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Cosmic lasers and. Photon-correlation Spectroscopy. Claudio Germanà and Dainis Dravins INAF Observatory of Padua Lund Observatory. OUTLINE. 1. Laser Emission in astrophysical sources 2. Photon-Correlation Spectroscopy: Resolving narrow spectral lines
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Cosmic lasers and Photon-correlation Spectroscopy Claudio Germanà and Dainis Dravins INAF Observatory of Padua Lund Observatory
OUTLINE 1. Laser Emission in astrophysical sources 2. Photon-Correlation Spectroscopy: Resolving narrow spectral lines 3. Signal – to – Noise ratio
Thermal Equilibrium conditions Energy level populations described by Boltzmann’s statistics Medium acts as an absorber
NON-Equilibrium Population inversion Medium acts as an amplifier ”Light amplification by stimulated emission of radiation” LASER
Astrophysical laser emission Lasers may be observed if: 1) Population inversion is feasible 2) Pumping mechanism for population inversion 3) Structures allow amplification (e.g., clouds)
...laser emission might be observed in: Fe II and O I lines in η Carinae (Johansson & Letokhov 2004, 2005) Wolf-Rayet stars He II He I lines (Varshni & Nasser 1975,1986) Mass – loosing stars
S. Johansson & V.S. Letokhov Astrophysical lasers operating in optical Fe II lines in stellar ejecta of Eta Carinae Astron.Astrophys. 428, 497 (2004)
Model of a compact gas condensation near η Car with its Strömgren boundary between photoionized (H II) and neutral (H I) regions S. Johansson & V. S. Letokhov Laser Action in a Gas Condensation in the Vicinity of a Hot Star JETP Lett. 75, 495 (2002) = Pis’ma Zh.Eksp.Teor.Fiz. 75, 591 (2002)
at 9997 Å A microsecond “bottle-neck” creates a population inversion in the 3 → 2 transition of Fe II S. Johansson & V.S. Letokhov Astrophysical lasers and nonlinear optical effects in space New Astron. Rev. 51, 443 (2007)
...how to confirm Laser emission? Expected extremely narrow linewidth < 1 mÅ (0.1 pm) (Johansson & Letokhov 2004) by Dravins et al. 2007 Spectral resolution 100 million!!
What about a spectral line? Electric field emitted from one atom which undergoes collisions: E n(t)= E0 cos(ω0t + φn (t)) φn (t) is a Gaussian (chaotic process) Total electric field from the system of n atoms (Loudon 1973): a(t) is a Gaussian
... signal in Fourier’s notation... exp(iωt) Fourier component E(t)TOT thermal light a(t) ≠ cost (Gaussian) E(t)TOT laser light a(t) ≈ cost
...spectral line profile... a(t) ≈ cost a(t)≠ cost (Gaussian)
...FWHM and time scale of intensity fluctuations Fourier’s temporal domain Fourier’s energy domain
Intensity interferometry Narrabri stellar intensity interferomter (R.Hanbury Brown, R.Q.Twiss et al., University of Sydney)
Required Telescope diameters has been set
S/N for laser spectral lines If there is laser emission, the coherence time of light is three or more orders of magnitude greater and so the S/N. The required telescope diameter is smaller!!
