Publications & Presentations

Refereed Publications

FEEDBACK is a SOFIA (Stratospheric Observatory for Infrared Astronomy) legacy program dedicated to study the interaction of massive stars with their environment. It performs a survey of 11 galactic high mass star-forming regions in the 158 μm (1.9 THz) line of [C II] and the 63 μm (4.7 THz) line of [O I]. We employ the 14 pixel Low Frequency Array and 7 pixel High Frequency Array upGREAT heterodyne instrument to spectrally resolve (0.24 MHz) these far-infrared fine structure lines. With a total observing time of 96h, we will cover ~6700 arcmin2 at 14.1" angular resolution for the [C II] line and 6.3" for the [O I] line. The observations started in spring 2019 (Cycle 7). Our aim is to understand the dynamics in regions dominated by different feedback processes from massive stars such as stellar winds, thermal expansion, and radiation pressure, and to quantify the mechanical energy injection and radiative heating efficiency. This is an important science topic because feedback of massive stars on their environment regulates the physical conditions and sets the emission characteristics in the interstellar medium (ISM), influences the star formation activity through molecular cloud dissolution and compression processes, and drives the evolution of the ISM in galaxies. The [C II] line provides the kinematics of the gas and is one of the dominant cooling lines of gas for low to moderate densities and UV fields. The [O I] line traces warm and high-density gas, excited in photodissociations regions with a strong UV field or by shocks. The source sample spans a broad range in stellar characteristics from single OB stars, to small groups of O stars, to rich young stellar clusters, to ministarburst complexes. It contains well-known targets such as Aquila, the Cygnus X region, M16, M17, NGC7538, NGC6334, Vela, and W43 as well as a selection of H II region bubbles, namely RCW49, RCW79, and RCW120. These [C II] maps, together with the less explored [O I] 63 μm line, provide an outstanding database for the community. They will be made publically available and will trigger further studies and follow-up observations.

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 (Stratospheric Observatory for Infrared Astronomy) 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 (Atacama Pathfinder Experiment) 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 time scales than hitherto thought (<0.15 million years), suggesting that positive feedback operates on short time periods.

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 data set 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 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 toward the observer. Comparing our observed data with the ancillary data at X-ray, infrared, submillimeter, 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 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 of ~0.27 Myr, making the Wolf-Rayet star WR 20a a likely candidate responsible for the shell's reacceleration.

Context. The interaction of expanding H II regions with their environmental clouds is one of the central questions driving the Stratospheric Observatory for Infrared Astronomy (SOFIA) legacy program FEEDBACK.
Aims: We want to understand the interaction of the prototypical NGC 7538 H II region with the neighboring molecular cloud hosting several active star-forming regions.
Methods: Using the SOFIA, we mapped an area of ~210 arcmin2 (~125 pc2) around NGC 7538 in the velocity-resolved ionized carbon fine-structure line [CII] at 1.9 THz (158 μm). Complementary observed atomic carbon [CI] at 492 GHz and high-J CO(8-7) data, as well as archival near- and far-infrared, cm continuum, CO(3-2), and HI data are folded into the analysis.
Results: The ionized carbon [CII] data reveal rich morphological and kinematic structures. While the overall morphology follows the general ionized gas that is also visible in the radio continuum emission, the channel maps show multiple bubble-like structures with sizes on the order of ~80-100 arcseconds (~1.0-1.28 pc). While at least one of them may be an individual feedback bubble driven by the main exciting sources of the NGC 7538 H II region (the O3 and O9 stars IRS6 and IRS5), the other bubble-like morphologies may also be due to the intrinsically porous structure of the H II region. An analysis of the expansion velocities around 10 km/s indicates that thermal expansion is not sufficient but that wind-driving from the central O-stars is required. The region exhibits a general velocity gradient across, but we also identify several individual velocity components. The most blue-shifted [CII] component has barely any molecular or atomic counterparts. At the interface to the molecular cloud, we find a typical photon-dominated region (PDR) with a bar-shape. Ionized C+, atomic C0 and molecular carbon CO show a layered structure in this PDR. The carbon in the PDR is dominated by its ionized C+ form with atomic C0 and molecular CO masses of ~0.45 0.1 MSUN and ~1.2 0.1 MSUN, respectively, compared to the ionized carbon C+ in the range of 3.6–9.7 MSUN. This bar-shaped PDR exhibits a velocity-gradient across, indicating motions along the line of sight toward the observer.
Conclusions: Even if it is shown to be dominated by two nearby exciting sources (IRS6 and IRS5), the NGC 7538 H II region exhibits a diverse set of substructures that interact with each other as well as with the adjacent cloud. Compared to other recent [CII] observations of H II regions (e.g., Orion Veil, RCW120, RCW49), the bubble-shape morphologies revealed in [CII] emission that are indicative of expanding shells are recurring structures of PDRs.

Additional integrated maps and individual spectra are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/659/A77

The [CII] 158 μm data are provided at the IRSA/IPAC Infrared science archive https://irsa.ipac.caltech.edu/Missions/sofia.html.


Aims: Revealing the 3D dynamics of H II region bubbles and their associated molecular clouds and H I envelopes is important for developing an understanding of the longstanding problem as to how stellar feedback affects the density structure and kinematics of the different phases of the interstellar medium.
Methods: We employed observations of the H II region RCW 120 in the [C II] 158 μm line, observed within the Stratospheric Observatory for Infrared Astronomy (SOFIA) legacy program FEEDBACK, and in the 12CO and 13CO(3-2) lines, obtained with the Atacama Pathfinder Experiment (APEX) to derive the physical properties of the gas in the photodissociation region (PDR) and in the molecular cloud. We used high angular resolution H I data from the Southern Galactic Plane Survey to quantify the physical properties of the cold atomic gas through H I self-absorption. The high spectral resolution of the heterodyne observations turns out to be essential in order to analyze the physical conditions, geometry, and overall structure of the sources. Two types of radiative transfer models were used to fit the observed [C II] and CO spectra. A line profile analysis with the 1D non-LTE radiative transfer code SimLine proves that the CO emission cannot stem from a spherically symmetric molecular cloud configuration. With a two-layer multicomponent model, we then quantified the amount of warm background and cold foreground gas. To fully exploit the spectral-spatial information in the CO spectra, a Gaussian mixture model was introduced that allows for grouping spectra into clusters with similar properties.
Results: The CO emission arises mostly from a limb-brightened, warm molecular ring, or more specifically a torus when extrapolated in 3D. There is a deficit of CO emission along the line-of-sight toward the center of the H II region which indicates that the H II region is associated with a flattened molecular cloud. Self-absorption in the CO line may hide signatures of infalling and expanding molecular gas. The [C II] emission arises from an expanding [C II] bubble and from the PDRs in the ring/torus. A significant part of [C II] emission is absorbed in a cool (~60-100 K), low-density (<500 cm-3) atomic foreground layer with a thickness of a few parsec.
Conclusions: We propose that the RCW 120 H II region formed in a flattened, filamentary, or sheet-like, molecular cloud and is now bursting out of its parental cloud. The compressed surrounding molecular layer formed a torus around the spherically expanding H II bubble. This scenario can possibly be generalized for other H II bubbles and would explain the observed "flat" structure of molecular clouds associated with H II bubbles. We suggest that the [C II] absorption observed in many star-forming regions is at least partly caused by low-density, cool, H I -envelopes surrounding the molecular clouds.

The 12CO and 13CO (3-2) data shown in Fig. 4 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/659/A36

The [C II] data are provided at the NASA/IPAC Infrared science archive at https://irsa.ipac.caltech.edu/Missions/sofia.html

Presentations

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.

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] 158μm square 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.

Numerical simulations predict that molecular cloud collisions are traced by the 158 micron far-infrared emission line of ionized carbon ([CII]) and low-J CO rotational lines. Observations so far concentrated on CO because of a lack of [CII] data, and possible examples of cloud-cloud collisions based on CO are announced in the literature. Here, we report on the detection of high-velocity [CII] emission at +15 km/s in the DR21 region in Cygnus X that stems from a cloud that impacts with the DR21 cloud at a rest velocity of -3 km/s. The data were taken in the context of the SOFIA legacy program FEEDBACK, and indicate a bridge of emission between these two clouds in spectra and position-velocity diagrams. This scenario differs from the one previously suggested, based on CO observations, that the DR21 cloud is in collision with a cloud at +9 km/s (the 'W75N cloud'). We advocate that the [CII] 158 micron line is the best tracer for cloud-cloud collisions and look for further detections of this phenomenon in other molecular cloud regions.

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, hν > 13.6 eV) and far-UV (FUV, 6 < hν < 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 the12CO 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.

Massive stars, often born in rich stellar clusters, weather away their natal environments with intense FUV and EUV radiation and powerful stellar winds, thereby pumping vast amounts of radiative and mechanical energy into what had been a cold, dense, and relatively serene molecular cloud complex. The strong winds shock nearby gas into an X-ray emitting plasma, the harsh EUV radiation ionizes an H II region, and an energetic bubble is blown into the cloud complex with the cluster at its center. Such reckless acts do not go unnoticed; at the edges of these wind-blown bubbles, intriguing elongated structures are carved out from the dense molecular gas, pointing back towards the responsible cluster like accusing fingers. These "pillars", with the iconic Pillars of Creation in M16 as their ambassador, are known to be indicators of massive star feedback on dense molecular gas, but the conditions and mechanisms of their formation are not well-studied.

In this poster, I present my ongoing study of the Pillars of Creation which I conduct as part of the FEEDBACK SOFIA C+ Legacy Project, whose goal is to better understand massive star feedback and its effect on the interstellar medium and to provide the community with valuable legacy data products. I also outline my thesis work, for which I propose to leverage the FEEDBACK Project's spatially and spectrally resolved maps of the [CII] 158 micron and [OI] 63 micron cooling lines, obtained with the 14-pixel upGREAT receiver on SOFIA, for 11 Galactic H II regions, in order to glean kinematic and energetic information about a diverse sample of pillars and other types of similarly carved structures. With this sample, I will be able to explore variables such as cluster mass/age/energetics, location in the Galaxy, or initial cloud conditions, and study how the pillars respond. My goal in establishing these connections is to "read" pillars as signposts to better understand the conditions by which they were formed. Until then, they remain pointed fingers, only able to tell us "who" irradiated their molecular cloud, but not how, or why.

The impact of stellar feedback on the evolution of the interstellar medium (ISM) plays a central role in galactic evolution as well as the star formation process at the molecular cloud and core scales. We present observations of the [CII] and [OI] fine-structure lines from the SOFIA FEEDBACK Legacy Survey towards the bipolar HII region RCW 36 in the Vela C molecular cloud, complemented with Chandra observations. RCW 36 is a prototypical bipolar HII region that consists of an OB cluster, with an estimated age of 1 Myr, at the center of a dense molecular ring and bipolar cavities. The SOFIA observations show [CII] and [OI] self-absorption in the dense molecular ring, and unveil blueshifted expanding shells in the cavities. Using this kinematic information, the expansion timescale for the ring and cavities can be estimated. The resulting expansion timescale for the cavities is significantly smaller than for the central ring, demonstrating that the expansion of RCW 36 proceeds in multiple stages. The observations further show that the stellar feedback locally breaks through the dense molecular ring and thus disrupts dense star forming gas. Lastly, the SOFIA observations unveil a powerful bipolar outflow in the cavities that is only detected in [CII]. The Chandra data demonstrates the presence of a hot plasma inside the ring and cavities that is excited by stellar winds from the OB cluster. This hot plasma also extends beyond the bipolar region, implying important leakage of the hot plasma from the HII region. This leakage provides an explanation for the relatively low expansion energetics of the ring and cavity. The combined data thus shows that stellar feedback drives inhomogeneous expansion that can simultaneously lead to the formation and disruption of dense star forming gas.

Massive stars, often born in rich stellar clusters, weather away their natal environments with intense FUV and EUV radiation and powerful stellar winds, thereby pumping vast amounts of radiative and mechanical energy into what had been a cold, dense, and relatively serene molecular cloud complex. The strong winds shock nearby gas into an X-ray emitting plasma, the harsh EUV radiation ionizes an H II region, and an energetic bubble is blown into the cloud complex with the cluster at its center. Such reckless acts do not go unnoticed; at the edges of these wind-blown bubbles, intriguing elongated structures are carved out from the dense molecular gas, pointing back towards the responsible cluster like accusing fingers. These "pillars", with the iconic Pillars of Creation in M16 as their ambassador, are known to be indicators of massive star feedback on dense molecular gas, but the conditions and mechanisms of their formation are still shrouded in darkness.

In this talk, I present my ongoing study of the Pillars of Creation which I conduct as part of the FEEDBACK SOFIA C+ Legacy Project, whose goal is to better understand massive star feedback and its effect on the interstellar medium and to provide the community with valuable legacy data products. With these stunning new velocity-resolved SOFIA upGREAT observations of the [CII] line, which traces directly the kinematics of the gas and which acts as a major coolant of the photodissociation regions (PDRs) separating the ionized and molecular gas, coupled with observations of molecular gas tracers HCO+ and CO(1-0), we reveal the complex spatial and dynamic relationship between the warmer, outer PDR layer and the denser molecular layers within the pillars in M16. I will discuss our analysis of these observations and their implications for both the history and the fate of the pillars.

Schneider, N. & Tielens, A. 2022

RCW 49, one of the most luminous star-forming regions in our Galaxy, is illuminated by a rich stellar cluster called the Westerlund 2 (Wd2). This cluster comprises many OB stars and a Wolf-Rayet star, whose radiative and mechanical energy has given rise to structures like shell, pillar, ridge and dense clouds. Using the SOFIA telescope we aim to quantify the radiative and mechanical feedback in RCW 49.

We started by disentangling the shell of RCW 49 from a complex set of gas components using the spatially and spectrally resolved SOFIA C+ observations towards RCW 49. We find that the shell of ~ 2.5 x 104 solar masses and radius 6 pc, is moving towards us with a speed of 13 km/s. Comparing the energetics and timescales, we find that the shell is driven by the stellar winds (mechanical feedback) of the OB stars and the Wolf-Rayet star of Wd2. This work is described in Tiwari et al., 2021.

Following up on the other interesting regions in RCW 49, we performed a detailed PDR analysis employing the newly updated models of the PDR ToolBox (Pound & Wolfire 2008) to quantify the physical conditions (FUV flux, densities, temperatures and pressure) in different regions of RCW 49. Determination of these parameters allowed us to explain the morphology of these regions and their observed star formation efficiency. We used the CII, OI and CO dataset for our analysis. We find that the regions closest to the Wd2 cluster have the highest FUV flux, while the regions with highest densities have been involved in the formation of the Wd2 cluster itself and are sites of ongoing star formation. This work is described in Tiwari et al., to be submitted.

Through my presentation, I will discuss these results and their implications to our understanding of stellar feedback.

The PhotoDissociation Region Toolbox is a science-enabling tool for the community, designed to help astronomers determine the physical parameters of photodissociation regions from observations. Typical observations of both Galactic and extragalactic PDRs come from ground- and space-based millimeter, submillimeter, and far-infrared telescopes such as SOFIA, JWST, ALMA, Spitzer, and Herschel. Given a set of observations of spectral line or continuum intensities, PDR Toolbox can compute best-fit FUV incident intensity and cloud density based on models of PDR emission. In addition, there are tools for H2 excitation fitting of gas temperature, column density, and ortho-to-para ratio, and for ionized gas line (e.g., [Fe II]) analysis. The PDR Toolbox is an open-source Python package (https://pypi.org/project/pdrtpy/) installable with pip and downloadable from github (https://github.com/mpound/pdrtpy). It includes example Jupyter notebook tutorials and extensive documentation for both users and developers (https://pdrtpy.readthedocs.io/). Our PDR models solve for the thermal equilibrium gas temperature and steady-state abundances in a layer of gas of density n, illuminated by a far-ultraviolet radiation field G0. They are based on those of Kaufman et al. 2006, and Wolfire et al. 2010, with many recent updates to the chemical reaction rates and cooling rates. The new models feature carbon isotope species and line intensities, grain surface chemistry, and a power-law dependence of cosmic-ray ionization rates. We also have recently added the KOSMA-tau PDR models, allowing users to compare analyses with models from different PDR codes.