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Register a new userSecular light curves of 25 members of the Themis family of asteroids, suspected of exhibiting low level cometary activity
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From 1996 to 2015 sixteen main belt asteroids were discovered exhibiting cometary activity (less than one per year), all of them during searches at the telescope. In this work we will explore another way to discover them. We reduced 192016 magnitude observations of 165 asteroids of the Themis family, using data from the astrometric-photometric database of the Minor Planet Center, MPCOBS, and measuring the absolute magnitudes from the phase plots. 25 objects of 165 (15.2%), exhibited bumps or enhancements in brightness that might indicate low level cometary activity. Since activity repeats at the same place in different orbits and in many occasions is centered at perihelion, activity might be due to water ice sublimation. As of September 2016, there are 717768 asteroids listed in the MPC files. If we assume that we do not have any false positives and the above percentage can be extrapolated to the whole Main Belt, the number of potentially active asteroid gets to the very large number of ~111.000. This number is much larger than the ones predicted in previous surveys and indicates one of three scenarios: A) there are many false positives in our detections and the real number of active asteroid is much smaller than we found, implying that the MPC astrometric-photometric database is only astrometric and not photometric. B) The location of active asteroids is restricted to the Themis family and an extrapolation to the whole belt is not possible. Or C) there are few false positives in our candidates and the main belt actually contains many low level active asteroids undetected by current surveys. Case C) would imply that the main belt is not a field of bare rocks but a graveyard of extinct comets, changing our current paradigm of the main belt. So it is of the outmost importance to verify observationally our candidates, and determine which of these scenarios is valid.
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Themis family is one of the largest and oldest asteroid populations in the main-belt. Water-ice may widely exist on the parent body (24) Themis. In this work, we employ the Advanced Thermophysical Model as well as mid-infrared measurements from NASAs Wide-Field Infrared Survey Explorer to explore thermal parameters of 20 Themis family members. Here we show that the average thermal inertia and geometric albedo are ~$39.5pm26.0 ~rm J m^{-2} s^{-1/2} K^{-1}$ and $0.067pm0.018$, respectively. The family members have a relatively moderate roughness fraction on their surfaces. We find that the relatively low albedos of Themis members are consistent with the typical values of B-type and C-type asteroids. As aforementioned, Themis family bears a very low thermal inertia, which indicates a fine and mature regolith on their surfaces. The resemblance of thermal inertia and geometric albedo of Themis members may reveal their close connection in origin and evolution. In addition, we present the compared results of thermal parameters for several prominent families.
Context. It has recently been proposed that the surface composition of icy main-belt asteroids (B-,C-,Cb-,Cg-,P-,and D-types) may be consistent with that of chondritic porous interplanetary dust particles (CPIDPs). Aims. In the light of this new association, we re-examine the surface composition of a sample of asteroids belonging to the Themis family in order to place new constraints on the formation and evolution of its parent body. Methods. We acquired NIR spectral data for 15 members of the Themis family and complemented this dataset with existing spectra in the visible and mid-infrared ranges to perform a thorough analysis of the composition of the family. Assuming end-member minerals and particle sizes (<2mum) similar to those found in CPIDPs, we used a radiative transfer code adapted for light scattering by small particles to model the spectral properties of these asteroids. Results. Our best-matching models indicate that most objects in our sample possess a surface composition that is consistent with the composition of CP IDPs.We find ultra-fine grained Fe-bearing olivine glasses to be among the dominant constituents. We further detect the presence of minor fractions of Mg-rich crystalline silicates. The few unsuccessfully matched asteroids may indicate the presence of interlopers in the family or objects sampling a distinct compositional layer of the parent body. Conclusions. The composition inferred for the Themis family members suggests that the parent body accreted from a mixture of ice and anhydrous silicates (mainly amorphous) and subsequently underwent limited heating. By comparison with existing thermal models that assume a 400km diameter progenitor, the accretion process of the Themis parent body must have occurred relatively late (>4Myr after CAIs) so that only moderate internal heating occurred in its interior, preventing aqueous alteration of the outer shell.
We obtained 877 images of active asteroid 6478 Gault on 41 nights from January 10th to June 8th, 2019, using several telescopes. We created the phase, secular and rotational light curves of Gault, from which several physical parameters can be derived. From the phase plot we find that no phase effect was evident. This implies that an optically thick cloud of dust surrounded the nucleus hiding the surface. The secular light curve (SLC) shows several zones of activity the origin of which is speculative. From the SLC plots a robust absolute magnitude can be derived and we find mV(1,1,alpha) = 16.11+-0.05. We also found a rotational period Prot = 3.360+-0.005 h and show evidence that 6478 might be a binary. The parameters of the pair are derived. Previous works have concluded that 6478 is in a state of rotational disruption and the above rotational period supports this result. Our conclusion is that 6478 Gault is a suffocated comet getting rid of its suffocation by expelling surface dust into space using the centrifugal force. This is an evolutionary stage in the lifetime of some comets. Besides being a main belt comet (MBC) the object is classified as a dormant Methuselah Lazarus comet.
Asteroid families are groups of minor planets which have a common origin in catastrophic disruptions. Young asteroid families are very interesting because they represent the product of their parent bodys fragmentation before orbital and physical evolutionary processes could have changed them. A group of minor asteroids associated with the largest body Datura(1270) is of particular interest because it has enough known members and resides in the inner part of the main asteroid belt and is easy to observe. Up to now, 7 members of this family are known. Here we discuss three new members of the Datura Family: 338309(2002 VR17), 2002 RH291 and 2014 OE206. To prove that these recently-discovered members belong to the Datura family, we conducted numeric integration with all gravitational perturbation over the last 800 kyrs. In the results, we have found that 338309(2002 VR17) and 2002 RH291 are very close to the mean orbit of this family throughout the calculation. In the case of 2014 OE206, it has a strongly chaotic orbit. The possible explanation of this is in the resonance character of its orbit.
The Karin family is a very young asteroid family created by an asteroid breakup 5.8 Myr ago. Since the members of this family probably have not experienced significant orbital or collisional evolution yet, it is possible that they still preserve properties of the original family-forming event in terms of their spin state. As we carried out a series of photometric observations of the Karin family asteroids, here we report an analysis result of lightcurves including the rotation period of eleven members. The mean rotation rate of the Karin family members turned out to be much lower than those of NEAs or smaller MBAs (diameter D<12 km), and even lower than that of larger MBAs (D>130 km). We investigated a correlation between the peak-to-peak variation magnitude reduced to zero solar phase angle and the rotation period of the eleven Karin family asteroids, and found a possible trend that elongated members have lower spin rate, and less elongated members have higher spin rate. However, this has to be confirmed by another series of future observations.
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