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