The VLT MUSE NFM view of outflows and externally photoevaporating discs near the orion bar★

Abstract

We present VLT/MUSE Narrow Field Mode (NFM) observations of a pair of disc-bearing young stellar objects towards the Orion Bar: 203-504 and 203-506. Both of these discs are subject to external photoevaporation, where winds are launched from their outer regions due to environmental irradiation. Intriguingly, despite having projected separation from one another of only 1.65″(660 au at 400 pc), 203-504 has a classic teardrop shaped “proplyd” morphology pointing towards θ2Ori A (indicating irradiation by the EUV of that star, rather than $\rm \theta ^1$ Ori C) but 203-506 has no ionisation front, indicating it is not irradiated by stellar EUV at all. However, 203-506 does show [C i] 8727 Å and [O i] 6300 Å in emission, indicating irradiation by stellar FUV. This explicitly demonstrates the importance of FUV irradiation in driving mass loss from discs. We conclude that shielding of 203-506 from EUV is most likely due to its position on the observers side of an ionized layer lying in the foreground of the Huygens Region. We demonstrate that the outflow HH 519, previously thought to be emanating from 203-504 is actually an irradiated cloud edge and identify a new compact outflow from that object approximately along our line of sight with a velocity ∼130 km s−1.

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

“…Recent ALMA surveys in Orion have also found indirect evidence of external photoevaporation in intermediate-UV environments, revealing that disks close to B-and A-type stars are on average ∼2-4 times less massive than the disks farther away from B-and A-type stars (van Terwisga et al 2019;van Terwisga & Hacar 2023). Despite these findings, identifying direct signatures of external photoevaporation has proven challenging outside of the >10 4 G 0 regions of the ONC and other O-star-hosting clusters, where many disks are exposed not only to intense FUV irradiation but also to intense EUV irradiation that can externally ionize FUV-driven photoevaporative winds to the point where large samples of ionized winds become bright and easy to see with the VLA (e.g., Churchwell et al 1987;Garay et al 1987;Felli et al 1993aFelli et al , 1993bForbrich et al 2016;Sheehan et al 2016), HST (e.g., O'dell & Wen 1994;Bally et al 1998Bally et al , 2000Ricci et al 2008;Smith et al 2010;Fang et al 2012;Haworth et al 2021), VLT/MUSE (e.g., Haworth et al 2023;Aru et al 2024), JWST (Berné et al 2022;Habart et al 2023;McCaughrean & Pearson 2023), and ALMA (Ballering et al 2023).…”

“…Given the potential prevalence of internally illuminated disk winds in NGC 1977, it is worth pointing out that externally driven but internally illuminated disk winds can have similar morphologies as externally driven and externally illuminated winds. The d203-506 disk in the Orion Bar, for example, is irradiated by a ∼10 4 G 0 FUV field but shielded from the external EUV field (e.g., Champion et al 2017), and recent infrared-and submillimeter-wavelength imaging of this disk has revealed that the photoevaporative wind surrounding d203-506 has the same cometary shape as the externally illuminated ONC proplyds (e.g., Haworth et al 2023;Berné et al 2024). This new imaging demonstrates that anisotropic FUV radiation fields play a dominant role in sculpting cometary wind morphologies when an external EUV radiation field is not present.…”

“…The presence and morphologies of proplyds are a direct result of the external photoevaporation process, in which strong far-UV (FUV) and extreme-UV (EUV) radiation from massive stars drives material off the disks in the form of ionized photoevaporative winds (e.g., Johnstone et al 1998;Störzer & Hollenbach 1999). The high surface brightnesses of the ONC proplyds have enabled large samples of photoevaporating disks in the ONC to be detected and characterized with a range of facilities, including NSF's Karl G. Jansky Very Large Array (VLA; e.g., Churchwell et al 1987;Garay et al 1987;Zapata et al 2004a;Forbrich et al 2016;Sheehan et al 2016), the Hubble Space Telescope (HST; e.g., O'dell & Wen 1994;Bally et al 1998Bally et al , 2000Ricci et al 2008), the Atacama Large Millimeter/submillimeter Array (ALMA; e.g., Eisner et al 2018;Ballering et al 2023), the Very Large Telescope (VLT; e.g., Haworth et al 2023;Aru et al 2024), and, more recently, JWST (e.g., Berné et al 2022Berné et al , 2024Habart et al 2023;McCaughrean & Pearson 2023).…”