Geoffrey C. Clayton

The relationship between infrared, optical, and ultraviolet extinction

The parameterized extinction data of Fitzpatrick and Massa (1986, 1988) for the ultraviolet and various sources for the optical and near-infrared are used to derive a meaningful average extinction law over the 3.5 micron to 0.125 wavelength range which is applicable to both diffuse and dense regions of the interstellar medium. The law depends on only one parameter R(V) = A(V)/E(B-V). An analytic formula is given for the mean extinction law which can be used to calculate color excesses or to deredden observations. The validity of the law over a large wavelength interval suggests that the processes which modify the sizes and compositions of grains are stochastic in nature and very efficient.

A Quantitative Comparison of the Small Magellanic Cloud, Large Magellanic Cloud, and Milky Way Ultraviolet to Near-Infrared Extinction Curves*

We present an exhaustive, quantitative comparison of all of the known extinction curves in the Small and Large Magellanic Clouds (SMC and LMC) with our understanding of the general behavior of Milky Way extinction curves. The R_V dependent CCM relationship and the sample of extinction curves used to derive this relationship is used to describe the general behavior of Milky Way extinction curves. The ultraviolet portion of the SMC and LMC extinction curves are derived from archival IUE data, except for one new SMC extinction curve which was measured using HST/STIS observations. The optical extinction curves are derived from new (for the SMC) and literature UBVRI photometry (for the LMC). The near-infrared extinction curves are calculated mainly from 2MASS photometry supplemented with DENIS and new JHK photometry. For each extinction curve, we give R_V = A(V)/E(B-V) and N(HI) values which probe the same dust column as the extinction curve. We compare the properties of the SMC and LMC extinction curves with the CCM relationship three different ways: each curve by itself, the behavior of extinction at different wavelengths with R_V, and behavior of the extinction curve FM fit parameters with R_V. As has been found previously, we find that a small number of LMC extinction curves are consistent with the CCM relationship, but majority of the LMC and all of the SMC curves do not follow the CCM relationship. For the first time, we find that the CCM relationship seems to form a bound on the properties of all of the LMC and SMC extinction curves. This result strengthens the picture of dust extinction curves exhibit a continuum of properties between those found in the Milky Way and the SMC Bar. (abridged)We present an exhaustive quantitative comparison of all the known extinction curves in the Small and Large Magellanic Clouds (SMC and LMC) with our understanding of the general behavior of Milky Way extinction curves. The RV-dependent CCM relationship of Cardelli, Clayton, and Mathis and the sample of extinction curves used to derive this relationship are used to describe the general behavior of Milky Way extinction curves. The ultraviolet portion of the SMC and LMC extinction curves are derived from archival IUE data, except for one new SMC extinction curve, which was measured using Hubble Space Telescope Space Telescope Imaging Spectrograph observations. The optical extinction curves are derived from new (for the SMC) and literature UBVRI photometry (for the LMC). The near-infrared extinction curves are calculated mainly from 2MASS photometry supplemented with DENIS and new JHK photometry. For each extinction curve, we give RV = A(V)/E(B - V) and N(H I) values that probe the same dust column as the extinction curve. We compare the properties of the SMC and LMC extinction curves with the CCM relationship three different ways: each curve by itself, the behavior of extinction at different wavelengths with RV, and the behavior of the extinction curve Fitzpatrick and Massa fit parameters with RV. As has been found previously, we find that a small number of LMC extinction curves are consistent with the CCM relationship, but the majority of the LMC and all the SMC curves do not follow the CCM relationship. For the first time, we find that the CCM relationship seems to form a bound on the properties of all the LMC and SMC extinction curves. This result strengthens the picture dust extinction curves exhibit of a continuum of properties between those found in the Milky Way and the SMC bar. Tentative evidence based on the behavior of the extinction curves with dust-to-gas ratio suggests that the continuum of dust extinction curves is possibly caused by the environmental stresses of nearby star formation activity.

Herschel detects a massive dust reservoir in supernova 1987A.

The large amount of dust produced by this supernova may help explain the dust observed in young galaxies. We report far-infrared and submillimeter observations of supernova 1987A, the star whose explosion was observed on 23 February 1987 in the Large Magellanic Cloud, a galaxy located 160,000 light years away. The observations reveal the presence of a population of cold dust grains radiating with a temperature of about 17 to 23 kelvin at a rate of about 220 times the luminosity of the Sun. The intensity and spectral energy distribution of the emission suggest a dust mass of about 0.4 to 0.7 times the mass of the Sun. The radiation must originate from the supernova ejecta and requires the efficient precipitation of all refractory material into dust. Our observations imply that supernovae can produce the large dust masses detected in young galaxies at very high redshifts.

THE R CORONAE BOREALIS STARS

The R Coronae Borealis (RCB) stars are rare hydrogen-deficient, carbon-rich, supergiants, best known for their spectacular declines in brightness at irregular intervals. Efforts to discover more RCB stars have more than doubled the number known in the last few years and they appear to be members of an old, bulge population. Two evolutionary scenarios have been suggested for producing an RCB star, a double degenerate merger of two white dwarfs, or a final helium shell flash in a planetary nebula central star. The evidence pointing toward one or the other is somewhat contradictory, but the discovery that RCB stars have large amounts of 18O has tilted the scales towards the merger scenario. If the RCB stars are the product of white dwarf mergers, this would be a very exciting result since RCB stars would then be low-mass analogs of type Ia supernovae. The predicted number of RCB stars in the Galaxy is consistent with the predicted number of He/CO WD mergers. But, so far, only about 65 of the predicted 5000 RCB stars in the Galaxy have been discovered. The mystery has yet to be solved.

Starburst-like Dust Extinction in the Small Magellanic Cloud

The recent discovery that the ultraviolet (UV) dust extinction in starburst galaxies is similar to that found in the Small Magellanic Cloud (SMC) motivated us to reinvestigate the UV extinction found in the SMC. We have been able to improve significantly on previous studies by carefully choosing pairs of well-matched reddened and unreddened stars. In addition, we benefited from the improved signal-to-noise ratio of the NEWSIPS International Ultraviolet Explorer (IUE) data and the larger sample of SMC stars now available. Searching the IUE Final Archive, we found only four suitable early-type stars that were significantly reddened and had well-matched comparison stars. The extinction for three of these stars is remarkably similar. The curves are roughly linear with λ-1 and have no measurable 2175 A bump. The fourth star has an extinction curve with a significant 2175 A bump and weaker far-UV extinction. The dust along all four sight lines is thought to be local to the SMC. There is no significant Galactic foreground component. The first three stars lie in the SMC bar, and the line of sight for each of them passes through regions of recent star formation. The fourth star belongs to the SMC wing, and its line of sight passes through a much more quiescent region. Thus, the behavior of the dust extinction in the SMC supports a dependence of dust properties on star formation activity. However, other environmental factors (such as galactic metallicity) must also be important. Dust in the 30 Dor region of the LMC, where much more active star formation is present, does not share the extreme extinction properties seen in SMC dust.

Massive-Star Supernovae as Major Dust Factories

We present late-time optical and mid-infrared observations of the Type II supernova 2003gd in the galaxy NGC 628. Mid-infrared excesses consistent with cooling dust in the ejecta are observed 499 to 678 days after outburst and are accompanied by increasing optical extinction and growing asymmetries in the emission-line profiles. Radiative-transfer models show that up to 0.02 solar masses of dust has formed within the ejecta, beginning as early as 250 days after outburst. These observations show that dust formation in supernova ejecta can be efficient and that massive-star supernovae could have been major dust producers throughout the history of the universe.

Surveying the Agents of Galaxy Evolution in the Tidally Stripped, Low Metallicity Small Magellanic Cloud (SAGE-SMC). I. Overview

The Small Magellanic Cloud (SMC) provides a unique laboratory for the study of the lifecycle of dust given its low metallicity (~1/5 solar) and relative proximity (~60 kpc). This motivated the SAGE-SMC (Surveying the Agents of Galaxy Evolution in the Tidally Stripped, Low Metallicity Small Magellanic Cloud) Spitzer Legacy program with the specific goals of studying the amount and type of dust in the present interstellar medium, the sources of dust in the winds of evolved stars, and how much dust is consumed in star formation. This program mapped the full SMC (30 deg^2) including the body, wing, and tail in seven bands from 3.6 to 160 μm using IRAC and MIPS on the Spitzer Space Telescope. The data were reduced and mosaicked, and the point sources were measured using customized routines specific for large surveys. We have made the resulting mosaics and point-source catalogs available to the community. The infrared colors of the SMC are compared to those of other nearby galaxies and the 8 μm/24 μm ratio is somewhat lower than the average and the 70 μm/160 μm ratio is somewhat higher than the average. The global infrared spectral energy distribution (SED) shows that the SMC has approximately 1/3 the aromatic emission/polycyclic aromatic hydrocarbon abundance of most nearby galaxies. Infrared color-magnitude diagrams are given illustrating the distribution of different asymptotic giant branch stars and the locations of young stellar objects. Finally, the average SED of H II/star formation regions is compared to the equivalent Large Magellanic Cloud average H II/star formation region SED. These preliminary results will be expanded in detail in subsequent papers.

A Reanalysis of the Ultraviolet Extinction from Interstellar Dust in the Large Magellanic Cloud

We have reanalyzed the Large Magellanic Clouds (LMC) ultraviolet (UV) extinction using data from the IUE final archive. Our new analysis takes advantage of the improved signal-to-noise ratio of the IUE NEWSIPS reduction, the exclusion of stars with very low reddening, the careful selection of well-matched comparison stars, and an analysis of the effects of Galactic foreground dust. Differences between the average extinction curves of the 30 Dor region and the rest of the LMC are reduced compared with previous studies. We find that there is a group of stars with very weak 2175 A bumps that lie in or near the region occupied by the supergiant shell, LMC 2, on the southeast side of 30 Dor. The average extinction curves inside and outside LMC 2 show a very significant difference in 2175 A bump strength, but their far-UV extinctions are similar. While it is unclear whether or not the extinction outside the LMC 2 region can be fitted with the relation of Cardelli, Clayton, & Mathis (CCM), sight lines near LMC 2 cannot be fitted with CCM because of their weak 2175 A bumps. While the extinction properties seen in the LMC lie within the range of properties seen in the Galaxy, the correlations of UV extinction properties with environment seen in the Galaxy do not appear to hold in the LMC.

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.

Ultraviolet Extinction Properties in the Milky Way

We have assembled a homogeneous database of 417 ultraviolet (UV) extinction curves for reddened sight lines having International Ultraviolet Explorer spectra. We have combined these with optical and Two Micron All Sky Survey photometry, allowing estimates of the ratio of total-to-selective extinction, RV, for the entire sample. Fitzpatrick-Massa (FM) parameters have also been found for the entire sample. This is the largest study of parameterized UV extinction curves yet published, and it covers a wide range of environments, from dense molecular clouds to the diffuse interstellar medium, with extinctions AV ranging from 0.50 to 4.80. It is the first to extend far beyond the solar neighborhood and into the Galaxy at large, with 30 sight lines having distances greater than 5 kpc. Previously, the longest sight lines with FM parameters and RV extended ~1 kpc. We find that (1) the Cardelli, Clayton, & Mathis (CCM) extinction law applies for 93% of the sight lines, implying that dust processing in the Galaxy is efficient and systematic, (2) the central wavelength of the 2175 A bump is constant, and (3) the 2175 A bump width is dependent on environment. Only four sight lines show systematic deviations from CCM: HD 29647, 62542, 204827, and 210121. These sight lines all sample dense, molecule-rich clouds. The new extinction curves and values of RV allow us to revise the CCM law.