arXiv subjects

Laurent Verstraete

Publications and source records attributed to Laurent Verstraete.

10 recordsLinked to original sources

arXiv2026

ALMA Band1 observations of the rhoOphW filament I. Enhanced power from excess microwave emission at high spatial frequencies

The rhoOphW photo-dissociation region (PDR) is an example source of bright excess microwave emission (EME), over synchrotron, free-free, and the Rayleigh-Jeans tail of the sub-millimetre (sub-mm) dust continuum. Its filamentary morphology follows roughly that of the IR poly-cyclic aromatic hydrocarbon (PAHs) bands. The EME signal in rhoOphW drops abruptly above ~30GHz and its spectrum can be interpreted in terms of electric-dipole radiation from spinning dust grains, or ``spinning dust''. Deep and high-fidelity imaging and spectroscopy of rhoOphW may reveal the detailed morphology of the EME signal, free from imaging priors, while also enabling a search for fine structure in its spectrum. The same observations may constrain the spectral index of the high-frequency drop. An ALMA Band1 mosaic yields a deep deconvolved image of the filament at 36-44GHz, which we use as template for the extraction of a spectrum via cross-correlation in the uv-plane. Simulations and cross-correlations on near-infrared ancillary data yield estimates of flux-loss and biases. The spectrum is a power law, with no detectable fine structure. It follows a spectral index alpha=-0.78+-0.05, in frequency, with some variations along the filament. Interestingly, the Band1 power at high spatial frequencies increases relative to that of the IR signal, with a factor of two more power in Band1 at ~20'' than at ~100'' (relative to IRAC3.6um). An extreme of such radio-only structures is a compact EME source, without IR counterpart. It is embedded in strong and filamentary Band1 signal, while the IRAC maps are smooth in the same region. We provide multi-frequency intensity estimates for spectral modelling.

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arXiv2024

THEMIS 2.0: A self-consistent model for dust extinction, emission, and polarisation

Recent observations in emission, extinction, and polarisation have at least partially invalidated most of the astronomical standard grain models for the diffuse ISM. Moreover, lab measurements on interstellar silicate analogues have shown differences with the optical properties used in these standard models. To address these issues, our objective is twofold: (i) to update the optical properties of silicates and (ii) to develop the THEMIS dust model to allow the calculation of polarised extinction and emission. Based on optical constants measured in the lab for amorphous silicates and on observational constraints in mid-IR extinction and X-ray scattering, we defined new optical constants for the THEMIS silicates. Absorption and scattering efficiencies for spheroidal grains were then derived with the discrete dipole approximation. These new optical properties make it possible to explain the dust emission and extinction, both total and polarised. The model is not yet pushed to its limits since it does not require the perfect alignment of all grains to explain the observations and it therefore has the potential to accommodate the highest polarisation levels inferred from extinction measures. Moreover, the dispersion of the optical properties of the different lab silicates naturally explain the variations in both the total and polarised emission and extinction observed in the diffuse ISM. A single, invariant model calibrated on one single set of observations is obsolete for explaining contemporary observations. We are proposing a completely flexible dust model based entirely on lab measurements that has the potential to make major advances in understanding the nature of ISM grains and how they evolve as a function of their environment. Even if challenging, this is also relevant for future missions that will aim to perform precise measurements of the CMB spectral distortions and polarisation.

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arXiv2024

PDRs4All VIII: Mid-IR emission line inventory of the Orion Bar

Mid-infrared emission features probe the properties of ionized gas, and hot or warm molecular gas. The Orion Bar is a frequently studied photodissociation region (PDR) containing large amounts of gas under these conditions, and was observed with the MIRI IFU aboard JWST as part of the "PDRs4All" program. The resulting IR spectroscopic images of high angular resolution (0.2") reveal a rich observational inventory of mid-IR emission lines, and spatially resolve the substructure of the PDR, with a mosaic cutting perpendicularly across the ionization front and three dissociation fronts. We extracted five spectra that represent the ionized, atomic, and molecular gas layers, and measured the most prominent gas emission lines. An initial analysis summarizes the physical conditions of the gas and the potential of these data. We identified around 100 lines, report an additional 18 lines that remain unidentified, and measured the line intensities and central wavelengths. The H I recombination lines originating from the ionized gas layer bordering the PDR, have intensity ratios that are well matched by emissivity coefficients from H recombination theory, but deviate up to 10% due contamination by He I lines. We report the observed emission lines of various ionization stages of Ne, P, S, Cl, Ar, Fe, and Ni, and show how certain line ratios vary between the five regions. We observe the pure-rotational H$_2$ lines in the vibrational ground state from 0-0 S(1) to 0-0 S(8), and in the first vibrationally excited state from 1-1 S(5) to 1-1 S(9). We derive H$_2$ excitation diagrams, and approximate the excitation with one thermal (~700 K) component representative of an average gas temperature, and one non-thermal component (~2700 K) probing the effect of UV pumping. We compare these results to an existing model for the Orion Bar PDR and highlight the differences with the observations.

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arXiv2022

Spinning nano-carbon grains: Viable origin for anomalous microwave emission

Context. Excess microwave emission, commonly known as anomalous microwave emission (AME), is now routinely detected in the Milky Way. Although its link with the rotation of interstellar (carbonaceous) nano-grains seems to be relatively well established at cloud scales, large-scale observations show a lack of correlation between the different tracers of nano-carbons and AME, which has led the community to question the viability of this link. Aims. Using ancillary data and spinning dust models for nano-carbons and nano-silicates, we explore the extent to which the AME that come out of the Galactic Plane might originate with one or another carrier. Methods. In contrast to previous large-scale studies, our method is not built on comparing the correlations of the different dust tracers with each other, but rather on comparing the poor correlations predicted by the models with observed correlations. This is based on estimates that are as realistic as possible of the gas ionisation state and grain charge as a function of the local radiation field and gas density. Results. First, nano-carbon dust can explain all the observations for medium properties, in agreement with the latest findings about the separation of cold and warm neutral medium in the diffuse interstellar medium. The dispersion in the observations can be accounted for with little variations in the dust size distribution, abundance, or electric dipole moment. Second, regardless of the properties and abundance of the nano-silicate dust we considered, spinning nano-silicates are excluded as the sole source of the AME. Third, the best agreement with the observations is obtained when the emission of spinning nano-carbons alone is taken into account. However, a marginal participation of nano-silicates in AME production cannot be excluded as long as their abundance does not exceed $Y_{\rm Si} \sim 1\%$.

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arXiv2014

Studies of Anomalous Microwave Emission (AME) with the SKA

In this chapter, we will outline the scientific motivation for studying Anomalous Microwave Emission (AME) with the SKA. AME is thought to be due to electric dipole radiation from small spinning dust grains, although thermal fluctuations of magnetic dust grains may also contribute. Studies of this mysterious component would shed light on the emission mechanism, which then opens up a new window onto the interstellar medium (ISM). AME is emitted mostly in the frequency range $\sim 10$--100\,GHz, and thus the SKA has the potential of measuring the low frequency side of the AME spectrum, particularly in band 5. Science targets include dense molecular clouds in the Milky Way, as well as extragalactic sources. We also discuss the possibility of detecting rotational line emission from Poly-cyclic Aromatic Hydrocarbons (PAHs), which could be the main carriers of AME. Detecting PAH lines of a given spacing would allow for a definitive identification of specific PAH species.

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arXiv2014

The cycling of carbon into and out of dust

Observational evidence seems to indicate that the depletion of interstellar carbon into dust shows rather wide variations and that carbon undergoes rather rapid recycling in the interstellar medium (ISM). Small hydrocarbon grains are processed in photo-dissociation regions by UV photons, by ion and electron collisions in interstellar shock waves and by cosmic rays. A significant fraction of hydrocarbon dust must therefore be re-formed by accretion in the dense, molecular ISM. A new dust model (Jones et al., Astron. Astrophys., 2013, 558, A62) shows that variations in the dust observables in the diffuse interstellar medium (nH = 1000 cm^3), can be explained by systematic and environmentally-driven changes in the small hydrocarbon grain population. Here we explore the consequences of gas-phase carbon accretion onto the surfaces of grains in the transition regions between the diffuse ISM and molecular clouds (e.g., Jones, Astron. Astrophys., 2013, 555, A39). We find that significant carbonaceous dust re-processing and/or mantle accretion can occur in the outer regions of molecular clouds and that this dust will have significantly different optical properties from the dust in the adjacent diffuse ISM. We conclude that the (re-)processing and cycling of carbon into and out of dust is perhaps the key to advancing our understanding of dust evolution in the ISM.

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arXiv2010

Excitation of H$_2$ in photodissociation regions as seen by Spitzer

We present spectroscopic observations obtained with the infrared Spitzer Space Telescope, which provide insight into the H$_2$ physics and gas energetics in photodissociation Regions (PDRs) of low to moderate far-ultraviolet (FUV) fields and densities. We analyze data on six well known Galactic PDRs (L1721, California, N7023E, Horsehead, rho Oph, N2023N), sampling a poorly explored range of excitation conditions ($χ\sim 5-10^3$), relevant to the bulk of molecular clouds in galaxies. Spitzer observations of H$_2$ rotational lines are complemented with H$_2$ data, including ro-vibrational line measurements, obtained with ground-based telescopes and ISO, to constrain the relative contributions of ultraviolet pumping and collisions to the H$_2$ excitation. The data analysis is supported by model calculations with the Meudon PDR code. The observed column densities of rotationally excited H$_2$ are observed to be much higher than PDR model predictions. In the lowest excitation PDRs, the discrepancy between the model and the data is about one order of magnitude for rotational levels $J \ge $3. We discuss whether an enhancement in the H$_2$ formation rate or a local increase in photoelectric heating, as proposed for brighter PDRs in former ISO studies, may improve the data-model comparison. We find that an enhancement in the H$_2$ formation rates reduces the discrepancy, but the models still fall short of the data. This large disagreement suggests that our understanding of the formation and excitation of H$_2$ and/or of PDRs energetics is still incomplete. We discuss several explanations, which could be further tested using the Herschel Space Telescope

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arXiv2009

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

The emission of cold dust grains at long wavelengths will soon be observed by the Planck and Herschel satellites and provide new constraints on the nature of interstellar dust. The microwave anomalous emission, proposed to be due to spinning PAHs, should help to better define these species. Moreover, understanding the fluctuations of the anomalous emission over the sky is crucial for CMB studies. We focus on the long wavelength emission of interstellar PAHs in their rovibrational and rotational transitions. The PAH emission spectrum from the IR to the microwave range is presented and compared to anomalous emission observations. To model their long wavelength emission, we treat PAHs as isolated systems and follow consistently their IR and rotational emissions. We consider several interstellar phases and discuss how the anomalous emission may constrain their size distribution. Our model of PAH emission accounts for the mid-IR spectra of the diffuse interstellar medium and of the Orion Bar. For lambda<3mm the PAH IR emission does not scale any more with the radiation field intensity (Go) unlike the mid-IR part of the spectrum. This emission represents less than 10% of the total dust emission at 100 GHz. We find the broadband emissivity of spinning PAHs per carbon atom to be rather constant for Go<100 and nH<100cm-3. Observations of anomalous emission in the Perseus molecular cloud are explained by our model of spinning PAH emission with a standard abundance, supporting PAHs as the origin of the anomalous emission. The behaviour of the spinning dust emissivity with Go provides a clear test that can be tested against observations of anomalous and dust mid-IR emissions. Comparison of these emissions will provide constraints on the size and/or electric dipole moment of interstellar PAHs.

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arXiv2009

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 $μ$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 $μ$m) than to that of big grains (at 100 $μ$m). In addition we show that the former correlation is significantly improved when the 12 $μ$m flux is divided by Go, as predicted by current models of spinning dust.

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arXiv2003

Determination of the gas-to-dust ratio in nearby dense clouds using X-ray absorption measurements

We present a comparison of the gas and dust properties of the dense interstellar matter in six nearby star-forming regions (d<500 pc): rho Oph, Cha I, R CrA, IC 348, NGC 1333, and Orion. We measure from Chandra and XMM-Newton observations the X-ray absorption toward pre-main sequence stars (PMS) without accretion disks (i.e., Class III sources) to obtain the total hydrogen column density N_{H,X}. For these sources we take from the literature the corresponding dust extinction in the near-infrared, A_J, or when unavailable we derive it from SED fitting using the available DENIS, 2MASS, ISOCAM and other data. We then compare N_{H,X} and A_J for each object, up to unprecedently high extinction. For the rho Oph dark cloud with a relatively large sample of 20 bona-fide Class III sources, we probe the extinction up to A_J <~ 14 (A_V <~ 45), and find a best-fit linear relation N_{H,X}/A_J = 5.6 (+/- 0.4)x10^{21} cm^{-2} mag^{-1}, adopting standard ISM abundances. The other regions reveal a large dispersion in the N_{H,X}/A_J ratio for each source but for lack of adequate IR data these studies remain limited to moderate extinctions (A_J <~ 1.5 or A_V <~5). For rho Oph, the N_{H,X}/A_J ratio is significantly lower (>~2 sigma) than the galactic value, derived using the standard extinction curve (R_V = 3.1). This result is consistent with the recent downwards revision of the metallicity of the Sun and stars in the solar vicinity. We find that the rho Oph dense cloud has the same metallicity than the local ISM when assuming that the galactic gas-to-dust ratio remains unchanged. The difference between galactic and local values of the gas-to-dust ratio can thus be attributed entirely to a difference in metallicity.

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