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Context.Most stars form in clusters or associations, but only a small number of these groups are expected to remain bound for longer than a few megayears. Once star formation has ended and the molecular gas around young stellar objects has been expelled via feedback processes, most initially bound young clusters lose the majority of their binding mass and begin to disperse into the Galactic field. Aims.This process can be investigated by analysing the structure and kinematic trends in nearby young clusters, particularly by analyzing the trend of expansion, which is a tell-tale sign that a cluster is no longer gravitationally bound and dispersing into the field. Methods.We combinedGaiaDR3 five-parameter astrometry with calibrated RVs for members of the nearby young clusterλOri (Collinder 69). Results.We characterised the plane-of-sky substructure of the cluster using theQ-parameter and the angular dispersion parameter. We find evidence that the cluster contains a significant substructure but that this is preferentially located away from the central cluster core, which is smooth and likely remains bound. We found strong evidence for expansion inλOri in the plane of sky by using a number of metrics, but we also found that the trends are asymmetric at the 5σsignificance level, with the maximum rate of expansion being directed nearly parallel to the Galactic plane. We subsequently inverted the maximum rate of expansion of 0.144_−0.003^+0.003kms⁻¹pc⁻¹to give an expansion timescale of 6.944_−0.142^+0.148Myr, which is slightly larger than the typical literature age estimates for the cluster. We also found asymmetry in the velocity dispersion as well as signatures of cluster rotation, and we calculated the kinematic ages for individual cluster members by tracing their motion back in time to their closest approach to the cluster centre. 

Abstract Star formation is a fundamental, yet poorly understood, process of the Universe. It is important to study how star formation occurs in different galactic environments. Thus, here, in the first of a series of papers, we introduce the Low-metallicity Star Formation (LZ-STAR) survey of the Sh2-284 (hereafter S284) region, which, atZ ∼  0.3–0.5Z_⊙, is one of the lowest-metallicity star-forming regions of our Galaxy. LZ-STAR is a multifacility survey, including observations with JWST, the Atacama Large Millimeter/submillimeter Array (ALMA), Hubble Space Telescope, Chandra, and Gemini. As a starting point, we report JWST and ALMA observations of one of the most massive protostars in the region, S284p1. The observations of shock-excited molecular hydrogen reveal a symmetric, bipolar outflow originating from the protostar, spanning several parsecs, and fully covered by the JWST field of view and ALMA observations of CO(2–1) emission. These allow us to infer that the protostar has maintained a relatively stable orientation of disk accretion over its formation history. The JWST near-infrared continuum observations detect a centrally illuminated bipolar outflow cavity around the protostar, as well as a surrounding cluster of low-mass young stars. We develop new radiative transfer models of massive protostars designed for the low metallicity of S284. Fitting these models to the protostar’s spectral energy distribution implies a current protostellar mass of ∼10M_⊙has formed from an initial ∼100M_⊙core over the last ∼3 × 10⁵yr. Overall, these results indicate that massive stars can form in an ordered manner in low-metallicity, protocluster environments. 

ABSTRACT Most stars are born in the crowded environments of gradually forming star clusters. Dynamical interactions between close-passing stars and the evolving ultraviolet radiation fields from proximate massive stars are expected to sculpt the protoplanetary discs (PPDs) in these clusters, potentially contributing to the diversity of planetary systems that we observe. Here, we investigate the impact of cluster environment on disc demographics by implementing simple PPD evolution models within N-body simulations of gradual star cluster formation, containing 50 per cent primordial binaries. We consider a range of star formation efficiency per free-fall time, $$\epsilon _{\rm ff}$$, and mass surface density of the natal cloud environment, $$\Sigma _{\rm cloud}$$, both of which affect the overall duration of cluster formation. We track the interaction history of all stars to estimate the dynamical truncation of the discs around stars involved in close encounters. We also track external photoevaporation of the discs due to the ionizing radiation field of the nearby high- and intermediate-mass ($$\gt 5\,{\rm M}_\odot$$) stars. We find that $$\epsilon _{\rm ff}$$, $$\Sigma _{\rm cloud}$$, and the presence of primordial binaries have major influences on the masses and radii of the disc population. In particular, external photoevaporation has a greater impact than dynamical interactions in determining the fate of discs in our clusters. 

ABSTRACT In their early, formative stages star clusters can undergo rapid dynamical evolution leading to strong gravitational interactions and ejection of “runaway” stars at high velocities. While O/B runaway stars have been well studied, lower-mass runaways are so far very poorly characterized, even though they are expected to be much more common. We carried out spectroscopic observations with MAG2-MIKE to follow-up 27 high priority candidate runaways consistent with having been ejected from the Orion Nebula Cluster (ONC) $$\gt 2.5$$ Myr ago, based on Gaia astrometry. We derive spectroscopic youth indicators (Li and H $$\alpha$$) and radial velocities, enabling detection of bona fide runaway stars via signatures of youth and 3D traceback. We successfully confirmed 11 of the candidates as low-mass Young Stellar Objects (YSOs) on the basis of our spectroscopic criteria and derived radial velocities (RVs) with which we performed 3D traceback analysis. Three of these confirmed YSOs have kinematic ejection ages $$\gt 4\:$$ Myr, with the oldest being 4.7 Myr. Assuming that these stars indeed formed in the ONC and were then ejected, this yields an estimate for the overall formation time of the ONC to be at least $$\sim 5\:$$ Myr, i.e. about 10 free-fall times, and with a mean star formation efficiency per free-fall time of $$\bar{\epsilon }_{\rm ff}\lesssim 0.05$$. These results favour a scenario of slow, quasi-equilibrium star cluster formation, regulated by magnetic fields and/or protostellar outflow feedback. 

Abstract Determining the physical processes that control galactic-scale star formation rates is essential for an improved understanding of galaxy evolution. The role of orbital shear is currently unclear, with some models expecting reduced star formation rates and efficiencies with increasing shear, e.g., if shear stabilizes gas against gravitational collapse, while others predicting enhanced rates, e.g., if shear-driven collisions between giant molecular clouds trigger star formation. Expanding on the analysis of 16 galaxies by C. Suwannajak et al., we assess the shear dependence of star formation efficiency (SFE) per orbital time (ϵ_orb) in 49 galaxies selected from the PHANGS-ALMA survey. In particular, we test a prediction of the shear-driven giant molecular cloud​​​​​​ (GMC) collision model thatϵ_orb∝ (1–0.7β), where $\beta \equiv d\ln v_{\mathrm{circ}}/d\ln r$, i.e., SFE per orbital time declines with decreasing shear. We fit the functionϵ_orb=ϵ_orb,0(1 −α_CCβ) findingα_CC≃ 0.76 ± 0.16; an alternative fit withϵ_orbnormalized by the median value in each galaxy yields ${\alpha }_{\mathrm{CC}}^{*}=0.80\pm 0.15$. These results are in good agreement with the prediction of the shear-driven GMC collision theory. We also examine the impact of a galactic bar onϵ_orbfinding a modest decrease in SFE in the presence of a bar, which can be attributed to lower rates of shear in these regions. We discuss the implications of our results for the GMC life cycle and environmental dependence of star formation activity. 

Abstract We present new Atacama Large Millimeter/submillimeter Array observations that, for the first time, detect hydrogen and helium radio recombination lines from a protoplanetary disk. We imaged the Orion Nebula Cluster at 3.1 mm with a spectral setup that covered then= 42 → 41 transitions of hydrogen (H41α) and helium (He41α). The unprecedented sensitivity of these observations enables us to search for radio recombination lines toward the positions of ∼200 protoplanetary disks. We detect H41αfrom 17 disks, all of which are HST-identified “proplyds.” The detected H41αemission is spatially coincident with the locations of proplyd ionization fronts, indicating that proplyd H41αemission is produced by gas that has been photoevaporated off the disk and ionized by UV radiation from massive stars. We measure the fluxes and widths of the detected H41αlines and find line fluxes of ∼30–800 mJy km s⁻¹and line widths of ∼30–90 km s⁻¹. The derived line widths indicate that the broadening of proplyd H41αemission is dominated by outflowing gas motions associated with external photoevaporation. The derived line fluxes, when compared with measurements of 3.1 mm free–free flux, imply that the ionization fronts of H41α-detected proplyds have electron temperatures of ∼6000–11,000 K and electron densities of ∼10⁶–10⁷cm⁻³. Finally, we detect He41αtoward one H41α-detected source and find evidence that this system is helium-rich. Our study demonstrates that radio recombination lines are readily detectable in ionized photoevaporating disks, providing a new way to measure disk properties in clustered star-forming regions. 

Context. Hydrogen recombination lines (HRLs) are valuable diagnostics of the physical conditions in ionized regions surrounding high-mass stars. Understanding these lines, including broadening mechanisms and intensity trends, can provide insights into HII region densities, temperatures, and kinematics. Aims. This study aims to investigate the physical properties of ionized gas around massive protostars by analysing the HRLs (Hαand Hβ) in the Q band. Methods. We carried out observations using the Yebes 40m radio telescope in the Q band (30.5–50 GHz) towards six high-mass protostars selected from the SOMA Survey (G45.12+0.13, G45.47+0.05, G28.20−0.05, G35.20−0.74, G19.08−0.29, and G31.28+0.06). The observed line profiles were analysed to assess broadening mechanisms, and electron densities and temperatures were derived. The results were compared with available Q-band data from the TianMa 65-m Radio Telescope (TMRT) that have been reported in the literature, and ALMA Band 1 (35–50 GHz) Science Verification observations towards Orion KL, analysed in this study. Results. A total of eight Hα(n = 51 to 58) and ten Hβ(n = 64 to 73) lines were detected towards G45.12+0.13, G45.47+0.05, and G28.20−0.05; there were no detections in other sources. We derived electron densities of ~1−5 × 10⁶cm⁻³and temperatures of 8000–10 000 K for the sources. However, for Orion KL, we obtained an electron density one order of magnitude lower, while its temperature was found to be more similar. Interestingly, G45.12 and G28.20 show an increasing intensity trend with frequency for both Hαand Hβtransitions, contrary to the decreasing trend observed in Orion KL. Conclusions. The line widths of the detected HRLs indicate contributions from both thermal and dynamical broadening, suggesting the presence of high-temperature ionized gas that is likely kinematically broadened (e.g. due to turbulence, outflows, rapid rotation, or stellar winds). Pressure broadening caused by electron density may also have a minor effect. We discuss different scenarios to explain the measured line widths of the HRLs. The contrasting intensity trends between the sources may reflect variations in local physical conditions or radiative transfer effects, highlighting the need for further investigation through higher-resolution observations and detailed modelling. 

Abstract We present JWST-NIRCam narrowband, 4.05μm Brαimages of the Sgr C Hiiregion, located in the central molecular zone (CMZ) of the Galaxy. Unlike any Hiiregion in the solar vicinity, the Sgr C plasma is dominated by filamentary structure in both Brαand the radio continuum. Some bright filaments, which form a fractured arc with a radius of about 1.85 pc centered on the Sgr C star-forming molecular clump, likely trace ionization fronts. The brightest filaments form a “π-shaped” structure in the center of the Hiiregion. Fainter filaments radiate away from the surface of the Sgr C molecular cloud. The filaments are emitting optically thin free–free emission, as revealed by spectral index measurements from 1.28 GHz (MeerKAT) to 97 GHz (Atacama Large Millimeter/submillimeter Array). But, the negative in-band 1 to 2 GHz spectral index in the MeerKAT data alone reveals the presence of a nonthermal component across the entire Sgr C Hiiregion. We argue that the plasma flow in Sgr C is controlled by magnetic fields, which confine the plasma to ropelike filaments or sheets. This results in the measured nonthermal component of low-frequency radio emission plasma, as well as a plasmaβ(thermal pressure divided by magnetic pressure) below 1, even in the densest regions. We speculate that all mature Hiiregions in the CMZ, and galactic nuclei in general, evolve in a magnetically dominated, low plasmaβregime. 

Abstract We present James Webb Space Telescope (JWST) Near Infrared Camera observations of the massive star-forming molecular cloud Sagittarius C (Sgr C) in the Central Molecular Zone (CMZ). In conjunction with ancillary mid-IR and far-IR data, we characterize the two most massive protostars in Sgr C via spectral energy distribution (SED) fitting, estimating that they each have current masses ofm_*∼ 20M_⊙and surrounding envelope masses of ∼100M_⊙. We report a census of lower-mass protostars in Sgr C via a search for infrared counterparts to millimeter continuum dust cores found with the Atacama Large Millimeter/submillimeter Array (ALMA). We identify 88 molecular hydrogen outflow knot candidates originating from outflows from protostars in Sgr C, the first such unambiguous detections in the infrared in the CMZ. About a quarter of these are associated with flows from the two massive protostars in Sgr C; these extend for over 1 pc and are associated with outflows detected in ALMA SiO line data. An additional ∼40 features likely trace shocks in outflows powered by lower-mass protostars throughout the cloud. We report the discovery of a new star-forming region hosting two prominent bow shocks and several other line-emitting features driven by at least two protostars. We infer that one of these is forming a high-mass star given an SED-derived mass ofm_*∼ 9M_⊙and associated massive (∼90M_⊙) millimeter core and water maser. Finally, we identify a population of miscellaneous molecular hydrogen objects that do not appear to be associated with protostellar outflows. 

Abstract The “tuning-fork” (TF) analysis of CO and Hαemission has been used to estimate the lifetimes of molecular clouds in nearby galaxies. With simple model calculations, we show that this analysis does not necessarily estimate cloud lifetimes, but instead captures a duration of the cloud evolutionary cycle, from dormant to star-forming, and then back to a dormant phase. We adopt a hypothetical setup in which molecular clouds (e.g., traced in CO) live forever and form stars (e.g., Hiiregions) at some frequency, which then drift away from the clouds. The TF analysis still returns a timescale for the immortal clouds. This model requires drifting motion to separate the newborn stars from the clouds, and we discuss its origin. We also discuss the physical origin of the characteristic spatial separation term in the TF analysis and a bias due to systematic error in the determination of the reference timescale.