Claudio Germanà and Dainis DravinsINAF 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 populationsdescribed by Boltzmann’sstatistics

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 starsHe II He I lines(Varshni & Nasser 1975,1986)

Mass – loosing stars

S. Johansson & V.S. LetokhovAstrophysical lasers operating in optical Fe II lines in stellar ejecta of Eta CarinaeAstron.Astrophys. 428, 497 (2004)

Model of a compact gas condensation near η Car with its Strömgren boundarybetween photoionized (H II) and neutral (H I) regions

S. Johansson & V. S. LetokhovLaser Action in a Gas Condensation in the Vicinity of a Hot StarJETP Lett. 75, 495 (2002) = Pis’ma Zh.Eksp.Teor.Fiz. 75, 591 (2002)

S. Johansson & V.S. LetokhovAstrophysical 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)= E0 cos(ω0t + φn (t))

φn (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 domain Fourier’s energy domain

Photon (intensity) – correlation Spectroscopy

Intensity interferometryIntensity interferometry

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

Required Telescope diameters2/1

0

21 2

||/

T

AnNS c

3/ NS 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!!