analysis of spectral features in tno and asteroid spectra
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 PresentationTRANSCRIPT
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