Galaxies, Volume 8, Issue 2 (June 2020) – 24 articles

We present preliminary results of our study of a small sample of planetary nebulae in the Galactic Bulge for which high-angular resolution Hubble Space Telescope imaging is available. From this and from archival spectroscopy, we were able to calculate temperatures and luminosities for their central stars. These were then correlated to up-to-date evolutionary tracks found in the literature to help us estimate stellar masses and therefore ages for the central stars. Our current analysis indicates that our sample appears to represent a somewhat mixed population of planetary nebulae central stars, while at least one of the nebulae might have been formed by a more massive progenitor (i.e., $M_{\mathrm{ZAMS}}\sim 4$ M ${}_{\odot }$ ). Full article

Nebular spectroscopy is a valuable tool for assessing the production of heavy elements by slow neutron(n)-capture nucleosynthesis (the s-process). Several transitions of n-capture elements have been identified in planetary nebulae (PNe) in the last few years, with the aid of sensitive, high-resolution, near-infrared spectrometers. Combined with optical spectroscopy, the newly discovered near-infrared lines enable more accurate abundance determinations than previously possible, and provide access to elements that had not previously been studied in PNe or their progenitors. Neutron-capture elements have also been detected in PNe in the Sagittarius Dwarf galaxy and in the Magellanic Clouds. In this brief review, I discuss developments in observational studies of s-process enrichments in PNe, with an emphasis on the last five years, and note some open questions and preliminary trends. Full article

We present the first results from comprehensive, near-UV-to-near-IR Hubble Space Telescope Wide Field Camera 3 (WFC3) emission-line imaging studies of two young planetary nebulae (PNe), NGC 7027 and NGC 6302. These two objects represent key sources for purposes of understanding PNe shaping processes. Both nebulae feature axisymmetric and point-symmetric (bipolar) structures and, despite hot central stars and high nebular excitation states, both harbor large masses of molecular gas and dust. The sweeping wavelength coverage of our Cycle 27 Hubble Space Telescope (HST)/WFC3 imaging surveys targeting these two rapidly evolving PNe will provide a battery of essential tests for theories describing the structural and chemical evolution of evolved star ejecta. Here, we present initial color overlays for selected images, and we highlight some of the first results gleaned from the surveys. Full article

Using 3.6-μm data from 2112 galaxies, we show that, contrary to widely held expectations of a continuous steep decline, radial surface brightness profiles of galaxies tend to flatten and form extended plateaus beyond 27–28 mag_AB/arcsec². This phenomenon could be explained by the presence of extended stellar populations dominated by low-mass stars in galactic outskirts. The flattening of radial brightness profiles questions the artificial exponential extrapolations of brightness data and the automatic assumption that light always declines considerably faster than mass density, presenting an empirical challenge for the dark matter hypothesis. Full article

Proposed explanations of galactic rotation curves (RC = tangential velocity vs. equatorial radius, determined from Doppler measurements) involve dramatically different assumptions. A dominant, original camp invoked huge amounts of unknown, non-baryonic dark matter (NBDM) in surrounding haloes to reconcile RC simulated using their Newtonian orbital models (NOMs) for billions of stars in spiral galaxies with the familiar Keplerian orbital patterns of the few, tiny planets in our Solar System. A competing minority proposed that hypothetical, non-relativistic, non-Newtonian forces govern the internal motions of galaxies. More than 40 years of controversy has followed. Other smaller groups, unsatisfied by explanations rooted in unknown matter or undocumented forces, have variously employed force summations, spin models, or relativistic adaptations to explain galactic rotation curves. Some small groups have pursued inverse models and found no need for NBDM. The successes, failures, and underlying assumptions of the above models are reviewed in this paper, focusing on their mathematical underpinnings. We also show that extractions of RC from Doppler measurements need revising to account for the effect of galaxy shape on flux-velocity profiles and for the possible presence of a secondary spin axis. The latter is indicated by complex Doppler shift patterns. Our findings, combined with independent evidence such as hadron collider experiments failing to produce non-baryonic matter, suggest that a paradigm shift is unfolding. Full article

In addition to large-scale structures, planetary nebulae (PNe) show small-scale structures that emit mainly in low-ionization species such as [N ii], [S ii], [O ii], and [O i], known as LISs. Here, we present the analysis of optical long-slit spectra, for three slit positions, of the PN IC 4593, which possesses a pair of knots and an isolated low-ionization knot. The motivation for this work is the need to characterize LISs completely to evaluate their impact on the PNe studies. These data allow us to derive the physical properties and ionization state for each morphological component of the nebula, including its pair of knots and individual knot. Due to the large uncertainties in the [S ii] derived electron densities, we cannot confirm any contrast between the LISs’ electron densities and the surrounding nebula, found in numerous other LISs. Though the lack of spatially-resolved physical parameters in the literature prevents further comparisons, in general, our results derived for the entire nebula agree with previous studies. Full article

In the Newtonian limit of general relativity a force acting on a test mass in a central gravitational field is conventionally defined by the attractive Newtonian gravity (inverse square) term plus a small repulsive cosmological force, which is proportional to the slow acceleration of the universe expansion. In this paper we considered the cosmological-force correction due to fast quantum oscillations of the universe scale factor as a potential solution of the cosmological constant problem. These fast fluctuations of the cosmological scale factor violate Lorentz invariance at the Planck scale, and they induce strong changes to the current sign and magnitude of the average cosmological force, thus making it one of the potential probable causes for the modification of Newtonian dynamics in galaxy-scale systems. The modified cosmological force may be responsible for the recently discovered “cosmic-clock” behavior of disk galaxies in the low-redshift universe. The obtained results have strong implications for astroparticle physics since they demonstrate that typical galaxy rotation curves may be obtained without (or almost without) dark-matter particles. Full article

In this paper, I present how near and mid-infrared observations can be used for the study of planetary nebulae and related objects. I present the main observing techniques, from the ground and space, highlighting main differences and how they can be complementary. I also highlight some new observing facilities and present the infrared observatories of the future to show that the future of infrared observations of planetary nebulae is bright. Full article

With the launch of JWST and the upcoming installation of extremely large telescopes, the first galaxies in our Universe will finally be revealed. Their light will be dominated by massive stars, which peak in in the ultra-violet (UV) part of the electromagnetic spectrum. Star formation is the key driver of the evolution of our Universe. At young ages, within 10 Million years, both high and low mass stars generate complex UV emission processes which are poorly understood yet are vital for interpreting high red-shift line emission. For these reasons, the Hubble Space Telescope (HST) will devote 1000 orbits to obtaining a UV Legacy Library of Young Stars as Essential Standards (ULLYSES). The purpose of this Overview is to outline the basic physical principles driving UV emission processes from local (within 100 parsecs of) star formation, ranging from huge star-forming complexes containing hundreds of massive and very-massive stars (VMS), such as 30 Doradus (the Tarantula Nebula) in the neighboring Magellanic Clouds (only 50 kpc away), to galaxies near and far, out to the epoch of Cosmic Reionization. Full article

We show that the Earth acts as a high-efficiency gravitational collector of low-velocity flow of dark matter (DM). The focal point appears on the Earth’s surface, when the DM flow speed is about 17 km/s with respect to the geo-center. We discuss diurnal modulation of the local DM density influenced by the Earth’s gravity. We also touch upon similar effects on galactic and solar system objects. Full article

(1) Background: We investigate the hypothesis that exoplanet engulfment can help explain the observed non-spherical planetary nebula population, as a complementary shaping mechanism to the binary hypothesis. The aim is to investigate the extent to which massive planets can explain the population of non-spherical planetary nebulae; (2) Methods: This research utilises a new tool to calculate the planet-fraction of planetary nebulae progenitor stars called simsplash; (3) Results: we conclude that ∼15–30% of non-spherical planetary nebulae around single stars will have a history in which they engulfed a massive planet on the AGB; and (4) Conclusions: Engulfment of massive exoplanets may contribute significantly to the formation of non-spherical planetary nebulae around single stars, yet appears to be insufficient to explain them all. Full article

To assess Mars’ potential for both harboring life and providing useable resources for future human exploration, it is of paramount importance to comprehend the water situation on the planet. Therefore, studies have been conducted to determine any evidence of past or present water existence on Mars. While the presence of abundant water on Mars very early in its history is widely accepted, on its modern form, only a fraction of this water can be found, as either ice or locked into the structure of Mars’ plentiful water-rich materials. Water on the planet is evaluated through various evidence such as rocks and minerals, Martian achondrites, low volume transient briny outflows (e.g., dune flows, reactivated gullies, slope streaks, etc.), diurnal shallow soil moisture (e.g., measurements by Curiosity and Phoenix Lander), geomorphic representation (possibly from lakes and river valleys), and groundwater, along with further evidence obtained by probe and rover discoveries. One of the most significant lines of evidence is for an ancient streambed in Gale Crater, implying ancient amounts of “vigorous” water on Mars. Long ago, hospitable conditions for microbial life existed on the surface of Mars, as it was likely periodically wet. However, its current dry surface makes it almost impossible as an appropriate environment for living organisms; therefore, scientists have recognized the planet’s subsurface environments as the best potential locations for exploring life on Mars. As a result, modern research has aimed towards discovering underground water, leading to the discovery of a large amount of underground ice in 2016 by NASA, and a subglacial lake in 2018 by Italian scientists. Nevertheless, the presence of life in Mars’ history is still an open question. In this unifying context, the current review summarizes results from a wide variety of studies and reports related to the history of water on Mars, as well as any related discussions on the possibility of living organism existence on the planet. Full article

Understanding how young stars gain their masses through disk-to-star accretion is of paramount importance in astrophysics. It affects our knowledge about the early stellar evolution, the disk lifetime and dissipation processes, the way the planets form on the smallest scales, or the connection to macroscopic parameters characterizing star-forming regions on the largest ones, among others. In turn, mass accretion rate estimates depend on the accretion paradigm assumed. For low-mass T Tauri stars with strong magnetic fields there is consensus that magnetospheric accretion (MA) is the driving mechanism, but the transfer of mass in massive young stellar objects with weak or negligible magnetic fields probably occurs directly from the disk to the star through a hot boundary layer (BL). The intermediate-mass Herbig Ae/Be (HAeBe) stars bridge the gap between both previous regimes and are still optically visible during the pre-main sequence phase, thus constituting a unique opportunity to test a possible change of accretion mode from MA to BL. This review deals with our estimates of accretion rates in HAeBes, critically discussing the different accretion paradigms. It shows that although mounting evidence supports that MA may extend to late-type HAes but not to early-type HBes, there is not yet a consensus on the validity of this scenario versus the BL one. Based on MA and BL shock modeling, it is argued that the ultraviolet regime could significantly contribute in the future to discriminating between these competing accretion scenarios. Full article

One of the key methods for determining the unknown nature of Type Ia supernovae (SNe Ia) is the search for traces of interaction between the SN ejecta and the circumstellar structures at the resulting supernova remnants (SNRs Ia). So far, the observables that we receive from well-studied SNRs Ia cannot be explained self-consistently by any model presented in the literature. In this study, we suggest that the circumstellar medium (CSM) being observed to surround several SNRs Ia was mainly shaped by planetary nebulae (PNe) that originated from one or both progenitor stars. Performing two-dimensional hydrodynamic simulations, we show that the ambient medium shaped by PNe can account for several properties of the CSM that have been found to surround SNe Ia and their remnants. Finally, we model Kepler’s SNR considering that the SN explosion occurred inside a bipolar PN. Our simulations show good agreement with the observed morphological and kinematic properties of Kepler’s SNR. In particular, our model reproduces the current expansion parameter of Kepler’s SNR, the partial interaction of the remnant with a dense CSM at its northern region and finally the existence of two opposite protrusions (‘ears’) at the equatorial plane of the SNR. Full article

We review the current status of the study of rotation curve (RC) of the Milky Way, and present a unified RC from the Galactic Center to the galacto-centric distance of about 100 kpc. The RC is used to directly calculate the distribution of the surface mass density (SMD). We then propose a method to derive the distribution of dark matter (DM) density in the in the Milky Way using the SMD distribution. The best-fit dark halo profile yielded a local DM density of ${\rho }_{\odot }=0.36\pm 0.02$ GeV cm ${}^{-3}$ . We also review the estimations of the local DM density in the last decade, and show that the value is converging to a value at ${\rho }_{\odot }=0.39\pm 0.09$ GeV cm ${}^{-3}$ . Full article

The aim of this paper is to test the need for non-baryonic dark matter in the context of galactic rotation and the apparent difference between distributions of galactic mass and luminosity. We present a set of rotation curves and 3.6 μm surface brightness profiles for a diverse sample of 214 galaxies. Using rotation curves as the sole input into our Newtonian disk model, we compute non-parametric radial profiles of surface mass density. All profiles exhibit lower density than parametric models with dark halos and provide a superior fit with observed rotation curves. Assuming all dynamical mass is in main-sequence stars, we estimate radial distributions of characteristic star mass implied by the corresponding pairs of density and brightness profiles. We find that for 132 galaxies or 62% of the sample, the relation between density and brightness can be fully explained by a radially declining stellar mass gradient. Such idealized stellar population fitting can also largely address density and brightness distributions of the remaining 82 galaxies, but their periphery shows, on average, 14 M_⊙/pc² difference between total density and light-constrained stellar density. We discuss how this density gap can be interpreted, by considering a low-luminosity baryonic matter, observational uncertainties, and visibility cutoffs for red dwarf populations. Lastly, we report tight correlation between radial density and brightness trends, and the discovered flattening of surface brightness profiles—both being evidence against dark matter. Our findings make non-baryonic dark matter unnecessary in the context of galactic rotation. Full article

A review is given of a priori predictions made for the dynamics of rotating galaxies. One theory—MOND—has had many predictions corroborated by subsequent observations. While it is sometimes possible to offer post hoc explanations for these observations in terms of dark matter, it is seldom possible to use dark matter to predict the same phenomena. Full article

We present preliminary results from measuring Galactic orientation angles of 766 elliptical and bipolar Planetary Nebulae (PNe) in the Hong Kong/Australian Astronomical Observatory/Strasbourg Observatory H-alpha Planetary Nebula research platform and database (HASH DB). For elliptical PNe the distribution of orientation angles is found to be more or less uniform. However, for bipolar PNe there is statistically significant evidence for preferred orientation angles (as tentatively reported before with smaller samples) across the whole Galaxy. Full article

Weakly magnetized, relativistic collisionless shock waves are not only the natural offsprings of relativistic jets in high-energy astrophysical sources, they are also associated with some of the most outstanding displays of energy dissipation through particle acceleration and radiation. Perhaps their most peculiar and exciting feature is that the magnetized turbulence that sustains the acceleration process, and (possibly) the secondary radiation itself, is self-excited by the accelerated particles themselves, so that the phenomenology of these shock waves hinges strongly on the microphysics of the shock. In this review, we draw a status report of this microphysics, benchmarking analytical arguments with particle-in-cell simulations, and extract consequences of direct interest to the phenomenology, regarding, in particular, the so-called microphysical parameters used in phenomenological studies. Full article

Background: Many planetary nebulae retain significant quantities of molecular gas and dust despite their signature hostile radiation environments and energetic shocks. Photoionization and dissociation by extreme UV and (often) X-ray emission from their central stars drive the chemical processing of this material. Their well-defined geometries make planetary nebulae ideal testbeds for modeling the effects of radiation-driven heating and chemistry on molecular gas in photodissociation regions. Methods: We have carried out IRAM 30m/APEX 12m/ALMA radio studies of the Helix Nebula and its molecule-rich globules, exploiting the unique properties of the Helix to follow up our discovery of an anti-correlation between HNC/HCN line intensity ratio and central star UV Luminosity. Results: Analysis of HNC/HCN across the Helix Nebula reveals the line ratio increases with distance from the central star, and thus decreasing incident UV flux, indicative of the utility of the HNC/HCN ratio as a tracer of UV irradiation in photodissociation environments. However, modeling of the observed regions suggests HNC/HCN should decrease with greater distance, contrary to the observed trend. Conclusion: HNC/HCN acts as an effective tracer of UV irradiation of cold molecular gas. Further model studies are required. Full article

The Multi-Unit Spectroscopic Explorer (MUSE) is a large integral field unit mounted on the ESO Very Large Telescope. Its spatial (60 arcsecond field) and wavelength (4800–9300Å) coverage is well suited to detailed imaging spectroscopy of extended planetary nebulae, such as in the Galaxy. An overview of the capabilities of MUSE applied to Planetary Nebulae (PNe) is provided together with the specific advantages and disadvantages. Some examples of archival MUSE observations of PNe are provided. MUSE datacubes for two targets (NGC 3132 and NGC 7009) are analyzed in detail, and they are used to show the advances achievable for planetary nebula studies. Prospects for further MUSE observations of PNe and a broader analysis of existing datasets are outlined. Full article

Molecular hydrogen (H ${}_2$ ) emission is commonly detected in planetary nebulae (PNe), specially in objects with bipolar morphologies. New studies showed that H ${}_2$ gas is also packed in microstructures embedded in PNe of any morphological type. Despite the presence of H ${}_2$ in cometary knots being known for years, only in the last five years, much deeper imagery of PNe have revealed that H ${}_2$ also exists in other types of low-ionisation microstructures (LISs). Significant differences are found between the host PNe of cometary knots and other types of LISs, such as nebula age, central star temperature (evolutionary stage) and the absolute sizes of the microstructure itself. Full article

The aim of this work is to examine distances to planetary nebulae (PNe) together with other properties that were derived from them, using the astrometry of Gaia Data Release 2 (DR2). We were able to identify 1571 objects classified as PNe, for which we assumed distances calculated following a Bayesian statistical approach. From those objects, we selected a sample of PNe with good quality parallax measurements and distance derivations, which we called Golden Astrometry PNe sample (GAPN). In this paper we will review the physical properties of the stars and nebulae in this subsample of PNe. Full article

It is now clear that a vast majority of intermediate-mass stars have stellar and/or sub-stellar companions, therefore it is no longer appropriate to consider planetary nebulae as a single-star phenomenon, although some single, isolated stars may well lead to planetary nebulae. As such, while understanding binary evolution is critical for furthering our knowledge of planetary nebulae, the converse is also true: planetary nebulae can be valuable tools with which to probe binary evolution. In this brief review, I attempt to summarise some of our current understanding with regards to the role of binarity in the formation of planetary nebulae, and the areas in which continued study of planetary nebulae may have wider ramifications for our grasp on the fundaments of binary evolution. Full article