Agreement between X-ray data and magnetically channeled wind shock model of q1 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 q1 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 Wade et al. (2006) Magnetic field obliquity, ~ 45o, inclination, i ~ 45o
Predictions from the MCWS model and MHD simulations: Bulk of hot plasma is in the closed field region (< Alfven radius; h(r) < 1) ~2R* for 1 Ori C Significant shock heating – plasma very hot (few 107 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 θ1 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 1st order, the depth of eclipse depends on how close the shock-heated plasma is to the star
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
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.
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
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
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. 1s2s 1S
The f/iratio 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. 1s2s 1S
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. 1s2s 1S
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 magnesiumMg XIin q1 Ori C single source radius assumed data constrain: 1.0 R* < Rfir < 2.1 R*
Rfir=1.2 R* Rfir=2.1 R* Rfir=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.
z Pup(O4 If) Line profiles: resolved, but narrow q1 Ori C: Ne X Ly-alpha Normal O star, with a much broader emission line, for comparison.
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) Note that the UV wind lines in 1 Ori C imply a wind terminal velocity in excess of 2500 km/s.
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
MHD simulations of magnetically channeled wind temperature emission measure simulations by A. ud-Doula; Gagné et al. (2005) Channeled collision is close to head-on – at 1000+ km s-1 : T = 107+ K
Emission measure contour encloses T > 106 K
MHD simulations show multi-106 K plasma, moving slowly, ~1R* above photosphere contour encloses T > 106 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.
Chandra grating spectra (R ~ 1000 ~ 300 km s-1) 1 Ori C z Pup 1 Ori C: hotter plasma, narrower emission lines zPup (O4 I): cooler plasma, broad emission lines
1 Ori C z Pup H-like/He-like ratio is temperature sensitive Mg XII Mg XI Si XIV Si XIII
1 Ori C z Pup The young O star – 1 Ori C – is hotter Mg XII Mg XI Si XIV Si XIII