Starburst Energy Feedback Seen through HCO+/HOC+ Emission in NGC 253 from ALCHEMI

Molecular abundances are sensitive to the UV photon flux and cosmic-ray ionization rate. In starburst environments, the effects of high-energy photons and particles are expected to be stronger. We examine these astrochemical signatures through multiple transitions of HCO+ and its metastable isomer HOC+ in the center of the starburst galaxy NGC 253 using data from the Atacama Large Millimeter/submillimeter Array large program ALMA Comprehensive High-resolution Extragalactic Molecular inventory. The distribution of the HOC+(1−0) integrated intensity shows its association with “superbubbles,” cavities created either by supernovae or expanding H ii regions. The observed HCO+/HOC+ abundance ratios are ∼10–150, and the fractional abundance of HOC+ relative to H2 is ∼1.5 × 10−11–6 × 10−10, which implies that the HOC+ abundance in the center of NGC 253 is significantly higher than in quiescent spiral arm dark clouds in the Galaxy and the Galactic center clouds. Comparison with chemical models implies either an interstellar radiation field of G 0 ≳ 103 if the maximum visual extinction is ≳5, or a cosmic-ray ionization rate of ζ ≳ 10−14 s−1 (3–4 orders of magnitude higher than that within clouds in the Galactic spiral arms) to reproduce the observed results. From the difference in formation routes of HOC+, we propose that a low-excitation line of HOC+ traces cosmic-ray dominated regions, while high-excitation lines trace photodissociation regions. Our results suggest that the interstellar medium in the center of NGC 253 is significantly affected by energy input from UV photons and cosmic rays, sources of energy feedback.

The Detection of a Hot Molecular Core in the Extreme Outer Galaxy

Interstellar chemistry in low-metallicity environments is crucial to understand chemical processes in the past metal-poor universe. Recent studies of interstellar molecules in nearby low-metallicity galaxies have suggested that metallicity has a significant effect on the chemistry of star-forming cores. Here we report the first detection of a hot molecular core in the extreme outer Galaxy, which is an excellent laboratory to study star formation and the interstellar medium in a Galactic low-metallicity environment. The target star-forming region, WB 89–789, is located at a galactocentric distance of 19 kpc. Our Atacama Large Millimeter/submillimeter Array observations in 241–246, 256–261, 337–341, and 349–353 GHz have detected a variety of carbon-, oxygen-, nitrogen-, sulfur-, and silicon-bearing species, including complex organic molecules (COMs) containing up to nine atoms, toward a warm (>100 K) and compact (<0.03 pc) region associated with a protostar (∼8 × 103 L ☉). Deuterated species such as HDO, HDCO, D2CO, and CH2DOH are also detected. A comparison of fractional abundances of COMs relative to CH3OH between the outer Galactic hot core and an inner Galactic counterpart shows a remarkable similarity. On the other hand, the molecular abundances in the present source do not resemble those of low-metallicity hot cores in the Large Magellanic Cloud. The results suggest that great molecular complexity exists even in the primordial environment of the extreme outer Galaxy. The detection of another embedded protostar associated with high-velocity SiO outflows is also reported.

First detection of C2H5NCO in the ISM and search of other isocyanates towards the G+0.693-0.027 molecular cloud

Context. Little is known about the chemistry of isocyanates (compounds with the functional group R-N=C=O) in the interstellar medium (ISM), as only four of them have been detected so far: isocyanate radical (NCO), isocyanic acid (HNCO), N-protonated isocyanic acid (H2NCO+), and methyl isocyanate (CH3NCO). The molecular cloud G+0.693-0.027, located in the Galactic Centre, represents an excellent candidate to search for new isocyanates since it exhibits high abundances of the simplest ones, HNCO and CH3NCO. Aims. After CH3NCO, the next most complex isocyanates are ethyl isocyanate (C2H5NCO) and vinyl isocyanate (C2H3NCO). Their detection in the ISM would enhance our understanding of the formation of these compounds in space. Methods. We have searched for C2H5NCO, H2NCO+, C2H3NCO, and cyanogen isocyanate (NCNCO) in a sensitive unbiased spectral survey carried out in the 2 mm and 7 mm radio windows using the IRAM 30m and Yebes 40m radio telescopes, respectively. Results. We have detected C2H5NCO and H2NCO+ towards G+0.693-0.027 (the former for the first time in the ISM) with molecular abundances of (4.7–7.3) × 10−11 and (1.0–1.5) × 10−11, respectively. A ratio of CH3NCO/C2H5NCO = 8 ± 1 is obtained; therefore, the relative abundance determined for HNCO:CH3NCO:C2H5NCO is 1:1/55:1/447, which implies a decrease by more than one order of magnitude, going progressively from HNCO to CH3NCO and to C2H5NCO. This is similar to what has been found for alcohols and thiols, for example, and suggests that C2H5NCO is likely formed on the surface of dust grains. In addition, we have obtained column density ratios of HNCO/NCO > 269, HNCO/H2NCO+ ∼ 2100, and C2H3NCO/C2H5NCO < 4. A comparison of the methyl/ethyl ratios for isocyanates (-NCO), alcohols (-OH), formiates (HCOO-), nitriles (-CN), and thiols (-SH) is performed and shows that ethyl derivatives may be formed more efficiently for the N-bearing molecules than for the O- and S-bearing molecules.

A Systematic Characterization of the HR8799 System with GRAVITY

&lt;p&gt;The four planets of the HR8799 system provide a benchmark for directly imaged exoplanets. As these planets share a formation history, variations between the planet&amp;#8217;s atmospheric properties - likely tracing their individual formation pathways - could provide insight into the details of the process of planet formation. In order explore these atmospheres and their evolution, we use new data obtained with the GRAVITY instrument at the VLTI as part of the ExoGRAVITY campaign, combined with data from SPHERE, GPI, CHARIS, ALES and OSIRIS in order to provide the best picture of the planetary atmospheres across a broad wavelength range. Using petitRADTRANS in a Bayesian retrieval framework, we compare a suite of state-of-the-art models applied to each of the targets in order to measure atmospheric properties such as metallicity, molecular abundances, and the C/O ratio, which is a well known tracer of the formation history. In this talk I will describe the data processing and modelling efforts which allow us to peer into the atmospheres of the HR8799 planets, and will outline the steps needed to tie the newly retrieved planetary properties to the formation history of the system.&lt;/p&gt;

Using HCO+ isotopologues as tracers of gas depletion in protoplanetary disk gaps

Context. The widespread rings and gaps seen in the dust continuum in protoplanetary disks are sometimes accompanied by similar substructures seen in molecular line emission. One example is the outer gap at ~100 au in AS 209, which shows that the H13CO+ and C18O emission intensities decrease along with the continuum in the gap, while the DCO+ emission increases inside the gap. Aims. We aim to study the behavior of DCO+/H13CO+ and DCO+/HCO+ ratios in protoplanetary disk gaps assuming the two scenarios: (A) the gas depletion follows the dust depletion and (B) only the dust is depleted. Methods. We first modeled the physical disk structure using the thermo-chemical model ANDES. This 1+1D steady-state disk model calculates the thermal balance of gas and dust and includes the far ultraviolet, X-rays, cosmic rays, and other ionization sources together with the reduced chemical network for molecular coolants. Afterward, this physical structure was adopted for calculations of molecular abundances with the extended gas-grain chemical network with deuterium fractionation. Ideal synthetic spectra and 0th-moment maps were produced with the LIne Modeling Engine. Results. We are able to qualitatively reproduce the increase in the DCO+ intensity and the decrease in the H13CO+ and C18O intensities inside the disk gap, which is qualitatively similar to what is observed in the outer AS 209 gap. The corresponding disk model (A) assumes that both the gas and dust are depleted in the gap. The model (B) with the gas-rich gap, where only the dust is depleted, produces emission that is too bright in all HCO+ isotopologues and C18O. Conclusions. The DCO+/H13CO+ line ratio can be used to probe gas depletion in dust continuum gaps outside of the CO snow line. The DCO+/C18O line ratio shows a similar, albeit weaker, effect; however, these species can be observed simultaneously with a single (sub)mm interferometer setup.

Alfnoor: assessing the information content of Ariel's low resolution spectra with planetary population studies.

&lt;p&gt;In the next decade the Ariel Space Telescope will provide the first statistical dataset of exoplanet spectra, performing spectroscopic observation of about 1000 exoplanets in the wavelength range 0.5&amp;#8594;7.8 &amp;#956;m thanks to its Reconnaissance Survey. About one half of these 1000 targets will be then selected for more accurate observations with higher spectral resolution.&lt;/p&gt; &lt;p&gt;We present a novel metric to assess the information content of the Ariel Reconnaissance Survey low resolution transmission spectra. The proposed strategy will not only allow us to select candidate planets to be re-observed in Ariel higher resolution Tiers, but also to classify exoplanets by their atmospheric composition and to put the basis for the statistical analysis of such a large exoplanetary sample.&lt;/p&gt; &lt;p&gt;To test our metric we use Alfnoor, a new package combining the TauRex spectral modelling with the ArielRad payload performance model, to produce populations of hundreds of exoplanets matching those presented in the Ariel Mission Reference Sample. For each of the planets in the Ariel candidate targets list we create an atmosphere with a randomised quantity of H&lt;sub&gt;2&lt;/sub&gt;O, CH&lt;sub&gt;4&lt;/sub&gt;, CO&lt;sub&gt;2&lt;/sub&gt;, NH&lt;sub&gt;3&lt;/sub&gt; and clouds.&amp;#160;&lt;/p&gt; &lt;p&gt;Our metric proves able to identify methane,&amp;#160; carbon&amp;#160; dioxide&amp;#160; and&amp;#160; water&amp;#160; rich&amp;#160; atmospheres in the cases of molecular abundances &gt; 10&lt;sup&gt;&amp;#8722;4&lt;/sup&gt; in mixing ratio,&amp;#160; but it shows its limits in separating water from ammonia.&amp;#160;&lt;/p&gt; &lt;p&gt;We compare our metric with four different Deep Learning algorithms, they show only &amp;#8764;10% better performance in identifying the molecular content.&lt;/p&gt;

The surprisingly carbon-rich environment of the S-type star W Aql,

Context. W Aql is an asymptotic giant branch (AGB) star with an atmospheric elemental abundance ratio C/O ≈ 0.98. It has previously been reported to have circumstellar molecular abundances intermediate between those of M-type and C-type AGB stars, which respectively have C/O < 1 and C/O > 1. This intermediate status is considered typical for S-type stars, although our understanding of the chemical content of their circumstellar envelopes is currently rather limited. Aims. We aim to assess the reported intermediate status of W Aql by analysing the line emission of molecules that have never before been observed towards this star. Methods. We performed observations in the frequency range 159−268 GHz with the SEPIA/B5 and PI230 instruments on the APEX telescope. We made abundance estimates through direct comparison to available spectra towards a number of well-studied AGB stars and based on rotational diagram analysis in the case of one molecule. Results. From a compilation of our abundance estimates and those found in the literature for two M-type (R Dor, IK Tau), two S-type (χ Cyg, W Aql), and two C-type stars (V Aql, IRC +10 216), we conclude that the circumstellar environment of W Aql appears considerably closer to that of a C-type AGB star than to that of an M-type AGB star. In particular, we detect emission from C2H, SiC2, SiN, and HC3N, molecules previously only detected towards the circumstellar environment of C-type stars. This conclusion, based on the chemistry of the gaseous component of the circumstellar environment, is further supported by reports in the literature on the presence of atmospheric molecular bands and spectral features of dust species which are typical for C-type AGB stars. Although our observations mainly trace species in the outer regions of the circumstellar environment, our conclusion matches closely that based on recent chemical equilibrium models for the inner wind of S-type stars: the atmospheric and circumstellar chemistry of S-type stars likely resembles that of C-type AGB stars much more closely than that of M-type AGB stars. Conclusions. Further observational investigation of the gaseous circumstellar chemistry of S-type stars is required to characterise its dependence on the atmospheric C/O. Non-equilibrium chemical models of the circumstellar environment of AGB stars need to address the particular class of S-type stars and the chemical variety that is induced by the range in atmospheric C/O.

Reproducing the CO-to-H2 conversion factor in cosmological simulations of Milky-Way-mass galaxies

ABSTRACT We present models of CO(1–0) emission from Milky-Way-mass galaxies at redshift zero in the FIRE-2 cosmological zoom-in simulations. We calculate the molecular abundances by post-processing the simulations with an equilibrium chemistry solver while accounting for the effects of local sources, and determine the emergent CO(1–0) emission using a line radiative transfer code. We find that the results depend strongly on the shielding length assumed, which, in our models, sets the attenuation of the incident UV radiation field. At the resolution of these simulations, commonly used choices for the shielding length, such as the Jeans length, result in CO abundances that are too high at a given H2 abundance. We find that a model with a distribution of shielding lengths, which has a median shielding length of ∼3 pc in cold gas (T &lt; 300 K) for both CO and H2, is able to reproduce both the observed CO(1–0) luminosity and inferred CO-to-H2 conversion factor at a given star formation rate compared with observations. We suggest that this short shielding length can be thought of as a subgrid model, which controls the amount of radiation that penetrates giant molecular clouds.

Tracing bulk elemental ratios in exoplanetary atmospheres with TiO chemistry

Deciphering the bulk elemental abundances of exoplanetary atmospheres is not an easy task, yet it is crucial to understanding the formation history of planets. The purpose of this work is to show that the observability of TiO features at optical wavelengths in the transmission spectra of hot Jupiter atmospheres is sensitive to the bulk chemical properties of the atmosphere. To this end, we ran a grid of chemical models, which include TiO formation and destruction, for the ultra-hot Jupiter WASP-19b and an ultra-hot version of HD 209458b. We take into account non-equilibrium chemistry and changes in the temperature and pressure structure of these atmospheres caused by different C/O ratios. We calculated synthetic transmission spectra for these models, and studied the relative strengths of TiO and H2O features quantitatively. To compare with observations, we used a model-independent metric for molecular abundances, ΔZTiO−H2O/Heq, which has previously been used in observational studies of exoplanetary atmospheres. We find that with this metric we can differentiate between different chemical models and place constraints on the bulk carbon and oxygen abundances of the atmosphere. From chemical considerations, we expected the TiO abundance to depend on the bulk nitrogen. However, we find that changes in N/H do not result in changes in the resulting TiO. We applied our method to a set of known exoplanets that have been observed in the relevant optical wavelengths and find good agreement between low-resolution observations and our model for WASP-121b, marginally good agreement with WASP-79b, WASP-76b, and WASP-19b, and poorer agreement with HD 209458b. Our method will be particularly helpful for indirect studies of the bulk abundances of carbon and oxygen.