Paola Caselli

The HCO ⁺ /HOC ⁺ abundance ratio has been observed to vary significantly in different astrophysical environments, making it an important diagnostic tool for exploring the interstellar medium. However, uncertainties in the reaction rates and branching ratios of key chemical reactions involving these ions affect the conclusions drawn from observations of either these species or other species that are derived on their basis. Here, we present isomer-resolved ion-molecule reaction rate coefficients of reactions involving HCO ⁺ and HOC ⁺ using a cryogenic ion trap experiment. We measured the rate coefficients of the isomerization reaction of HOC ⁺ to HCO ⁺ with H ₂ and CO, as well as three different reactions producing both isomers: CO ⁺ + H ₂ , H ₃ ⁺ + CO, and ArH ⁺ + CO. Our results show that the reaction of CO ⁺ with H ₂ predominantly produces HCO ⁺ with a negligible yield of HOC ⁺ . This result is in agreement with recent theoretical predictions, but in major contrast to the generally accepted branching ratio based on previous experimental results. We also find that the isomerization reaction of HOC ⁺ to HCO ⁺ is efficient with both H ₂ and CO. We were able to assess the impact of our new measured reaction rate coefficients on the predicted HCO ⁺ /HOC ⁺ abundance ratio using a state-of-the-art astrochemical model.

We present molecular emission line data from the massive young stellar object (MYSO) G28.28-0.36 (G28.28) obtained with the Atacama Large Millimeter/submillimeter Array Band 3. Cyanopolyynes (HC ₃ N and HC ₅ N) and three complex organic molecules (COMs; CH ₃ OH, CH ₃ CN, and CH ₃ CHO) are detected from the MYSO G28.28. In addition, strong emission regions of cyanopolyynes are identified between G28.28 and a nearby ultracompact H ii region. The HC ₅ N emission is coincident with the dust continuum peak, where an excitation temperature of 100 K is derived from CH ₃ CN. These results suggest that the hot carbon-chain chemistry (HCCC) mechanism produces cyanopolyynes in the hot region around G28.28. We find that G28.28 exhibits a unique chemical feature: cyanopolyynes are abundant, but COMs are deficient, unlike the other MYSOs studied previously. These results imply that G28.28 is a counterpart of the warm carbon-chain chemistry (WCCC) low-mass source L1527. G28.28 is the first HCCC source identified so far.

Context . The ionisation balance in molecular clouds is regulated by several ionising sources, including cosmic rays, X-rays, and ultraviolet radiation. Their relative importance depends on the local physical conditions and on the shielding column density. Aims . Our goal is to quantify the contribution of the large-scale diffuse X-ray radiation field to ionisation in molecular clouds in the absence of strong local X-ray sources, such as young stars, and to assess its significance relative to Galactic cosmic rays. Methods . We used measurements of diffuse X-ray emission from the first eROSITA all-sky survey to estimate the depth-dependent X-ray ionisation rate in molecular clouds across different Galactic environments. We considered both the observed diffuse field and a deabsorbed field obtained by correcting for foreground Galactic attenuation using HI4PI line-of-sight column densities in a pixel-by-pixel framework. Results . Diffuse Galactic X-rays are unlikely to dominate the ionisation balance in well-shielded molecular gas ( N _H > 5 × 10 ²¹ cm ⁻² ), where cosmic rays remain the primary ionising agent. At low column densities, the deabsorbed diffuse X-ray field produces an ionisation rate of ζ _X ∼ 10 ⁻¹⁸ s ⁻¹ at N _H ∼ 10 ¹⁹ cm ⁻² , remaining below the lower end of the Galactic cosmic ray ionisation rate (∼10 ⁻¹⁷ s ⁻¹ ). The X-ray contribution decreases rapidly with increasing shielding, falling to ζ _X ∼ 10 ⁻¹⁹ s ⁻¹ by N _H ∼ 10 ²¹ cm ⁻² and to ζ _X ∼ 5 × 10 ⁻²¹ s ⁻¹ by N _H ∼ 10 ²² cm ⁻² . Diffuse X-ray may therefore contribute to the thermal and chemical structure of low-extinction material and should be considered when interpreting ionisation tracers in diffuse or cloud-envelope environments.

Context. Isotopic fractionation can serve as a powerful tracer of chemical evolution during star and planet formation. To accurately interpret observations, it is crucial to identify the dominant pathways of nitrogen and carbon fractionation at different evolutionary stages. Aims. We aim to study nitrogen and carbon fractionation in a sample of young cores at the onset of star formation. Methods. We mapped H ¹³ CN and HC ¹⁵ N around one starless and three pre-stellar cores. We computed the N (H ¹³ CN)/ N (HC ¹⁵ N) column density ratio across the cores and compared the distribution with N (H ₂ ) maps from Herschel /SPIRE. In addition, we calculated ¹⁴ N/ ¹⁵ N maps using the double isotope method for comparison with earlier studies. We compared these results with astrochemical modeling of carbon and nitrogen fractionation for a 1D pre-stellar core model. Results. The computed N (H ¹³ CN)/ N (HC ¹⁵ N) ratio exhibits clear spatial variation across the maps. This variation correlates with N (H ₂ ) in three out of four cores. Conclusions. Our analysis reveals a correlation between the H ¹³ CN/HC ¹⁵ N ratios and the N (H ₂ ) maps. According to the astrochemical model, the correlation is mainly due to variations in the ¹² C/ ¹³ C ratio. Consequently, the results caution against applying the doubleisotope method to derive ¹⁴ N/ ¹⁵ N ratios without independently assessing possible spatial variations in ¹² C/ ¹³ C. Furthermore, the leading cause of the isotopic variation in the model is not isotope-selective photodissociation, but rather more efficient fractionation through exchange reactions at lower temperatures in the denser regions of the cores.

Context. Prestellar cores are the birthplace of stars and planetary systems, but they are short-lived objects since the initial stages of dense core evolution, collapse, and the formation of a protostellar seed are fast. In an effort to build a catalogue of bona-fide prestellar cores in the solar neighbourhood, we used APEX observations to identify dynamically evolved cores among dense cores observed with Herschel . One of them, Corona Australis 151, stood out because of its centrally peaked structure, with densities above 10 ⁷ cm ⁻³ in the central 500 − 1000 au, and high deuteration levels (N ₂ D ⁺ /N ₂ H ⁺ ∼ 0.5), which are suggestive of an evolved prestellar stage. Aims. Corona Australis 151 appears to be an evolved prestellar core, but the presence of broad wings in some of the detected lines and a tentative 70 μm detection with Herschel partially challenges its status of being in a prestellar stage. We therefore assessed its evolutionary stage. Methods. We analysed new ALMA data of the continuum emission and of several line tracers (including typical outflow tracers) at a resolution of ∼150 − 200 au, tracing the envelope. Results. We unveil the presence of a compact and young outflow (projected dynamical age: ∼500 yr) traced by SiO, CO, and H ₂ CO emission. The continuum emission traces an envelope structure of size ∼1000 au and mass 0.33 M _⊙ (assuming T _dust = 20 K). Conclusions. Corona Australis 151 is an extremely young protostar, possibly one of the youngest known in the solar neighbourhood, and it can thus serve a new laboratory to study the chemical and dynamical evolution at the dawn of star formation.

Erratum: “A Detailed Temperature Map of the Archetypal Protostellar Shocks in L1157” (2022, ApJL, 933, L35), Feng, S., Liu, H. B., Caselli, P., Burkhardt, A., Du, F., Bachiller, R., Codella, C., Ceccarelli, C. Erratum: “A Detailed Temperature Map of the Archetypal Protostellar Shocks in L1157” (2022, ApJL, 933, L35), Feng, S., Liu, H. B., Caselli, P., Burkhardt, A., Du, F., Bachiller, R., Codella, C., Ceccarelli, C.

The search for dithioformic acid (t–HC(S)SH) in star-forming regions is crucial for understanding interstellar sulfur chemistry and addressing the “missing sulfur” problem. Motivated by a recent claim of t–HC(S)SH detection in NGC 1333 IRAS 4A2, we independently reanalyzed the same Atacama Large Millimeter/submillimeter Array dataset using comprehensive spectral and chemical modeling. We find no credible evidence for t–HC(S)SH: all reported transitions are fully accounted for by known, abundant molecules, with no unblended features unique to t–HC(S)SH. We critically reassess the reported detection, deriving stringent upper limits on the column density ( N _t-HC(S)SH ≤ 4 × 10 ¹⁴ cm ⁻² ) and the fractional abundance (≤1× 10 ⁻¹⁰ relative to H ₂ ). Our astrochemical models place these limits in context, showing the claimed detection likely results from spectral blending and inconsistent modeling assumptions. The nondetection aligns with chemical expectations given the rarity of complex and doubly sulfur-substituted molecules in hot corino environments. Furthermore, our analysis establishes a rigorous framework to guide future searches for sulfur-bearing species and highlights the critical importance of thorough line identification and modeling practices in astrochemistry.

Context . The dynamical role of the magnetic field in the star formation process is tightly linked to the coupling between matter and the field. This coupling is due to the interaction between ions and neutrals in the partially ionized interstellar medium. When the ionization degree drops in the dense environment of prestellar cores, the magnetic field and matter may decouple, leading to differences in the infalling velocities of ions and neutrals known as ambipolar diffusion. Aims . The onset of gravitational collapse resulting from ion-neutral decoupling has never been observed. The aim of this work is to search for signatures of ambipolar diffusion within a prestellar core. Methods . We observed the deuterated N ₂ D ⁺ ion and the neutral para-NH ₂ D species toward the prototypical prestellar core L1544. These two species are ideal tracers of prestellar cores, sampling the same high densities in the core interior. We compared the velocity centroid and linewidth maps of the ion-neutral pair. Results . We find a mean ion-neutral velocity difference of ∼0.05 km/s toward the core. We interpret the observed ion-neutral velocity difference in L1544 as a signature of ambipolar diffusion by comparing it with predictions from self-consistent calculations of ambipolar resistivity, including dust grain growth. We do not detect a significant ion-neutral linewidth difference that may be attributed to the subsonic infall motions of the gas in L1544 and geometrical effects in the presence of inclination. Conclusions . These results emphasize the role of dust grain growth at the prestellar core stage in setting the ambipolar resistivity and regulating the dynamical evolution of dense cores toward their collapse into protostars. We propose that measurements of ion-neutral drift velocities provide new constraints on the total magnetic field strength and the dust size distribution within prestellar cores.

For over a century, the origin of low-energy cosmic rays (LECRs), the dominant heaters and ionizers of dense interstellar gas, has remained elusive owing to solar modulation and uncertain transport processes. In this study, we introduce a new astrophysical approach based on H i narrow self-absorption (HINSA) to obtain spatially resolved measurements of LECR ionization rates using high-fidelity H i observations toward the Orion region from the FAST telescope. The LECR ionization rate is found to scale with local star formation rate as log 10 ζ = ( 1.4 ± 0.70 ) log 10 SFR + ( − 10.5 ± 2.9 ) . Moreover, it increases with visual extinction, and is found to exceed, toward active star-forming regions, the value predicted for diffuse regions based on Voyager measurements and an external propagation model. These findings demonstrate that LECRs are generated in situ by star-forming activities rather than penetrating from the broader Galactic cosmic-ray population. This is further supported by Fermi-LAT gamma-ray observations toward the Orion region. Together, these results resolve a key uncertainty in cosmic-ray origin and establish a new avenue for quantifying the energetic feedback that regulates the interstellar medium.