Early Stars Research Articles

The Ancient Star Formation History of the Extremely Low-mass Galaxy Leo P: An Emerging Trend of a Post-reionization Pause in Star Formation

Isolated, low-mass galaxies provide the opportunity to assess the impact of reionization on their star formation histories (SFHs) without the ambiguity of environmental processes associated with massive host galaxies. There are very few isolated, low-mass galaxies that are close enough to determine their SFHs from resolved star photometry reaching below the oldest main-sequence turnoff. The James Webb Space Telescope (JWST) has increased the volume for which this is possible, and here we report on JWST observations of the low-mass, isolated galaxy Leo P. From NIRCam imaging in F090W, F150W, and F277W, we derive an SFH that shows early star formation followed by a pause subsequent to the Epoch of Reionization, which is then later followed by a reignition of star formation. This is very similar to the SFHs from previous studies of other dwarf galaxies in the “transition zone” between quenched very-low-mass galaxies and the more massive galaxies that show no evidence of the impact of reionization on their SFHs; this pattern is rarely produced in simulations of SFHs. The lifetime SFH reveals that Leo P’s stellar mass at the Epoch of Reionization was in the range that is normally associated with being totally quenched. The extended pause in star formation from z ∼ 5 to 1 has important implications for the contribution of low-mass galaxies to the ultraviolet photon budget at intermediate redshifts. We also demonstrate that, due to higher sensitivity and angular resolution, observing in two NIRCam short-wavelength filters is superior to observing in a combination of a short- and a long-wavelength filter.

The Molecular Cloud Life Cycle. II. Formation and Destruction of Molecular Clouds Diagnosed via H2 Fluorescent Emission

Molecular hydrogen (H2) formation and dissociation are key processes that drive the gas life cycle in galaxies. Using the SImulating the LifeCycle of Molecular Clouds zoom-in simulation suite, we explore the utility of future observations of H2 dissociation and formation for tracking the life cycle of molecular clouds. The simulations used in this work include nonequilibrium H2 formation, stellar radiation, sink particles, and turbulence. We find that at early times in the cloud evolution H2 formation rapidly outpaces dissociation and molecular clouds build their mass from the atomic reservoir in their environment. Rapid H2 formation is also associated with a higher early star formation rate. For the clouds studied here, H2 is strongly out of chemical equilibrium during the early stages of cloud formation but settles into a bursty chemical steady state about 2 Myr after the first stars form. At the latest stage of cloud evolution, dissociation outweighs formation and the clouds enter a dispersal phase. We discuss how theories of the molecular cloud life cycle and star formation efficiency may be distinguished with observational measurements of H2 fluorescence with a space-based high-resolution far-UV spectrometer, such as the proposed Hyperion and Eos NASA Explorer missions. Such missions would enable measurements of the H2 dissociation and formation rates, which we demonstrate can be connected to different phases in a molecular cloud’s star-forming life, including cloud building, rapidly star forming, H2 chemical equilibrium, and cloud destruction.

Exploring the Cosmos: Impact and Significance of the James Webb Space Telescope

The James Webb Space Telescope (JWST) marks a pivotal advancement in astronomical research, building upon the legacy of its predecessors by enhancing the observational capabilities and expanding the understanding of the universe. This study explores the transformative impact of JWST, emphasizing its role in studying the formation and evolution of early galaxies, stars, and planetary systems. By utilizing its advanced infrared capabilities, JWST can penetrate cosmic dust and observe celestial phenomena that were previously obscured, thereby providing unprecedented insights into the early universe and the origins of life’s building blocks. The findings from JWST are expected to challenge existing theories and assumptions in cosmology and astrophysics, facilitating a paradigm shift in the comprehension of cosmic structures and the fundamental processes governing them. Furthermore, the telescope’s ability to analyze exoplanetary atmospheres will contribute significantly to the search for life beyond Earth. Ultimately, the JWST not only represents a technological marvel but also serves as a cornerstone for future astronomical discoveries, shaping the conception of the cosmos for generations to come.

Low-mass Stellar and Substellar Candidate Companions around Massive Stars in Sco OB1 and M17

Massive stars are recognized for their high degree of multiplicity, yet the mass ratio regime below 0.1 remains insufficiently explored. It is therefore unknown whether extremely low-mass (possibly substellar) companions can form and survive in the direct UV-irradiated environment of massive stars. In this paper, we discuss Very Large Telescope/Spectro-Polarimetric High-contrast Exoplanet REsearch integral field spectrograph (0.″15–0.″85) observations of six massive O- and early B-type stars in Sco OB1 and M17 that each have a low-mass candidate companion (CC). Two targets have companions that are brown dwarf candidates. The other four have CCs in the low end of the stellar mass regime (≤0.30 M ⊙). For three of these, we have obtained a second epoch observation. At least two sources exhibit similar proper motion to that of their central star. However, given the expected proper motion of background objects, this does not imply certain companionship. We show how future follow-up observations of the brown dwarf CCs in the J, H, and L bands should allow for an unambiguous confirmation of their nature.

TOI 762 A b and TIC 46432937 b: Two Giant Planets Transiting M-dwarf Stars

We present the discovery of TOI 762 A b and TIC 46432937 b, two giant planets transiting M-dwarf stars. Transits of both systems were first detected from observations by the NASA TESS mission, and the transiting objects are confirmed as planets through high-precision radial velocity observations carried out with Very Large Telescope/ESPRESSO. TOI 762 A b is a warm sub-Saturn with a mass of 0.251 ± 0.042 M J, a radius of 0.744 ± 0.017 R J, and an orbital period of 3.4717 days. It transits a mid-M-dwarf star with a mass of 0.442 ± 0.025 M ☉ and a radius of 0.4250 ± 0.0091 R ☉. The star TOI 762 A has a resolved binary star companion, TOI 762 B, that is separated from TOI 762 A by 3.″2 (∼319 au) and has an estimated mass of 0.227 ± 0.010 M ☉. The planet TIC 46432937 b is a warm super-Jupiter with a mass of 3.20 ± 0.11 M J and radius of 1.188 ± 0.030 R J. The planet’s orbital period is P = 1.4404 days, and it undergoes grazing transits of its early M-dwarf host star, which has a mass of 0.563 ± 0.029 M ☉ and a radius of 0.5299 ± 0.0091 R ☉. TIC 46432937 b is one of the highest-mass planets found to date transiting an M-dwarf star. TIC 46432937 b is also a promising target for atmospheric observations, having the highest transmission spectroscopy metric or emission spectroscopy metric value of any known warm super-Jupiter (mass greater than 3.0 M J, equilibrium temperature below 1000 K).

The OATMEAL Survey. I. Low Stellar Obliquity in the Transiting Brown Dwarf System GPX-1

We introduce the OATMEAL survey, an effort to measure the obliquities of stars with transiting brown dwarf companions. We observed a transit of the close-in (P orb = 1.74 days) brown dwarf GPX-1 b using the Keck Planet Finder spectrograph to measure the sky-projected angle between its orbital axis and the spin axis of its early F-type host star (λ). We measured λ = 6.°9 ± 10.°0, suggesting an orbit that is prograde and well aligned with the stellar equator. Hot Jupiters around early F stars are frequently found to have highly misaligned orbits, with polar and retrograde orbits being commonplace. It has been theorized that these misalignments stem from dynamical interactions, such as von Zeipel–Kozai–Lidov cycles, and are retained over long timescales due to weak tidal dissipation in stars with radiative envelopes. By comparing GPX-1 to similar systems under the frameworks of different tidal evolution theories, we argued that the rate of tidal dissipation is too slow to have re-aligned the system. This suggests that GPX-1 may have arrived at its close-in orbit via coplanar high-eccentricity migration or migration through an aligned protoplanetary disk. Our result for GPX-1 is one of few measurements of the obliquity of a star with a transiting brown dwarf. By enlarging the number of such measurements and comparing them with hot-Jupiter systems, we will more clearly discern the differences between the mechanisms that dictate the formation and evolution of both classes of objects.

Boron depletion in Galactic early B-type stars reveals two different main sequence star populations

Context. The evolution and fate of massive stars are thought to be affected by rotationally induced internal mixing. The surface boron abundance is a sensitive tracer of this in early B-type main sequence stars. Aims. We test current stellar evolution models of massive main sequence stars which include rotational mixing through a systematic study of their predicted surface boron depletion. Methods. We construct a dense grid of rotating single star models using MESA, for which we employ a new nuclear network which follows all the stable isotopes up to silicon, including lithium, beryllium, boron, as well as the radioactive isotope aluminium-26. We also compile the measured physical parameters of the 90 Galactic early B-type stars with boron abundance information. We then compare each observed stars with our models through a Bayesian analysis, which yields the mixing efficiency parameter with which the star is reproduced the best, and the probability that it is represented by the stellar models. Results. We find that about two-thirds of the sample stars are well represented by the stellar models, with the best agreement achieved for a rotational mixing efficiency of ∼50% compared to the widely adopted value. The remaining one third of the stars, of which many are strongly boron depleted slow rotators, are largely incompatible with our models, for any rotational mixing efficiency. We investigate the observational incidence of binary companions and surface magnetic fields, and discuss their evolutionary implications. Conclusions. Our results confirm the concept of rotational mixing in radiative stellar envelopes. On the other hand, we find that a different boron depletion mechanism, and likely a different formation path, is required to explain about one-third of the sample stars. The large spread in the surface boron abundances of these stars may hold a clue to understanding their origin.

A binary supernova OB-runaway candidate inside Berkeley 97

OB-runaway stars ejected by the binary supernova mechanism can be found near young open star clusters. In this paper, we present an OB-runaway candidate as a pre-SN binary companion to the progenitor of the pulsar PSR J2238$+$5903 inside the young open star cluster Berkeley 97. We tried to find a kinematic outlier based on Gaia DR3 proper motions and parallaxes to be the pre-supernova binary companion to the progenitor of the pulsar. We took the spectra of two bright early B-type stars of the cluster, determined their effective temperature and surface gravity, and updated the parameters of the cluster. Through isochrone fitting of the color-magnitude diagram of the star cluster, we identified the members and determined the stellar parameters of the runaway star. Two bright members of the cluster, HD 240015 and HD 240016, are massive stars with spectral types of B0.5II and B1.5II and effective temperatures of $T_ eff K and $T_ eff K, respectively, as well as surface gravities of $ g cm/s^2 We find that Berkeley 97 is a star cluster with an age of $ age yr )=7.1$, an uncertainty of $<0.1$ dex, and an interstellar extinction of $A_ V mag. The runaway star has an effective temperature of $T_ eff K with a surface gravity of $ g cm/s^2 (B8V type star). By tracing back the proper motion of the runaway star, the explosion center was found for different possible pulsar ages of 10, 20, and 26.6 kyr. The pulsar moving out from the 20 kyr position must have a space velocity of $ km $, which is consistent with the general pulsar velocity distribution. This supports the idea that the pulsar originated from the cluster as a result of a binary supernova. Despite its young age, $ kyr, the supernova remnant is not visible.

Atomic Data for Astrophysically Important Spectral Lines of Singly Ionized Nitrogen

Nitrogen lines are widely observed in astrophysical spectra and provide important diagnostics for plasma properties. In this work, we present extended calculations for accurate energy levels, electric dipole radiative transition parameters, and lifetimes for the lowest 102 states of the 2s 22p 2, 2s2p 3, 2s2p 23s, 2s 22p{n 1 l, n 2 d, 4f}(n 1 = 3–5, l = s, p, n 2 = 3, 4), and 2p 4 configurations of N ii within the framework of the fully relativistic multiconfiguration Dirac–Hartree–Fock and relativistic configuration interaction methods. These data are useful for modeling astrophysical spectra, for example, for nitrogen abundance determinations in early B-type stars, and for studying the compositions and plasma properties of H ii regions and planetary nebulae. Our computed transition parameters are compared with available experimental and theoretical data. The accuracy of the calculations is also assessed via a statistical analysis of the differences between the transition rates in the Babushkin and Coulomb gauges and by consideration of cancellation factors. In this way, 201 of the 1656 transitions computed in this work are estimated to be from accurate to better than 3%, corresponding to an accuracy class of A.

Construction of New Female Image in Early Modern China - Review of <i>Performing China: Female Stars, Performance Culture, Visual Politics, 1910-1945 </i>by Huiling Zhou

The author of the book <i>Performing China: Female Stars, Performance Culture, Visual Politics, 1910-1945</i>, Huiling Zhou, through the method of careful cross-text reading of cultural and video data and the theory of performance cultural studies, studies the tortuous course of constructing new female images of Chinses women represented by female film stars in the early 1920s and 1930s. They are not only “theater actresses” on the screen, but also “social actresses” in social practice. In face of various social opinions, gossip and party comments at that time, early Chinese female stars found ways to release themselves in their screen roles and real lives in the real society. Their performances in the process of China’s modernization revealed the society’s conception of the country, urban scene and female image at that time. Hollywood’s influence on Chinese films in the 1930s was profound and extensive, from the portrayal of urban women in China to the demonstration of film marketing strategies. From the perspective of film performance, the author analyzes the process of cross-cultural communication and interaction between China and the United States in the 1930s, involving various aspects of film, literature, politics, painting and industry. The difficulty of reading this book lies in its strong theoretical nature. Then, the film documents and image data selected by the author in the early 1920s and 1930s are relatively unknown to many people today. The author’s cultural research method with visual as core of cross-cultural interaction research can make us have some new thinking about many popular cultural forms at present, and broaden our vision and thinking.

The Supersonic Project: Early Star Formation with the Streaming Velocity

At high redshifts (z ≳ 12), the relative velocity between baryons and dark matter (the so-called streaming velocity) significantly affects star formation in low-mass objects. Streaming substantially reduces the abundance of low-mass gas objects while simultaneously allowing for the formation of supersonically induced gas objects (SIGOs) and their associated star clusters outside of dark matter halos. Here, we present a study of the population-level effects of streaming on star formation within both halos and SIGOs in a set of simulations with and without streaming. Notably, we find that streaming actually enhances star formation within individual halos of all masses at redshifts between z = 12 and z = 20. This is demonstrated both as an increased star formation rate per object as well as an enhancement of the Kennicutt–Schmidt relation for objects with streaming. We find that our simulations are consistent with some observations at high redshift, but on a population level, they continue to underpredict star formation relative to the majority of observations. Notably, our simulations do not include feedback and so can be taken as an upper limit on the star formation rate, exacerbating these differences. However, simulations of overdense regions (both with and without streaming) agree with observations, suggesting a strategy for extracting information about the overdensity and streaming velocity in a given survey volume in future observations.

The radius distribution of M dwarf-hosted planets and its evolution

Abstract M dwarf stars are the most promising hosts for detection and characterization of small and potentially habitable planets, and provide leverage relative to solar-type stars to test models of planet formation and evolution. Using Gaia astrometry, adaptive optics imaging, and calibrated gyrochronologic relations to estimate stellar properties and filter binaries we refined the radii of 117 Kepler Objects of Interest (confirmed or candidate planets) transiting 74 single late K- and early M-type stars, and assigned stellar rotation-based ages to 113 of these. We constructed the radius distribution of 115 small (<4R⊕) planets and assessed its evolution. As for solar-type stars, the inferred distribution contains distinct populations of ‘super-Earths’ (at ∼1.3R⊕) and ‘sub-Neptunes’ (at ∼2.2 R⊕) separated by a gap or ‘valley’ at ≈1.7R⊕that has a period dependence that is significantly weaker (power law index of -0.03$^{+0.01}_{-0.03}$) than for solar-type stars. Sub-Neptunes are largely absent at short periods (<2 days) and high irradiance, a feature analogous to the ‘Neptune desert’ observed around solar-type stars. The relative number of sub-Neptunes to super-Earths declines between the younger and older halves of the sample (median age 3.86 Gyr), although the formal significance is low (p = 0.08) because of the small sample size. The decline in sub-Neptunes appears to be more pronounced on wider orbits and low stellar irradiance. This is not due to detection bias and suggests a role for H2O as steam in inflating the radii of sub-Neptunes and/or regulating the escape of H/He from them.

TESS observations of non-Be fast rotators

Context. The variability of fast-rotating Oe/Be stars has been reported in detail in recent years. However, much less is known about the behaviour of fast-rotating OB stars without known decretion disks, and hence it is difficult to identify the commonalities and differences in the photometric variability of these two populations, especially with regards to their pulsational properties and their link with the presence of circumstellar material. Aims. Via an in-depth literature search, we identified a set of fast-rotating (vsin(i) > 200 km s−1) early B-type stars not known to have disks. TESS and Kepler light curves were built for 58 stars that appear isolated (no bright neighbour within 1′ and no known companion) to avoid contamination of the light curves. Frequency spectra were calculated and then analysed to determine the noise level and the presence of significant signals above the noise. Methods. Red noise is detected in all targets, without obvious correlations between noise and stellar parameters. Long-term changes are much less frequent than in Be stars, with only 12% of our targets having the variability below 0.5 d−1 dominating their frequency spectrum. In contrast, strong frequency groups are detected in about a third of targets, as in Be stars. These groups generally occur in pairs with harmonic frequencies, as is usually seen in Be stars, but with the first group more often displaying larger amplitudes. Finally, the most frequent variability is due to isolated frequencies in the 0.5–6. d−1 range (which is found in two-thirds of cases and dominates the spectra in 42% of the sample). Higher-frequency signals (up to 40 d−1) are sometimes also detected but rarely (only 12% of stars) appear as the strongest ones of the frequency spectra. Overall, fast-rotating B-type stars, with or without disks, display similar photometric properties, except as regards their longer-term behaviour.

A Comparative Analysis of Illustris TNG and THESAN Cosmological Simulations

Cosmological simulations play a crucial role in understanding the evolution of the universe, offering valuable insights when compared with observational data. With advancements in computational capabilities and data-gathering techniques, simulations have evolved to model larger volumes and finer details of cosmic structures. This review compares two prominent simulations, Illustris TNG and THESAN, focusing on their algorithms and factors considered. Illustris TNG primarily employs smoothed particle hydrodynamics (SPH), while THESAN utilizes moving mesh hydrodynamics (MMH), each offering unique advantages in simulating different aspects of the universe. Furthermore, THESAN incorporates additional factors such as cosmological dust and sophisticated radiation transport algorithms, enhancing its accuracy in predicting early galaxies and stars. The comparison underscores the impact of algorithmic differences and the consideration of various factors on simulation accuracy. This analysis highlights the importance of understanding these specifications in designing simulations that effectively capture the complexities of the cosmos.

Long-term stellar activity of M dwarfs

Studies of the rotation and activity of M type stars are essential in order to enhance our understanding of stellar dynamos and angular momentum evolution. Using the outstanding photometric capabilities of space telescopes, even those rotation signals with low amplitudes can now be investigated in previously unrivaled detail. By combining data of K2 and the TESS prime mission, the star spot activity of M dwarfs can be monitored over a timescale of half a decade. In the framework of our study on the rotation–activity relation for bright and nearby M dwarfs, we also aim to investigate their long-term activity. While K2 was observing fields distributed around the ecliptic plane, the TESS prime mission was oriented along a line of ecliptic longitude, with one camera centered on an ecliptic pole. Due to these different observing strategies, the overlap between K2 and the TESS prime mission is marginal. However, 45 stars from our sample were observed with both missions, and of these, two early M-type stars that fulfill our selection criteria, EPIC 202059229 and EPIC 245919787, were analyzed in more detail. We find that, for both stars, the rotation period did not change, while the rotational phase did change for EPIC 245919787 by ∼0.2. The amplitude of the spot-induced variability changed for both stars but more significantly for EPIC 245919787. By comparing the cumulative flare frequency distributions, we find that the flare activity for EPIC 202059229 is unchanged, while it slightly changes for EPIC 245919787 between the K2 and TESS epochs. Using a combination of light curves from K2 and TESS that span a baseline of up to 4.5 yr, we are able to measure significant differential rotation for EPIC 245919787. Furthermore, we show that combining missions like K2 and TESS is a promising method for detecting stellar activity cycles.

KMT-2023-BLG-1866Lb: Microlensing super-Earth around an M dwarf host

Aims. We aim to investigate the nature of the short-term anomaly that appears in the lensing light curve of KMT-2023-BLG-1866. The anomaly was only partly covered due to its short duration of less than a day, coupled with cloudy weather conditions and a restricted nighttime duration. Methods. Considering the intricacy of interpreting partially covered signals, we thoroughly explored all potential degenerate solutions. Through this process, we identified three planetary scenarios that account for the observed anomaly equally well. These scenarios are characterized by the specific planetary parameters: (s, q)inner = [0.9740 ± 0.0083, (2.46 ± 1.07) × 10−5], (s, q)intermediate = [0.9779 ± 0.0017, (1.56 ± 0.25) × 10−5], and (s, q)outer = [0.9894 ± 0.0107, (2.31 ± 1.29) × 10−5], where s and q denote the projected separation (scaled to the Einstein radius) and mass ratio between the planet and its host, respectively. We identify that the ambiguity between the inner and outer solutions stems from the inner-outer degeneracy, while the similarity between the intermediate solution and the others is due to an accidental degeneracy caused by incomplete anomaly coverage. Results. Through Bayesian analysis utilizing the constraints derived from measured lensing observables and blending flux, our estimation indicates that the lens system comprises a very-low-mass planet orbiting an early M-type star situated approximately (6.2–6.5) kpc from Earth in terms of median posterior values for the different solutions. The median mass of the planet host is in the range of (0.48–0.51) M⊙, and that of the planet’s mass spans a range of (2.6–4.0) ME, varying across different solutions. The detection of KMT-2023-BLG-1866Lb signifies the extension of the lensing surveys to very-low-mass planets that have been difficult to detect in earlier surveys.

Searching for OB-type pre-supernova binary companions inside supernova remnants

ABSTRACT We searched for OB-runaway stars inside supernova remnants (SNRs) as a pre-supernova binary companion. As the majority of massive stars are found in close binary systems, a runaway star ejected by the orbital energy after the supernova (SN) is expected to be found. Considering a binary mass fraction, q = 0.25−1.0, the runaway star is likely to have an OB spectral type. We selected 12 SNRs at Galactic longitudes 109°−189°. Using Gaia astrometry, we selected stars having consistent distances with those of the SNRs and showing a peculiar proper motion directed away from the central region of the SNRs. We also determined the radial distribution of the extinction towards the SNRs and estimated the spectral types of the OB-runaway candidates through Gaia and Two Micron All Sky Survey photometry. We found two candidates among 12 SNRs. By spectroscopic observations, Gaia DR3 195632152560621440 inside SNR G166.0 + 4.3 was found to be an evolved A3 type star that cannot be the pre-SN binary companion to the progenitor. Gaia DR3 513927750767375872 inside SNR HB 3 is the only OB-runaway candidate (2D space velocity of 33.3 ± 3.3 km s−1). Based on photometric study, the star can be an early B-type main-sequence star with a slightly higher extinction relative to the SNR; however, it might instead be an evolved A-type star at the same distance. The other 11 SNRs do not host an OB-runaway star within their central regions. Although the runaway search was performed in a large interval of distance and extinction, we also estimated distances to the SNRs.

Probing the Relationship Between Early Star Formation and CO in the Dwarf Irregular Galaxy WLM with JWST

Wolf–Lundmark–Melotte (WLM) is a Local Group dwarf irregular (dIrr) galaxy with a metallicity 13% of solar. At 1 Mpc, the relative isolation of WLM provides a unique opportunity to investigate the internal mechanisms of star formation at low metallicities. The earliest stages of star formation in larger spirals occur in embedded clusters within molecular clouds, but dIrrs lack the dust, heavy metals, and organized structure of spirals believed necessary to collapse the molecular clouds into stars. Despite actively forming stars, the early stages of star formation in dIrrs is not well understood. We examine the relationship between early star formation and molecular clouds at low metallicities. We utilize ALMA-detected CO cores, JWST near-infrared (NIR) images (F090W, F150W, F250M, and F430M), and GALEX far-ultraviolet (FUV) images of WLM to trace molecular clouds, early star formation, and longer star formation timescales respectively. We compare clumps of NIR-bright sources (referred to as objects) categorized into three types based on their proximity to FUV sources and CO cores. We find objects, independent of their location, have similar colors and magnitudes and no discernible difference in temperature. However, we find that objects near CO have higher masses than objects away from CO, independent of proximity to FUV. Additionally, objects near CO are coincident with Spitzer 8 μm sources at a higher frequency than objects elsewhere in WLM. This suggests objects near CO may be embedded star clusters at an earlier stage of star formation, but accurate age estimates for all objects are required for confirmation.

The oldest stars with low neutron-capture element abundances and origins in ancient dwarf galaxies

ABSTRACT We present a detailed chemical abundance and kinematic analysis of six extremely metal-poor (−4.2 ≤ [Fe/H] ≤−2.9) halo stars with very low neutron-capture abundances ([Sr/H] and [Ba/H]) based on high-resolution Magellan/MIKE spectra. Three of our stars have [Sr/Ba] and [Sr/H] ratios that resemble those of metal-poor stars in ultra-faint dwarf galaxies (UFDs). Since early UFDs may be the building blocks of the Milky Way, extremely metal-poor halo stars with low, UFD-like Sr and Ba abundances may thus be ancient stars from the earliest small galactic systems that were accreted by the proto-Milky Way. We label these objects as Small Accreted Stellar System (SASS) stars, and we find an additional 61 similar ones in the literature. A kinematic analysis of our sample and literature stars reveals them to be fast-moving halo objects, all with retrograde motion, indicating an accretion origin. Because SASS stars are much brighter than typical UFD stars, identifying them offers promising ways towards detailed studies of early star formation environments. From the chemical abundances of SASS stars, it appears that the earliest accreted systems were likely enriched by a few supernovae whose light element yields varied from system to system. Neutron-capture elements were sparsely produced and/or diluted, with r-process nucleosynthesis playing a role. These insights offer a glimpse into the early formation of the Galaxy. Using neutron-capture elements as a distinguishing criterion for early formation, we have access to a unique metal-poor population that consists of the oldest stars in the universe.

Constraining the properties of Population III galaxies with multiwavelength observations

ABSTRACT The early Universe, spanning 400 000 to 400 million years after the big bang (z ≈ 1100–11), has been left largely unexplored as the light from luminous objects is too faint to be observed directly. While new experiments are pushing the redshift limit of direct observations, measurements in the low-frequency radio band promise to probe early star and black hole formation via observations of the hydrogen 21-cm line. In this work, we explore synergies between 21-cm data from the HERA and SARAS 3 experiments and observations of the unresolved radio and X-ray backgrounds using multiwavelength Bayesian analysis. We use the combined data set to constrain properties of Population II and Population III stars as well as early X-ray and radio sources. The joint fit reveals a 68 percentile disfavouring of Population III star formation efficiencies $\gtrsim 5.7~{{\ \rm per\ cent}}$. We also show how the 21-cm and the X-ray background data synergistically constrain opposite ends of the X-ray efficiency prior distribution to produce a peak in the 1D posterior of the X-ray luminosity per star formation rate. We find (at 68 per cent confidence) that early galaxies were likely 0.3–318 times as X-ray efficient as present-day starburst galaxies. We also show that the functional posteriors from our joint fit rule out global 21-cm signals deeper than ≲−203 mK and power spectrum amplitudes at k = 0.34 h Mpc−1 greater than $\Delta _{21}^2 \gtrsim 946\ \mathrm{mK}^2$ with 3σ confidence.