Dust Shell Research Articles

Asymmetries and Outflows in the Circumstellar Dust of Mira A

Asymmetries and motions in the dust shell surrounding Mira A (o Ceti) are reported. Measurements were taken with the UC Berkeley Infrared Spatial Interferometer (ISI), a three-element interferometer operating at 11.15 μm. At the time of these observations, it was in a linear, east-west configuration with a maximum baseline of 12 m and thus had a resolution of better than 100 mas. Three years of data (2003-2005) are presented and permit observation of the movement of dust shells over time. Fits are made to the visibility and closure-phase curves, which are then used to create one-dimensional profiles of the dust shells. Asymmetries in the circumstellar dust have been observed, and several possible explanations for these asymmetries are explored.

Compact mid-IR sources east of Galactic Center source IRS5

Aims. Mid-infrared observations of the Galactic Center show among the extended mini-spiral a number of compact sources. Their nature is of interest because they represent an interaction of luminous stars with the mini-spiral material or mass losing sources that are enshrouded in dust and gas shells. Characterizing their nature is necessary to obtain a complete picture of the different stellar populations and the star formation history of the cental stellar cluster in general. Prominent compact MIR sources in the Galactic Center are either clearly offset from the mini-spiral (e.g. the M2 super-giant IRS 7 and the bright dust enshrouded IRS 3) or have been identified earlier with bright bow shock sources (e.g. IRS 21, 1W, 10W and IRS 5). There are, however, four less prominent compact sources east of IRS 5, the natures of which were unclear until now. Methods. We present near-infrared K-band long slit spectroscopy of the four sources east of IRS 5 obtained with the ISAAC spectrograph at the ESO VLT in July 2005. We interpret the data in combination with high angular resolution NIR and MIR images obtained with ISAAC and NACO at the ESO VLT. Results. The K � -band images and proper motions show that the sources are multiple. For all but one source we find dominant contributions from late type stars with best overall fits to template stars with temperatures below 5000 K. Conclusions. The brightest sources contained in IRS 5NE, 5E and 5S may be asymptotic giant branch stars and a part of the MIR excess may be due to dust shells produced by the individual sources. However, in all cases an interaction with the mini-spiral cannot be excluded and their broad band infrared SEDs indicate that they could be lower luminosity counterparts of the identified bow shock sources. In fact, IRS 5SE is associated with a faint bow shock and its spectrum shows contributions from a hotter early type star which supports such a classification.

Gravastar energy conditions revisited

We consider the gravastar model where the vacuum phase transition between the de Sitter interior and the Schwarzschild or Schwarzschild–de Sitter exterior geometries takes place at a single spherical δ-shell. We derive sharp analytic bounds on the surface compactness (2m/r) that follow from the requirement that the dominant energy condition (DEC) holds at the shell. In the case of Schwarzschild exterior, the highest surface compactness is achieved with the stiff shell in the limit of vanishing (dark) energy density in the interior. In the case of Schwarzschild–de Sitter exterior, in addition to the gravastar configurations with the shell under surface pressure, gravastar configurations with vanishing shell pressure (dust shells), as well as configurations with the shell under surface tension, are allowed by the DEC. Respective bounds on the surface compactness are derived for all cases. We also consider the speed of sound on the shell as derived from the requirement that the shell is stable against the radial perturbations. The causality requirement (sound speed not exceeding that of light) further restricts the space of allowed gravastar configurations.

A massive, dusty toroid with large grains in the pre-planetary nebula IRAS22036+5306

Using the Submillimeter Array (SMA), we have obtained high angular-resolution (∼1″) interferometric maps of the submillimeter (0.88 mm) continuum and CO J=3–2 line from IRAS 22036+5306 (I 22036), a bipolar pre-planetary nebula (PPN) with knotty jets discovered in our HST SNAPshot survey of young PPNe. In addition, we have obtained supporting lower-resolution (∼10″) 2.6 mm continuum and CO, 13CO J=1–0 observations with the Owens Valley Radio Observatory (OVRO) interferometer. We find an unresolved source of submillimeter (and millimeter-wave) continuum emission in I 22036, implying a very substantial mass (0.02–0.04M ⊙) of large (i.e., radius ≳1 mm), cold (≲50 K) dust grains associated with I 22036’s toroidal waist. The CO J=3–2 observations show the presence of a very fast (∼220 km s−1), highly collimated, massive (0.03M ⊙) bipolar outflow with a very large scalar momentum (about 1039 g cm s−1), and the characteristic spatio-kinematic structure of bow-shocks at the tips of this outflow. The fast outflow in I 22036, as in most PPNe, cannot be driven by radiation pressure. The large mass of the torus suggests that it has most likely resulted from common-envelope evolution in a binary, however it remains to be seen whether or not the time-scales required for the growth of grains to millimeter sizes in the torus are commensurate with such a formation scenario. The presence of the torus should facilitate the formation of the accretion disk needed to launch the jet. We also find that the 13C/12C ratio in I 22036 is very high (0.16), close to the maximum value achieved in equilibrium CNO-nucleosynthesis (0.33). The combination of the high circumstellar mass (i.e., in the torus and an extended dust shell inferred from ISO far-infrared spectra) and the high 13C/12C ratio in I 22036 provides strong support for this object having evolved from a massive (≳4M ⊙) progenitor in which hot-bottom-burning has occurred.

The formation of a detached shell around the carbon star Y CVn

Y CVn is a carbon star surrounded by a detached dust shell that has been imaged by the Infrared Space Observatory at 90 microns. With the Nancay Radio Telescope we have studied the gaseous counterpart in the 21-cm HI emission line. New data have been acquired and allow to improve the signal to noise ratio on this line. The high spectral resolution line profiles obtained at the position of the star and at several offset positions set strong constraints on the gas temperature and kinematics within the detached shell; the bulk of the material should be at ~ 100-200 K and in expansion at ~ 1-2 km/s. In addition, the line profile at the central position shows a quasi-rectangular pedestal that traces an 8 km/s outflow of ~ 1.0 10^-7 Msol/yr, stable for about 2 10^4 years, which corresponds to the central outflow already studied with CO rotational lines. We present a model in which the detached shell results from the slowing-down of the stellar wind by surrounding matter. The inner radius corresponds to the location where the stellar outflow is abruptly slowed down from ~ 8 km/s to 2 km/s (termination shock). The outer radius corresponds to the location where external matter is compressed by the expanding shell (bow shock). In this model the mass loss rate of Y CVn has been set constant, at the same level of 1.0 10^-7 Msol/yr, for ~ 4.5 10^5 years. The gas temperature varies from ~ 1800 K at the inner limit to 165 K at the interface between circumstellar matter and external matter. Our modelling shows that the presence of a detached shell around an AGB star may not mean that a drastic reduction of the mass loss rate has occurred in the past. The inner radius of such a shell might only be the effect of a termination shock rather than of an interruption of the mass loss process.

The molecular and dusty composition of Betelgeuse's inner circumstellar environment

Context. The study of the atmosphere of red supergiant stars in general and of Betelgeuse (α Orionis) in particular is of prime importance to understand dust formation and how mass is lost to the interstellar medium in evolved massive stars. Aims. A molecular shell, the MOLsphere (Tsuji, 2000a), in the atmosphere of Betelgeuse has been proposed to account for the nearand mid-infrared spectroscopic observations of Betelgeuse. The goal is to further test this hypothesis and to identify some of the molecules in this MOLsphere. Methods. We report on measurements taken with the mid-infrared two-telescope beam combiner of the VLTI, MIDI, operated between 7.5 and 13.5� m. The data are compared to a simple geometric model of a photosphere surrounded by a warm absorbing and emitting shell. Physical characteristics of the shell are derived: s ize, temperature and optical depth. The chemical constituents are determined with an analysis consistent with available infrared spectr a and interferometric data. Results. The MIDI data are well modeled with a geometrically thin shell whose radius varies from 1.31 to 1.43 R⋆ across the N band with a typical temperature of 1550 K. We are able to account for the measured optical depth of the shell in the N band, the ISO-SWS spectrum and K and L band interferometric data with a shell whose inner and outer radii are given by the above range and with the following species and densities: H2O (7.1± 4.7× 10 19 cm −2 ), SiO (4.0± 1.1× 10 20 cm −2 ), Al2O3 (2.4± 0.5× 10 15 cm −2 ). Conclusions. These results confirm the MOLsphere model. We bring evidence for more constituents and for the presence of species participating in the formation of dust grains in the atmosphere of the star, i.e. well below the distance at which the dust shell is detected. We believe these results bring key elements to the understanding of mass loss in Betelgeuse and red supergiants in general and bring support to the dust-driven scenario.

ASpitzerStudy of the Mass-Loss Histories of Three Bipolar Preplanetary Nebulae

We present the results of far-infrared imaging of extended regions around three bipolar preplanetary nebulae, AFGL 2688, OH 231.8+4.2, and IRAS 16342-3814, at 70 and 160 μm with the MIPS instrument on the Spitzer Space Telescope. After a careful subtraction of the point-spread function of the central star from these images, we place constraints on the existence of extended shells and thus on the mass outflow rates as a function of radial distance from these stars. We find no apparent extended emission in AFGL 2688 and OH 231.8+4.2 beyond 100'' from the central source. In the case of AFGL 2688, this result is inconsistent with a previous report of two extended dust shells made on the basis of ISO observations. We derive upper limits of 2.1 × 10-7 and 1.0 × 10-7 M⊙ yr-1 for the dust mass-loss rates of AFGL 2688 and OH 231.8, respectively, at 200'' from each source. In contrast to these two sources, IRAS 16342-3814 does show extended emission at both wavelengths, which can be interpreted as a very large dust shell with a radius of ~400'' and a thickness of ~100'', corresponding to 4 and 1 pc, respectively, at a distance of 2 kpc. However, this enhanced emission may also be Galactic cirrus; better azimuthal coverage is necessary for confirmation of a shell. If the extended emission is a shell, it can be modeled, with some assumptions about its dust properties, as enhanced mass outflow at a dust mass outflow rate of 1.5 × 10-6 M⊙ yr-1 superimposed on a steady outflow with a dust mass outflow rate of 1.5 × 10-7 M⊙ yr-1. Because of the size of the possible shell, it is likely that this shell has swept up a substantial mass of interstellar gas during its expansion, so these estimates are upper limits to the stellar mass-loss rate. We find a constant color temperature of 32 K throughout the circumstellar envelope of IRAS 16342-3814, which is consistent with heating by the interstellar radiation field.

Unusual infrared fading of CH Cygni in 2006

Infrared observations of the unique symbiotic system CH Cyg in 2003–2006 are presented. Analysis of the observations has shown that a fairly dense dust structure (a cloud or a shell) appeared on the line of sight in August–November 2006. The dust grains in the new shell are similar in optical properties to graphite ones and their sizes are mostly within the range 0.14–0.16 µm. The dust shell is optically thick and its optical depth at 2.2 µm is τ(2.2) ≈ 0.97. The dust shell mass is M d(06) ≈ 8 × 10−6 M ⊙ and the rate of matter flow into the shell has reached ∼2 × 10−5 M ⊙ yr−1.

The M33 Variable Star Population Revealed bySpitzer

We analyze five epochs of Spitzer Space Telescope/Infrared Array Camera (IRAC) observations of the nearby spiral galaxy M33. Each epoch covered nearly a square degree at 3.6, 4.5, and 8.0 microns. The point source catalog from the full dataset contains 37,650 stars. The stars have luminosities characteristic of the asymptotic giant branch and can be separated into oxygen-rich and carbon-rich populations by their [3.6] - [4.5] colors. The [3.6] - [8.0] colors indicate that over 80% of the stars detected at 8.0 microns have dust shells. Photometric comparison of epochs using conservative criteria yields a catalog of 2,923 variable stars. These variables are most likely long-period variables amidst an evolved stellar population. At least one-third of the identified carbon stars are variable.

Quantum collapse of dust shells in 2 + 1 gravity

This paper considers the quantum collapse of infinitesimally thin dust shells in 2+1 gravity. In 2+1 gravity a shell is no longer a sphere but a ring of matter. The classical equation of motion has been considered by Peleg and Steif and Cristosomo and Olea. The minisuperspace quantum problem can be reduced to that of a harmonic oscillator in terms of the curvature radius of the shell, allowing the use of well-known methods to find the motion of coherent wave packets that give the quantum collapse of the shell. Classically, as the radius of the shell falls below a certain point, a forms. In the quantum problem one can define various quantities that give indications of formation. Without proper definitions of a horizon in quantum gravity, these can be nothing but indications.

Multispectral Observations of Lunar Occultations. I. Resolving the Dust Shell around AFGL 5440

We present observations and modeling of a lunar occultation of the dust-enshrouded carbon star AFGL 5440. The observations were made over a continuous range of wavelengths from 1 to 4 μm with a high-speed spectrophotometer designed expressly for this purpose. We find that the occultation fringes cannot be fit by any single-size model. We use the DUSTY radiative transfer code to model a circumstellar shell and fit both the observed occultation light curves and the spectral energy distribution described in the literature. We find a strong constraint on the inner radius of the dust shell, Tmax = 950 ± 50 K, and optical depth at 5 μm of 0.5 ± 0.1. The observations are best fit by models with a density gradient of ρ ∝ r-2 or the gradient derived by Ivezic & Elitzur for a radiatively driven hydrodynamic outflow. Our models cannot fit the observed IRAS 60 μm flux without assuming a substantial abundance of graphite or by assuming a substantially higher mass-loss rate in the past.

The Mira variable S Orionis: relationships between the photosphere, molecular layer, dust shell, and SiO maser shell at 4 epochs

Aims. We present the first multi-epoch study that includes concurrent mid-infrared and radio interferometry of an oxygen-rich Mira star. Methods. We obtained mid-infrared interferometry of S Ori with VLTI/MIDI at four epochs in December 2004, February/March 2005, November 2005, and December 2005. We concurrently observed v = 1, J = 1-0 (43.1 GHz), and v = 2, J = 1-0 (42.8 GHz) SiO mascr emission toward S Ori with the VLBA in January, February, and November 2005. The MIDI data are analyzed using self-excited dynamic model atmospheres including molecular layers, complemented by a radiative transfer model of the circumstellar dust shell. The VLBA data are reduced to the spatial structure and kinematics of the maser spots. Results. The modeling of our MIDI data results in phase-dependent continuum photospheric angular diameters of 9.0±0.3mas (phase 0.42), 7.9 ±0.1 mas (0.55), 9.7 ±0.1 mas (1.16), and 9.5±0.4mas (1.27). The dust shell can best be modeled with Al 2 O 3 grains using phase-dependent inner boundary radii between 1.8 and 2.4 photospheric radii. The dust shell appears to be more compact with greater optical depth near visual minimum (τ V ∼ 2.5), and more extended with lower optical depth after visual maximum (τ V ∼ 1.5). The ratios of the 43.1 GHz/42.8 GHz SiO maser ring radii to the photospheric radii are 2.2 ± 0.3/2.1 ± 0.2 (phase 0.44), 2.4 ±0.3/2.3 ±0.4 (0.55), and 2.1 ±0.3/1.9 ±0.2 (1.15). The maser spots mark the region of the molecular atmospheric layers just beyond the steepest decrease in the mid-infrared model intensity profile. Their velocity structure indicates a radial gas expansion. Conclusions. S Ori shows significant phase-dependences of photospheric radii and dust shell parameters. Al 2 O 3 dust grains and SiO mascr spots form at relatively small radii of ∼ 1.8-2.4 photospheric radii. Our results suggest increased mass loss and dust formation close to the surface near the minimum visual phase, when Al 2 O 3 dust grains are co-located with the molecular gas and the SiO maser shells, and a more expanded dust shell after visual maximum. Silicon does not appear to be bound in dust, as our data show no sign of silicate grains.

The complete quantum collapse scenario of a 2 + 1 dust shell: Preliminary calculations

If we consider the gravitational collapse of a material object to a black hole, we would expect, for ranges of mass where a black hole would form, the following scenario. A large enough object would collapse classically until an event horizon forms, and to an external observer the object would be lost from view. However, once the horizon has formed, the black hole will begin to emit Hawking radiation and the hole will lose mass and the horizon will shrink. The final state of this process could be either a zero-mass "black hole" with consequent information loss, or some sort of "quantum remnant." A complete investigation of this process would require: 1) A complete and consistent theory of quantum gravity coupled to some kind of field that would provide the Hawking radiation [1] (which could be the gravitational field itself-gravitons); 2) Some kind of definition of a "horizon" in this quantum gravity, and; 3) The calculational tools to achieve a description of the scenario. Lacking these, one may resort to toy models to try to give some sort of a preliminary answer. In this paper we will consider the collapse of an infinitesimally thin dust shell in 2 + 1 gravity, where an exact minisuperspace quantum solution exists, and try to make rough estimates of the collapse-Hawking radiationremnant formation process.

Temporal variations of the outer atmosphere and the dust shell of the carbon-rich Mira variable V Ophiuchi probed with VLTI/MIDI

Aims. We present the first multi-epoch N -band spectro-interferometric observations of the carbon-rich Mira variable V Oph using MIDI at the ESO's Very Large Telescope Interferometer. Our aim is to study temporal variations of physical properties of the outer atmosphere and the circumstellar dust shell based on spectrally-dispersed N -band visibilities over the C 2 H 2 (+HCN) features and the dust emission. Methods. Our MIDI observations were carried out at three different phases 0.18, 0.49, and 0.65, with three different baselines (projected baseline lengths of 42–124 m) using four 8.2 m Unit Telescopes (UT2-UT4, UT1-UT4, and UT2-UT3 baseline configurations). Results. The wavelength dependence of the uniform-disk diameters obtained at all epochs is characterized by a roughly constant region between 8 and 10 μ m with a slight dip centered at ~9.5 μ m and a gradual increase longward of 10 μ m. These N -band angular sizes are significantly larger than the estimated photospheric size of V Oph. The angular sizes observed at different epochs reveal that the object appears smaller at phase 0.49 (minimum light) with uniform-disk diameters of ~5–12 mas than at phases 0.18 (~12–20 mas) and 0.65 (~9–15 mas). We interpret these results with a model consisting of optically thick C 2 H 2 layers and an optically thin dust shell. Our modeling suggests that the C 2 H 2 layers around V Oph are more extended (~1.7–1.8 $R_{\star}$) at phases 0.18 and 0.65 than at phase 0.49 (~1.4 $R_{\star}$) and that the C 2 H 2 column densities appear to be the smallest at phase 0.49. We also find that the dust shell consists of amorphous carbon and SiC with an inner radius of ~2.5 $R_{\star}$, and the total optical depths of $\mbox{$\tau_{V}$} \! \approx \! 0.6$–0.9 ($\tau_{11.3~\mu{\rm m}} \! \approx \! 0.003$ and 0.004 for amorphous carbon and SiC, respectively) found at phases 0.18 and 0.65 are higher than the value obtained at phase 0.49, $\mbox{$\tau_{V}$} \! \approx \! 0.3$ ($\tau_{11.3~\mu{\rm m}} \! \approx \! 0.001$ and 0.002 for amorphous carbon and SiC, respectively). Conclusions. Our MIDI observations and modeling indicate that carbon-rich Miras also have extended layers of polyatomic molecules as previously confirmed in oxygen-rich Miras. The temporal variation of the N -band angular size is largely governed by the variations of the opacity and the geometrical extension of the C 2 H 2 layers and the dust shell, and consequently, this masks the size variation of the photosphere. Also, the observed weakness of the mid-infrared C 2 H 2 absorption in carbon-rich Miras can be explained by the emission from the extended C 2 H 2 layers and the dust shell.

Very Large Array Observations of Hiin the Circumstellar Envelopes of Asymptotic Giant Branch Stars

We have used the Very Large Array to search for neutral atomic hydrogen (H I) in the circumstellar envelopes of five asymptotic giant branch stars. We have detected H I 21 cm emission coincident in both position and velocity with the S-type semiregular variable star RS Cnc. The emission comprises a compact, slightly elongated feature centered on the star with a mean diameter of ~82'' (1.5 × 1017 cm), plus an additional filament extending ~6' to the northwest. If this filament is associated with RS Cnc, it would imply that a portion of its mass loss is highly asymmetric. We estimate M ≈ 1.5 × 10-3 M⊙ and a mass-loss rate ≈ 1.7 × 10-7 M⊙ yr-1. Toward three other stars (IRC+10216, EP Aqr, R Cas) we have detected arcminute-scale H I emission features at velocities consistent with the circumstellar envelopes, but spatially offset from the stellar positions. Toward R Cas, the emission is weak but peaks at the stellar systemic velocity and overlaps with the location of its circumstellar dust shell and thus is probably related to the star. In the case of IRC+10216, we were unable to confirm the detection of H I in absorption against the cosmic background previously reported by Le Bertre & Gerard. However, we detect arcs of emission at projected distances of r ~ 14'-18' (~ 2 × 1018 cm) to the northwest of the star. The large separation of the emission from the star is plausible, given its advanced evolutionary status, although it is unclear if the asymmetric distribution and complex velocity structure are consistent with a circumstellar origin. For EP Aqr, the detected H I emission comprises multiple clumps redward of the systemic velocity, but we are unable to determine unambiguously whether the emission arises from the circumstellar envelope or from interstellar clouds along the line of sight. Regardless of the adopted distance for the H I clumps, their inferred H I masses are at least an order of magnitude smaller than their individual gravitational binding masses. We did not detect any H I emission from our fifth target, R Aqr (a symbiotic binary), but measured a 1.4 GHz continuum flux density of 18.8 ± 0.7 mJy. R Aqr is a previously known radio source, and the 1.4 GHz emission likely arises primarily from free-free emission from an ionized circumbinary envelope.

The Mira star S Ori: SiO maser shells related to the stellar photosphere, the molecular layers, and the dust shell at three epochs

AbstractWe present recent results from our project of concurrent radio and infrared interferometry of oxygen-rich Mira stars. Our pilot study on the Mira variable S Ori included coordinated VLBA mapping of the v=1, J=1-0 43.1 GHz and v=2, J=1-0 42.8 GHz SiO maser radiation at one epoch and concurrent near-infrared K-band interferometry to constrain the stellar photospheric diameter. We recently obtained new maps of these SiO maser transitions toward S Ori at three additional epochs with the VLBA and concurrent mid-infrared interferometric data with VLTI/MIDI. The MIDI data are analyzed using recent self-excited dynamic model atmospheres including molecular shells close to continuum-forming layers, which we complement by a radiative transfer model of the dust shell. The modelling of our MIDI data results in phase-dependent continuum photospheric angular diameters. The dust shell can best be modelled with Al2O3 grains alone. We find that the mean SiO maser ring radii lie between about 1.9 and 2.4 stellar continuum radii. The maser spots mark the region of the molecular atmospheric layers shortly outward of the steepest decrease of the mid-infrared model intensity profile. Our results suggest that the SiO maser shells are co-located with the Al2O3 dust shell near minimum visual phase. Their velocity structure indicates a radial gas expansion. Preliminary results from a study of the Mira star GX Mon suggest a similar picture.

Accretion-induced quasinormal mode excitation of a Schwarzschild black hole

By combining the numerical solution of the nonlinear hydrodynamics equations with the solution of the linear inhomogeneous Zerilli-Moncrief and Regge-Wheeler equations, we investigate the properties of the gravitational radiation emitted during the axisymmetric accretion of matter onto a Schwarzschild black hole. The matter models considered include quadrupolar dust shells and thick accretion disks, permitting us to simulate situations which may be encountered at the end stages of stellar gravitational collapse or binary neutron star merger. We focus on the interference pattern appearing in the energy spectra of the emitted gravitational waves and on the amount of excitation of the quasinormal modes of the accreting black hole. We show that, quite generically in the presence of accretion, the black-hole ringdown is not a simple superposition of quasinormal modes, although the fundamental mode is usually present and often dominates the gravitational-wave signal. We interpret this as due to backscattering of waves off the nonexponentially decaying part of the black-hole potential and to the finite spatial extension of the accreting matter. Our results suggest that the black-hole QNM contributions to the full gravitational-wave signal should be extremely small and possibly not detectable in generic astrophysical scenarios involving the accretion of extended distributions of matter.

The Keck Aperture‐masking Experiment: Near‐Infrared Sizes of Dusty Wolf‐Rayet Stars

We report the results of a high angular resolution near-infrared survey of dusty Wolf-Rayet stars using the Keck I Telescope, including new multiwavelength images of the pinwheel nebulae WR 98a, WR 104, and WR 112. Angular sizes were measured for an additional eight dusty Wolf-Rayet stars using aperture-masking interferometry, allowing us to probe characteristic sizes down to � 20 mas (� 40 AU for typical sources). With angular sizes and specific fluxes, we can directly measure the wavelength-dependent surface brightness and size relations for our sample. We discovered tight correlations of these properties within our sample that could not be explained by simple spherically symmetric dust shells or even the more realistic ‘‘pinwheel nebula’’ (three-dimensional) radiative transfer model, when using Zubko’s optical constants. While the tightly correlated surface brightness relations we uncovered offer compelling indirect evidence of a shared and distinctive dust shell geometry among our sample, long-baseline interferometers should target the marginally resolved objects in our sample in order to conclusively establish the presence or absence of the putative underlying colliding-wind binaries thought to produce the dust shells around WC WolfRayet stars. Subject headingg binaries: general — circumstellar matter — instrumentation: interferometers —

Charged shells in Lovelock gravity: Hamiltonian treatment and physical implications

Using a Hamiltonian treatment, charged thin shells, static and dynamic, in spherically symmetric spacetimes, containing black holes or other specific types of solutions, in d dimensional Lovelock-Maxwell theory are studied. The free coefficients that appear in the Lovelock theory are chosen to obtain a sensible theory, with a negative cosmological constant appearing naturally. Using an Arnowitt-Deser-Misner (ADM) description, one then finds the Hamiltonian for the charged shell system. Variation of the Hamiltonian with respect to the canonical coordinates and conjugate momenta, and the relevant Lagrange multipliers, yields the dynamic and constraint equations. The vacuum solutions of these equations yield a division of the theory into two branches, namely d-2k-1>0 (which includes general relativity, Born-Infeld type theories, and other generic gravities) and d-2k-1=0 (which includes Chern-Simons type theories), where k is the parameter giving the highest power of the curvature in the Lagrangian. There appears an additional parameter {chi}=(-1){sup k+1}, which gives the character of the vacuum solutions. For {chi}=1 the solutions, being of the type found in general relativity, have a black hole character. For {chi}=-1 the solutions, being of a new type not found in general relativity, have a totally naked singularity character. Since there is a negative cosmologicalmore » constant, the spacetimes are asymptotically anti-de Sitter (AdS), and AdS when empty (for zero cosmological constant the spacetimes are asymptotically flat). The integration from the interior to the exterior vacuum regions through the thin shell takes care of a smooth junction, showing the power of the method. The subsequent analysis is divided into two cases: static charged thin shell configurations, and gravitationally collapsing charged dust shells (expanding shells are the time reversal of the collapsing shells). In the collapsing case, into an initially nonsingular spacetime with generic character or an empty interior, it is proved that the cosmic censorship is definitely upheld. Physical implications of the dynamics of such shells in a large extra dimension world scenario are also drawn. One concludes that, if such a large extra dimension scenario is correct, one can extract enough information from the outcome of those collisions as to know, not only the actual dimension of spacetime, but also which particular Lovelock gravity, general relativity or any other, is the correct one at these scales, in brief, to know d and k.« less

A compact dust shell in the symbiotic system HM Sagittae

Aims.We present high spatial resolution observations of the mid-infrared core of the dusty symbiotic system HM Sge.