94 research outputs found
Dust-enshrouded star near supermassive black hole: predictions for high-eccentricity passages near low-luminosity galactic nuclei
Supermassive black holes reside in cores of galaxies, where they are often
surrounded by a nuclear cluster and a clumpy torus of gas and dust. Mutual
interactions can set some stars on a plunging trajectory towards the black
hole. We model the pericentre passage of a dust-enshrouded star during which
the dusty envelope becomes stretched by tidal forces and is affected by the
interaction with the surrounding medium. In particular, we explore under which
conditions these encounters can lead to periods of enhanced accretion activity.
We discuss different scenarios for such a dusty source. To this end, we
employed a modification of the Swift integration package. Elements of the cloud
were modelled as numerical particles that represent the dust component that
interacts with the optically thin gaseous environment. We determine the
fraction of the total mass of the dust component that is diverted from the
original path during the passages through the pericentre at
Schwarzschild radii and find that the main part of the dust ( of
its mass) is significantly affected upon the first crossing. The fraction of
mass captured at the second passage generally decreases to very low values. As
an example, we show predictions for the dusty source evolution assuming the
current orbital parameters of the G2 cloud (also known as Dusty S-Cluster
Object, DSO) in our Galactic centre. Encounter of a core-less cloud with a
supermassive black hole is, most likely, a non-repeating event: the cloud is
destroyed. However, in the case of a dust-enshrouded star, part of the envelope
survives the pericentre passage. We discuss an offset of arcsec
between the centre of mass of the diverted part and the star along the
eccentric orbit. Finally, we examine an interesting possibility of a binary
star embedded within a common wind envelope that becomes dispersed at the
pericentre passage.Comment: 18 pages, 15 figures, Astronomy and Astrophysics accepte
An outbreak of food poisoning among children attending an international sports event in Johannesburg
Objectives. To describe an outbreak of food poisoning at a major international sports event in Johannesburg and to determine the likely cause and source of the outbreak.Design. A descriptive, case-control study.Setting. An international sports event in Johannesburg.Methods. A questionnaire survey of involved children was used to conduct a case-control study. Microbiological and chemical analysis of the implicated food was undertaken. Site visits to the premises involved in food preparation were conducted.Results. A total of 578 children were involved. Of the 361 children who returned questionnaires, 134 were affected by an acute-onset emetic-type illness, while 53 children developed diarrhoea. Consumption of fruit juice was associated with acute illness, while diarrhoea was associated with the consumption of maize-meal porridge (pap) and chicken stew. Microbiological analysis revealed high bacterial loads in samples of the fruit juice and the presence of Shigella flexneri in the maize-meal porridge. Visits to the suppliers of the implicated foods revealed several deficiencies in terms of food hygiene precautions.Conclusion. The likely vehicles and causes of this outbreak are elucidated. Guidelines for monitoring the supply and distribution of food to future similar events should be established. Furthermore, hospitals should have protocols in place to deal with such outbreaks in a manner that facilitates epidemiological investigation
Depletion of bright red giants in the Galactic center during its active phases
Observations in the near-infrared domain showed the presence of the flat core
of bright late-type stars inside from the Galactic center
supermassive black hole (Sgr A*), while young massive OB/Wolf-Rayet stars form
a cusp. Several dynamical processes were proposed to explain this apparent
paradox of the distribution of the Galactic center stellar populations. Given
the mounting evidence about a significantly increased activity of Sgr A* during
the past million years, we propose a scenario based on the interaction between
the late-type giants and a nuclear jet, whose past existence and energetics can
be inferred from the presence of -ray Fermi bubbles and bipolar radio
bubbles. Extended, loose envelopes of red giant stars can be ablated by the jet
with kinetic luminosity in the range of - within the inner of
Sgr A* (S cluster region), which would lead to their infrared luminosity
decrease after several thousand jet-star interactions. The ablation of the
atmospheres of red giants is complemented by the process of tidal stripping
that operates at distances of , and by the direct
mechanical interaction of stars with a clumpy disc at ,
which can explain the flat density profile of bright late-type stars inside the
inner half parsec from Sgr A*.Comment: 24 pages, 10 figures, 3 tables; accepted for publication by the
Astrophysical Journa
Polarimetry and strong gravity effects from spots orbiting near a black hole
We study the modulation of the observed radiation flux and the associated
changes in the polarization degree and angle that are predicted by the orbiting
spot model for flares from accreting black holes. The geometric shape of the
emission region influences the resulting model lightcurves, namely, the
emission region of a spiral shape can be distinguished from a simpler geometry
of a small orbiting spot.Comment: 5 pages, 2 figures; to appear in Proceedings of the 15th Marcel
Grossman Meeting on General Relativity - the session AC1 on "Spectral and
Temporal properties of Black Holes and Neutron Stars and the Theoretical
Models" by Sandip Chakrabarti (Rome, 1-7 July 2018), edited by Elia
Battistelli, Robert T. Jantzen, and Remo Ruffini, in preparatio
Effect of Electromagnetic Interaction on Galactic Center Flare Components
Recently, near-infrared GRAVITY@ESO observations at have
announced the detection of three bright "flares" in the vicinity of the
Galactic center supermassive black hole (SMBH) that exhibited orbital motion at
a distance of about gravitational radii from an black hole. There are indications of the presence of a large-scale,
organized component of the magnetic field at the Galactic center.
Electromagnetic effects on the flare dynamics were previously not taken into
account despite the relativistic motion of a plasma in magnetic field leading
to the charge separation and nonnegligible net charge density in the plasma.
Applying various approaches, we find the net charge number density of the flare
components of the order of cm, while the particles'
total number density is of the order of cm. However,
even such a tiny excess of charged particles in the quasi-neutral plasma can
significantly affect the dynamics of flare components, which can then lead to
the degeneracy in the measurements of spin of the SMBH. Analyzing the dynamics
of recent flares in the case of the rapidly rotating black hole, we also
constrain the inclination angle between the magnetic field and spin axis to
, as for larger angles, the motion of the hot spot is
strongly chaotic.Comment: 28 pages, 7 figures; to appear in the Astrophysical Journal; accepted
versio
The Evaporating Massive Embedded Stellar Cluster IRS 13 Close to Sgr A*. I. Detection of a rich population of dusty objects in the IRS 13 cluster
A detailed analysis of the Nuclear Stellar Cluster (NSC) concedes not only
the existence of the Scluster with its fast-moving stars and the supermassive
black hole (SMBH) Sgr A*. It also reveals an embedded region of gas and dust
with an exceptionally high stellar density called IRS 13. The IRS 13 cluster
can be divided into the northern and the eastern counterparts, called IRS 13N
and IRS 13E, respectively. This work will focus on both regions and study their
most prominent members using rich infrared and radio/submm data baselines.
Applying a multiwavelength analysis enables us to determine a comprehensive
photometric footprint of the investigated cluster sample. Using the
raytracing-based radiative transfer model HYPERION, the spectral energy
distribution of the IRS 13 members suggests a stellar nature of the dusty
sources. These putative Young Stellar Objects (YSOs) have a comparable
spectroscopic identification to the D and G sources in or near the S cluster.
Furthermore, we report the existence of a population of dusty sources in IRS 13
that can be mostly identified in the H-, K-, and Lband. Together with the
objects reported in literature, we propose that this population is the outcome
of a recent star formation process. Furthermore, we report that these
presumably young objects are arranged in a disk structure. Although it cannot
be excluded that the intrinsic arrangement of IRS 13 does show a disk
structure, we find indications that the investigated cluster sample might be
related to the counterclockwise disk.Comment: 59 pages, 44 figures, accepted and published by the Ap
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