Far-ultraviolet Research Articles (Page 1)

Molecular hydrogen (̋mol) is the most abundant molecule in the interstellar medium. Because of its excited form in irradiated regions, it is a useful tool for studying photodissociation regions (PDRs), where radiative feedback from massive stars on molecular clouds is dominant. The James Webb Space Telescope (JWST), with its high spatial resolution, sensitivity, and wavelength coverage, provides unique access to the detection of most of the ̋mol rotational and rovibrational lines, as well as the analysis of their spatial morphology. Our goal is to use ̋mol line emission detected with JWST in the Horsehead nebula to constrain the physical parameters (e.g., extinction, gas temperature, and thermal pressure) throughout the PDR and its geometry. We used spectro-imaging data acquired using both the NIRSpec and MIRI-MRS instruments on board JWST to study the ̋mol spatial distribution at very small scales (down to 0.1"). From the ̋mol line ratios, we constrained the extinction throughout the PDR. We then studied the excitation of ̋mol levels in detail and used this analysis to derive the physical parameters. We detect hundreds of ̋mol rotational and rovibrational lines in the Horsehead nebula. The ̋mol morphology reveals a spatial separation between ̋mol lines (∼ 0.5") across the PDR interface. Far-ultraviolet (FUV)-pumped lines (v=0 J_u>6, v>0) peak closer to the edge of the PDR than thermalized lines. From ̋mol lines arising from the same upper level, we estimated the value of extinction throughout the PDR. We find that A_V increases from the edge of the PDR to the second and third ̋mol filaments. We find A_V = 0.3 ± 1.3 in the first filament and A_V = 6.1 ± 1.4 in the second and third filaments. We then studied the ̋mol excitation in different regions across the PDR. The excitation diagrams were fit by two excitation temperatures. As the first levels of ̋mol are thermalized, the colder temperature corresponds to the gas temperature. The second, hotter component corresponds to the FUV-pumped levels. In each filament, we derive a gas temperature of T ∼ 500 K. The temperature profile shows that the observed gas temperature remains nearly constant throughout the PDR, with a slight decrease in each of the dissociation fronts. The spatial distribution of ̋mol reveals that most of the ̋mol column density is concentrated in the second and third filaments. The column density in the first filament is approximately N(̋mol) = (3.8 ± 0.8) cm^-2, while in the second and third filaments it is N(̋mol) = (1.9 ± 0.4) cm^-2 , about five times higher. The ortho-to-para ratio (OPR) is far from equilibrium, varying from 2-2.5 at the edge of each dissociation front to 1.3-1.5 deeper into the PDR. We observe a clear spatial separation between the para and ortho rovibrational levels, as well as between 0--0 S(2) and 0--0 S(1), indicating that efficient ortho-para conversion and preferential ortho self-shielding are driving the spatial variations of the OPR. Finally, we derive a thermal pressure in the first filament of about P_ ̊m gas ≥ 6 10^6 K ç, which is approximately ten times higher than that of the ionized gas. We highlight that template stationary 1D PDR models cannot account for the intrinsic 2D structure and the very high temperature observed in the Horsehead nebula. We argue that the highly excited, over-pressurized ̋mol gas at the edge of the PDR interface could originate from mixing between the cold and hot phases induced by photo-evaporation of the cloud. The analysis of ̋mol lines detected with JWST provides unique access to the geometry and physical conditions in the Horsehead nebula at very small scales and reveals, for the first time, the possible importance of dynamical effects at the edge of the PDR. This study nevertheless highlights the need for extended modeling of these dynamical effects.

X-ray burst oscillations are quasi-coherent periodic signals at frequencies close to the neutron star spin frequency. They are observed during thermonuclear Type I X-ray bursts from a number of low-mass X-ray binaries (LMXBs) hosting a fast-spinning, weakly magnetic neutron star. Besides measuring the spin frequencies, burst oscillations hold the potential to accurately measure neutron star mass and radius, and thus provide constraints on the equation of state of matter at nuclear densities. Based on far-ultraviolet (FUV) observations of the X-ray binary taken with the Hubble Space Telescope in 2003, we report a possible indication of ultraviolet burst oscillations at the neutron star spin frequency (∼552,Hz), potentially the first such case for an LMXB. The candidate signal is observed during an ∼8,s interval in the rising phase of an FUV burst, which occurred ∼4,s after a Type I X-ray burst. Through simulations, we estimated that the probability of detecting the observed signal power from pure random noise is $3.7%$, decreasing to $0.3%$ if only the burst rise interval is considered, during which X-ray burst oscillations had already been observed in this source. The background-subtracted folded pulse profile of the candidate FUV oscillations in the (120--160,nm) band is nearly sinusoidal with a ∼ 16% pulsed fraction, corresponding to a pulsed luminosity of ∼ 8 erg,s^-1. Interpreting the properties of these candidate FUV burst oscillations in the light of current models for optical-ultraviolet emission from neutron star LMXBs faces severe problems. If signals of this kind are confirmed in future observations, they might point to an unknown coherent emission process as the origin of the FUV burst oscillations observed in

High purity aluminum in its bulk form has intrinsicallyhigh reflectancein the far-ultraviolet (FUV) regime and finds utility in astrophysicalinstrumentation applications. However, bulk Al oxidizes rapidly inthe atmosphere, and its native oxide strongly absorbs and severelydegrades the observed FUV properties relative to bare Al. Varioustechniques have been investigated to produce coatings that inhibitaluminum oxide formation and lead to high FUV mirror reflectance.This work examines the development and use of a uniquely modified,hybrid plasma-enhanced atomic layer deposition (PEALD) system to passivatealuminum mirrors with metal fluoride films. This system combines twoplasma sources in a commercial atomic layer deposition (ALD) reactor.The first is a conventional inductively coupled plasma (ICP) sourceoperated as a remote plasma, and the second is an electron beam (e-beam)driven plasma near the mirror surface. To establish the operatingconditions for the in situ e-beam plasma source, the effects of samplegrounding, SF6/Ar flow, and sample temperature on resultingAlF3 films were investigated. Optimal operating conditionsproduced mirrors with excellent FUV reflectivity, 92% at 121 nm and42% at 103 nm wavelengths, which is comparable to state-of-the-artAlF3-based passivation coatings and matches that of previouslyreported ex situ e-beam plasma-processed mirrors. This optimized insitu e-beam process, along with XeF2 passivation, is thenexplored to produce a clean seed layer (unoxidized Al surface) forsubsequent PEALD of AlF3. Both approaches are demonstratedas valid pretreatments before PEALD of AlF3, showing apromising pathway for the deposition of other fluoride-based layers,such as MgF2 or LiF, with ALD or PEALD.

The Orion Bar photodissociation region (PDR), which is shaped by far-ultraviolet (FUV) radiation, displays filamentary substructures and non-thermal gas motions across its ionized, atomic, and molecular zones, as observed by ALMA, JWST, and other telescopes, suggesting dynamic processes beyond static equilibrium. Our goal is to test isentropic instability as a mechanism for these features, focusing on the atomic zone’s shockwave structures and their alignment with observational data. We adapted gasdynamic equations to derive analytical solutions for shockwave pulse properties and complemented them with numerical simulations to track their evolution under varying initial conditions while incorporating a heat-loss function reflecting Orion Bar conditions. Our analytical and numerical results yield pulse widths (∼ 0.001 pc) and periods (∼ 0.02 pc) that closely match the observed substructure sizes (0.002–0.004 pc) and separations (0.01 pc), while the velocity amplitudes ($0.72 c_S$) agree with non-thermal estimates. The leading pulse amplitude reaches a stationary value within 5,000-40,000 years, which is well within the estimated ∼10^5-year lifetime of the Orion Bar PDR. This study demonstrates that isentropic instability can reproduce the Orion Bar’s dynamic features, bridging theoretical predictions and high-resolution observations from ALMA and JWST.

Context. HD 49798 is a bright (mV = 8.287), hot (effective temperature Teff = 45 000 K) subdwarf star of the spectral type O (sdO). It is the only confirmed sdO-type mass-donor star of an X-ray binary that has a high-mass (1.28 M☉) white-dwarf or neutron-star primary with a spin period of only 13.2 s. Aims. Since a high-quality spectrum of HD 49798, obtained with the Tübingen Ultraviolet Echelle Spectrometer (TUES), that has never been analyzed before is available in the database of the Orbiting and Retrievable Far and Extreme Ultraviolet Spectrometer (ORFEUS), we performed a spectral analysis based on observations from the far ultraviolet (FUV) to the optical wavelength range. Methods. We used advanced non-local thermodynamic equilibrium (NLTE) model atmospheres of the Tübingen Model-Atmosphere Package (TMAP) to determine the effective temperature, the surface gravity (log g), and the abundances of those elements that exhibit lines in the available observed spectra. Results. We determined Teff = 45 000 ± 1000 K, log g = 4.46 ± 0.10, and re-analyzed the previously determined photospheric abundances of H, He, N, O, Mg, Al, Si, Fe, and Ni. For the first time, we measured the abundances of C, Ne, P, S, Cr, and Mn. Conclusions. Our panchromatic spectral analysis of HD 49798 – from the FUV to the optical – allowed us to reduce the error limits of the photospheric parameters and to precisely measure the metal abundances. HD 49798 is a stripped, intermediate-mass (zero-age main sequence mass of MZAMS ≈ 7.15 M☉) He star with a mass of 1.14−0.24+0.30 M⊙. At its surface, it exhibits abundances that are the result of CNO-cycle and 3 α burning nucleosynthesis as well as enhanced Cr, Mn, Fe, and Ni abundances.

Context. M stars are preferred targets for upcoming studies of terrestrial exoplanets aimed at obtaining their atmosphere spectra over the next decade. However, M dwarfs have long been known for their strong magnetic activity and the ability to frequently produce optical and broadband emission flares. Aims. We aim to characterise the flaring behaviour of young M dwarfs in the temporal, spectral, and energy dimensions, as well as examine the stellar parameters governing this behaviour. In this way, we aim to improve our understanding of the energy and frequency of the flare events capable of shaping the exoplanet atmosphere. Methods. Members of young moving groups (YMGs) provide a unique age-based perspective on stellar activity. By examining their flare behaviour in conjunction with rotation, mass, and Hα data, we can obtain a comprehensive understanding of flare-activity drivers in young stars. Results. We demonstrate that young stars sharing similar stellar parameters could also exhibit a broad range of flare frequency distributions. We also find that the flare behaviour shows indications of difference between optical and far ultraviolet (FUV). We propose that the period of rotation (and not the age of the star) can serve as a good proxy for assessing flaring activity. Furthermore, we recommend that instead of a simple power law for describing the flare frequency distribution, a piecewise power law can be used to describe mid-size and large flare distributions in young and active M dwarfs. Conclusions. Using known periods of rotation and fine-tuned power laws governing the flare frequency, we can produce a realistic sequence of flare events to study whether the atmosphere of small exoplanets orbiting M dwarf could withstand such activity until the emergence of life.

Abstract Protoplanetary disk evolution can be deeply influenced by the UV radiation emitted by neighboring massive stars (mainly of spectral types O and B). We show that the process of external photoevaporation, which causes an outside-in depletion of disk material due to environmental UV radiation, can lead to a significant decrease in disk size, disk mass, and lifetime even at moderate irradiation levels (1–10 G0). In this work, we investigate the role of external photoevaporation in shaping the masses and sizes of the 10 AGE-PRO disks in the Upper Scorpius (Upper Sco) region, which we estimate to be subject to far-ultraviolet (FUV) fluxes ranging between ∼2 and ∼12 G0, on average. We compare the disk masses and sizes resulting from 1D numerical viscous evolution simulations, in which the effect of external photoevaporation is included, to the values retrieved from the AGE-PRO observations. While the pure viscous framework fails in adequately explaining the observed disk properties in Upper Sco, with the inclusion of external photoevaporation, we can successfully reproduce gas disk sizes for seven out of 10 sources within a factor <2, when the initial disk mass is 1%–10% of the stellar mass. We emphasize the importance of accounting for the environmental irradiation when comparing star-forming regions of different ages, even when moderate FUV irradiation fields are experienced, as in the case of Upper Sco.

The ionization fraction ( $=n_ e /n_ H $) represents a fundamental parameter of the gas in the interstellar medium (ISM). However, estimating relies on a deep knowledge of the underlying chemistry of molecular gas as well as observations of atomic recombination lines and electron-sensitive molecular emission, such as deuterated isotopologs of ̋COP, and which are only detectable in the dense cores. Until now, it has been challenging to constrain the ionization fraction in the interstellar gas over a large areas because of the observational limitations on these tracers and chemistry models. Recent models have provided a set of molecular lines whose ratios (intensities and column densities) can be used to trace in different environments of molecular clouds. Here, we use a set of various molecular lines typically detected in the 3-4 mm range to constrain the ionization fraction across the Orion B giant molecular cloud. In this work, we derived the ionization fraction for dense and translucent gas, and we investigated its variation with the density of the gas, n, and the strength of the far-ultraviolet (FUV) radiation field with their ratio We present our results for the ionization fraction across one square degree in Orion B derived using analytical models as well as observational intensity and column density ratios of þCO and in the dense and shielded medium ( ≥10,mag). We also used ratios of and in the translucent gas ($2$,mag,łeq łeq6,mag). We find that the ionization fraction is within the range of 10^-5.5 to 10^-4 for the translucent medium and 10^-8 to 10^-6 for the dense medium. Our results show that the inferred values are sensitive to the value of especially in the dense, highly UV-illuminated gas. We also find that the ionization fraction in dense and translucent gas decreases with an increasing volume density (f_e∝ n^ for dense gas and f_e∝ n^ in translucent gas). It increases with which is a consequence of how sensitive the emission of selected molecular lines (e.g., CN and ̋COP) is to the UV radiation field. In the case of the translucent medium, we did not find any significant difference in the ionization fraction computed from different line ratios. The range of values found in translucent gas implies that the electron excitation of HCN and HNC becomes significant in this regime. In dense and shielded gas, we recommend using to derive an upper limit on the ionization fraction along with to set constraints on the lower limit. In a translucent medium serves as a good tracer of The moderately high values found in translucent gas are consistent with the C^+/ transition regime, while the values we find in the dense gas are sufficient to couple the gas with the magnetic field.

We investigate the James Webb Space Telescope (JWST) MIRI MRS gas molecular content of an externally irradiated Herbig disk, the F-type XUE 10 source, in the context of the eXtreme UV Environments (XUE) program. XUE 10 belongs to the massive star cluster NGC 6357 (1.69 kpc), where it is exposed to an external far-ultraviolet (FUV) radiation ≈ 10^3 times stronger than in the solar neighborhood. We modeled the molecular features in the mid-infrared spectrum with local thermodynamic equilibrium (LTE) 0D slab models. We derived basic parameters of the stellar host from a VLT FORS2 optical spectrum using PHOENIX stellar templates. We detected bright CO_2 gas with the first simultaneous detection ($>$ 5σ) of four isotopologues (^12CO_2, ^13CO_2, ^16O^12C^18O, ^16O^12C^17O) in a protoplanetary disk. We also detected faint CO emission (2σ) and the HI Pfα line (8σ). We placed strict upper limits on the water content, finding a total column density of łesssim 10^18 cm^-2. The CO_2 species trace low gas temperatures (300--370 K) with a range of column densities of 7.4 times 10^17 cm^-2 (^16O^12C^17O)--1.3 times 10^20 cm^-2 (^12CO_2) in an equivalent emitting radius of 1.15 au. The emission of ^13CO_2 is likely affected by line optical depth effects. The ^16O^12C^18O and ^16O^12C^17O abundances may be isotopically anomalous compared to the ^16O/^18O and ^16O/^17O ratios measured in the interstellar medium and the Solar System. We propose that the mid-infrared spectrum of XUE 10 is explained by H_2O removal either via advection or strong photo-dissociation by stellar UV irradiation and enhanced local CO_2 gas phase production. Outer disk truncation supports the observed CO_2--H_2O dichotomy. A CO_2 vapor enrichment in ^18O and ^17O can be explained by means of external UV irradiation and early (10^4-5 yr) delivery of isotopically anomalous water ice to the inner disk.

Most protoplanetary disks experience a phase in which they are subjected to strong ultraviolet radiation from nearby massive stars. This UV radiation can substantially alter their chemistry by producing numerous radicals and molecular ions. In this Letter we present a detailed analysis of the JWST-NIRSpec spectrum of the d203-506 obtained as part of the PDRs4All Early Release Science program. Using state-of-the-art spectroscopic data, we searched for species using a multi-molecule fitting tool, PAHTATMOL, which we developed for this purpose. Based on this analysis, we report the clear detection of ro-vibrational emission of the CH radical and the likely detection of the H3+ molecular ion, with estimated abundances of a few times 10−7 and approximately 10−8, respectively. The presence of CH is predicted by gas-phase models and is well explained by hydrocarbon photochemistry. Interstellar H3+ is usually formed through reactions of H2 with H3+ originating from cosmic ray ionization of H2. However, recent theoretical studies suggest that H3+ also forms through far-UV (FUV)-driven chemistry in strongly irradiated (G0 > 103), dense (nH > 106 cm−3) gas. The latter is favored as an explanation for the presence of hot H3+ (Tex ≳ 1000 K) in the outer disk layers of d203-506, coinciding with the emission of FUV-pumped H2 and other photodissociation region (PDR) species, such as CH+, CH3+, and OH. Our detection of infrared emission from vibrationally excited H3+ and CH raises questions about their excitation mechanisms and underscores that FUV radiation can have a profound impact on the chemistry of planet-forming disks. They also demonstrate the power of JWST to push the limit of the detection of elusive species in protoplanetary disks.

Abstract We present a pilot method to estimate the high-mass initial mass function (IMF) across the arm, interarm, and spur regions in galaxies and apply it to NGC 628. We extracted star-forming complexes (SFCs) from the Hα Very Large Telescope/Multi Unit Spectroscopic Explorer and Ultraviolet Imaging Telescope (far-ultraviolet (FUV) and near-ultraviolet (NUV)) observations of NGC 628 and used Atacama Large Millimeter/submillimeter Array observations to define the molecular gas distribution. We find that the extinction-corrected Hα and FUV luminosities correlate well. Using the fact that O stars have a shorter lifetime (107 yr) compared to B stars (108 yr), we estimated the approximate number of O stars from Hα emission, and the number of B0 (M * > 10M ⊙), and B1 (10M ⊙ ≥ M * ≥ 3M ⊙) stars using FUV and NUV observations. We derived the IMF index (α) for different regions using O to B0 (α 1) and B0 to B1 (α 2) stellar ratios. Our findings indicate that if we assume Hα arises only from O8-type stars, the resulting α 1 value is consistent with the canonical IMF index. It steepens when we assume O stars with masses up to 100 M ⊙ with mean α 1 = 3.16 ± 0.62. However, the α 2 does not change for large variations in the O-star population, and the mean α = 2.64 ± 0.14. When we include only blue SFCs (FUV − NUV ≤ 0.3), mean α 2 is 2.43 ± 0.06. The IMF variation for SFCs in arms and spurs is insignificant. We also find that α 2 correlates with different properties of the SFCs, the most prominent being the extinction-corrected UV color (FUV − NUV).

We present the neutral atomic hydrogen ( distribution, kinematics, mass modelling, and gravitational stability of the dwarf irregular galaxies ESO444--G084 and KKS2000 23 using high spatial, spectral, and column density sensitivity data from the MeerKAT observations of nearby galactic objects: Observing Southern Emitters (MHONGOOSE) survey obtained with MeerKAT. ESO444--G084 exhibits centrally concentrated emission, while KKS2000 23 has irregularly distributed high-density pockets. The total fluxes measured down to column density thresholds of cm ) and cm ) are nearly the same, suggesting that the increase in the diameter at lower column densities is primarily due to the larger beam size, and that no significant additional emission is detected. The total masses are ((1.1 ± 0.1) ⊙ ) for ESO444--G084 and ((6.1 ± 0.3) ⊙ ) for KKS2000 23. We derived rotation curves using 3D kinematic modelling tools Python fully automated TiRiFiC (PyFAT) and tilted ring fitting code (TiRiFiC), which allow us to fully capture the gas kinematics. Both galaxies exhibit disk-like rotation, with ESO444--G084 showing a kinematic warp beyond (∼1.8) kpc. Its relatively fast-rising rotation curve suggests a more centrally concentrated dark matter distribution, whereas KKS2000 3.4,μ m = 0.20) for ESO444--G084 and (0.18) for KKS2000 gas /Σ_ crit ), linking these with recent star formation traced by hydrogen-alpha (H(α)) and far-ultraviolet (FUV) emission. ESO444--G084 supports localised star formation despite global stability, while KKS2000 23 is gravitationally unstable yet lacks strong H(α) emission, suggesting that turbulence, gas depletion, or past feedback suppresses star formation. The absence of detectable inflows or outflows implies that internal processes regulate star formation. This study highlights the interplay between morphology, kinematics, dark matter distribution, and disk stability, demonstrating how internal mechanisms shape dwarf galaxy evolution.

ABSTRACT During the fall of late-type galaxies into clusters, they can experiment a variety of evolutionary mechanisms according to their local environment. Consequently, studying the ultraviolet (UV) emission and the cold gas of late-type galaxies provide key insights in the evolution of short-lived starburst and galaxy quenching. In this work, we conducted a study of two 28 arcmin fields observed with Ultra Violet Imaging Telescope-AstroSat in the central region of the Abell cluster A496 ($z=0.033$), including H i data from National Radio Astronomy Observatory (NRAO) Very Large Array (VLA). We reported 22 cluster members detected in far-ultraviolet (FUV); all of them are detected in H i, or have upper limits for the H i-mass. We find our FUV detected galaxies generally have higher specific star formation rates (sSFRs) than other star-forming galaxies. Most of the FUV galaxies with masses above 10$^9$$\mathrm{M}_{\odot }$ and showing high sSFR have no close neighbours, pointing at RPS as the dominant mechanism affecting them. In contrast, most of the low-mass FUV objects present at least one companion, suggesting that tidal interactions also play an important role in the triggering of infalling galaxies. Combining the FUV–SFR with the H i properties of the observed galaxies in A496, we identify an evolutionary sequence consisting of five stages: (1) Pre-triggering, (2) Initial SF-triggering, (3) Peak of star-formation, (4) SF-fading, and (5) SF-quenching. During this path, normal gas-rich objects reach a gas-deficiency phase with SFR well below the main sequence. This process, prior to becoming a full passive galaxy, can be accomplished within a few 10$^{8}$ yr.

Abstract We present the variations in far-ultraviolet (FUV) and Hα star formation rates (SFR), SFRUV and SFRHα , respectively, at subkiloparsec scales in 11 galaxies as part of the Deciphering the Interplay between the Interstellar Medium, Stars, and the Circumgalactic medium survey. Using archival GALEX FUV imagery and Hα+[N ii] narrowband images obtained with the Vatican Advanced Technology Telescope, we detect a total of 1335 (FUV-selected) and 1474 (Hα-selected) regions of recent high-mass star formation, respectively. We find the Hα-to-FUV SFR ratios tend to be lower primarily for FUV-selected regions, where SFRHα generally underestimates the SFR by an average factor of 2–3, for SFR < 10−4 M ⊙ yr−1. In contrast, the SFRs are generally observed to be consistent for Hα-selected regions. This discrepancy arises from morphological differences between the two indicators. Extended FUV morphologies and larger areas covered by FUV-only regions, along with decreasing overlap between FUV clumps and compact H ii regions with R/R 25 suggest that stochastic sampling of the initial mass function may be more pronounced in the outer regions of galaxies. Our observed Hα-to-FUV SFR ratios are also consistent with stochastic star formation model predictions. However, using larger apertures that include diffuse FUV emission results in an offset of 1 dex between SFRHα and SFRUV, suggesting that the observed low Hα-to-FUV SFR ratios in galaxies are likely caused by diffuse FUV emission, which can contribute ∼60%–90% to the total FUV flux.

Abstract We study star formation rate (SFR) indicators and dust attenuation of 74 nearby star-forming galaxies on kiloparsec scales, based on GALEX far-ultraviolet (FUV) and WISE mid-infrared (MIR) images with CALIFA optical integral field spectroscopic data. We obtain hybrid SFR indicators by combining the observed FUV and MIR luminosities and calibrate them using the dust-corrected Hα luminosity as a reference SFR. The simple linear combination appears to follow well the reference SFR, but the calibration residual shows a significant dependence on the specific SFR (sSFR), which can be removed by employing the combination coefficient or conversion offset that varies with the sSFR. In the plane of gas versus stellar attenuation, the median trend line’s slope (≈stellar-to-gas attenuation ratio) changes from 0.44 to 1.0 with increasing attenuation. The differential attenuation, defined as the deviation of stellar attenuation from the median trend line, is strongly correlated with the SFR surface density and sSFR, compatible with the two-component dust model. The differential attenuation seems to be affected by both local and global factors.

Abstract UV-upturn galaxies are characterized by unusually excessive flux in the far-ultraviolet (FUV) band, observed in some elliptical galaxies and the bulges of disk galaxies. We examine UV-upturn galaxies within the semi-analytic model gabe, which embeds the formation of extreme horizontal branch (EHB) stars—proposed as key candidates responsible for the UV-upturn phenomenon. We have analyzed all related physical processes, including stellar evolution, initial mass functions (IMFs), dust attenuation, galaxy age, metallicity, and binary fractions, in an effort to determine which processes play significant roles. Our findings reveal two categories of UV-upturn galaxies in the semi-analytic model, each with distinct formation channels: old metal-rich quenched elliptical galaxies, which are intrinsic UV-upturn galaxies induced by EHB stars within their old stellar populations, and dusty star-forming galaxies, which are relatively young and may also be photometrically identified as UV-upturn galaxies when accounting for dust attenuation. Dust attenuation contributes to 20%-60% of the UV-upturn galaxies, depending on the specific dust attenuation models adopted. With the binary star formation model of EHB stars, both of these formation channels exhibit strong preferences for high stellar metallicity. The high-mass end slope of the IMFs is found to have a marginal effect, indicating that a universal IMF is adequate for studying the UV-upturn phenomenon.

The far-UV (FUV) reflectance of the state-of-the-art, broadband UV/optical/IR mirrors of XeF2-passivated LiF on Al (Al + XeLiF) is promising for future space telescope missions. To reach their potential, dependable cleaning procedures and storage methods for such reflective surfaces need to be developed. First Contact™ polymer (FCP) formulations have proven to be a reliable method for cleaning conventional mirror surfaces coated with oxides or bare metal and for protecting them in storage. We report here on studies of the cleaning and storage of Al + XeLiF samples using customized FCP formulations designed by Photonic Cleaning Technologies. Cleaning of such mirrors is demanding since fluoride coatings are softer than oxides and can be moisture sensitive. Any damage that marks the overcoat can lead to catastrophic loss of FUV reflectance due to surface roughening and formation of aluminum oxide, which is FUV opaque. We discovered that one formulation could be successfully applied to and removed from Al + XeLiF coatings multiple times. The coatings retained low roughness, minimal aluminum oxide thickness, and high far-UV reflectance. Another of the four FCP formulations successfully cleaned the Al + XeLiF coatings several times. Variable-angle, spectroscopic ellipsometry, tapping-mode atomic force microscopy, x-ray photoelectron spectroscopy, and FUV reflectance allowed us to observe any changes in reflectance and surface roughness, the formation of aluminum oxide, and damage to coating integrity. From the studies of the range of FCP-fluoride interactions, we noted that too much polymer-to-surface adhesion or exposure to trace water in the polymer can result in coating damage.

ABSTRACT We present magnetohydrodynamic simulations of star formation in the multiphase interstellar medium (ISM) to quantify the impact of non-ionizing far-ultraviolet (FUV) radiation within the Silcc Project simulation framework. Our study incorporates the radiative transfer of ionizing radiation and self-consistent modelling of variable FUV radiation from star clusters, advancing beyond previous studies using static or simplified FUV fields. This enables a more accurate capture of the dynamic interaction between radiation and the evolving ISM alongside other stellar feedback channels. The interstellar radiation field (ISRF) near young star clusters can reach $G_0 \approx 10^4$ (in Habing units), far exceeding the solar neighbourhood value of $G_0 = 1.7$. Despite these high intensities, FUV radiation minimally impacts the integrated star formation rate compared to ionizing radiation, stellar winds, and supernovae. A slight reduction in star formation burstiness is linked to increased photoelectric (PE) heating efficiency by the variable FUV field. Dust near star-forming regions can be heated up to 60 K via the PE effect, with a broad temperature distribution. PE heating rates in variable FUV models exhibit higher peaks but lower averages than static ISRF models. Simulations under solar neighbourhood conditions without stellar winds or ionizing radiation but with supernovae yield unexpectedly high star formation rates of $\sim 0.1~\mathrm{M_\odot ~yr^{-1}~kpc^{-2}}$. Our analysis reveals increased cold neutral medium volume-filling factors (VFF) outside stellar clusters, reduced thermally unstable gas, and sharper warm–cold gas separation. The variable FUV field also promotes a cold diffuse gas phase with a molecular component, exhibiting a VFF of $\sim 5{-}10$ per cent.

Understanding the molecular gas content in the interstellar medium (ISM) is crucial for studying star formation and galaxy evolution. The CO-to-H2 (XCO) and the [CI]-to-H2 (XCI) conversion factors are widely used to estimate the molecular mass content in galaxies. However, these factors depend on many environmental parameters in the ISM, such as metallicity, cosmic-ray ionization rate, and far-ultraviolet (FUV) radiation field, in particular, in the low-metallicity ISM that is found at large galactocentric radii and in early-type galaxies. This work investigates the dependence of XCO and XCI on the environmental parameters of the ISM, with a focus on the low-metallicity α-enhanced ISM ([C/O] < 0), to provide improved tracers of molecular gas under diverse conditions. We used the statistical algorithm PDFCHEM, coupled with a database of photodissociation region (PDR) models generated with the 3D-PDR astrochemical code. The models account for a wide range of metallicities, dust-to-gas mass ratios, FUV intensities, and cosmic-ray ionization rates. The conversion factors were computed by integrating the PDR properties over log-normal column density distributions (AV-PDFs) that represent various cloud types. The XCO factor increases significantly with decreasing metallicity. It exceeds ∼1000 times the Galactic value at [O/H] = −1.0 under α-enhanced conditions, as opposed to ∼300 times under non-α-enhanced conditions ([C/O] = 0). In contrast, XCI varies more gradually with metallicity, which makes it a more reliable tracer of molecular gas in metal-poor environments under most conditions. The fraction of CO-dark molecular gas increases dramatically in low-metallicity regions, where it exceeds 90% at [O/H] = −1.0, in particular, in diffuse clouds and environments with strong FUV radiation fields. The results highlight the limitations of CO as a molecular gas tracer in the metal-poor ISM and demonstrate the potential of [CI] (1–0) as a complementary tracer. The use of metallicity-dependent XCO and XCI factors as provided by this study is recommended for accurately estimating molecular gas masses in diverse environments. We recommend the use of the log10 XCO ≃ −2.41 Z + 41.3 relation for the CO-to-H2 conversion factor and the log10 XCI ≃ −0.99 Z + 29.7 relation for the [CI]-to-H2 conversion factor, where Z = 12 + log10(O/H).

Abstract We present the star formation histories (SFHs) of 10 metal-poor (≲12% Z ⊙), star-forming dwarf galaxies from the Local Ultraviolet to Infrared Treasury survey. The derived SFHs exhibit significant variability, consistent with the irregular star formation expected for dwarf galaxies. Using synthetic near-ultraviolet (UV) and optical color–magnitude diagrams (CMDs) with various yet targeted configurations for dust and input SFHs, we quantitatively demonstrate that simultaneous modeling of the UV and optical CMDs (“UVopt” case) improves the precision of SFH measurements in recent time bins up to ∼1 Gyr, compared to the classical single optical CMD modeling (“Opt-only” case). The UVopt case reduces uncertainties relative to the Opt-only case by ∼4%–8% over the past 10 Myr, ∼8%–20% over 100 Myr, and ∼8%–14% over 1 Gyr, across various dust configurations and input SFHs. Additionally, we demonstrate discrepancies in stellar models for blue core helium-burning (BHeB) stars at the low-metallicity regime. This discrepancy can artificially inflate star formation rate (SFR) estimates in younger age bins by misinterpreting the evolved BHeB stars as reddened upper main-sequence (MS) stars. Incorporating UV data improves BHeB-MS separation and mitigates the limitations of current low-metallicity stellar models. Comparisons of the UVopt SFHs with Hα and far-UV (FUV)-based SFRs reconfirm that Hα is an unreliable tracer over its nominal 10 Myr timescale for low-SFR galaxies, while FUV provides a more reliable tracer but yields SFRFUV values up to twice those of CMD-based 〈SFR〉100 Myr. Our findings underscore the importance of UV data in refining recent SFHs in low-metallicity environments.