Pressure broadening of alkali-metal resonance lines in the presence of helium or molecular hydrogen. James Babb

Transcription

1 Pressure broadening of alkali-metal resonance lines in the presence of helium or molecular hydrogen James Babb Harvard-Smithsonian Center for Astrophysics CfA Lab. Astro. Symposium September 20, 2010

2 Introduction 1995 Scientific Achievements BEC Brown dwarfs Extrasolar planets 51 Peg b, Debivort (Wikipedia)

3 Update Now more than 400 planets known, 113 hot Jupiters (e.g. 51 Pegasi b)(planetquest.jpl.nasa.gov) Hundreds of L-dwarfs and about 200 T-dwarfs (Gliese 229B) known (June 2010, from dwarfarchives.org) Brown dwarfs = failed stars Effective temperatures: L dwarf to T dwarf K

4 Astrophysics Sun M L T Jupiter Courtesy NASA/IPAC/R. Hurt

5 Introduction Broadened alkali-metal atom lines are prominent in L and T dwarf spectra in the optical and near infrared Na lines at 589 nm and K lines at 770 nm are prevalent Important ingredient of atmospheric models L dwarf 2MASSW J1507, Reid et al., 2000; Burrows et al., 2001

6 Broadening Collisions with He and H 2 beyond Lorentzian shape Profound broadening, need accurate data because the wings extend hundreds of nm to each side of line cores Data is scarce. How to obtain it? A) Calculate alkali-metal interaction with He or H 2 using quantum chemistry and then calculate line profile/opacity. B) Measure line absorption profile at about 900 K in the lab.

7 Data needed Atomic energies and oscillators strengths Thermal properties, partition functions Molecular potentials, transition dipole moments Long-range potentials (van der Waals)... some of which come from ultra-cold atom studies. Calculations: Cross sections, emission & absorption spectra

8 Potential energies Start with Na-He. Accurate molecular potential energy data from various sources. Spectroscopy and/or quantum chemistry calculations. C. Zhu, Babb, Dalgarno 06

9 Absorption Start with Na-He. Accurate molecular potential energy data. Quantum mechanical treatment of radiative absorption and molecular degrees of freedom. Note: 1000 K = au C. Zhu, 2004 calculation

10 Compare to photoassociation Heinzen, ICAP 94.

11 Wavelengths Start with Na-He. Accurate molecular potential energy data from various sources. Difference potentials are photon wavelengths C. Zhu, Babb, Dalgarno 06

12 Long-range Start with Na-He. Accurate molecular potential energy data. Long-range (R>20 a 0 ). C. Zhu, Dalgarno, Porsev, Derevianko, 04. Calculation.

13 Transition dipole moments Atomic case. Absorption with a, b electronic levels. Molecular case. Now a,b electronic, vibrational, rotational levels; weights, bands. Theodorakopoulos & Petsalakis, 93

14 Calculations Free-bound absorption Also free-free absorption

15 Results Completed NaHe, KHe calculations for absorption at various temperatures. H 2 cases underway. Are they accurate? NaHe KHe C. Zhu, Babb, Dalgarno 06; K-He molecular data from Santra & Kirby 05

16 Experiment Little, if any, Na-H 2, K-He and K-H 2 exptl. data. AMP lab at CfA Dr. François Shindo, Dr. K. Yoshino Important feature - we measured the alkali-metal atom density and the spectra.

17 Experimental set-up Furnace Shutters Gas cell Plane ruled grating (1200 l/mm) 3m McPherson Spectrometer Tungsten lamp (24V, 250W) Collimator Beam splitter Mach-Zehnder interferometer Mirror CCD Camera Compensating plate (MgOX2+QuartzX2 windows) Stepper control +motor Focusing Mirrors Specifications: Wavelength range: 370 to 920 nm, through 116 positions of stepper control-grating system Resolution: 0.02 nm at λ=500 nm CCD: 1024x256 pixels, pixel size 26 µm CCD acquisition spectrometer and shutters controls

18 Gas cell design Furnace: three heating zones, max temp K Gas cell features: Total length: 43 cm Inside diameter: 2.5 cm Tubing and body material: Stainless steel SS330, 317 Gas chamber: Magnesium oxide (MgO) windows Sealing gasket in graphite (GRAFOIL Applied Technology) Optical pathlength: 20 cm (room temperature) Max pressure: 700 torr

19 Experimental spectrum K 2 band Β 1 Π u -X 1 Σ + g K lines 4 2 S 1/2-4 2 P 3/2,1/2 ( nm, nm) K lines 4 2 S 1/2-5 2 P 3/2,1/2 ( nm, nm) K 2 band Α 1 Σ + u -X1 Σ + g K-He satellite feature K: 7.6x10 21 m torr He K: 9.7x10 21 m torr He

20 Reduced absorption coefficient for the wings of the K doublet lines at 770 nm. On the blue wing, the satellite feature predicted by the calculations is visible. The spectra were measured at 950 K, with a potassium density around cm -3 and He pressure between 170 to 700 torr. To bring the curves into accord, the calculations were multiplied by a factor of 2.5. We are investigating the discrepancy.

21 Opacities Calculations For extrasolar giant planets (Sudarsky et al 03) Roaster planet Numerous atoms, molecules

22 Opacities Calculations For L and T dwarfs (Sudarsky et al 06) FeH, CrH, VO, TiO

23 Conclusions 1. Theoretical and experimental studies for Na and K with He and H Will lead to improved diagnostics of brown dwarf gravities and effective temperatures. 3. Improve models of extrasolar giant planet atmospheres. 4. Future work: Focus on K-H 2 5. Future missions: Direct spectroscopy? Only a handful so far. Recent: Potassium by narrowband transit spectrophotometry

24 Direct 2002: detection of Na in HD b (Charbonneau et al.); Na doublet 2008: HD b (Redfield et al.). 2010: detection of K in HD 80606b (Colón et al.); XO-2b (Sing et al.) NASA/JPL-Caltech/G. Laughlin et al

25 Acknowledgments Collaborators: François Shindo, Cheng Zhu, K. Yoshino, Robin Santra, Alex Dalgarno, Kate Kirby. NASA, Science Mission Directorate, Universe Division ITAMP (NSF) Thanks to organizers: Randall, Nancy, and Nick

26 End of talk