1. Cosmic lasers and Photon-correlation Spectroscopy Claudio Germanà and Dainis Dravins INAF Observatory of Padua Lund Observatory

2. OUTLINE 1. Laser Emission in astrophysical sources 2. Photon-Correlation Spectroscopy: Resolving narrow spectral lines 3. Signal – to – Noise ratio

3. Thermal Equilibrium conditions Energy level populations described by Boltzmann’s statistics Medium acts as an absorber

4. NON-Equilibrium Population inversion Medium acts as an amplifier ”Light amplification by stimulated emission of radiation” LASER

5. 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)

6. ...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

7. S. Johansson & V.S. Letokhov Astrophysical lasers operating in optical Fe II lines in stellar ejecta of Eta Carinae Astron.Astrophys. 428, 497 (2004)

8. 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)

9. 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)

10. ...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!!

11. 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

12. ... 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

13. ...spectral line profile... a(t) ≈ cost a(t)≠ cost (Gaussian)

14. ...FWHM and time scale of intensity fluctuations Fourier’s temporal domain Fourier’s energy domain

15. Photon (intensity) – correlation Spectroscopy

16. Intensity interferometry Narrabri stellar intensity interferomter (R.Hanbury Brown, R.Q.Twiss et al., University of Sydney)

17. Required Telescope diameters has been set

18. 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!!