Cloudy Models Research Articles

A cloudy fuzzy economic order quantity model for imperfect-quality items with allowable proportionate discounts

In the traditional economic order quantity/economic production quantity model, most of the items considered are of perfect type. But this situation rarely takes place in practice. Thus, in this paper, an economic order quantity model with imperfect-quality items is developed. 100% screening process is performed, and the items of imperfect quality are sold as a single batch. A proportionate rate of discount for the items of imperfect quality has also been studied. Moreover, a case study has been incorporated to comprehend the model. To nullify the issues of non-random uncertainties of demand rate in business scenario, cloudy fuzzy method has been utilized here. Numerical study reveals that cloud model along with its new defuzzification methods can give maximum profit of the model all the time instead of deterministic ones. Finally, sensitivity analysis and graphical illustrations are made to justify the novelty of the model.

Mass outflow of the X-ray emission line gas in NGC 4151

AbstractWe have analyzed Chandra/High Energy Transmission Grating spectra of the X-ray emission line gas in the Seyfert galaxy NGC 4151. The zeroth-order spectral images show extended H- and He-like O and Ne, up to a distance r ˜ 200 pc from the nucleus. Using the 1st-order spectra, we measure an average line velocity ˜230 km s–1, suggesting significant outflow of X-ray gas. We generated Cloudy photoionization models to fit the 1st-order spectra; the fit required three distinct emission-line components. To estimate the total mass of ionized gas (M) and the mass outflow rates, we applied the model parameters to fit the zeroth-order emission-line profiles of Ne IX and Ne X. We determined an M ≍ 5.4 × 105Mʘ. Assuming the same kinematic profile as that for the [O III] gas, derived from our analysis of Hubble Space Telescope/Space Telescope Imaging Spectrograph spectra, the peak X-ray mass outflow rate is approximately 1.8 Mʘ yr–1, at r ˜ 150 pc. The total mass and mass outflow rates are similar to those determined using [O III], implying that the X-ray gas is a major outflow component. However, unlike the optical outflows, the X-ray emitting mass outflow rate does not drop off at r > 100pc, which suggests that it may have a greater impact on the host galaxy.

From Cold to Hot Irradiated Gaseous Exoplanets: Toward an Observation-based Classification Scheme

AbstractA carbon-to-oxygen ratio (C/O) of around unity is believed to act as a natural separator of water- and methane-dominated spectra when characterizing exoplanet atmospheres. In this paper, we quantify the C/O ratios at which this separation occurs by calculating a large self-consistent grid of cloud-free atmospheric models in chemical equilibrium using the latest version ofpetitCODE. Our study covers a broad range of parameter space: 400 K < Teff < 2600 K, 2.0 < log(g) < 5.0, −1.0 < [Fe/H] < 2.0, 0.25 < C/O < 1.25, and stellar types from M to F. We make the synthetic transmission and emission spectra, as well as the temperature structures, publicly available. We find that the transition C/O ratio depends on many parameters, such as effective temperature, surface gravity, metallicity, and spectral type of the host star, and could have values less than, equal to, or higher than unity. By mapping all of the transition C/O ratios, we propose a “four-class” classification scheme for irradiated planets in this temperature range. We find a parameter space where methane always remains the cause of dominant spectral features. Detection of CH4in this region, or the lack of it, provides a diagnostic tool to identify the prevalence of cloud formation and nonequilibrium chemistry. As another diagnostic tool, we construct syntheticSpitzerInfrared Array Camera color diagrams showing two distinguishable populations of planets. Since most of the exoplanet atmospheres appear cloudy when studied in transmission, we regard this study as a starting point of how such a C/O-sensitive observation-based classification scheme should be constructed. This preparatory work will have to be refined by future cloudy and nonequilibrium modeling to further investigate the existence and exact location of the classes, as well as the color-diagram analysis.

Planetary nebula IC 5148 and its ionized halo

Context. Many round or roundish planetary nebulae (PNe) show multiple shells and halo structures during their evolutionary stage near the maximum temperature of their central star. Controversial debate is ongoing if these structures are recombination halos, as suggested by hydrodynamic modeling efforts, or ionized material. Recently, we discovered a halo with somewhat unusual structures around the sparsely studied PN IC 5148 and present for the first time spectroscopy going out to the halo of such a PN. Aims. We investigate the spatial distribution of material and its ionization state from the center of the nebula up to the very outskirts of the halo. Methods. We obtained long-slit low resolution spectroscopy (FORS2 at VLT) of the nebula in two position angles, which we used to investigate the nebular structure and its halo in the optical range from 450 to 880 nm. In addition we used medium resolution spectra taken with X-shooter at VLT ranging from 320 nm to 2.4 μm to derive atmospheric parameters for the central star. We obtained the distance and position in the Galaxy from various methods combined with Gaia DR2 data. We also applied Cloudy models to the nebula in order to derive physical parameters of the various regions. Results. We obtained spatially resolved structures and detailed descriptions of the outrunning shock front and a set of unusual halo structures denoted to further shock. The halo structures appears clearly as hot ionized material. Furthermore, we derived a reliable photometric value for the central star at a Gaia distance of D = 1.3 kpc. Considering the large distance z = 1.0 kpc from the galactic plane together to its non-circular motion in the galaxy and, a metallicity only slightly below that of typical disk PNe, most likely IC 5148 originates from a thick disk population star.

On the nature and physical conditions of the luminous Ly α emitter CR7 and its rest-frame UV components

We present new HST/WFC3 observations and re-analyse VLT data to unveil the continuum, variability and rest-frame UV lines of the multiple UV clumps of the most luminous Ly$\alpha$ emitter at z=6.6, CR7. Our re-reduced, flux calibrated X-SHOOTER spectra of CR7 reveal a HeII emission line in observations obtained along the major axis of Lyman-alpha (Lya) emission with the best seeing conditions. HeII is spatially offset by +0.8'' from the peak of Lya emission, and it is found towards clump B. Our WFC3 grism spectra detects the UV continuum of CR7's clump A, yielding a power law with $\beta=-2.5^{+0.6}_{-0.7}$ and $M_{UV}=-21.87^{+0.25}_{-0.20}$. No significant variability is found for any of the UV clumps on their own, but there is tentative (~2.2$\sigma$) brightening of CR7 in F110W as a whole from 2012 to 2017. HST grism data fail to robustly detect rest-frame UV lines in any of the clumps, implying fluxes <2x10$^{-17}$ erg s$^{-1}$ cm$^{-2}$ (3 $\sigma$). We perform CLOUDY modelling to constrain the metallicity and the ionising nature of CR7. CR7 seems to be actively forming stars without any clear AGN activity in clump A, consistent with a metallicity of ~0.05-0.2 Z$_{\odot}$. Component C or an inter-clump component between B and C may host a high ionisation source. Our results highlight the need for spatially resolved information to study the formation and assembly of early galaxies.

The GTC exoplanet transit spectroscopy survey

Exoplanets with relatively clear atmospheres are prime targets for detailed studies of chemical compositions and abundances in their atmospheres. Alkali metals have long been suggested to exhibit broad wings due to pressure broadening, but most of the alkali detections only show very narrow absorption cores, probably because of the presence of clouds. We report the strong detection of the pressure-broadened spectral profiles of Na, K, and Li absorption in the atmosphere of the super-Neptune WASP-127b, at 4.1σ, 5.0σ, and 3.4σ, respectively. We performed a spectral retrieval modeling on the high-quality optical transmission spectrum newly acquired with the 10.4 m Gran Telescopio Canarias (GTC), in combination with the re-analyzed optical transmission spectrum obtained with the 2.5 m Nordic Optical Telescope (NOT). By assuming a patchy cloudy model, we retrieved the abundances of Na, K, and Li, which are super-solar at 3.7σ for K and 5.1σ for Li (and only 1.8σ for Na). We constrained the presence of haze coverage to be around 52%. We also found a hint of water absorption, but cannot constrain it with the global retrieval owing to larger uncertainties in the probed wavelengths. WASP-127b will be extremely valuable for atmospheric characterization in the era of James Webb Space Telescope.

Top-of-atmosphere reflectance over homogeneous Lambertian and non-Lambertian surfaces.

In this study, the effects of the Lambertian assumption on the top-of-atmosphere reflectance are evaluated through comparison with calculations derived using a more detailed bidirectional reflectance distribution function under different atmospheric, lighting, and viewing conditions. The numerical experiments are performed for background, dusty, and cloudy models of the atmosphere in spectral channels of 0.44 and 0.87μm. In the case of a background or dusty medium over the terrestrial surface, the overestimation of the top-of-atmosphere reflectance in the forward-scatter viewing direction and underestimation in the backscatter one are observed. The angular range as well as magnitude of the discrepancy is noticeably narrower and lower, respectively, when the atmosphere is more turbid and the wavelength is shorter. The use of the Lambertian assumption instead of "true" ocean reflectance leads to a significant underestimation of the top-of-atmosphere reflectance in the forward-scatter direction and a moderate overestimation of reflectance in the backscatter one. The ocean reflectance generally exhibits a high dependence on wind, which affects the reflected solar radiation distribution around the forward-scatter direction. Analysis of simulation results for an overcast sky showed that, in general, the multiple scattered radiation smoothes the anisotropy effects. However, there are conditions at which the choice of a bidirectional reflectance distribution function model is significant: in the case of thin cirrus cloudiness over the ocean at large solar zenith angles and stratus cloudiness with an optical thickness of at least 5 over a vegetation cover or ocean in the near-infrared region of the spectrum.

HST/WFPC2 imaging analysis and cloudy modelling of the multiple-shell planetary nebulae NGC 3242, NGC 6826, and NGC 7662

We performed a detailed photometric analysis and photoionisation modelling on three high excitation multiple shell planetary nebulae: NGC 3242, NGC 6826 and NGC 7662. Archival HST/WFPC2 narrow band filter images were used to investigate shocked regions by two independent methods: using low excitation ions (via H alpha/[N II] vs H alpha/[S II] extended diagnostic diagrams) and by means of high excitation species looking for regions of enhanced [O III]/H alpha line ratios. Shocked region analysis via low excitation ions shows that major deviations from the general inside to outside ionisation trend correspond only to regions where FLIERs or LISs are located. The reduction on the SNR at the outskirts of the [O III]/H alpha ratio maps, made us unable to unambiguously identified for an enhancement on the [O III]/H alpha line ratio as an indicator for shocks. For non-shocked regions we performed a photoionisation modelling using Cloudy. Fittings to the [O III] and H alpha observed radial profiles lead us to constrain on the free parameters of the density laws and filling factors together to temperatures and luminosities for the CSPNe. Best fit models show a very well representation of the [O III] and H alpha emission. Discrepancies in the model fittings to the [N II] and [S II] profiles at NGC 6826 and NGC 3242, can be attributed in the former case, due to a contamination by the light of the CSPN and, in the latter case, either due to gas inhomogeneities within the clumps or to a leaking of UV radiation.

WR galaxies and PAH: observations and modeling

This report analyzes the archival data of the Spitzer observatory for 9 WR galaxies showing PAH emission lines at 6.2 and 7.7 µ. For these galaxies, the effective radii of the emitting regions and the total number of hydrogen ionizing quanta were calculated on the base of the fluxes corrected for interstellar extinction. Using the observed values (which are obtained using SDSS data), and taking as an example stars of the WNh type (Ts = 50000 K, R = 20 R☉), the number of massive WR stars in the cluster and the distance to the nearest neighbor are determined. With these data, the theoretical Cloudy models, in particular, the distribution of H, and He, C, O, Ne atoms and ions along the radius are calculated. The theoretical intensities of the characteristic PAH bands at 6.2 and 7.7 µ were obtained for two types of PAH (containing 15 and 120 carbon atoms, respectively). The corpuscular radiation doses accumulated in PAH molecules due to hard irradiation by MeV protons were calculated and it was shown that such molecules had to decay for a time less than the lifetime of the cluster. A conclusion is drawn about the permanent source of PAH, most likely, the remnants of supernova explosions.

The Prototypical Young L/T-Transition Dwarf HD 203030B Likely Has Planetary Mass

Abstract Upon its discovery in 2006, the young L7.5 companion to the solar analog HD 203030 was found to be K cooler than older late-L dwarfs, which is quite unusual. HD 203030B offered the first clear indication that the effective temperature at the L-to-T spectral type transition depends on surface gravity: now a well-known characteristic of low-gravity ultra-cool dwarfs. An initial age analysis of the G8V primary star indicated that the system was 130–400 Myr old, and so the companion would be between 12 and 31 . Using moderate-resolution near-infrared spectra of HD 203030B, we now find features of very low gravity comparable to those of 10–150 Myr old L7–L8 dwarfs. We also obtained more accurate near-infrared and Spitzer/IRAC photometry, and we find a color of 2.56 ± 0.13 mag—comparable to those observed in other young planetary-mass objects—and a luminosity of log( ) = −4.75 ± 0.04 dex. We further re-assess the evidence for the young age of the host star, HD 203030, with a more comprehensive analysis of the photometry and updated stellar activity measurements and age calibrations. Summarizing the age diagnostics for both components of the binary, we adopt an age of 100 Myr for HD 203030B and an age range of 30–150 Myr. Using cloudy evolutionary models, the new companion age range and luminosity result in a mass of 11 with a range of 8–15 , and an effective temperature of 1040 ± 50 K.

Convenient models of the atmosphere: optics and solar radiation

Simple optical models of clear and cloudy atmosphere are proposed. Four versions of atmospheric aerosols content are considered: a complete lack of aerosols in the atmosphere, low background concentration (500 cm−3), high concentrations (2000 cm−3) and very high content of particles (5000 cm−3). In a cloud scenario, the model of external mixture is assumed. The values of optical thickness and single scattering albedo for 13 wavelengths are calculated in the short wavelength range of 0.28–0.90 µm, with regard to the molecular absorption bands, that is simulated with triangle function. A comparison of the proposed optical parameters with results of various measurements and retrieval (lidar measurement, sampling, processing radiation measurements) is presented. For a cloudy atmosphere models of single-layer and two-layer atmosphere are proposed. It is found that cloud optical parameters with assuming the “external mixture” agrees with retrieved values from airborne observations. The results of calculating hemispherical fluxes of the reflected and transmitted solar radiation and the radiative divergence are obtained with the Delta-Eddington approach. The calculation is done for surface albedo values of 0, 0.5, 0.9 and for spectral values of the sandy surface. Four values of solar zenith angle: 0°, 30°, 40° and 60° are taken. The obtained values are compared with data of radiative airborne observations. Estimating the local instantaneous radiative forcing of atmospheric aerosols and clouds for considered models is presented together with the heating rate.

Spectral and atmospheric characterization of 51 Eridani b using VLT/SPHERE

51 Eridani b is an exoplanet around a young (20 Myr) nearby (29.4 pc) F0-type star, recently discovered by direct imaging. Being only 0.5" away from its host star it is well suited for spectroscopic analysis using integral field spectrographs. We aim to refine the atmospheric properties of this and to further constrain the architecture of the system by searching for additional companions. Using the SPHERE instrument at the VLT we extend the spectral coverage of the planet to the complete Y- to H-band range and provide photometry in the K12-bands (2.11, 2.25 micron). The object is compared to other cool and peculiar dwarfs. Furthermore, the posterior probability distributions of cloudy and clear atmospheric models are explored using MCMC. We verified our methods by determining atmospheric parameters for the two benchmark brown dwarfs Gl 570D and HD 3651B. For probing the innermost region for additional companions, archival VLT-NACO (L') SAM data is used. We present the first spectrophotometric measurements in the Y- and K-bands for the planet and revise its J-band flux to values 40% fainter than previous measurements. Cloudy models with uniform cloud coverage provide a good match to the data. We derive the temperature, radius, surface gravity, metallicity and cloud sedimentation parameter f_sed. We find that the atmosphere is highly super-solar (Fe/H~1.0) with an extended, thick cloud cover of small particles. The model radius and surface gravity suggest planetary masses of about 9 M_jup. The evolutionary model only provides a lower mass limit of >2 M_jup (for pure hot-start). The cold-start model cannot explain the planet's luminosity. The SPHERE and NACO/SAM detection limits probe the 51 Eri system at Solar System scales and exclude brown-dwarf companions more massive than 20 M_jup beyond separations of ~2.5 au and giant planets more massive than 2 M_jup beyond 9 au.

The Y-type Brown Dwarfs: Estimates of Mass and Age from New Astrometry, Homogenized Photometry, and Near-infrared Spectroscopy

Abstract The survey of the mid-infrared sky by the Wide-field Infrared Survey Explorer (WISE) led to the discovery of extremely cold, low-mass brown dwarfs, classified as Y dwarfs, which extend the T class to lower temperatures. Twenty-four Y dwarfs are known at the time of writing. Here we present improved parallaxes for four of these, determined using Spitzer images. We give new photometry for four late-type T and three Y dwarfs and new spectra of three Y dwarfs, obtained at Gemini Observatory. We also present previously unpublished photometry taken from HST, ESO, Spitzer, and WISE archives of 11 late-type T and 9 Y dwarfs. The near-infrared data are put onto the same photometric system, forming a homogeneous data set for the coolest brown dwarfs. We compare recent models to our photometric and spectroscopic data set. We confirm that nonequilibrium atmospheric chemistry is important for these objects. Nonequilibrium cloud-free models reproduce well the near-infrared spectra and mid-infrared photometry for the warmer Y dwarfs with 425 ≤ T eff (K) ≤ 450. A small amount of cloud cover may improve the model fits in the near-infrared for the Y dwarfs with 325 ≤ T eff (K) ≤ 375. Neither cloudy nor cloud-free models reproduce the near-infrared photometry for the T eff = 250 K Y dwarf W0855. We use the mid-infrared region, where most of the flux originates, to constrain our models of W0855. We find that W0855 likely has a mass of 1.5–8 Jupiter masses and an age of 0.3–6 Gyr. The Y dwarfs with measured parallaxes are within 20 pc of the Sun and have tangential velocities typical of the thin disk. The metallicities and ages we derive for the sample are generally solar-like. We estimate that the known Y dwarfs are 3 to 20 Jupiter-mass objects with ages of 0.6–8.5 Gyr.

Emission Line Ratios of FE III as Astrophysical Plasma Diagnostics

Abstract Recent, state-of-the-art calculations of A-values and electron impact excitation rates for Fe iii are used in conjunction with the Cloudy modeling code to derive emission-line intensity ratios for optical transitions among the fine-structure levels of the 3d6 configuration. A comparison of these with high-resolution, high signal-to-noise spectra of gaseous nebulae reveals that previous discrepancies found between theory and observation are not fully resolved by the latest atomic data. Blending is ruled out as a likely cause of the discrepancies, because temperature- and density-independent ratios (arising from lines with common upper levels) match well with those predicted by theory. For a typical nebular plasma with electron temperature K and electron density , cascading of electrons from the levels , and plays an important role in determining the populations of lower levels, such as , which provide the density diagnostic emission lines of Fe iii, such as - at 4658 Å. Hence, further work on the A-values for these transitions is recommended, ideally including measurements if possible. However, some Fe iii ratios do provide reliable -diagnostics, such as 4986/4658. The Fe iii cooling function, calculated with Cloudy using the most recent atomic data, is found to be significantly greater at T e 30,000 K than predicted with the existing Cloudy model. This is due to the presence of additional emission lines with the new data, particularly in the 1000–4000 Å wavelength region.

Identification of Near-infrared [Se iii] and [Kr vi] Emission Lines in Planetary Nebulae∗

Abstract We identify [Se iii] 1.0994 μm in the planetary nebula (PN) NGC 5315 and [Kr vi] 1.2330 μm in three PNe from spectra obtained with the Folded-Port InfraRed Echellette (FIRE) spectrometer on the 6.5 m Baade Telescope. Se and Kr are the two most widely detected neutron-capture elements in astrophysical nebulae, and can be enriched by s-process nucleosynthesis in PN progenitor stars. The detection of [Se iii] 1.0994 μm is particularly valuable when paired with observations of [Se iv] 2.2864 μm, as it can be used to improve the accuracy of nebular Se abundance determinations, and allows Se ionization correction factor (ICF) schemes to be empirically tested for the first time. We present new effective collision strength calculations for Se2+ and Kr5+, which we use to compute ionic abundances. In NGC 5315, we find that the Se abundance computed from Se3+/H+ is lower than that determined with ICFs that incorporate Se2+/H+. We compute new Kr ICFs that take Kr5+/H+ into account, by fitting correlations found in grids of Cloudy models between Kr ionic fractions and those of more abundant elements, and use these to derive Kr abundances in four PNe. Observations of [Se iii] and [Kr vi] in a larger sample of PNe, with a range of excitation levels, are needed to rigorously test the ICF prescriptions for Se and our new Kr ICFs.

Excitation of Molecular Hydrogen in the Orion Bar PhotodissociationRegion from a Deep Near-infrared IGRINS Spectrum

Abstract We present a deep near-infrared spectrum of the Orion Bar Photodissociation Region (PDR) taken with the Immersion Grating INfrared Spectrometer (IGRINS) on the 2.7 m telescope at the McDonald Observatory. IGRINS has high spectral resolution ( R ∼ 45,000 ) and instantaneous broad wavelength coverage (1.45–2.45 μm), enabling us to detect 87 emission lines from rovibrationally excited molecular hydrogen (H2) that arise from transitions out of 69 upper rovibration levels of the electronic ground state. These levels cover a large range of rotational and vibrational quantum numbers and excitation energies, making them excellent probes of the excitation mechanisms of H2 and physical conditions within the PDR. The Orion Bar PDR is thought to consist of cooler high density clumps or filaments ( T = 50 – 250 K, n H = 10 5 – 10 7 cm−3) embedded in a warmer lower density medium ( T = 250 – 1000 K, n H = 10 4 – 10 5 cm−3). We fit a grid of constant temperature and density Cloudy models, which recreate the observed H2 level populations well, to constrain the temperature to a range of 600–650 K and the density to n H = 2.5 × 10 3 – 10 4 cm−3. The best-fit model gives T = 625 K and n H = 5 × 10 3 cm−3. This well-constrained warm temperature is consistent with kinetic temperatures found by other studies for the Orion Bar’s lower density medium. However, the range of densities well fit by the model grid is marginally lower than those reported by other studies. We could be observing lower density gas than the surrounding medium, or perhaps a density-sensitive parameter in our models is not properly estimated.

Which Stars Are Ionizing the Orion Nebula?

Abstract The common assumption that is the dominant ionizing source for the Orion Nebula is critically examined. This assumption underlies much of the existing analysis of the nebula. In this paper we establish through comparison of the relative strengths of emission lines with expectations from Cloudy models and through the direction of the bright edges of proplyds that , which lies beyond the Bright Bar, also plays an important role. does dominate ionization in the inner part of the Orion Nebula, but outside of the Bright Bar as far as the southeast boundary of the Extended Orion Nebula, is the dominant source. In addition to identifying the ionizing star in sample regions, we were able to locate those portions of the nebula in 3D. This analysis illustrates the power of MUSE spectral imaging observations to identify sources of ionization in extended regions.

ULTRAVIOLET EMISSION LINES OF Si ii IN QUASARS—INVESTIGATING THE “Si ii DISASTER”

ABSTRACT The observed line intensity ratios of the Si ii λ1263 and λ1307 multiplets to that of Si ii λ1814 in the broad-line region (BLR) of quasars are both an order of magnitude larger than the theoretical values. This was first pointed out by Baldwin et al., who termed it the “Si ii disaster,” and it has remained unresolved. We investigate the problem in the light of newly published atomic data for Si ii. Specifically, we perform BLR calculations using several different atomic data sets within the CLOUDY modeling code under optically thick quasar cloud conditions. In addition, we test for selective pumping by the source photons or intrinsic galactic reddening as possible causes for the discrepancy, and we also consider blending with other species. However, we find that none of the options investigated resolve the Si ii disaster, with the potential exception of microturbulent velocity broadening and line blending. We find that a larger microturbulent velocity ( ) may solve the Si ii disaster through continuum pumping and other effects. The CLOUDY models indicate strong blending of the Si ii λ1307 multiplet with emission lines of O i, although the predicted degree of blending is incompatible with the observed λ1263/λ1307 intensity ratios. Clearly, more work is required on the quasar modeling of not just the Si ii lines but also nearby transitions (in particular those of O i) to fully investigate whether blending may be responsible for the Si ii disaster.

THE FIRST DETECTION OF PHOTOMETRIC VARIABILITY IN A Y DWARF: WISE J140518.39+553421.3

ABSTRACT We present the first detection of the photometric variability in a spectroscopically confirmed Y dwarf. The Infrared Array Camera on board the Spitzer Space Telescope was used to obtain time series photometry of WISE J140518.39+553421.3 at 3.6 and 4.5 μm over a 24-hr period at two different epochs separated by 149 days. Variability is evident at 4.5 μm in the first epoch and at 3.6 and 4.5 μm in the second epoch, which suggests that the underlying cause or causes of this variability change on the timescales of months. The second-epoch [3.6] and [4.5] light curves are nearly sinusoidal in form, in phase, have periods of roughly 8.5 hr, and have semi-amplitudes of 3.5%. We find that a simple geometric spot model with a single bright spot reproduces these observations well. We also compare our measured semi-amplitudes of the second-epoch light curves to predictions of the static, one-dimensional, partly cloudy, and hot spot models of Morley and collaborators, and find that neither set of models can reproduce the observed [3.6] and [4.5] semi-amplitudes simultaneously. Therefore, more advanced two-dimensional or three-dimensional models that include time-dependent phenomena like vertical mixing, cloud formation, and thermal relaxation are sorely needed in order to properly interpret our observations.

ORION’S VEIL. IV. H2 EXCITATION AND GEOMETRY

ABSTRACT The foreground Veil of material that lies in front of the Orion Nebula is the best studied sample of the interstellar medium because we know where it is located, how it is illuminated, and the balance of thermal and magnetic energy. In this work, we present high-resolution STIS observations toward the Trapezium, with the goal of better understanding the chemistry and geometry of the two primary Veil layers, along with ionized gas along the line of sight. The most complete characterization of the rotational/vibrational column densities of H2 in the almost purely atomic components of the Veil are presented, including updates to the Cloudy model for H2 formation on grain surfaces. The observed H2 is found to correlate almost exclusively with Component B. The observed H2, observations of CI, CI*, and CI**, and theoretical calculations using Cloudy allow us to place the tightest constraints yet on the distance, density, temperature, and other physical characteristics for each cloud component. We find the H2 excitation spectrum observed in the Veil is incompatible with a recent study that argued that the Veil was quite close to the Trapezium. The nature of a layer of ionized gas lying between the Veil and the Trapezium is characterized through the emission and absorption lines it produces, which we find to be the blueshifted component observed in S iii and P iii absorption. We deduce that, within the next 30–60 thousand years, the blueshifted ionized layer and Component B will merge, which will subsequently merge with Component A in the next one million years.