9,544 research outputs found
Accretion and activity on the post-common-envelope binary RR~Cae
Current scenarios for the evolution of interacting close binaries - such as
cataclysmic variables (CVs) - rely mainly on our understanding of low-mass star
angular momentum loss (AML) mechanisms. The coupling of stellar wind with its
magnetic field, i.e., magnetic braking, is the most promising mechanism to
drive AML in these stars. There are basically two properties driving magnetic
braking: the stellar magnetic field and the stellar wind. Understanding the
mechanisms that drive AML therefore requires a comprehensive understanding of
these two properties. RRCae is a well-known nearby (d=20pc) eclipsing DA+M
binary with an orbital period of P=7.29h. The system harbors a metal-rich cool
white dwarf (WD) and a highly active M-dwarf locked in synchronous rotation.
The metallicity of the WD suggests that wind accretion is taking place, which
provides a good opportunity to obtain the mass-loss rate of the M-dwarf
component. We analyzed multi-epoch time-resolved high-resolution spectra of
RRCae in search for traces of magnetic activity and accretion. We selected a
number of well-known activity indicators and studied their short and long-term
behavior. Indirect-imaging tomographic techniques were also applied to provide
the surface brightness distribution of the magnetically active M-dwarf, and
reveals a polar feature similar to those observed in fast-rotating solar-type
stars. The blue part of the spectrum was modeled using a atmosphere model to
constrain the WD properties and its metal enrichment. The latter was used to
improve the determination of the mass-accretion rate from the M-dwarf wind. The
presence of metals in the WD spectrum suggests that this component arises from
accretion of the M-dwarf wind. A model fit to the WD gives Teff=(7260+/-250)K
and logg=(7.8+/-0.1) dex with a metallicity of =(-2.8+/-0.1)dex,
and a mass-accretion rate of dotMacc=(7+/-2)x1e-16Msun/yr.Comment: 14 pages, 7 Figures, 6 Table
Asteroseismic test of rotational mixing in low-mass white dwarfs
We exploit the recent discovery of pulsations in mixed-atmosphere (He/H),
extremely low-mass white dwarf precursors (ELM proto-WDs) to test the
proposition that rotational mixing is a fundamental process in the formation
and evolution of low-mass helium core white dwarfs. Rotational mixing has been
shown to be a mechanism able to compete efficiently against gravitational
settling, thus accounting naturally for the presence of He, as well as traces
of metals such as Mg and Ca, typically found in the atmospheres of ELM
proto-WDs. Here we investigate whether rotational mixing can maintain a
sufficient amount of He in the deeper driving region of the star, such that it
can fuel, through HeII-HeIII ionization, the observed pulsations in this type
of stars. Using state-of-the-art evolutionary models computed with MESA, we
show that rotational mixing can indeed explain qualitatively the very existence
and general properties of the known pulsating, mixed-atmosphere ELM proto-WDs.
Moreover, such objects are very likely to pulsate again during their final WD
cooling phase.Comment: accepted for publication in A&A Letter
Just how hot are the Centauri extreme horizontal branch pulsators?
Past studies based on optical spectroscopy suggest that the five Cen
pulsators form a rather homogeneous group of hydrogen-rich subdwarf O stars
with effective temperatures of around 50 000 K. This places the stars below the
red edge of the theoretical instability strip in the log Teff diagram,
where no pulsation modes are predicted to be excited. Our goal is to determine
whether this temperature discrepancy is real, or whether the stars' effective
temperatures were simply underestimated. We present a spectral analysis of two
rapidly pulsating extreme horizontal branch (EHB) stars found in Cen.
We obtained Hubble Space Telescope/COS UV spectra of two Cen
pulsators, V1 and V5, and used the ionisation equilibrium of UV metallic lines
to better constrain their effective temperatures. As a by-product we also
obtained FUV lightcurves of the two pulsators. Using the relative strength of
the N IV and N V lines as a temperature indicator yields Teff values close to
60 000 K, significantly hotter than the temperatures previously derived. From
the FUV light curves we were able to confirm the main pulsation periods known
from optical data. With the UV spectra indicating higher effective temperatures
than previously assumed, the sdO stars would now be found within the predicted
instability strip. Such higher temperatures also provide consistent
spectroscopic masses for both the cool and hot EHB stars of our previously
studied sample.Comment: 9 pages, accepted for publication in Astronomy and Astrophysic
Multiwavelength Observations of the Hot DB Star PG 0112+104
We present a comprehensive multiwavelength analysis of the hot DB white dwarf
PG 0112+104. Our analysis relies on newly-acquired FUSE observations, on
medium-resolution FOS and GHRS data, on archival high-resolution GHRS
observations, on optical spectrophotometry both in the blue and around Halpha,
as well as on time-resolved photometry. From the optical data, we derive a
self-consistent effective temperature of 31,300+-500 K, a surface gravity of
log g = 7.8 +- 0.1 (M=0.52 Msun), and a hydrogen abundance of log N(H)/N(He) <
-4.0. The FUSE spectra reveal the presence of CII and CIII lines that
complement the previous detection of CII transitions with the GHRS. The
improved carbon abundance in this hot object is log N(C)/N(He) = -6.15 +- 0.23.
No photospheric features associated with other heavy elements are detected. We
reconsider the role of PG 0112+104 in the definition of the blue edge of the
V777 Her instability strip in light of our high-speed photometry, and contrast
our results with those of previous observations carried out at the McDonald
Observatory.Comment: 10 pages in emulateapj, 9 figures, accepted for publication in Ap
One Dimensional Oxygen Ordering in YBa2Cu3O(7-delta)
A model consisting of oxygen-occupied and -vacant chains is considered, with
repulsive first and second nearest-neighbor interactions V1 and V2,
respectively. The statistical mechanics and the diffraction spectrum of the
model is solved exactly and analytically with the only assumption V1 >> V2. At
temperatures T ~ V1 only a broad maximum at (1/2,0,0) is present, while for
ABS(delta - 1/2) > 1/14 at low enough T, the peak splits into two. The simple
expression for the diffraction intensity obtained for T << V1 represents in a
more compact form previous results of Khachaturyan and Morris[1],extends them
to all delta and T/V2 and leads to a good agreement with experiment. [1]
A.G.Khachaturyan and J.W.Morris, Jr., Phys.Rev.Lett. 64,76(1990)Comment: 13 pages,Revtex,3 figures available upon request but can be plotted
using simple analytical functions,CNEA-CAB 92/04
EISCAT and Cluster observations in the vicinity of the dynamical polar cap boundary
The dynamics of the polar cap boundary and auroral oval in the nightside ionosphere are studied during late expansion and recovery of a substorm from the region between Tromsø (66.6&deg; cgmLat) and Longyearbyen (75.2&deg; cgmLat) on 27 February 2004 by using the coordinated EISCAT incoherent scatter radar, MIRACLE magnetometer and Cluster satellite measurements. During the late substorm expansion/early recovery phase, the polar cap boundary (PCB) made zig-zag-type motion with amplitude of 2.5&deg; cgmLat and period of about 30 min near magnetic midnight. We suggest that the poleward motions of the PCB were produced by bursts of enhanced reconnection at the near-Earth neutral line (NENL). The subsequent equatorward motions of the PCB would then represent the recovery of the merging line towards the equilibrium state (Cowley and Lockwood, 1992). The observed bursts of enhanced westward electrojet just equatorward of the polar cap boundary during poleward expansions were produced plausibly by particles accelerated in the vicinity of the neutral line and thus lend evidence to the Cowley-Lockwood paradigm. <br><br> During the substorm recovery phase, the footpoints of the Cluster satellites at a geocentric distance of 4.4 <I>R<sub>E</sub></I> mapped in the vicinity of EISCAT measurements. Cluster data indicate that outflow of H<sup>+</sup> and O<sup>+</sup> ions took place within the plasma sheet boundary layer (PSBL) as noted in some earlier studies as well. We show that in this case the PSBL corresponded to a region of enhanced electron temperature in the ionospheric F region. It is suggested that the ion outflow originates from the F region as a result of increased ambipolar diffusion. At higher altitudes, the ions could be further energized by waves, which at Cluster altitudes were observed as BBELF (broad band extra low frequency) fluctuations. <br><br> The four-satellite configuration of Cluster revealed a sudden poleward expansion of the PSBL by 2&deg; during ~5 min. The beginning of the poleward motion of the PCB was associated with an intensification of the downward FAC at the boundary. We suggest that the downward FAC sheet at the PCB is the high-altitude counterpart of the Earthward flowing FAC produced in the vicinity of the magnetotail neutral line by the Hall effect (Sonnerup, 1979) during a short-lived reconnection pulse
Theory of temperature dependence of the Fermi surface-induced splitting of the alloy diffuse-scattering intensity peak
The explanation is presented for the temperature dependence of the fourfold
intensity peak splitting found recently in diffuse scattering from the
disordered Cu3Au alloy. The wavevector and temperature dependence of the
self-energy is identified as the origin of the observed behaviour. Two
approaches for the calculation of the self-energy, the high-temperature
expansion and the alpha-expansion, are proposed. Applied to the Cu3Au alloy,
both methods predict the increase of the splitting with temperature, in
agreement with the experimental results.Comment: 4 pages, 3 EPS figures, RevTeX, submitted to J. Phys. Condens. Matter
(Letter to the Editor
Study of Phase Stability in NiPt Systems
We have studied the problem of phase stability in NiPt alloy system. We have
used the augmented space recursion based on the TB-LMTO as the method for
studying the electronic structure of the alloys. In particular, we have used
the relativistic generalization of our earlier technique. We note that, in
order to predict the proper ground state structures and energetics, in addition
to relativistic effects, we have to take into account charge transfer effects
with precision.Comment: 22 pages, 7 figures. Accepted for publication in JPC
Improving pikeperch larviculture by combining environmental, feeding and populational factors
Pikeperch (Sander lucioperca) has a high potential for inland aquaculture diversification in Europe. Bottlenecks (i.e. low survival, cannibalism and deformity rates) hamper further expansion of pikeperch culture, because of the weak production performances at the nursery level. To improve the production of pikeperch juveniles under recirculation system we used a pilot scale larval rearing system (700 L tanks) and multifactorial designs. Three successive larval rearing trials (duration: 35-49 days) were conducted to identify the best combination of environmental, feeding and population factors. Considering the main significant effects observed on survival, growth, swim bladder inflation rates and biomass gain, a favourable combination of twelve factor (F) modalities was proposed (F1- initial density: 100 larvae.L-1, F2- no sorting of fish jumper, F3- no sibling population, F4- eggs from large females, F5- discontinuous feeding, F6- no co-feeding, F7- light intensity: 50 lx, F8- beginning of the weaning at 16 dph (days post-hatching), F9- weaning duration: 9 days, F10- water renewal rate of 100 % per hour, F11- tank cleaning during morning and F12- tank bottom-up water current). A final validation step was realized over a last trial (seven replicates, duration: 49 days), and validated with the best productive results obtained over the global experimental period (2015-2018). These results were: a final body weight of 815.64 ± 95.34 mg, a survival rate of 16.9 ± 1.7 %, a specific growth rate of 15.1 ± 5.9 %.d-1, a final fish biomass of 9.55 ± 0.23 kg, a swim bladder inflation rate of 92.6 ± 3.2 % and a food conversion rate of 0.65 ± 0.02 (dry food). The final stocking density was 13.6 kg.m-3 of rearing volume. Authors were able to validate and provide a reliable basic protocol for pikeperch larval rearing using recirculating units.info:eu-repo/semantics/publishedVersio
- …