H II Regions Research Articles

Analytical strong line diagnostics and their redshift evolution

Abstract The JWST is allowing new measurements of gas-phase metallicities in galaxies between cosmic noon and cosmic dawn. The most robust approach uses luminosity ratios between the excited auroral transition, [O iii] 4364 Å, and the lower [O iii] 5008 Å/4960 Å lines to determine the gas temperature. The ratio of the luminosities in the latter transitions to those in hydrogen Balmer series lines then yield relatively clean metallicity estimates. In the absence of detection of the [O iii] auroral line, the ratios of various [O iii], [O ii], [N ii], and Balmer lines are used to determine metallicities. Here we present a refined approach for extracting metallicities from these “strong line diagnostics”. Our method exploits empirical correlations between the temperature of O iii/O ii regions and gas-phase metallicity. We then show, from first principles, how to extract metallicities and break degeneracies in these estimates using traditional strong line diagnostics, R2, R3, R23, and O3O2, and N2O2. We show that these ratios depend also on volume correction factors, i.e. on accounting for the fraction of the volume of HII regions that are in O iii and O ii, but that these can be determined self-consistently along with the metallicities. We quantify the success of our method using metallicities derived from galaxies with auroral line determinations and show that it generally works better than previous empirical approaches. The scatter in the observed line ratios and redshift evolution are largely explained by O3O2 variations. We provide publicly available routines for extracting metallicities from strong line diagnostics using our methodology.

Dynamical accretion flows

Context. Investigating the flow of material along filamentary structures towards the central core can help provide insights into high-mass star formation and evolution. Aims. Our main motivation is to answer the question of what the properties of accretion flows are in star-forming clusters. We used data from the ALMA Evolutionary Study of High Mass Protocluster Formation in the Galaxy (ALMAGAL) survey to study 100 ALMAGAL regions at a ∼1″ resolution, located between ∼2 and 6 kpc. Methods. Making use of the ALMAGAL ∼1.3 mm line and continuum data, we estimated flow rates onto individual cores. We focus specifically on flow rates along filamentary structures associated with these cores. Our primary analysis is centered around position velocity cuts in H2CO (30, 3–20, 2), which allow us to measure the velocity fields surrounding these cores. Combining this work with column density estimates, we were able to derive the flow rates along the extended filamentary structures associated with cores in these regions. Results. We selected a sample of 100 ALMAGAL regions, covering four evolutionary stages from quiescent to protostellar, young stellar objects (YSOs), and HII regions (25 each). Using a dendrogram and line analysis, we identify a final sample of 182 cores in 87 regions. In this paper, we present 728 flow rates for our sample (4 per core), analysed in the context of evolutionary stage, distance from the core, and core mass. On average, for the whole sample, we derived flow rates on the order of ∼10−4 M⊙ yr−1 with estimated uncertainties of ±50%. We see increasing differences in the values among evolutionary stages, most notably between the less evolved (quiescent and protostellar) and more evolved (YSO and HII region) sources and we also see an increasing trend as we move further away from the centre of these cores. We also find a clear relationship between the calculated flow rates and core masses ∼M2/3, which is in line with the result expected from the tidal-lobe accretion mechanism. The significance of these relationships is tested with Kolmogorov–Smirnov and Mann-Whitney U tests. Conclusions. Overall, we see an increasing trend in the relationships between the flow rate and the three investigated parameters, namely: evolutionary stage, distance from the core, and core mass.

Contribution of astrophysical events to the chemical evolution of a dwarf irregular galaxy

ABSTRACT We study the contribution of various astrophysical events asymptotic giant branch (AGB stars, core collapse and thermonuclear supernovae, neutron star mergers, and collapsars) to the abundances of elements both up to and heavier than the iron peak in a Local Group dwarf irregular galaxy, Wolf–Lundmark–Melotte (WLM). Its star formation history has been recently determined by observations with the JWST, and we use it to estimate the occurrence of the astrophysical sources. The rates of the gas accretion and the outflow are roughly determined based on the stellar metallicity distribution, the oxygen abundance of H ii regions, and the gas fraction. As discussed in the literature, the difference in time-scales on which the astrophysical events release the nucleosynthesis products is seen in the occurrence of the events and the evolution of abundance ratios. WLM has an extended star formation history and massive stars are recently and currently formed. Thus, the contribution of rotating massive stars through the weak s-process appears in the abundance ratios of light trans-iron elements to iron at later time of the evolution.

Optical polarimetry study of the Lambda-Orionis star-forming region

We present an optical polarimetry study of the nearby star-forming region Lambda-Orionis to map the plane-of-the-sky magnetic field geometry to understand the magnetized evolution of the HII region and the associated small molecular clouds. We made multiwavelength polarization observations of 34 bright stars distributed across the region. We also present the R-band polarization measurements that focused on the small molecular clouds, bright-rimmed clouds (BRC), BRC 17, and BRC 18, which are located at the periphery of the HII region. The magnetic field lines exhibit a large-scale ordered orientation consistent with the Planck submillimeter polarization measurements. The magnetic field lines in the two BRCs are found to be roughly in north-south directions. However, a larger dispersion is noted in the orientation for BRC 17 compared to BRC 18. Using a structure-function analysis, we estimate the strength of the plane-of-the-sky component of the magnetic field as ∼28 μG for BRC 17 and ∼40 μG for BRC 18. The average dust grain size and the mean value of the total-to-selective extinction ratio (RV) in the HII region are found to be ∼0.51 ± 0.05 μm and ∼2.9 ± 0.3, respectively. The distance of the whole HII region is estimated as ∼392 ± 8 pc by combining astrometry information from Global Astrometric Interferometer for Astrophysics (Gaia) early data release 3 (EDR3) for young stellar objects associated with BRCs and confirmed members of the central cluster Collinder 69.

HOMERUN: A new approach to photoionization modeling

We present HOMERUN (Highly Optimized Multi-cloud Emission-line Ratios Using photo-ionizatioN), a new approach to modeling emission lines from photoionized gas that can simultaneously reproduce all observed line intensities from a wide range of ionization levels with high accuracy. Our approach is based on the weighted combination of multiple single-cloud photoionization models, and contrary to previous works, the novelty of our approach consists of using the weights as free parameters of the fit and constraining them with the observed data. One of the main applications of HOMERUN is the accurate determination of gas-phase metallicities, and we show that a critical point is to allow for a variation of the N/O and S/O abundance ratios, as this can significantly improve the quality of the fit and the accuracy of the results. Moreover, our approach provides a major improvement compared to the single-cloud constant-pressure models commonly used in the literature. By using high-quality spectra from the literature of H II regions, where 10 to 20 emission lines (including several auroral lines) are detected with a high signal-to-noise ratio, we show that all lines are reproduced by the model with an accuracy better than 10%. In particular, the model is able to simultaneously reproduce [O I]λλ6300, 6363; [O II]λλ3726, 3729; [O III]λλ4959, 5007; [S II]λλ6717, 6731; and [S III]λλ9069, 9532 emission lines, which to our knowledge is an unprecedented result. Finally, we show that the gas metallicities estimated with our models for HII regions in the Milky Way are in better agreement with the stellar metallicities than the estimates based on the Te method. Overall, our method provides a new accurate tool to estimate the metallicity and the physical conditions of the ionized gas. It can be applied to many different science cases, from HII regions to active galactic nuclei, and wherever there are emission lines from photoionized gas.

Revisiting the massive star-forming complex RCW 122: New millimeter and submillimeter study

In this paper, we present a new multifrequency study of the giant star-forming complex RCW 122. We used molecular data obtained with the ASTE 10 m and the APEX 12 m telescopes, along with infrared observations spanning from 3.6 µm to 870 µm, obtained from available databases. We also incorporated a range of public datasets, including the radio continuum at 3 GHz, narrowband Ha images, and deep JHK photometry. Our analysis focuses mostly on cataloged ATLASGAL sources, showcasing a spectrum of evolutionary stages from infrared dark cloud (IRDC)/high-mass protostellar object (HMPO) to ultra-compact HII region (UCHII), as inferred from preliminary inspections of the public dataset. Based on ASTE HCO+(4−3) and CO(3−2) data, we identified five molecular clumps, designated A, B, C, D, and E, as molecular counterparts of the ATLASGAL sources. These clumps have radial velocities ranging from ~−15 km s−1 to −10 km s−1, confirming their association with RCW 122. In addition, we report the detection of 20 transitions from 11 distinct molecules in the APEX spectra in the frequency ranges from 258.38 GHz to 262.38 GHz, 228.538 GHz to 232.538 GHz, and 218.3 GHz to 222.3 GHz, unveiling a diverse chemical complexity among the clumps. Utilizing CO(2−1) and C18O(2−1) data taken from the observations with the APEX telescope, we estimated the total LTE molecular mass, ranging from 200 M⊙ (clump A) to 4400 M⊙ (clump B). Our mid- to far-infrared (MIR-FIR) flux density analysis yielded minimum dust temperatures of 23.7 K (clump A) to maximum temperatures of 33.9 K (clump B), indicating varying degrees of internal heating among the clumps. The bolometric luminosities span 1.7×103 L⊙ (clump A) to 2.4×105 L⊙ (clump B), while the total (dust+gas) mass ranges from 350 M⊙ (clump A) to 3800 M⊙ (clump B). Our analysis of the molecular line richness, L/M ratios, and CH3CCH and dust temperatures reveals an evolutionary sequence of A/E→C→D/B, consistent with preliminary inferences of the ATLASGAL sources. In this context, clumps A and E exhibit early stages of collapse, with clump A likely in an early HMPO phase, which is supported by identifying a candidate molecular outflow. Clump E appears to be in an intermediate stage between IRDC and HMPO. Clumps D and B show evidence of being in the UCHII phase, with clump B likely more advanced. Clump C likely represents an intermediate stage between HMPO and HMC. Our findings suggest clump B is undergoing ionization and heating by multiple stellar and protostellar members of the stellar cluster DBS 119. Meanwhile, other cluster members may be responsible for ionizing other regions of RCW 122 that have evolved into fully developed HII regions, beyond the molecular dissociation stage.

Extended CO(1–0) survey and ammonia measurements towards two bubble regions in W5

The feedback effect of massive stars can either accelerate or inhibit star formation activity within molecular clouds. Studying the morphology of molecular clouds near W5 offers an excellent opportunity to examine this feedback effect. We conducted a comprehensive survey of the W5 complex using the Purple Mountain Observatory 13.7 m millimeter telescope. This survey includes 12CO, 13CO, and C18O (J = 1 − 0), with a sky coverage of 6.6 deg2 (136.0° < l < 138.75°, 0° < b < 2.4°). Furthermore, we performed simultaneous observations of the NH3 (1,1) and NH3 (2,2) lines in the four densest star-forming regions of W5, using the 26 m radio telescope of the Xinjiang Astronomy Observatory (XAO). Our analysis of the morphological distribution of the molecular clouds, distribution of high-mass young stellar objects (HMYSOs), 13CO/C18O abundance ratio, and the stacked average spectral line distribution at different 8 μm thresholds provide compelling evidence of triggering. Within the mapped region, we identified a total of 212 molecular clumps in the 13CO cube data using the astrodendro algorithm. Remarkably, approximately 26.4% (56) of these clumps demonstrate the potential to form massive stars and 42.9% (91) of them are gravitationally bound. Within clumps that are capable of forming high-mass stars, there is a distribution of class I YSOs, all located in dense regions near the boundaries of the HII regions. The detection of NH3 near the most prominent cores reveals moderate kinetic temperatures and densities (as CO). Comparing the Tkin and Tex values reveals a reversal in trends for AFGL 4029 (higher Tex and lower Tkin) and W5-W1, indicating the inadequacy of optically thick CO for dense region parameter calculations. Moreover, a comparison of the intensity distributions between NH3 (1,1) and C18O (1–0) in the four densest region reveals a notable depletion effect in AFGL 4029, characterised by a low Tkin (9 K) value and a relatively high NH3 column density, 2.5 × 1014 cm−2. By classifying the 13CO clumps as: “feedback,” “non-feedback,” “outflow,” or “non-outflow” clumps, we observe that the parameters of the “feedback” and “outflow” clumps exhibit variations based on the intensity of the internal 8 μm flux and the outflow energy, respectively. These changes demonstrate a clear linear correlation, which distinctly separate them from the parameter distributions of the “non-feedback” and “non-outflow” clumps, thus providing robust evidence to support a triggering scenario.

MagMaR III—Resisting the Pressure, Is the Magnetic Field Overwhelmed in NGC6334I?

We report on Atacama Large Millimeter/submillimeter Array observations of polarized dust emission at 1.2 mm from NGC6334I, a source known for its significant flux outbursts. Between five months, our data show no substantial change in total intensity and a modest 8% variation in linear polarization, suggesting a phase of stability or the conclusion of the outburst. The magnetic field, inferred from this polarized emission, displays a predominantly radial pattern from northwest to southeast with intricate disturbances across major cores, hinting at spiral structures. Energy analysis of CS (J = 5 → 4) emission yields an outflow energy of approximately 3.5 × 1045 erg, aligning with previous interferometric studies. Utilizing the Davis–Chandrasekhar–Fermi method, we determined magnetic field strengths ranging from 1 to 11 mG, averaging at 1.9 mG. This average increases to 4 ± 1 mG when incorporating Zeeman measurements. Comparative analyses using gravitational, thermal, and kinetic energy maps reveal that magnetic energy is significantly weaker, possibly explaining the observed field morphology. We also find that the energy in the outflows and the expanding cometary HII region is also larger than the magnetic energy, suggesting that protostellar feedback may be the dominant driver behind the injection of turbulence in NGC6334I at the scales sampled by our data. The gas in NGC6334I predominantly exhibits supersonic and trans-Alfvenic conditions, transitioning towards a super-Alfvenic regime, underscoring a diminished influence of the magnetic field with increasing gas density. These observations are in agreement with prior polarization studies at 220 GHz, enriching our understanding of the dynamic processes in high-mass star-forming regions.

Identification of the young stellar clusters in the G345.5+1.5 Star-Forming Region

Star-forming regions, their formation and evolution are a subject of active study. The subject of our research is the G345.5+1.5 region, which is composed of two dusty ring-like structures referred to as G345.45+1.5 and G345.10+1.35. The region is located at a distance of 1.8 kpc and classified as HII regions. According to previous studies, G345.45+1.5 could have been created by a supernova explosion represented by the 36.6 cm source J165920-400424 in its center. Despite the considerable interest, only a few works are devoted to this region, and there is very little information about its stellar population. Our work focuses specifically on the stellar population in this region. Using 2MASS astrometric and photometric data, we identified three well-defined clusters of young stellar objects (YSOs) in the vicinity of the IRAS 16561-4006, 16571-4029, and 16545-4012 sources.

Stellar Membership of RCW 34 HII Star-Forming Region

The Vel OB1 association, located at a distance of 1.5-1.9 kpc, is one of the most actively studied star formation regions in the southern hemisphere. Our study focuses on an “Arclike” structure within the Vel OB1 association, which may have been formed from a shock wave caused by a supernova explosion about 2-2.5 Myr ago. Optical Gaia data revealed probable stellar members only in the southern part of the Arclike structure, where the RCW34 HII (IRAS 08546-4254) region is located. In contrast, the northeastern part of the Arclike structure, in the vicinity of the IRAS 08563-4225 source, hosts more embedded stellar population. Using near-infrared astrometric and photometric data from the 2MASS survey, we identified well-defined clusters of young stellar objects in the vicinity of both IRAS 08546-4254 and IRAS 08563-4225. The members of the cluster associated with IRAS 08563-4225 exhibit stronger infrared excess.

Unraveling the birthplaces of NGC 2070’s massive stars, tracked with MUSE and revealed with JWST

The formation of massive O-type stars cannot be simply explained as a scaled-up version of the accretion mechanisms observed in lower-mass stars. Understanding these processes necessitates systematic studies of their early stages, which are challenging to identify. Forming massive stars remain embedded in their dense nursery clouds, and IR instruments with high spatial resolution capabilities are needed to better observe them. Despite these challenges, MUSE optical observations of the massive cluster NGC 2070 successfully detected potential star-forming regions through spatially resolved electron density maps. To further explore these regions, the James Webb Space Telescope (JWST) utilized its NIRCam and MIRI instruments to penetrate optically obscured areas. This study examines two specific regions in the southeast part of the NGC 2070 MUSE density map, where tracks of highly dense point sources were identified. NIRCam, partially overlapped with MIRI, resolved these MUSE findings, revealing a procession of stellar point sources in the projected images. The detections are associated with elongated clouds, suggesting greater proper motions compared to the surrounding interstellar medium. These findings may indicate the presence of runaway candidates in the early stages of their evolution that are following common escape routes. This would support the notion that dynamical ejection is an efficient mechanism for the formation of massive runaway stars during early stages and likely plays a significant role in the origin of O-type field stars. However, additional data are required to confirm this scenario and rule out other ionizing feedback mechanisms, such as those observed in the formation of pillar-like structures around HII regions in the Milky Way. MUSE electron density mapping effectively captures the complexity of NGC 2070’s interstellar medium and highlights targets for subsequent spectroscopic follow-ups, as demonstrated by the JWST data in the two fields studied.

Particle acceleration at the bow shock of runaway star LS 2355: non-thermal radio emission but no γ-ray counterpart

ABSTRACT Massive stars that travel at supersonic speeds can create bow shocks as their stellar winds interact with the surrounding interstellar medium (ISM). These bow shocks – prominent sites for mechanical feedback of individual massive stars – are predominantly observed in the infrared (IR) band. Confirmed high-energy emission from stellar bow shocks has remained elusive and confirmed radio counterparts, while rising in recent years, remain rare. Here, we present an in-depth multiwavelength exploration of the bow shock driven by LS 2355, focusing on its non-thermal properties. Using the most recent Fermi source catalogue, we rule out its previously proposed association with an unidentified γ-ray source. Furthermore, we use deep Australian Square Kilometre Array Pathfinder (ASKAP) observations from the Rapid ASKAP Continuum Survey and the Evolutionary Map of the Universe survey to identify a non-thermal radio counterpart: the third spectrally confirmed non-thermal bow shock counterpart after BD+43°3654 and BD+60°2522. We finally use Wide-field Infrared Survey Explorer (WISE) IR data and Gaia to study the surrounding ISM and update the motion of LS 2355. Specifically, we derive a substantially reduced stellar velocity, $v_* = 7.0\pm 2.5$ km s−1, compared to previous estimates. The observed non-thermal properties of the bow shock can be explained by an interaction between the wind of LS 2355 and a dense H ii region, at a magnetic field close to the maximum magnetic field strength allowed by the compressibility of the ISM. Similar to earlier works, we find that the thermal radio emission of the shocked ISM is likely to be substantially suppressed for it to be consistent with the observed radio spectrum.

Core to ultracompact HII region evolution in the W49A massive protocluster

Aims. We aim to identify and characterize cores in the high-mass protocluster W49A, determine their evolutionary stages, and measure the associated lifetimes. Methods. We built a catalog of 129 cores extracted from an ALMA 1.3 mm continuum image at 0.26″ (2900 au) angular resolution. The association between cores and hypercompact or ultracompact HII (H/UC HII) regions was established from the analysis of VLA 3.3 cm continuum and H30α line observations. We also looked for emission of hot molecular cores (HMCs) using the methyl formate doublet at 218.29 GHz. Results. We identified 40 cores associated with an H/UC HII region and 19 HMCs over the ALMA mosaic. The 52 cores with an H/UC HII region and/or an HMC are assumed to be high-mass protostellar cores, while the rest of the core population likely consists of prestellar cores and low-mass protostellar cores. We found a good agreement between the two tracers of ionized gas, with 23 common detections and only four cores detected at 3.3 cm and not in H30α. The spectral indexes from 3.3 cm to 1.3 mm range from 1, for the youngest cores with partially optically thick free-free emission, to about −0.1, which is for the optically thin free-free emission obtained for cores that are likely more evolved. Conclusions. Using the H/UC HII regions as a reference, we found the statistical lifetimes of the HMC and massive protostellar phases in W49N to be about 6 × 104 yr and 1.4 × 105 yr, respectively. We also showed that HMCs can coexist with H/UC HII regions during a short fraction of the core lifetime, about 2 × 104 yr. This indicates a rapid dispersal of the inner molecule envelope once the HC HII is formed.

PDRs4All

Context. One of the main problems in astrochemistry is determining the amount of sulfur in volatiles and refractories in the interstellar medium. The detection of the main sulfur reservoirs (icy H2S and atomic gas) has been challenging, and estimates are based on the reliability of models to account for the abundances of species containing less than 1% of the total sulfur. The high sensitivity of the James Webb Space Telescope provides an unprecedented opportunity to estimate the sulfur abundance through the observation of the [S I] 25.249 µm line. Aims. Our aim is to determine the amount of sulfur in the ionized and warm molecular phases toward the Orion Bar as a template to investigate sulfur depletion in the transition between the ionized gas and the molecular cloud in HII regions. Methods. We used the [S III] 18.7 µm, [S IV] 10.5 µm, and [S l] 25.249 µm lines to estimate the amount of sulfur in the ionized and molecular gas along the Orion Bar. For the theoretical part, we used an upgraded version of the Meudon photodissociation region (PDR) code to model the observations. New inelastic collision rates of neutral atomic sulfur with ortho-and para- molecular hydrogen were calculated to predict the line intensities. Results. The [S III] 18.7 µm and [S IV] 10.5 µm lines are detected over the imaged region with a shallow increase (by a factor of 4) toward the HII region. This suggests that their emissions are partially coming from the Orion Veil. We estimate a moderate sulfur depletion, by a factor of ~2, in the ionized gas. The corrugated interface between the molecular and atomic phases gives rise to several edge-on dissociation fronts we refer to as DF1, DF2, and DF3. The [S l] 25.249 µm line is only detected toward DF2 and DF3, the dissociation fronts located farthest from the HII region. This is the first ever detection of the [S l] 25.249 µm line in a PDR. The detailed modeling of DF3 using the Meudon PDR code shows that the emission of the [S l] 25.249 µm line is coming from warm (>40 K) molecular gas located at AV ~1–5 mag from the ionization front. Moreover, the intensity of the [S l] 25.249 µm line is only accounted for if we assume the presence of undepleted sulfur. Conclusions. Our data show that sulfur remains undepleted along the ionic, atomic, and molecular gas in the Orion Bar. This is consistent with recent findings that suggest that sulfur depletion is low in massive star-forming regions because of the interaction of the UV photons coming from the newly formed stars with the interstellar matter.

Modeling Ionized Gas in the Small Magellanic Cloud: The Wolf–Rayet Nebula N76

We present Cloudy modeling of infrared emission lines in the Wolf–Rayet (WR) nebula N76 caused by one of the most luminous and hottest WR stars in the low metallicity Small Magellanic Cloud. We use spatially resolved mid-infrared Spitzer/InfRared Spectrograph and far-infrared Herschel/PACS spectroscopy to establish the physical conditions of the ionized gas. The spatially resolved distribution of the emission allows us to constrain properties much more accurately than using spatially integrated quantities. We construct models with a range of constant hydrogen densities between nH = 4–10 cm−3 and a stellar wind-blown cavity of 10 pc, which reproduces the intensity and shape of most ionized gas emission lines, including the high ionization lines [O iv] and [Ne v], as well as [S iii], [S iv], [O iii], and [Ne iii]. Our models suggest that the majority of [Si ii] emission (91%) is produced at the edge of the H ii region around the transition between ionized and atomic gas while very little of the [C ii] (<5%) is associated with the ionized gas. The physical conditions of N76 are characterized by a hot HII region with a maximum electron temperature of T e ∼ 24,000 K, electron densities that range from n e ∼ 4 to 12 cm−3, and high ionization parameters of log(U)∼−1.15to−1.77. By analyzing a low-metallicity WR nebula with a single ionization source, this work gives valuable insights into the impact WR stars have on the galaxy-integrated ionized gas properties in nearby dwarf galaxies.

Constraining the physical properties of gas in high-z galaxies with far-infrared and submillimetre line ratios

Optical emission line diagnostics, which are a common tool for constraining the properties of the interstellar medium (ISM) of galaxies, become progressively inaccessible at higher redshifts for ground-based facilities. Far-infrared (FIR) emission lines, which are redshifted into atmospheric windows that are accessible for ground-based submillimetre facilities, could provide ISM diagnostics alternative to optical emission lines. We investigated FIR line ratios involving CII OIII OIII NII and NIII using synthetic emission lines applied to a high-resolution gas $= 883.4 M$_ odot $) cosmological zoom-in simulation, including radiative transfer post-processing with the code at z = 6.5. We find that the CII NII 122 ratio is sensitive to the temperature and density of photodissociation regions. It might therefore be a useful tool for tracing the properties of this gas phase in galaxies. We also find that NII NIII is a good tracer of the temperature and that OIII OIII 88 is a good tracer of the gas density of HII regions. Emission line ratios containing the OIII line are sensitive to high-velocity outflowing gas.

JWST observations of the Horsehead photon-dominated region

Context. The James Webb Space Telescope (JWST) has captured the sharpest infrared images ever taken of the Horsehead nebula, a prototypical moderately irradiated photon-dominated region (PDR) that is fully representative of most of the UV-illuminated molecular gas in the Milky Way and star-forming galaxies. Aims. We investigate the impact of far-ultraviolet (FUV) radiation emitted by a massive star on the edge of a molecular cloud in terms of photoevaporation, ionization, dissociation, H2 excitation, and dust heating. We also aim to constrain the structure of the edge of the PDR and its illumination conditions. Methods. We used NIRCam and MIRI to obtain 17 broadband and 6 narrowband maps of the illuminated edge of the Horsehead across a wide spectral range from 0.7 to 28 µm. We mapped the dust emission, including the aromatic and aliphatic infrared (IR) bands, scattered light, and several gas phase lines (e.g., Paa, Brα, H2 1-0 S(1) at 2.12 µm). For our analysis, we also associated two HST-WFC3 maps at 1.1 and 1.6 µm, along with HST-STIS spectroscopic observations of the Ha line. Results. We probed the structure of the edge of the Horsehead and resolved its spatial complexity with an angular resolution of 0.1 to 1″ (equivalent to 2 × 10−4 to 2 × 10−3 pc or 40 to 400 au at the distance of 400 pc). We detected a network of faint striated features extending perpendicularly to the PDR front into the HII region in NIRCam and MIRI filters sensitive to nano-grain emission, as well as in the HST filter at 1.1 µm, which traces light scattered by larger grains. This may indeed figure as the first detection of the entrainment of dust particles in the evaporative flow. The filamentary structure of the 1-0 S(1) line of H2 at the illuminated edge of the PDR presents numerous sharp sub-structures on scales as small as 1.5″. An excess of H2 emission compared to dust emission is found all along the edge, in a narrow layer (width around 1″, corresponding to 2 × 10−3 pc or 400 au) directly illuminated by σ-Orionis. The ionization front and the dissociation front appear at distances 1–2″ behind the external edge of the PDR and seem to spatially coincide, indicating a very small thickness of the neutral atomic layer (below 100 au). All broadband maps present strong color variations between the illuminated edge and the internal regions. This can be explained by dust attenuation in a scenario where the illuminating star σ-Orionis is slightly inclined compared to the plane of the sky, so that the Horsehead is illuminated from behind at an oblique angle. The deviations from predictions of the measured emissions in the Hα, Paα, and Brα lines also indicate dust attenuation. With a very simple model, we used the data to derive the main spectral features of the extinction curve. A small excess of extinction at 3 µm may be attributed to icy H2O mantles onto grains formed in dense regions. We also derived attenuation profiles from 0.7 to 25 µm across the PDR. In all lines of sight crossing the inner regions of the Horsehead, especially around the IR peak position, it appears that dust attenuation is non-negligible over the entire spectral range of the JWST.

Spectral characterisation of the extinction properties of NGC 3603 using JWST NIRSpec.

A necessary ingredient in understanding the star formation history of a young cluster is knowledge of the extinction towards the region. This has typically been done by making use of the colour-difference method with photometry, or similar methods utilising the colour-colour diagram. These approaches rely on adopting an extinction law with a given total-to-selective extinction ratio $R(V)$, or determining a value of $R(V)$ through empirical relationships. They also rely upon accurate spectral classification, reliable stellar isochrones, and separating field stars from genuine cluster members. The colour excess $E(B-V)$ can be independently determined by studying the decrements of the recombination lines produced by the nebular gas. Having access to many recombination lines from the same spectral series removes the need of adopting an extinction curve. Rather, different extinction curves can be trialled and the most appropriate one selected based on a minimum $ procedure. Using the Micro-Shutter Assembly (MSA) on board the Near InfraRed Spectrograph (NIRSpec), multi-object spectroscopy was performed, yielding 600 nebular spectra from the Galactic massive star formation region NGC 3603. The recombination line intensity ratios were used to determine independent values of $E(B-V)$. A series of extinction curves were trialled ranging from $R(V) = 2$ to $R(V) = 8$. The appropriate value of $R(V)$ was adopted based on the minimum $ procedure. The extinction characteristics of NGC 3603 are similar to other Galactic HII regions like Orion, as well as starburst regions such 30 Doradus in the Large Magellanic Cloud, in that we find a relatively large value of $R(V) = 4.8 1.06$, larger than the Galactic average of $3.1$. We find a typical value of $E(B-V) = 0.64 0.27$, significantly lower than values determined in previous studies. We also present a stacked nebular spectrum with a typical continuum S/N = $70$. This spectrum highlights the recombination lines of the HII region, several s-process elements such as Kr $III$ and Se $IV$, and molecular $H_2$ emission lines. This high S/N spectrum can act as a helpful template for identifying nebular emission lines. Using ratios of hydrogen recombination lines, we calculated the value of $R(V)$, $E(B-V)$ and $A(V)$ for $&gt; 200$ lines of sight across NGC 3603. An extinction curve with a typical value $R(V) = 4.8 1.06$ is required to explain the colour excess observed in the nebular spectra. This corresponds to a typical $E(B-V) = 0.64 0.27$. This is significantly lower than what has been found in previous extinction studies of NGC 3603.

Multiwavelength study of the HII region LHA 120-N11 in the Large Magellanic Cloud with eROSITA

Aims. We studied the diffuse X-ray emission around the HII region LHA 120-N11, which is one of the most active star-forming regions in the Large Magellanic Cloud. We want to determine the nature of the diffuse X-ray emission and improve our understanding of its origin including related interactions with the cold interstellar medium. Methods. We analyzed the diffuse X-ray emission observed with the extended Roentgen Survey with an Imaging Telescope Array (eROSITA) on the Spectrum-Roentgen-Gamma mission to determine the physical properties of the hot diffuse X-ray emission. Four spectral extraction regions were defined based on the morphology of the X-ray emission. We also studied HI and CO data, as well as Hα line emission in the optical, and compared them with the properties of the diffuse X-ray emission. Results. The X-ray emission in the four regions is well fitted with an absorbed model consisting of thermal plasma models (vapec) yielding temperatures of kT = ~0.2 keV and kT = 0.8–1.0 keV. The comparison of the X-ray absorption column density and the hydrogen column density shows that the X-ray dark lane located north of N11 is apparently caused by the absorption by HI and CO clouds. By estimating the energy budget of the thermal plasma, we also investigated the heating mechanism of the X-ray emitting plasma. The energy of the diffuse X-ray emission in the superbubble which is a star-forming bubble with a radius of ~120 pc including OB associations LH9, LH10, LH11, and LH13 can be explained by heating from high-mass stars. In the surrounding regions we find that the energy implied by the X-ray emission suggests that additional heating might have been caused by shocks generated by cloud–cloud collisions.

Spectroscopic evidence of a possible young stellar cluster at the Galactic Center

Context. The nuclear stellar disk has been the most prolific star-forming region in the Milky Way over the past ∼30 million years. Notably, the cumulative mass of the three clusters currently found in the nuclear stellar disk, the Quintuplet, the Arches, and the Nuclear clusters, amounts to just 10% of the total anticipated mass of young stars that formed in this period. This discrepancy, known as the missing cluster problem, is attributed to factors such as high stellar density and tidal forces. Traces of dissolving clusters may exist as comoving groups of stars, providing insights into the star formation history of the region. Recently, a new cluster candidate associated with an HII region was reported through the analysis of kinematic data Aims. Our aim is to determine whether the young and massive stellar objects in the region share proper motion, positions in the plane of the sky, and line-of-sight distances. We use reddening as a proxy for the distances. Methods. We reduced and analyzed integral field spectroscopy data from the KMOS instrument at the ESO VLT to locate possible massive young stellar objects in the field. Then, we identified young massive stars with astrophotometric data from the two different catalogs to analyze their extinction and kinematics. Results. We present a group of young stellar objects that share velocities, are close together in the plane of the sky, and are located at a similar depth in the nuclear stellar disk. Conclusions. The results presented here offer valuable insights into the missing clusters problem. They indicate that not all young massive stars in the Galactic center form in isolation; some of them seem to be the remnants of dissolved clusters or stellar associations.