Advances in Astronomy and Space Physics, 10, 48-54 (2020) doi: 10.17721/2227-1451.10.40-54 Astrometric and photometric observations of six brightest trans-Neptunian objects at the Kyiv comet station 1 Astronomical Observatory of Taras Shevchenko National University of Kyiv, 3 Observatorna Str., 04053 Kyiv, Ukraine 2Taras Shevchenko National University of Kyiv, 4 Glushkova Ave., 03127, Kyiv, Ukraine Main Astronomical Observatory of the NAS of Ukraine, 27 Akademika Zabolotnoho Str., 03143, Kyiv, Ukraine In this work we focused on observations of six trans-Neptunian objects (TNOs) whose apparent magnitudes are brighter than 20m. We present the results of astrometric and photometric observations of (134340) Pluto, (136108) Haumea, (136472) Makemake, (136199) Eris, (90482) Orcus and (20000) Varuna obtained at the Kyiv comet station (Code MPC 585) in 2017-2019. For observations we used the 0.7-m (f/4) reflector AZT-8 with the FLI PL4710 CCD camera and filters of Johnson – Cousins photometric system. From our images we measured the objects’ astrometric positions, calculated apparent magnitudes in the BVRI (mostly R) bands using the aperture photometry method, and found the absolute magnitudes together with the colour indices in several bands. Analysing our results, we investigate the limitation on the astrometry and photometry of faint objects with the 0.7-m telescope. Key words: Kuiper belt objects: individual: Pluto, Haumea, Makemake, Eris, Orcus, Varuna; astrometry; techniques: photometric INTRODUCTION aperture under 1-m are still widely used for astro- nomical observations, in particular for monitoring Trans-Neptunian objects (TNOs) are the Solar programs. One of many examples is the 0.7-m (f/4) System bodies which orbit the Sun at a greater aver- reflector AZT-8, which is one of a few sources of age distance than Neptune. Although the properties unique astronomical observations in Ukraine. Work- of the bright TNOs are well-defined, the peripheral ing with such equipment, it is important to know as region of the Solar System generally remains poorly much as possible about its limitations and the quality studied. Ongoing astrometric and photometric mon- of the results obtained on the edge of observational itoring of TNOs enables better determination of their ability. Due to the small sizes and large distances to orbits, detection of variations in surface properties the Sun and the Earth of TNOs, the vast majority of and investigation of their orbital and photometric them lie close to or beyond the limit of observation for evolution. Also, TNOs are the indicators of possi- AZT-8. In this paper we present the results of our first ble gravitational influence from undiscovered bodies observations of several TNOs using 0.7-m telescope at including the hypothetical Ninth Planet [1]. During the Kyiv comet station. the last decade, multiple objects were discovered and observed both from ground-based and space obser- vatories as a part of the focused monitoring of sin- TARGET DESCRIPTION gle bodies and the large-scale surveys. In this work Pluto is a Kuiper belt object and a namesake for a we have chosen six bright objects (Pluto, Haumea, group of TNOs in the 2:3 orbital resonance with Makemake, Eris, Orcus, Varuna) that are possible to Neptune, plutinos. It has a system of 5 satellites, observe with the equipment at the Kyiv comet and Charon is the only moon of Pluto that is massive station (MPC code 585) because of their predicted m m enough to be in hydrostatic equilibrium, also caus- apparent magnitude of 14 – 20 . ing the barycenter of the system to be outside Pluto. Despite the growing number of space and large Beyond Charon, there are four much smaller moons, ground-based telescopes, small telescopes with the which are, in ascending order of distance from Pluto, [email protected] [email protected] A. Baransk y, O. Lukina, S. Borysenko, 2020 48 Advance s in Astronomy and Space Physics A. Baransky, 0. Lukina, S. Borysenko Styx, Nix, Kerberos, and Hydra [32]. In 2015, the Uoruno is a classical Kuiper belt object. In 2019, observations by the New Horizons spacecraft were it was proposed that changes in the rotational light- conducted using a remote sensing package that in- curve shape of Varuna may be caused by the pres- cluded imaging instruments and a radio science in- ence of a large undiscovered satellite 9]. Multi-band vestigation tool, as well as spectroscopic and other thermal observations from the Herschel Space Ob- experiments. Using the data from the New Horizons servatory in 2013 proposed the mean diameter of Long-Range Reconnaissance Imager, the diameter of Varuna equal to 668 km 15]. The rotational period Pluto was found to be 2377 km [20], making Pluto of Varuna is calculated to be 6.344 hours [2], and it the largest known TNO. Pluto's period of rotation is is a possible reason for its elongated shape. Varuna known to be 6.387 days [19]. It passed perihelion in will approach aphelion in 2071. 1989. Haumea is a resonance Kuiper belt object in the OBSERVATIONS weak 7:12 orbital resonance with Neptune. It has two small satellites, Hi'iaka and Namaka, with the In the period 2017-2019, the observations of se- compound mass of approximately 0.5 percent of the lected trans-Neptunian objects were conducted by mass of the system [24]. The data from the 2017 Alexander Baransky and Oleksandra Lukina at the stellar occultation indicate the presence of a narrow Kyiv Comet Station (MPC code 585), Lisnyky, and dense ring with the radius close to the 3:1 mean- Ukraine (see Figure 1). The standard Johnson- motion resonance with Haumea's spin period [21]. It Cousins broadband filters BVRI were used with is the object of an unusual elongated shape with the the FLI PL 4710 CCD camera at the prime focus largest axis estimated to be at least 2322 km [21]. F = 2828 mm) of the 0.7—m (f/4) reflector AZT—8. The possible reasons for Haumea's shape are the very The detector consists of the 1024 x 1024 array of short period of rotation, 3.915 hours [14] and mass 13 μm pixels, which corresponds to the scale of 0.947" concentrations towards the nucleus [21]. It passed per pixel. aphelion in 1992. The log of observations is listed in Table 3, where Makemake Ni is the number of images per observational night; is a classical Kuiper belt object close Exp. to the 7:13 orbital resonance with Neptune, and it (s) is the exposition in seconds; r (AU) is the has a single known satellite [22]. Makemake has a high heliocentric distance; Δ (AU) is the geocentric dis- orbital inclination and its orbital properties are tance; α (deg) is the phase angle of the object; SNR generally close to the ones of Haumea (see Table l). is the signal-to-noise ratio. Time of the observations The diameter of Makemake was investigated dur- presented in Table 3 is a mean of exposure. ing the 2013 stellar occultation and estimated to be Firstly, using the software SkyMap, The Sky, and 1434 km [5]. Visible and near-infrared spectral ob- JPL ephemerides, we planned the observations and servations of Makemake showed that this object has prepared equatorial coordinates of the targets. Then an overall homogeneous surface [31]. Recent results we used the MaxIm DL program to cool down the of the long-term monitoring of Makemake suggested camera and control the process of observation. We that it has an almost spherical shape and estimated took series (from 5 to 30 per filter) of images in mul- its rotational period as 22.826 hours [12]. Makemake tiple filters during one session. passed aphelion in 1992. The limitation for astrometric analysis is the abil- ity to see the target on the image. With the 0.7-m Eris is classified as a scattered disk object due telescope it can be performed for faint objects with to its high orbital eccentricity, and it has one known the apparent magnitude up to 20m in case if the large moon Dysnomia [6]. The orbit of Eris has the incli- enough number of images is available and the addi- nation of 44 degrees, which is much higher than the tion tool is used. We managed to obtain the astro- values of Kuiper belt objects. The results of the 2011 metric positions for all cases, except the observations occultation estimated its diameter to be 2326 km, of Eris in V and B bands (18.5m in R band) and the making Eris the second-largest TNO after Pluto. It observation of Varuna in V band (19.8m in R band). also indicated the albedo of 0.96, one of the high- As a conclusion, the observations of targets with the est values in the Solar System [29]. The rotational R magnitudes in the range 18m—20m generally gave period of Eris estimated from the space-based pho- satisfactory results only when observed in the R tometric observations is 1.08 days [26]. Eris passed band. The limitation of photometric analysis is aphelion around 1977. mainly determined by the signal-to-noise ratio given Orcus is a Kuiper belt object in the 2:3 or- in Table 3. bital resonance with Neptune which is classified as plutino. Vanth, the large satellite of Orcus, is ap- ASTROMETRIC AND PHOTOMETRIC proximately half of its diameter with the albedo ap- proximately three times smaller. In 2018, the diam- ANALYSIS eter of Orcus was calculated to be 910 km |6] , and the rotational period of this object was found to be For astrometric measurements, the Astrometrica 4 10.47 hours 16].