836 research outputs found
Candidate hypervelocity stars of spectral type G and K revisited
Hypervelocity stars (HVS) move so fast that they are unbound to the Galaxy.
When they were first discovered in 2005, dynamical ejection from the
supermassive black hole (SMBH) in the Galactic Centre (GC) was suggested as
their origin. The two dozen HVSs known today are young massive B stars, mostly
of 3-4 solar masses. Recently, 20 HVS candidates of low mass were discovered in
the Segue G and K dwarf sample, but none of them originates from the GC. We
embarked on a kinematic analysis of the Segue HVS candidate sample using the
full 6D phase space information based on new proper motion measurements. Their
orbital properties can then be derived by tracing back their trajectories in
different mass models of our Galaxy. We present the results for 14 candidate
HVSs, for which proper motion measurements were possible. Significantly lower
proper motions than found in the previous study were derived. Considering three
different Galactic mass models we find that all stars are bound to the Galaxy.
We confirm that the stars do not originate from the GC. The distribution of
their proper motions and radial velocities is consistent with predictions for
runaway stars ejected from the Galactic disk by the binary supernova mechanism.
However, their kinematics are also consistent with old disk membership.
Moreover, most stars have rather low metallicities and strong -element
enrichment as typical for thick disk and halo stars, whereas the metallicity of
the three most metal-rich stars could possibly indicate that they are runaway
stars from the thin disk. One star shows halo kinematics.Comment: A&A letter accepte
Hot subdwarf stars and their connection to thermonuclear supernovae
Hot subdwarf stars (sdO/Bs) are evolved core helium-burning stars with very
thin hydrogen envelopes, which can be formed by common envelope ejection. Close
sdB binaries with massive white dwarf (WD) companions are potential progenitors
of thermonuclear supernovae type Ia (SN Ia). We discovered such a progenitor
candidate as well as a candidate for a surviving companion star, which escapes
from the Galaxy. More candidates for both types of objects have been found by
crossmatching known sdB stars with proper motion and light curve catalogues.
The Gaia mission will provide accurate astrometry and light curves of all the
stars in our hot subdwarf sample and will allow us to compile a much larger
all-sky catalogue of those stars. In this way we expect to find hundreds of
progenitor binaries and ejected companions.Comment: Proceedings of the 11th Pacific Rim Conference on Stellar
Astrophysics, Hong Kong 2015, Journal of Physics: Conference Series, in pres
The enigmatic He-sdB pulsator LS IV14116: new insights from the VLT
The intermediate Helium subdwarf B star LS IV14116 is a unique
object showing extremely peculiar atmospheric abundances as well as long-period
pulsations that cannot be explained in terms of the usual opacity mechanism.
One hypothesis invoked was that a strong magnetic field may be responsible. We
discredit this possibility on the basis of FORS2 spectro-polarimetry, which
allows us to rule out a mean longitudinal magnetic field down to 300 G.
Using the same data, we derive the atmospheric parameters for LS
IV14116 to be = 35,150111 K, =
5.880.02 and = 0.620.01. The high
surface gravity in particular is at odds with the theory that LS
IV14116 has not yet settled onto the Helium Main Sequence, and that
the pulsations are excited by an mechanism acting on the
Helium-burning shells present after the main Helium flash.
Archival UVES spectroscopy reveals LS IV14116 to have a radial
velocity of 149.12.1 km/s. Running a full kinematic analysis, we find that
it is on a retrograde orbit around the Galactic centre, with a Galactic radial
velocity component =13.238.28 km/s and a Galactic rotational velocity
component =55.5622.13 km/s. This implies that LS IV14116
belongs to the halo population, an intriguing discovery.Comment: accepted for publication in A&
Voluntary activation of quadriceps femoris in patients with unilateral anterior cruciate ligament rupture within 6 months of injury: a cross-sectional observational study
How does the Structure of Spherical Dark Matter Halos Affect the Types of Orbits in Disk Galaxies?
The main objective of this work is to determine the character of orbits of
stars moving in the meridional plane of an axially symmetric
time-independent disk galaxy model with a central massive nucleus and an
additional spherical dark matter halo component. In particular, we try to
reveal the influence of the scale length of the dark matter halo on the
different families of orbits of stars, by monitoring how the percentage of
chaotic orbits, as well as the percentages of orbits of the main regular
resonant families evolve when this parameter varies. The smaller alignment
index (SALI) was computed by numerically integrating the equations of motion as
well as the variational equations to extensive samples of orbits in order to
distinguish safely between ordered and chaotic motion. In addition, a method
based on the concept of spectral dynamics that utilizes the Fourier transform
of the time series of each coordinate is used to identify the various families
of regular orbits and also to recognize the secondary resonances that bifurcate
from them. Our numerical computations reveal that when the dark matter halo is
highly concentrated, that is when the scale length has low values the vast
majority of star orbits move in regular orbits, while on the other hand in less
concentrated dark matter halos the percentage of chaos increases significantly.
We also compared our results with early related work.Comment: Published in Baltic Astronomy journal. arXiv admin note: previous
papers with related context: arXiv:1404.4194, arXiv:1404.3961,
arXiv:1309.560
Revealing the influence of dark matter on the nature of motion and the families of orbits in axisymmetric galaxy models
An axially symmetric galactic gravitational model composed of a dense,
massive and spherical nucleus with an additional dark matter halo component was
used, to distinguish between the regular and chaotic character of orbits of
stars that move in the meridional plane (R,z). We investigated two different
cases: (i) a flat-disk galaxy (ii) an elliptical galaxy. It is of particular
interest to reveal how the portion of the dark matter inside the main body of
the galaxy influences the ordered or chaotic nature of motion. Varying the
ratio of dark matter to stellar mass, we monitored the evolution not only of
the percentage of chaotic orbits, but also of the percentages of orbits that
compose the main regular resonant families, by classifying regular orbits into
different families. Moreover we tried, to reveal how the starting position of
the parent periodic orbits of each regular family changes with respect to the
fractional portion of dark matter. We compared our results with previous
similar work.Comment: Published in Astronomy & Astrophysics (A&A) journal. arXiv admin
note: previous paper with related context: arXiv:1309.560
The population of hot subdwarf stars studied with Gaia I. The catalogue of known hot subdwarf stars
In preparation for the upcoming all-sky data releases of the Gaia mission we
compiled a catalogue of known hot subdwarf stars and candidates drawn from the
literature and yet unpublished databases. The catalogue contains 5613 unique
sources and provides multi-band photometry from the ultraviolet to the far
infrared, ground based proper motions, classifications based on spectroscopy
and colours, published atmospheric parameters, radial velocities and light
curve variability information. Using several different techniques we removed
outliers and misclassified objects. By matching this catalogue with astrometric
and photometric data from the Gaia mission, we will develop selection criteria
to construct a homogeneous, magnitude-limited all-sky catalogue of hot subdwarf
stars based on Gaia data.Comment: 11 pages, A&A accepte
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