Far-ultraviolet Research Articles

A novel method for estimating the far-ultraviolet flux, and a catalogue for star-hosting discs in nearby star-forming regions

When protoplanetary discs are externally irradiated by far-ultraviolet (FUV) photons from OBA-type stars, they lose material through photoevaporative winds. This reduces the amount of material that is available to form planets. Understanding the link between the environmental irradiation and the observed disc properties requires accurately evaluating the FUV flux at star-hosting discs, which can be challenging because of the uncertainty in stellar parallax. We addressed this issue by proposing a novel approach: using the local density distribution of a star-forming region (i.e. 2D pairwise star separation distribution) and assuming isotropy, we inferred the 3D separation between star-hosting discs and massive stars. We tested this approach on synthetic clusters and showed that it significantly improves accuracy compared to previous methods. We computed the FUV fluxes for numerous star-bearing discs in seven regions within ∼200 pc, six regions in Orion and in Serpens sub-regions. We provided a publicly accessible catalogue. We found that discs in regions hosting late-type B and early-type A stars can reach non-negligible FUV radiation levels for the disc evolution (10-100 mathrm G ). We investigated dust disc masses relative to FUV fluxes and detected indications of a negative correlation when we restricted the investigation to average region ages. However, we emphasize the need for more stellar and disc measurements at >$10^ G $ to probe the dependence of disc properties on environmental irradiation. The method presented in this work is a powerful tool that can be expanded to additional regions.

Time-resolved Hubble Space Telescope UV Observations of an X-Ray Quasiperiodic Eruption Source

Abstract X-ray quasiperiodic eruptions (QPEs) are a novel mode of variability in nearby galactic nuclei whose origin remains unknown. Their multiwavelength properties are poorly constrained, as studies have focused almost entirely on the X-ray band. Here, we report on time-resolved, coordinated Hubble Space Telescope far-ultraviolet (FUV) and XMM-Newton X-ray observations of the shortest period X-ray QPE source currently known, eRO-QPE2. We detect a bright UV point source (L FUV ≈ few × 1041 erg s−1) that does not show statistically significant variability between the X-ray eruption and quiescent phases. This emission is unlikely to be powered by a young stellar population in a nuclear stellar cluster. The X-ray-to-UV spectral energy distribution can be described by a compact accretion disk ( R out = 34 3 − 138 + 202 R g ). Such compact disks are incompatible with typical disks in active galactic nuclei, but form naturally following the tidal disruption of a star. Our results rule out models (for eRO-QPE2) invoking (i) a classic active galactic nucleus accretion disk and (ii) no accretion disk at all. For orbiter models, the expected radius derived from the timing properties would naturally lead to disk-orbiter interactions for both quasi-spherical and eccentric trajectories. We infer a black hole mass of log10(M BH) = 5.9 ± 0.3 M ⊙ and an Eddington ratio of 0.13 − 0.07 + 0.18 ; in combination with the compact outer radius, this is inconsistent with existing disk instability models. After accounting for the quiescent disk emission, we constrain the ratio of X-ray to FUV luminosity of the eruption component to be L X/L FUV > 16−85 (depending on the intrinsic extinction).

GOLD Observations of Thermospheric Neutral Temperature Variability During the 14 October 2023 Annular Solar Eclipse

AbstractThe 14 October 2023 annular solar eclipse was visible from the US Pacific coast to Brazil's east coast. NASA's Global‐scale Observations of Limb and Disk (GOLD) mission observed the first synoptic thermospheric temperature changes from a geo‐stationary orbit above 47.5°W longitude between 17 and 20 UT during the eclipse. These daytime thermospheric changes were derived using GOLD's disk far ultraviolet (FUV) measurements. A significant decrease in the daytime disk temperatures (∼100 K) was seen near the peak annularity compared to the day before (baseline). The temperature reduction's spatial morphology is also like that of the eclipse shadow. Previous modeling studies of other eclipses typically show a much smaller temperature decrease (∼30–40 K; a factor of 2–3 lower) compared to GOLD observations. These first of kind results provide new insight into the dynamic response of the coupled thermosphere and ionosphere system to transient solar events, including eclipses.

The Effect of Stellar Evolution on Dispersal of Protoplanetary Disks: Disk Fraction in Star-forming Regions

Abstract We study the effect of stellar evolution on the dispersal of protoplanetary disks by performing one-dimensional simulations of long-term disk evolution. Our simulations include viscous disk accretion, magnetohydrodynamic winds, and photoevaporation as important disk dispersal processes. We consider a wide range of stellar mass of 0.1–7 M ⊙ and incorporate the luminosity evolution of the central star. For solar-mass stars, stellar evolution delays the disk dispersal time as the far-ultraviolet (FUV) luminosity decreases toward the main sequence. In the case of intermediate-mass stars, the FUV luminosity increases significantly over a few million years, driving strong photoevaporation and enhancing disk mass loss during the later stages of disk evolution. This highlights the limitations of assuming a constant FUV luminosity throughout a simulation. Photoevaporation primarily impacts the outer regions of the disk and is the dominant disk dispersal process in the late evolutionary stage. Based on the results of a large set of simulations, we study the evolution of a population of star–disk systems and derive the disk fraction as a function of time. We demonstrate that the inclusion of stellar luminosity evolution can alter the disk fraction by several tens of percent, bringing the simulations into closer agreement with recent observations. We argue that it is important to include the stellar luminosity evolution in simulations of the long-term dispersal of protoplanetary disks.

A Be star that turned bright and blue during a major outburst

Classical Be stars are rapidly rotating B-type stars exhibiting Balmer emission lines originating from circumstellar Keplerian gaseous disks, which likely form through episodic mass-ejection events. The currently favored model for Be star disks is the viscous decretion disk (VDD) model. However, the mechanism behind the mass ejection process during the formation of a VDD is a mystery. We present peculiar behavior of the Be star KIC 9715425, which exhibited several minor outbursts (MIBs) and one major outburst (MAB) observed as brightenings in broadband photometry, as well as transient H line emission. The variability may provide valuable keys to understanding the nature of Be outbursts. Based on Kepler the All-Sky Automated Survey for Supernovae (ASAS-SN) and the Transiting Exoplanet Survey Satellite (TESS) time-series photometry, the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) spectroscopy, multi-wavelength observations in the near-ultraviolet (NUV) from the Galaxy Evolution Explorer (GALEX) and optical from Gaia Xinglong 2.16m telescope and Isaac Newton Telescope (INT) of KIC,9715425 covering its outbursts, we determined fundamental parameters of the central star and the circumstellar disk. A frequency analysis and spectral modeling were carried out to characterize the events. During the major outburst, we find a 36% flux increase in the GALEX NUV band compared to 25% in the Kepler band, suggesting that whatever produced the flux increase should be hotter than the central B-type star. This is contradictory to the conventional scenario that the VDD should be cooler. In addition, such a high flux increase can only be accounted for by a luminous disk, which should produce a much stronger Hα emission than observed. The origin of this anomalously hot component remains unexplained. Except for the bluing, the available observations of KIC 9715425 are perfectly compatible with being a normal Be star. If the bluing was physically related to the MAB, the most economical effort to identify the nature of the underlying process could be adaptive NUV and far-ultraviolet (FUV) monitoring of Be stars with cyclically repeating MABs.

The Blue Lurker WOCS 14020: A Long-period Post-common-envelope Binary in M67 Originating from a Merger in a Triple System

Abstract We present Hubble Space Telescope far-ultraviolet (FUV) spectra of a blue lurker–white dwarf (BL–WD) binary system in the 4 Gyr open cluster M67. We fit the FUV spectrum of the WD, determining it is a C/O WD with a mass of 0.7 2 − 0.04 + 0.05 M ⊙ and a cooling age of ~400 Myr. This requires a WD progenitor of ~3 M ⊙, significantly larger than the current cluster turnoff mass of 1.3 M ⊙. We suggest the WD progenitor star formed several hundred megayears ago via the merger of two stars near the turnoff of the cluster. In this scenario, the original progenitor system was a hierarchical triple consisting of a close, near-equal-mass inner binary, with a tertiary companion with an orbit of a few thousand days. The WD is descended from the merged inner binary, and the original tertiary is now the observed BL. The likely formation scenario involves a common envelope while the WD progenitor is on the AGB, and thus the observed orbital period of 359 days requires an efficient common envelope ejection. The rapid rotation of the BL indicates it accreted some material during its evolution, perhaps via a wind prior to the common envelope. This system will likely undergo a second common envelope in the future and thus could result in a short-period double WD binary or merger of a 0.72 M ⊙ C/O WD and a 0.38 M ⊙ helium WD, making this a potential progenitor of an interesting transient such as a sub-Chandrasekhar Type Ia supernova.

Ultraviolet Photometry and Habitable Zones of over 2700 Planet-hosting Stars

The ongoing discovery of exoplanets has sparked significant interest in finding suitable worlds that could potentially support life. Stellar ultraviolet (UV; 100–3000 Å) radiation may play a crucial role in determining the habitability of their planets. In this paper, we conducted a detailed analysis of the UV photometry of over 2700 host stars with confirmed planets, using observational data from the Galaxy Evolution Explorer and Swift Ultraviolet/Optical Telescope missions. We performed aperture photometry on single-exposure images and provided photometric catalogs that can be used to explore a wide range of scientific questions, such as stellar UV activity and planet habitability. By calculating the circumstellar habitable zone and UV habitable zone, we found that fewer than 100 exoplanets fall within both of these zones, with the majority being gas giants. We also examined stellar activity based on their far-UV (FUV) and near-UV (NUV) emissions. We found the FUV−NUV color more effectively represents stellar activity compared to the RFUV′ and RNUV′ indices. The Sun’s low FUV emission and moderate NUV emission highlight its uniqueness among (solar-like) stars.

Uranus’ hydrogen upper atmosphere: Insights from pre- and post-equinox HST Lyman-α images

We present the first spatially resolved images of Lyman-α (Lyα) emissions from Uranus taken by the Hubble Space Telescope (HST). The observations were carried out using HST’s Space Telescope Imaging Spectrograph instrument as part of two far-ultraviolet (FUV) observing campaigns in 1998 and 2011, before and after Uranus’ equinox in 2007. The average intensities (± uncertainties) on Uranus’ disk were 860 ± 6 and 725 ± 9 R, respectively. The images reveal widely extended emissions, detectable up to ~4 Uranus radii (RU). We performed simulations of the Lyα radiative transfer in the atmosphere, considering resonant scattering by H, Rayleigh scattering by H2, and absorption by CH4. We considered only solar Lyα fluxes at Uranus as the Lyα source for simulations. The effects of hydrogen in the interplanetary medium and Earth’s exosphere on Uranus’ Lyα emissions were taken into account. We find a good agreement between on-disk brightnesses from simulations and the HST observations assuming the (H, H2, and CH4) atmosphere profile derived from Voyager 2 measurements. Only slight adjustments of the H or H2 densities were required in some of the simulation cases, in particular, for the 1998 observations. To match the off-disk HST brightnesses in both years, a substantial exosphere of gravitationally bound hot H is required, which we modelled assuming the hot H number density has a Chapman profile. We find that compared to 1998, the hot H abundance required for 2011 is lower and the inferred hot H profiles seem to be more extended. This bound hot H is likely to be a persistent part of Uranus’ upper atmosphere and is distinct from the escaping hot H population derived from Voyager 2 observations. We discuss the possible production mechanisms involving solar EUV radiation and study the sensitivity of the modelled brightness to the parameters of the hot H profile. We find that solar EUV radiation is not a sufficient source to explain the hot H in the exosphere of Uranus.

Rising Near-ultraviolet Spectra in Stellar Megaflares

Flares from M dwarf stars can attain energies up to 104 times larger than solar flares but are generally thought to result from similar processes of magnetic energy release and particle acceleration. Larger heating rates in the low atmosphere are needed to reproduce the shape and strength of the observed continua in stellar flares, which are often simplified to a blackbody model from the optical to the far-ultraviolet (FUV). The near-ultraviolet (NUV) has been woefully undersampled in spectral observations despite this being where the blackbody radiation should peak. We present Hubble Space Telescope NUV spectra in the impulsive phase of a flare with E TESS ≈ 7.5 × 1033 erg and a flare with E TESS ≈ 1035 erg and the largest NUV flare luminosity observed to date from an M star. The composite NUV spectra are not well represented by a single blackbody that is commonly assumed in the literature. Rather, continuum flux rises toward shorter wavelengths into the FUV, and we calculate that an optical T = 104 K blackbody underestimates the short-wavelength NUV flux by a factor of ≈6. We show that rising NUV continuum spectra can be reproduced by collisionally heating the lower atmosphere with beams of E ≳ 10 MeV protons or E ≳ 500 keV electrons and flux densities of 1013 erg cm−2 s−1. These are much larger than the canonical values describing accelerated particles in solar flares.

Tracing hierarchical star formation out to kiloparsec scales in nearby spiral galaxies with UVIT

abstract Molecular clouds fragment under the action of supersonic turbulence and gravity, which results in a scale-free hierarchical distribution of star formation within galaxies. Recent studies suggest that the hierarchical distribution of star formation in nearby galaxies shows a dependence on host galaxy properties. In this context, we study the hierarchical distribution of star formation from a few tens of parsecs up to several kiloparsecs in four nearby spiral galaxies: NGC 1566, NGC 5194, NGC 5457, and NGC 7793, by leveraging large-field-of-view and high-resolution far-ultraviolet (FUV) and near-ultraviolet (NUV) observations from the UltraViolet Imaging Telescope (UVIT). Using the two-point correlation function, we infer that the young star-forming clumps (SFCs) in the galaxies are arranged in a fractal-like hierarchical distribution, but only up to a maximum scale. This largest scale of hierarchy ($l_ corr $) is ubiquitous in all four galaxies and ranges from 0.5 kpc to 3.1 kpc. The flocculent spiral NGC 7793 has roughly five times smaller $l_ corr $ than the other three grand design spirals, possibly due to its lower mass, lower pressure environment, and a lack of strong spiral arms. $l_ corr $ being much smaller than the galaxy size suggests that the star formation hierarchy does not extend to the full galaxy size and it is likely an effect set by multiple physical mechanisms in the galaxy. The hierarchical distribution of SFCs dissipates almost completely within 10$-$50 Myr in our galaxy sample, signifying the migration of SFCs away from their birthplaces with increasing age. The fractal dimension of the hierarchy for our galaxies is found to be between 1.05 and 1.50. We also find that depending upon the star formation environment, significant variations can exist in the local and global hierarchy parameters of a galaxy. Overall, our results suggest that the global hierarchical properties of star formation in galaxies are not universal. This study also demonstrates the capabilities of UVIT in characterising the star formation hierarchy in nearby galaxies. In the future, a bigger sample can be employed to better understand the role of large-scale galaxy properties such as morphology and environment as well as physical processes like feedback, turbulence, shear, and interstellar medium conditions in determining the non-universal hierarchical properties of star formation in galaxies. abstract

Analysis of FUV Emission in the Vicinity of R136 Cluster in 30 Doradus

Abstract The intense diffuse radiation of galactic systems in the far ultraviolet (FUV) range is primarily due to dust scattering, with properties such as albedo, scattering phase function, and optical depth playing a crucial role. Utilizing observed data from the Far Ultraviolet Spectroscopic Explorer (FUSE) telescope, we investigated dust-scattered diffuse FUV emission from the 30 Doradus nebula in the Large Magellanic Cloud (LMC), offering insight into interstellar dust properties essential for understanding starburst galaxies. Employing a simple spherical shell model in the Monte Carlo based radiative transfer tool SKIRT, we simulated the dust-scattered FUV emissions, and compared them with the observed data. We predicted an excess of FUV photons being scattered near the star cluster R136 (formerly RMC 136) and ascertained a robust linear correlation between observations and model. Additionally, our best-fit model predicted the hydrogen column density near R136 to be approximately ~ 4.36 × 1021cm−2, corresponding to a color excess of E(B − V ) = 0.2, with the contribution of diffuse scattered FUV emissions estimated at 13% of the total radiation. This model can be further extended to better understand dust properties of similar starburst environments in the local universe and beyond.

CLASSY. XI. Tracing Neutral Gas Properties Using UV Absorption Lines and 21 cm Observations* ∗ Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

Rest-frame far-ultraviolet (FUV) observations from JWST are revolutionizing our understanding of the high-z galaxies that drove reionization and the mechanisms by which they accomplished it. To fully interpret these observations, we must be able to diagnose how properties of the interstellar medium (ISM; e.g., column density, covering fraction, and outflow velocity) directly relate to the absorption features produced. Using the high-signal-to-noise and high-resolution FUV spectra of 45 nearby star-forming galaxies from the Cosmic Origins Spectrograph Legacy Spectroscopic Survey, we present the largest uniform, simultaneous characterization of neutral and low-ionization state (LIS) interstellar UV absorption lines (O i, Si ii, S ii, C ii, and Al ii) across a wide range of galaxy properties. We also present 21 cm H i observations for 35 galaxies, multiple of which are gas-poor or nondetected, possibly indicating the onset of a post-starburst phase. We find that our simultaneous one-component Voigt profile fits are capable of accurately modeling the LIS absorption for ∼75% of galaxies, mitigating challenges associated with saturation, infilling, and degeneracies. While the most massive galaxies require additional components, our one-component fits return average properties of the absorbing gas and follow the scaling relations described by a single gas cloud. We explore connections between LIS absorption and direct tracers of the neutral ISM (O i, Lyα, and H i 21 cm), finding that C ii most closely traces the neutral gas trends, while other ions exhibit weaker correlations. Given the challenges with directly observing H i at higher-z, we demonstrate that LIS absorption can be a powerful means to study the neutral ISM and present empirical relationships for predicting neutral gas properties.

UVIT Study of the MAgellanic Clouds (U-SMAC). II. A Far-UV Catalog of the Small Magellanic Cloud: Morphology and Kinematics of Young Stellar Population

The Small Magellanic Cloud (SMC) is an irregular dwarf galaxy that has recently undergone an interaction with the Large Magellanic Cloud. The young massive stars in the SMC formed in the disturbed low-metallicity environment are important targets in astrophysics. We present a catalog of ∼76,800 far-ultraviolet (FUV) sources toward the SMC detected using the Ultra Violet Imaging Telescope onboard AstroSat. We created an FUV catalog with ∼62,900 probable SMC members which predominantly comprise main-sequence, giant, and subgiant stars. We selected four young populations (Young 1, Young 2, Young 3, and Blue Loop (BL) stars) identified from the Gaia optical color–magnitude diagram to study the morphology and kinematics of the young SMC using this catalog. We detect a clumpy morphology with a broken bar, a shell-like structure, and the inner SMC Wing for the four stellar populations. The eastern region and the northeastern regions are mainly populated by Young 1, 2, and 3 stars. The central region predominantly has the Young 2 and 3 populations, whereas the SW has BL stars, and Young 2 and 3 stars. The 2D kinematic study using proper motion (PM) reveals that Young 2 and 3 populations show two kinematically distinct subpopulations with low and high PM dispersion, whereas the Young 1 and BL stars show two kinematically distinct populations with low dispersion. Our analysis points to a kinematic disturbance along the R.A. direction for stars younger than ∼150 Myr located in the eastern region, with no significant disturbance along the decl.

Revealing the Elusive Companion of the Red Giant Binary 2MASS J05215658+4359220 from UV HST and Astrosat/UVIT Data

Abstract Black hole (BH) demographics in different environments is critical in view of recent results on massive star binarity, and of the multimessenger detectability of compact object mergers. But the identification and characterization of noninteracting BHs are elusive, especially in the sparse field stellar population. A candidate noninteractive BH + red giant (RG) binary system, 2MASS J05215658+4359220, was identified by T. A. Thompson et al. We obtained Astrosat/UVIT far-ultraviolet (FUV) imaging and Hubble Space Telescope (HST) UV−optical imaging and spectroscopy of the source to test possible scenarios for the optically elusive companion. HST/STIS spectra from ≈1600 to 10230 Å are best fit by the combination of two stellar sources, a RG with T eff = 4250 ± 150 K, log g = 2.0, R RG ∼ 27.8 R ⊙ (assuming a single-temperature atmosphere), and a subgiant companion with T eff = 6000 K, R comp = 2.7 R ⊙, or T eff = 5270 K, R comp = 4.2 R ⊙ using models with one-tenth or one-third solar metallicity, respectively, log g = 3.0, extinction E B−V = 0.50 ± 0.2, adopting the Data Release 3 Gaia distance D = 2463 ± 120 pc. No FUV data existed prior to our programs. STIS spectra give an upper limit of 10−17 erg cm−2 s−1 Å−1 shortwards of 2300 Å; an upper limit of ≳25.7 ABmag was obtained in two UVIT FUV broad bands. The nondetection of FUV flux rules out a compact companion such as a hot white dwarf. The STIS spectrum shows strong Mg ii λ2800 Å emission, typical of chromospherically active RGs. The masses inferred by comparison with evolutionary tracks, ∼1M ⊙ for the RG and between 1.1 and 1.6 M ⊙ for the subgiant companion, suggest past mass transfer, although the RG currently does not fill its Roche lobe. WFC3 imaging in F218W, F275W, F336W, F475W, and F606W shows an unresolved source in all filters.

Molecular Hydrogen Line Identifications in Solar Flares Observed by IRIS: Lower Atmospheric Structure from Radiometric Analysis

A rich spectrum of molecular hydrogen (H2) emission lines is seen in sensitive observations from the far-ultraviolet (FUV) channels of the Interface Region Imaging Spectrograph (IRIS) during flare activity in solar active region NOAA Active Region 11861. Based on this observation, we have determined 37 new line identifications by comparing synthetic spectra produced using 1D modeling of H2 fluorescence. To avoid misidentification of the H2 lines, we have also compiled a complete list of atomic line identifications for the IRIS FUV bandpasses from previous work. We carry out analysis of the spatially resolved H2 emission that occurs during the flares and find the following: (1) in spatially resolved observations the H2 line ratios may show optically thick line formation, contrary to previous results; (2) comparison of the spatial distribution of H2 Doppler velocities with those measured from other species reveals that H2 remote sensing probes an intermediate depth in the atmosphere between the photosphere and chromosphere, consistent with expectations from modeling; (3) the relationship between H2 line intensity and the observed intensity of its exciter is related to the atmospheric stratification; however, (4) H2 fluorescence can sometimes occur in response to radiation from distant sources many megameters away across the solar surface.

Predicting Photospheric Ultraviolet Emission from Stellar Evolutionary Models

Stellar ultraviolet (UV) emission serves as a crucial indicator for estimating magnetic activity and evaluating the habitability of exoplanets orbiting stars. In this paper, we present a straightforward method to derive stellar photospheric UV emission for F to M main-sequence stars. By using PARSEC models, we establish relations between near-UV (NUV) and far-UV (FUV) magnitudes from the Galaxy Evolution Explorer, NUV magnitudes from the China Space Station Telescope, and stellar effective temperatures and Gaia blue photometer–red photometer colors for different metallicities. Together with the observed sample, we find that for NUV emission, the photospheric contribution to the observed flux is less than 20% for M stars, around 10%–70% for K stars, and in a range from 30% to 85% for G and F stars. For FUV emission, the photospheric contribution is less than 10−6 for M stars, below 10−4 for K stars, around 10−4–10% for G stars, and between 6% and 50% for F stars. Our work enables the simple and effective determination of stellar excess UV emission and the exploration of magnetic activity.

Ultraviolet Radiation Fields in Star-forming Disk Galaxies: Numerical Simulations with TIGRESS-NCR

With numerical simulations that employ adaptive ray-tracing (ART) for radiative transfer at the same time as evolving gas magnetohydrodynamics, thermodynamics, and photochemistry, it is possible to obtain a high-resolution view of ultraviolet (UV) fields and their effects in realistic models of the multiphase interstellar medium. Here, we analyze results from TIGRESS-NCR simulations, which follow both far-UV (FUV) wavelengths, important for photoelectric heating and polycyclic aromatic hydrocarbon excitation, and the Lyman continuum (LyC), which photoionizes hydrogen. Considering two models, representing solar neighborhood and inner-galaxy conditions, we characterize the spatial distribution and time variation of UV radiation fields, and quantify their correlations with gas. We compare four approximate models for the FUV to simulated values to evaluate alternatives when full ART is infeasible. By convolving FUV radiation with density, we produce mock maps of dust emission. We introduce a method to calibrate mid-IR observations, for example from JWST, to obtain high-resolution gas surface density maps. We then consider the LyC radiation field, finding most of the gas exposed to this radiation to be in ionization–recombination equilibrium and to have a low neutral fraction. Additionally, we characterize the ionization parameter as a function of the environment. Using a simplified model of the LyC radiation field, we produce synthetic maps of emission measure (EM). We show that the simplified model can be used to extract an estimate of the neutral fraction of the photoionized gas and mean free path of ionizing radiation from observed EM maps in galaxies.

Vuv/vis absorption spectroscopy of different PAHs

The present study provides new solid-phase absorption data of different Polycyclic Aromatic Hydrocarbon (PAHs) molecules in the Far Ultraviolet (FUV) and UV/Visible regions. Two features of interstellar extinction curves fall in this region: the FUV rise and the UV bump. Here, thin films of PAHs were prepared by means of the spin coating method onto a transparent substrate, and their solid phase absorption spectra were recorded. Spectra of isolated molecules simulated by Time-Dependent Density Functional Theory (TD-DFT) calculations were also produced for comparison. The results fit quite well with the FUV rise but not with the UV bump. Given the scarcity of FUV experimental absorption data of PAHs in the literature and especially in solid phase, these results are also interesting for reference for other studies ranging from thin films applications to interstellar medium models.

Comparison of terrestrial exospheric hydrogen 3D distributions at solar minimum and maximum using TWINS Lyman-α observations

Remote sensing observations of far-ultraviolet (FUV) emissions have been used to estimate 3D neutral hydrogen (H) density models of the terrestrial exosphere under solar minimum (2008) and solar maximum (2013 and 2015) conditions. Specifically, we used Lyman-alpha (Lyman-α, FUV at 121.6 nm) radiance data acquired by the Lyman-alpha detectors (LADs) onboard NASA’s TWINS satellites, collected over several days for each of the 3 years to provide sufficient coverage of the exospheric region. The datasets included Lyman-α measurements taken only above Earth radii (Re) of 3.75, assumed to be the optically thin region of the exosphere, where the measured Lyman-α intensity along a line of sight (LOS) is linearly proportional to the atomic hydrogen column density. Based on the calibration using multiple UV-bright stars, a significant decrease in TWINS1 LAD1/2 sensitivities was found for 2013 (LAD2 ∼1/2, LAD ∼1/3) and 2015 (LAD2 ∼1/3, LAD ∼1/7) compared to 2008. The calibration uncertainty was derived to be, on average, 5%. We estimated the 3D global hydrogen density distributions from these radiance data using two different tomographic inversion methods: first, a parametric fitting method based on a spherical harmonic function of order 3 and second, a high degree-of-freedom retrieval approach to validate our retrievals of H density. Both inversion methods consistently incorporate the effects of Lyman-α absorption within the exosphere and Lyman-α re-emission from Earth’s albedo. Our results reveal that H densities during solar maximum conditions are, on average, 30%–40% higher than those during solar minimum. All models showed a high concentration of atomic H on the dayside and nightside near the Sun–Earth line, which determines a nose/geotail structure consistent with theoretical effects from the solar radiation pressure. Furthermore, we identified that H-density enhancements during solar maximum with respect to solar minimum conditions occur at mid-to-high latitudes, particularly on the dusk side, while no significant enhancement seems to occur near the dayside nose.

Exploring Filament Galaxies Using AstroSat/UVIT

Abstract We present results from our deep far-ultraviolet (FUV) survey using AstroSat/UVIT of a filamentary structure at z ∼0.072. A total of four filaments comprising 58 galaxies were probed in our study. We detect 18 filament galaxies in our FUV observation. All filament galaxies are further classified based on their photometric color, nuclear activity, and morphology. The filaments contain galaxies with mixed stellar population types and structures. We do not detect galaxies in our UVIT survey up to a distance of 0.4 Mpc h −1 from the filament axis, implying a lack of recent star formation in the inner region of filaments. The FUV star formation rate (SFR) for star-forming galaxies agrees well with the SFR144MHz calculated using Low-Frequency Array radio-continuum observations. We witness an increase in the FUV specific-SFR (sSFR) of filament galaxies with increasing distance from the filament spine (D fil). The intermediate-to-high stellar mass filament galaxies were more star-forming than cluster galaxies in a fixed stellar mass bin. The FUV morphology of some filament galaxies detected in the filament outskirts (D fil ≳ 0.7 Mpc h −1) is comparable to or slightly extended than their optical counterpart. The mass assembly of galaxies examined by estimating (FUV − r) color gradients shows that more “red-cored’ galaxies reside in the outer region of the filaments. Our results prove that the likelihood of merger interaction and gas starvation increases when approaching the filament spine. We report a definitive and inhomogeneous impact of filaments on the galaxies residing inside them.