Transcript hotstar_xrays

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 t Sco.
I have observed the unusual hot star t Scorpii (t 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 t Sco -- right in the middle of Scorpius the
scorpion -- as seen at 10 PM on a June evening from Swarthmore.
t 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.
Solar-type magnetic heating?
Or massive stellar wind shock heating?
ROSAT (1993), resolution E/DE ~ 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 t Sco.
ASCA (1997) resolution E/DE ~ 40 - Some of the strongest line complexes
are just visible above the pseudo-continuum of blended weak lines.
Chandra (2000) resolution E/DE ~ 800 - We can now see numerous emission
lines; and not just see them, but resolve them, measuring their intrinsic widths.
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 t 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=cDl/lov) 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 t 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.
Coronal
Wind-shock
Instrumental broadening
s/v = 0, 0.1, 0.3, 0.5
lx
t*=0.01
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