Adolf N. Witt

Interstellar Depletions Updated: Where All the Atoms Went

Measures of the depletion of interstellar elements from the gas phase are usually derived by assuming that the general composition of the interstellar medium is identical to that of the Sun. A compilation of stellar composition data, including B stars as well as field F and G stars, however, calls this assumption into question. In this Letter we consider the impact on derived depletions if the reference abundances are derived from stars in the solar neighborhood rather than from the Sun, and we discuss the implications for current models of the interstellar dust. Using recent, accurate gas-phase column densities for ζ Ophiuchi, we show that the systematically lower depletions resulting from our revised cosmic abundances are in conflict with most dust models because insufficient quantities of raw materials are available to explain the observed extinction. A Kramers-Kronig analysis for the ζ Oph line of sight shows that the revised depletions are consistent with the required opacity of interstellar dust only if the density of the grain material is near 1 g cm-3, suggesting that the grain structure must be open (i.e., the grains must by fluffy, porous, or fractal in structure).

Dust and the transfer of stellar radiation within galaxies

We present and discuss models for the transfer of ultraviolet, visual, and near-infrared radiation within a variety of spherical geometries which are intended to approximate different types of dusty environments within galaxies. Among those objects that our models approximate are normal and starburst galaxies, active galactic nuclei and QSOs, colliding systems with a widely dispersed interstellar medium, and elliptical galaxies. Our mathematical technique is a complete spherically symmetric, three-dimensional Monte Carlo simulation which covers a wide range of dust optical depths for 10 photometric bands

Dust in Starburst Galaxies

To investigate the nature of starburst dust, we constructed a model of the stars and dust in starburst galaxies and applied it to 30 observed starburst spectral energy distributions (SEDs). The starburst model was constructed by combining two stellar evolutionary synthesis models with a model describing the radiative transfer of stellar photons through dust. The stellar evolutionary synthesis models were used to compute the dust-free SEDs for stellar populations with ages between 1 ? 106 and 15 ? 109 yr. Using a Monte Carlo radiative transfer model, the effects of dust were computed for average Milky Way (MW) and Small Magellanic Cloud (SMC) dust, two different star/dust geometries, and locally homogeneous or clumpy dust. Using color-color plots, the starburst model was used to interpret the behavior of 30 starbursts with aperture-matched UV and optical SEDs (and IR for 19 of the 30) from previous studies. From the color-color plots, it was evident that the dust in starbursts has an extinction curve lacking a 2175 ? bump, like the SMC curve, and a steep far-UV rise, intermediate between the MW and SMC curves. The star/dust geometry that is able to explain the distribution of the 30 starbursts in various color-color plots has an inner dust-free sphere of stars surrounded by an outer star-free shell of clumpy dust. When combined with other work from the literature on the Orion region and the 30 Dor region of the Large Magellanic Cloud, this work implies a trend in dust properties with star formation intensity.

Dust in the Local Interstellar Wind

The gas-to-dust mass ratios found for interstellar dust within the solar system, versus values determined astronomically for the cloud around the solar system, suggest that large and small interstellar grains have separate histories and that large interstellar grains preferentially detected by spacecraft are not formed exclusively by mass exchange with nearby interstellar gas. Observations by the Ulysses and Galileo satellites of the mass spectrum and flux rate of interstellar dust within the heliosphere are combined with information about the density, composition, and relative flow speed and direction of interstellar gas in the cloud surrounding the solar system to derive an in situ value for the gas-to-dust mass ratio, Rg/d = 94. This ratio is dominated by the larger near-micron-sized grains. Including an estimate for the mass of smaller grains, which do not penetrate the heliosphere owing to charged grain interactions with heliosheath and solar wind plasmas, and including estimates for the mass of the larger population of interstellar micrometeorites, the total gas-to-dust mass ratio in the cloud surrounding the solar system is half this value. Based on in situ data, interstellar dust grains in the 10-14 to 10-13 g mass range are underabundant in the solar system, compared to a Mathis, Rumple, & Nordsiek mass distribution scaled to the local interstellar gas density, because such small grains do not penetrate the heliosphere. The gas-to-dust mass ratios are also derived by combining spectroscopic observations of the gas-phase abundances in the nearest interstellar clouds. Measurements of interstellar absorption lines formed in the cloud around the solar system, as seen in the direction of CMa, give Rg/d = 427 for assumed solar reference abundances and Rg/d = 551 for assumed B star reference abundances. These values exceed the in situ value suggesting either that grain mixing or grain histories are not correctly understood or that sweptup stardust is present. Such high values for diffuse interstellar clouds are strongly supported by diffuse cloud data seen toward λ Sco and 23 Ori, provided B star reference abundances apply. If solar reference abundances prevail, however, the surrounding cloud is seen to have greater than normal dust destruction compared to higher column density diffuse clouds. The cloud surrounding the solar system exhibits enhanced gas-phase abundances of refractory elements such as Fe+ and Mg+, indicating the destruction of dust grains by shock fronts. The good correlation locally between Fe+ and Mg+ indicates that the gas-phase abundances of these elements are dominated by grain destruction, while the poor correlation between Fe+ and H0 indicates either variable gas ionization or the decoupling of neutral gas and dust over parsec scale lengths. These abundances, combined with grain destruction models, indicate that the nearest interstellar material has been shocked with shocks of velocity ~150 km s-1. If solar reference abundances are correct, the low Rg/d value toward λ Sco may indicate that at least one cloud component in this direction contains dust grains that have retained their silicate mantles and are responsible for the polarization of the light from nearby stars seen in this general region. Weak frictional coupling between gas and dust in nearby low density gas permit inhomogeneities to be present.

The Interstellar Carbon Budget and the Role of Carbon in Dust and Large Molecules

Published data on stellar composition show that carbon in the sun is substantially more abundant than in other stars. A carbon abundance of 225 carbon atoms per 106 hydrogen atoms is representative of galactic stars, whereas published values for the sun range from 350 to 470 carbon atoms per 106 hydrogen atoms. Other elements are also present in enhanced quantities in the solar system, consistent with suggestions that a supernova event was closely associated with the formation of the solar system. The overabundance of carbon in the solar system has many important implications, including new constraints on nucleosynthesis models for supernovae and substantial modification of the so-called “cosmic” composition normally adopted in discussions of galactic and interstellar abundances. A reduction in the galactic carbon budget, as suggested by the stellar composition data, strongly constrains the quantity of carbon that is available for the formation of interstellar dust, and some dust models now appear implausible because they require more carbon than is available.

The DIRTY Model. I. Monte Carlo Radiative Transfer through Dust

We present the DIRTY (DustI Radiative Transfer, Yeah!) radiative transfer model in this paper and a companion paper. This model computes the polarized radiative transfer of photons from arbitrary distributions of stars through arbitrary distributions of dust using Monte Carlo techniques. The dust re-emission is done self-consistently with the dust absorption and scattering and includes all three important emission paths: equilibrium thermal emission, nonequilibrium thermal emission, and the aromatic features emission. The algorithm used for the radiative transfer allows for the efficient computation of the appearance of a model system as seen from any viewing direction. We present a simple method for computing an upper limit on the output quantity uncertainties for Monte Carlo radiative transfer models that use the weighted photon approach.

Detection of Extended Red Emission in the Diffuse Interstellar Medium

Extended red emission (ERE) has been detected in many dusty astrophysical objects, raising the question of whether ERE is present only in discrete objects or if it is an observational feature of all dust, i.e., present in the diffuse interstellar medium. In order to answer this question, we determined the blue and red intensities of the radiation from the diffuse interstellar medium (ISM) and examined the red intensity for the presence of an excess above that expected for scattered light. The diffuse ISM blue and red intensities were obtained by subtracting the integrated star and galaxy intensities from the blue and red measurements made by the Imaging Photopolarimeters (IPPs) aboard the Pioneer 10 and 11 spacecraft. The unique characteristic of the Pioneer measurements is that they were taken outside the zodiacal dust cloud and, therefore, are free from zodiacal light. The color of the diffuse ISM was found to be redder than the Pioneer intensities. If the diffuse ISM intensities were entirely caused by scattering from dust (i.e., diffuse Galactic light or DGL), the color of the diffuse ISM would be bluer than the Pioneer intensities. Finding a redder color implies the presence of an excess red intensity. Using a model for the DGL, we found the blue diffuse ISM intensity to be entirely attributable to the DGL. The red DGL was calculated using the blue diffuse ISM intensities and the approximately invariant color of the DGL calculated with the DGL model. Subtracting the calculated red DGL from the red diffuse ISM intensities resulted in the detection of an excess red intensity with an average value of ~10 S10(V)G2 V. This represents the likely detection of ERE in the diffuse ISM since Hα emission cannot account for the strength of this excess and the only other known emission process applicable to the diffuse ISM is ERE. Thus, ERE appears to be a general characteristic of dust. The correlation between NH I and ERE intensity is (1.43 ± 0.31) × 10-29 ergs s-1 A-1 sr-1 H atom-1, from which the ERE photon conversion efficiency was estimated at 10% ± 3%.

Dust Attenuation in Late-Type Galaxies. I. Effects on Bulge and Disk Components

We present results of new Monte Carlo calculations made with the DIRTY code of radiative transfer of stellar and scattered radiation for a dusty giant late-type galaxy like the Milky Way, which illustrate the effect of the attenuation of stellar light by internal dust on the integrated photometry of the individual bulge and disk components. Here we focus on the behavior of the attenuation function, the color excess, and the fraction of light scattered or directly transmitted toward the outside observer as a function of the total amount of dust and the inclination of the galaxy, and the structure of the dusty interstellar medium (ISM) of the disk. We confirm that dust attenuation produces qualitatively and quantitatively different effects on the integrated photometry of bulge and disk, whatever the wavelength. In addition, we find that the structure of the dusty ISM affects more sensitively the observed magnitudes than the observed colors of both bulge and disk. Finally, we show that the contribution of the scattered radiation to the total monochromatic light received by the outside observer is significant, particularly at UV wavelengths, even for a two-phase, clumpy, dusty ISM. Thus, understanding dust scattering properties is fundamental for the interpretation of extragalactic observations in the rest-frame UV.

Silicon Nanoparticles: Source of Extended Red Emission?

We have reviewed the characteristics of the extended red emission (ERE) as observed in many dusty astronomical environments, in particular, the diffuse interstellar medium of the Galaxy. The spectral nature and the photon conversion efficiency of the ERE identify the underlying process as highly efficient photoluminescence by an abundant component of interstellar dust. We have compared the photoluminescence properties of a variety of carbon- and silicon-based materials proposed as sources for the ERE with the observationally established constraints. We found that silicon nanoparticles provide the best match to the spectrum and to the efficiency requirement of the ERE. If present in interstellar space with an abundance sufficient to explain the intensity of the ERE, silicon nanoparticles will also contribute to the interstellar 9.7 μm Si—O stretch feature in absorption, to the near- and mid-IR nonequilibrium thermal background radiation, and to the continuum extinction in the near- and far-UV. About 36% of the interstellar silicon that is depleted into the dust phase would be needed in the form of silicon nanoparticles, amounting to less than 5% of the interstellar dust mass. We propose that silicon nanoparticles form through the nucleation of SiO in oxygen-rich stellar mass outflows and that they represent an important small-grain component of the interstellar dust spectrum.

The Excitation of Extended Red Emission: New Constraints on Its Carrier from Hubble Space Telescope Observations of NGC 7023*

The carrier of the dust-associated photoluminescence process causing ERE in many dusty interstellar environments remains unidentified. Several competing models are more or less able to match the observed broad, unstructured ERE band. We now constrain the character of the ERE carrier further by determining the wavelengths of the radiation that initiates the ERE. Using the imaging capabilities of the HST, we have resolved the width of narrow ERE filaments appearing on the surfaces of externally illuminated molecular clouds in the bright reflection nebula NGC 7023 and compared them with the depth of penetration of radiation of known wavelengths into the same cloud surfaces. We identify photons with wavelengths shortward of 118 nm as the source of ERE initiation, not to be confused with ERE excitation, however. There are strong indications from the well-studied ERE in the Red Rectangle Nebula and in the high-|b| Galactic cirrus that the photon flux with wavelengths shortward of 118 nm is too small to actually excite the observed ERE, even with 100% quantum efficiency. We conclude, therefore, that ERE excitation results from a two-step process. The first, involving far-UV photons with E > 10.5 eV, leads to the creation of the ERE carrier, most likely through photoionization or photodissociation of an existing precursor. The second, involving more abundant near-UV/optical photons, consists of the optical pumping of the previously created carrier, followed by subsequent deexcitation via photoluminescence. The latter process can occur many times for a single particle, depending upon the lifetime of the ERE carrier in its active state. While none of the previously proposed ERE models can match these new constraints, we note that under interstellar conditions most PAH molecules are ionized to the dication stage by photons with E > 10.5 eV and that the electronic energy level structure of PAH dications is consistent with fluorescence in the wavelength band of the ERE. Therefore, PAH dications deserve further study as potential carriers of the ERE.