Extinction Feature Research Articles

Infrared dark clouds as precursors to star clusters

Infrared dark clouds (IRDCs) are cold, dense molecular clouds identified as extinction features against the bright mid-infrared Galactic background. Our recent 1.2 mm continuum emission survey of IRDCs reveals many compact (<0.5 pc) and massive (10-2100 M ⊙ ) cores within them. These prestellar cores hold the key to understanding IRDCs and their role in star formation. Here, we present high angular resolution spectral-line and mm/sub-mm continuum images obtained with the IRAM Plateau de Bure Interferometer and the Sub-Millimeter Array towards three high-mass IRDC cores. The high angular resolution images reveal that two of the cores are resolved into multiple, compact protostellar condensations, while the remaining core contains a single, compact protostellar condensation with a very rich molecular spectrum, indicating that it is a hot molecular core. The derived gas masses for these condensations suggest that each core is forming at least one high-mass protostar, while two of the cores are also forming lower-mass protostars. The close proximity of multiple protostars of disparate mass indicates that these IRDCs are in the earliest evolutionary states in the formation of stellar clusters.

NANODIAMONDS ENVELOPED IN GLASSY CARBON SHELLS AND THE ORIGIN OF THE 2175 Å OPTICAL EXTINCTION FEATURE

It is shown that small nanodiamond particles encapsulated into cages of glassy carbon may contribute to the 2175 A (5.67 eV) interstellar extinction feature, in the normalized interstellar extinction curve, for a ratio between the size of the diamond core to its overall exterior radius in the range 0.6-0.9. Results for optical absorption by diamond nanoclusters encased in multishell fullerenes are also presented. These are also possible candidates to explain the feature.

PROBING COSMIC DUST OF THE EARLY UNIVERSE THROUGH HIGH-REDSHIFT GAMMA-RAY BURSTS

We explore the extinction properties of the dust in the distant universe through the afterglows of high-redshifted GRBs based on the Drude model which, unlike previous studies, does not require a prior assumption of template extinction laws. We select GRB070802 at z~2.45 (which shows clear evidence for the 2175\AA extinction bump) and GRB050904 at z~6.29, the 2nd most distant GRB observed to date. We fit their afterglow spectra to determine the extinction of their host galaxies. We find that (1) their extinction curves differ substantially from that of the Milky Way, the Small and Large Magellanic Clouds (which were widely adopted as template extinction laws in literature); (2) the 2175\AA extinction feature appears to be also present in GRB050904 at z~6.29; and (3) there does not appear to show strong evidence for a dependence of dust extinction on redshifts. The inferred extinction curves are closely reproduced in terms of a mixture of amorphous silicate and graphite, both of which are expected supernova condensates and have been identified in primitive meteorites as presolar grains originating from supernovae (which are considered as the main source of dust at high-z).

Detection of the 2175 Å extinction feature and 21-cm absorption in two Mg ii systems at z ∼ 1.3

Abstract We have discovered two dusty intervening Mg ii absorption systems at z∼ 1.3 in the Sloan Digital Sky Survey (SDSS) data base. The overall spectra of both quasi-stellar objects (QSOs) are red (u−K &amp;gt; 4.5 mag) and are well modelled by the composite QSO spectrum reddened by the extinction curve from the Large Magellanic Cloud (LMC2) Supershell redshifted to the rest frame of the Mg ii systems. In particular, we detect clearly the presence of the ultraviolet (UV) extinction bump at λrest∼ 2175Å. Absorption lines of weak transitions like Si iiλ1808, Cr iiλ2056, Cr ii+ Zn iiλ2062, Mn iiλ2594, Ca iiλ3934 and Ti iiλ1910 from these systems are detected even in the low signal-to-noise ratio and low-esolution SDSS spectra, suggesting high column densities of these species. The depletion pattern inferred from these absorption lines is consistent with that seen in the cold neutral medium of the LMC. Using the LMC AV versus N(H i) relationship, we derive N(H i) ∼ 6 × 1021 cm−2 in both systems. Metallicities are close to solar. Giant Metrewave Radio Telescope (GMRT) observations of these two relatively weak radio QSOs (fν∼ 50 mJy) resulted in the detection of 21-cm absorption in both the cases. We show that the spin temperature of the gas is of the order of or smaller than 500 K. These systems provide a unique opportunity to search for molecules and diffuse interstellar bands at z &amp;gt; 1.

On buckyonions as an interstellar grain component

Abstract The carrier of the 2175 Å interstellar extinction feature remains unidentified since its first detection over 40 yr ago. In recent years, carbon buckyonions have been proposed as a carrier of this feature, based on the close similarity between the electronic transition spectra of buckyonions and the 2175 Å interstellar feature. We examine this hypothesis by modelling the interstellar extinction with buckyonions as a dust component. It is found that dust models containing buckyonions (in addition to amorphous silicates, polycyclic aromatic hydrocarbon molecules, graphite) can closely reproduce the observed interstellar extinction curve. To further test this hypothesis, we call for experimental measurements and/or theoretical calculations of the infrared vibrational spectra of hydrogenated buckyonions. By comparing the infrared emission spectra predicted for buckyonions vibrationally excited by the interstellar radiation with the observed emission spectra of the diffuse interstellar medium, we will be able to derive (or place an upper limit on) the abundance of interstellar buckyonions.

A12CO,13CO, and C18O Survey of Infrared Dark Clouds

Infrared dark clouds (IRDCs) are extinction features against the Galactic infrared background, mainly in the mid-infrared band. Recently they were proposed to be potential sites of massive star formation. In this work we have made a12CO,13CO, and C18O (J = 1→ 0) survey of 61 IRDCs, 52 of which are in the first Galactic quadrant, selected from a catalog given by Simon and coworkers, while the others are in the outer Galaxy, selected by visually inspecting the MSX images. Detection rates in the three CO lines are 90%, 71%, and 62%, respectively. The distribution of IRDCs in the first Galactic quadrant is consistent with the 5 kpc molecular ring picture, while a slight trace of a spiral pattern is also noticeable, and needs to be further examined. The IRDCs have a typical excitation temperature of 10 K and typical column density of several 1022 cm−2. Their typical physical size is estimated to be several parsecs using angular sizes from the Simon catalog. Typical volume density and typical LTE mass are ~5000 cm−3 and ~5000 M☉, respectively. The IRDCs are in or near virial equilibrium. The properties of IRDCs are similar to those of star-forming molecular clumps, and they seem to be intermediate between giant molecular clouds and Bok globules; thus they may represent early stages of massive star formation.

On buckyonions as a carrier of the 2175 Å interstellar extinction feature

AbstractIn recent years buckyonions have been suggested as a carrier of the 2175 Å interstellar extinction feature, based on the close similarity between the electronic transition spectra of buckyonions and the 2175 Å interstellar extinction feature. We examine this hypothesis by calculating the interstellar extinction with buckyonions as a dust component. It is found that dust models containing buckyonions (in addition to amorphous silicates, PAHs, graphite or amorphous carbon) can closely reproduce the observed interstellar extinction curve. However, a more severe challenge to the buckyonion hypothesis is provided by the non-detection of the ~7–8 μm C–H stretching bands expected from buckyonions in the diffuse interstellar medium. This will allow us to place an upper limit on the abundance of buckyonions.

The Rich Mid-Infrared Environments of Two Highly Obscured X-Ray Binaries: Spitzer Observations of IGR J16318-4848 and GX 301-2

We present the results of Spitzer spectroscopic observations of two highly obscured massive X-ray binaries: IGR J16318-4848 and GX 301-2. Our observations reveal for the first time the extremely rich mid-infrared environments of this type of source, including multiple continuum emission components (a hot component with T > 700 K and a warm component with T ~ 180 K) with apparent silicate absorption features, numerous H I recombination lines, many forbidden lines of low ionization potential, and pure rotational H2 lines. This indicates that both sources have hot and warm circumstellar dust, ionized stellar winds, extended low-density ionized regions, and photodissociated regions. It appears difficult to attribute the total optical extinction of both sources to the hot and warm dust, which suggests that there could be an otherwise observable colder dust component responsible for most of the optical extinction and silicate absorption features. The observed mid-infrared spectra are similar to those from luminous blue variables, indicating that the highly obscured massive X-ray binaries may represent a previously unknown evolutionary phase of X-ray binaries with early-type optical companions. Our results highlight the importance and utility of mid-infrared spectroscopy for investigating highly obscured X-ray binaries.

The Detection of Protostellar Condensations in Infrared Dark Cloud Cores

Infrared dark clouds (IRDCs) are a distinct class of interstellar molecular cloud identified as dark extinction features against the bright mid-infrared Galactic background. Here we present high angular resolution millimeter continuum images obtained with the IRAM Plateau de Bure Interferometer toward four high-mass (200-1800 M?) IRDC cores that show evidence for active high-mass star formation (M > 8 M?). We detect twelve bright (>7 mJy), compact (2'', 0.024 pc) condensations toward these cores. Two of the cores (G024.60+00.08 MM1 and G024.60+00.08 MM2) are resolved into multiple protostellar condensations, while one core (G022.35+00.41 MM1) shows two condensations. The remaining core (G024.33+00.11 MM1) contains a single, compact protostellar condensation with a very rich molecular spectrum, indicating that this is a hot molecular core associated with an early stage in the formation of a high-mass protostar. The derived gas masses for these condensations suggest that each core is forming at least one high-mass protostar (Mgas > 8 M?), and three cores are also forming lower mass protostars (Mgas ~ 2-5 M?). A comparison of the ratios of the gas masses (MG) to the Jeans masses (MJ) for IRDCs, cores, and condensations, provides broad support for the idea of hierarchical fragmentation. The close proximity of multiple protostars of disparate mass indicates that these IRDCs are in the earliest evolutionary states in the formation of stellar clusters.

Response to comments on Species diversity can drive speciation

This issue of Ecography features three responses to our paper Species diversity can drive (Emerson and Kolm 2005a), each presenting arguments of a different nature as to why our original analyses and interpretations may be flawed. We welcome the opportunity to respond in turn to each of these, and to clarify some points of confusion that may have emerged from our earlier paper. The distinction between speciation rate and diversification rate was perhaps not made clear in our earlier paper (Emerson and Kolm 2005a). In several instances we referred interchangeably to both speciation rate and diversification rate. However, in a subsequent paper we did explicitly refer to speciation rate, not diversification rate (see Fig. 1, Emerson and Kolm 2005b). Rate of species origin can be considered a measure of speciation rate, while rate of species accumulation can be considered a measure of diversification rate (Futuyma 2005, p. 400). Our model makes no explicit prediction regarding diversification rate, only about speciation rate. Whittaker et al. (2007) criticise our approach because the index we used is not a rate of species production. They argue that the proportion of single endemics (our index of speciation rate) does not measure rate of species production, and is inconclusive as to the mechanisms leading to the relationships between species richness and the proportion of single endemics. To be clear, the index we used is not a rate of speciation. It is the proportion of single endemics that we use as an index of relative speciation rate on an the higher the proportion the greater the probability of a species on an undergoing anagenetic or cladogenetic change. An analogous approach has been taken by MacArthur and Wilson (1967, p. 173) where they related percentage endemism to turnover rate (see below). Whittaker et al. (2007) are prepared to accept that the proportion of single endemics reveals something of the opportunities for speciation, reflecting an increasing opportunity for speciation with age, followed by eventual loss of endemic species on older isl s through the progression rule. Exactly how extinction features in their model is unclear as it is not depicted in their Fig. 1, but outside of abiotic cata trophes they expect it to range between extremely low and very low. To this end they put forward a conceptual model they call the island immaturity sp ciation pulse model (IISP) that appears to predict a specific relationship between age and speciation rate. Implicit within their model is that speciation is the product of opportunity, and opportunity presents itself in he form of vacant niche space and competitive release, important primers for adaptive radiation. There is no doubt that these are particularly important variables contributing to speciation rate in young islands. However, it is interesting to note that in a meta-analysis of arthropod phylogenetic data from the central and western Canary Islands, Emerson and Oromi (2005) found little evidence for adaptive radiation on young islands. They did however find increasing evidence for evolutionary patterns consistent with adaptive radiation on increasingly older islands, a result that sits more comfortably with mechanisms of competitive interactions rather than competitive release, phenomena that are increasingly being recognised as drivers of evolutionary change (e.g. Grant and Grant 2006, Nosil and Crespi 2006, Jordal et al. 2006, Meyer and Kassen 2007). Some of the criticism from Whittaker et al. (2007) is directed toward apparent contradictions between the writings of MacArthur and Wilson (1963, 1967) and ourselves. In their original model MacArthur and

Blowing-up of deterministic fixed points in stochastic population dynamics

We discuss the stochastic dynamics of biological (and other) populations presenting a limit behaviour for large environments (called deterministic limit) and its relation with the dynamics in the limit. The discussion is circumscribed to linearly stable fixed points of the deterministic dynamics, and it is shown that the cases of extinction and non-extinction equilibriums present different features. Mainly, non-extinction equilibria have associated a region of stochastic instability surrounded by a region of stochastic stability. The instability region does not exist in the case of extinction fixed points, and a linear Lyapunov function can be associated with them. Stochastically sustained oscillations of two subpopulations are also discussed in the case of complex eigenvalues of the stability matrix of the deterministic system.

The shape and composition of interstellar silicate grains

We investigate the composition and shape distribution of silicate dust grains in the interstellar medium. The effect of the amount of magnesium in the silicate lattice is studied. We fit the spectral shape of the interstellar 10 mu extinction feature as observed towards the galactic center. We use very irregularly shaped coated and non-coated porous Gaussian Random Field particles as well as a statistical approach to model shape effects. For the dust materials we use amorphous and crystalline silicates with various composition and SiC. The results of our analysis of the 10 mu feature are used to compute the shape of the 20 mu silicate feature and to compare this with observations. By using realistic particle shapes we are, for the first time, able to derive the magnesium fraction in interstellar silicates. We find that the interstellar silicates are highly magnesium rich (Mg/(Fe+Mg)>0.9) and that the stoichiometry lies between pyroxene and olivine type silicates. This composition is not consistent with that of the glassy material found in GEMS in interplanetary dust particles indicating that these are, in general, not unprocessed remnants from the interstellar medium. Also, we find a significant fraction of SiC (~3%). We discuss the implications of our results for the formation and evolutionary history of cometary and circumstellar dust. We argue that the fact that crystalline silicates in cometary and circumstellar grains are almost purely magnesium silicates is a natural consequence of our findings that the amorphous silicates from which they were formed were already magnesium rich.

Spatially extended host–parasite interactions: The role of recovery and immunity

Techniques for determining the long-term dynamics of host–parasite systems are well established for mixed populations. The field of spatial modelling in ecology is more recent but a number of key advances have been made. In this paper, we use state-of-the-art approximation techniques, supported by simulations, in order to investigate the role of recovery and immunity in spatially structured populations. Our approach is to use correlation models, namely pair-wise models, to capture the spatial relationships of contacts and interactions between individuals. We use the pair-wise framework to address a number of key ecological questions; including, the persistence of endemic limit cycles and regions of parasite-driven extinction—features which differentiate spatial from non-spatial models—and the effects on invasion fitness. We demonstrate a loss of limit cycle behaviour, in addition to an increase in the critical transmissibility and extinction thresholds, when recovery is included. This approach allows for a better analytical understanding of the dynamics of host–parasite interactions and demonstrates the importance of recovery and immunity in local interactions.

Aggregation Properties of Hyperporphyrins with Hydroxyphenyl Substituents

Acidification of tetrakis(4-hydroxyphenyl)porphyrin (THPP) and tetrakis(3,5-dihydroxyphenyl)porphyrin (OHPP) in dichloromethane solutions has been investigated as a function of the nature of the counteranion. These porphyrins exhibit different patterns of behavior, and extended aggregates are formed displaying broad extinction features together with intense components due to resonant light scattering. Especially in the case of haloacids, J-aggregated species are obtained exhibiting large bathochromic shifts both for B- and Q-bands, which extend in the far red region. The optical characteristics of the aggregated and monomeric protonated species are strongly influenced by the nature of the counteranions. A comparison with tetrakis(4-methoxyphenyl)porphyrin (TMPP), which remains always in a monomeric form, demonstrates the key role played by the peripheral hydroxyl groups in stabilizing various porphyrin aggregates.

Infrared Dark Clouds: Precursors to Star Clusters

Infrared Dark Clouds (IRDCs) are dense molecular clouds seen as extinction features against the bright mid-infrared Galactic background. Millimeter continuum maps toward 38 IRDCs reveal extended cold dust emission to be associated with each of the IRDCs. IRDCs range in morphology from filamentary to compact and have masses of 120 to 16,000 Msun, with a median mass of ~940 Msun. Each IRDC contains at least one compact (<=0.5 pc) dust core and most show multiple cores. We find 140 cold millimeter cores unassociated with MSX 8um emission. The core masses range from 10 to 2,100 Msun, with a median mass of ~120 Msun. The slope of the IRDC core mass spectrum (alpha ~ 2.1 +/- 0.4) is similar to that of the stellar IMF. Assuming that each core will form a single star, the majority of the cores will form OB stars. IRDC cores have similar sizes, masses, and densities as hot cores associated with individual, young high-mass stars, but they are much colder. We therefore suggest that IRDC represent an earlier evolutionary phase in high-mass star formation. In addition, because IRDCs contain many compact cores, and have the same sizes and masses as molecular clumps associated with young clusters, we suggest that IRDCs are the cold precursors to star clusters. Indeed, an estimate of the star formation rate within molecular clumps with similar properties to IRDCs (~2 Msun/yr) is comparable to the global star formation rate in the Galaxy, supporting the idea that all stars may form in such clumps.

On the Unusual Depletions toward Sk 155, or What Are the Small Magellanic Cloud Dust Grains Made Of?

The dust in the Small Magellanic Cloud (SMC), an ideal analog of primordial galaxies at high redshifts, differs markedly from that in the Milky Way by exhibiting a steeply rising far-ultraviolet extinction curve, an absence of the 2175 Angstrom extinction feature, and a local minimum at ~12 micron in its infrared emission spectrum, suggesting the lack of ultrasmall carbonaceous grains (i.e. polycyclic aromatic hydrocarbon molecules) which are ubiquitously seen in the Milky Way. While current models for the SMC dust all rely heavily on silicates, recent observations of the SMC sightline toward Sk 155 indicated that Si and Mg are essentially undepleted and the depletions of Fe range from mild to severe, suggesting that metallic grains and/or iron oxides, instead of silicates, may dominate the SMC dust. However, in this Letter we apply the Kramers-Kronig relation to demonstrate that neither metallic grains nor iron oxides are capable of accounting for the observed extinction; silicates remain as an important contributor to the extinction, consistent with current models for the SMC dust.

A Catalog ofMidcourse Space ExperimentInfrared Dark Cloud Candidates

We use 8.3 μm mid-infrared images acquired with the Midcourse Space Experiment satellite to identify and catalog infrared dark clouds (IRDCs) in the first and fourth quadrants of the Galactic plane. Because IRDCs are seen as dark extinction features against the diffuse Galactic infrared background, we identify them by first determining a model background from the 8.3 μm images and then searching for regions of high decremental contrast with respect to this background. IRDC candidates in our catalog are defined by contiguous regions bounded by closed contours of a 2 σ decremental contrast threshold. Although most of the identified IRDCs are actual cold dark clouds, some as yet unknown fraction may be spurious identifications. For large high-contrast clouds, we estimate the reliability to be 82%. Low-contrast clouds should have lower reliabilities. Verification of the reality of individual clouds will require additional data. We identify 10,931 candidate IRDCs. For each IRDC, we also catalog cores. These cores, defined as localized regions with at least 40% higher extinction than the cloud's average extinction, are found by iteratively fitting two-dimensional elliptical Gaussian functions to the contrast peaks. We identify 12,774 cores. The catalog contains the position, angular size, orientation, area, peak contrast, peak contrast signal-to-noise, and integrated contrast of the candidate IRDCs and their cores. The distribution of IRDCs closely follows the Galactic diffuse mid-infrared background and peaks toward prominent star-forming regions, spiral arm tangents, and the so-called 5 kpc Galactic molecular ring.

Low-resolution VLT spectroscopy of GRBs 991216, 011211 and 021211

We present low-resolution VLT spectroscopy of the afterglow of the gamma-ray bursts (GRBs) 991216, 011211 and 021211. Our spectrum of GRB991216 is the only optical spectrum for this afterglow. It shows two probable absorption systems at z=0.80 and z=1.02, where the highest redshift most likely reflects the distance to the host galaxy. A third system may be detected at z=0.77. HST imaging of the field, obtained 4 months after the burst, has resulted in the detection of two amorphous regions of emission, one at the projected afterglow position, and the other 0.6" away. The spectrum shows a depression in flux in between 4000 A and 5500 A. This could be the result of a 2175 A-type extinction feature in the host of GRB991216, but at a rather red wavelength of 2360 A. If this interpretation is correct, it is the first time the extinction feature is seen in a GRB afterglow spectrum. It is centered at a wavelength similar to that of the ultra-violet (UV) bumps inferred from observations of a few UV-strong, hydrogen-poor stars in the Galaxy. All significant absorption lines (except for one) detected in the spectrum of GRB011211 are identified with lines originating in a single absorption system at z=2.142+/-0.002, the redshift of the GRB011211 host galaxy. We also detect the Lyman alpha absorption line in the host, to which we fit a neutral hydrogen column density of log N(HI)=20.4+/-0.2, which indicates that it is a damped Lyman alpha system. Using a curve-of-growth analysis, we estimate the Si, Fe and Al metallicity at the GRB011211 redshift. For GRB021211, we detect a single emission line in a spectrum obtained tens of days after the burst, which we identify as [OII] 3727 at z=1.006. The corresponding unobscured [OII] star-formation rate is 1.4 Msun/yr.

Polycyclic aromatic hydrocarbons, carbon nanoparticles and the diffuse interstellar bands

Observational data on the appearance and properties of the diffuse interstellar bands (DIBs) are reviewed in the context of a model in which the proposed carriers of these bands are large carbon molecules and carbon nanoparticles containing between 30 and several hundred carbon atoms. The abundance of these carriers, as estimated from the observed strengths of the DIBs, place strong constraints on their rates of formation and destruction, and suggest that the strongest bands, including that at 4428 A, could be produced via the decomposition of larger carbon particles, possibly those particles that have been postulated to be the source of the 2175 A extinction feature. Such particles are of mixed sp2 and sp3 carbon composition, with sizes between that of large molecules and small macroscopic solids. Any description of their characteristics must combine aspects of molecular and condensed matter physics, and this is incorporated in the present discussion. I discuss recent experimental and theoretical data related to these matters.

FUSEMeasurements of Far‐Ultraviolet Extinction. II. Magellanic Cloud Sight Lines

We present an extinction analysis of nine reddened/comparison star pairs in the Large and Small Magellanic Clouds (LMC and SMC) based on Far-Ultraviolet Spectroscopic Explorer (FUSE) FUV observations. To date, just two LMC sight lines have probed dust grain composition and size distributions in the Magellanic Clouds using spectral data for wavelengths as short as 950 A. We supplement these two with data from four regions distinguished by their IR through UV extinction curves and grouped as LMCAvg, LMC2, SMC bar, and SMC wing. Despite the distinct characters of extinction in the Clouds and Milky Way, our results are generally analogous to those found for Galactic curves—namely, that the FUSE portions of each extinction curve are described reasonably well by Fitzpatrick & Massa curves fitted only to longer wavelength data and lack any dramatic new extinction features, and any deviations from the Cardelli, Clayton, & Mathis (CCM) formalism continue into FUV wavelengths. A maximum entropy method analysis of all of these curves suggests that LMCAvg and SMC wing sight lines, whose extinction parameters more closely resemble those for Galactic paths, require more silicon and/or carbon in dust than current abundance measurements would indicate are available. The requirements for LMC2 and SMC bar sight lines do not fully tax the available reservoirs, in part because large grains contribute less to the extinction in these directions. An intermediate product of this extinction analysis is the measurement of new H2 abundances in the Magellanic Clouds. Collectively considering Cloud sight lines that possess significant H2 column densities, E(B - V)/N(H ) ratios are reduced by significant factors relative to the Galactic mean, whereas the corresponding E(B - V)/N(H2) values more closely resemble their Galactic counterpart. These trends reflect the fact that among these sight lines f(H2) values are lower than those common in the Milky Way for paths with similar degrees of reddening.