Equatorial Disk Research Articles

THE EXPANDING BIPOLAR SHELL OF THE HELIUM NOVA V445 PUPPIS

From multi-epoch adaptive optics imaging and integral field unit spectroscopy we report the discovery of an expanding and narrowly confined bipolar shell surrounding the helium nova V445 Puppis (Nova Puppis 2000). An equatorial dust disc obscures the nova remnant, and the outflow is characterised by a large polar outflow velocity of 6720 +/- 650 km/s and knots moving at even larger velocities of 8450 +/- 570 km/s. We derive an expansion parallax distance of 8.2 +/- 0.5 kpc and deduce a pre-outburst luminosity of the underlying binary of log L/L_Sun = 4.34 +/- 0.36. The derived luminosity suggests that V445 Puppis probably contains a massive white dwarf accreting at high rate from a helium star companion making it part of a population of binary stars that potentially lead to supernova Ia explosions due to accumulation of helium-rich material on the surface of a massive white dwarf.

Correlated optical/X-ray variability in the high-mass X-ray binary SAX J2103.5+4545

SAX J2103.5+4545 is the Be/X-ray binary with the shortest orbital period. It shows extended bright and faint X-ray states that last for a few hundred days. The main objective of this work is to investigate the relationship between the X-ray and optical variability and to characterise the spectral and timing properties of the bright and faint states. We have found a correlation between the spectral and temporal parameters that fit the energy and power spectra. Softer energy spectra correspond to softer power spectra. That is to say, when the energy spectrum is soft the power at high frequencies is suppressed. We also present the results of our monitoring of the Halpha line of the optical counterpart since its discovery in 2003. There is a correlation between the strength and shape of the Halpha line, originated in the circumstellar envelope of the massive companion and the X-ray emission from the vicinity of the neutron star. Halpha emission, indicative of an equatorial disc around the B-type star, is detected whenever the source is bright in X-rays. When the disc is absent, the X-ray emission decreases significantly. The long-term variability of SAX J2103.5+4545 is characterised by fast episodes of disc loss and subsequent reformation. The time scales for the loss and reformation of the disc (about 2 years) are the fastest among Be/X-ray binaries.

Novel explanation for the shape of the lenticular galaxy bulge and its implication for red spiral galaxy evolution

Aims. According to Hubble’s classification scheme, lenticular galaxies represent an intermediate evolutionary step between elliptical and spiral galaxies. This evolutionary path predicts that the aspect ratios of both lenticular and spiral galaxies should be smaller than the aspect ratios of their E6 or E7 elliptical predecessors. In contradiction to this prediction, observation has demonstrated that the aspect ratio of lenticular galaxies is larger than its immediate elliptical predecessor. In this paper, we suggest a novel explanation for this inconsistency. Methods. The approach described in this paper is primarily based on analytical methods; however, some numerical methods are also used. Results. Our idea comes from theoretical and experimental results, which show that a small increase in the equatorial diameter of an oblate spheroid with an aspect ratio ≤0.6 surprisingly causes its minor axis to also increase. We demonstrate that the same phenomenon occurs in the isodensity contours of elliptical galaxies given by Miyamoto & Nagai (Miyamoto M., & Nagai R., 1975, PASJ, 27, 533) and in a Maclaurin spheroidal mass in response to the gravitational force generated by a circumferential equatorial disk. Conclusions. The result of this paper is our explanation for the transformation of a disky elliptical galaxy into a lenticular galaxy which in response to rotation and equatorial diameter expansion evolves into a red spiral galaxy. This evolutionary path is consistent with the common environmental location of disky ellipticals, lenticular and red spiral galaxies and explains why elliptical galaxies are generally ≤E4. The proposed evolutionary path is opposite to the generally accepted formation of lenticular galaxies from the merger of spiral galaxies.

Non-linear evolution of the diocotron instability in a pulsar electrosphere: two-dimensional particle-in-cell simulations

<i>Context. <i/>The physics of the pulsar magnetosphere near the neutron star surface remains poorly constrained by observations. Indeed, little is known about its emission mechanism, from radio to high-energy X-ray and gamma-rays. Nevertheless, it is believed that large vacuum gaps exist in this magnetosphere, and a non-neutral plasma partially fills the neutron star surroundings to form an electrosphere in differential rotation.<i>Aims. <i/>According to several of our previous works, the equatorial disk in this electrosphere is diocotron and magnetron unstable, at least in the linear regime. To better assess the long term evolution of these instabilities, we study the behavior of the non-neutral plasma using particle simulations.<i>Methods. <i/>We designed a two-dimensional electrostatic particle-in-cell (PIC) code in cylindrical coordinates, solving Poisson equation for the electric potential. In the diocotron regime, the equation of motion for particles obeys the electric drift approximation. As in the linear study, the plasma is confined between two conducting walls. Moreover, in order to simulate a pair cascade in the gaps, we add a source term feeding the plasma with charged particles having the same sign as those already present in the electrosphere.<i>Results. <i/>First we checked our code by looking for the linear development of the diocotron instability in the same regime as the one used in our previous work, for a plasma annulus and for a typical electrosphere with differential rotation. To very good accuracy, we retrieve the same growth rates, supporting the correctness of our PIC code. Next, we consider the long term non-linear evolution of the diocotron instability. We found that particles tend to cluster together to form a small vortex of high charge density rotating around the axis of the cylinder with only little radial excursion of the particles. This grouping of particles generates new low density or even vacuum gaps in the plasma column. Finally, in more general initial configurations, we show that particle injection into the plasma can drastically increase the diffusion of particles across the magnetic field lines. The newly formed vacuum gaps cannot be replenished by simply invoking diocotron instability.<i>Conclusions. <i/>Diocotron instability offers a new possibility to solve the current closure problem in a pulsar magnetosphere. It is a promising mechanism leading to highly unstable flows in the pulsar inner magnetosphere. When flowing towards the light cylinder, some relativistic and particle inertia effects (not included in this study) appear. Nevertheless, the system should remain unstable because of the relativistic diocotron or the magnetron instability. Therefore, we expect an electric current to circulate in the closed magnetosphere and to feed the base of the wind with charged particles.

OPTICAL PHENOMENA IN THE FIELD OF BRANEWORLD KERR BLACK HOLES

We study the influence of the tidal charge parameter of the braneworld models on some optical phenomena in rotating black hole space–times. The escape photon cones are determined for special families of locally nonrotating, circular geodetical and radially free-falling observers. The silhouette of a rotating black hole, the shape of an equatorial thin accretion disk and the time delay effect for direct and indirect images of a radiating hot spot orbiting the black hole are given and classified in terms of the black hole rotational and tidal parameters. It is shown that increase of the negatively valued tidal parameter, with the rotational parameter fixed, generally strengthens the relativistic effects and suppresses the rotation-induced asymmetries in the optical phenomena.

SUZAKUOBSERVATION OF A HARD EXCESS IN 1H 0419 – 577: DETECTION OF A COMPTON-THICK PARTIAL-COVERING ABSORBER

We present results from a 200 ks Suzaku observation of 1H 0419-577 taken during 2007 July. The source shows a strong excess of counts above 10 keV compared to the extrapolation of models based on previous data in the 0.5-10 keV band. The 'hard excess' in 1H 0419-577 can be explained by the presence of a Compton-thick partial-covering absorber that covers ~ 70% of the source. The Compton-thick gas likely originates from a radius inside of the optical BLR and may form part of a clumpy disk wind. The fluorescent Fe Ka luminosity measured by Suzaku is consistent with that expected from an equatorial disk wind.

An alternative symbiotic channel to Type Ia supernovae

By assuming an aspherical stellar wind with an equatorial disc from a red giant, we investigate the production of Type Ia supernovae (SNe Ia) via a symbiotic channel. We estimate that the Galactic birthrate of SNe Ia via the symbiotic channel is between 1.03 x 10(-3) and 2.27 x 10(-5) yr(-1), while the delay time of SNe Ia has a wide range from similar to 0.07 to 5 Gyr. The results are greatly affected by the outflow velocity and mass-loss rate of the equatorial disc. Using our model, we discuss the progenitors of SN 2002ic and SN 2006X.

Thickness of Saturn's ring current determined from north‐south Cassini passes through the current layer

We investigate magnetic field and plasma electron data from six Cassini orbits during which, unique within the mission to date, the spacecraft passed almost directly north‐south through Saturn's equatorial plane within the ring current region of the middle magnetosphere. Three passes took place in the postnoon sector and three took place in the postmidnight sector, each at radial distances of ∼9, ∼12, and ∼15 RS. (RS is Saturn's radius, equal to 60,268 km). These data allow for the first time an essentially direct determination of the north‐south field and plasma structure in this region at approximately constant radius and local time. The dayside data indicate the presence of an equatorial current disk with a near constant half‐thickness of ∼1.5 RS. Comparison of the magnetic data with simultaneous electron spectrograms shows that the current layer is located within a significantly broader layer of hot plasma. More variable conditions are found on the nightside. In the inner region a thin equatorial current layer of half‐thickness ∼0.5 RS is found embedded at the center of a much broader layer of current and plasma. In the central and outer passes the current sheet corresponds to essentially the whole of the hot equatorial electron layer, with a half‐thickness of ∼2.5 RS for the central pass, falling strongly to ∼0.4 RS for the outer pass. The data also provide confirmatory evidence of northward displacements of the center of the current layer from Saturn's equatorial plane in the outer regions, on both the dayside and the nightside of the planet.

Analytic solutions to the accretion of a rotating finite cloud towards a central object - I. Newtonian approach

We construct a steady analytic accretion flow model for a finite rotating gas cloud that accretes material to a central gravitational object. The pressure gradients of the flow are considered to be negligible, and so the flow is ballistic. We also assume a steady flow and consider the particles at the boundary of the spherical cloud to be rotating as a rigid body, with a fixed amount of inwards radial velocity. This represents a generalization to the traditional infinite gas cloud model described by Ulrich. We show that the streamlines and density profiles obtained deviate largely from the ones calculated by Ulrich. The extra freedom in the choice of the parameters on the model can naturally account for the study of protostars formed in dense clusters by triggered mechanisms, where a wide variety of external physical mechanisms determine the boundary conditions. Also, as expected, the model predicts the formation of an equatorial accretion disc about the central object with a radius different from the one calculated by Ulrich.

On geometric distance determination to the Cepheid RS Puppis from its light echoes

The luminous Galactic Cepheid RS Puppis is unique in being surrounded by a dust nebula illuminated by the variable light of the Cepheid. In a recent paper in this journal, Kervella et al. (2008) report a very precise geometric distance to RS Pup, based on measured phase lags of the light variations of individual knots in the reflection nebula. In this commentary, we examine the validity of the distance measurement, as well as the reality of the spatial structure of the nebula determined by Feast (2008) based upon the phase lags of the knots. {Kervella et al. assumed that the illuminated dust knots lie, on average, in the plane of the sky (otherwise it is not possible to derive a geometric distance from direct imaging of light echoes). We consider the biasing introduced by the high efficiency of forward scattering. We conclude that most of the knots are in fact likely to lie in front of the plane of the sky, thus invalidating the Kervella et al. result. We also show that the flat equatorial disk structure determined by Feast is unlikely; instead, the morphology of the nebula is more probably bipolar, with a significant tilt of its axis with respect to the plane of the sky. Although the Kervella et al. distance result is invalidated, we show that high-resolution polarimetric imaging has the potential to yield a valid geometric distance to this important Cepheid.

Spacetime symmetry and mass of a lepton

The Einstein equations admit the class of regular solutions generated by stress-energy tensors representing vacuum with the reduced symmetry as compared with the maximally symmetric de Sitter vacuum. In the spherically symmetric case they describe, in particular, gravitational vacuum solitons with the de Sitter center whose mass is related to the de Sitter vacuum trapped inside and smooth breaking of spacetime symmetry from the de Sitter group in the origin to the Poincaré group at infinity. In nonlinear electrodynamics coupled to gravity and satisfying the weak energy condition, an electrovacuum soliton has an obligatory de Sitter center where the electric field vanishes while the energy density of the electromagnetic vacuum achieves its maximal finite value which gives a natural cutoff on self-energy. By the Gürses–Gürsey algorithm based on the Trautman–Newman technique it is transformed into a spinning electrovacuum soliton asymptotically Kerr–Newman for a distant observer, with the gyromagnetic ratio g = 2. The de Sitter center becomes the de Sitter equatorial disk which has properties of a perfect conductor and ideal diamagnetic. The interior de Sitter vacuum disk displays superconducting behavior within a single spinning particle. This behavior is generic for the class of spinning electrovacuum solitons. The de Sitter vacuum supplies a particle with the finite electromagnetic mass related to breaking of spacetime symmetry.

The nebulosity and distance of the Cepheid RS Puppis

Abstract Adopting a distance for RS Pup derived from a period-luminosity relation based on Cepheid parallaxes, it is shown that the phase-lag observations of the surrounding nebulosity by Kervella et al. are well fitted by a model of an equatorial disc at an angle of to the plane of the sky. The astrophysical implications of this are briefly mentioned.

Disk and wind evolution of Achernar: the breaking of the fellowship

Aims. We use spectral energy distributions (SEDs), Hα line profiles, and visibilities available in the literature to study Achernar's envelope geometry and to propose a possible scenario for its circumstellar disk formation and dissipation.Methods. We use the SIMECA code to investigate possible geometries of the circumstellar environment by comparing our synthetic results with spectroscopic and high angular resolution data from the VLTI/VINCI instrument. We compute three different kinds of models: an equatorial disk, a polar wind, and a disk+wind model.Results. We develop a 2D axial symmetric kinematic model to study the variation of the observed Hα line profiles, which provides clear evidence of Achernar's equatorial disk formation and dissipation between 1991 and 2002. Our model can reproduce the polar-wind extension greater than 10 and a possible equatorial disk (≤ 5 ) but we were unable to estimate the wind opening angle. We reproduce the Hα line-profile variations using an outburst scenario, but the disk final contraction requieres an additional physical effect to be taken into account. The polar stellar wind does not appear to be linked to the presence of a disk or a ring around the star. We test the possibility of a binary companion to Achernar, as found by Kervella & Domiciano de Souza (2007, A&A, 474, L49), but conclude that the VLTI/VINCI visibilities cannot be explained by a rotationally-distorted Be star and a companion alone. The presence of a polar jet provides an important component to reproduce the observations even if it is not excluded that the companion could partially influence the observations. New interferometric observations at short baselines (5 ≤ B ≤ 20 m) are mandatory to constrain Achernar's circumstellar envelope, as well as spectroscopic long-term follow-up observational programs to link Achernar's mass-loss episodes with its circumstellar disk formation.

Near-infrared integral field spectroscopy of the Homunculus nebula around η Carinae using Gemini/cirpass

Abstract This work presents the first integral field spectroscopy of the Homunculus nebula around η Carinae in the near-infrared spectral region (J band). We confirmed the presence of a hole on the polar region of each lobe, as indicated by previous near-IR long-slit spectra and mid-IR images. The holes can be described as a cylinder of height (i.e. the thickness of the lobe) and diameter of 6.5 and 6.0 × 1016 cm, respectively. We also mapped the blue-shifted component of He i λ10830 seen towards the NW lobe. Contrary to previous works, we suggested that this blue-shifted component is not related to the Paddle but it is indeed in the equatorial disc. We confirmed the claim of N. Smith and showed that the spatial extent of the Little Homunculus matches remarkably well the radio continuum emission at 3 cm, indicating that the Little Homunculus can be regarded as a small H ii region. Therefore, we used the optically thin 1.3 mm radio flux to derive a lower limit for the number of Lyman-continuum photons of the central source in η Car. In the context of a binary system, and assuming that the ionizing flux comes entirely from the hot companion star, the lower limit for its spectral type and luminosity class ranges from O5.5 iii to O7 i. Moreover, we showed that the radio peak at 1.7 arcsec NW from the central star is in the same line-of-sight of the ‘Sr-filament’ but they are obviously spatially separated, while the blue-shifted component of He iλ10830 may be related to the radio peak and can be explained by the ultraviolet radiation from the companion star.

The Dust Halo of Saturn's Largest Icy Moon, Rhea

Saturn's moon Rhea had been considered massive enough to retain a thin, externally generated atmosphere capable of locally affecting Saturn's magnetosphere. The Cassini spacecraft's in situ observations reveal that energetic electrons are depleted in the moon's vicinity. The absence of a substantial exosphere implies that Rhea's magnetospheric interaction region, rather than being exclusively induced by sputtered gas and its products, likely contains solid material that can absorb magnetospheric particles. Combined observations from several instruments suggest that this material is in the form of grains and boulders up to several decimetres in size and orbits Rhea as an equatorial debris disk. Within this disk may reside denser, discrete rings or arcs of material.

On the Determination of the Rotational Oblateness of Achernar

The recent interferometric study of Achernar, leading to the conclusion that its geometrical oblateness cannot be explained by the Roche approximation, has stirred substantial interest in the community, in view of its potential impact on many fields of stellar astrophysics. It is the purpose of this Letter to reinterpret the interferometric observations with a fast-rotating, gravity-darkened central star surrounded by a small equatorial disk, whose presence is consistent with contemporaneous spectroscopic data. We find that we can fit the available data only assuming a critically rotating central star. We identified two different disk models that simultaneously fit the spectroscopic, polarimetric, and interferometric observational constraints: a tenuous disk in hydrostatic equilibrium (i.e., with small scale height) and a smaller, scale height enhanced disk. We believe that these relatively small disks correspond to the transition region between the photosphere and the circumstellar environment and that they are probably perturbed by some photospheric mechanism. The study of this interface between photosphere and circumstellar disk for near-critical rotators is crucial to our understanding of the Be phenomenon and the mass and angular momentum loss of stars in general. This work shows that it is nowadays possible to directly study this transition region from simultaneous multitechnique observations.

Disks around Massive Young Stellar Objects: Are They Common?

We present K-band polarimetric images of several massive young stellar objects at resolutions $\sim$ 0.1-0.5 arcsec. The polarization vectors around these sources are nearly centro-symmetric, indicating they are dominating the illumination of each field. Three out of the four sources show elongated low-polarization structures passing through the centers, suggesting the presence of polarization disks. These structures and their surrounding reflection nebulae make up bipolar outflow/disk systems, supporting the collapse/accretion scenario as their low-mass siblings. In particular, S140 IRS1 show well defined outflow cavity walls and a polarization disk which matches the direction of previously observed equatorial disk wind, thus confirming the polarization disk is actually the circumstellar disk. To date, a dozen massive protostellar objects show evidence for the existence of disks; our work add additional samples around MYSOs equivalent to early B-type stars.

The magnetron instability in a pulsar's cylindrical electrosphere

(abridged) The physics of the pulsar magnetosphere remains poorly constrained by observations. Little is known about their emission mechanism. Large vacuum gaps probably exist, and a non-neutral plasma partially fills the neutron star surroundings to form an electrosphere. We showed that the differentially rotating equatorial disk in the pulsar's electrosphere is diocotron unstable and that it tends to stabilise when relativistic effects are included. However, when approaching the light cylinder, particle inertia becomes significant and the electric drift approximation is violated. In this paper, we study the most general instability, i.e. by including particle inertia effects, as well as relativistic motions. This general non-neutral plasma instability is called the magnetron instability. We linearise the coupled relativistic cold-fluid and Maxwell equations. The non-linear eigenvalue problem for the perturbed azimuthal electric field component is solved numerically. The spectrum of the magnetron instability in a non-neutral plasma column confined between two cylindrically conducting walls is computed for several cylindrical configurations. For a pulsar electrosphere, no outer wall exists. In this case, we allow for electromagnetic wave emission propagating to infinity. When the self-field induced by the plasma becomes significant, it can first increase the growth rate of the magnetron instability. However, equilibrium solutions are only possible when the self-electric field, measured by the parameter $s_{\rm e}$ and tending to disrupt the plasma configuration, is bounded to an upper limit, $s_{\rm e,max}$. For $s_{\rm e}$ close to but smaller than this value $s_{\rm e,max}$, the instability becomes weaker or can be suppressed as was the case in the diocotron regime.

An alternative hypothesis for the outburst mechanism in supergiant fast X-ray transients: the case of IGR J11215–5952

Context. The physical mechanism responsible for the short outbursts in a recently recognised class of high mass X-ray binaries, the supergiant fast X-ray transients (SFXTs), is still unknown. Two main hypotheses have been proposed to date: the sudden accretion by the compact object of small ejections originating in a clumpy wind from the supergiant donor, or outbursts produced at (or near) the periastron passage in wide and eccentric orbits, to explain the low (∼10 32 erg s −1 ) quiescent emission. Neither proposed mechanisms explain the whole phenomenology of these sources. IGR J11215-5952, discovered in April 2005 by the INTEGRAL satellite, is a SFXT which undergoes an outburst every 329 days, a periodicity likely associated with the orbital period of the binary system. Aims. We propose a new explanation for the outburst mechanism, based on the X-ray observations of the unique SFXT known to display periodic outbursts, IGR J11215-5952. Methods. We performed three Target of Opportunity (ToO) observations with Swift, XMM-Newton and INTEGRAL at the time of the fifth outburst, expected on 2007 February 9. Swift observations of the February 2007 outburst have been reported elsewhere. Another ToO with Swift was performed in July 2007, to monitor the supposed “apastron” passage. Results. XMM-Newton observed the source on 2007 February 9, for 23 ks, at the peak of the outburst, while INTEGRAL started the observation two days later, failing to detect the source, which had already undergone the decaying phase of the fast outburst. XMM-Newton data show large variability, with a bright flare at the beginning of the observation (lasting about 1 h), followed by a lower intensity phase (about one order of magnitude fainter) with a large variability as well as low level flaring activity. The spin periodicity discovered by RXTE is confirmed, and a spin-phase spectral variability is observed and studied in detail. The Swift campaign performed in July 2007 reveals a second outburst on 2007 July 24, as bright as that observed about 165 days before. Conclusions. The new X-ray observations allow us to propose an alternative hypothesis for the outburst mechanism in SFXTs, linked to the possible presence of a second wind component, in the form of an equatorial disc from the supergiant donor. We discuss the applicability of the model to the short outburst durations of all other SFXTs, where a clear periodicity in the outbursts has not been found yet. The new outburst from IGR J11215–5952 observed in July suggests that the true orbital period is ∼165 days, instead of 329 days, as previously thought.

The Formation of Massive Stars: Accretion, Disks, and the Development of Hypercompact HiiRegions

The hypothesis that massive stars form by accretion can be investigated by simple analytical calculations that describe the effect that the formation of a massive star has on its own accretion flow. Within a simple accretion model that includes angular momentum, that of gas flow on ballistic trajectories around a star, the increasing ionization of a massive star growing by accretion produces a three-stage evolutionary sequence. The ionization first forms a small quasi-spherical HII region gravitationally trapped within the accretion flow. At this stage the flow of ionized gas is entirely inward. As the ionization increases, the HII region transitions to a bipolar morphology in which the inflow is replaced by outflow within a narrow range of angle with about the bipolar axis. At higher rates of ionization, the opening angle of the outflow region progressively increases. Eventually, in the third stage, the accretion is confined to a thin region about an equatorial disk. Throughout this early evolution, the HII region is of hypercompact to ultracompact size depending on the mass of the enclosed star or stars. These small HII regions whose dynamics are dominated by stellar gravitation and accretion are different than compact and larger HII regions whose dynamics are dominated by the thermal pressure of the ionized gas.