J/A+A/618/A154 SPHERE/ZIMPOL (89) Julia images (Vernazza+, 2018)
The impact crater at the origin of the Julia family detected with VLT/SPHERE?
Vernazza P., Brozv M., Drouard A., Hanusv J., Viikinkoski M., Marsset M.,
Jorda L., Fetick R., Carry B., Marchis F., Birlan M., Fusco T.,
Santana-Ros T., Podlewska-Gaca E., Jehin E., Ferrais M., Bartczak P.,
Dudzinski G., Berthier J., Castillo-Rogez J., Cipriani F., Colas F.,
Dumas C., Durech J., Kaasalainen M., Kryszczynska A., Lamy P.,
Le Coroller H., Marciniak A., Michalowski T., Michel P., Pajuelo M.,
Tanga P., Vachier F., Vigan A., Warner B., Witasse O., Yang B., Asphaug E.,
Richardson D.C., Svevevek P., Gillon M., Benkhaldoun Z.
<Astron. Astrophys. 618, A154 (2018)>
=2018A&A...618A.154V 2018A&A...618A.154V (SIMBAD/NED BibCode)
ADC_Keywords: Solar system ; Minor planets
Keywords: minor planets, asteroids: individual: (89) Julia -
methods: observational - techniques: high angular resolution -
methods: numerical
Abstract:
The vast majority of the geophysical and geological constraints (e.g.,
internal structure, cratering history) for main-belt asteroids have so
far been obtained via dedicated interplanetary missions (e.g., ESA
Rosetta, NASA Dawn). The high angular resolution of SPHERE/ZIMPOL, the
new-generation visible adaptive-optics camera at ESO VLT, implies that
these science objectives can now be investigated from the ground for a
large fraction of D≥100km main-belt asteroids. The sharp images
acquired by this instrument can be used to accurately constrain the
shape and thus volume of these bodies (hence density when combined
with mass estimates) and to characterize the distribution and
topography of D≥30km craters across their surfaces.
Here, via several complementary approaches, we evaluated the recently
proposed hypothesis that the S-type asteroid (89) Julia is the parent
body of a small compact asteroid family that formed via a cratering
collisional event.
We observed (89) Julia with VLT/SPHERE/ZIMPOL throughout its rotation,
derived its 3D shape, and performed a reconnaissance and
characterization of the largest craters. We also performed numerical
simulations to first confirm the existence of the Julia family and to
determine its age and the size of the impact crater at its origin.
Finally, we utilized the images/3D shape in an attempt to identify the
origin location of the small collisional family.
On the one hand, our VLT/SPHERE observations reveal the presence of a
large crater (D∼75km) in Julia's southern hemisphere. On the other
hand, our numerical simulations suggest that (89) Julia was impacted
30-120Myrs ago by a D∼8km asteroid, thereby creating a D≥60km
impact crater at the surface of Julia. Given the small size of the
impactor, the obliquity of Julia and the particular orientation of the
family in the (a,i) space, the imaged impact crater is likely to be
the origin of the family.
New doors into ground-based asteroid exploration, namely, geophysics
and geology, are being opened thanks to the unique capabilities of
VLT/SPHERE. Also, the present work may represent the beginning of a
new era of asteroid-family studies. In the fields of geophysics,
geology, and asteroid family studies, the future will only get
brighter with the forthcoming arrival of 30-40m class telescopes
like ELT, TMT, and GMT.
Description:
Julia was observed with the SPHERE instrument (ESO/VLT) around its
opposition at eight different epochs. We used ZIMPOL in narrowband
imaging mode (N_R filter; filter central wavelength = 645.9nm,
width = 56.7nm). Each observational sequence consisted of a series of
five images, where each image corresponded to a series of detector
integration times (DITs) of 10s, during which Julia was used as a
natural guide star for adaptive optics (AO) corrections. Observations
were performed under good seeing conditions (≤0.8") with an airmass
usually below 1.6. After every asteroid observation, we observed a
nearby star for deconvolution purposes to estimate the instrument
point spread function (PSF). Finally, standard calibrations, which
include detector flat-fields and darks, were acquired in the morning
as part of the instrument calibration plan.
objects:
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Planet Name H Diam i e a
mag km deg AU
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89 Julia 6.60 151.5 16.127006 0.18459430 2.55051892
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
list.dat 127 80 List of fits images
fits/* . 80 Individual fits images
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Byte-by-byte Description of file: list.dat
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Bytes Format Units Label Explanations
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1- 9 F9.5 deg RAdeg Right Ascension of center (J2000)
10- 18 F9.5 deg DEdeg Declination of center (J2000)
20- 26 F7.5 arcsec/pix scale Scale of the image
28- 31 I4 --- Nx Number of pixels along X-axis
33- 36 I4 --- Ny Number of pixels along Y-axis
38- 60 A23 --- Obs.Date Observation date (YYYY-MM-DDThh:mm:ss.sss)
62- 65 I4 Kibyte size Size of FITS file
67-127 A61 --- FileName Name of FITS file, in subdirectory fits
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Acknowledgements:
Pierre Vernazza, pierre.vernazza(at)lam.fr
(End) Patricia Vannier [CDS] 12-Jul-2018