The spectral resolution of x-ray telescopes has improved many hundred-fold over the past decade, enabling us to detect and resolve emission lines in hot stars like Sco. I have observed the unusual hot star Scorpii ( Sco, for short) with a succession of X-ray telescopes over the past 7 years. The series of X-ray spectra presented here is a good demonstration of the improvement in the quality of astrophysical X-ray data overall (not just for hot stars) in the past decade, and gives an indication of the richness of the information provided by the new generation of X-ray telescopes.
Here’s a view of Sco -- right in the middle of Scorpius the scorpion -- as seen at 10 PM on a June evening from Swarthmore. Sco is a B0 V star -- with a surface temperature of about 30,000 K (5 times the sun’s temperature), and with about 50,000 times the sun’s luminosity. It has unusual ultraviolet absorption lines, a very low projected rotational velocity, and -- perhaps most relevant -- seems to be very young.
The context of X-ray spectral observations of hot stars Hot stars are thought not to have outer convection zones, magnetic fields, or the associated magnetic dynamo and corona that our sun has. Thus their discovery 20 years ago as relatively strong soft X-ray sources was a surprise. Hot stars do have strong radiation-driven winds. These winds are subject to a line-driving instability which can lead to shock heating of the wind plasma. Although this mechanism has been assumed to produce the observed X-rays, the numerical simulations do not do a very good job of quantitatively reproducing the observed X-rays. The new high-resolution X-ray spectra of hot stars holds out the potential to discriminate between these two general theories of stellar X-ray emission; or possibly to inspire the development of a new theory.
ROSAT (1993), resolution E/E ~ 2 - The shape of this smooth-looking spectrum reflects the sensitivity of the detector; we were able to fit two plasma temperatures to this low-resolution data, but knew that the spectrum was full of emission lines that contained a lot of quantitative information about the large amounts of hot gas on Sco.
energy 1s2p 1P 1s2p 3P 1s2s 3S R I F 1s2 1S A partial energy level diagram for helium-like ions, such as Si +12 (see the FIR panel inset on the Chandra spectrum figure). The resonance transition (R) is strongest, but the intercombination (I) and forbidden (F) lines can also be strong. Electrons in the longlived 3S state can be collisionally or radiatively excited to the 3P level (transition energy of order 10 eV), making the F/I ratio a good diagnostic of density and radiation field (effectively distance from the star). The forbidden line of the silicon feature in the Sco spectrum is strong, indicating that the X-ray emitting plasma is both far from the star and low density…but how far and how low?
Line profiles are affected by the hot plasma’s spatial and velocity distribution, as well as the degree of attenuation by an overlying cold stellar wind. In the next panel I show synthetic line profiles for a family of coronal models (left) and wind-shock models (right). Line profiles as a function of the scaled wind velocity (x=c/ov) are shown for different instrumental resolutions in each sub-panel. The panels have wind attenuation increasing downward. Note that as the wind attenuation increases less and less of the red side of the profiles are seen. This is because the redshifted wind necessarily comes from the back side of the star, which suffers the most absorption. The coronal models assume an X-ray source that is strongly concentrated at the base of a slowly accelerating wind, while the wind-shock models assume that the source is distributed within the supersonic outflow beyond some minimum radius. The narrow features seen in Sco would seem to point to a mostly coronal origin to these X-rays; but the strong forbidden lines indicate that the hot plasma is not very close to the surface of the star.