Far-ultraviolet Research Articles

Using Multiwavelength Variability to Explore the Connection among X-Ray Emission, the Far-ultraviolet H2 Bump, and Accretion in T Tauri Stars

Abstract The high-energy radiation fields of T Tauri stars (TTS) should affect the surrounding circumstellar disk, having implications for disk transport and heating. Yet observational evidence of the effect of high-energy fields on disks is scarce. Here we investigate the connection between X-ray emission and the innermost gas disk by leveraging the variability of TTS. We obtained multiple epochs of coordinated data (taken either simultaneously or within a few hours) of accreting TTS with the Hubble Space Telescope, the Neil Gehrels Swift Observatory, and the Chandra X-ray Observatory. We measured the far-ultraviolet (FUV) H2 bump feature at 1600 Å, which traces gas <1 au from the star; the near-ultraviolet emission, from which we extract the accretion luminosity; and also the X-ray luminosity. We do not find a correlation between the FUV H2 bump and X-ray luminosity. Therefore, an observable tracer of the effect of X-ray ionization in the innermost disk remains elusive. We report a correlation between the FUV H2 bump and accretion luminosity, linking this feature to the disk surface density. We also see a correlation between the X-ray luminosity and the accretion column density, implying that flaring activity may influence accretion. These results stress the importance of coordinated multiwavelength work to understand TTS.

Optical design for CETUS: a wide-field 1.5-m aperture UV payload being studied for a NASA probe class mission study

As part of a study funded by NASA headquarters, we are developing a probe-class mission concept called the Cosmic Evolution through UV Spectroscopy (CETUS). CETUS includes a 1.5-m aperture diameter telescope with a large field of view (FOV). CETUS includes three scientific instruments: a far ultraviolet (FUV) and near ultraviolet (NUV) imaging camera (CAM); a NUV multiobject spectrograph (MOS); and a dual-channel point/slit spectrograph (PSS) in the Lyman ultraviolet (LUV), FUV, and NUV spectral regions. The large FOV three-mirror anastigmatic (TMA) optical telescope assembly (OTA) simultaneously feeds the three separate scientific instruments. That is, the instruments view separate portions of the TMA image plane, enabling parallel operation by the three instruments. The field viewed by the MOS, whose design is based on an Offner-type spectrographic configuration to provide wide FOV correction, is actively configured to select and isolate numerous field sources using a next-generation micro-shutter array. The two-channel CAM design is also based on an Offner-like configuration. The PSS performs high spectral resolution spectroscopy on unresolved objects over the NUV region with spectral resolving power, R ∼ 40,000, in an echelle mode. The PSS also performs long-slit imaging spectroscopy at R ∼ 20,000 in the LUV and FUV spectral regions with two aberration-corrected, blazed, holographic gratings used in a Rowland-like configuration. The optical system also includes two fine guidance sensors, and wavefront sensors that sample numerous locations over the full OTA FOV. In-flight wavelength calibration is performed by a wavelength calibration system, and flat-fielding is also performed, both using in-flight calibration sources. We describe the current optical design of CETUS and the major trade studies leading to the design.

Ultra-diffuse Galaxies at Ultraviolet Wavelengths

Abstract We measure near-ultraviolet (NUV) aperture magnitudes from Galaxy Evolution Explorer images for 258 ultra-diffuse galaxy (UDG) candidates drawn from the initial Systematically Measuring Ultra-Diffuse Galaxies (SMUDGes) survey of ∼300 square degrees surrounding, and including, the Coma galaxy cluster. For the vast majority, 242 of them, we present flux upper limits due either to a lack of significant flux in the aperture or confusion with other objects projected within the aperture. These limits often place interesting constraints on the UDG candidates, indicating that they are non-star-forming or quiescent. In particular, we identify field, quiescent UDG candidates, which are a challenge for formation models and are, therefore, compelling prospects for spectroscopic follow-up and distance determinations. We present far-ultraviolet (FUV) and NUV magnitudes for 16 detected UDG candidates and compare those galaxies to the local population of galaxies on color–magnitude and specific star formation rate diagrams. The NUV-detected UDG candidates form mostly an extension toward lower stellar masses of the star-forming galaxy sequence, and none of these lie within regions of high local galaxy density. UDG candidates span a range of properties, although almost all are consistent with being quiescent, low surface brightness galaxies, regardless of environment.

UV Study of the Fourth Positive Band System of CO and O i 135.6 nm From Electron Impact on CO and CO2

AbstractWe have measured the 30 and 100 eV far ultraviolet (FUV) emission cross sections of the optically allowed Fourth Positive Group (4PG) band system (A1Π → X1Σ+) of CO and the optically forbidden O (5So → 3P) 135.6 nm atomic transition by electron‐impact‐induced‐fluorescence of CO and CO2. We present a model excitation cross section from threshold to high energy for the A1Π state, including cascade by electron impact on CO. The A1Π state is perturbed by triplet states leading to an extended FUV glow from electron excitation of CO. We derive a model FUV spectrum of the 4PG band system from dissociative excitation of CO2, an important process observed on Mars and Venus. Our unique experimental setup consists of a large vacuum chamber housing an electron gun system and the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission Imaging Ultraviolet Spectrograph optical engineering unit, operating in the FUV (110–170 nm). The determination of the total O i (5So) at 135.6 nm emission cross section is accomplished by measuring the cylindrical glow pattern of the metastable emission from electron impact by imaging the glow intensity about the electron beam from nominally zero to ~400 mm distance from the electron beam. The study of the glow pattern of O i (135.6 nm) from dissociative excitation of CO and CO2 indicates that the O i (5So) state has a kinetic energy of ~1 eV by modeling the radial glow pattern with the published lifetime of 180 μs for the O i (5So) state.

Recent Advances in Molecular Spectroscopy of Electronic and Vibrational Transitions in Condensed Phase and Its Application to Chemistry

Abstract This review outlines recent progress in molecular spectroscopy of electronic and vibrational transitions in a condensed phase. Among various spectroscopies, in this review, I have selected far-ultraviolet (FUV) spectroscopy, near-infrared (NIR) spectroscopy, Raman spectroscopy, and Terahertz (THz)/far-infrared (FIR) spectroscopy because our group has been very much involved in the development of these spectroscopies. For each spectroscopy I describe its brief history, characteristics, and applications to a variety of chemistry. One of the purposes of this review article is to show how the advances in molecular spectroscopy have brought about progress in various fields of chemistry. Some emphasis is put on the explanation of the development of instruments in our group because it induced new advances in FUV, NIR and Raman (particularly surface-enhanced Raman scattering and tip-enhanced Raman scattering). Moreover, through this article, new introduction of quantum chemical calculations to each spectroscopy is highlighted.

Star formation and gas in the minor merger UGC 10214

Minor mergers play a crucial role in galaxy evolution. UGC 10214 (the Tadpole galaxy) is a prime example of this process in which a dwarf galaxy has interacted with a large spiral galaxy ∼250 Myr ago and produced a perturbed disc and a giant tidal tail. We used a multi-wavelength dataset that partly consists of new observations (Hα, HI, and CO) and partly of archival data to study the present and past star formation rate (SFR) and its relation to the gas and stellar mass at a spatial resolution down to 4 kpc. UGC 10214 is a high-mass (stellar mass M⋆ = 1.28 × 1011 M⊙) galaxy with a low gas fraction (Mgas/M⋆ = 0.24), a high molecular gas fraction (MH2/MHI = 0.4), and a modest SFR (2–5 M⊙ yr−1). The global SFR compared to its stellar mass places UGC 10214 on the galaxy main sequence (MS). The comparison of the molecular gas mass and current SFR gives a molecular gas depletion time of about ∼2 Gyr (based on Hα), comparable to those of normal spiral galaxies. Both from a comparison of the Hα emission, tracing the current SFR, and far-ultraviolet (FUV) emission, tracing the recent SFR during the past tens of Myr, and also from spectral energy distribution fitting with CIGALE, we find that the SFR has increased by a factor of about 2–3 during the recent past. This increase is particularly noticeable in the centre of the galaxy where a pronounced peak of the Hα emission is visible. A pixel-to-pixel comparison of the SFR, molecular gas mass, and stellar mass shows that the central region has had a depressed FUV-traced SFR compared to the molecular gas and the stellar mass, whereas the Hα-traced SFR shows a normal level. The atomic and molecular gas distribution is asymmetric, but the position-velocity diagram along the major axis shows a pattern of regular rotation. We conclude that the minor merger has most likely caused variations in the SFR in the past that resulted in a moderate increase of the SFR, but it has not perturbed the gas significantly so that the molecular depletion time remains normal.

Globular clusters in the outer halo of M 31

In this paper, we present photometry of 53 globular clusters (GCs) in the M 31 outer halo, including the GALEX far-ultraviolet (FUV) and near-ultraviolet (NUV), SDSS ugriz, 15 intermediate-band filters of BATC, and 2MASS JHKs bands. By comparing the multicolour photometry with stellar population synthesis models, we determine the metallicities, ages, and masses for these GCs, aiming to probe the merging/accretion history of M 31. We find no clear trend of metallicity and mass with the de-projected radius. The halo GCs younger than ∼8 Gyr are mostly located at the de-projected radii around 100 kpc, but this may be due to a selection effect. We also find that the halo GCs have consistent metallicities with their spatially associated substructures, which provides further evidence of the physical association between them. Both the disc and halo GCs in M 31 show a bimodal luminosity distribution. However, we should emphasise that there are more faint halo GCs which are not seen in the disc. The bimodal luminosity function of the halo GCs may reflect a different origin or evolution environment in their original hosts. The M 31 halo GCs include one intermediate metallicity group (−1.5 < [Fe/H] < −0.4) and one metal-poor group ([Fe/H] < −1.5), while the disc GCs have one metal-rich group more. There are considerable differences between the halo GCs in M 31 and the Milky Way (MW). The total number of GCs in M 31 is approximately three times greater than in the MW, however M 31 has about six times more halo GCs than the MW. Compared to the halo GCs of M 31, those of the MW are mostly metal-poor. Both the numerous halo GCs and the higher-metallicity component are suggestive of an active merger history of M 31.

Investigation of the globular cluster NGC 2808 with the Ultra-Violet Imaging Telescope

Globular clusters represent stellar laboratories where observations can be used to validate models of stellar evolution. In this study, we put forth new ultraviolet (UV) photometric results of stars in the Galactic globular cluster NGC 2808. NGC 2808 is known to host multiple stellar populations that include at least four distinct groups of horizontal Branch (HB) stars. We have observed this cluster with the AstroSat-UltraViolet Imaging Telescope in two far-UV (FUV) and five near-UV (NUV) filters, respectively. These UV filters enable the identification of HB populations of stars. The results from four NUV filters exhibit bimodal distributions in magnitude histograms. The nature of bimodality has been investigated on the basis of distinct stellar types contributing to those bands. The color-magnitude diagrams constructed using FUV and NUV filters enable the location of hot stellar populations, viz. stars belonging to Red HB (RHB), Blue HB, Extreme HB, Blue Hook branch and post-Asymptotic Giant Branch. Prominent gaps are observed in the UV color-magnitude diagrams. We report for the first time, a photometric gap in a NUV color-magnitude diagram, that segregates the RHB population of this cluster into two groups, that are likely to be associated with distinct generations of stars. We have constructed and examined the GAIA color-magnitude diagram of the optical counterparts of the hot UV stars. We also investigate the spatial density distributions of various groups of stars in the cluster and comment on the proposed formation models of multiple populations.

External photoevaporation of protoplanetary discs in Cygnus OB2: linking discs to star formation dynamical history

Many stars form in regions of enhanced stellar density, wherein the influence of stellar neighbours can have a strong influence on a protoplanetary disc (PPD) population. In particular, far ultraviolet (FUV) flux from massive stars drives thermal winds from the outer edge of PPDs, accelerating disc destruction. In this work, we present a novel technique for constraining the dynamical history of a star forming environment using PPD properties in a strongly FUV irradiated environment. Applying recent models for FUV induced mass loss rates to the PPD population of Cygnus OB2, we constrain how long ago primordial gas was expelled from the region; $ 0.5$ Myr ago if the Shakura & Sunyaev $\alpha$-viscosity parameter is $\alpha = 10^{-2}$ (corresponding to a viscous timescale of $\tau_\mathrm{visc} \approx 0.5$ Myr for a disc of scale radius $40$ au around a $1\, M_\odot$ star). This value of $\alpha$ is effectively an upper limit, since it assumes efficient extinction of FUV photons throughout the embedded phase. With this gas expulsion timescale we are able to produce a full dynamical model that fits kinematic and morphological data as well as disc fractions. We suggest Cygnus OB2 was originally composed of distinct massive clumps or filaments, each with a stellar mass $\sim 10^4 \, M_\odot$. Finally we predict that in regions of efficient FUV induced mass loss, disc mass $M_\mathrm{disc}$ as a function of stellar host mass $m_\mathrm{star}$ follows a power law with $M_\mathrm{disc} \propto m_\mathrm{star}^\beta$, where $\beta \gtrsim 2.7$ (depending on disc initial conditions and FUV exposure). This is steeper than observed correlations in regions of moderate FUV flux ($1 < \beta <1.9$), and offers a promising diagnostic to establish the influence of external photoevaporation in a given region.

Molecular tracers of radiative feedback in Orion (OMC-1) Widespread CH+ (J = 1-0), CO (10-9), HCN (6-5), and HCO+ (6-5) emission.

Young massive stars regulate the physical conditions, ionization, and fate of their natal molecular cloud and surroundings. It is important to find tracers that help quantifying the stellar feedback processes that take place at different spatial scales. We present ~85 arcmin2 (~1.3 pc2) velocity-resolved maps of several submillimeter molecular lines, taken with Herschel/HIFI, toward the closest high-mass star-forming region, the Orion molecular cloud 1 core (OMC-1). The observed rotational lines include probes of warm and dense molecular gas that are difficult, if not impossible, to detect from ground-based telescopes: CH+ (J = 1-0), CO (J = 10-9), HCO+ (J = 6-5) and HCN (J = 6-5), and CH (N, J =1, 3/2-1, 1/2). These lines trace an extended but thin layer (A V ≃3-6 mag or ~1016 cm) of molecular gas at high thermal pressure, P th = n H · T k ≈ 107 - 109 cm-3 K, associated with the far ultraviolet (FUV) irradiated surface of OMC-1. The intense FUV radiation field, emerging from massive stars in the Trapezium cluster, heats, compresses and photoevaporates the cloud edge. It also triggers the formation of specific reactive molecules such as CH+. We find that the CH+ (J = 1-0) emission spatially correlates with the flux of FUV photons impinging the cloud: G 0 from ~103 to ~105. This correlation is supported by constant-pressure photodissociation region (PDR) models in the parameter space P th/G 0 ≈ [5 · 103 - 8 · 104] cm-3 K where many observed PDRs seem to lie. The CH+ (J = 1-0) emission spatially correlates with the extended infrared emission from vibrationally excited H2 (v ≥ 1), and with that of [C ii] 158 μm and CO J = 10-9, all emerging from FUV-irradiated gas. These correlations link the presence of CH+ to the availability of C+ ions and of FUV-pumped H2 (v ≥ 1) molecules. We conclude that the parsec-scale CH+ emission and narrow-line (Δv ≃ 3 km s-1) mid-J CO emission arises from extended PDR gas and not from fast shocks. PDR line tracers are the smoking gun of the stellar feedback from young massive stars. The PDR cloud surface component in OMC-1, with a mass density of 120-240 M ⊙ pc-2, represents ~5% to ~10% of the total gas mass, however, it dominates the emitted line luminosity; the average CO J = 10-9 surface luminosity in the mapped region being ~35 times brighter than that of CO J = 2-1. These results provide insights into the source of submillimeter CH+ and mid-J CO emission from distant star-forming galaxies.

CIGALE: a python Code Investigating GALaxy Emission

Context. Measuring how the physical properties of galaxies change across cosmic times is essential to understand galaxy formation and evolution. With the advent of numerous ground-based and space-borne instruments launched over the past few decades we now have exquisite multi-wavelength observations of galaxies from the far-ultraviolet (FUV) to the radio domain. To tap into this mine of data and obtain new insight into the formation and evolution of galaxies, it is essential that we are able to extract information from their spectral energy distribution (SED). Aims. We present a completely new implementation of Code Investigating GALaxy Emission (CIGALE). Written in python, its main aims are to easily and efficiently model the FUV to radio spectrum of galaxies and estimate their physical properties such as star formation rate, attenuation, dust luminosity, stellar mass, and many other physical quantities. Methods. To compute the spectral models, CIGALE builds composite stellar populations from simple stellar populations combined with highly flexible star formation histories, calculates the emission from gas ionised by massive stars, and attenuates both the stars and the ionised gas with a highly flexible attenuation curve. Based on an energy balance principle, the absorbed energy is then re-emitted by the dust in the mid- and far-infrared domains while thermal and non-thermal components are also included, extending the spectrum far into the radio range. A large grid of models is then fitted to the data and the physical properties are estimated through the analysis of the likelihood distribution. Results. CIGALE is a versatile and easy-to-use tool that makes full use of the architecture of multi-core computers, building grids of millions of models and analysing samples of thousands of galaxies, both at high speed. Beyond fitting the SEDs of galaxies and parameter estimations, it can also be used as a model-generation tool or serve as a library to build new applications.

A Hot Ultraviolet Flare on the M Dwarf Star GJ 674

Abstract As part of the Mega-Measurements of the Ultraviolet Spectral Characteristics of Low-Mass Exoplanetary Systems Hubble Space Telescope (HST) Treasury program, we obtained time-series ultraviolet spectroscopy of the M2.5V star, GJ 674. During the far-ultraviolet (FUV) monitoring observations, the target exhibited several small flares and one large flare (E FUV = 1030.75 erg) that persisted over the entirety of an HST orbit and had an equivalent duration &gt;30,000 s, comparable to the highest relative amplitude event previously recorded in the FUV. The flare spectrum exhibited enhanced line emission from chromospheric, transition region, and coronal transitions and a blue FUV continuum with an unprecedented color temperature of TC ≃ 40,000 ± 10,000 K. In this Letter, we compare the flare FUV continuum emission with parameterizations of radiative hydrodynamic model atmospheres of M star flares. We find that the observed flare continuum can be reproduced using flare models but only with the ad hoc addition of a hot, dense emitting component. This observation demonstrates that flares with hot FUV continuum temperatures and significant extreme-ultraviolet/FUV energy deposition will continue to be of importance to exoplanet atmospheric chemistry and heating, even as the host M dwarfs age beyond their most active evolutionary phases.

Study of the 13CO/C18O abundance ratio towards the filamentary infrared dark cloud IRDC 34.43 + 0.24

Abstract Nowadays, there are several observational studies about the 13CO/C18O abundance ratio ( $X^{13/18}$ ) towards nearby molecular clouds. These works give observational support to the C18O selective photodissociation due to the interaction between the far ultraviolet (FUV) radiation and the molecular gas. It is necessary to increase the sample of molecular clouds located at different distances and affected in different ways by nearby or embedded H ii regions and OB associations to study the selective photodissociation. Using 12CO, 13CO, and C18O J = 1–0 data obtained from the FOREST unbiased Galactic plane imaging survey performed with the Nobeyama 45-m telescope, we analyse the filamentary infrared dark cloud IRDC $34.43+0.24$ located at the distance of about 3.9 kpc. This infrared dark cloud (IRDC) is related to several H ii regions and young stellar objects. Assuming local thermodynamic equilibrium, we obtain: $0.8 \times 10^{16} &lt;$ N(13CO) $&lt;4 \times 10^{17}$ cm–2 (average value $= 4.2 \times 10^{16}$ cm–2), $0.6 \times 10^{15} &lt;$ N(C18O) $&lt;4.4 \times 10^{16}$ cm–2 (average value $= 5.0 \times 10^{15}$ cm–2), and 3 $&lt;$ $X^{13/18}$ $&lt;$ 30 (average $= 8$ ) across the whole IRDC. Larger values of $X^{13/18}$ were found towards portions of the cloud related to the H ii regions associated with the N61 and N62 bubbles and with the photodissociation regions, precisely the regions in which FUV photons are strongly interacting with the molecular gas. Our result represents an observational support to the C18O selectively photodissociation phenomenon occurring in a quite distant filamentary IRDC. Additionally, based on IR data from the Hi-GAL survey, the FUV radiation field was estimated in Habing units, and the dust temperature (T $_{dust}$ ) and H2 column density (N(H2)) distribution were studied. Using the average of N(H2), values in close agreement with the ‘canonical’ abundance ratios [H2]/[13CO] and [H2]/[C18O] were derived. However, the obtained ranges in the abundance ratios show that if an accurate analysis of the molecular gas is required, the use of the ‘canonical’ values may introduce some bias. Thus, it is important to consider how the gas is irradiated by the FUV photons across the molecular cloud. The analysis of $X^{13/18}$ is a good tool to perform that. Effects of beam dilution and clumpiness were studied.

Comprehensive comparison of models for spectral energy distributions from 0.1μm to 1 mm of nearby star-forming galaxies

We have fit the far-ultraviolet (FUV) to sub-millimeter (850μm) spectral energy distributions (SEDs) of the 61 galaxies from the Key Insights on Nearby Galaxies: A Far-Infrared Survey withHerschel(KINGFISH). The fitting has been performed using three models: the Code for Investigating GALaxy Evolution (CIGALE), the GRAphite-SILicate approach (GRASIL), and the Multiwavelength Analysis of Galaxy PHYSical properties (MAGPHYS). We have analyzed the results of the three codes in terms of the SED shapes, and by comparing the derived quantities with simple “recipes” for stellar mass (Mstar), star-formation rate (SFR), dust mass (Mdust), and monochromatic luminosities. Although the algorithms rely on different assumptions for star-formation history, dust attenuation and dust reprocessing, they all well approximate the observed SEDs and are in generally good agreement for the associated quantities. However, the three codes show very different behavior in the mid-infrared regime: in the 5–10μm region dominated by PAH emission, and also between 25 and 70μm where there are no observational constraints for the KINGFISH sample. We find that different algorithms give discordant SFR estimates for galaxies with low specific SFR, and that the standard recipes for calculating FUV absorption overestimate the extinction compared to the SED-fitting results. Results also suggest that assuming a “standard” constant stellar mass-to-light ratio overestimatesMstarrelative to the SED fitting, and we provide new SED-based formulations for estimatingMstarfrom WISE W1 (3.4μm) luminosities and colors. From a principal component analysis ofMstar, SFR,Mdust, and O/H, we reproduce previous scaling relations amongMstar, SFR, and O/H, and find thatMdustcan be predicted to within ∼0.3 dex using onlyMstarand SFR.

To be or not to be: the case of the hot WHIM absorption in the blazar PKS 2155–304 sight line

The cosmological missing baryons at z &lt; 1 most likely hide in the hot (T ≳ 105.5 K) phase of the warm hot intergalactic medium (WHIM). While the hot WHIM is hard to detect due to its high ionisation level, the warm (T ≲ 105.5 K) phase of the WHIM has been very robustly detected in the far-ultraviolet (FUV) band. We adopted the assumption that the hot and warm WHIM phases are co-located and therefore used the FUV-detected warm WHIM as a tracer for the cosmologically interesting hot WHIM. We performed an X-ray follow-up in the sight line of the blazar PKS 2155–304 at the redshifts where previous FUV measurements of O VI, Si IV, and broad Lyman-alpha (BLA) absorption have indicated the existence of the warm WHIM. We looked for the O VII Heα and O VIII Lyα absorption lines, the most likely hot WHIM tracers. Despite the very large exposure time (≈1 Ms), the Reflection Grating Spectrometer unit 1 (RGS1) on-board XMM-Newton data yielded no significant detection which corresponds to upper limits of log N(O VII(cm−2)) ≤ 14.5−15.2 and log N(O VIII(cm−2)) ≤ 14.5−15.2. An analysis of the data obtained with the combination of the Low Energy Transmission Grating (LETG) and the High Resolution Camera (HRC) on-board Chandra yielded consistent results. However, the data obtained with the LETG, combined with the Advanced CCD Imaging Spectrometer (ACIS) lead to the detection of an feature resembling an absorption line at λ ≈ 20 Å at simple one-parameter confidence level of 3.7σ, consistent with several earlier LETG/ACIS reports. Given the high statistical quality of the RGS1 data, the possibility of RGS1 accidentally missing the true line at λ ∼ 20 Å is very low: 0.006%. Neglecting this, the LETG/ACIS detection can be interpreted as Lyα transition of O VIII at one of the redshifts (z ≈ 0.054) of FUV-detected warm WHIM. Given the very convincing X-ray spectral evidence for and against the existence of the λ ∼ 20 Å feature, we cannot conclude whether or not it is a true astrophysical absorption line. Considering cosmological simulations, the probability of the LETG/ACIS λ ∼ 20 Å feature being due to the astrophysical O VIII absorber co-located with the FUV-detected O VI absorber is at the very low level of ≲0.1%. We cannot completely rule out the very unlikely possibility that the LETG/ACIS 20 Å feature is due to a transient event located close to the blazar.

The Nature of Turbulence in the Outer Regions of Protoplanetary Disks

We carry out a series of local, shearing box simulations of the outer regions of protoplanetary disks, where ambipolar diffusion is important due to low ionization levels, to better characterize the nature of turbulence and angular momentum transport in these disks. These simulations are divided into two groups, one with far ultraviolet (FUV) ionization included, and one without FUV. In both cases, we explore a large range in diffusivity values. We find that in the simulations without FUV, the properties of the turbulence are similar to the unstratified simulations of Bai & Stone (2011); for a given diffusivity, the MRI can still be present so long as the magnetic field is sufficiently weak. Furthermore, the dynamics of the mid-plane in these simulations are primarily controlled by the MRI. In the FUV simulations on the other hand, the MRI-active FUV layers transport strong toroidal magnetic flux to the mid-plane, which shuts off the MRI. Instead, angular momentum transport at the mid-plane is dominated by laminar magnetic fields, resulting in lower levels of turbulent Maxwell stress compared to the no-FUV simulations. Finally, we perform a temporal correlation analysis on the FUV simulations, confirming our result that the dynamics in the mid-plane region is strongly controlled by the FUV-ionized layers.

Planetary nebulae with UVIT

The high excitation planetary nebula, NGC 6302, has been imaged in two far-ultraviolet (FUV) filters, F169M (Sapphire; λeff: 1608 Å) and F172M (Silica; λeff: 1717 Å) and two near-UV (NUV) filters, N219M (B15; λeff: 2196 Å) and N279N (N2; λeff: 2792 Å) with the Ultra Violet Imaging Telescope (UVIT). The FUV F169M image shows faint emission lobes that extend to about 5 arcmin on either side of the central source. Faint orthogonal collimated jet-like structures are present on either side of the FUV lobes through the central source. These structures are not present in the two NUV filters or in the FUV F172M filter. Optical and infrared (IR) images of NGC 6302 show bright emission bipolar lobes in the east-west direction with a massive torus of molecular gas and dust seen as a dark lane in the north-south direction. The FUV lobes are much more extended and oriented at a position angle of 113°. They and the jet-like structures might be remnants of an earlier evolutionary phase, prior to the dramatic explosive event that triggered the Hubble type bipolar flows approximately 2200 years ago. The source of the FUV lobe and jet emission is not known, but is likely due to fluorescent emission from H2 molecules. The cause of the difference in orientation of optical and FUV lobes is not clear and, we speculate, could be related to two binary interactions.

Gaia calibrated UV luminous stars in LAMOST

We take advantage of Gaia Data Release 2 to present 275 and 1774 ultraviolet luminous stars in the far ultraviolet (FUV) and near ultraviolet (NUV), respectively. These stars exceed their expected values by 5σ with respect to over one million ultraviolet stars in the log g vs. Teff diagram. Galactic extinction is corrected with a 3D dust map. In order to limit the Lutz-Kelker bias to an insignificant level, we select stars with relative uncertainties in luminosity less than 40% and trigonometric parallaxes less than 20%. We cross-identified our sample with the catalogs of RR Lyr stars and possible white dwarf main-sequence binaries, and find they compose ∼62% and ∼16% of our sample in the FUV and NUV, respectively. This catalog provides a unique sample to study stellar activity, spectrally unresolved compact main-sequence binaries and variable stars.

Multiwavelength observations of the EUV variable metal-rich white dwarf GD 394

We present new Hubble Space Telescope (HST) ultraviolet and ground-based optical observations of the hot, metal-rich white dwarf GD 394. Extreme-ultraviolet (EUV) observations in 1992-1996 revealed a 1.15d periodicity with a 25 percent amplitude, hypothesised to be due to metals in a surface accretion spot. We obtained phase-resolved HST/Space Telescope Imaging Spectrograph (STIS) high-resolution far-ultraviolet (FUV) spectra of GD 394 that sample the entire period, along with a large body of supplementary data. We find no evidence for an accretion spot, with the flux, accretion rate and radial velocity of GD 394 constant over the observed timescales at ultraviolet and optical wavelengths. We speculate that the spot may have no longer been present when our observations were obtained, or that the EUV variability is being caused by an otherwise undetected evaporating planet. The atmospheric parameters obtained from separate fits to optical and ultraviolet spectra are inconsistent, as is found for multiple hot white dwarfs. We also detect non-photospheric, high-excitation absorption lines of multiple volatile elements, which could be evidence for a hot plasma cocoon surrounding the white dwarf.

Electronic Spectra of Graphene in Far- and Deep-Ultraviolet Region: Attenuated Total Reflection Spectroscopy and Quantum Chemical Calculation Study

We measured the electronic spectra of graphene nanostructures (flakes and platelets) extending into the far-ultraviolet (FUV) region by attenuated total reflection far- and deep-ultraviolet (ATR–FUV–DUV) spectroscopy in the region of 2.76–8.55 eV (450–145 nm). The major absorption of graphene appears in the DUV region (4.7 eV), as already reported; however, we observed a new peak in the FUV region, visible clearly in the case of flakes at 7.5–7.7 eV (165–161 nm) and less pronounced in the spectrum of the platelets at 6.6–6.7 eV (188–185 nm). Graphene flakes (thickness 1–2 nm; sub-micrometers of side dimension) and nanoplatelets (thickness 6–8 nm; several micrometers of side dimension) give notably different ATR absorbance spectra in the spectral region studied. This discrepancy is reduced upon applying mechanical pressure on the samples. These observations can evidence that the morphology as well as electronic structure of graphene can be manifested in the FUV–DUV region. Quantum chemical calculations wer...