Circumstellar Dust Shells Research Articles

Modelling of oxygen-rich envelopes using corundum and silicate grains

A set of 31 oxygen-rich stars has been modelled using corundum and silicate grains. These stars were selected according to their dust-envelope class, as suggested by Little-Marenin and Little in 1990. Then 16 stars classified as Sil were modelled using silicate grains; 10 Broad class stars using corundum (Al2O3) grains; and 5 ‘Intermediate’ class stars using two kinds of grain simultaneously: corundum and silicate. The temperature of the central stars and some characteristics of their circumstellar envelopes such as their extinction opacities and extensions were determined by fitting the flux curves. The corundum/silicate ratios as well as the energy distributions and temperature laws have been obtained. Based on the authors’ results they suggest the existence of chemical and structural evolution of the modelled circumstellar dust shells. The temperature of the central stars and the temperature of the hottest grains decrease from Broad to Intermediate to Sil classes, while the inner radii and optical depths increase in this sequence.

A search of the IRAS database for evidence of Dyson Spheres

One of the forms of astroengineering activity that a very advanced civilization could possibly carry on is the constructions of huge “buildings” in space around the central star. Historically such constructions are called Dyson Spheres. We would like to introduce a new name — Astroengineering Constructions (AC) — to mean a more general type of construction not necessarily related to any specific star. AC absorb energy from different types of activity and re-emit it as infrared radiation, i.e. radiation lying in the submillimeter and millimeter range. Further, AC are expect to have spectra similar to the black-body spectra because they re-emit all the energy that they absorb, although in the infrared range, as already mentioned. Thus, the effective temperature of these Planckian distributions is expected to lie between 3–300 K with the spectrum peaking between 10 μm and 10 mm. We have analyzed the IRAS database and extracted a catalog of sources whose spectra are similar to the black-body emission. The catalog of these sources and their preliminary parameters are discussed. The distribution of the color temperatures of IRAS sources and the sky distribution of sources are also considered. The possibility of the distinction of AC from thick circumstellar dust shells around red giant stars is discussed.

Analysis of correlations between polarimetric and photometric characteristics of young stars

We present the results of the investigation of correlations between the polarimetric and photometric characteristics of a sample (496 objects) of young Herbig Ae/Be (HAEBE) stars and T Tauri (TT) stars. It is shown that, for 85% of the sample stars there is a gen- eral relation between the degree of optical polarization and the infrared colour index (V L)obs and the colour excess E(V L) due to the contribution of a circumstellar dust shell. Polarimetric data were also compared with the value of v sini to search for a possible correlation between the polarization and an inclination of circumstellar disks. Polarimetric data as well as IR excesses are considered and compared for dierent subgroups of young stars namely: HAEBE and TT stars with Algol{like minima of bright- ness (26 objects), Vega{type stars and post HAEBE stars (114 objects) and young solar{type stars (58 objects). For statistical purposes the data for young stars were com- pared with those collected for dierent groups of evolved objects such as: classical Be stars (300 objects), Mira Ceti stars (39 objects), early{type supergiants from the Serkowski et al. (1975) catalogue (120 objects) and main sequence (MS) stars within 50 pc from the Sun from the Leroy (1993) catalogue (68 objects). The value of polar- ization is discussed in context with the stages of evolution of circumstellar shells which were established by compari- son of spectral energy distribution in the far IR (using the IRAS data). It is shown that most young stars have sta- tistically larger value of polarization in comparison with the stars which are on a stage of evolution close to MS. We are able to contend that the changes in polarimetric behaviour of young stars are connected with evolution of their circumstellar shells.

Near‐ and Mid‐Infrared Subarcsecond Structure of the Dusty Symbiotic Star R Aquarii

The results of a high-resolution interferometric campaign targeting the symbiotic long-period variable (LPV) R Aqr are reported. With both near-infrared measurements on baselines out to 10 m and mid-infrared data extending to 32 m, we have been able to measure the characteristic sizes of regions from the photosphere of the LPV and its extended molecular atmosphere out to the cooler circumstellar dust shell. The near-infrared data were taken using aperture-masking interferometry on the Keck I telescope and show R Aqr to be partially resolved for wavelengths out to 2.2 μm but with a marked enlargement, possibly due to molecular opacity, at 3.1 μm. Mid-infrared interferometric measurements were obtained with the Univeristy of California, Berkeley, infrared spatial interferometer (ISI) operating at 11.15 μm from 1992 to 1999. Although this data set is somewhat heterogeneous with incomplete coverage of the Fourier plane and sampling of the pulsation cycle, clear changes in the mid-infrared brightness distribution were observed, both as a function of position angle on the sky and as a function of pulsation phase. Spherically symmetric radiative transfer calculations of uniform-outflow dust shell models produce brightness distributions and spectra which partially explain the data; however, limitations to this approximation are noted. Evidence for significant deviation from circular symmetry was found in the mid-infrared and more tentatively at 3.08 μm in the near-infrared; however, no clear detection of binarity or of non-LPV elements in the symbiotic system is reported.

Circumstellar dust shells of carbon stars resolved with ISO

We present far-infrared images of very extended dust shells of carbon stars observed with ISOPHOT on board the Infrared Space Observatory. A prototypical J-type carbon star Y CVn is surrounded by a very extended, detached dust shell, which shows that once there was a period with an intense mass-loss on the order of 10 −5 M Θ yr −1 followed by a swift switch to a phase with a mass-loss rate lower by two orders of magnitude about (1–2)×10 4 years ago. An N-type carbon star U Ant shows a double shell structure, a compact but resolved dust shell surrounded by a very extended one. The observed structure as well as the previously known thin CO gas shell indicates that this star experienced two cycles of “a high mass-loss phase of the order of 10 −5 M Θ yr −1 followed by a phase with a fainter mass-loss by two orders of magnitude” in the last (1–2)×10 4 years.

The Unusual Infrared Object HDF‐N J123656.3+621322

We describe an object in the Hubble Deep Field North with very unusual near-infrared properties. It is readily visible in Hubble Space Telescope NICMOS images at 1.6 ?m and from the ground at 2.2 ?m, but it is undetected (with S/N 2) in very deep WFPC2 and NICMOS data from 0.3 to 1.1 ?m. The f? flux density drops by a factor 8.3 (97.7% confidence) from 1.6 to 1.1 ?m. The object is compact but may be slightly resolved in the NICMOS 1.6 ?m image. In a low-resolution, near-infrared spectrogram, we find a possible emission line at 1.643 ?m, but a reobservation at higher spectral resolution failed to confirm the line, leaving its reality in doubt. We consider various hypotheses for the nature of this object. Its colors are unlike those of known Galactic stars, except perhaps the most extreme carbon stars or Mira variables with thick circumstellar dust shells. It does not appear to be possible to explain its spectral energy distribution as that of a normal galaxy at any redshift without additional opacity from either dust or intergalactic neutral hydrogen. The colors can be matched by those of a dusty galaxy at z 2, by a maximally old elliptical galaxy at z 3 (perhaps with some additional reddening), or by an object at z 10 whose optical and 1.1 ?m light have been suppressed by the intergalactic medium. Under the latter hypothesis, if the luminosity results from stars and not an AGN, the object would resemble a classical, unobscured protogalaxy, with a star formation rate 100 M? yr-1. Such UV-bright objects are evidently rare at 2 < z < 12.5, however, with a space density several hundred times lower than that of present-day L* galaxies.

Optical Appearance of Dynamical Models for Circumstellar Dust Shells around Long-Period Variables: AFGL 3068

Optical Appearance of Dynamical Models for Circumstellar Dust Shells around Long-Period Variables: AFGL 3068

Dynamical Models of Circumstellar Dust Shells around Long-Period Variables

We present dynamical models of circumstellar dust shells around long-period variables which include time-dependent hydrodynamics and a detailed treatment of dust formation, growth and evaporation. Important effects due to the complex interaction between the dynamics of the pulsating atmosphere and the dust complex are demonstrated.

Condensation of SiC in Circumstellar Dust Shells of C-Rich Red Giants

Condensation of SiC in Circumstellar Dust Shells of C-Rich Red Giants

HR 8799: A Link between γ Doradus Variables and λ Bootis Stars

On the basis of a new classification-resolution spectrum, we find that HR 8799, a known member of the newly discovered γ Doradus variable star class, is a λ Bootis star. Spectral synthesis, in conjunction with fluxes from visible spectrophotometry and the TD-1 satellite, yields Teff = 7430 K, log g = 4.35, microturbulent velocity ξt = 2.7 km s-1, and metallicity [M/H] = -0.47 for HR 8799, confirming its metal-weak nature. HR 8799 is also a Vega-like star in that it shows excess flux at 60 μm, probably due to a circumstellar dust shell or disk. Thus, this star links three astrophysically interesting classes of stars and may provide potentially important constraints on the physics of, and the interconnections between, the λ Bootis phenomenon and the γ Doradus pulsation phenomenon.

The science potential of ALFA: Adaptive optics with natural and laser guide stars

Adaptive optics with laser guide stars is manda- tory in order to make use of the full capabilities of 8-m class telescopes. However, progress has been slow in two particular areas: techniques for spectroscopy at diraction limited resolution and wavefront sensing on laser guide stars. ALFA { currently the only European laser guide star adaptive optics system, and the only one in the world open to guest observers { has made signicant advances in both of these. In this paper we report on our rst results from summer 1998, representing signicant improvements over previous performance. We report on observations us- ing natural guide stars which demonstrate that for bright stars (mV < 8) ALFA can now reach K-band Strehl ra- tios in excess of 60% and easily resolve binaries at the diraction limit of the telescope. We then present some of the rst integral eld spectroscopy at diraction lim- ited scales, showing we are able to distinguish spectra of binary stars with a separation of only 0.26 00 . We also dis- cuss results from a wide eld image, which indicate that useful correction (allowing binary stars and circumstellar dust shells to be resolved) can be achieved from a rela- tively faint star to a radius of at least 1 0 . Our last set of results include a correction on a galaxy using the laser guide star as the reference. The best result to date is of the galaxy UGC 1347 in Abell 262. Correcting tip-tilt on a star 41 00 away and higher orders on the laser, we achieved an increase in peak intensity of 2.5, and a reduction in FWHM from 1.07 00 to 0.40 00 .

A Mid‐infrared Imaging Survey of Proto–Planetary Nebula Candidates

We present the data from a mid-infrared (MIR) imaging survey of 66 proto-planetary nebula candidates using two MIR cameras (MIRAC2 and Berkcam) at the NASA Infrared Telescope Facility and the United Kingdom Infrared Telescope. The goal of this survey is to determine the size, flux, and morphology of the MIR emission regions, which sample the inner regions of the circumstellar dust shells of proto-planetary nebulae. We imaged these proto-planetary nebulae with narrowband filters(Δλ/λ~10%) at wavelengths of notable dust features. With our typical angular resolution of 1'', we resolve 17 sources, find 48 objects unresolved, and do not detect one source. For several sources we checked optical and infrared associations and positions of the sources. In table format, we list the size and flux measurements for all of the detected objects and show figures of all of the resolved sources. The proto-planetary nebula candidate sample includes, in addition to the predominant proto-planetary nebulae, extreme asymptotic giant branch stars, young planetary nebulae, a supergiant, and a luminous blue variable. We find that dust shells that are cooler (T~150 K) and brighter in the infrared are more easily resolved. Eleven of the seventeen resolved sources are extended and fall into one of two types of MIR morphological classes: core/elliptical or toroidal. Core/elliptical structures show unresolved cores with lower surface brightness elliptical nebulae. Toroidal structures show limb-brightened peaks suggesting equatorial density enhancements. We argue that core/ellipticals have denser dust shells than toroidals.

Mid-Infrared Visibility Measurements of Evolved Stars

We have used Denver University's Ten-and-Twenty Micron Camera at the Wyoming Infrared Observatory to image 12 evolved stars at 11.5 μm. We have obtained both visibility data and photometric magnitudes from these images. We have partially resolved the circumstellar dust shell for eight of these 12 stars: α Ori, VY CMa, IRC +10216, χ Cyg, IK Tau, NML Cyg, μ Cep, and IRC +10420. We compare our data to published data and to visibility curves predicted by radiative transfer models in the literature.

The Last Gasps of VY Canis Majoris: Aperture Synthesis and Adaptive Optics Imagery

We present new observations of the red supergiant VY CMa at 1.25, 1.65, 2.26, 3.08, and 4.8 km. Two complementary observational techniques were utilized: nonredundant aperture masking on the 10 m Keck I telescope, yielding images of the innermost regions at unprecedented resolution, and adaptive optics imaging on the ESO 3.6 m telescope at La Silla, attaining an extremely high (D105) peak-to-noise dynamic range over a wide —eld. For the —rst time the inner dust shell has been resolved in the near- infrared to reveal a one-sided extension of circumstellar emission within (D15 of the star. The 0A.1 R * ) line-of-sight optical depths of the circumstellar dust shell at 1.65, 2.26, and 3.08 km have been estimated to be 1.86 ^ 0.42, 0.85 ^ 0.20, and 0.44 ^ 0.11, respectively. These new results allow the bolometric lumi- nosity of VY CMa to be estimated independent of the dust shell geometry, yielding A L * B 2 ) 105 L _ . variety of dust condensations, including a large scattering plume and a bow-shaped dust feature, were observed in the faint, extended nebula up to 4A from the central source. While the origin of the nebulous plume remains uncertain, a geometrical model is developed assuming the plume is produced by radially driven dust grains forming at a rotating —ow insertion point with a rotational period between 1200 and 4200 yr, which is perhaps the stellar rotational period or the orbital period of an unseen companion. Subject headings: circumstellar matterdust, extinctionstars: individual (VY Canis Majoris) ¨ infrared: starstechniques: interferometric

Infrared interferometry of circumstellar envelopes

This paper will review the technical progress of interferometric infrared observing techniques from the first 2-element interferometer 25 years ago to the 3+ element arrays now coming into service. To date, only the Infrared Spatial Interferometer (ISI) has published separate-element interferometric data on circumstellar dust shells in the infrared and many of these scientific results will be discussed. Speckle interferometry has also evolved significantly over the last few decades as slit-scanning techniques over single-pixel detectors have largely been replaced by fast-readout of large format detector arrays. Important near-infrared and mid-infrared results derived from speckle data will also be reviewed.Until recently, two-dimensional information about circumstellar dust distributions has been sorely lacking, hence detections of dust shell asymmetries have been difficult and uncertain. New speckle observations using modern, 10-m class telescopes have yielded surprising results, demonstrating the importance of accurate closure phase information in interpreting interferometric data. These discoveries hopefully precursor those to be made from closure-phase imaging with the new generation of separate-element, interferometric arrays.

Dust formation in oxygen-rich circumstellar shells around long–period variables

We present the first steps of our work aimed at a consistent time-dependent modeling of oxygen–rich circumstellar dust shells (CDS) around pulsating AGB stars. The nature of the most likely nucleation seeds is investigated for this situation and we find that TiO2 is a most promising candidate to serve as the primary condensate, forming already at temperatures well above 1 000 K. These nuclei evolve to macroscopic dust grains by heterogeneous growth processes involving several chemical species. We investigate the varying chemical composition of the resulting dust grains as they evolve as a function of time in a fluid element moving through the CDS of an oxygen–rich long-period variable star (LPV).

Imaging of Stellar Disks and Mass Loss Envelopes in Evolved Stars

Interferometric imaging of evolved stars has been performed in the near-IR at the Keck Observatory revealing diffraction-limited structure at unprecedented spatial resolution. By studying the hottest regions of the circumstellar dust shells of a number of bright prototypical objects, we have revealed complex and anisotropic structures in the inner regions where dust nucleation occurs. We find that dramatic dust plumes and clumps dominate the morphology of the circumstellar environment at a scale of a few stellar radii for extreme carbon stars such as IRC+10216 and CIT 6. Furthermore our observations have yielded near-IR diameters and images of the stellar photospheres of nearby red giant and supergiant stars. With the additional benefit of multi-epoch observations, we have begun to investigate the time-evolution of our enshrouded objects as dust is created and accelerated away from the nucleation zone. Armed with such a powerful new set of observational constraints, we are able to address contemporary problems such as the origin of structure in PNe, the physics of mass loss, and the dynamical properties of Mira pulsation.

Extended Dust Shells Surrounding AGB Stars Revealed with ISO

We obtained high resolution far-infrared images of the circumstellar dust shells of several AGB carbon stars using ISOPHOT on board the ISO. We used the C-100 and C-200 detector arrays in PHT32 oversampling mode at 90 μm and 160 μm except for U Ant, which was observed at 60 μm and 90 μm. AFGL 3068 is very compact in both bands. R Scl is marginally resolved at 90 μm. U Ant shows a double shell structure, a compact but well resolved shell surrounded by a very extended envelope. Y CVn exhibits a very extended, hollow dust shell. We analyse the structure of the dust shells using a simple dust shell model and discuss the mass-loss behaviors, particularly in conjunction with the thermal pulses on the AGB.

Dynamical Modeling of Circumstellar Outflows

We review dynamical models of circumstellar dust shells around long-period variables which include time-dependent hydrodynamics and a detailed treatment of dust formation, growth and evaporation. Important effects caused by the complex interaction between the dynamics of the pulsating atmosphere and the dust complex which only can be revealed in the dynamical approach are summarized. Special emphasis is given to the treatment of the dust and gas opacity.

Midinfrared Imaging of α Orionis

We have imaged the M2Iab supergiant star α Ori at 11.7 and 17.9 μm using the Hale 5 m telescope and an array camera with subarcsecond angular resolution. Our images reveal the circumstellar dust shell extending out to ~5'' from the star. By fitting two multiparameter models to the data, we find values for the dust shell inner radius, the temperature at this radius, the percentage flux produced by the circumstellar material at each of our wavelengths, and the radial dependence of the dust mass density. We present profiles of the mean surface brightness as a function of radius and the power detected from within annular rings as a function of radius.