non-LTE Radiative Transfer Model Research Articles

Molecular isotopologue measurements toward super star clusters and the relation to their ages in NGC 253 with ALCHEMI

Determining the evolution of the CNO isotopes in the interstellar medium (ISM) of starburst galaxies can yield important constraints on the ages of super star clusters (SSCs), or on other aspects and factors contributing to their evolution, such as the initial mass function (IMF). Due to the time-dependent nature of the abundances of isotopes within the ISM ---as they are supplied from processes such as nucleosynthesis or chemical fractionation---, this provides the opportunity to test whether or not isotope ratios trace the ages of highly star-forming regions, such as SSCs. The goal of this study is to investigate whether the isotopic variations in SSC regions within NGC 253 are correlated with their different ages as derived from stellar population modelling. We measured abundance ratios of CO, HCN, and HCO isotopologues in six regions containing SSCs within NGC 253 using high-spatial-resolution (1.6",$ 28$pc) data from the ALCHEMI (ALma Comprehensive High-resolution Extragalactic Molecular Inventory) ALMA Large program. We then analysed these ratios using RADEX radiative transfer modelling, with the parameter space sampled using the nested sampling Monte Carlo algorithm MLFriends. These abundance ratios were then compared to ages predicted in each region via the fitting of observed star-formation tracers (such as Brgamma ) to Starburst99 starburst stellar population evolution models. We determined the isotopic column density ratios across multiple regions of SSC activity in NGC 253 using non-LTE radiative transfer modelling. We do not find any significant trend with age for the CO and HCN isotopologue ratios on timescales of the ages of the SSC* regions observed. However, HCO may show a correlation with age over these timescales in C/ C. The driving factors of these ratios within SSCs could be the IMF or fractionation effects. To further probe these effects in SSCs over time, a larger sample of SSCs must be observed spanning a larger age range.

Modeling of the High-velocity Jet Powered by the Massive Star MWC 349A

MWC 349A is a massive star with a well-known circumstellar disk rotating following a Keplerian law, and an ionized wind launched from the disk surface. Recent observations with the Atacama Large Millimeter/submillimeter Array (ALMA) carried out toward this system, however, have revealed an additional high-velocity component in the strong, maser emission of hydrogen radio recombination lines (RRLs), suggesting the presence of a high-velocity ionized jet. In this work, we present 3D non-LTE radiative transfer modeling of the emission of the H30α and H26α maser lines, and of their associated radio continuum emission, toward MWC 349A. By using the MORELI code, we reproduce the spatial distribution and kinematics of the high-velocity emission of the H30α and H26α maser lines with a high-velocity ionized jet expanding at a velocity of ∼250 km s−1, surrounded by MWC 349A’s wide-angle ionized wind. The bipolar jet, which is launched from MWC 349A’s disk, is poorly collimated and slightly misaligned with respect to the disk rotation axis. Thanks to the unprecedented sensitivity and spatial accuracy provided by ALMA, we also find that the already known, wide-angle ionized wind decelerates as it expands radially from the ionized disk. We briefly discuss the implications of our findings for understanding the formation and evolution of massive stars. Our results show the huge potential of RRL masers as powerful probes of the innermost ionized regions around massive stars and of their high-velocity jets.

Unveiling the Evolutionary State of Three B Supergiant Stars: PU Gem, ϵ CMa, and η CMa

We aim to combine asteroseismology, spectroscopy, and evolutionary models to establish a comprehensive picture of the evolution of Galactic blue supergiant stars (BSG). To start such an investigation, we selected three BSG candidates for our analysis: HD 42087 (PU Gem), HD 52089 (ϵ CMa), and HD 58350 (η CMa). These stars show pulsations and were suspected to be in an evolutionary stage either preceding or succeding the red supergiant (RSG) stage. For our analysis, we utilized the 2-min cadence TESS data to study the photometric variability, and we obtained new spectroscopic observations at the CASLEO observatory. We used non-LTE radiative transfer models calculated with CMFGEN to derive their stellar and wind parameters. For the fitting procedure, we included CMFGEN models in the iterative spectral analysis pipeline XTgrid to determine their CNO abundances. The spectral modeling was limited to changing only the effective temperature, surface gravity, CNO abundances, and mass-loss rates. Finally, we compared the derived metal abundances with prediction from Geneva stellar evolution models. The frequency spectra of all three stars show stochastic oscillations and indications of one nonradial strange mode, fr= 0.09321 d−1 in HD 42087 and a rotational splitting centred in f2= 0.36366 d−1 in HD 52089. We conclude that the rather short sectoral observing windows of TESS prevent establishing a reliable mode identification of low frequencies connected to mass-loss variabilities. The spectral analysis confirmed gradual changes in the mass-loss rates, and the derived CNO abundances comply with the values reported in the literature. We were able to achieve a quantitative match with stellar evolution models for the stellar masses and luminosities. However, the spectroscopic surface abundances turned out to be inconsistent with the theoretical predictions. The stars show N enrichment, typical for CNO cycle processed material, but the abundance ratios did not reflect the associated levels of C and O depletion. We found HD 42087 to be the most consistent with a pre-RSG evolutionary stage, HD 58350 is most likely in a post-RSG evolution and HD 52089 shows stellar parameters compatible with a star at the TAMS.

Sub-kiloparsec empirical relations and excitation conditions of HCN and HCO+ J = 3–2 in nearby star-forming galaxies

We present new HCN and HCO+ (J = 3–2) images of the nearby star-forming galaxies (SFGs) NGC 3351, NGC 3627, and NGC 4321. The observations, obtained with the Morita ALMA Compact Array, have a spatial resolution of ∼290–440 pc and resolve the inner Rgal ≲ 0.6–1 kpc of the targets, as well as the southern bar end of NGC 3627. We complement this data set with publicly available images of lower excitation lines of HCN, HCO+, and CO and analyse the behaviour of a representative set of line ratios: HCN(3–2)/HCN(1–0), HCN(3–2)/HCO+(3–2), HCN(1–0)/CO(2–1), and HCN(3–2)/CO(2–1). Most of these ratios peak at the galaxy centres and decrease outwards. We compare the HCN and HCO+ observations with a grid of one-phase, non-local thermodynamic equilibrium (non-LTE) radiative transfer models and find them compatible with models that predict subthermally excited and optically thick lines. We study the systematic variations of the line ratios across the targets as a function of the stellar surface density (Σstar), the intensity-weighted CO(2–1) (⟨ICO⟩), and the star formation rate surface density (ΣSFR). We find no apparent correlation with ΣSFR, but positive correlations with the other two parameters, which are stronger in the case of ⟨ICO⟩. The HCN/CO–⟨ICO⟩ relations show ≲0.3 dex galaxy-to-galaxy offsets, with HCN(3–2)/CO(2–1)–⟨ICO⟩ being ∼2 times steeper than HCN(1–0)/CO(2–1). In contrast, the HCN(3–2)/HCN(1–0)–⟨ICO⟩ relation exhibits a tighter alignment between galaxies. We conclude that the overall behaviour of the line ratios cannot be ascribed to variations in a single excitation parameter (e.g., density or temperature).

The opaque heart of the galaxy IC 860: Analogous protostellar, kinematics, morphology, and chemistry

Compact Obscured Nuclei (CONs) account for a significant fraction of the population of luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs). These galaxy nuclei are compact, with radii of 10–100 pc, with large optical depths at submm and far-infrared wavelengths, and characterized by vibrationally excited HCN emission. It is not known what powers the large luminosities of the CON host galaxies because of the extreme optical depths towards their nuclei. CONs represent an extreme phase of nuclear growth, hiding either a rapidly accreting supermassive black hole or an abnormal mode of star formation. Regardless of their power source, the CONs allow us to investigate the processes of nuclear growth in galaxies. Here we apply principal component analysis (PCA) tomography to high-resolution (0″​​.06) ALMA observations at frequencies 245 to 265 GHz of the nearby CON (59 Mpc) IC 860. PCA is a technique to unveil correlation in the data parameter space, and we apply it to explore the morphological and chemical properties of species in our dataset. The leading principal components reveal morphological features in molecular emission that suggest a rotating, infalling disk or envelope, and an outflow analogous to those seen in Galactic protostars. One particular molecule of astrochemical interest is methanimine (CH2NH), a precursor to glycine, three transitions of which have been detected towards IC 860. We estimate the average CH2NH column density towards the nucleus of IC 860 to be ∼1017cm−2, with an abundance exceeding 10−8 relative to molecular hydrogen, using the rotation diagram method and non-LTE radiative transfer models. This CH2NH abundance is consistent with those found in hot cores of molecular clouds in the Milky Way. Our analysis suggests that CONs are an important stage of chemical evolution in galaxies, that are chemically and morphologically similar to Milky Way hot cores.

Non-LTE spectroscopy of the tetradecad region of methane recorded in a hypersonic flow

Laboratory spectroscopic data is essential for the modeling of hot exoplanet atmospheres, since molecules such as methane, a major component of hot-Jupiter-type exoplanet atmospheres, have a complex vibrational energy structure that makes computational predictions difficult at high temperatures for ro-vibrational transitions involving highly excited vibrational sates. To better inform line lists used in radiative transfer modeling, the ro-vibrational spectrum of methane has been recorded in the tetradecad region between 1.7 and 1.65 μm (5880–6060 cm−1) through non-local thermodynamic equilibrium (non-LTE) cavity ringdown spectroscopy (CRDS). Non-LTE conditions, characterized by a low rotational temperature (∼39 K) and a high vibrational temperature (up to 1130 K), have been obtained by hypersonic expansion of a pre-heated mixture of argon and methane in a contoured Laval nozzle. The high vibrational temperature increases the intensity of new hot bands, while the very low rotational temperature greatly simplifies their rotational structure, thus facilitating their identification. A close comparison of the recorded CRDS data to the TheoReTS database reveals both inefficient vibrational relaxation between polyads and efficient vibrational relaxation between vibrational states forming a polyad. These effects result in an overpopulation of the lowest vibrational energy level of each polyad, an effect not widely currently incorporated in non-LTE radiative transfer models. A series of new hot band transitions originating from the pentad and octad polyads were assigned and are provided as a line list for use in future databases.

Investigating Anomalous Photochemistry in the Inner Wind of IRC+10216 through Interferometric Observations of HC3N

In recent years, many questions have arisen regarding the chemistry of photochemical products in the carbon-rich winds of evolved stars. To address them, it is imperative to constrain the distributions of such species through high-angular-resolution interferometric observations covering multiple rotational transitions. We used archival Atacama Large Millimeter/submillimeter Array observations to map rotational lines involving high energy levels of cyanoacetylene (HC3N) toward the inner envelope (radius < 8″/1000 au) of the carbon star IRC+10216. The observed lines include the J = 28 − 27, J = 30 − 29, and J = 38 − 37 transitions of HC3N in its ground vibrational state. In contrast to previous observations of linear carbon chains toward this asymptotic giant branch (AGB) star that show extended, hollow emission at 15″–20″ radii (e.g., C4H, C6H, and HC5N), the maps of the HC3N lines here show compact morphologies comprising various arcs and density enhancements, with significant emission from gas clumps at an angular distance of ∼3″ (350 au) from the central AGB star. We compared visibility sampled non-LTE radiative transfer models with the observed brightness distributions, and derive a fractional abundance with respect to H2 of 10−8 for HC3N at the radii probed by these lines. These results are consistent with enhanced photochemistry occurring in warm (∼200 K) regions of the circumstellar envelope. After application of a specialized chemical model for IRC+10216, we find evidence that the enhanced HC3N abundances in the inner wind are most likely due to a solar-type binary companion initiating photochemistry in this region.

Laboratory and Astronomical Detection of the SiP Radical (X2Π i ): More Circumstellar Phosphorus

The millimeter-wave spectrum of the SiP radical (X2Πi) has been measured in the laboratory for the first time using direct-absorption methods. SiP was created by the reaction of phosphorus vapor and SiH4 in argon in an AC discharge. Fifteen rotational transitions (J + 1 ← J) were measured for SiP in the Ω = 3/2 ladder in the frequency range 151–533 GHz, and rotational, lambda doubling, and phosphorus hyperfine constants determined. Based on the laboratory measurements, SiP was detected in the circumstellar shell of IRC+10216, using the Submillimeter Telescope and the 12 m antenna of the Arizona Radio Observatory at 1 mm and 2 mm, respectively. Eight transitions of SiP were searched: four were completely obscured by stronger features, two were uncontaminated (J = 13.5 → 12.5 and 16.5 → 15.5), and two were partially blended with other lines (J = 8.5 → 7.5 and 17.5 → 16.5). The SiP line profiles were broader than expected for IRC+10216, consistent with the hyperfine splitting. From non-LTE radiative transfer modeling, SiP was found to have a shell distribution with a radius ∼300 R *, and an abundance, relative to H2, of f ∼ 2 × 10−9. From additional modeling, abundances of 7 × 10−9 and 9 × 10−10 were determined for CP and PN, respectively, both located in shells at 550–650 R *. SiP may be formed from grain destruction, which liberates both phosphorus and silicon into the gas phase, and then is channeled into other P-bearing molecules such as PN and CP.

Doppler-velocity Drifts Detected in a Solar Prominence

We analyzed multiline observations of a quiescent prominence from the slit spectrograph located at the Ondřejov Observatory. Dopplergrams and integrated intensity maps of the whole prominence were obtained from observations in six spectral lines: Ca ii H, Hϵ, Hβ, He i D3, Hα, and Ca ii IR. By combining integrated intensity maps with non-LTE radiative-transfer modeling, we carefully identified areas in an optically thin regime. The comparison of the Doppler-velocity maps and scatterplots from different lines shows the existence of differences in the velocity of ions and neutrals called velocity drift. The drift is of a local nature, present mostly at prominence edges in the area with a large velocity gradient, as can be tentatively expected based on multifluid MHD models. We could not explore the time evolution of the drift, since our data set consists of a single scan only. Our paper brings another contribution to a rather controversial problem of the detection of multifluid effects in solar prominences.

Non-LTE modelling of the HC2NC and HNC3abundance in astrophysical environments

The isomers of HC3N, namely HC2NC and HNC3, are widely observed in the interstellar medium and in circumstellar envelopes. Their abundance has been determined under the assumption of local thermodynamic equilibrium (LTE) conditions or non-LTE radiative transfer models, but in considering the collisional excitation of HC3N as the same for all isomers. Chemical models for the prototypical cold cores, TMC-1 and L1544, reproduced the abundance of HC3N fairly well, but they tend to overestimate the abundances of HC2NC and HNC3with respect to the observations. It is therefore worth revisiting the interpretation of the observational spectra of these isomers using a rigorous non-LTE modelling. The abundance of HC2NC and HNC3were then determined using non-LTE radiative transfer calculations based on the proper rate coefficients for the first time in this work. Modelling the brightness temperature of HC2NC and HNC3when using their proper collision rate coefficients shows that models based on LTE or non-LTE with approximate collision data may lead to deviations of up to a factor of ~1.5. Reinterpreting the observational spectra led us to significant differences relative to the observed abundances previously determined. Our findings suggest quite similar abundance ratios for the TMC-1 and L1544 cold cores as well as the L483 protostar. This work will encourage further modelling with more robust non-LTE radiative transfer calculations and future studies to revisit the chemistry of HC3N and its isomers in cold molecular clouds.

Collisional Excitation and Non-LTE Modeling of Interstellar Chiral Propylene Oxide

Abstract The first set of theoretical rotational cross sections for propylene oxide (CH3CHCH2O) colliding with cold He atoms has been obtained at the full quantum level using a high-accuracy potential energy surface. By scaling the collision reduced mass, rotational rate coefficients for collisions with para-H2 are deduced in the temperature range 5–30 K. These collisional coefficients are combined with radiative data in a non-LTE radiative transfer model in order to reproduce observations of propylene oxide made toward the Sagittarius B2(N) molecular cloud with the Green Bank and Parkes radio telescopes. The three detected absorption lines are found to probe the cold (∼10 K) and translucent (n H ∼ 2000 cm−3) gas in the outer edges of the extended Sgr B2(N) envelope. The derived column density for propylene oxide is N tot ∼ 3 × 1012 cm−2, corresponding to a fractional abundance relative to total hydrogen of ∼2.5 × 10−11. The present results are expected to help our understanding of the chemistry of propylene oxide, including a potential enantiomeric excess, in the cold interstellar medium.

Balmer continuum enhancement detected in a mini flare observed with IRIS

Context. Optical and near-UV continuum emissions in flares contribute substantially to the flare energy budget. Two mechanisms play an important role for continuum emission in flares: hydrogen recombination after sudden ionization at chromospheric layers, and transportation of the energy radiatively from the chromosphere to lower layers in the atmosphere, the so-called back-warming. Aims. The aim of the paper is to distinguish between these two mechanisms for the excess of the Balmer continuum observed in a flare. Methods. We combined the observations of the Balmer continuum obtained with the Interface Region Imaging Spectrograph (IRIS) (spectra and slit-jaw images (SJIs) 2832 Å) and hard X-ray (HXR) emission detected by the Fermi/Gamma Burst Monitor (GBM) during a mini flare. The calibrated Balmer continuum was compared to non-local thermodynamic equilibrium (LTE) radiative transfer flare models, and the radiated energy was estimated. Assuming thick target HXR emission, we calculated the energy of the nonthermal electrons detected by the Fermi/GBM and compared it to the radiated energy. Results. The favorable argument of a relation between the Balmer continuum excess and the HXR emission is that there is a good time coincidence between them. In addition, the shape of the maximum brightness in the 2832 SJIs, which is mainly due to this Balmer continuum excess, is similar to that of the Fermi/GBM light curve. The electron-beam flux estimated from Fermi/GBM between 109 and 1010 erg s−1 cm−2 is consistent with the beam flux required in non-LTE radiative transfer models to obtain the excess of Balmer continuum emission observed in this IRIS spectra. Conclusions. The low-energy input by nonthermal electrons above 20 keV is sufficient to produce the enhancement in the Balmer continuum emission. This could be explained by the topology of the reconnection site. The reconnection starts in a tiny bald-patch region, which is transformed dynamically into an X-point current sheet. The size of the interacting region would be below the spatial resolution of the instrument.

Solar prominence diagnostics from non-LTE modelling of Mg II h&amp;k line profiles

Aims. We investigate a new method to for obtaining the plasma parameters of solar prominences observed in the Mg II h&amp;k spectral lines by comparing line profiles from the IRIS satellite to a bank of profiles computed with a one-dimensional non-local thermodynamic equilibrium (non-LTE) radiative transfer code. Methods. Using a grid of 1007 one-dimensional non-LTE radiative transfer models, some including a prominence-corona transition region (PCTR), we carry out this new method to match computed spectra to observed line profiles while accounting for line core shifts not present in the models. The prominence observations were carried out by the IRIS satellite on 19 April 2018. Results. The prominence is very dynamic with many flows, including a large arm extending from the main body seen near the end of the observation. This flow is found to be redshifted, as is the prominence overall. The models are able to recover satisfactory matches in areas of the prominence where single line profiles are observed. We recover: mean temperatures of 6000–50 000 K; mean pressures of 0.01–0.5 dyne cm−2; column masses of 3.7 × 10−8–5 × 10−4 g cm−2; a mean electron density of 7.3 × 108–1.8 × 1011 cm−3; and an ionisation degree nHII/nHI = 0.03 − 4500. The highest values for the ionisation degree are found in areas where the line of sight crosses mostly plasma from the PCTR, correlating with high mean temperatures and correspondingly no Hα emission. Conclusions. This new method naturally returns information on how closely the observed and computed profiles match, allowing the user to identify areas where no satisfactory match between models and observations can be obtained. The inclusion of the PCTR was found to be important when fitting models to data as regions where satisfactory fits were found were more likely to contain a model encompassing a PCTR. The line core shift can also be recovered from this new method, and it shows a good qualitative match with that of the line core shift found by the quantile method. This demonstrates the effectiveness of the approach to line core shifts in the new method.

Mapping the H2D+ and N2H+ emission toward prestellar cores. Testing dynamical models of the collapse using gas tracers

Context. The study of prestellar cores is critical as they set the initial conditions in star formation and determine the final mass of the stellar object. To date, several hypotheses have described their gravitational collapse. Deriving the dynamical model that fits both the observed dust and the gas emission from such cores is therefore of great importance. Aims. We perform detailed line analysis and modeling of H2D+ 110–111 and N2H+ 4–3 emission at 372 GHz, using 2′ × 2′ maps (James Clerk Maxwell Telescope; JCMT). Our goal is to test the most prominent dynamical models by comparing the modeled gas kinematics and spatial distribution (H2D+ and N2H+) with observations toward four prestellar (L1544, L183, L694-2, L1517B) and one protostellar core (L1521f). Methods. We fit the line profiles at all offsets showing emission using single Gaussian distributions. We investigate how the line parameters (VLSR, FWHM and TA*) change with offset to examine the velocity field, the degree of nonthermal contributions to the line broadening, and the distribution of the material in these cores. To assess the thermal broadening, we derive the average gas kinetic temperature toward all cores using the non-LTE radiative transfer code RADEX. We perform a more detailed non-LTE radiative transfer modeling using RATRAN, where we compare the predicted spatial distribution and line profiles of H2D+ and N2H+ with observations toward all cores. To do so, we adopt the physical structure for each core predicted by three different dynamical models taken from literature: quasi-equilibrium Bonnor–Ebert sphere (QE-BES), singular isothermal sphere (SIS), and Larson–Penston (LP) flow. In addition, we compare these results to those of a static sphere, whose density and temperature profiles are based on the observed dust continuum. Lastly, we constrain the abundance profiles of H2D+ and N2H+ toward each core. Results. We find that variable nonthermal contributions (variations by a factor of 2.5) are required to explain the observed line width of both H2D+ and N2H+, while the nonthermal contributions are found to be 50% higher for N2H+. The RADEX modeling results in average core column densities of ~9 × 1012 cm−2 for H2D+ and N2H+. The LP flow seems to be the dynamical model that can reproduce the observed spatial distribution and line profiles of H2D+ on a global scale of prestellar cores, while the SIS model systematically and significantly overestimates the width of the line profiles and underestimates the line peak intensity. We find similar abundance profiles for the prestellar cores and the protostellar core. The typical abundances of H2D+ vary between 10−9 and 10−10 for the inner 5000 au and drop by about an order of magnitude for the outer regions of the core (2 × 10−10–6 × 10−11). In addition, a higher N2H+ abundance by about a factor of 4 compared to H2D+ is found toward the two cores with detected emission. The presence of N2H+ 4–3 toward the protostellar core and toward one of the prestellar cores reflects the increasing densities as the core evolves. Conclusions. Our analysis provides an updated picture of the physical structure of prestellar cores. Although the dynamical models account for mass differences by up to a factor of 7, the velocity structure drives the shape of the line profiles, allowing for a robust comparison between the models. We find that the SIS model can be clearly excluded in explaining the gas emission toward the cores, but a larger sample is required to differentiate clearly between the LP flow, the QE-BES, and the static models. All models of collapse underestimate the intensity of the gas emission by up to several factors toward the only protostellar core in our sample, indicating that different dynamics take place in different evolutionary core stages. If the LP model is confirmed toward a larger sample of prestellar cores, it would indicate that they may form by compression or accretion of gas from larger scales. If the QE-BES model is confirmed, it means that quasi-hydrostatic cores can exist within turbulent ISM.

Arp 220: New Observational Insights into the Structure and Kinematics of the Nuclear Molecular Disks and Surrounding Gas

Abstract ALMA cycle 3 observations of 12CO J = 3 → 2, 13CO J = 4 → 3, SiO J = 8 → 7, and HCN J = 5 → 4 are presented. Significant extended emission is detected in 12CO J = 3 → 2 with a morphology that is indicative of m = 2 tidal features, suggesting gas inflow. In addition, outflows for both nuclei are found in 12CO J = 3 → 2. Significant SiO absorption is detected in the western nucleus. HCN that is morphologically distinct from CO is detected in both nuclei. These observations are compared to non-LTE radiative transfer models created using the Line Modeling Engine for simple gas dynamics to gain insight into how physical parameters, such as rotational velocity, turbulent velocity, gas temperature, dust temperature, and gas mass, can reproduce the observed kinematic and spatial features. The eastern nucleus is found to be best modeled with an inclusion of a temperature asymmetry from one side of the disk to the other. It is also found that the western nucleus is optically thick even in the less abundant species of 13CO, absorbing significant amounts of continuum radiation.

Ammonia in circumstellar environment of V Cyg

The HIFI instrument on board of the Herschel Space Observatory (HSO) has been very successful in detecting molecular lines from the circumstellar envelopes around evolved stars, like massive red supergiants, Asymptotic Giant Branch (AGB) and post-AGB stars, as well as the planetary nebulae. Among others, ammonia have been found in the circumstellar envelopes of C-rich AGB stars in amounts that significantly exceeded the theoretical predictions for C-rich stars. Few scenarios have been proposed to resolve this problem: formation of ammonia behind the shock front and photochemical processes in the inner part of the envelope partly transparent to UV background radiation due to the clumpy structure of the gas and formation of ammonia on dust grains. Careful analysis of observations may help to put the constraints on one or another mechanism of ammonia formation. Here, we present results of the non-LTE radiative transfer modeling of ammonia transitions including the crucial process of radiative pumping via the v2=1 vibrational band (at ∼10 μm) for V Cyg. Only the ground-based ammonia transition NH3 J = 10-00 at 572.5 GHz has been observed by HIFI. Therefore, to determine the abundance of ammonia we estimate the photodissociation radius of NH3 using chemical model of the envelope consistent with the dust grain properties concluded from the spectral energy distribution.

High-velocity hot CO emission close to Sgr A*: Herschel/HIFI★ , ★★ submillimeter spectral survey toward Sgr A.

The properties of molecular gas, the fuel that forms stars, inside the cavity of the circumnuclear disk (CND) are not well constrained. We present results of a velocity-resolved submillimeter scan (~480 to 1250 GHz) and [C ii] 158 μm line observations carried out with Herschel/HIFI toward Sgr A*; these results are complemented by a ~2'×2' 12CO (J=3-2) map taken with the IRAM 30 m telescope at ~7″ resolution. We report the presence of high positive-velocity emission (up to about +300 km s-1) detected in the wings of 12CO J=5-4 to 10-9 lines. This wing component is also seen in H2O (11,0-10,1), a tracer of hot molecular gas; in [C ii]158 μm, an unambiguous tracer of UV radiation; but not in [C i] 492, 806 GHz. This first measurement of the high-velocity 12CO rotational ladder toward Sgr A* adds more evidence that hot molecular gas exists inside the cavity of the CND, relatively close to the supermassive black hole (< 1 pc). Observed by ALMA, this velocity range appears as a collection of 12CO (J=3-2) cloudlets lying in a very harsh environment that is pervaded by intense UV radiation fields, shocks, and affected by strong gravitational shears. We constrain the physical conditions of the high positive-velocity CO gas component by comparing with non-LTE excitation and radiative transfer models. We infer T k≃400 K to 2000 K for n H≃(0.2-1.0)·105 cm-3. These results point toward the important role of stellar UV radiation, but we show that radiative heating alone cannot explain the excitation of this ~10-60 M ⊙ component of hot molecular gas inside the central cavity. Instead, strongly irradiated shocks are promising candidates.

Ammonia in C-rich stars

AbstractHIFI instrument onboard the Herschel satellite provided an unprecedented number of detections of rotational transitions of ammonia in circumstellar envelopes of evolved stars including massive red supergiants, Asymptotic Giant Branch (AGB), and post-AGB stars. The chemistry of ammonia formation in the circumstellar envelopes of evolved stars is poorly understood. The mechanisms proposed for its formation are processes behind the shock front, photochemistry in the inner part of the clumpy envelope, and formation on dust grains. We present results of the non-local thermodynamical equilibrium (non-LTE) radiative transfer modeling of ammonia transitions, mainly of the ground-state rotational one NH3 JK = 10 – 00 at 572.5 GHz, in selected AGB stars, aiming at the quantitative estimation of the NH3 abundance. The model of ammonia includes IR radiative pumping via v2 = 1 vibrational band at 10 μm.

Major impact from a minor merger

Context. Minor mergers are important processes contributing significantly to how galaxies evolve across the age of the Universe. Their impact on the growth of supermassive black holes and star formation is profound – about half of the star formation activity in the local Universe is the result of minor mergers. Aims. The detailed study of dense molecular gas in galaxies provides an important test of the validity of the relation between star formation rate and HCN luminosity on different galactic scales – from whole galaxies to giant molecular clouds in their molecular gas-rich centers. Methods. We use observations of HCN and HCO+ 1−0 with NOEMA and of CO3−2 with the SMA to study the properties of the dense molecular gas in the Medusa merger (NGC 4194) at 1′′ resolution. In particular, we compare the distribution of these dense gas tracers with CO2−1 high-resolution maps in the Medusa merger. To characterize gas properties, we calculate the brightness temperature ratios between the three tracers and use them in conjunction with a non-local thermodynamic equilibrium (non-LTE) radiative line transfer model. Results. The gas represented by HCN and HCO+ 1−0, and CO3−2 does not occupy the same structures as the less dense gas associated with the lower-J CO emission. Interestingly, the only emission from dense gas is detected in a 200 pc region within the “Eye of the Medusa”, an asymmetric 500 pc off-nuclear concentration of molecular gas. Surprisingly, no HCN or HCO+ is detected for the extended starburst of the Medusa merger. Additionally, there are only small amounts of HCN or HCO+ associated with the active galactic nucleus. The CO3−2/2−1 brightness temperature ratio inside “the Eye” is ~2.5 – the highest ratio found so far – implying optically thin CO emission. The CO2−1/HCN 1−0 (~9.8) and CO2−1/HCO+ 1−0 (~7.9) ratios show that the dense gas filling factor must be relatively high in the central region, consistent with the elevated CO3−1/2−1 ratio. Conclusions. The line ratios reveal an extreme, fragmented molecular cloud population inside the Eye with large bulk temperatures (T &gt; 300 K) and high gas densities (n(H2) &gt; 104 cm-3). This is very different from the cool, self-gravitating structures of giant molecular clouds normally found in the disks of galaxies. The Eye of the Medusa is found at an interface between a large-scale minor axis inflow and the central region of the Medusa. Hence, the extreme conditions inside the Eye may be the result of the radiative and mechanical feedback from a deeply embedded, young and massive super star cluster formed due to the gas pile-up at the intersection. Alternatively, shocks from the inflowing gas entering the central region of the Medusa may be strong enough to shock and fragment the gas. For both scenarios, however, it appears that the HCN and HCO+ dense gas tracers are not probing star formation, but instead a post-starburst and/or shocked ISM that is too hot and fragmented to form newstars. Thus, caution is advised in taking the detection of emission from dense gas tracers as evidence of ongoing or imminent star formation.

A Thorough View of the Nuclear Region of NGC 253: Combined Herschel, SOFIA, and APEX Data Set

Abstract We present a large set of spectral lines detected in the 40″ central region of the starburst galaxy NGC 253. Observations were obtained with the three instruments SPIRE, PACS, and HIFI on board the Herschel Space Observatory, upGREAT on board the SOFIA airborne observatory, and the ground-based Atacama Pathfinder EXperiment telescope. Combining the spectral and photometry products of SPIRE and PACS, we model the dust continuum spectral energy distribution (SED) and the most complete 12CO line SED reported so far toward the nuclear region of NGC 253. The properties and excitation of the molecular gas were derived from a three-component non-LTE radiative transfer model, using the SPIRE 13CO lines and ground-based observations of the lower-J 13CO and HCN lines, to constrain the model parameters. Three dust temperatures were identified from the continuum emission, and three components are needed to fit the full CO line SED. Only the third CO component (fitting mostly the HCN and PACS 12CO lines) is consistent with a shock-/mechanical-heating scenario. A hot core chemistry is also argued as a plausible scenario to explain the high-J 12CO lines detected with PACS. The effect of enhanced cosmic-ray ionization rates, however, cannot be ruled out and is expected to play a significant role in the diffuse and dense gas chemistry. This is supported by the detection of ionic species like OH+ and H2O+, as well as the enhanced fluxes of the OH lines with respect to those of H2O lines detected in both PACS and SPIRE spectra.