Galactic Field Research Articles

Evolution of the mass-radius relation of expanding very young star clusters

The initial mass–radius relation of embedded star clusters is an essential boundary condition for understanding the evolution of embedded clusters in which stars form to their release into the galactic field via an open star cluster phase. The initial mass–radius relation of embedded clusters deduced by Marks & Kroupa (2012, A&A, 543, A8) is significantly different from the relation suggested by Pfalzner et al. (2016, A&A, 586, A68). Here, we use direct N-body simulations to model the early expansion of embedded clusters after the expulsion of their residual gas. The observationally deduced radii of clusters up to a few million years old, compiled from various sources, are well fitted by N-body models, implying that these observed very young clusters are most likely in an expanding state. We show that the mass–radius relation of Pfalzner et al. (2016) reflects the expansion of embedded clusters following the initial mass–radius relation of Marks & Kroupa (2012). We also suggest that even the embedded clusters in ATLASGAL clumps with HII regions are probably already in expansion. All the clusters collected here from different observations show a mass-radius relation with a similar slope, which may indicate that all clusters were born with a profile resembling that of the Plummer phase-space distribution function.

The masses of open star clusters and their tidal tails and the stellar initial mass function

Unresolved binaries have a strong influence on the observed parameters of SC We quantify this influence and compute the resulting mass underestimates and MF N-body simulations of realistic SC were used to investigate the evolution of the binary population in a SC and its tidal tails. Together with an empirically gauged stellar mass-luminosity relation, the results were then used to determine how the presence of binaries changes the photometric mass and MF of the SC and its tails as deduced from observations. T1 which is the tidal tail caused by gas expulsion, contains a larger fraction of binaries than both the SC and T2 which forms after gas expulsion. Additionally T1 has a larger velocity dispersion. Using the luminosity of an unresolved binary, an observer would underestimate its mass. This bias sensitively depends on the companion masses due to the structure of the stellar mass-luminosity relation. Combining the effect of all binaries in the simulation, the total photometric mass of the SC is underestimated by 15<!PCT!>. Dark objects (black holes and neutron stars) increase the difference between the real and observed mass of the SC further. For both the SC and the tails, the observed power-law index of the MF between a stellar mass of 0.3 and 0.7 $M_ is smaller by up to 0.2 than the real one, the real IMF being steeper by this amount. This difference is larger for stars with a larger velocity dispersion or binary fraction. Since the stars formed in SC are the progenitors of the Galactic field stars, this work suggests that the binary fractions of different populations of stars in the Galactic disc will differ as a function of the velocity dispersion. However, the direction of this correlation is currently unclear, and a complete population synthesis will be needed to investigate this effect. Variations in the binary fractions of different clusters can lead to perceived variations of the deduced stellar MF

The Imprint of Dark Matter on the Galactic Acceleration Field

Measurements of the accelerations of stars enabled by time-series extreme-precision spectroscopic observations, pulsar timing, and eclipsing binary stars in the solar neighborhood offer insights into the mass distribution of the Milky Way that do not rely on traditional equilibrium modeling. Given the measured accelerations, we can determine a total mass density and infer the amount of dark matter (DM) by accounting for the mass in stars, gas, and dust. Leveraging FIRE-2 simulations of Milky Way–mass galaxies we compare vertical acceleration profiles between cold DM (CDM) and self-interacting DM (SIDM) with a constant cross section of 1 cm2 g−1 across three halos with diverse assembly histories. Notably, significant asymmetries in vertical acceleration profiles near the midplane at fixed radii are observed in both CDM and SIDM, particularly in halos recently affected by mergers with satellites of Sagittarius/SMC-like masses or greater. These asymmetries offer a unique window into exploring the merger history of a galaxy. We show that SIDM halos manifest a more oblate shape and consistently exhibit higher local stellar and DM densities and steeper vertical acceleration gradients, up to 10%–30% steeper near the solar neighborhood. However, similar magnitude changes can arise from azimuthal variations in the baryonic components at a fixed radius and external influences like mergers, making it difficult to distinguish between CDM and SIDM using acceleration measurements in a single galaxy.

Spatially–resolved gas-phase metallicity in Seyfert galaxies

Abstract We explore the relations between the gas-phase metallicity radial profiles (few hundred inner parsec) and multiple galaxy properties for 15 Seyfert galaxies from the AGNIFS (Active Galactic Nuclei Integral Field Spectroscopy) sample using optical Integral Field Unit (IFU) observations from Gemini Multi-Object Spectrographs (GMOS) and Multi Unit Spectroscopic Explorer (MUSE) processed archival data. The data were selected at z ≲ 0.013 within black hole mass range [6 < log (MBH/M⊙) < 9] with moderate 14–150 keV X-ray luminosities $\left[42\, \lesssim \, \log L_X (\rm erg\, s^{-1})\, \lesssim \, 44\right]$. We estimated the gas-phase metallicity using the strong-line methods and found mean values for the oxygen dependent (Z ∼ 0.75 Z⊙) and nitrogen dependent (Z ∼ 1.14 Z⊙) calibrations. These estimates show excellent agreement with ΔZ ≈ 0.19 dex and ΔZ ≈ 0.18 dex between the mean values from the two strong-line calibrations for GMOS and MUSE respectively, consistent with the order of metallicity uncertainty via the strong-line methods. We contend that our findings align with a scenario wherein local Seyferts have undergone seamless gas accretion histories, resulting in positive metallicity profile over an extended period of time, thereby providing insights into galaxy evolution and the chemical enrichment or depletion of the universe. Additionally, we argue that metal-poor gas inflow from the local interstellar medium (ISM) and accreted through the circumgalactic medium (CGM) onto the galaxy systems regulates the star formation processes by diluting their central metallicity and inverting their metallicity gradients, producing a more prominent anti-correlation between gas-phase metallicity and Eddington ratio.

Combined Gemini-South and HST photometric analysis of the globular cluster NGC 6558

Context. NGC 6558 is a low-galactic-latitude globular cluster projected in the direction of the Galactic bulge. Due to high reddening, this region presents challenges in deriving accurate parameters, which require meticulous photometric analysis. We present a combined analysis of near-infrared and optical photometry from multi-epoch high-resolution images collected with Gemini-South/GSAOI+GeMS (in the J and KS filters) and HST/ACS (in the F606W and F814W filters). Aims. We aim to refine the fundamental parameters of NGC 6558, utilising high-quality Gemini-South/GSAOI and HST/ACS photometries. Additionally, we intend to investigate its role in the formation of the Galactic bulge. Methods. We performed a meticulous differential reddening correction to investigate the effect of contamination from Galactic bulge field stars. To derive the fundamental parameters – age, distance, reddening, and the total-to-selective coefficient – we employed a Bayesian isochrone fitting. The results from high-resolution spectroscopy and RR Lyrae stars were implemented as priors. For the orbital parameters, we employed a barred Galactic mass model. Furthermore, we analysed the age-metallicity relation to contextualise NGC 6558 within the Galactic bulge’s history. Results. We studied the impact of two differential reddening corrections on the age derivation. When removing as much as possible of the Galactic bulge field star contamination, the isochrone fitting combined with synthetic colour-magnitude diagrams gives a distance of 8.41−0.10+0.11 kpc, an age of 13.0 ± 0.9 Gyr, and a reddening of E(B − V) = 0.34 ± 0.02. We derived a total-to-selective coefficient of RV = 3.2 ± 0.2 thanks to the simultaneous near-infrared–optical synthetic colour-magnitude diagram fitting, which, aside from errors, agrees with the commonly used value. The orbital parameters showed that NGC 6558 is confined within the inner Galaxy and it is not compatible with a bar-shape orbit, indicating that it is a bulge member. Assembling the old and moderately metal-poor ([Fe/H] ∼ −1.1) clusters in the Galactic bulge, we derived their age-metallicity relation with star formation starting at 13.6 ± 0.2 Gyr and effective yields of ρ = 0.05 ± 0.01 Z⊙. Conclusions. The old age derived for NGC 6558 is compatible with other clusters with similar metallicity and a blue horizontal branch in the Galactic bulge, which constitute the moderately metal-poor globular clusters. The age-metallicity relation shows that the starting age of star formation is compatible with the age of NGC 6558, and the chemical enrichment is ten times faster than the ex situ globular cluster branch.

An `alien' called the Oosterhoff dichotomy?

In this Letter we investigate the origin of the Oosterhoff dichotomy in light of recent discoveries related to several ancient merging events of external galaxies with the Milky Way (MW). In particular, we aim to clarify if the subdivision in terms of the Oosterhoff type between Galactic globular clusters (GGCs) and field RR Lyrae (RRLs) can be traced back to one or more ancient galaxies that merged with the MW in its past. We first explored the association of GGCs with the past merging events according to different literature studies. Subsequently, we compiled the positions, proper motions, and radial velocities of 10,138 field RRL variables from Gaia Data Release 3. To infer the distances, we adopted the $M_G$-- Fe/H relation, with Fe/H values estimated via empirical relationships involving individual periods and Fourier parameters. We then calculated the orbits and the integrals of motion using the Python library Galpy for the whole sample. By comparing the location of the field RRLs in the energy--angular momentum diagram with that of the GGCs, we determined their likely origin. Finally, using Gaia G-band light curves, we determined the Oosterhoff types of our RRL stars based on their location in the Bailey diagram. The analysis of the Bailey diagrams for Galactic RRL stars and GGCs associated with an `in situ' versus `accreted' halo origin shows remarkable differences. The in situ sample shows a wide range of metallicities with a continuous distribution and no sign of the Oosterhoff dichotomy. Conversely, the accreted RRLs clearly show the Oosterhoff dichotomy and a significantly smaller dispersion in metallicity. Our results suggest that the Oosterhoff dichotomy was imported into the MW by the merging events that shaped the Galaxy.

Close Encounters of Wide Binaries Induced by the Galactic Tide: Implications for Stellar Mergers and Gravitational-wave Sources

A substantial fraction of stars can be found in wide binaries with projected separations between ∼102 and 105 au. In the standard lore of binary physics, these would evolve as effectively single stars that remotely orbit one another on stationary Keplerian ellipses. However, embedded in their Galactic environment, the low binding energy of wide binaries makes them exceptionally prone to perturbations from the gravitational potential of the Milky Way and encounters with passing stars. Employing a fully relativistic N-body integration scheme, we study the impact of these perturbations on the orbital evolution of wide binaries along their trajectory through the Milky Way. Our analysis reveals that the torques exerted by the Galaxy can cause large-amplitude oscillations of the binary eccentricity to 1 − e ≲ 10−8. As a consequence, the wide binary members pass close to each other at periapsis, which, depending on the type of binary, potentially leads to a mass transfer or collision of stars or to an inspiral and subsequent merger of compact remnants due to gravitational-wave radiation. Based on a simulation of 105 wide binaries across the Galactic field, we find that this mechanism could significantly contribute to the rate of stellar collisions and binary black hole mergers as inferred from observations of luminous red novae and gravitational-wave events by LIGO/Virgo/Kagra. We conclude that the dynamics of wide binaries, despite their large mean separation, can give rise to extreme interactions between stars and compact remnants.

Period-luminosity and period-luminosity-metallicity relations for Galactic RR Lyrae stars in the Sloan bands

Context. RR Lyrae stars are excellent tracers of the old population II due to their period-luminosity (PL) and period-luminosity-metallicity (PLZ) relations. While these relations have been investigated in detail in many photometric bands, there are few comprehensive studies about them in Sloan-like systems. Aims. We present PL and PLZ relations (as well as their counterparts in Wesenheit magnitudes) in the Sloan–Pan-STARSS gP1rP1iP1 bands obtained for Galactic RR Lyrae stars in the vincinity of the Sun. Methods. The data used in this paper were collected with the network of 40 cm telescopes of the Las Cumbres Observatory, and geometric parallaxes were adopted from Gaia Data Release 3. Results. We derived PL and PLZ relations separately for RRab and RRc-type stars, as well as for the mixed population of RRab+RRc stars. Conclusions. To our knowledge, these are the first PL and PLZ relations in the Sloan bands determined using RR Lyrae stars in the Galactic field.

An all-sky catalogue of stellar reddening values

Context. When observing astronomical objects, we have to deal with extinction (i.e. the absorption and scattering of the emitted radiation by dust and gas between the source and the observer). Interstellar extinction depends on the location of the object and the wavelength. The different extinction laws describing these effects are difficult to estimate for a small sample of stars. Aims. Many sophisticated and automatic methods have recently been developed for estimating astrophysical parameters (age and metallicity, for example) depending on the reddening, which is normally treated as a free parameter within the corresponding estimations. However, many reddening values for stars have been published over the last few decades, most of which include observations in the ultraviolet, which are essential for a good estimation but are essentially no longer taken into account. Methods. We searched the literature through the end of 2022 for published independent reddening values of stellar objects based on various methods that exclude estimates from reddening maps. In addition, we present new reddening estimates based on the classical photometric indices in the Geneva, Johnson, and Strömgren-Crawford systems. These are based on well-established and reliable calibrations. Results. After a careful identification procedure and quality assessment of the data, we calculated the mean reddening values of 157 631 individual available measurements for 97 826 objects. We compared our results with the ones from recent automatic pipeline values, including those from the Gaia consortium. In addition, we chose star cluster members to compare their mean values with estimates for the corresponding aggregates. Within the different references, we find several statistically significant offsets and trends and discuss possible explanations for them. Conclusions. Our new catalogue can serve as a starting point for calibrating and testing automatic tools such as isochrone and spectral energy distribution fitting. Our sample covers the whole sky, including the Galactic field, star clusters, and Magellanic Clouds, and so can be used for a variety of astrophysical studies.

Dynamics of star clusters with tangentially anisotropic velocity distribution

Recent high-precision observations with HST and Gaia enabled new investigations of the internal kinematics of star clusters (SCs) and the dependence of kinematic properties on the stellar mass. These studies raised new questions about the dynamical evolution of self-gravitating stellar systems. We aim to develop a more complete theoretical understanding of how the various kinematical properties of stars affect the global dynamical development of their host SCs. We perform $N$-body simulations of globular clusters with isotropic, radially anisotropic, and tangentially anisotropic initial velocity distributions. We also study the effect of an external Galactic tidal field. We obtain three main results. First, compared to the conventional, isotropic case, the relaxation processes are accelerated in the tangentially anisotropic models and, in agreement with our previous investigations, are slower in the radially anisotropic ones. This leads to, for example, more rapid mass segregation in the central regions of the tangential models or their earlier core collapse. Second, although all SCs become isotropic in the inner regions after several relaxation times, we observe differences in the anisotropy profile evolution in the outer cluster regions --- all tidally filling models gain tangential anisotropy there, while the underfilling models become radially anisotropic. Third, we observe different rates of evolution towards energy equipartition (EEP). While all SCs evolve towards EEP in their inner regions (regardless of the filling factor), the outer regions of the tangentially anisotropic and isotropic models are evolving to an `inverted' EEP (i.e.\ with the high-mass stars having higher velocity dispersion than the low-mass ones). The extent (both spatial and temporal) of this inversion can be attributed to the initial velocity anisotropy --- it grows with increasing tangential anisotropy and decreases as the radial anisotropy rises.

Importance of Cosmic-Ray Propagation on Sub-GeV Dark Matter Constraints

We study sub-GeV dark matter (DM) particles that may annihilate or decay into Standard Model particles producing an exotic injection component in the Milky Way that leaves an imprint in both photon and cosmic-ray (CR) fluxes. Specifically, the DM particles may annihilate or decay into e + e −, μ + μ −, or π + π − and may radiate photons through their e ± products. The resulting e ± products can be directly observed in probes such as Voyager 1. Alternatively, the e ± products may produce bremsstrahlung radiation and upscatter the low-energy Galactic photon fields via the inverse Compton process, generating a broad emission from X-ray to γ-ray energies observable in experiments such as XMM-Newton. We find that we get a significant improvement in the DM annihilation and decay constraints from XMM-Newton (excluding thermally averaged cross sections of 10−31 cm3 s−1 ≲ 〈σ v〉 ≲ 10−26 cm3 s−1 and decay lifetimes of 1026 s ≲ τ ≲ 1028 s, respectively) by including best-fit CR propagation and diffusion parameters. This yields the strongest astrophysical constraints for this mass range of DM of 1 MeV to a few GeV and even surpasses cosmological bounds across a wide range of masses as well.

Testing MOND on Small Bodies in the Remote Solar System

Modified Newtonian dynamics (MOND), which postulates a breakdown of Newton's laws of gravity/dynamics below some critical acceleration threshold, can explain many otherwise puzzling observational phenomena on galactic scales. MOND competes with the hypothesis of dark matter, which successfully explains the cosmic microwave background and large-scale structure. Here we provide the first solar system test of MOND that probes the subcritical acceleration regime. Using the Bekenstein–Milgrom “aquadratic Lagrangian” (or AQUAL) formulation, we simulate the evolution of myriads of test particles (planetesimals or comets) born in the trans-Neptunian region and scattered by the giant planets over the lifetime of the Sun to heliocentric distances of 102–105 au. We include the effects of the Galactic tidal field and passing stars. While Newtonian simulations reproduce the distribution of binding energies of long-period and Oort-cloud comets detectable from Earth, MOND-based simulations do not. This conclusion is robust to plausible changes in the migration history of the planets, the migration history of the Sun, the MOND transition function, effects of the Sun's birth cluster, and the fading properties of long-period comets. For the most popular version of AQUAL, characterized by a gradual transition between the Newtonian and MOND regimes, our MOND-based simulations also fail to reproduce the orbital distribution of trans-Neptunian objects in the detached disk (perihelion q > 38 au). Our results do not rule out some MOND theories more elaborate than AQUAL, in which non-Newtonian effects are screened on small spatial scales, at small masses, or in external gravitational fields comparable in strength to the critical acceleration.

Charting the Galactic Acceleration Field. II. A Global Mass Model of the Milky Way from the STREAMFINDER Atlas of Stellar Streams Detected in Gaia DR3

We present an atlas and follow-up spectroscopic observations of 87 thin stream-like structures detected with the STREAMFINDER algorithm in Gaia DR3, of which 28 are new discoveries. Here, we focus on using these streams to refine mass models of the Galaxy. Fits with a double-power-law halo with the outer power-law slope set to −β h = −3 yield an inner power-law slope of −γh=-(0.97−0.21+0.17) , a scale radius of r0,h=14.7−1.0+4.7kpc , a halo density flattening q m,h = 0.75 ± 0.03, and a local dark matter density of ρ h,⊙ = 0.0114 ± 0.0007 M ⊙ pc−3. Freeing β yields β=2.53−0.16+0.42 , but this value is heavily influenced by our chosen virial mass limit. The stellar disks are found to have a combined mass of 4.20−0.53+0.44×1010M⊙ , with the thick disk contributing 12.4% ± 0.7% to the local stellar surface density. The scale lengths of the thin and thick disks are 2.17−0.08+0.18 and 1.62−0.13+0.72kpc , respectively, while their scale heights are 0.347−0.010+0.007 and 0.86−0.02+0.03kpc , respectively. The virial mass of the favored model is M200=1.09−0.14+0.19×1012M⊙ , while the mass inside of 50 kpc is M R<50 = 0.46 ± 0.03 × 1012 M ⊙. We introduce the Large Magellanic Cloud (LMC) into the derived potential models, and fit the Orphan stream therein, finding a mass for the LMC that is consistent with recent estimates. Some salient highlights include the nearby trailing arm of ω Cen, and a nearby very metal-poor stream that was once a satellite of the Sagittarius dwarf galaxy. Finally, we unambiguously detect a hot component around the GD-1 stream, consistent with it having been tidally preprocessed within its own dark matter subhalo.

Multimessenger search for electrophilic feebly interacting particles from supernovae

We study MeV-scale electrophilic feebly interacting particles (FIPs), that may be abundantly produced in supernova explosions, escape the star and decay into electrons and positrons. This exotic injection of leptons in the Milky Way leaves an imprint in both photon and cosmic ray fluxes. Specifically, positrons lose energy and annihilate almost at rest with background electrons, producing photons with 511 keV energy. In addition, electrons and positrons radiate photons through bremsstrahlung emission and upscatter the low-energy galactic photon fields via the inverse Compton process generating a broad emission from x-ray to γ ray energies. Finally, electrons and positrons are directly observable in cosmic ray experiments. In order to describe the FIP-induced lepton injection in full generality, we use a model-independent parametrization which can be applied to a host of FIPs such as axionlike particles, dark photons and sterile neutrinos. Theoretical predictions are compared to experimental data to robustly constrain FIP-electron interactions with an innovative multimessenger analysis. Published by the American Physical Society 2024

Sub-GeV Dark Matter and X-rays

I present bounds on Dark Matter (DM) in the MeV to GeV mass range, obtained by using X-ray measurements from the Integral, XMM-Newton and other telescopes. A crucial element, which allows to derive bounds competitive or stronger than existing ones from other techniques, resides in the inclusion of the contribution from inverse Compton scattering on galactic radiation fields and on the CMB. This contribution is based on [1,2], to which I refer for additional details.

The S-PLUS Fornax Project (S+FP): SExtractor detection and measurement of nearby galaxies in large photometric surveys

ABSTRACT All-sky multiband photometric surveys represent a unique opportunity of exploring rich nearby galaxy clusters up to several virial radii, reaching the filament regions where pre-processing is expected to occur. These projects aim to tackle a large number of astrophysical topics, encompassing both the Galactic and extragalactic fields. In that sense, generating large catalogues with homogeneous photometry for both resolved and unresolved sources that might be interesting to achieve specific goals, imposes a compromise when choosing the set of parameters to automatically detect and measure such a plethora of objects. In this work, we present the acquired experience on studying the galaxy content of the Fornax cluster using large catalogues obtained by the Southern Photometric Local Universe Survey (S-PLUS). We realized that some Fornax bright galaxies are missed in the S-PLUS iDR4 catalogues. In addition, Fornax star-forming galaxies are included as multiple detections due to overdeblending. To solve those issues, we performed specific SExtractor runs to identify the proper set of parameters to recover as many Fornax galaxies as possible with confident photometry and avoiding duplications. From that process, we obtained new catalogues containing 12-band improved photometry for ∼ 3 × 106 resolved and unresolved sources in an area of ∼ 208 deg2 in the direction of the Fornax cluster. Together with identifying the main difficulties to carry out the study of nearby groups and clusters of galaxies using S-PLUS catalogues, we also share possible solutions to face issues that seem to be common to other ongoing photometric surveys.

The impact of a top-heavy IMF on the formation and evolution of dark star clusters

ABSTRACT The Spitzer instability leads to the formation of a black hole subsystem (BHSub) at the centre of a star cluster providing energy to luminous stars (LSs) and increasing their rate of evaporation. When the self-depletion time of the BHSub exceeds the evaporation time of the LSs, a dark star cluster (DSC) will appear. Using the nbody7 code, we performed a comprehensive set of direct N-body simulations over a wide range of initial conditions to study the pure effect of the top-heaviness of the initial mass function (IMF) on the formation of the DSC phase. In the Galactic tidal field, top-heavy IMFs lead to the fast evaporation of LSs and the formation of DSCs. Therefore, DSCs can be present even in the outer region of the Milky Way (MW). To successfully transition to the DSC phase, the MW Globular Clusters (GCs) must possess an initial black hole (BH) mass fraction of $\widetilde {\mathit {M}}_\mathrm{BH}(0)\gt 0.05$. For star clusters with $\widetilde {\mathit {M}}_\mathrm{BH}(0)\gt 0.08$, the DSC phase will be created for any given initial density of the cluster and Galactocentric distance. The duration of the cluster’s lifetime spent in the DSC phase shows a negative (positive) correlation with the initial density, and Galactocentric distance of the star cluster if $\widetilde {\mathit {M}}_\mathrm{BH}(0)\le 0.12$ ($\widetilde {\mathit {M}}_\mathrm{BH}(0)\ge 0.15$). Considering the canonical IMF, it is unlikely for any MW GCs to enter the DSC phase. We discuss the BH retention fraction in view of the observed properties of the GCs of the MW.

Prototype Faraday Rotation Measure Catalogs from the Polarisation Sky Survey of the Universe’s Magnetism (POSSUM) Pilot Observations

The Polarisation Sky Survey of the Universe’s Magnetism (POSSUM) will conduct a sensitive ∼1 GHz radio polarization survey covering 20,000 deg2 of the southern sky with the Australian Square Kilometre Array Pathfinder. In anticipation of the full survey, we analyze pilot observations of low-band (800–1087 MHz), mid-band (1316–1439 MHz), and combined-band observations for an extragalactic field and a Galactic plane field (low-band only). Using the POSSUM processing pipeline, we produce prototype rotation measure (RM) catalogs that are filtered to construct prototype RM grids. We assess typical RM grid densities and RM uncertainties and their dependence on frequency, bandwidth, and Galactic latitude. We present a median filter method for separating foreground diffuse emission from background components and find that after application of the filter, 99.5% of the measured RMs of simulated sources are within 3σ of their true RM, with a typical loss of polarized intensity of 5% ± 5%. We find RM grid densities of 35.1, 30.6, 37.2, and 13.5 RMs per square degree and median uncertainties on RM measurements of 1.55, 12.82, 1.06, and 1.89 rad m−2 for the median-filtered low-band, mid-band, combined-band, and Galactic observations, respectively. We estimate that the full POSSUM survey will produce an RM catalog of ∼775,000 RMs with median-filtered low-band observations and ∼877,000 RMs with median-filtered combined-band observations. We construct a structure function from the Galactic RM catalog, which shows a break at 0.°7, corresponding to a physical scale of 12–24 pc for the nearest spiral arm.

Cracking the Relation between Mass and 1P Star Fraction of Globular Clusters. I. Present-day Cluster Masses as a First Tool

The phenomenon of multiple stellar populations is exacerbated in massive globular clusters, with the fraction of first-population (1P) stars a decreasing function of the cluster present-day mass. We decipher this relation in far greater detail than has been done so far. We assume (i) a fixed stellar mass threshold for the formation of second-population (2P) stars, (ii) a power-law scaling F1P∝mecl−1 between the mass m ecl of newly formed clusters and their 1P star fraction F 1P, and (iii) a constant F 1P over time. The F 1P(m ecl) relation is then evolved up to an age of 12 Gyr for tidal field strengths representative of the entire Galactic halo. The 12 Gyr old model tracks cover the present-day distribution of Galactic globular clusters in the (mass, F 1P) space extremely well. The distribution is curtailed on its top right side by the scarcity of clusters at large Galactocentric distances and on its bottom left side by the initial scarcity of very high-mass clusters and dynamical friction. Given their distinct dissolution rates, “inner” and “outer” model clusters are offset from each other, as observed. The locus of Magellanic Clouds clusters in the (mass, F 1P) space is as expected for intermediate-age clusters evolving in a gentle tidal field. Given the assumed constancy of F 1P, we conclude that 2P stars do not necessarily form centrally concentrated. We infer a minimum mass of 4 · 105 M ⊙ for multiple-population clusters at secular evolution onset. This high-mass threshold severely limits the number of 2P stars lost from evolving clusters, thereby fitting the low 2P star fraction of the Galactic halo field.

Dynamics of Fluids in the Cavity of a Rotating Body: A Review of Analytical Solutions

Since the middle of the 20th century, an understanding of the diversity of the natural magnetohydrodynamic phenomena surrounding us has begun to emerge. Magnetohydrodynamic nature manifests itself in such seemingly heterogeneous processes as the flow of water in the world’s oceans, the movements of Earth’s liquid core, the dynamics of the solar magnetosphere and galactic electromagnetic fields. Their close relationship and multifaceted influence on human life are becoming more and more clearly revealed. The study of these phenomena requires the development of theory both fundamental and analytical, unifying a wide range of phenomena, and specialized areas that describe specific processes. The theory of translational fluid motion is well developed, but for most natural phenomena, this condition leads to a rather limited model. The fluid motion in the cavity of a rotating body such that the Coriolis forces are significant has been studied much less. A distinctive feature of the problems under consideration is their significant nonlinearity, (i.e., the absence of a linear approximation that allows one to obtain nontrivial useful results). From this point of view, the studies presented here were selected. This review presents studies on the movements of ideal and viscous fluids without taking into account electromagnetic phenomena (non-conducting, non-magnetic fluid) and while taking them into account (conducting fluid). Much attention is payed to the macroscopic movements of sea water (conducting liquid) located in Earth’s magnetic field, which spawns electric currents and, as a result, an induced magnetic field. Exploring the processes of generating magnetic fields in the moving turbulent flows of conducting fluid in the frame of dynamic systems with distributed parameters allows better understanding of the origin of cosmic magnetic fields (those of planets, stars, and galaxies). Various approaches are presented for rotational and librational movements. In particular, an analytical solution of three-dimensional unsteady magnetohydrodynamic equations for problems in a plane-parallel configuration is presented.