Circumstellar Dust Grains Research Articles

Infrared spectroscopy of IRC + 10420

High resolution 1.6, 3.3 and 4.6 micron spectra were obtained on the F8 supergiant star IRC + 10420 on June 9-11, 1984, using the 4 m Mayall telescope and Fourier Transform Spectrometer at Kitt Peak. IRC + 10420 is distinguished by its hydrogen maser, unusual for stars of this type. The measured column densities of CO led to an estimated isotopic carbon ratio of 7-11, supporting a model of the object as a highly evolved star. The redshift is interpreted as due to scattering by outward-moving circumstellar dust grains. The IR spectrum of IRC + 10420 suggests that the object is a post-asymptotic branch star still surrounded by the thick dust shell it produced while it was much cooler.

Random fluctuations versus Poynting–Robertson drag on interplanetary dust grains

A particle moving through a sea of stochastic impulses gains energy from the sea; this is a long-established property of brownian motion, given theoretical backing in the fluctuation–dissipation theorem1. However, this energy gain is generally neglected in describing circumstellar dust grains as slowly spiralling into the central star under Poynting–Robertson (P–R) drag. Stochastic forces on circumsolar grains arise from low-frequency fluctuations in surface charge and interplanetary fields, as has long been recognized2,3, but previous studies have treated the mean orbital energy as constant4,5 or slowly decreasing6. I show here that stochastic heating can balance and indeed overwhelm the dissipative P–R drag for small grains (≲1 µm scale radius). Furthermore, the consequent drift to the outer Solar System may cancel the strong collisional source inferred7 for micrometre and sub-micrometre fragments, explaining how their present distribution is stable in time. More generally, the stochastic heating mechanism combined with collisional fragmentation sets limits on dust accretion onto cool stars.

On the far-infrared excess of Vega

The Infrared Astronomy Satellite (IRAS) has recently detected a compact far-infrared emission source associated with Vega (α Lyrae)1. This flux appears to be thermal emission from dust heated by the visual/UV luminosity of Vega. Although circum-stellar dust is commonly associated with young, early-type stars2, the characteristics of the emission from Vega are quite atypical. The essential difference is the relatively low temperature of the dust for the observed source size. This low temperature implies that the dust grains producing the far-infrared emission must be very efficient 60/100-µm emitters and must, therefore, be much larger than typical interstellar and circumstellar dust grains. To place more limits on the properties of this far-infrared emission region, we observed Vega at 47 and 95 µm with the Kuiper Airborne Observatory (KAO) on the night of 12–13 August 1983. We report here confirmation of the IRAS results and that the source size may be as large as 46 arc s along some axis.

Observationally-based infrared efficiencies and planck means for circumstellar dust grains

In this paper the mean relative efficiency factors for absorption of dust grains in the wavelength range from 8 to 22 μm have been derived from IR spectra of the optically thin dust shells surrounding the O-rich supergiants μ Cep, αOri, X Her, and R Cas. The resulting absorption efficiency curve has been extrapolated into the near as well as the far infrared regions, having regard to restrictions set by experience. Planck mean efficiency factors have been calculated for the temperature range from 30 to 1500 K. The curveQ P(T), which shows a maximum atT=300 K, has been approximated conveniently by a power law on either side of this maximum.

Linear polarization of T Tauri stars

Linear polarization observations over the wavelength range 0.44−2.2 µm are presented for six T Tauri type stars. All of the stars studied show substantial polarization in the infrared, while T Tau itself (and probably SU Aur) show a 90° change in position angle in the infrared. The data are discussed in terms of scattering from circumstellar dust grains with a component of large (∼ 1 µm) grains being required to fit the observed spectral dependence.

Winds in late-type stars: Mechanisms of mass outflow

AbstractFour basic mechanisms have been proposed to explain the acceleration of winds in late-type stars –– thermal pressure gradients, radiation pressure on circumstellar dust grains, momentum addition by Alfvén waves, and momentum addition by periodic shock waves. In this review I describe recent work in applying these mechanisms to stars, and consider whether these mechanisms can work even in principle and whether they are consistent with recent ultraviolet and X-ray data from the IUE and Einstein spacecraft. Thermally-driven winds are likely important for late-type dwarfs, where the mass loss rates are small, and perhaps also in G giants and supergiants, but they cannot operate alone in the K and M giants and supergiants. Radiatively-driven winds are probably unimportant for all cool stars, even the M supergiants with dusty circumstellar envelopes. In principle, Alfvén waves can accelerate winds to high speeds provided the field lines are initially open or forced open by some mechanism, but detailed calculations are needed. Magnetic reconnection is an interesting suggestion for an acceleration mechanism when the field lines are initially closed. For the Mras and semiregular variable supergiants, periodic shock waves provide a simple way of producing rapid mass loss. Thus we are making some progress in understanding mass loss mechanisms for the cool half of the H-R diagram.

16-39 micron spectroscopy of oxygen-rich stars

view Abstract Citations (87) References (36) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS 16 - 39 micron spectroscopy of oxygen-rich stars. Forrest, W. J. ; McCarthy, J. F. ; Houck, J. R. Abstract Airborne observations of the 16-39-micron spectra of 10 oxygen-rich stars with excess emission in the infrared have been obtained. The stars show excess emission attributed to circumstellar dust grains in the 16-39-micron region in the form of a broad hump peaking near 18 microns and falling smoothly to longer wavelengths. The emission is similar in character to the emission from the Trapezium region of the Orion Nebula, indicating the grain materials are quite similar in these objects. The existence of a feature in the 20-micron region is consistent with the O-Si-O bending resonance expected for silicate material. The lack of any sharp structure in the spectra indicates the silicate is in an amorphous, disordered form. A simple model of small grains of carbonaceous chondrite silicate material in a diffuse circumstellar envelope is shown to give a good qualitative fit to the observed 8-39-micron circumstellar spectra. Comparison of the observed spectra with the model spectra indicates the grain emissivity falls as the inverse square of wavelength from 20 to 40 microns. Publication: The Astrophysical Journal Pub Date: October 1979 DOI: 10.1086/157422 Bibcode: 1979ApJ...233..611F Keywords: Astronomical Spectroscopy; Infrared Astronomy; Infrared Spectra; Oxygen; Stellar Spectra; Supergiant Stars; Abundance; Carbonaceous Chondrites; Cosmic Dust; Late Stars; Silicates; Stellar Envelopes; Astrophysics; Circumstellar Envelopes:Infrared Spectra; Circumstellar Envelopes:Models; Infrared Radiation:Late-Type Stars full text sources ADS | data products SIMBAD (9)

Laboratory analogues to cosmic dust

A pulsed laser has been used to vaporize olivine, pyroxene, nickel-iron alloy, Al2O3, carbon, calcium carbonate, and silicon carbide, as well as mixtures of immiscible phases (Au−Al2O3 and Au-olivine) in oxidizing, reducing, and inert atmospheres. The collected condensates usually consist of strings of grains which have a median diameter of 20–30 nm, which is comparable to the calculated sizes of some interstellar and circumstellar dust grains. The silicate minerals vaporized in O2 as well as calcium carbonate and carbon vaporized in Ar or H2, are collected as glassy grains while the other materials produced crystalline grains. The systems of immiscible phases when vaporized produced condensates consisting of intermixed 2–50 nm grains of both components. The type of size distribution, crystal structures, and qualitiative elemental analyses of the condensates are given. Possible similarities between the mechanism of grain growth, structure, morphology, and chemistry of laboratory grains compared to interstellar and circumstellar grains, phases in meteorites and extraterrestrial dust collected in the stratosphere are examined. Applications of the experimental technique include the production of grain systems to serve as laboratory analogues for spectral studies of grain materials believed to exist in astronomical environments, and studies of the structure of grains condensed from complex gas mixtures.

Possible identification of a circumstellar 33-micron silicate emission band in cool-star spectra

Infrared photometry of S Per, $mu$ Cep, and HD 11979 reveals that these stars have substantially larger flux excesses at 33 $mu$ than do $alpha$ Ori and o Cet. These prominent excesses may be attributable to a distinct lattice band in circumstellar dust grains composed of silicates such as $gamma$-Ca$sub 2$SiO$sub 4$, Fe$sub 2$SiO$sub 4$, or Mn$sub 2$SiO$sub 4$. The observations show that the abundance of this silicate material varies dramatically from one star to another among the class of M stars. (AIP)

Dust, Star, and Hydrogen Distributions in the Orion Association

From the star counts on the Palomar Sky Survey prints, dust and star distributions in the Orion Association are studied. The Hydrogen distribution in the same region is obtained from an Hα photograph. It is found that a considerable part of the interstellar absorption on the line of sight is due to the circumstellar dust grains in the Orion Association.