Properties of slowly rotating asteroids from the Convex Inversion Thermophysical Model (original) (raw)
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Thermal properties of slowly rotating asteroids: results from a targeted survey
Astronomy & Astrophysics
Context. Earlier work suggests that slowly rotating asteroids should have higher thermal inertias than faster rotators because the heat wave penetrates deeper into the subsurface. However, thermal inertias have been determined mainly for fast rotators due to selection effects in the available photometry used to obtain shape models required for thermophysical modelling (TPM). Aims. Our aims are to mitigate these selection effects by producing shape models of slow rotators, to scale them and compute their thermal inertia with TPM, and to verify whether thermal inertia increases with the rotation period. Methods. To decrease the bias against slow rotators, we conducted a photometric observing campaign of main-belt asteroids with periods longer than 12 h, from multiple stations worldwide, adding in some cases data from WISE and Kepler space telescopes. For spin and shape reconstruction we used the lightcurve inversion method, and to derive thermal inertias we applied a thermophysical mo...
Thermal properties of large main-belt asteroids observed byHerschelPACS
Astronomy & Astrophysics, 2020
Non-resolved thermal infrared observations enable studies of thermal and physical properties of asteroids via thermo-physical models provided the shape and rotational properties of the target are well determined. We used calibration-programmeHerschelPACS data (70, 100, 160μm) and state-of-the-art shape models derived from adaptive-optics observations and/or optical light curves to constrain for the first time the thermal inertia of twelve large main-belt asteroids. We also modelled previously well-characterised targets such as (1) Ceres or (4) Vesta as they constitute important benchmarks. Using the scale as a free parameter, most targets required a re-scaling ~5% consistent with what would be expected given the absolute calibration error bars. This constitutes a good cross-validation of the scaled shape models, although some targets required larger re-scaling to reproduce the IR data. We obtained low thermal inertias typical of large main belt asteroids studied before, which contin...
Photometric survey, modelling, and scaling of long-period and low-amplitude asteroids
Astronomy & Astrophysics
Context. The available set of spin and shape modelled asteroids is strongly biased against slowly rotating targets and those with low lightcurve amplitudes. This is due to the observing selection effects. As a consequence, the current picture of asteroid spin axis distribution, rotation rates, radiometric properties, or aspects related to the object’s internal structure might be affected too. Aims. To counteract these selection effects, we are running a photometric campaign of a large sample of main belt asteroids omitted in most previous studies. Using least chi-squared fitting we determined synodic rotation periods and verified previous determinations. When a dataset for a given target was sufficiently large and varied, we performed spin and shape modelling with two different methods to compare their performance. Methods. We used the convex inversion method and the non-convex SAGE algorithm, applied on the same datasets of dense lightcurves. Both methods search for the lowest devi...
Thermophysical Modeling of Asteroid Surfaces Using Ellipsoid Shape Models
The Astronomical Journal
Thermophysical Models (TPMs), which have proven to be a powerful tool in the interpretation of the infrared emission of asteroid surfaces, typically make use of a priori obtained shape models and spin axes for use as input boundary conditions. We test then employ a TPM approach-under an assumption of an ellipsoidal shape-that exploits the combination of thermal multi-wavelength observations obtained at preand post-opposition. Thermal infrared data, when available, at these observing circumstances are inherently advantageous in constraining thermal inertia and sense of spin, among other physical traits. We show that, despite the lack of a priori knowledge mentioned above, the size, albedo, and thermal inertia of an object are well-constrained with precision comparable to that of previous techniques. Useful estimates of the surface roughness, shape, and spin direction can also be made, to varying degrees of success. Applying the method to WISE observations, we present best-fit size, albedo, thermal inertia, surface roughness, shape elongation and sense of spin direction for 21 asteroids. We explore the thermal inertia's correlation with asteroid diameter, after accounting for its dependence on the heliocentric distance.
Against the biases in spins and shapes of asteroids
Planetary and Space Science, 2015
Physical studies of asteroids depend on an availability of lightcurve data. Targets that are easy to observe and analyse naturally have more data available, so their synodic periods are confirmed from multiple sources. Also, thanks to availability of data from a number of apparitions, their spin and shape models can often be obtained, with a precise value of sidereal period and spin axis coordinates. Almost half of bright (H≤11 mag) main-belt asteroid population with known lightcurve parameters have rotation periods considered long (P≥12 hours) and are rarely chosen for photometric observations. There is a similar selection effect against asteroids with low lightcurve amplitudes (a max ≤0.25 mag). As a result such targets, though numerous in this brightness range, are underrepresented in the sample of spin and shape modelled asteroids. In the range of fainter targets such effects are stronger. These selection effects can influence what is now known about asteroid spin vs. size distribution, on asteroid internal structure and densities and on spatial orientation of asteroid spin axes. To reduce both biases at the same time, we started a photometric survey of a substantial sample of those bright main-belt asteroids that displayed both features: periods longer than 12 hours, and amplitudes that did not exceed 0.25 magnitude. First we aim at finding synodic periods of rotation, and after a few observed apparitions, obtaining spin and shape models of the studied targets. As an initial result of our survey we found that a quarter of the studied sample (8 out of 34 targets) have rotation periods different from those widely accepted. We publish here these newly found period values with the lightcurves. The size/frequency plot might in reality look different in the long-period range. Further studies of asteroid spins, shapes, and structure should take into account serious biases that are present in the parameters available today. Photometric studies should concentrate on such difficult targets to remove the biases and to complete the sample.
Using observations made during the second quarter of 2016, the rotation periods and the semi-axis a/b ratio of the projected shape for six main-belt asteroids were For about the last four decades, photometric analysis of main-belt asteroids has been growing steadily. As of mid-2016, rotation periods for more than five thousand asteroids are reported in the asteroid lightcurve database (LCDB; Warner et al., 2009). In the LCDB, the quality of almost all lightcurve period solutions is indicated with a quality code (U). The U values range from 0 to 3, with U = 0 indicating that the reported period has been found to be wrong while U = 3 indicates a secure solution without any ambiguities. Some asteroids have no quality code assigned, which means that a valid estimate of the period and amplitude could not be made from the available data.
Photometry and Spin Rate Distribution of Small-Sized Main Belt Asteroids
2008
Photometry results of 32 asteroids are reported from only seven observing nights on only seven fields, consisting of 34.11 cumulative hours of observations. The data were obtained with a wide-field CCD (40.5'x27.3') mounted on a small, 46-cm telescope at the Wise Observatory. The fields are located within ±1.5 0 from the ecliptic plane and include a region within the main asteroid belt.
A refined “standard” thermal model for asteroids based on observations of 1 Ceres and 2 Pallas
Icarus, 1986
We present ground-based thermal infrared observations of asteroids 1 Ceres and 2 Pallas made over a period of 2 years. By analyzing these data in light of the recently determined occultation diameter of Ceres (933-945 km) and Pallas (538 km) and their known small-amplitude lightcurves, we have determined a new value for the infrared beaming parameter used in the "standard" thermal emission model for asteroids. The new value is significantly lower than that previously used, and when applied in the reduction of thermal infrared observations of other asteroids, should yield model diameters that are closer to actual diameters. In our formulation, we also incorporate the recently adopted IAU magnitude convention for asteroids, which uses the zero-phase magnitudes (including the opposition effect) the same as is used for satellites. oid whose surface elements are in instantaneous equilibrium with solar insolation, equivalent to a nonrotating body. In reality these conditions are not true, but given the uncertainties in the infrared calibration, absolute visual magnitudes, etc., these assumptions were acceptable. Although models developed by Morrison and his colleagues (Morrison, 1973; Jones and Morrison, 1974) differed somewhat from those of Matson and Lebofsky (Matson, 1971a,
Icarus, 2012
We collected mid-IR spectra from 5.2 to 38 lm using the Spitzer Space Telescope Infrared Spectrograph of 28 asteroids representative of all established types of binary groups. Photometric lightcurves were also obtained for 14 of them during the Spitzer observations to provide the context of the observations and reliable estimates of their absolute magnitudes. The extracted mid-IR spectra were analyzed using a modified standard thermal model (STM) and a thermophysical model (TPM) that takes into account the shape and geometry of the large primary at the time of the Spitzer observation. We derived a reliable estimate of the size, albedo, and beaming factor for each of these asteroids, representing three main taxonomic groups: C, S, and X. For large (volume-equivalent system diameter D eq > 130 km) binary asteroids, the TPM analysis indicates a low thermal inertia (C 6 100JsAˋ1/2KAˋ1mAˋ2)andtheiremissivityspectradisplaystrongmineralfeatures,implyingthattheyarecoveredwithathicklayerofthermallyinsulatingregolith.Thesmaller(surface−equivalentsystemdiameterDeff<17km)asteroidsalsoshowsomeemissionlinesofminerals,buttheyaresignificantlyweaker,consistentwithregolithswithcoarsergrains,thanthoseofthelargebinaryasteroids.Theaveragebulkdensitiesofthesemultipleasteroidsvaryfrom0.7−1.7g/cm3(P−,C−type)to100 J s À1/2 K À1 m À2 ) and their emissivity spectra display strong mineral features, implying that they are covered with a thick layer of thermally insulating regolith. The smaller (surface-equivalent system diameter D eff < 17 km) asteroids also show some emission lines of minerals, but they are significantly weaker, consistent with regoliths with coarser grains, than those of the large binary asteroids. The average bulk densities of these multiple asteroids vary from 0.7-1.7 g/cm 3 (P-, C-type) to 100JsAˋ1/2KAˋ1mAˋ2)andtheiremissivityspectradisplaystrongmineralfeatures,implyingthattheyarecoveredwithathicklayerofthermallyinsulatingregolith.Thesmaller(surface−equivalentsystemdiameterDeff<17km)asteroidsalsoshowsomeemissionlinesofminerals,buttheyaresignificantlyweaker,consistentwithregolithswithcoarsergrains,thanthoseofthelargebinaryasteroids.Theaveragebulkdensitiesofthesemultipleasteroidsvaryfrom0.7−1.7g/cm3(P−,C−type)to2 g/cm 3 (S-type). The highest density is estimated for the M-type (22) Kalliope (3.2 ± 0.9 g/cm 3 ). The spectral energy distributions (SEDs) and emissivity spectra, made available as a supplement document, could help to constrain the surface compositions of these asteroids. Icarus j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / i c a r u s from mid-IR observations. If the asteroid has a close passage with Earth or is sufficiently large, radar observations can also directly determine the size of the components.