Claudio Germanà and Dainis Dravins INAF Observatory of Padua Lund Observatory

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

Energy level populations described by Boltzmann’s statistics Medium acts as an absorber

Medium acts as an amplifier ”Light amplification by stimulated emission of radiation” LASER Population inversion

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)

S. Johansson & V.S. Letokhov Astrophysical lasers and nonlinear optical effects in space New Astron. Rev. 51, 443 (2007) A microsecond “bottle-neck” creates a population inversion in the 3 → 2 transition of Fe II at 9997 Å

...how to confirm Laser emission? Expected extremely narrow linewidth < 1 mÅ (0.1 pm) (Johansson & Letokhov 2004) Spectral resolution  100 million!! by Dravins et al. 2007

What about a spectral line? Electric field emitted from one atom which undergoes collisions: E n (t)= E 0 cos(ω 0 t + φ n (t)) φ n (t) is a Gaussian (chaotic process) a(t) is a Gaussian Total electric field from the system of n atoms (Loudon 1973):

exp(iωt) Fourier component... signal in Fourier’s notation... E(t) TOT thermal light a(t) ≠ cost (Gaussian) E(t) TOT laser light a(t) ≈ cost

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

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

Photon (intensity) – correlation Spectroscopy

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!!