Atomic Spectroscopy for Space Applications: Galactic Evolutionl

M. P. Ruffoni, J. C. Pickering, G. Nave, C. Allende-Prieto

APOGEE is one of 4 instruments formingthe third Slone Digital Sky Survey (SDSS3)

It will conduct a spectroscopic survey of all stellar populations in the Milky Way

It will measure in the near-IR where galactic dust extinction is ~1/6 of that at visible wavelengths

It will measure chemical abundances and radial velocities of 100,000 evolved stars to help explain galactic evolution

Duration Spring 2011 to Summer 2014

Spectra Measuring 1.51µm < < 1.7µmResolving power ~30,000S/N Ratio greater than 100

Targets 100,000 evolved stars

15 elements - Fe most important

Precision Metal abundances to ~0.1 dexRadial velocities to <0.3 km s-1

Detecting elements in stars

Photosphere

Hot, denseinterior

Emission contains absorption lines

Section of a star

Visible spectra for different star types

Absorption lines indicate the presence of an element.

Line strength is mainly linked to:

•Stellar properties (e.g. temperature)•Absorption transition probability•Chemical abundance

TemperatureType

Simulated H-band spectrum for different Fe abundances. All other parameters fixed.

Finding chemical abundances

1) Use a 2 fit to stellar models to obtain• Stellar temperature• Surface gravity• Microturbulence parameter• Abundance of important elements

[Fe/H], [C/H], and [O/H]

2) Fix these then fit other abundances

The results are only as good as the model!

Measuring Transition Probabilities

E2

E1

11

1

1

A21 B12 B21

Einstein coefficients

Spontaneous Absorption Stimulated Emission Emission

21312

3

1

212

8A

h

cgg

B

Transition probabilities can be obtained from emission spectra

Number of experimentally measured transition probabilities in the IR:

J. C. Pickering et al. Can J Phys 89 pp. 387 (2011)

Sc Ti V Cr Mn Fe Co Ni Cu Zn

– 45 7 – 26 51 – 4 – 1

Better experimental transition probabilities are needed

221

1

A

Decay to a single level

Decay to multiple levels

E2

E1

E2

E1

I I

I

I

12

Branching Fractions

i ii i II

AA

BF2

21

2

2121

2

2121

BFA

12

12 12

BF = Branching fractionI = Integrated line intensity

Complications

1.0

0.0

Spectrometer Response

Determined by measuring a calibrated continuum source

• Tungsten lamp (IR to UV)• Deuterium lamp (UV and vacuum UV)

I

1.0

0.0

I

Norm

alis

ed

re

spon

se

0 4000 8000 12000 35000 45000 55000Wavenumber / cm-1

1.0

0.8

0.6

0.4

0.2

0.0Norm

alis

ed

Resp

on

se

W lamp D2 Lamp

Free spectral range

Spectral range determined by

• Spectrometer optics• Detector sensitivity• Filter combinations• Measurement electronics

Either

Select range to measure all upper level branches

or

Use overlapping spectra to carry calibration

Measuring Upper Level Lifetimes

Lifetimes are commonly measured with Laser Induced Fluorescence (LIF)

E2

E1

1) A laser pulse is used to excite electrons in a populated lower level.

2)The upper level is populated

3)After time the electrons de-exciteu

Critical Fe I transitions for the APOGEE project

There are no transitions to populated lower levels

No lines in the visible/UV

LIF lifetimes are unavailable

BFs are unavailableNo lines to carry intensity calibration

0 4000 8000 12000 35000 45000 55000Wavenumber / cm-1

1.0

0.8

0.6

0.4

0.2

0.0Norm

alis

ed

Resp

on

se

205 - 720 nmUV - visible

Catch-22: Branching fractions or level lifetimes

2

2121

BFA

Situation BF21 2All lines in the IR

At least 1 line in vis/UV

Critical Fe I transitions for the APOGEE project

Solution: Invert the problem

Solution: Invert the Problem

Line strength is mainly linked to:

•Stellar properties (e.g. temperature)•Absorption transition probability•Chemical abundance

Recall from slide 2

APOGEE needs transition probabilities to find abundances

The Solar Fe abundance is known. Use it to find transition probabilities

2

22

ii

BFA i

ii A

BF

2

22

i22

Results

A section of our results table:

Consistency Checking

Relative transition probabilities can be found by combining absorption and emission data (Ladenburg 1933).

j

i

j

i

j

i

BFBF

II

AA

2

2

2

2

2

2

k

j

k

j

k

j

I

I

A

A

B

B

2

2

2

2

2

2

Ratio of line strengths in emission:

Ratio of line strengths in absorption: A2i A2j

B2j B2k

Lifetimes not needed

Results

A section of our results table:

1) We have found a reliable method for obtaining IR transition probabilities

2) Present study has almost doubled the number of Fe transition probabilities available in the IR.

3) We have the data to quickly provide many more transition probabilities

Conclusions

Number of experimentally measured transition probabilities in the IR:

J. C. Pickering et al. Can J Phys 89 pp. 387 (2011)

Sc Ti V Cr Mn Fe Co Ni Cu Zn

– 45 7 – 26 51 – 4 – 1

For more information:

Technique: M. P. Ruffoni, Comp. Phys. Comm., accepted (2012)

Results for APOGEE: M. P. Ruffoni et al., ApJ, submitted (2013)

Results for GaiaESO: M. P. Ruffoni et al., ApJ, in preparation (2013)