Driving spiral arms in the circumstellar disks of HD 100546 and HD 141569A

With 2D hydrodynamical simulations of disks perturbed externally by stars, brown dwarfs or planets we investigate possible scenarios that can account for the spiral structure in circumstellar disks. We consider two scenarios, spiral structure driven by an external bound planet or low mass star and that excited by a previous stellar close encounter or flyby. We find that both scenarios produce morphology similar to that observed in the outer disks of HD 141569A and HD 100546; moderately open 2-armed outer spiral structure. The outer two-armed spiral structure observed in the disk of HD 141569A is qualitatively reproduced with tidal perturbations from its companion binary HD 141569B,C on a prograde orbit near periapse. Our simulation accounts for the outer spiral arms, but is less successful than the secular model of Augereau and Papaloizou at matching the lopsidedness or asymmetry of the disk edge at 300AU. The disk has been previously truncated by the tidal force from the binary. A bound object (stellar or planetary) is unlikely to explain the spiral structure in HD 100546. A co-eval planet or brown dwarf in the disk of sufficient mass to account for the amplitude of the spiral structure would be detectable in NICMOS and STIS images, however existing images reveal no such object. A previous encounter could explain the observed structure, provided that the encounter occurred less than a few thousand year ago. The object responsible for causing the spiral structure must then be within a few arcminutes of the star. However, the USNO-B proper motion survey reveals no candidate object. Moreover, the probability that a field star encountered HD 100546 in the past few thousand years is very low.Comment: accepted to A

Magnetic Fields in Stellar Jets

Although several lines of evidence suggest that jets from young stars are driven magnetically from accretion disks, existing observations of field strengths in the bow shocks of these flows imply that magnetic fields play only a minor role in the dynamics at these locations. To investigate this apparent discrepancy we performed numerical simulations of expanding magnetized jets with stochastically variable input velocities with the AstroBEAR MHD code. Because the magnetic field B is proportional to the density n within compression and rarefaction regions, the magnetic signal speed drops in rarefactions and increases in the compressed areas of velocity-variable flows. In contrast, B ~ n^0.5 for a steady-state conical flow with a toroidal field, so the Alfven speed in that case is constant along the entire jet. The simulations show that the combined effects of shocks, rarefactions, and divergent flow cause magnetic fields to scale with density as an intermediate power 1 > p > 0.5. Because p > 0.5, the Alfven speed in rarefactions decreases on average as the jet propagates away from the star. This behavior is extremely important to the flow dynamics because it means that a typical Alfven velocity in the jet close to the star is significantly larger than it is in the rarefactions ahead of bow shocks at larger distances, the one place where the field is a measurable quantity. We find that the observed values of weak fields at large distances are consistent with strong fields required to drive the observed mass loss close to the star. For a typical stellar jet the crossover point inside which velocity perturbations of 30 - 40 km/s no longer produce shocks is ~ 300 AU from the source

QPO Frequency - Color Radius Connection in GRS 1915+105: a Possible Turnover supporting AEI predictions

It is widely believed that the low frequency quasi-periodic X-ray oscillations observed in microquasars are correlated to, but do not originate at, the physical radius of the inner edge of the accretion disk. Models relating the QPO frequency and color radius are hindered by observations showing contradicting trend correlations between the microquasars GRO 1655-40, XTE J1550-564 and GRS 1915+105. The first shows a negative correlation and the latter two a positive one. By taking into account relativistic rotation in the accretion disk, the Accretion-Ejection Instability (AEI) model predicts a turnover in the frequency-radius relationship, and has been successfully compared with observations of GRO J1655-40 and GRS 1915+105. We present further evidence supporting the AEI model prediction by using observations of the microquasar GRS 1915+105. By combining a data set including $\theta$-, $\beta$- and $\alpha$-class X-ray light curves, we observe positive, negative and null correlations in the frequency-radius relationship. This is the first time a single source has shown a possible inversion in the QPO frequency-color radius curve predicted by the AEI model

X-ray Spectral Analysis of the Steady States of GRS 1915+105

We report on the X-ray spectral behavior within the steady states of GRS 1915+105. Our work is based on the full data set on the source obtained using the Proportional Counter Array on the Rossi X-ray Timing Explorer and 15 GHz radio data obtained using the Ryle Telescope. The steady observations within the X-ray data set naturally separated into two regions in the color-color diagram and we refer to them as steady-soft and steady-hard. GRS 1915+105 displays significant curvature in the coronal component in both the soft and hard data within the {\it RXTE}/PCA bandpass. A majority of the steady-soft observations displays a roughly constant inner disk radius (R_in), while the steady-hard observations display an evolving disk truncation which is correlated to the mass accretion rate through the disk. The disk flux and coronal flux are strongly correlated in steady-hard observations and very weakly correlated in the steady-soft observations. Within the steady-hard observations we observe two particular circumstances when there are correlations between the coronal X-ray flux and the radio flux with log slopes \eta~0.68 +/- 0.35 and \eta ~ 1.12 +/- 0.13. They are consistent with the upper and lower tracks of Gallo et al. (2012), respectively. A comparison of model parameters to the state definitions show that almost all steady-soft observations match the criteria of either thermal or steep power law state, while a large portion of the steady-hard observations match the hard state criteria when the disk fraction constraint is neglected.Comment: 21 pages, 15 figures, 2 tables. Accepted for publication in Ap

The Evolution of Protoplanetary Disk Edges

We investigate gap formation in gaseous protostellar disks by a planet in a circular orbit in the limit of low disk viscosity. This regime may be appropriate to an aging disk after the epoch of planet formation. We find that the distance of planet to the gap outer boundary can be between the location of the $m=2$ and $m=1$ outer Lindblad resonances. This distance is weakly dependent upon both the planet's mass and disk viscosity. We find that the evolution of the disk edge takes place on two timescales. The first timescale is set by the spiral density waves driven by the planet. The second timescale depends on the viscosity of the disk. The disk approaches a state where the outward angular momentum flux caused by the disk viscosity is balanced by the dissipation of spiral density waves which are driven at the Lindblad resonances. This occurs inefficiently however because of the extremely low gas density near the planet. We find that the distance between the planet and the peak density at the disk outer edge is only weakly dependent on the viscosity and planet mass, however the ratio of the gas density near the planet to that in the disk (or the slope of density along the disk edge) is strongly dependent upon both quantities. We find that the disk density profile along the edge scales approximately with disk viscosity divided by the square of the planet mass. We account for this behavior with a simple scenario in which the dissipation of angular momentum from the spiral density waves is balanced against diffusion in the steep edge of the disk.Comment: Accepted for publication in ApJ, 11 figures, 13 page

Accretion-Ejection Instability, MHD Rossby Wave Instability, diskoseismology, and the high-frequency QPO of microquasars

We present a possible explanation for the high-frequency Quasi-Periodic Oscillations of microquasars by an MHD instability that combines the physics developed, in different contexts, for the Accretion-Ejection Instability, the Rossby-Wave Instability, and the normal modes of diskoseismic models (which rely on the properties of the relativistic rotation curve in the vicinity of the Marginally Stable Orbit). This instability can appear as modes of azimuthal wavenumbers m=2, 3,... that have very similar pattern speeds \omega/m, while the m=1 mode, which would appear as the fundamental of this discrete spectrum, is less unstable. This would readily explain the 2:3 (and sometimes higher) frequency ratio observed between these QPO. These instabilites form eigenmodes, i.e. standing wave patterns at a constant frequency in the disk; they are strongly unstable, and thus do not need an external excitation mechanism to reach high amplitudes. Furthermore, they have the property that a fraction of the accretion energy can be emitted toward the corona: this would explain that these QPO are seen in a spectral state where Comptonized emission from the corona is always present. Their existence depends critically on the existence of a magnetic structure, formed by poloidal flux advected in the accretion process, in the central region between the disk and the black hole.Comment: To be published in Ap.

Planetary Migration in Protoplanetary Disks

The known exoplanet population displays a great diversity of orbital architectures, and explaining the origin of this is a major challenge for planet formation theories. The gravitational interaction between young planets and their protoplanetary disks provides one way in which planetary orbits can be shaped during the formation epoch. Disk-planet interactions are strongly influenced by the structure and physical processes that drive the evolution of the protoplanetary disk. In this review we focus on how disk-planet interactions drive the migration of planets when different assumptions are made about the physics of angular momentum transport, and how it drives accretion flows in protoplanetary disk models. In particular, we consider migration in discs where: (i) accretion flows arise because turbulence diffusively transports angular momentum; (ii) laminar accretion flows are confined to thin, ionised layers near disk surfaces and are driven by the launching of magneto-centrifugal winds, with the midplane being completely inert; (iii) laminar accretion flows pervade the full column density of the disc, and are driven by a combination of large scale horizontal and vertical magnetic fields

Looking for the Elusive 3:2 Ratio of High-frequency Quasi-periodic Oscillations in the Microquasar XTE J1550−564

International audienceUsing the two main XTE J1550−564 outbursts (1998–99 and 2000), we gathered about 30 observations with confirmed detections of high-frequency quasi-periodic oscillations (HFQPOs). While this is a small sample it is enough to start looking at the generic properties of these oscillations, especially focusing on their frequencies and their potential harmonic relationship. This then will provide us with a list of constraints, which are necessary for any attempt to model their origin. We defined five groups based on their similarities in the Fourier domain, namely the continuum of their power-density spectra (PDS) and the HFQPO frequencies. We then combined the individual PDS of each family to obtain a PDS with higher statistics to search for other potential, previously undetected, weaker peaks. While we have two 3σ potential detections of a pair of HFQPOs in our combined PDS, none of them show HFQPOs with frequencies in a previously claimed 3:2 ratio. Using the results presented here we propose an updated list of requirements for any model trying to explain the HFQPOs in microquasars

Gravitational waves or X-ray counterpart? No need to choose

Binary black holes emit gravitational waves as they inspiral towards coalescence. Searches for electromagnetic counterparts to these gravitational waves rely on looking for common sources producing both signals. In this paper, we take a different approach: we investigate the impact of radiation zone effects, including retardation effects and gravitational wave propagation onto the circumbinary disk around stellar-mass, spinning black holes, using general relativistic hydrodynamical simulations. Then we used a general relativistic ray-tracing code to extract its X-ray spectrum and lightcurve. This allowed us to show that radiation zone effects leave an imprint onto the disk, leading to quasi-periodic patterns in the X-ray lightcurve. The amplitude of the modulation is weak (<1%) but increases with time and is strongly dependent on the inclination angle

GR simulations of the Rossby Wave Instability: what impacts HFQPOs’ observables

International audienceThe Rossby-Wave Instability (RWI) has been proposed to be at the origin of the highfrequency QPOs observed in black-hole systems. Here we are presenting the first full GR simulations of the instability around a Kerr black-hole which allow us to explore the impact of the spin on the instability. Those simulations, coupled with a full GR ray-tracing, allow us to directly compare our simulation with the observables we get through X-ray observations