Context. 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. Aims. 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. Methods. 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 Phaethon’s orbit and spin axis with a numerical orbital and rotation-state integrator. Results. 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°, −39°) with a 5° uncertainty. Moreover, we derive its size (D = 5.1 ± 0.2 km), thermal inertia (Γ = 600 ± 200 J m–2 s–1/2 K–1), geometric visible albedo (pV = 0.122 ± 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.
•Asteroid Apophis was found in a non-principal axis rotation state (tumbling).•Its dynamical parameters and a shape model were derived.•Its retrograde spin calls for a further assessment of its ...impact probability.•Recent estimates of asteroid nutational damping times were reviewed and applied.•Parameters of tumbling asteroids place constraints on their evolution and properties.
Our photometric observations of Asteroid (99942) Apophis from December 2012 to April 2013 revealed it to be in a state of non-principal axis rotation (tumbling). We constructed its spin and shape model and found that it is in a moderately excited Short Axis Mode (SAM) state with a ratio of the rotational kinetic energy to the basic spin state energy E/E0=1.024±0.013. (All quoted uncertainties correspond to 3σ.) The greatest and intermediate principal moments of inertia are nearly the same with I2/I3=0.965-0.015+0.009, but the smallest principal moment of inertia is substantially lower with I1/I3=0.61-0.08+0.11; the asteroid’s dynamically equivalent ellipsoid is close to a prolate ellipsoid. The precession and rotation periods are Pϕ=27.38±0.07h and Pψ=263±6h, respectively; the strongest observed lightcurve amplitude for the SAM case is in the 2nd harmonic of P1=Pϕ-1-Pψ-1-1=30.56±0.01h. The rotation is retrograde with the angular momentum vector’s ecliptic longitude and latitude of 250° and -75° (the uncertainty area is approximately an ellipse with the major and minor semiaxes of 27° and 14°, respectively). An implication of the retrograde rotation is a somewhat increased probability of the Apophis’ impact in 2068, but it is still very small with the risk level on the Palermo Scale remaining well below zero. Apophis is a member of the population of slowly tumbling asteroids. Applying the theory of asteroid nutational damping by Breiter et al. (Breiter, S., Rożek, A., Vokrouhlický, D. 2012. Mon. Not. R. Astron. Soc. 427, 755–769), we found that slowly tumbling asteroids predominate in the spin rate–size range where their estimated damping times are greater than about 0.2Gyr. The appearance that the PA/NPA rotators transition line seems to follow a line of constant damping time may be because there are two or more asteroid spin evolution mechanisms in play, or the factor of μQ (the elastic modulus times the quality factor) is not constant but it may decrease with decreasing asteroid size, which would oppose the trend due to decreasing collisional age or excitation time.
Context. The rotation states of small asteroids are affected by a net torque arising from an anisotropic sunlight reflection and thermal radiation from the asteroids’ surfaces. On long timescales, ...this so-called YORP effect can change asteroid spin directions and their rotation periods. Aims. We analyzed lightcurves of four selected near-Earth asteroids with the aim of detecting secular changes in their rotation rates that are caused by YORP or at least of putting upper limits on such changes. Methods. We use the lightcurve inversion method to model the observed lightcurves and include the change in the rotation rate dω/ dt as a free parameter of optimization. To enlarge the time line of observations and to increase the sensitivity of the method, we collected more than 70 new lightcurves. For asteroids Toro and Cacus, we used thermal infrared data from the WISE spacecraft and estimated their size and thermal inertia by means of a thermophysical model. We also used the currently available optical and radar astrometry of Toro, Ra-Shalom, and Cacus to infer the Yarkovsky effect. Results. We detected a YORP acceleration of dω/ dt = (1.9 ± 0.3) × 10-8 rad d-2 for asteroid Cacus. The current astrometric data set is not sufficient to provide detection of the Yarkovsky effect in this case. For Toro, we have a tentative (2σ) detection of YORP from a significant improvement of the lightcurve fit for a nonzero value of dω/ dt = 3.0 × 10-9 rad d-2. We note an excellent agreement between the observed secular change of the semimajor axis da/ dt and the theoretical expectation for densities in the 2–2.5 g cm-3 range. For asteroid Eger, we confirmed the previously published YORP detection with more data and updated the YORP value to (1.1 ± 0.5) × 10-8 rad d-2. We also updated the shape model of asteroid Ra-Shalom and put an upper limit for the change of the rotation rate to | dω/ dt | ≲ 1.5 × 10-8 rad d-2. Ra-Shalom has a greater than 3σ Yarkovsky detection with a theoretical value consistent with observations assuming its size and/or density is slightly larger than the nominally expected values. Using the convex shape models and spin parameters reconstructed from lightcurves, we computed theoretical YORP values and compared them with those measured. They agree with each other within the expected uncertainties of the model.
•Data on rotations of 46 binary and triple asteroid systems were collected.•An anti-correlation of secondary synchroneity with orbital eccentricity observed.•Libration angles of synchronous ...secondaries are less than 20° on most epochs.•A paucity of chaotic rotations among asynchronous secondaries is apparent.•An upper limit on the secondary equatorial axis ratios of 1.5 was found.
We collected data on rotations and elongations of 46 secondaries of binary and triple systems among near-Earth, Mars-crossing and small main belt asteroids. 24 were found or are strongly suspected to be synchronous (in 1:1 spin–orbit resonance), and the other 22, generally on more distant and/or eccentric orbits, were found or are suggested to have asynchronous rotations. For 18 of the synchronous secondaries, we constrained their librational angles, finding that their long axes pointed to within 20° of the primary on most epochs. The observed anti-correlation of secondary synchroneity with orbital eccentricity and the limited librational angles agree with the theories by Ćuk and Nesvorný (Ćuk, M., Nesvorný, D. 2010. Icarus 207, 732–743) and Naidu and Margot (Naidu, S.P., Margot, J.-L. 2015. Astron. J. 149, 80). A reason for the asynchronous secondaries being on wider orbits than synchronous ones may be longer tidal circularization time scales at larger semi-major axes. The asynchronous secondaries show relatively fast spins; their rotation periods are typically <10 h. An intriguing observation is a paucity of chaotic secondary rotations; with an exception of (35107) 1991 VH, the secondary rotations are single-periodic with no signs of chaotic rotation and their periods are constant on timescales from weeks to years. The secondary equatorial elongations show an upper limit of a2/b2∼1.5. The lack of synchronous secondaries with greater elongations appears consistent, considering uncertainties of the axis ratio estimates, with the theory by Ćuk and Nesvorný that predicts large regions of chaotic rotation in the phase space for a2/b2≳2. Alternatively, secondaries may not form or stay very elongated in gravitational (tidal) field of the primary. It could be due to the secondary fission mechanism suggested by Jacobson and Scheeres (Jacobson, S.A., Scheeres, D.J. 2011. Icarus 214, 161–178), as its efficiency is correlated with the secondary elongation. Sharma (Sharma, I. 2014. Icarus 229, 278–294) found that rubble-pile satellites with a2/b2≲1.5 are more stable to finite structural perturbations than more elongated ones. It appears that more elongated secondaries, if they originally formed in spin fission of parent asteroid, are less likely to survive intact and they more frequently fail or fission.
Context.
The study of non-principal axis (NPA) rotators can provide important clues to the evolution of the spin state of asteroids. However, very few studies to date have focused on NPA-rotating ...main belt asteroids (MBAs). One MBA known to be in an excited rotation state is asteroid (5247) Krylov.
Aims.
By using disk-integrated photometric data, we construct a physical model of (5247) Krylov including shape and spin state.
Methods.
We applied the light curve convex inversion method employing optical light curves obtained by using ground-based telescopes in three apparitions during 2006, 2016, and 2017, along with infrared light curves obtained by the Wide-field Infrared Survey Explorer satellite in 2010.
Results.
Asteroid (5247) Krylov is spinning in a short axis mode characterized by rotation and precession periods of 368.7 and 67.27 h, respectively. The angular momentum vector orientation of Krylov is found to be
λ
L
= 298° and
β
L
= −58°. The ratio of the rotational kinetic energy to the basic spin-state energy
E
∕
E
0
≃ 1.02 shows that the (5247) Krylov is about 2% excited state compared to the principal axis rotation state. The shape of (5247) Krylov can be approximated by an elongated prolate ellipsoid with a ratio of moments of inertia of
I
a
:
I
b
:
I
c
= 0.36 : 0.96 : 1. This is the first physical model of an NPA rotator among MBAs. The physical processes that led to the current NPA rotation cannot be unambiguously reconstructed.
Context. Asteroid-pair age estimations are usually performed by backward integration of possible orbits of pair components, taking planetary perturbations into account along with the Yarkovsky ...effect. It is assumed that uncertainties coming from the backward integration process itself are small, so to reduce uncertainties in estimations, effort usually focuses on reducing uncertainties in the initial orbital elements and the dimensions of the pair components. Aims. This work aims to evaluate the role of the frequently ignored nonplanetary perturbers in asteroid-pair age estimation. When their role is not negligible, the ages of the youngest known pairs can be roughly re-estimated. Methods. The orbital evolution of several asteroid-pair components and the close approaches between components during the last ≈43 kyr are investigated. Initially, the force equations consisted of only planetary perturbers. The three largest main belt perturbers were added afterwards. Finally, as many as 262 massive main belt perturbers were included. The effect of main belters on age estimation is assessed by comparing the dates of the closest approaches between pair components. The range of the Yarkovsky effect is simulated very roughly, only for comparison purposes. Results. The estimated age of the youngest known pair (6070) Rheinland – (54827) 2001 NQ8 including the Yarkovsky effect seems to be either 16.3 ± 0.1 kyr for the retrograde rotation or 16.0 ± 0.2 kyr for the prograde rotation of the second component, respectively. This is younger by about 0.7−0.9 kyr than the previous estimate. Several more pairs, namely (88259) 2001 HJ7 − 1999 VA117, (111962) 2002 GP75 − (280008) 2001 UR224, (180906) 2005 KB6 − (217266) 2003 YR67, (229401) 2005 SU152 − 2005 UY97, and 2005 GS180 − 2008 FK107, had relative encounter velocities within 1.0 m s-1, suggesting they might also have formed within the interval studied. Some other pairs, including (5026) Martes – 2005 WW113, which was previously reported as very young, had slightly higher encounter velocities and longer nominal minimum distances, calling into question whether they could have formed within the interval studied or in some previous mutual encounter of pair components. Conclusions. The effect of main belt perturbers on asteroid-pair age estimation is buried in the huge age uncertainties caused by the Yarkovsky effect until the rotation direction of pair components is recognized.
A Survey of Novae in M83 Shafter, A. W.; Hornoch, K.; Benáček, J. ...
Astrophysical journal/The Astrophysical journal,
12/2021, Letnik:
923, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Abstract
The results of the first synoptic survey of novae in the barred spiral and starburst galaxy, M83 (NGC 5236), are presented. A total of 19 novae and one background supernova were discovered ...during the course of a nearly 7 year survey comprised of over 200 individual nights of observation between 2012 December 12 and 2019 March 14. After correcting for the limiting magnitude and the spatial and temporal coverage of the survey, the nova rate in M83 was found to be
R
= 19
+5
−3
yr
−1
. This rate, when normalized to the
K
-band luminosity of the galaxy, yields a luminosity-specific nova rate,
ν
K
= 3.0
+0.9
−0.6
× 10
−10
yr
−1
L
⊙,K
−1
. The spatial distribution of the novae is found to be more extended than the overall galaxy light suggesting that the observed novae are likely dominated by a disk population. This result is consistent with the observed novae light curves, which reveals that the M83 novae are on average more luminous at maximum light and fade faster when compared with novae observed in M31. Generally, the more luminous M83 novae were observed to fade more rapidly, with the complete sample being broadly consistent with a linear maximum magnitude versus rate of decline relation.
Pairs of asteroids sharing similar heliocentric orbits, but not bound together, were found recently. Backward integrations of their orbits indicated that they separated gently with low relative ...velocities, but did not provide additional insight into their formation mechanism. A previously hypothesized rotational fission process may explain their formation-critical predictions are that the mass ratios are less than about 0.2 and, as the mass ratio approaches this upper limit, the spin period of the larger body becomes long. Here we report photometric observations of a sample of asteroid pairs, revealing that the primaries of pairs with mass ratios much less than 0.2 rotate rapidly, near their critical fission frequency. As the mass ratio approaches 0.2, the primary period grows long. This occurs as the total energy of the system approaches zero, requiring the asteroid pair to extract an increasing fraction of energy from the primary's spin in order to escape. We do not find asteroid pairs with mass ratios larger than 0.2. Rotationally fissioned systems beyond this limit have insufficient energy to disrupt. We conclude that asteroid pairs are formed by the rotational fission of a parent asteroid into a proto-binary system, which subsequently disrupts under its own internal system dynamics soon after formation.
Context. Pairs of asteroids, that is, couples of single bodies on tightly similar heliocentric orbits, were recently postulated as a new category of objects in the solar system. They are believed to ...be close twins to binary and multiple systems. Aims. Ages of the known pairs range from about 15 kyr to nearly a million years. Beyond the upper limit, the pairs disperse in the background population of asteroids and become difficult to detect. Below the lower limit, the pairs should be easily recognizable if they exist and are discovered by surveys. Using the available data, we analyze the possible existence of very young asteroid pairs with clearly proven ages ≤ 10 kyr. Methods. We searched for candidate very young asteroid pairs in the current catalog of asteroid orbits. After a preliminary analysis, we selected the most promising case of the small asteroids (87887) 2000 SS286 and (415992) 2002 AT49. We collected photometric observations to determine their rotation periods and absolute magnitudes. Results. The rotation period of (87887) 2000 SS286 is 5.7773 ± 0.0004 h. Analysis of the data for (415992) 2002 AT49 indicates as the most probable period 2.6366 ± 0.0003 h, but other solutions are still possible. The composite light curves of the two asteroids have very low amplitudes, 0.22 and 0.12 mag, suggesting roundish shapes. Our observations also allow us to determine the absolute magnitude in R band HR = 14.99 ± 0.04 and HR = 16.24 ± 0.03 for the primary and secondary components. A transformation to the visible band provides H = 15.44 ± 0.05 and H = 16.69 ± 0.04. These two asteroids experienced a very close encounter, probably a formation event, some 7.4 ± 0.3 kyr ago. The formal extension of our numerical runs backward in time reveal that these close encounters may have continued, starting from ≃ 45 kyr ago. However, based on tests using synthetic fission events, we argue that the older age solutions might be the true solution only at ≃ (10−15)% level, assuming their low initial separation velocity is of between 10−20 cm s-1. This means that 87887–415992 probably is the youngest known asteroid pair in our dataset with a reliable determined age.
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