Agreement between X-ray data and magnetically channeled wind shock model of  1 Ori C David Cohen, Marc Gagné for Massive Star research group

context Gagné et al., 2005, ApJ, 628, 986 (and the erratum: 2005, ApJ, 634, 712) discusses four separate Chandra grating spectra, covering different rotational phases of the star. A coherent picture of magnetically channeled, shock heated magnetospheric plasma is presented, integrating several aspects of the X-ray data with UV, H- alpha, and magnetic field measurements. 2-D MHD modeling of MCWS (by Asif ud-Doula) is presented in this paper. The overall agreement between these models and the data is very good; and the detailed dynamical models confirm much of the qualitative physical picture of the magnetospheric properties.

outline Context: we would like individual annotated images showing the agreement between the x-ray data and the MCWS modeling, for group members to use in their presentations. The points of agreement include: phase-dependence of the x-ray flux line widths temperature distribution (DEM) forbidden-to-intercombination line ratios

Central 4 arcminutes of the Orion Nebula Cluster as seen by Chandra ACIS-I arrow pointing at  1 Ori C Red: < 1.5 keV Green: 1.5 keV < E < 2.5 keV Blue: > 2.5 keV Note: there is no diffuse x-ray emission; you’re just seeing the wings of the point spread function; and the black dot at the center of the source is an effect of pileup in the ACIS detector.

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

Predictions from the MCWS model and MHD simulations: Bulk of hot plasma is in the closed field region (< Alfven radius;  (r) < 1) ~2R * for  1 Ori C 7 Significant shock heating – plasma very hot (few 10 7 K) Post-shock plasma moving quite slowly Four separate diagnostics address these three predicted properties

The phase dependence of the x-ray flux constrains the spatial distribution of the x-ray emitting plasma in the magnetosphere. A deeper eclipse at edge-on viewing angles if the hot plasma is closer to the star.

Fortuitous access to all viewing angles of the magnetic field Cartoon showing viewing angles of θ 1 Ori C for Chandra observations in Gagné et al. (2005). 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) Gagné et al. (2005)

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

Simulation EM (10 56 cm -3 ) θ 1 Ori C ACIS-I count rate (s -1 ) Rotational phase (P= days) Chandra broadband count rate vs. rotational phase Model from MHD simulation

Note that the assumption in the last two figures is that the only thing that modulates the x-ray flux is occultation of the magnetosphere by the star itself. Fig. 14 and Table 6 in Gagné et al. (2005) indicate that there might be a modest excess of absorbing column when the magnetosphere is viewed edge-on.

The ratio of the intensity of the helium-like forbidden to intercombination lines also constrains the spatial distribution of the x-ray emitting plasma in the magnetosphere. A weaker forbidden line indicates the plasma is closer to the star, since the upper level of the forbidden line is depopulated by the local UV radiation field.

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

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

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

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

1s2s 3 S 1s2p 3 P 1s2p 1 P resonance (r) intercombination (i) forbidden (f) g.s. 1s2s 1 S 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. UV

Model of f/i ratio dependence on the radius (via the dilution factor) model constraints from data implied constraints on location

R I F helium-like magnesium Mg XI in  1 Ori C single source radius assumed data constrain: 1.0 R * < R fir < 2.1 R *

R fir =1.2 R * R fir =2.1 R * R fir =4 R *

Temperature distribution in the X-ray emitting plasma is predicted to be skewed toward high temperatures, due to the strong, head-on shocks near the magnetic equator. The temperature distribution (‘differential emission measure’) is derived from fitting thermal equilibrium models (e.g. APEC) to the dispersed spectrum.

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

Emission line widths trace the line-of-sight velocity distribution in the hot plasma. The magnetic confinement of the post-shock plasma predicts small, but not infinitely narrow, line widths.

 Pup (O4 If) Line profiles: resolved, but narrow  1 Ori C: Ne X Ly-alpha Normal O star, with a much broader emission line, for comparison.

Distribution of X-ray line widths in  1 Ori C hotter lines: narrow, but resolved cooler lines: broad (LDI wind shocks) Gagné et al. (2005) Note that the UV wind lines in  1 Ori C imply a wind terminal velocity in excess of 2500 km/s.

z-axis (stellar radii) x-axis (stellar radii) EM per unit volume (1110 ks) Line profile (1110 ks) – tilt: 0 deg Line-of-Sight Velocity (km/s) EM (cm -3 ) 0 2x x x x x10 55 Line profile (1110 ks) – tilt: 90 deg Line-of-Sight Velocity (km/s) EM (cm -3 ) 0 2x x x x x10 55 MHD sims: HWHM ~ 200 km/s No viewing angle dependence

The following slides demo the basic results from the MHD sims

temperature emission measure MHD simulations of magnetically channeled wind Channeled collision is close to head-on – at km s -1 : T = K simulations by A. ud-Doula; Gagné et al. (2005)

Emission measure contour encloses T > 10 6 K

MHD simulations show multi-10 6 K plasma, moving slowly, ~1R * above photosphere contour encloses T > 10 6 K

The following slides show the Chandra grating (MEG) data, with that of  Pup for comparison. The helium-like complexes of several ions are visible; The high temperatures are apparent in the line ratios (and strong continuum); The modest line widths are easily seen when compared to  Pup’s.

 Pup  1 Ori C Chandra grating spectra (R ~ 1000 ~ 300 km s -1 )  1 Ori C: hotter plasma, narrower emission lines  Pup (O4 I): cooler plasma, broad emission lines

 Pup  1 Ori C Si XIII Si XIV Mg XI Mg XII H-like/He-like ratio is temperature sensitive

 Pup  1 Ori C Si XIII Si XIV The young O star –  1 Ori C – is hotter Mg XI Mg XII