J/A+A/623/A132 SPHERE/ZIMPOL (41) Daphne images (Carry+, 2019)
The homogeneous internal structure of CM-like asteroid (41) Daphne.
Carry B., Vachier F., Berthier J., Marsset M., Vernazza P., Grice J.,
Merline W. J., Lagadec E., Fienga A., Conrad A., Podlewska-Gaca E.,
Santana-Ros T., Viikinkoski M., Hanus J., Dumas C., Drummond J. D.,
Tamblyn P. M., Chapman C. R., Behrend R., Bernasconi L., Bartczak P.,
Benkhaldoun Z., Birlan M., Castillo-Rogez J., Cipriani F., Colas F.,
Drouard A., Durech J., Enke B. L., Fauvaud S., Ferrais M., Fetick R.,
Fusco T., Gillon M., Jehin E., Jorda L., Kaasalainen M., Keppler M.,
Kryszczynska A., Lamy P., Marchis F., Marciniak A., Michalowski T.,
Michel P., Pajuelo M., Tanga P., Vigan A., Warner B., Witasse O.,
Yang B., Zurlo A.
<Astron. Astrophys. 623, A132 (2019)>
=2019A&A...623A.132C 2019A&A...623A.132C (SIMBAD/NED BibCode)
ADC_Keywords: Solar system ; Minor planets
Keywords: minor planets, asteroids: general -
minor planets, asteroids: individual: (41) Daphne -
methods: observational - techniques: high angular resolution
Abstract:
CM-like asteroids (Ch and Cgh classes) are a major population within
the broader C-complex, encompassing about 10% of the mass of the main
asteroid belt. Their internal structure has been predicted to be
homogeneous, based on their compositional similarity as inferred from
spectroscopy (Vernazza et al., 2016AJ....152..154G 2016AJ....152..154G) and numerical
modeling of their early thermal evolution (Bland & Travis, 2017, Sci.
Adv. 3, e1602514).
Here we aim to test this hypothesis by deriving the density of the
CM-like asteroid (41) Daphne from detailed modeling of its shape and
the orbit of its small satellite.
We observed Daphne and its satellite within our imaging survey with
the Very Large Telescope extreme adaptive-optics SPHERE/ZIMPOL camera
(ID 199.C-0074, PI P. Vernazza) and complemented this data set with
earlier Keck/NIRC2 and VLT/NACO observations. We analyzed the dynamics
of the satellite with our Genoid meta-heuristic algorithm. Combining
our high-angular resolution images with optical lightcurves and
stellar occultations, we determine the spin period, orientation, and
3-D shape, using our ADAM shape modeling algorithm.
The satellite orbits Daphne on an equatorial, quasi-circular, prograde
orbit, like the satellites of many other large main-belt asteroids.
The shape model of Daphne reveals several large flat areas that could
be large impact craters. The mass determined from this orbit combined
with the volume computed from the shape model implies a density for
Daphne of 1.77±0.26g/cm3 (3σ). This density is consistent
with a primordial CM-like homogeneous internal structure with some
level of macroporosity (∼17%).
Based on our analysis of the density of Daphne and 75 other
Ch/Cgh-type asteroids gathered from the literature, we conclude that
the primordial internal structure of the CM parent bodies was
homogeneous.
Description:
Daphne 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.
For a full description of the parameters, see:
https://astro.troja.mff.cuni.cz/projects/asteroids3D/web.php?page=db_description
and Durech et al. (2010A&A...513A..46D 2010A&A...513A..46D)
object:
-------------------------------------------------------------------
Planet Name H Diam i e a
mag km deg AU
-------------------------------------------------------------------
41 Daphne 7.12 187.0 15.792100 0.27541753 2.76043870
-------------------------------------------------------------------
File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
spin.dat 47 1 Spin parameter of the shape model
list.dat 105 98 List of fits images
fits/* . 98 Individual fits images
daphne.ver 30 2703 Shape model in lightcurve-inversion format
daphne.obj 29 2702 Shape model in OBJ format
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See also:
http://astro.troja.mff.cuni.cz/projects/asteroids3D/web.php : DAMIT Home Page
Byte-by-byte Description of file: spin.dat
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Bytes Format Units Label Explanations
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1- 8 F8.4 deg LongP Ecliptic longitude of the pole (J2000)
10- 17 F8.4 deg LatP Ecliptic latitude of the pole (J2000)
19- 28 F10.8 h period Sidereal rotation period
30- 43 F14.6 d t0 Reference epoch (Julian date, JD)
45- 47 I3 deg phi0 Ecliptic latitude of the pole (J2000)
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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)
11- 19 F9.5 deg DEdeg Declination of center (J2000)
22- 28 F7.5 arcsec/pix scale Scale of the image
31- 34 I4 --- Nx Number of pixels along X-axis
37- 40 I4 --- Ny Number of pixels along Y-axis
44- 66 A23 "datime" Obs.Date Observation date (YYYY-MM-DDThh:mm:ss.sss)
68- 71 I4 Kibyte size Size of FITS file
74-105 A32 --- FileName Name of FITS file, in subdirectory fits
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Description of file: daphne.obj
First column:
Flag for vertices "v" or facets "f" line
Columns 2 to 4:
For vertices lines: Cartesian coordinates (x,y,z) of the vertex
For facet lines: Index of the three vertices in the triangular facet
Description of file: daphne.ver
First line: Number of vertices (#v) and number of facets (#f)
Then, for #v lines: Cartesian coordinates (x,y,z) of the vertex
Then, for #f lines: Index of the three vertices in the triangular facet
Acknowledgements:
Benoit Carry, benoit.carry(at)oca.eu
(End) Patricia Vannier [CDS] 06-Mar-2019