Bright Component Research Articles

Lopsidedness in Early-Type Disk Galaxies

We quantify the mean asymmetry of 54 face-on, early-type disk galaxies (S0 to Sab) using the amplitude of the m = 1 azimuthal Fourier component of the R-band surface brightness. We find that the median lopsidedness, A1/A0, of our sample is 0.11 and that the most lopsided 20% of our galaxies have A1/A0 ≥ 0.19. Asymmetries in early-type disks appear to be of similar frequency and strength as in late-type disk galaxies. We have observed our early-type disks in a bandpass (R band) in which the light is dominated by stars with ages greater than 109 yr and therefore are seeing azimuthal asymmetries in the stellar mass distribution. The similar degree of lopsidedness seen in disks of very different star formation rates indicates that the lopsidedness in all galactic disks is primarily due to azimuthal mass asymmetries. Hence, 20% of all disk galaxies (regardless of Hubble type) have azimuthal asymmetries, A1/A0 ≥ 0.19, in their stellar disk mass distribution, confirming lopsidedness as a dynamical phenomenon.

A differential method of maximum entropy

We consider a differential method of maximum entropy that is based on the linearity of Fourier transform and involves reconstruction of images from the differences of the visibility function. The efficiency of the method is demonstrated with respect to the recovery of source images with bright components against the background of a sufficiently weak extended base. The simulation results are given along with the maps of an extragalactic radio source 0059+581, which were obtained using the standard and differential methods of maximum entropy for three observation dates and show that the principle of differential mapping allows us to increase considerably the dynamic interval of images.

The formation of double degenerates and related objects

I systematically investigate the formation of double degenerates (DDs) via binary interactions. I consider three evolutionary channels for their formation [stable Roche lobe overflow (RLOF) plus common envelope (CE), CE plus CE, exposed core plus CE], and carry out Monte Carlo simulations. I explore the effects of model parameters, such as the tidal-enhancement parameter for stellar wind, the mass transfer efficiency for stable RLOF and the CE ejection efficiency, on my results. I also explore the effects of various assumptions about age, metallicity, mass ratio distribution and wind velocity. My results show that the model is successful in the explanation of the formation of DDs. I explain satisfactorily the distributions of masses, mass ratios, orbital periods and birth rate of the observed DDs. The main conclusions are the following. (i) Stable RLOF plus CE and CE plus CE are the main evolutionary scenarios leading to the formation of DDs. (ii) The Galactic birth rate of DDs is 0.03 yr(-1), and the birth rate of DDs with helium (He) white dwarfs (WDs) as brighter components is 0.017 yr(-1). (iii) The number of detectable DDs in our Galaxy is 3 x 10(6), and DDs with brighter He WDs make up 56 per cent. (iv) The distribution of orbital periods for detectable DDs peaks around 6 h. (v) WD 0957-666 and WD 1101+364 are formed through the stable RLOF plus CE scenario, and WD 0135-052 is possibly a carbon-oxygen (CO) WD pair rather than a helium (He) WD pair. (vi) The Galactic birth rates of close i, respectively. (vii) The mergers of two WD binaries and DD mergers are 0.074 and 0.029 yr(-1), respectively. (vii) The merges of two He WDs and the mergers of He and CO WDs have masses of 0.61 +/- 0.09 and 0.96 +/- 0.13 M., respectively. (viii) Mass transfer during stable RLOF is not conservative. (ix) A tidally enhanced stellar wind exists. I also investigate the formation of Type Ia supernovae (SNe Ia), cataclysmic variables (CVs), subdwarf O-type (sdO) stars and R Coronae Borealis (R CrB) stars. The birth rates of SNe Ia and CVs are successfully explained in the above model. The model also shows that CVs with long orbital periods tend to have CO WDs. The birth rates of the mergers of two He WDs (sdO stars) and the mergers of He and CO WDs (R CrB stars) are 0.006 and 0.018 yr(-1) in the Galaxy, respectively. The birth rates of CVs, DDs, DD mergers and SNe Ia are more sensitive to the recent stellar formation history than to the past one.

Radio jet interactions in the radio galaxy PKS 2152—699

We present radio observations of the radio galaxy PKS 2152–699 obtained with the Australia Telescope Compact Array. The much higher resolution and signal-to-noise ratio of the new radio maps reveal the presence of a bright radio component about 10 arcsec north-east of the nucleus. This lies close to the highly ionized cloud previously studied in the optical and here shown in a broad-band red snapshot image with the HST PC 2. It suggests that PKS 2152–699 may be a jet/cloud interaction similar to 3C 277.3. This could cause the change in the position angle (of ∼ 20°) of the radio emission from the inner to the outer regions. On the large scale, the source has Fanaroff & Riley type II morphology although the presence of the two hotspots in the centres of the lobes is unusual. The northern lobe shows a particularly relaxed structure while the southern one has an edge-brightened, arc-like structure.

The puzzling detection of the 22 GHz water emission line in Comet Hyakutake at perihelion

We observed Comet Hyakutake C/1996 B2 during the period 26 April–4 May 1996 around perihelion searching for the 22 GHz neutral water line by using a fast multichannel spectrometer coupled to the 32 m dish of the Medicina radiotelescope. We report here not only the first certain detection of this radio spectral line in a comet and the unique detection of water in a celestial body at a distance of 0.23 AU from the Sun, but also strong physical anomalies in the behavior of the water molecule velocities. The line is split into two clear and bright components and is positively shifted with respect to the comet rest velocity at velocities varying from 22 to 44 km/s during the four days of observation. As far as we know, a similar behavior of neutral molecules in comets has never been observed before as the outflow velocity should normally not exceed 1 km/s. A qualitative analysis of the data seems to support sublimation of excited water molecules from accelerated icy grains which form an extended source around the nucleus. The strong coronal activity observed by the SOHO spacecraft in the same period may have contributed to the acceleration mechanism. A future quantitative analysis would be very useful for the study of the complex plasma-grain interaction problem and of the cometary activity at very close distances from the Sun.

A Subparsec Radio Jet or Disk in NGC 4151

We have imaged the classical Seyfert galaxy NGC 4151 with the VLBA at wavelengths of 6 and 18 cm, achieving resolutions of 2 and 5 mas (0.16 and 0.40 pc), respectively. At 18 cm, four radio components, spread over 05 (40 pc) are detected. The easternmost pair comprises a bright component elongated in position angle 20° (component E) and an extension to the northeast (component F) which curves into position angle 75°, similar to that of the 35 (280 pc) scale radio jet and the narrow-line region. At the higher resolution achieved at 6 cm, component E is found to be a linear radio source with a length of ~13 mas (1.0 pc). This source has a length/width ratio of 4, and therefore fulfills one of the classical criteria for a radio jet, but its radio luminosity is only ~1038 ergs s-1, several orders of magnitude less than the parsec-scale jets in radio galaxies. It is also misaligned by 55° from the arcsecond-scale radio jet. Possible reasons for the misalignment include deflection by the inner narrow-line region, buoyancy forces, and a change in the plane of the accretion disk presumably responsible for jet collimation. We also discuss an alternative interpretation, in which component E is a disk or torus viewed edge-on and emitting flat-spectrum, possibly thermal, radio emission. Component E appears to be the only part of the radio emission that contains a significant flat-spectrum component, and we favor identification of the central, brightest, unresolved subcomponent of E as the ultraviolet (UV) and optical nucleus. In order to reconcile the much smaller column density of H I toward the nucleus found by Lyman absorption than by 21 cm absorption measurements, we argue that a 0.01 pc thick gas disk surrounds the nucleus and is ionized out to a radius of 2 pc. The large 21 cm absorption column observed then results from off-nuclear radio components shining through the outer, neutral part of this disk. The flat spectrum of the nuclear radio source may indicate synchrotron self-absorption or free-free absorption by the inner, ionized part of the accretion disk. Interestingly, the apparent nuclear source and a radio subcomponent 0.2 pc to the southeast align precisely perpendicular to the arcsec-scale radio jet, suggesting that they may outline the large-scale structure of the accretion disk responsible for jet collimation. Comparison of the 18 cm image with the European VLBI network image acquired by Harrison et al. in 1984 provides upper limits of 0.14c and 0.25c for the apparent speeds of the radio components at distances of 7 and 36 pc, respectively, from the galaxy nucleus.

Full Adaptive Optics Images of Asteroids Ceres and Vesta; Rotational Poles and Triaxial Ellipsoid Dimensions

Adaptive optics (AO) images of asteroids Ceres and Vesta were obtained on September 18–20, 1993, with the 1.5-m telescope at the Starfire Optical Range of the USAF Phillips Laboratory located near Albuquerque, NM. The light source for higher-order wavefront correction was a Rayleigh laser beacon focused at a range of 10.5 km generated by a copper vapor laser. On April 27 and May 11, 1996, Vesta was again imaged, this time at its perihelic opposition using Vesta itself as the beacon for the AO. Images obtained at an effective wavelength of 0.85 μm were analyzed with a new reconstruction technique, called parametric blind deconvolution. The technique allows fits for relevant parameters in the frequency domain, where the convolution of the asteroid ellipse with the variable Lorentz-shaped point spread function produced by the AO system during these observations can be separated into a multiplication of analytic functions.The triaxial ellipsoid dimensions and rotational pole (with a two-fold ambiguity) of Ceres were obtained from 17 images and found to be in excellent agreement with its 1983 stellar occultation outline (975882) and the 1991 AO image produced by the COME-ON system (975882). A similar analysis of eight images of Vesta in 1993, 19 images in April 1996, and 28 images in May 1996, and combined with results from four previous speckle interferometry (SI) sets, yield dimensions and a rotational pole in reasonable agreement with the Hubble Space Telescope's results (975882). Differences may arise from a non-alignment between principal axes of inertia and the spin axis. The AO data from May 11, 1996, is of sufficient quality that Vesta's lightcurve can be decomposed into its cross-sectional area and surface brightness components, proving analytically that Vesta's lightcurve minimum is caused by a dark hemisphere. Combining the AO and SI observations with previous lightcurves yields a sidereal period of 0.22258874 days with an uncertainty of 4 in the last decimal place (3.5 ms) and shows that lightcurve minimum occurs 6° before a maximum in cross-section area.

Structure and Variability of Sources from Geodetic VLBI-Data

Abstract8 GHz maps of 39 radiosources have been obtained with ≈ 0.5 mas resolution from geodetic VLBI-data. Structure variability has been investigated on ≈ monthly interval during the bursts of radio emission of two sources 0059+581 and 0202+149 in period from June 1994 to February 1996. Bright jet components in the sources show no steady outward motion, but may be some oscillations near stationary positions.

The Radio Jet of Quasar 0153+744

AbstractWe highlight a few aspects of the radio jet of quasar 0153+744. Some challenge the relativistic beaming model to explain a stationary jet of short length and ending in a bright secondary component, extreme bending of the jet, and the lack of any emission on the counter jet side. We discuss a model of a precessing mildly relativistic jet for 0153+744.

The Nearby Young Dwarf Galaxy SBS 0335-052: Primordial Gas and Lyman-Alpha Emission

Ever since their discovery, blue compact dwarf (BCD) galaxies have been thought of as excellent candidates for being young galaxies, i.e. systems presently undergoing one of their very first bursts of star formation. This is mainly because BCDs are very metal-deficient, the metallicities of their ionized gas ranging between Zʘ/50 and Zʘ/3, which makes them the least chemically evolved galaxies in the Universe. Other evidence, such as the very high fractional neutral hydrogen gas content and the lack of an evident underlying old stellar population on optical images, also point to the relative youth of some BCDs.Thuan, Izotov h Lipovetsky (1997) have argued SBS 0335-052 to be a young galaxy on the basis of the following evidence: 1) HST imaging of the BCD shows its underlying extended low surface brightness component to have an irregular and filamentary structure, suggesting that a significant part of the emission (~1/3) comes not from an underlying stellar population, but from ionized gas. Any underlying stellar population must be younger than ~108yr. Propagating star formation occurs in a chain of 6 super-star clusters with ages ranging between 4 and 30 Myr. 2) The underlying component shows unusually blue colors consistent with gaseous emission, in contrast to most BCDs which possess an underlying red component. 3) VLA 21 cm observations show the BCD to be embedded in an extraordinarily large HI cloud with dimensions some 64 × 24 kpc (the typical size of HI envelopes around BCDs is more like a few kpc in each dimension).

An Ultraviolet and Near‐Infrared View of NGC 4214: A Starbursting Core Embedded in a Low Surface Brightness Disk

During the Astro-2 Spacelab mission in 1995 March, the Ultraviolet Imaging Telescope (UIT) obtained far-UV (λ = 1500 A) imagery of the nearby Sm/Im galaxy NGC 4214. The UIT images have a spatial resolution of ~3'' and a limiting surface brightness, μ1500 > 25 mag arcsec-2, permitting detailed investigation of the intensity and spatial distribution of the young, high-mass stellar component. These data provide the first far-UV imagery covering the full spatial extent of NGC 4214. Comparison with a corresponding I-band image reveals the presence of a starbursting core embedded in an extensive low surface brightness disk. In the far-UV (FUV), NGC 4214 is resolved into several components: a luminous, central knot; an inner region (r 2.5 kpc) with ~15 resolved sources embedded in bright, diffuse emission; and a population of fainter knots extending to the edge of the optically defined disk (r ≈ 5 kpc). The FUV light, which traces recent massive star formation, is observed to be more centrally concentrated than the I-band light, which traces the global stellar population. The FUV radial light profile is remarkably well represented by an R1/4 law, providing evidence that the centrally concentrated massive star formation in NGC 4214 is the result of an interaction, possibly a tidal encounter, with a dwarf companion(s). The brightest FUV source produces ~8% of the global FUV luminosity. This unresolved source, corresponding to the Wolf-Rayet knot described by Sargent & Filippenko, is located at the center of the FUV light distribution, giving NGC 4214 an active galactic nucleus-like morphology. Another strong source is present in the I band, located 19'' west, 10'' north of the central starburst knot, with no FUV counterpart. The I-band source may be the previously unrecognized nucleus of NGC 4214 or an evolved star cluster with an age greater than ~200 Myr. The global star formation rate derived from the total FUV flux is consistent with rates derived using data at other wavelengths and lends support to the scenario of roughly constant star formation during the last few hundred million years at a level significantly enhanced relative to the lifetime averaged star formation rate. The hybrid disk/starburst-irregular morphology evident in NGC 4214 emphasizes the danger of classifying galaxies based on their high surface brightness components at any particular wavelength.

ITAL]Hubble Space Telescope[/ITAL] Imaging of [CLC][ITAL]z[/ITAL] ≥ 0.4[/CLC] Quasar Host Galaxies Selected by Quasar Radio and Optical Properties

A sample of 16 quasars selected from the Large Bright Quasar Survey in the redshift range 0.4 < z < 0.5 has been imaged in the R band with the Planetary Camera on the WFPC2 instrument of the Hubble Space Telescope. The host galaxy magnitudes are mostly similar to or brighter than L*, and the host luminosity is positively correlated with the luminosity of the quasar nuclear component. There is no distinction in host galaxy magnitude between radio-loud and radio-quiet quasars, assuming they are all of the same galaxy type. Many of the host galaxies in the sample have small axial ratios, which may indicate that they are inclined disk systems. Alternatively, this elongated appearance may be due to bars or other distinctive morphological features which are visible while the bulk of the underlying lower surface brightness components of the host galaxy are not.

ITAL]ASCA[/ITAL] Observations of the Supernova Remnant VRO 42.05.01

We present the results of our ASCA SIS and GIS observations of the supernova remnant VRO 42.05.01. Our spectral fits indicate that the SNR is isothermal (~8.3 × 106 K), consistent with our ROSAT analysis results. The absorbing column density (~2.9 × 1021 cm-2) obtained from these spectral fits is much smaller than expected for the nominal distance of 5 kpc, which indicates that the line of sight toward VRO 42.05.01 has an unusually low gas density. The spectral resolution of ASCA allows us to determine elemental abundances for the hot X-ray-emitting plasma in the bright wing component of this remnant. We find that Mg, Si, and Fe are underabundant and attribute these low abundances to the Galactic metallicity gradient and to the location of the remnant in the outer Milky Way.

Hubble Space TelescopeObservations of the Blue Compact Dwarf SBS 0335−052: A Probable Young Galaxy

We present HST WFPC2 V and I images and GHRS UV spectrophotometry of the spectral regions around Lyα and O I λ1302 of the extremely metal-deficient (Z ~ Z☉/41) blue compact dwarf galaxy (BCD) SBS 0335-052. All the star formation in the BCD occurs in six super-star clusters (SSCs) with ages ≤25 Myr, within a region of ~2'' or 520 pc in size. Dust is clearly present and mixed spatially with the SSCs. The SSCs are roughly aligned in the southeast-northwest direction, and there is a systematic increase in reddening of the clusters away from the brightest one. The observed color dependence on position may be the combined effects of differential extinction by dust and color evolution with time due to sequential propagating star formation. There is a supershell of radius ~380 pc, delineating a large supernova cavity. The instantaneous star formation rate is ~0.4 M☉ yr-1. Strong narrow Lyα emission is not observed. Rather there is low-intensity broad (FWZI = 20 A) Lyα emission superposed on even broader Lyα absorption by the H I envelope. This broad low-intensity emission is caused by resonant scattering of Lyα photons. The absence of strong Lyα emission may be due partly to dust absorption, but is due mainly to multiple scattering that removes Lyα photons from the small HST aperture. As the H I cloud is seen nearly edge-on, geometrical effects may also play a role as photons escape more easily in a direction perpendicular to the plane than along it. The BCD appears to be a young galaxy, undergoing one of its very first bursts of star formation. This conclusion is based on the following evidence: (1) The underlying extended low surface brightness component is irregular and filamentary, suggesting that a significant part of the emission comes from ionized gas. Any underlying stellar population must be younger than ~108 yr. (2) The underlying component has very blue colors [-0.34 ≤ (V-I)0 ≤ 0.16], consistent with gaseous emission colors. (3) The O I λ1302 line is not detected in absorption in the Goddard High Resolution Spectrograph spectrum, setting an upper limit for N(O)/N(H) in the H I envelope of the BCD of more than 3000 times smaller than the value in Orion.

Helium and Carbon‐Oxygen White Dwarfs in Close Binaries

We present a scenario model of the population of close binaries in which the brighter component is a helium or a carbon-oxygen white dwarf. The other, dimmer component can be a helium white dwarf, a carbon-oxygen white dwarf, or a low-mass (≤0.3 M☉) main-sequence star. The model takes into account the effects of observational selection related to the cooling of white dwarfs and the relative brightness of components. The total birthrate of such binaries in our Galaxy is estimated to be ~0.17 yr-1. In the model, in 63% of all cases, the dimmer component is also a white dwarf. In 82% of the systems that consist of two close white dwarfs, the brighter component is a helium white dwarf. This explains why, in at least seven out of the eight white dwarf pairs found in recent years, the primary is composed of helium. We estimate that close white dwarf pairs may constitute of the total sample of observed white dwarfs. Systems that have a total mass exceeding 1.4 M☉ and in which the components are close enough to merge in a Hubble time may constitute ~1/40 of all close white dwarf pairs. This means that the sample of observed white dwarf pairs must be at least quadrupled before one may hope to find a hypothetical Type Ia supernova (SN Ia) precursor. The total number of SN Ia precursors is estimated to exceed the observable number by a factor of about 20. The merger frequency of close binary helium white dwarfs in the Galaxy is estimated to be ~0.02 yr-1. This number is consistent with the fact that, in the total sample of known white dwarf pairs, there are three in which the components are close enough to merge in a Hubble time. In one system (WD 1101+364), the merger may result in the formation of a helium subdwarf (nondegenerate helium star), and in two others (WD 2331+290 and WD 0957-666), probably in the formation of a hydrogen-deficient subgiant and possibly later of an R CrB star. The predicted merger frequency is also consistent with the fact that two out of seven white dwarfs selected for their low mass are apparently single, while five are in close binaries.

Surface Photometry of Low Surface Brightness Dwarf Irregular Galaxies

We present CCD B and I surface brightness and color profiles for a sample of 51 dwarf and low surface brightness (LSB) galaxies. The galaxies were drawn mainly from the sample of UGC dwarf and LSB galaxies detected in H I by Schneider et al. We focus our attention on extreme dwarf irregulars (dIs) where turbulent motions are as important as or more important than rotational motions, so that the majority of the dwarfs were chosen to have narrow H I line widths ({DELTA}V_20_ &lt; 100 km s^-1^), in addition to being nearby (v_0_ &lt; 1500 km s^-1^).For each dI galaxy, we derive structural parameters for the underlying low surface brightness disk component on which are superimposed the star-forming regions. We found that the central surface brightness of the underlying exponential component of dIs is similar to that of dwarf ellipticals (dEs) but ~1.5 mag fainter than that of blue compact dwarfs (BCDs). As for the scale lengths, the dIs divide into two groups; one where, for a given B luminosity, the scale length of the underlying exponential component is comparable to that of dEs and a factor of ~2 larger than that of BCDs, and one group where the scale length of the dIs is comparable to that of BCDs but a factor of 2 smaller than that of dEs. These differences in structural parameters put strong constraints on evolutionary scenarios among the three types of dwarfs. We use the CCD images to set up a tentative morphological classification scheme for LSB dIs based of the shape of the underlying low surface brightness component and the morphology and location of the star-forming regions on top of it. The division of the dIs into two groups appears to be correlated with morphology. There are a few dwarf spirals in our sample, which show fragments of low surface brightness spiral arms. We found that Nilson and Zwicky magnitudes are systematically too faint for dwarfs and LSB galaxies by 1-2 mag. The underlying disks in dIs have B-I colors ~1.5, corresponding to a stellar population of G and K main-sequence or giant stars and are redder than those of BCDs. The color profiles of the galaxies in the sample are generally flat, with no strong gradient.

Large-Scale Outflows in Edge-on Seyfert Galaxies. II. Kiloparsec-Scale Radio Continuum Emission

We present deep images of the kpc-scale radio continuum emission in 14 edge-on galaxies (ten Seyfert and four starburst galaxies). Observations were taken with the VLA at 4.9~GHz (6~cm). The Seyfert galaxies were selected from a distance-limited sample of 22 objects (defined in paper~I). The starburst galaxies were selected to be well-matched to the Seyferts in radio power, recessional velocity and inclination angle. All four starburst galaxies have a very bright disk component and one (NGC~3044) has a radio halo that extends several kpc out of the galaxy plane. Six of the ten Seyferts observed have large-scale (radial extent $\gapprox$1 kpc) radio structures extending outward from the nuclear region, indicating that large-scale outflows are quite common in Seyferts. Large-scale radio sources in Seyferts are similar in radio power and radial extent to radio halos in edge-on starburst galaxies, but their morphologies do not resemble spherical halos observed in starburst galaxies. The sources have diffuse morphologies, but, in general, they are oriented at skewed angles with respect to the galaxy minor axes. This result is most easily understood if the outflows are AGN-driven jets that are somehow diverted away from the galaxy disk on scales $\gapprox$1 kpc. Starburst-driven winds, however, cannot be ruled out. More observational work is needed to determine whether massive star formation is present at high enough rates to drive galactic winds out to kpc scales in Seyfert galaxies.

New optical and MERLIN images of the quadruple gravitational lens B1422+231

B1422+231, a multiple component quasar at a redshift of 3.62, is believed to be a gravitationally lensed system (Patnaik et al. 1992). It has 4 components with maximum image separation of 1.3 arcsec. The three brighter components, A, B and C have similar polarization properties at 8.4 GHz. The radio spectra between 5 and 8.4 GHz of A, B and C are similar; since D is very weak its spectrum is not accurately determined.

Interstellar Scattering And The Einstein Ring PKS 1830-211

The remarkably strong radio gravitational lens PKS 1830-211 consists of a one arcsecond diameter Einstein ring with two bright compact components located on opposite sides of the ring. We have obtained 22 GHz VLBA data on this source to determine the intrinsic angular sizes of the compact components. Previous VLBI observations at lower frequencies indicate that the brightness temperatures of these components are significantly lower than 1010 K (Jauncey et al. 1991), less than is typical for compact synchrotron radio sources and less than is implied by flux density variations. A possible explanation is that interstellar scattering is broadening the apparent angular size of the source and thereby reducing the observed brightness temperature. Our VLBA data support this hypothesis.

Radio Emission from SN 1987A

We review the first six years of radio observations of Supernova 1987A. The evolution can be divided into two phases: the initial radio outburst which lasted a few weeks, and the period from mid-1990 to the present, during which the radio emission has steadily increased. Both phases can be explained by a small fraction (0.1-0.5%) of the post-shock thermal energy being converted to energy in relativistic particles and magnetic fields, which give rise to synchrotron radiation. The optical depths, densities and density profiles for the pre-shocked circumstellar material are somewhat different for the two phases, but consistent with models of the density structure of the material within the circumstellar ring. New high-resolution radio observations show that the SN shock front is already at about three-quarters of the radius of the circumstellar ring, and that there exists a bright equatorial component of emission aligned with this ring which is probably due to a polar density gradient in the ‘hourglass’ structure.