Andrew S. RivkinJHU/APL

The Fraction of Ch asteroids in the

C complex from SDSS observations

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

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

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)

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)

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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)

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)

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

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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

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Lines from SMASS survey, points from SDSS

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

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Lines from SMASS survey, points from SDSS

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

“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

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“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.

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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

Population Approach Ch fraction σ N

All1 0.30 0.01 3591

2a 0.16

2c 0.14 0.01

Inner(2.06-2.50 AU)

1 0.31 0.02 6972a 0.14

2c 0.14 0.02

Mid(2.50-2.82 AU)

1 0.33 0.01 1076

2a 0.29

2c 0.23 0.02

Outer(2.82-3.28)

1 0.27 0.01 1769

2a 0.08

2c 0.09 0.02

H 10.5-14.01 0.28 0.01 9002c 0.06

H 14.0-15.01 0.29 0.01 11732c 0.15

H 15.0-16.01 0.31 0.01 1014

2c 0.18

H 16.0-18.01 0.31 0.02 462

2c 0.23

Themis fam1 0.20 0.02 324

2c 0.00 0.09

Hygiea fam1 0.19 0.02 294

2c 0.01 0.03

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.

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19% Ch class 10% Cgh

class

…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.

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…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.

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Cloutis et al., (in press/on the web)

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?

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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

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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

Contributions to size ranges from regions of asteroid belt

H range Inner Mid Outer

10.5-14.0 4% 20% 75%

14.0-15.0 9% 29% 63%

15.0-16.0 26% 42% 32%

16.0-18.0 62% 29% 9%

SMASS/S3OS2

15% 45% 40%

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

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?

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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

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)

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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~