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

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? 1.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 2.Are there trends with size? Semi-major axis?  Such trends would (could?) speak to the “ice line” and alteration timescales and processes 3.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. Motivation  0.7-µm absorption correlated with 3-µm hydration band Used in Bus-DeMeo taxonomy (Ch/Cgh groups) Much easier to observe, fainter objects Use SDSS sample to search for Ch-like objects (here termed “Ch” to avoid confusion with formally- classified taxonomy)  Use large sample size, take advantage of statistics ~

6. 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) ~ ~

7. The 0.7-µm proxy band  This band is correlated with 3- µm band: 1.Objects with proxy band will also have 3- µm band 2.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)

8. 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 S 3 OS 2 (with 85 in common)

9. Sloan Digital Sky Survey  Sloan survey aiming to cover about a quarter of the sky in five filters, with goal of producing a 3-D map of ~10 6 galaxies and quasars  204,305 moving objects in most recent data release  67,367 observations of 43,424 known objects  4301 observations with C-complex colors, 3594 unique objects (compared to <400 for SMASS + S3OS2 surveys combined)  1940 with H > 14.75 (D smaller than ~ 5 km), 161 smaller than ~ 2 km. Ivezi´c et al (2002)

10. 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 ~ ~

11. 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

12. 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

13. Two (independent, I think) approaches to the problem 1.Do chi-sq comparison of a spectrum to Bus/Tholen class averages 2.Use distribution of band depths to estimate relative contributions of populations

14. “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 ~

15. MENTION  CLOUTIS CM SPECTRA- 0.7 = CM?  WELL-MIXED -> GRAND TACK?  WAYS TO INTERPRET/OVERI NTERPRET GAUSSIANS

16. “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. ~ ~ ~

17. 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

18. PopulationApproachCh fractionσN All 10.300.013591 2a0.16 2c0.140.01 Inner (2.06-2.50 AU) 10.310.02697 2a0.14 2c0.140.02 Mid (2.50-2.82 AU) 10.330.011076 2a0.29 2c0.230.02 Outer (2.82-3.28) 10.270.011769 2a0.08 2c0.090.02 H 10.5-14.0 10.280.01900 2c0.06 H 14.0-15.0 10.290.011173 2c0.15 H 15.0-16.0 10.310.011014 2c0.18 H 16.0-18.0 10.310.02462 2c0.23 Themis fam 10.200.02324 2c0.000.09 Hygiea fam 10.190.02294 2c0.010.03

19. 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

20. …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. ~

21. …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)

22.  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. …and those Gaussians? ~

23. 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 ~

24. 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

25. Contributions to size ranges from regions of asteroid belt H rangeInnerMidOuter 10.5-14.04%20%75% 14.0-15.09%29%63% 15.0-16.026%42%32% 16.0-18.062%29%9% SMASS/S 3OS2 15%45%40%

26. 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

27. 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? ~

28. 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

29. 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) ~~ ~

31. 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 ~ ~ ~ ~