L. Verstraete

The global dust SED: tracing the nature and evolution of dust with DustEM

The Planck and Herschel missions are currently measuring the far-infrared to millimeter emission of dust, which combined with existing IR data, will for the first time provide the full spectral energy distribution (SED) of the galactic interstellar medium dust emission, from the mid-IR to the mm range, with an unprecedented sensitivity and down to spatial scales ∼30 �� . Such a global SED will allow a systematic study of the dust evolution processes (e.g. grain growth or fragmentation) that directly affect the SED because they redistribute the dust mass among the observed grain sizes. The dust SED is also affected by variations of the radiation field intensity. Here we present a versatile numerical tool, DustEM, that predicts the emission and extinction of dust grains given their size distribution and their optical and thermal properties. In order to model dust evolution, DustEM has been designed to deal with a variety of grain types, structures and size distributions and to be able to easily include new dust physics. We use DustEM to model the dust SED and extinction in the diffuse interstellar medium at high-galactic latitude (DHGL), a natural reference SED that will allow us to study dust evolution. We present a coherent set of observations for the DHGL SED, which has been obtained by correlating the IR and HI-21 cm data. The dust components in our DHGL model are (i) polycyclic aromatic hydrocarbons; (ii) amorphous carbon and (iii) amorphous silicates. We use amorphous carbon dust, rather than graphite, because it better explains the observed high abundances of gas-phase carbon in shocked regions of the interstellar medium. Using the DustEM model, we illustrate how, in the optically thin limit, the IRAS/Planck HFI (and likewise Spitzer/Herschel for smaller spatial scales) photometric band ratios of the dust SED can disentangle the influence of the exciting radiation field intensity and constrain the abundance of small grains (a < 10 nm) relative to the larger grains. We also discuss the contributions of the different grain populations to the IRAS, Planck (and similarly to Herschel) channels. Such information is required to enable a study of the evolution of dust as well as to systematically extract the dust thermal emission from CMB data and to analyze the emission in the Planck polarized channels. The DustEM code described in this paper is publically available. Dust plays a key role in the physics (e.g. heating of the gas, coupling to the magnetic field) and chemistry (formation of H2, shielding of molecules from dissociative radiation) of the interstellar medium (ISM). Heated by stellar photons, dust grains radiate away the absorbed energy by emission in the near-IR to mm range. Dust emission can thus be used as a tracer of the radiation field intensity and, hence, of star formation activity. Assuming a constant dust abundance, the far-IR to mm dust emission is also used to derive the total column density along a line of sight and to provide mass estimates. The impact of dust on the ISM and the use of its emission as a tracer of the local conditions depends on the dust properties and abundances. It is therefore of major importance to understand dust properties and their evolution throughout the ISM. The instruments onboard the Herschel and Planck satel

Spitzer/IRAC and ISOCAM/CVF insights on the origin of the near to mid-IR Galactic diffuse emission

Spitzer/IRAC images of extended emission provide a new insight on the nature of small dust particles in the Galactic diffuse interstellar medium. We measure IRAC colors of extended emission in several fields covering a range of Galactic latitudes and longitudes outside of star forming regions. We determine the nature of the Galactic diffuse emission in Spitzer/IRAC images by combining them with spectroscopic data. We show that PAH features make the emission in the IRAC 5.8 and 8.0 μ m channels, whereas the 3.3 μ m feature represents only 20 to 50% of the IRAC 3.6 μ m channel. A NIR continuum is necessary to account for IRAC 4.5 μ m emission and the remaining fraction of the IRAC 3.6 μ m emission. This continuum cannot be accounted by scattered light. It represents 9% of the total power absorbed by PAHs and 120% of the interstellar UV photon flux. The 3.3 μ m feature is observed to vary from field-to-field with respect to the IRAC 8.0 μ m channel. The continuum and 3.3 μ m feature intensities are not correlated.
We present model calculations which relate our measurements of the PAHs spectral energy distribution to the particles size and ionization state. Cation and neutral PAHs emission properties are inferred empirically from NGC 7023 observations. PAHs caracteristics are best constrained in a line of sight towards the inner Galaxy, dominated by the Cold Neutral Medium phase: we find that the PAH cation fraction is about 50% and that their mean size is about 60 carbon atoms. A significant field-to-field dispersion in the PAH mean size, from 40 to 80 carbon atoms, is necessary to account for the observed variations in the 3.3 μ m feature intensity relative to the IRAC 8.0 μ m flux. However, one cannot be secure about the feature interpretation as long as the continuum origin remains unclear. The continuum and 3.3 μ m feature emission process could be the same even if they do not share carriers.

The long-wavelength emission of interstellar PAHs: characterizing the spinning dust contribution

Context. The emission of cold dust grains at long wavelengths will soon be observed by the Planck and Herschel satellites and will provide new constraints on the nature of interstellar dust. In particular, the microwave galactic anomalous foreground detected between 10 to 90 GHz, proposed as coming from small spinning grains (PAHs), should help to define these species better. Moreover, understanding the fluctuations of the anomalous foreground quantitatively over the sky is crucial for CMB studies. Aims. We focus on the long-wavelength emission of interstellar PAHs in their vibrational and rotational transitions. We present here the first model that coherently describes the PAH emission from the near-IR to microwave range. Methods. We take quantum effects into account to describe the rotation of PAHs and compare our results to current models of spinning dust to assess the validity of the classical treatment used. Between absorptions of stellar photons, we followed the rovibrational radiative cascade of PAHs. We used the exact-statistical method of Draine & Li to derive the distribution of PAH internal energy and followed a quantum approach for the rotational excitation induced by vibrational (IR) transitions. We also examined the influence of the vibrational relaxation scheme and of the low-energy cross-section on the PAH emission. We study the emissivity of spinning PAHs in a variety of physical conditions (radiation field intensity and gas density), search for specific signatures in this emission that can be looked for observationally, and discuss how the anomalous foreground may constrain the PAH size distribution. Results. Simultaneously predicting the vibrational and rotational emission of PAHs, our model can explain the observed emission of the Perseus molecular cloud from the IR to the microwave range with plausible PAH properties. We show that for λ ≥ 3m m the PAH vibrational emission no longer scales with the radiation field intensity (G0), unlike the mid-IR part of the spectrum (which scales withG0). This emission represents less than 10% of the total dust emission at 100 GHz. Similarly, we find the broadband emissivity of spinning PAHs per carbon atom to be rather constant for G0 ≤ 100 and for proton densities nH < 100 cm −3 . In the diffuse ISM, photon exchange and gas-grain interactions play comparable roles in exciting the rotation of PAHs, and the emissivity of spinning PAHs is dominated by the contribution of small species (bearing less than 100 C atoms). We show that the classical description of rotation used in previous works is a good approximation and that unknowns in the vibrational relaxation scheme and low-energy cross-section affect the PAH rotational emissivity around 30 GHz by less than 15%. Conclusions. The contrasted behaviour of the PAH vibrational and rotational emissivities withG0 provides a clear prediction that can be tested against observations of anomalous and dust mid-IR emissions: this is the subject of a companion paper. Comparison of these emissions complemented with radio observations (21 cm or continuum) will provide constraints on the fraction of small species and the electric dipole moment of interstellar PAHs.

H2 infrared line emission across the bright side of the ρ Ophiuchi main cloud

We present imaging and spectroscopic observations of dust and gas (H2) emission, obtained with ISO, from the western edge of the ρ Ophiuchi molecular cloud illuminated by the B2 star HD147889 (χ ∼ 400). This photodissociation region (PDR) is one of the nearest PDRs to the Sun (d = 135 ± 15 pc from the stellar parallax) and the layer of UV light penetration and of H2 emission is spatially resolved. It is therefore an ideal target to test the prediction of models on the integrated fluxes but also on the spatial distribution. The emission from dust heated by the external UV radiation, from collisionally excited and fluorescent H2 are observed to coincide spatially. The spectroscopic data, obtained with ISO-SWS, allows us to estimate the gas temperature to be 300-345 K in the H2 emitting layer, in which the ortho-to-para H2 ratio is about 1 or significantly smaller than the equilibrium ratio (∼3 at that temperature). We interpret this data with an equilibrium PDR model. In this low excitation PDR, the gas heat budget is dominated by the contribution of the photoelectric heating from very small grains and polycyclic aromatic hydrocarbons (PAHs). With the standard PAH abundance ((C/H)PAH � 5 × 10 −5 ), we find that the H2 formation rate Rf must be high in warm gas (∼6 times the rate derived by Jura, 1975), in order to account for the observed H2 emission. This result and the spatial coincidence between the PAHs and H2 emission suggest that H2 forms efficiently by chemisorption on the PAHs surface. If the latter interpretation is correct, the enhancement in Rf may also result from an increased PAH abundance: assuming that Rf scales with the PAH abundance, the observed H2 excitation is well explained with Rf � 1 × 10 −16 cm 3 s −1 at Tgas = 330 K (∼3 times the rate derived by Jura 1975) and (C/H)PAH � 7.5 × 10 −5 .

Probing the origin of the microwave anomalous foreground

The galactic anomalous microwave emission detected between 10 and 90 GHz is a major foreground to CMB fluctuations. Well correlated to dust emission at 100

Excitation of H2 in photodissociation regions as seen by Spitzer

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Photoelectric effect on dust grains across the L1721 cloud in the

m, the anomalous emission is interstellar but its origin is still debated. Some possible explanations relate it to dust: emission of spinning, small (nanometric) grains carrying a permanent electric dipole or magnetic fluctuations in larger (submicronic) grains. To probe the origin of the anomalous emission, we compare microwave data to dust IR emission and search for specific signatures predicted by models of spinning dust. For the anomalous emission, we use the 23 GHz all-sky map deduced from WMAP data by Miville-Deschenes et al. (2008). The dust emission is traced by IRAS data. Models show that spinning dust emission is little sensitive to the intensity of the radiation field (Go) for 10<nu<30 GHz while the corresponding mid-IR emission is proportional to Go. To test this behaviour in our comparison, we derive Go from the dust temperature maps of Schlegel et al. (1998). From all-sky maps, we show that the anomalous emission is better correlated to the emission of small grains (at 12

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Ophiuchi molecular complex

m) than to that of big grains (at 100

Modelling the spinning dust emission from dense interstellar clouds

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