Venus Research Articles

The Hottest Neptunes Orbit Metal-rich Stars

Abstract The Neptune desert is no longer empty. A handful of close-in planets with masses between those of Neptune and Saturn have now been discovered, and their puzzling properties have inspired a number of interesting theories on the formation and evolution of desert-dwellers. While some studies suggest that Neptune desert planets form and evolve similarly to longer-period Neptunes, others argue that they are products of rare collisions between smaller planets, or that they are the exposed interiors of giant planets (i.e., “hot Jupiters gone wrong”). These origin stories make different predictions for the metallicities of Neptune desert host stars. In this paper, we use the homogeneous catalog of stellar metallicities from Gaia Data Release 3 to investigate the origins of Neptune desert dwellers. We find that planets in the Neptune desert orbit stars that are significantly more metal rich than the hosts of longer-period Neptunes (p = 0.0016) and smaller planets (p = 0.00014). In contrast, Neptune desert host star metallicities are statistically indistinguishable from those of hot Jupiter host stars (p = 0.55). Therefore, we find it relatively unlikely that Neptune desert planets formed and evolved similarly to longer-period Neptunes, or that they resulted from collisions between smaller planets, at least without another metallicity-selective process involved. A more straightforward explanation for this result is that planets in the desert truly are the exposed interiors of larger planets. Atmospheric spectroscopy of Neptune desert worlds may therefore provide a rare glimpse into the interiors of giant exoplanets.

Multiepoch Observations of the Nearby Spiral Galaxy NGC 3938 with the Chandra X-Ray Observatory

Abstract We present an analysis of two epochs of ACIS observations of the SA(s)c spiral galaxy NGC 3938 with the Chandra X-ray Observatory. The total exposure time of the observations was 95 ks, with a limiting unabsorbed luminosity of ≈1038 erg s−1 assuming a distance of 22 Mpc. A total of 47 discrete merged sources from both epochs were detected at the ≈3σ level or greater with the D25 radius. We demonstrate that at the time of the Chandra observations the nucleus was not detected. We connect the detected sources to counterparts in other wavebands to the degree possible. Based on the two epochs, we identify three variable sources and an additional two that may have varied between the two observations. We do not formally detect any of the five historical supernovae that have occurred in NGC 3938. The luminosity function of NGC 3938 is compared to a recent compilation of 38 galaxies, and we identify a potentially significant problem with the “known” distance to NGC 3938. Star formation rate and metallicity values are also computed; the star formation rate is highly dependent on the adopted distance. The metallicity appears to lie in the range of 8.2–9.2, consistent with values from other work. We include in an appendix a short discussion of the sources that lie in Chandra’s field of view but lie outside of NGC 3938.

The ALPINE-ALMA [CII] Survey: Unveiling the baryon evolution in the interstellar medium of z ∼ 5 star-forming galaxies

Context. Recent observations suggest a significant and rapid buildup of dust in galaxies at high redshift (z > 4); this presents new challenges to our understanding of galaxy formation in the early Universe. Although our understanding of the physics of dust production and destruction in a galaxy’s interstellar medium (ISM) is improving, investigating the baryonic processes in the early universe remains a complex task owing to the inherent degeneracies in cosmological simulations and chemical evolution models. Aims. In this work we characterized the evolution of 98 z ∼ 5 star-forming galaxies observed as part of the ALMA Large Program ALPINE by constraining the physical processes underpinning the gas and dust production, consumption, and destruction in their ISM. Methods. We made use of chemical evolution models to simultaneously reproduce the observed dust and gas content of our galaxies, obtained respectively from spectral energy distribution (SED) fitting and ionized carbon measurements. For each galaxy we constrained the initial gas mass, gas inflows and outflows, and efficiencies of dust growth and destruction. We tested these models with both the canonical Chabrier and a top-heavy initial mass function (IMF); the latter allowed rapid dust production on shorter timescales. Results. We successfully reproduced the gas and dust content in most of the older galaxies (≳600 Myr) regardless of the assumed IMF, predicting dust production primarily through Type II supernovae (SNe) and no dust growth in the ISM, as well as moderate inflow of primordial gas. In the case of intermediate-age galaxies (300−600 Myr), we reproduced the gas and dust content through Type II SNe and dust growth in ISM, though we observed an overprediction of dust mass in older galaxies, potentially indicating an unaccounted dust destruction mechanism and/or an overestimation of the observed dust masses. The number of young galaxies (≲300 Myr) reproduced, increases for models assuming top-heavy IMF but with maximal prescriptions of dust production. Galactic outflows are required (up to a mass-loading factor of 2) to reproduce the observed gas and dust mass, and to recover the decreasing trend of gas and dust over stellar mass with age. Assuming the Chabrier IMF, models are able to reproduce ∼65% of the total sample, while with top-heavy IMF the fraction increases to ∼93%, alleviating the tension between the observations and the models. Observations from the James Webb Space Telescope (JWST) will allow us to remove degeneracies in the diverse intrinsic properties of these galaxies (e.g., star formation histories and metallicity), thereby refining our models.

Nanoscale heat generation in a single Si nanowire

Abstract We develop a nanoheater utilizing a single Si nanowire with a porous segment that produces localized heat. The 19-fold higher resistivity of the porous segment compared to the solid segment in the nanowire facilitates the substantial confinement of heat to the porous segment by Joule heating. The heat profiles of the nanowire are examined using scanning thermal microscopy, a direct thermal imaging technique. The profiles recorded along the longitudinal and cross-sectional axes of the nanowire reveal that heat is concentrated in the sub-micrometer region of the porous segment, whereas it is uniformly distributed along the whole axis of the homogeneous solid Si nanowire. Moreover, the HfO2-passivated nanowire device exhibits a temperature increase above 10 °C within a 0.4 × 1 μm2 area, which is advantageous compared to the 3.3 °C increase observed in the hBN-passivated device. These point heaters demonstrate considerable potential for future applications in biomedical engineering and optoelectronics.

Planetary nebulae of the Large Magellanic Cloud II. The connection with the progenitors' properties

The study of planetary nebulae (PNe) offers the opportunity to evaluate the efficiency of the dust production mechanism during the very late asymptotic giant branch (AGB) phases, which allows us to assess the role played by AGB stars as dust manufacturers. We studied the relationship between the properties of PNe, particularly the gas and dust content, and the mass and metallicity of the progenitor stars to understand how dust production works in the late AGB phases and to shed new light on the physical processes the stars and the material in their surroundings are subject to in the period between the departure from the AGB and the start of the PN phase. We considered a sample of nine PNe in the Large Magellanic Cloud, seven of which are characterised by the presence of carbonaceous dust and the remaining two the presence of silicates. For these stars, we estimated the masses and the metallicity of their progenitor stars. We combined results from stellar evolution and dust formation modelling with results from analyses of the spectral energy distribution to determine the relation between the dust and gas mass of the PNe considered and the mass and metallicity of the progenitors. The physical properties of carbon-rich PNe are influenced by the mass of the progenitor star. Specifically, the dust-to-gas ratio in the nebula increases from $5 $ to $6 $ as the progenitor star's mass increases from approximately rm 0.9 M_ ⊙ to rm 2 M_ ⊙ . This change is partly influenced by the effective temperature of the PNe, and it occurs because higher-mass carbon stars are more efficient at producing dust. Consequently, as the progenitor's mass increases, the gas mass of the PN decreases since the larger amounts of dust lead to greater effects from radiation pressure, which pushes the gas outwards. No meaningful conclusions can be drawn from the study of the PNe with silicate-type dust, because the subsample comprises two PNe only, one of which is almost dust-free.

Chemically peculiar stars as members of open clusters

ABSTRACT The chemically peculiar (CP) stars of the upper main sequence are excellent astrophysical laboratories to test the diffusion, mass-loss, rotational mixing, and pulsation in the (non-)presence of a stable local magnetic field. These processes are time-dependent. The age estimation of Galactic field stars suffers from several limitations. Therefore, studying members of star clusters overcomes these difficulties. We matched the most recently published catalogues of star clusters and CP stars. For the matching, we used the newest Gaia Data Release. We also used the $\Delta$a photometry tool to further distinguish between the CP subgroups. We found 595 CP stars in 408 star clusters of all ages. Furthermore, we report on misclassified metallic line stars (Am or CP1) and objects with no CP classification. The distribution of magnetic and non-magnetic CP stars on the main sequence seems different. We do not detect very young and very old CP stars showing rotationally induced variability. CP members of star clusters help to study all relevant processes responsible for this phenomenon in more detail. Still, a larger sample is desired to put tighter constraints on models.

Photometric detection of internal gravity waves in upper main-sequence stars

Context. Massive main-sequence stars have convective cores and radiative envelopes, but can also have sub-surface convection zones caused by partial ionisation zones. However, the convective properties of such regions strongly depend on opacity and therefore a star’s metallicity. Non-rotating 1D evolution models of main-sequence stars between 7 ≤ M ≤ 40 M⊙ and the metallicity of the Small Magellanic Cloud (SMC) galaxy suggest tenuous (if any) sub-surface convection zones when using the Rayleigh number as a criterion for convection owing to their substantially lower metallicity compared to Galactic massive stars. Aims. We test whether massive stars of different metallicities both inside and outside of asteroseismically calibrated stability windows for sub-surface convection exhibit different properties in stochastic low-frequency (SLF) variability. Thus, we aim to constrain the metallicity dependence of the physical mechanism responsible for SLF variability commonly found in light curves of massive stars. Methods. We extracted customised light curves from the ongoing NASA Transiting Exoplanet Survey Satellite (TESS) mission for a sample of massive stars using an effective point spread function (ePSF) method, and compared their morphologies in terms of characteristic frequency, νchar, and amplitude using a Gaussian process (GP) regression methodology. Results. We demonstrate that the properties of SLF variability observed in time series photometry of massive stars are generally consistent across the metallicity range from the Milky Way down to the SMC galaxy, for stars both inside and outside of the sub-surface stability windows based on the Rayleigh number as a criterion for convection. Conclusions. We conclude that non-rotating 1D stellar structure models of sub-surface convection cannot alone be used to explain the mechanism giving rise to SLF variability in light curves of massive stars. Additionally, the similar properties of SLF variability across a wide range of metallicity values, which follow the same trends in mass and age in the Hertzsprung–Russell (HR) diagram at both high and low metallicity, support a transition in the dominant mechanism causing SLF variability from younger to more evolved stars. Specifically, core-excited internal gravity waves (IGWs) are favoured for younger stars lacking sub-surface convection zones, especially at low metallicity, and sub-surface convection zones are favoured for more evolved massive stars.

Zoomies: A Tool to Infer Stellar Age from Vertical Action in Gaia Data

Stellar age measurements are fundamental to understanding a wide range of astronomical processes, including Galactic dynamics, stellar evolution, and planetary system formation. However, extracting age information from main-sequence stars is complicated, with techniques often relying on age proxies in the absence of direct measurements. The Gaia data releases have enabled detailed studies of the dynamical properties of stars within the Milky Way, offering new opportunities to understand the relationship between stellar age and dynamics. In this study, we leverage high-precision astrometric data from Gaia DR3 to construct a stellar age prediction model based only on stellar dynamical properties, namely the vertical action. We calibrate two distinct, hierarchical stellar age–vertical action relations, first employing asteroseismic ages for red-giant-branch stars, then isochrone ages for main-sequence turn-off stars. We describe a framework called zoomies based on this calibration, by which we can infer ages for any star given its vertical action. This tool is open-source and intended for community use. We compare dynamical age estimates from zoomies with age measurements from open clusters and asteroseismology. We use zoomies to generate and compare dynamical age estimates for stars from the Kepler, K2, and TESS exoplanet transit surveys. While dynamical age relations are associated with large uncertainty, they are generally mass independent and depend on homogeneously measured astrometric data. These age predictions are uniquely useful for large-scale demographic investigations, especially in disentangling the relationship between planet occurrence, metallicity, and age for low-mass stars.

Chemo-dynamics of the stellar component of the Sculptor dwarf galaxy

Aims. Recently, both the presence of multiple stellar chemo-kinematic components and rotation in the Sculptor dwarf spheroidal galaxy have been put into question. Therefore, we re-examine the chemo-kinematic properties of this galaxy, making use of the best spectroscopic dataset available containing both the line-of-sight velocities and metallicities of individual stars. Methods. We carried out a detailed, quantitative analysis on a recent spectroscopic dataset from the literature that contains high precision velocities and metallicities for 1339 members of Sculptor. In particular, we assessed whether Sculptor is best represented by a single stellar population with a negative metallicity gradient or by the super-position of two or more components with a different mean metallicity, spatial distribution, and kinematic properties. For this analysis, we also include the incompleteness of the spectroscopic dataset. Results. We find that Sculptor is better described by a two-population model than by a single-population model with a metallicity gradient. Moreover, given the assumptions of the current modeling, we find evidence of a third population, composed of few stars, that is more extended and metal-poor than the two other populations. This very metal-poor group of stars shows a shift of ~15 km s−1 in its average line-of-sight velocity (vlos) with respect to the rest of the galaxy. We discuss several possible origins for this new population, finding a minor merger as the most likely one. We also find a vlos gradient of 4.0−1.5+1.5 km s−1 deg−1 but its statistical evidence is inconclusive and, moreover, its detection is partially driven by the group of stars with off-set velocities.

Chemodynamic evolution of Sun-like stars in nearby moving groups

ABSTRACT Sun-like stars are well represented in the solar neighbourhood but are currently underutilized, with many studies of chemical and kinematic evolution focusing on red giants (which can be observed further away) or turn-off stars (which have well-measured ages). Recent surveys (e.g. GALAH) provide spectra for large numbers of nearby Sun-like stars, which provides an opportunity to apply our newly developed method for measuring metallicities, temperatures, and surface gravities – the ${\small EPIC}$ algorithm – which yields improved ages via isochrone fitting. We test this on moving groups, by applying it to the large GALAH DR3 sample. This defines a sample of $72\,288$ solar analogue targets for which the stellar parameter measurements are most precise and reliable. These stars are used to estimate, and test the accuracy and precision of, age measurements derived with the samd isochrone fitting algorithm. Using these ages, we recover the age–metallicity relationships for nearby ($\lesssim 1 \,{\rm kpc}$) moving groups, traced by solar analogues, and analyse them with respect to the stellar kinematics. In particular, we found that the age–metallicity relationships of all moving groups follow a particular trend of young ($\textrm {age}\lt 6 \,{\rm Gyr}$) stars having constant metallicity and older ($\textrm {age} \ge 6 \,{\rm Gyr}$) stars decreasing in metallicity with increasing age. The Hercules stream carries the highest fraction of metal-rich young stars ($\sim 0.1 \,{\rm dex}$) in our sample, which is consistent with a migrating population of stars from the inner Galaxy, and we discuss the possible causes of this migration in the context of our results.

Values and facts and fancies

Abstract An examination of the apparent gap – familiar in many branches of philosophy – between ‘the facts’ and ‘values’, focusing especially on Sam Gamgee’s perception of ‘Earendil’s Star’ and the real nature of ‘the planet Venus’: Is it possible to trust in the awe and admiration we may feel towards ‘the heavens’ in the light of current astronomical theory about the wider world? How can humane values, including love of beauty, survive in an inhumanly indifferent world? Can obvious fictions have more than allegorical significance? Must we rely on fictions to survive as humane creatures, or may those seeming fictions, and our initial emotional response, provide true guidance to the way things are, and how we might be?

The importance of stochasticity in determining galaxy emissivities and UV LFs during cosmic dawn and reionization

The stochastic nature of star formation and photon propagation in high-redshift galaxies can result in sizable galaxy-to-galaxy scatter in their properties. Ignoring this scatter by assuming mean quantities can bias estimates of their emissivity and corresponding observables. We constructed a flexible, semi-empirical model, sampling scatter around the following mean relations: (i) the conditional halo mass function (CHMF); (ii) the stellar-to-halo mass relation (SHMR); (iii) the galaxy star formation main sequence (SFMS); (iv) the fundamental metallicity relation (FMR); (v) the conditional intrinsic luminosity; and (vi) the photon escape fraction. In our fiducial model, ignoring scatter in these galaxy properties overestimates the duration of the Epoch of Reionization (EoR), delaying its completion by $ z 1--2. We quantified the relative importance of each of the above sources of scatter in determining the ionizing, soft-band X-ray, and Lyman Werner (LW) emissivities as a function of scale and redshift. We find that scatter around the SFMS is important for all bands, especially at the highest redshifts where the emissivity is dominated by the faintest, most "bursty" galaxies. Ignoring this scatter would underestimate the mean emissivity and its standard deviation computed over 5 cMpc regions by factors of up to sim 2--10 at $5 z 15$. The scatter around the X-ray luminosity to star formation rate and metallicity relation is important for determining X-ray emissivity, accounting for roughly half of its mean and standard deviation. The importance of scatter in the ionizing escape fraction depends on its functional form, while scatter around the SHMR contributes at the level of sim 10--20<!PCT!>. Other sources of scatter have a negligible contribution to the emissivities. Although scatter does flatten the UV luminosity functions, shifting the bright end by 1--2 magnitudes, the level of scatter in our fiducial model is insufficient to fully explain recent estimates from JWST photometry (consistent with previous studies). We conclude that models of the EoR should account for the burstiness of star formation, while models for the cosmic 21cm signal should additionally account for scatter in intrinsic X-ray production.

Grazing-incidence small-angle neutron scattering at high pressure (HP-GISANS): a soft matter feasibility study on grafted brush films

Grazing-incidence small-angle neutron scattering (GISANS) under pressure (HP-GISANS) at the solid (Si)–liquid (D2O) interface is demonstrated for the pressure-induced lateral morphological characterization of the nanostructure in thin (<100 nm) soft matter films. We demonstrate feasibility by investigating a hydrophobic {poly[(2,2,3,3,4,4,5,5-octafluoro)pentyl methacrylate]} (POFPMA)–hydrophilic {poly[2-(dimethylamino)ethyl methacrylate]} (PDMAEMA) brush mixture of strong incompatibility between the homopolymers, anchored on Si, at T = 45°C for two pressures, P = 1 bar and P = 800 bar. Our GISANS results reveal nanostructural rearrangements with increasing P, underlining P-induced effects in tethered polymer brush layers swollen with bulk solvent.

Mixed-mode coupling in the red clump

The investigation of global, resonant oscillation modes in red giant stars offers valuable insights into their internal structures. In this study, we investigate in detail the information we can recover on the structural properties of core-helium burning (CHeB) stars by examining how the coupling between gravity- and pressure-mode cavities depends on several stellar properties, including mass, chemical composition, and evolutionary state. Using the structure of models computed with the stellar evolution code MESA, we calculate the coupling coefficient implementing analytical expressions, which are appropriate for the strong coupling regime and the structure of the evanescent region in CHeB stars. Our analysis reveals a notable anti-correlation between the coupling coefficient and both the mass and metallicity of stars in the regime M ≲ 1.8 M⊙, in agreement with Kepler data. We attribute this correlation primarily to variations in the density contrast between the stellar envelope and core. The strongest coupling is expected thus for red-horizontal branch stars, partially stripped stars, and stars in the higher-mass range exhibiting solar-like oscillations (M ≳ 1.8 M⊙). While our investigation emphasises some limitations of current analytical expressions, it also presents promising avenues. The frequency dependence of the coupling coefficient emerges as a potential tool for reconstructing the detailed stratification of the evanescent region.

The Extreme Low-mass End of the Mass–Metallicity Relation at z ∼ 7

The mass–metallicity relation provides crucial insights into the baryon cycle in galaxies and strong constraints on galaxy formation models. We use JWST NIRSpec observations from the UNCOVER program to measure the gas-phase metallicity in a sample of eight galaxies during the epoch of reionization at z = 6–8. Thanks to the strong lensing of the galaxy cluster Abell 2744, we are able to probe extremely low stellar masses between 106 and 108 M ⊙. Using strong-line diagnostics and the most recent JWST calibrations, we derive extremely low oxygen abundances in the range of 12 + log(O/H) = 6.7–7.8. By combining this sample with more massive galaxies at similar redshifts, we derive a best-fit relation of 12 + log(O/H) = −0.076−0.03+0.03×(log(M⋆))2+1.61−0.52+0.52 × log(M⋆)−0.26−0.10+0.10 , which becomes steeper than determinations at z ∼ 3–6 toward low-mass galaxies. Our results show a clear redshift evolution in the overall normalization of the relation, galaxies at higher redshift having significantly lower metallicities at a given mass. A comparison with theoretical models provides important constraints on which physical processes, such as metal mixing, star formation or feedback recipes, are important in reproducing the observations. Additionally, these galaxies exhibit star formation rates that are higher by a factor of a few to tens compared to extrapolated relations at similar redshifts or theoretical predictions of main-sequence galaxies, pointing to a recent burst of star formation. All these observations are indicative of the highly stochastic star formation and interstellar medium enrichment expected in these low-mass systems, suggesting that feedback mechanisms in high-z dwarf galaxies might be different from those in place at higher masses.

Tracking Outflow Using Line Locking (TOLL). I. The Case Study of Quasar J221531-174408

Investigating line-locked phenomena within quasars is crucial for understanding the dynamics of quasar outflows, the role of radiation pressure in astrophysical flows, and the star formation history and metallicity of the early Universe. We have initiated the Tracking Outflow by Line Locking project to study quasar outflow by studying line-locking signatures using high-resolution high-signal-to-noise-ratio quasar spectra. In this paper, we present a case study of the line-locking signatures from QSO J221531-174408. The spectrum was obtained using the Very Large Telescope’s UV Visual Echelle Spectrograph. We first identify associated absorbers in the spectrum using C iv, N v, and Si iv doublets and measure their velocity shifts, covering fractions, and column densities through a line-profile-fitting technique. Then we compare the velocity separations between different absorbers, and detect nine pairs of line-locked C iv doublets, three pairs of line-locked N v doublets, and one pair of line-locked Si iv doublets. This is one of the four quasars known to possess line-locked signatures in C iv, Si iv, and N v at the same time. We also find three complex line-locked systems, where three to five absorbers are locked together through multi-ion doublets. Our study suggests that line locking is a common phenomenon in the quasar outflows, and theoretical models involving more than two clouds and one ionic doublet are needed in the future to explain the formation of these complex line-locking signatures.

The impact of UV spectra on searches for extremely metal-poor stars: A study for future CSST observations

Extremely metal-poor (EMPs, [Fe/H] < −3.0) and carbon-enhanced EMP (CE-EMP, [Fe/H] < −3.0 and [C/Fe]> + 1.0) stars are crucial for understanding the chemical evolution and early formation of the galaxies. Current research on EMP stars is limited by small samples, and ultraviolet (UV) band spectra are lacking. The China Space Station Telescope (CSST) will provide high-quality, low-resolution spectra across wavelengths from near-ultraviolet to near-infrared, with a limiting magnitude of about 21 mag. These will help in identifying EMPs and CE-EMP candidates in the distant Milky Way and nearby galaxies. The present study first uses the simulated CSST spectra to quantitatively evaluate the contribution of UV band spectra in predicting [Fe/H], [α/Fe], and [C/Fe]. The results indicate that UV band spectra reduce the mean absolute error by 0.04, 0.04, and 0.03, respectively, with a σ of 0.02, 0.03, and 0.04, respectively. Further spectral analysis reveals that the EMP stars show unique spectral-line features in the UV band, with significant differences compared to stars of higher metallicity, which could help in identifying EMP stars. Additionally, we tested the impact of UV band spectra on identifying EMP and CE-EMP stars at different noise levels and find that models including UV band spectra improve the identification of EMP and CE-EMP stars in terms of accuracy, recall, and F1 score. At low signal-to-noise ratio (S/N), the model including the UV band achieves a recall of 0.94, significantly higher than the model without the UV band (Recall = 0.48), doubling the capability to identify EMP stars. As the S/N increases, the inclusion of the UV band maintains high recall. This suggests that UV spectra in future large surveys could reduce the risk of missing potentially interesting stellar candidates, ensuring a more comprehensive identification of all possible EMP and CE-EMP star candidates.

X-ray observations of Blueberry galaxies

Context. Compact star-forming galaxies were dominant galaxy types in the early Universe. Blueberry galaxies (BBs) represent their local analogues, being very compact and having intense star formation. Aims. Motivated by high X-ray emission recently found in other analogical dwarf galaxies, called Green Peas, we probed the X-ray properties of BBs to determine if their X-ray emission is consistent with the empirical laws for star-forming galaxies. Methods. We performed the first X-ray observations of a small sample of BBs with the XMM-Newton satellite. Spectral analysis for detected sources and upper limits measured via Bayesian-based analysis for very low-count measurements were used to determine the X-ray properties of our galaxy sample. Results. Clear detection was obtained for only two sources, with one source exhibiting an enhanced X-ray luminosity to the scaling relations. For the remaining five sources, only an upper limit was constrained, suggesting BBs to be rather underluminous as a whole. Our analysis shows that the large scatter cannot be easily explained by the stochasticity effects. While the bright source is above (and inconsistent with) the expected distribution at almost the 99% confidence level, the upper limits of the two sources are below the expected distribution. Conclusions. These results indicate that the empirical relations between the star formation rate, metallicity, and X-ray luminosity might not hold for BBs with uniquely high specific star formation rates. One possible explanation could be that the BBs may not be old enough to have a significant X-ray binary population. The high luminosity of the only bright source can then be caused by an additional X-ray source, such as a hidden active galactic nucleus or more extreme ultraluminous X-ray sources.

The Initial-Final Mass Relation from Carbon Stars in Open Clusters

Recently, Marigo et al, identified a kink in the initial-final mass relation around initial masses of Mini≈1.65−2.10M⊙, based on Gaia DR2 and EDR3 data for white dwarfs in open clusters aged 1.5–2.5 Gyr. Notably, the white dwarfs associated with this kink, all from NGC 7789, exhibit masses of ∼0.70–0.74 M⊙, usually associated with stars of Mini∼ 3–4 M⊙. This kink in the Mini mass range coincides with the theoretically accepted solar metallicity lowest-mass stars evolving into carbon stars during the AGB phase. According to Marigo et al., these carbon stars likely experienced shallow third dredge-up events, resulting in low photospheric C/O ratios and, consequently, middle stellar winds. Under such conditions, the AGB phase is prolonged, allowing for further core mass growth beyond typical predictions. If this occurs, it might provoke other anomalies, such as a non-standard surface chemical composition. We have conducted a chemical analysis of several carbon stars belonging to open clusters within the above cluster ages. Our chemical analysis reveals that the carbon stars found within the kink exhibit C/O ratios only slightly above the unity and the typical chemical composition expected for carbon stars of near solar metallicity, partially validating the above theoretical predictions. We also show that this kink in the IMFR strongly depends on the method used to derived the distances (luminosity) of these carbon stars.

Comparing silicon mineral species of different crystallinity using Fourier transform infrared spectroscopy

In soils, various solid silica (Si) species exhibit different weathering behaviors and surface reactivities, which are among other characteristics related to the crystallinity of the silicate tetrahedral network. Amorphous species exhibit faster weathering and generally possess a larger specific surface area in comparison to crystalline species. However, the characterization of these different species is commonly based on wet chemical extraction methods, which lack selectivity. While Fourier transform infrared spectroscopy (FTIR) in the mid-infrared range can differentiate between short-range ordered aluminosilicates (SROAS) and pure amorphous silica (ASi), few systematic studies are found on the IR spectral features that distinguish solid Si species by crystallinity. This study aims to identify FTIR absorption bands that can differentiate Si species based on their crystallinity. Our data clearly indicate that ASi can be distinguished from very crystalline silica (quartz) and sea sand. The absorption band at approximately 800 cm−1 in the FTIR spectra allows determining the degree of crystallinity of the studied ASi species since the band becomes smaller and the band maximum shifted toward lower wavenumbers with increasing degree of crystallinity. Hence, FTIR spectra may be used to differentiate certain Si species in complex samples like soils, allowing the estimation of weatherability and surface reactivity of those species.