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Register a new userThe phase-polarization curve of asteroid (3200) Phaethon
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A multi-colour phase-polarization curve of asteroid (3200)~Phaethon has been obtained during the December 2017 apparition by merging measurements taken at the observing station of Calern (France) and at the Rhozen observatory (Bulgaria). All the observations were obtained in the positive polarization branch, the phase angle ranging from 36$^circ$ to 116$^circ$. The measured values of linear polarization are among the highest ever observed for a Solar system body. The covered interval of phase angle was not sufficiently extended to derive a firm determination of the $P_{rm max}$ parameter, but this appears to occur at a phase angle around 130$^circ$ and reaches more than 45% of linear polarization. Phaethon is the parent body of the Geminid meteor shower, and the real physical nature of this object (asteroid or comet) has been a long-debated subject. Our polarimetric measurements seem to support the asteroid hypothesis with a phase-polarization curve similar to the asteroid (2)~Pallas, but further observations at smaller phase angles are needed to draw definitive conclusions.
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The polarimetric observations of asteroid 3200 Phaethon, the target of international observation campaign, did not cover a proper phase angle interval to provide estimating all the attributes of the asteroid polarization curve. Based on present discrete observation data for Phaethon, its full polarimetric curves in BVRI bandpasses were reproduced. The polarimetric properties of the asteroid correspond to a notion on surface structure as thermally altered regolith particles mixed with lager rock fragments like a coarse pebble.
Asteroid (3200) Phaethon is an active near-Earth asteroid and the parent body of the Geminid Meteor Shower. Because of its small perihelion distance, Phaethons surface reaches temperatures sufficient to destabilize hydrated materials. We conducted rotationally resolved spectroscopic observations of this asteroid, mostly covering the northern hemisphere and the equatorial region, beyond 2.5-micron to search for evidence of hydration on its surface. Here we show that the observed part of Phaethon does not exhibit the 3-micron hydrated mineral absorption (within 2-sigma). These observations suggest that Phaethons modern activity is not due to volatile sublimation or devolatilization of phyllosilicates on its surface. It is possible that the observed part of Phaethon was originally hydrated and has since lost volatiles from its surface via dehydration, supporting its connection to the Pallas family, or it was formed from anhydrous material.
The near-Earth asteroid (3200) Phaethon is an intriguing object: its perihelion is at only 0.14 au and is associated with the Geminid meteor stream. We aim to use all available disk-integrated optical data to derive a reliable convex shape model of Phaethon. By interpreting the available space- and ground-based thermal infrared data and Spitzer spectra using a thermophysical model, we also aim to further constrain its size, thermal inertia, and visible geometric albedo. We applied the convex inversion method to the new optical data obtained by six instruments and to previous observations. The convex shape model was then used as input for the thermophysical modeling. We also studied the long-term stability of Phaethons orbit and spin axis with a numerical orbital and rotation-state integrator. We present a new convex shape model and rotational state of Phaethon: a sidereal rotation period of 3.603958(2) h and ecliptic coordinates of the preferred pole orientation of (319$^{circ}$, $-$39$^{circ}$) with a 5$^{circ}$ uncertainty. Moreover, we derive its size ($D$=5.1$pm$0.2 km), thermal inertia ($Gamma$=600$pm$200 J m$^{-2}$ s$^{-1/2}$ K$^{-1}$), geometric visible albedo ($p_{mathrm{V}}$=0.122$pm$0.008), and estimate the macroscopic surface roughness. We also find that the Sun illumination at the perihelion passage during the past several thousand years is not connected to a specific area on the surface, which implies non-preferential heating.
The JAXA/ISAS spacecraft DESTINY$^+$ will be launched to the active asteroid (3200) Phaethon in 2022. Among the proposed core payload is the DESTINY+ Dust Analyzer (DDA) which is an upgrade of the Cosmic Dust Analyzer flown on the Cassini spacecraft to Saturn (Srama et al. 2011). We use two up-to-date computer models, the ESA Interplanetary Meteoroid Engineering Model (IMEM, Dikarev et al. 2005), and the interstellar dust module of the Interplanetary Meteoroid environment for EXploration model (IMEX; Sterken2013 et al., Strub et al. 2019) to study the detection conditions and fluences of interplanetary and interstellar dust with DDA. Our results show that a statistically significant number of interplanetary and interstellar dust particles will be detectable with DDA during the 4-years interplanetary cruise of DESTINY+. The particle impact direction and speed can be used to descriminate between interstellar and interplanetary particles and likely also to distinguish between cometary and asteroidal particles.
Apollo-type NEA (3200) Phaethon, classified at the B/F-type taxonomy, probably the main mass of the Phaethon-Geminid stream complex (PGC), can be the most metamorphic C-complex asteroid in our solar system, since it is heated up to ~1000 K by the solar radiation around its perihelion passages. Hence, its surface material may be easily decomposed in near-sun environment. Phaethons spectrum exhibits extremely blue-slope in the VIS-NIR region (so-called Phaethon Blue). Another candidate large member of the PGC, (155140) 2005 UD, shows a B/F-type color, however with a C-type-like red color over its ~1/4 rotational part, which implies an exposition of less metamorphosed primordial internal structure of the PGC precursor by a splitting or breakup event long ago. If so, some rotational part of Phaethon should show the C-type color as well as 2005 UD. Hence, we carried out the time-series VIS-spectroscopic observations of Phaethon using 1-m telescope in order to detect such a signature. Also, R-band photometries were simultaneously performed in order to complement our spectroscopy. Consequently, we obtained a total of 68 VIS-spectrophotometric data, 78% of which show the B-type blue-color, as against the rest of 22% showing the C-type red-color. We successfully acquired rotationally time-resolved spectroscopic data, of which particular rotational phase shows a red-spectral slope as the C-type color, as 2005 UD does, suggesting longitudinal inhomogeneity on Phaethons surface. We constrained this C-type red-colored area in the mid-latitude in Phaethons southern hemisphere based on the rotationally time-resolved spectroscopy along with Phaethons axial rotation state, of which size suggests the impact-induced origin of the PGC. We also surveyed the meteoritic analog of Phaethons surface blue-color, and found thermally metamorphosed CI/CM chondrites as likely candidates.
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