Keywords

We observationally investigate star formation process occurring in AFGL 5157 (area ∼13.5 pc × 13.5 pc) using a multi-wavelength approach. Embedded filaments are seen in the Herschel column density map, and one of them is identified as an elongated filamentary feature (FF) (length ∼8.3 pc; mass ∼1170 M_⊙). Five Herschel clumps (M_clump ∼45–300 M_⊙) are traced in the central part of the FF, where an extended temperature structure (T_d ∼13.5–26.5 K) is observed. In the direction of the central part of the FF, the warmer region at T_d ∼20–26.5 K spatially coincides with a mid-infrared shell surrounding a previously known evolved infrared cluster. Diffuse Hα emission is traced inside the infrared shell, suggesting the presence of massive stars in the evolved cluster. Based on the surface density analysis of young stellar objects (YSOs), embedded clusters of YSOs are traced toward the central part of the FF, and are distributed around the infrared shell. Previously detected H₂O masers, H₂ knots, massive protostar candidates, and an H ii region are also seen toward the embedded clusters. Using the ¹²CO and ¹³CO line data, the central part of the FF is observed at the overlapping zones of two filamentary molecular clouds (length ∼12.5 pc) around −20 and −17 km s⁻¹, which are also connected in velocity. Our observational results suggest that the formation of massive stars appears to be triggered by a collision of two filamentary molecular clouds, which might have also influenced the birth of YSOs in AFGL 5157.

Although the ${}^{2}{{\rm{P}}}_{3/2}-{}^{2}{{\rm{P}}}_{1/2}$ transition of [C ii] at λ ≃  158 $\,\mu {\rm{m}}$ is known to be an excellent tracer of active star formation, we still do not have a complete understanding of where within star formation regions the emission originates. Here, we use SOFIA upGREAT observations of [C ii] emission toward the H ii region complex Sh2-235 (S235) to better understand in detail the origin of [C ii] emission. We complement these data with a fully sampled Green Bank Telescope radio recombination line map tracing the ionized hydrogen gas. About half of the total [C ii] emission associated with S235 is spatially coincident with ionized hydrogen gas, although spectroscopic analysis shows little evidence that this emission is coming from the ionized hydrogen volume. Velocity-integrated [C ii] intensity is strongly correlated with Wide-field Infrared Survey Explorer (WISE) 12 $\,\mu {\rm{m}}$ intensity across the entire complex, indicating that both trace ultraviolet radiation fields. The 22 $\,\mu {\rm{m}}$ and radio continuum intensities are only correlated with [C ii] intensity in the ionized hydrogen portion of the S235 region and the correlations between the [C ii] and molecular gas tracers are poor across the region. We find similar results for emission averaged over a sample of external galaxies, although the strength of the correlations is weaker. Therefore, although many tracers are correlated with the strength of [C ii] emission, only WISE 12 $\,\mu {\rm{m}}$ emission is correlated on small scales of the individual H ii region S235 and also has a decent correlation at the scale of entire range of galaxies. Future studies of a larger sample of Galactic H ii regions would help to determine whether these results are truly representative.

We present detections of the CO(4–3) and [C i] 609 μm spectral lines, as well as the dust continuum at 480.5 GHz (rest frame), in 3C 368, a Fanaroff–Riley class II (FR-II) galaxy at redshift (z) 1.131. 3C 368 has a large stellar mass, ∼ 3.6 × 10¹¹ M_⊙, and is undergoing an episode of vigorous star formation, at a rate of ∼ 350 M_⊙ yr⁻¹, and active galactic nucleus activity, with radio-emitting lobes extended over ∼ 73 kpc. Our observations allow us to inventory the molecular-gas reservoirs in 3C 368 by applying three independent methods: (1) using the CO(4–3)-line luminosity, excitation state of the gas, and an α_CO conversion factor, (2) scaling from the [C i]-line luminosity, and (3) adopting a gas-to-dust conversion factor. We also present gas-phase metallicity estimates in this source, both using far-infrared fine-structure lines together with radio free–free continuum emission and independently employing the optical [O iii] 5007 Å and [O ii] 3727 Å lines (R₂₃ method). Both methods agree on a subsolar gas-phase metallicity of ∼ 0.3 Z_⊙. Intriguingly, comparing the molecular-gas mass estimated using this subsolar metallicity, M_gas ∼ 6.4 × 10¹⁰ M_⊙, to dust-mass estimates from multicomponent spectral energy distribution modeling, M_dust ∼ 1.4 × 10⁸ M_⊙, yields a gas-to-dust ratio within ∼ 15% of the accepted value for a metallicity of 0.3 Z_⊙. The derived gas mass puts 3C 368 on a par with other galaxies at z ∼ 1 in terms of specific star formation rate and gas fraction. However, it does not explain how a galaxy can amass such a large stellar population while maintaining such a low gas-phase metallicity. Perhaps 3C 368 has recently undergone a merger, accreting pristine molecular gas from an external source.

Second only to initial mass, the rate of wind-driven mass loss determines the final mass of a massive star and the nature of its remnant. Motivated by the need to reconcile observational values and theory, we use a recently vetted technique to analyze the mass-loss rates in a sample of OB stars that generate bow shock nebulae. We measure peculiar velocities from new Gaia parallax and proper motion data and their spectral types from new optical and infrared spectroscopy. For our sample of 70 central stars in morphologically selected bow shock nebulae, 67 are OB stars. The median peculiar velocity is 11 km s⁻¹, significantly smaller than classical "runaway star" velocities. Mass-loss rates for these O and early B stars agree with recently lowered theoretical predictions, ranging from ≃10⁻⁷ M_⊙ yr⁻¹ for mid-O dwarfs to 10⁻⁹ M_⊙ yr⁻¹ for late O dwarfs—a factor of about 2.7 lower than the often-used Vink et al. formulation. Our results provide the first observational mass-loss rates for B0–B3 dwarfs and giants—10⁻⁹ to 10⁻⁸ M_⊙ yr⁻¹. We find evidence for an increase in the mass-loss rates below a critical effective temperature, consistent with predictions of the bistability phenomenon in the range T_eff = 19,000–27,000 K. The sample exhibits a correlation between modified wind momentum and luminosity, consistent in slope but lower by 0.43 dex in magnitude compared to canonical wind–luminosity relations. We identify a small subset of objects deviating most significantly from theoretical expectations as probable radiation-driven bow wave nebulae by virtue of their low stellar-to-nebular luminosity ratios. For these, the inferred mass-loss rates must be regarded as upper limits.

We present analytical expressions for direct evaluation of ℓ-mixing rate coefficients in proton-excited hydrogen atom collisions and describe a software package for efficient numerical evaluation of the collisional rate coefficients. Comparisons between rate coefficients calculated with various levels of approximation are discussed, highlighting their range of validity. These rate coefficients are benchmarked for radio recombination lines for hydrogen, evaluating the corresponding departure coefficients from local thermal equilibrium.

Conditions in super star clusters (SSCs) lead to the formation of dozens of massive stars in close proximity. However, SSCs are rare in the local universe. H72.97-69.39, located in the N79 region of the Large Magellanic Cloud (LMC), is an SSC candidate. In this paper we report the ALMA observations of the potential SSC. ALMA reveals colliding filaments, outflows, an H ii region, and a C ii region associated with this cluster. The timescale of the outflow is 65,000 yr, which is consistent with this being a young cluster. The molecular gas around this potential early-stage SSC candidate is complex in nature on small scales (as seen with ALMA) and large scales (as seen with Herschel).

Electron excitation collision strengths for a wide range of transitions giving rise to infrared, optical, ultraviolet, and extreme ultraviolet lines of S iii have been calculated using the B-spline Breit–Pauli R-matrix method. The term-dependent non-orthogonal orbitals have been employed for the accurate representation of target wave functions and the electron plus S iii target scattering system. The multiconfiguration Hartree–Fock method has been utilized for the calculation of 198 S iii fine-structure level energies belonging to the $3{s}^{2}3{p}^{2},3s3{p}^{3},3{p}^{4},3{s}^{2}3p3d$, $3{s}^{2}3p4s,4p,4d,4f,3{s}^{2}3p5s,5p,5d$, $3{s}^{2}3p6s,3s3{p}^{2}3d,3s3{p}^{2}4s,4p,4d,4f$, and 3s3p²5s configurations. The transition probabilities between fine-structure levels have also been calculated and compared with available other calculations. The close-coupling expansion includes these 198 fine-structure levels of S iii in the electron collision calculations. The effective collision strengths are calculated at electron temperatures in the range of 10³–10⁶ K for all possible transitions between the 198 fine-structure levels. The present calculation includes a larger number of levels in the close-coupling expansion and improved target description than previous calculations and should be useful for the analysis of measured spectra from various astrophysical objects. Comparison with other calculations is used to assess likely uncertainties in the existing collision and radiative rates for S iii. The collision and radiative rates are estimated to be accurate to about 20% or better for most main transitions of astrophysical importance.

The Sgr A-H H ii regions are a collection of sources that may represent one of the most recent sites of star formation in the Galactic center (GC). Examining these H ii regions provides important information on the prevalence and distribution of massive stars in the region and may provide clues on the origin of other massive field stars throughout the GC. In this work, we present infrared imaging observations of the Sgr A-H H ii regions taken with the Faint Object Infrared Camera for the SOFIA Telescope (FORCAST) at 19.7, 25.2, 31.5, and 37.1 μm. These data provide high angular resolution maps (∼4'') of the sources, which allow us to study the morphology of spatially extended H ii regions in detail. The wavelength coverage of FORCAST also supplies important constraints on the dust mass, temperature, and luminosity of the sources. We produce spectral energy distribution models using DustEM to measure the luminosity and dust mass, which range from ∼3 × 10³ to 6 × 10⁵ L_⊙ and from ∼6 × 10⁻⁴ to 3 × 10⁻¹ M_⊙, respectively. Analysis of dust temperatures in the four spatially extended H ii regions (H1, H2, H3, and H5) shows that three of these objects require multiple heating sources, and we identify potential stellar candidates. We also compare the size and morphology of the H ii regions and demonstrate that the properties of H2 are consistent with in situ star formation. Lastly, we identify nine additional sources that may be part of the H complex and provide initial characterizations of their infrared emission.

We report the first high-resolution (subarcminute) large-scale mapping ¹²CO and ¹³CO observations of the molecular clouds associated with the giant outer Galaxy H ii region CTB 102 (KR 1). These observations were made using a newly commissioned receiver system on the 13.7 m radio telescope at the Taeduk Radio Astronomy Observatory. Our observations show that the molecular clouds have a spatial extent of 60 × 35 pc and a total mass of 10^4.8–10^5.0 M_⊙. Infrared data from the Wide-field Infrared Survey Explorer and Two Micron All Sky Survey were used to identify and classify the young stellar object (YSO) population associated with ongoing star formation activity within the molecular clouds. We directly detect 18 class I/class II YSOs and six transition disk objects. Moving away from the H ii region, there is an age/class gradient consistent with sequential star formation. The infrared and molecular-line data were combined to estimate the star formation efficiency (SFE) of the entire cloud as well as the SFE for various subregions of the cloud. We find that the overall SFE is between ∼5% and 10%, consistent with previous observations of giant molecular clouds. One of the subregions, region 1a, is a clear outlier, with a SFE of 17%–35% on a 5 pc spatial scale. This high SFE is more typical for much smaller (subparsec scale) star-forming cores, and we think region 1a is likely an embedded massive protocluster.

We discuss the first results from our mid-infrared (MIR) imaging survey of Milky Way Giant H ii (GH ii) regions with our detailed analysis of W51A, which is one of the largest GH ii regions in our Galaxy. We used the FORCAST instrument on the Stratospheric Observatory For Infrared Astronomy (SOFIA) to obtain 20 and 37 μm images of the central 10' × 20' area, which encompasses both of the G49.5–0.4 and G49.4–0.3 subregions. Based on these new data, and in conjunction with previous multiwavelength observations, we conjecture on the physical nature of several individual sources and subcomponents within W51A. We find that extinction seems to play an important role in the observed structures we see in the near- to MIR, both globally and locally. We used the SOFIA photometry combined with Spitzer–IRAC and Herschel–PACS photometry data to construct spectral energy distributions (SEDs) of the subcomponents and point sources detected in the SOFIA images. We fit those SEDs with young stellar object models and found 41 sources that are likely to be massive young stellar objects, many of which are identified as such in this work for the first time. Close to half of the massive young stellar objects do not have detectable radio continuum emission at centimeter wavelengths, implying a very young state of formation. We derived luminosity-to-mass ratio and virial parameters of the extended radio subregions of W51A to estimate their relative ages.