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Far-infrared/submillimetre properties of pre-stellar cores L1521E, L1521F and L1689B as revealed by the Herschel SPIRE instrument - I. Central positions
(Oxford University Press, 2016) Makiwa, G.; Naylor, David A.; van der Wiel, M. H. D.; Ward-Thompson, D.; Kirk, J. M.; Eyres, S.; Abergel, A.; Köhler, M.
Dust grains play a key role in the physics of star-forming regions, even though they constitute only ∼1 per cent of the mass of the interstellar medium. The derivation of accurate dust parameters such as temperature (Td), emissivity spectral index (β) and column density requires broad-band continuum observations at far-infrared wavelengths. We present Herschel-Spectral and Photometric Imaging Receiver Array (SPIRE) Fourier Transform Spectrometer (FTS) measurements of three starless cores: L1521E, L1521F and L1689B, covering wavelengths between 194 and 671 µm. This paper is the ﬁrst to use our recently updated SPIRE-FTS intensity calibration, yielding a direct match with SPIRE photometer measurements of extended sources. In addition, we carefully assess the validity of calibration schemes depending on-source extent and on the strength of background emission. The broad-band far-infrared spectra for all three sources peak near 250 µm. Our observations therefore provide much tighter constraints on the spectral energy distribution (SED) shape than measurements that do not probe the SED peak. The spectra are ﬁtted using modiﬁed blackbody functions, allowing both Td and β to vary as free parameters.This yields Td of9.8±0.2,15.6±0.5and10.9±0.2K and corresponding β of 2.6∓0.9, 0.8∓0.1 and 2.4∓0.8 for L1521E, L1521F and L1689B, respectively.Thederivedcoremassesare1.0±0.1,0.10±0.01and0.49±0.05M ,respectively. The core mass/Jeans mass ratios for L1521E and L1689B exceed unity indicating that they are unstable to gravitational collapse, and thus pre-stellar cores. By comparison, the elevated temperature and gravitational stability of L1521F support previous arguments that this source is more evolved and likely a protostar.
Physical structure of the photodissociation regions in NGC 7023: observations of gas and dust emission with Herschel*
(EDP Sciences, 2014) Kohler, M.; Habart, E.; Arab, H.; Bernard-Salas, J.; Ayasso, H.; Abergel, A.; Zavagno, A.; Polehampton, E.; van der Wiel, M.H.D.; Naylor, David A.; Makiwa, Gibion; Dassas, K.; Joblin, C.; Pilleri, P.; Berne, O.; Fuente, A.; Gerin, M.; Goicoechea, J.R.; Teyssier, D.
Context. The determination of the physical conditions in molecular clouds is a key step towards our understanding of their formation and evolution of associated star formation. We investigate the density, temperature, and column density of both dust and gas in the photodissociation regions (PDRs) located at the interface between the atomic and cold molecular gas of the NGC 7023 reflection nebula. We study how young stars a ect the gas and dust in their environment. Aims. Several Herschel Space Telescope programs provide a wealth of spatial and spectral information of dust and gas in the heart of PDRs. We focus our study on Spectral and Photometric Image Receiver (SPIRE) Fourier-Transform Spectrometer (FTS) fully sampled maps that allow us for the first time to study the bulk of cool/warm dust and warm molecular gas (CO) together. In particular, we investigate if these populations spatially coincide, if and how the medium is structured, and if strong density and temperature gradients occur, within the limits of the spatial resolution obtained with Herschel. Methods. The SPIRE FTS fully sampled maps at di erent wavelengths are analysed towards the northwest (NW) and the east (E) PDRs in NGC 7023. We study the spatial and spectral energy distribution of a wealth of intermediate rotational 12CO 4 Ju 13 and 13CO 5 Ju 10 lines. A radiative transfer code is used to assess the gas kinetic temperature, density, and column density at di erent positions in the cloud. The dust continuum emission including Spitzer, the Photoconductor Array Camera and Spectrometer (PACS), and SPIRE photometric and the Institute for Radio Astronomy in the Millimeter Range (IRAM) telescope data is also analysed. Using a single modified black body and a radiative transfer model, we derive the dust temperature, density, and column density. Results. The cloud is highly inhomogeneous, containing several irradiated dense structures. Excited 12CO and 13CO lines and warm dust grains localised at the edge of the dense structures reveal high column densities of warm/cool dense matter. Both tracers give a good agreement in the local density, column density, and physical extent, leading to the conclusion that they trace the same regions. The derived density profiles show a steep gradient at the cloud edge reaching a maximum gas density of 10^5 -10^6 cm^-3 in the PDR NGC 7023 NW and 10^4 -10^5 cm^-3 in the PDR NGC 7023 E and a subsequent decrease inside the cloud. Close to the PDR edges, the dust temperature (30 K and 20 K for the NW and E PDRs, respectively) is lower than the gas temperature derived from CO lines (65-130 K and 45-55 K, respectively). Further inside the cloud, the dust and gas temperatures are similar. The derived thermal pressure is about 10 times higher in NGC 7023NWthan in NGC 7023 E. Comparing the physical conditions to the positions of known young stellar object candidates in NGC 7023 NW, we find that protostars seem to be spatially correlated with the dense structures. Conclusions. Our approach combining both dust and gas delivers strong constraints on the physical conditions of the PDRs. We find dense and warm molecular gas of high column density in the PDRs.
Spatial variation of the cooling lines in the Orion Bar from Hersehel/PACS
(EDP Sciences, 2012) Bernard-Salas, J.; Habart, E.; Arab, H.; Abergel, A.; Dartois, E.; Martin, P.; Bontemps, S.; Joblin, C.; White, G. J.; Bernard, J.-P.; Naylor, David A.
Context. The energetics in photo-dissociation regions (PDRs) are mainly regulated by the balance between the heating from the photo-electric eﬀect acting on dust grains, and the cooling via the copious emission of photons in far-infrared lines. The Orion Bar is a luminous and nearby PDR, which presents to the observer an ideal edge-on orientation in which to study this energy balance. Spatially resolved studies of such a nearby system are essential as they enable us to characterise the physical processes that control the energetics of the regions and can serve as templates for distant systems where these processes cannot be disentangled. Aims. We characterise the emission of the far-infrared ﬁne-structure lines of [Cii](158 μm),[Oi](63and145 μm),and[Nii](122 μm) that trace the gas local conditions, via spatially resolved observations of the Orion Bar. The observed distribution and variation of the lines are discussed in relation to the underlying geometry and linked to the energetics associated with the Trapezium stars. Methods. Herschel/PACS observations are used to map the spatial distribution of these ﬁne-structure lines across the Bar, with a spatial resolution between 4 and 11 and covering a total square area of about 120 ×105 . The spatial proﬁle of the emission lines are modelled using the radiative transfer code Cloudy. Results. TheHerschel observations reveal in unprecedented detail the morphology of the Bar.The spatial distribution of the [Cii] line coincides with that of the [Oi] lines. The [Nii] line peaks closer to the ionising star than the other three lines, but with a small region of overlap. We can distinguish several knots of enhanced emission within the Bar indicating the presence of an in homogenous and structured medium. The emission proﬁles cannot be reproduced by a single PDR, clearly indicating that, besides the Bar, there is a signiﬁcant contribution from additional PDR(s)over the area studied. The combination of both the [Nii] and [Oi] 145 μm lines can be used to estimate the [Cii] emission and distinguish between it sionised or neutral origin. We have calculated how much[Cii] emission comes from the neutral and ionised region, and ﬁnd that at least ∼82% originates from the photo-dissocciation region. Together, the [Cii] 158 μm and [Oi] 63 and 145 μm lines account for∼90% of the power emitted by the main cooling lines in the Bar (including CO, H2, etc.), with [Oi] 63μm alone accounting for 72% of the total.
Three-dimensional distribution of hydrogen fluoride gas toward NGC 6334 I and I(N)
(EDP Sciences, 2016) van der Wiel, M. H. D.; Naylor, David A.; Makiwa, Gibion; Satt, M.; Abergel, A.
Context. The HF molecule has been proposed as a sensitive tracer of diﬀuse interstellar gas, while at higher densities its abundance could be inﬂuenced heavily by freeze-out onto dust grains. Aims. We investigate the spatial distribution of a collection of absorbing gas clouds, some associated with the dense, massive star-forming core NGC6334I, and others with diﬀuse foreground clouds elsewhere along the line of sight. For the former category, we aim to study the dynamical properties of the clouds in order to assess their potential to feed the accreting protostellar cores. Methods. We use far-infrared spectral imaging from the Herschel SPIRE iFTS to construct a map of HF absorption at 243 µm in a 60 ×30 .5 region surrounding NGC6334 I and I(N). Results.The combination of new mapping that is fully sampled spatially, but is spectrally unresolved with a previous, single-pointing, spectrally resolved HF signature yields a three-dimensional picture of absorbing gas clouds in the direction of NGC6334. Toward core I, the HF equivalent width matches that of the spectrally resolved observation. At angular separations &2000 from core I, the HF absorption becomes weaker, which is consistent with three of the seven components being associated with this dense star-forming envelope. Of the remaining four components, two disappear beyond∼10 distance from the NGC6334 ﬁlament, suggesting that these clouds are spatially associated with the star-forming complex. Our data also implies a lack of gas-phase HF in the envelope of core I(N). Using a simple description of adsorption onto and desorption from dust grain surfaces, we show that the overall lower temperature of the envelope of source I(N) is consistent with freeze-out of HF, while it remains in the gas phase in source I. Conclusions. We use the HF molecule as a tracer of column density in diﬀuse gas(nH ≈102–103 cm−3),and ﬁnd that it may uniquely trace a relatively low-density portion of the gas reservoir available for star formation that otherwise escapes detection. At higher densities prevailing in protostellar envelopes (&104 cm−3), we ﬁnd evidence of HF depletion from the gas phase under suﬃciently cold conditions.