1,670 research outputs found
The stars of the galactic center
We consider the origin of the so-called S stars orbiting the supermassive
black hole at the very center of the Galaxy. These are usually assumed to be
massive main-sequence stars. We argue instead that they are the remnants of
low-to-intermediate mass red giants which have been scattered on to near-radial
orbits and tidally stripped as they approach the central black hole. Such stars
retain only low-mass envelopes and thus have high effective temperatures. Our
picture simultaneously explains why S stars have tightly-bound orbits, and the
observed depletion of red giants in the very center of the Galaxy.Comment: 9 pages, 1 figure, ApJ Letters, in pres
Naturwissenschaftliches Vorverständnis von Kindern zum Zeitpunkt der Einschulung:eine Studie zu außerschulisch erworbenen Kenntnissen und Vorstellungen
Ziel der Studie war es ein möglichst umfassendes Bild naturwissenschaftlicher Kenntnisse und Vorstellungen von Kindern zu Beginn ihrer schulischen Laufbahn zu zeichnen. Dabei sollten die Kenntnisse in Abhängigkeit von der sozialen Herkunft und der intellektuellen Leistungsfähigkeit ermittelt werden. Aus den Befunden ergaben sich zahlreiche Hinweise auf vorhandene konzeptionelle Vorstellungen der Kinder über alle Naturwissenschaften hinweg. Zur Ermittlung vorhandener Kenntnisse wurde ein 73 Fragen umfassender Interviewbogen entwickelt, der sich an die inhaltlichen Vorgaben des baden-württembergischen Bildungsplans für Grundschulen hielt. Die intellektuelle Leistungsfähigkeit wurde durch einen standardisierten Tests (CPM), die Fertigkeiten durch Hands-on Aufgaben sowie der soziale Hintergrund durch eine entsprechende Stichprobenauswahl und einem Elternfragebogen erhoben. Insgesamt wurden innerhalb der Hauptstudie 130 Kinder untersucht.
Die Ergebnisse legen dar, dass das Vorwissen der Schulanfänger im Bereich der stofflichen Natur am geringsten ist. Dabei sind die Kenntnisse wesentlich von der sozialen Herkunft geprägt. Hingegen konnte kein signifikanter Zusammenhang zwischen dem ermittelten naturwissenschaftlichen Wissen und der intellektuellen Leistungsfähigkeit festgestellt werden. Die Schulanfänger verfügen über verhältnismäßig rudimentäre konzeptionelle Vorstellungen in Bereichen, die sich mit „inneren“, nicht sichtbaren, stofflichen Vorgängen und Prozessen beschäftigen. Dabei wird in der Regel vom äußeren Eindruck auf die innere Beschaffenheit unbelebter Objekte sowie Lebewesen geschlossen
Ultrasonic assisted milling of reinforced plastics
The milling of glass and carbon fibre reinforced plastics provides manufacturers from the automotive and aerospace industry with major challenges. The high carbon and glass fibre content increases the risk of insufficient production qualities. The abrasive fibres cause cutting edge rounding which results in the issue that the comparatively thick glass fibre cannot be reliably cut, while the carbon fiber is being less of a challenge. One approach to improve the production quality is the use of ultrasonic assisted milling. At the IWF tests have been undertaken to study the influence of ultrasonic assistance on workpiece quality, cutting forces and dust generation
Star Formation in the vicinity of Nuclear Black Holes: Young Stellar Objects close to Sgr A*
It is often assumed that the strong gravitational field of a super-massive
black hole disrupts an adjacent molecular cloud preventing classical star
formation in the deep potential well of the black hole. Yet, young stars have
been observed across the entire nuclear star cluster of the Milky Way including
the region close (0.5~pc) to the central black hole, Sgr A*. Here, we focus
particularly on small groups of young stars, such as IRS 13N located 0.1 pc
away from Sgr A*, which is suggested to contain about five embedded massive
young stellar objects (1 Myr). We perform three dimensional hydrodynamical
simulations to follow the evolution of molecular clumps orbiting about a
black hole, to constrain the formation and the physical
conditions of such groups. The molecular clumps in our models assumed to be
isothermal containing 100 in 0.2 pc radius. Such molecular
clumps exist in the circumnuclear disk of the Galaxy. In our highly
eccentrically orbiting clump, the strong orbital compression of the clump along
the orbital radius vector and perpendicular to the orbital plane causes the gas
densities to increase to values higher than the tidal density of Sgr A*, which
are required for star formation. Additionally, we speculate that the infrared
excess source G2/DSO approaching Sgr A* on a highly eccentric orbit could be
associated with a dust enshrouded star that may have been formed recently
through the mechanism supported by our models.Comment: 18 pages, 11 figures, accepted for publication in MNRA
Sgr A^*: A supermassive black hole or a spatially extended object?
We report here on a calculation of possible orbits of the fast moving
infrared source S1 which has been recently observed by Eckart and Genzel (1997)
near the Galactic center. It is shown that tracking of the orbit of S1 or any
other fast moving star near Sgr A^* offers a possibility of distinguishing
between the supermassive black hole and extended object scenarios of Sgr A^*.
In our calculations we assumed that the extended object at the Galactic center
is a non-baryonic ball made of degenerate, self-gravitating heavy neutrino
matter, as it has been recently proposed by Tsiklauri & Viollier (1998a,b).Comment: AASTEX, 5 postscript figs., submitted to ApJ Let
High Proper Motion Stars in the Vicinity of Sgr A*: Evidence for a Supermassive Black Hole at the Center of Our Galaxy
Over a two year period (1995-1997), we have conducted a diffraction-limited
imaging study at 2.2 microns of the inner 6"x6" of the Galaxy's central stellar
cluster using the Keck 10-m telescope. The K band images obtained reveal a
large population of faint stars. We use an unbiased approach for identifying
and selecting stars to be included in this proper motion study, which results
in a sample of 90 stars with brightness ranging from K=9-17 and velocities as
large as 1,400+-100 km/sec. Compared to earlier work (Eckart et al. 1997;
Genzel et al. 1997), the source confusion is reduced by a factor of 9, the
number of stars with proper motion measurement in the central 25 arcsec^2 of
our galaxy is doubled, and the accuracy of the velocity measurements in the
central 1 arcsec^2 is improved by a factor of 4. The peaks of both the stellar
surface density and the velocity dispersion are consistent with the position of
the unusual radio source and blackhole candidate, Sgr A*, suggesting that Sgr
A* is coincident (+-0."1) with the dynamical center of the Galaxy. As a
function of distance from Sgr A*, the velocity dispersion displays a falloff
well fit by Keplerian motion about a central dark mass of 2.6(+-0.2)x10^6 Mo
confined to a volume of at most 10^-6 pc^3, consistent with earlier results.
Although uncertainties in the measurements mathematically allow for the matter
to be distributed over this volume as a cluster, no realistic cluster is
physically tenable. Thus, independent of the presence of Sgr A*, the large
inferred central density of at least 10^12 Mo/pc^3, which exceeds the
volume-averaged mass densities found at the center of any other galaxy, leads
us to the conclusion that our Galaxy harbors a massive central black hole.Comment: 19 pages, 8 figures, accepted for publications in the Astrophysical
Journa
Red giant collisions in the galactic centre
We simulate collisions involving red-giant stars in the centre of our galaxy.
Such encounters may explain the observed paucity of highly luminous red giants
within the central 0.2pc. The masses of the missing stars are likely to be in
the range 2 to 8 solar masses. Recent models of the galactic centre cluster's
density and velocity distributions are used to calculate two-body collision
rates. In particular we use stellar-evolution models to calculate the number of
collisions a star will have during different evolutionary phases. We find that
the number of two-body collisions per star is \lo 1 in the central 0.1 to 0.2
pc, depending strongly on the galactocentric radius. Using a 3D numerical
hydrodynamics code (SPH) we simulate encounters involving cluster stars of
various masses with 2 and 8 solar-mass red giants. The instantaneous mass loss
in such collisions is rarely enough to destroy either giant. A fraction of the
collisions do, however, lead to the formation of common envelope systems where
the impactor and giant's core are enshrouded by the envelope of the giant. Such
systems may evolve to expel the envelope, leaving a tight binary; the original
giant is destroyed. The fraction of collisions that produce common envelope
systems is sensitive to the local velocity dispersion and hence galactocentric
radius. Using our collision-rate calculations we compute the time-scales for a
giant star to suffer such a collision within the galactic centre. These
time-scales are >10^{9-10}years and so are longer than the lifetimes of stars
more-massive than 2 solar masses. Thus the observed paucity of luminous giants
is unlikely to be due to the formation of common envelope systems as a result
of two-body encounters involving giant stars.Comment: 10 pages, 11 figures, MNRAS (in press
XMM-Newton observation of the brightest X-ray flare detected so far from SgrA*
We report the high S/N observation on October 3, 2002 with XMM-Newton of the
brightest X-ray flare detected so far from SgrA* with a duration shorter than
one hour (~ 2.7 ks). The light curve is almost symmetrical with respect to the
peak flare, and no significant difference between the soft and hard X-ray range
is detected. The overall flare spectrum is well represented by an absorbed
power-law with a soft photon spectral index of Gamma=2.5+/-0.3, and a peak 2-10
keV luminosity of 3.6 (+0.3-0.4) x 10^35 erg/s, i.e. a factor 160 higher than
the Sgr A* quiescent value. No significant spectral change during the flare is
observed. This X-ray flare is very different from other bright flares reported
so far: it is much brighter and softer. The present accurate determination of
the flare characteristics challenge the current interpretation of the physical
processes occuring inside the very close environment of SgrA* by bringing very
strong constraints for the theoretical flare models.Comment: Accepted for publication in A&A Letters. 4 pages, 2 figures, 1 table
Near-infrared proper motions and spectroscopy of infrared excess sources at the Galactic Center
There are a number of faint compact infrared excess sources in the central
stellar cluster of the Milky Way. Their nature and origin is unclear. In
addition to several isolated objects of this kind we find a small but dense
cluster of co-moving sources (IRS13N) about 3" west of SgrA* just 0.5" north of
the bright IRS13E cluster of WR and O-type stars. Based on their color and
brightness, there are two main possibilities: (1) they may be dust embedded
stars older than few Myr, or (2) extremely young, dusty stars with ages less
than 1Myr. We present fist H- and Ks-band identifications or proper motions of
the IRS13N members, the high velocity dusty S-cluster object (DSO), and other
infrared excess sources in the central field. We also present results of NIR H-
and Ks-band ESO-SINFONI integral field spectroscopy of ISR13N. We show that
within the uncertainties, the proper motions of the IRS13N sources in Ks- and
L'-band are identical. This indicates that the bright L'-band IRS13N sources
are indeed dust enshrouded stars rather than core-less dust clouds. The proper
motions show that the IRS13N sources are not strongly gravitationally bound to
each other implying that they have been formed recently. We also present a
first H- and Ks-band identification as well as proper motions and HKsL'-colors
of a fast moving DSO which was recently found in the cluster of high speed
S-stars that surround the super-massive black hole Sagittarius A* (SgrA*). Most
of the compact L'-band excess emission sources have a compact H- or Ks-band
counterpart and therefore are likely stars with dust shells or disks. Our new
results and orbital analysis from our previous work favor the hypothesis that
the infrared excess IRS13N members and other dusty sources close to SgrA* are
very young dusty stars and that star formation at the GC is a continuously
ongoing process.Comment: 20 pages, 18 figures, 4 tables plus appendix with 16 figures and 3
tables accepted by A&
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