832 Karin Shows No Rotational Spectral Variations

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832 Karin Shows No Rotational Spectral Variations
Clark R. Chapman, B. Enke, W.J. Merline, D.
Nesvorný, P. Tamblyn, and E.F. Young
Southwest Research Institute Boulder, Colorado,
USA
Presentation No. 71.08, Fri. 130-300 pm
38th Annual AAS/DPS Meeting Pasadena,
California USA 8-13 October 2006
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Spatial Variations of Color (hence Composition)
on Asteroids?

• 1928 Bobrovnikoff first found spectral
  variations on Vesta, determined 5h rotation
  period from their periodicity
• Since then No other asteroid has been found to
  have such significant color variations
• Many that have been reported (e.g. for 433 Eros)
  have been found later not to be real
• By far the most prominent color variations
  reported have been for 832 Karin (Sasaki et al.
  2004)

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832 Karin, a Very Young Asteroid

• Karin is the brightest member of a cluster
  within the large, well-known Koronis family
• Nesvorný et al. (2002) integrated the orbits of
  Karin cluster members backwards in time and
  showed that orbital parameters converged at 5.8
  My ago, precisely defining the age of the
  break-upvery young!
• Thus Karin cluster members became obvious
  targets, in comparison with regular Koronis
  members, for studying time-variable phenomena
  like space-weathering

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Sasaki et al. (ApJ, 2004) Big Color Differences!

• IR spectra biggest ever color variation on any
  asteroid
• Rotational phases plotted on Yoshida et al.
  lightcurve
• Green spectrum (identical to blue) just 15
  from red
• Interpretation blue fresh, red
  space-weathered

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Our IRTF SpeX Observations of Karin, 7-14 January
2006

• IRTF SpeX
• 0.8 - 2.4 µm
• Good rotation- al phase coverage, analyzed in 7
  50 intervals
• Solar analog stars for airmass cor. and solar
  calibration
• Generally excellent skies during 5 nights
• Remote on-site observing
• Funding from NASA PAST

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Result No Significant Spectral Variations with
Rotation

• Average spectra for 7 bins of rotational phase
• Error bars shown for (a) noisiest and (b) best
  spectrum (sta-tistical errors only)
• There is no obvious-ly significant differences
  between these spectra
• Certainly, no dramatic differences are apparent

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Comparison of Sasaki et al. and Chapman et al.
Results

• Two results are overlaid
• Fair agreement between our average spectrum
  (black) and their blue spectrum
• Their red result is dramatically inconsistent
  with our result.

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Observing Geometry

• Only way to explain the spectral constancy of our
  observations, if there were a real large reddish
  area, would be if we were looking pole-on while
  Sasaki et al. were not.
• But we observed just 0.5 Karin-year later, in
  opposite direction from where Sasaki and Yoshida
  et al. observations showed 0.7 mag. lightcurve
  amplitude.
• It is not plausible that we were observing nearly
  pole-on.
• There is no plausible way Karin could show a
  red hemisphere and we would have missed it in
  our rotational phase sampling.

From JPL orbit viewer website
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Karins Reflectance SpectrumPreliminary
Interpretation

• As previous observers have noted, Karins
  spectrum is
• Slightly less red than other S-types
• Has slightly weaker pyroxene absorption bands
• Possibly Karin, as a relatively fresh asteroid,
  has had time for space-weathering to reduce
  spectral contrast but not enough time to show
  substantial reddening
• We are currently reducing our other SpeX data on
  Koronis family members

Average spectral reflectance for 832 Karin
Wavelength (µm)