|Sažetak (engleski)|| |
Extended abstract - Besides the Sun and the planets, our Solar System contains nearly 1 million discovered small Solar System bodies. Due to their small size, they belong to objects whose research often requires large-sized telescopes or the application of modern observation methods. One of today’s modern observational methods used for determining the physical characteristics of the small Solar System bodies, allowing several orders of magnitude better resolutions than any other Earth-based method, is the stellar occultations method. By composition and orbital parameters, small Solar System bodies can be classified into asteroids and comets. Nevertheless, there is a group of asteroid-like objects that show cometlike activities. This doctoral thesis consists of 3 parts. In part one, the observed micro activity of the small Solar System body 95P/(2060) Chiron was investigated. In part two, as an extensive example of the application of the stellar occultation method to investigate the physical characteristics of small Solar System bodies, the determination of the dwarf planet’s Haumea size, shape, albedo, and density was described. In part three, in order to improve the temporal resolution of observations of stellar occultations with small bodies, the potential of using the atmospheric Cherenkov telescopes MAGIC for observing stellar occultations was investigated. Activity of 95P/(2060) Chiron The minor planet 95P/(2060) Chiron belongs to the asteroid group of centaurs, which according to dynamic parameters belongs to asteroids. Chiron has an orbit far away from the Sun, in which sublimation of water ice is not considered as a possible cause for triggering mass loss mechanisms. The expected rate of collisions among small bodies in the region of centaurs is not expected to be high. Regardless of that, since its discovery, Chiron has repeatedly shown indications of cometary activity (Kowal et al. 1979; Tholen et al. 1988; Meech Belton 1990). In 2015, Ortiz et al. (2015) found indications that (2060) Chiron possesses rings. Motivated by these findings, additional observations and interpretations of the cause of its activity were made and described in this doctoral thesis (and published in Cikota et al., 2018). Observations were carried out in three different observation campaigns between 2014 and 2016, using three different instruments located at the Calar Alto Observatory (CAHA) in Almeria, Spain - 3.5 m MOSCA, 1.23 m DLR-MKIII and 2.2 m CAFOS. From the collected observations, amplitudes of Chiron’s rotational light curves and measurements of Chiron’s absolute magnitudes were determined. The measured amplitudes are consistent with the model proposed by Ortiz et al. (2015) which is taking into account both, Chiron’s triangular ellipsoid-like shape and the light contribution of rings around Chiron. Although the frequency of collisions among small bodies in the centaur region is very low, iv due to the recent indication that Chiron possesses rings, and assuming that there is debris orbiting around Chiron, it is well possible that some of this debris may be continually falling and impacting on its surface, producing outbursts and forming a bound or quasi-bound coma. Consequently, a dust release mechanism triggered by impacts should not be excluded as a possible cause for its activity. To study this hypothesis, the scatter in Chiron’s photometric data was compared to the scatter of comparison stars of similar brightness as Chiron. The scatter of Chiron’s photometric measurements for the observations carried out on July 23 and 24, 2014, with the DLR-MKIII 1.23 m CAHA telescope was found to be 0.027 mag, while the comparison star’s scatter is 0.014 mag. During the photometric campaign in 2015, containing data collected with CAFOS 2.2 m CAHA telescope on September 11 and September 13, the scatter of Chiron’s photometric measurements was determined to be 0.029 mag, while the comparison star’s scatter values were 0.018 mag. The noticeable difference between the comparison star’s and Chiron’s photometric scatter supports our hypothesis of continuous micro activity on Chiron. That mechanism could also be a plausible explanation for impulsive brightening on a timescale of hours that have been observed by Luu Jewitt (1990). We hypothesize that some activity outbursts on regular comets could also be triggered by collisions, either with debris orbiting them or with meteoroids. Additionally, the data collected with the CAFOS 2.2 m CAHA telescope over 3 nights between September 11 and September 14, 2015, was used to search for comet-like activity signs like coma or tail. The data was combined to a false-color image of Chiron that contains 95 integrations of 300 s each, resulting in a total integration time of 28500 s (475 min). The combined image of Chiron shows that the coma itself is not detectable, but a very faint asymmetric shape with a measured position angle of ∼ 87◦ was observed. The feature can be explained by a ∼5 arcsec long comet-like tail, of a determined surface brightness of 25.3 mag(R)/arcsec2. Determination of the dwarf planet Haumea’s physical characteristics - Five dwarf planets are currently known in our Solar system. (1) Ceres is the closest dwarf planet to us, and the only dwarf planet orbiting in the inner part of the Solar system, within the main belt of asteroids. Four dwarf planets - (134340) Pluto, (136199) Eris, (136108) Haumea, and (136472) Makemake, are members of the Kuiper belt, located in the outer part of the Solar system beyond Neptune’s orbit. Due to the large distances of dwarf planets in the outer Solar system, indirect observation methods are necessary to accurately determine their physical characteristics. Among trans-Neptunian objects, the dwarf planet Haumea stands out in its elongated shape and extremely short rotation period (Brown et al. 2005; Rabinowitz et al. 2006; Brown et al. 2007). Unlike other dwarf planets (Sicardy et al. 2011; Ortiz et al. 2012; Stern et al. v 2015), Haumea’s physical characteristics were not precisely determined. In this doctoral thesis, the determination of Haumea’s size, shape, albedo, and density was described, as well as the discovery and characterization of its rings (published in Ortiz et al. 2017). On January 21, 2017, a stellar occultation of the dwarf planet Haumea was observed. The occulted star URAT1 533-182543 with its apparent brightness of 𝑚𝑅 = 17.6 mag made it observable already for small- and mid-sized telescopes. Therefore, the occultation event was observed from many locations and resulted in 12 positive detections from 10 different sites across Europe. From the collected sequences of CCD images, photometric measurements of the synthetic aperture were performed. In the case of data collected from the Črni Vrh Observatory by the drift method, a special procedure of photometric analysis was applied to the stellar trail. By fitting a square well model to the collected photometric measurements, the times of the star’s disappearance and reappearance were determined, and occultation chords were generated for each observation location. To the chords in the projected plane, an elliptical limb was fitted by minimizing a 𝜒2 function. By that, the apparent edge of the shape of the ellipse, which represents the outline of Haumea’s shadow, was determined. The determined parameters of the major and minor axis of the ellipse are 1 704 ± 4 km and 1 138 ± 26 km respectively. The position angle of the minor axis is -76.3◦ ± 1.2◦. A search for an atmosphere was performed and the upper value of Haumea’s atmospheric pressure was determined by using the best available data set collected by the Asiago Observatory. From Haumea’s mass, and assuming that the body is in hydrostatic equilibrium, an average surface gravity of 0.39 ms−2 was determined. By comparing a modelled occultation light curve with the observed light curve, the upper value of Haumea’s pure isothermal global atmosphere of 𝑁2 in thermal equilibrium can be determined. Assuming a surface temperature of 40 K, we find a 1 𝜎 upper limit for 𝑝surface(𝑁2) = 3 nbar, and a 3 𝜎 upper limit for 𝑝surface(𝑁2) = 15 nbar. In addition to the main occultation, there are brief dimmings prior to and after the main event. These dips are consistently explained by a narrow and dense ring around Haumea that absorbed about 50% of the incoming stellar flux. A distinct cross-sectional profile of the rings collected in an observation with the 1 m telescope of the Konkoly Observatory reveals a radial ring width of 𝑊ring ≈ 74 km at the occultation ingress and 𝑊ring ≈ 44 km at the occultation egress, with corresponding apparent transparency of p’ = 0.55 and p’ = 0.56. The locations of twelve detected secondary events, projected in the sky plane, allow the retrieval of the full ring orbit, assuming an apparent elliptical shape, and using the same approach as used for fitting Haumea’s limb. The parameters of the elliptical fit to the rings are 𝑎0 ring = 2287+75 −45 km for the apparent semimajor axis, and 𝑏0 ring = 541 ± 23 km for a semiminor axis. The position angle of the minor vi axis is 𝑃ring = -74.3◦ ± 1.3◦. Assuming the rings are circular, it implies a radius of the ring of 𝑟ring = 2287+75 −45 km and an inclination angle of 𝐵ring = 13,8◦ ± 0.5◦. The circular ring assumption is supported by the fact that the center of the ring ellipse coincides with the order of uncertainty with the center of the limb of Haumea’s main body. The position angle of the ring minor axis 𝑃ring = -74.3◦ ±1.3◦ coincides with that of the limb minor axis 𝑃limb = -76.3◦ ± 1.2◦, which is another strong argument that we are observing a ring that settled into Haumea’s equatorial plane. Furthermore, it was found that the radius of the rings coincides with the area where 3:1 spin-orbit resonances are expected. From the determined parameters of Haumea’s limb, a three-dimensional shape and density of Haumea were determined. The obtained parameters of the triaxial ellipsoid a × b × c are (1 161 ± 30) km × (852 ± 2) km × (513 ± 16) km. The mean value of the Haumea density, determined through the parameters of the triangular ellipsoid, and the mass determined by the orbital period of the Hi’iake satellite, is 1 885 ± 80 kg m−3. The geometric albedo of Haumea determined by our observations is 0.51, which is significantly smaller than the value of 0.804, as available in the most recent literature (Fornasier et al. 2013). Application of the atmospheric Cherenkov telescopes MAGIC for observing stellar occultations - In order to accurately determine the physical characteristics of small bodies by using the occultation method, a high temporal resolution of the detector is required. In order to improve the observational methods of stellar occultations, the use of the MAGIC (Major Atmospheric Gamma Imaging Cherenkov) telescopes was considered. With its sensitive central pixel, the MAGIC-II telescope has shown to be an excellent instrument for detecting millisecond flashes in the visible spectra. Compared to any other optical telescope used for observing stellar occultations, its central pixel with a temporal resolution of 10 kHz, combined with a large 17 m aperture, the MAGIC-II telescope enables reaching two orders higher resolutions. However, some of its limiting characteristics are the inability to control the sampling frequency, sensitivity only in the visible spectrum between 300 and 400 nm, and a low angular resolution (due to physically large pixels) responsible for a limitation in the apparent brightness of m = 13.5 ± 0.6 mag. In addition to determining the physical characteristics of small bodies, using the stellar occultation method enables us to determine the angular sizes of the occulted stars. Since stars are not point sources (they have a finite angular diameter), during the occultation of a stellar disk, its light curve does not show a sharp brightness drop but describes a Fresnel interference pattern whose details depend on the angular diameter of the occulted star. High sampling rates make it possible to record these interference patterns. Therefore, the high temporal resolution vii of the detector is the key for direct measurements of stellar radii. In this doctoral thesis, the method of determining stellar radii by observing Fresnel diffraction patterns that can be detected during stellar occultations is described. Models of the expected signal were performed, taking into account the characteristics of the MAGIC telescopes and the sensitivity of its detectors. Through observational proposals, telescope time was granted and first observations of stellar occultations with the MAGIC telescopes were acquired. Additionally, the effects of limb darkening of stellar disks were investigated. It was found that diffraction patterns formed by occultations of large disks with angular diameters of 0.7 mas and high limb darkening factors of u = 1.0 deviate in intensity up to approximately 5% from diffraction patterns of homogeneously illuminated stellar disks (u = 0.0). For stellar disks of smaller diameters or smaller limb darkening factors, the intensity deviations are smaller. These deviations in the intensity of diffraction patterns are reflected in the determination of the angular diameters of stellar disks. It was found that the angular radius of a homogeneously illuminated stellar disk, compared to a disk with a limb darkening factor of u = 1.0 taken into account, deviates by approximately 12.4%. Taking the limb darkening factors into account contributes to the accuracy of measurements of the angular diameters of stellar disks, but to achieve this accuracy, photometric measurements with uncertainties below 5% are required. Historically, almost all measurements of the stellar radii smaller than ∼1 arc millisecond have been performed by interferometric measurements. Measurements of stellar radii by observing occultations with asteroids allow reaching angular resolutions up to 0.1 milliarcseconds, which is at least a factor of two larger than the one available through interferometric measurements, and a factor of 10 larger than what can be achieved by observing lunar occultations. Therefore, the asteroid occultation method is extremely useful for measuring the angular diameters of stars below 1 milliarcsecond. In the period from December 2017 to February 2020, a total of 6 occultation observations with the MAGIC-II telescope were scheduled. Four observations were not successfully acquired due to bad weather or technical reasons. Two occultation events, those by the small bodies (83) Beatrix and (28) Bellona, were successfully acquired but resulted in a negative detection. Although the occultation events of (83) Beatrix and (28) Bellona occurred over sufficiently bright sources, and their probability of observation was estimated at 99%, the negative detections can be explained by a too small brightness drops, ∼1.5 mag for the (83) Beatrix event (2.5 𝜎), and ∼0.9 mag for the (28) Bellona event (1.5 𝜎). Based on the analysis presented in this thesis, we expect that any event with a brightness drop larger than 1.8-3.0 mag (3-5 𝜎 above background noise) should be detected. Therefore, the observation campaign of occultations on MAGIC telescopes will be continued in the future.