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Volume 10
Issue 2
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Galaxies, Volume 10, Issue 2 (April 2022) – 22 articles

Cover Story (view full-size image): A printout of the Beacon in the Galaxy (BITG) Message. It includes basic mathematical and physical concepts to establish a universal means of communication followed by information on the biochemical composition of life on Earth, the solar system’s time-stamped position in the milky way relative to known globular clusters, as well as digitized depictions of the solar system and Earth’s surface. The message concludes with digitized images of the human form, along with an invitation for any receiving intelligences to respond. View this paper
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14 pages, 344 KiB  
Article
On the VHE Spectrum and Formation of the Teraelectronvolt Pulsed Emission of the Crab Pulsar
by Nino Chkheidze
Galaxies 2022, 10(2), 59; https://doi.org/10.3390/galaxies10020059 - 12 Apr 2022
Viewed by 2138
Abstract
In the present paper, a model for the pulsed γ-ray emission of the Crab pulsar from 0.01 GeV to 1 TeV in the context of synchrotron emission generated in the vicinity of a light cylinder is developed. The generation of such high [...] Read more.
In the present paper, a model for the pulsed γ-ray emission of the Crab pulsar from 0.01 GeV to 1 TeV in the context of synchrotron emission generated in the vicinity of a light cylinder is developed. The generation of such high energies through the synchrotron process requires the existence of very energetic plasma particles in pulsar magnetospheres. It is assumed that the emitting particles are ultra-relativistic primary beam electrons re-accelerated to very high energies due to the Landau damping process of a special type of parametrically driven Langmuir waves. This type of Langmuir wave carries energy released through the rotational slow-down of a pulsar and is very effective in supplying the resonant particles with energy from a natural reservoir. The model provides simultaneous generation of energetic γ-ray and low-frequency radio (0.1–1 GHz) emission in the same location of the pulsar magnetosphere. These two radiations processes are triggered by a single plasma process, namely excitation of the cyclotron instability. This provides a natural explanation for the observed coincidence of radio and γ-ray signals observed from the Crab pulsar. Full article
(This article belongs to the Special Issue Observations of Gamma-Ray Pulsars)
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11 pages, 32615 KiB  
Article
Dynamics and Kinematics of the EUV Wave Event on 6 May 2019
by Ramesh Chandra, P. F. Chen, Pooja Devi, Reetika Joshi and Y. W. Ni
Galaxies 2022, 10(2), 58; https://doi.org/10.3390/galaxies10020058 - 10 Apr 2022
Cited by 5 | Viewed by 2398
Abstract
We present here the kinematics of the EUV wave associated with a GOES M1.0-class solar flare, which originates in NOAA AR 12740. The event is thoroughly observed with Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory (SDO) with high spatio-temporal resolutions. This event [...] Read more.
We present here the kinematics of the EUV wave associated with a GOES M1.0-class solar flare, which originates in NOAA AR 12740. The event is thoroughly observed with Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory (SDO) with high spatio-temporal resolutions. This event displays many features of EUV waves, which are very decisive for the understanding of the nature of EUV waves. These features include: a fast-mode wave, a pseudo wave, a slow-mode wave and stationary fronts, probably due to mode conversion. One fast-mode wave also propagates towards the coronal hole situated close to the north pole and the wave speed does not change when it encounters the coronal hole. We intend to provide self-consistent interpretations for all these different features. Full article
(This article belongs to the Special Issue Extreme Ultraviolet Waves in Solar Corona)
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14 pages, 460 KiB  
Article
Modelling Quintessential Inflation in Palatini-Modified Gravity
by Konstantinos Dimopoulos, Alexandros Karam, Samuel Sánchez López and Eemeli Tomberg
Galaxies 2022, 10(2), 57; https://doi.org/10.3390/galaxies10020057 - 5 Apr 2022
Cited by 13 | Viewed by 2406
Abstract
We study a model of quintessential inflation constructed in R2-modified gravity with a non-minimally coupled scalar field, in the Palatini formalism. Our non-minimal inflaton field is characterised by a simple exponential potential. We find that successful quintessential inflation can be achieved [...] Read more.
We study a model of quintessential inflation constructed in R2-modified gravity with a non-minimally coupled scalar field, in the Palatini formalism. Our non-minimal inflaton field is characterised by a simple exponential potential. We find that successful quintessential inflation can be achieved with no fine-tuning of the model parameters. Predictions of the characteristics of dark energy will be tested by observations in the near future, while contrasting with existing observations provides insights on the modified gravity background, such as the value of the non-minimal coupling and its running. Full article
(This article belongs to the Special Issue Latest Developments in the Quest for the Unification of Cosmic Inflation and Dark Energy)
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11 pages, 664 KiB  
Article
A Study of the Dusty Disks and Shells around Post-RGB Stars in the LMC
by Geetanjali Sarkar and Raghvendra Sahai
Galaxies 2022, 10(2), 56; https://doi.org/10.3390/galaxies10020056 - 2 Apr 2022
Viewed by 2153
Abstract
A new class of dusty post-Red Giant Branch (post-RGB) stars has recently been identified in the Magellanic Clouds. Their spectral energy distributions (SEDs) suggest that their mass-ejecta are similar to dusty post-Asymptotic Giant Branch (post-AGB) stars. We modeled the SEDs of a select [...] Read more.
A new class of dusty post-Red Giant Branch (post-RGB) stars has recently been identified in the Magellanic Clouds. Their spectral energy distributions (SEDs) suggest that their mass-ejecta are similar to dusty post-Asymptotic Giant Branch (post-AGB) stars. We modeled the SEDs of a select sample of post-RGB and post-AGB stars in the Large Magellanic Cloud (LMC), quantified the total dust mass in the disks and shells and set rough constraints on the dust grain compositions and sizes. The shells were significantly more massive than the disks. Our models suggest that circumstellar disks, when present, are geometrically thick with a substantial opening angle, which is consistent with numerical simulations of CE evolution (CEE). Comparison of our model dust mass values with the predictions of dust production during CEE on the RGB suggest that CEE occurred near or at the tip of the RGB for the post-RGB sources in our sample. Amorphous silicate emission features at 10 and 18 μm are seen in the model spectra of several post-RGBs. A surprising result is that the ejected dust in certain post-RGB sources appears to be carbon-rich, thus, providing independent support for the hypothesis of binary interactions leading to the formation of dusty post-RGB objects. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae 8e)
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28 pages, 10588 KiB  
Article
A Beacon in the Galaxy: Updated Arecibo Message for Potential FAST and SETI Projects
by Jonathan H. Jiang, Hanjie Li, Matthew Chong, Qitian Jin, Philip E. Rosen, Xiaoming Jiang, Kristen A. Fahy, Stuart F. Taylor, Zhihui Kong, Jamilah Hah and Zong-Hong Zhu
Galaxies 2022, 10(2), 55; https://doi.org/10.3390/galaxies10020055 - 25 Mar 2022
Cited by 8 | Viewed by 22747
Abstract
An updated, binary-coded message has been developed for transmission to extraterrestrial intelligences in the Milky Way galaxy. The proposed message includes basic mathematical and physical concepts to establish a universal means of communication followed by information on the biochemical composition of life on [...] Read more.
An updated, binary-coded message has been developed for transmission to extraterrestrial intelligences in the Milky Way galaxy. The proposed message includes basic mathematical and physical concepts to establish a universal means of communication followed by information on the biochemical composition of life on Earth, the Solar System’s time-stamped position in the Milky Way relative to known globular clusters, as well as digitized depictions of the Solar System, and Earth’s surface. The message concludes with digitized images of the human form, along with an invitation for any receiving intelligences to respond. Calculation of the optimal timing during a given calendar year is specified for potential future transmission from both the Five-hundred-meter Aperture Spherical radio Telescope in China and the SETI Institute’s Allen Telescope Array in northern California to a selected region of the Milky Way which has been proposed as the most likely location for life to have developed. These powerful new beacons, the successors to the Arecibo radio telescope which transmitted the 1974 message upon which this expanded communication is in part based, can carry forward Arecibo’s legacy into the 21st century with this equally well-constructed communication from Earth’s technological civilization. Full article
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17 pages, 584 KiB  
Article
Isolating an Outflow Component in Single-Epoch Spectra of Quasars
by Paola Marziani, Alice Deconto-Machado and Ascension Del Olmo
Galaxies 2022, 10(2), 54; https://doi.org/10.3390/galaxies10020054 - 24 Mar 2022
Cited by 7 | Viewed by 2532
Abstract
Gaseous outflows appear to be a universal property of type-1 and type-2 active galactic nuclei (AGN). The main diagnostic is provided by emission features shifted to higher frequencies via the Doppler effect, implying that the emitting gas is moving toward the observer. However, [...] Read more.
Gaseous outflows appear to be a universal property of type-1 and type-2 active galactic nuclei (AGN). The main diagnostic is provided by emission features shifted to higher frequencies via the Doppler effect, implying that the emitting gas is moving toward the observer. However, beyond the presence of blueshift, the observational signatures of the outflows are often unclear, and no established criteria exist to isolate the outflow contribution in the integrated, single-epoch spectra of type-1 AGN. The emission spectrum collected the typical apertures of long-slit spectroscopy or of fiber optics sample contributions over a broad range of spatial scales, making it difficult to analyze the line profiles in terms of different kinematical components. Nevertheless, hundred of thousands of quasars spectra collected at moderate resolution demand a proper analysis of the line profiles for proper dynamical modeling of the emitting regions. In this small contribution, we analyze several profiles of the Hi Balmer line Hβ from composite and individual spectra of sources radiating at moderate Eddington ratio (Population B). Features and profile shapes that might be traced to outflow due to narrow-line region gas are detected over a wide range of luminosity. Full article
(This article belongs to the Special Issue Global Understanding of Accretion and Ejection around Black Holes)
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11 pages, 3591 KiB  
Article
Cylindrical Planetary Nebulae. I. Flow from an Irradiated Ring
by Vincent Icke
Galaxies 2022, 10(2), 53; https://doi.org/10.3390/galaxies10020053 - 24 Mar 2022
Cited by 2 | Viewed by 2148
Abstract
Many bipolar nebulae with a pronounced cylindrical shape, such as Henize 3-401, show no indication whatsoever of interaction between a disk and a stellar wind, or a jet on the nebular axis. I propose that the disk that is observed at the base [...] Read more.
Many bipolar nebulae with a pronounced cylindrical shape, such as Henize 3-401, show no indication whatsoever of interaction between a disk and a stellar wind, or a jet on the nebular axis. I propose that the disk that is observed at the base of the bipolar is itself the source of the outflow. In particular, I assume that irradiation from the central star causes the disk to evaporate. I have performed numerical hydrodynamical calculations of outflows driven by evaporation of a pseudo-barotropic ring around a hot central star. The first results show that the outflow shapes are cylindrical, and the internal structures are similar to what is observed in some of these nebulae. Since shape is only the first step in the assessment of a model, synthetic observations should be made. For the moment I merely verify that the scalar quantities observed in the archetypical cylindrical nebula Hen 3-401 can be accommodated in my models. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae 8e)
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8 pages, 435 KiB  
Article
Why Masses of Binary Black Hole Mergers Are Overestimated?
by Michal Křížek and Lawrence Somer
Galaxies 2022, 10(2), 52; https://doi.org/10.3390/galaxies10020052 - 23 Mar 2022
Cited by 3 | Viewed by 2573
Abstract
We show that masses of binary black hole mergers are overestimated, since a large gravitational redshift is not taken into account. Such a phenomenon occurs due to time dilation in a close neighborhood of any black hole. This fact allows us to explain [...] Read more.
We show that masses of binary black hole mergers are overestimated, since a large gravitational redshift is not taken into account. Such a phenomenon occurs due to time dilation in a close neighborhood of any black hole. This fact allows us to explain a high mass gap between observed binary neutron stars and calculated binary black hole mergers. We also present other reasons why masses of black hole mergers are determined incorrectly. Full article
(This article belongs to the Collection A Trip across the Universe: Our Present Knowledge and Future Perspectives)
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36 pages, 1683 KiB  
Article
What Is Needed of a Scalar Field If It Is to Unify Inflation and Late Time Acceleration?
by Nur Jaman and Mohammad Sami
Galaxies 2022, 10(2), 51; https://doi.org/10.3390/galaxies10020051 - 23 Mar 2022
Cited by 13 | Viewed by 3433
Abstract
Quintessential inflation refers to scenarios in which a single scalar field is used to describe inflation and late time acceleration. This review is dedicated to the framework of quintessential inflation, with a focus on the building blocks of formalism. Consistent unification of inflation [...] Read more.
Quintessential inflation refers to scenarios in which a single scalar field is used to describe inflation and late time acceleration. This review is dedicated to the framework of quintessential inflation, with a focus on the building blocks of formalism. Consistent unification of inflation and late time acceleration using a single scalar field asks for a shallow field potential initially followed by steep behaviour thereafter and shallow again around the present epoch. The requirement of non-interference of the scalar field with thermal history dictates the steep nature of potential in the post-inflationary era, with a further restriction that late time physics be independent of initial conditions. We describe, in detail, the scaling and asymptotic scaling solutions and the mechanism of exit from the scaling regime to late time acceleration. The review includes a fresh look at scaling solutions that are central to the theme of unification of inflation and late time acceleration. As for the exit mechanism, special attention is paid to the coupling of massive neutrino matter to the scalar field, which builds up dynamically and can give rise to late time acceleration. We present a detailed analytical treatment of scalar field dynamics in the presence of coupling. We briefly discuss the distinguishing feature of quintessential inflation, namely the blue spectrum of gravity waves produced during the transition from inflation to the kinetic regime. Full article
(This article belongs to the Special Issue Latest Developments in the Quest for the Unification of Cosmic Inflation and Dark Energy)
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19 pages, 564 KiB  
Article
The Quantum Gravity Connection between Inflation and Quintessence
by Christof Wetterich
Galaxies 2022, 10(2), 50; https://doi.org/10.3390/galaxies10020050 - 17 Mar 2022
Cited by 14 | Viewed by 2608
Abstract
Inflation and quintessence can both be described by a single scalar field. The cosmic time evolution of this cosmon field realizes a crossover from the region of an ultraviolet fixed point in the infinite past to an infrared fixed point in the infinite [...] Read more.
Inflation and quintessence can both be described by a single scalar field. The cosmic time evolution of this cosmon field realizes a crossover from the region of an ultraviolet fixed point in the infinite past to an infrared fixed point in the infinite future. This amounts to a transition from early inflation to late dynamical dark energy, with intermediate radiation and matter domination. The scaling solution of the renormalization flow in quantum gravity connects the two fixed points. It provides for the essential characteristics of the scalar potential needed for the crossover cosmology and solves the cosmological constant problem dynamically. The quantum scale symmetry at the infrared fixed point protects the tiny mass of the cosmon and suppresses the cosmon coupling to atoms without the need of a non-linear screening mechanism, thereby explaining apparent issues of fine tuning. For a given content of particles, the scaling solution of quantum gravity is a predictive framework for the properties of inflation and dynamical dark energy. Full article
(This article belongs to the Special Issue Latest Developments in the Quest for the Unification of Cosmic Inflation and Dark Energy)
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6 pages, 243 KiB  
Article
Weyl Conformal Symmetry Model of the Dark Galactic Halo
by R. K. Nesbet
Galaxies 2022, 10(2), 49; https://doi.org/10.3390/galaxies10020049 - 15 Mar 2022
Viewed by 2006
Abstract
The postulate of universal conformal (local Weyl scaling) symmetry modifies both general relativity and the Higgs scalar field model. The conformal Higgs model (CHM) generates an effective cosmological constant that fits the observed accelerating Hubble expansion for redshifts z≤1 (7.33 Gyr) [...] Read more.
The postulate of universal conformal (local Weyl scaling) symmetry modifies both general relativity and the Higgs scalar field model. The conformal Higgs model (CHM) generates an effective cosmological constant that fits the observed accelerating Hubble expansion for redshifts z≤1 (7.33 Gyr) accurately with only one free parameter. Growth of a galaxy is modeled by the central accumulation of matter from an enclosing empty spherical halo whose radius expands with depletion. Details of this process account for the nonclassical, radial centripetal acceleration observed at excessive orbital velocities in galactic haloes. There is no need for dark matter. Full article
(This article belongs to the Special Issue Dark Cosmology: Shedding Light on Our Current Universe)
6 pages, 269 KiB  
Article
Millimetre Observations of Maser-Emitting Planetary Nebulae
by Lucero Uscanga, José R. Rizzo, Miguel Santander-García, José F. Gómez, Luis F. Miranda, Olga Suárez, Panayotis Boumis, Mónica I. Rodríguez, Gerardo Ramos-Larios and Roldán A. Cala
Galaxies 2022, 10(2), 48; https://doi.org/10.3390/galaxies10020048 - 11 Mar 2022
Cited by 1 | Viewed by 2196
Abstract
Observations in the millimetre bands of maser-emitting planetary nebulae (PNe) are crucial to study their circumstellar molecular gas at the beginning of the PN phase. Maser-emitting PNe are in the earliest phases of PN formation; therefore, these sources are key objects to study [...] Read more.
Observations in the millimetre bands of maser-emitting planetary nebulae (PNe) are crucial to study their circumstellar molecular gas at the beginning of the PN phase. Maser-emitting PNe are in the earliest phases of PN formation; therefore, these sources are key objects to study the molecular content during the early evolution of PNe. These circumstellar envelopes are active sites for the formation of molecules. We present preliminary results of millimetre observations with the IRAM 30 m telescope towards one PN (IRAS 17393−2727) of a sample of five maser-emitting PNe, where we detect 12CO and 13CO lines in both J=1→0 and J=2→1 transitions. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae 8e)
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9 pages, 1218 KiB  
Article
M 1–92 Revisited: New Findings and Open Questions: New NOEMA Observations of Minkowski’s Footprint
by Javier Alcolea, Marcelino Agúndez, Valentín Bujarrabal, Arancha Castro-Carrizo, Jean-François Desmurs, John-Eduard Martínez-Fernández, Carmen Sánchez Contreras and Miguel Santander-García
Galaxies 2022, 10(2), 47; https://doi.org/10.3390/galaxies10020047 - 10 Mar 2022
Cited by 3 | Viewed by 2621
Abstract
PN M 1–92, also known as Minkowski’s Footprint, is a textbook example of a massive pre-planetary nebula. It presents all the characteristics of this type of source: non-spherical symmetry (bipolar cylindrical symmetry in this case), high-velocity gas emission, large amounts of linear momentum [...] Read more.
PN M 1–92, also known as Minkowski’s Footprint, is a textbook example of a massive pre-planetary nebula. It presents all the characteristics of this type of source: non-spherical symmetry (bipolar cylindrical symmetry in this case), high-velocity gas emission, large amounts of linear momentum and kinetic energy (momentum excess), and a self-similar growing structure. We have revisited this object by performing new NOEMA observations (with half arc-second resolution) of a wealth of molecules, including the rare isotopologues of CO, as well as other less abundant species. These maps provide new insights into the origin of this source. Our findings include the discovery of molecular species in the ionised regions of the nebula, confirming its shocked origin; the structure of the massive equatorial component, including the presence of active wind collisions; and the strong evidence that the AGB evolution of the source was terminated prematurely, probably due to the huge mass loss event that resulted in the formation/acceleration of the present nebula. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae 8e)
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16 pages, 4182 KiB  
Review
Squeezing in Gravitational Wave Detectors
by Sheila E. Dwyer, Georgia L. Mansell and Lee McCuller
Galaxies 2022, 10(2), 46; https://doi.org/10.3390/galaxies10020046 - 9 Mar 2022
Cited by 16 | Viewed by 5004
Abstract
Injecting optical squeezed states of light, a technique known as squeezing, is now a tool for gravitational wave detection. Its ability to reduce quantum noise is helping to reveal more gravitational wave transients, expanding the catalog of observations in the last observing run. [...] Read more.
Injecting optical squeezed states of light, a technique known as squeezing, is now a tool for gravitational wave detection. Its ability to reduce quantum noise is helping to reveal more gravitational wave transients, expanding the catalog of observations in the last observing run. This review introduces squeezing and its history in the context of gravitational-wave detectors. It overviews the benefits, limitations and methods of incorporating squeezing into advanced interferometers, emphasizing the most relevant details for astrophysics instrumentation. Full article
(This article belongs to the Special Issue Present and Future of Gravitational Wave Astronomy)
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11 pages, 997 KiB  
Article
Morphologies of Wolf–Rayet Planetary Nebulae Based on IFU Observations
by Ashkbiz Danehkar
Galaxies 2022, 10(2), 45; https://doi.org/10.3390/galaxies10020045 - 8 Mar 2022
Viewed by 2578
Abstract
Integral field unit (IFU) spectroscopy of planetary nebulae (PNe) provides a plethora of information about their morphologies and ionization structures. An IFU survey of a sample of PNe around hydrogen-deficient stars has been conducted with the Wide Field Spectrograph (WiFeS) on the ANU [...] Read more.
Integral field unit (IFU) spectroscopy of planetary nebulae (PNe) provides a plethora of information about their morphologies and ionization structures. An IFU survey of a sample of PNe around hydrogen-deficient stars has been conducted with the Wide Field Spectrograph (WiFeS) on the ANU 2.3-m telescope. In this paper, we present the Hα kinematic observations of the PN M 2-42 with a weak emission-line star (wels), and the compact PNe Hen 3-1333 and Hen 2-113 around Wolf–Rayet ([WR]) stars from this WiFeS survey. We see that the ring and point-symmetric knots previously identified in the velocity [N ii] channels of M 2-42 are also surrounded by a thin exterior ionized Hα halo, whose polar expansion is apparently faster than the low-ionization knots. The velocity-resolved Hα channel maps of Hen 3-1333 and Hen 2-113 also suggest that the faint multipolar lobes may get to a projected outflow velocity of ∼100 ± 20 km s−1 far from the central stars. Our recent kinematic studies of the WiFeS/IFU survey of other PNe around [WR] and wels mostly hint at elliptical morphologies, while collimated outflows are present in many of them. As the WiFeS does not have adequate resolution for compact (≤6 arcsec) PNe, future high-resolution spatially-resolved observations are necessary to unveil full details of their morpho-kinematic structures. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae 8e)
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9 pages, 3167 KiB  
Article
Further Studies of the Association of Planetary Nebula BMP J16135406 with Galactic Open Cluster NGC 6067
by Vasiliki Fragkou, Quentin A. Parker, Albert A. Zijlstra, Lisa Crause, Laurence Sabin and Roberto Vázquez
Galaxies 2022, 10(2), 44; https://doi.org/10.3390/galaxies10020044 - 7 Mar 2022
Cited by 2 | Viewed by 2540
Abstract
Planetary Nebulae (PNe) that are physical members of Galactic open clusters are powerful probes that allow precise determination of their distance and crucially their initial mass on the main sequence. Here, we revisit the physical association of the PN BMP J1613–5406 with the [...] Read more.
Planetary Nebulae (PNe) that are physical members of Galactic open clusters are powerful probes that allow precise determination of their distance and crucially their initial mass on the main sequence. Here, we revisit the physical association of the PN BMP J1613–5406 with the open cluster NGC 6067 and present our preliminary results based on our new ESO/VLT FORS2 data. Our PN spectral data permit the calculation of a precise radial velocity and reddening to the PN that shows a tight consistency with the literature corresponding cluster parameters including importantly the radial velocity. Our measurements, combined with the agreement between the distances of the two objects and the fact that the PN is located well within the cluster boundaries, confirm that the PN is physically associated with the cluster. The cluster has a turn-off mass of around 5 solar masses that indicates a PN initial mass of around 5.6 solar masses. This is closer to the theoretical lower limit of core-collapse supernova formation than has ever been previously observed, providing a unique opportunity for further stellar and Galactic chemical evolution studies using this system. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae 8e)
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13 pages, 560 KiB  
Review
The Accreting White Dwarfs in Cataclysmic Variables
by Edward M. Sion and Patrick Godon
Galaxies 2022, 10(2), 43; https://doi.org/10.3390/galaxies10020043 - 7 Mar 2022
Cited by 4 | Viewed by 3246
Abstract
Accreting white dwarfs (WDs) in cataclysmic variables (CVs) provide crucial insights about the accretion of mass and angular momentum in all types of binaries, including accreting NSs and BHs. Accreting WDs are the critical component in the single degenerate pathway to SNe Ia, [...] Read more.
Accreting white dwarfs (WDs) in cataclysmic variables (CVs) provide crucial insights about the accretion of mass and angular momentum in all types of binaries, including accreting NSs and BHs. Accreting WDs are the critical component in the single degenerate pathway to SNe Ia, along with the double degenerate merger pathway, they are the standard candles of cosmology proving that the universe is accelerating and the existence of dark energy. Another key question is whether the WD in a CV can grow in mass despite the mass loss due to thousands of nova explosions in its lifetime. Angular momentum loss drives CV evolution and accreting WDs offer critically needed WD masses from Gaia distances and reliable surface temperatures to derive the most accurate accretion rates. We review the studies on accreting WDs, including WD masses, accurate rotational velocities and chemical abundances of elements. Most of the progress that has been made is based upon Hubble Space Telescope spectroscopy and FUSE spectroscopy in the UV spectral region during dwarf nova quiescence and the low states of novalike variables, when the accreting WD dominates the UV spectral range. Full article
(This article belongs to the Special Issue What’s New under the Binary Suns)
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14 pages, 882 KiB  
Article
Toward Calibration of the Global Network of Gravitational Wave Detectors with Sub-Percent Absolute and Relative Accuracy
by Sudarshan Karki, Dripta Bhattacharjee and Richard L. Savage
Galaxies 2022, 10(2), 42; https://doi.org/10.3390/galaxies10020042 - 5 Mar 2022
Cited by 3 | Viewed by 2330
Abstract
The detection of gravitational-wave signals by the LIGO and Virgo observatories during the past few years has ushered us into the era of gravitational-wave astronomy, shifting our focus from detection to source parameter estimation. This has imposed stringent requirements on calibration in order [...] Read more.
The detection of gravitational-wave signals by the LIGO and Virgo observatories during the past few years has ushered us into the era of gravitational-wave astronomy, shifting our focus from detection to source parameter estimation. This has imposed stringent requirements on calibration in order to maximize the astrophysical information extracted from these detected signals. Current detectors rely on photon radiation pressure from auxiliary lasers to achieve required calibration accuracy. These photon calibrators have made significant improvements over the last few years, realizing fiducials displacements with sub-percent accuracy. This achieved accuracy is directly dependent on the laser power calibration. For the next observing campaign, scheduled to begin at the end of 2022, a new scheme is being implemented to achieve improved laser power calibration accuracy for all of the GW detectors in the global network. It is expected to significantly improve absolute and relative calibration accuracy for the entire network. Full article
(This article belongs to the Special Issue Present and Future of Gravitational Wave Astronomy)
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10 pages, 8583 KiB  
Article
Follow-Up of Extended Shells around B[e] Stars
by Tiina Liimets, Michaela Kraus, Alexei Moiseev, Nicolas Duronea, Lydia Sonia Cidale and Cecilia Fariña
Galaxies 2022, 10(2), 41; https://doi.org/10.3390/galaxies10020041 - 1 Mar 2022
Cited by 9 | Viewed by 2745
Abstract
B[e] stars are massive B type emission line stars in different evolutionary stages ranging from pre-main sequence to post-main sequence. Due to their mass loss and ejection events these objects deposit huge amounts of mass and energy into their environment and enrich it [...] Read more.
B[e] stars are massive B type emission line stars in different evolutionary stages ranging from pre-main sequence to post-main sequence. Due to their mass loss and ejection events these objects deposit huge amounts of mass and energy into their environment and enrich it with chemically processed material, contributing significantly to the chemical and dynamical evolution of their host galaxies. However, the large-scale environments of these enigmatic objects have not attracted much attention. The first and so far only catalog reporting the detection of extended shells around a sample of B[e] stars was an Hα imaging survey carried out in the year 2001, and was limited to bright targets in the northern hemisphere. We have recently started a follow-up of those targets to detect possible evolution of their nebulae in the plane of the sky over a baseline of two decades. Furthermore, we extend our survey to southern targets and fainter northern ones to complement and complete our knowledge on large-scale ejecta surrounding B[e] stars. Besides imaging in Hα and selected nebular lines, we utilize long-slit and 3D spectral observations across the nebulae to derive their physical properties. We discovered pronounced nebula structures around 15 more objects, resulting in a total of 27 B[e] stars with a large-scale nebula. Here we present our (preliminary) results for three selected objects: the two massive supergiants MWC137 and MWC 314, and the unclassified B[e] star MWC 819. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae 8e)
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13 pages, 331 KiB  
Article
Cylindrical Gravastar Like-Structures in f(G) Gravity
by M. Z. Bhatti, Z. Yousaf and A. Rehman
Galaxies 2022, 10(2), 40; https://doi.org/10.3390/galaxies10020040 - 23 Feb 2022
Cited by 7 | Viewed by 2294
Abstract
The aim of this manuscript is to explore singularity-free solution for a specific self-gravitating highly dense object known as gravastar suggested by Mazur and Mottola, in the context of f(G) gravity theory. Gravastars are regarded as a possible alternate to [...] Read more.
The aim of this manuscript is to explore singularity-free solution for a specific self-gravitating highly dense object known as gravastar suggested by Mazur and Mottola, in the context of f(G) gravity theory. Gravastars are regarded as a possible alternate to black hole. To derive modified field equations and law of conservation related to Gauss-Bonnet gravity, we assume cylindrically symmetric irrotational configuration. Particular equation of states are used for the illustration of three sectors of gravastar model. Furthermore, we are intended to obtain a regular solution for our model and graphs will be used to elaborate various substantial characteristics of it. Full article
(This article belongs to the Special Issue Self-Gravitating Systems and Fuzzification of Geometric Structures)
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28 pages, 990 KiB  
Review
Gamma-Ray Cosmology and Tests of Fundamental Physics
by Jonathan Biteau and Manuel Meyer
Galaxies 2022, 10(2), 39; https://doi.org/10.3390/galaxies10020039 - 22 Feb 2022
Cited by 15 | Viewed by 3950
Abstract
The propagation of gamma-rays over cosmological distances is the subject of extensive theoretical and observational research at GeV and TeV energies. The mean free path of gamma-rays in the cosmic web is limited above 100 GeV due to the production of electrons and [...] Read more.
The propagation of gamma-rays over cosmological distances is the subject of extensive theoretical and observational research at GeV and TeV energies. The mean free path of gamma-rays in the cosmic web is limited above 100 GeV due to the production of electrons and positrons on the cosmic optical and infrared backgrounds. Electrons and positrons cool in the intergalactic medium while gyrating in its magnetic fields, which could cause either its global heating or the production of lower-energy secondary gamma-rays. The energy distribution of gamma-rays surviving the cosmological journey carries observed absorption features that gauge the emissivity of baryonic matter over cosmic time, constrain the distance scale of ΛCDM cosmology, and limit the alterations of the interaction cross section. Competitive constraints are, in particular, placed on the cosmic star-formation history as well as on phenomena expected from quantum gravity and string theory, such as the coupling to hypothetical axion-like particles or the violation of Lorentz invariance. Recent theoretical and observational advances offer a glimpse of the multi-wavelength and multi-messenger path that the new generation of gamma-ray observatories is about to open. Full article
(This article belongs to the Special Issue Extragalactic TeV Astronomy)
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20 pages, 739 KiB  
Review
The GRB Prompt Emission: An Unsolved Puzzle
by Željka Bošnjak, Rodolfo Barniol Duran and Asaf Pe’er
Galaxies 2022, 10(2), 38; https://doi.org/10.3390/galaxies10020038 - 22 Feb 2022
Cited by 12 | Viewed by 3888
Abstract
The recent multi-messenger and multi-wavelength observations of gamma-ray bursts (GRBs) have encouraged renewed interest in these energetic events. In spite of the substantial amount of data accumulated during the past few decades, the nature of the prompt emission remains an unsolved puzzle. We [...] Read more.
The recent multi-messenger and multi-wavelength observations of gamma-ray bursts (GRBs) have encouraged renewed interest in these energetic events. In spite of the substantial amount of data accumulated during the past few decades, the nature of the prompt emission remains an unsolved puzzle. We present an overview of the leading models for their prompt emission phase, focusing on the perspective opened by future missions. Full article
(This article belongs to the Special Issue Gamma-Ray Burst Science in 2030)
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