Observations with the
Herschel
Space Telescope have established that most star forming gas is organised in filaments, a finding that is supported by numerical simulations of the supersonic ...interstellar medium (ISM) where dense filamentary structures are ubiquitous. We aim to understand the formation of these dense structures by performing observations covering the
12
CO(4→3),
12
CO(3→2), and various CO(2–1) isotopologue lines of the Musca filament, using the APEX telescope. The observed CO intensities and line ratios cannot be explained by PDR (photodissociation region) emission because of the low ambient far-UV field that is strongly constrained by the non-detections of the C
II
line at 158
μ
m and the O
I
line at 63
μ
m, observed with the upGREAT receiver on SOFIA, as well as a weak C
I
609
μ
m line detected with APEX. We propose that the observations are consistent with a scenario in which shock excitation gives rise to warm and dense gas close to the highest column density regions in the Musca filament. Using shock models, we find that the CO observations can be consistent with excitation by J-type low-velocity shocks. A qualitative comparison of the observed CO spectra with synthetic observations of dynamic filament formation simulations shows a good agreement with the signature of a filament accretion shock that forms a cold and dense filament from a converging flow. The Musca filament is thus found to be dense molecular post-shock gas. Filament accretion shocks that dissipate the supersonic kinetic energy of converging flows in the ISM may thus play a prominent role in the evolution of cold and dense filamentary structures.
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 m (1.9 THz) line of C ii and the 63 m (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 m line, provide an outstanding database for the community. They will be made publically available and will trigger further studies and follow-up observations.
Abstract
We present C
ii
158
μ
m and O
i
63
μ
m observations of the bipolar H
ii
region RCW 36 in the Vela C molecular cloud, obtained within the SOFIA legacy project FEEDBACK, which is ...complemented with APEX
12/13
CO (3–2) and Chandra X-ray (0.5–7 keV) data. This shows that the molecular ring, forming the waist of the bipolar nebula, expands with a velocity of 1–1.9 km s
−1
. We also observe an increased line width in the ring, indicating that turbulence is driven by energy injection from the stellar feedback. The bipolar cavity hosts blueshifted expanding C
ii
shells at 5.2 ± 0.5 ± 0.5 km s
−1
(statistical and systematic uncertainty), which indicates that expansion out of the dense gas happens nonuniformly and that the observed bipolar phase might be relatively short (∼0.2 Myr). The X-ray observations show diffuse emission that traces a hot plasma, created by stellar winds, in and around RCW 36. At least 50% of the stellar wind energy is missing in RCW 36. This is likely due to leakage that is clearing even larger cavities around the bipolar RCW 36 region. Lastly, the cavities host high-velocity wings in C
ii
, which indicates relatively high mass ejection rates (∼5 × 10
−4
M
⊙
yr
−1
). This could be driven by stellar winds and/or radiation but remains difficult to constrain. This local mass ejection, which can remove all mass within 1 pc of RCW 36 in 1–2 Myr, and the large-scale clearing of ambient gas in the Vela C cloud indicate that stellar feedback plays a significant role in suppressing the star formation efficiency.
We present a new multi-pixel high resolution (R ≳ 107) spectrometer for the Stratospheric Observatory for Far-Infrared Astronomy (SOFIA). The receiver uses 2 × 7-pixel subarrays in orthogonal ...polarization, each in an hexagonal array around a central pixel. We present the first results for this new instrument after commissioning campaigns in May and December 2015 and after science observations performed in May 2016. The receiver is designed to ultimately cover the full 1.8−2.5 THz frequency range but in its first implementation, the observing range was limited to observations of the CII line at 1.9 THz in 2015 and extended to 1.83−2.07 THz in 2016. The instrument sensitivities are state-of-the-art and the first scientific observations performed shortly after the commissioning confirm that the time efficiency for large scale imaging is improved by more than an order of magnitude as compared to single pixel receivers. An example of large scale mapping around the Horsehead Nebula is presented here illustrating this improvement. The array has been added to SOFIA’s instrument suite already for ongoing observing cycle 4.
Abstract
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
μ
m line. The complementary data set of the
...12
CO and
13
CO
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
−1
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.
Abstract
We report SOFIA/GREAT observations of high-
J
CO lines and C
ii
observations of the super star cluster candidate H72.97-69.39 in the Large Magellanic Cloud (LMC), which is in its very early ...formation stage. We use our observations to determine if shocks are heating the gas or if photon-dominated regions (PDRs) are being heated by local far-UV radiation. We use a PDR model and a shock model to determine whether the CO and C
ii
lines arise from PDRs or shocks. We can reproduce the observed high-
J
CO and C
ii
emission with a clumpy PDR model with the following properties: a density of 10
4.7
cm
−3
, a mass of 10
4
M
⊙
, and UV radiation of 10
3.5
in units of Draine field. Comparison with the ALMA beam-filling factor suggests a higher density within the uncertainty of the fit. We find the lower-limit C
ii
/total infrared (TIR) ratio (
ϵ
) traced by C
ii
/TIR to be 0.026%, lower than other known young star-forming regions in the LMC. Our shock models may explain the CO (16−15) and CO (11−10) emission lines with shock velocity of 8–11 km s
−1
, pre-shock density of 10
4
–10
5
cm
−3
, and
G
UV
= 0 in units of Draine field. However, the C
ii
line emission cannot be explained by a shock model, thus it is originating in a different gas component. Observations of O
i
63
μ
m predicted to be 1.1 × 10
−13
W m
−2
by PDR models and 7.8 × 10
−15
W m
−2
by shock models will help distinguish between the PDR and shock scenarios.
FEEDBACK from the NGC 7538 H II region Beuther, H.; Schneider, N.; Simon, R. ...
Astronomy and astrophysics (Berlin),
3/2022, Letnik:
659
Journal Article
Recenzirano
Odprti dostop
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′
2
(~125 pc
2
) 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″ (~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
−1
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 C
0
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 C
0
and molecular CO masses of ~0.45 ± 0.1
M
⊙
and ~1.2 ± 0.1
M
⊙
, respectively, compared to the ionized carbon C
+
in the range of 3.6−9.7
M
⊙
. 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.
Power spectra of deprojected images of late-type galaxies in gas or dust emission are very useful diagnostics of the dynamics and stability of their interstellar medium. Previous studies have shown ...that the power spectra can be approximated as two power laws, a shallow one on large scales (larger than 500 pc) and a steeper one on small scales, with the break between the two corresponding to the line-of-sight thickness of the galaxy disk. The break separates the 3D behavior of the interstellar medium on small scales, controlled by star formation and feedback, from the 2D behavior on large scales, driven by density waves in the disk. The break between these two regimes depends on the thickness of the plane, which is determined by the natural self-gravitating scale of the interstellar medium. We present a thorough analysis of the power spectra of the dust and gas emission at several wavelengths in the nearby galaxy M 33. In particular, we use the recently obtained images at five wavelengths by PACS and SPIRE onboard Herschel. The wide dynamical range (2–3 dex in scale) of most images allows us to clearly determine the change in slopes from −1.5 to −4, with some variations with wavelength. The break scale increases with wavelength from 100 pc at 24 and 100 μm to 350 pc at 500 μm, suggesting that the cool dust lies in a thicker disk than the warm dust, perhaps because of star formation that is more confined to the plane. The slope on small scales tends to be steeper at longer wavelength, meaning that the warmer dust is more concentrated in clumps. Numerical simulations of an isolated late-type galaxy, rich in gas and with no bulge, such as M 33, are carried out to better interpret these observed results. Varying the star formation and feedback parameters, it is possible to obtain a range of power spectra, with two power-law slopes and breaks, that nicelybracket the data. The small-scale power-law does indeed reflect the 3D behavior of the gas layer, steepening strongly while the feedback smoothes the structures by increasing the gas turbulence. M 33 appears to correspond to a fiducial model with an SFR of ~ 0.7 M⊙/yr, with 10% supernovae energy coupled to the gas kinematics.
It has long been discussed whether stellar feedback in the form of winds and/or radiation can shred the nascent molecular cloud, thereby controlling the star formation rate. However, directly probing ...and quantifying the impact of stellar feedback on the neutral gas of the nascent clouds is challenging. We present an investigation of this impact toward the RCW 79 H II region using the ionized carbon line at 158 μm (C II) from the FEEDBACK Legacy Survey. We combine this data with information on the dozen ionizing O stars responsible for the evolution of the region, and observe in C II for the first time both blue- and redshifted high-velocity gas that reaches velocities of up to 25 km s
−1
relative to the bulk emission of the molecular cloud. This high-velocity gas mostly contains neutral gas, and partly forms a fragmented shell, similar to recently found shells in a few Galactic H II regions. However, this shell does not account for all of the observed neutral high-velocity gas. We also find high-velocity gas streaming out of the nascent cloud through holes, and obtain a range of dynamical timescales below 1.0 Myr for the high-velocity gas that is well below the 2.3 ± 0.5 Myr age of the OB cluster. This suggests a different scenario for the evolution of RCW 79, where the high-velocity gas does not solely stem from a spherical expanding bubble, but also from gas recently ablated at the edge of the turbulent molecular cloud into the surrounding interstellar medium through low-pressure holes or chimneys. The resulting mass ejection rate estimate for the cloud is 0.9–3.5 × 10
−2
M
⊙
yr
−1
, which leads to short erosion timescales (< 5 Myr) for the nascent molecular cloud. This finding provides direct observational evidence of rapid molecular cloud dispersal.
Abstract
We quantified the effects of stellar feedback in RCW 49 by determining the physical conditions in different regions using the C
ii
158
μ
m and O
i
63
μ
m observations from SOFIA, the
12
CO ...(3–2) observations from APEX, and the H
2
line observations from Spitzer telescopes. Large maps of RCW 49 were observed with the SOFIA and APEX telescopes, while the Spitzer observations were only available toward three small areas. From our qualitative analysis, we found that the H
2
0–0
S
(2) emission line probes denser gas compared to the H
2
0–0
S
(1) line. In four regions (“northern cloud,” “pillar,” “ridge,” and “shell”), we compared our observations with the updated PDR Toolbox models and derived the integrated far-ultraviolet flux between 6 and 13.6 eV (
G
0
), H nucleus density (
n
), temperatures, and pressures. We found the ridge to have the highest
G
0
(2.4 × 10
3
Habing units), while the northern cloud has the lowest
G
0
(5 × 10
2
Habing units). This is a direct consequence of the location of these regions with respect to the Wd2 cluster. The ridge also has a high density (6.4 × 10
3
cm
−3
), which is consistent with its ongoing star formation. Among the Spitzer positions, we found the one closest to the Wd2 cluster to be the densest, suggesting an early phase of star formation. Furthermore, the Spitzer position that overlaps with the shell was found to have the highest
G
0
, and we expect this to be a result of its proximity to an O9V star.
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