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Henrik Hartman, Lund University, Sweden
Current Development in Atomic, Molecular and Optical Physics, Delhi 2011
Fluorescence and resonant ionization in Eta Carinae - an astrophysical laboratory
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Laboratory Astrophysics @ Lund University
- Stellar high resolution spectroscopy (VLT; HST) - High-resolution FTS measurements of dishcarges (Edlén lab) - Lifetime measurements: LIF on laser produced plasmas (LLC)
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Collaborators
Lund/Malmö: H.Nilsson, H.Lundberg, L.Engström, S.Huldt, T.Lennartsson, T.Brage, P.Jönsson, N.Ryde, (S.Johansson & V.Letokhov) Eta Carinae: T.Gull (NASA), M.Bautista, V.Fivet (U Mich), J.Groh(MPIfI) Lifetimes: E.Biemont group (th, Liege), S.Mannervik group (exp, Stockholm) Thanks to: CDAMOP2011 organizers Swedish Science Council (VR), Swedish National Space Board (SNSB) Johansson & Letokhov (deceased 2009)
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Outline
- Introduction, Eta Carinae
- Fluorescence
- Selective ionization (RETPI – Resonance Enhanced Two-Photon Ionization) for SiIII and FeIII
- Outlook
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Eta Carinae
HST / WFPC2 image
Homunculus
Central stars
Strontium filament
10” 1”
Weigelt blobs
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5.54 year periodicity observed in X-ray
RXTE-obsverations, courtesy of M.Corcoran.
K. Nielsen
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Eta Carinae
Maximum state – the radiation from the central star is driving the photoprocesses in the Weigelt condensation.
Minimum state – the radiation from the central star is blocked for several months. The transition into the minimum is referred to as the spectroscopic event.
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The Bowen mechanism (fluorescence)
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Fluorescence lines in HST spectra of WB
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Temporal variations of HI and FeII lines
The temporal variations during the spectroscopic event is different for different kinds of lines. Hydrogen lines and fluroescent lines vary quicker, whereas the (most) forbidden lines excited by collisions have a much slower response set by the recombination time scale, i.e. removal of electrons.
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Variations of SiIII] 1892 Å line
Spatial scale maximum
minimum
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Variations of SiIII] 1892 Å line
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RETPI of Si+, enhancing the 1892Å SiIII] line
The 2-photon process involving HLyα and HLyγ leaves the atom doubly ionized and in an excited state, producing the 1892 Å radiation as it decays.
Johansson & Letokhov, Science (2001) Johansson, Hartman, Letokhov A&A (2006)
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Estimation of the RETPI rate in the Weigelt blobs
The 2-photon process involving HLyα and HLyγ depends on the difference between the real level and the virtual level, and in the intensity of the H Lyman lines. With estimates of the conditions in the WB, the observed intensity of the 1892 Å feature, 7x10-12 erg cm-2 s-1 Å-1, can be explained buy the RETPI process.
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Ionization balance in the Weigelt blobs
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Possibilities for RETPI producing [FeIII]
The double H Lyα energy corresponds to the difference between the 3d5(6S) 4s 5S in FeIII and 3d6(5D) 4s 4D in FeII, with the 5p levels in between.
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Conclusion and outlook • In the Weigelt blobs of Eta Carinae (as in many other astrophysical plasmas),
fluorescence is an important process for line formation. The temporal variations in the incident radiation from the central star allow us to study these processes separate from the collisional excitation and ionization.
• The same HST spectra indicate that RETPI (Resonance Enhanced Two-Photon Ionization) can be responsible for the formation of strongly varying lines. During the high state (maximum), the RETPI process can control the ionization equilibrium for many ions.
• RETPI often leaves the ions in an excited state. With the RETPI producing strong forbidden lines, e.g. Si III] and [FeIII], care must be taken when using simple line rations for determination of physical conditions.
• Forbidden lines are often strong and used for diagnostics in emission line objects like stellar winds, AGNs and Quasars. Eta Carinae is an ideal and rare object to study these processes, but the importance of these processes are probably much wider.
