Context.
The rotation state of small asteroids is affected by the Yarkovsky–O’Keefe–Radzievskii–Paddack (YORP) effect, which is a net torque caused by solar radiation directly reflected and thermally ...reemitted from the surface. Due to this effect, the rotation period slowly changes, which can be most easily measured in light curves because the shift in the rotation phase accumulates over time quadratically.
Aims.
By new photometric observations of selected near-Earth asteroids, we want to enlarge the sample of asteroids with a detected YORP effect.
Methods.
We collected archived light curves and carried out new photometric observations for asteroids (10115) 1992 SK, (1620) Geographos, and (1685) Toro. We applied the method of light curve inversion to fit observations with a convex shape model. The YORP effect was modeled as a linear change of the rotation frequency
υ
≡ d
ω
∕d
t
and optimized together with other spin and shape parameters.
Results.
We detected the acceleration
υ
= (8.3 ± 0.6) × 10
−8
rad d
−2
of the rotation for asteroid (10115) 1992 SK. This observed value agrees well with the theoretical value of YORP-induced spin-up computed for our shape and spin model. For (1685) Toro, we obtained
υ
= (3.3 ± 0.3) × 10
−9
rad d
−2
, which confirms an earlier tentative YORP detection. For (1620) Geographos, we confirmed the previously detected YORP acceleration and derived an updated value of
υ
with a smaller uncertainty. We also included the effect of solar precession into our inversion algorithm, and we show that there are hints of this effect in Geographos’ data.
Conclusions.
The detected change of the spin rate of (10115) 1992 SK has increased the total number of asteroids with YORP detection to ten. In all ten cases, the d
ω
∕d
t
value is positive, so the rotation of these asteroids is accelerated. It is unlikely to be just a statistical fluke, but it is probably a real feature that needs to be explained.
ABSTRACT Asteroid (3200) Phaethon is a Near-Earth Apollo asteroid with an unusual orbit that brings it closer to the Sun than any other known asteroid. Its last close approach to the Earth was in ...2017 mid-December and the next one will be on 2026 October. Previous rotationally time-resolved spectroscopy of Phaethon showed that its spectral slope is slightly bluish, in agreement with its B/F taxonomic classification, but at some rotational phases, it changes to slightly reddish. Motivated by this result, we performed time-resolved imaging polarimetry of Phaethon during its recent close approach to the Earth. Phaethon has a spin period of 3.604 h, and we found a variation of the linear polarization with rotation. This seems to be a rare case in which such variation is unambiguously found, also a consequence of its fairly large amplitude. Combining this new information with the brightness and colour variation as well as previously reported results from Arecibo radar observations, we conclude that there is no variation of the mineralogy across the surface of Phaeton. However, the observed change in the linear polarization may be related to differences in the thickness of the surface regolith in different areas or local topographic features.
Context.The Jupiter family comet 73P/Schwassmann-Wachmann 3 (SW3) split into several pieces in 1995. Some of the original fragments were observed during the next apparition of the comet in 1999-2001. ...The last return of the comet in 2005-2006 was accompanied by tremendous further splitting of some SW3 components – in particular component B – into a large number of subfragments. Aims.We present observations of components B and C during their closest approach to Earth in the first half of May 2006. These results aim at characterizing the properties of dust particles released from the fragments of comet SW3 and at identifying dusty and gaseous structures in the comae and tail regions of components B and C, which could be useful for conclusions about the presence of active regions and break-up events of the components. Methods.We used narrowband imaging polarimetry and an analysis of the broadband surface brightness for the characterization of the dust comae of the SW3 components. Coma and tail structures in components B and C were enhanced by numerical methods. Results.The degree of linear polarization of components B and C follows the empirical phase-angle dependence as derived from other comets. Fragment B was found in outburst on May 8. The brightness outburst was accompanied by changes in the shape of the coma. During the outburst, the spatial distribution of the linear polarization showed some peculiarities that had disappeared again on May 9. Arc-like signatures of a temporary nature, typical of fragmentation events, are seen in broadband filter images of component B on 3, 8, and 9 May 2006. The secondary dust tail, seen in component B on 3 and 4 May 2006, marks the synchrone of an earlier splitting event around 25 April 2006. Fan-like coma structures are found in the sunward coma hemisphere of components B and C that might be related to localized enhanced activity on the rotating nuclei.
Context. In late 2010, a Jupiter family comet 103P/Hartley 2 was the subject of an intensive world-wide investigation. On UT October 20.7, the comet approached the Earth within only 0.12 AU, and on ...UT November 4.6 it was visited by the NASA EPOXI spacecraft. Aims. We joined this international effort and organized a ground-based observing campaign with three key goals to: (1) measure the parameters of the nucleus rotation in a time series of CN; (2) investigate the compositional structure of the coma by comparing the CN images with nightly snapshots of C3; and (3) investigate the photochemical relation of CN to HCN, using the HCN data collected nearly simultaneously with our images. Methods. The images were obtained through narrowband filters using the two-meter telescope of the Rozhen National Astronomical Observatory. They were taken over four nights about the moment of the EPOXI encounter. Image processing methods and periodicity analysis techniques were used to identify transient coma structures and investigate their repeatability and kinematics. Results. We observe shells, arc-, jet- and spiral-like patterns that are very similar for the CN and C3 comae. The CN features expanded outwards with the sky-plane projected velocities of between 0.1 to 0.3 km s-1. A corkscrew structure, observed on November 6, evolved with a much higher velocity of 0.66 km s-1. The photometry of the inner coma of CN shows variability with a period of 18.32 ± 0.30 h (valid for the middle moment of our run, UT 2010 Nov 5.0835), which we attribute to the nucleus rotation. This result is fully consistent with independent determinations around the same time by other teams. The pattern of repeatability is, however, imperfect, which is understendable given the suggested excitation of the rotation state, and the variability detected in CN correlates well with the cyclic changes in HCN, but only in the active phases. The identified coma structures, along with the snapshot of the nucleus orientation obtained by EPOXI, enable us to estimate the spin axis orientation. We obtain RA = 122°, Dec = +16° (epoch J2000.0), neglecting at this point the rotational excitation.
► We present models that retrieve the dependence of cometary gas flux on direction. ► We determine the rotation rate of Comet 2P/Encke from observations of its CN coma. ► We determine the sense of ...rotation of Comet 2P/Encke from HCN spectra. ► We find evidence in Comet 2P/Encke for gas flow across its terminator. ► Comet 2P/Encke’s outgassing before perihelion is likely to come from deeper layers.
Axisymmetric models of the outgassing of a cometary nucleus have been constructed. Such models can be used to describe a nucleus with a single active region. The models may include a solar zenith angle dependence of the outgassing. They retrieve the outgassing flux at distances from the nucleus where collisions between molecules are unimportant, as function of the angle with respect to the outgassing axis. The observed emissions must be optically thin. Furthermore the models assume that the outflow speed at large distance from the nucleus does not depend on direction. The value of the outflow speed is retrieved. The models are applied to CN images and HCN spectra of Comet 2P/Encke, obtained nearly simultaneously in November 2003 with the 2
m optical telescope on Mount Rozhen, Bulgaria, and with the 10
m Heinrich Hertz Submillimeter Telescope on Mount Graham, Arizona, USA. According to
Sekanina (1988), Astron. J. 95, 911–924, at that time a single outgassing source was active. Input parameters to the models like the rotation period of the nucleus and a small correction to Sekanina’s rotation axis are determined from a simpler jet position angle model. The rotation is prograde with a sideric period of 11.056
±
0.024
h, in agreement with literature values. The best fit model has an outflow speed of 0.95
±
0.04
km
s
−1. The same value has been derived from the corkscrew appearing in the CN images. The location of the outgassing axis is at colatitude
δ
a
=
7.4°
±
2.9° and longitude
λ
a
=
235°
±
17° (a definition of zero longitude is provided). Comet Encke’s outgassing corresponds approximately to the longitudinally averaged solar input on a spherical nucleus (i.e. very likely comes from deeper layers) but with some deficiency of outgassing at mid-latitudes and non-zero outgassing from the dark polar cap. The presence of gas flow from the dark polar cap is explained as evidence of gas flow across the terminator. The models rely mostly on the CN images. The HCN spectra are more noisy. They provide information how to determine the best fit outflow velocity and the sense of rotation. The model HCN spectra are distinctly non-Gaussian. Within error limits they are consistent with the observations. Models based solely on the HCN spectra are also presented but, because of the lower quality of the data and the unfavorable observing geometry, yield inferior results. As a by-product we determine the CN parent life time from our CN observations. The solar EUV and Ly
α radiation field at the time of our observations is taken into account.
We present results of imaging and aperture polarimetry of the dust of comet 2P/Encke at phase angles 91–105°, obtained during the 2003 apparition. We investigate how strongly molecular emissions ...transmitted by the filters used in the observations can affect the resulting polarization of cometary dust. This problem is of particular importance for so-called gas-rich comets like comet 2P/Encke which has particularly strong molecular emission as compared to its dust continuum. Aperture polarimetry in the wide-band UBVR filters was performed at the 2.6-m Shain telescope and 1.25-m telescope of the Crimean Astrophysical Observatory on November 17–24. From these measurements a dust polarization of ≈$ 8$% is derived, which puts the comet in the class of comets with low polarization. The imaging observations of comet 2P/Encke were carried out at the 2-m telescope of the Bulgarian National Astronomical Observatory on November 20–22, 2003. Narrow-band filters centered on the 0–7–0 transition of the Ã2A_1 – $\tilde{X}$2B_1 electronic band system of NH2 (662 nm) and on an adjacent red continuum at 642 nm were employed. The polarization of NH2 averaged over the 0–7–0 vibronic transition amounts to ≈$ 7$% at phase angles close 90°, similar to the polarization of the two-atomic molecules CN and C2. The dust polarization however, when corrected for the effect of molecular emissions, is larger than 30%. We conclude that the division of comets into two polarimetric classes with one class having in the visual wavelength range a maximum polarization less than 20% is caused by ignoring the contribution of molecular emission and therefore is an artifact. Whether the comet displays a strong silicate feature (i.e. its dust grains are small) or not, the dust polarization is high.
We present preliminary results of the narrow-band CN observations of comet 8P/Tuttle from early January 2008, realized as part of a project consisting of near-simultaneous spectroscopic monitoring of ...HCN at millimeter-wavelengths and optical imaging of the coma. The mean-image subtraction method revealed low-contrast CN envelopes. Using the image cross-correlation technique we measured the projected velocity of these shells. For the sunward part we found it to be equal to 0.96 ± 0.03 km s
−1
on January 4 and 1.10 ± 0.01 km s
−1
on January 9, whereas the anti-sunward part reached 0.73 ± 0.05 and 0.80 ± 0.02 km s
−1
, respectively. The periodicity of gas emission was investigated using a repeatability of the shells, their kinematics, and an aperture photometry of the near nucleus region. We found a period of 5.70 ± 0.07 h (along with multiples), consistent with previous findings by other authors. A toy Monte Carlo model was implemented to reproduce the time-series of the CN images. We show that emission of HCN into a relatively wide cone by a single active region on a rotating nucleus is the most probable scenario.
On 4 July 2005 at 05:52 UT, the impactor of NASA's Deep Impact (DI) mission crashed into comet 9P/Tempel 1 with a velocity of about 10 ${\rm km\,s}^{-1}$. The material ejected by the impact expanded ...into the normal coma, produced by ordinary cometary activity. The La Silla and Paranal sites of the European Southern Observatory (ESO) in Chile participated in the world-wide campaign to observe this event.
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