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1. A Massive Star Is Born: How Feedback from Stellar Winds, Radiation Pressure, and Collimated Outflows Limits Accretion onto Massive Stars

   https://doi.org/10.3847/1538-4357/ac9f3d  

   Rosen, Anna L. (December 2022, The Astrophysical Journal)

   Abstract Massive protostars attain high luminosities as they are actively accreting and the radiation pressure exerted on the gas in the star’s atmosphere may launch isotropic high-velocity winds. These winds will collide with the surrounding gas producing shock-heated ( T ∼ 10 7 K) tenuous gas that adiabatically expands and pushes on the dense gas that may otherwise be accreted. We present a suite of 3D radiation-magnetohydrodynamic simulations of the collapse of massive prestellar cores and include radiative feedback from the stellar and dust-reprocessed radiation fields, collimated outflows, and, for the first time, isotropic stellar winds to model how these processes affect the formation of massive stars. We find that winds are initially launched when the massive protostar is still accreting and its wind properties evolve as the protostar contracts to the main sequence. Wind feedback drives asymmetric adiabatic wind bubbles that have a bipolar morphology because the dense circumstellar material pinches the expansion of the hot shock-heated gas. We term this the “wind tunnel effect.” If the core is magnetized, wind feedback is less efficient at driving adiabatic wind bubbles initially because magnetic tension delays their growth. We find that wind feedback eventually quenches accretion onto ∼30 M ⊙ protostars that form from the collapse of the isolated cores simulated here. Hence, our results suggest that ≳30 M ⊙ stars likely require larger-scale dynamical inflows from their host cloud to overcome wind feedback. Additionally, we discuss the implications of observing adiabatic wind bubbles with Chandra while the massive protostars are still highly embedded. 

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2. Panchromatic HST/WFC3 Imaging Studies of Young, Rapidly Evolving Planetary Nebulae. I. NGC 6302

   https://doi.org/10.3847/1538-4357/ac51cd  

   Kastner, Joel H.; Moraga Baez, Paula; Balick, Bruce; Bublitz, Jesse; Montez, Rodolfo; Frank, Adam; Blackman, Eric (March 2022, The Astrophysical Journal)

   Abstract We present the results of a comprehensive, near-UV-to-near-IR Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) imaging study of the young planetary nebula (PN) NGC 6302, the archetype of the class of extreme bilobed, pinched-waist PNe that are rich in dust and molecular gas. The new WFC3 emission-line image suite clearly defines the dusty toroidal equatorial structure that bisects NGC 6302's polar lobes, and the fine structures (clumps, knots, and filaments) within the lobes. The most striking aspect of the new WFC3 image suite is the bright, S-shaped 1.64 μ m [Fe ii ] emission that traces the southern interior of the east lobe rim and the northern interior of the west lobe rim, in point-symmetric fashion. We interpret this [Fe ii ] emitting region as a zone of shocks caused by ongoing, fast (∼100 km s −1 ), collimated, off-axis winds from NGC 6302's central star(s). The [Fe ii ] emission and a zone of dusty, N- and S-rich clumps near the nebular symmetry axis form wedge-shaped structures on opposite sides of the core, with boundaries marked by sharp azimuthal ionization gradients. Comparison of our new images with earlier HST/WFC3 imaging reveals that the object previously identified as NGC 6302's central star is a foreground field star. Shell-like inner lobe features may instead pinpoint the obscured central star’s actual position within the nebula’s dusty central torus. The juxtaposition of structures revealed in this HST/WFC3 imaging study of NGC 6302 presents a daunting challenge for models of the origin and evolution of bipolar PNe. 

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3. Shocked jets in CCSNe can power the zoo of fast blue optical transients

   https://doi.org/10.1093/mnras/stac910  

   Gottlieb, Ore; Tchekhovskoy, Alexander; Margutti, Raffaella (April 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Evidence is mounting that recent multiwavelength detections of fast blue optical transients (FBOTs) in star-forming galaxies comprise a new class of transients, whose origin is yet to be understood. We show that hydrogen-rich collapsing stars that launch relativistic jets near the central engine can naturally explain the entire set of FBOT observables. The jet–star interaction forms a mildly relativistic shocked jet (inner cocoon) component, which powers cooling emission that dominates the high velocity optical signal during the first few weeks, with a typical energy of ∼1050–1051 erg. During this time, the cocoon radial energy distribution implies that the optical light curve exhibits a fast decay of $$L \,\, \buildrel\propto \over \sim \,\,t^{-2.4}$$. After a few weeks, when the velocity of the emitting shell is ∼0.01 c, the cocoon becomes transparent, and the cooling envelope governs the emission. The interaction between the cocoon and the dense circumstellar winds generates synchrotron self-absorbed emission in the radio bands, featuring a steady rise on a month time-scale. After a few months the relativistic outflow decelerates, enters the observer’s line of sight, and powers the peak of the radio light curve, which rapidly decays thereafter. The jet (and the inner cocoon) becomes optically thin to X-rays ∼day after the collapse, allowing X-ray photons to diffuse from the central engine that launched the jet to the observer. Cocoon cooling emission is expected at higher volumetric rates than gamma-ray bursts (GRBs) by a factor of a few, similar to FBOTs. We rule out uncollimated outflows, however, both GRB jets and failed collimated jets are compatible with all observables. 

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4. Expanding Bipolar X-Ray Structure After the 2006 Eruption of RS Oph

   https://doi.org/10.3847/1538-4357/ac4583  

   Montez, R.; Luna, G. J.; Mukai, K.; Sokoloski, J. L.; Kastner, J. H. (February 2022, The Astrophysical Journal)

   Abstract We report on the detection and analysis of extended X-ray emission by the Chandra X-ray Observatory stemming from the 2006 eruption of the recurrent nova RS Oph. The extended emission was detected 1254 and 1927 days after the start of the 2006 eruption and is consistent with a bipolar flow oriented in the East–West direction of the sky with opening angles of approximately 70°. The length of both lobes appeared to expand from 1.″3 in 2009 to 2.″0 in 2011, suggesting a projected expansion rate of 1.1 ± 0.1 mas day −1 and an expansion velocity of 4600 km s −1 ( D /2.4 kpc) in the plane of the sky. This expansion rate is consistent with previous estimates from optical and radio observations of material in a similar orientation. The X-ray emission does not show any evidence of cooling between 2009 and 2011, consistent with free expansion of the material. This discovery suggests that some mechanism collimates ejecta away from the equatorial plane, and that after that material passes through the red giant wind, it expands freely into the cavity left by the 1985 eruption. We expect similar structures to arise from the latest eruption and to expand into the cavity shaped by the 2006 eruption. 

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5. Cooling and instabilities in colliding flows

   https://doi.org/10.1093/mnras/stab2577  

   Markwick, R N; Frank, A; Carroll-Nellenback, J; Liu, B; Blackman, E G; Lebedev, S V; Hartigan, P M (October 2021, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Collisional self-interactions occurring in protostellar jets give rise to strong shocks, the structure of which can be affected by radiative cooling within the flow. To study such colliding flows, we use the AstroBEAR AMR code to conduct hydrodynamic simulations in both one and three dimensions with a power-law cooling function. The characteristic length and time-scales for cooling are temperature dependent and thus may vary as shocked gas cools. When the cooling length decreases sufficiently and rapidly, the system becomes unstable to the radiative shock instability, which produces oscillations in the position of the shock front; these oscillations can be seen in both the one- and three-dimensional cases. Our simulations show no evidence of the density clumping characteristic of a thermal instability, even when the cooling function meets the expected criteria. In the three-dimensional case, the nonlinear thin shell instability (NTSI) is found to dominate when the cooling length is sufficiently small. When the flows are subjected to the radiative shock instability, oscillations in the size of the cooling region allow NTSI to occur at larger cooling lengths, though larger cooling lengths delay the onset of NTSI by increasing the oscillation period. 

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