Transcript E/DE

Introductory Summer Research Group Meeting
Spectroscopy and X-rays
28 May 2001
Members of the group:
David Cohen
Allison Adelman
David Conners
Carie Cardamone
Elliot Reed
Rachel Sapiro
and
Matti Klock
Geneviève de Messières
Kate Penrose
Project Summaries
Allison – ROSAT X-ray survey of O stars
David – Inertial fusion modeling, ultraviolet spectroscopy of hot
star winds
Carie – spectral analysis of Chandra spectrum of t Sco and
XMM spectrum of e CMa
Elliot – spectral analysis of Chandra spectrum of t Sco
Rachel – line ratio analysis and modeling of UV spectral data
from SSX (Michael Brown’s experiment), hydrodynamical
and spectral modeling of gas cell photoionization
experiments on the Z-machine at Sandia National Lab
Matti – hydrodynamical modeling of table-top laser ablation
experiments both in the Moscatelli lab and at Sandia
National Lab
contact information
David: [email protected]
Allison: [email protected]
David: [email protected]
Carie: [email protected]
Elliot: [email protected]
Rachel: [email protected]
Matti: [email protected]
Geneviève: [email protected]
Kate: [email protected]
Geneviève (Chandra) and Kate (fusion) are at home this summer, but are
both continuing to work part-time on their projects -- those of you whose
work overlaps with theirs should feel free to get in touch with them if you
have any questions
Our individual projects are part of a bigger pursuit
Understanding the interactions of very strong electromagnetic
radiation fields (i.e. light) with matter. Contexts are (1) hot star
winds, (2) laser-produced fusion plasmas, (3) X-ray photoionized
plasmas.
We want to use spectroscopy to determine the properties of (i.e.
diagnose) experimental and astrophysical systems. In practice,
this means determining temperatures, ionization states, densities,
velocities, incident radiation field spectra, …
This is an intermediate step in gaining a global understanding of
the physical systems in question.
The physical systems we’re trying to understand are
• (1) stellar winds (supersonic outflows from the surfaces of
stars)
• (2) X-ray binaries/X-ray nebulae (gas clouds surrounding
black holes and neutron stars)
• (3) laser fusion experiments
On the stellar wind side, the big specific question we’re trying to
answer is:
How do hot stars make their X-rays?
magnetic/coronal mechanism vs. wind shocks
Solar-type magnetic heating?
Or massive stellar wind shock heating?
High intensity light invariably means that things get hot…
Hot things (above about half-a-million degrees) means X-rays
(remember, there’s a relationship between color/wavelength and temperature)
Spectroscopy provides information, through its dependence on
atomic states and the Doppler shift, about the microphysics, as
well as bulk properties, of gases (plasmas, if they’re ionized).
A spectrum carries a huge amount of information:
“If a picture is worth a thousand words, then a spectrum is
worth a thousand pictures.” -- anon.
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.
Spectral resolution,
defined:

E


 E
Where  is the width of the narrowest feature
detectable, and  is the wavelength of that feature.
Thus, the resolution is a (rough) measure of the total
number of independent “channels” or pieces of
information in a spectrum.
ROSAT (1993), resolution E/E ~ 2 - With only a few independent pieces of
information, the shape of this smooth-looking spectrum tells us only about
the relative amount of ‘hard’ to ‘soft’ X-rays (i.e. those with energies above
0.5 keV to those with energies below this level).
ASCA (1997) resolution E/E ~ 40 - Some of the strongest line
complexes are just visible above the pseudo-continuum of blended
weak lines. The number of independent elements of information is
just not great enough to distinguish individual emission lines.
Chandra (2000) resolution E/E ~ 800 - We can now see numerous emission
lines; and not just see them, but resolve them, measuring their intrinsic widths.
Background
Astronomers measure wavelength, , in Angstrom Units (Å)
Remember, frequency, n, and wavelength of any wave phenomenon are related
by:
n=c
Where c is the characteristic wave velocity (speed of light for light, duh…)
Frequency is directly proportional to photon energy: E=hn
And inversely proportional to wavelength: E=hc/
When we talk about single photons, the unit of energy is the electron volt (eV)
12.4 Å = 1000 eV or 1 keV; X-rays are taken to be any photons with energies
greater than 100 eV (0.1 keV) and wavelengths, therefore, below 124 Å
Note that since temperature is proportional to energy, we can relate light of a
given wavelength to a characteristic temperature through Boltzmann’s constant,
k:
E=kT
Where 1 keV corresponds to 11.6 million degrees
The photons of different wavelength, that together constitute a
spectrum, are produced by a handful of specific atomic processes
Much of the power of spectroscopy comes from this macro-micro
connection (observed spectrum vs. movement of electrons inside
atoms) -- What is an atom? To us, it is a central electrostatic
potential plus a bunch of quantum mechanically determined bound
energy levels and a “continuum” of unbound states. Think potential
energy:
an atom
continuum
energy
is like a staircase
ground state
In both these cases, energy is released
Let’s consider some specific atomic processes and see which ones
lead to either the production of photons (emission) or the
destruction of photons (absorption)
key
previous position of an electron
current position of an electron
movement of an electron
g
photon
movement of a photon
e
external electron
simplified 2-level atom
(plus continuum)
key
1. Radiative excitation
previous position of an electron
current position of an electron
movement of an electron
g
photon
movement of a photon
e
external electron
•Photon must be of exactly the right
energy
•Photon is destroyed (absorbed)
g
2. Radiative de-excitation (“spontaneous emission”)
key
previous position of an electron
current position of an electron
movement of an electron
g
photon
movement of a photon
e
external electron
•Photon energy reflects energy
difference of electron bound states
•Photon is produced (emitted)
g
3. Collisional excitation
key
previous position of an electron
current position of an electron
movement of an electron
g
photon
movement of a photon
e
external electron
•Free electron need only have more
energy than the energy difference
between states
•Free electron is not destroyed, but
loses some kinetic energy (note
potential for cooling…)
e
e
4. Collisional de-excitation
key
previous position of an electron
current position of an electron
movement of an electron
g
photon
movement of a photon
e
external electron
•Free electron is not destroyed, but
gains some kinetic energy
e
e
Collisional excitation followed by spontaneous (radiative) emission
is the most important process in X-ray-emitting cosmic plasmas
It is a cooling process (taking kinetic energy from free
electrons -- i.e. heat) and converting it to photons, which
can escape the plasma (and bring information to our
telescopes about the conditions in the emitting plasma).
g
e
e
These basic atomic processes lead to a wealth of
phenomena in X-ray spectra.
An X-ray spectrum reflects the physical properties of the
source (the electron density and temperature, radiation flux
and temperature, bulk motions).
These properties can be used to discriminate among
different physical models of a laboratory or astrophysical
object.
The following slides are supplemental – read at your
own risk
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.
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, or metastable, 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=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 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
1
3
5
-1
0
1
10
-1
0
1
x