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1. Isolating Dust and Free–Free Emission in ONC Proplyds with ALMA Band 3 Observations

   https://doi.org/10.3847/1538-4357/ace901  

   Ballering, Nicholas P.; Cleeves, L. Ilsedore; Haworth, Thomas J.; Bally, John; Eisner, Josh A.; Ginsburg, Adam; Boyden, Ryan D.; Fang, Min; Kim, Jinyoung Serena (August 2023, The Astrophysical Journal)

   Abstract The Orion Nebula Cluster (ONC) hosts protoplanetary disks experiencing external photoevaporation by the cluster’s intense UV field. These “proplyds” are comprised of a disk surrounded by an ionization front. We present ALMA Band 3 (3.1 mm) continuum observations of 12 proplyds. Thermal emission from the dust disks and free–free emission from the ionization fronts are both detected, and the high-resolution (0.″057) of the observations allows us to spatially isolate these two components. The morphology is unique compared to images at shorter (sub)millimeter wavelengths, which only detect the disks, and images at longer centimeter wavelengths, which only detect the ionization fronts. The disks are small (r_d= 6.4–38 au), likely due to truncation by ongoing photoevaporation. They have low spectral indices (α≲ 2.1) measured between Bands 7 and 3, suggesting the dust emission is optically thick. They harbor tens of Earth masses of dust as computed from the millimeter flux using the standard method although their true masses may be larger due to the high optical depth. We derive their photoevaporative mass-loss rates in two ways: first, by invoking ionization equilibrium and second, by using the brightness of the free–free emission to compute the density of the outflow. We find decent agreement between these measurements and $\dot{M}$= 0.6–18.4 × 10⁻⁷M_⊙yr⁻¹. The photoevaporation timescales are generally shorter than the ∼1 Myr age of the ONC, underscoring the known “proplyd lifetime problem.” Disk masses that are underestimated due to being optically thick remains one explanation to ease this discrepancy. 

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2. Discovery of Radio Recombination Lines from Proplyds in the Orion Nebula Cluster

   https://doi.org/10.3847/1538-4357/adbbcc  

   Boyden, Ryan D; Emig, Kimberly L; Ballering, Nicholas P; Law, Charles J; Haworth, Thomas J; Tan, Jonathan C; Cleeves, L Ilsedore; Li, Zhi-Yun (April 2025, The Astrophysical Journal)

   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. 

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3. A tell-tale tracer for externally irradiated protoplanetary disks: Comparing the [C I] 8727 Å line and ALMA observations in proplyds

   https://doi.org/10.1051/0004-6361/202451737  

   Aru, M-L; Maucó, K; Manara, C F; Haworth, T J; Ballering, N; Boyden, R; Campbell-White, J; Facchini, S; Rosotti, G P; Winter, A; et al (December 2024, Astronomy & Astrophysics)

   The evolution of protoplanetary disks in regions with massive OB stars is influenced by externally driven winds that deplete the outer parts of these disks. The winds have previously been studied via forbidden oxygen emission lines, which also arise in isolated disks in low-mass star-forming regions (SFRs) with weak external UV fields in photoevaporative or magnetic (internal) disk winds. It is crucial to determine how to disentangle external winds from internal ones. Here, we report a proxy for unambiguously identifying externally driven winds with a forbidden line of neutral atomic carbon, [CI] 8727 Å. We compare for the first time the spatial location of the emission in the [OI] 5577 Å, [OI] 6300 Å, and [CI] 8727 Å lines traced by VLT/MUSE-NFM with the ALMA Band 7 continuum disk emission in a sample of 12 proplyds in the Orion Nebula Cluster (ONC). We confirm that the [OI] 5577 Å emission is co-spatial with the disk emission, whereas that of [OI] 6300 Å is emitted both on the disk surface and on the ionization front of the proplyds. We show for the first time that the [CI] 8727 Å line is also co-spatial with the disk surface in proplyds, as seen in the MUSE and ALMA data comparison. The peak emission is compatible with the stellar location in all cases, apart from one target with high relative inclination with respect to the ionizing radiation, where the peak emission is located at the disk edge in the direction of the ionizing radiation. To verify whether the [CI] 8727 Å line is detected in regions where external photoevaporation is not expected, we examined VLT/X-Shooter spectra for young stars in low-mass SFRs. Although the [OI] 5577 Å and 6300 Å lines are well detected in all these targets, the total detection rate is ≪10% in the case of the [CI] 8727 Å line. This number increases substantially to a ∼40% detection rate inσ-Orionis, a region with higher UV radiation than in low-mass SFRs, but lower than in the ONC. The spatial location of the [CI] 8727 Å line emission and the lack of its detection in isolated disks in low-mass SFRs strongly suggest that this line is a tell-tale tracer of externally driven photoevaporative winds, which agrees with recent excitation models. 

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4. Incoherence and fibering of many free-by-free groups

   https://doi.org/10.5802/aif.3494  

   Kropholler, Robert P; Walsh, Genevieve S (January 2022, Annales de l'Institut Fourier)

   A group is called free-by-free if it is the semi-direct product of two finitely generated free groups. A group is coherent if any finitely generated subgroup is finitely presented, and incoherent otherwise. In this paper, the authors provide evidence towards the conjecture (due independently to the authors and Dani Wise) that every free-by-free group is incoherent. To do this, they give a homological condition which lets them conclude that the free-by-free group has a finite index subgroup which surjects onto ℤ with finitely generated kernel; standard arguments imply that this kernel cannot be finitely presented. As an important special case, they show that if the free-by-free group is hyperbolic and virtually special, then it is incoherent. 

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5. On hyperbolicity of free splitting and free factor complexes

   Kapovich, Ilya; Rafi, Kasra (August 2014, Groups, geometry, and dynamics)