Galina V. Ustyugova

Publications (8)0 Total impact

• Article: Propeller outflows from an MRI disc

  Patrick S Lii, Marina M. Romanova, Galina V. Ustyugova, Alexander V. Koldoba, Richard V. E. Lovelace
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  ABSTRACT: We present the results of axisymmetric simulations of MRI-driven accretion onto a rapidly rotating, magnetized star accreting in the propeller regime. The stellar magnetosphere corotates with the star, forming a centrifugal barrier at the disc-magnetosphere boundary which inhibits matter accretion onto the star. Instead, the disc matter accumulates at the disc-magnetosphere interface and slowly diffuses into the inner magnetosphere where it picks up angular momentum and is quickly ejected from the system as an outflow. Due to the interaction of the matter with the magnetosphere, this wind is discontinuous and is launched as discrete plasmoids. If the ejection rate is lower than the disc accretion rate, the matter accumulates at the disc-magnetosphere boundary faster than it can be ejected. In this case, accretion onto the star proceeds through the episodic accretion instability in which episodes of matter accumulation are followed by simultaneous accretion and ejection. During the accretion phase of this instability in which matter flows onto the star in funnel streams, we observe a corresponding rise in the outflow rate. Both the accretion and ejection processes observed in our simulations are highly non-stationary. The stars undergo strong spin-down due to the coupling of the stellar field with the disc and corona and we measure the spin-down timescales of around 1 Myr for a typical CTTS in the propeller regime.
  04/2013;
• Source

  Available from: Richard V. E. Lovelace

  Article: MRI-driven Accretion onto Magnetized stars: Axisymmetric MHD Simulations

  Marina M. Romanova, Galina V. Ustyugova, Alexander V. Koldoba, Richard V. E. Lovelace
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  ABSTRACT: We present the first results of a global axisymmetric simulation of accretion onto rotating magnetized stars from a turbulent, MRI-driven disk. The angular momentum is transported outward by the magnetic stress of the turbulent flow with a rate corresponding to a Shakura-Sunyaev viscosity parameter alpha\approx 0.01-0.04. The result of the disk-magnetosphere interaction depends on the orientation of the poloidal field in the disk relative to that of the star at the disk-magnetosphere boundary. If fields have the same polarity, then the magnetic flux is accumulated at the boundary and blocks the accretion which leads to the accumulation of matter at the boundary. Subsequently, this matter accretes to the star in outburst before accumulating again. Hence, the cycling, `bursty' accretion is observed. If the disc and stellar fields have opposite polarity, then the field reconnection enhances the penetration of the disk matter towards the deeper field lines of the magnetosphere. However, the magnetic stress at the boundary is lower due to the field reconnection. This decreases the accretion rate and leads to smoother accretion at a lower rate. Test simulations show that in the case of higher accretion rate corresponding to alpha=0.05-0.1, accretion is bursty in cases of both polarities. On the other hand, at much lower accretion rates corresponding to alpha < 0.01, accretion is not bursty in any of these cases. We conclude that the episodic, bursty accretion is expected during periods of higher accretion rates in the disc, and in some cases it may alternate between bursty and smooth accretion, if the disk brings the poloidal field of alternating polarity. We find that a rotating, magnetically-dominated corona forms above and below the disk, and that it slowly expands outward, driven by the magnetic force.
  02/2011;
• Source

  Available from: Richard V. E. Lovelace

  Article: 3D MHD Simulations of Disk Accretion onto Magnetized Stars: Numerical Approach and Sample Simulations

  Marina M. Romanova, Alexander V. Koldoba, Galina V. Ustyugova, Akshay K. Kulkarni, Min Long, Richard V. E. Lovelace
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  ABSTRACT: We present results of global 3D MHD simulations of disk accretion to a rotating star with dipole and more complex magnetic fields using a Godunov-type code based on the "cubed sphere" grid developed earlier in our group. We describe the code and the grid and show examples of simulation results.
  02/2009;
• Chapter: Disk-Magnetosphere Interaction and Outflows: Conical Winds and Axial Jets

  Marina M. Romanova, Galina V. Ustyugova, Alexander V. Koldoba, Richard V. E. Lovelace
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  ABSTRACT: We investigate outflows from the disk-magnetosphere boundary of rotating magnetized stars in cases where the magnetic field of a star is bunched into an X-type configuration using axisymmetric and full 3D MHD simulations. Such configuration appears if viscosity in the disk is larger than diffusivity, or if the accretion rate in the disk is enhanced. Conical outflows flow from the inner edge of the disk to a narrow shell with an opening angle 30–45∘. Outflows carry 0.1–0.3 of the disk mass and part of the disk’s angular momentum outward. Conical outflows appear around stars of different periods, however in case of stars in the “propeller” regime, an additional – much faster component appears: an axial jet, where matter is accelerated up to very high velocities at small distances from the star by magnetic pressure force above the surface of the star. Exploratory 3D simulations show that conical outflows are symmetric about rotational axis of the disk even if magnetic dipole is significantly misaligned. Conical outflows and axial jets may appear in different types of young stars including Class I young stars, classical T Tauri stars, and EXors.
  12/2008: pages 153-163;
• Source

  Available from: Richard V. E. Lovelace

  Article: The Propeller Regime of Disk Accretion to a Rapidly Rotating Magnetized Star

  Marina M. Romanova, Galina V. Ustyugova, Alexander V. Koldoba, Richard V. E. Lovelace
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  ABSTRACT: The propeller regime of disk accretion to a rapidly rotating magnetized star is investigated here for the first time by axisymmetric 2.5D magnetohydrodynamic simulations. An expanded, closed magnetosphere forms in which the magnetic field is predominantly toroidal. A smaller fraction of the star's poloidal magnetic flux inflates vertically, forming a magnetically dominated tower. Matter accumulates in the equatorial region outside magnetosphere and accretes to the star quasi-periodically through elongated funnel streams which cause the magnetic field to reconnect. The star spins-down owing to the interaction of the closed magnetosphere with the disk. For the considered conditions, the spin-down torque varies with the angular velocity of the star omega* as omega*^1.3 for fixed mass accretion rate. The propeller stage may be important in the evolution of X-ray pulsars, cataclysmic variables and young stars. In particular, it may explain the present slow rotation of the classical T Tauri stars. Comment: 5 pages with 4 figures, LaTeX, macros: emulapj.sty, avi movies are available at http://www.astro.cornell.edu/us-russia/disk_prop.htm
  02/2005;
• Source

  Available from: Richard V. E. Lovelace

  Article: Three-dimensional Simulations of Disk Accretion to an Inclined Dipole: II. Hot Spots and Variability

  Marina M. Romanova, Galina V. Ustyugova, Alexander V. Koldoba, Richard V. E. Lovelace
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  ABSTRACT: The physics of the "hot spots" on stellar surfaces and the associated variability of accreting magnetized rotating stars is investigated for the first time using fully three-dimensional magnetohydrodynamic simulations. The magnetic moment of the star is inclined relative to its rotation axis by an angle Theta. A sequence of misalignment angles was investigated, between Theta=0 and 90 degrees.Typically at small Theta the spots are observed to have the shape of a bow which is curved around the magnetic axis, while at largest Theta the spots have a shape of a bar, crossing the magnetic pole. The physical parameters (density, temperature, etc.) increase toward the central regions of the spots. At relatively low density and temperature, the spots occupy approximately 10-20 % of the stellar surface, while at the highest values of these parameters this area may be less than 1 % of the area of the star. The light curves were calculated for different Theta and inclination angles of the disk i. They show a range of variability patterns, including one maximum-per-period curves (at most of angles Theta and i), and two maximum-per-period curves (at large Theta and i). At small Theta, the funnel streams may rotate faster/slower than the star, and this may lead to quasi-periodic variability of the star. The results are of interest for understanding the variability and quasi-variability of Classical T Tauri Stars, millisecond pulsars and cataclysmic variables. Comment: 15 pages, 13 figures, LaTeX, macros: emulapj.sty, avi simulations are available at http://www.astro.cornell.edu/us-rus/spots.htm
  04/2004;
• Source

  Available from: Richard V. E. Lovelace

  Article: Three-dimensional Simulations of Disk Accretion to an Inclined Dipole: I. Magnetospheric Flow at Different Theta

  Marina M. Romanova, Galina V. Ustyugova, Alexander V. Koldoba, Justin V. Wick, Richard V. E. Lovelace
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  ABSTRACT: We present results of fully three-dimensional MHD simulations of disk accretion to a rotating magnetized star with its dipole moment inclined at an angle Theta to the rotation axis of the disk. We observed that matter accretes from the disk to a star in two or several streams depending on Theta. Streams may precess around the star at small Theta. The inner regions of the disk are warped. The warping is due to the tendency of matter to co-rotate with inclined magnetosphere. The accreting matter brings positive angular momentum to the (slowly rotating) star tending to spin it up. The corresponding torque N_z depends only weakly on Theta. The angular momentum flux to the star is transported predominantly by the magnetic field; the matter component contributes < 1 % of the total flux. Results of simulations are important for understanding the nature of classical T Tauri stars, cataclysmic variables, and X-ray pulsars. Comment: 26 pages, 22 figures, LaTeX, macros: emulapj.sty, avi simulations are available at http://www.astro.cornell.edu/us-rus/inclined.htm
  01/2004;
• Article: 2D and 3D MHD simulations of disk accretion by rotating magnetized stars: Search for variability

  Marina M. Romanova, Akshay Kulkarni, Min Long, Richard V.E. Lovelace, Justin V. Wick, Galina V. Ustyugova, Alexander V. Koldoba
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  ABSTRACT: We performed 2D and full 3D magnetohydrodynamic simulations of disk accretion to a rotating star with an aligned or misaligned dipole magnetic field. We investigated the rotational equilibrium state and derived from simulations the ratio between two main frequencies: the spin frequency of the star and the orbital frequency at the inner radius of the disk. In 3D simulations we observed different features related to the non-axisymmetry of the magnetospheric flow. These features may be responsible for high-frequency quasi-periodic oscillations (QPOs). Variability at much lower frequencies may be connected with restructuring of the magnetic flux threading the inner regions of the disk. Such variability is specifically strong at the propeller stage of evolution.
  Advances in Space Research.