319 research outputs found
Results of the ROTOR-program. I. The long-term photometric variability of classical T Tauri stars
We present a unique, homogeneous database of photometric measurements for
Classical T Tauri stars extending up to 20 years. The database contains more
than 21,000 UBVR observations of 72 CTTs. All the data were collected within
the framework of the ROTOR-program at Mount Maidanak Observatory (Uzbekistan)
and together they constitute the longest homogeneous, accurate record of TTS
variability ever assembled. We characterize the long term photometric
variations of 49 CTTs with sufficient data to allow a robust statistical
analysis and propose an empirical classification scheme. Several patterns of
long term photometric variability are identified. The most common pattern,
exhibited by a group of 15 stars which includes T Tau itself, consists of low
level variability (Delta(V)<=0.4mag) with no significant changes occurring from
season to season over many years. A related subgroup of 22 stars exhibits a
similar stable long term variability pattern, though with larger amplitudes (up
to Delta(V)~1.6 mag). Besides these representative groups, we identify three
smaller groups of 3-5 stars each which have distinctive photometric properties.
The long term variability of most CTTs is fairly stable and merely reflects
shorter term variability due to cold and hot surface spots. Only a small
fraction of CTTs undergo significant brightness changes on the long term
(months, years), which probably arise from slowly varying circumstellar
extinction.Comment: 16 pages, 11 figures. Astron. Astrophys., in pres
Spatial random multiple access with multiple departure
We introduce a new model of spatial random multiple access systems with a
non-standard departure policy: all arriving messages are distributed uniformly
on a finite sphere in the space, and when a successful transmission of a single
message occurs, the transmitted message leaves the system together with all its
neighbours within a ball of a given radius centred at the message's location.
We consider three classes of protocols: centralised protocols and decentralised
protocols with either ternary or binary feedback; and analyse their stability.
Further, we discuss some asymptotic properties of stable protocols
Quantum optical non-linearities induced by Rydberg-Rydberg interactions: a perturbative approach
In this article, we theoretically study the quantum statistical properties of
the light transmitted through or reflected from an optical cavity, filled by an
atomic medium with strong optical non-linearity induced by Rydberg-Rydberg van
der Waals interactions. Atoms are driven on a two-photon transition from their
ground state to a Rydberg level via an intermediate state by the combination of
a weak signal field and a strong control beam. By using a perturbative
approach, we get analytic results which remain valid in the regime of weak
feeding fields, even when the intermediate state becomes resonant. Therefore
they allow us to investigate quantitatively new features associated with the
resonant behaviour of the system. We also propose an effective non-linear
three-boson model of the system which, in addition to leading to the same
analytic results as the original problem, sheds light on the physical processes
at work in the system
Diagrammatic treatment of few-photon scattering from a Rydberg blockaded atomic ensemble in a cavity
In a previous letter we studied the giant optical nonlinearities of a Rydberg
atomic medium within an optical cavity, in the Schwinger-Keldysh formalism. In
particular, we calculated the non-linear contributions to the spectrum of the
light transmitted through the cavity. In this article we spell out the
essential details of this calculation, and we show how it can be extended to
higher input photon numbers, and higher order correlation functions. As a
relevant example, we calculate and discuss the three-photon correlation
function of the transmitted light, and discuss its physical significance in
terms of the polariton energy levels of the Rydberg medium within the optical
cavity
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