Simulation of a Protoplanetary Disk Accretion Activity Due to a Collision with a Gas Stream

Using smoothed particle hydrodynamics, we numerically simulate steady state accretion discs for Cataclysmic Variable Dwarf Novae systems that have a secondary-to-primary mass ratio (0.35 \le q \le 0.55). After these accretion discs have come to quasi-equilibrium, we rotate each disc out of the orbital plane by (\delta = (1, 2, 3, 4, 5,) or (20)^{o}) to induce negative superhumps. For accretion discs tilted $5^{o}$, we generate light curves and associated Fourier transforms for an atlas on negative superhumps and retrograde precession. Our simulation results suggest that accretion discs need to be tilted more than three degrees for negative superhumps to be statistically significant. We also show that if the disc is tilted enough such that the gas stream strikes a disc face, then a dense cooling ring is generated near the radius of impact. In addition to the atlas, we study these artificially tilted accretion discs to find the source to negative superhumps. Our results suggest that the source is additional light from innermost disc annuli, and this additional light waxes and wanes with the amount of gas stream overflow received as the secondary orbits. The nodes, where the gas stream transitions from flowing over to under the disc rim (and vice versa), precess in the retrograde direction.

Time-resolved eclipse spectroscopy of the nova-like variable UX UMa obtained with the HST/FOS on 1994 August and November is analysed with eclipse mapping techniques to produce spatially resolved spectra of its accretion disc and gas stream as a function of distance from the disc centre. The inner accretion disc is characterized by a blue continuum filled with absorption bands and lines, which cross over to emission with increasing disc radius, similar to that reported by Rutten et al. at optical wavelengths. The comparison of spatially resolved spectra at different azimuths reveals a significant asymmetry in the disc emission at ultraviolet (UV) wavelengths, with the disc side closest to the secondary star showing pronounced absorption by an ‘iron curtain’ and a Balmer jump in absorption. These results suggest the existence of an absorbing ring of cold gas whose density and/or vertical scale increase with disc radius. The spectrum of the infalling gas stream is noticeably different from the disc spectrum at the same radius suggesting that gas overflows through the impact point at the disc rim and continues along the stream trajectory, producing distinct emission down to 0.1 RL1. The spectrum of the uneclipsed light shows prominent emission lines of Lyα, N vλ 1241, Si ivλ 1400, C ivλ 1550, He iiλ 1640, and Mg iiλ 2800, and a UV continuum rising towards longer wavelengths. The Balmer jump appears clearly in emission indicating that the uneclipsed light has an important contribution from optically thin gas. The lines and optically thin continuum emission are most probably emitted in a vertically extended disc chromosphere + wind. The radial temperature profiles of the continuum maps are well described by a steady-state disc model in the inner and intermediate disc regions (R ≤ 0.3 RL1). There is evidence of an increase in the mass accretion rate from August to November (from M. = 10−8.3 ± 0.1 to 10−8.1 ± 0.1 M⊙ yr−1), in accordance with the observed increase in brightness. Since the UX UMa disc seems to be in a high mass accretion, high-viscosity regime in both epochs, this result suggests that the mass transfer rate of UX UMa varies substantially (≃ 50 per cent) on time-scales of a few months. It is suggested that the reason for the discrepancies between the prediction of the standard disc model and observations is not an inadequate treatment of radiative transfer in the disc atmosphere, but rather the presence of additional important sources of light in the system besides the accretion disc (e.g. optically thin continuum emission from the disc wind and possible absorption by circumstellar cool gas).

Time-resolved eclipse spectroscopy of the nova-like variable UX UMa obtained with the HST/FOS on 1994 August and November is analysed with eclipse mapping techniques to produce spatially resolved spectra of its accretion disc and gas stream as a function of distance from the disc centre. The inner accretion disc is characterized by a blue continuum filled with absorption bands and lines, which cross over to emission with increasing disc radius, similar to that reported by Rutten et al. at optical wavelengths. The comparison of spatially resolved spectra at different azimuths reveals a significant asymmetry in the disc emission at ultraviolet (UV) wavelengths, with the disc side closest to the secondary star showing pronounced absorption by an ‘iron curtain’ and a Balmer jump in absorption. These results suggest the existence of an absorbing ring of cold gas whose density and/or vertical scale increase with disc radius. The spectrum of the infalling gas stream is noticeably different from the disc spectrum at the same radius suggesting that gas overflows through the impact point at the disc rim and continues along the stream trajectory, producing distinct emission down to 0.1 RL1. The spectrum of the uneclipsed light shows prominent emission lines of Lyα, N vλ 1241, Si ivλ 1400, C ivλ 1550, He iiλ 1640, and Mg iiλ 2800, and a UV continuum rising towards longer wavelengths. The Balmer jump appears clearly in emission indicating that the uneclipsed light has an important contribution from optically thin gas. The lines and optically thin continuum emission are most probably emitted in a vertically extended disc chromosphere + wind. The radial temperature profiles of the continuum maps are well described by a steady-state disc model in the inner and intermediate disc regions (R ≤ 0.3 RL1). There is evidence of an increase in the mass accretion rate from August to November (from M. = 10−8.3 ± 0.1 to 10−8.1 ± 0.1 M⊙ yr−1), in accordance with the observed increase in brightness. Since the UX UMa disc seems to be in a high mass accretion, high-viscosity regime in both epochs, this result suggests that the mass transfer rate of UX UMa varies substantially (≃ 50 per cent) on time-scales of a few months. It is suggested that the reason for the discrepancies between the prediction of the standard disc model and observations is not an inadequate treatment of radiative transfer in the disc atmosphere, but rather the presence of additional important sources of light in the system besides the accretion disc (e.g. optically thin continuum emission from the disc wind and possible absorption by circumstellar cool gas).

AU Mon is a long-period (11.113 d) Algol-type binary system with a persistent accretion disk that is apparent as double-peaked H-alpha emission. We present previously unpublished optical spectra of AU Mon which were obtained over several years with dense orbital phase coverage. We utilize these data, along with archival UV spectra, to model the temperature and structure of the accretion disk and the gas stream. Synthetic spectral profiles for lines including H-alpha, H-beta, and the Al III and Si IV doublets were computed with the Shellspec program. The best match between the model spectra and the observations is obtained for an accretion disk of inner/outer radius 5.1/23 R_sun, thickness of 5.2 R_sun, density of 1.0e-13 g/cm^3, and maximum temperature of 14000 K, along with a gas stream at a temperature of ~8000 K transferring ~2.4e-9 M_sun/yr. We show H-alpha Doppler tomograms of the velocity structure of the gas, constructed from difference profiles calculated through sequentially subtracting contributions from the stars and accretion structures. The tomograms provide independent support for the Shellspec modeling, while also illustrating that residual emission at sub-Keplerian velocities persists even after subtracting the disk and stream emission. Spectral variability in the H-alpha profile beyond that expected from either the orbital or the long-period cycle is present on both multi-week and multi-year timescales, and may reflect quasi-random changes in the mass transfer rate or the disk structure. Finally, a transient UV spectral absorption feature may be modeled as an occasional outflow launched from the vicinity of the disk-stream interaction region.

We present the results of 3D-hydrodynamical simulations of accretion flow in\nthe eclipsing dwarf nova V1239 Her in quiescence. The model includes the\noptical star filling its Roche lobe, a gas stream emanating from the inner\nLagrangian point of the binary system, and the accretion disc structure. A cold\nhydrogen gas stream is initially emitted towards a point-like gravitational\ncentre. A stationary accretion disc is formed in about 15 orbital periods after\nthe beginning of accretion. The model takes into account partial ionization of\nhydrogen and uses realistic cooling function for hydrogen. The light curve of\nthe system is calculated as the volume emission of optically thin layers along\nthe line of sight up to the optical depth \\tau =2/3 calculated using\nPlanck-averaged opacities. The calculated eclipse light curves show good\nagreement with observations, with the changing shape of pre-eclipse and\npost-eclipse light curves being explained entirely due to ~ 50% variations in\nthe mass accretion rate through the gas stream.\n

We demonstrate through smoothed particle hydrodynamics simulations that a circumbinary disk can supply mass to the central binary through gas streams that penetrate the disk gap without closing it. The conditions for an efficient flow typically require the disk thickness-to-radius ratio z/r 0.05, if the turbulent viscosity parameter α is greater than 0.01. This mass flow may be important for both the individual systems and their statistics. It occurs preferentially onto the lower mass object and acts toward equalization of component masses. The less massive component may be more luminous and easier to detect, owing to its larger accretion luminosity. For eccentric binaries, the mass flow is strongly modulated in time, providing diagnostics for both the disk and the binary. In the protostellar disks, the flow could be detected as shock emission phased with the binary orbit, resulting from stream impact with the circumstellar disks and/or the young stars. In the (super)massive black hole binaries in nuclei of galaxies, the flow may result from the surrounding interstellar medium and produce nearly periodic emission, as observed in quasar OJ 287. For star-planet-disk systems, our results show that the opening of a gap around a planet is not always sufficient for the termination of its growth. This suggests that planets supplied by gas streams from protoplanetary disks may outgrow Jupiter to become superplanets with properties heretofore reserved for stars.

The results of a long-term photometric observations of the cataclysmic variable EX Dra acquired between 2014 and 2016 at the Crimean Station of the Sternberg Astronomical Institute (24 nights, more than 10 500 measurements) are presented. The observations were performed using CCD photometers mounted on 50-cm and 60-cm telescopes in the visible and red, during both quiescence and the active state. For completeness, photometric observations obtained at the Ondrejov Observatory in 2010 in the V and R Johnson filters are also used in the analysis. The new observations of EX Dra are used to derive the orbital period of the system, which agrees well with earlier determinations. A combined model that takes into account the radiation fluxes from the gaseous stream and a hot spot on the lateral surface of the accretion disk is used to determine the parameters of the system components (white dwarf, red dwarf, accretion disk and hot spot, and gaseous stream). Variations of the parameters when the system changes from one activity state to the other are considered. Six light curves displaying unsatisfactory agreement between the observed and theoretical light curves can be successfully fitted using a version of the combined model that includes hot spots on the secondary’s surface. This model is able to qualitatively reproduce a secondary minima in the light curves that exhibits shifts of this minimum from phase 0.5. The parameters of dark spots on the red-dwarf surface were determined. The data obtained indicate that the outbursts in the EX Dra system are related to instability of the matter outflow from the secondary.

The technique of Doppler tomography has been influential in the study of mass transfer in Algol‐type interacting binaries. The Algols contain a hot blue dwarf star with a magnetically‐active late‐type companion. In the close Algols, the gas stream flows directly into the photosphere of the blue mass‐gaining star because it does not have enough room to avoid impact with that star. Doppler tomograms of the Algols have been produced from over 2500 time‐resolved spectra at wavelengths corresponding to Hα, Hβ, He I (6678 Å), Si II (6371 Å) and Si IV (1394 ° A). These tomograms display images of accretion structures that include a gas stream, accretion annulus, accretion disk, stream‐star impact region, and occasionally a source of chromospheric emission associated with the cool, mass‐losing companion. Some Algol systems alternate between streamlike and disk‐like states, and provide direct evidence of active mass transfer within the Algols. This work produced the very first images of the gas stream for the entire class of interacting binaries, and demonstrated that the Algols are far more active than formerly believed, with variability on time scales of weeks to months. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The various types and classes of X-ray binary are reviewed high-lighting recent results. The high mass X-ray binaries (HMXRBs) can be used to probe the nature of the mass loss from the OB star in these systems. Absorption measurements through one orbital cycle of the supergiant system X1700-37 are well modelled by a radiation driven wind and also require a gas stream trailing behind the X-ray source. In Cen X-3 the gas stream is accreted by the X-ray source via an accretion disk. Changes in the gas stream can cause the disk to thicken and the disk to obscure the X-ray source. How close the supergiant is to corotation seems to be as much a critical factor in these systems as how close it is to filling its Roche lobe. In the Be star X-ray binaries a strong correlation between the neutron stars rotation period and its orbital period has been explained as due to the neutron star being immersed in a dense, slow moving equatorial wind from the Be star. For the X-ray pulsars in the transient Be X-ray binaries a centrifugal barrier to accretion is important in determining the X-ray lightcurve and the spin evolution. The X-ray orbital modulations from the low mass X-ray binaries, LMXRBs, include eclipses by the companion and/or periodic dipping behaviour from structure at the edge of the disk. The corresponding optical modulations show a smooth sinusoidal like component and in some cases a sharp eclipse by the companion. The orbital period of the LMXRB XB1916-05 is 1% longer in the optical compared to that given by the X-ray dip period. The optical period has been interpreted as the orbital period, but this seems inconsistent with the well established view of the origin of the X-ray modulations in LMXRB. A new model is presented that assumes the X-ray dip period is the true orbital period. The 5.2 h eclipsing LMXRB XB2129+47 recently entered a low state and optical observations unexpectedly reveal an F star which is too big to fit into the binary. This is probably the first direct evidence that an X-ray binary is part of a hierarchical triple. Finally the class of X-ray binaries containing black hole candidates is reviewed focusing on the value of using X-ray signatures to identify new candidates.

The application of tomography to the study of gas flows in interacting binaries has led to fascinating images of the Cataclysmic Variables and Algol-type binaries. Such detailed images are currently unachievable using direct-imaging techniques. Numerous images of accretion flows have now been derived from optical and ultraviolet spectra and they have been used to identify multiple emission sources including the gas stream, accretion disk, accretion annulus, shock regions, and the chromosphere of the mass loser. It was difficult to distinguish between the separate sources of emission since these sources have overlapping velocities in the Doppler tomogram. However, with the aid of a new spectral synthesis code, we can now systematically extract the individual emission sources to sequentially isolate the images of the disk and gas stream. With these new tools, we have begun to extract the critical properties of the disk and gas stream more accurately than previously possible.

We present multi-epoch, time-resolved optical spectroscopic observations of\nthe dwarf nova HT Cas, obtained during 1986, 1992, 1995 and 2005 with the aim\nto study the properties of emission structures in the system. We determined\nthat the accretion disc radius, measured from the double-peaked emission line\nprofiles, is persistently large and lies within the range of 0.45-0.52a, where\na is the binary separation. This is close to the tidal truncation radius\nr_max=0.52a. This result contradicts with previous radius measurements. An\nextensive set of Doppler maps has revealed a very complex emission structure of\nthe accretion disc. Apart from a ring of disc emission, the tomograms display\nat least three areas of enhanced emission: the hot spot from the area of\ninteraction between the gas stream and the disc, which is superposed on the\nelongated spiral structure, and the extended bright region on the leading side\nof the disc, opposite to the location of the hot spot. The position of the hot\nspot in all the emission lines is consistent with the trajectory of the gas\nstream. However, the peaks of emission are located in the range of distances\n0.22-0.30a, which are much closer to the white dwarf than the disc edge. This\nsuggests that the outer disc regions have a very low density, allowing the gas\nstream to flow almost freely before it starts to be seen as an emission source.\nWe have found that the extended emission region in the leading side of the disc\nis always observed at the very edge of the large disc. Observations of other\ncataclysmic variables, which show a similar emission structure in their\ntomograms, confirm this conclusion. We propose that the leading side bright\nregion is caused by irradiation of tidally thickened sectors of the outer disc\nby the white dwarf and/or hot inner disc regions.\n

We report an eclipse-mapping analysis of an ensemble of light curves of the dwarf nova V2051 Oph, with the aim to study the spatial distribution of its steady-light and flickering sources. The data are combined to derive the orbital dependence of the steady-light and the flickering components at two different brightness levels, named the and states. The differences in brightness are caused by long-term variations in the mass transfer rate from the secondary star. The white dwarf is hardly affected by the long-term changes; its flux increases by only 10% from the faint to the bright state, whereas the disk flux raises by a factor of 2. Eclipse maps of the steady light show asymmetric brightness distributions with enhanced emission along the ballistic stream trajectory, clear evidence of gas stream overflow. Comparison between the steady-light maps of the faint and bright states suggests that the quiescent disk responds to changes in mass transfer rate in a homologous way. The ability to separate the orbital dependence of the low- and high-frequency flickering components allows us to identify the existence of two different and independent sources of flickering in V2051 Oph. The low-frequency flickering arises mainly in the overflowing gas stream and is associated with the mass transfer process. It maximum emission occurs at the position of closest approach of the gas stream to the white dwarf, and its spatial distribution changes in response to variations in the mass transfer rate. The high-frequency flickering originates in the accretion disk, showing a radial distribution similar to that of the steady-light maps and no evidence of emission from the hot spot, gas stream, or white dwarf. This disk flickering component has a relative amplitude of about 3% of the steady disk light, independent of disk radius and brightness state. If the disk flickering is caused by fluctuations in the energy dissipation rate induced by magnetohydrodynamic turbulence, its relative amplitude leads to a viscosity parameter αcool 0.1–0.2 at all radii for the quiescent disk. This value seems uncomfortably high to be accommodated by the disk-instability model.

Surface brightness distributions for accretion disks in eclipsing cataclysmic binaries may be reconstructed from observed eclipse light curves by use maximum entropy imaging methods. Three-color eclipse photometry of six eclipsing cataclysmic binaries provides the basis for an observational study of the local and global structure of their accretion disks. Observations from several eclipse cycles are averaged to form mean eclipse profiles with diminished contamination by flickering. Flickering is shown to be widely distributed over the face of the accretion disks in RW Tri and LX Ser. Accretion disk maps are derived which give satisfying fits to the observed eclipse profiles and which are maximally symmetric about the center of the disk. Disk maps for nova DQ Her and for four nova-like systems show broad intensity distributions with bright spots, associated with the gas streams that feed the disks, in evidence at a radii comparable to the disk tidal radius. The HT Cas disk is smaller and is dominated by a compact source at its center. Analysis of disk intensity maps at U, B and R show that the brighter parts of the accretion disks are optically-thick thermal radiators with temperatures which decrease with radius. The angular scale of each disk is fixed by a method analogous to cluster main sequence fitting. Distance estimates are derived which are free of assumptions about the global structure of the disks, but which depend on currently uncertain dyamical parameters of the binaries. The temperature in the disk is observed to fall much more slowly with radius than the R-3/4 law predicted by a steady-state mass-conserving viscous accretion disk model. Apparently, much of the visible light from the disk is produced by reprocessing of hard radiation from the center of the disk. Upper limits are found for the mass transfer rates which are one to two orders of magnitude smaller than the canonical mass transfer rate for novae and nova-like systems and which thus are in better agreement with mass transfer driven by gravitational radiation alone. Mass transfer rates have previously been systematically overestimated by the neglect of reprocessing. The accretion disk in HT Cas is shown to be optically thin in the Paschen continuum. The compact bright source at the center of its disk is a composite of light from the photosphere of the white dwarf and of free-free emission from hot gas in the accretion boundary layer.

We present Atacama Large Millimeter/submillimeter Array observations of the CO (J = 2 − 1) line emission from a protoplanetary disk around T-Tauri star SU Aurigae (hereafter SU Aur). Previous observations in optical and near-infrared wavelengths find a unique structure in SU Aur. One of the highlights of the observational results is that an extended tail-like structure is associated with the disk, indicating mass transfer from or into the disk. Here we report the discovery of the counterpart of the tail-like structure in CO gas extending more than 1000 au long. Based on geometric and kinematic perspectives, both of the disk and the tail-like structure components physically connect to each other. Several theoretical studies predicted the observed tail-like structure via the following possible scenarios: (1) a gaseous stream from the molecular cloud remnant, (2) collision with a (sub)stellar intruder or a gaseous blob from the ambient cloud, and (3) ejection of a planetary or brown dwarf mass object due to gravitational instability via multibody gravitational interaction. Since the tail-like structures associated with the SU Aur disk are a new example following RW Aurigae, some disks may experience the internal or external interaction and drastically lose mass during disk evolution.

view Abstract Citations (42) References (13) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Hydrodynamic Simulations of the Mass Transfer in Algol Blondin, John M. ; Richards, Mercedes T. ; Malinowski, Michael L. Abstract We have conducted two-dimensional hydrodynamic simulations of the mass transfer of the short-period binary Algol (beta Persei). Optical H alpha line observations suggest that the Algol system possesses a transient accretion disk, in contrast to long-period systems in which a steady accretion disk is inferred. We have used our model to explore the dependence of the circumstellar flow on three parameters: the velocity, angle, and density of the tidal stream of gas flowing from the evolved companion star toward the more massive main-sequence star. The flow is relatively insensitive to the initial speed and direction of the tidal stream, but varies substantially depending on the density of the stream. For high values of the stream density, ns is greater than or approximately 109/cu cm, the gas is effectively isothermal and does not bounce off the surface of the primary star. For low values of the stream density, ns is less than or approximately 108/cu cm, the gas is effectively adiabatic and the high thermal pressure generated when the stream strikes the stellar results in an extended accretion disk. In the intermediate range of stream density, radiative cooling is only partially effective, and the tidal stream produces a variable accretion disk reminiscent of the transient accretion disk suggested by the optical observations. In all of our simulations the region of highest thermal pressure corresponded to the region of interaction between the gas stream from the secondary and the gas which had circled the primary star. This interaction region near the surface of the primary star at the line of centers may be the source of the localized H alpha emission observed in Algol. Publication: The Astrophysical Journal Pub Date: June 1995 DOI: 10.1086/175753 Bibcode: 1995ApJ...445..939B Keywords: Accretion Disks; Eclipsing Binary Stars; Hydrodynamic Equations; Mass Transfer; Stellar Envelopes; Two Dimensional Models; Cooling; Gas Flow; H Alpha Line; Radiative Transfer; X Ray Spectra; Astrophysics; ACCRETION; ACCRETION DISKS; HYDRODYNAMICS; STARS: BINARIES: ECLIPSING; STARS: CIRCUMSTELLAR MATTER; STARS: INDIVIDUAL CONSTELLATION NAME: BETA PERSEI full text sources ADS | data products SIMBAD (2)