θ1 Ori C: Chandra spectral analysis
This big project involves the analysis of roughly 15 separate Chandra HETGS observations of the prototype magnetic O star, θ1 Ori C, and eventually the numerical modeling of its magnetosphere. The ultimate goal is to use the phase-resolved spectroscopy to constrain models of X-ray production and more globally, of the star's magnetospheric physics.
In some sense, this study is a follow-up to the original study of four HETGS observations, published by Gagné et al. in 2005.
The entire HETGS spectra: MEG and HEG
MEG
HEG
These are the 15 separate observations, coadded. For now, this is a quick-and-dirty extraction using TGCAT. They are sufficient for starting the preliminary analysis, but eventually we'll analyze carefully extracted separate, phase-depended observations as well as the co-added spectra.
The star and its X-ray emission
θ1 Ori C is a mid-O star, quite hot (although its apparent spectral type varies with rotational phase and viewing angle - the Gagné et al. paper has some background), and very young (less than a million years). The brightest star in the Trapezium it provides the majority of the UV photons that illuminate the Orion Nebula in the constellation Orion.
It was the first O star with a magnetic field directly detected, via the Zeeman effect, detected in polarized light measurements in spectral lines (so, via spectropolarimetry). The discovery papers are Donati et al. 1999 and Donati et al. 2002. The field is very stable in the ten years since its discovery, is consistent with a pure dipole a little more than a kilogauss in strength, and is tilted with respect to the rotation axis by about 45 degrees. Combined with the rotation axis's 45 degree inclination with respect to the line-of-sight from the Earth, this affords a wide range of viewing angles of the magnetic field over the course of one 15 day rotation period (see this movie of the rotation-phase dependent view of the magnetic field or this one, that also shows the density of circumstellar material trapped in the magnetosphere).
Such a large-scale magnetic field will channel and confine the ionized stellar wind driven off the surface of the O star via the radiation force - the momentum in the light of these very luminous stars. This magnetically confined circumstellar environment is referred to as a magnetosphere. The strong wind flows up the closed magnetic loop's footpoints collide at the top of the loop, shock heating the wind plasma to many millions of degrees (see these simulation movies showing density and temperature made by our colleague Asif ud-Doula). This hot gas radiates X-rays as it cools, and these X-rays provide potentially useful diagnostics of the shock physics, the wind properties, and the dynamics of the magnetosphere. We have recently published an extensive hydrodynamical study of radiatively cooling magnetically confined wind shocks in O stars (see ud-Doula et al. 2014).
Because θ1 Ori C is the prototype of magnetic O stars, because its magnetic field is well characterized and relatively simple, and because it is so X-ray bright, analyzing and modeling its X-rays provides an important opportunity to test our understanding of this entire class of magnetic massive stars and their X-ray emission.
The X-ray spectrum and X-ray spectral modeling
The spectra shown above were recorded by the ACIS detector aboard Chandra using the HETGS (High Energy Transmission Grating Spectrometer) which has two separate grating arrays - the Medium Energy Grating and the High Energy Grating - producing two separate spectra for each pointing. The Proposers' Observatory Guide is an excellent resource for information about the telescope and its instruments. It is the source of the various topical links in this paragraph.
We will mostly be using the XSPEC software environment for modeling and analyzing our Chandra spectral data. The manual is informative, and has information about spectral models and about the techniques and philosophy of model-fitting in addition to have explanations of various commands and command syntax.
Presentation for Paris MiMeS meeting
I presented some of our preliminary results - via Skype - at the Magnetism in Massive Stars meeting, in Paris, on Wed., July 23 [pdf].
last modified: 28 July 2014