Charge Exchange Models for X-ray Spectroscopy with AtomDB v2.0
Randall Smith, Adam Foster & Nancy Brickhouse Smithsonian
Astrophysical Observatory Slide 2 Back to basics! The total
emissivity from a cubic centimeter is just given by the densities,
velocities, and cross sections of various processes involved:
Considering recombination only, we get: Building a CX model for
X-ray Astrophysics Dielectronic Recombination Radiative
RecombinationCharge Exchange Slide 3 In 1969, Allen & Dupree
pointed out that dielectronic recombination dominated collisional
ionization at high temperatures, although radiative recombination
picks up at low temperatures. (there are exceptions) Building a CX
model for X-ray Astrophysics Slide 4 Regardless of the exact value,
CX swamps other processes by factors of 100 1000 If neutral H or He
mixes with highly-ionized material, CX dominates (until the H, He
is ionized) Building a CX model for X-ray Astrophysics Slide 5
Question: Why recombination only? Answer: If electron excitation is
even remotely possible, it will tend to dominate the x-ray
emission. Building a CX model for X-ray Astrophysics Slide 6
Upshot: An astrophysical plasma dominated by CX will have: ionized
metals neutral hydrogen a rapid transition region Typical
circumstances might involve a stellar wind CXing into the
surrounding ISM, or shocked SNe ejecta (unshocked is cold) hitting
a molecular cloud. Building a CX model for X-ray Astrophysics Slide
7 Highly-ionized plasmas dont wait Smith & Hughes 2010 Consider
an ion Z +q created in a stellar wind or a supernova shock,
travelling at 300 km/s. At a minimum, it has q electrons nearby to
ensure charge neutrality. At a density of 0.1 cm -3 it will travel
Building a CX model for X-ray Astrophysics At T e > 410 4 K,
hydrogen is 99.9% ionized. At T e ~10 6 K, electrons outnumber
neutral hydrogen by more than a million to one. In thermal
equilibrium v e ~ 43 v H If the electrons are in a Boltzmann
distribution and the plasma is ionized, then and, dominating the
fact that : Slide 26 Are we Boltzmann yet? or What are the relevant
time constants? For CX, scaled to v=300 km/s, = 10 -15 cm 2 :
Electrons & protons reach a Boltzmann distribution and Hydrogen
ionizes in: (where kT e = 35 eV thermal equivalent of 100 km/s
velocity) Summers+ 2006 Slide 27 What about Do proper cross
sections matter? Not that much, unless you have detailed
information about the distribution of the ion and neutral
components.* We do know that the CX cross section is sufficiently
large that when ions and neutrals are able to CX, they will. If CX
can happen, it will. * Relative rates do matter Slide 28
Recombining plasmas are inefficient Creating a 0.654 keV O VIII
photon via: Electron excitation: requires 0.654 keV of electron
energy, O 7+ ion remains Recombination: requires 0 keV electron
energy, but destroys the O 8+ ion that required 0.87 keV to create.
But most importantly: Electron excitation and recombination can
co-exist in a plasma; Electron excitation and CX cannot.
