2,283 research outputs found
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&
Population of hot subdwarf stars studied with Gaia III. Catalogue of known hot subdwarf stars: Data Release 2
In light of substantial new discoveries of hot subdwarfs by ongoing
spectroscopic surveys and the availability of new all-sky data from
ground-based photometric surveys and the Gaia mission Data Release 2, we
compiled an updated catalogue of the known hot subdwarf stars. The catalogue
contains 5874 unique sources including 528 previously unknown hot subdwarfs and
provides multi-band photometry, astrometry from Gaia, and classifications based
on spectroscopy and colours. This new catalogue provides atmospheric parameters
of 2187 stars and radial velocities of 2790 stars from the literature. Using
colour, absolute magnitude, and reduced proper motion criteria, we identified
268 previously misclassified objects, most of which are less luminous white
dwarfs or more luminous blue horizontal branch and main-sequence stars.Comment: 8 pages, A&A accepte
Binary sdB Stars with Massive Compact Companions
Original paper can be found at: http://astrosociety.org/pubs/cs/381.html Copyright ASPThe masses of compact objects like white dwarfs, neutron stars and black holes are fundamental to astrophysics, but very difficult to measure. We present the results of an analysis of subluminous B (sdB) stars in close binary systems with unseen compact companions to derive their masses and clarify their nature. Radial velocity curves were obtained from time resolved spectroscopy. The atmospheric parameters were determined in a quantitative spectral analysis. Based on high resolution spectra we were able to measure the projected rotational velocity of the stars with high accuracy. In the distribution of projected rotational velocities signs of tidal locking with the companions are visible. By detecting ellipsoidal variations in the lightcurve of an sdB binary we were able to show that subdwarf binaries with orbital periods up to 0.6 d are most likely synchronized. In this case, the inclination angles and companion masses of the binaries can be tightly constrained. Five invisible companions have masses that are compatible with that of normal white dwarfs or late type main sequence stars. However, four sdBs have compact companions massive enough to be heavy white dwarfs (> 1M⊙), neutron stars or even black holes. Such a high fraction of massive compact companions is not expected from current models of binary evolution
Optical spectroscopy of candidate Alpha Persei white dwarfs
As part of an investigation into the high mass end of the initial mass-final
mass relation we performed a search for new white dwarf members of the nearby
(172.4 pc), young (80-90 Myr) Persei open star cluster. The
photometric and astrometric search using the UKIRT Infrared Deep Sky Survey and
SuperCOSMOS sky surveys discovered 14 new white dwarf candidates. We have
obtained medium resolution optical spectra of the brightest 11 candidates using
the William Herschel Telescope and confirmed that while 7 are DA white dwarfs,
3 are DB white dwarfs and one is an sdOB star, only three have cooling ages
within the cluster age, and from their position on the initial mass-final mass
relation, it is likely none are cluster members. This result is disappointing,
as recent work on the cluster mass function suggests that there should be at
least one white dwarf member, even at this young age. It may be that any white
dwarf members of Per are hidden within binary systems, as is the case
in the Hyades cluster, however the lack of high mass stars within the cluster
also makes this seem unlikely. One alternative is that a significant level of
detection incompleteness in the legacy optical image survey data at this
Galactic latitude has caused some white dwarf members to be overlooked. If this
is the case, Gaia will find them.Comment: 8 pages, 7 Figures, 3 Tables. Accepted for publication in MNRA
Frequency and Phase Synchronization in Stochastic Systems
The phenomenon of frequency and phase synchronization in stochastic systems
requires a revision of concepts originally phrased in the context of purely
deterministic systems. Various definitions of an instantaneous phase are
presented and compared with each other with special attention payed to their
robustness with respect to noise. We review the results of an analytic approach
describing noise-induced phase synchronization in a thermal two-state system.
In this context exact expressions for the mean frequency and the phase
diffusivity are obtained that together determine the average length of locking
episodes. A recently proposed method to quantify frequency synchronization in
noisy potential systems is presented and exemplified by applying it to the
periodically driven noisy harmonic oscillator. Since this method is based on a
threshold crossing rate pioneered by S.O. Rice the related phase velocity is
termed Rice frequency. Finally, we discuss the relation between the phenomenon
of stochastic resonance and noise-enhanced phase coherence by applying the
developed concepts to the periodically driven bistable Kramers oscillator.Comment: to appear in the Chaos focus issue on "Control, communication, and
synchronization in chaotic dynamical systems
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