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16 pages, 310 KiB

Open AccessArticle

Non-Zero Coriolis Field in Ehlers’ Frame Theory

by Federico Re and Oliver F. Piattella

Galaxies 2025, 13(2), 38; https://doi.org/10.3390/galaxies13020038 (registering DOI) - 5 Apr 2025

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Ehlers’ Frame Theory is a class of geometric theories parameterized by

λ : = 1 / c^{2}

and identical to the General Theory of Relativity for

λ \neq 0

. The limit

λ \to 0

does not recover Newtonian gravity, as one [...] Read more.

Ehlers’ Frame Theory is a class of geometric theories parameterized by

λ : = 1 / c^{2}

and identical to the General Theory of Relativity for

λ \neq 0

. The limit

λ \to 0

does not recover Newtonian gravity, as one might expect, but yields the so-called Newton–Cartan theory of gravity, which is characterized by a second gravitational field

ω

, called the Coriolis field. Such a field encodes at a non-relativistic level the dragging feature of general spacetimes, as we show explicitly for the case of the

(η, H)

geometries. Taking advantage of the Coriolis field, we apply Ehlers’ theory to an axially symmetric distribution of matter, mimicking, for example, a disc galaxy, and show how its dynamics might reproduce a flattish rotation curve. In the same setting, we further exploit the formal simplicity of Ehlers’ formalism in addressing non-stationary cases, which are remarkably difficult to treat with the General Theory of Relativity. We show that the time derivative of the Coriolis field gives rise to a tangential acceleration which allows for studying a possible formation in time of the rotation curve’s flattish feature. Full article

(This article belongs to the Special Issue From Tides to Waves: Understanding the Formation Mechanisms of Galactic Spirals)

6 pages, 226 KiB

Open AccessArticle

Stellar Wind Parameters of Massive Stars in Accretion-Powered High-Mass X-Ray Binary Pulsars

by Nina Beskrovnaya, Nazar Ikhsanov and Vitaliy Kim

Galaxies 2025, 13(2), 37; https://doi.org/10.3390/galaxies13020037 (registering DOI) - 5 Apr 2025

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Abstract

The process of mass exchange between the components of High-Mass X-ray Binary (HMXB) systems with neutron stars undergoing wind-fed accretion is discussed. The X-ray luminosity of these systems allows us to evaluate the mass capture rate by the neutron star from the stellar [...] Read more.

The process of mass exchange between the components of High-Mass X-ray Binary (HMXB) systems with neutron stars undergoing wind-fed accretion is discussed. The X-ray luminosity of these systems allows us to evaluate the mass capture rate by the neutron star from the stellar wind of its massive companion and set limits on the relative velocity between the neutron star and the wind. We found that the upper limit to the wind velocity in the orbital plane during the high state of the X-ray source is in the range of 120–1000

km s^{- 1}

, which is by a factor of 2–4 lower than both the terminal wind velocity and the speed of the wind flowing out from the polar regions of massive stars for all the objects under investigation. This finding is valid not only for the systems with Be stars, but also for the systems in which the optical components do not exhibit the Be phenomenon. We also show that the lower limit to the radial wind velocity in these systems can unlikely be smaller than a few percent of the orbital velocity of the neutron star. This provides us with a new constraint on the mass transfer process in the outflowing disks of Be-type stars. Full article

(This article belongs to the Special Issue Circumstellar Matter in Hot Star Systems)

8 pages, 1341 KiB

Open AccessReview

Chemical Synthesis in the Circumstellar Environment

by Sun Kwok

Galaxies 2025, 13(2), 36; https://doi.org/10.3390/galaxies13020036 - 3 Apr 2025

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Abstract

We discuss the spectral distinctions between B[e] stars and compact planetary nebulae. The differentiation between proto-planetary nebulae, transition objects between the asymptotic giant branch and planetary nebulae, and reflection nebulae in binary systems is also discussed. Infrared and millimeter-wave observations have identified many [...] Read more.

We discuss the spectral distinctions between B[e] stars and compact planetary nebulae. The differentiation between proto-planetary nebulae, transition objects between the asymptotic giant branch and planetary nebulae, and reflection nebulae in binary systems is also discussed. Infrared and millimeter-wave observations have identified many inorganic and organic molecules, as well as solid-state minerals, in the circumstellar environment. There is evidence that complex organics in the form of mixed aromatic/aliphatic nanoparticles (MAONs) are synthesized during the proto-planetary nebulae phase of evolution. Their ejection into the interstellar medium may have enriched the primordial Solar System, and the complex organics found in comets, asteroids, and planetary satellites could be stellar in origin. Full article

(This article belongs to the Special Issue Circumstellar Matter in Hot Star Systems)

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10 pages, 1540 KiB

Open AccessArticle

Why Jet Power and Star Formation Are Uncorrelated in Active Galaxies

by David Garofalo, Brent McDaniel and Max North

Galaxies 2025, 13(2), 35; https://doi.org/10.3390/galaxies13020035 - 3 Apr 2025

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Abstract

Jet luminosity from active galaxies and the rate of star formation have recently been found to be uncorrelated observationally. We show how to understand this in the context of a model in which powerful AGN jets enhance star formation for up to hundreds [...] Read more.

Jet luminosity from active galaxies and the rate of star formation have recently been found to be uncorrelated observationally. We show how to understand this in the context of a model in which powerful AGN jets enhance star formation for up to hundreds of millions of years while jet power decreases in time, followed by a longer phase in which star formation is suppressed but coupled to jet power increasing with time. We also highlight characteristic differences, depending on environment richness in a way that is compatible with the observed SEDs of high redshift radio galaxies. While the absence of a direct correlation between jet power and star formation rate emerges naturally, our framework allows us to also predict the environment richness, range of excitation, and redshift values of radio AGN in the jet power-star formation rate plane. Full article

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15 pages, 685 KiB

Open AccessReview

An Updated Repository of Sub-mJy Extragalactic Source-Count Measurements in the Radio Domain

by Vincenzo Galluzzi, Meriem Behiri, Marika Giulietti and Andrea Lapi

Galaxies 2025, 13(2), 34; https://doi.org/10.3390/galaxies13020034 - 2 Apr 2025

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Abstract

We present an updated repository of sub-mJy extragalactic radio source counts between 150 MHz and 10 GHz, incorporating recent advances in radio surveys and observational techniques. By compiling and refining previous datasets, we provide a comprehensive catalog that enhances the understanding of faint [...] Read more.

We present an updated repository of sub-mJy extragalactic radio source counts between 150 MHz and 10 GHz, incorporating recent advances in radio surveys and observational techniques. By compiling and refining previous datasets, we provide a comprehensive catalog that enhances the understanding of faint radio-source populations, including Dusty Star-Forming Galaxies (DSFGs) and Radio-Quiet Active Galactic Nuclei (RQAGNs), from intermediate to high redshifts. Our analysis accounts for observational biases, such as resolution effects and Eddington bias, ensuring improved accuracy in flux-density estimations. We also discuss the implications of new-generation radio telescopes, such as the Square-Kilometer Array Observatory (SKAO) and its precursors and pathfinders, to further resolve these populations. Our collection contributes to constraining evolutionary models of radio sources, highlighting the increasing role of polarization studies in distinguishing different classes. This work serves as a key reference for future deep radio surveys targeting the faintest end of the extragalactic radio sky. Full article

(This article belongs to the Special Issue The Observation and Detection of Dusty Star-Forming Galaxies)

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24 pages, 1031 KiB

Open AccessReview

Red Supergiants as Supernova Progenitors

by Schuyler D. Van Dyk

Galaxies 2025, 13(2), 33; https://doi.org/10.3390/galaxies13020033 - 2 Apr 2025

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Abstract

The inevitable fate of massive stars in the initial mass range of ≈8–

30 M_{⊙}

in the red supergiant (RSG) phase is a core-collapse supernova (SN) explosion, although some stars may collapse directly to a black hole. We know that this is [...] Read more.

The inevitable fate of massive stars in the initial mass range of ≈8–

30 M_{⊙}

in the red supergiant (RSG) phase is a core-collapse supernova (SN) explosion, although some stars may collapse directly to a black hole. We know that this is the case, since RSGs have been directly identified and characterized for a number of supernovae (SNe) in pre-explosion archival optical and infrared images. RSGs likely all have some amount of circumstellar matter (CSM), through nominal mass loss, although evidence exists that some RSGs must experience enhanced mass loss during their lifetimes. The SNe from RSGs are hydrogen-rich Type II-Plateau (II-P), and SNe II-P at the low end of the luminosity range tend to arise from low-luminosity RSGs. The typical spectral energy distribution (SED) for such RSGs can generally be fit with a cool photospheric model, whereas the more luminous RSG progenitors of more luminous SNe II-P tend to require a greater quantity of dust in their CSM to account for their SEDs. The SN II-P progenitor luminosity range is

log (L_{bol} / L_{⊙}) \sim 4.0

–5.2. The fact RSGs are known up to

log (L_{bol} / L_{⊙}) \sim 5.7

leads to the so-called “RSG problem”, which may, in the end, be a result of small number of available statistics to date. Full article

(This article belongs to the Special Issue The Red Supergiants: Crucial Signposts for the Fate of Massive Stars)

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11 pages, 2062 KiB

Open AccessArticle

IRAS 17449+2320: A Possible Binary System with the B[e] Phenomenon and a Strong Magnetic Field

by Sergey Zharikov, Anatoly Miroshnichenko, Inna Reva, Raushan Kokumbaeva, Chingis Omarov, Steve Danford, Alicia Aarnio, Nadine Manset, Ashish Raj, S. Drew Chojnowski and Joseph Daglen

Galaxies 2025, 13(2), 32; https://doi.org/10.3390/galaxies13020032 - 31 Mar 2025

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Abstract

We report the recent results of a long-term spectroscopic and photometric monitoring of IRAS 17449+2320, a member of the least studied group of objects with the B[e] phenomenon called FS CMa-type objects. The main hypothesis for explaining the strong emission-line spectra and infrared [...] Read more.

We report the recent results of a long-term spectroscopic and photometric monitoring of IRAS 17449+2320, a member of the least studied group of objects with the B[e] phenomenon called FS CMa-type objects. The main hypothesis for explaining the strong emission-line spectra and infrared excesses of these objects assumes an ongoing or past mass transfer between the components in binary systems. The object is the only star with a gaseous and dusty envelope, where a strong and variable magnetic field (5.5–7.2 kG) was found through the splitting of some spectral lines. Additionally, we discovered the regular appearance of a red-shifted absorption component in spectral lines of neutral hydrogen, helium, and oxygen as well as one of ionized silicon with a period of 36.13 ± 0.20 days. We show that the magnetic field strength also followed this period. The process was accompanied by increasing emission component strengths for the hydrogen lines as well as the helium and metallic absorption lines. We refined the fundamental parameters of the optical counterpart of IRAS 17449+2320 (

T_{eff} = 9800 \pm 300

K, log L/

L_{⊙} = 1.86 \pm 0.06

,

v sin i = 9 \pm 2

km s⁻¹) and concluded that the star was slightly metal-deficient and viewed nearly pole-on. No signs of a secondary component were found. Possible interpretations of the observed phenomena are suggested, and some earlier findings about the object’s nature are revised. Full article

(This article belongs to the Special Issue Circumstellar Matter in Hot Star Systems)

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11 pages, 520 KiB

Open AccessArticle

The Photometric Variability and Spectrum of the Hot Post-AGB Star IRAS 21546+4721

by Natalia Ikonnikova, Marina Burlak and Alexander Dodin

Galaxies 2025, 13(2), 31; https://doi.org/10.3390/galaxies13020031 - 31 Mar 2025

Viewed by 48

Abstract

We present the results of photometric and spectroscopic observations of a poorly studied B-type supergiant with infrared excess, the hot post-AGB star IRAS 21546+4721. Based on our photometric observations in the

U B V R_{C} I_{C}

bands, we detected rapid, night-to-night, [...] Read more.

We present the results of photometric and spectroscopic observations of a poorly studied B-type supergiant with infrared excess, the hot post-AGB star IRAS 21546+4721. Based on our photometric observations in the

U B V R_{C} I_{C}

bands, we detected rapid, night-to-night, non-periodic brightness variations in the star with peak-to-peak amplitudes up to

0 .^{m} 3

in the V band, as well as color–color and color–brightness correlations. Based on its variability characteristics, IRAS 21546+4721 appears similar to other hot post-AGB stars. Possible causes of the photometric variability are discussed. Additionally, we acquired low-resolution spectra in a wavelength range from 3500 to 7500 Å. The spectrum contains absorption lines typical of an early B-type star, along with a set of emission lines of H I, He I, [O I], [O II], [N II], [S II], and C II originating from an ionized circumstellar envelope. An analysis of the emission spectrum allowed us to estimate the parameters of the gas envelope (

N_{e}

∼ 10⁴ cm⁻³,

T_{e}

∼ 10,000 K) and the star’s temperature (∼26,500 K). The radial velocity measured from the emission lines was

V_{r} = - 141 \pm 7

km s⁻¹. Full article

(This article belongs to the Special Issue Circumstellar Matter in Hot Star Systems)

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13 pages, 2295 KiB

Open AccessArticle

Nonlinear Stability of the Bardeen–De Sitter Wormhole in f(R) Gravity

by A. Eid

Galaxies 2025, 13(2), 30; https://doi.org/10.3390/galaxies13020030 - 28 Mar 2025

Viewed by 55

Abstract

This paper discusses the nonlinear stability of a thin-shell wormhole from a regular black hole in Bardeen–de Sitter spacetime in the

f (R)

gravity framework. The stability is examined under the linear perturbation about static solution and a nonlinear variable equation [...] Read more.

This paper discusses the nonlinear stability of a thin-shell wormhole from a regular black hole in Bardeen–de Sitter spacetime in the

f (R)

gravity framework. The stability is examined under the linear perturbation about static solution and a nonlinear variable equation of state, such as the modified generalized Chaplygin gas. The stability solutions for a suitable choice of different parameters included in the variable equation of state and

f (R)

gravity models, as well as the metric space–time, are illustrated. Full article

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18 pages, 1207 KiB

Open AccessArticle

Multiplicity of Luminous Blue Variable Stars

by A. Lobel and N. Gorlova

Galaxies 2025, 13(2), 29; https://doi.org/10.3390/galaxies13020029 - 28 Mar 2025

Viewed by 171

Abstract

The study of Luminous Blue Variables (LBVs) is critical to understanding the mechanisms behind their recurring S Dor-like outbursts, which can span decades and feature dramatic spectral changes. These outbursts may result from atmospheric instability or interactions with a companion star, but their [...] Read more.

The study of Luminous Blue Variables (LBVs) is critical to understanding the mechanisms behind their recurring S Dor-like outbursts, which can span decades and feature dramatic spectral changes. These outbursts may result from atmospheric instability or interactions with a companion star, but their causes remain poorly understood. Detecting binarity in LBVs through long-term radial velocity (RV) measurements, which can identify orbital motion via shifts in absorption lines, is a promising method. Periodic line shifts and variability can reveal the presence of a companion star. We report on the monitoring of four LBVs and two candidate LBVs (cLBVs), observing high-resolution spectra from 2009 to 2024. Although we do not find a clear periodic RV signal for LBVs MWC 930, P Cyg, or HD 168607, our long-term monitoring campaign previously detected the binarity of cLBV MWC 314 in 2013. For the first time, we detected a periodic signal in the RV data of the cLBV Schulte 12. In addition, in LBV HD 168607, we observed Discrete Absorption Components, indicative of large-scale structures in a rotating wind. These findings advance our understanding of the binary nature of LBVs and their complex outburst behaviors. Full article

(This article belongs to the Special Issue Circumstellar Matter in Hot Star Systems)

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36 pages, 538 KiB

Open AccessReview

Advanced Suspension Techniques in Interferometric Gravitational Wave Detectors: An Overview

by Vishnu G. Nair

Galaxies 2025, 13(2), 28; https://doi.org/10.3390/galaxies13020028 - 26 Mar 2025

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Abstract

Interferometric gravitational wave (GW) detectors are sophisticated instruments that require suspended mirrors to be effectively isolated from all forms of vibrations and noise. This isolation is crucial for enabling the detectors to function efficiently at low frequencies, which directly impacts their capacity to [...] Read more.

Interferometric gravitational wave (GW) detectors are sophisticated instruments that require suspended mirrors to be effectively isolated from all forms of vibrations and noise. This isolation is crucial for enabling the detectors to function efficiently at low frequencies, which directly impacts their capacity to detect distant events from the universe’s past. To address this challenge, various suspension systems have been developed, utilizing passive, active, or hybrid control mechanisms. The effectiveness of these systems in suppressing noise determines the lowest detectable frequencies. Designing and managing mirror suspensions present significant challenges across all interferometric GW detectors. Detectors such as LIGO, VIRGO, TAMA300, KAGRA, and GEO600 implement unique suspension designs and techniques to enhance their performance. A comprehensive comparison of these systems would offer valuable insights. This paper provides an overview of the different suspension systems employed in major global interferometric GW detectors, alongside a brief examination of proposed future detectors. It discusses the rationale behind each design, the materials utilized, and other relevant details, serving as a useful resource for the gravitational wave detector community. Full article

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30 pages, 10621 KiB

Open AccessArticle

A Comprehensive Analysis on the Nature of the Spiral Arms in NGC 3686, NGC 4321, and NGC 2403

by Valeria Kostiuk, Alexander Marchuk, Alexander Gusev and Ilia V. Chugunov

Galaxies 2025, 13(2), 27; https://doi.org/10.3390/galaxies13020027 - 24 Mar 2025

Viewed by 88

Abstract

In theoretical investigations, various mechanisms have been put forward to explain the emergence of spiral patterns in galaxies. One of the few ways to find out the nature of spirals in a particular galaxy is to consider the so-called corotation radius, or corotation [...] Read more.

In theoretical investigations, various mechanisms have been put forward to explain the emergence of spiral patterns in galaxies. One of the few ways to find out the nature of spirals in a particular galaxy is to consider the so-called corotation radius, or corotation resonance. A distinctly defined corotation resonance is likely to indicate the existence of a spiral density wave, while the chaotic distribution of their positions may suggest a dynamic nature to the spiral structure. In this study, we analyzed measurements of the corotation radius obtained using several methods for three galaxies (NGC 3686, NGC 4321, and NGC 2403) that exhibit different morphologies of spiral structures. We also performed independent measurements to estimate the location of the resonance, which allowed us to determine whether each galaxy has a clear corotation radius position. This examination, along with other tests such as stellar age gradient, interlocking resonances, and the radial distribution of metallicity, enables us to understand the mechanism that may be responsible for the formation of spiral arms in the studied galaxies. Full article

(This article belongs to the Special Issue From Tides to Waves: Understanding the Formation Mechanisms of Galactic Spirals)

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11 pages, 479 KiB

Open AccessArticle

HR 4049: A Spectroscopic Analysis of a Post-AGB Object

by Shakhida T. Nurmakhametova, Nadezhda L. Vaidman, Anatoly S. Miroshnichenko, Azamat A. Khokhlov, Aldiyar T. Agishev, Berik S. Yermekbayev, Stephen Danford and Alicia N. Aarnio

Galaxies 2025, 13(2), 26; https://doi.org/10.3390/galaxies13020026 - 22 Mar 2025

Viewed by 150

Abstract

A new spectroscopic study of HR 4049, a post-AGB star in a binary system, based on échelle spectra taken between 2019 and 2025 with the 0.81 m telescope of the Three College Observatory (North Carolina, USA) at a resolution of R ≈ 12,000 [...] Read more.

A new spectroscopic study of HR 4049, a post-AGB star in a binary system, based on échelle spectra taken between 2019 and 2025 with the 0.81 m telescope of the Three College Observatory (North Carolina, USA) at a resolution of R ≈ 12,000 is reported. A cross-correlation analysis of 73 spectra of a single C i multiplet in the 4760–4780 Å range yielded the following orbital parameters: the orbital period

P = 428.474 \pm 0.002

days, eccentricity

e = 0.29 \pm 0.01

, argument of periastron

ω = {242.3}^{\circ} \pm {0.3}^{\circ}

, epoch of periastron

T_{0} = 2,458,383.2 \pm 0.6

, heliocentric systemic radial velocity

γ = - 30.12 \pm 0.09

km s⁻¹, and semi-amplitude of the radial velocity curve

K_{1} = 15.52 \pm 0.13

km s⁻¹. Phase-dependent variations of the Hα line profile indicate dynamic processes in the circumstellar environment. The luminosity of HR 4049 was refined using the Gaia EDR3 parallax (

0.71 \pm 0.10

mas), corresponding to a distance of

1397 \pm 170

pc, and the average visual magnitude in the brightest state (

m_{V} = 5.35

mag). The derived luminosity,

log (L / L_{⊙}) = 4.22 \pm 0.12

, suggests an initial mass of 3.0–4.0

M_{⊙}

. Analysis of the mass function and most probable orbital inclinations (60°–75°) leads to current masses of

\sim 0.75 M_{⊙}

for the primary and

0.70

–

0.82 M_{⊙}

for the secondary component. The results confirm the system’s long-term orbital stability and provide further insights for future research into the nature of post-AGB binaries. Full article

(This article belongs to the Special Issue Circumstellar Matter in Hot Star Systems)

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12 pages, 1218 KiB

Open AccessReview

Red Supergiants—The Other Side of the H-R Diagram

by Roberta M. Humphreys

Galaxies 2025, 13(2), 25; https://doi.org/10.3390/galaxies13020025 - 20 Mar 2025

Viewed by 73

Abstract

Red supergiants are the largest stars known with some of the highest mass loss rates observed. They are the final stage in the evolution of the majority of massive stars. The unexpected discovery of high mass loss episodes in many red supergiants have [...] Read more.

Red supergiants are the largest stars known with some of the highest mass loss rates observed. They are the final stage in the evolution of the majority of massive stars. The unexpected discovery of high mass loss episodes in many red supergiants have posed questions about the role of mass loss on their final stages. The papers in this volume are timely reviews of our current understanding of this often surprising population of massive stars. This introductory paper is a brief summary of their observed properties and a historical perspective on some of the current problems on mass loss, their circumstellar environments, and their evolutionary state. Full article

(This article belongs to the Special Issue The Red Supergiants: Crucial Signposts for the Fate of Massive Stars)

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10 pages, 1232 KiB

Open AccessArticle

The Relative Position of the Solar Magnetic Dipole Axis and Rotation Axis of the Sun

by Alexandr Riehokainen, Victoria Smirnova, Alexander Solov’ev and Polina Strekalova

Galaxies 2025, 13(2), 24; https://doi.org/10.3390/galaxies13020024 - 19 Mar 2025

Viewed by 166

Abstract

We estimated the relative location of the solar rotation axis and the magnetic axis of the solar dipole, which were defined as centers of polar coronal holes. We used observations of polar coronal hole data, which were originally obtained with Solar Dynamic Observatory [...] Read more.

We estimated the relative location of the solar rotation axis and the magnetic axis of the solar dipole, which were defined as centers of polar coronal holes. We used observations of polar coronal hole data, which were originally obtained with Solar Dynamic Observatory (SDO) spacecraft. To calculate the tilt of the magnetic axis relative to the rotation axis of the Sun, an empirical method for the estimation of the coronal hole centers is proposed. As a result, it was found that these axes do not coincide. The average deviation of the magnetic dipole axis from the rotation axis is ∼5 degrees of latitude. Using the wavelet transform method, it was found that the magnetic axis rotates around the rotation axis with a main period of 15–16 days. This period is related to the sector structure of the global magnetic field in the polar zones of the Sun. Full article

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