arXiv subjects

S. Bovino

Publications and source records attributed to S. Bovino.

At least 19 recordsLinked to original sources

arXiv2026

End-to-end differentiable retrieval of molecular spectra using hydrodynamics, chemistry, and radiative transfer

We aim to reproduce observed molecular line emission using a pipeline that couples hydrodynamics, chemistry, and radiative transfer, capable of simultaneously optimizing all relevant physical and chemical parameters. We developed an end-to-end differentiable Jax pipeline consisting of a custom hydrodynamical code, a modified version of the differentiable chemical code Carbox, and a custom radiative transfer code. We tested the framework using controlled synthetic data. We demonstrate that the framework can recover the parameters of hydrodynamical shock models directly from molecular line spectra and optimize selected chemical reaction rate coefficients through gradient-based optimization. The differentiable formulation enables efficient optimization of the coupled physical and chemical model while preserving the full time-dependent evolution.

astro-ph.IM↗

arXiv2026

Interstellar Formation of Thioethanal (CH$_{3}$CHS). Gas-Phase and Ice-Surface Mechanisms involving Secondary Sulfur Products

The formation pathways of sulfur-bearing species in the interstellar medium are crucial to understand astrochemical processes in cold molecular clouds and to gain new insights about the sulfur budget in these regions. We aim to explore the recently detected, thioethanal (CH$_{3}$CHS) formation mechanisms from thioethanol (CH$_{3}$CH$_{2}$SH) as a precursor in addition to secondary sulfur products. The electronic structure methods and density functional theory for both gas-phase and ice-grain surface environments is employed. To mimic interstellar ice-mantles, we use medium (W6) and large amorphized (W22) water clusters as implemented in Binding Energy Evaluation protocol. A barrierless formation mechanism for CH$_{3}$CHS under low-temperature interstellar conditions is identified, in the gas phase. Surface environments modulate activation barriers in a site-specific manner, elucidated through both Langmuir-Hinshelwood and Eley-Rideal initiated surface reaction pathways. Compared to oxygen analogs, sulfur chemistry enables alternate pathways due to weaker S-H bonding, with a competing route forming ethane-1,1-di-thiol (CH$_{3}$CH(SH)SH), on the ice-grain surface, potentially reducing CH$_{3}$CHS yields. The first accurate binding energy for thioethanol on water ice is also reported, confirming its greater volatility than ethanol. The proposed mechanism offers a tentative hypothesis for the apparent mutual exclusive detections of the CH$_{3}$CH$_{2}$SH and CH$_{3}$CHS in TMC-1, Orion, and Sgr B2(N), that further requires validation through quantitative astrochemical modeling and also to distinguish this chemical differentiation from observational sensitivity limitations. These qualitative findings highlight the multifaceted chemical behavior of sulfur-bearing organics in the interstellar medium and support CH$_{3}$CH(SH)SH as promising astro-chemical targets.

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arXiv2025

Cosmic-ray ionisation rate in low-mass cores: the role of the environment

Context: Cosmic rays drive several key processes for the chemistry and dynamical evolution of star-forming regions. Their effect is quantified mainly by means of the cosmic-ray ionisation rate $ζ_2$. Aims: We aim to obtain a sample of $ζ_2$ measurements in 20 low-mass starless cores embedded in different parental clouds, to assess the average level of ionisation in this kind of sources and to investigate the role of the environment in this context. The warmest clouds in our sample are Ophiuchus and Corona Australis, where star formation activity is higher than in the Taurus cloud and the other isolated cores we targeted. Methods: We compute $ζ_2$ using an analytical method based on the {column density} of ortho-$\rm H_2D^+$, the CO abundance, and the deuteration level of HCO$^+$. To estimate these quantities, we analysed new, high-sensitivity molecular line observations obtained with the Atacama Pathfinder EXperiment (APEX) single-dish telescope and archival continuum data from Herschel. Results: We report $ζ_2$ estimates in 17 cores in our sample and provide upper limits on the three remaining sources. The values span almost two orders of magnitude, from $1.3 \times 10^{-18}\, \rm s^{-1}$ to $8.5 \times 10^{-17}\, \rm s^{-1}$. Conclusions: We find no significant correlation between $ζ_2$ and the core's column densities $N\rm (H_2)$. On the contrary, we find a positive correlation between $ζ_2$ and the cores' temperature, estimated via Herschel data: cores embedded in warmer environments present higher ionisation levels. The warmest clouds in our sample are Ophiuchus and Corona Australis, where star formation activity is higher than in the other clouds we targeted. The higher ionisation rates in these regions support the scenario that low-mass protostars in the vicinity of our targeted cores contribute to the re-acceleration of local cosmic rays.

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arXiv2025

Impact of cosmic-ray propagation on the chemistry and ionisation fraction of dark clouds

A proper modelling of the cosmic-ray ionisation rate within gas clouds is crucial to describe their chemical evolution accurately. However, this modelling is computationally demanding because it requires the propagation of cosmic rays throughout the cloud over time. We present a more efficient approach that simultaneously guarantees a reliable estimate of the cosmic-ray impact on the chemistry of prestellar cores. We introduce a numerical framework that mimics the cosmic-ray propagation within gas clouds and applies it to magnetohydrodynamic simulations performed with the code GIZMO. It simulates the cosmic-ray attenuation by computing the effective column density of H$_2$ that is traversed, which is estimated using the same kernel weighting approach as employed in the simulation. The obtained cosmic-ray ionisation rate is then used in post-processing to study the chemical evolution of the clouds. We found that cosmic-ray propagation affects deuterated and non-deuterated species significantly and that it depends on the assumed cosmic-ray spectrum. We explored correlations between the electron abundance, the cosmic-ray ionisation rate, and the abundance of the most relevant ions (HCO$^+$, N$_2$H$^+$, DCO$^+$, N$_2$D$^+$, and o-H$_2$D$^+$), with the purpose of finding simple expressions that link them. We provide an analytical formula to estimate the ionisation fraction, X(e$^-$), from observable tracers and applied it to existing observations of high-mass clumps. We obtained values of about 10$^{-8}$, which is in line with previous works and with expectations for dense clouds. We also provide a linear fit to calculate the cosmic-ray ionisation rate from the local H$_2$ density, which is to be employed in three-dimensional simulations that do not include cosmic-ray propagation.

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arXiv2025

The ALMA-CRISTAL survey: Gas, dust, and stars in star-forming galaxies when the Universe was ~1 Gyr old I. Survey overview and case studies

We present the ALMA-CRISTAL survey, an ALMA Cycle 8 Large Program designed to investigate the physical properties of star-forming galaxies at $4 \lesssim z \lesssim 6$ through spatially resolved, multi-wavelength observations. This survey targets 19 star-forming main-sequence galaxies selected from the ALPINE survey, using ALMA Band 7 observations to study [CII] 158 $μ$m line emission and dust continuum, complemented by JWST/NIRCam and HST imaging to map stellar and UV emission. The CRISTAL sample expanded to 39 after including newly detected galaxies in the CRISTAL fields, archival data, and pilot study targets. The resulting dataset provides a detailed view of gas, dust, and stellar structures on kiloparsec scales at the end of the era of reionization. The survey reveals diverse morphologies and kinematics, including rotating disks, merging systems, [CII] emission tails from potential interactions, and clumpy star formation. Notably, the [CII] emission in many cases extends beyond the stellar light seen in HST and JWST imaging. Scientific highlights include CRISTAL-10, exhibiting an extreme [CII] deficit similar to Arp 220; and CRISTAL-13, where feedback from young star-forming clumps likely causes an offset between the stellar clumps and the peaks of [CII] emission. CRISTAL galaxies exhibit global [CII]/FIR ratios that decrease with increasing FIR luminosity, similar to trends seen in local galaxies but shifted to higher luminosities, likely due to their higher molecular gas content. CRISTAL galaxies also span a previously unexplored range of global FIR surface brightness at high-redshift, showing that high-redshift galaxies can have elevated [CII]/FIR ratios. These elevated ratios are likely influenced by factors such as lower metallicity gas, the presence of significant extraplanar gas, and contributions from shock-excited gas.

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arXiv2025

Mapping Synthetic Observations to Prestellar Core Models: An Interpretable Machine Learning Approach

Observations of molecular lines are a key tool to determine the main physical properties of prestellar cores. However, not all the information is retained in the observational process or easily interpretable, especially when a larger number of physical properties and spectral features are involved. We present a methodology to link the information in the synthetic spectra with the actual information in the simulated models (i.e., their physical properties), in particular, to determine where the information resides in the spectra. We employ a 1D gravitational collapse model with advanced thermochemistry, from which we generate synthetic spectra. We then use neural network emulations and the SHapley Additive exPlanations (SHAP), a machine learning technique, to connect the models' properties to the specific spectral features. Thanks to interpretable machine learning, we find several correlations between synthetic lines and some of the key model parameters, such as the cosmic-ray ionization radial profile, the central density, or the abundance of various species, suggesting that most of the information is retained in the observational process. Our procedure can be generalized to similar scenarios to quantify the amount of information lost in the real observations. We also point out the limitations for future applicability.

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arXiv2024

A hidden active galactic nucleus powering bright [O III] nebulae in a protocluster at $z=4.5$ revealed by JWST

Galaxy protoclusters are sites of rapid growth, with a high density of massive galaxies driving elevated rates of star formation and accretion onto supermassive black holes. Here, we present new JWST/NIRSpec IFU observations of the J1000+0234 group at $z=4.54$, a dense region of a protocluster hosting a massive, dusty star forming galaxy (DSFG). The new data reveal two extended, high-equivalent-width (EW$_0>1000Å$) [O III] nebulae that appear at both sides of the DSFG along its minor axis (namely O3-N and O3-S). On one hand, the spectrum of O3-N shows a broad and blueshifted component with a full width at half maximum (FWHM) of 1300 km/s, suggesting an outflow origin. On the other hand, O3-S stretches over 8.6 kpc, and has a velocity gradient that spans 800 km/s, but shows no evidence of a broad component. However, both sources seem to be powered by an active galactic nucleus (AGN), so we classified them as extended emission-line regions (EELRs). The strongest evidence comes from the detection of the high-ionization [Ne V] $λ3427$ line toward O3-N, which paired with the lack of hard X-rays implies an obscuring column density above the Compton-thick regime. The [Ne V] line is not detected in O3-S, but we measure a He II $λ4687$/H$β$=0.25, which is well above the expectation for star formation. Despite the remarkable alignment of O3-N and O3-S with two radio sources, we do not find evidence of shocks from a radio jet that could be powering the EELRs. We interpret this as O3-S being externally irradiated by the AGN, akin to the famous Hanny's Voorwerp object in the local Universe. In addition, classical line ratio diagnostics (e.g., [O III]/H$β$ vs [N II]/H$α$) put the DSFG itself in the AGN region of the diagrams, and therefore suggest it to be the most probable AGN host. These results showcase the ability of JWST to unveil obscured AGN at high redshifts.

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arXiv2024

Time evolution of o-H$_2$D$^+$, N$_2$D$^+$, and N$_2$H$^+$ during the high-mass star formation process

Deuterium fractionation is a well-established evolutionary tracer in low-mass star formation, but its applicability to the high-mass regime remains an open question. The abundances and ratios of deuterated species have often been proposed as reliable evolutionary indicators for different stages of the high-mass star formation. We investigate the role of N$_2$H$^+$ and key deuterated molecules as tracers of the different stages of the high-mass star formation, and test whether their abundance ratios can serve as reliable evolutionary indicators. We conducted APEX observations of o-H$_2$D$^+$ (1$_{10}$-1$_{11}$), N$_2$H$^+$ (4-3), and N$_2$d$^+$ (3-2) in 40 high-mass clumps at different evolutionary stages, selected from the ATLASGAL survey. Molecular column densities ($N$) and abundances ($X$), were derived through spectral line modelling, both under local thermodynamic equilibrium (LTE) and non-LTE conditions. The $N$(o-H$_2$D$^+$) show the smallest deviation from LTE results when derived under non-LTE assumptions. In contrast, N$_2$D$^+$ shows the largest discrepancy between the $N$ derived from LTE and non-LTE. In all the cases discussed, we found that $X$(o-H$_2$D$^+$) decreases more significantly with time than in the case of $X$(N$_2$D$^+$); whereas $X$(N$_2$H$^+$) increases slightly. Therefore, the validity of the recently proposed $X$(o-H$_2$D$^+$)/$X$(N$_2$D$^+$) ratio as a reliable evolutionary indicator was not observed for this sample. While the deuteration fraction derived from N$_2$D$^+$ and N$_2$H$^+$ clearly decreases with clump evolution, the interpretation of this trend is complex, given the different distribution of the two tracers. Our results suggest that a careful consideration of the observational biases and beam-dilution effects are crucial for an accurate interpretation of the evolution of the deuteration process during the high-mass star formation process.

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arXiv2024

The ALMA-CRISTAL survey: Dust temperature and physical conditions of the interstellar medium in a typical galaxy at z=5.66

We present new $λ_{\rm rest}=77$ $μ$m dust continuum observations from the ALMA of HZ10 (CRISTAL-22), a dusty main-sequence galaxy at $z$=5.66 as part of the [CII] Resolved Ism in STar-forming Alma Large program, CRISTAL. The high angular resolution of the ALMA Band 7 and new Band 9 data($\sim{0}''.4$) reveals the complex structure of HZ10, which comprises two main components (HZ10-C and HZ10-W) and a bridge-like dusty emission between them (the Bridge). We model the dust spectral energy distribution (SED) to constrain the physical conditions of the interstellar medium (ISM) and its variations among the different components identified in HZ10. We find that HZ10-W (the more UV-obscured component) has an SED dust temperature of $T_{\rm SED}$$\sim$51.2$\pm13.1$ K; this is $\sim$5 K higher (although still consistent) than that of the central component and previous global estimations for HZ10. Our new ALMA data allow us to reduce by a factor of $\sim$2.3 the uncertainties of global $T_{\rm SED}$ measurements compared to previous studies. Interestingly, HZ10-W shows a lower [CII]/FIR ratio compared to the other two components (although still within the uncertainties), suggesting a harder radiation field destroying polycyclic aromatic hydrocarbon associated with [CII] emission (e.g., active galactic nuclei or young stellar populations). While HZ10-C appears to follow the tight IRX-$β_{\rm UV}$ relation seen in local UV-selected starburst galaxies and high-$z$ star-forming galaxies, we find that both HZ10-W and the Bridge depart from this relation and are well described by dust-screen models with holes in front of a hard UV radiation field. This suggests that the UV emission (likely from young stellar populations) is strongly attenuated in the more dusty components of the HZ10 system.

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arXiv2024

Parsec-scale cosmic-ray ionisation rate in Orion

Cosmic rays regulate the dynamics and the chemical processes in the densest and coldest regions of the ISM. Still, the determination of the cosmic-ray ionisation rate of H$_2$ (${ζ^{\rm ion}_{{\rm H}_2}}$) is plagued by uncertainties in the adopted chemical networks and the analysis techniques. This work aims to homogeneously estimate the ${ζ^{\rm ion}_{{\rm H}_2}}$ at parsec scales towards the Orion Molecular Clouds OMC-2 and OMC-3, probing its variation across a whole star-forming region and a range of column densities never explored before. The most recent ${ζ^{\rm ion}_{{\rm H}_2}}$ estimates are based on o$-$H$_2$D$^+$, whose abundance we proxy through CO depletion taking advantage of the existing correlation between the two parameters. We therefore employ observations of C$^{18}$O (2$-$1), HCO$^+$ (1$-$0) and DCO$^+$ (3$-$2) towards OMC-2 and OMC-3 to determine the depletion factor, the deuteration fraction and, ultimately, a map of ${ζ^{\rm ion}_{{\rm H}_2}}$ in these two regions. The depletion factors and deuteration fractions correlate with the total column density of H$_2$, the N$_2$H$^+$ emission and the coldest fields across OMC-2 and OMC-3. The cosmic-ray ionisation rate shows values of ${ζ^{\rm ion}_{{\rm H}_2}}\sim5\times10^{-18}-10^{-16}$~s$^{-1}$, in agreement with previous o$-$H$_2$D$^+$-based estimates. In addition, it shows an overall decrease for increasing $N(\mathrm{H_2}$), consistently with the predictions from theoretical models. Our approach provides results comparable with theoretical predictions and previous independent studies, confirming the robustness of the analytical framework and the viability of CO depletion as proxy for o$-$H$_2$D$^+$. By exploring the major limitations of the method, we suggest interferometric observations as mandatory to reliably constrain the ${ζ^{\rm ion}_{{\rm H}_2}}$ also at parsec scales.

astro-ph.GA↗

arXiv2024

The ALMA-CRISTAL survey: Extended [CII] emission in an interacting galaxy system at z ~ 5.5

The ALMA [CII] Resolved Ism in STar-forming gALaxies (CRISTAL) survey is a Cycle 8 ALMA Large Programme that studies the cold gas component of high-redshift galaxies. Its sub-arcsecond resolution observations are key to disentangling physical mechanisms that shape galaxies during cosmic dawn. In this paper, we explore the morphology and kinematics of the cold gas, star-forming, and stellar components in the star-forming main-sequence galaxy CRISTAL-05/HZ3, at z = 5.54. Our analysis includes 0.3" spatial resolution (~2 kpc) ALMA observations of the [CII] line. While CRISTAL-05 was previously classified as a single source, our observations reveal that the system is a close interacting pair surrounded by an extended component of carbon-enriched gas. This is imprinted in the disturbed elongated [CII] morphology and the separation of the two components in the position-velocity diagram (~100 km/s). The central region is composed of two components, named C05-NW and C05-SE, with the former being the dominant one. A significant fraction of the [CII] arises beyond the close pair up to 10 kpc, while the regions forming new massive stars and the stellar component seem compact (r_[CII] ~ 4 r_UV), as traced by rest-frame UV and optical imaging obtained with the Hubble Space Telescope and the James Webb Space Telescope. Our kinematic model, using the DYSMALpy software, yields a minor contribution of dark matter of C05-NW within a radius of ~2x Reff. Finally, we explore the resolved [CII]/FIR ratios as a proxy for shock-heating produced by this merger. We argue that the extended [CII] emission is mainly caused by the merger, which could not be discerned with lower-resolution observations. Our work emphasizes the need for high-resolution observations to fully characterize the dynamic stages of infant galaxies and the physical mechanisms that drive the metal enrichment of the circumgalactic medium.

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arXiv2024

Testing analytical methods to derive the cosmic-ray ionisation rate in cold regions via synthetic observations

Cosmic rays (CRs) heavily impact the chemistry and physics of cold and dense star-forming regions. However, characterising their ionisation rate is still challenging from an observational point of view. In the past, a few analytical formulas have been proposed to infer the cosmic-ray ionization rate $ζ_2$ from molecular line observations. These have been derived from the chemical kinetics of the involved species, but they have not been validated using synthetic data processed with a standard observative pipeline. We aim to bridge this gap. We perform the radiative transfer on a set of three-dimensional magneto-hydrodynamical simulations of prestellar cores, exploring different initial $ζ_2$, evolutionary stages, types of radiative transfer (e.g. assuming local-thermodynamic-equilibrium conditions), and telescope responses. We then compute the column densities of the involved tracers to determine $ζ_2$, using, in particular, the equation proposed by Bovino et. al (2020) and by Caselli et al. (1998) both used nowadays. Our results confirm that the method of Bovino et al. (2020) accurately retrieves the actual $ζ_2$ within a factor of $2-3$, in the physical conditions explored in our tests. Since we also explore a non-local thermodynamic equilibrium radiative transfer, this work indirectly offers insights into the excitation temperatures of common transitions at moderate volume densities ($n\approx 10^5 \, \rm cm^{-3}$). We have also performed a few tests using the formula proposed by Caselli et al. (1998), which overestimates the actual $ζ_2$ by at least two orders of magnitudes. We also consider a new derivation of this method, which, however, still leads to large overestimates.

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arXiv2022

The core population and kinematics of a massive clump at early stages: an ALMA view

High-mass star formation theories make distinct predictions on the properties of the prestellar seeds of high-mass stars. Observations of the early stages of high-mass star formation can provide crucial constraints, but they are challenging and scarce. We investigate the properties of the prestellar core population embedded in the high-mass clump AGAL014.492-00.139, and we study the kinematics at the clump and the clump-to-core scales. We have analysed an extensive dataset acquired with the ALMA interferometer. Applying a dendrogram analysis to the Band o-$\rm H_2D^+$ data, we identified 22 cores. We have fitted their average spectra in local-thermodinamic-equilibrium conditions, and we analysed their continuum emission at $0.8 \, \rm mm$. The cores have transonic to mildly supersonic turbulence levels and appear mostly low-mass, with $M_\mathrm{core}< 30 \, \rm M_\odot$. Furthermore, we have analysed Band 3 observations of the $\rm N_2H^+$ (1-0) transition, which traces the large scale gas kinematics. Using a friend-of-friend algorithm, we identify four main velocity coherent structures, all of which are associated with prestellar and protostellar cores. One of them presents a filament-like structure, and our observations could be consistent with mass accretion towards one of the protostars. In this case, we estimate a mass accretion rate of $ \dot{M}_\mathrm{acc}\approx 2 \times 10^{-4} \rm \, M_\odot \, yr^{-1}$. Our results support a clump-fed accretion scenario in the targeted source. The cores in prestellar stage are essentially low-mass, and they appear subvirial and gravitationally bound, unless further support is available for instance due to magnetic fields.

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arXiv2021

Establishing the evolutionary timescales of the massive star formation process through chemistry

(Abridged) Understanding the details of the formation process of massive (i.e. M<8-10M$_\odot$) stars is a long-standing problem in astrophysics. [...] We present a method to derive accurate timescales of the different evolutionary phases of the high-mass star formation process. We model a representative number of massive clumps of the ATLASGAL-TOP100 sample which cover all the evolutionary stages. The models describe an isothermal collapse and the subsequent warm-up phase, for which we follow their chemical evolution. The timescale of each phase is derived by comparing the results of the models with the properties of the sources of the ATLASGAL-TOP100 sample, taking into account the mass and luminosity of the clumps, and the column densities of methyl acetylene (CH$_3$CCH), acetonitrile (CH$_3$CN), formaldehyde (H$_2$CO) and methanol (CH$_3$OH). We find that the chosen molecular tracers are affected by the thermal evolution of the clumps, showing steep ice evaporation gradients from 10$^3$ to 10$^5$ AU during the warm-up phase. We succeed in reproducing the observed column densities of CH$_3$CCH and CH$_3$CN, while H$_2$CO and CH$_3$OH show a poorer agreement with the observed values. The total (massive) star formation time is found to be $\sim5.2\times10^5$ yr, which is defined by the timescales of the individual evolutionary phases of the ATLASGAL-TOP100 sample: $\sim5\times10^4$ yr for 70-$μ$m weak, $\sim1.2\times10^5$ yr for mid-IR weak, $\sim2.4\times10^5$ yr for mid-IR bright and $\sim1.1\times10^5$ yr for HII-regions phases. Our models, with an appropriate selection of molecular tracers that can act as chemical clocks, allow to get robust estimates of the duration of the individual phases of the high-mass star formation process, with the advantage of being capable to include additional tracers aimed at increasing the accuracy of the estimated timescales.

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arXiv2021

Reducing the complexity of chemical networks via interpretable autoencoders

In many astrophysical applications, the cost of solving a chemical network represented by a system of ordinary differential equations (ODEs) grows significantly with the size of the network, and can often represent a significant computational bottleneck, particularly in coupled chemo-dynamical models. Although standard numerical techniques and complex solutions tailored to thermochemistry can somewhat reduce the cost, more recently, machine learning algorithms have begun to attack this challenge via data-driven dimensional reduction techniques. In this work, we present a new class of methods that take advantage of machine learning techniques to reduce complex data sets (autoencoders), the optimization of multi-parameter systems (standard backpropagation), and the robustness of well-established ODE solvers to to explicitly incorporate time-dependence. This new method allows us to find a compressed and simplified version of a large chemical network in a semi-automated fashion that can be solved with a standard ODE solver, while also enabling interpretability of the compressed, latent network. As a proof of concept, we tested the method on an astrophysically-relevant chemical network with 29 species and 224 reactions, obtaining a reduced but representative network with only 5 species and 12 reactions, and a x65 speed-up.

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arXiv2021

Molecular gas properties of Q1700-MD94: a massive, main-sequence galaxy at $z\approx2$

We use a combination of new NOrthern Extended Millimeter Array (NOEMA) observations of the pair of [CI] transitions, the CO(7-6) line, and the dust continuum, in addition to ancillary CO(1-0) and CO(3-2) data, to study the molecular gas properties of Q1700-MD94, a massive, main-sequence galaxy at $z\approx2$. We find that for a reasonable set of assumptions for a typical massive star-forming galaxy, the CO(1-0), the [CI](1-0) and the dust continuum yield molecular gas masses that are consistent within a factor of $\sim2$. The global excitation properties of the molecular gas as traced by the [CI] and CO transitions are similar to those observed in other massive, star-forming galaxies at $z\sim2$. Our large velocity gradient (LVG) modeling using RADEX of the CO and [CI] spectral line energy distributions (SLEDs) suggests the presence of relatively warm ($T_{\rm kin}=41$K), dense ($n_{\rm H_2}=8\times10^{3}~{\rm cm}^{-3}$) molecular gas, comparable to the high-excitation molecular gas component observed in main-sequence, star-forming galaxies at $z\sim1$. The galaxy size in the CO(1-0) and CO(7-6) line emission are comparable, which suggests that the highly-excited molecular gas is distributed throughout the disk powered by intense star formation activity. To confirm this scenario will require spatially resolved observations of the CO and [CI] lines which can now be obtained with NOEMA upgraded capabilities.

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arXiv2021

Identification of prestellar cores in high-mass star forming clumps via $\rm H_2D^+$ observations with ALMA

Context. The different theoretical models concerning the formation of high-mass stars make distinct predictions regarding their progenitors, i.e. the high-mass prestellar cores. However, so far no conclusive observation of such objects has been made. Aims. We aim to study the very early stages of high-mass star formation in two infrared-dark, massive clumps, to identify the core population that they harbour. Methods. We obtained ALMA observations of continuum emission at 0.8mm and of the ortho-$\rm H_2D^+$ transition at 372GHz towards the two clumps. We use the SCIMES algorithm to identify cores in the position-position-velocity space, finding 16 cores. We model their observed spectra in the LTE approximation, deriving the centroid velocity, linewidth, and column density maps. We also study the correlation between the continuum and molecular data, which in general do not present the same structure. Results. We report for the first time the detection of ortho-$\rm H_2D^+$ in high-mass star-forming regions performed with an interferometer. The molecular emission shows narrow and subsonic lines, suggesting that locally the temperature of the gas is less than 10K. From the continuum emission we estimate the cores' total masses, and compare them with the respective virial masses. We also compute the volume density values, which are found to be higher than $10^{6}\, \rm cm^{-3}$. Conclusions. Our data confirm that ortho-$\rm H_2D^+$ is an ideal tracer of cold and dense gas. Interestingly, almost all the $\rm H_2D^+$-identified cores are less massive than 13M_sun , with the exception of one core in AG354. Furthermore, most of them are subvirial and larger than their Jeans masses. These results are difficult to explain in the context of the turbulent accretion models, which predict massive and virialised prestellar cores.

astro-ph.GA↗

arXiv2020

Survey of ortho-H$_2$D$^+$ in high-mass star-forming regions

(Abridged) We present a large sample of o-H$_2$D$^+$ observations in high-mass star-forming regions and discuss possible empirical correlations with relevant physical quantities to assess its role as a chronometer of star-forming regions through different evolutionary stages. APEX observations of the ground-state transition of o-H$_2$D$^+$ were analysed in a sample of massive clumps selected from ATLASGAL at different evolutionary stages. Column densities and beam-averaged abundances of o-H$_2$D$^+$ with respect to H$_2$, $X$(o-H$_2$D$^+$), were obtained by modelling the spectra under the assumption of local thermodynamic equilibrium. We detect 16 sources in o-H$_2$D$^+$ and find clear correlations between $X$(o-H$_2$D$^+$) and the clump bolometric luminosity and the dust temperature, while only a mild correlation is found with the CO-depletion factor. In addition, we see a clear correlation with the luminosity-to-mass ratio, which is known to trace the evolution of the star formation process. This would indicate that the deuterated forms of H$_3^+$ are more abundant in the early stages of the star formation process and that deuteration is influenced by the time evolution of the clumps. In this respect, our findings would suggest that the $X$(o-H$_2$D$^+$) abundance is mainly affected by the thermal changes rather than density changes in the gas. We have employed these findings together with observations of H$^{13}$CO$^+$, DCO$^+$, and C$^{17}$O to provide an estimate of the cosmic-ray ionisation rate in a sub-sample of eight clumps based on recent analytical work. Our study presents the largest sample of o-H$_2$D$^+$ in star-forming regions to date. The results confirm that the deuteration process is strongly affected by temperature and suggests that o-H$_2$D$^+$ can be considered a reliable chemical clock during the star formation processes, as proved by its strong temporal dependence.

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