8,642 research outputs found
Towards an Empirical Determination of the ZZ Ceti Instability Strip
We present atmospheric parameters for a large sample of DA white dwarfs that
are known to be photometrically constant. For each star, we determine the
effective temperature and surface gravity by comparing high signal-to-noise
ratio optical spectra to the predictions of detailed model atmosphere
calculations. We also report the successful prediction and detection of
photometric variability in G232-38 based on similar Teff and log g
determinations. The atmospheric parameters derived for this sample of constant
stars as well as those for the known sample of bright ZZ Ceti stars (now
boosted to a total of 39) have been obtained in a highly homogeneous way. We
combine them to study the empirical red and blue edges as well as the purity of
the ZZ Ceti instability strip. We find that the red edge is rather well
constrained whereas there exists a rather large range of possibilities for the
slope of the blue edge. Furthermore, the ZZ Ceti instability strip that results
from our analysis contains no nonvariable white dwarfs. Our sample of constant
stars is part of a much broader spectroscopic survey of bright (V < 17) DA
white dwarfs, which we have recently undertaken. We also present here some
preliminary results of this survey. Finally, we revisit the analysis by Mukadam
et al. of the variable and nonvariable DA stars uncovered as part of the Sloan
Digital Sky Survey. Their erroneous conclusion of an instability strip
containing several nonvariable stars is traced back to the low signal-to-noise
ratio spectroscopic observations used in that survey.Comment: 43 pages, 2 tables, 14 figures, accepted for publication in the
Astrophysical Journa
Spectral analysis and abundances of the post-HB star HD 76431
HD76431 is a slow rotating post-HB star that shows an underabundance of
helium by 0.5 dex relative to the solar value. These observational facts
suggest that atomic diffusion could be active in its atmosphere. We have used
the MMT and Bok spectra to estimate the atmospheric parameters of the target
star using the model atmospheres and synthetic spectra calculated with TLUSTY
and SYNSPEC. The derived values of the effective temperature, surface gravity,
helium abundance are consistent with those obtained by Ramspeck et al. (2001b).
It appears that NLTE effect are not important for HD76431. We have used Stokes
I spectra from ESPaDOnS at CFHT to perform an abundance analysis and a search
for observational evidence of vertical stratification of the abundance of
certain elements. The results of our abundance analysis are in good agreement
with previously published data with respect to average abundances. Our
numerical simulations show that carbon and nitrogen reveal signatures of
vertical abundance stratification in the atmosphere of HD76431. It appears that
the carbon abundance increases toward the deeper atmospheric layers. Nitrogen
also shows a similar behaviour, but in deeper atmospheric layers we obtain a
significant dispersion for the estimates of its abundance. To our knowledge,
this is the first demonstration of vertical abundance stratification of metals
in a post-HB star and up to now it is the hottest star to show such
stratification features. We also report the detection of two SiIII and one
TiIII emission lines in the spectra of HD76431 that were not detected in
previous studies.Comment: 7 pages, 5 figures, accepted for publication in MNRA
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
Signatures of electron-boson coupling in half-metallic ferromagnet MnGe: study of electron self-energy obtained from infrared spectroscopy
We report results of our infrared and optical spectroscopy study of a
half-metallic ferromagnet MnGe. This compound is currently being
investigated as a potential injector of spin polarized currents into germanium.
Infrared measurements have been performed over a broad frequency (50 - 50000
cm) and temperature (10 - 300 K) range. From the complex optical
conductivity we extract the electron self-energy
. The calculation of is based on novel
numerical algorithms for solution of systems of non-linear equations. The
obtained self-energy provides a new insight into electron correlations in
MnGe. In particular, it reveals that charge carriers may be coupled to
bosonic modes, possibly of magnetic origin
Diluted Random Fields in Mixed Cyanide Crystals
A percolation argument and a dilute compressible random field Ising model are
used to present a simple model for mixed cyanide crystals. The model reproduces
quantitatively several features of the phase diagrams altough some crude
approximations are made. In particular critical thresholds x_c at which
ferroelastic first order transitions disappear, are calculated. Moreover,
transitions are found to remain first order down to x_c for all mixtures except
for bromine, for which the transition becomes continuous. All the results are
in full agreement with experimental data.Comment: 8 pages, late
Detailed Spectroscopic and Photometric Analysis of DQ White Dwarfs
We present an analysis of spectroscopic and photometric data for cool DQ
white dwarfs based on improved model atmosphere calculations. In particular, we
revise the atmospheric parameters of the trigonometric parallax sample of
Bergeron et al.(2001), and discuss the astrophysical implications on the
temperature scale and mean mass, as well as the chemical evolution of these
stars. We also analyze 40 new DQ stars discovered in the first data release of
the Sloan Digital Sky Survey.Comment: 6 pages,3 figures, 14th European Workshop on White Dwarfs, ASP
Conference Series, in pres
Hot DAVs : a probable new class of pulsating white dwarf stars
We have discovered a pulsating DA white dwarf at the lower end of the temperature range 45 000–30 000 K where a few helium atmosphere white dwarfs are known. There are now three such pulsators known, suggesting that a new class of theoretically predicted pulsating white dwarf stars exists. We name them the hot DAV stars. From high-speed photometric observations with the ULTRACAM photometer on the 4.2-m William Herschel Telescope, we show that the hydrogen atmosphere white dwarf star WD1017−138 pulsates in at least one mode with a frequency of 1.62 mHz (a period of 624 s). The amplitude of that mode was near 1 mmag at a 10σ confidence level on one night of observation and an 8.4σ confidence level on a second night. The combined data have a confidence level of 11.8σ. This supports the two other detections of hot DAV stars previously reported. From three Very Large Telescope Ultraviolet and Visual Echelle Spectrograph spectra we confirm also that WD1017−138 is a hydrogen atmosphere white dwarf with no trace of helium or metals with Teff = 32 600 K, log g = 7.8 (cgs) and M = 0.55 M⊙. The existence of pulsations in these DA white dwarfs at the cool edge of the 45 000–30 000 K temperature range supports the thin hydrogen layer model for the deficit of helium atmosphere white dwarfs in this range. DA white dwarfs with thick hydrogen layers do not have the superadiabatic, chemically inhomogeneous (μ-gradient) zone that drives pulsation in this temperature range. The potential for higher amplitude hot DAV stars exists; their discovery would open the possibility of a direct test of the explanation for the deficit of helium atmosphere white dwarfs at these temperatures by asteroseismic probing of the atmospheric layers of the hot DAV stars. A search for pulsation in a further 22 candidates with ULTRACAM on the European Southern Observatory New Technology Telescope gave null results for pulsation at precisions in the range 0.5–3 mmag, suggesting that the pulsation amplitudes in such stars are relatively low, hence near the detection limit with the ground-based telescopes used in the surve
An Orthogonal Modular Approach to Macromonomers Using Clickable Cyclobutenyl Derivatives and RAFT Polymerization
A series of cyclobutene-based macromonomers
derived from monomethyl ether poly(ethylene oxide) (PEO),
poly(ethyl acrylate) (PEA), poly(N-isopropylacrylamide)
(PNIPAM), and PEO-b-PNIPAM were synthesized by
“click” copper-catalyzed azide−alkyne cycloaddition
(CuAAC) and reversible addition−fragmentation chain transfer
(RAFT) polymerization. First, original di- and trifunctional
cyclobutene precursors with azido, alkyne and/or chain
transfer agent were successfully obtained and fully characterized.
Azido- and alkyne-functionalized cyclobutenes were then conjugated with modified PEO bearing azido or alkyne groups,
resulting in cyclobutene-based PEOs in quantitative conversions as ascertained by NMR spectroscopy and MALDI−TOF mass
spectrometry. The new chain transfer agent-terminated cyclobutene was used to mediate the RAFT polymerization of ethyl
acrylate and N-isopropylacrylamide. Well-defined polymers with controlled molecular weights (Mn = 3700−11 500 g·mol−1) and
narrow molecular weight distributions (PDI = 1.06−1.14) were thus obtained that retain the cyclobutene functionality,
demonstrating the orthogonality of the RAFT process toward the cyclobutenyl insaturation. Combination of CuACC and RAFT
polymerization was used to afford PEO-b-PNIPAM block copolymer functionalized by a cyclobutene end-group
Observations and asteroseismological analysis of the rapid subdwarf B pulsator EC 09582-1137
We made photometric and spectroscopic observations of the rapidly pulsating
subdwarf B star EC 09582-1137 with the aim of determining the target's
fundamental structural parameters from asteroseismology. The new data comprise
~ 30 hours of fast time-series photometry obtained with SUSI2 at the NTT on La
Silla, Chile, as well as 1 hour of low-resolution spectroscopy gathered with
EMMI, also mounted on the NTT. From the photometry we detected 5 independent
harmonic oscillations in the 135-170 s period range with amplitudes up to 0.5%
of the mean brightness of the star. In addition, we extracted two periodicities
interpreted as components of a rotationally split multiplet that indicate a
rotation period of the order of 2-5 days. We also recovered the first harmonic
of the dominant pulsation, albeit at an amplitude below the imposed 4-sigma
detection threshold. The spectroscopic observations led to the following
estimates of the atmospheric parameters of EC 09582-1137: Teff = 34,806+-233 K,
log g = 5.80+-0.04, and log[N(He)/N(H)] = - 1.68+-0.06.
Using the observed oscillations as input, we searched in model parameter
space for unique solutions that present a good fit to the data. Under the
assumption that the two dominant observed periodicities correspond to radial or
dipole modes, we were able to isolate a well-constrained optimal model that
agrees with the atmospheric parameters derived from spectroscopy. The inferred
structural parameters of EC 09582-1137 are Teff = 34,806 K (from spectroscopy),
log g = 5.788+-0.004, M = 0.485+-0.011 M_solar, log(M_env/M_star) = -
4.39+-0.10, R = 0.147+-0.002 R_solar, and L = 28.6+-1.7 L_solar. We
additionally derive the absolute magnitude M_V = 4.44+-0.05 and the distance d
= 1460+-66 pc.Comment: accepted for publication in Astronomy and Astrophysic
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