Michel Perault

A calculation of confusion noise due to infrared cirrus

General expressions are developed for the statistical errors to be expected in photometric measurements due to confusion in a background of fluctuating surface brightness. Backgrounds actually observed in the far infrared by the IRAS satellite are used to calculate tables of these error expressions for two simple measurement techniques. The confusion noise limited sensitivities for NASAs planned Space Infrared Telescope Facility and the European Space Agencys Infrared Space Observatory are estimated at a wavelength of 100 μm from these tables

Herschel observations of EXtra-Ordinary Sources (HEXOS):The present and future of spectral surveys with Herschel/HIFI

We present initial results from the Herschel GT key program: Herschel observations of EXtra-Ordinary Sources (HEXOS) and outline the promise and potential of spectral surveys with Herschel/HIFI. The HIFI instrument offers unprecedented sensitivity, as well as continuous spectral coverage across the gaps imposed by the atmosphere, opening up a largely unexplored wavelength regime to high-resolution spectroscopy. We show the spectrum of Orion KL between 480 and 560 GHz and from 1.06 to 1.115 THz. From these data, we confirm that HIFI separately measures the dust continuum and spectrally resolves emission lines in Orion KL. Based on this capability we demonstrate that the line contribution to the broad-band continuum in this molecule-rich source is ~20-40% below 1 THz and declines to a few percent at higher frequencies. We also tentatively identify multiple transitions of HD18O in the spectra. The first detection of this rare isotopologue in the interstellar medium suggests that HDO emission is optically thick in the Orion hot core with HDO/H2O ~ 0.02. We discuss the implications of this detection for the water D/H ratio in hot cores. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Figure 2 (page 6) is also available in electronic form at http://www.aanda.org

The young stellar population in the Serpens Cloud Core: An ISOCAM survey

We present results from an ISOCAM survey in the two broad band filters LW2 (5-8.5 μm) and LW3 (12-18 μm) of a 0.13 square degree coverage of the Serpens Main Cloud Core. A total of 392 sources were detected in the 6.7 μm band and 139 in the 14.3 μm band to a limiting sensitivity of ~2 mJy. We identified 53 Young Stellar Objects (YSOs) with mid-IR excess from the single colour index [14.3/6.7], and 8 additional YSOs from the H – K/K – m_(6.7) diagram. Only 32 of these 61 sources were previously known to be YSO candidates. Only about 50% of the mid-IR excess sources show excesses in the near-IR J – H/H – K diagram. In the 48 square arcmin field covering the central Cloud Core the Class I/Class II number ratio is 19/18, i.e. about 10 times larger than in other young embedded clusters such as ρ Ophiuchi or Chamaeleon. The mid-IR fluxes of the Class I and flat-spectrum sources are found to be on the average larger than those of Class II sources. Stellar luminosities are estimated for the Class II sample, and its luminosity function is compatible with a coeval population of about 2 Myr which follows a three segment power-law IMF. For this age about 20% of the Class IIs are found to be young brown dwarf candidates. The YSOs are in general strongly clustered, the Class I sources more than the Class II sources, and there is an indication of sub-clustering. The sub-clustering of the protostar candidates has a spatial scale of 0.12 pc. These sub-clusters are found along the NW-SE oriented ridge and in very good agreement with the location of dense cores traced by millimeter data. The smallest clustering scale for the Class II sources is about 0.25 pc, similar to what was found for ρ Ophiuchi. Our data show evidence that star formation in Serpens has proceeded in several phases, and that a “microburst” of star formation has taken place very recently, probably within the last 10^5 yrs.

Evolution of very small particles in the southern part of Orion B observed by ISOCAM

We present ISOCAM observations (5 18 m) of the southern part of Orion B, including the reflection nebula NGC 2023 and the Horsehead nebula illuminated by the B star HD 37903 and the O star Orionis, respectively. Due to the limited radiation eld, the emission in these wavelengths is due to very small particles which are heated each time they absorb a UV photon. A lamentary structure is detected at small angular scales (down to the angular resolution of 6 00 ) on top of a smoother background. The particular case of the Horsehead nebula suggests that the laments in general result from the illuminated surfaces of dense structures, while the smoother background comes from lower density matters probably ionised. Striking spatial variations of the infrared colour (5 8.5 m/12{18 m) are also detected. Spectroscopic observations show that they are due to variations of the intensity of the aromatic features (especially at 7:7 m) relative to a continuum emission increasing in intensity towards longer wavelengths. The contribution of the continuum relative to the aromatic features appears signicantly higher at the illuminated surfaces of dense structures than in lower density matter. This eect could be the signature of the evolution of the very small particles from shielded molecular material to photo-dissociated and photo-ionised matter. We also show that size segregation due to grain dynamics in uni-directional radiation elds may play a major role.

Infrared dark clouds from the ISOGAL survey ? Constraints on the interstellar extinction curve

The ISO galactic survey provides images of the inner disk in two broad lters (around 7 and 15m) over some 15 square degrees, away from the brightest star forming regions. A multiresolution analysis of the images leads to a catalogue of infrared dark clouds, most of which are condensed cores of large molecular clouds, several kpc away from the Sun, seen in absorption in front of the diuse galactic emission. The longitude distributions of the background emission and of the dark clouds correlate with known tracers of young population components. We analyse the morphology of the dark clouds and the intensity fluctuations within the cloud boundaries at the two wavelengths. The 7 to 15 m contrast ratio is 0:75 0:15 for the clouds located away from the Galactic Centre (jlj > 1 )a nd 1:05 0:15 for the clouds closest to the Galactic Centre (jlj < 1;jbj < 0:2). Using a simple absorption model, we derive a 7 to 15 m opacity ratio equal to 0:7 0:1 for the clouds located away from the Galactic Centre and estimate the opacity, , of a few objects at 15 m in the range 1 to 4. Several explanations for the variation of the contrast ratio, including absorption along the line of sight and local variations of the extinction curve are discussed.

ISOCAM and molecular observations of the edge of the Horsehead nebula

We present ISOCAM observations (5-18 µm) of the Horsehead nebula, together with observations of the (J = 1−0) and (J = 2−1) transitions of 12 CO, 13 CO and C 18 O taken at the IRAM 30-m telescope. The Horsehead nebula presents a typical photodissociation region illuminated by the O9.5 V system σ Ori. The ISOCAM emission is due to very small particles transiently heated to high temperature each time they absorb a UV photon. A very sharp filament (width: ∼10 �� or ∼0.02 pc) is detected by ISOCAM at the illuminated edge of the nebula. This filament is due to a combined effect of steep increase of the column density and extinction of incident radiation, on typical sizes below ∼0.01 pc. Both the three-dimensional shape and the local density of the illuminated interface are strongly constrained. The dense material forming the edge of the Horsehead nebula appears illuminated edge-on by σ Ori, and the particles located beyond the border should not be affected by the incident radiation field. This structure may be due to dense filaments in the parental cloud which have shielded the material located in their shadow from the photo-dissociating radiations. The measurement of the penetration depth of the incident radiation from the infrared data (∼0.01 pc) gives a density of a few 10 4 cm −3 just behind the bright filament. This value is comparable to the estimate of the density beyond the edge and deduced from our molecular observations, and also to the density behind the ionization front calculated in the stationary case. The material behind the illuminated edge could also be non-homogeneous, with clump sizes significantly smaller than the observed penetration depth of ∼0.01 pc. In that case no upper limit on the average density just behind the illuminated edge can be given.

Herschel observations of EXtra-Ordinary Sources (HEXOS): Detection of hydrogen fluoride in absorption towards Orion KL

We report a detection of the fundamental rotational transition of hydrogen fluoride in absorption towards Orion KL using Herschel/HIFI. After the removal of contaminating features associated with common molecules (“weeds”), the HF spectrum shows a P-Cygni profile, with weak redshifted emission and strong blue-shifted absorption, associated with the low-velocity molecular outflow. We derive an estimate of 2.9 × 10^(13) cm^(-2) for the HF column density responsible for the broad absorption component. Using our best estimate of the H_2 column density within the low-velocity molecular outflow, we obtain a lower limit of ~1.6 × 10^(-10) for the HF abundance relative to hydrogen nuclei, corresponding to ~0.6% of the solar abundance of fluorine. This value is close to that inferred from previous ISO observations of HF J = 2–1 absorption towards Sgr B2, but is in sharp contrast to the lower limit of 6 × 10^(-9) derived by Neufeld et al. for cold, foreground clouds on the line of sight towards G10.6-0.4.

Detection of OH+ and H2O+ towards Orion KL

We report observations of the reactive molecular ions OH+, H2O+, and H3O+ towards Orion KL with Herschel/HIFI. All three N = 1-0 fine-structure transitions of OH+ at 909, 971, and 1033 GHz and both fine-structure components of the doublet ortho-H2O+ 111-000 transition at 1115 and 1139 GHz were detected; an upper limit was obtained for H3O+. OH+ and H2O+ are observed purely in absorption, showing a narrow component at the source velocity of 9 km s-1, and a broad blueshifted absorption similar to that reported recently for HF and para-H218O, and attributed to the low velocity outflow of Orion KL. We estimate column densities of OH+ and H2O+ for the 9 km s-1 component of 9 ± 3 × 1012 cm-2 and 7 ± 2 × 1012 cm-2, and those in the outflow of 1.9 ± 0.7 × 1013 cm-2 and 1.0 ± 0.3 × 1013 cm-2. Upper limits of 2.4 × 1012 cm-2 and 8.7 × 1012 cm-2 were derived for the column densities of ortho and para-H3O+ from transitions near 985 and 1657 GHz. The column densities of the three ions are up to an order of magnitude lower than those obtained from recent observations of W31C and W49N. The comparatively low column densities may be explained by a higher gas density despite the assumption of a very high ionization rate.

Jeans collapse of turbulent gas clouds: tentative theory

The linear Jeans stability problem in a turbulent medium is treated using a description of the large-scale motions, with the response of turbulence on the small scales being treated using a renormalization approach. This treatment shows how turbulence at scales smaller than the potentially collapsing scale builds up a turbulent pressure force which effectively resists compression, if the kinetic energy is sufficient to balance the gravitational attraction.

Herschel observations of EXtra-Ordinary Sources (HEXOS): Methanol as a probe of physical conditions in Orion KL

We have examined methanol emission from Orion KL with the Herschel/HIFI instrument, and detected two methanol bands centered at 524 GHz and 1061 GHz. The 524 GHz methanol band (observed in HIFI band 1a) is dominated by the isolated ΔJ = 0, K = −4 →− 3, vt = 0 Q branch, and includes 25 E-type and 2 A-type transitions. The 1061 GHz methanol band (observed in HIFI band 4b) is dominated by the ΔJ = 0, K = 7 → 6, vt = 0 Q branch transitions which are mostly blended. We have used the isolated E-type vt = 0 methanol transitions to explore the physical conditions in the molecular gas. With HIFI’s high velocity resolution, the methanol emission contributed by different spatial components along the line of sight toward Orion KL (hot core, low velocity flow, and compact ridge) can be distinguished and studied separately. The isolated transitions detected in these bands cover a broad energy range (upper state energy ranging from 80 K to 900 K), which provides a unique probe of the thermal structure in each spatial component. The observations further show that the compact ridge is externally heated. These observations demonstrate the power of methanol lines as probes of the physical conditions in warm regions in close proximity to young stars.