) g.s. 1s21S

  26. The upper level of the forbidden line is very long lived – metastable (the transition is dipole-forbidden) 1s2p 1P 10-20 eV 1s2p 3P 1s2s 3S resonance (r) forbidden (f) 1-2 keV intercombination (i) g.s. 1s21S

  27. While an electron is sitting in the metastable 3S level, an ultraviolet photon from the star’s photosphere can excite it to the 3P level – this decreases the intensity of the forbidden line and increases the intensity of the intercombination line. 1s2p 1P 1s2p 3P UV 1s2s 3S resonance (r) forbidden (f) intercombination (i) g.s. 1s21S

  28. The f/i ratio is thus a diagnostic of the strength of the local UV radiation field. 1s2p 1P 1s2p 3P UV 1s2s 3S resonance (r) forbidden (f) intercombination (i) g.s. 1s21S

  29. If you know the UV intensity emitted from the star’s surface, it thus becomes a diagnostic of the distance that the x-ray emitting plasma is from the star’s surface. 1s2p 1P 1s2p 3P UV 1s2s 3S resonance (r) forbidden (f) intercombination (i) g.s. 1s21S

  30. Model of f/i ratio dependence on dilution factor (radius)

  31. helium-like magnesiumMg XIin q1 Ori C R I F Single source radius assumed Data constrain: 1.0 < Rfir < 2.1 R*

  32. Rfir=2.1 R* Rfir=1.2 R* Rfir=4.0 R*

  33. He-like f/i ratios have the potential for discriminating MCWS from wind-wind sources – close to photosphere in the former case, not so much in the latter

  34. MHD with rotation revealed the potential for breakout-driven magnetic reconnection …source of x-ray flaring in s Ori E (B2Vp)?

  35. MHD simulation of MCWS: higher magnetic confinement and rapid rotation Note, though: confinement parameter of s Ori E is much higher than in this MHD simulation

  36. temperature

  37. Not much emission measure at very high temperatures from the reconnection (but maybe with larger confinement parameter?)

  38. Another application: slowly rotating magnetic B star with a more complex field – t Scorpii (B0.2 V) Donati et al. (2006) f/i ratios imply location of hot plasma between 2 and 3 R*… T ~ 20 MK – is there enough room in the closed field region for wind to accelerate to the required velocity? Rotational modulation? MHD? RFHD?

  39. Conclusions • Magnetic OB stars with strong, large-scale dipole fields have distinctive X-ray properties: • High X-ray luminosities • Hard emission • Narrow lines • Rotational modulation (if magnetic obliquity .ne. 0) • Specific, quantitative diagnostics for studying MCWS (but only some utility for identification)

  40. More… Smaller scale magnetic structures…may have different effects on X-rays Centrifugally driven breakout and reconnection? But X-rays may not be very sensitive to it

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