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Carbon 1s excitation of methane, CH4, has been studied in the gas phase using the ion trap integrated with the photon–ion instrument at PETRA III/DESY and soft X-rays from the beamline P04. The created photoions are stored within the ion trap so that in further steps the photoions can undergo reactions with neutral methane molecules. The ionic photoproducts as well as reaction products created thereby are mass-over-charge analyzed by an ion time-of-flight spectrometer. Besides the photoions, product ions with up to three carbon atoms are found. In contrast to experiments using vacuum ultraviolet radiation, especially highly reactive product ions with a small number of hydrogen atoms such as C 2 H 2 + and C 2 H 3 + are found, which are important precursors for larger hydrocarbons such as C6H6. Possible production routes of the product ions are analyzed on the basis of a model that considers the probabilities for photofragmentation and the first subsequent chemical reaction step. The model indicates that the high degree of fragmentation by photons with energies around 280 eV is favoring these products. The results of the measurements show that the products like C 2 H 2 + and C 2 H 3 + can be generated by a single collision of the ionization product with neutral methane. The results suggest that soft X-rays might be important for chemical reactions in planetary atmospheres, which has usually not been taken into account. However, due to the high degree of fragmentation and large cross sections involved, they can have a large influence even when the corresponding photon flux is rather small. [ABSTRACT FROM AUTHOR]

A correction to the article "Fuse Measurements of Far-Ultraviolet Extinction. III. The Dependence on R(V) and Discrete Feature Limits From 75 Galactic Sightlines" by Karl D. Gordon, Stefan Cartledge and Geoffrey C. Clayton in a 2009 issue of the journal is presented.

The diffuse cosmic background radiation in the Galaxy Evolution Explorer far-ultraviolet (FUV, 1300-1700 Å) is deduced to originate only partially in the dust-scattered radiation of FUV-emitting stars: the source of a substantial fraction of the FUV background radiation remains a mystery. The radiation is remarkably uniform at both far northern and far southern Galactic latitudes and increases toward lower Galactic latitudes at all Galactic longitudes. We examine speculation that this might be due to interaction of the dark matter with the nuclei of the interstellar medium, but we are unable to point to a plausible mechanism for an effective interaction. We also explore the possibility that we are seeing radiation from bright FUV-emitting stars scattering from a “second population” of interstellar grains—grains that are small compared with FUV wavelengths. Such grains are known to exist, and they scatter with very high albedo, with an isotropic scattering pattern. However, comparison with the observed distribution (deduced from their 100 μm emission) of grains at high Galactic latitudes shows no correlation between the grains' location and the observed FUV emission. Our modeling of the FUV scattering by small grains also shows that there must be remarkably few such “smaller” grains at high Galactic latitudes, both north and south; this likely means simply that there is very little interstellar dust of any kind at the Galactic poles, in agreement with Perry and Johnston. We also review our limited knowledge of the cosmic diffuse background at ultraviolet wavelengths shortward of Lyα—it could be that our “second component” of the diffuse FUV background persists shortward of the Lyman limit and is the cause of the reionization of the universe. [ABSTRACT FROM AUTHOR]

We report measurements of the vacuum ultraviolet (VUV) emission spectra of a microwave-discharge hydrogen-flow lamp (MDHL), a common tool in astrochemistry laboratories working on ice VUV photoprocessing. The MDHL provides hydrogen Ly-α (121.6 nm) and H2 molecular emission in the 110-180 nm range. We show that the spectral characteristics of the VUV light emitted in this range, in particular the relative proportion of Ly-α to molecular emission bands, strongly depend on the pressure of H2 inside the lamp, the lamp geometry (F type versus T type), the gas used (pure H2 versus H2 seeded in He), and the optical properties of the window used (MgF2 versus CaF2). These different configurations are used to study the VUV irradiation of CO ice at 14 K. In contrast to the majority of studies dedicated to the VUV irradiation of astrophysical ice analogs, which have not taken into consideration the emission spectrum of the MDHL, our results show that the processes induced by photons in CO ice from a broad energy range are different and more complex than the sum of individual processes induced by monochromatic sources spanning the same energy range, as a result of the existence of multistate electronic transitions and discrepancy in absorption cross sections between parent molecules and products in the Ly-α and H2 molecular emission ranges. [ABSTRACT FROM AUTHOR]

A correction to the article "Gas Motion Study of Lyα Emitters at z ∼ 2 Using Far-Ultraviolet and Optical Spectral Lines" by Takuya Hashimoto et al., published in the September 16, 2013 issue is presented.

We present an analysis of 98 galaxies of low-dust content, selected from the Spitzer Local Volume Legacy survey, aimed at examining the relation between the ultraviolet (UV) color and dust attenuation in normal star-forming galaxies. The IRX–β diagram relates the total dust attenuation in a galaxy, traced by the far-IR (FIR) to UV ratio, to the observed UV color, indicated by β. Previous research has indicated that while starburst galaxies exhibit a relatively tight IRX–β relation, normal star-forming galaxies do not, and have a much larger spread in the total-IR to far-UV (FUV) luminosity for a fixed UV color. We examine the role that the age of the stellar population plays as the “second parameter” responsible for the observed deviation and spread of star-forming galaxies from the starburst relation. We model the FUV to FIR spectral energy distribution of each galaxy according to two broad bins of star formation history (SFH): constant and instantaneous burst. We find clear trends between stellar population mean age estimators (extinction-corrected FUV/NIR, U−B, and EW(Hα)) and the UV color β; the trends are mostly driven by the galaxies best-described by instantaneous burst populations. We also find a significant correlation between β and the mean age directly determined from the best-fit instantaneous models. As already indicated by other authors, the UV attenuation in star-forming galaxies may not be recovered with the UV color alone and is highly influenced by the stellar population’s mean age and SFH. Overall, the scatter in the IRX–β diagram is better correlated with β than with the perpendicular distance, dp. [ABSTRACT FROM AUTHOR]

The Galactic Center black hole (Sgr A*) provides an ideal laboratory for astronomical tests of new gravitational physics. This work reports that curvature correction (f(R)) to quantum vacuum fluctuations naturally yields a Yukawa-type scalar fifth force with potential , where Mψ is the mass of the f(R) scalarons. Estimating the UV and IR cutoff scales of vacuum fluctuations, the Yukawa coupling strength is connected to the scalaron field amplitude. Whereas recently constrained Yukawa coupling and range correspond to light scalarons with Mψ = (1.37 × 10−21–5.49 × 10−20) eV, vacuum fluctuations yield a massive scalaron with Mψ = 10−16 eV. Scalaron-induced periastron shift of stellar orbits near Sgr A* has been studied with respect to the semimajor axis in the range a = 10–1000 au. It is found that the scalarons resulting from quantum fluctuations affect the precession of orbits with a = 128–256 Rs. The possibility of future constraints on massive scalarons in observations near Sgr A* is discussed. This is a new and independent effort to express a prototype quantum gravity effect in terms of astronomically accessible quantities. [ABSTRACT FROM AUTHOR]

Thanks to the Cassini–Huygens mission, it is now established that the first aerosols in Titan’s upper atmosphere are found from an altitude of ∼1200 km. Once they are formed and through their descent toward the surface, these nanoparticles are submitted to persistent far-ultraviolet (FUV) radiation that can reach lower atmospheric layers. Such an interaction has an impact, especially on the chemistry and charge budget of the atmospheric compounds. Models are useful to understand this photoprocessing, but they lack important input data such as the photoemission threshold or the absolute photoabsorption/emission cross sections of the aerosols. In order to quantify the photoemission processes, analogs of Titan’s aerosols have been studied with the DESIRS FUV beamline at the synchrotron SOLEIL as isolated substrate-free nanoparticles. We present here the corresponding angle-resolved photoelectron spectroscopy data recorded at different FUV photon energies. The results show a very low photoionization threshold (6.0 ± 0.1 eV ∼ 207 nm) and very high absolute ionization cross sections (∼106 Mb), indicating that FUV photoemission from aerosols is an intense source of slow electrons that has to be taken into account in photochemical models of Titan’s atmosphere. [ABSTRACT FROM AUTHOR]

We report on a Hubble Space Telescope detection of the nearby, old pulsar B0950+08 ( pc, spin-down age 17.5 Myr) in two far-ultraviolet (FUV) bands. We measured the mean flux densities nJy and 83 ± 14 nJy in the F125LP and F140LP filters (pivot wavelengths 1438 and 1528 Å). Using the FUV data together with previously obtained optical–UV data, we conclude that the optical–FUV spectrum consists of two components—a nonthermal (presumably magnetospheric) power-law spectrum () with slope and a thermal spectrum emitted from the bulk of the neutron star (NS) surface with a temperature in the range of (1–3) × 105 K, depending on interstellar extinction and NS radius. These temperatures are much higher than predicted by NS cooling models for such an old pulsar, which means that some heating mechanisms operate in NSs. A plausible mechanism responsible for the high temperature of PSR B0950+08 is the interaction of vortex lines of the faster rotating neutron superfluid with the slower rotating normal matter in the inner NS crust (vortex creep heating). [ABSTRACT FROM AUTHOR]

From our Secret Lives of Cepheids program, the prototype Classical Cepheid, δ Cep, is found to be an X-ray source with periodic pulsation-modulated X-ray variations. This finding complements our earlier reported phase-dependent FUV–UV emissions of the star that increase ∼10–20 times with highest fluxes at , just prior to maximum brightness. Previously δ Cep was found as potentially X-ray variable, using XMM-Newton observations. Additional phase-constrained data were secured with Chandra near X-ray emission peak, to determine if the emission and variability were pulsation-phase-specific to δ Cep and not transient or due to a possible coronally active, cool companion. The Chandra data were combined with prior XMM-Newton observations, and were found to very closely match the previously observed X-ray behavior. From the combined data set, a ∼4 increase in X-ray flux is measured, reaching a peak = 1.7 × 1029 erg s−1 near 0.45ϕ. The precise X-ray flux phasing with the star’s pulsation indicates that the emissions arise from the Cepheid and not from a companion. However, it is puzzling that the maximum X-ray flux occurs ∼0.5ϕ (∼3 days) later than the FUV–UV maximum. There are several other potential Cepheid X-ray detections with properties similar to δ Cep, and comparable X-ray variability is indicated for two other Cepheids: β Dor and V473 Lyr. X-ray generating mechanisms in δ Cep and other Cepheids are discussed. If additional Cepheids are confirmed to show phased X-ray variations, then δ Cep will be the prototype of a new class of pulsation-induced X-ray variables. [ABSTRACT FROM AUTHOR]

We present Herschel observations of the far-infrared (FIR) fine-structure (FS) lines [C ii]158 μm, [O i]63 μm, [O iii]52 μm, and [Si ii]35 μm in the z = 2.56 Cloverleaf quasar, and combine them with published data in an analysis of the dense interstellar medium (ISM) in this system. Observed [C ii]158 μm, [O i]63 μm, and FIR continuum flux ratios are reproduced with photodissociation region (PDR) models characterized by moderate far-ultraviolet (FUV) radiation fields with 0.3–1 × 103 and atomic gas densities 3–5 × 103 cm−3, depending on contributions to [C ii]158 μm from ionized gas. We assess the contribution to the [C ii]158 μm flux from an active galactic nucleus (AGN) narrow line region (NLR) using ground-based measurements of the [N ii]122 μm transition, finding that the NLR can contribute at most 20%–30% of the observed [C ii]158 μm flux. The PDR density and far-UV radiation fields inferred from the atomic lines are not consistent with the CO emission, indicating that the molecular gas excitation is not solely provided via UV heating from local star formation (SF), but requires an additional heating source. X-ray heating from the AGN is explored, and we find that X-ray-dominated region (XDR) models, in combination with PDR models, can match the CO cooling without overproducing the observed FS line emission. While this XDR/PDR solution is favored given the evidence for both X-rays and SF in the Cloverleaf, we also investigate alternatives for the warm molecular gas, finding that either mechanical heating via low-velocity shocks or an enhanced cosmic-ray ionization rate may also contribute. Finally, we include upper limits on two other measurements attempted in the Herschel program: [C ii]158 μm in FSC 10214 and [O i]63 μm in APM 08279+5255. [ABSTRACT FROM AUTHOR]

We have imaged GM Aurigae with the Hubble Space Telescope, detected its disk in scattered light at 1400 and 1650 Å, and compared these with observations at 3300 Å, 5550 Å, 1.1 μm, and 1.6 μm. The scattered light increases at shorter wavelengths. The radial surface brightness profile at 3300 Å shows no evidence of the 24 au radius cavity that has been previously observed in submillimeter observations. Comparison with dust grain opacity models indicates that the surface of the entire disk is populated with submicron grains. We have compiled a spectral energy distribution from 0.1 μm to 1 mm and used it to constrain a model of the star + disk system that includes the submillimeter cavity using the Monte Carlo radiative transfer code by Barbara Whitney. The best-fit model image indicates that the cavity should be detectable in the F330W bandpass if the cavity has been cleared of both large and small dust grains, but we do not detect it. The lack of an observed cavity can be explained by the presence of submicron grains interior to the submillimeter cavity wall. We suggest one explanation for this that could be due to a planet of mass <9 MJ interior to 24 au. A unique cylindrical structure is detected in the far-UV data from the Advanced Camera for Surveys/Solar Blind Channel. It is aligned along the system semiminor axis, but does not resemble an accretion-driven jet. The structure is limb brightened and extends 190 ± 35 au above the disk midplane. The inner radius of the limb brightening is 40 ± 10 au, just beyond the submillimeter cavity wall. [ABSTRACT FROM AUTHOR]

We present new Herschel observations using the Photodetector Array Camera and Spectrometer of 32 T Tauri stars in the young (∼3 Myr) σ Ori cluster. Most of our objects are K and M stars with large excesses at 24 μm. We used irradiated accretion disk models of D’Alessio et al. to compare their spectral energy distributions with our observational data. We arrive at the following six conclusions. (i) The observed disks are consistent with irradiated accretion disk systems. (ii) Most of our objects (60%) can be explained by significant dust depletion from the upper disk layers. (iii) Similarly, 61% of our objects can be modeled with large disk sizes (Rd ≥ 100 au). (iv) The masses of our disks range between 0.03 and 39 MJup, where 35% of our objects have disk masses less than 1 MJup. Although these are lower limits, high-mass (>0.05 ) disks, which are present in, e.g., Taurus, are missing. (v) By assuming a uniform distribution of objects around the brightest stars at the center of the cluster, we found that 80% of our disks are exposed to external FUV radiation of , which can be strong enough to photoevaporate the outer edges of the closer disks. (vi) Within 0.6 pc from σ Ori we found forbidden emission lines of [N ii] in the spectrum of one of our large disks (SO662), but no emission in any of our small ones. This suggests that this object may be an example of a photoevaporating disk. [ABSTRACT FROM AUTHOR]

Investigating the molecular gas in the inner regions of protoplanetary disks (PPDs) provides insight into how the molecular disk environment changes during the transition from primordial to debris disk systems. We conduct a small survey of molecular hydrogen (H2) fluorescent emission, using 14 well-studied Classical T Tauri stars at two distinct dust disk evolutionary stages, to explore how the structure of the inner molecular disk changes as the optically thick warm dust dissipates. We simulate the observed Hi-Lyman α-pumped H2 disk fluorescence by creating a 2D radiative transfer model that describes the radial distributions of H2 emission in the disk atmosphere and compare these to observations from the Hubble Space Telescope. We find the radial distributions that best describe the observed H2 FUV emission arising in primordial disk targets (full dust disk) are demonstrably different than those of transition disks (little-to-no warm dust observed). For each best-fit model, we estimate inner and outer disk emission boundaries (rin and rout), describing where the bulk of the observed H2 emission arises in each disk, and we examine correlations between these and several observational disk evolution indicators, such as n13–31, rin, CO, and the mass accretion rate. We find strong, positive correlations between the H2 radial distributions and the slope of the dust spectral energy distribution, implying the behavior of the molecular disk atmosphere changes as the inner dust clears in evolving PPDs. Overall, we find that H2 inner radii are ∼4 times larger in transition systems, while the bulk of the H2 emission originates inside the dust gap radius for all transitional sources. [ABSTRACT FROM AUTHOR]

We report the results of a pilot survey for X-ray emission from a newly discovered class of AGB stars with far-ultraviolet excesses (fuvAGB stars) using XMM-Newton and Chandra. We detected X-ray emission in three of six fuvAGB stars observed—the X-ray fluxes are found to vary in a stochastic or quasi-periodic manner on roughly hour-long timescales, and simultaneous UV observations using the Optical Monitor on XMM for these sources show similar variations in the UV flux. These data, together with previous studies, show that X-ray emission is found only in fuvAGB stars. From modeling the spectra, we find that the observed X-ray luminosities are ∼(0.002–0.2) L⊙ and the X-ray-emitting plasma temperatures are ∼(35–160) × 106 K. The high X-ray temperatures argue against the emission arising in stellar coronae, or directly in an accretion shock, unless it occurs on a WD companion. However, none of the detected objects is a known WD-symbiotic star, suggesting that if WD companions are present, they are relatively cool (<20,000 K). In addition, the high X-ray luminosities specifically argue against emission originating in the coronae of main-sequence companions. We discuss several models for the X-ray emission and its variability and find that the most likely scenario for the origin of the X-ray (and FUV) emission involves accretion activity around a companion star, with confinement by strong magnetic fields associated with the companion and/or an accretion disk around it. [ABSTRACT FROM AUTHOR]

We model the extinction profiles observed in the Small and Large Magellanic clouds with a synthetic population of dust grains consisting of core-mantle particles and a collection of free-flying polycyclic aromatic hydrocarbons (PAHs). All different flavors of the extinction curves observed in the Magellanic Clouds (MCs) can be described by the present model, which has been previously (successfully) applied to a large sample of diffuse and translucent lines of sight in the Milky Way. We find that in the MCs the extinction produced by classical grains is generally larger than absorption by PAHs. Within this model, the nonlinear far-UV rise is accounted for by PAHs, whose presence in turn is always associated with a gap in the size distribution of classical particles. This hints either at a physical connection between (e.g., a common cause for) PAHs and the absence of middle-sized dust particles or the need for an additional component in the model that can account for the nonlinear far-UV rise without contributing to the UV bump at ∼217 nm such as, e.g., nanodiamonds. [ABSTRACT FROM AUTHOR]

Molecular oxygen (O2) has been the target of ground-based and space-borne searches for decades. Of the thousands of lines of sight surveyed, only those toward Rho Ophiuchus and Orion H2 Peak 1 have yielded detections of any statistical significance. The detection of the O2NJ = 33–12 and 54–34 lines at 487.249 GHz and 773.840 GHz, respectively, toward Rho Ophiuchus has been attributed to a short-lived peak in the time-dependent, cold-cloud O2 abundance, while the detection of the O2NJ = 33–12, 54–34 lines, plus the 76–56 line at 1120.715 GHz, toward Orion has been ascribed to time-dependent preshock physical and chemical evolution and low-velocity (12 km s−1) non-dissociative C-type shocks, both of which are fully shielded from far-ultraviolet (FUV) radiation, plus a postshock region that is exposed to an FUV field. We report a re-interpretation of the Orion O2 detection based on new C-type shock models that fully incorporate the significant effects the presence of even a weak FUV field can have on the preshock gas, shock structure, and postshock chemistry. In particular, we show that a family of solutions exists, depending on the FUV intensity, that reproduces both the observed O2 intensities and O2 line ratios. The solution in closest agreement with the shock parameters inferred for H2 Peak 1 from other gas tracers assumes a 23 km s−1 shock impacting gas with a preshock density of 8 × 104 cm−3 and = 1, substantially different from that inferred for the fully shielded shock case. As pointed out previously, the similarity between the LSR velocity of all three O2 lines (11 km s−1) and recently measured H2O 532–441 maser emission at 620.701 GHz toward H2 Peak 1 suggests that the O2 emission arises behind the same shocks responsible for the maser emission, though the O2 emission is almost certainly more extended than the localized high-density maser spots. Since maser emission arises along lines of sight of low-velocity gradient, indicating shock motion largely perpendicular to our line of sight, we note that this geometry can explain not only the narrow (≲3 km s−1) observed O2 line widths despite their excitation behind a shock but also why such O2 detections are rare. [ABSTRACT FROM AUTHOR]

We present a model explaining the elemental enrichments in Jupiter's atmosphere, particularly the noble gases Ar, Kr, and Xe. While He, Ne, and O are depleted, seven other elements show similar enrichments (∼3 times solar, relative to H). Being volatile, Ar is difficult to fractionate from H2. We argue that external photoevaporation by far-ultraviolet (FUV) radiation from nearby massive stars removed H2, He, and Ne from the solar nebula, but Ar and other species were retained because photoevaporation occurred at large heliocentric distances where temperatures were cold enough (≲ 30 K) to trap them in amorphous water ice. As the solar nebula lost H, it became relatively and uniformly enriched in other species. Our model improves on the similar model of Guillot & Hueso. We recognize that cold temperatures alone do not trap volatiles; continuous water vapor production is also necessary. We demonstrate that FUV fluxes that photoevaporated the disk generated sufficient water vapor in regions ≲ 30 K to trap gas-phase species in amorphous water ice in solar proportions. We find more efficient chemical fractionation in the outer disk: whereas the model of Guillot & Hueso predicts a factor of three enrichment when only <2% of the disk mass remains, we find the same enrichments when 30% of the disk mass remains. Finally, we predict the presence of ∼0.1 M⊕ of water vapor in the outer solar nebula and protoplanetary disks in H II regions. [ABSTRACT FROM AUTHOR]

We present results of a Chandra High-Energy Transmission Grating (HETG) observation of the carbon-rich Wolf-Rayet (WR) star WR 48a. These are the first high-resolution spectra of this object in X-ray. Blueshifted centroids of the spectral lines of ∼ – 360 km s–1 and line widths of 1000-1500 km s–1 (FWHM) were deduced from the analysis of the line profiles of strong emission lines. The forbidden line of Si XIII is strong and not suppressed, indicating that the rarified 10-30 MK plasma forms far from strong sources of far-ultraviolet emission, most likely in a wind collision zone. Global spectral modeling showed that the X-ray spectrum of WR 48a suffered higher absorption in the 2012 October Chandra observation compared with a previous 2008 January XMM-Newton observation. The emission measure of the hot plasma in WR 48a decreased by a factor ∼3 over the same period of time. The most likely physical picture that emerges from the analysis of the available X-ray data is that of colliding stellar winds in a wide binary system with an elliptical orbit. We propose that the unseen secondary star in the system is another WR star or perhaps a luminous blue variable. [ABSTRACT FROM AUTHOR]

We present the results of far ultraviolet (FUV) observations of the broad region around the Aquila Rift including the Galactic plane. As compared with various wavelength data sets, dust scattering is found to be the major origin of the diffuse FUV continuum in this region. The FUV intensity clearly correlates with the dust extinction level for E(B – V) < 0.2, while this correlation disappears for E(B – V) > 0.2 due to heavy dust extinction combined with the effect of nonuniform interstellar radiation fields. The FUV intensity also correlates well with Hα intensity, implying that at least some fraction of the observed Hα emission could be the dust-scattered light of Hα photons originating elsewhere in the Galaxy. Most of the Aquila Rift region is seen devoid of diffuse FUV continuum due to heavy extinction while strong emission is observed in the surrounding regions. Molecular hydrogen fluorescent emission lines are clearly seen in the spectrum of “Aquila-Serpens,” while “Aquila-East” does not show any apparent line features. CO emission intensity is also found to be higher in the “Aquila-Serpens” region than in the “Aquila-East” region. In this regard, we note that regions of star formation have been found in “Aquila-Serpens” but not in “Aquila-East.” [ABSTRACT FROM AUTHOR]