International Astronomical Union Research Articles

Quantitative Criteria for Defining Planets

The current International Astronomical Union (IAU) definition of “planet” is problematic because it is vague and excludes exoplanets. Here, we describe aspects of quantitative planetary taxonomy and examine the results of unsupervised clustering of solar system bodies to guide the development of possible classification frameworks. Two unsurprising conclusions emerged from the clustering analysis: (1) satellites are distinct from planets and (2) dynamical dominance is a natural organizing principle for planetary taxonomy. To generalize an existing dynamical dominance criterion, we adopt a universal clearing timescale applicable to all central bodies (brown dwarfs, stars, and stellar remnants). Then, we propose two quantitative, unified frameworks to define both planets and exoplanets. The first framework is aligned with both the IAU definition of planet in the solar system and the IAU working definition of an exoplanet. The second framework is a simpler mass-based framework that avoids some of the difficulties ingrained in current IAU recommendations.

How Well is the International Celestial Reference System Maintained in Official IAU Implementations?

The International Celestial Reference Frame (ICRF) based on the VLBI-derived positions of 608 extragalactic radio sources was adopted by the International Astronomical Union (IAU) in 1998 as the first realization of the International Celestial Reference System (ICRS). Later, in 2009 and 2020, two extended ICRF versions, ICRF2 and ICRF3, respectively, were released. The latter is adopted by the IAU as the current implementation of the ICRF in the radio band. In the meantime, the Gaia mission delivered three versions of the optical ICRS realization in 2016, 2018, and 2022 with an accuracy similar to that achieved by VLBI-based ICRF. The Gaia-CRF catalogs were linked to the ICRF under no-rotation conditions and thus may suffer from ICRF systematic instability if the latter is substantial. In this work, a new analysis was performed to assess the long-term stability of radio and optical ICRS realizations. Based on the 16-parameter vector spherical harmonics expansion of the differences between the three ICRF catalogs, it can be concluded that the mutual orientation between them is at a level of a few tens of microarcseconds, while the components of the glide vector and E 2,0 term are several times greater. A comparison of the three Gaia-CRF catalogs with the ICRF3-SX showed that for the latest Gaia-CRF catalog, Gaia-CRF3, all rotational and deformation components are below 20 μas except for the E 2,0 term, which is several times greater. For both ICRF and Gaia-CRF catalogs, the evolution of the source position errors is also tracked.

Water Ice Sublimation Distribution on the Surface of Short Period Comet

Comets are the primitive planetesimals left in the solar system. Studying the evolution of comet nucleus is of great significance for understanding the formation and evolution history of other celestial bodies in the solar system. Under the action of solar radiation, the volatile components of comets sublimate and drive the dust movement, resulting in the loss of comet nucleus material. Therefore, the activity of comet nucleus affects the surface morphology and even the overall shape evolution. The orbit data were obtained from IAU (International Astronomical Union) MPC (Minor Planet Center), and the spin and precession of comet nucleus were taken into account. The shape evolution model of Mass loss-driven shape evolution model (MONET) was used to simulate the short period comet. The distribution of solar radiation energy and surface erosion depth of short-period comet 1P/Halley, 9P/Tempel 1, 19P/Borrelly, 67P/C-G (Churyumov-Gerasimenko), 81P/Wild 2, and 103P/Hartley 2 in one orbital period is calculated. Combined with its dynamic parameters, the effects of rotation, precession, and revolution on the sublimation distribution of surface water ice and the possibility of causing the difference in erosion between north and south are discussed.

On more than two decades of Celestial Reference Frame VLBI observations in the deep south: IVS-CRDS (1995–2021)

Abstract The International VLBI Service for Geodesy and Astrometry (IVS) regularly provides high-quality data to produce Earth Orientation Parameters (EOP), and for the maintenance and realisation of the International Terrestrial and Celestial Reference Frames, ITRF and ICRF. The first iteration of the celestial reference frame (CRF) at radio wavelengths, the ICRF1, was adopted by the International Astronomical Union (IAU) in 1997 to replace the FK5 optical frame. Soon after, the IVS began official operations and in 2009 there was a significant increase in data sufficient to warrant a second iteration of the CRF, ICRF2. The most recent ICRF3, was adopted by the IAU in 2018. However, due to the geographic distribution of observing stations being concentrated in the Northern hemisphere, CRFs are generally weaker in the South due to there being fewer Southern Hemisphere observations. To increase the Southern Hemisphere observations, and the density, precision of the sources, a series of deep South observing sessions was initiated in 1995. This initiative in 2004 became the IVS Celestial Reference Frame Deep South (IVS-CRDS) observing programme. This paper covers the evolution of the CRDS observing programme for the period 1995–2021, details the data products and results, and concludes with a summary of upcoming improvements to this ongoing project.

The Accuracy Assessment of the PREM and AK135-F Radial Density Models

The Earth’s synthetic density and gravitational models can be used to validate numerical methods for global (or large-scale) gravimetric forward and inverse modelling formulated either in the spatial or spectral domains. The Preliminary Reference Earth Model (PREM) density parameters can be adopted as a 1-D reference density model and further refined using more detailed 2-D or 3-D crust and mantle density models. Alternatively, the AK135-F density parameters can be used for this purpose. In this study, we investigate options for a refinement of the Earth’s synthetic density model by assessing the accuracy of available 1-D density models, specifically the PREM and AK135-F radial density parameters. First, we use density parameters from both models to estimate the Earth’s total mass and compare these estimates with published results. We then estimate the Earth’s gravity field parameters, particularly the geoidal geopotential number W0 and the mean gravitational attraction and compare them with published values. According to our results, the Earth’s total mass from the two models (the PREM and the AK135-F) differ less than 0.02% and 0.01%, respectively, when compared with the value adopted by the International Astronomical Union (IAU). The geoidal geopotential values of the two models differ from the value adopted by the IAU by less than 0.1% and 0.04%, respectively. The values of the mean gravitational attraction of the two models differ less than 0.02% and 0.08%, respectively, when compared with the value obtained from the geocentric gravitational constant and the Earth’s mean radius. These numerical findings ascertain that the PREM and AK135-F density parameters are suitable for defining a 1-D reference density model.

Development of astronomy teaching materials to overcome visual impairment. IAU Working Group, Astronomy for Equity and Inclusion IAU Symposium358

It is important that science is inclusive and communication is prioritised. To make science accessible to all, the gap between highly specific research and people with a general interest in science must be bridged. Effective science communication also helps to engage young people and inspire them to pursue a career in science. In Tokyo, Japan, Dr Kumiko Usuda-Sato, National Astronomical Observatory of Japan (NAOJ), and Dr Lina Canas of the International Astronomical Union (IAU) Office for Astronomy Outreach (OAO), are working to achieve effective communication and full inclusion. The researchers are interested in ensuring that science reaches diverse people, including people with a disability such as blind and visually impaired (BVI) people, as well as promoting diversity and inclusivity within international astronomical research communication. To this end, Usuda-Sato is engaging with the BVI community and tactility experts at the Japan Braille Library. A key focus is on developing resources for helping people to understand the Subaru Telescope, such as 3D printed models, braille and 2D explanations, as well as adapted communication methods. Canas is working to make the research community aware of accessibility issues. A key component of this was the IAU Symposium358, which brought together experts in accessibility and science communication, fostering discussion and promoting international collaboration.

The third realization of the International Celestial Reference Frame by very long baseline interferometry

A new realization of the International Celestial Reference Frame (ICRF) is presented based on the work achieved by a working group of the International Astronomical Union (IAU) mandated for this purpose. This new realization follows the initial realization of the ICRF completed in 1997 and its successor, ICRF2, adopted as a replacement in 2009. The new frame, referred to as ICRF3, is based on nearly 40 years of data acquired by very long baseline interferometry at the standard geodetic and astrometric radio frequencies (8.4 and 2.3 GHz), supplemented with data collected at higher radio frequencies (24 GHz and dual-frequency 32 and 8.4 GHz) over the past 15 years. State-of-the-art astronomical and geophysical modeling has been used to analyze these data and derive source positions. The modeling integrates, for the first time, the effect of the galactocentric acceleration of the solar system (directly estimated from the data) which, if not considered, induces significant deformation of the frame due to the data span. The new frame includes positions at 8.4 GHz for 4536 extragalactic sources. Of these, 303 sources, uniformly distributed on the sky, are identified as “defining sources” and as such serve to define the axes of the frame. Positions at 8.4 GHz are supplemented with positions at 24 GHz for 824 sources and at 32 GHz for 678 sources. In all, ICRF3 comprises 4588 sources, with three-frequency positions available for 600 of these. Source positions have been determined independently at each of the frequencies in order to preserve the underlying astrophysical content behind such positions. They are reported for epoch 2015.0 and must be propagated for observations at other epochs for the most accurate needs, accounting for the acceleration toward the Galactic center, which results in a dipolar proper motion field of amplitude 0.0058 milliarcsecond yr−1(mas yr−1). The frame is aligned onto the International Celestial Reference System to within the accuracy of ICRF2 and shows a median positional uncertainty of about 0.1 mas in right ascension and 0.2 mas in declination, with a noise floor of 0.03 mas in the individual source coordinates. A subset of 500 sources is found to have extremely accurate positions, in the range of 0.03–0.06 mas, at the traditional 8.4 GHz frequency. Comparing ICRF3 with the recently releasedGaiaCelestial Reference Frame 2 in the optical domain, there is no evidence for deformations larger than 0.03 mas between the two frames, in agreement with the ICRF3 noise level. Significant positional offsets between the three ICRF3 frequencies are detected for about 5% of the sources. Moreover, a notable fraction (22%) of the sources shows optical and radio positions that are significantly offset. There are indications that these positional offsets may be the manifestation of extended source structures. This third realization of the ICRF was adopted by the IAU at its 30th General Assembly in August 2018 and replaced the previous realization, ICRF2, on January 1, 2019.

The κ-Cygnid Meteor Shower and Its Relationship with Near-Earth Asteroids

This study is focused on the relationships between the κ-Cygnid meteor shower and different groups of asteroids crossing the Earth’s orbit (the so-called near-Earth objects (NEOs)). The analysis is based on the visible structure of the meteor shower and a comprehensive strategy for estimating the proximity of orbits of two bodies. The κ-Cygnid meteor shower (abbreviated as KCG) is observed from August 3 to August 25 every year; it is classified as a minor shower, and the size of its mean orbit is around 3.2 AU, while the geocentric velocity is 20.9 km/s. No parent body (PB) of the shower has been found among comets so far. Relationships between the shower and asteroids, as potential PBs of the shower, are under active examination; some hypotheses on this subject can be found on the website of the Meteor Data Center (MDC) of the International Astronomical Union (IAU).

Nutation of the non-rigid Earth: Effect of the mass redistribution

In this research, we computed the nutation of the figure axis for a non-rigid Earth model due to the mass redistribution resulting from the lunisolar attraction on the deformable Earth, thus extending our previous work on the precessional motion. The basic Earth model is a two-layer structure composed of a fluid core and an anelastic mantle. We used the Hamiltonian approach, leading to closed-form analytical formulae that describe the nutations in longitude and obliquity of the figure axis as a sum of Poisson and Oppolzer terms. Those formulae were evaluated assuming different Earth rheologies by means of the Love number formalism. In particular, we first computed the effect using the standard model of the International Earth Rotation and Reference Systems Service Conventions (2010) solid tides, and then the Love numbers computed by Williams and Boggs, accounting for the complete oceanic tide contribution, which should provide more consistent and updated values for the nutations. The main amplitudes correspond to the 18.6 yr nutation component and reach 201 μas and −96 μas in the in-phase components in longitude and obliquity, respectively. The obtained values differ greatly from those considered in the current nutation model, IAU2000, of the International Astronomical Union (IAU) – and later similar studies – which includes this effect under the denomination of non-linear terms and derives its numerical contribution on the basis of the Sasao, Okubo, and Saito framework. The differences are significant and reach more than 30μas for some nutation amplitudes. They can be likely attributed to several factors: an incomplete modelling of the redistribution potential; a different treatment of the permanent tide; and the use of different oceanic tide models.

Forced nutations of a two-layer Earth in canonical formulation with dissipative Hori-like kernel

In this research, a modification of the Lie-Hori perturbation method developed by the authors in a recent investigation is used to compute the forced nutations of a non-rigid Earth model, including dissipative processes at the core-mantle boundary. The study is tackled within the Hamiltonian formalism of a two-layer Earth, where the viscous and electromagnetic couplings between mantle and core are introduced via generalized forces. The modified Lie-Hori method is introduced within the framework of the generalized Hamiltonian formalism. It, therefore, allows for calculating first-order perturbations in both conservative and non-conservative systems, while the classical Lie-Hori procedure is not designed to include generalized forces in the kernel to account for dissipative processes. Unlike other methods, this one presents the advantage of keeping the same dimensionality of the original problem, avoiding the doubling of the dimension of the phase space. With this mathematical refinement, differences in the derived nutation amplitudes at the microarcsecond level have been found when compared with the former, first approximation for dissipative systems based on damped oscillators —the only existing previous solution. Those figures are of relevance according to recent recommendations of the International Astronomical Union (IAU) and the International Association of Geodesy (IAG) based on the final report of the Joint Working Group on Theory of Earth rotation and validation.

A GLOBAL SHAPE MODEL FOR SATURN’S MOON ENCELADUS FROM A DENSE PHOTOGRAMMETRIC CONTROL NETWORK

Abstract. A planetary body’s global shape provides both insight into its geologic evolution, and a key element of any Planetary Spatial Data Infrastructure (PSDI). NASA’s Cassini mission to Saturn acquired more than 600 moderate- to high-resolution images (<500 m/pixel) of the small, geologically active moon Enceladus. The moon’s internal global ocean and intriguing geology mark it as a candidate for future exploration and motivates the development of a PSDI. Recently, two PSDI foundational data sets were created: geodetic control and orthoimages. To provide the third foundational data set, we generate a new shape model for Enceladus from Cassini images and a dense photogrammetric control network (nearly 1 million tie points) using the U.S. Geological Survey’s Integrated Software for Imagers and Spectrometers (ISIS) and the Ames Stereo Pipeline (ASP). The new shape model is near-global in extent and gridded to 2.2 km/pixel, ∼50 times better resolution than previous global models. Our calculated triaxial shape, rotation rate, and pole orientation for Enceladus is consistent with current International Astronomical Union (IAU) values to within the error; however, we determined a new prime meridian offset (Wo) of 7.063°. We calculate Enceladus’ long-wavelength topography by subtracting the best-fit triaxial ellipsoid from our shape model. The result is comparable to previous global models but can resolve topographic features as small as 5–7 km across in certain areas. To evaluate the spatially varying quality of the model, we calculate the point density (variable from 5 to more than 50 points per pixel), normalized median absolute deviation of the points within each pixel (typically less than 100 m), and the minimum expected vertical precision of each point (ranging from 29 m to 2 km).

IAU South-West and Central Asian Regional Office of Astronomy for Development

The International Astronomical Union (IAU) announced its Strategic Plan on Astronomy for Development in 2009, during the International Year of Astronomy (IYA). One of its main components was the creation of the Office of Astronomy for Development (OAD) and corresponding Regional Offices (ROADs) for implementation and coordination of its aims. The OAD was created in Cape Town, South Africa and later on ROADs were created in 11 regions. Since 2015, Armenia hosts one of them, IAU South West Asian (SWA), later renamed to South West and Central Asian (SWCA) ROAD. At present, already 6 countries have officially joined (Armenia, Georgia, Iran, Kazakhstan, Tajikistan, and Turkey), but the Office serves for a rather broad region, from Eastern Europe to Central Asia. Armenia's geographical location and its historical role in astronomy (both for well-known archaeoastronomical heritage and the presence of the famous Byurakan Astrophysical Observatory (BAO) founded by Viktor Ambartsumian in 1946) serve as a link between Europe and Eastern Partnership countries, Middle East and Asia in general. We run activities in 3 directions, Task Forces (TF): TF1 Universities and Research, TF2 Children and Schools and TF3 Public Outreach. We present our projects and all other accomplishments and discuss the role of our ROAD in maintaining contacts and development of astronomy in the region, as well as contacts between Europe and the Eastern Partnership countries. Most up-to-date information about the IAU SWCA ROAD is available on its webpage at http://iau-swa-road.aras.am/eng/index.php.

Australia and the International Astronomical Union: the 1973 Sydney general assembly

Formed in 1919, the International Astronomical Union is the international body representing professional astronomers. Australia joined the union soon after its formation but, due to financial difficulties, dropped out for a few years until re-joining just before World War 2. The main non-financial contribution any country can make to the union is to host one of its general assemblies that are held in different countries and cities every three years. After Australia’s bid to host a general assembly in 1967 or 1970 was unsuccessful, another bid was made for 1973. This second bid was accepted by the union’s executive council and confirmed in a letter from the union’s general secretary. The five years of planning and organisation for the assembly were made difficult by several external threats. The main one was the late proposal from Poland to move the 1973 assembly to Warsaw to celebrate the 500th anniversary of the birth of Copernicus. A compromise of an extraordinary general assembly in Poland following the ordinary one in Australia led to reduced numbers of overseas participants in Sydney. Despite this and other problems, the 1973 general assembly was regarded as highly successful.

Távoli csillagok és bolygóik új elnevezései

The International Astronomical Union (IAU) celebrated its 100th anniversary in 2019. To celebrate the occasion, the organisation announced a campaign for countries to name exoplanets, that is planets outside of our solar system and their stars. A total of 113 countries and autonomous regions took part in the campaign, choosing names by way of public votes. The IAU announced the 225 names chosen at a press conference on 17 December 2019 and accepted these as official (alongside the scientific names already in use). Following a brief description of exoplanets, the study categorises the planet names according to onomastic aspects and offers an overview of the names submitted to the Hungarian committee.

Correction to: Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2015

We point out some errors in the most recent report from the International Astronomical Union (IAU) Working Group on Cartographic Coordinates and Rotational Elements (Archinal et al. 2018).

Science strategy of the African Astronomical Society (AfAS): An outcome of the Science Business held in synergy with the IAUS 356

AbstractPresented here, is a summary of discussions at African Astronomical Society (AfAS) Science Business Meeting, Addis Ababa, Ethiopia, 10-11 October 2019. This summary was deliberated with delegates of the International Astronomical Union (IAU) Symposium 356, during a lunch session of the meeting.

POWER, POLITICS AND PERSONALITIES IN AUSTRALIAN ASTRONOMY: WILLIAM ERNEST COOKE AND THE TRIANGULATION OF THE PACIFIC BY WIRELESS TIME SIGNALS

In 1916 the New South Wales Government Astronomer, Professor Ernest Cooke, proposed the triangulation of the Pacific by wireless time signals, in order to improve mapping. The world was at war, and this scientific advancement was urgently required. The State Government gave Cooke authority to proceed, but later rescinded this decision. It also prevented Cooke from attending the first International Astronomical Union (IAU) General Assembly in Rome in 1919. Although Cooke became Chairman of the Longitude Committee of the Australian National Research Council in 1922, attended the Pan-Pacific Science Association Congress in 1923, joined the IAU's Commission 18 (Longitude by Wireless) in 1925, and continued to promote triangulation of the Pacific by wireless, the Sydney Observatory Board of Visitors, bureaucrats and politicians all continued to block him. This paper examines the interplay between Federal and State politics in international astronomy, using Cooke's triangulation of the Pacific project as a case study.

Astronomy for development in Portuguese-speaking countries

The PLOAD — Portuguese Language Office of Astronomy for Development — was established in 2015 by the International Astronomical Union (IAU) with the goal of promoting astronomy as a tool for sustainable development in Portuguese-speaking countries.

A robust method to identify meteor showers new parent bodies from the SonotaCo and EDMOND meteoroid orbit databases

Context. Several new meteor showers are added to the International Astronomical Union (IAU) list of meteor showers every year. Given the multiplication of video meteor measurements new parent bodies are to be found in addition to new showers. Such an endeavor is usually performed by comparing orbital elements, using a high threshold single-linking Dsh-criterion. However, questions remain about the accuracy of the method and the veracity of the newly associated parent bodies. Aims. Our goal is to find the presence of new parent bodies in a statistical meaningful way. Methods. A search for parent bodies was performed among SonotaCo and EDMOND databases. The association of asteroids with meteors was based on different methods, discussed and compared below. In addition, a thorough statistical test was performed in order to investigate the possible random associations. Results. A list of potential new parent bodies associated with at least 50 meteors is found. A statistical test was used to show whether the group of meteor orbits and the asteroid is a random coincidence or not. Out of 54 potential new parent bodies, only three new parent bodies are not excluded by the statistical test: 2014 BN14, 2015 TX24 and 2015 QT3, with a probability of chance occurrence of 37, 10 and 13%, respectively. This shows the need for rigorous method when searching for the existence of meteor showers and parent bodies. Conclusions. Ideally, such a test (or even better, tests) should be conducted in order to confirm (or not) the current IAU list of meteor showers. Similarly, all meteor orbit data in our databases should ideally be revisited using the latest methods providing a better estimate of the real uncertainty and accuracy of the derived orbits.

Experience in Asteroid Search using Astrometrica Software

The present work of this research consists of the analysis of telescopic images provided by the International Astronomical Search Collaboration (IASC) to find asteroids that can be named. The concern in searching for asteroids helps the scientific community that promotes the collaboration of young students and astronomy fans get experience in finding asteroids through campaigns related to these. The Space Generation Advisory Council (SGAC) campaign in partnership with IASC has found around 1500 asteroids since the beginning of October 2006 as each year more than 1000 teams from different countries participate. The Astrometrica software was used which is in charge of receiving the images in FITS format. The configuration of the selected telescope is carried out so that they are later analyzed in greater detail. Finally, a clean and precise Minor Planet Center (MPC) report is made, which is what the campaign requires so that it can go on to a preliminary phase and subsequently be accepted by the international astronomical union. The asteroids that become named will be registered in the catalog of official minor planets of the world. In the campaign related to this study, one finds 28 possible asteroids.