Upload
topaz
View
27
Download
0
Tags:
Embed Size (px)
DESCRIPTION
Analysis of spectral features in TNO and asteroid spectra. S. Erard, D. Despan, F. Merlin. Spectral observation of TNOs. Dark objects (≥ 18th mag) Shallow spectral features (in the NIR) => Very long exposure times required to access compositional information - PowerPoint PPT Presentation
Citation preview
S. Erard et al. — Workshop 3e zone, Nantes, 11-12 janvier 2007
Analysis of spectral features in TNO and asteroid spectra
S. Erard, D. Despan, F. Merlin
S. Erard et al. — Workshop 3e zone, Nantes, 11-12 janvier 2007
Spectral observation of TNOs
1998 Cruikshank et al.
Dark objects (≥ 18th mag)
Shallow spectral features (in the NIR)
=> Very long exposure times requiredto access compositional information
implication for observing strategyand for analysis methods
S. Erard et al. — Workshop 3e zone, Nantes, 11-12 janvier 2007
Spectral observation of TNOs
Methanol is the second most easily detected ice (after H2O)
Detection requires SNR ≥ 70 for pure ice
Ammonia detection requires SNR ≥ 125
Requirements are higher if only a fraction of the surface is covered, or mixture with other ices
C. Trujillo, Catania 2006 meeting
8-10m telescope, mag 18:1 h exposure <=> SNR ~100
Only 2005 FY9 has been observed with SNR allowing detection of N2, CO, CO2, or ethane ice (only ethane is detected)
S. Erard et al. — Workshop 3e zone, Nantes, 11-12 janvier 2007
Spectral observation of TNOs
Trujillo’s conclusion, Catania 2006 meeting(excerpt)
The Good News:
- About 25 KBOs could be observed by an international team of collaborators using the world's largest telescopes.
The Bad News:
- Don't bother observing any of the brightest 15 KBOs unless you spend at least 4 hours of exposure time on a 8m – 10m telescope in good conditions.
Tips for observers:-Don't repeat objects that are already done!-Observe in good conditions and at low airmass!-Take high (80-100) S/N spectra!
S. Erard et al. — Workshop 3e zone, Nantes, 11-12 janvier 2007
Spectral detection /characterization methods
Simulation + spectral fit, inversion:
- The first step is to identify the components- Extra components just add noise to the fits- Continuum is always an issue
Spectral ratios:
- Historically important, but very crude
MGM:
- Adapted only to specific minerals (pyroxenes, olivines, feldpars…)
Tetracorder, etc…:
- Rely on a more or less complete data base, - Not really adapted to ices
Geographic mixture
Hapke model Shkuratov model
H2Oa - - - - 6% 5µm
H2Oc 1% 5µm 14% 5µm 5% 5µm
Carbon
83% 15µm 50% 15µm 68% 15µm
Ice-Th
5% 5µm 7% 31µm 7% 5µm
Tit-Th 5% 5µm - - - -
Tri-Th 6% 5µm 29% 12µm 14% 15µm
Merlin & Barucci,Catania 2006 meeting
S. Erard et al. — Workshop 3e zone, Nantes, 11-12 janvier 2007
Multiresolution spectral analysis
Purpose:
- Detection method adapted to low SNR situations
Output:
- Characteristics of absorptions features (center, depth, width) - Detection thresholds in terms of S/N and proximity to the edges
Basis:
- Wavelet decomposition + multiscale grouping (based on imaging algorithms)- Uses a dyadic algorithm to avoid band reconstruction
Performances:
- Separates bands within Rayleigh criterion (if slightly different)- Accuracy on band properties ~10% for Gaussians- Correctly identifies bands at SNR = 3 in I/F- Robust to asymmetrical band shapes- Separates continuum variations from resolved bands
S. Erard et al. — Workshop 3e zone, Nantes, 11-12 janvier 2007
Orthopyroxene (laboratory spectrum)
The two bands are correctly detected
at all scales
Grouping and identification of a
dominant scale provides accurate band
characteristics
Simulations — High SNR
S. Erard et al. — Workshop 3e zone, Nantes, 11-12 janvier 2007
Simulated spectrum + noise
3 wide bands and a narrow one,
correctly detected
Simulations — medium SNR
S. Erard et al. — Workshop 3e zone, Nantes, 11-12 janvier 2007
Jarosite (lab. spectrum)
Many narrow bands on varying
structure, correctly detected
Bands near the edge (uncomplete) are
detected with a low statistical weight
Simulations — very tilted continuum
S. Erard et al. — Workshop 3e zone, Nantes, 11-12 janvier 2007
VLT / Naco resolved observations
Bright, extended object (mag ~8)
• Ice features?
• Clay features?
Ceres, 2.1-2.4 µmCH3OH
CH4
H2O
N2
NH3
S. Erard et al. — Workshop 3e zone, Nantes, 11-12 janvier 2007
• 19 structures detected, mainly small
telluric (with atm. counterparts) and
solar bands
• No ice absorption above 5
(disk centre or pole)
• Possible feature at 2.11 µm
• Improvement of telluric correction
pending
Ceres, 2.1-2.4 µm
Erard et al.,EGU 2006
S. Erard et al. — Workshop 3e zone, Nantes, 11-12 janvier 2007
Observations by Barucci et al 2005
(VLT), R~3000
• 6 structures detected, mostly telluric
correction remants
• Positive detection at 2.142 µm,
corresponding to N2 ice but
significantly narrower
Sedna, 1.9-2.5 µm
Erard et al.,DPS 2005
S. Erard et al. — Workshop 3e zone, Nantes, 11-12 janvier 2007
• Multiscale analysis methods, coupled with noise filtering algorithm, are very
efficient in low SNR situations
• TNO spectral studies require this kind of analysis
• The present one, based on a very redundant algorithm, may still be improved
with band reconstruction
• Tests are still being performed on laboratory spectra + observations
• First article with full description and tests to be submitted in 2007 (hopefully)
Conclusion
S. Erard et al. — Workshop 3e zone, Nantes, 11-12 janvier 2007