Diffuse Interstellar Clouds Research Articles

SERENDIPITY OBSERVATIONS OF FAR INFRARED CIRRUS EMISSION IN THE SPITZER INFRARED NEARBY GALAXIES SURVEY: ANALYSIS OF FAR-INFRARED CORRELATIONS

We present an analysis of far-infrared dust emission from diffuse cirrus clouds. This study is based on serendipitous observations at 160 microns at high galactic latitude with the Multiband Imaging Photometer (MIPS) onboard the Spitzer Space Telescope by the Spitzer Infrared Nearby Galaxies Survey (SINGS). These observations are complemented with IRIS data at 100 and 60 microns and constitute one of the most sensitive and unbiased samples of far infrared observations at small scale of diffuse interstellar clouds. Outside regions dominated by the cosmic infrared background fluctuations, we observe a substantial scatter in the 160/100 colors from cirrus emission. We compared the 160/100 color variations to 60/100 colors in the same fields and find a trend of decreasing 60/100 with increasing 160/100. This trend can not be accounted for by current dust models by changing solely the interstellar radiation field. It requires a significant change of dust properties such as grain size distribution or emissivity or a mixing of clouds in different physical conditions along the line of sight. These variations are important as a potential confusing foreground for extragalactic studies.

INFRARED EMISSION BY DUST AROUND λ BOOTIS STARS: DEBRIS DISKS OR THERMALLY EMITTING NEBULAE?

We present a model that describes stellar infrared excesses due to heating of the interstellar (IS) dust by a hot star passing through a diffuse IS cloud. This model is applied to six lambda Bootis stars with infrared excesses. Plausible values for the IS medium (ISM) density and relative velocity between the cloud and the star yield fits to the excess emission. This result is consistent with the diffusion/accretion hypothesis that lambda Bootis stars (A- to F-type stars with large underabundances of Fe-peak elements) owe their characteristics to interactions with the ISM. This proposal invokes radiation pressure from the star to repel the IS dust and excavate a paraboloidal dust cavity in the IS cloud, while the metal-poor gas is accreted onto the stellar photosphere. However, the measurements of the infrared excesses can also be fit by planetary debris disk models. A more detailed consideration of the conditions to produce lambda Bootis characteristics indicates that the majority of infrared-excess stars within the Local Bubble probably have debris disks. Nevertheless, more distant stars may often have excesses due to heating of interstellar material such as in our model.

THE IMPLICATIONS OF A HIGH COSMIC-RAY IONIZATION RATE IN DIFFUSE INTERSTELLAR CLOUDS

Diffuse interstellar clouds show large abundances of H_3^+ which can be maintained only by a high ionization rate of H_2. Cosmic rays are the dominant ionization mechanism in this environment, so the large ionization rate implies a high cosmic-ray flux, and a large amount of energy residing in cosmic rays. In this paper we find that the standard propagated cosmic-ray spectrum predicts an ionization rate much lower than that inferred from H_3^+. Low-energy (~10 MeV) cosmic rays are the most efficient at ionizing hydrogen, but cannot be directly detected; consequently, an otherwise unobservable enhancement of the low-energy cosmic-ray flux offers a plausible explanation for the H_3^+ results. Beyond ionization, cosmic rays also interact with the interstellar medium by spalling atomic nuclei and exciting atomic nuclear states. These processes produce the light elements Li, Be, and B, as well as gamma-ray lines. To test the consequences of an enhanced low-energy cosmic-ray flux, we adopt two physically-motivated cosmic-ray spectra which by construction reproduce the ionization rate inferred in diffuse clouds, and investigate the implications of these spectra on dense cloud ionization rates, light element abundances, gamma-ray fluxes, and energetics. One spectrum proposed here provides an explanation for the high ionization rate seen in diffuse clouds while still appearing to be broadly consistent with other observables, but the shape of this spectrum suggests that supernovae remnants may not be the predominant accelerators of low-energy cosmic rays.

Physical properties of amorphous solid interstellar material from X-ray absorption spectroscopy of Scorpius X-1

High-quality, high-resolution X-ray spectra were obtained of Sco X-1 with the Reflection Grating Spectrometer (RGS) onboard the XMM-Newton satellite. The spectrum around the oxygen K-edge is searched for signatures indicative for EXAFS. Analysis shows a clear indication of EXAFS with photo-electron scattering distances in the absorbing medium in a range that applies to many solids. When scattering of the photo-electron on Oxygen atoms is assumed, the oxygen-to-oxygen atom distances found are 2.75 A. This fits with distances as found in amorphous water ice, although water ice is thought to be an unlikely constituent of the diffuse interstellar clouds that form the absorbing medium towards Sco X-1.

The dissociative recombination of [formula omitted] – a saga coming to an end?

Major progress has been made in the understanding of how H 3 + recombines with thermal and sub-thermal electrons, a process of great importance to the chemistry of diffuse interstellar clouds. Two independent ion storage ring experiments with rovibrationally cold H 3 + ions are in excellent agreement, and quantum mechanical calculations agree with the storage ring results quantitatively for the thermal rate constant, if not in all details concerning the cross section. The recombination mechanism is understood. A direct consequence of this progress is that the cosmic-ray ionization rate in diffuse clouds must be shifted upwards to a value larger than 10 −16 s −1.

Extended CO emission in the field of the light echo of V838 Monocerotis

Context. V838 Mon erupted at the beginning of 2002, becoming an extremely luminous star with L = 10 6 $L_{\odot}$. The outburst was followed by a spectacular light echo that revealed that the star is immersed in a diffuse and dusty medium, plausibly interstellar in nature. Low angular-resolution observations of the star and its closest vicinity in the lowest CO rotational transitions revealed a molecular emission from the direction of V838 Mon. The origin of this CO emission has not been estalished. Aims. The main aim of this paper is to better constrain the nature of the CO emission. In particular, we investigate the idea that the molecular emission originates in the material responsible for the optical light echo. Methods. We performed observations of 13 positions within the light echo in the two lowest rotational transitions of 12 CO using the IRAM 30 m telescope. Results. Emission in CO J = 1-0 and J = 2-1 was detected in three positions. In three other positions only weak J = 1-0 lines were found. The lines appear at two different velocities $V_{\rm LSR}$ = 53.3 km s -1 and V LSR = 48.5 km s -1 , and both components are very narrow with FWHM ≈ 1 km s -1 . Conclusions. The molecular emission from the direction of V838 Mon is extended and has a complex distribution. We identify the emission as arising from diffuse interstellar clouds. A rough estimate of the mass of the molecular matter in those regions gives a few tens of solar masses. The radial velocity of the emission at 53.3 km s -1 suggests that the CO-bearing gas and the echoing dust are collocated in the same interstellar cloud.

Analysis of Molecular Hydrogen Formation on Low-Temperature Surfaces in Temperature Programmed Desorption Experiments

The study of the formation of molecular hydrogen on low-temperature surfaces is of interest both because it enables the exploration of elementary steps in the heterogeneous catalysis of a simple molecule and because of its applications in astrochemistry. Here, we report results of experiments of molecular hydrogen formation on amorphous silicate surfaces using temperature-programmed desorption (TPD). In these experiments, beams of H and D atoms are irradiated on the surface of an amorphous silicate sample. The desorption rate of HD molecules is monitored using a mass spectrometer during a subsequent TPD run. The results are analyzed using rate equations, and the energy barriers of the processes leading to molecular hydrogen formation are obtained from the TPD data. We show that a model based on a single isotope provides the correct results for the activation energies for diffusion and desorption of H atoms. These results are used in order to evaluate the formation rate of H2 on dust grains under the actual conditions present in interstellar clouds. It is found that, under typical conditions in diffuse interstellar clouds, amorphous silicate grains are efficient catalysts of H2 formation when the grain temperatures are between 9 and 14 K. This temperature window is within the typical range of grain temperatures in diffuse clouds. It is thus concluded that amorphous silicates are good candidates to be efficient catalysts of H2 formation in diffuse clouds.

9.7 μm Silicate Absorption in a Damped Lyα Absorber at z = 0.52

We report a detection of the 9.7 μm silicate absorption feature in a damped Lyα absorber (DLA) at zabs = 0.524 toward AO 0235+164, using the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope. The feature shows a broad shallow profile over ≈8-12 μm in the absorber rest frame and appears to be >15 σ significant in equivalent width. The feature is fit reasonably well by the silicate absorption profiles for laboratory amorphous olivine or diffuse Galactic interstellar clouds. To our knowledge, this is the first indication of 9.7 μm silicate absorption in a DLA. We discuss potential implications of this finding for the nature of the dust in quasar absorbers. Although the feature is relatively shallow (τ9.7 ≈ 0.08-0.09), it is ≈2 times deeper than expected from extrapolation of the τ9.7 versus E(B - V) relation known for diffuse Galactic interstellar clouds. Further studies of the 9.7 μm silicate feature in quasar absorbers will open a new window on the dust in distant galaxies.

Molecular Hydrogen Formation on Amorphous Silicates under Interstellar Conditions

Experimental results on the formation of molecular hydrogen on amorphous silicate surfaces are presented for the first time and analyzed using a rate equation model. The energy barriers for the relevant diffusion and desorption processes are obtained. They turn out to be significantly higher than those obtained earlier for polycrystalline silicates, demonstrating the importance of grain morphology. Using these barriers, we evaluate the efficiency of molecular hydrogen formation on amorphous silicate grains under interstellar conditions. It is found that unlike polycrystalline silicates, amorphous silicate grains are efficient catalysts of H2 formation within a temperature range that is relevant to diffuse interstellar clouds. The results also indicate that the hydrogen molecules are thermalized with the surface and desorb with low kinetic energy. Thus, they are unlikely to occupy highly excited states.

Synthetic Observations of Carbon Lines of Turbulent Flows in the Diffuse Multiphase Interstellar Medium

We examine observational characteristics of multiphase turbulent flows in the diffuse interstellar medium (ISM) using a synthetic radiation field of atomic and molecular lines. We consider the multiphase ISM, which is formed by thermal instability under the irradiation of UV photons with moderate visual extinction AV ~ 1. Radiation field maps of C+, C0, and CO line emissions were generated by calculating the non-local thermodynamic equilibrium (non-LTE) level populations from the results of high-resolution hydrodynamic simulations of diffuse ISM models. By analyzing the synthetic radiation field of carbon lines of [C II] 158 μm, [C I] 3P2-3P1 (809 GHz), 3P1-3P0 (492 GHz), and CO rotational transitions, we found a high ratio between the lines of high- and low-excitation energies in the diffuse multiphase ISM. This shows that simultaneous observations of the lines of warm- and cold-gas tracers will be useful in examining the thermal structure, and hence the origin, of diffuse interstellar clouds.

Tracing magnetic fields with aligned grains

Magnetic fields play a crucial role in various astrophysical processes, including star formation, accretion of matter, transport processes (e.g., transport of heat), and cosmic rays. One of the easiest ways to determine the magnetic field direction is via polarization of radiation resulting from extinction or/and emission by aligned dust grains. Reliability of interpretation of the polarization maps in terms of magnetic fields depends on how well we understand the grain-alignment theory. Explaining what makes grains aligned has been one of the big issues of the modern astronomy. Numerous exciting physical effects have been discovered in the course of research undertaken in this field. As both the theory and observations matured, it became clear that the grain-alignment phenomenon is inherent not only in diffuse interstellar medium or molecular clouds but also is a generic property of the dust in circumstellar regions, interplanetary space and cometary comae. Currently the grain-alignment theory is a predictive one, and its results nicely match observations. Among its predictions is a subtle phenomenon of radiative torques. This phenomenon, after having stayed in oblivion for many years after its discovery, is currently viewed as the most powerful means of alignment. In this article, I shall review the basic physical processes involved in grain alignment, and the currently known mechanisms of alignment. I shall also discuss possible niches for different alignment mechanisms. I shall dwell on the importance of the concept of grain helicity for understanding of many properties of grain alignment, and shall demonstrate that rather arbitrarily shaped grains exhibit helicity when they interact with gaseous and radiative flows.

Effective rate coefficients for molecular hydrogen formation in diffuse interstellar clouds

Aims. A method to determine effective rate coefficients for H 2 formation on grain surfaces is proposed based on the continuous-time, random-walk Monte Carlo simulation. Methods. Our Monte Carlo simulation is used to calculate the efficiency of molecular hydrogen formation via recombination of H atoms on flat and rough surfaces of olivine and amorphous carbon under a variety of diffuse cloud conditions. The results are then fitted to two types of rate laws to determine effective rate coefficients for use in rate equation treatments. These rate coefficients can be utilized in pure gas-phase models, where the rate of molecular formation is associated with a grain collision rate for H atoms multiplied by an efficiency factor, and in gas-grain models, where the actual diffusion of H atoms on a grain surface is considered. Results. The effective rate coefficients are tabulated as a function of incoming atomic hydrogen flux and surface temperature, over temperature ranges for which the surfaces show a reasonable efficiency of molecular hydrogen formation. For the flat surfaces studied, corrections to the standard rate treatment do not exceed a factor of three.

Diffuse Atomic and Molecular Clouds

Diffuse interstellar clouds have long been thought to be relatively devoid of molecules, because of their low densities and high radiation fields. However, in the past ten years or so, a plethora of polyatomic molecules have been observed in diffuse clouds, via their rotational, vibrational, and electronic transitions. In this review, we propose a new systematic classification method for the different types of interstellar clouds: diffuse atomic, diffuse molecular, translucent, and dense. We review the observations of molecules (both diatomic and polyatomic) in diffuse clouds and discuss how molecules can be utilized as indicators of the physical and chemical conditions within these clouds. We review the progress made in the modeling of the chemistry in these clouds, and the (significant) challenges that remain in this endeavor. We also review the evidence for the existence of very large molecules in diffuse clouds, and discuss a few specific clouds of particular interest.

Monte Carlo studies of surface chemistry and nonthermal desorption involving interstellar grains.

Although still poorly understood, the chemistry that occurs on the surfaces of interstellar dust particles profoundly affects the growth of molecules in the interstellar medium. The most important surface reaction is the conversion of atomic to molecular hydrogen, which is a precursor for all subsequent molecular development and which occurs both in diffuse and dense interstellar clouds. Another set of surface reactions produces icy mantles of many monolayers in cold and dense regions of the interstellar medium. The monolayers are dominated by water ice but also contain CO, CO(2), and occasionally methanol. In this work, we first review both our stochastic approach to the surface chemistry that can occur on small dust particles and how it has been applied to the problem of the formation of molecular hydrogen. This latter problem is strongly affected by the pulsed heating of smaller grains by photons. Photons are not the only source of pulsed heating; cosmic rays also can heat interstellar grains in a pulsed manner. Here, we calculate the heating by cosmic rays for different grain sizes and cosmic ray components. It is then shown that this mechanism is an important one for desorption of ice mantles.

Synthetic observations of turbulent flows in diffuse multiphase interstellar medium

AbstractWe examined observational characteristics of multi-phase turbulent flows in the diffuse interstellar medium (ISM) by calculating atomic and molecular carbon lines. Radiation field maps of C+, C0, and CO line emissions were generated by calculating the non-local thermodynamic equilibrium (nonLTE) level populations and high resolution hydrodynamic simulations of diffuse ISM. By analyzing synthetic line emission, we found a high ratio between the lines of high- and low-excitation energies in the diffuse multi-phase interstellar medium. Our results shows that simultaneous observations of the lines of warm- and cold-gas tracers will be useful in examining the thermal structure, and hence the origin of diffuse interstellar clouds.

Optical and Infrared Observations of Diffuse Clouds

In the past several years, great progress has been made on the spectroscopy of polyatomic molecules in diffuse interstellar clouds. In this talk, I will review recent developments involving H (Thorburn et al. 2003) as well as some well-correlated pairs of DIBs. I will discuss our recent results.

The Gas Phase Spectrum of Cyclic C18and the Diffuse Interstellar Bands

The gas phase spectrum of the cyclic C18 molecule recorded in the laboratory at a temperature typical of diffuse interstellar clouds is compared with absorption features toward ζ Oph and HD 204827 in the 5730-5934 A region. For the origin band at 5928.5 A an upper limit to the column density of ≤1.8 × 1011 cm-2 is inferred. The origin band pattern in the laboratory spectrum changes on lowering the internal temperature in the 100-20 K range and bears a striking resemblance to the observed structure of a number of DIBs at other wavelengths. This suggests that platelike molecules or ions, comprising a couple of dozen to hundred carbon atoms, could be responsible for some of the latter absorptions.

H2 excitation in turbulent interstellar clouds

We discuss the observational differences between lines of sight that intercept a group of turbulent dissipative structures and lines of sight that cross less-active regions. Using time-dependent calculations we show that the energy level distribution of the hydrogen molecule evolves in time in response to the local thermal phase. We find that relatively simple models can explain the observed properties of molecular hydrogen in diffuse interstellar clouds in terms of time evolution induced by collisional excitation in a low-density, high-temperature gas.

FUSE observations of molecular hydrogen on the line of sight towards HD 141569A

We present an analysis of the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of HD 141569A, a transitional object known to possess a circumstellar disk. We observe two components of gas at widely different temperatures along the line of sight. We detect cold H 2 , which is thermalized up to J = 2 at a kinetic temperature of ∼51 K. Such low temperatures are typical of the diffuse interstellar medium. Since the line of sight to HD 141569A does not pass through its disk. it appears that we are observing the cold H 2 in a low extinction envelope associated with the high Galactic latitude dark cloud complex L134N, which is in the same direction and at nearly the same distance as HD 141569A. The column densities of the higher J-levels of H 2 suggest the presence of warm gas along the line of sight. The excitation conditions do not seem to he consistent with what is generally observed in diffuse interstellar clouds. The observed radial velocity of the gas implies that the UV spectral lines we observe are likely interstellar in origin rather than circumstellar, although our absorption line study does not definitely rule out the possibility that the warm gas is close to the star. The discovery of such warm gas along the line of sight may provide evidence for turbulent phenomena in the dark cloud L134N.

A quantitative study of proton irradiation and UV photolysis of benzene in interstellar environments

Benzene is an essential intermediate in the formation pathways of polycyclic aromatic hydrocarbons (PAHs) and carbon dust. Therefore, it is important to understand the interplay of formation and destruction in order to assess the lifetime of benzene in space. We performed UV photolysis and proton (0.8 MeV) bombardment experiments on benzene (C6H6) isolated in inert argon matrices and in oxygen-rich solid mixtures in the laboratory. The destruction of benzene in different chemical environments was measured for both methods of energetic processing. Additionally, we quantitatively determined the absorbed photon fraction in the sample layers when exposed to our UV lamp with actinometry. This enabled us to derive destruction cross sections for benzene for both UV photolysis and proton bombardment allowing us to compare these two ways of energetic processing. The laboratory data were extrapolated to different interstellar environments and we found that benzene is efficiently destroyed in diffuse interstellar clouds, but could survive dense cloud environments longer than the average lifetime of the cloud. Benzene is likely to survive in the dense parts of circumstellar envelopes around carbon-rich AGB stars but only in a very finite region where UV photons are attenuated.