May 17, 2021 Masarykova Univerzita, Brno, CZ, Astronomický seminář
Institute of Astronomy and Astrophysics
Heavy metals in hot white dwarf stars
Klaus Werner
Contents
Introduction: Astrophysical significance of element abundance
measurements in stars
Stellar evolution: neutron-capture nucleosynthesis
Measured element abundances; comparison with predictions of stellar
evolution models
Atomic spectroscopy of laboratory plasmas
Summary
Introduction
Chemical evolution of the Universe is driven by nucleosynthesis of
chemical elements in stars
Evolved stars return a significant fraction of their mass (up to 95%) to
the interstellar matter (stellar winds, supernova explosions)
This material is enriched with heavy elements produced in the stellar
interior and dredged up to the surface by convective motions
For quantitative modeling of Galactic chemical evolution we must
know: The stellar yields of chemical elements, i.e., how much
elements are produced by which stars?
Metal yields are computed with stellar evolution models, but
uncertainties in numerical modeling
Biggest problem: Mixing processes (convection) and some nuclear
reaction rates
Only solution: Compare surface abundances, predicted by evolution
models, with observations, i.e.:
Quantitative spectroscopy is the only possibility to “calibrate“ stellar
models
wikipedia
The “standard“: element abundances in the Sun
wikipedia
light metals | heavy metals →
The “standard“: element abundances in the Sun
wikipedia
light metals | heavy metals →
Fe group
(Sc-Ni)
trans-Fe elements →
The “standard“: element abundances in the Sun
Red giant star: interior structure
+CO core material
(dredged up)
from Lattanzio (2003),
modified
All stars < 8 solar masses end their life with cessation of H and He
burning, becoming white dwarfs.
wikipedia
Heavy elements in stars produced by
• nuclear fusion (up to iron)
• neutron-captures on heavy nuclei
n-captures in red giants: “s-process”
s = slow, i.e., time between n-captures long compared to half-life for beta-decay
nuclide map detail Ag-Sb
protons
neutrons
s-process in red giants
Main neutron source is reaction starting from 12C nuclei (from He-burning
shell):
12C(p,)13N(β+ν)13C(α,n)16O protons mixed down from H envelope
depth
H-burning
He-burning
Lattanzio 1998
Products of s-process nucleosynthesis in intershell region are not
directly observable; hidden below massive, convective hydrogen-rich
stellar envelope
Dredge-up (convection) of s-processed material to the surface of red
giants; spectroscopically detectable
But: difficult interpretation, because additional burning and mixing
processes in the convective H envelope blurr the picture
Fortunately: Nature, in some cases, allows a direct view onto the
processed material: hydrogen-deficient (pre-) white dwarfs have lost
their hydrogen-envelope
Hydrogen-deficient (pre-) white dwarfs
Ca. 20% of all (pre-) white dwarfs are free of hydrogen
Atmospheres dominated by He=60%, C=30%, O=10% (mass fractions)
= chemistry of material between H and He burning shells in red giants
(intershell abundances)
Origin: these stars were already white dwarfs, but re-ignite He-fusion,
“helium-shell flash”, “born-again” stars
Consequence: flash-induced
envelope convection
H is ingested and burned,
He-rich intershell material
lifted up
Measurement of element abundances by quantitative
spectroscopic analysis
Abundances of main atmospheric constituents (He, C, O) can be
determined from optical spectra
Heavy elements only accessible with ultraviolet spectroscopy (Hubble
and FUSE Space Telescopes)
Model atmospheres: plane-parallel, hydrostatic, radiative equilibrium,
non-local thermodynamic equilibrium
Fluorine (19F)
Interesting element, origin is unclear: formed by nucleosynthesis in
red giants stars or massive stars? Or by neutrino spallation of 20Ne
in supernovae?
Interesting to know intershell abundance of F, use H-deficient stars as
“probes”
Discovery of F V and F VI lines
fluorine
overabundant by
factor 200!
(Werner et al. 2005)
Trans-iron elements
Low-mass stars have produced ~50% of all elements heavier than
iron in our Galaxy.
Produced by neutron captures (s process)
Large overabundances expected in hydrogen-deficient (pre-) white
dwarfs
Would be interesting to find these elements, and to compare their
abundances with nucleosynthesis models
Since 2012: discovery of 18 trans-iron elements in the heliumdominated
white dwarf RE 0503-289
• FUSE space telescope observed UV spectra in 2000/2001
-- Large number of unidentified spectral lines; not seen in any
other white dwarf star
-- Problem unsolved for a decade
Identification of krypton und xenon
• 20 lines detected from Kr VI - VII and Xe VI - VII
• Abundance determination possible, because atomic data
available (energy levels, f-values [oscillator strengths])
• For atomic models: all f-values required (not only those for
observed lines)
Werner, Rauch, Ringat, Kruk (2012)
Arsenic!
In a dead star!!
Identification of 18 heavy metals, highly ionised
21 | K. Werner EUROWD14 Montréal
Trans-Fe elements
in RE0503-289
Relevant ionisation stages: V – VII
Problem: lack of atomic data. The luckier cases are ions, for which
energy levels are known: can compute line positions and f-values
One of these lucky cases: zinc
> 2000 f-values computed
(relativistic Hartree-Fock approach; Cowan 1981)
→ (almost) all the >100 Zn lines in RE 0503 can be matched
Rauch, Werner, Quinet, Kruk (2014)
Similar work was done for copper, gallium, germanium, selenium,
krypton, strontium, zirconium, molybdenum, tellurium, iodine, xenon,
and barium
(Rauch et al. 2014-20)
Abundance analysis of trans – Fe elements
2014
strongest zinc lines
in RE0503-289
Trans-iron element abundances in RE 0503-289: current state
• For other species (Cd, Ir, …): even energy levels unavailable
• Badly needed:
laboratory measurements of line positions → level energies
mass fractions ~10-5 – 10-3
up to 100,000 times solar
Open question:
• Why are trans-iron elements so abundant?
Could be result of:
• either s-process nucleosynthesis in red-giant phase
• or radiative pressure (“metal clouds“)
• or both
26 | K. Werner EUROWD14 Montréal
Trans-Fe elements
in RE0503-289
s-process path through 99Tc
From: Norbert Langer “Leben und Sterben der Sterne”
Search for technetium in RE0503-289
• Tc is a key element to decide whether s-process played an
important role to shape the abundance pattern
• Only unstable isotopes, hence, any Tc in the WD must have been
produced during previous red giant phase
(a milestone: discovery of Tc in red giants, Merrill 1952)
• Half-live of 99Tc is 210,000 yrs
• Post-red giant age of RE0503 is 650,000 yrs, i.e. ~ 3 half-lives
• Tc could still be present in the atmosphere
• Problem: Atomic data completely lacking for Tc III and higher ions
Search for technetium
• 1st step:
Quantum mechanical computation of energy levels and f-values
Line positions uncertain, no identification possible, but: we can
check, at what abundance level Tc has detectable lines
• 2nd step:
Observe laboratory spectra of Tc and derive energy levels
Example: computed atomic data for Tc V
Werner, Rauch, Kučas, Kruk (2015)
level energies line positions and gf-values
Tc VI lines in RE0503 model (Tc = 10-4 mass fraction)
In preparation: Laboratory spectroscopy of technetium
• Electron Beam Ion Traps (EBIT) facility at MPI for Nuclear Physics
in Heidelberg to produce Tc plasma (only minute quantities of Tc required,
some 10-12 g; radioactive!)
• 3m UV spectrograph attached (lent from BESSY, Berlin)
• MCP detector from Tübingen (flight spare of ORFEUS space
telescope)
• Measurements ongoing
Electron Beam Ion Trap (EBIT)
Summary:
Element abundances in hot H-deficient white dwarfs
Stellar atmospheres mainly composed of He,C,O: Ashes of H- and
He-burning, mixed up by final He-shell ignition
We indeed see the direct outcome of nucleosynthesis that was at
work in previous phases of stellar evolution (red giant).
The observed element abundances are hard tests for stellar
models and predicted metal yields.
Light metals (up to iron): Abundances in accordance with models
Heavy metals (trans-iron elements): new territory. Atomic data lacking.
Laboratory plasma spectroscopy in preparation.
May 17, 2021 Masarykova Univerzita, Brno, CZ, Astronomický seminář
Heavy metals (trans-iron elements): new territory. Atomic data lacking.
Laboratory plasma spectroscopy in preparation.
Díky za pozornost !
May 17, 2021 Masarykova Univerzita, Brno, CZ, Astronomický seminář