Context.
The Orion Molecular Cloud is the nearest massive-star forming region. Massive stars have profound effects on their environment due to their strong radiation fields and stellar winds. Stellar ...feedback is one of the most crucial cosmological parameters that determine the properties and evolution of the interstellar medium in galaxies.
Aims.
We aim to understand the role that feedback by stellar winds and radiation play in the evolution of the interstellar medium. Velocity-resolved observations of the C
II
158
μ
m fine-structure line allow us to study the kinematics of UV-illuminated gas. Here, we present a square-degree-sized map of C
II
emission from the Orion Nebula complex at a spatial resolution of 16′′ and high spectral resolution of 0.2 km s
−1
, covering the entire Orion Nebula (M 42) plus M 43 and the nebulae NGC 1973, 1975, and 1977 to the north. We compare the stellar characteristics of these three regions with the kinematics of the expanding bubbles surrounding them.
Methods.
We use C
II
158
μ
m line observations over an area of 1.2 deg
2
in the Orion Nebula complex obtained by the upGREAT instrument onboard SOFIA.
Results.
The bubble blown by the O7V star
θ
1
Ori C in the Orion Nebula expands rapidly, at 13 km s
−1
. Simple analytical models reproduce the characteristics of the hot interior gas and the neutral shell of this wind-blown bubble and give us an estimate of the expansion time of 0.2 Myr. M 43 with the B0.5V star NU Ori also exhibits an expanding bubble structure, with an expansion velocity of 6 km s
−1
. Comparison with analytical models for the pressure-driven expansion of H
II
regions gives an age estimate of 0.02 Myr. The bubble surrounding NGC 1973, 1975, and 1977 with the central B1V star 42 Orionis expands at 1.5 km s
−1
, likely due to the over-pressurized ionized gas as in the case of M 43. We derive an age of 0.4 Myr for this structure.
Conclusions.
We conclude that the bubble of the Orion Nebula is driven by the mechanical energy input by the strong stellar wind from
θ
1
Ori C, while the bubbles associated with M 43 and NGC 1977 are caused by the thermal expansion of the gas ionized by their central later-type massive stars.
We present a systematic survey of multiple velocity-resolved H2O spectra using Herschel/Heterodyne Instrument for the Far Infrared (HIFI) toward nine nearby actively star-forming galaxies. The ...ground-state and low-excitation lines (Eup ≤ 130 K) show profiles with emission and absorption blended together, while absorption-free medium-excitation lines (130 K ≤ Eup ≤ 350 K) typically display line shapes similar to CO. We analyze the HIFI observation together with archival SPIRE/PACS H2O data using a state-of-the-art 3D radiative transfer code that includes the interaction between continuum and line emission. The water excitation models are combined with information on the dust and CO spectral line energy distribution to determine the physical structure of the interstellar medium (ISM). We identify two ISM components that are common to all galaxies: a warm ( K), dense ( ) phase that dominates the emission of medium-excitation H2O lines. This gas phase also dominates the far-IR emission and the CO intensities for . In addition, a cold ( K), dense ( ), more extended phase is present. It outputs the emission in the low-excitation H2O lines and typically also produces the prominent line absorption features. For the two ULIRGs in our sample (Arp 220 and Mrk 231) an even hotter and more compact (Rs ≤ 100 pc) region is present, which is possibly linked to AGN activity. We find that collisions dominate the water excitation in the cold gas and for lines with K and K in the warm and hot component, respectively. Higher-energy levels are mainly excited by IR pumping.
The Fourier power spectrum is one of the most widely used statistical tools to analyze the nature of magnetohydrodynamic (MHD) turbulence in the interstellar medium. Lazarian & Pogosyan predicted ...that the spectral slope should saturate to -3 for an optically thick medium and many observations exist in support of their prediction. However, there have not been any numerical studies to date for testing these results. We analyze the spatial power spectrum of MHD simulations with a wide range of sonic and Alfvenic Mach numbers, which include radiative transfer effects of the super(13)CO transition. We numerically confirm the predictions of Lazarian & Pogosyan that the spectral slope of line intensity maps of an optically thick medium saturates to -3. Furthermore, for very optically thin supersonic CO gas, where the density or CO abundance values are too low to excite emission in all but the densest shock compressed gas, we find that the spectral slope is shallower than expected from the column density. Finally, we find that mixed optically thin/thick CO gas, which has average optical depths on the order of unity, shows mixed behavior: for super-Alfvenic turbulence, the integrated intensity power spectral slopes generally follow the same trend with sonic Mach number as the true column density power spectrum slopes. However, for sub-Alfvenic turbulence the spectral slopes are steeper with values near -3 which are similar to the very optically thick regime.
We describe the design and construction of GREAT (German REceiver for Astronomy at Terahertz frequencies) operated on the Stratospheric Observatory For Infrared Astronomy (SOFIA). GREAT is a modular ...dual-color heterodyne instrument for high-resolution far-infrared (FIR) spectroscopy. Selected for SOFIA’s Early Science demonstration, the instrument has successfully performed three Short and more than a dozen Basic Science flights since first light was recorded on its April 1, 2011 commissioning flight. We report on the in-flight performance and operation of the receiver that – in various flight configurations, with three different detector channels – observed in several science-defined frequency windows between 1.25 and 2.5 THz. The receiver optics was verified to be diffraction-limited as designed, with nominal efficiencies; receiver sensitivities are state-of-the-art, with excellent system stability. The modular design allows for the continuous integration of latest technologies; we briefly discuss additional channels under development and ongoing improvements for Cycle 1 observations. GREAT is a principal investigator instrument, developed by a consortium of four German research institutes, available to the SOFIA users on a collaborative basis.
We present SOFIA/FIFI-LS observations of the C ii 158 m cooling line across the nearby spiral galaxy NGC 6946. We combine these with UV, IR, CO, and H i data to compare C ii emission to dust ...properties, star formation rate (SFR), H2, and H i 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 C ii luminosity to arms, and 19% and 8% to the center and interarm region, respectively. C ii/TIR, C ii/CO, and C ii/PAH radial profiles are largely constant, but rise at large radii ( 8 kpc) and drop in the center ("C ii deficit"). This increase at large radii and the observed decline with the 70 m/100 m dust color are likely driven by radiation field hardness. We find a near proportional C ii-SFR scaling relation for beam-sized regions, though the exact scaling depends on methodology. C ii 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 C ii as an H2 and SFR tracer in such regimes. Finally, based on the observed C ii and CO radial profiles and different models, we find CO to increase with radius, in line with the observed metallicity gradient. The low CO (galaxy average 2 M pc−2 (K km s−1)−1) and low C ii/CO ratios (∼400 on average) imply little CO-dark gas across NGC 6946, in contrast to estimates in the Milky Way.
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.
Context. Deuterated ions, especially H2D+ and N2D+, are abundant in cold (~10 K), dense (~105 cm-3) regions, in which CO is frozen out onto dust grains. In such environments, the N2D+/N2H+ ratio can ...exceed the elemental abundance ratio of D/H by a factor of $\simeq$104. Aims. We use deuterium fractionation to investigate the evolutionary state of Class 0 protostars. In particular, we expect the N2D+/N2H+ ratio to decrease as temperature (a sign of the evolution of the protostar) increases. Methods. We observed N2H+ 1-0, N2D+ 1-0, 2-1 and 3-2, C18O 1-0 and HCO+ 3-2 in a sample of 20 Class 0 and borderline Class 0/I protostars. We determined the deuteration fraction and searched for correlations between the N2D+/N2H+ ratio and well-established evolutionary tracers, such as TDust and the CO depletion factor. In addition, we compared the observational result with a chemical model. Results. In our protostellar sample, the N2H+ 1-0 optical depths are significantly lower than those found in prestellar cores, but the N2H+ column densities are comparable, which can be explained by the higher temperature and larger line width in protostellar cores. The deuterium fractionation of N2H+ in protostellar cores is also similar to that in prestellar cores. We found a clear correlation between the N2D+/N2H+ ratio and evolutionary tracers. As expected, the coolest, i.e. the youngest, objects show the largest deuterium fractionation. Furthermore, we find that sources with a high N2D+/N2H+ ratio show clear indications of infall (e.g. $ \delta v$ < 0). With decreasing deuterium fraction, the infall signature disappears and $ \delta v$ tends to be positive for the most evolved objects. The deuterium fractionation of other molecules deviates clearly from that of N2H+. The DCO+/HCO+ ratio stays low at all evolutionary stages, whereas the NH2D/NH3 ratio is >0.15 even in the most evolved objects. Conclusions. The N2D+/N2H+ ratio is known to trace the evolution of prestellar cores. We show that this ratio can be used to trace core evolution even after star formation. Protostars with an N2D+/N2H+ ratio above 0.15 are in a stage shortly after the beginning of collapse. Later on, deuterium fractionation decreases until it reaches a value of ~0.03 at the Class 0/I borderline.
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.
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.
Context.
The C
II
158
μ
m far-infrared fine-structure line is one of the most important cooling lines of the star-forming interstellar medium (ISM). It is used as a tracer of star formation ...efficiency in external galaxies and to study feedback effects in parental clouds. High spectral resolution observations have shown complex structures in the line profiles of the C
II
emission.
Aims.
Our aim is to determine whether the complex profiles observed in
12
C
II
are due to individual velocity components along the line-of-sight or to self-absorption based on a comparison of the
12
C
II
and isotopic
13
C
II
line profiles.
Methods.
Deep integrations with the SOFIA/upGREAT 7-pixel array receiver in the sources of M43, Horsehead PDR, Monoceros R2, and M17 SW allow for the detection of optically thin
13
C
II
emission lines, along with the
12
C
II
emission lines, with a high signal-to-noise ratio. We first derived the
12
C
II
optical depth and the C
II
column density from a single component model. However, the complex line profiles observed require a double layer model with an emitting background and an absorbing foreground. A multi-component velocity fit allows us to derive the physical conditions of the C
II
gas: column density and excitation temperature.
Results.
We find moderate to high
12
C
II
optical depths in all four sources and self-absorption of
12
C
II
in Mon R2 and M17 SW. The high column density of the warm background emission corresponds to an equivalent
A
v
of up to 41 mag. The foreground absorption requires substantial column densities of cold and dense C
II
gas, with an equivalent
A
v
ranging up to about 13 mag.
Conclusions.
The column density of the warm background material requires multiple photon-dominated region surfaces stacked along the line of sight and in velocity. The substantial column density of dense and cold foreground C
II
gas detected in absorption cannot be explained with any known scenario and we can only speculate on its origins.
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