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Volume 6
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Galaxies, Volume 6, Issue 3 (September 2018) – 34 articles

Cover Story (view full-size image): Most of the visible matter in our Galaxy and other galaxies is in the form of stars. If two stars started their lives close together, their life stories could be changed dramatically by an interaction between them. An example of a fate-changing interaction is a common envelope event, during which two stars temporarily orbit within a shared envelope. The episode ends with an exciting outburst, leaving behind either a significantly shrunken binary, or a single merged star. The outburst could be observed as recently discovered Luminous Red Novae. The expelled envelope starts its expansion and could appear in a future as a planetary nebulae. Physics of these violent interactions is a key for to understand life-stories of X-ray binaries, Type Ia supernovae, and progenitors of gravitational wave sources recently observed by the Laser Interferometer Gravitational-Wave Observatory (LIGO). View the paper
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8 pages, 1483 KiB  
Article
AGBs, Post-AGBs and the Shaping of Planetary Nebulae
by Eric Lagadec
Galaxies 2018, 6(3), 99; https://doi.org/10.3390/galaxies6030099 - 17 Sep 2018
Cited by 5 | Viewed by 2886
Abstract
During the last decades, observations, mostly with the Hubble Space Telescope, have revealed that round Planetary Nebulae were the exception rather than rule. A huge variety of features are observed, such as jets, discs, tori, showing that the ejection of material is not [...] Read more.
During the last decades, observations, mostly with the Hubble Space Telescope, have revealed that round Planetary Nebulae were the exception rather than rule. A huge variety of features are observed, such as jets, discs, tori, showing that the ejection of material is not due to isotropic radiation pressure on a spherical shell and that more physics is involved. This shaping process certainly occur early in the evolution of these low and intermediate mass stars and must leave imprints in the evolutionary stages prior the PN phase. Thanks to news instruments on the most advanced telescopes (e.g., the VLTI, SPHERE/VLT and ALMA), high angular resolution observations are revolutionising our view of the ejection of gas and dust during the AGB and post-AGB phases. In this review I will present the newest results concerning the mass loss from AGB stars, post-AGB stars and related objects. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae VII)
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7 pages, 441 KiB  
Article
X-ray Shaping of Planetary Nebulae
by Martín A. Guerrero
Galaxies 2018, 6(3), 98; https://doi.org/10.3390/galaxies6030098 - 11 Sep 2018
Viewed by 3587
Abstract
The stellar winds of the central stars of planetary nebulae play an essential role in the shaping of planetary nebulae. In the interacting stellar winds model, the fast stellar wind injects energy and momentum, which are transferred to the nebular envelope through an [...] Read more.
The stellar winds of the central stars of planetary nebulae play an essential role in the shaping of planetary nebulae. In the interacting stellar winds model, the fast stellar wind injects energy and momentum, which are transferred to the nebular envelope through an X-ray-emitting hot bubble. Together with other physical processes, such as the ionization of the nebular envelope, the asymmetrical mass-loss in the asymptotic giant branch (AGB), and the action of collimated outflows and magnetic fields, the pressurized hot gas determines the expansion and evolution of planetary nebulae. Chandra and XMM-Newton have provided us with detailed information of this hot gas. Here in this talk I will review our current understanding of the effects of the fast stellar wind in the shaping and evolution of planetary nebulae and give some hints of the promising future of this research. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae VII)
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7 pages, 320 KiB  
Article
Post-AGB Discs from Common-Envelope Evolution
by Robert G. Izzard and Adam S. Jermyn
Galaxies 2018, 6(3), 97; https://doi.org/10.3390/galaxies6030097 - 11 Sep 2018
Cited by 10 | Viewed by 2740
Abstract
Post-asymptotic giant branch (post-AGB) stars with discs are all binaries. Many of these binaries have orbital periods between 100 and 1000 days so cannot have avoided mass transfer between the AGB star and its companion, likely through a common-envelope type interaction. We report [...] Read more.
Post-asymptotic giant branch (post-AGB) stars with discs are all binaries. Many of these binaries have orbital periods between 100 and 1000 days so cannot have avoided mass transfer between the AGB star and its companion, likely through a common-envelope type interaction. We report on preliminary results of our project to model circumbinary discs around post-AGB stars using our binary population synthesis code binary_c. We combine a simple analytic thin-disc model with binary stellar evolution to estimate the impact of the disc on the binary, and vice versa, fast enough that we can model stellar population and hence explore the rather uncertain parameter space involved with disc formation. We find that, provided the discs form with sufficient mass and angular momentum, and have an inner edge that is relatively close to the binary, they can both prolong the life of their parent post-AGB star and pump the eccentricity of orbits of their inner binaries. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae VII)
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8 pages, 999 KiB  
Article
The Morphology of the Outflow in the Grazing Envelope Evolution
by Sagiv Shiber
Galaxies 2018, 6(3), 96; https://doi.org/10.3390/galaxies6030096 - 06 Sep 2018
Cited by 7 | Viewed by 2424
Abstract
We study the grazing envelope evolution (GEE), where a secondary star, which orbits the surface of a giant star, accretes mass from the giant envelope and launches jets. We conduct simulations of the GEE with different half-opening angles and velocities, and simulate the [...] Read more.
We study the grazing envelope evolution (GEE), where a secondary star, which orbits the surface of a giant star, accretes mass from the giant envelope and launches jets. We conduct simulations of the GEE with different half-opening angles and velocities, and simulate the onset phase and the spiralling-in phase. We discuss the resulting envelope structure and the outflow geometry. We find in the simulations of the onset phase with narrow jets that a large fraction of the ejected mass outflows along the polar directions. The mass ejected at these directions has the fastest velocity and the highest angular momentum magnitude. In the simulations of the spiralling-in phase, a large fraction of the ejected mass concentrates around the orbital plane. According to our findings, the outflow with the highest velocity is closer to the poles as we launch narrower jets. The outflow has a toroidal shape accompanied by two faster rings, one ring at each side of the equatorial plane. The interaction of the jets with the giant envelope causes these outflow structures, as we do not include in our simulations the secondary star gravity and the envelope self-gravity. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae VII)
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19 pages, 11591 KiB  
Article
SpArcFiRe: Enhancing Spiral Galaxy Recognition Using Arm Analysis and Random Forests
by Pedro Silva, Leon T. Cao and Wayne B. Hayes
Galaxies 2018, 6(3), 95; https://doi.org/10.3390/galaxies6030095 - 05 Sep 2018
Cited by 8 | Viewed by 3837
Abstract
Automated quantification of galaxy morphology is necessary because the size of upcoming sky surveys will overwhelm human volunteers. Existing classification schemes are inadequate because (a) their uncertainty increases near the boundary of classes and astronomers need more control over these uncertainties; (b) galaxy [...] Read more.
Automated quantification of galaxy morphology is necessary because the size of upcoming sky surveys will overwhelm human volunteers. Existing classification schemes are inadequate because (a) their uncertainty increases near the boundary of classes and astronomers need more control over these uncertainties; (b) galaxy morphology is continuous rather than discrete; and (c) sometimes we need to know not only the type of an object, but whether a particular image of the object exhibits visible structure. We propose that regression is better suited to these tasks than classification, and focus specifically on determining the extent to which an image of a spiral galaxy exhibits visible spiral structure. We use the human vote distributions from Galaxy Zoo 1 (GZ1) to train a random forest of decision trees to reproduce the fraction of GZ1 humans who vote for the “Spiral” class. We prefer the random forest model over other black box models like neural networks because it allows us to trace post hoc the precise reasoning behind the regression of each image. Finally, we demonstrate that using features from SpArcFiRe—a code designed to isolate and quantify arm structure in spiral galaxies—improves regression results over and above using traditional features alone, across a sample of 470,000 galaxies from the Sloan Digital Sky Survey. Full article
(This article belongs to the Special Issue Application of Machine-Learning Techniques in Astronomical Data Analysis)
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9 pages, 2612 KiB  
Article
ALMA’s Acute View of pPNe: Through the Magnifying Glass... and What We Found There
by Carmen Sánchez Contreras, Javier Alcolea, Valentín Bujarrabal and Arancha Castro-Carrizo
Galaxies 2018, 6(3), 94; https://doi.org/10.3390/galaxies6030094 - 04 Sep 2018
Cited by 2 | Viewed by 3221
Abstract
We present recent Atacama Large Millimeter/submillimeter Array (ALMA)-based studies of circumstellar envelopes (CSEs) around Asymptotic Giant Branch (AGB) stars and pre-Planetary Nebulae (pPNe). In only a few years of operation, ALMA is revolutionising the field of AGB-to-PN research by providing unprecedentedly detailed information [...] Read more.
We present recent Atacama Large Millimeter/submillimeter Array (ALMA)-based studies of circumstellar envelopes (CSEs) around Asymptotic Giant Branch (AGB) stars and pre-Planetary Nebulae (pPNe). In only a few years of operation, ALMA is revolutionising the field of AGB-to-PN research by providing unprecedentedly detailed information on the complex nebular architecture (at large but also on small scales down to a few ∼10 AU from the centre), dynamics and chemistry of the outflows/envelopes of low-to-intermediate mass stars in their late stages of the evolution. Here, we focus on continuum and molecular line mapping studies with high angular resolution and sensitivity of some objects that are key to understanding the complex PN-shaping process. In particular, we offer (i) a brief summary of ALMA observations of rotating disks in post-AGB objects and (ii) report on ALMA observations of OH 231.8+4.2 providing the most detailed and accurate description of the global nebular structure and kinematics of this iconic object to date. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae VII)
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8 pages, 1569 KiB  
Article
VALD: The Meeting Point of Data Producers and Data Users
by Tatiana Ryabchikova, Yury Pakhomov and Nikolai Piskunov
Galaxies 2018, 6(3), 93; https://doi.org/10.3390/galaxies6030093 - 03 Sep 2018
Cited by 3 | Viewed by 3548
Abstract
Vienna Atomic Line Database (VALD) contains data on atomic and molecular energy levels and parameters of spectral lines required for stellar spectra analysis. Hundreds of millions of lines for fine spectral synthesis and for opacity calculations are collected in the present version of [...] Read more.
Vienna Atomic Line Database (VALD) contains data on atomic and molecular energy levels and parameters of spectral lines required for stellar spectra analysis. Hundreds of millions of lines for fine spectral synthesis and for opacity calculations are collected in the present version of VALD (VALD3). Critical evaluation of the data and the diversity of extraction tools support the high popularity of VALD among users. The data model of VALD3 incorporates obligatory links to the bibliography making our database more attractive as a publishing platform for data producers. The VALD data quality and completeness are constantly improving allowing better reproduction of stellar spectra. To illustrate continuous evolution of the data content we present a comparative analysis of the recent experimental and theoretical atomic data for Fe-group elements, which will be included in the next VALD release. This release will also include a possibility for extracting the line data with full isotopic and hyperfine structures. Full article
(This article belongs to the Special Issue Atomic and Molecular Data Needs for Astronomy and Astrophysics)
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8 pages, 266 KiB  
Article
Relieving Tensions Related to the Dark Matter Interpretation of the Fermi-LAT Data
by Man Ho Chan
Galaxies 2018, 6(3), 92; https://doi.org/10.3390/galaxies6030092 - 29 Aug 2018
Cited by 2 | Viewed by 2779
Abstract
Recently, many studies indicate that the GeV gamma ray excess signal from the central Milky Way can be best explained by ∼40–50 GeV dark matter annihilating via the b b ¯ channel. However, this model appears to be disfavored by the recent Fermi-LAT [...] Read more.
Recently, many studies indicate that the GeV gamma ray excess signal from the central Milky Way can be best explained by ∼40–50 GeV dark matter annihilating via the b b ¯ channel. However, this model appears to be disfavored by the recent Fermi-LAT data for dwarf spheroidal galaxies and the constraint from synchrotron radiation. In this article, we describe a consistent picture to relieve the tensions between the dark matter annihilation model and the observations. We show that a baryonic feedback process is the key to alleviate the tensions and the ∼40–50 GeV dark matter model is still the best one to account for the GeV gamma ray excess in the Milky Way. Full article
(This article belongs to the Special Issue The Dark Side of the Universe: Dark Energy, Dark Matter and Modified Gravity)
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6 pages, 223 KiB  
Article
Towards the Provision of Accurate Atomic Data for Neutral Iron
by Andrew Conroy, Catherine Ramsbottom, Connor Ballance and Francis Keenan
Galaxies 2018, 6(3), 91; https://doi.org/10.3390/galaxies6030091 - 25 Aug 2018
Cited by 1 | Viewed by 3045
Abstract
The rich emission and absorption line spectra of Fe I may be used to extract crucial information on astrophysical plasmas, such as stellar metallicities. There is currently a lack, in quality and quantity, of accurate level-resolved effective electron-impact collision strengths and oscillator strengths [...] Read more.
The rich emission and absorption line spectra of Fe I may be used to extract crucial information on astrophysical plasmas, such as stellar metallicities. There is currently a lack, in quality and quantity, of accurate level-resolved effective electron-impact collision strengths and oscillator strengths for radiative transitions. Here, we discuss the challenges in obtaining an accurate model of the neutral iron atom and compare our theoretical fine-structure energy levels with observation for several increasingly large models. Radiative data is presented for several transitions for which the atomic data is accurately known. Full article
(This article belongs to the Special Issue Atomic and Molecular Data Needs for Astronomy and Astrophysics)
11 pages, 8887 KiB  
Article
Benchmarking Current Capabilities for the Generation of Excitation and Photoionisation Atomic Data
by Catherine Ramsbottom, Connor Ballance, Ryan Smyth, Andrew Conroy, Luis Fernández-Menchero, Michael Turkington and Francis Keenan
Galaxies 2018, 6(3), 90; https://doi.org/10.3390/galaxies6030090 - 21 Aug 2018
Cited by 2 | Viewed by 3104
Abstract
The spectra currently emerging from modern ground- and space-based astronomical instruments are of exceptionally high quality and resolution. To meaningfully analyse these spectra, researchers utilise complex modelling codes to replicate the observations. The main inputs to these codes are atomic data such as [...] Read more.
The spectra currently emerging from modern ground- and space-based astronomical instruments are of exceptionally high quality and resolution. To meaningfully analyse these spectra, researchers utilise complex modelling codes to replicate the observations. The main inputs to these codes are atomic data such as excitation and photoionisation cross sections, as well as radiative transition probabilities, energy levels, and line strengths. In this publication, the current capabilities of the numerical methods and computer packages used in the generation of these data are discussed. Particular emphasis is given to Fe-peak species and the heavy systems of tungsten and molybdenum. Some of the results presented to highlight certain issues and/or advances have already been published in the literature, while other sections present new recently evaluated atomic data for the first time. Full article
(This article belongs to the Special Issue Atomic and Molecular Data Needs for Astronomy and Astrophysics)
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7 pages, 501 KiB  
Article
Jsolated Stars of Low Metallicity
by Efrat Sabach
Galaxies 2018, 6(3), 89; https://doi.org/10.3390/galaxies6030089 - 15 Aug 2018
Viewed by 2699
Abstract
We study the effects of a reduced mass-loss rate on the evolution of low metallicity Jsolated stars, following our earlier classification for angular momentum (J) isolated stars. By using the stellar evolution code MESA we study the evolution with different mass-loss rate efficiencies [...] Read more.
We study the effects of a reduced mass-loss rate on the evolution of low metallicity Jsolated stars, following our earlier classification for angular momentum (J) isolated stars. By using the stellar evolution code MESA we study the evolution with different mass-loss rate efficiencies for stars with low metallicities of Z = 0.001 and Z = 0.004 , and compare with the evolution with solar metallicity, Z = 0.02 . We further study the possibility for late asymptomatic giant branch (AGB)—planet interaction and its possible effects on the properties of the planetary nebula (PN). We find for all metallicities that only with a reduced mass-loss rate an interaction with a low mass companion might take place during the AGB phase of the star. The interaction will most likely shape an elliptical PN. The maximum post-AGB luminosities obtained, both for solar metallicity and low metallicities, reach high values corresponding to the enigmatic finding of the PN luminosity function. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae VII)
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7 pages, 641 KiB  
Article
Revealing the True Nature of Hen 2-428
by Nicole Reindl, Nicolle L. Finch, Veronika Schaffenroth, Martin A. Barstow, Sarah L. Casewell, Stephan Geier, Marcelo M. Miller Bertolami and Stefan Taubenberger
Galaxies 2018, 6(3), 88; https://doi.org/10.3390/galaxies6030088 - 14 Aug 2018
Cited by 6 | Viewed by 3992
Abstract
The nucleus of Hen 2-428 is a short orbital period (4.2 h) spectroscopic binary, whose status as potential supernovae type Ia progenitor has raised some controversy in the literature. We present preliminary results of a thorough analysis of this interesting system, which combines [...] Read more.
The nucleus of Hen 2-428 is a short orbital period (4.2 h) spectroscopic binary, whose status as potential supernovae type Ia progenitor has raised some controversy in the literature. We present preliminary results of a thorough analysis of this interesting system, which combines quantitative non-local thermodynamic (non-LTE) equilibrium spectral modelling, radial velocity analysis, multi-band light curve fitting, and state-of-the art stellar evolutionary calculations. Importantly, we find that the dynamical system mass that is derived by using all available He II lines does not exceed the Chandrasekhar mass limit. Furthermore, the individual masses of the two central stars are too small to lead to an SN Ia in case of a dynamical explosion during the merger process. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae VII)
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6 pages, 219 KiB  
Article
R-Matrix Scattering Calculations for Iron-Peak Species: Photoionisation of Fe I and Electron-Impact Excitation of Fe II
by Ryan T. Smyth, Catherine A. Ramsbottom and Connor P. Ballance
Galaxies 2018, 6(3), 87; https://doi.org/10.3390/galaxies6030087 - 10 Aug 2018
Cited by 2 | Viewed by 3256
Abstract
An abundance of absorption and emission lines of iron-peak species such as Fe I and Fe II can be seen in the spectra of many astrophysical objects. Thus, the accurate modelling of such spectra requires sets of high quality atomic data for these [...] Read more.
An abundance of absorption and emission lines of iron-peak species such as Fe I and Fe II can be seen in the spectra of many astrophysical objects. Thus, the accurate modelling of such spectra requires sets of high quality atomic data for these species. In this paper, we present preliminary results from the present electron-impact excitation calculations for Fe II and fine-structure resolved photoionisation calculations for Fe I employing the Dirac atomic R-matrix and Breit–Pauli R-matrix methods. For the Fe II excitation, we compare results with all existing calculations, and for the Fe I photoionisation, we present a sample of level-resolved cross-sections. The calculations and results described throughout will be of use to those requiring high quality atomic data for modelling a wide variety of astrophysical objects. Full article
(This article belongs to the Special Issue Atomic and Molecular Data Needs for Astronomy and Astrophysics)
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9 pages, 270 KiB  
Review
Molecular Data Needs for Modelling AGB Stellar Winds and Other Molecular Environments
by Taïssa Danilovich, Leen Decin and Marie Van de Sande
Galaxies 2018, 6(3), 86; https://doi.org/10.3390/galaxies6030086 - 07 Aug 2018
Cited by 2 | Viewed by 2621
Abstract
The modern era of highly sensitive telescopes is enabling the detection of more and more molecular species in various astronomical environments. Many of these are now being carefully examined for the first time. However, to move beyond detection to more detailed analysis such [...] Read more.
The modern era of highly sensitive telescopes is enabling the detection of more and more molecular species in various astronomical environments. Many of these are now being carefully examined for the first time. However, to move beyond detection to more detailed analysis such as radiative transfer modelling, certain molecular properties need to be properly measured and calculated. The importance of contributions from vibrationally excited states or collisional (de-)excitations can vary greatly, depending on the specific molecule and the environment being studied. Here, we discuss the present molecular data needs for detailed radiative transfer modelling of observations of molecular rotational transitions, primarily in the (sub-)millimetre and adjacent regimes, and with a focus on the stellar winds of AGB stars. Full article
(This article belongs to the Special Issue Atomic and Molecular Data Needs for Astronomy and Astrophysics)
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8 pages, 276 KiB  
Article
Surveying Planetary Nebulae Central Stars for Close Binaries: Constraining Evolution of Central Stars Based on Binary Parameters
by Todd Hillwig
Galaxies 2018, 6(3), 85; https://doi.org/10.3390/galaxies6030085 - 06 Aug 2018
Cited by 8 | Viewed by 3045
Abstract
The increase in discovered close binary central stars of planetary nebulae is leading to a sufficiently large sample to begin to make broader conclusions about the effect of close binary stars on common envelope evolution and planetary nebula formation. Herein I review some [...] Read more.
The increase in discovered close binary central stars of planetary nebulae is leading to a sufficiently large sample to begin to make broader conclusions about the effect of close binary stars on common envelope evolution and planetary nebula formation. Herein I review some of the recent results and conclusions specifically relating close binary central stars to nebular shaping, common envelope evolution off the red giant branch, and the total binary fraction and double degenerate fraction of central stars. Finally, I use parameters of known binary central stars to explore the relationship between the proto-planetary nebula and planetary nebula stages, demonstrating that the known proto-planetary nebulae are not the precursors of planetary nebulae with close binary central stars. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae VII)
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6 pages, 1186 KiB  
Article
Analysis of Multiple Shell Planetary Nebulae Based on HST/WFPC2 Extended 2D Diagnostic Diagrams
by Daniela Barría and Stefan Kimeswenger
Galaxies 2018, 6(3), 84; https://doi.org/10.3390/galaxies6030084 - 03 Aug 2018
Cited by 1 | Viewed by 2929
Abstract
The investigation of gaseous nebulae, emitting in forbidden lines, is often based extensively on diagnostic diagrams. The special physics of these lines often allows for disentangling with a few line ratios normally coupled thermodynamic parameters like electron temperature, density and properties of the [...] Read more.
The investigation of gaseous nebulae, emitting in forbidden lines, is often based extensively on diagnostic diagrams. The special physics of these lines often allows for disentangling with a few line ratios normally coupled thermodynamic parameters like electron temperature, density and properties of the photo-ionizing radiation field. Diagnostic diagrams are usually used for the investigation of planetary nebulae as a total. We investigated the extension of such integrated properties towards spatially resolved 2D diagnostics, using Hubble Space Telescope/Wide Field Planetary Camera 2 (HST/WFPC2) narrow band images. For this purpose, we also derived a method to isolate pure Hα emission from the [N ii] contamination as normally suffering in the F656N HST/WFPC2 filter. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae VII)
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26 pages, 4533 KiB  
Article
Applications of a Particular Four-Dimensional Projective Geometry to Galactic Dynamics
by Jacques L. Rubin
Galaxies 2018, 6(3), 83; https://doi.org/10.3390/galaxies6030083 - 03 Aug 2018
Cited by 3 | Viewed by 3437 | Correction
Abstract
Relativistic localizing systems that extend relativistic positioning systems show that pseudo-Riemannian space-time geometry is somehow encompassed in a particular four-dimensional projective geometry. The resulting geometric structure is then that of a generalized Cartan space (also called Cartan connection space) with projective connection. The [...] Read more.
Relativistic localizing systems that extend relativistic positioning systems show that pseudo-Riemannian space-time geometry is somehow encompassed in a particular four-dimensional projective geometry. The resulting geometric structure is then that of a generalized Cartan space (also called Cartan connection space) with projective connection. The result is that locally non-linear actions of projective groups via homographies systematically induce the existence of a particular space-time foliation independent of any space-time dynamics or solutions of Einstein’s equations for example. In this article, we present the consequences of these projective group actions and this foliation. In particular, it is shown that the particular geometric structure due to this foliation is similar from a certain point of view to that of a black hole but not necessarily based on the existence of singularities. We also present a modified Newton’s laws invariant with respect to the homographic transformations induced by this projective geometry. Consequences on galactic dynamics are discussed and fits of galactic rotational velocity curves based on these modifications which are independent of any Modified Newtonian Dynamics (MOND) or dark matter theories are presented. Full article
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11 pages, 482 KiB  
Article
Simulations and Modeling of Intermediate Luminosity Optical Transients and Supernova Impostors
by Amit Kashi
Galaxies 2018, 6(3), 82; https://doi.org/10.3390/galaxies6030082 - 01 Aug 2018
Cited by 9 | Viewed by 3563
Abstract
More luminous than classical novae, but less luminous than supernovae, lies the exotic stellar eruptions known as Intermediate luminosity optical transients (ILOTs). They are divided into a number of sub-groups depending on the erupting progenitor and the properties of the eruption. A large [...] Read more.
More luminous than classical novae, but less luminous than supernovae, lies the exotic stellar eruptions known as Intermediate luminosity optical transients (ILOTs). They are divided into a number of sub-groups depending on the erupting progenitor and the properties of the eruption. A large part of the ILOTs is positioned on the slanted Optical Transient Stripe (OTS) in the Energy-Time Diagram (ETD) that shows their total energy vs. duration of their eruption. We describe the different kinds of ILOTs that populate the OTS and other parts of the ETD. The high energy part of the OTS hosts the supernova impostors—giant eruptions (GE) of very massive stars. We show the results of the 3D hydrodynamical simulations of GEs that expose the mechanism behind these GEs and present new models for recent ILOTs. We discuss the connection between different kinds of ILOTs and suggest that they have a common energy source—gravitational energy released by mass transfer. We emphasize similarities between Planetary Nebulae (PNe) and ILOTs, and suggest that some PNe were formed in an ILOT event. Therefore, simulations used for GEs can be adapted for PNe, and used to learn about the influence of the ILOT events on the central star of the planetary nebula. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae VII)
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8 pages, 245 KiB  
Article
Renormalizable Gravitational Action That Reduces to General Relativity on the Mass-Shell
by Peter D. Morley
Galaxies 2018, 6(3), 81; https://doi.org/10.3390/galaxies6030081 - 30 Jul 2018
Cited by 2 | Viewed by 3109
Abstract
We derive the equation that relates gravity to quantum mechanics: R|mass-shell=8πGc4LSM, where R is the scalar curvature, G is the gravitational constant, [...] Read more.
We derive the equation that relates gravity to quantum mechanics: R|mass-shell=8πGc4LSM, where R is the scalar curvature, G is the gravitational constant, c is the speed of light and LSM is the Standard Model Lagrangian, or its future replacement. Implications of this equation are discussed in the paper. In particular, we show (in the last section) that this equation is the transformation that relates four-dimensional physics to two-dimensional physics. Full article
4 pages, 519 KiB  
Article
Simulations of the Formation and X-ray Emission from Hot Bubbles in Planetary Nebulae
by Jesus A. Toalá and S. Jane Arthur
Galaxies 2018, 6(3), 80; https://doi.org/10.3390/galaxies6030080 - 30 Jul 2018
Viewed by 2537
Abstract
High-quality X-ray observations of planetary nebulae (PNe) have demonstrated that the X-ray-emitting gas in their hot bubbles have temperatures in the small range TX = (1 − 3) × 106 K. However, according to theoretical expectations, adiabatically-shocked wind-blown bubbles should have [...] Read more.
High-quality X-ray observations of planetary nebulae (PNe) have demonstrated that the X-ray-emitting gas in their hot bubbles have temperatures in the small range TX = (1 − 3) × 106 K. However, according to theoretical expectations, adiabatically-shocked wind-blown bubbles should have temperatures up to two orders of magnitude higher. Numerical simulations show that instabilities at the interface between the hot bubble and the nebular material form clumps and filaments that generate an intermediate-temperature turbulent mixing layer. We describe the X-ray properties resulting from simulations of PNe in our Galaxy and the Magellanic Clouds. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae VII)
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6 pages, 467 KiB  
Article
The Real-Time Evolution of V4334 Sgr
by Peter A. M. Van Hoof, Stefan Kimeswenger, Griet C. Van de Steene, Adam Avison, Albert A. Zijlstra, Lizette Guzman-Ramirez, Falk Herwig and Marcin Hajduk
Galaxies 2018, 6(3), 79; https://doi.org/10.3390/galaxies6030079 - 26 Jul 2018
Cited by 5 | Viewed by 3059
Abstract
V4334 Sgr (Sakurai’s object) is an enigmatic evolved star that underwent a very late thermal pulse a few years before its discovery in 1996. It ejected a new hydrogen-deficient nebula in the process. The source has been observed continuously since, at many wavelengths [...] Read more.
V4334 Sgr (Sakurai’s object) is an enigmatic evolved star that underwent a very late thermal pulse a few years before its discovery in 1996. It ejected a new hydrogen-deficient nebula in the process. The source has been observed continuously since, at many wavelengths ranging from the optical to the radio regime. In this paper we evaluate these data and discuss the evolution of this object. We reach the conclusion that we have seen no evidence for photoionization of the nebula yet and that the spectral features we see are caused either by shocks or by dust. These shocks are an integral part of the hydrodynamic shaping that is now producing a new bipolar nebula inside the old planetary nebula (PN), implying that we have a detailed observational record of the very early stages of the shaping of a bipolar nebula. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae VII)
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9 pages, 255 KiB  
Article
The Belgian Repository of Fundamental Atomic Data and Stellar Spectra (BRASS) Identifying Fruitful Methods for Producing Atomic Data
by Mike Laverick, Alex Lobel, Pierre Royer, Christophe Martayan, Thibault Merle, Peter A. M. Van Hoof, Mathieu Van de Swaelmen, Marc David, Herman Hensberge and Emmanuel Thienpont
Galaxies 2018, 6(3), 78; https://doi.org/10.3390/galaxies6030078 - 25 Jul 2018
Cited by 5 | Viewed by 2657
Abstract
The Belgian repository of fundamental atomic data and stellar spectra (BRASS) aims to provide the largest systematic and homogeneous quality assessment to-date of input atomic data required for stellar spectral synthesis. In addition to quality-assessed atomic data, BRASS shall also provide of a [...] Read more.
The Belgian repository of fundamental atomic data and stellar spectra (BRASS) aims to provide the largest systematic and homogeneous quality assessment to-date of input atomic data required for stellar spectral synthesis. In addition to quality-assessed atomic data, BRASS shall also provide of a range of extremely high-quality benchmark stellar spectra spanning late B-type stars to early K-type stars. In this paper, we summarise the project’s progress and available results to-date. We provide a brief comparison between our results and the BRASS project’s compiled and cross-matched atomic literature, with the goal of providing useful feedback to the atomic community on which methods may produce more reliable and accurate atomic data. We hope that the examples presented here stimulate further investigation by the atomic physics community. Full article
(This article belongs to the Special Issue Atomic and Molecular Data Needs for Astronomy and Astrophysics)
12 pages, 275 KiB  
Article
Successes and Difficulties in Calculating Atomic Oscillator Strengths and Transition Rates
by Alan Hibbert
Galaxies 2018, 6(3), 77; https://doi.org/10.3390/galaxies6030077 - 24 Jul 2018
Cited by 7 | Viewed by 2676
Abstract
There is an on-going need for accurate oscillator strengths to be used in astrophysical applications, particularly in plasma diagnostics and in the modelling of stellar atmospheres and the interstellar medium. There are several databases in regular use which contain some of the required [...] Read more.
There is an on-going need for accurate oscillator strengths to be used in astrophysical applications, particularly in plasma diagnostics and in the modelling of stellar atmospheres and the interstellar medium. There are several databases in regular use which contain some of the required data, although often insufficiently complete, and sometimes not sufficiently accurate. In addition, several atomic structure packages are available through the literature, or from their individual authors, which would allow further calculations to be undertaken. Laboratory measurements provide an important check on the accuracy of calculated data, and the combined efforts of theorists and experimentalists have succeeded in providing data of an accuracy sufficient for some astrophysical applications. However, the insufficiency or inadequacy of atomic data is a continuing problem. We discuss in the context of appropriate examples some of the principal steps which researchers have taken to calculate accurate oscillator strengths, including both ab initio results and also various extrapolation processes which attempt to improve such results. We also present some examples of the main causes of difficulty in such calculations, particularly for complex (many-electron) ions, and indicate ways in which the difficulties might be overcome. Full article
(This article belongs to the Special Issue Atomic and Molecular Data Needs for Astronomy and Astrophysics)
16 pages, 572 KiB  
Article
Strongly Magnetized Sources: QED and X-ray Polarization
by Jeremy Heyl and Ilaria Caiazzo
Galaxies 2018, 6(3), 76; https://doi.org/10.3390/galaxies6030076 - 21 Jul 2018
Cited by 20 | Viewed by 2987
Abstract
Radiative corrections of quantum electrodynamics cause a vacuum threaded by a magnetic field to be birefringent. This means that radiation of different polarizations travels at different speeds. Even in the strong magnetic fields of astrophysical sources, the difference in speed is small. However, [...] Read more.
Radiative corrections of quantum electrodynamics cause a vacuum threaded by a magnetic field to be birefringent. This means that radiation of different polarizations travels at different speeds. Even in the strong magnetic fields of astrophysical sources, the difference in speed is small. However, it has profound consequences for the extent of polarization expected from strongly magnetized sources. We demonstrate how the birefringence arises from first principles, show how birefringence affects the polarization state of radiation and present recent calculations for the expected polarization from magnetars and X-ray pulsars. Full article
(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)
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8 pages, 619 KiB  
Article
Planetary Nebulae Embryo after a Common Envelope Event
by Natalia Ivanova and Jose L. A. Nandez
Galaxies 2018, 6(3), 75; https://doi.org/10.3390/galaxies6030075 - 19 Jul 2018
Cited by 6 | Viewed by 2863
Abstract
In the centers of some planetary nebulae are found close binary stars. The formation of those planetary nebulae was likely through a common envelope event, which transformed an initially-wide progenitor binary into the currently observed close binary, while stripping the outer layers away. [...] Read more.
In the centers of some planetary nebulae are found close binary stars. The formation of those planetary nebulae was likely through a common envelope event, which transformed an initially-wide progenitor binary into the currently observed close binary, while stripping the outer layers away. A common envelope event proceeds through several qualitatively different stages, each of which ejects matter at its own characteristic speed, and with a different degree of symmetry. Here, we present how typical post-common envelope ejecta looks kinematically a few years after the start of a common envelope event. We also show some asymmetric features we have detected in our simulations (jet-like structures, lobes, and hemispheres). Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae VII)
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7 pages, 2208 KiB  
Article
Estimating the Level of Uncertainty in Electron-Impact Excitation and Photoionisation Rates of Low-Ionised Stages of Ni Ions
by Luis Fernández-Menchero, Catherine A. Ramsbottom and Connor P. Ballance
Galaxies 2018, 6(3), 74; https://doi.org/10.3390/galaxies6030074 - 18 Jul 2018
Viewed by 2483
Abstract
Ni iv lines can be used as diagnostics for temperature and density for various astrophysical objects. In addition, ionization of Ni2+ is one of the causes of the opacity in the interstellar medium. We calculate the photoionization of Ni2+ [...] Read more.
Ni iv lines can be used as diagnostics for temperature and density for various astrophysical objects. In addition, ionization of Ni2+ is one of the causes of the opacity in the interstellar medium. We calculate the photoionization of Ni2+ and the electron-impact excitation of Ni3+. We use a fully-relativistic Dirac Atomic R-Matrix Code (DARC) method. We include a large set of configurations in the expansion of the wave functions of the target, up to the n=6 atomic shell. We show preliminary results for the photoionization cross-sections of Ni2+ and the electron-impact excitation collision strengths of Ni3+. The expected final results can be implemented in the available software packages for astrophysical plasma simulation, such as CLOUDY. We also show a preliminary estimation of the error of the data by the comparison of different sets of calculations. Full article
(This article belongs to the Special Issue Atomic and Molecular Data Needs for Astronomy and Astrophysics)
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8 pages, 2824 KiB  
Article
Spectroscopy of Planetary Nebulae with Herschel: A Beginners Guide
by Katrina Exter
Galaxies 2018, 6(3), 73; https://doi.org/10.3390/galaxies6030073 - 17 Jul 2018
Viewed by 2976
Abstract
A brief overview of the Herschel Space Telescope PACS and SPIRE spectrographs is given, pointing out aspects of working with the data products that should be considered by anyone using them. Some preliminary results of Planetary Nebulae (PNe) taken from the Herschel Planetary [...] Read more.
A brief overview of the Herschel Space Telescope PACS and SPIRE spectrographs is given, pointing out aspects of working with the data products that should be considered by anyone using them. Some preliminary results of Planetary Nebulae (PNe) taken from the Herschel Planetary Nebula Survey (HerPlaNs) programme are then used to demonstrate what can be done with spectroscopy observations made with PACS. The take-home message is that using the full 3D information that PACS spectroscopy observations give will greatly aid in the interpretation of PNe. Full article
(This article belongs to the Special Issue Asymmetric Planetary Nebulae VII)
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10 pages, 548 KiB  
Article
The Collisional Atomic Processes of Rydberg Hydrogen and Helium Atoms: Astrophysical Relevance
by Vladimir A. Srećković, Milan S. Dimitrijević, Ljubinko M. Ignjatović, Nikolai N. Bezuglov and Andrey N. Klyucharev
Galaxies 2018, 6(3), 72; https://doi.org/10.3390/galaxies6030072 - 16 Jul 2018
Cited by 10 | Viewed by 3146
Abstract
Elementary processes in astrophysical environments traditionally attract researchers’ attention. We present the data needed for the inclusion of the specific atomic collisional processes in the investigation of the optical and kinetic properties of weakly ionized stellar atmosphere layers. The first type of processes [...] Read more.
Elementary processes in astrophysical environments traditionally attract researchers’ attention. We present the data needed for the inclusion of the specific atomic collisional processes in the investigation of the optical and kinetic properties of weakly ionized stellar atmosphere layers. The first type of processes are collisional ionisation (chemi-ionization) processes, and the second ones are excitation and de-excitation (i.e., ( n - n ′ )-mixing processes). We give the rate coefficients of the aforementioned processes for the conditions that exist in the solar photosphere, the atmosphere of DB white dwarfs, M-type red dwarfs, etc. Full article
(This article belongs to the Special Issue Atomic and Molecular Data Needs for Astronomy and Astrophysics)
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12 pages, 454 KiB  
Article
Photoelectric Polarimetry and the Gas Pixel Detector Yesterday, Today and Tomorrow
by Enrico Costa
Galaxies 2018, 6(3), 71; https://doi.org/10.3390/galaxies6030071 - 16 Jul 2018
Viewed by 2530
Abstract
Since the very beginning of X-ray Astronomy, polarimetry has been suggested as a tool of diagnostics, of great potentiality. While almost all measurements of X-rays were based on detectors using the photoelectric effect, the first attempt to perform polarimetry were based on Compton [...] Read more.
Since the very beginning of X-ray Astronomy, polarimetry has been suggested as a tool of diagnostics, of great potentiality. While almost all measurements of X-rays were based on detectors using the photoelectric effect, the first attempt to perform polarimetry were based on Compton scattering and Bragg diffraction. The use of photoelectric effect also for polarimetry has been hypothesized and attempted for many years but never accomplished. Only 40 years from the start of X-ray astronomy, the Gas Pixel Detector (GPD) was developed, compatible with an X-ray optics, and capable of measuring energy, time, position and polarization simultaneously. Only after 20 more years, the Imaging X-ray Polarimetry Explorer, based on the GPD detectors, will be launched. I present the story of the development of photoelectric polarimetry that arrived to the Gas Pixel Detector, and discuss the possible future evolutions. Full article
(This article belongs to the Special Issue The Bright Future of Astronomical X-ray Polarimetry)
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13 pages, 1376 KiB  
Article
A Simple Model for Explaining Galaxy Rotation Curves
by Aneta Wojnar, Ciprian A. Sporea and Andrzej Borowiec
Galaxies 2018, 6(3), 70; https://doi.org/10.3390/galaxies6030070 - 13 Jul 2018
Cited by 9 | Viewed by 8023
Abstract
A new simple expression for the circular velocity of spiral galaxies is proposed and tested against HI Nearby Galaxy Survey (THINGS) data set. Its accuracy is compared with the one coming from MOND. Full article
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