Young Star-forming Regions Research Articles

The Internal Proper Motion Kinematics of NGC 346: Past Formation and Future Evolution

We investigate the internal kinematics of the young star-forming region NGC 346 in the Small Magellanic Cloud (SMC). We used two epochs of deep F555W and F814W Hubble Space Telescope Advanced Camera for Surveys observations with an 11 yr baseline to determine proper motions and study the kinematics of different populations, as identified by their color–magnitude diagram and spatial distribution characteristics. The proper motion field of the young stars shows a complex structure with spatially coherent patterns. NGC 346’s upper main sequence and pre-main sequence stars follow very similar motion patterns, with the outer parts of the cluster being characterized both by outflows and inflows. The proper motion field in the inner ∼10 pc shows a combination of rotation and inflow, indicative of inspiraling motion. The rotation velocity in this regions peaks at ∼3 km s−1, whereas the inflow velocity peaks at ∼1 km s−1. Subclusters and massive young stellar objects in NGC 346 are found at the interface of significant changes in the coherence of the proper motion field. This suggests that turbulence is the main star formation driver in this region. Similar kinematics observed in the metal-poor NGC 346 and in the Milky Way’s star-forming regions suggest that the differences in the cooling conditions due to different amounts of metallicity and dust density between the SMC and our galaxy are too small to alter significantly the process of star cluster assembly and growth. The main characteristics of our findings are consistent with various proposed star cluster formation models.

First Peek with JWST/NIRCam Wide-field Slitless Spectroscopy: Serendipitous Discovery of a Strong [O iii]/Hα Emitter at z = 6.11

We report the serendipitous discovery of an [O iii] λ λ4959/5007 and Hα line emitter in the Epoch of Reionization (EoR) with the James Webb Space Telescope (JWST) commissioning data taken in the NIRCam wide-field slitless spectroscopy (WFSS) mode. Located ∼55″ away from the flux calibrator P330-E, this galaxy exhibits bright [O iii] λ λ4959/5007 and Hα lines detected at 3.7σ, 9.9σ, and 5.7σ, respectively, with a spectroscopic redshift of z = 6.112 ± 0.001. The total Hβ+[O iii] equivalent width is 664 ± 98 Å (454 ± 78 Å from the [O iii] λ5007 line). This provides direct spectroscopic evidence for the presence of strong rest-frame optical lines (Hβ+[O iii] and Hα) in EoR galaxies as inferred previously from the analyses of the Spitzer/IRAC spectral energy distributions (SEDs). Two spatial and velocity components are identified in this source, possibly indicating that this system is undergoing a major merger, which might have triggered the ongoing starburst with strong nebular emission lines over a timescale of ∼2 Myr, as our SED modeling suggests. The tentative detection of He ii λ4686 line (1.9σ), if real, may indicate the existence of very young and metal-poor star-forming regions with a hard UV radiation field. Finally, this discovery demonstrates the power and readiness of the JWST/NIRCam WFSS mode, and marks the beginning of a new era for extragalactic astronomy, in which EoR galaxies can be routinely discovered via blind slitless spectroscopy through the detection of rest-frame optical emission lines.

The evolution of protoplanetary disc radii and disc masses in star-forming regions

ABSTRACT Protoplanetary discs are crucial to understanding how planets form and evolve, but these objects are subject to the vagaries of the birth environments of their host stars. In particular, photoionizing radiation from massive stars has been shown to be an effective agent in disrupting protoplanetary discs. External photoevaporation leads to the inward evolution of the radii of discs, whereas the internal viscous evolution of the disc causes the radii to evolve outwards. We couple N-body simulations of star-forming regions with a post-processing analysis of disc evolution to determine how the radius and mass distributions of protoplanetary discs evolve in young star-forming regions. To be consistent with observations, we find that the initial disc radii must be of the order of 100 au, even though these discs are readily destroyed by photoevaporation from massive stars. Furthermore, the observed disc radius distribution in the Orion Nebula Cluster (ONC) is more consistent with moderate initial stellar densities (100 M⊙ pc−3), in tension with dynamical models that posit much higher initial densities for the ONC. Furthermore, we cannot reproduce the observed disc radius distribution in the Lupus star-forming region if its discs are subject to external photoevaporation. A more detailed comparison is not possible due to the well-documented uncertainties in determining the ages of pre-main-sequence (disc-hosting) stars.

Quantifying kinematic substructure in star-forming regions with statistical tests of spatial autocorrelation

ABSTRACT We investigate whether spatial-kinematic substructure in young star-forming regions can be quantified using Moran’s I statistic. Its presence in young star clusters would provide an indication that the system formed from initially substructured conditions, as expected by the hierarchical model of star cluster formation, even if the cluster were spatially smooth and centrally concentrated. Its absence, on the other hand, would be evidence that star clusters form monolithically. The Moran’s I statistic is applied to N-body simulations of star clusters with different primordial spatial-velocity structures, and its evolution over time is studied. It is found that this statistic can be used to reliably quantify spatial-kinematic substructure, and can be used to provide evidence as to whether the spatial-kinematic structure of regions with ages ≲6 Myr is best reproduced by the hierarchical or monolithic models of star formation. Moran’s I statistic is also able to conclusively say whether the data are not consistent with initial conditions that lack kinematic substructure, such as the monolithic model, in regions with ages up to, and potentially beyond, 10 Myrs. This can therefore provide a kinematic signature of the star cluster formation process that is observable for many Myr after any initial spatial structure has been erased.

Early planet formation in embedded protostellar disks

Recent surveys of young star formation regions have shown that the dust mass of the average class II object is not high enough to make up the cores of giant planets. Younger class O/I objects have enough dust in their embedded disk, which raises the question whether the first steps of planet formation occur in these younger systems. The first step is building the first planetesimals, which are generally thought to be the product of the streaming instability. Hence the question can be restated to read whether the physical conditions of embedded disks are conducive to the growth of the streaming instability. The streaming instability requires moderately coupled dust grains and a dust-to-gas mass ratio near unity. We model the collapse of a dusty proto-stellar cloud to show that if there is sufficient drift between the falling gas and dust, regions of the embedded disk can become sufficiently enhanced in dust to drive the streaming instability. We include four models to test a variety of collapse theories: three models with different dust grain sizes, and one model with a different initial cloud angular momentum. We find a sweet spot for planetesimal formation for grain sizes of a few 10s of micron because they fall sufficiently fast relative to the gas to build a high dust-to-gas ratio in the disk midplane, but their radial drift speeds are slow enough in the embedded disk to maintain the high dust-to-gas ratio. Unlike the gas, which is held in hydrostatic equilibrium for a time as a result of gas pressure, the dust can begin to collapse from all radii at a much earlier time. The dust mass flux in class O/I systems can thus be higher than the gas flux. This builds an embedded dusty disk with a global dust-to-gas mass ratio that exceeds the inter-stellar mass ratio by at least an order of magnitude. The streaming instability can produce at least between 7 and 35 M⊕ of planetesimals in the class O/I phase of our smooth embedded disks, depending on the size of the falling dust grains. This mass is sufficient to build the core of the first giant planet in the system, and could be further enhanced by dust traps and/or pebble growth. This first generation of planetesimals could represent the first step in planet formation. It occurs earlier in the lifetime of the young star than is traditionally thought.

Multiwavelength and Multi-CO View of the Minor Merger Driven Star Formation in the Nearby LIRG NGC 3110

We present Atacama Large Millimeter/submillimeter Array observations of multiple 12CO, 13CO, and C18O lines and 2.9 mm and 1.3 mm continuum emission toward the nearby interacting luminous infrared galaxy NGC 3110, supplemented with similar spatial resolution Hα, 1.4 GHz continuum, and K-band data. We estimate the typical CO-to-H2 conversion factor of 1.7 M ⊙ (K km s−1 pc2)−1 within the disk using local thermal equilibrium-based and dust-based H2 column densities, and measure the 1 kpc scale surface densities of the star formation rate (ΣSFR), super star clusters (ΣSSC), molecular gas mass, and star formation efficiency (SFE) toward the entire gas disk. These parameters show a peak in the southern part of the southern spiral arm (SFE ∼ 10−8.2 yr−1, ΣSFR ∼ 10−0.6 M ⊙ kpc−2 yr−1, ΣSSC ∼ 6.0 kpc−2), which is likely attributable to the ongoing tidal interaction with the companion galaxy MCG-01-26-013, as well as toward the circumnuclear region. We also find that thermal free–free emission contributes to a significant fraction of the millimeter continuum emission at the position of the southern peak. These measurements imply that the peak of the southern arm is an active and young star-forming region, whereas the central part of NGC 3110 is a site of long-continued star formation. We suggest that during the early stage of the galaxy–galaxy interaction in which the mass ratio was high in NGC 3110, fragmentation along the main galaxy arms is an important driver of merger-induced star formation, and that massive gas inflow results in dusty nuclear starbursts.

Stellar multiplicity affects the correlation between protoplanetary disc masses and accretion rates: binaries explain high accretors in Upper Sco

ABSTRACT In recent years, a correlation between mass accretion rates onto new-born stars and their protoplanetary disc masses was detected in nearby young star-forming regions. Although such a correlation can be interpreted as due to viscous-diffusion processes in the disc, highly accreting sources with low disc masses in more evolved regions remain puzzling. In this paper, we hypothesize that the presence of a stellar companion truncating the disc can explain these outliers. First, we searched the literature for information on stellar multiplicity in Lupus, Chamaeleon I, and Upper Sco, finding that roughly 20 per cent of the discs involved in the correlation are in binaries or higher order multiple stellar systems. We prove with high statistical significance that at any disc mass these sources have systematically higher accretion rates than those in single-stars, with the bulk of the binary population being clustered around $M_\mathrm{disc}/\dot{M}_\mathrm{acc}\approx 0.1\, \mathrm{Myr}$. We then run coupled gas and dust one-dimensional evolutionary models of tidally truncated discs to be compared with the data. We find that these models are able to reproduce well most of the population of observed discs in Lupus and Upper Sco, even though the unknown eccentricity of each binary prevents an object by object comparison. In the latter region, the agreement improves if the grain coagulation efficiency is reduced, as may be expected in discs around close binaries. Finally, we mention that thermal winds and sub-structures can be important in explaining few outlying sources.

ALMA chemical survey of disk-outflow sources in Taurus (ALMA-DOT)

Planet-forming disks are not isolated systems. Their interaction with the surrounding medium affects their mass budget and chemical content. In the context of the ALMA-DOT program, we obtained high-resolution maps of assorted lines from six disks that are still partly embedded in their natal envelope. In this work, we examine the SO and SO2 emission that is detected from four sources: DG Tau, HL Tau, IRAS 04302+2247, and T Tau. The comparison with CO, HCO+, and CS maps reveals that the SO and SO2 emission originates at the intersection between extended streamers and the planet-forming disk. Two targets, DG Tau and HL Tau, offer clear cases of inflowing material inducing an accretion shock on the disk material. The measured rotational temperatures and radial velocities are consistent with this view. In contrast to younger Class 0 sources, these shocks are confined to the specific disk region impacted by the streamer. In HL Tau, the known accreting streamer induces a shock in the disk outskirts, and the released SO and SO2 molecules spiral toward the star in a few hundred years. These results suggest that shocks induced by late accreting material may be common in the disks of young star-forming regions with possible consequences for the chemical composition and mass content of the disk. They also highlight the importance of SO and SO2 line observations in probing accretion shocks from a larger sample.

Clustered star formation at early evolutionary stages

Context. The process of high-mass star formation during the earliest evolutionary stages and the change over time of the physical and chemical properties of individual fragmented cores are still not fully understood. Aims. We aim to characterize the physical and chemical properties of fragmented cores during the earliest evolutionary stages in the very young star-forming regions ISOSS J22478+6357 and ISOSS J23053+5953. Methods. NOrthern Extended Millimeter Array 1.3 mm data are used in combination with archival mid- and far-infrared Spitzer and Herschel telescope observations to construct and fit the spectral energy distributions of individual fragmented cores. The radial density profiles are inferred from the 1.3 mm continuum visibility profiles, and the radial temperature profiles are estimated from H2CO rotation temperature maps. Molecular column densities are derived with the line fitting tool XCLASS. The physical and chemical properties are combined by applying the physical-chemical model MUlti Stage ChemicaL codE in order to constrain the chemical timescales of a few line-rich cores. The morphology and spatial correlations of the molecular emission are analyzed using the histogram of oriented gradients (HOG) method. Results. The mid-infrared data show that both regions contain a cluster of young stellar objects. Bipolar molecular outflows are observed in the CO 2−1 transition toward the strong millimeter (mm) cores, indicating protostellar activity. We find strong molecular emission of SO, SiO, H2CO, and CH3OH in locations that are not associated with the mm cores. These shocked knots can be associated either with the bipolar outflows or, in the case of ISOSS J23053+5953, with a colliding flow that creates a large shocked region between the mm cores. The mean chemical timescale of the cores is lower (~20 000 yr) compared to that of the sources of the more evolved CORE sample (~60 000 yr). With the HOG method, we find that the spatial emission of species that trace the extended emission and of shock-tracing molecules are well correlated within transitions of these groups. Conclusions. Clustered star formation is observed toward both regions. Comparing the mean results of the density and temperature power-law index with the results of the original CORE sample of more evolved regions, it appears that neither change significantly from the earliest evolutionary stages to the hot molecular core stage. However, we find that the 1.3 mm flux, kinetic temperature, H2 column density, and core mass of the cores increase in time, which can be traced both in the M/L ratio and the chemical timescale, τchem.

Constraining the initial conditions of NGC 2264 using ejected stars found in Gaia DR2

ABSTRACT Fast, ejected stars have been found around several young star-forming regions, such as the Orion Nebula Cluster. These ejected stars can be used to constrain the initial density, spatial and kinematic substructure when compared to predictions from N-body simulations. We search for runaway and slower walkaway stars using Gaia Data Release 2 (DR2) within 100 pc of NGC 2264, which contains subclustered regions around higher mass OB stars (S Mon, IRS 1, and IRS 2). We find five runaways and nine walkaways that trace back to S Monocerotis (S Mon) and six runaways and five walkaways that trace back to IRS 1/2 based on their 3D kinematics. We compare these numbers to a range of N-body simulations with different initial conditions. The number of runaways/walkaways is consistent with initial conditions with a high initial stellar density (∼10 000 M⊙ pc−3), a high initial amount of spatial substructure, and either a subvirial or virialized ratio for all subclusters. We also confirm the trajectories of our ejected stars using the data from Gaia Early Data Release 3 (EDR3), which reduces the number of runaways from IRS 1/2 from six to four but leaves the number of runaways from S Mon unchanged. The reduction in runaways is due to smaller uncertainties in the proper motion and changes in the parallax/distance estimate for these stars in Gaia EDR3. We find further runaway/walkaway candidates based on proper motion alone in Gaia DR2, which could increase these numbers once radial velocities are available. We also expect further changes in the candidate list with upcoming Gaia data releases.

On the secular evolution of the ratio between gas and dust radii in protoplanetary discs

ABSTRACT A key problem in protoplanetary disc evolution is understanding the efficiency of dust radial drift. This process makes the observed dust disc sizes shrink on relatively short time-scales, implying that discs started much larger than what we see now. In this paper, we use an independent constraint, the gas radius (as probed by CO rotational emission), to test disc evolution models. In particular, we consider the ratio between the dust and gas radius, RCO/Rdust. We model the time evolution of protoplanetary discs under the influence of viscous evolution, grain growth, and radial drift. Then, using the radiative transfer code radmc with approximate chemistry, we compute the dust and gas radii of the models and investigate how RCO/Rdust evolves. Our main finding is that, for a broad range of values of disc mass, initial radius, and viscosity, RCO/Rdust becomes large (>5) after only a short time (<1 Myr) due to radial drift. This is at odds with measurements in young star-forming regions such as Lupus, which find much smaller values, implying that dust radial drift is too efficient in these models. Substructures, commonly invoked to stop radial drift in large, bright discs, must then be present, although currently unresolved, in most discs.

KMOS study of the mass accretion rate from Class I to Class II in NGC 1333

Context. The mass accretion rate (Ṁacc) is the fundamental parameter to understand the process of mass assembly that results in the formation of a low-mass star. This parameter has been largely studied in Classical T Tauri stars in star-forming regions with ages of ∼1 − 10 Myr. However, little is known about the accretion properties of young stellar objects (YSOs) in younger regions and early stages of star formation, such as in the Class 0/I phases. Aims. We present new near-infrared spectra of 17 Class I/Flat and 35 Class II sources located in the young (< 1 Myr) NGC 1333 cluster, acquired with the KMOS instrument at the Very Large Telescope. Our goal is to study whether the mass accretion rate evolves with age, as suggested by the widely adopted viscous evolution model, by comparing the properties of the NGC 1333 members with samples of older regions. Methods. For the Class II sources in our sample, we measured the stellar parameters (SpT, AV, and L⋆) through a comparison of the IR spectra with a grid of non-accreting Class III stellar templates. We then computed the accretion luminosity by using the known correlation between Lacc and the luminosity of HI lines (Paβ and Brγ). For the Class I sample, where the presence of a large IR excess makes it impossible to use the same spectral typing method, we applied a procedure that allowed us to measure the stellar and accretion luminosity in a self-consistent way. Mass accretion rates Ṁacc were then measured once masses and radii were estimated adopting suitable evolutionary tracks. Results. The NGC 1333 Class II sources of our sample have Lacc ∼ 10−4 − 1 L⊙ and Ṁacc ∼ 10−11 − 10−7 M⊙ yr−1. We find a correlation between accretion and stellar luminosity in the form of log Lacc = (1.5 ± 0.2)log L⋆ + ( − 1.0 ± 0.1), and a correlation between the mass accretion rate and stellar mass in the form of log Ṁacc = (2.6 ± 0.9) log M⋆ + (−7.3 ± 0.7). Both correlations are compatible within the errors with the older Lupus star-forming region, while only the latter is consistent with results from Chamaeleon I. The Class I sample shows larger accretion luminosities (∼10−2 − 102 L⊙) and mass accretion rates (∼10−9 − 10−6 M⊙ yr−1) with respect to the Class II stars of the same cloud. However, the derived mass accretion rates are not sufficiently high to build up the inferred stellar masses, assuming steady accretion during the Class I lifetime. This suggests that the sources are not in their main accretion phase and that most of their mass has already been accumulated during a previous stage and/or that the accretion is an episodic phenomenon. We show that some of the targets originally classified as Class I through Spitzer photometry are in fact evolved or low accreting objects. This evidence can have implications for the estimated protostellar phase lifetimes. Conclusions. The accretion rates of our sample are larger in more embedded and early stage YSOs. Further observations of larger samples in young star-forming regions are needed to determine if this is a general result. In addition, we highlight the importance of spectroscopic surveys of YSOs to confirm their classification and perform a more correct estimate of their lifetime.

PENELLOPE: The ESO data legacy program to complement the Hubble UV Legacy Library of Young Stars (ULLYSES)

The evolution of young stars and disks is driven by the interplay of several processes, notably the accretion and ejection of material. These processes, critical to correctly describe the conditions of planet formation, are best probed spectroscopically. Between 2020 and 2022, about 500orbits of the Hubble Space Telescope (HST) are being devoted in to the ULLYSES public survey of about 70 low-mass (M⋆ ≤ 2 M⊙) young (age < 10 Myr) stars at UV wavelengths. Here, we present the PENELLOPE Large Program carried out with the ESO Very Large Telescope (VLT) with the aim of acquiring, contemporaneously to the HST, optical ESPRESSO/UVES high-resolution spectra for the purpose of investigating the kinematics of the emitting gas, along with UV-to-NIR X-shooter medium-resolution flux-calibrated spectra to provide the fundamental parameters that HST data alone cannot provide, such as extinction and stellar properties. The data obtained by PENELLOPE have no proprietary time and the fully reduced spectra are being made available to the whole community. Here, we describe the data and the first scientific analysis of the accretion properties for the sample of 13 targets located in the Orion OB1 association and in the σ-Orionis cluster, observed in November–December 2020. We find that the accretion rates are in line with those observed previously in similarly young star-forming regions, with a variability on a timescale of days (≲3). The comparison of the fits to the continuum excess emission obtained with a slab model on the X-shooter spectra and the HST/STIS spectra shows a shortcoming in the X-shooter estimates of ≲10%, which is well within the assumed uncertainty. Its origin can be either due to an erroneous UV extinction curve or to the simplicity of the modeling and, thus, this question will form the basis of the investigation undertaken over the course of the PENELLOPE program. The combined ULLYSES and PENELLOPE data will be key in attaining a better understanding of the accretion and ejection mechanisms in young stars.

External Photoevaporation of Protoplanetary Disks: Does Location Matter?

Abstract Many theoretical studies have shown that external photoevaporation from massive stars can severely truncate, or destroy altogether, the gaseous protoplanetary disks around young stars. In tandem, several observational studies report a correlation between the mass of a protoplanetary disk and its distance to massive ionizing stars in star-forming regions, and cite external photoevaporation by the massive stars as the origin of this correlation. We present N-body simulations of the dynamical evolution of star-forming regions and determine the mass loss in protoplanetary disks from external photoevaporation due to far-ultraviolet and extreme-ultraviolet radiation from massive stars. We find that projection effects can be significant, in that low-mass disk-hosting stars that appear close to the ionizing sources may be fore- or background stars in the star-forming region. We find very little evidence in our simulations for a trend in increasing disk mass with increasing distance from the massive star(s), even when projection effects are ignored. Furthermore, the dynamical evolution of these young star-forming regions moves stars whose disks have been photoevaporated to far-flung locations, away from the ionizing stars, and we suggest that any correlation between disk mass and distance from the ionizing star is either coincidental, or due to some process other than external photoevaporation.

Searching for the brightest stars in galaxies outside the Local Group

This paper introduces a technique for searching for bright massive stars in galaxies beyond the Local Group. To search for massive stars, we processed the results of stellar photometry from the Hubble Space Telescope (HST) images using the DAOPHOT and DOLPHOT packages. The results of such searches are demonstrated with examples of the galaxies DDO 68, M94 and NGC 1672. In the galaxy DDO 68, the LBV star changes its brightness, and massive stars in M94 can be identified by excess in the Hα band. For the galaxy NGC 1672, we measure the distance for the first time by the TRGB method, which enabled determining the luminosities of the brightest stars, likely hypergiants, in the young star formation region. So far, we have performed stellar photometry on HST images of 320 northern sky galaxies located at a distance less than 12 Mpc. This allowed us to identify 53 galaxies with probable hypergiants. Further photometric and spectral observations of these galaxies are planned to search for massive stars.

Fragmentation and kinematics in high-mass star formation

Context. The formation of high-mass star-forming regions from their parental gas cloud and the subsequent fragmentation processes lie at the heart of star formation research. Aims. We aim to study the dynamical and fragmentation properties at very early evolutionary stages of high-mass star formation. Methods. Employing the NOrthern Extended Millimeter Array and the IRAM 30 m telescope, we observed two young high-mass star-forming regions, ISOSS22478 and ISOSS23053, in the 1.3 mm continuum and spectral line emission at a high angular resolution (~0.8″). Results. We resolved 29 cores that are mostly located along filament-like structures. Depending on the temperature assumption, these cores follow a mass-size relation of approximately M ∝ r2.0 ± 0.3, corresponding to constant mean column densities. However, with different temperature assumptions, a steeper mass-size relation up to M ∝ r3.0 ± 0.2, which would be more likely to correspond to constant mean volume densities, cannot be ruled out. The correlation of the core masses with their nearest neighbor separations is consistent with thermal Jeans fragmentation. We found hardly any core separations at the spatial resolution limit, indicating that the data resolve the large-scale fragmentation well. Although the kinematics of the two regions appear very different at first sight – multiple velocity components along filaments in ISOSS22478 versus a steep velocity gradient of more than 50 km s−1 pc−1 in ISOSS23053 – the findings can all be explained within the framework of a dynamical cloud collapse scenario. Conclusions. While our data are consistent with a dynamical cloud collapse scenario and subsequent thermal Jeans fragmentation, the importance of additional environmental properties, such as the magnetization of the gas or external shocks triggering converging gas flows, is nonetheless not as well constrained and would require future investigation.

Tracing Young Star-forming Clumps in the Nearby Flocculent Spiral Galaxy NGC 7793 with UVIT Imaging

Abstract Star formation in galaxies is a hierarchical process with a wide range of scales, from smaller clusters to larger stellar complexes. Here, we present an ultra-violet imaging study of the nearby flocculent spiral galaxy NGC 7793, observed using the Ultra-Violet Imaging Telescope (UVIT). We find that the disk scale length estimated in Far-UV (2.64 ± 0.16 kpc) is larger than that in Near-UV (2.21 ± 0.21 Kpc) and optical (1.08 Kpc), which supports the inside-out growth scenario of the galaxy disk. The star-forming UV disk is also found to be contained within the extent of H i gas, with a column density greater than 1021 cm−2. With the spatial resolution of UVIT (1 pixel ∼ 6.8 pc), we identified 2046 young star-forming clumps in the galaxy with radii between ∼12 and 70 pc, which matches well with the size of giant molecular clouds (GMCs) detected in the galaxy. Around 61% of the regions identified in our study are younger than 20 Myr, which points to a recent enhancement in star formation across the galaxy. We also note that the youngest star-forming regions, with an age of <10 Myr, distinctly trace the flocculent arms of the galaxy. The estimated mass of the clumps covers a range between 103 and 106 M ⊙. We notice a gradient in the mass distribution of identified clumps along the spiral arms. We also study the nuclear star cluster of the galaxy, finding that the stellar populations in the cluster outskirts are younger than the inner part.

Searching for Active Low-mass Stars in the CMa Star-forming Region: Multi-band Photometry with T80S

Abstract An exotic environment surrounds the young stellar groups associated with the Canis Major (CMa) OB1/R1 region, which probably was formed under feedback from at least three supernova events having occurred a few million years ago. We use astrometric data from the Gaia-DR2 to confirm the membership of the stars in CMa R1, based on proper motion and parallax, which revealed 514 new members and candidates. The mean age of 5 Myr estimated from the color–magnitude diagram characterizes the sources as likely pre-main-sequence candidates. In total, a sample of 694 stars detected with the T80-South telescope was analyzed according to different color–color diagrams, which were compared with theoretical colors from evolutionary models, aiming to reveal the objects that exhibit color excess due to accretion processes. Accretion and magnetic activity were also explored on the basis of empirical flux–flux relation, such as F 660 and F 861 that are related to Hα and Ca ii triplet emission, respectively. A low fraction (∼3 percent) of the sample have Hα excess and other colors expected for stars exhibiting chromospheric activity. The number of Class I and Class II objects, identified by the infrared (Wide-field Infrared Survey Explorer) colors, indicates a disk fraction of ∼6 percent, which is lower than that expected for stellar clusters with a similar age. Such a large sample of objects associated with CMa R1 without evidences of circumstellar accretion can be interpreted as a lack of disk-bearing stars, unusual for young star-forming regions. However, this may be explained as the result of supernova events.

Regulating Star Formation in Nearby Dusty Galaxies: Low Photoelectric Efficiencies in the Most Compact Systems

Abstract Star formation in galaxies is regulated by heating and cooling in the interstellar medium (ISM). In particular, the processing of molecular gas into stars will depend strongly on the ratio of gas heating to gas cooling in the neutral gas around sites of recent star formation. In this work, we combine mid-infrared (mid-IR) observations of polycyclic aromatic hydrocarbons (PAHs), the dominant heating mechanism of gas in the ISM, with [C ii], [O i], and [Si ii] fine-structure emission, the strongest cooling channels in dense, neutral gas. The ratio of IR cooling line emission to PAH emission measures the photoelectric efficiency, a property of the ISM which dictates how much energy carried by ultraviolet photons gets transferred into the gas. We find that star-forming, IR-luminous galaxies in the Great Observatories All-Sky LIRG Survey with high IR surface densities have low photoelectric efficiencies. These systems also have, on average, higher ratios of radiation field strength to gas densities, and larger average dust grain size distributions. The data support a scenario in which the most compact galaxies have more young star-forming regions per unit area that exhibit less efficient gas heating. These conditions may be more common at high z, and may help explain the higher star formation rates at cosmic noon. We make predictions on how this can be investigated with the James Webb Space Telescope.

Stellar Feedback on the Earliest Stage of Massive Star Formation

Abstract We report SOFIA/GREAT observations of high-J CO lines and [C ii] observations of the super star cluster candidate H72.97-69.39 in the Large Magellanic Cloud (LMC), which is in its very early formation stage. We use our observations to determine if shocks are heating the gas or if photon-dominated regions (PDRs) are being heated by local far-UV radiation. We use a PDR model and a shock model to determine whether the CO and [C ii] lines arise from PDRs or shocks. We can reproduce the observed high-J CO and [C ii] emission with a clumpy PDR model with the following properties: a density of 104.7 cm−3, a mass of 104 M ⊙, and UV radiation of 103.5 in units of Draine field. Comparison with the ALMA beam-filling factor suggests a higher density within the uncertainty of the fit. We find the lower-limit [C ii]/total infrared (TIR) ratio (ϵ) traced by [C ii]/TIR to be 0.026%, lower than other known young star-forming regions in the LMC. Our shock models may explain the CO (16−15) and CO (11−10) emission lines with shock velocity of 8–11 km s−1, pre-shock density of 104–105 cm−3, and G UV = 0 in units of Draine field. However, the [C ii] line emission cannot be explained by a shock model, thus it is originating in a different gas component. Observations of [O i] 63 μm predicted to be 1.1 × 10−13 W m−2 by PDR models and 7.8 × 10−15 W m−2 by shock models will help distinguish between the PDR and shock scenarios.