Interstellar Dust Research Articles

The NOEMA observations of GN-z11: constraining the neutral interstellar medium and dust formation in the heart of cosmic reionization at z = 10.6

ABSTRACT We present results of dust continuum and [C ii]$\, 158\, {\rm \mu m}$ emission line observations of a remarkably UV luminous (MUV = −21.6) galaxy at z = 10.603: GN-z11. Using the Northern Extended Millimeter Array (NOEMA), observations have been carried out over multiple observing cycles. We achieved a high sensitivity resulting in a $\lambda _{\rm rest}=160\, {\rm \mu m}$ continuum $1\, \sigma$ sensitivity of $13.0\, \rm {\mu Jy \, beam}^{ -1}$ and a [C ii] emission line $1\, \sigma$ sensitivity of $31\, \rm {mJy\, beam^{ -1}\, km \, s}^{ -1}$ using $50\, \rm {km \, s}^{ -1}$ binning with a $\sim 2\, {\rm arcsec}$ synthesized beam. Neither dust continuum nor [C ii]$\, 158\, {\rm \mu m}$ line emission are detected at the expected frequency of ν[C ii]$= 163.791\, \rm {GHz}$ and the sky location of GN-z11. The upper limits show that GN-z11 is neither luminous in LIR nor L[C ii], with a dust mass $3\, \sigma$ limit of ${\rm log}\, (M_{\rm dust}/{\rm {\rm M}_{\odot }}) \, \lt\, 6.5-6.9$ and with a [C ii] based molecular gas mass $3\, \sigma$ limit of log (Mmol, [C ii]$/{\rm {\rm M}_{\odot }}) \, \lt \, 9.3$. Together with radiative transfer calculations, we also investigated the possible cause of the dust poor nature of the GN-z11 showed by the blue colour in the UV continuum of GN-z11 (βUV = −2.4), and found that ≳3 × deeper observations are crucial to study dust production at very high-redshift. Nevertheless, our observations show the crucial role of deep mm/submm observations of very high-redshift galaxies to constrain multiple phases in the interstellar medium.

Cosmic dust fertilization of glacial prebiotic chemistry on early Earth

AbstractEarth’s surface is deficient in available forms of many elements considered limiting for prebiotic chemistry. In contrast, many extraterrestrial rocky objects are rich in these same elements. Limiting prebiotic ingredients may, therefore, have been delivered by exogenous material; however, the mechanisms by which exogeneous material may be reliably and non-destructively supplied to a planetary surface remains unclear. Today, the flux of extraterrestrial matter to Earth is dominated by fine-grained cosmic dust. Although this material is rarely discussed in a prebiotic context due to its delivery over a large surface area, concentrated cosmic dust deposits are known to form on Earth today due to the action of sedimentary processes. Here we combine empirical constraints on dust sedimentation with dynamical simulations of dust formation and planetary accretion to show that localized sedimentary deposits of cosmic dust could have accumulated in arid environments on early Earth, in particular glacial settings that today produce cryoconite sediments. Our results challenge the widely held assumption that cosmic dust is incapable of fertilizing prebiotic chemistry. Cosmic dust deposits may have plausibly formed on early Earth and acted to fertilize prebiotic chemistry.

Near-ultraviolet radiation toward molecular cloud N4 in W 50/SS 433: Evidence for direct interaction of the jet with molecular cloud

Abstract We compared the molecular clouds in the western part of SS 433 with near-ultraviolet radiation data obtained from GALEX. Near-ultraviolet radiation is prominently confirmed toward only N4, while no near-ultraviolet radiation is detected toward N1, N2, or N3. The radiative region of near-ultraviolet radiation is nearly the same as the CO-emitting region in N4, and does not extend beyond the jet seen in X-ray radiation. Near-ultraviolet radiation cannot be explained solely by broad-band continuous radiation and may originate from line emissions. The intensity of near-ultraviolet radiation exhibits an anti-correlation with that of 13CO(J = 3–2) emission. This anti-correlation, along with strong far-infrared radiation in the region with weaker near-ultraviolet radiation intensity compared to its surroundings, suggests that near-ultraviolet radiation originates from behind the molecular cloud, heating up the interstellar dust in N4. Subsequently, the dust in N4 reradiates in the far-infrared band. In the same region, a high peak TMB ratio of 12CO(J = 3–2)$/$12CO(J = 1–0) of ∼0.9, and a high kinetic temperature of Tk ∼ 56 K in the molecular cloud, indicate that CO molecules are highly excited, and the molecular cloud is heated through photoelectric heating. This heating results from electrons released due to the photoelectric effect caused by the phenomenon where interstellar dust absorbs near-ultraviolet radiation. In terms of the timescale of near-ultraviolet radiation originating from line emissions, near-ultraviolet radiation towards N4 cannot be explained by the shock of the blast wave from a supernova that created W 50. These findings also suggest that N4 directly interacts with the jet from SS 433. As a result of this direct interaction, near-ultraviolet radiation is emitted from an interacting layer between the jet and N4.

Constructing the three-dimensional extinction density maps using V-net

ABSTRACT One of the major challenges we face is how to quickly and accurately create the three-dimensional (3D) density distributions of interstellar dust in the Milky Way using extinction and distance measurements of large samples of stars. In this study, we introduce a novel machine-learning approach that utilizes a convolution neural network, specifically a V-net, to infer the 3D distribution of dust density. Experiments are performed within two regions located towards the Galactic anticentre. The neural network is trained and tested using 10 000 simulations of dust density and line-of-sight extinction maps. Evaluation of the test sample confirms the successful generation of dust density maps from extinction maps by our model. Additionally, the performance of the trained network is evaluated using data from the literature. Our results demonstrate that our model is capable of capturing detailed dust density variations and can recover dust density maps while reducing the ‘fingers of god’ effect. Moving forward, we plan to apply this model to real observational data to obtain the fine distribution of dust at large and small scales in the Milky Way.

Oxygen isotopic compositions of fresh rooftop micrometeorites from the Budel collection—Insights into the contemporary cosmic dust flux

AbstractCosmic dust particles originate from a wide variety of solar system and interstellar objects, including sources not identified among meteorite collections. Particles that survive atmospheric entry are retrieved on the Earth's surface as micrometeorites. The recovery of these micrometeorites has recently advanced to rooftop sites. Here, we present the results of an extensive isotopic study on this type of rooftop micrometeorite from the Budel collection, the Netherlands, accreted to the Earth between October 31, 2018 and June 16, 2021. The triple oxygen isotopic compositions of 80 silica‐dominated cosmic spherules (CSs) with diameters ranging between 105 and 515 μm are obtained relying on 213 in situ spot analyses determined using ion microprobe. Our analyzed population spans a large range of isotopic compositions and is dominated by carbonaceous chondritic sources. In situ measurements on several CSs support a possible continuum between 16O‐rich and 16O‐poor compositions following the CM mixing line, showing that 16O‐poor CSs may be genetically related to aqueously altered carbonaceous chondrites. We demonstrate that weathering in the terrestrial environment has negligible effects on the isotopic compositions of the studied CSs and attempt to quantify the effects of kinetic mass‐dependent fractionation and admixture of terrestrial oxygen during atmospheric entry. The results further corroborate previously suggested relations between CS texture and the duration and intensity of the heating pulse experienced during atmospheric deceleration. Finally, the young and well‐constrained terrestrial age of the collection provides insights into the most recent flux of cosmic dust. Our results indicate no major recent changes in the global flux compared with collections sampled over thousand‐ to million‐year time scales and demonstrate that 16O‐poor material is still represented in the modern‐day cosmic dust flux at a relative abundance of ~13%–15%. As such, rooftop micrometeorites represent a valuable reservoir to study the characteristics of the contemporary cosmic dust flux.

From total destruction to complete survival: dust processing at different evolutionary stages in the supernova remnant Cassiopeia A

ABSTRACT The expanding ejecta of supernova remnants (SNRs) are believed to form dust in dense clumps of gas. Before the dust can be expelled into the interstellar medium and contribute to the interstellar dust budget, it has to survive the reverse shock that is generated through the interaction of the preceding supernova blast wave with the surrounding medium. The conditions under which the reverse shock hits the clumps change with remnant age and define the dust survival rate. To study the dust destruction in the SNR Cassiopeia A, we conduct magnetohydrodynamical simulations of the evolution of a supernova blast wave and of the reverse shock. In a second step, we use these evolving conditions to model clumps that are disrupted by the reverse shock at different remnant ages. Finally, we compute the amount of dust that is destroyed by the impact of the reverse shock. We find that most of the dust in the SNR is hit by the reverse shock within the first 350 yr after the SN explosion. While the dust destruction in the first 200 yr is almost complete, we expect greater dust survival rates at later times and almost total survival for clumps that are first impacted at ages beyond 1000 yr. Integrated over the entire evolution of the SNR, the dust mass shows the lowest survival fraction (17 per cent) for the smallest grains (1 nm) and the highest survival fraction (28 per cent) for the largest grains (1000 nm).

Dust enrichment in the circum-galactic medium

ABSTRACT To understand the origin of dust in the circum-galactic medium (CGM), we develop a dust enrichment model. We describe each of the central galaxy and its CGM as a single zone, and consider the mass exchange between them through galactic inflows and outflows. We calculate the evolution of the gas, metal, and dust masses in the galaxy and the CGM. In the galaxy, we include stellar dust production and interstellar dust processing following our previous models. The dust in the galaxy is transported to the CGM via galactic outflows, and it is further processed by dust destruction (sputtering) in the CGM. We parametrize the time-scale or efficiency of each process and investigate the effect on the dust abundance in the CGM. We find that the resulting dust mass is sensitive to the dust destruction in the CGM, and the dust supply from galactic outflows, both of which directly regulate the dust abundance in the CGM. The inflow time-scale also affects the dust abundance in the CGM because it determines the gas mass evolution (thus, the star formation history) in the galaxy. The dust abundance in the CGM, however, is insensitive to stellar dust formation in the galaxy at later epochs because the dust production is dominated by dust growth in the interstellar medium. We also find that the resulting dust mass in the CGM is consistent with the value derived from a large sample of SDSS galaxies.

Understanding the Dust Environment at Mercury: From Surface to Exosphere

We provide an overview of our understanding of the dust environment at Mercury and the role that dust plays in shaping the planet's surface and exosphere. Our understanding of the role that dust impacts play in the generation of Mercury's atmosphere has evolved considerably with continued analysis of results from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission. Recent models have provided evidence for the probable release of refractory species into Mercury's exosphere via impacts. However, there remain significant questions regarding the relative contributions of atoms released via impacts versus other mechanisms (e.g., photon-stimulated desorption) to the overall exospheric budget. We also discuss the state of observational and modeling efforts to constrain the dust environment at Mercury, including sources from the zodiacal cloud, cometary trails, and interstellar dust. We describe the advancements that will be made in our characterization of dust at Mercury with BepiColombo, providing observational constraints on the dust clouds themselves and the role that impacts play in exospheric generation. On Mercury's surface, there remain outstanding questions regarding the role that dust impacts play in the regolith cycling and development. We review how improved modeling efforts to understand grain lifetimes as a function of impactor flux will further our understanding of Mercury's regolith. Finally, there are few constraints on the role of dust impacts on the space weathering of Mercury's surface, particularly the expected chemical, physical, and spectral alterations to the regolith. Here we discuss the importance of laboratory experiments to simulate these processes for the interpretation of data from MESSENGER and BepiColombo.

Formation and Evolution Mechanisms for Organic Matter in Space

Organic compounds are a major component of dust in molecular clouds, alongside silicates and water ice, due to the high abundances of elements that make up these compounds in the Galaxy. The initial molecular inventory of the Solar System, inherited from the molecular cloud, was modified and new complex molecules were formed in the protoplanetary disk and planetesimals. Because astronomical observations mainly target gas, while cosmochemical evidence deals with solid phases, it is crucial to link discrepant knowledge on organic species through state-of-the-art modeling. This chapter reviews the latest understanding of surface reactions on inter-stellar dusts, gas–dust reactions in the protoplanetary disk, and alteration processes on planetesimals in the early Solar System.

Dust–gas dynamics driven by the streaming instability with various pressure gradients

ABSTRACT The streaming instability, a promising mechanism to drive planetesimal formation in dusty protoplanetary discs, relies on aerodynamic drag naturally induced by the background radial pressure gradient. This gradient should vary in discs, but its effect on the streaming instability has not been sufficiently explored. For this purpose, we use numerical simulations of an unstratified disc to study the non-linear saturation of the streaming instability with mono-disperse dust particles and survey a wide range of gradients for two distinct combinations of the particle stopping time and the dust-to-gas mass ratio. As the gradient increases, we find most kinematic and morphological properties increase but not always in linear proportion. The density distributions of tightly coupled particles are insensitive to the gradient whereas marginally coupled particles tend to concentrate by more than an order of magnitude as the gradient decreases. Moreover, dust–gas vortices for tightly coupled particles shrink as the gradient decreases, and we note higher resolutions are required to trigger the instability in this case. In addition, we find various properties at saturation that depend on the gradient may be observable and may help reconstruct models of observed discs dominated by streaming turbulence. In general, increased dust diffusion from stronger gradients can lower the concentration of dust filaments and can explain the higher solid abundances needed to trigger strong particle clumping and the reduced planetesimal formation efficiency previously found in vertically stratified simulations.

Variations of Interstellar Gas-to-dust Ratios at High Galactic Latitudes

Interstellar dust at high Galactic latitudes can influence astronomical foreground subtraction, produce diffuse scattered light, and soften the UV spectra of quasars. In a sample of 94 sight lines toward quasars at high latitude and low extinction, we evaluate the interstellar “gas-to-dust ratio” N H/E(B − V), using hydrogen column densities (H i and H2) and far-IR (FIR) estimates of dust reddening. In the Galactic plane, this ratio is 6.0 ± 0.2 (in units of 1021 cm−2 mag−1). On average, recent Planck estimates of E(B − V) in low-reddening sight lines are 12% higher than those from Schlafly & Finkbeiner, and N H I exhibits significant variations when measured at different radio telescopes. In a sample of 51 quasars with measurements of both H i and H2 and 0.01 ≤ E(B − V) ≲ 0.1, we find mean ratios 10.3 ± 0.4 (gas at all velocities) and 9.2 ± 0.3 (low-velocity only) using Planck E(B − V) data. High-latitude H2 fractions are generally small (2%–3% on average), although nine of 39 sight lines at ∣b∣ ≥ 40° have f H2 of 1%–17%. Because FIR-inferred E(B − V) is sensitive to modeled dust temperature T d and emissivity index β, gas-to-dust ratios have large, asymmetric errors at low E(B − V). The ratios are elevated in sight lines with high-velocity clouds, which contribute N H but little reddening. In Complex C, the ratio decreases by 40% when high-velocity gas is excluded. Decreases in dust content are expected in low-metallicity gas above the Galactic plane, resulting from grain destruction in shocks, settling to the disk, and thermal sputtering in hot halo gas.

New Horizons Venetia Burney Student Dust Counter Observes Higher than Expected Fluxes Approaching 60 au

The NASA New Horizons Venetia Burney Student Dust Counter (SDC) measures dust particle impacts along the spacecraft’s flight path for grains with mass ≥10−12 g, mapping out their spatial density distribution. We present the latest SDC dust density, size distribution, and flux measurements through 55 au and compare them to numerical model predictions. Kuiper Belt objects (KBOs) are thought to be the dominant source of interplanetary dust particles in the outer solar system due to both collisions between KBOs and their continual bombardment by interstellar dust particles. Continued measurements through 55 au show higher than model-predicted dust fluxes as New Horizons approaches the putative outer edge of the Kuiper Belt (KB). We discuss potential explanations for the growing deviation: radiation pressure stretches the dust distribution to further heliocentric distances than its parent body distribution; icy dust grains undergo photosputtering that rapidly increases their response to radiation pressure forces and pushes them further away from the Sun; and the distribution of KBOs may extend much further than existing observations suggest. Ongoing SDC measurements at even larger heliocentric distances will continue to constrain the contributions of dust production in the KB. Continued SDC measurements remain crucial for understanding the Kuiper Belt and the interpretation of dust disks around other stars.

Hypervelocity impact induced light flash experiments on single and dual layer Kapton targets to develop a time of flight space dust and debris detector

The impact flash from hypervelocity impact on thin (12.5 µm) Kapton film was observed. The projectile sizes ranged from 0.1 to 1 mm, with speeds from 2 to 5 km s−1 and penetrated the Kapton intact, leaving holes the same size as the projectile (to within measurement errors). The flash intensity (normalised to impactor mass) scaled with impact speed to the power 5.5. However, the data also suggest that at constant speed the intensity scales with the area of the hole in the Kapton and not the projectile mass (i.e. with some property of the target and not as a function of the projectile energy or momentum). Using two layers of Kapton, it was possible to construct a Time of Flight (TOF) system, which used the time of the onset of the flash in each layer to produce flight speeds accurate to within typically 1%. When compared to the projectile speed pre-impact, there was no indication of projectile deceleration during passage through the Kapton film. In addition, when PVDF acoustic sensors were placed on the Kapton film, they exhibited an electromagnetic “pick-up” signal from the impact of projectile on the Kapton, confirming suspicions of signal interference from past work with acoustic sensors. The ability of the light flash to provide accurate impact timing signals suggests the TOF system would be suitable for use as a cosmic dust or debris impact detector in space (e.g. Low Earth Orbit).

Evaporative cooling and reaction of carbon dioxide clusters by low-energy electron attachment.

Anionic carbonate CO3- has been found in interstellar space and the Martian atmosphere, but its production mechanism is in debate so far. To mimic the irradiation-induced reactions on icy micrograins in the Martian atmosphere and the icy shell of interstellar dust, here we report a laboratory investigation on the dissociative electron attachments to the molecular clusters of CO2. We find that anionic species (CO2)n-1O- and (CO2)n- (n = 2, 3, 4) are produced in the concerted reaction and further stabilized by the evaporative cooling after the electron attachment. We further propose a dynamics model to elucidate their competitive productions: the (CO2)n- yields survive substantially in the molecular evaporative cooling at the lower electron attachment energy, while the reactions leading to (CO2)n-1O- are favored at the higher attachment energy. This work provides new insights into physicochemical processes in CO2-rich atmospheres and interstellar space.

Cassini’s CDA observes a variety of dust populations just outside Saturn’s main rings

ABSTRACT Before the end of its mission, the Cassini spacecraft orbited Saturn in a series of highly inclined elliptical ‘Ring-Grazing’ orbits (RGO). During the RGO, the spacecraft passed repeatedly through the ring plane outside the F ring, near the orbits of Janus and Epimetheus, at an average relative speed of ∼20 km s–1. For the first time, Cassini’s Cosmic Dust Analyser (CDA) directly sampled dust particles from this region. Here, we analyse the compositions of dust grains sampled within ±15 min relative to nine ring plane crossings of the RGO. The compositions of most analysed RGO grains are similar to those of E ring ice grains, implying that the E ring extends to within at least 2.45 Saturn radii (RS) of Saturn. The compositional distribution of these grains point at a similar average period (decades) since ejection from Enceladus as of particles in the outer E ring (beyond 8 RS). Higher fractions of larger grains are found near the orbits of Janus and Epimetheus, which probably represent ejecta from these moons. Most of these grains have compositions similar to the background E ring grains, indicating that E ring material is coating the surfaces of Janus and Epimetheus. We also report the detection of several types of mineral grains on prograde orbits, one of which, a water ice/silicate mixture, has never been observed by CDA elsewhere. These mineral grains appear to have a different origin from the E ring, and may arise from nearby moons, the F ring, or main rings.

Spectroscopic sizing of interstellar icy grains with JWST

Spectroscopic sizing of interstellar icy grains with JWST

The Atacama Cosmology Telescope: Galactic Dust Structure and the Cosmic PAH Background in Cross-correlation with WISE

We present a cross-correlation analysis between resolution total intensity and polarization observations from the Atacama Cosmology Telescope (ACT) at 150 and 220 GHz and 15″ mid-infrared photometry from the Wide-field Infrared Survey Explorer (WISE) over 107 12.°5 × 12.°5 patches of sky. We detect a spatially isotropic signal in the WISE×ACT TT cross-power spectrum at 30σ significance that we interpret as the correlation between the cosmic infrared background at ACT frequencies and polycyclic aromatic hydrocarbon (PAH) emission from galaxies in WISE, i.e., the cosmic PAH background. Within the Milky Way, the Galactic dust TT spectra are generally well described by power laws in ℓ over the range 103 < ℓ < 104, but there is evidence both for variability in the power-law index and for non-power-law behavior in some regions. We measure a positive correlation between WISE total intensity and ACT E-mode polarization at 1000 < ℓ ≲ 6000 at >3σ in each of 35 distinct ∼100 deg2 regions of the sky, suggesting that alignment between Galactic density structures and the local magnetic field persists to subparsec physical scales in these regions. The distribution of TE amplitudes in this ℓ range across all 107 regions is biased to positive values, while there is no evidence for such a bias in the TB spectra. This work constitutes the highest-ℓ measurements of the Galactic dust TE spectrum to date and indicates that cross-correlation with high-resolution mid-infrared measurements of dust emission is a promising tool for constraining the spatial statistics of dust emission at millimeter wavelengths.

The significant role of water in reactions occurring on the surface of interstellar ice grains: Hydrogenation of pure ketene H2CCO ice versus hydrogenation of mixed H2CCO/H2O ice at 10 K.

Water ice plays an important role in reactions taking place on the surface of interstellar ice grains, ranging from catalytic effects that reduce reaction barrier heights to effects that stabilize the reaction products and intermediates formed, or that favor one reaction pathway over another, passing through water-involvement in the reaction to produce more complex molecules that cannot be formed without water or water-derived fragments H, O and OH. In this context, we have combined experimental and theoretical studies to investigate ketene (CH2CO) + H solid-state reaction at 10 K in the presence and absence of water molecules under interstellar conditions, through H-bombardment of CH2CO and CH2CO/H2O ices. We show in the present study that with or without water, the ketene molecule reacts with H atoms to form four reaction products, namely CO, H2CO, CH4 and CH3CHO. Based on the amounts of CH2CO consumed during the hydrogenation processes, the CH2CO + 2H reaction appears to be more efficient in the presence of water. This underlines the catalytic role of water ice in reactions occurring on the surface of interstellar ice grains. However, if we refer to the yields of reaction products formed during the hydrogenation of CH2CO and CH2CO/H2O ices, we find that water molecules favor the reaction pathway to form CH3CHO and deactivate that leading to CH4 and H2CO. These experimental results are in good agreements with the theoretical predictions that highlight the catalytic effect of H2O on the CH2CO + H reaction, whose potential energy barrier drops from 4.6 kcal mol-1 (without water) to 3.8 and 3.6 kcal mol-1 with one and two water molecules respectively.

An investigation into transition states of cyclic tetra-atomic silicon and germanium interstellar dust compounds: SixC4-x, GexC4-x, and GexSi4-x (x ∈ {1,2,3}).

Presented in this work is a thorough determination of the transition states between the different isomers of cyclic tetra-atomic silicon carbide, germanium carbide, and germanium silicide clusters. Through use of density functional theory (B3LYP-D3BJ, M06-2X, ωB97X-D4, and B2GP-PLYP) in conjunction with the aug-cc-pVTZ basis set, transition state structures and their barrier heights are determined for the interconversions between the various isomers for the family of tetra-atomic SiC, GeC, and GeSi compounds. SiC dust grains are known to be prevalent in interstellar dust, and among this group, so far only diamond-shaped (d-)SiC3 has been detected in the interstellar medium (ISM). Determining which other structures might be detectable not only depends on their intrinsic spectroscopic features, but whether or not they are likely to exist as isomers in interstellar environments. By examining the energy barrier heights for transitions between isomers, we determined that many of these structures are unlikely to exhibit interconversion in the ISM, outside of hotter circumstellar environments. Although Boltzmann population ratios at approximate circumstellar temperatures suggest the presence of higher energy minima, it is likely that once interconversion happens, as molecules travel away from a star and cool, they will get kinetically trapped in the potential energy well they inhabit, making how the ratios freeze out dependent on the time and pathways the molecules take to cool down. As such, many of these higher energy minima may still be good candidates for detection including (rhomboidal) r-SiC3, r-GeC3, r-GeSi3, (trapezoidal) t-Si2C2, r-Ge2C2, and d-Si3C.

Sensitivity of Polarization to Grain Shape. I. Convex Shapes

Aligned interstellar grains produce polarized extinction (observed at wavelengths from the far-ultraviolet to the mid-infrared) and polarized thermal emission (observed at far-infrared and submm wavelengths). The grains must be quite nonspherical, but the actual shapes are unknown. The relative efficacy for aligned grains to produce polarization at optical versus infrared wavelengths depends on particle shape. The discrete dipole approximation is used to calculate polarization cross sections for 20 different convex shapes, for wavelengths from 0.1 to 100 μm, and grain sizes a eff from 0.05 to 0.3 μm. Spheroids, cylinders, square prisms, and triaxial ellipsoids are considered. Minimum aspect ratios required by the observed starlight polarization are determined. Some shapes can also be ruled out because they provide too little or too much polarization at far-infrared and submm wavelengths. The ratio of 10 μm polarization to integrated optical polarization is almost independent of grain shape, varying by only ±8% among the viable convex shapes; thus, at least for convex grains, uncertainties in grain shape cannot account for the discrepancy between predicted and observed 10 μm polarization toward Cyg OB2-12.