Transcript PowerPoint - Astronomy at Swarthmore College

The Young Magnetic O Star
1

Ori C:
Multi-phase Chandra High-Resolution Grating Spectra
Mary Oksala, Marc Gagné (West Chester University),
David Cohen, Stephanie Tonnesen (Swarthmore College),
Asif ud-Doula (North Caroline State University),
Stanley Owocki (Bartol Research Institute),
Joseph MacFarlane (Prism Computational Sciences)
ABSTRACT
Chandra High-Energy Grating spectra obtained at four rotational phases of the oblique magnetic
rotator, 1 Ori C (O6 V), corresponding to four different viewing angles with respect to the
magnetic axis, are used to constrain the temperature, spatial location, and kinematics of the hot
plasma on this very young hot star with a strong (1100 G) dipole field. The plasma is moving, but
only at speeds of a few 100 km s-1, much slower than the terminal wind velocity. It is close to the
star (within 1.8 R* of the surface) and hot (peak temperature ~30 MK). We analyze these
diagnostics in conjunction with new MHD simulations of the magnetically channeled wind shock
mechanism on 1 Ori C. This model fits all the data surprisingly well, reproducing the very high
temperatures, relatively narrow and unshifted lines, and the near-star source location.
Figure 1- First-order MEG spectra of 1 Ori C obtained at viewing
angles α=4, α=40, α=80, and α=87 (the viewing geometry is shown
in Figure 3). The bottom panel shows the combined MEG
spectrum from all four observations. HEG and MEG spectra of 1
Ori C show strong, narrow emission lines and a strong 2-15 Å
brehmsstrahlung continuum. Variable-abundance, multitemperature APEC model fits to the MEG spectra indicate that
most of the plasma is hotter than 10 MK, with a peak in the
emission- measure distribution at log T = 7.5.
Introduction
The brightest star in the Trapezium and primary source of ionization radiation in the Orion nebula,
the magnetic star 1 Ori C is very young (<400,000 yr). It has a relatively strong wind (dM/dt 
10-7 M, v  2500 km s-1) in addition to a strong dipole field (Donati et al. 2002). Its strong,
hard, variable X-ray emission suggests it may be the prototype of a new class of stellar X-ray
source: an oblique magnetic rotator with magnetically confined wind shock (Babel & Montmerle
1997). We have calculated 2-D MHD models for 1 Ori C, extending the work of ud-Doula &
Owocki (2002) to include radiative and adiabatic cooling.
ray torus is occulted by the star (magnetic equator-on).
Equator-on =90°
Figure 2 - To examine wavelength-dependent time variability, the
four MEG spectra in Fig. 1 were binned in 1A bins and divided by
the binned average spectrum. The emission at most wavelengths is
strongest at alpha=4 and alpha=40 (see Fig. 3). X-ray emission is
weaker by approximately 30% at alpha=87 with an apparent
increase in absorption with increasing wavelength. This pattern is
consistent with an X-ray emitting torus above the magnetic
equator, partially occulted by the star at alpha=80 and 87. The
increased absorption from 5-20A at alpha=87 may be due to
higher column density along the equator’s line of sight. MHD
models predict that the distorted wind and magnetic field will
funnel some wind material out the magnetic equator.
Figure 4 - Phase-folded light curves of 1 Ori C. Open circles
represent the excess C IV 1548, 1551 Å equivalent width (left
axis) taken from Walborn & Nichols (1994) and phased to the
ephemeris of Stahl et al. (1996) with period 15.422 days and
MJD0 = 48832.50. Maximum C IV absorption occurs near phase
0.0 (alpha=0deg). Filled circles represent the longitudinal
magnetic field strength (right axis) from Donati et al. (2002)
phased to the Stahl et al. (1996) ephemeris. Bl is maximum near
phase 0.0 when the star is viewed pole-on. The solid line
corresponds to the equivalent width of H from Stahl et al.
(1996). Phase 0.0 corresponds to H-emission maximum. The
X-ray emission maximum (see top panel of Fig. 2) occurs near
phase 0.0. Since the X-ray emission is not significantly
absorbed, X-ray and H-alpha maxima occur when the entire Xtorus above the magnetic equator is visible (low viewing
angles). X-ray and H-alpha minima occur when part of the X-
Pole-on =0°
Figure 3 - Geometry of the O6 V star 1 Ori C where the magnetic dipole is B
and the rotation axis is omega. 1 Ori C has a measured longitudinal
magnetic field which varies from 360 G at viewing angle φ=0 (X-ray
maximum) to ~0 G at viewing angle φ =90 (X-ray minimum). The
corresponding dipole field strength is 1100 G (Donati et al. 2002). The
inclination of the system is i=45° and the obliquity of the dipole is β=42°. As
a result, the viewing angle between the line of sight and the magnetic pole
varies between alpha=0 and alpha=90 during its 15.422 day period (Stahl et
al. 1996). Chandra HETG spectra were obtained at viewing angles α=40˚,
α=80˚, α=4˚, and α=87˚ (phases 0.84, 0.38, 0.01, and 0.47). X-ray maximum
corresponds to phase 0.0 (magnetic pole-on) and x-ray minimum corresponds
to phase 0.5 (magnetic equator-on).
Figure 5 - First-order HEG He-like line profiles for Ar XVII, S XV, Si XIII,
and Mg XI (solid histograms) . Show are the resonant (left), intercombination
(middle) and forbidden (right) lines. In hot stars, the forbidden-line strength
of He-like ions is reduced by photoexcitation of electrons in the 3S1 state by
photospheric UV radiation. We do not detect the Mg XI f line. The Si XIII f
line is blended with H-like Mg XII, and the Ar XVII f line is blended with Hlike S XVI. The Ca XIX and Fe XXV r and i lines are blended, but the f lines
are resolved. Hence, we limit our discussion to the reliably measured f/i ratio
of S XV and the upper limit for Mg XI. For S XV and Mg XI photoexcitation
occurs at 743 and 1036 A, respectively. For the 743 A flux, we customcomputed non-LTE models of the excitation kinematics of the relevant ions.
For the 1036 A flux, we used archival Copernicus data. This analysis
suggests that, on average, the X-ray emitting plasma is located 1.5-1.8 R
above the photosphere. At this radius, the X-ray emitting torus would be 30%
occulted around alpha=90deg, consistent with observation.
Figure 6 - Line widths for the strongest lines in the Chandra spectra
plotted against the temperature of peak line emissivity, taken from
APED. The open circles represent the Doppler width measured by
SHERPA. The filled diamonds are the rms velocity as measured by
ISIS. The mean rms velocity and standard deviations are indicated
by the horizontal lines. Note that two of the OVIII and Fe XVII
lines formed in the coolest plasma are significantly broader than the
mean and may be formed by shocks in the far wind. The hotter Xray the lines are narrow: much narrower than the terminal wind
speed. The f/i ratios and the hot line widths suggest strongly that
the X-rays are produced in a confined plasma close to the
photosphere.
Conclusions
1 Ori C’s emission lines and f/i ratios show that most
of the very hot plasma is located within 1.8 R of the
photosphere. The amount of occultation and its phase
dependence suggest this plasma is concentrated near the
magnetic equator. The narrow emission lines show that
the hottest, densest plasma is moving at only 10 to 15 %
of the wind terminal velocity. MHD simulations of
magnetically confined wind shocks produce very hot
plasma (T  30 MK) and narrow emission lines. The
data analysis show, in conjunction with the MHD
simulations, that 1 Ori C is consistent with the general
picture of the MCWS model.