Silicate Emission Bands Research Articles

The Evolution of Dust Opacity in Core Collapse Supernovae and the Rapid Formation of Dust in Their Ejecta

Infrared (IR) observations of core collapse supernovae (CCSNe) have been used to infer the mass of dust that has formed in their ejecta. A plot of inferred dust masses versus supernova (SN) ages shows a trend of increasing dust mass with time, spanning a few decades of observations. This trend has been interpreted as evidence for the slow and gradual formation of dust in CCSNe. Observationally, the trend exhibits a $ t^2$ behavior, exactly what is expected from an expanding optically thick ejecta. In this case, the observed dust resides in the infrared (IR)-thin "photosphere" of the ejecta, and constitutes only a fraction of the total dust mass. We therefore propose that dust formation proceeds very rapidly, condensing most available refractory elements within two years after the explosion. At early epochs, only a fraction of the dust emission escapes the ejecta accounting for the low observed dust mass. The ejecta's entire dust content is unveiled only a few decades after the explosion, with the gradual decrease in its IR opacity. Corroborating evidence for this picture includes the early depletions of refractory elements in the ejecta of SN1987A and the appearance of a silicate emission band around day 300 in SN~2004et.

The Dust and Gas Around β Pictoris

We have obtained Spitzer IRS 5.5-35 μm spectroscopy of the debris disk around β Pictoris. In addition to the 10 μm silicate emission feature originally observed from the ground, we also detect the crystalline silicate emission bands at 28 and 33.5 μm. This is the first time that the silicate bands at wavelengths longer than 10 μm have ever been seen in the β Pictoris disk. The observed dust emission is well reproduced by a dust model consisting of fluffy cometary and crystalline olivine aggregates. We searched for line emission from molecular hydrogen and atomic [S I], Fe II, and Si II gas but detected none. We place a 3 σ upper limit of <17 M⊕ on the H2 S(1) gas mass, assuming an excitation temperature of Tex = 100 K. This suggests that there is less gas in this system than is required to form the envelope of Jupiter. We hypothesize that some of the atomic Na I gas observed in Keplerian rotation around β Pictoris may be produced by photon-stimulated desorption from circumstellar dust grains.

Silicates in D-Type Symbiotic Stars: AnInfrared Space ObservatoryOverview

We investigate the IR spectral features of a sample of D-type symbiotic stars in order to constrain the emitting properties of coupled dust-gas particles across the whole system. In particular, by analyzing unexploited ISO Short Wavelength Spectrometer data, deriving the basic observational parameters of dust bands, and comparing them with respect to those observed in other astronomical sources, we try to highlight the effect of environment on grain chemistry and physics. We find strong amorphous silicate emission bands at ~10 and ~18 ?m in a large fraction of the sample. The analysis of the ~10 ?m band, along with a direct comparison with several astronomical sources, reveals that silicate dust in symbiotic stars shows features between the characteristic circumstellar environments and the interstellar medium. This indicates an increasing reprocessing of grains in relation to specific symbiotic behavior of the objects. A correlation between the central wavelengths of the ~10 and ~18 ?m dust bands is found. By modeling IR spectral lines we also investigate dust grain conditions within the shocked nebulae. Both the unusual depletion values and the high sputtering efficiency might be explained by the formation of SiO molecules, which are known to be a very reliable shock tracer. We conclude that the signature of dust chemical disturbance due to symbiotic activity should be looked for in the outer, circumbinary, expanding shells where the environmental conditions for grain processing might be achieved. Symbiotic stars are thus attractive targets for new mid-IR and millimeter observations.

Probing Protoplanetary Disks with Silicate Emission: Where Is the Silicate Emission Zone?

Recent results indicate that the grain size and crystallinity inferred from observations of silicate features may be correlated with spectral type of the central star and/or disk geometry. In this paper, we show that grain size, as probed by the 10 um silicate feature peak-to-continuum and 11.3-to-9.8 um flux ratios, is inversely proportional to log L_star. These trends can be understood using a simple two-layer disk model for passive irradiated flaring disks, CGPLUS. We find that the radius, R_10, of the 10 um silicate emission zone in the disk goes as (L_star/L_sun)^0.56, with slight variations depending on disk geometry (flaring angle, inner disk radius). The observed correlations, combined with simulated emission spectra of olivine and pyroxene mixtures, imply a grain size dependence on luminosity. Combined with the fact that R_10 is smaller for less luminous stars, this implies that the apparent grain size of the emitting dust is larger for low-luminositysources. In contrast, our models suggest that the crystallinity is only marginally affected, because for increasing luminosity, the zone for thermal annealing (assumed to be at T>800 K) is enlarged by roughly the same factor as the silicate emission zone. The observed crystallinity is affected by disk geometry, however, with increased crystallinity in flat disks. The apparent crystallinity may also increase with grain growth due to a corresponding increase in contrast between crystalline and amorphous silicate emission bands.

C2dSpitzerIRS Spectra of Disks around T Tauri Stars. I. Silicate Emission and Grain Growth

Infrared ~5-35 μm spectra for 40 solar mass T Tauri stars and 7 intermediate-mass Herbig Ae stars with circumstellar disks were obtained using the Spitzer Space Telescope as part of the c2d IRS survey. This work complements prior spectroscopic studies of silicate infrared emission from disks, which were focused on intermediate-mass stars, with observations of solar mass stars limited primarily to the 10 μm region. The observed 10 and 20 μm silicate feature strengths/shapes are consistent with source-to-source variations in grain size. A large fraction of the features are weak and flat, consistent with micron-sized grains indicating fast grain growth (from 0.1 to 1.0 μm in radius). In addition, approximately half of the T Tauri star spectra show crystalline silicate features near 28 and 33 μm, indicating significant processing when compared to interstellar grains. A few sources show large 10-to-20 μm ratios and require even larger grains emitting at 20 μm than at 10 μm. This size difference may arise from the difference in the depth into the disk probed by the two silicate emission bands in disks where dust settling has occurred. The 10 μm feature strength versus shape trend is not correlated with age or Hα equivalent width, suggesting that some amount of turbulent mixing and regeneration of small grains is occurring. The strength versus shape trend is related to spectral type, however, with M stars showing significantly flatter 10 μm features (larger grain sizes) than A/B stars. The connection between spectral type and grain size is interpreted in terms of the variation in the silicate emission radius as a function of stellar luminosity, but could also be indicative of other spectral-type-dependent factors (e.g., X-rays, UV radiation, and stellar/disk winds).

3–14 Micron Spectroscopy of Comets C/2002 O4 (Honig), C/2002 V1 (NEAT), C/2002 X5 (Kudo‐Fujikawa), C/2002 Y1 (Juels‐Holvorcem), and 69P/Taylor and the Relationships among Grain Temperature, Silicate Band Strength, and Structure among Comet Families

We report 3-13 μm spectroscopy of four comets observed between 2002 August and 2003 February: C/2002 O4 (Honig) on 2002 August 1, C/2002 V1 (NEAT) on 2003 January 9 and 10, C/2002 X5 (Kudo-Fujikawa) on 2003 January 9 and 10, and C/2002 Y1 (Juels-Holvorcem) on 2003 February 20. In addition, we include data obtained much earlier on 69P/Taylor (1998 February 9) but not previously published. For comets Taylor, Honig, NEAT, and Kudo-Fujikawa, the silicate emission band was detected, being approximately 23%, 12%, 15%, and 10%, respectively, above the continuum. The data for comet Juels-Holvorcem were of insufficient quality to detect the presence of a silicate band of comparable strength to the other three objects, and we place an upper limit of 24% on this feature. The silicate features in both NEAT and Kudo-Fujikawa contained structure indicating the presence of crystalline material. The shape of the silicate feature at a projected distance of 1900 km from the nucleus of Kudo-Fujukawa was nearly identical to that centered on the nucleus, indicating that the grain size population had not been measurably modified by the time it had reached that distance. Combining these data with those of other comets, we confirm the correlation between silicate band strength and grain temperature of Gehrz & Ney and Williams and coworkers for dynamically new and long-period comets, but the majority of Jupiter family objects may deviate from this relation. Despite the weakness of the silicate band in Kudo-Fujikawa, its structure resembles the bands seen in dynamically new and long-period objects with substantially stronger features. The limited data available on Jupiter family objects suggest that they may have silicate bands that are slightly different from the former objects. Finally, when compared to the silicate emission bands observed in pre-main-sequence stars, the dynamically new and long-period comets most closely resemble the more evolved stellar systems, while the limited data (in quantity and quality) on Jupiter family objects seem to suggest that these have spectra more like the less evolved stars. Higher quality data on a larger number of Jupiter family objects are needed to confirm (or reject) this trend.

Mid-IR Spectroscopy of the Debris Disks in the TW Hydrae Association

Spectroscopic observation between 3-14 μm of 3 stars in the TW Hya Association were obtained using the Aerospace Corporation Broadband Array Spectrograph System (BASS) on the NASA IRTF telescope. The targets observed were TW Hya, HR 4796A, and HD 98800. Both of the late-type stars, TW Hya and HD 98800, exhibit a strong 10 μm silicate emission band. The strong emission indicates the presence of small dust grains close to the star, at angular distances not currently accessible to coronagraphic techniques. The spectral structure due to crystalline material that is present in some young early-type stars (HD 163296, HD 31648, etc.) is not apparent in these two objects. For HR 4796A, the dust emission is weak.

Mid-IR spectroscopy of T Tauri stars in Chamealeon I: Evidence for processed dust at the earliest stages

We present mid-IR spectroscopy of three T Tauri stars in the young Chamealeon I dark cloud obtained with TIMMI2 on the ESO 3.6 m telescope. In these three stars, the silicate emission band at 9.7 μ m is prominent. We model it with a mixture of amorphous olivine grains of different size, crystalline silicates and silica. The fractional mass of these various components change widely from star to star. While the spectrum of CR Cha is dominated by small amorphous silicates, in VW Cha (and in a lesser degree in Glass I), there is clear evidence of a large amount of processed dust in the form of crystalline silicates and large amorphous grains. This is the first time that processed dust has been detetected in very young T Tauri stars (~1 Myr).

Time variation of SWS spectra of M-type Mira variables

The M-type Mira variable star, Z Cyg, was observed with the Short-Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO) 7 times at roughly 60 day intervals over one and a half period. The infrared spectrum (2.38{45.2m) of Z Cyg shows prominent silicate emission bands at 10 ma nd 18m and displays quite large variations over the observed period. The variation in the infrared spectrum of Z Cyg is synchronized with the visual light curve. The circumstellar emission and the 10 mt o 18m silicate band ratio increases at maximum and decreases at minimum, indicating a variation in the dust temperature with phase. Apart from minor emission features which may be partly due to oxide dust, the observed spectra can be tted by optically thin dust shell models with one single silicate dust emissivity prole. Thus silicate is the dominant dust component in the circumstellar shell of Z Cyg. The variation in the integrated infrared flux and in the dust temperature derived from the observed spectra can be interpreted in terms of the variation in the luminosity of the central star if proper dust optical properties are adopted. Conversely, the dust emissivity can be estimated from the variations in the infrared spectrum. The derived optical properties are relatively insensitive to the assumptions made in the model analysis because of the optically thin nature of the dust shell. Possible evidence for dust formation near minimum is discussed. The observed variation of the dust shell spectrum of Z Cyg is tted most consistently with a model in which the inner dust shell temperature is 700 100 K at maximum. From the model ts we derive a ratio of the 18 ma nd 10m emission eciencies of Q(18 m)=Q(10 m) = 0:510:08. The Z Cyg dust also has a broad feature in the 20{25m region in addition to the 18m silicate band. The optical properties of dust grains around Z Cyg are compared with those in circumstellar shells of other oxygen-rich late-type stars and it is shown that there are variations in the 20m emissivity of circumstellar dust, possibly related to the presence of another dust component.

Temperature effects on the 15-85 m spectra of olivines and pyroxenes

Far-infrared spectra of laboratory silicates are normally obtained at room temperature even though the grains responsible for astronomical silicate emission bands seen at wavelengths >20 micron are likely to be at temperatures below ~150 K. In order to investigate the effect of temperature on silicate spectra, we have obtained absorption spectra of powdered forsterite and olivine, along with two orthoenstatites and diopside clinopyroxene, at 3.5+-0.5 K and at room temperature (295+-2K). To determine the changes in the spectra the resolution must be increased from 1 to 0.25 cm^-1 at both temperatures since a reduction in temperature reduces the phonon density, thereby reducing the width of the infrared peaks. Several bands observed at 295 K split at 3.5 K. At 3.5 K the widths of isolated single bands in olivine, enstatites and diopside are ~ 90% of their 295 K-widths. However, in forsterite the 3.5-K-widths of the 31-, 49- and 69-micron bands are, respectively, 90%, 45% and 31% of their 295 K widths. Due to an increase in phonon energy as the lattice contracts, 3.5-K-singlet peaks occur at shorter wavelengths than do the corresponding 295-K peaks; the magnitude of the wavelength shift increases from \~ 0-0.2 micron at 25 micron to ~0.9 micron at 80 micron. Changes in the relative absorbances of spectral peaks are also observed. The temperature dependence of lambda_pk and bandwidth shows promise as a means to deduce characteristic temperatures of mineralogically distinct grain populations. In addition, the observed changes in band strength with temperature will affect estimates of grain masses and relative mineral abundances inferred using room-temperature laboratory data.

Spectral Energy Distributions of Passive T Tauri and Herbig Ae Disks: Grain Mineralogy, Parameter Dependences, and Comparison withInfrared Space ObservatoryLWS Observations

We improve upon the radiative, hydrostatic equilibrium models of passive circumstellar disks constructed by Chiang & Goldreich. New features include (1) an account for a range of particle sizes, (2) employment of laboratory-based optical constants of representative grain materials, and (3) numerical solution of the equations of radiative and hydrostatic equilibrium within the original two-layer (disk surface plus disk interior) approximation. We systematically explore how the spectral energy distribution (SED) of a face-on disk depends on grain size distributions, disk geometries and surface densities, and stellar photospheric temperatures. Observed SEDs of three Herbig Ae and two T Tauri stars, including spectra from the Long Wavelength Spectrometer (LWS) aboard the Infrared Space Observatory (ISO), are fitted with our models. Silicate emission bands from optically thin, superheated disk surface layers appear in nearly all systems. Water ice emission bands appear in LWS spectra of two of the coolest stars. Infrared excesses in several sources are consistent with significant vertical settling of photospheric grains. While this work furnishes further evidence that passive reprocessing of starlight by flared disks adequately explains the origin of infrared-to-millimeter wavelength excesses of young stars, we emphasize by explicit calculations how the SED alone does not provide sufficient information to constrain particle sizes and disk masses uniquely.

Silicate and ice emission bands in the ISO spectrum of the PAH-emitting carbon-rich planetary nebula CPD-56°8032

Combined ISO SWS and LWS spectroscopy is presented of the late WC-type planetary nebula nucleus CPD-56†8032 and its carbon-rich nebula. The extremely broad coverage (2.4–197 μm) enables us to recognize the clear and simultaneous presence of emission features from both oxygen- and carbon-rich circumstellar materials. Removing a smooth continuum highlights bright emission bands characteristic of polycyclic aromatic hydrocarbons (hereafter PAHs) in the 3–15 μm region, bands from crystalline silicates longwards of 18 μm, and the 43- and 62-μm bands of crystalline water ice. We discuss the probable evolutionary state and history of this unusual object in terms of (a) a recent transition from an O-rich to a C-rich outflow following a helium shell flash; or (b) a carbon-rich nebular outflow encountering an O-rich comet cloud orbiting in a Kuiper-belt-like distribution.

IRTS observation of the mid-infrared spectrum of the zodiacal emission

We present the mid-infrared spectrum (3–12 μm) of the zodiacal emission obtained by the Infrared Telescope in Space (IRTS), the first Japanese cryogenically cooled orbital infrared telescope. The Near-Infrared Spectrometer (NIRS) on board IRTS provided the spectrum of 3–4 μm, while that of 4.5–11.7 μm has been observed by the Mid-Infrared Spectrometer (MIRS). In this paper we present the data reduction and results of the observations by MIRS. Spectra of the background emission at high galactic latitudes (¦b¦ > 30°) have been extracted from the MIRS observations by excluding point sources. The observed sky brightness has a clear dependence on the ecliptic latitude, indicating that the zodiacal emission dominates in the mid-infrared sky brightness. On the other hand, the spectral shape does not show any appreciable dependence on the ecliptic latitude for ß = 0°–75°. The spectrum combining the NIRS and MIRS observations can be fitted by a grey body radiation at 250 K, but excess emission is seen in the 3–6 μm range. Alternatively, the spectrum of the zodiacal emission can be reproduced fairly well by a grey body at 280 K with an excess around 10 μm. In this case the excess may be attributed to a silicate emission band. Other than these excesses, no spectral features above the 10% level are seen in the MIRS spectrum.

Circumstellar environments – II. The silicate emission band and cool stellar mass loss rates

Etude des raies de silicate a 9,7 μm dans la poussiee circumstellaire de geantes rouges et supergeantes riches en oxygene dont on a determine les taux de perte de masse