APOGEE: The Apache Point Observatory Galactic Evolution Experiment l M. P. Ruffoni 1, J. C....
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Transcript of APOGEE: The Apache Point Observatory Galactic Evolution Experiment l M. P. Ruffoni 1, J. C....
APOGEE:The Apache Point Observatory Galactic Evolution Experiment l
M. P. Ruffoni1, J. C. Pickering1, E. Den Hartog2, G. Nave3, J. Lawler2, C. Allende-Prieto4
1.Imperial College London, UK
2.University of Wisconsin, Madison, WI, USA
3.NIST, Gaithersburg, MD, USA
4.University of Texas, Austin, TX, USA
1
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-12
APOGEE is one of 4 instruments formingthe third Sloan 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
Detecting elements in stars
Photosphere
Hot, denseinterior
Emission contains absorption lines
Section of a star
Visible spectra for different star types
n = E = 0n = 4 E = -0.85 eVn = 3 E = -1.51 eV
n = 2 E = -3.40 eV
n = 1 E = -13.6 eV
Type
Absorption lines indicate the presence of an element.
Line strength is mainly linked to:
•Stellar properties (e.g. temperature)•Absorption transition probability•Chemical abundance
Temperature/KType
K A O
Effect of lower level population on H
3
Why look at QSOs when studying ?
Determining chemical abundances
1) Use a 2 fit to stellar models to find• Stellar temperature• Surface gravity• Microturbulence parameter• Abundance of important elements
[Fe/H], [C/H], and [O/H]
2) Fix these parameters then fit other abundances
Simulated H-band spectra for an Fe-poor (black) and Fe-rich (grey) star. All other parameters fixed.
4
Experimentally measured transition probabilities in the literature:
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
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
2
2121
BFA
5
To vacuumpump High voltage
~600 V at50-1000 mA
Ne or Ne/Argas at 1-3 mbar
Glass GlassAnode Cathode Anode
Decay to a single level
Decay to multiple levels
E2
E1
E2
E1
I I
I
I
12
Branching Fractions
i iEWEW
BF2
2121
2
2121
BFA
12
12 12
BF = Branching fractionEW = Line equivalent width
6
Free spectral range
Spectral range determined by
• Spectrometer optics• Detector sensitivity• Filter combinations• Measurement electronics
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
Either
Select a range to measureall upper level branches
or
Use overlapping spectrato carry calibration
7
Branching Fractions for APOGEE
A21 for Neutral Fe (Fe I) is of the greatest importance
Wavenumber / cm-1
S/N
S/N
x C
alib
rati
on f
unct
ion
/ cm-1 EW BF / %
6124.10169 2413.49953 7 ± 3
6395.40329 10599.58897 32 ± 2
6742.87349 19624.82794 60 ± 2
• Target H-band lines in a single spectral range
• Extract all lines from a single upper level
• Calibrate line intensities
• Fit line profiles to get BFs
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Catch-22: Branching fractions or level lifetimes
Laser induced fluorescence (LIF) is used to measure 2
E2
E1
205 - 720 nmUV - visible
No significantly populated lower level to excite
Not possible to measure 2
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
Change target levels so they are LIF compatible
No lines to carry calibration
Not possible to measure BFs
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Solutions
Some levels contain visible lines to link IR spectra to UV spectra
• LIF has a low lying level available• FTS line intensities can be calibrated
For FTS compatible upper levels
• Use theory to calculate 2 (check against known levels)• Constrain models with stellar spectra
For LIF compatible upper levels
• Use lines from similar upper levels as calibration proxies• Link spectral regions with theory calculations• Use stellar spectra to estimate missing calibration factors
The work continues ... (with the support of the STFC)
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