Detailed Star Research Articles

The JWST-SUSPENSE Ultradeep Spectroscopic Program: Survey Overview and Star Formation Histories of Quiescent Galaxies at 1 < z < 3

We present an overview and first results from the Spectroscopic Ultradeep Survey Probing Extragalactic Near-infrared Stellar Emission (SUSPENSE), executed with NIRSpec on JWST. The primary goal of the SUSPENSE program is to characterize the stellar, chemical, and kinematic properties of massive quiescent galaxies at cosmic noon. In a single deep NIRSpec/MSA configuration, we target 20 distant quiescent galaxy candidates (z = 1–3, H AB ≤ 23), as well as 53 star-forming galaxies at z = 1–4. With 16 hr of integration and the G140M-F100LP dispersion-filter combination, we observe numerous Balmer and metal absorption lines for all quiescent candidates. We derive stellar masses (logM */M ⊙ ∼ 10.2–11.5) and detailed star formation histories (SFHs) and show that all 20 candidate quiescent galaxies indeed have quenched stellar populations. These galaxies show a variety of mass-weighted ages (0.8–3.3 Gyr) and star formation timescales (∼0.5–4 Gyr), and four out of 20 galaxies were already quiescent by z = 3. On average, the z > 1.75 [z < 1.75] galaxies formed 50% of their stellar mass before z = 4 [z = 3]. Furthermore, the typical SFHs of the galaxies in these two redshift bins (z mean = 2.2 [1.3]) indicate that galaxies at higher redshift formed earlier and over shorter star formation timescales compared to lower redshifts. Although this evolution is naturally explained by the growth of the quiescent galaxy population over cosmic time, number density calculations imply that mergers and/or late-time star formation also contribute to the evolution. In future work, we will further unravel the early formation, quenching, and late-time evolution of these galaxies by extending this work with studies on their chemical abundances, resolved stellar populations, and kinematics.

Closeby Habitable Exoplanet Survey (CHES). II. An Observation Strategy for the Target Stars

The Closeby Habitable Exoplanet Survey (CHES) constitutes a mission intricately designed to systematically survey approximately 100 solar-type stars located within the immediate proximity of the solar system, specifically within a range of 10 pc. The core objective of this mission is the detection and characterization of potentially habitable Earth-like planets or super-Earths within the habitable zone of these stars. The CHES mission obtains high-precision astrometric measurements of planets orbiting the target stars by observing angular distance variations between the target star and reference stars. As a result, we surveyed the relevant parameters of both target and reference stars in detail, conducting a thorough analysis and calculation of the required observation accuracy, the number of observations, and the priority assigned to each target star. Observational emphasis will be concentrated on targets considered of higher priority, ensuring the effectiveness of their observation capabilities. Through this approach, we formulate a 5 yr observation strategy that will cover all the target stars within a 6 month time frame. The strategy not only fulfills the required observing capability but also exhibits high efficiency simultaneously, providing an executable program for future mission. Over the span of the mission’s 5 yr duration, a cumulative observation time of 29,220 hr will be available. Approximately 86% of this, totaling 25,120 hr, is allocated for the observation of target stars. This allocation leaves approximately 4100 hr for extended scientific observation programs. We have also performed simulated observations based on this strategy and verified its observational capability for exoplanets.

The impact of asteroseismically calibrated internal mixing on nucleosynthetic wind yields of massive stars

Context. Asteroseismology gives us the opportunity to look inside stars and determine their internal properties, such as the radius and mass of the convective core. Based on these observations, estimations can be made for the amount of the convective boundary mixing and envelope mixing of such stars and for the shape of the mixing profile in the envelope. However, these results are not typically included in stellar evolution models. Aims. We aim to investigate the impact of varying convective boundary mixing and envelope mixing in a range based on asteroseismic modelling in stellar models up to the core collapse, both for the stellar structure and for the nucleosynthetic yields. In this first study, we focus on the pre-explosive evolution and we evolved the models to the final phases of carbon burning. This set of models is the first to implement envelope mixing based on internal gravity waves for the entire evolution of the star. Methods. We used the MESA stellar evolution code to simulate stellar models with an initial mass of 20 M⊙ from zero-age main sequence up to a central core temperature of 109 K, which corresponds to the final phases of carbon burning. We varied the convective boundary mixing, implemented as ‘step-overshoot’, with the overshoot parameter (αov) in the range 0.05−0.4. We varied the amount of envelope mixing (log(Denv/cm2 s−1)) in the range 0−6 with a mixing profile based on internal gravity waves. To study the nucleosynthesis taking place in these stars in great detail, we used a large nuclear network of 212 isotopes from 1H to 66Zn. Results. Enhanced mixing according to the asteroseismology of main-sequence stars, both at the convective core boundary and in the envelope, has significant effects on the nucleosynthetic wind yields. This is especially the case for 36Cl and 41Ca, whose wind yields increase by ten orders of magnitude compared to those of the models without enhance envelope mixing. Our evolutionary models beyond the main sequence diverge in yields from models based on rotational mixing, having longer helium-burning lifetimes and lighter helium-depleted cores. Conclusions. We find that the asteroseismic ranges of internal mixing calibrated from core hydrogen-burning stars lead to similar wind yields as those resulting from the theory of rotational mixing. Adopting the seismic mixing levels beyond the main sequence, we find earlier transitions to radiative carbon burning compared to models based on rotational mixing because they have lower envelope mixing in that phase. This influences the compactness and the occurrence of shell mergers, which may affect the supernova properties and explosive nucleosynthesis.

Frequency analysis of the first-overtone RR Lyrae stars based on Extended Aperture Photometry from K2 data

Context. Additional low-amplitude signals have been observed in many RR Lyrae stars separate from pulsations in radial modes. The most common of these are short-period signals forming a period ratio of around 0.60–0.65 with the first overtone and long-period signals forming a period ratio of around 0.68. The RR Lyrae stars may also exhibit quasi-periodic modulation in their light curves, the so-called Blazhko effect. Aims. We used the extensive sample of the first-overtone RR Lyrae stars observed by the Kepler telescope during the K2 mission to search for and characterize additional low-amplitude signals. The K2 data provides space-based photometry for a statistically significant sample. Hence, this data is excellent for studying the pulsation properties of RR Lyrae stars in detail. Methods. We used K2 space-based photometry for RR Lyrae candidates from Campaigns 0–19. We selected RR Lyrae stars pulsating in the first overtone and performed a frequency analysis for each star to characterize their frequency contents. Results. We classified 452 stars as first-overtone RR Lyrae. From that sample, we selected 281 RR0.61 stars, 67 RR0.68 stars, and 68 Blazhko stars. We found particularly interesting stars that show all of the above phenomena simultaneously. We detected signals in RR0.61 stars that form period ratios lower than those observed for the majority of stars of this type. These signals likely form a new sequence in the Petersen diagram, around a period ratio of 0.60. In 32 stars, we detected additional signals that form a period ratio close to that expected in RRd stars, but the classification of these stars as RRd is uncertain. We also report a discovery of additional signals in eight stars that form a new group in the Petersen diagram around the period ratio of 0.465–0.490. The nature of this periodicity remains unknown.

The IACOB project

Context. Blue supergiants (BSGs) are key objects for studying the intermediate phases of massive star evolution because they are very useful to constrain evolutionary models. However, the lack of a holistic study of a statistically significant and unbiased sample of these objects has lead to several long-standing questions about their physical properties and evolutionary nature to remain unsolved. Aims. This paper and other upcoming papers of the IACOB series are focused on studying from a pure empirical point of view a sample of about 500 Galactic O9–B9 stars with luminosity classes I and II (plus 250 late O- and early B-type stars with luminosity classes III, IV, and V) that cover distances up to ≈4 kpc from the Sun. Methods. We compiled an initial set of ≈11 000 high-resolution spectra from ≈1600 Galactic late O- and B-type stars. We used a novel spectroscopic strategy based on a simple fitting of the Hβ line to select stars in a specific region of the spectroscopic Hertzsprung–Russel diagram. We evaluated the completeness of our sample using the Alma Luminous Star catalog (ALS III) and Gaia-DR3 data. Results. We show the benefits of the proposed strategy for identifying BSGs that are descended in the context of single star evolution from stellar objects that are born as O-type stars. The resulting sample reaches a high level of completeness with respect to the ALS III catalog, gathering ≈80% of all-sky targets brighter than Bmag &lt; 9 located within 2 kpc. However, we identify the need for new observations in specific regions of the southern hemisphere. Conclusions. We have explored a very fast and robust method for selecting BSGs. This provides a valuable tool for large spectroscopic surveys such as WEAVE-SCIP or 4MIDABLE-LR, and it highlights the risk of using spectral classifications from the literature. Upcoming studies will make use of this large and homogeneous spectroscopic sample to study the specific properties of these stars in detail. We initially provide first results for their rotational properties (in terms of projected rotational velocities, v sin i).

Light Curve and Period Change Analysis of KIC3347485

High precision photometry with the space-based Kepler telescope offers an opportunity to analyze the physical processes occurring in stars in detail. In this study, we examined the light curve of the Kepler binary system KIC 3347485, which is a semi-detached binary system. We calculated minima and maxima times of four years of Kepler observation of this system using our newly developed Python code. We did period change analysis (O − C) of this system, and we discuss the existence of a possible third or even fourth component.

The early evolution of young massive clusters

Context. Young massive clusters provide the opportunity to study the outcome of the star formation process and the early evolution of star clusters. In the first few million years, the (massive) stars dynamically interact, producing runaways and affecting the initial (binary) population. Aims. Observing and interpreting the dynamics of young massive clusters is key to our understanding of the star formation process and predicting the outcome of stellar evolution, for example the number of gravitational wave sources. Methods. We have studied NGC 6611 in the Eagle Nebula (M16), a young massive cluster hosting ∼19 O stars. We used Gaia EDR3 data to determine the membership, age, cluster dynamics, and the kinematics of the massive stars including runaways. Results. The membership analysis yields 137 members located at a mean distance of 1706 ± 7 pc. The colour – absolute magnitude diagram reveals a blue and a red population of pre-main-sequence stars, consistent with two distinct populations of stars. In line with earlier studies, the youngest (reddest) population has a mean extinction of AV = 3.6 ± 0.1 mag and an age of 1.3 ± 0.2 Myr, while the older population of stars has a mean extinction of AV = 2.0 ± 0.1 mag and an age of 7.5 ± 0.4 Myr. The latter population is more spatially extended than the younger generation of stars. We argue that most of the OB stars belong to the younger population. We identify eight runaways originating from the centre of NGC 6611, consistent with the dynamical ejection scenario. Conclusions. We have studied the kinematics of the O stars in detail and show that ≳50% of the O stars have velocities comparable to or greater than the escape velocity. These O stars can be traced back to the centre of NGC 6611 with kinematic ages ranging from 0 to 2 Myr. These results suggest that dynamical interactions played an important role in the early evolution of NGC 6611, which is surprising considering the relatively low current stellar density (0.1–1 × 103 M⊙ pc−3). Comparing our results to simulations of young massive clusters, the initial radius of 0.1–0.5 pc (needed to produce the observed O star runaway fraction) is not consistent with that of NGC 6611. We propose a scenario where the O stars initially form in wide binaries or higher order systems and possibly harden through dynamical interactions.

Beyond the two-infall model

Context. The recent Gaia Data Release 3 (DR3) represents an unparalleled revolution in Galactic archaeology, providing numerous radial velocities and chemical abundances for millions of stars as well as all-sky coverage. Aims We present a new chemical evolution model for the Galactic disc components (high- and low- α sequence stars) designed to reproduce the new abundance ratios provided by the General Stellar Parametriser-spectroscopy module for the Gaia DR3 and constrained by the detailed star formation (SF) histories for both the thick and thin disc stars inferred from previous Gaia releases. Methods. Sophisticated modelling based on previous Gaia releases have found evidence for narrow episodes of enhanced SF inferred in recent time. Additionally, Gaia DR3 indicated the presence of young (massive) low-α disc stars that show evidence of a recent chemical impoverishment in several elements. In order to reproduce these observables, we propose a new chemical evolution model in which the low-α sequence is generated by two distinct infall episodes. Hence, in this study we compare Gaia DR3 chemical abundances with the predictions of a three-infall chemical evolution model for the high- and low-α components. Results The proposed three-infall chemical evolution model nicely reproduces the main features of the abundance ratio [X/Fe] versus [M/H] (X=Mg, Si, Ca, Ti, α) of Gaia DR3 stars in different age bins for the considered α elements. Moreover, the most recent gas infall – which started ∼2.7 Gyr ago – allowed us to predict accurately predict the Gaia DR3 young population which has experienced a recent chemical impoverishment. Conclusions. We extended previous chemical evolution models designed to reproduce APOGEE and APOKASC data in order to predict new Gaia DR3 chemical abundances. To this aim, we proposed a three-infall chemical evolution model to better trace both (i) the young population in Gaia DR3 with evidence of chemical impoverishment and (ii) the SF history from previous Gaia releases.

Clues to the Formation of Liller 1 from Modeling Its Complex Star Formation History

Liller 1 and Terzan 5 are two massive systems in the Milky Way bulge hosting populations characterized by significantly different ages (Δt > 7–8 Gyr) and metallicities (Δ[Fe/H] ∼ 1 dex). Their origin is still strongly debated in the literature and all formation scenarios proposed so far require some level of fine-tuning. The detailed star formation histories of these systems may represent an important piece of information to assess their origin. Here we present the first attempt to perform such an analysis for Liller 1. The first key result we find is that Liller 1 has been forming stars over its entire lifetime. More specifically, three broad star formation episodes are clearly detected: (1) a dominant one, occurring some 12–13 Gyr ago with a tail extending for up to ∼3 Gyr; (2) an intermediate burst, between 6 and 9 Gyr ago; and (3) a recent one, occurring between 1 and 3 Gyr ago. The old population contributes to about 70% of the total stellar mass, and the remaining fraction is almost equally split between the intermediate and young populations. If we take these results at face value, they would suggest that this system unlikely formed through the merger between an old globular cluster and a giant molecular cloud, as recently proposed. On the contrary, our findings provide further support to the idea that Liller 1 is the surviving relic of a massive primordial structure that contributed to the Galactic bulge formation, similarly to the giant clumps observed in star-forming high-redshift galaxies.

A detailed star formation history for the extremely diffuse Andromeda XIX dwarf galaxy

ABSTRACT We present deep imaging of the ultradiffuse Andromeda XIX dwarf galaxy from the Advance Camera for Surveys on the Hubble Space Telescope which resolves its stellar populations to below the oldest main-sequence turn-off. We derive a full star formation history for the galaxy using match, and find no evidence of star formation in the past 8 Gyr. We calculate a quenching time of τ90 = 9.7 ± 0.2 Gyr, suggesting And XIX ceased forming stars very early on. This early quenching, combined with its extremely large half-light radius, low-density dark matter halo and lower than expected metallicity make it a unique galaxy within the Local Group and raises questions about how it formed. The early quenching time allows us to rule out feedback from bursty star formation as a means to explain its diffuse stellar population and low-density dark matter halo. We find that the extended stellar population, low-density halo, and star formation could be explained by either tidal interactions (such as tidal shocking) or by late dry mergers, with the latter also explaining its low metallicity. Proper motions and detailed abundances would allow us to distinguish between these two scenarios.

The (im)possibility of strong chemical tagging

Context.The possibility of identifying co-natal stars that have dispersed into the Galactic disc based on chemistry alone is called strong chemical tagging. It has been debated for a long time whether this is indeed feasible; it holds the promise of reconstructing the detailed star formation history of a large fraction of stars in the Galactic disc.Aims.We investigate the feasibility of strong chemical tagging using known member stars of open clusters.Methods.We analysed the largest sample of cluster members that have been homogeneously characterised with high-resolution differential abundances for 16 different elements. We also investigated the possibility of finding the known clusters in the APOGEE DR16 red clump sample with 18 chemical species. For both purposes, we used a clustering algorithm and an unsupervised dimensionality reduction technique to blindly search for groups of stars in chemical space.Results.Even if the internal coherence of the stellar abundances in the same cluster is high, typically 0.03 dex, the overlap in the chemical signatures of the clusters is large. In the sample with the highest precision and no field stars, we only recover 9 out of the 31 analysed clusters at a 40% threshold of homogeneity and precision. This ratio slightly increases when we only use clusters with 7 or more members. In the APOGEE sample, field stars are present along with four populated clusters. In this case, only one of the open clusters was moderately recovered.Conclusions.In our best-case scenario, more than 70% of the groups of stars are in fact statistical groups that contain stars belonging to different real clusters. This indicates that the chances of recovering the majority of birth clusters dissolved in the field are slim, even with the most advanced clustering techniques. We show that different stellar birth sites can have overlapping chemical signatures, even when high-resolution abundances of many different nucleosynthesis channels are used. This is substantial evidence against the possibility of strong chemical tagging. However, we can hope to recover some particular birth clusters that stand out at the edges of the chemical distribution.

Charged analogues of isotropic compact stars model with buchdahl metric in general relativity

In this work, we examine a spherically symmetric compact body with isotropic pressure profile, in this context we obtain a new class of exact solutions of Einstein's-Maxwell field equation for compact stars with uniform charged distributions on the basis of Pseudo-spheroidal space-time with a particular form of electric field intensity and the metric potential g_rr. Indicating these two parameters takes into account further examination to be done in deciding unknown constants and depicts the compact strange star candidates likes PSR J1614-2230, 4U 1608-52, SAX J1808.4-3658, 4U 1538-52, SMC X-1, Her X-1, and Cen X-3. By the isotropic Tolman-Oppenhimer-Volkoff(TOV)equation, We explore the equilibrium among hydrostatic, gravitational and electric forces. Then, we analyze the stability of the model through the adiabatic index(gamma) and velocity of sound (0 <dp/c^2dr< 1). we additionally talk about other physical features of this model like, for example, the pressure, redshift, density, energy conditions and mass-radius of the stars in detail and demonstrate that our results are satisfying all the basic prerequisites of a physically legitimate stellar model. We have seen the measurement of basic physical parameter such as pressure, density, energy and redshift are satisfied the reality condition. All the physical quantities such as density, pressure pressure-density ratio and speed of sound is monotonically decreasing. The outcomes acquired are valuable in exploring the strength of other compact objects like white dwarfs, gravastars and neutron stars. Finally, we have shown that the obtained solutions are compatible with observational data for compact objects.

Comparison of stellar populations in simulated and real post-starburst galaxies in MaNGA

ABSTRACT Recent integral field spectroscopic (IFS) surveys have revealed radial gradients in the optical spectral indices of post-starburst (PSB) galaxies, which can be used to constrain their formation histories. We study the spectral indices of post-processed mock IFS datacubes of binary merger simulations, carefully matched to the properties of the MaNGA IFS survey, with a variety of black hole (BH) feedback models, progenitor galaxies, orbits, and mass ratios. Based on our simulation sample, we find that only major mergers on prograde–prograde or retrograde–prograde orbits in combination with a mechanical BH feedback model can form galaxies with weak enough ongoing star formation, and therefore absent H α emission, to be selected by traditional PSB selection methods. We find strong fluctuations in nebular emission line strengths, even within the PSB phase, suggesting that H α selected PSBs are only a subsample of the underlying population. The global PSB population can be more robustly identified using stellar continuum-based approaches. The difficulty in reproducing the very young PSBs in simulations potentially indicates that new sub-resolution star formation recipes are required to properly model the process of star formation quenching. In our simulations, we find that the starburst peaks at the same time at all radii, but is stronger and more prolonged in the inner regions. This results in a strong time evolution in the radial gradients of the spectral indices that can be used to estimate the age of the starburst without reliance on detailed star formation histories from spectral synthesis models.

The recurrent impact of the Sagittarius dwarf on the star formation history of the Milky Way

Satellites orbiting disk galaxies can induce phase space features such as spirality, vertical heating and phase-mixing in their disks. Such features have also been observed in our own Galaxy, but the complexity of the Milky Way disk has only recently been fully mapped by Gaia Data Release 2 (DR2) data. This complex behaviour is mainly ascribed to repeated perturbations induced by the Sagittarius dwarf galaxy (Sgr) along its orbit, pointing to this satellite as the main dynamical architect of the Milky Way disk. Here, we model Gaia DR2-observed colour–magnitude diagrams to obtain a detailed star formation history of the ~2 kpc bubble around the Sun. It reveals three conspicuous and narrow episodes of enhanced star formation that we can precisely date as having occurred 5.7, 1.9 and 1.0 Gyr ago. The timing of these episodes coincides with proposed Sgr pericentre passages according to (1) orbit simulations, (2) phase space features in the Galactic disk and (3) Sgr stellar content. These findings most probably suggest that Sgr has also been an important actor in the build-up of the stellar mass of the Milky Way disk, with the perturbations from Sgr repeatedly triggering major episodes of star formation. Galactic close encounters can induce gravitational effects in the participants. Here Ruiz-Lara et al. have reconstructed the star formation history of the region of our Galaxy close to the Sun, finding that three recent visits of the neighbouring Sagittarius dwarf galaxy have resulted in well-defined episodes of star formation.

Superflares and variability in solar-type stars with TESS in the Southern hemisphere

ABSTRACT Superflares on solar-type stars have been a rapidly developing field ever since the launch of Kepler. Over the years, there have been several studies investigating the statistics of these explosive events. In this study, we present a statistical analysis of stellar flares on solar-type stars made using photometric data in 2-min cadence from Transiting Exoplanet Survey Satellite of the whole Southern hemisphere (sectors 1–13). We derive rotational periods for all the stars in our sample from rotational modulations present in the light curve as a result of large star-spot(s) on the surface. We identify 1980 stellar flares from 209 solar-type stars with energies in the range of 1031–1036 erg (using the solar flare classification, this corresponds to X1–X100 000) and conduct an analysis into their properties. We investigate the rotational phase of the flares and find no preference for any phase, suggesting the flares are randomly distributed. As a benchmark, we use GOES data of solar flares to detail the close relationship between solar flares and sunspots. In addition, we also calculate approximate spot areas for each of our stars and compare this to flare number, rotational phase, and flare energy. Additionally, two of our stars were observed in the continuous viewing zone with light-curves spanning 1 yr; as a result we examine the stellar variability of these stars in more detail.

Early formation and recent starburst activity in the nuclear disk of the Milky Way

The nuclear disc is a dense stellar structure at the centre of the Milky Way, with a radius of $\sim$150 pc. It has been a place of intense star formation in the past several tens of millions of years but its overall formation history has remained unknown up to now. Here we report the first detailed star formation history of this region. The bulk of its stars formed at least eight billion years ago. This initial activity was followed by a long period of quiescence that was ended by an outstanding event about 1 Gyr ago, during which roughly 5% of its mass formed in a time window $\sim$100 Myr, in what may arguably have been one of the most energetic events in the history of the Milky Way. Star formation continued subsequently on a lower level, creating a few percent of the stellar mass in the past $\sim$500 Myr, with an increased rate up to $\sim$30 Myr ago. Our findings contradict the previously accepted paradigm of quasi-continuous star formation at the centre of the Milky Way. The long quiescent phase agrees with the overall quiescent history of the Milky Way and suggests that our Galaxy's bar may not have existed until recently, or that gas transport through the bar was extremely inefficient during a long stretch of the Milky Way's life, and that the central black hole may have acquired most of its mass already in the early days of the Milky Way.

Star formation quenching imprinted on the internal structure of naked red nuggets

ABSTRACT The formation and assembly process of massive galaxies is a combination of two phases: an initial in-situ-dominated one followed by an ex-situ-dominated evolution. Separating these two contributions is therefore crucial to understand the baryonic cycle within massive haloes. A recently discovered population of so-called naked red nuggets, galaxies that shortcut the ex-situ stage preserving their pristine properties, presents a unique opportunity to study in detail star formation in massive galaxies without the confounding effect of later accretion. We investigate the spatially resolved star formation histories of a sample of 12 naked red nuggets. We measure how their radial light distributions, star formation rates, and central densities evolved in time. We find that, while forming stars, red nuggets become gradually more concentrated, reaching a maximum concentration at quenching. After being quenched, they kept forming stars in a more discy-like configuration. Our measurements suggest that supermassive black holes and host galaxies grow their mass in a self-regulated way until a characteristic M•/Mhalo is reached. Once black holes are massive enough, red nuggets get quenched and depart from the star formation main sequence. While in the main sequence, red nuggets evolve at roughly constant star formation rate. This can explain up to ∼0.3 dex of the scatter of the star formation main sequence, as well as its higher normalization observed in the early Universe. Hence, our results suggest that the main sequence is composed of populations of galaxies at different evolutionary stages and that the scatter is therefore due to secular processes.

The Chemical Evolution of Carbon, Nitrogen, and Oxygen in Metal-poor Dwarf Galaxies*

Abstract Ultraviolet nebular emission lines are important for understanding the time evolution and nucleosynthetic origins of their associated elements, but the underlying trends of their relative abundances are unclear. We present UV spectroscopy of 20 nearby low-metallicity, high-ionization dwarf galaxies obtained using the Hubble Space Telescope. Building upon previous studies, we analyze the C/O relationship for a combined sample of 40 galaxies with significant detections of the UV O+2/C+2 collisionally excited lines and direct-method oxygen abundance measurements. Using new analytic carbon ionization correction factor relationships, we confirm the flat trend in C/O versus O/H observed for local metal-poor galaxies. We find an average log(C/O) = −0.71 with an intrinsic dispersion of σ = 0.17 dex. The C/N ratio also appears to be constant at log(C/N) = 0.75, plus significant scatter (σ = 0.20 dex), with the result that carbon and nitrogen show similar evolutionary trends. This large and real scatter in C/O over a large range in O/H implies that measuring the UV C and O emission lines alone does not provide a reliable indicator of the O/H abundance. By modeling the chemical evolution of C, N, and O of individual targets, we find that the C/O ratio is very sensitive to both the detailed star formation history and to supernova feedback. Longer burst durations and lower star formation efficiencies correspond to low C/O ratios, while the escape of oxygen atoms in supernovae winds produces decreased effective oxygen yields and larger C/O ratios. Further, a declining C/O relationship is seen with increasing baryonic mass due to increasing effective oxygen yields.

Is it possible to reconcile extragalactic IMF variations with a universal Milky Way IMF?

Abstract One of the most robust observations of the stellar initial mass function (IMF) is its near-universality in the Milky Way and neighbouring galaxies. But recent observations of early-type galaxies can be interpreted to imply a ‘bottom-heavy’ IMF, while others of ultrafaint dwarfs could imply a ‘top-heavy’ IMF. This would impose powerful constraints on star formation models. We explore what sort of ‘cloud-scale’ IMF models could possibly satisfy these constraints. We utilize simulated galaxies that reproduce (broadly) the observed galaxy properties, while they also provide the detailed star formation history and properties of each progenitor star-forming cloud. We then consider generic models where the characteristic mass of the IMF is some arbitrary power-law function of progenitor cloud properties, along with well-known literature IMF models which scale with Jeans mass, ‘turbulent Bonnor–Ebert mass’, temperature, the opacity limit, metallicity, or the ‘protostellar heating mass’. We show that no IMF models currently in the literature – nor any model where the turnover mass is an arbitrary power-law function of a combination of cloud temperature/density/size/metallicity/velocity dispersion/magnetic field – can reproduce the claimed IMF variation in ellipticals or dwarfs without severely violating observational constraints in the Milky Way. Specifically, they predict too much variation in the ‘extreme’ environments of the Galaxy compared to that observed. Either the IMF varies in a more complicated manner, or alternative interpretations of the extragalactic observations must be explored.

A SALT Spectral Study of S0s Hosting Pseudobulges

We present a SALT-RSS spectroscopic study of a sample of S0 galaxies established by Vaghmare et al. (2015) as having pseudobulges using a combination of photometric criteria. We extract the spectra of various regions along the galaxy major axis using standard long-slit spectroscopic reduction procedures and model the spectra using STARLIGHT to derive detailed star formation histories. The central spectra of galaxies without bars in our sample reveal a complex star formation history, which is consistent with the belief that pseudobulges have a history of star formation distributed over extended periods of time. The spectra of the unbarred galaxies contain strong emission lines such as H $\alpha$, indicating active star formation, which appears to be in contradiction with the expectation that S0 galaxies have been stripped of gas. In the case of the two barred galaxies in the sample, the spectrum is dominated by light from a much older stellar population. This seems to suggest an accelerated formation of the pseudobulge made possible by the action of the bar. One of these galaxies appears to have exhausted its reservoir of gas and thus has no signature of a recently formed population of stars while the other galaxy has managed to give rise to new stars through a recent funnelling action. We have also confirmed the influence of bars on the nature of the stellar population in a pseudobulge using an alternate sample based on the SDSS