Ultraviolet Extinction Curve Research Articles

DIFFUSE-BAND OBSERVATIONS RELATED TO THE INTERSTELLAR EXTINCTION LAW

ABSTRACT This paper discusses the relation between the observed intensity ratios of the unidentified diffuse interstellar bands at 5780 A and 5797 A and the shape of the ultraviolet extinction curve determined along the same lines of sight. High-resolution, low-noise spectroscopic observations are presented for nine lightly reddened stars chosen to exhibit very different ultraviolet extinction characteristics. It is shown that the strengths of these diffuse bands depend strongly on the extinction law, but that the intrinsic profiles do not; however, a physical interpretation of the observed phenomena is still very uncertain.

An analysis of the shapes of ultraviolet extinction curves. III - an atlas of ultraviolet extinction curves

The basic data needed to produce UV extinction curves for a sample of 78 stars, including spectral types, visual photometry, parameters used to produce fits to the UV extinction curves, distances to the program stars, and line-of-sight H I column densities, are presented. The observational and numerical techniques used to obtain the results are discussed in detail. The UV extinction curves and their analytic fits are shown graphically.

On the size distribution of newly formed grains in red supergiant atmospheres

Theoretical ultraviolet extinction curves have been calculated for comparison with observed curves for circumstellar dust in M supergiants. The theoretical curves assume a silicate grain composition, because silicate grains are expected in the oxygen-rich environments that are observed. Calculations were performed with and without the inclusion of scattering into the beam, with largely similar results. A comparison of the computed curves with the observed ultraviolet extinction curve for circumstellar dust in Alpha Scorpii indicates that the size distribution of the circumstellar grains must cut off near 800 A; that is, there are few or no grains smaller than this. The conclusion is that smaller interstellar silicate grains, where they exist, must come from other sources such as grain fragmentation in shocks.

Ultraviolet extinction differences within NGC 6530

Ultraviolet extinction curves are presented for 14 stars in the very young cluster NGC 6530. The variation of the short-wavelength extinction is consistent with that expected based on random errors alone, although the weakest extinction is found only toward the cluster center. Variations significantly larger than the measurement uncertainty are found in the area, width, and central peak position of the 2200 A extinction bump. These parameters are largest in the cluster center and decrease systematically outwards. Physical processes capable of modifying the grains in the H II region containing the cluster are discussed.

The Composition of Dust in Stellar Ejecta

I review the observational evidence relevant to the composition of dust ejected from circumstellar shells. Emphasis is placed on those types of stars believed to be the principal sources of refractory stardust in the interstellar medium. In stars with normal abundances (O > C), the predominant species appears to be an amorphous silicate, characterised by smooth spectral features at 9.7 and 19μm, a far infrared emissivity close to unity, and a featureless ultraviolet extinction curve. Similarly, the characteristics of carbon stars (C > O) are consistent with the presence of amorphous carbon rather than graphite. Silicon carbide is a minor constituent of carbon-rich stardust and is therefore unlikely to be an important ingredient of interstellar grains. A major goal for future studies will be to establish the evolutionary link between amorphous carbon stardust and the carriers of the unidentified infrared emission features observed in planetary nebulae.

The ultraviolet extinction from interstellar graphitic onions

Carbon particles in interstellar dust cause absorption of starlight at certain wavelengths. Kroto and McKay1 have suggested that such particles have the form of quasi-icosahedral spiral shells, with the hexagonal graphitic planes perpendicular to the radius, like the layers of an onion. Using the discrete dipole approximation (DDA)2 I have calculated the ultraviolet extinction curves of small spherical particles with the tensor dielectric constant of graphite but with the c-axis always running parallel to the radius vector. The feature at a wavelength of 2,200 A is doubled in width and moved to a wavelength about 80 A shorter in comparison to the parameters derived from the usual Mie theory approximation for a mixture of particles with scalar dielectric constants. For larger particles, the peak shifts to longer wavelengths, but for a radius of ∼300 A which puts the peak at 2,200 A (the same position as the mean interstellar absorption peak3), the width of absorption by quasi-icosahedral particles is too large to fit the data.

An analysis of the shapes of ultraviolet extinction curves. II - The far-UV extinction

In this paper the properties of interstellar extinction in the far-ultraviolet region are examined utilizing IUE extinction curves for a primary data sample of 45 reddened Milky Way OB stars. These results are combined with those derived for the 2175 A bump in a previous study. It is found that IUE extinction curves can be represented by linear combinations of a Lorentzian-like 2175 A bump profile, a well-determined FUV curvature term, and an underlying linear component. The parameters of the linear component are strongly correlated and therefore only five free parameters are required to fit all of the curves in our sample. Three parameters describe the 2175 A bump, one parameter describes the linear background, and one parameter describes the strength of the FUV curvature term. The shape of the FUV curvature is found to be identical, to within the observational errors, for all the curves in the sample - which spans virtually the entire range of extinction curve morphologies observed in the Milky Way.

Ultraviolet interstellar extinction toward NGC 6530 and the intrinsic energy distribution of 9 Sagittarii and HD 165052

The ultraviolet interstellar extinction curve is determined toward 12 early B main-sequence stars and one O8 V star in NGC 6530. The extinction is found to be uniform across the cluster. At short wavelengths, the extinction curve of NGC 6530 falls below the average Galactic curve by a full magnitude, indicating that the population of small grains relative to larger grains is lower than normal. Longward of 2830 A the curve is almost coincident with the mean Galactic curve. The 2175 A feature is narrower than the average. The derived extinction curve is used to deredden the observed fluxes of two early O-type stars in NGC 6530:9 Sgr and HD 165052. The intrinsic continua are compared to model atmospheres in the region 1200-8200 A. The LTE, line-blanketed models of Kurucz (1979) fit the observations better than the non-LTE, unblanketed models of Mihalas (1972) suggesting that line-blanketing effects are more important than the non-LTE effects in the atmospheres of these stars. The temperatures of the models that best fit the observations are lower than the Zanstra temperatures and the temperatures estimated from the strengths of the He I and He II lines, by 10,000 to 20,000 K.

An analysis on the shapes of ultraviolet extinction curves. I - The 2175 A bump

IUE data are used to investigate the properties of the 2175 A interstellar extinction bump toward 45 reddened Milky Way stars. Using analytic fitting functions, parameters are derived which describe the central position, FWHM, and strength of the bump. The bump position is very stable, with an extreme deviation which is still significantly larger than the measurement uncertainty, indicating true variability. The bump FWHM has a large range of intrinsic values, 0.77/micron to 1.25/micron. The normalized bump strength varies by more than a factor of three. There are no convincing correlations among the three bump parameters, but the width of the bump is strongly correlated with the dust grain environments. Dense quiescent regions yield broad bumps, while diffuse interstellar medium and regions of recent star formation yield narrower bumps. The absence of any correlation between bump width and central position argues strongly against graphite grains in the size range prescribed by the Mathis, Rumpl, and Nordsieck (1977) model as carriers of the bump.

International Ultraviolet Explorer observations of stars in 30 Doradus - Extinction and stellar continua

A study of ultraviolet interstellar extinction in and near the core of the 30 Doradus nebula is presented. The pair method is used to determine the shape of the ultraviolet extinction curve, and reddened stars from within 80 pc of the core and unreddened stars from a variety of locations in the Large Magellanic Cloud (LMC) are considered. All stars near the core appear to be reddened by E(B-V) = 0.09-0.16 with an extinction law similar to previous LMC extinction laws. Some stars, including R136a, R145, and R147, are additionally reddened by E(B-V) = 0.18 with a 'nebular-type' extinction law. A model consisting of a layer of 'LMC foreground dust' which affects all of the stars and a deeper layer of 'nebular dust' which affects some of the stars is used for an explanation of the extinction properties. The extinction curves are then applied to the ultraviolet energy distribution of R136a for a determination of its intrinsic continuum shape. Evidence showing that the brightest LMC OB supergiants have intrinsic (B-V) colors and UV-to-visual continuum slopes which are redder than expected is presented.

Interstellar Dust in the LMC

New IUE observations confirm the differences between the Galactic and LMC ultraviolet extinction curves and show some evidence for variations within the LMC. Visual and infrared photometry and polarimetry show that the anomalous properties of the LMC dust do not extend to longer wavelengths. Despite the much different dust to gas ratios in the Galaxy and the Magellanic clouds, the dust formation efficiency is similar when the abundances are included.

Shock processing of interstellar grains

Theoretical and observational evidence is presented that shock processing of interstellar dust grains by supernova blast waves affects both heavy element depletions and ultraviolet extinction curves. By coupling a realistic model of grain sizes and populations with a radiative shock code, significant grain destruction at velocities as low as 40 km/s is demonstrated. Nonthermal sputtering and grain-grain collisions destroy relatively more large grains than small, and more silicates than graphite. Consequently, both the 2175 A extinction 'bump' and the far-ultraviolet normalized extinction are increased in strength. Ultraviolet extinction studies with the International Ultraviolet Explorer of nine stars near three supernova remnants (the Monoceros Loop, Shajn 147, and Vela) exhibit strong 2175 A bumps and normal or high far-ultraviolet extinction. Diffuse bands, if they are created by small grains, should show little correlation with such activity.

Observation of ice mantles toward HD 29647

The water ice absorption band at 3.1 microns has been observed in HD 29647, which is a late B type star with an anomalous ultraviolet extinction curve. The interpretation is that ice mantles are present on the dust grains in the intervening cloud and that these mantles are responsible for suppressing the 2200 A interstellar absorption feature. This is the first observation of both the 2200 A region and the 3.1 micron interstellar absorption feature toward the same star. A comparison of recent UV laboratory data of water ice with the UV spectrum of HD 29647 further supports the presence of water ice mantles in the same column density as that observed in the infrared. The diffuse interstellar features have been reported to be weakened in this star, consistent with their origin being on grain core surfaces, rather than in mantles or gaseous molecules.

Ultraviolet extinction curves derived from IUE data

Data from the International Ultraviolet Explorer (IUE) have been used to derive extinction curves for several stars chosen because of unusual diffuse band strength or location within dense clouds. Both the strength and position of the 2200 A extinction maximum were assessed, as well as the level of extinction in the far-ultraviolet. A variety of combinations of these features were found, ranging from near or complete absence of a 2200 A bump (HD 29647), to low far-ultraviolet extinction (Rho Oph), to unusually steep, far-ultraviolet extinction (HD 169454). Anomalous extinction apparently shows up frequently in stars associated with nebulosity or dense clouds. The apparent independence of 2200 A and far-ultraviolet extinction supports earlier suggestions that these two features are created by distinct populations of grains.

An anomalous ultraviolet extinction curve in the Taurus dark cloud

view Abstract Citations (57) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An anomalous ultraviolet extinction curve in the Taurus dark cloud. Snow, T. P., Jr. ; Seab, C. G. Abstract Data from IUE have been used to derive an ultraviolet extinction curve for HD 29647, a B star embedded in the Taurus dark cloud complex. The curve appears normal, except that the 2200 A extinction bump is absent. Arguments are developed to show that this is most likely due to a modification of the grain optical properties through the accretion of mantles in this dense cloud. If this conclusion is correct, it will have some impact on models for the formation of the 2200 A bump. Publication: The Astrophysical Journal Pub Date: December 1980 DOI: 10.1086/183408 Bibcode: 1980ApJ...242L..83S Keywords: Interstellar Extinction; Molecular Clouds; Nebulae; Ultraviolet Absorption; Anomalies; B Stars; Iue; Astrophysics full text sources ADS | data products SIMBAD (1) MAST (1)

Ultraviolet observations of interstellar molecules and grains fromSpacelab

Observational results obtained to date on interstellar grains and molecules are briefly reviewed, and several promising areas for further research withSpacelab are suggested. Regarding grains, useful data can be expected on the shape of the ultraviolet extinction curve for new interstellar regions; the nature of UV extinction at short wavelengths, down to the Lyman limit; the presence or absence of structure in the UV extinction curve comparable to the visible-wavelength diffuse bands; the scattering properties of grains in new kinds of clouds and nebulae; and the polarization properties of grains in UV wavelengths. The principal advances which may be expected in observations of molecules will include the ability to probe more heavily-obscured regions, where molecular species are more abundant than in the diffuse clouds observed to date; coverage of wavelength regions (such as λλ 1400–3200) not well-studied with previous instruments such asCopernicus; and the capability of observing in optical absorption species detected in the same line of sight in radio emission, which provides unique information on cloud geometry and physical conditions.

The Problem of the UV Interstellar Absorption Band at 2200A

The ultraviolet interstellar extinction curve from 2740Å to 1350Å has been obtained using data from the S2/68 satellite experiment. The extinction increases into the ultraviolet and shows a pronounced peak at 2200Å. This is interpreted as a general scattering continuum with a strong absorption feature superposed on it at 2200Å. The profile of the feature appears to be symmetrical and has a half-width of 360Å. There is a strong correlation between the strength of the feature and the scattering part of the curve in both the ultraviolet and the visible. On a broad scale the shape of the extinction curve is constant showing no variation with distance from the sun, direction around the galaxy, and height above the galactic plane.

Progress in ultraviolet stellar spectrophotometry with S59

A progress report is given of investigations related to observations of stellar spectra obtained with the ultraviolet stellar spectrophotometer S59 aboard the ESRO TD-1A satellite. We describe first the processing of the observations: intensity and wavelength calibration, identification of lines, classification of spectra. Thereafter some important groups of lines are dealt with: non-LTE computation of the Mgii lines are presented: apart from the peculiar emission line and shell stars they compare well with the observations; intensity ratios, of Feii and Feiii lines are an important temperature classification criterion. Interstellar UV lines indicate large deficiencies of some metals, as compared to solar values. A new ultraviolet continuous extinction curve is determined. We discuss the strong outstreaming motions observed in α Cygni, and the consequent mass loss of this star (< 3 × 10−10\(\mathfrak{M}_ \odot \) yr−1and finally we describe the composite spectrum of the (WC8+09I) bnary γ2 Velorum; the ultraviolet continuous spectrum of the WC star is about one magnitude brighter than any theory predicts.

Optical properties of particulates

Optical properties of small particles of olivine (less than 0.1 μ) have been studied in the ultraviolet as an example of an insulating solid. Very little structure survives in the ultraviolet extinction curves for such small particles. By contrast ‘surface modes’, observed for graphite small particles in the ultraviolet and for olivine particles in the infrared, produce dominant and persistent structure in extinction. The general trend of optical properties of graphite is surprisingly similar to the behavior required to explain all features of the interstellar extinction and albedo curves from near visible to 1000 A. Measured extinction of small olivine particles in the infrared agrees with calculations based on newly measured optical constants, but dominant sharp structure in the 10μ region still presents a bit of a problem in explaining ‘silicate’ features in astronomical data.

Ultraviolet photometry of stars from OSO II

Ultraviolet stellar fluxes from 1500 to 3200 Å were from February through August 1965 on OSO II. A 15-cm diameter Gregorian telescope with a stepped grating spectrophotometer provided flux measurements in ten adjacent 180 Å wide band passes.By comparing fluxes of stars showing B-U color excesses smaller than 0.19 and larger than 0.26, an ultraviolet extinction curve has been derived, which agrees with others which have been published as far as the magnitude effect is concerned, but the shape differs somewhat, the slope at 1/λ = 3.8 μ−1 being somewhat greater than at 1/λ = 5.0 μ−1.If the stars' fluxes are corrected for reddening, it is found that there is a good agreement between observation and models for stars earlier than B3, but that many of the later type stars have a residual apparently intrinsic reddening.