Stellar Archaeology Research Articles

BD+44°493: Chemo-dynamical Analysis and Constraints on Companion Planetary Masses from WIYN/NEID Spectroscopy* *The WIYN Observatory is a joint facility of the University of Wisconsin Madison, Indiana University, NSF NOIRLab, the Pennsylvania State University, Purdue University, and Princeton University. Based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. Based on observations made

In this work, we present high-resolution (R ∼ 100,000), high signal-to-noise ratio (S/N ∼ 800) spectroscopic observations for the well-known, bright, extremely metal-poor, carbon-enhanced star BD+44°493. We determined chemical abundances and upper limits for 17 elements from WIYN/NEID data, complemented with 11 abundances redetermined from Subaru and Hubble data, using the new, more accurate, stellar atmospheric parameters calculated in this work. Our analysis suggests that BD+44°493 is a low-mass (0.83 M ⊙), old (12.1–13.2 Gyr) second-generation star likely formed from a gas cloud enriched by a single metal-free 20.5 M ⊙ Population III star in the early Universe. With a disk-like orbit, BD+44°493 does not appear to be associated with any major merger event in the early history of the Milky Way. From the precision radial-velocity NEID measurements (median absolute deviation = 16 m s−1), we were able to constrain companion planetary masses around BD+44°493 and rule out the presence of planets as small as msini=2 M J out to periods of 100 days. This study opens a new avenue of exploration for the intersection between stellar archaeology and exoplanet science using NEID.

Peculiarities of the chemical enrichment of metal-poor stars in the Milky Way Galaxy

The oldest stars in the Milky Way are metal-poor with Fe/H $<$ -- 1.0, displaying peculiar elemental abundances compared to solar values. The relative variations in the chemical compositions among stars is also increasing with decreasing stellar metallicity, allowing for the pure signature of unique nucleosynthesis processes to be revealed. The study of the $r$-process is, for instance, one of the main goals of stellar archaeology and metal-poor stars exhibit an unexpected complexity in the stellar production of the $r$-process elements in the early Galaxy. In this work, we report the atmospheric parameters, main dynamic properties, and the abundances of four metal-poor stars: HE 1523-–0901, HD 6268, HD 121135, and HD 195636 (--1.5 $>$ Fe/H The abundances were derived from spectra obtained with the HRS echelle spectrograph at the Southern African Large Telescope, using both local and non-local thermodynamic equilibrium (LTE and NLTE) approaches, with the average error between 0.10 and 0.20 dex. Based on their kinematical properties, we show that HE 1523--0901 and HD 195636 are halo stars with typical high velocities. In particular, HD 121135 displays a peculiar kinematical behaviour, making it unclear whether it is a halo or an accreted star. Furthermore, HD 6268 is possibly a rare prototype of very metal-poor thick disk stars. The abundances derived for our stars are compared with theoretical stellar models and with other stars with similar metallicity values from the literature. HD 121135 is Al-poor and Sc-poor, compared to stars observed in the same metallicity range (--1.62 $>$ Fe/H $>$--1.12). The most metal-poor stars in our sample, HE 1523 -- 0901, HD 6268, and HD 195636, exhibit anomalies that are better explained by supernova models from fast-rotating stellar progenitors for elements up to the Fe group. Compared to other stars in the same metallicity range, their common biggest anomaly is represented by the low Sc abundances. If we consider the elements beyond Zn, HE 1523--0901 can be classified as an r-II star, HD 6268 as an r-I candidate, and HD 195636 and HD 121135 exhibiting a borderline $r$-process enrichment between limited-r and r-I star. Significant relative differences are observed between the $r$-process signatures in these stars.

Unraveling the Origins and Development of the Galactic Disk through Metal-Poor Stars

The Milky Way is a spiral galaxy comprising three main components: the Bulge, the Disk, and the Halo. Of particular interest is the Galactic disk, which holds a significant portion of the baryonic matter angular momentum and harbors at least two primary stellar populations: the thin and thick disks. Understanding the formation and evolution of the Galactic disk is crucial for comprehending the origins and development of our Galaxy. Stellar archaeology offers a means to probe the disk’s evolution by listening to the cosmological narratives of its oldest and most pristine stars, specifically the metal-poor stars. In this study, we employed accurate photometric metallicity estimates and Gaia Early Data Release 3 astrometry to curate a pure sample of the oldest Galactic stars. This proceeding presents a summary of our primary findings.

Hydrodynamics and Nucleosynthesis of Jet-driven Supernovae. I. Parameter Study of the Dependence on Jet Energetics

Rotating massive stars with initial progenitor masses M prog ∼ 25–140 M ⊙ can leave rapidly rotating black holes to become collapsars. The black holes and the surrounding accretion disks may develop powerful jets by magnetohydrodynamics instabilities. The propagation of the jet in the stellar envelope provides the necessary shock heating for triggering nucleosynthesis unseen in canonical core-collapse supernovae. However, the energy budget of the jet and its effects on the final chemical abundance pattern are unclear. In this exploratory work, we present a survey on the parameter dependence of collapsar nucleosynthesis on jet energetics. We use the zero-metallicity star with M prog ∼ 40 M ⊙ as the progenitor. The parameters include the jet duration, its energy deposition rate, deposited energy, and the opening angle. We examine the correlations of the following observables: (1) the ejecta and remnant masses; (2) the energy deposition efficiency; (3) the 56Ni production and its correlation with the ejecta velocity, deposited energy, and the ejected mass; (4) the Sc–Ti–V correlation as observed in metal-poor stars; and (5) the [Zn/Fe] ratio as observed in some metal-poor stars. We also provide the chemical abundance table of these explosion models for the use of the galactic chemical evolution and stellar archeology.

Machine Learning Detects Multiplicity of the First Stars in Stellar Archaeology Data

In unveiling the nature of the first stars, the main astronomical clue is the elemental compositions of the second generation of stars, observed as extremely metal-poor (EMP) stars, in the Milky Way. However, no observational constraint was available on their multiplicity, which is crucial for understanding early phases of galaxy formation. We develop a new data-driven method to classify observed EMP stars into mono- or multi-enriched stars with support vector machines. We also use our own nucleosynthesis yields of core-collapse supernovae with mixing fallback that can explain many of the observed EMP stars. Our method predicts, for the first time, that 31.8% ± 2.3% of 462 analyzed EMP stars are classified as mono-enriched. This means that the majority of EMP stars are likely multi-enriched, suggesting that the first stars were born in small clusters. Lower-metallicity stars are more likely to be enriched by a single supernova, most of which have high carbon enhancement. We also find that Fe, Mg. Ca, and C are the most informative elements for this classification. In addition, oxygen is very informative despite its low observability. Our data-driven method sheds a new light on solving the mystery of the first stars from the complex data set of Galactic archeology surveys.

Starbursts with suppressed velocity dispersion revealed in a forming cluster at z = 2.51

One of the most prominent features of galaxy clusters is the presence of a dominant population of massive ellipticals in their cores. Stellar archaeology suggests that these gigantic beasts assembled most of their stars in the early Universe via starbursts. However, the role of dense environments and their detailed physical mechanisms in triggering starburst activities remain unknown. Here we report spatially resolved Atacama Large Millimeter/submillimeter Array (ALMA) observations of the CO J = 3−2 emission line, with a resolution of about 2.5 kpc, toward a forming galaxy cluster core with starburst galaxies at z = 2.51. In contrast to starburst galaxies in the field often associated with galaxy mergers or highly turbulent gaseous disks, our observations show that the two starbursts in the cluster exhibit dynamically cold (rotation-dominated) gas-rich disks. Their gas disks have extremely low velocity dispersion (σ0 ∼ 20−30 km s−1), which is three times lower than their field counterparts at similar redshifts. The high gas fraction and suppressed velocity dispersion yield gravitationally unstable gas disks, which enables highly efficient star formation. The suppressed velocity dispersion, likely induced by the accretion of corotating and coplanar cold gas, might serve as an essential avenue to trigger starbursts in massive halos at high redshifts.

The DIVING3D survey – Deep Integral Field Spectrograph View of Nuclei of Galaxies – I. Definition and sample presentation

ABSTRACT We present the Deep Integral Field Spectrograph View of Nuclei of Galaxies (DIVING3D) survey, a seeing-limited optical 3D spectroscopy study of the central regions of all 170 galaxies in the Southern hemisphere with B < 12.0 and |b| > 15○. Most of the observations were taken with the Integral Field Unit of the Gemini Multi-Object Spectrograph, at the Gemini South telescope, but some are also being taken with the Southern Astrophysical Research Telescope (SOAR) Integral Field Spectrograph. The DIVING3D survey was designed for the study of nuclear emission-line properties, circumnuclear (within scales of hundreds of pc) emission-line properties, stellar and gas kinematics, and stellar archaeology. The data have a combination of high spatial and spectral resolution not matched by previous surveys and will result in significant contributions for studies related to, for example, the statistics of low-luminosity active galactic nuclei, the ionization mechanisms in low-ionization nuclear emission-line regions, and the nature of transition objects, among other topics.

First light: switching on stars at the dawn of time

The Era of the First Stars is one of the last unknown frontiers for exploration: a poorly understood billion years missing from our cosmological timeline. We have now developed several methods for finally filling in the lost billion years of the history of our Universe: stellar archaeology, detecting primordial hydrogen using 21 cm cosmological emission, and observing the earliest galaxies, most recently using the James Webb Space Telescope. This review will summarise why the first stars and galaxies are unique and worthy of observation, and the methods employed by the groundbreaking telescopes aiming to detect them. Abbreviations: LOFAR: low frequency array; SKA: square kilometre array; MWA: Murchison widefield array; EDGES: experiment to detect the global EoR signature; EoR: epoch of reionisation; JWST: James Webb space telescope.

Weighing stars from birth to death: mass determination methods across the HRD

The mass of a star is the most fundamental parameter for its structure, evolution, and final fate. It is particularly important for any kind of stellar archaeology and characterization of exoplanets. There exists a variety of methods in astronomy to estimate or determine it. In this review we present a significant number of such methods, beginning with the most direct and model-independent approach using detached eclipsing binaries. We then move to more indirect and model-dependent methods, such as the quite commonly used isochrone or stellar track fitting. The arrival of quantitative asteroseismology has opened a completely new approach to determine stellar masses and to complement and improve the accuracy of other methods. We include methods for different evolutionary stages, from the pre-main sequence to evolved (super)giants and final remnants. For all methods uncertainties and restrictions will be discussed. We provide lists of altogether more than 200 benchmark stars with relative mass accuracies between $[0.3,2]\%$ for the covered mass range of $M\in [0.1,16]\,\msun$, $75\%$ of which are stars burning hydrogen in their core and the other $25\%$ covering all other evolved stages. We close with a recommendation how to combine various methods to arrive at a "mass-ladder" for stars.

The nuclear environment of NGC 2442: a Compton-thick low-luminosity AGN

ABSTRACT The detailed study of nuclear regions of galaxies is important because it can help understanding the active galactic nucleus (AGN) feedback mechanisms, the connections between the nuclei and their host galaxies, and ultimately the galaxy formation processes. We present the analysis of an optical data cube of the central region of the galaxy NGC 2442, obtained with the integral field unit (IFU) of the Gemini Multi-Object Spectrograph (GMOS). We also performed a multiwavelength analysis, with Chandra data, XMM–Newton and NuSTAR spectra, and Hubble Space Telescope (HST) images. The analysis revealed that the nuclear emission is consistent with a Low Ionization Nuclear Emission-line Region (LINER) associated with a highly obscured compact hard X-ray source, indicating a Compton-thick AGN. The HST image in the F658N filter (H α) reveals an arched structure corresponding to the walls of the ionization cone of the AGN. The gas kinematic pattern and the high gas velocity dispersion values in the same region of the ionization cone suggest an outflow emission. The stellar archaeology results indicate the presence of only old stellar populations (∼ 10 Gyr), with high metallicity (z = 0.02 and 0.05), and the absence of recent star formation in the central region of NGC 2442, which is possibly a consequence of the AGN feedback, associated with the detected outflow, shutting off star formation. NGC 2442 is a late-type galaxy similar to the Milky Way, and comparisons show that the main difference between them is the presence of a low-luminosity AGN.

Explosive Nucleosynthesis in Near-Chandrasekhar Mass White Dwarf Models for Type Iax Supernovae: Dependence on Model Parameters

Abstract The recently observed diversity of Type Ia supernovae (SNe Ia) has motivated us to conduct the theoretical modeling of SNe Ia for a wide parameter range. In particular, the origin of Type Iax supernovae (SNe Iax) has been obscure. Following our earlier work on the parameter dependence of SN Ia models, we focus on SNe Iax in the present study. For a model of SNe Iax, we adopt the currently leading model of pure turbulent deflagration of near-Chandrasekhar mass C+O white dwarfs (WDs). We carry out two-dimensional hydrodynamical simulations of the propagation of the deflagration wave, which leaves a small WD remnant behind and ejects nucleosynthesis materials. We show how the explosion properties, such as nucleosynthesis and explosion energy, depend on the model parameters, such as central densities and compositions of the WDs (including the hybrid WDs), turbulent flame prescription, and initial flame geometry. We extract the associated observables in our models and compare with the recently discovered low-mass WDs with unusual surface abundance patterns and the abundance patterns of some SN remnants. We provide the nucleosynthesis yield tables for applications to stellar archeology and galactic chemical evolution. Our results are compared with the representative models in the literature.

Implications of Inhomogeneous Metal Mixing for Stellar Archaeology

Abstract The first supernovae enrich the previously pristine gas with metals, out of which the next generation of stars form. Based on hydrodynamical simulations, we develop a new stochastic model to predict the metallicity of star-forming gas in the first galaxies. On average, in internally enriched galaxies, the metals are well mixed with the pristine gas. However, in externally enriched galaxies, the metals cannot easily penetrate into the dense gas, which yields a significant metallicity difference between the star-forming and average gas inside a halo. To study the consequences of this effect, we apply a semianalytical model to Milky Way–like dark matter merger trees and follow stellar fossils from high redshift until the present day with a novel realistic metal mixing recipe. We calibrate the model to reproduce the metallicity distribution function (MDF) at low metallicities and find that a primordial initial mass function (IMF) with a slope of from 2 to best reproduces the MDF. Our improved model for inhomogeneous mixing can have a large impact for individual minihalos but does not significantly influence the modeled MDF at [Fe/H] ≳ −4 or the best-fitting Population III IMF.

The nuclear region of NGC 613 – II. Kinematics and stellar archaeology

ABSTRACT In this work, we continue the study of the central region of NGC 613 by da Silva, Menezes & Steiner (Paper I), by analysing the stellar and gas kinematics and the stellar archaeology in optical and near-infrared data cubes. The high spatial resolution of the Gemini Multi-Object Spectrograph (GMOS) data cube allowed the detection, using spectral synthesis methods, of an inner circumnuclear ring, with a radius of ∼1 arcsec, composed of ∼109-yr stellar populations. Such a ring is located between the nucleus and the circumnuclear ring composed by H ii regions detected in previous works. Besides that, there is a stellar rotation around the nucleus and the rings follow the same direction of rotation with different velocities. The intensity-weighted average stellar velocity dispersion at the centre is 92 ± 3 km s−1. Three distinct gas outflow components were detected. The direction of the outflow observed with the H α emission line is compatible with the direction of the previously observed radio jet. The direction of one of the outflows detected in the [O iii]λ5007 emission coincides with the axis of the ionization cone. There is no difference regarding the stellar populations and the stellar kinematics along the double stellar emission, probably separated by a dust lane as mentioned in Paper I, confirming that they are part of the same structure.

Gas kinematics and stellar archaeology of the Seyfert galaxy NGC 5643

AbstractIn this work we derive stellar archaeology and kinematics of the central 400 pc of NGC 5643. The star formation history (SFH) reveals nuclear contribution of stellar populations older (20% older than 3.5 Gyr) and younger (60% younger than 320 Myr) as compared to the circumnuclear region. The [OIII] 5007 Å kinematics reveals the eastern ionization cone with an outflow (−60 km/s ⩽ v ⩽ 120 km/s).

The DIVING3D Survey - Deep IFS View of Nuclei of Galaxies

AbstractThe DIVING3D Survey (Deep Integral Field Spectrograph View of Nuclei of Galaxies) aims to observe, with high signal/noise and high spatial resolution, a statistically complete sample of southern galaxies brighter than B = 12.0 The main objectives of this survey are to study: 1) the nuclear emission line properties; 2) the circumnuclear emission line properties; 3) the central stellar kinematics and 4) the central stellar archaeology. Preliminary results of individual or small groups of galaxies have been published in 18 papers.

Very Metal-Poor Stars and the Early Universe

AbstractVery metal-poor stars ([Fe/H] < –2.0) inform our understanding of the formation and evolution of the Galaxy, and the physical conditions in the earliest star-forming environments of the Universe. They play an integral part in the paradigms of stellar populations, stellar archaeology, and near-field cosmology. We review the carbon-rich and carbon-normal sub-populations of the most iron-poor stars, providing insight into chemical enrichment at the earliest times in the Universe. We also discuss the role of very metal-poor stars in providing insight into the Galaxy’s halo, thick disk, and bulge, and the promise they hold for the future. A comparison between the abundances obtained for the nine most Fe-poor stars ([Fe/H] < –4.5) (all but one of which is C-rich) with abundances obtained from far-field cosmology suggests that the former are the most chemically primitive objects yet observed and probably older than the DLA- and sub-DLA systems for which data are currently available from far-field studies.

Preserving chemical signatures of primordial star formation in the first low-mass stars

We model early star forming regions and their chemical enrichment by Population III (Pop III) supernovae with nucleosynthetic yields featuring high [C/Fe] ratios and pair-instability supernova (PISN) signatures. We aim to test how well these chemical abundance signatures are preserved in the gas prior to forming the first long-lived low-mass stars (or second-generation stars). Our results show that second-generation stars can retain the nucleosynthetic signature of their Pop III progenitors, even in the presence of nucleosynthetically normal Pop III core-collapse supernovae. We find that carbon-enhanced metal-poor stars are likely second-generation stars that form in minihaloes. Furthermore, it is likely that the majority of Pop III supernovae produce high [C/Fe] yields. In contrast, metals ejected by a PISN are not concentrated in the first star forming haloes, which may explain the absence of observed PISN signatures in metal-poor stars. We also find that unique Pop III abundance signatures in the gas are quickly wiped out by the emergence of Pop II supernovae. We caution that the observed fractions of stars with Pop III signatures cannot be directly interpreted as the fraction of Pop III stars producing that signature. Such interpretations require modelling the metal enrichment process prior to the second-generation stars' formation, including results from simulations of metal mixing. The full potential of stellar archaeology can likely be reached in ultra-faint dwarf galaxies, where the simple formation history may allow for straightforward identification of second-generation stars.

Near-Field Cosmology with Extremely Metal-Poor Stars

The oldest, most metal-poor stars in the Galactic halo and satellite dwarf galaxies present an opportunity to explore the chemical and physical conditions of the earliest star-forming environments in the Universe. We review the fields of stellar archaeology and dwarf galaxy archaeology by examining the chemical abundance measurements of various elements in extremely metal-poor stars. Focus on the carbon-rich and carbon-normal halo star populations illustrates how these provide insight into the Population III star progenitors responsible for the first metal enrichment events. We extend the discussion to near-field cosmology, which is concerned with the formation of the first stars and galaxies, and how metal-poor stars can be used to constrain these processes. Complementary abundance measurements in high-redshift gas clouds further help establish the early chemical evolution of the Universe. The data appear consistent with the existence of two distinct channels of star formation at the earliest times.

Investigating the early universe through “stellar archaeology”

The New York Times calls attention to a metaphorical way of understanding certain astrophysics work.

The rapid assembly of an elliptical galaxy of 400 billion solar masses at a redshift of 2.3

Stellar archaeology shows that massive elliptical galaxies formed rapidly about ten billion years ago with star-formation rates of above several hundred solar masses per year. Their progenitors are probably the submillimetre bright galaxies at redshifts z greater than 2. Although the mean molecular gas mass (5 × 10(10) solar masses) of the submillimetre bright galaxies can explain the formation of typical elliptical galaxies, it is inadequate to form elliptical galaxies that already have stellar masses above 2 × 10(11) solar masses at z ≈ 2. Here we report multi-wavelength high-resolution observations of a rare merger of two massive submillimetre bright galaxies at z = 2.3. The system is seen to be forming stars at a rate of 2,000 solar masses per year. The star-formation efficiency is an order of magnitude greater than that of normal galaxies, so the gas reservoir will be exhausted and star formation will be quenched in only around 200 million years. At a projected separation of 19 kiloparsecs, the two massive starbursts are about to merge and form a passive elliptical galaxy with a stellar mass of about 4 × 10(11) solar masses. We conclude that gas-rich major galaxy mergers with intense star formation can form the most massive elliptical galaxies by z ≈ 1.5.