Non-local Thermodynamic Equilibrium Radiative Transfer Research Articles

New predictions for radiation-driven, steady-state mass-loss and wind-momentum from hot, massive stars

Context. Reliable predictions of mass-loss rates are important for massive-star evolution computations. Aims. We aim to provide predictions for mass-loss rates and wind-momentum rates of O-type stars, while carefully studying the behaviour of these winds as functions of stellar parameters, such as luminosity and metallicity. Methods. We used newly developed steady-state models of radiation-driven winds to compute the global properties of a grid of O-stars. The self-consistent models were calculated by means of an iterative solution to the equation of motion using full non-local thermodynamic equilibrium radiative transfer in the co-moving frame to compute the radiative acceleration. In order to study winds in different galactic environments, the grid covers main-sequence stars, giants, and supergiants in the Galaxy and both Magellanic Clouds. Results. We find a strong dependence of mass-loss on both luminosity and metallicity. Mean values across the grid are Ṁ~L*2.2 and Ṁ~L*0.95; however, we also find a somewhat stronger dependence on metallicity for lower luminosities. Similarly, the mass loss-luminosity relation is somewhat steeper for the Small Magellanic Cloud (SMC) than for the Galaxy. In addition, the computed rates are systematically lower (by a factor 2 and more) than those commonly used in stellar-evolution calculations. Overall, our results are in good agreement with observations in the Galaxy that properly account for wind-clumping, with empirical Ṁ versus Z* scaling relations and with observations of O-dwarfs in the SMC. Conclusions. Our results provide simple fit relations for mass-loss rates and wind momenta of massive O-stars stars as functions of luminosity and metallicity, which are valid in the range Teff = 28 000–45 000 K. Due to the systematically lower values for Ṁ, our new models suggest that new rates might be needed in evolution simulations of massive stars.

Non-local Thermodynamic Equilibrium Radiative Transfer Simulations of Sub-Chandrasekhar-mass White Dwarf Detonations

Abstract Type Ia supernovae (SNe Ia) span a range of luminosities and timescales, from rapidly evolving subluminous to slowly evolving overluminous subtypes. Previous theoretical work has, for the most part, been unable to match the entire breadth of observed SNe Ia with one progenitor scenario. Here, for the first time, we apply non-local thermodynamic equilibrium radiative transfer calculations to a range of accurate explosion models of sub-Chandrasekhar-mass white dwarf detonations. The resulting photometry and spectra are in excellent agreement with the range of observed nonpeculiar SNe Ia through 15 days after the time of B-band maximum, yielding one of the first examples of a quantitative match to the entire Phillips relation. The intermediate-mass element velocities inferred from theoretical spectra at maximum light for the more massive white dwarf explosions are higher than those of bright observed SNe Ia, but these and other discrepancies likely stem from the one-dimensional nature of our explosion models and will be improved upon by future non-local thermodynamic equilibrium radiation transport calculations of multidimensional sub-Chandrasekhar-mass white dwarf detonations.

Atomic line radiative transfer with MCFOST

Aims. We present MCFOST-art, a new non-local thermodynamic equilibrium radiative transfer solver for multilevel atomic systems. The code is embedded in the 3D radiative transfer code MCFOST and is compatible with most of the MCFOST modules. The code is versatile and designed to model the close environment of stars in 3D. Methods. The code solves for the statistical equilibrium and radiative transfer equations using the Multilevel Accelerated Lambda Iteration method. We tested MCFOST-art on spherically symmetric models of stellar photospheres as well as on a standard model of the solar atmosphere. We computed atomic level populations and outgoing fluxes and compared these values with the results of the TURBOspectrum and RH codes. Calculations including expansion and rotation of the atmosphere were also performed. We tested both the pure local thermodynamic equilibrium and the out-of-equilibrium problems. Results. In all cases, the results from all codes agree within a few percent at all wavelengths and reach the sub-percent level between RH and MCFOST-art. We still note a few marginal discrepancies between MCFOST-art and TURBOspectrum as a result of different treatments of background opacities at some critical wavelength ranges.

Non-LTE modelling of cyanoacetylene: evidence for isomer-specific excitation

ABSTRACT Cyanoacetylene molecules are widespread in the interstellar medium (ISM) and several of its isomers have been detected in cold molecular clouds and circumstellar gas. Accurate estimates of the abundance ratio of cyanoacetylene isomers may provide deep insight into their environment. Such knowledge requires rigorous modelling of the emission spectra based on non-local thermodynamic equilibrium (LTE) radiative transfer calculations. To this end, we computed excitation cross-sections of HC2NC and HNC3 induced by collision with para- and ortho-H2, using a quantum mechanical close-coupling method. Then, by thermally averaging these data, we derived rate coefficients for the first 31 low-lying rotational levels of each isomer for temperatures up to 80 K. For the para-H2 collider, the propensity rules are in favour of rotational transitions involving Δj1 = 2 for both isomers, while for the ortho-H2 collider, Δj1 = 2 and Δj1 = 1 rotational transitions are favoured for HC2NC and HNC3, respectively. A comparison of rate coefficients for the HC3N isomers shows differences up to an order of magnitude, especially at low temperatures. Finally, we performed non-LTE radiative transfer calculations to assess the impact of such variations in the analysis of observations. Our simulation suggests that the lack of collisional data specific to each isomer could lead to errors up to a factor of 2–3 in the excitation temperatures. We expect that these data could help in better understanding the cyanoacetylene chemistry and constraining the nitrogen chemistry in the ISM.

Radiative-transfer modeling of supernovae in the nebular-phase

Supernova (SN) explosions play a pivotal role in the chemical evolution of the Universe and the origin of life through the metals they release. Nebular phase spectroscopy constrains such metal yields, for example through forbidden line emission associated with O I, Ca II, Fe II, or Fe III. Fluid instabilities during the explosion produce a complex 3D ejecta structure, with considerable macroscopic, but no microscopic, mixing of elements. This structure sets a formidable challenge for detailed nonlocal thermodynamic equilibrium radiative transfer modeling, which is generally limited to 1D in grid-based codes. Here, we present a novel and simple method that allows for macroscopic mixing without any microscopic mixing, thereby capturing the essence of mixing in SN explosions. With this new technique, the macroscopically mixed ejecta are built by shuffling the shells from the unmixed coasting ejecta in mass space, or equivalently in velocity space. The method requires no change to the radiative transfer, but it necessitates high spatial resolution to resolve the rapid variation in composition with depth inherent to this shuffled-shell structure. We show the results for a few radiative-transfer simulations for a Type II SN explosion from a 15 M⊙ progenitor star. Our simulations capture the strong variations in temperature or ionization between the various shells that are rich in H, He, O, or Si. Because of nonlocal energy deposition, γ rays permeate through an extended region of the ejecta, making the details of the shell arrangement unimportant. The greater physical consistency of the method delivers spectral properties at nebular times that are more reliable, in particular in terms of individual emission line strengths, which may serve to constrain the SN yields as well as the progenitor mass for core collapse SNe. The method works for all SN types.

Excitation and acceleration of molecular outflows in LIRGs: The extended ESO 320-G030 outflow on 200-pc scales

We used high-spatial resolution (70 pc; 0$ {{\overset{\prime\prime}{.}}} $3) CO multi-transition (J = 1–0, 2–1, 4–3, and 6–5) ALMA data to study the physical conditions and kinematics of the cold molecular outflow in the local luminous infrared galaxy (LIRG) ESO 320-G030 (d = 48 Mpc, LIR/L⊙ = 1011.3). ESO 320-G030 is a double-barred isolated spiral, but its compact and obscured nuclear starburst (SFR ∼ 15 M⊙ yr−1; AV ∼ 40 mag) resembles those of ultra-luminous infrared galaxies (LIR/L⊙ > 1012). In the outflow, the CO(1–0)/CO(2–1) ratio is enhanced with respect to the rest of the galaxy and the CO(4–3) transition is undetected. This indicates that the outflowing molecular gas is less excited than the molecular gas in the nuclear starburst (i.e., outflow launching site) and in the galaxy disk. Non-local thermodynamic equilibrium radiative transfer modeling reveals that the properties of the molecular clouds in the outflow differ from those of the nuclear and disk clouds: The kinetic temperature is lower (Tkin ∼ 9 K) in the outflow, and the outflowing clouds have lower column densities. Assuming a 10−4 CO abundance, the large internal velocity gradients, 60−45+250 km s−1 pc−1, imply that the outflowing molecular clouds are not bound by self-gravity. All this suggests that the life-cycle (formation, collapse, dissipation) of the galaxy disk molecular clouds might differ from that of the outflowing molecular clouds which might not be able to form stars. The low kinetic temperature of the molecular outflow remains constant at radial distances between 0.3 and 1.7 kpc. This indicates that the heating by the hotter ionized outflow phase is not efficient and may favor the survival of the molecular gas phase in the outflow. The spatially resolved velocity structure of the outflow shows a 0.8 km s−1 pc−1 velocity gradient between 190 pc and 560 pc and then a constant maximum outflow velocity of about 700–800 km s−1 up to 1.7 kpc. This could be compatible with a pure gravitational evolution of the outflow, which would require coupled variations of the mass outflow rate and the outflow launching velocity distribution. Alternatively, a combination of ram pressure acceleration and cloud evaporation could explain the observed kinematics and the total size of the cold molecular phase of the outflow.

Radiative-transfer modeling of nebular-phase type II supernovae

Nebular phase spectra of core-collapse supernovae (SNe) provide critical and unique information on the progenitor massive star and its explosion. We present a set of one-dimensional steady-state non-local thermodynamic equilibrium radiative transfer calculations of type II SNe at 300 d after explosion. Guided by the results obtained from a large set of stellar evolution simulations, we craft ejecta models for type II SNe from the explosion of a 12, 15, 20, and 25 M⊙ star. The ejecta density structure and kinetic energy, the 56Ni mass, and the level of chemical mixing are parametrized. Our model spectra are sensitive to the adopted line Doppler width, a phenomenon we associate with the overlap of Fe II and O I lines with Ly α and Ly β. Our spectra show a strong sensitivity to 56Ni mixing since it determines where decay power is absorbed. Even at 300 d after explosion, the H-rich layers reprocess the radiation from the inner metal rich layers. In a given progenitor model, variations in 56Ni mass and distribution impact the ejecta ionization, which can modulate the strength of all lines. Such ionization shifts can quench Ca II line emission. In our set of models, the [O I] λλ 6300, 6364 doublet strength is the most robust signature of progenitor mass. However, we emphasize that convective shell merging in the progenitor massive star interior can pollute the O-rich shell with Ca, which would weaken the O I doublet flux in the resulting nebular SN II spectrum. This process may occur in nature, with a greater occurrence in higher mass progenitors, and this may explain in part the preponderance of progenitor masses below 17 M⊙ that are inferred from nebular spectra.

Umbral chromospheric fine structure and umbral flashes modelled as one: The corrugated umbra

Context.The chromosphere of the umbra of sunspots features an assortment of dynamic fine structures that are poorly understood and often studied separately. Small-scale umbral brightenings (SSUBs), umbral microjets, spikes or short dynamic fibrils (SDFs), and umbral dark fibrils are found in any observation of the chromosphere with sufficient spatial resolution performed at the correct umbral flash stage and passband. Understanding these features means understanding the dynamics of the umbral chromosphere.Aims.We aim to fully understand the dynamics of umbral chromosphere through analysis of the relationships between distinct observed fine features and to produce complete models that explain both spectral profiles and the temporal evolution of the features. We seek to relate such understanding to umbral flashes.Methods.We studied the spatial and spectral co-evolution of SDFs, SSUBs, and umbral flashes in Ca II8542 Å spectral profiles. We produced models that generate the spectral profiles for all classes of features using non-local thermodynamic equilibrium radiative transfer with a recent version of the NICOLE inversion code.Results.We find that both bright SSUBs and dark SDF structures are described with a continuous feature in the parameter space that is distinct from the surroundings even in pixel-by-pixel inversions. We find a phase difference between such features and umbral flashes in both inverted line-of-sight velocities and timing of the brightenings. For umbral flashes themselves we resolve, for the first time in inversion-based semi-empirical modelling, the pre-flash downflows, post-flash upflows, and the counter-flows present during the umbral flash phase. We further present a simple time-dependent cartoon model that explains the dynamics and spectral profiles of both fine structure, dark and bright, and umbral flashes in umbral chromospheres.Conclusions.The similarity of the profiles between the brightenings and umbral flashes, the pattern of velocities obtained from the inversions, and the phase relationships between the structures all lead us to put forward that all dynamic umbral chromospheric structures observed to this date are a locally delayed or locally early portion of the oscillatory flow pattern that generates flashes, secondary to the steepening large-scale acoustic waves at its source. Essentially, SSUBs are part of the same shock or merely compression front responsible for the spatially larger umbral flash phenomenon, but out of phase with the broader oscillation.

Non-local thermodynamic equilibrium transmission spectrum modelling of HD 209458b

Context. Exoplanetary upper atmospheres are low density environments where radiative processes can compete with collisional ones and introduce non-local thermodynamic equilibrium (NLTE) effects into transmission spectra. Aims. We develop a NLTE radiative transfer framework capable of modelling exoplanetary transmission spectra over a wide range of planetary properties. Methods. We adapted the NLTE spectral synthesis code Cloudy to produce an atmospheric structure and atomic transmission spectrum in both NLTE and local thermodynamic equilibrium (LTE) for the hot Jupiter HD 209458b, given a published T–P profile and assuming solar metallicity. Selected spectral features, including Hα, NaI D, HeI λ10 830, FeI and II ultra-violet (UV) bands, and C, O, and Si UV lines, are compared with literature observations and models where available. The strength of NLTE effects are measured for individual spectral lines to identify which features are most strongly affected. Results. The developed modelling framework that computes NLTE synthetic spectra reproduces literature results for the HeI λ10 830 triplet, the NaI D lines, and the forest of FeI lines in the optical. Individual spectral lines in the NLTE spectrum exhibit up to 40% stronger absorption relative to the LTE spectrum.

Modelling spectra of MN112

ABSTRACT MN112 is a Galactic luminous blue variable (LBV) candidate with a circumstellar nebula. P Cygni was the first LBV discovered, and was recorded during major eruptions in the 17th century. The stars have similar spectra with strong emission hydrogen lines, He i, N ii, Si ii, and Fe iii lines. We present the results of spectroscopic analysis and modelling of MN112 spectra. We obtained the main stellar parameters and chemical abundances of MN112 and compared them with those of P Cygni. Atmosphere models were calculated using the non-local thermodynamic equilibrium radiative transfer code cmfgen. We have used spectra of MN112 obtained with the 3.5-m telescope at the Observatory of Calar Alto and 3.5-m ARC telescope at the Apache Point Observatory. P Cygni spectra were taken with the 6-m BTA telescope. We have found the best fit of the observed spectrum with the model at temperature $T_{\text{eff}}= 15\, 200$ K, clumping-corrected mass-loss rate $\dot{M}f^{-0.5}=5.74 \times 10^{-5}\, \mathrm{M}_{\odot }\text{yr}^{-1}$, filling factor f = 0.1, luminosity $L=5.77 \times 10^5\, \mathrm{L}_{\odot }$ for MN112. The ratio of helium to hydrogen He/H is 0.27 (by the number of atoms) with nitrogen overabundance (XN/X⊙ = 6.8) and an underabundance of carbon (XC/X⊙ < 0.1).

CO and HCN isotopologue ratios in the outflows of AGB stars

Context. Isotopologue line intensity ratios of circumstellar molecules have been widely used to trace the photospheric elemental isotopic ratios of evolved stars. However, depending on the molecular species and the physical conditions of the environment, the isotopologue ratio in the circumstellar envelope (CSE) may deviate considerably from the stellar atmospheric value. Aims. In this paper, we aim to examine how the 12CO/13CO and H12CN/H13CN abundance ratios vary radially due to chemical reactions in the outflows of asymptotic giant branch (AGB) stars and the effect of excitation and optical depth on the resulting line intensity ratios. We study both carbon-rich (C-type) and oxygen-rich (O-type) CSEs. Methods. We performed chemical modeling to derive radial abundance distributions of our selected species in the CSEs over a wide range of mass-loss rates (10−8 < Ṁ < 10−4 M⊙ yr−1). We used these as input in a non-local thermodynamic equilibrium radiative transfer code to derive the line intensities of several ground-state rotational transitions. We also test the influence of stellar parameters, physical conditions in the outflows, the intensity of the interstellar radiation field, and the importance of considering the chemical networks in our model results. Results. We quantified deviations from the atmospheric value for typical outflows. We find that the circumstellar value of 12CO/13CO can deviate from its atmospheric value by up to 25–94% and 6–60% for C- and O-type CSEs, respectively, in radial ranges that depend on the mass-loss rate. We show that variations of the intensity of the interstellar radiation field and the gas kinetic temperature can significantly influence the CO isotopologue abundance ratio in the outer CSEs of both C-type and O-type. On the contrary, the H12CN/H13CN abundance ratio is stable throughout the CSEs for all tested mass-loss rates. The radiative transfer modeling shows that the integrated line intensity ratio I12CO/I13CO of different rotational transitions varies significantly for stars with mass-loss rates in the range from 10−7 to 10−6 M⊙ yr−1 due to combined chemical and excitation effects. In contrast, the excitation conditions for the HCN isotopologues are the same for both isotopologues. Conclusions. We demonstrate the importance of using the isotopologue abundance profiles from detailed chemical models as inputs to radiative transfer models in the interpretation of isotopologue observations. Previous studies of circumstellar CO isotopologue ratios are based on multi-transition data for individual sources and it is difficult to estimate the errors in the reported values due to assumptions that are not entirely correct according to this study. If anything, previous studies may have overestimated the circumstellar 12CO/13CO abundance ratio. The use of the HCN molecule as a tracer of C isotope ratios is affected by fewer complicating problems, but we note that the corrections for high optical depths are very large in the case of high-mass-loss-rate C-type CSEs; and in O-type CSEs the H13CN lines may be too weak to detect.

Effect of the 3D distribution on water observations made with the SWI

Context. Characterising and understanding the atmospheres of Jovian icy moons is one of the key exploration goals of the Submillimetre Wave Instrument (SWI), which is to be flown on ESA’s Jupiter Icy Moons Explorer (JUICE) mission. Aims. The aim of this paper is to investigate how and under which conditions a 3D asymmetric distribution of the atmosphere may affect the SWI observations. In this work we target the role of phase angle for both nadir and limb geometries for unresolved and partially resolved disc observations from large distances. Methods. We adapted the LIME software package, a 3D non-local thermodynamical equilibrium radiative transfer model, to evaluate ortho-H2O populations and synthesise the simulated SWI beam spectra for different study cases of Ganymede’s atmosphere. The temperature and density vertical distributions were adopted from a previous work. The study cases presented here were selected according to the distance and operational scenarios of moon monitoring anticipated for SWI during the Jupiter phase of the JUICE mission. Results. We demonstrate that nadir and limb observations at different phase angles will modify the line amplitude and width. Unresolved observations where both absorption against surface continuum and limb emission contributes within the beam will lead to characteristic line wing emission, which may also appear in pure nadir geometry for specific phase angles. We also find that for Ganymede, the 3D non-local thermodynamical equilibrium populations are more highly excited in the upper atmosphere near the sub-solar region than they are in 1D spherically symmetric models. Finally, the 3D radiative transfer is better suited to properly simulate spectral lines for cases where density or population gradients exist along the line of sight.

Spectroscopic patch model for massive stars using PHOEBE II and FASTWIND

Context. Massive stars play an important role in the mechanical and chemical evolution of galaxies. Understanding the internal processes of these stars is vital to our understanding of their evolution and eventual end products. Deformations from spherical geometry are common for massive stars; however, the tools that are currently available for the study of these systems are almost exclusively one-dimensional. Aims. We present a new spectroscopic analysis tool tailored for massive stars that deviate from spherical symmetry. This code (entitled SPAMMS) is a spectroscopic patch model that takes the three-dimensional surface geometry of the system into account to produce spectral profiles at given phases and orientations. Methods. In using the Wilson–Devinney-like code PHOEBE in combination with the nonlocal thermodynamic equilibrium radiative transfer code FASTWIND, we created a three-dimensional mesh that represents the surface geometry of our system and we assigned FASTWIND emergent intensity line profiles to each mesh triangle, which take the local parameters such as temperature, surface gravity, and radius into account. These line profiles were then integrated across the visible surface, where their flux contribution and radial velocity are taken into account, thus returning a final line profile for the visible surface of the system at a given phase. Results. We demonstrate that SPAMMS can accurately reproduce the morphology of observed spectral line profiles for overcontact systems. Additionally, we show how line profiles of rapidly-rotating single stars differ when taking rotational distortion into account, and the effects that these can have on the determined parameters. Finally, we demonstrate the code’s ability to reproduce the Rossiter–Mclaughlin and Struve–Sahade effects.

Observational constraints on the origin of the elements

Context. The pursuit of more realistic spectroscopic modelling and consistent abundances has led us to begin a new series of papers designed to improve current solar and stellar abundances of various atomic species. To achieve this, we have begun updating the three-dimensional (3D) non-local thermodynamic equilibrium (non-LTE) radiative transfer code, MULTI3D, and the equivalent one-dimensional (1D) non-LTE radiative transfer code, MULTI 2.3. Aims. We examine our improvements to these codes by redetermining the solar barium abundance. Barium was chosen for this test as it is an important diagnostic element of the s-process in the context of galactic chemical evolution. New Ba II + H collisional data for excitation and charge exchange reactions computed from first principles had recently become available and were included in the model atom. The atom also includes the effects of isotopic line shifts and hyperfine splitting. Methods. A grid of 1D LTE barium lines were constructed with MULTI 2.3 and fit to the four Ba II lines available to us in the optical region of the solar spectrum. Abundance corrections were then determined in 1D non-LTE, 3D LTE, and 3D non-LTE. A new 3D non-LTE solar barium abundance was computed from these corrections. Results. We present for the first time the full 3D non-LTE barium abundance of A(Ba) = 2.27 ± 0.02 ± 0.01, which was derived from four individual fully consistent barium lines. Errors here represent the systematic and random errors, respectively.

New predictions for radiation-driven, steady-state mass-loss and wind-momentum from hot, massive stars

Context. Radiation-driven mass loss plays a key role in the life cycles of massive stars. However, basic predictions of such mass loss still suffer from significant quantitative uncertainties. Aims. We develop new radiation-driven, steady-state wind models for massive stars with hot surfaces, suitable for quantitative predictions of global parameters like mass-loss and wind-momentum rates. Methods. The simulations presented here are based on a self-consistent, iterative grid solution to the spherically symmetric, steady-state equation of motion, using full non-local thermodynamic equilibrium radiative transfer solutions in the co-moving frame to derive the radiative acceleration. We do not rely on any distribution functions or parametrization for computation of the line force responsible for the wind driving. The models start deep in the subsonic and optically thick atmosphere and extend up to a large radius at which the terminal wind speed has been reached. Results. In this first paper, we present models representing two prototypical O-stars in the Galaxy, one with a higher stellar mass M*∕M⊙ = 59 and luminosity log10L*∕L⊙ = 5.87 (spectroscopically an early O supergiant) and one with a lower M*∕M⊙ = 27 and log10L*∕L⊙ = 5.1 (a late O dwarf). For these simulations, basic predictions for global mass-loss rates, velocity laws, and wind momentum are given, and the influence from additional parameters like wind clumping and microturbulent speeds is discussed. A key result is that although our mass-loss rates agree rather well with alternative models using co-moving frame radiative transfer, they are significantly lower than those predicted by the mass-loss recipes normally included in models of massive-star evolution. Conclusions. Our results support previous suggestions that Galactic O-star mass-loss rates may be overestimated in present-day stellar evolution models, and that new rates might therefore be needed. Indeed, future papers in this series will incorporate our new models into such simulations of stellar evolution, extending the very first simulations presented here toward larger grids covering a range of metallicities, B supergiants across the bistability jump, and possibly also Wolf-Rayet stars.

High-sensitivity maps of molecular ions in L1544

Context. The deuterium fraction in low-mass prestellar cores is a good diagnostic indicator of the initial phases of star formation, and is also a fundamental quantity to infer the ionisation degree in these objects. Aims. With the analysis of multiple transitions of N2H+, N2D+, HC18O+, and DCO+ we are able to determine the molecular column density maps and the deuterium fraction in N2H+ and HCO+ toward the prototypical prestellar core L1544. This is the preliminary step to derive the ionisation degree in the source. Methods. We used a non-local thermodynamic equilibrium (non-LTE) radiative transfer code combined with the molecular abundances derived from a chemical model to infer the excitation conditions of all the observed transitions. This allowed us to derive reliable maps of the column density of each molecule. The ratio between the column density of a deuterated species and its non-deuterated counterpart gives the sought-after deuteration level. Results. The non-LTE analysis confirms that, for the molecules analysed, higher-J transitions are characterised by excitation temperatures that are ≈1–2 K lower than those of the lower-J transitions. The chemical model that provides the best fit to the observational data predicts the depletion of N2H+ and to a lesser extent of N2D+ in the innermost region. The peak values for the deuterium fraction that we find are D/HN2H+ = 0.26−0.14+0.15 and D/HHCO+=0.035−0.012+0.015, in good agreement with previous estimates in the source.

Launch of a CME-associated eruptive prominence as observed with IRIS and ancillary instruments

Aims. In this paper we focus on the possible observational signatures of the processes which have been put forward for explaining eruptive prominences. We also try to understand the variations in the physical conditions of eruptive prominences and estimate the masses leaving the Sun versus the masses returning to the Sun during eruptive prominences.Methods. As far as velocities are concerned, we combined an optical flow method on the Atmospheric Imaging Assembly (AIA) 304 Å and Interface Region Imaging Spectrograph (IRIS). Mg IIh&k observations in order to derive the plane-of-sky velocities in the prominence, and a Doppler technique on the IRIS Mg IIh&k profiles to compute the line-of-sight velocities. As far as densities are concerned, we compared the absolute observed intensities with values derived from non-local thermodynamic equilibrium radiative transfer computations to derive the total (hydrogen) density and consequently compute the mass flows.Results. The derived electron densities range from 1.3 × 109to 6.0 × 1010cm−3and the derived total hydrogen densities range from 1.5 × 109to 2.4 × 1011cm−3in different regions of the prominence. The mean temperature is around 1.1 × 104K, which is higher than in quiescent prominences. The ionization degree is in the range of 0.1–10. The total (hydrogen) mass is in the range of 1.3 × 1014–3.2 × 1014g. The total mass drainage from the prominence to the solar surface during the whole observation time of IRIS is about one order of magnitude smaller than the total mass of the prominence.

Herschel water maps towards the vicinity of the black hole Sgr A*

Aims. We study the spatial distribution and kinematics of water emission in a ~8 × 8 pc2 region of the Galactic center (GC) that covers the main molecular features around the supermassive black hole Sagittarius A* (Sgr A*). We also analyze the water excitation to derive the physical conditions and water abundances in the circumnuclear disk (CND) and the “quiescent clouds”. Methods. We presented the integrated line intensity maps of the ortho 110 − 101, and para 202 − 111 and 111 − 000 water transitions observed using the On the Fly mapping mode with the Heterodyne Instrument for the Far Infrared (HIFI) on board Herschel. To study the water excitation, we used HIFI observations of the ground state ortho and para H218O transitions toward three selected positions in the vicinity of Sgr A*. In our study, we also used dust continuum measurements of the CND, obtained with the Spectral and Photometric Imaging REceiver (SPIRE) instrument. Using a non-local thermodynamical equilibrium (LTE) radiative transfer code, the water line profiles and dust continuum were modeled, deriving H2O abundances (XH2O), turbulent velocities (V t), and dust temperatures (Td). We also used a rotating ring model to reproduce the CND kinematics represented by the position velocity (PV) diagram derived from para 202 − 111 H2O lines. Results. In our H2O maps we identify the emission associated with known features around Sgr A*: CND, the Western Streamer, and the 20 and 50 km s−1 clouds. The ground-state ortho water maps show absorption structures in the velocity range of [−220,10] km s−1 associated with foreground sources. The PV diagram reveals that the 202 − 111 H2O emission traces the CND also observed in other high-dipole molecules such as SiO, HCN, and CN. Using the non-LTE code, we derive high XH2O of ~(0.1–1.3) × 10−5, V t of 14–23 km s−1 , and Td of 15–45 K for the CND, and the lower XH2O of 4 × 10−8 and V t of 9 km s−1 for the 20 km s−1 cloud. Collisional excitation and dust effects are responsible for the water excitation in the southwest lobe of the CND and the 20 km s−1 cloud, whereas only collisions can account for the water excitation in the northeast lobe of the CND. We propose that the water vapor in the CND is produced by grain sputtering by shocks of 10–20 km s−1, with some contribution of high temperature and cosmic-ray chemistries plus a photon-dominated region chemistry, whereas the low XH2O derived for the 20 km s−1 cloud could be partially a consequence of the water freeze-out on grains.

Depth of Ellerman Burst Derived from High-resolution Hα and Ca ii 8542 Å Spectra

Abstract High-resolution spectra of an Ellerman burst (EB) sampling the Hα and the Ca ii 8542 Å lines obtained with the Fast Imaging Solar Spectrograph (FISS) installed on the 1.6 m Goode Solar Telescope at the Big Bear Solar Observatory are compared with synthetic line profiles constructed using the RH code for nonlocal thermodynamical equilibrium radiative transfer. The EB heating is modeled by a local temperature hump above the quiet-Sun temperature. Our first finding is that FISS Hα and Ca ii 8542 Å intensity profiles cannot be reproduced simultaneously by a single hump model as far as the hump is thicker than ≥100 km. Simultaneous reproduction of both line profiles is possible when the EB temperature enhancement is confined to a layer as thin as ≤20 km in the photosphere where the Hα wing response is high and that of the Ca ii 8542 Å is not. Moreover, when we examine the EB spectra at different times, we find that the EB at a time of weaker appearance is located at lower heights, ∼50 km, and moves upward to ∼120 km at the time of maximum intensity. Complementary calculations of the Na i D1 and Mg i b2 lines as well as that of UV continuum at 1600 and 1700 Å with the deduced EB atmosphere are also performed to test the result, which allows us to discuss the shortcomings of this plane–parallel static model atmosphere for understanding the physical properties of EBs.

Multiple-line study of molecular gas in spiral galaxy NGC 2903

Multiple molecular lines with radiative transfer modelling are a powerful tool to probe the physics of star-forming gas in galaxies. We investigate the gas properties in the centre of the spiral galaxy NGC 2903 using low-J CO lines, i.e. 12CO(1-0), 12CO(2-1), 12CO(3-2), 13CO(1-0), and HCN(1-0). We apply a nonlocal thermodynamic equilibrium radiative transfer code to derive beam-averaged molecular gas properties. We use two methods (i.e. chi-square minimizations and likelihood) to define the best model representing the observed line ratios best. The line ratio diagnostics suggest that CO gas in the centre of NGC 2903 is thinner and the dense gas fraction is similar compared to that of spirals, starburst, and early-type galaxies (ETGs), while the gas in the centre of the galaxy is warmer than that of ETGs and colder than that of starbursts. Based on the best-fitting model results, we find that the beam-averaged gas kinetic temperature is T_K = 20 K, H_2 volume number density is log(n(H_2)) = 4.2 cm-3 , and CO column number density is log(N(CO)) = 19.0 cm-2 in the centre of NGC 2903. Both methods, i.e. the line ratio diagnostics and modelling, indicate an ISM in the centre of NGC 2903 having intermediate temperature and star formation activity (also supported by star formation rates), thinner CO gas with similar dense gas fraction, and higher H_2 volume number density compared to that of spirals, ETGs, and starbursts.