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We present a new model of interstellar dust in which large grains are a single composite material, “astrodust,” and nanoparticle-sized grains come in distinct varieties including polycyclic aromatic hydrocarbons (PAHs). We argue that a single-composition model for grains larger than ∼0.02 μm most naturally explains the lack of frequency dependence in the far-infrared (FIR) polarization fraction and the characteristic ratio of optical to FIR polarization. We derive a size distribution and alignment function for 1.4:1 oblate astrodust grains that, with PAHs, reproduce the mean wavelength dependence and polarization of Galactic extinction and emission from the diffuse interstellar medium while respecting constraints on solid-phase abundances. All model data and Python-based interfaces are made publicly available.

Dust extinction is one of the fundamental measurements of dust grain sizes, compositions, and shapes. Most of the wavelength-dependent variations seen in Milky Way extinction are strongly correlated with the single parameter R(V) = A(V)/E(B − V). Existing R(V)-dependent extinction relationships use a mixture of spectroscopic and photometry observations, and hence do not fully capture all the important dust features or continuum variations. Using four existing samples of spectroscopically measured dust extinction curves, we consistently measure the R(V)-dependent extinction relationship spectroscopically from the far-ultraviolet (FUV) to mid-infrared for the first time. Linear fits of A(λ)/A(V) dependent on R(V) are done using a method that fully accounts for their significant and correlated uncertainties. These linear parameters are fit with analytic wavelength-dependent functions to determine the smooth R(V) (2.3–5.6) and wavelength (912 Å–32 μm) dependent extinction relationship. This relationship shows that the FUV rise, 2175 Å bump, and the three broad optical features are dependent on R(V), but the 10 and 20 μm features are not. Existing literature relationships show significant deviations compared to this relationship especially in the FUV and infrared (IR). Extinction curves that clearly deviate from this relationship illustrate that this relationship only describes the average behavior versus R(V). We find tentative evidence that the relationship may not be linear with R(V)−1 especially in the ultraviolet (UV). For the first time, this relationship provides measurements of dust extinction that spectroscopically resolve the continuum and features in the UV, optical, and IR as a function of R(V), enabling detailed studies of dust grain properties and full spectroscopic accounting for the effects of dust extinction on astrophysical objects.

Interstellar dust extinction curves provide valuable information about dust properties, including the composition and size of the dust grains, and are essential to correct observations for the effects of interstellar dust. In this work, we measure a representative sample of near-infrared (NIR; 0.8–5.5 μm) spectroscopic extinction curves for the first time, enabling us to investigate the extinction at wavelengths where it is usually only measured in broad photometric bands. We use IRTF/SpeX spectra of a sample of reddened and comparison stars to measure 15 extinction curves with the pair method. Our sample spans A(V) values from 0.78 to 5.65 and R(V) values from 2.43 to 5.33. We confirm that the NIR extinction curves are well fit by a power law, with indices and amplitudes differing from sight line to sight line. Our average diffuse NIR extinction curve can be represented by a single power law with index α = 1.7, but because of the sight line-to-sight line variations, the shape of any average curve will depend on the parental sample. We find that most of the variation in our sample can be linked to the ratio of total-to-selective extinction R(V), a rough measurement of the average dust grain size. Two sight lines in our sample clearly show the ice extinction feature at 3 μm, which can be fitted by a modified Drude profile. We find tentative ice detections with slightly over 3σ significance in two other sight lines. In our average diffuse extinction curve, we measure a 3σ upper limit of A(ice)/A(V) = 0.0021 for this ice feature.

We present a comprehensive 3D dust-reddening map covering the entire Milky Way, constructed by combining reddening estimates based on Large Sky Area Multi-Object Fiber Spectroscopy Telescope (LAMOST) low-resolution spectra (E(B − V)LAMOST) with those derived from Gaia XP spectra (E(B − V)XP), along with revised Gaia distances. E(B − V)LAMOST values of ∼4.6 million unique sources were obtained with the standard-pair analysis using LAMOST DR11 stellar parameters and synthesized B-/V-band photometry from Gaia XP spectra, showing a typical precision of ∼0.01 mag. The E(B − V)XP from the catalog of X. Zhang et al., which was derived using forward modeling of Gaia XP spectra, were cross-validated with E(B − V)LAMOST, leading to the selection of ∼150 million high-reliability measurements. The combined data set achieves a median precision of ∼0.03 mag for E(B − V). To model the reddening–distance relationship along various lines of sight, we implemented a parametric approach that accounts for contributions from the Local Bubble, diffuse interstellar medium, and multiple potential molecular clouds. The sky was adaptively partitioned based on stellar density, resulting in angular resolutions ranging from 3 .′ 4 to 58′, with about half of the sky having a resolution better than 6 .′ 9. The reddening precision of our 3D map for individual stars reaches ∼0.01 mag in most regions at ∣b∣ > 20°, but degrades to 0.01–0.05 mag at ∣b∣ < 20°. The map reaches a maximum distance of 3–5 kpc in high-extinction regions with ∣b∣ < 5°, and extends to 10–15 kpc elsewhere. An interactive platform and Python package have been developed for utilization of the 3D dust map.

In the coldest (10–20 K) regions of the interstellar medium, the icy surfaces of interstellar grains serve as solid-state supports for chemical reactions. Among their plausible roles, that of third body is advocated, in which the reaction energies of surface reactions dissipate throughout the grain, stabilizing the product. This energy dissipation process is poorly understood at the atomic scale, although it can have a high impact on astrochemistry. Here we study, by means of quantum mechanical simulations, the formation of NH3 via successive H-additions to atomic N on water ice surfaces, paying special attention to the third-body role. We first characterize the hydrogenation reactions and the possible competitive processes (i.e., H-abstractions), in which the H-additions are more favorable than the H-abstractions. Subsequently, we study the fate of the hydrogenation reaction energies by means of ab initio molecular dynamics simulations. Results show that around 58%–90% of the released energy is quickly absorbed by the ice surface, inducing a temporary increase of the ice temperature. Different energy dissipation mechanisms are distinguished. One mechanism, more general, is based on the coupling of the highly excited vibrational modes of the newly formed species and the libration modes of the icy water molecules. A second mechanism, exclusive during the NH3 formation, is based on the formation of a transient H3O+/NH2 − ion pair, which significantly accelerates the energy transfer to the surface. Finally, the astrophysical implications of our findings relative to the interstellar synthesis of NH3 and its chemical desorption into the gas are discussed.

We measure the CO-to-H2 conversion factor (α CO) in 37 galaxies at 2 kpc resolution, using the dust surface density inferred from far-infrared emission as a tracer of the gas surface density and assuming a constant dust-to-metal ratio. In total, we have ∼790 and ∼610 independent measurements of α CO for CO (2–1) and (1–0), respectively. The mean values for α CO (2–1) and α CO (1–0) are 9.3−5.4+4.6 and 4.2−2.0+1.9M⊙pc−2(Kkms−1)−1 , respectively. The CO-intensity-weighted mean is 5.69 for α CO (2–1) and 3.33 for α CO (1–0). We examine how α CO scales with several physical quantities, e.g., the star formation rate (SFR), stellar mass, and dust-mass-weighted average interstellar radiation field strength ( U¯ ). Among them, U¯ , ΣSFR, and the integrated CO intensity (W CO) have the strongest anticorrelation with spatially resolved α CO. We provide linear regression results to α CO for all quantities tested. At galaxy-integrated scales, we observe significant correlations between α CO and W CO, metallicity, U¯ , and ΣSFR. We also find that α CO in each galaxy decreases with the stellar mass surface density (Σ⋆) in high-surface-density regions (Σ⋆ ≥ 100 M ⊙ pc−2), following the power-law relations αCO(2–1)∝Σ⋆−0.5 and αCO(1–0)∝Σ⋆−0.2 . The power-law index is insensitive to the assumed dust-to-metal ratio. We interpret the decrease in α CO with increasing Σ⋆ as a result of higher velocity dispersion compared to isolated, self-gravitating clouds due to the additional gravitational force from stellar sources, which leads to the reduction in α CO. The decrease in α CO at high Σ⋆ is important for accurately assessing molecular gas content and star formation efficiency in the centers of galaxies, which bridge “Milky Way–like” to “starburst-like” conversion factors.

We make use of Atacama Large Millimeter/submillimeter Array continuum observations of 15 luminous Lyman-break galaxies at z ∼ 7–8 to probe their dust-obscured star formation. These observations are sensitive enough to probe obscured star formation rates (SFRs) of 20 M ⊙ yr−1 (3σ). Six of the targeted galaxies show significant (≥3σ) dust-continuum detections, more than doubling the number of known dust-detected galaxies at z > 6.5. Their IR luminosities range from 2.7 × 1011 L ⊙ to 1.1 × 1012 L ⊙, equivalent to obscured SFRs of 25 to 101 M ⊙ yr−1. We use our results to quantify the correlation of the infrared excess (IRX) on the UV-continuum slope β UV and stellar mass. Our results are most consistent with a Small Magellanic Cloud (SMC) attenuation curve for intrinsic UV-slopes βUV,intr of −2.63 and most consistent with an attenuation curve in between SMC and Calzetti for βUV,intr slopes of −2.23, assuming a dust temperature T d of 50 K. Our fiducial IRX–stellar mass results at z ∼ 7–8 are consistent with marginal evolution from z ∼ 0. We then show how both results depend on T d . For our six dust-detected sources, we estimate their dust masses and find that they are consistent with dust production from supernovae if the dust destruction is low (<90%). Finally we determine the contribution of dust-obscured star formation to the SFR density for UV luminous (H<−21.5 mag: ≳1.7 L * UV) z ∼ 7–8 galaxies, finding that the total SFR density at z ∼ 7 and z ∼ 8 from bright galaxies is 0.20−0.10+0.10 dex and 0.23−0.09+0.06 dex higher, respectively; i.e., ∼ 13 of the star formation in ≳1.7 L * UV galaxies at z ∼ 7–8 is obscured by dust.

The total-to-selective extinction ratio, R V, is a key parameter for tracing the properties of interstellar dust, as it directly determines the variation of the extinction curve with wavelength. By utilizing accurate color excess measurements from the optical to the mid-infrared range, we have derived R V values for approximately three million stars from LAMOST data release 7 using a forward-modeling technique. This extensive data set enables us to construct a comprehensive 2D R V map of the Milky Way within the LAMOST footprint at a spatial resolution of ∼27.′5. Based on reliable sight lines of E(B − V) > 0.1, we find that R V exhibits a Gaussian distribution centered around 3.25 with a standard deviation of 0.25. The spatial variability of R V in the Galactic disk exhibits a wide range, spanning from small scales within individual molecular clouds to large scales up to kiloparsecs. A striking correlation is observed between the distribution of R V and molecular clouds. Notably, we observe lower R V values within the regions of nearby molecular clouds compared to their surrounding areas. Furthermore, we have investigated the relationships between R V and various parameters, including dust temperature, dust emissivity spectral index, column densities and ratios of atomic and molecular hydrogen, as well as the gas-to-dust ratio. We find that these relationships vary with the level of extinction. These analyses provide new insights into the properties and evolution of dust grains in diverse interstellar environments and also hold significant importance for achieving accurate extinction corrections.

We have obtained accurate dust reddening from the far-ultraviolet to the mid-infrared for up to 5 million stars by the star-pair algorithm based on LAMOST stellar parameters along with Galaxy Evolution Explorer, Pan-STARRS 1, Gaia, Sloan Digital Sky Survey, Two Micron All Sky Survey, and Wide-field Infrared Survey Explorer photometric data. The typical errors are between 0.01 and 0.03 mag for most colors. We derived the empirical reddening coefficients for 21 colors both in the traditional (single-valued) way and as a function of T eff and E(B − V) by using the largest samples of accurate reddening measurements, together with the extinction values from Schlegel et al. The corresponding extinction coefficients have also been obtained. The results are compared with model predictions and generally in good agreement. Comparisons with measurements in the literature show that the T eff- and E(B − V)-dependent coefficients explain the discrepancies between different measurements naturally, i.e., using sample stars of different temperatures and reddening. Our coefficients are mostly valid in the extinction range of 0–0.5 mag and the temperature range of 4000–10,000 K. We recommend that the new T eff- and E(B − V)-dependent reddening and extinction coefficients should be used in the future. A Python package is also provided for the usage of the coefficients (https://github.com/vnohhf/extinction_coeffcient/).

The Small Magellanic Cloud (SMC) shows a large variation in ultraviolet (UV) dust extinction curves, ranging from Milky Way (MW) like to significantly steeper curves with no detectable 2175 Å bump. This result is based on a sample of only nine sight lines. From Hubble Space Telescope Space Telescope Imaging Spectrograph and IUE spectra of OB stars, we have measured UV extinction curves along 32 SMC sight lines where eight of these curves were published previously. We find 16 sight lines with steep extinction with no detectable 2175 Å bump, four sight lines with MW-like extinction with a detectable 2175 Å bump, two sight lines with fairly flat UV extinction and weak/absent 2175 Å bumps, and 10 sight lines with unreliable curves due to low SMC dust columns. Our expanded sample shows that the sight lines with and without the 2175 Å bump are located throughout the SMC and not limited to specific regions. The average extinction curve of the 16 bump-less sight lines is very similar to the previous average based on four sight lines. We find no correlation between dust column and the strength of the 2175 Å bump. We test the hypothesis that the 2175 Å bump is due to the same dust grains that are responsible for the mid-infrared carbonaceous (polycyclic aromatic hydrocarbon) emission features and find they are correlated, confirming recent work in the MW. Overall, the slope of the UV extinction increases as the amplitudes of the 2175 Å bump and far-ultraviolet curvature decrease. Finally, the UV slope is correlated with N(H i)/A(V) and the 2175 Å bump and nonlinear far-ultraviolet rise amplitudes are anticorrelated with N(H i)/A(V).