Young Stellar Objects Research Articles

A population of mid-infrared large-amplitude variable young stellar objects from unTimely

ABSTRACT Utilizing a decade-long unTimely data set, supplemented by multiband data from archives, we search for young stellar objects (YSOs) with variations larger than one magnitude in W1 band within a region of 110 square degrees in the Galactic plane, covered by VISTA Variables in the Via Lactea. A total of 641 candidate YSOs have been identified. We classified them into bursts, dips, faders, seculars, and irregulars. Within the burst category, 18 sources were identified as FUor candidates and 1 as an EXor candidate. Irregulars are the most prevalent in the sample. In both bursts and faders, the redder sources tend to show a pattern of bluer when brighter, whereas the bluer sources display the opposite trend, possibly related to the accretion structure of YSOs at different stages. Finally, we obtained the recurrence time-scale for FUor eruptions at various stages of YSO evolution. Our findings indicate that younger YSOs generally experience more frequent eruptions compared to older ones.

Probing the physics of star formation (ProPStar)

Context. The detections of narrow channels of accretion toward protostellar disks, known as streamers, have increased in number in the last few years. However, it is unclear whether streamers are a common feature around protostars that were previously missed, or if they are a rare phenomenon. Aims. Our goals are to obtain the incidence of streamers toward a region of clustered star formation and to trace the origins of their gas to determine whether they originate within the filamentary structure of molecular clouds or from beyond. Methods. We used combined observations of the nearby NGC 1333 star-forming region, carried out with the NOEMA interferometer and the IRAM 30m single dish. Our observations cover the area between the systems IRAS 4 and SVS 13. We traced the chemically fresh gas within NGC 1333 with HC3N molecular gas emission and the structure of the fibers in this region with N2H+ emission. We fit multiple velocity components in both maps and used clustering algorithms to recover velocity-coherent structures. Results. We find streamer candidates toward 7 out of 16 young stellar objects within our field of view. This represents an incidence of approximately 40% of young stellar objects with streamer candidates in a clustered star-forming region. The incidence increases to about 60% when we only considered embedded protostars. All streamers are found in HC3N emission. Conclusions. Given the different velocities between HC3N and N2H+ emission, and because by construction, N2H+ traces the fiber structure, we suggest that the gas that forms the streamers comes from outside the fibers. This implies that streamers can connect cloud material that falls onto the filaments with protostellar disk scales.

The Galactic Center Lobe as an H ii Region

The Galactic center lobe (GCL) is an object ∼1° across that is located north of the Galactic center. In the mid-infrared (MIR) the GCL appears as two 8.0 μm filaments between which there is strong 24 μm and radio continuum emission. Due to its morphology and location in the sky, previous authors have argued that the GCL is located in the Galactic center region, created by outflows from star formation or by activity of the central black hole Sagittarius A*. In an associated paper, low-frequency radio emission indicates that the GCL must instead lie foreground to the Galactic center. If the GCL is foreground to the Galactic center, it is likely to be a type of object common throughout the Galactic disk; we here investigate whether its properties are similar to those of Galactic H ii regions. We find that the GCL’s MIR morphology, MIR flux densities, dust temperatures, and radio recombination line properties as traced by the Green Bank Telescope Diffuse Ionized Gas Survey are consistent with those of known Galactic H ii regions, although the derived electron temperature is low. We search for the ionizing source(s) of the possible H ii region and identify a stellar cluster candidate (Camargo #1092/Ryu & Lee #532) and a cluster of young stellar objects (the SPICY cluster G359.3+0.3) whose members have Gaia parallaxes distances of 1.7 ± 0.4 kpc. Taken together, the results of our companion paper and those shown here suggest that the GCL has properties consistent with those of an H ii region located ∼2 kpc from the Sun.

JWST Observations of Young protoStars (JOYS)

Context. Accretion and ejection dictate the outcomes of star and planet formation processes. The mid-infrared (MIR) wavelength range offers key tracers of processes that have been difficult to detect and spatially resolve in protostars until now. Aims. We aim to characterize the interplay between accretion and ejection in the low-mass Class I protobinary system TMC1, comprising two young stellar objects: TMC1-W and TMC1-E at a 85 au separation. Methods. Using the James Webb Space Telescope (JWST) Mid-Infrared Instrument (MIRI) observations in 5–28 μm range, we measured the intensities of emission lines of H2, atoms, and ions, for instance, the [Fe II] and [Ne II], and HI recombination lines. We analyzed the spatial distribution of the different species using the MIRI Medium Resolution Spectrometer (MRS) capabilities to spatially resolve emission at 0″​​.2–0″​​.7 scales. we compared these results with the corresponding Atacama Large Millimeter/submillimeter Array (ALMA) maps tracing cold gas and dust. Results. We detected H2 outflow coming from TMC1-E, with no significant H2 emission from TMC1-W. The H2 emission from TMC1-E outflow appears narrow and extends to wider opening angles with decreasing Eup from S(8) to S(1) rotational transitions, indicating the disk wind as its origin. The outflow from TMC1-E protostar shows spatially extended emission lines of [Ne II], [Ne III], [Ar II], and [Ar III], with their line ratios consistent with UV radiation as a source of ionization. With ALMA, we detected an accretion streamer infalling from > 1000 au scales onto the TMC1-E component. The TMC1-W protostar powers a collimated jet, detected with [Fe II] and [Ni II], making it consistent with energetic flow. A much weaker ionized jet is observed from TMC1-E, and both jets appear strikingly parallel to each other, indicating that the disks are co-planar. TMC1-W is associated with strong emission from hydrogen recombination lines, tracing the accretion onto the young star. Conclusions. MIRI-MRS observations provide an unprecedented view of protostellar accretion and ejection processes on 20 au scales. Observations of a binary Class I protostellar system show that the two processes are clearly intertwined, with accretion from the envelope onto the disk influencing a wide-angle wind ejected on disk scales. Finally, the accretion from the protostellar disk onto the protostar is associated with the source launching a collimated high-velocity jet within the innermost regions of the disk.

Imaging of I Zw 18 by JWST. I. Detecting Dusty Stellar Populations

We present a JWST imaging survey of I Zw 18, the archetypal extremely metal-poor, star-forming (SF), blue compact dwarf galaxy. With an oxygen abundance of only ∼3% Z ⊙, it is among the lowest-metallicity systems known in the local Universe, and is, therefore, an excellent accessible analog for the galactic building blocks which existed at early epochs of ionization and star formation. These JWST data provide a comprehensive infrared (IR) view of I Zw 18 with eight filters utilizing both Near Infrared Camera (F115W, F200W, F356W, and F444W) and Mid-Infrared Instrument (F770W, F1000W, F1500W, and F1800W) photometry, which we have used to identify key stellar populations that are bright in the near- and mid-IR. These data allow for a better understanding of the origins of dust and dust-production mechanisms in metal-poor environments by characterizing the population of massive, evolved stars in the red supergiant (RSG) and asymptotic giant branch (AGB) phases. In addition, it enables the identification of the brightest dust-enshrouded young stellar objects (YSOs), which provide insight into the formation of massive stars at extremely low metallicities typical of the very early Universe. This paper provides an overview of the observational strategy and data processing, and presents first science results, including identifications of dusty AGB, RSG, and bright YSO candidates. These first results assess the scientific quality of JWST data and provide a guide for obtaining and interpreting future observations of the dusty and evolved stars inhabiting compact dwarf SF galaxies in the local Universe.

The backreaction of stellar wobbling on accretion discs of massive protostars

black In recent years, it has been demonstrated that massive stars see their infant circumstellar medium shaped into a large irradiated, gravitationally unstable accretion disc during their early formation phase. Such discs constitute the gas reservoir from which nascent high-mass stars gain a substantial fraction of their mass by episodic accretion of dense gaseous circumstellar clumps, simultaneously undergoing accretion-driven bursts and producing close-orbit spectroscopic companions of the young high-mass stellar object. We aim to evaluate the effects of stellar motion caused by the disc non-axisymmetric gravitational field on the disc evolution and its spatial morphology. In particular, we analyse the disc's propensity to gravitational instability and fragmentation and the disc's appearance in synthetic millimetre band images pertinent to the ALMA facility. We employed three-dimensional radiation-hydrodynamical simulations of the surroundings of a young massive star in the non-inertial spherical coordinate system, adopting the highest spatial resolution to date and including the indirect star-disc gravitational potential caused by the asymmetries in the circumstellar disc. The resulting disc configurations were post-processed with the radiation transfer tool RADMC-3D and CASA software to obtain synthetic images of the disc. We confirm that the early evolution of the accretion disc is notably different when stellar wobbling is taken into account. The redistribution of angular momentum in the system makes the disc smaller and rounder, reduces the number of circumstellar gaseous clumps formed via disc gravitational fragmentation, and prevents the ejection of gaseous clumps from the disc. The synthetic predictive images at millimetre wavelengths of the accretion disc that includes stellar wobbling are in better agreement with the observations of the surroundings of massive young stellar objects, namely black AFGL 4176 mm1, G17.64+0.16, and G353.273 than our simulations of numerical hydrodynamics that omit this physical mechanism. Our work confirms that stellar wobbling is an essential ingredient to account for in numerical simulations of accretion discs of massive protostars.

Co-evolution of dust grains and protoplanetary disks. II. Structure and evolution of protoplanetary disks: An analytical approach

Abstract In our previous study (Tsukamoto et al. 2023b, PASJ, 75, 835), we investigated the formation and early evolution of protoplanetary disks with 3D non-ideal magnetohydrodynamics simulations considering dust growth, and found that the modified equations of the conventional steady accretion disk model that consider magnetic braking, dust growth, and ambipolar diffusion reproduce the disk structure (such as density and vertical magnetic field) obtained from simulations very well. In this paper, as a sequel to our previous study, we analytically investigate the structure and evolution of protoplanetary disks corresponding to Class 0/I young stellar objects using the modified steady accretion disk model combining an analytical model of envelope accretion. We estimate that the disk radius is several astronomical units at the disk formation epoch and increases to several hundred astronomical units at the end of the accretion phase. The disk mass is estimated to be $0.01 \lesssim M_{\rm disk} \lesssim 0.1 \, M_\odot$ for a disk with a radius of several tens of astronomical units and a mass accretion rate of $\dot{M}_{\rm disk} \sim 10^{-6} \, M_\odot \,\, {\rm yr^{-1}}$. These estimates seems to be consistent with recent observations. We also found that, with typical disk ionization rates (ζ ≳ 10−19 s−1) and a moderate mass accretion rate ($\dot{M}_{\rm disk}\gtrsim 10^{-8} \, M_\odot \,\, {\rm yr^{-1}}$), magnetorotational instability is suppressed in the disk because of low plasma β and efficient ambipolar diffusion. We argue that the radial profile of specific angular momentum (or rotational velocity) at the disk outer edge should be continuously connected to that of the envelope if the disk evolves by magnetic braking, and should be discontinuous if the disk evolves by an internal angular momentum transport process such as gravitational instability or magnetorotational instability. Future detailed observations of the specific angular momentum profile around the disk outer edge are important for understanding the angular momentum transport mechanism of protoplanetary disks.

Atacama Large Aperture Submillimeter Telescope (AtLAST) science: Our Galaxy.

As we learn more about the multi-scale interstellar medium (ISM) of our Galaxy, we develop a greater understanding for the complex relationships between the large-scale diffuse gas and dust in Giant Molecular Clouds (GMCs), how it moves, how it is affected by the nearby massive stars, and which portions of those GMCs eventually collapse into star forming regions. The complex interactions of those gas, dust and stellar populations form what has come to be known as the ecology of our Galaxy. Because we are deeply embedded in the plane of our Galaxy, it takes up a significant fraction of the sky, with complex dust lanes scattered throughout the optically recognizable bands of the Milky Way. These bands become bright at (sub-)millimetre wavelengths, where we can study dust thermal emission and the chemical and kinematic signatures of the gas. To properly study such large-scale environments, requires deep, large area surveys that are not possible with current facilities. Moreover, where stars form, so too do planetary systems, growing from the dust and gas in circumstellar discs, to planets and planetesimal belts. Understanding the evolution of these belts requires deep imaging capable of studying belts around young stellar objects to Kuiper belt analogues around the nearest stars. Here we present a plan for observing the Galactic Plane and circumstellar environments to quantify the physical structure, the magnetic fields, the dynamics, chemistry, star formation, and planetary system evolution of the galaxy in which we live with AtLAST; a concept for a new, 50m single-dish sub-mm telescope with a large field of view which is the only type of facility that will allow us to observe our Galaxy deeply and widely enough to make a leap forward in our understanding of our local ecology.

Gaia DR3 Data of Very Young Open Cluster NGC 6530: Distance and Evolutionary Stage of Selected Stars

Gaia EDR3 and Gaia DR3 data of selected stars in the very young (1.5 million years old) open cluster NGC 6530 are presented. All the selected stars have very accurate Gaia DR3 distances. Most of the selected stars are very young B stars and several of them are young stellar objects. From the analysis of selected stars, the distance to NGC 6530 is found to be 1318 pc. The two very red stars VAJ 304 and 338 have little bit less proper motion than other members of the NGC 6530 and their Mv values are found to be −3.57 and −3.68 respectively. They are much brighter than K5 III RGB stars and less luminous than massive K5 red supergiants. It is likely VAJ 304 and VAJ 338 are not members of NGC 6530 they may be field K5 type AGB or post-AGB stars. Further study of these two stars is needed.

Young stellar objects from the LAMOST and ZTF surveys: Physical properties, classification, and light curve analysis

Context. The study of young stellar objects (YSOs) not only enhances our understanding of star formation and stellar evolution, but also contributes to broader areas of astrophysics, including planetary science, galactic dynamics, and astrochemistry. Aims. We aimed to comprehensively analyse 657 YSOs and provide their physical parameter measurements using data from Zwicky Transient Facility (ZTF) g- and r-band light curves and the Gaia, WISE, 2MASS, and LAMOST databases. Specifically, we sought to identify periodicity in the light curves and classify the YSOs based on the Q – M variability plane, which enabled us to quantify flux asymmetry and quasi-periodicity. Methods. To achieve our objectives, we conducted a meticulous examination of the light curves obtained from the ZTF and estimated the physical parameters of the YSOs. These parameters were discerned by integrating stellar model atmosphere grids, photometric data, Gaia DR3 parallaxes, and pre-main-sequence evolutionary tracks. We employed the Q – M variability plane to classify the YSOs and determine the presence of periodic patterns. Additionally, we analysed the distribution of variability slope angles in the colour-magnitude diagram (CMD) to discern patterns associated with extinction-driven and accretion-related variability. Results. Our analysis revealed significant findings regarding the variability patterns and physical characteristics of the YSOs. Among the 657 objects analysed, 37 exhibited periodic variability and 2 displayed multi-period behaviour. Furthermore, we identified distinct variability patterns, including quasi-periodic symmetry, quasi-periodic dipping, aperiodic dipping, bursting behaviour, stochastic variability, and long-timescale variations. Notably, the distribution of variability slope angles in the CMD varied between dippers and bursters, indicating different underlying variability drivers. Additionally, we observed that YSOs classified as classical T Tauri stars and weak-line T Tauri stars exhibited contrasting light curve characteristics, with Class II YSOs displaying asymmetry and Class III YSOs showing (quasi-)periodic variations. These findings underscore the importance of considering variability patterns when classifying and determining the nature of YSOs.

Probing the physics of star formation (ProPStar)

Context. Electron fraction and cosmic-ray ionization rates in star-forming regions are important quantities in astrochemical modeling and are critical to the degree of coupling between neutrals, ions, and electrons, which regulates the dynamics of the magnetic field. However, these are difficult quantities to estimate. Aims. We aim to derive the electron fraction and cosmic-ray ionization rate maps of an active star-forming region. Methods. We combined observations of the nearby NGC 1333 star-forming region carried out with the NOEMA interferometer and IRAM 30 m single dish to generate high spatial dynamic range maps of different molecular transitions. We used the DCO+ and H13CO+ ratio (in addition to complementary data) to estimate the electron fraction and produce cosmic-ray ionization rate maps. Results. We derived the first large-area electron fraction and cosmic-ray ionization rate resolved maps in a star-forming region, with typical values of 10−65 and 10−16.5 s−1, respectively. The maps present clear evidence of enhanced values around embedded young stellar objects (YSOs). This provides strong evidence for locally accelerated cosmic rays. We also found a strong enhancement toward the northwest region in the map that might be related either to an interaction with a bubble or to locally generated cosmic rays by YSOs. We used the typical electron fraction and derived a magnetohydrodynamic (MHD) turbulence dissipation scale of 0.054 pc, which could be tested with future observations. Conclusions. We found a higher cosmic-ray ionization rate compared to the canonical value for N(H2) = 1021−1023 cm−2 of 10−17 s−1 in the region, and it is likely generated by the accreting YSOs. The high value of the electron fraction suggests that new disks will form from gas in the ideal-MHD limit. This indicates that local enhancements of ζ(H2), due to YSOs, should be taken into account in the analysis of clustered star formation.

Sample and Statistical Analysis on NEOWISE Variability of ATLASGAL Sources

This study focuses on the analysis of mid-infrared variability in a sample of high-mass young stellar objects (YSOs) associated with the cataloged sources from the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL). The Near-Earth Object Wide-field Infrared Survey Explorer Reactivation Mission (NEOWISE) database was used to explore the long-term mid-infrared variability of these high-mass YSOs at a half-year scale. After matching with NEOWISE photometric measurements, a total of 2230 ATLASGAL sources were selected for the variability analysis, out of which 717 were identified as variables. The derived proportions of variables at different evolutionary stages show that the variability rate of high-mass YSOs is highest during the YSO stage and decreases with evolution toward the H ii region stage, resembling the behavior of low-mass YSOs. The variables can be classified into six types based on their light curves, divided into two categories: secular (linear, sin, sin+linear) and stochastic variables (burst, drop, and irregular). The magnitude–color variations observed in ∼160 secular variables can be mainly divided into “bluer when brighter/redder when dimming” and “redder when brighter/bluer when dimming,” likely originating from changes in accretion rate or the effect of extinction due to obscuration. Moreover, several episodic accretion candidates were selected for further observational studies.

Accretion Funnel Reconfiguration during an Outburst in a Young Stellar Object: EX Lupi

EX Lupi, a low-mass young stellar object, went into an accretion-driven outburst in 2022 March. The outburst caused a sudden phase change of ∼112° ± 5° in periodically oscillating multiband lightcurves. Our high-resolution spectra obtained with the High Resolution Spectrograph (HRS) on board the Southern African Large Telescope also revealed a consistent phase change in the periodically varying radial velocities (RVs), along with an increase in the RV amplitude of various emission lines. The phase change and increase in RV amplitude morphologically translates to a change in the azimuthal and latitudinal location of the accretion hotspot over the stellar surface, which indicates a reconfiguration of the accretion funnel geometry. Our three-dimensional magnetohydrodynamic simulations reproduce the phase change for EX Lupi. To explain the observations, we explored the possibility of forward shifting of the dipolar accretion funnel as well as the possibility of the emergence of a new accretion funnel. During the outburst, we also found evidence of the hotspot’s morphology extending azimuthally asymmetrically with a leading hot edge and cold tail along the stellar rotation. Further, our high-cadence photometry showed that the accretion flow has clumps. We also detected possible clumpy accretion events in the HRS spectra that showed episodically highly blueshifted wings in the Ca ii IR triplet and Balmer H lines.

The role of heating in the formation and the dynamics of YSO jets

Context. Theoretical arguments as well as observations of young stellar objects (YSOs) support the presence of a diversified circumstellar environment. A stellar jet is thought to account for most of the stellar spin down and disk wind outflow for the observed high mass-loss rate, thus playing a major role in the launching of powerful jets. RY Tau, for instance, is an extensively studied intermediate mass pre-main sequence star. Observational data reveal a small-scale jet called micro-jet. Nevertheless, it is not clear how the micro-jet shapes the jet observed at a large scale. Aims. The goal is to investigate the spatial stability and structure of the central jet at a large scale by mixing the stellar and disk components. Methods. Two existing analytical self-similar models for the disk and the stellar winds to build the initial setups. Instead of using a polytropic equation of state, we mapped the heating and cooling sources from the analytical solutions. The heating exchange rate was controlled by two parameters, its spatial extent and its intensity. Results. The central jet and the surrounding disk are strongly affected by these two parameters. We separate the results into three categories, which show different emissivity, temperature, and velocity maps. We reached this categorization by looking at the opening angle of the stellar solution. For cylindrically, well-collimated jets, we have opening angles as low as 10° between 8 − 10 au, and for the wider jets, we can reach 30° with a morphology closer to radial solar winds. Conclusions. Our parametric study shows that the less heated the outflow is, the more collimated it appears. We also show that recollimation shocks appear consistently with UV observations in terms of temperature but not density.

Chemical Differentiation and Gas Kinematics around Massive Young Stellar Objects in RCW 120

Chemical Differentiation and Gas Kinematics around Massive Young Stellar Objects in RCW 120

Candidate young stellar objects in the S-cluster: Kinematic analysis of a subpopulation of the low-mass G objects close to Sgr A*

Candidate young stellar objects in the S-cluster: Kinematic analysis of a subpopulation of the low-mass G objects close to Sgr A*

High Mass Inner Regions Found in Five Outbursting Sources

Young stellar objects are thought to commonly undergo sudden accretion events that result in a rise in bolometric luminosity. These outbursts likely coincide with the onset of planet formation and could impact the formation of planets. The reason behind this dramatic enhancement of accretion is an active area of research, and the mass of the system is a critical parameter. Using the Northern Extended Millimeter Array, we survey five outbursting sources (three FU Ori, one EX Or, and one “peculiar” source) with the primary goal of determining the system’s mass using an optically thin line of CO. We estimate the mass of a central region for each object that using both continuum emission and C17O J = 2-1. The C17O emission likely includes both disk and inner envelope material, thus acts as an upper limit on the disk mass, ranging from 0.33 to 3.4 M⊙ for our sources. These derived masses suggest that the inner ∼1000 au contains enough mass along the line of sight for these sources to be gravitationally unstable.

JWST detections of amorphous and crystalline HDO ice toward massive protostars

Tracing the origin and evolution of interstellar water is key to understanding many of the physical and chemical processes involved in star and planet formation. Deuterium fractionation offers a window into the physicochemical history of water due to its sensitivity to local conditions. The aim of this work is to utilize the increased sensitivity and resolution of the James Webb Space Telescope (JWST) to quantify the HDO/H$_ $O ratio in ices toward young stellar objects (YSOs) and to determine if the HDO/H$_ $O ratios measured in the gas phase toward massive YSOs (MYSOs) are representative of the ratios in their ice envelopes. Two protostars observed in the Investigating Protostellar Accretion (IPA) program using JWST NIRSpec were analyzed: HOPS 370, an intermediate-mass YSO (IMYSO), and IRAS 20126+4104, a MYSO. The HDO ice toward these sources was quantified via its 4.1 mu m band. The contributions from the CH$_ $OH combination modes to the observed optical depth in this spectral region were constrained via the CH$_ $OH 3.53 mu m band to ensure that the integrated optical depth of the HDO feature was not overestimated. H$_ $O ice was quantified via its 3 mu m band. New laboratory IR spectra of ice mixtures containing HDO, H$_ $O, CH$_ $OH, and CO were collected to aid in the fitting and chemical interpretation of the observed spectra. HDO ice is detected above the 3sigma level in both sources. It requires a minimum of two components, one amorphous and one crystalline, to obtain satisfactory fits. The H$_ $O ice band at 3 mu m similarly requires both amorphous and crystalline components. The observed peak positions of the crystalline HDO component are consistent with those of annealed laboratory ices, which could be evidence of heating and subsequent recooling of the ice envelope (i.e., thermal cycling). The CH$_ $OH 3.53 mu m band is fit best with two cold components, one consisting of pure CH$_ $OH and the other of CH$_ $OH in an H$_ $O-rich mixture. From these fits, ice HDO/H$_ $O abundance ratios of 4.6pm 1.8times 10$^ $ and 2.6pm 1.2times 10$^ $ are obtained for HOPS 370 and IRAS 20126+4104, respectively. The simultaneous detections of both crystalline HDO and crystalline H$_ $O corroborate the assignment of the observed feature at 4.1 mu m to HDO ice. The ice HDO/H$_ $O ratios are similar to the highest reported gas HDO/H$_ $O ratios measured toward MYSOs and the hot inner regions of isolated low-mass protostars, suggesting that at least some of the gas HDO/H$_ $O ratios measured toward massive hot cores are representative of the HDO/H$_ $O ratios in ices. The need for an H$_ $O-rich CH$_ $OH component in the CH$_ $OH ice analysis supports recent experimental and observational results that indicate that some CH$_ $OH ice may form prior to the CO freeze-out stage in H$_ $O-rich ice layers.

A JWST/MIRI and NIRCam Analysis of the Young Stellar Object Population in the Spitzer I Region of NGC 6822

We present an imaging survey of the Spitzer I star-forming region in NGC 6822 conducted with the NIRCam and MIRI instruments on board JWST. Located at a distance of 490 kpc, NGC 6822 is the nearest non-interacting low-metallicity (∼0.2 Z ⊙) dwarf galaxy. It hosts some of the brightest known H ii regions in the local universe, including recently discovered sites of highly embedded active star formation. Of these, Spitzer I is the youngest and most active, and houses 90 color-selected candidate young stellar objects (YSOs) identified from Spitzer Space Telescope observations. We revisit the YSO population of Spitzer I with these new JWST observations. By analyzing color–magnitude diagrams constructed with NIRCam and MIRI data, we establish color selection criteria and construct spectral energy distributions to identify candidate YSOs and characterize the full population of young stars, from the most embedded phase to the more evolved stages. In this way, we have identified 140 YSOs in Spitzer I. Comparing to previous Spitzer studies of the NGC 6822 YSO population, we find that the YSOs we identify are fainter and less massive, indicating that the improved resolution of JWST allows us to resolve previously blended sources into multiple objects.

Constraining Free–Free Emission and Photoevaporative Mass-loss Rates for Known Proplyds and New VLA–identified Candidate Proplyds in NGC 1977

We present NSF's Karl G. Jansky Very Large Array observations covering the NGC 1977 region at 3.0, 6.4, and 15.0 GHz. We search for compact radio sources and detect continuum emission from 34 NGC 1977 cluster members and 37 background objects. Of the 34 radio-detected cluster members, 3 are associated with known proplyds in NGC 1977, 22 are associated with additional young stellar objects in NGC 1977, and 9 are newly identified cluster members. We examine the radio spectral energy distributions, circular polarization, and variability of the detected NGC 1977 sources and identify 10 new candidate proplyds whose radio fluxes are dominated by optically thin free–free emission. We use measurements of free–free emission to calculate the mass-loss rates of known proplyds and new candidate proplyds in NGC 1977, and find values ∼10−9 to 10−8 M ⊙ yr−1, which are lower than the mass-loss rates measured toward proplyds in the Orion Nebula Cluster but consistent with the mass-loss rates predicted by external photoevaporation models for spatially extended disks that are irradiated by the typical external ultraviolet (UV) fields encountered in NGC 1977. Finally, we show that photoevaporative disk winds in NGC 1977 may be illuminated by internal or external sources of ionization, depending on their positions within the cluster. This study provides new constraints on disk properties in a clustered star-forming region with a weaker UV environment than the Orion Nebula Cluster but a stronger UV environment than low-mass star-forming regions like Taurus. Such intermediate UV environments represent the typical conditions of Galactic star and planet formation.