Galactic Stars Research Articles

Radio continuum properties of OH megamaser galaxies

ABSTRACT We present a study of the radio continuum properties of two luminous/ultraluminous infrared galaxy samples: the OH megamaser (OHM) sample (74 objects) and the control sample (128 objects) without detected maser emission. We carried out pilot observations for 140 objects with the radio telescope RATAN-600 at 1.2, 2.3, 4.7, 8.2, 11.2, and 22.3 GHz in 2019–2021. The OHM sample has two times more flat-spectrum sources (32 per cent) than the control sample. Steep radio spectra prevail in both samples. The median spectral index at 4.7 GHz α4.7 = −0.59 for the OHM sample, and α4.7 = −0.71 for the non-OHM galaxies. We confirm a tight correlation of the far-infrared (FIR) and radio luminosities for the OHM sample. We found correlations between isotropic OH line luminosity LOH and the spectral index α4.7 (ρ = 0.26, p-val. = 0.04) and between LOH and radio luminosity P1.4 (ρ = 0.35, p-val. = 0.005). Reviewing subsamples of masers powered by active galactic nuclei (AGNs) and star formation revealed insignificant differences for their FIR and radio properties. None the less, AGN-powered galaxies exhibit larger scatter in a range of parameters and their standard deviations. The similarities in the radio and FIR properties in the two samples are presumably caused by the presence of a significant amount of AGN sources in both samples (47 and 30 per cent in the OHM and control samples) and/or possibly by the presence of undetected OH emission sources in the control sample.

Abundance of barium in the atmospheres of red giants in the Galactic globular cluster NGC 104 (47 Tuc)

Context. While most (if not all) Type I Galactic globular clusters (GGCs) are characterised by spreads in the abundances of light chemical elements (e.g. Li, N, O, Na, Mg, Al), it is not yet well established whether similar spreads may exist in s-process elements as well. Aims. We investigated the possible difference in Ba abundance between the primordial (1P) and polluted (2P) stars in the Galactic globular cluster (GGC) 47 Tuc (NGC 104). For this purpose, we obtained homogeneous abundances of Fe, Na, and Ba in a sample of 261 red giant branch (RGB) stars, which comprises the largest sample used for Na and Ba abundance analysis in any GGC so far. Methods. Abundances of Na and Ba were determined using archival GIRAFFE/VLT spectra and 1D non-local thermodynamic equilibrium (NLTE) abundance analysis methodology. Results. Contrary to the finding of Gratton et al. (2013, A&A, 549, A41), we did not detect any significant Ba–Na correlation or 2P–1P Ba abundance difference in the sample of 261 RGB stars in 47 Tuc. This corroborates the result of D’Orazi et al. (2010, ApJ, 719, L213), who found no statistically significant Ba–Na correlation in 110 RGB stars in this GGC. The average barium-to-iron ratio obtained in the sample of 261 RGB stars, ⟨[Ba/Fe]1D NLTE⟩ = −0.01 ± 0.06, agrees well with those determined in Galactic field stars at this metallicity and may therefore represent the abundance of primordial proto-cluster gas that has not been altered during the subsequent chemical evolution of the cluster.

A Preliminary Calibration of the JAGB Method Using Gaia EDR3

Abstract The recently developed J-region asymptotic giant branch (JAGB) method has extraordinary potential as an extragalactic standard candle, capable of calibrating the absolute magnitudes of locally accessible Type Ia supernovae, thereby leading to an independent determination of the Hubble constant. Using Gaia Early Data Release 3 (EDR3) parallaxes, we calibrate the zero-point of the JAGB method, based on the mean luminosity of a color-selected subset of carbon-rich AGB stars. We identify Galactic carbon stars from the literature and use their near-infrared photometry and Gaia EDR3 parallaxes to measure their absolute J-band magnitudes. Based on these Milky Way parallaxes we determine the zero-point of the JAGB method to be M J = −6.14 ± 0.05 (stat) ± 0.11 (sys) mag. This Galactic calibration serves as a consistency check on the JAGB zero-point, agreeing well with previously published, independent JAGB calibrations based on geometric, detached eclipsing binary distances to the LMC and SMC. However, the JAGB stars used in this study suffer from the high parallax uncertainties that afflict the bright and red stars in EDR3, so we are not able to attain the higher precision of previous calibrations, and ultimately will rely on future improved DR4 and DR5 releases.

The GALAH Survey: dependence of elemental abundances on age and metallicity for stars in the Galactic disc

ABSTRACT Using data from the GALAH survey, we explore the dependence of elemental abundances on stellar age and metallicity among Galactic disc stars. We find that the abundance of most elements can be predicted from age and [Fe/H] with an intrinsic scatter of about 0.03 dex. We discuss the possible causes for the existence of the abundance–age–metallicity relations. Using a stochastic chemical enrichment scheme that takes the volume of supernovae remnants into account, we show the intrinsic scatter is expected to be small, about 0.05 dex or even smaller if there is additional mixing in the ISM. Elemental abundances show trends with both age and metallicity and the relationship is well described by a simple model in which the dependence of abundance ([X/Fe]) on age and [Fe/H] are additively separable. Elements can be grouped based on the direction of their abundance gradient in the (age,[Fe/H]) plane and different groups can be roughly associated with three distinct nucleosynthetic production sites, the exploding massive stars, the exploding white dwarfs, and the AGB stars. However, the abundances of some elements, like Co, La, and Li, show large scatter for a given age and metallicity, suggesting processes other than simple Galactic chemical evolution are at play. We also compare the abundance trends of main-sequence turn-off (MSTO) stars against that of giants, whose ages were estimated using asteroseismic information from the K2 mission. For most elements, the trends of MSTO stars are similar to that of giants. The existence of abundance relations implies that we can estimate the age and birth radius of disc stars, which is important for studying the dynamic and chemical evolution of the Galaxy.

Modeling the Galactic Neutron Star Population for Use in Continuous Gravitational-wave Searches

Abstract Searches for continuous gravitational waves (GW) from unknown Galactic neutron stars provide limits on the shapes of neutron stars. A rotating neutron star will produce GW if asymmetric deformations exist in its structure that are characterized by the star’s ellipticity. In this study, we use a simple model of the spatial and spin distribution of Galactic neutron stars to estimate the total number of neutron stars probed, using GW, to a given upper limit on the ellipticity. This may help optimize future searches with improved sensitivity. The improved sensitivity of third-generation GW detectors may increase the number of neutron stars probed, to a given ellipticity, by factors of 100 to 1000.

Stringent Tests of Gravity with Highly Relativistic Binary Pulsars in the Era of LISA and SKA

Abstract At present, 19 double neutron star (DNS) systems are detected by radio timing and two merging DNS systems are detected by kilohertz gravitational waves. Because of selection effects, none of them has an orbital period P b in the range of a few tens of minutes. In this paper we consider a multimessenger strategy proposed by Kyutoku et al., jointly using the Laser Interferometer Space Antenna and the Square Kilometre Array to detect and study Galactic pulsar-neutron star (PSR-NS) systems with P b ∼ 10–100 minutes. We assume that we will detect PSR-NS systems by this strategy. We use standard pulsar-timing software to simulate times of arrival of pulse signals from these binary pulsars. We obtain the precision of timing parameters of short-orbital-period PSR-NS systems with orbital period P b ∈ (8, 120) minutes. We use the simulated uncertainty of the orbital decay, P ̇ b , to predict future tests for a variety of alternative theories of gravity. We show quantitatively that highly relativistic PSR-NS systems will significantly improve the constraint on parameters of specific gravity theories in the strong field regime. We also investigate the orbital periastron advance caused by the Lense−Thirring effect in a PSR-NS system with P b = 8 minutes, and show that the Lense−Thirring effect will be detectable to a good precision.

The structure and characteristic scales of the H I gas in galactic disks

The spatial distribution of the H I gas in galactic disks holds important clues about the physical processes that shape the structure and dynamics of the interstellar medium (ISM). The structure of the ISM could be affected by a variety of perturbations internal and external to the galaxy, and the unique signature of each of these perturbations could be visible in the structure of interstellar gas. In this work, we quantify the structure of the H I gas in a sample of 33 nearby galaxies taken from the HI Nearby Galaxy Survey (THINGS) using the delta-variance (Δ-variance) spectrum. The THINGS galaxies display a large diversity in their spectra, but there are a number of recurrent features. In many galaxies, we observe a bump in the spectrum on scales of a few to several hundred parsec. We find the characteristic scales associated with the bump to be correlated with the galactic star formation rate (SFR) for values of the SFR ≳0.5 M⊙ yr−1 and also with the median size of the H I shells detected in these galaxies. We interpret this characteristic scale as being associated with the effects of feedback from supernova explosions. On larger scales, we observe in most galaxies two self-similar, scale-free regimes. The first regime, on intermediate scales (≲0.5R25), is shallow, and the power law that describes this regime has an exponent in the range [0.1–1] with a mean value of 0.55 that is compatible with the density field that is generated by supersonic turbulence in the cold phase of the H I gas. The second power law is steeper, with a range of exponents between 0.5 and 2.3 and a mean value of ≈1.5. These values are associated with subsonic to transonic turbulence, which is characteristic of the warm phase of the H I gas. The spatial scale at which the transition between the two self-similar regimes occurs is found to be ≈0.5R25, which is very similar to the size of the molecular disk in the THINGS galaxies. Overall, our results suggest that on scales ≲0.5R25, the structure of the ISM is affected by the effects of supernova explosions. On larger scales (≳0.5R25), stellar feedback has no significant impact, and the structure of the ISM is determined by large-scale processes that govern the dynamics of the gas in the warm neutral medium, such as the flaring of the H I disk at large galactocentric radii and the effects of ram pressure stripping.

Multiphase Powerful Outflows Detected in High-z Quasars

Abstract We present results from a comprehensive study of ultrafast outflows (UFOs) detected in a sample of 14 quasars, 12 of which are gravitationally lensed, in a redshift range of 1.41–3.91, near the peak of the active galactic nucleus (AGN) and star formation activity. New XMM-Newton observations are presented for six of them, which were selected to be lensed and contain a narrow absorption line (NAL) in their UV spectra. Another lensed quasar was added to the sample, albeit already studied because it was not searched for UFOs. The remaining seven quasars of our sample are known to contain UFOs. The main goals of our study are to infer the outflow properties of high-z quasars, constrain their outflow induced feedback, study the relationship between the outflow properties and the properties of the ionizing source, and compare these results to those of nearby AGN. Our study adds six new detections (> 99% confidence) of UFOs at z > 1.4, almost doubling the current number of cases. Based on our survey of six quasars selected to contain a NAL and observed with XMM-Newton, the coexistence of intrinsic UV NALs and UFOs is found to be significant in >83% of these quasars suggesting a link between multiphase AGN feedback properties of the meso- and microscale. The kinematic luminosities of the UFOs of our high-z sample are large compared to their bolometric luminosities (median of L K/L Bol ≳ 50%). This suggests they provide efficient feedback to influence the evolution of their host galaxies and that magnetic driving may be a significant contributor to their acceleration.

Mapping the “invisible” circumgalactic medium around a z ∼ 4.5 radio galaxy with MUSE

In this paper we present Multi Unit Spectroscopic Explorer integral field unit spectroscopic observations of the ∼70 × 30 kpc2 Lyα halo around the radio galaxy 4C04.11 at z = 4.5077. High-redshift radio galaxies are hosted by some of the most massive galaxies known at any redshift and are unique markers of concomitant powerful active galactic nucleus (AGN) activity and star formation episodes. We map the emission and kinematics of the Lyα across the halo as well as the kinematics and column densities of eight H I absorbing systems at −3500 < Δv < 0 km s−1. We find that the strong absorber at Δv ∼ 0 km s−1 has a high areal coverage (30 × 30 kpc2), being detected across a large extent of the Lyα halo, a significant column density gradient along the southwest to northeast direction, and a velocity gradient along the radio jet axis. We propose that the absorbing structure, which is also seen in C IV and N V absorption, represents an outflowing metal-enriched shell driven by a previous AGN or star formation episode within the galaxy and is now caught up by the radio jet, leading to jet-gas interactions. These observations provide evidence that feedback from AGN in some of the most massive galaxies in the early Universe may play an important role in redistributing material and metals in their environments.

Simulating the Eclipsing Binary Yields of the Rubin Observatory in the Galactic Field and Star Clusters

Abstract We present a study of the detection and recovery efficiency of the Rubin Observatory for detached eclipsing binaries (EBs) in the galactic field, globular clusters (GCs), and open clusters (OCs), with a focus on comparing two proposed observing strategies: a standard cadence (baseline) and a cadence that samples the galactic plane more evenly (colossus). We generate realistic input binary populations in all observing fields of the Rubin Observatory, simulate the expected observations in each filter, and attempt to characterize the EBs using these simulated observations. In our models, we predict that the baseline cadence will enable the Rubin Observatory to observe about three million EBs; our technique could recover and characterize nearly one million of these in the field and thousands within star clusters. If the colossus cadence is used, the number of recovered EBs would increase by an overall factor of about 1.7 in the field and in globular clusters, and a factor of about 3 in open clusters. Including semidetached and contact systems could increase the number of recovered EBs by an additional factor of about 2.5 to 3. Regardless of the cadence, observations from the Rubin Observatory could reveal statistically significant physical distinctions between the distributions of EB orbital elements between the field, GCs, and OCs. Simulations such as these can be used to bias correct the sample of Rubin Observatory EBs to study the intrinsic properties of the full binary populations in the field and star clusters.

The Galactic neutron star population – I. An extragalactic view of the Milky Way and the implications for fast radio bursts

ABSTRACT A key tool astronomers have to investigate the nature of extragalactic transients is their position on their host galaxies. Galactocentric offsets, enclosed fluxes, and the fraction of light statistic are widely used at different wavelengths to help infer the nature of transient progenitors. Motivated by the proposed link between magnetars and fast radio bursts (FRBs), we create a face-on image of the Milky Way using best estimates of its size, structure, and colour. We place Galactic magnetars, pulsars, low-mass, and high-mass X-ray binaries on this image, using the available distance information. Galactocentric offsets, enclosed fluxes, and fraction of light distributions for these systems are compared to extragalactic transient samples. We find that FRBs follow the distributions for Galactic neutron stars closest, with 24 (75 per cent) of the Anderson–Darling tests we perform having a p-value greater than 0.05. This suggests that FRBs are located on their hosts in a manner consistent with Galactic neutron stars on the Milky Way’s light, although we cannot determine which specific neutron star population is the best match. The Galactic distributions are consistent with other extragalactic transients much less often across the range of comparisons made, with type Ia SNe in second place, at only 33 per cent of tests exceeding 0.05. Overall, our results provide further support for FRB models invoking isolated young neutron stars, or binaries containing a neutron star.

AGN and star formation at cosmic noon: comparison of data to theoretical models

ABSTRACT In theoretical models of galaxy evolution, active galactic nucleus (AGN) and star formation (SF) activity are closely linked and AGN feedback is routinely invoked to regulate galaxy growth. In order to constrain such models, we compare the hydrodynamical simulations IllustrisTNG and SIMBA, and the semi-analytical model SAG to the empirical results on AGN and SF at cosmic noon (0.75 < z < 2.25) reported in Florez et al. (2020). The empirical results are based on a large mass-complete sample drawn from 93 307 galaxies with and without high X-ray luminosity AGN (LX ≳ 1044 erg s−1), selected from a 11.8 deg2 area (∼0.18 Gpc3 comoving volume at z = 0.75–2.25). The main results of our comparisons are: (i) SAG and IllustrisTNG both qualitatively reproduce the empirical result that galaxies with high X-ray luminosity AGN have higher mean SFR, at a given stellar mass, than galaxies without such AGN. SAG, however, strongly overproduces the number density of high X-ray luminosity AGN by a factor of 10–100, while IllustrisTNG shows a lack of high X-ray luminosity AGN at high stellar mass ($M* \gt 10^{11} \, \mathrm{M}_{\odot }$) at z ∼ 2. (ii) In SIMBA, the mean SFR of galaxies with high X-ray luminosity AGN is lower than the SFR of galaxies without such AGN. Contrary to the data, many high X-ray luminosity AGN in SIMBA have quenched SF, suggesting that AGN feedback, or other feedback modes in galaxies with such AGN, might be too efficient in SIMBA.

Faint polarised sources in the Lockman Hole field at 1.4 GHz

Context. In the context of structure formation and galaxy evolution, the contribution of magnetic fields is not well understood. Feedback processes originating from active galactic nucleus (AGN) activity and star formation can be actively influenced by magnetic fields, depending on their strength and morphology. One of the best tracers of magnetic fields is polarised radio emission. Tracing this emission over a broad redshift range therefore allows an investigation of these fields and their evolution. Aims. We aim to study the nature of the faint, polarised radio source population whose source composition and redshift dependence contain information about the strength, morphology, and evolution of magnetic fields over cosmic timescales. Methods. We use a 15-pointing radio continuum L-band mosaic of the Lockman Hole, observed in full polarisation, generated from archival data of the Westerbork Synthesis Radio Telescope. The data were analysed using the rotation measure synthesis technique. We achieved a noise of 7 μJy beam−1 in polarised intensity, with a resolution of 15″. Using infrared and optical images and source catalogues, we were able to cross-identify and determine redshifts for one-third of our detected polarised sources. Results. We detected 150 polarised sources, most of which are weakly polarised with a mean fractional polarisation of 5.4%. No source was found with a fractional polarisation higher than 21%. With a total area of 6.5 deg2 and a detection threshold of 6.25σ, we find 23 polarised sources per deg2. Based on our multi-wavelength analysis, we find that our sample consists of AGN only. We find a discrepancy between archival number counts and those present in our data, which we attribute to sample variance (i.e. large-scale structures). Considering the absolute radio luminosity, we find a general trend of increased probability of detecting weak sources at low redshift and strong sources at high redshift. We attribute this trend to a selection bias. Further, we find an anti-correlation between fractional polarisation and redshift for our strong-source sample at z ≥ 0.6. Conclusions. A decrease in the fractional polarisation of strong sources with increasing redshift cannot be explained by a constant magnetic field and electron density over cosmic scales; however, the changing properties of cluster environments over cosmic time may play an important role. Disentangling these two effects requires deeper and wider polarisation observations as well as better models of the morphology and strength of cosmic magnetic fields.

Massive young stellar objects in the Local Group irregular galaxy NGC 6822 identified using machine learning

ABSTRACT We present a supervised machine learning methodology to classify stellar populations in the Local Group dwarf-irregular galaxy NGC 6822. Near-IR colours (J − H, H − K, and J − K), K-band magnitudes and far-IR surface brightness (at 70 and 160 $\mu$m) measured from Spitzer and Herschel images are the features used to train a Probabilistic Random Forest (PRF) classifier. Point-sources are classified into eight target classes: young stellar objects (YSOs), oxygen- and carbon-rich asymptotic giant branch stars, red giant branch and red supergiant stars, active galactic nuclei, massive main-sequence stars, and Galactic foreground stars. The PRF identifies sources with an accuracy of ∼ 90 per cent across all target classes rising to ∼96 per cent for YSOs. We confirm the nature of 125 out of 277 literature YSO candidates with sufficient feature information, and identify 199 new YSOs and candidates. Whilst these are mostly located in known star-forming regions, we have also identified new star formation sites. These YSOs have mass estimates between ∼15 and 50 M⊙, representing the most massive YSO population in NGC 6822. Another 82 out of 277 literature candidates are definitively classified as non-YSOs by the PRF analysis. We characterize the star formation environment by comparing the spatial distribution of YSOs to those of gas and dust using archival images. We also explore the potential of using (unsupervised) t-distributed stochastic neighbour embedding maps for the identification of the same stellar population classified by the PRF.

A Parametric Galactic Model toward the Galactic Bulge Based on Gaia and Microlensing Data

Abstract We developed a parametric Galactic model toward the Galactic bulge by fitting to spatial distributions of the Gaia DR2 disk velocity, VVV proper motion, BRAVA radial velocity, OGLE-III red clump star count, and OGLE-IV star count and microlens rate, optimized for use in microlensing studies. We include the asymmetric drift of Galactic disk stars and the dependence of velocity dispersion on Galactic location in the kinematic model, which has been ignored in most previous models used for microlensing studies. We show that our model predicts a microlensing parameter distribution significantly different from those typically used in previous studies. We estimate various fundamental model parameters for our Galaxy through our modeling, including the initial mass function (IMF) in the inner Galaxy. Combined constraints from star counts and the microlensing event timescale distribution from the OGLE-IV survey, in addition to a prior on the bulge stellar mass, enable us to successfully measure IMF slopes using a broken power-law form over a broad mass range, α bd = 0.22 − 0.55 + 0.20 for M < 0.08 M ⊙, α ms = 1.16 − 0.15 + 0.08 for 0.08 M ⊙ ≤ M < M br, and α hm = 2.32 − 0.10 + 0.14 for M ≥ M br, as well as a break mass at M br = 0.90 − 0.14 + 0.05 M ⊙ . This is significantly different from the Kroupa IMF for local stars, but similar to the Zoccali IMF measured from a bulge luminosity function. We also estimate the dark matter mass fraction in the bulge region of 28% ± 7% which could be larger than a previous estimate. Because our model is purely parametric, it can be universally applied using the parameters provided in this paper. (A tool for microlensing simulation using our Galactic model has been published, Koshimoto & Ranc 2021, and can be downloaded at https://github.com/nkoshimoto/genulens.)

The cosmic ray-driven streaming instability in astrophysical and space plasmas

Energetic non-thermal particles, or cosmic rays, are a major component of astrophysical plasmas next to magnetic fields, radiation, and thermal gas. Cosmic rays are usually sub-dominant in density but carry as much pressure as the thermal plasma background. In some cases, cosmic rays drift at faster speeds with respect to the normal modes' phase speeds of the background plasma. Because of this, cosmic rays are a strong source of free energy that causes new classes of kinetic or convective instabilities. Recent years have seen the development of intense analytical and numerical efforts to analyze the onset of an instability produced by the motion of these particles at fast bulk speeds: this is the streaming instability. The streaming instability has been applied to different space plasmas and astrophysical contexts like strong shocks, jets, or in interstellar and intergalactic medium studies. Streaming instabilities participate in the production of magnetic turbulence at scales corresponding to the gyroradius of the particles. By scattering off their self-generated waves, cosmic rays are coupled to the background thermal plasma. This mechanism is able to self-confine cosmic rays around sources and launch winds out of the disk of the galaxy, hence impacting galactic matter dynamics and ultimately the galactic star formation rate. We discuss a few science cases, which should be accessible in the near future for analytical calculations and numerical simulations.

Anatomy of Galactic Star Formation: Roles of Different Modes of Gas Accretion, Feedback, and Recycling

AbstractExistence of the cold-mode gas accretion along with the hot-mode accretion can explain the diversity in the galactic star formation history across galaxy mass. We examine the role of various physical processes in producing the observed diversity.

Revisiting the Galactic Double Neutron Star merger and LIGO detection rates

ABSTRACT We revisit the merger rate for Galactic double neutron star (DNS) systems in light of recent observational insight into the longitudinal and latitudinal beam shape of the relativistic DNS PSR J1906 + 0746. Due to its young age and its relativistic orbit, the pulsar contributes significantly to the estimate of the joint Galactic merger rate. We follow previous analyses by modelling the underlying pulsar population of nine merging DNS systems and study the impact and resulting uncertainties when replacing simplifying assumptions made in the past with actual knowledge of the beam shape, its extent, and the viewing geometry. We find that the individual contribution of PSR J1906 + 0746 increases to $\mathcal {R} = 6^{+28}_{-5} \, \mathrm{Myr}^{-1}$ although the values are still consistent with previous estimates, given the uncertainties. We also compute contributions to the merger rates from the other DNS systems by applying a generic beam shape derived from that of PSR J1906 + 0746, evaluating the impact of previous assumptions. We derive a joint Galactic DNS merger rate of $\mathcal {R}^{\mathrm{gen}}_{\mathrm{MW}} = 32^{+19}_{-9}\, \mathrm{Myr}^{-1}$, leading to a LIGO detection rate of $\mathcal {R}^{\mathrm{gen}}_{\mathrm{LIGO}} = 3.5^{+2.1}_{-1.0}\, \mathrm{yr}^{-1}$ (90 per cent conf. limit), considering the upcoming O3 sensitivity of LIGO. As these values are in good agreement with previous estimates, we conclude that the method of estimating the DNS merger and LIGO detection rates via the study of the radio pulsar DNS population is less prone to systematic uncertainties than previously thought.

Simulations of globular clusters within their parent galaxies: Metallicity spreads and anomalous precursor populations

ABSTRACT Recent observations of globular clusters (GCs) suggest that elemental abundance variations may exist between first-generation (1G) stars. We propose that metal abundance (‘metallicity’) spreads within GC forming giant molecular clouds (GMCs) can influence the iron abundances of future cluster members. To investigate this, we use original hydrodynamical simulations to model GMC formation in a high redshift dwarf galaxy. Our simulations self-consistently model physical processes such as stellar feedback, dust formation and destruction, and molecular gas formation on dust grains, making them well suited to the study of GMC formation. We conclude that iron abundance variations in GMCs are due to the merging of gas clumps and self-enrichment processes. The metallicity dispersions of GC forming clumps is ∼0.1 dex, reflecting a growing number of studies that claim a non-zero dispersion within GCs. The galactic gas fraction is a key parameter for the formation of clumps and the metallicity ‘floor’ observed for both Galactic and extragalactic GCs are associated with the parent galaxy’s capacity to form massive GMCs. Finally, we argue that GMCs have the potential to trap surrounding metal-poor galactic disc stars, which we interpret as a precursor population (0G). These low metallicity stars are representative of the [Fe/H] value of the host dwarf and thus the chemistry of this 0G may be a fossilized record of the parent galaxy. These results depend on the initial metallicity and radial gradient of the galaxy, the threshold gas density for star formation, and the star formation prescription.

New magnetic chemically peculiar stars and candidates in the ATLAS first catalogue of variable stars

ABSTRACT The number of known variable stars has increased by several magnitudes over the last decade, and automated classification routines are becoming increasingly important to cope with this development. Here we show that the ‘upside-down CBH variables’, which were proposed as a potentially new class of variable stars by Heinze et al. in the ATLAS First Catalogue of Variable Stars, are, at least to a high percentage, made up of α2 Canum Venaticorum (ACV) variables – that is, photometrically variable magnetic chemically peculiar (CP2/He-peculiar) stars – with distinct double-wave light curves. Using suitable selection criteria, we identified 264 candidate ACV variables in the ATLAS variable star catalogue. 62 of these objects were spectroscopically confirmed with spectra from the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (all new discoveries except for nine stars) and classified on the MK system. The other 202 stars are here presented as ACV star candidates that require spectroscopic confirmation. The vast majority of our sample of stars are main-sequence objects. Derived masses range from 1.4 M⊙ to 5 M⊙, with half our sample stars being situated in the range from 2 M⊙ to 2.4 M⊙, in good agreement with the spectral classifications. Most stars belong to the thin or thick disc; four objects, however, classify as members of the halo population. With a peak magnitude distribution at around 14th magnitude, the here presented stars are situated at the faint end of the known Galactic mCP star population. Our study highlights the need to consider rare variability classes, like ACV variables, in automated classification routines.