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Characterizing C-Complex Asteroids: SDSS Observations & Estimations

Explore the predominant C-complex asteroids in the outer and asteroid belt, associated with carbonaceous chondrites. Learn about trends in hydrated minerals, proxy bands, and sample analysis methods from SDSS data. Investigate the presence of the 0.7µm proxy band, its correlation with the 3µm band, and the estimation of C-asteroid populations through color matching and band depth distributions. Understand the nuances and interpretations of band observations.

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Characterizing C-Complex Asteroids: SDSS Observations & Estimations

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  1. The Fraction of Ch asteroids in the C complex from SDSS observations Andrew S. Rivkin JHU/APL

  2. C-complex asteroids • Dominate outer belt, and asteroid belt as a whole • Most of the largest asteroids (and at least one dwarf planet!) are classified in this group • Associated with carbonaceous chondrites • C class/complex traditionally (if unfortunately) divided into subclasses, including one also named C. • Some with hydrated/hydroxylated minerals, some not. • Absorption band near 3 µm diagnostic for hydrated/hydroxylated minerals

  3. C-complex asteroids • Dominate outer belt, and asteroid belt as a whole • Most of the largest asteroids (and at least one dwarf planet!) are classified in this group • Associated with carbonaceous chondrites • C class/complex traditionally (if unfortunately) divided into subclasses, including one also named C. • Some with hydrated/hydroxylated minerals, some not. • Absorption band near 3 µm diagnostic for hydrated/hydroxylated minerals • Would be useful for all sorts of reasons to at least get a ballpark estimate of what’s out there, hydrated/hydroxylated mineral-wise

  4. Estimation du “ballpark”? Qu'est-ce que c'est? • Does the amount of hydrated material vary greatly among C asteroids? • Some dynamical models would predict a well-mixed asteroid belt w/r/t/ C asteroid types • Are there trends with size? Semi-major axis? • Such trends would (could?) speak to the “ice line” and alteration timescales and processes • Can we use hydrated minerals to trace meteorite types? • Could be used as an independent measure of the bias of the meteorite collection (also, see #1)

  5. The 0.7-µm proxy band • Due to the inconvenience of observing near 3 µm and the limited number of suitable telescope/instrument combinations, “proxy” band desirable • Vilas and Howell have put lots of effort into study and analysis of band near 0.7 µm • Bus/DeMeo Taxa with proxy band: Ch, Cgh called “Ch” in this talk • Bus/DeMeo Taxa without: C, B, Cg, Cb called “C” in this talk ~ ~ Vilas and Sykes (1996)

  6. The 0.7-µm proxy band • This band is correlated with 3- µm band: • Objects with proxy band will also have 3- µm band • Those without have ~50% chance of having 3- µm band • Using proxy band on an individual object could be difficult, but should be hunky-dory for large survey • Luckily, there’s a large survey floating around… Vilas and Sykes (1996)

  7. Target Sample • SDSS Moving Object Catalog 3 • 67637 observations of 43424 known objects • Photometric nights • Filter out sample to most C-like using a* (Ivezic et al.) and limits on colors • 3951 observations of 3102 objects • 1476 with H > 15 (D < ~5 km) • Compare: 405 C-complex objects in SMASS, 193 in S3OS2 (with 85 in common)

  8. Making the call • The presence/absence of the 0.7-µm band can be approximated by looking at the position of the i’ measurement relative to r’ and z’ • Not a perfect measure, but a reasonable start, eh? • Below the line = “Ch” • Above the line = “C” • We’ll punt the error bars for the moment ~ ~ Lines from SMASS survey, points from SDSS

  9. Nuances to keep in mind • Can’t just look for BD>0 • Half of objects on continuum will look like BD>0 • Biased s.t. Ch too high • Can’t just look for BD > 1σ • Now potentially biased against Ch • Can’t exclude 1σ > BD > 0 • Throw out too many objects • Also probably still biased ~ ~ Lines from SMASS survey, points from SDSS

  10. Nuances to keep in mind • Can’t just look for BD>0 • Half of objects on continuum will look like BD>0 • Biased s.t. Ch too high • Can’t just look for BD > 1σ • Now potentially biased against Ch • Can’t exclude 1σ > BD > 0 • Throw out too many objects • Also probably still biased ~ ~ Lines from SMASS survey, points from SDSS

  11. Two (independent, I think) approaches to the problem • Do chi-sq comparison of a spectrum to Bus/Tholen class averages • Use distribution of band depths to estimate relative contributions of populations

  12. “Color Matching” • Compare g’r’i’z’ colors to convolved SMASS spectra • Assign to closest class (B/C/Cb/Cg/Ch/Cgh), minimizing square of errors • Test on SMASS/S3OS2 overlap with SDSS, recovered correct Ch fraction within uncertainty • However, while group values look good, individual values may give wrong results ~

  13. “Histogram Symmetry” • Measure band depth distribution • Assume C asteroids have BD=0, symmetrical scatter around • Ch asteroids are excess after C asteroid population removed • Variations from full-up two-Gaussian fits to simply comparing number of objects with BD > and < 0. ~ ~ ~

  14. What do these Gaussians mean? • Interpretation A: • There’s a fixed(ish) band depth for the 0.7-µm band, scatter is observational • But we do see different depths in meteorites • Interpretation B: • There’s a distribution of band depths in the real material • That suggests error bars all work themselves out • Interpretation C: • This is overthinking a plate of beans

  15. How about results, not in an unreadable table? ~ Ch fraction, Belt as a whole: • Chi sq: 0.30 • Symmetry: 0.16 • Average 0.23 +/- 0.08 For comparison, SMASS + S3OS2 has Ch fraction of ~0.38 +/- 0.02 Chi sq. suggests C complex is • 38% B class • 13% C class • 18% Cb class But, you know, don’t go crazy with that. • 19% Ch class • 10% Cgh class

  16. …and those Gaussians? • For belt as a whole, sum of gaussian with BD=0 and one with BD ~3-4%, both with scatter ~2% . • Other subsets give similar best-fit band depths for Ch gaussian • This is, admittedly suprisingly, consistent with what’s seen in meteorites • Still might be overthinking it, though. ~

  17. …and those Gaussians? • For belt as a whole, sum of gaussian with BD=0 and one with BD ~3-4%, both with scatter ~2% . • Other subsets give similar best-fit band depths for Ch gaussian • This is, admittedly suprisingly, consistent with what’s seen in meteorites • Still might be overthinking it, though. ~ Cloutis et al., (in press/on the web)

  18. …and those Gaussians? • For belt as a whole, sum of gaussian with BD=0 and one with BD ~3-4%, both with scatter ~2% . • Other subsets give similar best-fit band depths for Ch gaussian • This is, admittedly suprisingly, consistent with what’s seen in meteorites • Still might be overthinking it, though. ~

  19. Trends vs. semi-major axis • Divided belt into inner, outer, middle • Mid-belt has higher Ch fraction, as seen in other work • Outer belt has lowest fraction • Two approaches show different size of variation ~

  20. Trends vs. H magnitude • Symmetry approach sensitive to a, so split that out • General decline in Ch fraction with size seen in earlier work • SDSS data shows (slight?) rise with H > ~12.5 • Chi-sq more well-behaved than symmetry

  21. Contributions to size ranges from regions of asteroid belt

  22. Trends vs. H magnitude • Symmetry approach sensitive to a, so split that out • General decline in Ch fraction with size seen in earlier work • SDSS data shows (slight?) rise with H > ~12.5 • Chi-sq more well-behaved than symmetry

  23. NEO Implications/Speculation • SMASS Ch/Cgh NEO fraction 1/23 • Mars-crossers 3/10 • Ch/Cgh fraction of NEOs < 1/3 that of Ch fraction of similar-sized MBA • Marchi et al., Delbó et al. suggest orbital evolution  low-q orbits  destruction of 0.7-µm band • If so, estimate this happens to > 2/3 of NEOs? ~

  24. Dynamical Families • Bus and Binzel (2002) found asteroid families to be homogeneous spectrally • Two C-complex families appear in SDSS sample in large numbers: Themis and Hygiea • Both approaches agree: fewer Ch objects than general population • Approach 2 consistent with Ch fraction ≈ 0

  25. 3-µm Implications/Speculation(the original point of this exercise) • Basically all C-complex objects with 0.7-µm band have a 3-µm band • Roughly half of C-complex objects without a 0.7-µm band also have a 3-µm band • So hydrated fraction ≈ Ch +0.5 × C • With overall Ch fraction ~ 0.25 and C fraction ~ 0.75, hydrated fraction ≈ 60-65% • CM ~38% of carbonaceous chondrite falls • Also note smallest fraction of objects has Ch more like 0.3 than 0.25 • Hydrated CC fall fraction ~60?% (but hard to really say) ~ ~ ~

  26. Conclusions • Fraction of “Ch-like” ( ) asteroids ≈ 23 ± 8% of C complex • Themis and Hygiea families have fewer asteroids than the background population • The middle asteroid belt has a higher fraction than the inner or outer belts • The fraction reaches an apparent minimum near H ≈ 12, and either remains steady or slowly increases at smaller sizes ~ ~ ~ ~ Ch Ch Ch Ch

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