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.
Aims. We observationally investigate the relation between the photoelectric heating efficiency in photodissociation regions (PDRs) and the charge of polycyclic aromatic hydrocarbons (PAHs), which are ...considered to play a key role in photoelectric heating. Methods. Using PACS onboard Herschel, we observed six PDRs spanning a wide range of far-ultraviolet radiation fields (G0 = 100−105). To measure the photoelectric heating efficiency, we obtained the intensities of the main cooling lines in these PDRs, i.e., the O i 63 μm, 145 μm, and C ii 158 μm, as well as the far-infrared (FIR) continuum intensity. We used Spitzer/IRS spectroscopic mapping observations to investigate the mid-infrared (MIR; 5.5−14 μm) PAH features in the same regions. We decomposed the MIR PAH emission into that of neutral (PAH0) and positively ionized (PAH+) species to derive the fraction of the positively charged PAHs in each region, and compare it to the photoelectric heating efficiency. Results. The heating efficiency traced by (O i 63 μm + O i 145 μm + C ii 158 μm)/TIR, where TIR is the total infrared flux, ranges between 0.1% and 0.9% in different sources, and the fraction of PAH+ relative to (PAH0+ PAH+) spans from 0 (+11)% to 87 (±10)%. All positions with a high PAH+ fraction show a low heating efficiency, and all positions with a high heating efficiency have a low PAH+ fraction, supporting the scenario in which a positive grain charge results in a decreased heating efficiency. Theoretical estimates of the photoelectric heating efficiency show a stronger dependence on the charging parameter γ = G0T1/2/ne than the observed efficiency reported in this study, and the discrepancy is significant at low γ. The photoelectric heating efficiency on PAHs, traced by (O i 63 μm + O i 145 μm + C ii 158 μm)/(PAH-band emission + O i 63 μm + O i 145 μm + C ii 158 μm), shows a much better match between the observations and the theoretical estimates. Conclusions. The good agreement of the photoelectric heating efficiency on PAHs with a theoretical model indicates the dominant contribution of PAHs to the photoelectric heating. This study demonstrates the fundamental role that PAHs have in photoelectric heating. More studies of their charging behavior are crucial to understand the thermal balance of the interstellar medium.
Context. In bright photodissociation regions (PDR) associated with massive star formation, the presence of dense “clumps” that are immersed in a less dense interclump medium is often proposed to ...explain the difficulty of models to account for the observed gas emission in high-excitation lines. Aims. We aim to present a comprehensive view of the modelling of the CO rotational ladder in PDRs, including the high-J lines that trace warm molecular gas at PDR interfaces. Methods. We observed the 12CO and 13CO ladders in two prototypical PDRs, the Orion Bar and NGC 7023 NW using the instruments onboard Herschel. We also considered line emission from key species in the gas cooling of PDRs (C+, O, and H2) and other tracers of PDR edges such as OH and CH+. All the intensities are collected from Herschel observations, the literature and the Spitzer archive and were analysed using the Meudon PDR code. Results. A grid of models was run to explore the parameter space of only two parameters: thermal gas pressure and a global scaling factor that corrects for approximations in the assumed geometry. We conclude that the emission in the high-J CO lines, which were observed up to Jup = 23 in the Orion Bar (Jup = 19 in NGC 7023), can only originate from small structures with typical thicknesses of a few 10−3 pc and at high thermal pressures (Pth ~ 108 K cm−3). Conclusions. Compiling data from the literature, we find that the gas thermal pressure increases with the intensity of the UV radiation field given by G0, following a trend in line with recent simulations of the photoevaporation of illuminated edges of molecular clouds. This relation can help to rationalise the analysis of high-J CO emission in massive star formation and provides an observational constraint for models which study stellar feedback on molecular clouds.
Context. L1630 in the Orion B molecular cloud, which includes the iconic Horsehead Nebula, illuminated by the star system σ Ori, is an example of a photodissociation region (PDR). In PDRs, stellar ...radiation impinges on the surface of dense material, often a molecular cloud, thereby inducing a complex network of chemical reactions and physical processes. Aims. Observations toward L1630 allow us to study the interplay between stellar radiation and a molecular cloud under relatively benign conditions, that is, intermediate densities and an intermediate UV radiation field. Contrary to the well-studied Orion Molecular Cloud 1 (OMC1), which hosts much harsher conditions, L1630 has little star formation. Our goal is to relate the C ii fine-structure line emission to the physical conditions predominant in L1630 and compare it to studies of OMC1. Methods. The C ii 158 μm line emission of L1630 around the Horsehead Nebula, an area of 12′ × 17′, was observed using the upgraded German Receiver for Astronomy at Terahertz Frequencies (upGREAT) onboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). Results. Of the C ii emission from the mapped area 95%, 13 L⊙, originates from the molecular cloud; the adjacent H ii region contributes only 5%, that is, 1 L⊙. From comparison with other data (CO (1 − 0)-line emission, far-infrared (FIR) continuum studies, emission from polycyclic aromatic hydrocarbons (PAHs)), we infer a gas density of the molecular cloud of nH ~ 3 × 103 cm-3, with surface layers, including the Horsehead Nebula, having a density of up to nH ~ 4 × 104 cm-3. The temperature of the surface gas is T ~ 100 K. The average C ii cooling efficiency within the molecular cloud is 1.3 × 10-2. The fraction of the mass of the molecular cloud within the studied area that is traced by C ii is only 8%. Our PDR models are able to reproduce the FIR-C ii correlations and also the CO (1 − 0)-C ii correlations. Finally, we compare our results on the heating efficiency of the gas with theoretical studies of photoelectric heating by PAHs, clusters of PAHs, and very small grains, and find the heating efficiency to be lower than theoretically predicted, a continuation of the trend set by other observations. Conclusions. In L1630 only a small fraction of the gas mass is traced by C ii. Most of the C ii emission in the mapped area stems from PDR surfaces. The layered edge-on structure of the molecular cloud and limitations in spatial resolution put constraints on our ability to relate different tracers to each other and to the physical conditions. From our study, we conclude that the relation between C ii emission and physical conditions is likely to be more complicated than often assumed. The theoretical heating efficiency is higher than the one we calculate from the observed C ii emission in the L1630 molecular cloud.
CONFIRMATION OF CIRCUMSTELLAR PHOSPHINE AGUNDEZ, M; Cernicharo, J; Decin, L ...
Astrophysical journal. Letters,
08/2014, Letnik:
790, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Phosphine (PH sub(3)) was tentatively identified a few years ago in the carbon star envelopes IRC +10216 and CRL 2688 from observations of an emission line at 266.9 GHz attributable to the J = 1-0 ...rotational transition. We report the detection of the J = 2-1 rotational transition of PH sub(3) in IRC +10216 using the HIFI instrument on board Herschel, which definitively confirms the identification of PH sub(3). Radiative transfer calculations indicate that infrared pumping in excited vibrational states plays an important role in the excitation of PH sub(3) in the envelope of IRC +10216, and that the observed lines are consistent with phosphine being formed anywhere between the star and 100 Rlow * from the star, with an abundance of 10 super(-8) relative to H sub(2). The detection of PH sub(3) challenges chemical models, none of which offer a satisfactory formation scenario. Although PH sub(3) holds just 2% of the total available phosphorus in IRC +10216, it is, together with HCP, one of the major gas phase carriers of phosphorus in the inner circumstellar layers, suggesting that it could also be an important phosphorus species in other astronomical environments. This is the first unambiguous detection of PH sub(3) outside the solar system, and is a further step toward a better understanding of the chemistry of phosphorus in space.
Context. Asymptotic giant branch (AGB) stars are in one of the latest evolutionary stages of low to intermediate-mass stars. Their vigorous mass loss has a significant effect on the stellar ...evolution, and is a significant source of heavy elements and dust grains for the interstellar medium. The mass-loss rate can be well traced by carbon monoxide (CO) line emission. Aims. We present new Herschel/HIFI and IRAM 30 m telescope CO line data for a sample of 53 galactic AGB stars. The lines cover a fairly large range of excitation energy from the J = 1 → 0 line to the J = 9 → 8 line, and even the J = 14 → 13 line in a few cases. We perform radiative transfer modelling for 38 of these sources to estimate their mass-loss rates. Methods. We used a radiative transfer code based on the Monte Carlo method to model the CO line emission. We assume spherically symmetric circumstellar envelopes that are formed by a constant mass-loss rate through a smoothly accelerating wind. Results. We find models that are consistent across a broad range of CO lines for most of the stars in our sample, i.e., a large number of the circumstellar envelopes can be described with a constant mass-loss rate. We also find that an accelerating wind is required to fit, in particular, the higher-J lines and that a velocity law will have a significant effect on the model line intensities. The results cover a wide range of mass-loss rates (~10-8 to 2 × 10-5 M⊙ yr-1) and gas expansion velocities (2 to 21.5 km s-1) , and include M-, S-, and C-type AGB stars. Our results generally agree with those of earlier studies, although we tend to find slightly lower mass-loss rates by about 40%, on average. We also present “bonus” lines detected during our CO observations.
We present the first ~7.5'×11.5' velocity-resolved (~0.2 km s
) map of the C ii 158
m line toward the Orion molecular cloud 1 (OMC 1) taken with the
/HIFI instrument. In combination with far-infrared ...(FIR) photometric images and velocity-resolved maps of the H41
hydrogen recombination and CO
=2-1 lines, this data set provides an unprecedented view of the intricate small-scale kinematics of the ionized/PDR/molecular gas interfaces and of the radiative feedback from massive stars. The main contribution to the C ii luminosity (~85 %) is from the extended, FUV-illuminated face of the cloud (
>500,
>5×10
cm
) and from dense PDRs (
≳10
,
≳10
cm
) at the interface between OMC 1 and the H ii region surrounding the Trapezium cluster. Around ~15 % of the C ii emission arises from a different gas component without CO counterpart. The C ii excitation, PDR gas turbulence, line opacity (from
C ii) and role of the geometry of the illuminating stars with respect to the cloud are investigated. We construct maps of the
C ii/
and
/
ratios and show that
C ii/
decreases from the extended cloud component (~10
-10
) to the more opaque star-forming cores (~10
-10
). The lowest values are reminiscent of the "C ii deficit" seen in local ultra-luminous IR galaxies hosting vigorous star formation. Spatial correlation analysis shows that the decreasing
C ii/
ratio correlates better with the column density of dust through the molecular cloud than with
/
. We conclude that the C ii emitting column relative to the total dust column along each line of sight is responsible for the observed
C ii/
variations through the cloud.
Context.
Fast outflows and their interaction with slow shells (generally known as the fossil circumstellar envelope of asymptotic giant branch stars) play an important role in the structure and ...kinematics of protoplanetary and planetary nebulae (pPNe, PNe). To properly study their effects within these objects, we also need to observe the intermediate-temperature gas, which is only detectable in the far-infrared and submillimetre (submm) transitions.
Aims.
We study the physical conditions of the outflows presented in a number of pPNe and PNe, with a focus on their temperature and excitation states.
Methods.
We carried out
Herschel
/HIFI observations in the submm lines of
12
CO in nine pPNe and nine PNe and complemented them with low-
J
CO spectra obtained with the IRAM 30m telescope and taken from the literature. The spectral resolution of HIFI allows us to identify and measure the different nebular components in the line profiles. The comparison with large velocity gradient model predictions was used to estimate the physical conditions of the warm gas in the nebulae, such as excitation conditions, temperature, and density.
Results.
We found high kinetic temperatures for the fast winds of pPNe, typically reaching between 75 K and 200 K. In contrast, the high-velocity gas in the sampled PNe is colder, with characteristic temperatures between 25 K and 75 K, and it is found in a lower excitation state. We interpret this correlation of the kinetic temperature and excitation state of fast outflows with the amount of time elapsed since their acceleration (probably driven by shocks) as a consequence of the cooling that occurred during the pPN phase.
We present maps at high spatial and spectral resolution in emission lines of CCH, c-C3H2, C4H, 12CO and C18O of the edge of the Horsehead nebula obtained with the IRAM Plateau de Bure Interferometer ...(PdBI). The edge of the Horsehead nebula is a one-dimensional Photo-Dissociation Region (PDR) viewed almost edge-on. All hydrocarbons are detected at high signal-to-noise ratio in the PDR where intense emission is seen both in the H2 ro-vibrational lines and in the PAH mid-infrared bands. C18O peaks farther away from the cloud edge. Our observations demonstrate that CCH, c-C3H2 and C4H are present in UV-irradiated molecular gas, with abundances nearly as high as in dense, well-shielded molecular cores. PDR models i) need a large density gradient at the PDR edge to correctly reproduce the offset between the hydrocarbons and H2 peaks; and ii) fail to reproduce the hydrocarbon abundances. We propose that a new formation path of carbon chains, in addition to gas phase chemistry, should be considered in PDRs: because of intense UV-irradiation, large aromatic molecules and small carbon grains may fragment and feed the interstellar medium with small carbon clusters and molecules in significant amounts.
PDF
![VELOCITY-RESOLVED [C ii] EM...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/0004-637X_1538-4357/small "VELOCITY-RESOLVED [C ii] EMISSION AND [C ii]/FIR MAPPING ALONG ORION WITH HERSCHEL")
