4,757 research outputs found
Accretion Discs Trapped Near Corotation
We show that discs accreting onto the magnetosphere of a rotating star can
end up in a 'trapped' state, in which the inner edge of the disc stays near the
corotation radius, even at low and varying accretion rates. The accretion in
these trapped states can be steady or cyclic; we explore these states over wide
range of parameter space. We find two distinct regions of instability, one
related to the buildup and release of mass in the disk outside corotation, the
other to mass storage within the transition region near corotation. With a set
of calculations over long time scales we show how trapped states evolve from
both nonaccreting and fully accreting initial conditions, and also calculate
the effects of cyclic accretion on the spin evolution of the star. Observations
of cycles such as found here would provide important clues on the physics of
magnetospheric accretion. Recent observations of cyclic and other unusual
variability in T Tauri stars (EXors) and X-ray binaries are discussed in this
context.Comment: 14 pages, 10 figures, accepted to MNRA
Episodic Accretion on to Strongly Magnetic Stars
Some accreting neutron stars and young stars show unexplained episodic flares
in the form of quasi-periodic oscillations or recurrent outbursts. In a series
of two papers we present new work on an instability that can lead to episodic
outbursts when the accretion disc is truncated by the star's strong magnetic
field close to the corotation radius (where the Keplerian frequency matches the
star's rotational frequency). In this paper we outline the physics of the
instability and use a simple parameterization of the disc-field interaction to
explore the instability numerically, which we show can lead to repeated bursts
of accretion as well as steady-state solutions, as first suggested by Sunyaev
and Shakura. The cycle time of these bursts increases with decreasing accretion
rate. These solutions show that the usually assumed `propeller' state, in which
mass is ejected from the system, does not need to occur even at very low
accretion rates.Comment: 13 pages, 8 figures, accepted to MNRAS, minor corrections following
referee repor
Screening by coral green fluorescent protein (GFP)-like chromoproteins supports a role in photoprotection of zooxanthellae
Green fluorescent protein (GFP)-like pigments are responsible for the vivid colouration of many reef-building corals and have been proposed to act as photoprotectants. Their role remains controversial because the functional mechanism has not been elucidated. We provide direct evidence to support a photoprotective role of the non-fluorescent chromoproteins (CPs) that form a biochemically and photophysically distinct group of GFP-like proteins. Based on observations of Acropora nobilis from the Great Barrier Reef, we explored the photoprotective role of CPs by analysing five coral species under controlled conditions. In vitro and in hospite analyses of chlorophyll excitation demonstrate that screening by CPs leads to a reduction in chlorophyll excitation corresponding to the spectral properties of the specific CPs present in the coral tissues. Between 562 and 586 nm, the CPs maximal absorption range, there was an up to 50 % reduction of chlorophyll excitation. The screening was consistent for established and regenerating tissue and amongst symbiont clades A, C and D. Moreover, among two differently pigmented morphs of Acropora valida grown under identical light conditions and hosting subclade type C3 symbionts, high CP expression correlated with reduced photodamage under acute light stress
Closed-loop approach to thermodynamics
We present the closed loop approach to linear nonequilibrium thermodynamics
considering a generic heat engine dissipatively connected to two temperature
baths. The system is usually quite generally characterized by two parameters:
the output power and the conversion efficiency , to which we add a
third one, the working frequency . We establish that a detailed
understanding of the effects of the dissipative coupling on the energy
conversion process, necessitates the knowledge of only two quantities: the
system's feedback factor and its open-loop gain , the product of
which, , characterizes the interplay between the efficiency, the
output power and the operating rate of the system. By placing thermodynamics
analysis on a higher level of abstraction, the feedback loop approach provides
a versatile and economical, hence a very efficient, tool for the study of
\emph{any} conversion engine operation for which a feedback factor may be
defined
1 Hz Flaring in the Accreting Millisecond Pulsar NGC 6440 X-2: Disk Trapping and Accretion Cycles
The dynamics of the plasma in the inner regions of an accretion disk around
accreting millisecond X-ray pulsars is controlled by the magnetic field of the
neutron star. The interaction between an accretion disk and a strong magnetic
field is not well-understood, particularly at low accretion rates (the
so-called ``propeller regime'). This is due in part to the lack of clear
observational diagnostics to constrain the physics of the disk-field
interaction. Here we associate the strong ~1 Hz modulation seen in the
accreting millisecond X-ray pulsar NGC 6440 X-2 with an instability that arises
when the inner edge of the accretion disk is close to the corotation radius
(where the stellar rotation rate matches the Keplerian speed in the disk). A
similar modulation has previously been observed in another accreting
millisecond X-ray pulsar (SAX J1808.4-3658) and we suggest that the two
phenomena are related and that this may be a common phenomenon among other
magnetized systems. Detailed comparisons with theoretical models suggest that
when the instability is observed, the interaction region between the disk and
the field is very narrow -- of the order of 1 km. Modelling further suggests
that there is a transition region (~1-10 km) around the corotation radius where
the disk-field torque changes sign from spin up to spin down. This is the first
time that a direct observational constraint has been placed on the width of the
disk-magnetosphere interaction region, in the frame of the trapped-disk
instability model.Comment: Accepted by ApJ, minor revisio
Photo-ionization of planetary winds: case study HD209458b
Close-in hot Jupiters are exposed to a tremendous photon flux that ionizes
the neutral escaping material from the planet leaving an observable imprint
that makes them an interesting laboratory for testing theoretical models. In
this work we present 3D hydrodynamic simulations with radiation transfer
calculations of a close-in exoplanet in a blow-off state. We calculate the
Ly- absorption and compare it with observations of HD 209458b an
previous simplified model results.Our results show that the hydrodynamic
interaction together with a proper calculation of the photoionization proccess
are able to reproduce the main features of the observed Ly- absorption,
in particular at the blue-shifted wings of the line. We found that the ionizing
stellar flux produce an almost linear effect on the amount of absorption in the
wake. Varying the planetary mass loss rate and the radiation flux, we were able
to reproduce the absorption observed at .Comment: 9 pages, 6 figure
A population study of type II bursts in the Rapid Burster
Type II bursts are thought to arise from instabilities in the accretion flow
onto a neutron star in an X-ray binary. Despite having been known for almost 40
years, no model can yet satisfactorily account for all their properties. To
shed light on the nature of this phenomenon and provide a reference for future
theoretical work, we study the entire sample of Rossi X-ray Timing Explorer
data of type II bursts from the Rapid Burster (MXB 1730-335). We find that type
II bursts are Eddington-limited in flux, that a larger amount of energy goes in
the bursts than in the persistent emission, that type II bursts can be as short
as 0.130 s, and that the distribution of recurrence times drops abruptly below
15-18 s. We highlight the complicated feedback between type II bursts and the
NS surface thermonuclear explosions known as type I bursts, and between type II
bursts and the persistent emission. We review a number of models for type II
bursts. While no model can reproduce all the observed burst properties and
explain the source uniqueness, models involving a gating role for the magnetic
field come closest to matching the properties of our sample. The uniqueness of
the source may be explained by a special combination of magnetic field
strength, stellar spin period and alignment between the magnetic field and the
spin axis.Comment: Accepted 2015 February 12. Received 2015 February 10; in original
form 2014 December 1
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