Special AAS session 'Assessing the Impact of Stellar Feedback'

Tuesday, January 12, 2021 - 1:10pm to 2:40pm PST
Event Type: 

The SOFIA Science Center is organizing the special session 'Assessing the Impact of Stellar Feedback' at the 237th AAS meeting (online). The oral session will be held on Tuesday January 12, from 4:10 pm to 5:40 pm (Eastern Time), with talks by the following invited speakers:

  • Xander Tielens (U. Leiden) - PI of SOFIA Legacy program FEEDBACK
  • Laura Lopez (OSU) 
  • Mélanie Chevance (U. Heidelberg)
  • Hector Arce (Yale) 
  • Susanna Widicus Weaver (U. Wisconsin-Madison)
  • Crystal Martin (UCSB)

Session description:

Observational evidence shows that feedback - stellar winds, heating, and radiation - from protostars, low-mass and high-mass stars is a major contributor to the dynamics, morphology and chemistry of the interstellar medium. Stellar feedback leads to the creation and expansion of HII regions and drives shock chemistry throughout the ISM. At a larger scale, galactic-scale winds, fountains and chimneys can also have a profound impact on galactic structure and evolution. Understanding why and how stellar feedback affects its environment is one of the key questions in modern astrophysics. In particular, one standing problem is the relative importance of radiative as opposed to mechanical feedback. Another outstanding problem is how feedback varies with different ISM conditions (e.g., metallicity and density). This session will include recent theoretical and observational results which contribute to the understanding of the role of stellar feedback across different scales, from protostellar clouds to galaxies. We will place a particular focus on the exploration of the link between feedback and star formation, the shaping of ISM structures, and the chemistry in shocks regions and ionization fronts. Recent observations (IR surveys and high-resolution mm-wave images) of the kinematics and temperature gradients in stellar environments, displaying in detail the interaction of various types of stars and their surrounding ISM, will be presented.

L. Lopez: Assessing the Dynamical Role of Stellar Feedback using Multiwavelength Observations 

 Stellar feedback is dynamically important on the small scales of star clusters up to the large scales of whole galaxies. Despite its profound influence, a major challenge in feedback studies is setting observational constraints on the relative role of the many stellar feedback modes (such as direct radiation, dust-processed radiation, photoionization heating, protostellar outflows, stellar winds, and supernovae). Fortunately, high quality mutltiwavelength survey data are now available for the Milky Way and other Local Group galaxies, facilitating observational assessment of stellar feedback mechanisms in a variety of sources. I will present results from these multiwavelength programs, including new results showing how the pressures associated with different feedback modes vary between young, compact HII regions and more evolved HII regions. In particular, the dust-processed radiation pressure dominates in 95% of the younger sources, whereas the photoionized gas pressure is the dominant term in evolved sources. I will discuss the implications regarding the dynamical evolution of the HII regions and the lessons for feedback prescriptions in star and galaxy formation simulations. Finally, I will highlight the complementary constraints that can be obtained from integral field spectroscopy.

C. Martin: galactic winds and supernovae 

Gaseous outflows appear to be a ubiquitous property of star-forming galaxies across cosmic time. Cosmological hydrodynamical simulations require higher outflow efficiencies in lower mass galaxies in order to explain the observed distribution of galaxy stellar masses and gas-phase metallicities. Yet typical dwarf galaxies have very low specific star formation rates today, raising the question "Can we observe strong feedback in low mass galaxies?" Studying feedback in dwarf galaxies with extreme emission-line spectra provides some answers and offers insight into the interplay between radiative and mechanical feedback in young galaxies. These galaxies have an extremely dense star cluster, and I will present new integral field spectroscopy that reveals how the young, low-metallicity stars change the gas kinematics and excitation of the surrounding interstellar medium. The results suggest that the EUV spectrum is harder than that produced by typical star clusters, the escape of Lyman continuum radiation is anisotropic, and fast outflows may be impacting the densest interstellar gas.

M. Chevance: The lifecycle of molecular clouds in nearby galaxies 

The cycling of matter in galaxies between molecular clouds, stars and feedback is a major driver of galaxy evolution. However, it remains a major challenge to derive a theory of how galaxies turn their gas into stars and how stellar feedback affects the subsequent star formation on the cloud scale, as a function of the galactic environment. Star formation in galaxies is expected to be highly dependent on the galactic structure and dynamics, because it results from a competition between mechanisms such as gravitational collapse, shear, spiral arm passages, cloud-cloud collisions, and feedback processes such as supernovae, stellar winds, photoionization and radiation pressure. A statistically representative sample of galaxies is therefore needed to probe the wide range of conditions under which stars form. I will present the first systematic characterisation of the evolutionary timeline of the giant molecular cloud (GMC) lifecycle, star-formation and feedback in the PHANGS sample of star-forming disc galaxies. I will show that GMC are short-lived (10-30 Myr) and are dispersed after about one dynamical timescale by stellar feedback, between 1 and 5 Myr after massive stars emerge. Although the coupling efficiency of early feedback mechanisms such as radiation and stellar winds is limited to a few tens of percent, it is sufficient to disperse the parent molecular cloud prior to supernova explosions. This limits the integrated star formation efficiencies of GMCs to 2 to 10 per cent. These findings reveal that star formation in galaxies is fast and inefficient, and is governed by cloud-scale, environmentally-dependent, dynamical processes. These measurements constitute a fundamental test for numerical sub-grid recipes of star-formation and feedback in simulations of galaxy formation and evolution.

H. Arce: Outflow feedback from low-mass protostars   

Jets and winds from young stars are among the most prominent signposts of star formation. Protostellar winds originate within a few au (or less) of the forming star and may reach linear sizes of a few parsecs. As they travel through the circumstellar envelope, the surrounding core and host cloud, they push and accelerate the ambient gas, thereby injecting energy and momentum into their surroundings at various size and density scales. This has considerable impact on the dynamics, distribution, and chemical composition of the gas in the star-forming environment, and also helps decrease the star formation efficiency of the region. I will show recent millimeter interferometer (ALMA and SMA) observations of low- and intermediate-mass protostars at different evolutionary stages that reveal how outflows interact with their circumstellar environment (within a few 100 to a few 1000 au of the forming star), and discuss how outflows influence the star formation process.

X. Tielens: The C+ Universe

The interaction of massive stars with their environments regulates the evolution of galaxies. Mechanical and radiative energy input by massive stars stir up and heat the gas and control cloud and intercloud phases of the interstellar medium. Stellar feedback also governs the star formation efficiency of molecular clouds. On the one hand, stellar feedback can lead to ashredding of the nascent molecular cloud within a few cloud free-fall times thereby halting star formation. On the other hand, massive stars can also provide positive feedback to star formation as gravity can more easily overwhelm cloud-supporting forces in swept-up compressed shells. Moreover, stellar feedback is an important source of turbulence in the interstellar medium. The combination of sensitive THz heterodyne receiver arrays with a nimble telescope on SOFIA enables large scale, [CII] 158µm surveys of regions of massive star formation. This line is the main cooling line of neutral gas in the interstellar medium and therefore a key diagnostic of interstellar gas energy balance. In addition, the high spectral resolution inherent to heterodyne techniques allows a detailed study of the kinematics of photodissociation regions, which separate ionized from molecular gas. I will present results of the [CII] 158µmsquare degree Orion Survey and the SOFIA/upGREAT FEEDBACK Legacy Program, their analysis and implications for the interaction of massive stars with their environment and their role in the evolution of galaxies.

S. Widicus Weaver: UV Photodissociation and Thermal Processing in Interstellar Ice

It has been shown through both laboratory and observational studies that direct and cosmic-ray induced UV photodissociation drives a complex network of chemistry in interstellar ices. Astrochemical models have demonstrated that gradual heating of these UV-processed ices during star formation can lead to a wide variety of complex organic molecules. Additional modeling and observational studies have shown that these molecules are likely incorporated into protoplanetary disks and participate in the chemical processes leading to planet formation. Stellar feedback mechanisms are therefore directly involved in the chemical evolution of chemistry in the interstellar medium and could potentially serve as molecular starting points for prebiotic chemistry in the universe. Our observations of the chemistry of star-forming regions have shown that there is wide variation in the chemical compositions of hot cores, and that methanol photodissociation on icy grains may be the key process feeding the formation of larger prebiotic molecules. To test possible chemical routes in interstellar ices, we have built a novel laboratory experiment that couples the traditional tools of ice studies -- FTIR spectroscopy and mass spectrometry -- with the structure specificity of rotational spectroscopy. Such measurements can provide the "ground truth" to guide observations of star- and planet-forming zones. In this talk we will present both the observational and laboratory studies and discuss these results in the context of stellar feedback mechanisms.

J. Jackson: Expansion of an H II Region Bubble in the Nessie Nebula Triggers Star Formation in the IRDC Filament

Using the GREAT instrument aboard the airborne SOFIA observatory, we have imaged [C II] 157.74 micron and [O I] 63.18 micron line emission from a bright photodissociation region (PDR) associated with an ionized ``bubble'' located in the Nessie Nebula, a filamentary infrared dark cloud. A comparison of SOFIA data with ATCA radio data shows that the bubble has a classic PDR structure, with a uniform progression from ionized gas (traced by radio continuum), to photodissociated gas (traced by {C II] and [O I]), and on to molecular gas (traced by NH$_3$) from the interior to the exterior of the bubble. The bubble is expanding into the Nessie Infrared Dark Cloud filament. At the location of the interaction, a luminous YSO has formed, indicating that feedback from the expanding bubble has triggered the formation of the YSO. Successive triggering of star-formation along the filament as the bubble expands may explain the large cluster of stars toward the western edge of the bubble and its unusual tear-drop shape.

M. Luisi: Stellar feedback and triggered star formation in the prototypical bubble RCW 120

Radiative and mechanical feedback of massive stars regulates star formation and galaxy evolution. Positive feedback triggers the creation of new stars by collecting dense shells of gas, while negative feedback disrupts star formation by shredding molecular clouds. Although key to understanding star formation, their relative importance is unknown. Here we report velocity-resolved observations from the SOFIA legacy program FEEDBACK of the massive star-forming region RCW 120 in the [CII] 1.9 THz fine-structure line, revealing a gas shell expanding at 15 km/s. Complementary APEX CO J=3-2 345 GHz observations exhibit a ring-structure of molecular gas, fragmented into clumps that are actively forming stars. Our observations demonstrate that triggered star formation can occur on much shorter timescales than hitherto thought (<0.15 Myr), suggesting that positive feedback operates on short time periods.

M. Tiwari: SOFIA FEEDBACK survey: Exploring the kinetics of the stellar wind driven shell of RCW 49

One of the most important problems in modern astrophysics is to understand the role of massive stars in driving various physical and chemical processes in the Interstellar Medium (ISM). Massive stars inject an immense amount of mechanical and radiative energy into their immediate vicinity. Stellar winds are responsible for the mechanical energy input, which can push the gas into shell-like structures (as in the Rosette Nebula, Waering et al. 2018 and in the Orion Nebula, Pabst et al. 2019). The radiative energy input comes from the heating of gas through stellar extreme-ultraviolet (EUV, hv > 13.6 eV) and far-UV (FUV, 6 < hv < 13.6 eV) photons that can ionize atoms, dissociate molecules and heat the gas giving rise to H II regions and photodissociation regions (PDRs). These stellar feedback mechanisms power the expansion of H II regions and shock fronts causing morphological features that appear as shells or bubbles in the ISM. We unveil the stellar wind driven shell of the luminous massive star-forming region of RCW 49 using SOFIA FEEDBACK observations of the [C II] 158 μm line. The complementary dataset of the 12CO and 13CO J = 3 -2 transitions is observed by the APEX telescope and probes the dense gas toward RCW 49. Using the high spatial and spectral resolution provided by the SOFIA and APEX telescopes, we disentangle the shell from a complex set of individual components of gas centered around RCW 49. We find that the shell of radius ~ 6 pc is expanding at a velocity of 13 km s-1 toward the observer. Comparing our observed data with the ancillary data in X-Ray, infrared, sub-millimeter and radio wavelengths, we investigate the morphology of the region. The shell has a well defined eastern arc, while the western side is blown open and is venting plasma further into the west. Though the stellar cluster, which is ~ 2 Myr old gave rise to the shell, it only gained momentum relatively recently as we calculate the shell's expansion lifetime ~ 0.27 Myr, making the Wolf-Rayet star WR20a a likely candidate responsible for the shell's re-acceleration.

R. Klein: CO-dark Molecular Gas and Star Formation across the Nearby Spiral Galaxy NGC 6946

We present SOFIA/FIFI-LS observations of the [CII] 158 μm cooling line across the nearby spiral galaxy NGC 6946. We combine these with UV, IR, CO and HI data to compare [CII] emission to dust properties, SFR, H2 and HI at 560 pc scales via stacking by environment (spiral arms, interarm and center), radial profiles, and individual, beam-sized measurements. We attribute 73% of the [CII] luminosity to arms, 19% and 8% to center and interarm region, respectively. [CII]/TIR, [CII]/CO and [CII]/PAH radial profiles are largely constant, but rise at large radii (&gt 8kpc) and drop in the center ("[CII]-deficit”). This increase at large radii and the observed decline with the 70/100 μm dust color are likely driven by radiation field hardness. We find a near proportional [CII]-SFR scaling relation for beam-sized regions, though the exact scaling depends on methodology. [CII] also becomes increasingly luminous relative to CO at low SFR (interarm or large radii), likely indicating more efficient photodissociation of CO and emphasizing the importance of [CII] as an H2 and SFR tracer in such regimes.Based on the observed [CII] and CO radial profiles and different models, we find αCOto increase with radius, in line with the observed metallicity gradient. The low αCO and low [CII]/CO ratios imply little CO-dark gas across NGC 6946, in contrast to estimates in the Milky Way.

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