7,576 research outputs found
On the formation of hot DQ white dwarfs
We present the first full evolutionary calculations aimed at exploring the
origin of hot DQ white dwarfs. These calculations consistently cover the whole
evolution from the born-again stage to the white dwarf cooling track. Our
calculations provide strong support to the diffusive/convective-mixing picture
for the formation of hot DQs. We find that the hot DQ stage is a short-lived
stage and that the range of effective temperatures where hot DQ stars are found
can be accounted for by different masses of residual helium and/or different
initial stellar masses. In the frame of this scenario, a correlation between
the effective temperature and the surface carbon abundance in DQs should be
expected, with the largest carbon abundances expected in the hottest DQs. From
our calculations, we suggest that most of the hot DQs could be the cooler
descendants of some PG1159 stars characterized by He-rich envelopes markedly
smaller than those predicted by the standard theory of stellar evolution. At
least for one hot DQ, the high-gravity white dwarf SDSS J142625.70+575218.4, an
evolutionary link between this star and the massive PG1159 star H1504+65 is
plausible.Comment: 4 pages, 2 figures. To be published in The Astrophysical Journal
Letter
A Comprehensive Spectroscopic Analysis of DB White Dwarfs
We present a detailed analysis of 108 helium-line (DB) white dwarfs based on
model atmosphere fits to high signal-to-noise optical spectroscopy. We derive a
mean mass of 0.67 Mo for our sample, with a dispersion of only 0.09 Mo. White
dwarfs also showing hydrogen lines, the DBA stars, comprise 44% of our sample,
and their mass distribution appears similar to that of DB stars. As in our
previous investigation, we find no evidence for the existence of low-mass (M <
0.5 Mo) DB white dwarfs. We derive a luminosity function based on a subset of
DB white dwarfs identified in the Palomar-Green survey. We show that 20% of all
white dwarfs in the temperature range of interest are DB stars, although the
fraction drops to half this value above Teff ~ 20,000 K. We also show that the
persistence of DB stars with no hydrogen features at low temperatures is
difficult to reconcile with a scenario involving accretion from the
interstellar medium, often invoked to account for the observed hydrogen
abundances in DBA stars. We present evidence for the existence of two different
evolutionary channels that produce DB white dwarfs: the standard model where DA
stars are transformed into DB stars through the convective dilution of a thin
hydrogen layer, and a second channel where DB stars retain a helium-atmosphere
throughout their evolution. We finally demonstrate that the instability strip
of pulsating V777 Her white dwarfs contains no nonvariables, if the hydrogen
content of these stars is properly accounted for.Comment: 74 pages including 30 figures, accepted for publication in the
Astrophysical Journa
Geometrical Considerations for the Design of Liquid-phase Biochemical Sensors Using a Cantilever\u27s Fundamental In-plane Mode
The influence of the beam geometry on the quality factor and resonance frequency of resonant silicon cantilever beams vibrating in their fundamental in-plane flexural mode in water has been investigated. Compared to cantilevers vibrating in their first out-of-plane flexural mode, utilizing the in-plane mode results in reduced damping and reduced mass loading by the surrounding fluid. Quality factors as high as 86 have been measured in water for cantilevers with a 20 μm thick silicon layer. Based on the experimental data, design guidelines are established for beam dimensions that ensure maximal Q-factors and minimal mass loading by the surrounding fluid, thus improving the limit-of-detection of mass-sensitive biochemical sensors. Elementary theory is also presented to help explain the observed trends. Additional discussion focuses on the tradeoffs that exist in designing liquid-phase biochemical sensors using in-plane cantilevers
Thermal Excitation and Piezoresistive Detection of Cantilever In-Plane Resonance Modes for Sensing Applications
Thermally excited and piezoresistively detected bulk-micromachined cantilevers vibrating in their in-plane flexural resonance mode are presented. By shearing the surrounding fluid rather than exerting normal stress on it, the in-plane mode cantilevers exhibit reduced added fluid mass effects and improved quality factors in a fluid environment. In this letter, different cantilever geometries with in-plane resonance frequencies from 50 kHz to 2.2 MHz have been tested, with quality factors as high as 4200 in air and 67 in water
Pulsation in carbon-atmosphere white dwarfs: A new chapter in white dwarf asteroseismology
We present some of the results of a survey aimed at exploring the
asteroseismological potential of the newly-discovered carbon-atmosphere white
dwarfs. We show that, in certains regions of parameter space, carbon-atmosphere
white dwarfs may drive low-order gravity modes. We demonstrate that our
theoretical results are consistent with the recent exciting discovery of
luminosity variations in SDSS J1426+5752 and some null results obtained by a
team of scientists at McDonald Observatory. We also present follow-up
photometric observations carried out by ourselves at the Mount Bigelow 1.6-m
telescope using the new Mont4K camera. The results of follow-up spectroscopic
observations at the MMT are also briefly reported, including the surprising
discovery that SDSS J1426+5752 is not only a pulsating star but that it is also
a magnetic white dwarf with a surface field near 1.2 MG. The discovery of
-mode pulsations in SDSS J1426+5752 is quite significant in itself as it
opens a fourth asteroseismological "window", after the GW Vir, V777 Her, and ZZ
Ceti families, through which one may study white dwarfs.Comment: 7 pages, 4 figures, to appear in Journal of Physics Conference
Proceedings for the 16th European White Dwarf Worksho
Quantum sensitivity limit of a Sagnac hybrid interferometer based on slow-light propagation in ultra-cold gases
The light--matter-wave Sagnac interferometer based on ultra-slow light
proposed recently in (Phys. Rev. Lett. 92, 253201 (2004)) is analyzed in
detail. In particular the effect of confining potentials is examined and it is
shown that the ultra-slow light attains a rotational phase shift equivalent to
that of a matter wave, if and only if the coherence transfer from light to
atoms associated with slow light is associated with a momentum transfer and if
an ultra-cold gas in a ring trap is used. The quantum sensitivity limit of the
Sagnac interferometer is determined and the minimum detectable rotation rate
calculated. It is shown that the slow-light interferometer allows for a
significantly higher signal-to-noise ratio as possible in current matter-wave
gyroscopes.Comment: 12 pages, 6 figure
An Analytical Model of a Thermally Excited Microcantilever Vibrating Laterally in a Viscous Fluid
To achieve higher quality factors (Q) for microcantilevers used in liquid-phase sensing applications, recent studies have explored the use of the lateral (in-plane) flexural mode. In particular, we have recently shown that this mode may be excited electrothermally using integrated heating resistors near the micro cantilever support, and that the resulting increase in Q helps to make low-ppb limits of detection a possibility in liquids. However, because the use of electrothermally excited, liquid-phase, microcantilever-based sensors in lateral flexure is relatively new, theoretical models are lacking. Therefore, we present here a new analytical model for predicting the vibratory response of these devices. The model is also used to successfully confirm the validity of our previously derived Q formula, which was based on a single-degree-of-freedom (SDOF) model and a harmonic tip force. Comparisons with experimental data show that the present model and, thus, the analytical formula provide excellent Q estimates for sufficiently thin beams vibrating laterally in water and reasonable upper-bound estimates for thicker beams
SDSS J142625.71+575218.3: A Prototype for A New Class of Variable White Dwarf
We present the results of a search for pulsations in six of the recently discovered carbon-atmosphere white dwarf ("hot DQ") stars. On the basis of our theoretical calculations, the star SDSS J142625.71 + 575218.3 is the only object expected to pulsate. We observe this star to be variable, with significant power at 417.7 s and 208.8 s ( first harmonic), making it a strong candidate as the first member of a new class of pulsating white dwarf stars, the DQVs. Its folded pulse shape, however, is quite different from that of other white dwarf variables and shows similarities with that of the cataclysmic variable AM CVn, raising the possibility that this star may be a carbon-transferring analog of AM CVn stars. In either case, these observations represent the discovery of a new and exciting class of object.NSF AST-0507639, AST-0602288, AST-0607480, AST-0307321Astronom
Timoshenko Beam Model for Lateral Vibration of Liquid-Phase Microcantilever-Based Sensors
Dynamic-mode microcantilever-based devices are potentially well suited to biological and chemical sensing applications. However, when these applications involve liquid-phase detection, fluid-induced dissipative forces can significantly impair device performance. Recent experimental and analytical research has shown that higher in-fluid quality factors (Q) are achieved by exciting microcantilevers in the lateral flexural mode. However, experimental results show that, for microcantilevers having larger width-to-length ratios, the behaviors predicted by current analytical models differ from measurements. To more accurately model microcantilever resonant behavior in viscous fluids and to improve understanding of lateral-mode sensor performance, a new analytical model is developed, incorporating both viscous fluid effects and “Timoshenko beam” effects (shear deformation and rotatory inertia). Beam response is examined for two harmonic load types that simulate current actuation methods: tip force and support rotation. Results are expressed in terms of total beam displacement and beam displacement due solely to bending deformation, which correspond to current detection methods used with microcantilever-based devices (optical and piezoresistive detection, respectively). The influences of the shear, rotatory inertia, and fluid parameters, as well as the load/detection scheme, are investigated. Results indicate that load/detection type can impact the measured resonant characteristics and, thus, sensor performance, especially at larger values of fluid resistance
- …