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Tartu Observatory telescopes offer unique guaranteed access to objects in the Northern hemisphere. The observational facilities include a 1.5- m and 0.6-m classic Cassegrain reflectors, and 0.31-m remotely controllable telescope . The 1.5 m telescope is currently equipped with a long-slit Cassegrain spectrograph used for stellar characterisation. Historically, the objects of interest have been massive stars but we are now developing new research direction and expanding the list of targets to exoplanet- and disk-hosting stars. We have started evaluating our capabilities to characterise host stars spectroscopically to determine their parameters and composition. In 2020, we carried out a pilot study of a TESS candidate planet host, which we found to have a rare, strong chemical peculiarity [1]. This also allowed us to prepare our tools, workflow, and end-to-end analysis. We are also contributing to the European Space Agency Ariel space mission by offering stellar activity monitoring. The 0.6-m and 0.31-m telescopes are utilised for photometric measurements and the 0.31-m one in particular has been a workhorse for exoplanet transit monitoring. Since 2020, we have made significant preparations to develop and prove our transit observation capabilities: we have observed more than 70 transit light curves. About half of them have been submitted to ExoClock to contribute to Ariel mission planning. Concerning future upgrades, Tartu Observatory will have new instruments by the middle of 2023. The upgrades include procuring a medium resolution echelle spectrograph (projected bandwidth 390 nm to 750 nm, R= 25 000) and new photometer (Johnson-Cousins BVRI and SDSS filters) for the 1.5-m telescope, which will not only enhance our capabilities in both spectroscopic and photometric data retrieval of host stars. In addition, a new remote control system of the telescope will be installed and improvements on instrumentation for the 0.6-m and 0.31-m photometric telescopes will be made. This presentation will give an overview of our facilities, and of current and future spectroscopic and photometric capabilities. References: “A rare phosphorus-rich star in an eclipsing binary from TESS”, Colin P. et al., A&A 658 A105 (2022), DOI: 10.1051/0004-6361/202142124

Context: Few exoplanets around hot stars with radiative envelopes have been discovered, although new observations from the TESS mission are improving this. Stars with radiative envelopes have little mixing at their surface, and thus their surface abundances provide a sensitive test case for a variety of processes including potentially star-planet interactions. Atomic diffusion is particularly important in these envelopes, producing chemically peculiar objects such as Am and HgMn stars. Aims: An exoplanet candidate around the B6 star HD 235349 was identified by TESS. Here we determine the nature of this transiting object and identify possible chemical peculiarities in the star. Methods: HD 235349 was observed using the long-slit spectrograph at Tartu Observatory, as well as photometrically by the TESS mission. The spectra were modeled to determine stellar parameters and chemical abundances. The photometric light curve was then analyzed in the context of the stellar parameters to determine properties of the transiting object. Results: We find the transiting object is a low-mass stellar companion, not a planet. However, the primary of this eclipsing binary is a rare type of chemically peculiar star. A strong overabundance of P is found with overabundances of Ne and Nd, and mild overabundances of Ti and Mn, while He is mildly underabundant. There is also clear evidence for vertical stratification of P in the atmosphere of the star. The lack of Hg and weak Mn overabundance suggests that this is not a typical HgMn star. It may be in the class of helium-weak phosphorus-gallium (He-weak PGa) stars, or an intermediate between these two classes. Conclusions: We show that HD 235349 is a rare type of chemically peculiar star (He-weak PGa) in an eclipsing binary system with a low-mass stellar companion. This appears to be the first He-weak PGa star discovered in an eclipsing binary., Accepted for publication in A&A. 15 pages (corrected minor typos, two references modified)

Massive stars typically undergo short-lived post-main sequence evolutionary phases with strong mass loss and occasional mass eruptions. Many of such massive stars in transition phases have been identified based on their dusty envelopes. The ejected material often veils the stellar photospheres so that the central stars cannot be assigned proper spectral types and evolutionary stages. The infrared spectral range has proved to be ideal for the classification of evolved massive stars and for the characterization of their environments. To improve our knowledge on the central stars of four such dust enshrouded objects: [GKF2010] MN 83, [GKF2010] MN 108, [GKF2010] MN 109, and [GKF2010] MN 112, we collect and present their first medium resolution K-band spectra in the $2.3\,-\,2.47\,\mu$m region and discuss the location of the stars in the JHK color-color diagram. We find that the emission-line spectra of both MN 83 and MN 112 show characteristics typically seen in Luminous Blue Variable (LBV) stars. In addition, we propose that the presence and strength of the newly reported Mg II lines might be used as a new complementary criterion to identify LBV candidates. The spectra of the other two objects imply that MN 108 is an O-type supergiant, whereas MN 109 could be an LBV candidate in its active phase. We derive lower limits for the reddening toward the stars and find that three of all de-reddened fall into the region of confirmed LBVs., Facultad de Ciencias Astronómicas y Geofísicas, Instituto de Astrofísica de La Plata

Yellow hypergiants are evolved massive stars that were suggested to be in post-red supergiant stage. Post-red supergiants that evolve back to the blue, hot side of the Hertzsprung-Russell diagram can intersect a temperature domain in which their atmospheres become unstable against pulsations (the Yellow Void or Yellow Wall), and the stars can experience outbursts with short, but violent mass eruptions. The yellow hypergiant Rho Cas is famous for its historical and recent outbursts, during which the star develops a cool, optically thick wind with a very brief but high mass-loss rate, causing a sudden drop in the light curve. Here we report on a new outburst of Rho Cas which occurred in 2013, accompanied by a temperature decrease of ~3000 K and a brightness drop of 0.6 mag. During the outburst TiO bands appear, together with many low excitation metallic atmospheric lines characteristic for a later spectral type. With this new outburst, it appears that the time interval between individual events decreases, which might indicate that Rho Cas is preparing for a major eruption that could help the star to pass through the Yellow Void. We also analysed the emission features that appear during phases of maximum brightness and find that they vary synchronous with the emission in the prominent [CaII] lines. We conclude that the occasionally detected emission in the spectra of Rho Cas, as well as certain asymmetries seen in the absorption lines of low to medium-excitation potential, are circumstellar in nature, and we discuss the possible origin of this material., 19 pages, 13 figures, accepted for publication in MNRAS

ESTCube-1 is a student satellite project lead by the University of Tartu, Estonia, and supported by the European Space Agency (ESA) via Plan for European Cooperating States (PECS). Development of ESTCube-1 has been a collaborative effort with many international partners. The satellite is shown on Figure 1 [1].

The stellar evolution theory of massive stars remains uncalibrated with high-precision photometric observational data mainly due to a small number of luminous stars that are monitored from space. Automated all-sky surveys have revealed numerous variable stars but most of the luminous stars are often overexposed. Targeted campaigns can improve the time base of photometric data for those objects. The aim of this investigation is to study the variability of luminous stars at different timescales in young open clusters and OB associations. We monitored 22 open clusters and associations from 2011 to 2013 using a 0.25-m telescope. Variable stars were detected by comparing the overall light-curve scatter with measurement uncertainties. Variability was analysed by the light curve feature extraction tool FATS. Periods of pulsating stars were determined using the discrete Fourier transform code SigSpec. We then classified the variable stars based on their pulsation periods and available spectral information. We obtained light curves for more than 20000 sources of which 354 were found to be variable. Amongst them we find 80 eclipsing binaries, 31 $\alpha$ Cyg, 13 $\beta$ Cep, 62 Be, 16 slowly pulsating B, 7 Cepheid, 1 $\gamma$ Doradus, 3 Wolf-Rayet and 63 late-type variable stars. Up to 55% of these stars are potential new discoveries as they are not present in the Variable Star Index (VSX) database. We find the cluster membership fraction for variable stars to be 13% with an upper limit of 35%., Comment: 36 pages, 11 figures, catalogue in appendix A

This paper presents a novel low-power imaging system for nanosatellite proximity operations. A robust independent camera module with on-board image processing, based on the ARM Cortex-M3 microcontroller and fast static random access memory, has been developed and characterized for the requirements of the ESTCube-1 mission. The imaging system, optimized for use in a single unit CubeSat, utilizes commercial off-the-shelf components and standard interfaces for a cost-effective reusable design. The resulting 43.3 mm £ 22 mm £ 44.2 mm (W£ H£ D) aluminium camera module weighs 30 g and consumes on the average of 118 mW of power, with peaks of 280 mW during image capture. Space qualification and stress tests have been performed. A detailed case study for the ESTCube-1 10 m tether deployment monitoring and Earth imaging mission is presented. For this purpose a 4.4 mm telecentric lens, 10 bit 640£ 480 pixel CMOS image sensor, 700 nm infrared cut-off filter and a 25% neutral density filter are used. The resolution of the assembled system is 12.7 mm and 1 km per pixel at distances of 10 m and 700 km, respectively. Custom on-board image evaluation and high dynamic range imaging algorithms for ESTCube-1 have been implemented and tested. Optical calibration of the assembled system has been performed.

This paper presents the design, development, and pre-launch characterization of the ESTCube-1 Attitude Determination and Control System (ADCS). The design driver for the ADCS has been the mission requirement to spin up the satellite to 360 deg¢s i1 with controlled orientation of the spin axis and to acquire the angular velocity and the attitude during the scientific experiment. ESTCube-1 is a one-unit CubeSat launched on 7 May 2013, 2:06 UTC on board the Vega VV02 rocket. Its primary mission is to measure the Coulomb drag force exerted by a natural plasma stream on a charged tether and, therefore, to perform the basic proof of concept measurement and technology demonstration of electric solar wind sail technology. The attitude determination system uses three-axis magnetometers, three-axis gyroscopic sensors, and two-axis Sun sensors, a Sun sensor on each side of the satellite. While commercial off-the-shelf components are used for magnetometers and gyroscopic sensors, Sun sensors are custom- built based on analogue one-dimensional position sensitive detectors. The attitude of the satellite is estimated on board using an Unscented Kalman Filter. An ARM 32-bit processor is used for ADCS calculations. Three electromagnetic coils are used for attitude control. The system is characterized through tests and simulations. Results include mass and power budgets, estimated uncertainties as well as attitude determination and control performance. The system fulfils all mission requirements.

This paper presents two technology experiments – the plasma brake for deorbiting and the electric solar wind sail for interplanetary propulsion – on board the ESTCube-2 and FORESAIL-1 satellites. Since both technologies employ the Coulomb interaction between a charged tether and a plasma flow, they are commonly referred to as Coulomb drag propulsion. The plasma brake operates in the ionosphere, where a negatively charged tether deorbits a satellite. The electric sail operates in the solar wind, where a positively charged tether propels a spacecraft, while an electron emitter removes trapped electrons. Both satellites will be launched in low Earth orbit carrying nearly identical Coulomb drag propulsion experiments, with the main difference being that ESTCube-2 has an electron emitter and it can operate in the positive mode. While solar-wind sailing is not possible in low Earth orbit, ESTCube-2 will space-qualify the components necessary for future electric sail experiments in its authentic environment. The plasma brake can be used on a range of satellite mass classes and orbits. On nanosatellites, the plasma brake is an enabler of deorbiting – a 300-m-long tether fits within half a cubesat unit, and, when charged with - 1 kV, can deorbit a 4.5-kg satellite from between a 700- and 500-km altitude in approximately 9–13 months. This paper provides the design and detailed analysis of low-Earth-orbit experiments, as well as the overall mission design of ESTCube-2 and FORESAIL-1.

ESTCube-2 on Eesti Tudengisatelliidi Sihtasutuse satelliidi programm, mille eesmärgiks on tehnoloogiline testimine järgmiste missioonide tarbeks. Peale satelliidi orbiidile saatmist toimub suhtlemine satelliidiga maapealsete sidejaamade abil, aga kontakti aeg satelliidiga ühest maapealsest sidejaamast on väga limiteeritud . Käesoleva töö eesmärk on disainida ja implementeerida kontakti automatiseerimise tarkvaramoodul, mis võimaldab minimaalse inimese sekkumisega planeerida ja läbi viia kontakte satelliitidega. Esmane funktsioneeriv tarkvaramoodul on loodud autori poolt. Töös selgitatakse tarkvara disaini ja implementatsiooni ning ühildamist olemasoleva missioonijuhtimissüstemiga, mis on kombinatsioon tark- ja riistvarast, mille abil jälgitakse ja juhitakse orbiidil olevat satelliiti., ESTCube-2 is the first student satellite project of the Estonian Student Satellite Foundation and it's objective is the technological demonstration of scientific payloads for upcoming missions. After a satellite's launch, the communication can be done through ground stations, but the contact time is very short over a limited area at a given point in time over the earth. The aim of this thesis is to design and implement a contact automation module which can schedule and execute automatic contacts with the satellite without minimal/no human intervention. The author aims to implement the first functional version of the required software module. This thesis explains the thought process, design and implementation of the software module, including the integration with the existing ESTCube Mission Control System—a combination of software and hardware that is used for monitoring and controlling the satellite after launch.