Hydrogen-rich Material Research Articles

The formation of a helium white dwarf in a close binary system with diffusion

We study the evolution of a system composed of a 1.4-M⊙ neutron star and a normal, solar composition star of 2 M⊙ in orbit with a period of 1 d. Calculations were performed employing the binary hydro code presented by Benvenuto & De Vito that handle the mass transfer rate in a fully implicit way. We then included the main standard physical ingredients together with the diffusion processes and a proper outer boundary condition. We have assumed fully non-conservative mass transfer episodes. In order to study the interplay of mass loss episodes and diffusion we considered evolutionary sequences with and without diffusion in which all Roche lobe overflows (RLOFs) produce mass transfer. Another two sequences in which thermonuclearly driven RLOFs were not allowed to drive mass transfer have been computed with and without diffusion. As far as we are aware, this study represents the first binary evolution calculations in which diffusion is considered. The system produces a helium white dwarf of ∼0.21 M⊙ in an orbit with a period of ∼4.3 d for the four cases. We find that mass transfer episodes induced by hydrogen thermonuclear flashes drive a tiny amount of mass transfer. As diffusion produces stronger flashes, the amount of hydrogen-rich matter transferred is slightly higher than in the models without diffusion. We find that diffusion is the main agent in determining the evolutionary time-scale of low-mass white dwarfs even in the presence of mass transfer episodes.

Direct hydrogen production from alcohol using pulse-electron emission in an unsymmetrical electric field

We report a means of instantaneously producing hydrogen directly from alcohol using pulse-electron emission in an unsymmetrical electric field. We selected 1-butanol as a hydrogen-rich material for producing hydrogen. A 1-butanol molecule has more than twice as many hydrogen atoms as the methanol molecule and is a good candidate for a hydrogen source. The direct electron emission on the surface of volatile 1-butanol prevented intense discharge and produced hydrogen at room temperature in air.

On the Hydrogen Emission from the Type Ia Supernova SN 2002ic

The discovery of SN 2002ic by the Supernova Factory and the subsequent spectroscopic studies have led to the surprising finding that SN 2002ic is a type Ia supernova with strong ejecta-circumstellar interaction. Here we show that nearly 1 year after the explosion the supernova has become fainter overall, but the H-alpha emission has brightened and broadened dramatically compared to earlier observations. We have obtained spectropolarimetry data which show that the hydrogen-rich matter is highly aspherically distributed. These observations suggest that the supernova exploded inside a dense, clumpy, disk-like circumstellar environment.

RX J0513.9-6951 and V Sagittae: Accretion Wind Evolution, A Key Evolutionary Process to Type IA Supernovae

AbstractA new self-sustained model for long-term light-curve variations of the supersoft X-ray sources RX J0513.9–6951 and V Sge is proposed based on an optically thick wind model of mass-accreting white dwarfs (WDs). When the mass accretion rale to a WD exceeds the critical rate of ~ 1 x 10–1M⊙. yr–1, optically thick strong winds begin to blow from the WD so that a formation of common envelope is avoided. The WD can accrete and burn hydrogen-rich matter on the surface of the WD at the critical rate. The excess matter transferred to the WD above the critical rate is expelled by winds. This is called “accretion wind evolution.” This ejection process, however, occurs intermittently because the mass transfer is attenuated by strong winds: the strong winds collide with the secondary surface and strip oil the surface layer of the secondary. Properly formulating the mass-stripping effect and the ensuing decay of mass-transfer rate, we are able to reproduce the transition between the optical high/X-ray off and optical low/X-ray -on states of the supersoft X-ray sources RX J0513.9–6951 and V Sge. Thus these two objects are the lirst examples of accretion wind evolution, which is a key evolutionary process in a recently developed evolutionary path to Type la supernovae.

The central star of the planetary nebula N 66 in the Large Magellanic Cloud: A detailed analysis of its dramatic evolution 1983–2000

The central star of the planetary nebula N 66 (alias WS 35, SMP 83 and HV 5967) in the Large Magellanic Cloud enhanced its brightness dramatically in 1993 and 1994. Within the subsequent four years it returned to the previous level. Its spectrum resembles that of a Wolf-Rayet star of the nitrogen sequence (WN4.5). We monitored the object intensively from ground and with the Hubble Space Telescope. Now we present the complete set of spectroscopic observations from the different epochs before, during and after the brightness outburst of N66. The stellar spectra from the different epochs are analyzed in detail by means of most advanced non-LTE models for expanding stellar atmospheres. The main results are: the luminosity, log.L/L O . = 4.6, before and after the outburst is exceptionally high for a central star of a planetary nebula. During the outburst in 1994, it even climbed up to log L/L O . = 5.4 for about one year. The effective temperature of about 112 kK remained roughly constant, i.e. the luminosity mainly increased because of a larger effective stellar radius. The mass loss rate increased from 10 - 5 . 7 M O . yr - 1 in the quiet state to 10 - 5 . 0 M O . yr - 1 during the outburst. The chemical composition of the stellar atmosphere is that of incompletely CNO-processed matter: it is dominated by helium with a rest of hydrogen, nitrogen being slightly enhanced and carbon strongly depleted. We extensively discuss possible scenarios for the nature and evolutionary origin of N 66, which should explain the exceptional stellar parameters, the atmospheric composition, the outburst mechanism, and the existence of the bipolar nebula which was ejected only a few thousand years ago and contains about 0.6 solar masses of hydrogen-rich matter. If being a single star, N 66 might be (i) a low-mass star after the Asymptotic Giant Branch, as usually adopted for central stars of planetary nebulae, (ii) a massive, i.e. non-degenerate star, or (iii) a merger produced from two white dwarfs. Although there are no direct indications for binarity, we alternatively discuss whether N66 might be (iv) a massive star which lost its hydrogen envelope in a recent common-envelope phase with a less massive companion, or (v) a white dwarf accreting mass from a companion with a high rate. None of the scenarios is free of any contradiction to at least one of the observational facts. However, the binary scenarios pose less severe problems. If N 66 is a white dwarf accreting matter in a close-binary system, its present accretion rate would bring it to the Chandrasekhar limit within a few hundred thousand years. Thus N 66 might be a candidate for a future type la supernova explosion in our cosmic neighborhood.

RX J0513.9−6951: The First Example of Accretion Wind Evolution, a Key Evolutionary Process to Type Ia Supernovae

A new self-sustained model for long-term light-curve variations of RX J0513.9-6951 is proposed based on an optically thick wind model of mass-accreting white dwarfs (WDs). When the mass rate to a WD exceeds the critical rate of ~1 × 10-6 M☉ yr-1, optically thick strong winds begin to blow from the WD so that a formation of common envelope is avoided. The WD can accrete and burn hydrogen-rich matter on the surface of the WD at the critical rate. The excess matter transferred to the WD above the critical rate is expelled by winds. This is called accretion wind evolution. This ejection process, however, occurs intermittently because the mass transfer is attenuated by strong winds: the strong winds collide with the secondary surface and strip off the surface layer of the secondary. The stripped-off matter is lost from the binary system. Properly formulating this mass-stripping effect and the ensuing decay of mass-transfer rate, we are able to reproduce, in a self-sustained manner, the transition between the optical high/X-ray-off and optical low/X-ray-on states of RX J0513.9-6951. Thus RX J0513.9-6951 is the first example of wind evolution, which is a key evolutionary process in a recently developed evolutionary path to Type Ia supernovae.

Chemical–structural characterization of solvent and thermal extractable material from perhydrous vitrinites

In this work, the characterization of the non-covalently bonded compounds present in a set of perhydrous coals of different age and geographical location (Cretaceous coals: UCV and TCV and Jurassic coals: AJV, PGJV, WJVh and WJVl) was carried out by means of a combination of the analyses of the material soluble in chloroform and the thermal extract. The extract in chloroform was studied through GC/MS and NMR and the thermovaporized fraction was obtained by means of flash pyrolysis at the Curie temperature of 350 °C and quantified by on-line GC-MS. The results obtained for the Cretaceous coals confirm that the substances responsible for the hydrogenation are those covalently bonded to the vitrinite network as a result of the modifications undergone by the botanical precursors. Despite the striking similarity in the global characteristics of these two coals (TCV and UCV) significant differences between the material non-covalently bonded to their coal matrices were found. These differences are attributed to the type of resins present in the coals and/or to their different degree of evolution. With respect to the Jurassic coals, the present study allows the process by which the hydrogenated substances were assimilated into their structure to be established. The characterization of the assimilated substances in the WJVl coal reveals an unexpectedly high incorporation of alkanes given the humic origin of this sample. From these results the assimilation of hydrogen-rich substances from the decomposition of the organic remains in the sedimentary environment in which WJVl precursor were deposited, is proposed. The incorporation of products derived from the primary decomposition of organic material is not evident for the WJVh and AJV coals. The substances assimilated into the coal matrices show a higher aromaticity, the aromatic structures of WJVh being more condensed than those found in AJV. The compositional differences between them probably arise from the different source of the hydrogen-rich material. In the case of AJV coal the source was the adsorption of hydrocarbons generated and migrated from the Pliensbachian source-rocks whereas in WJVh, assimilated compounds come from the material generated by the thermal transformation (coalification) of the adjacent organic rocks of similar age. Finally, PGJV shows two types of non-covalently bonded compounds. Solvent extractable material is mainly composed of the first type of compounds, which is predominantly aliphatic in nature with a preponderance of alkanes. The second type of compounds is more aromatic and they are probably located in the close porosity of this coal. They are not accessible to chloroform but they are the most abundant in the thermal extract.

Past and Future Evolution of Sakurai's Object

We present a stellar evolution model sequence of the past and future evolution of the post-AGB born again star Sakurai's object (V 4334 Sgr). In order to match the short evolutionary time scale we have to assume that the convective ingestion of hydrogen-rich envelope material into the He-flash convection zone proceeds slower than predicted by the mixing length theory. For the future we predict a swift second evolution through the central star region before a second born-again evolution occurs.

GEOCHEMICAL CHARACTERIZATION OF CENOMANIAN/TURONIAN BLACK SHALES FROM THE TARFAYA BASIN (SW MOROCCO)

Organic geochemical and petrological investigations were carried out on Cenomanian/Turonian black shales from three sample sites in the Tarfaya Basin (SW Morocco) to characterize the sedimentary organic matter. These black shales have a variable bulk and molecular geochemical composition reflecting changes in the quantity and quality of the organic matter. High TOC contents (up to 18wt%) and hydrogen indices between 400 and 800 (mgHC/gTOC) indicate hydrogen‐rich organic matter (Type I‐II kerogen) which qualifies these laminated black shale sequences as excellent oil‐prone source rocks. Low Tmax values obtained from Rock‐Eval pyrolysis (404–425 MC) confirm an immature to early mature level of thermal maturation.Organic petrological studies indicate that the kerogen is almost entirely composed of bituminite particles. These unstructured organic aggregates were most probably formed by intensive restructuring of labile biopolymers (lipids and/or carbohydrates), with the incorporation of sulphur into the kerogen during early diagenesis. Total lipid analyses performed after desulphurization of the total extract shows that the biomarkers mostly comprise short‐chain n‐alkanes (C16–C22) and long‐chain (C25–C35) n‐alkanes with no obvious odd‐over‐even predominance, together with steranes, hopanoids and acyclic isoprenoids. The presence of isorenieratane derivatives originating from green sulphur bacteria indicates that dissolved sulphide had reached the photic zone at shallow water depths (∼100m) during times of deposition. These conditions probably favoured intensive sulphurization of the organic matter. Flash pyrolysis GC‐MS analysis of the kerogen indicates the aliphatic nature of the bulk organic carbon. The vast majority of pyrolysis products are sulphur‐containing components such as alkylthiophenes, alkenylthiophenes and alkybenzothiophenes. Abundant sulphurization of the Tarfaya Basin kerogen resulted from excess sulphide and metabolizable organic matter combined with a limited availability of iron during early diagenesis. The observed variability in the intensity of OM sulphurization may be attributed to sea level‐driven fluctuations in the palaeoenvironment during sedimentation.

Rebirth of novae as distance indicators due to efficient, large telescopes.

A nova is a close binary system, where one component is a white dwarf. A nova exhibits a sudden and rapid increase in its brightness because of thermonuclear reactions on the surface of the white dwarf that is accreting hydrogen-rich material from its smaller mass companion star. These explosions

Spectral analysis of the sdO K 648, the exciting star of the planetary nebula Ps 1 in the globular cluster M 15 (NGC 7078)

We present a spectral analysis of the sdO central star K 648 based on medium-resolution optical and high-resolution UV spectra. The photospheric parameters are determined by means of state-of-the-art NLTE model atmosphere techniques. We found and . The helium () and oxygen () abundances are about solar while carbon is enriched by a factor of 2.5 (). Nitrogen (, ) appears at a sub-solar value. However, these metal abundances are much higher than the cluster's metallicity (M 15: ). The surface composition appears to be a mixture of the original hydrogen-rich material and products of helium burning ( process) which have been mixed up to the surface. The abundances of He, C, and N are consistent with the nebular abundance, while O is considerably more abundant in the photosphere than in the nebula. From a comparison of its position in the – plane with evolutionary calculations a mass of and a luminosity of are deduced. Our spectroscopic distance is in agreement with the distance of M 15 as determined by Alves et al. (2000). From the GHRS spectra we measure a radial velocity of .

Characterisation of sedimentary organic matter by bulk and molecular geochemical proxies: an example from Oligocene maar-type Lake Enspel, Germany

Abstract Characterisation of organic matter composition of fossil lake sediments is important in reconstruction of fossil ecosystems and paleoclimates. High time-resolved organic matter analysis is needed to characterize sediments from small lake environments, due to their rapidly changing environmental conditions. A suitable technique for rapid analysis of organic matter in such sediments is Rock Eval pyrolysis. In small lakes, however, routine interpretation of Rock Eval data can be misleading. Lack of tributaries capable of transporting larger woody fragments into small lakes causes a bias towards the input of lipid-rich leaf waxes and cuticles. This hydrogen-rich and oxygen-depleted terrigenous organic matter (HI=250–600 [mg HC/g TOC] and OI=20–40 [mg CO 2 /g TOC]) can easily be misinterpreted as being of algal origin. Differentiation of terrigenous plant-derived hydrogen-rich organic matter from algal organic matter is readily achieved by including C/N-elemental ratios and lipid biomarker data.

Impact and perspectives of radioactive beam experiments for the rp-process

Nuclear Astrophysics is concerned with the study of nuclear processes at stellar temperature and density conditions and its influence on nucleosynthesis and energy generation in stars and stellar explosions. Of particular interest is the understanding and interpretation of hot and explosive stellar scenarios and the concomitant nucleosynthesis involving nuclear processes far off β-stability. Neutron induced explosive processes occur in the neutrino-wind driven shock front of supernova explosions or can be triggered by the merging of two neutron stars. Hydrogen induced explosive processes occur in the thermonuclear runaway on the surface of an accreting white dwarf (novae) and in the hydrogen rich electron degenerate accreted material in the atmosphere of neutron stars (X-ray bursts, X-ray pulsars). The associated nuclear reaction and decay processes are far away from β-stability and need to be measured by using radioactive ion beams or radioactive targets. Limitations for these experiments are low beam intensities and high background radiation. Several radioactive beam facilities are already in operation and more are expected based on improved technological developments allowing reliable studies of reaction processes far off stability. We will discuss recent results of radioactive beam experiments and their implications for explosive stellar hydrogen burning in novae, X-ray bursts, and X-ray pulsars. We will also identify specific needs for nuclear reaction and decay data of specific importance for explosive hydrogen burning for future radioactive ion beam experiments.

Low temperature growth of silicon nitride by electron cyclotron resonance plasma enhanced chemical vapour deposition

Silicon nitride (a-SiN) is extensively used as the gate insulator layer in thin films transistors (TFTs) for active matrix liquid crystal displays. Such displays are currently produced on thin, glass substrates; plastic substrates offer a cheaper and more robust alternative, but have maximum process temperatures below 150°C. The deposition of a-SiN by rf plasma enhanced chemical vapour deposition (PECVD) requires the production of SiNxHy radicals in the plasma. However, as N2 is very stable, a gas mixture of SiH4 with NH3 is required for this to take place, which yields a hydrogen-rich material. High substrate temperatures (∼300°C) are therefore required to drive out the excess hydrogen. In electron cyclotron resonance (ECR)-PECVD, a highly ionised plasma (∼1016 m−3) with very low ion energies (∼10 eV) can be produced. This allows the dissociation of N2 into atomic N, and hence a gas phase reaction can take place directly with SiH4, which results in a material with a lower hydrogen content than is obtained with NH3 gas. Energy for further H removal is provided by the bombardment of He ions, allowing a reduction in the deposition temperature to 80°C. In this way a-SiN has been produced with a resistivity of 3×1014 Ω cm and an average breakdown strength of 4.5 MV cm−1.

Hydrogen‐Accreting Carbon‐Oxygen White Dwarfs of Low Mass: Thermal and Chemical Behavior of Burning Shells

Numerical experiments have been performed to investigate the thermal behavior of a cooled-down white dwarf of initial mass MWD = 0.516 M☉ that accretes hydrogen-rich matter with Z = 0.02 at the rate = 10-8 M☉ yr-1, typical for a recurrent hydrogen shell flash regime. The evolution of the main physical quantities of a model during a pulse cycle is examined in detail. From selected models in the mass range MWD = 0.52-0.68 M☉, we derive the borders in the MWD- plane of the steady state accretion regime when hydrogen is burned at a constant rate as rapidly as it is accreted. The physical properties during a hydrogen shell flash in white dwarfs accreting hydrogen-rich matter with metallicities Z = 0.001 and Z = 0.0001 are also studied. For a fixed accretion rate, a decrease in the metallicity of the accreted matter leads to an increase in the thickness of the hydrogen-rich layer at outburst and a decrease in the hydrogen-burning shell efficiency. In the MWD- plane, the borders of the steady state accretion band are critically dependent on the metallicity of the accreted matter: on decreasing the metallicity, the band is shifted to lower accretion rates and its width in is reduced.

On the Very Long Term Evolutionary Behavior of Hydrogen-accreting Low-Mass CO White Dwarfs

Hydrogen-rich matter has been added to a CO white dwarf of initial mass 0.516 \msun at the rates $10^{-8}$ and $2\times 10^{-8}$ \msun \yrm1, and results are compared with those for a white dwarf of the same initial mass which accretes pure helium at the same rates. For the chosen accretion rates, hydrogen burns in a series of recurrent mild flashes and the ashes of hydrogen burning build up a helium layer at the base of which a He flash eventually occurs. In previous studies involving accretion at higher rates and including initially more massive WDs, the diffusion of energy inward from the H shell-flashing region contributes to the increase in the temperature at the base of the helium layer, and the mass of the helium layer when the He flash begins is significantly smaller than in a comparison model accreting pure helium; the He shell flash is not strong enough to develop into a supernova explosion. In contrast, for the conditions adopted here, the temperature at the base of the He layer becomes gradually independent of the deposition of energy by H shell flashes, and the mass of the He layer when the He flash occurs is a function only of the accretion rate, independent of the hydrogen content of the accreted matter. When the He flash takes place, due to the high degeneracy at the base of the He layer, temperatures in the flashing zone will rise without a corresponding increase in pressure, nuclear burning will continue until nuclear statistical equilibrium is achieved; the model will become a supernova, but not of the classical type Ia variety.

On the Evolution of Stars that Form Electron‐degenerate Cores Processed by Carbon Burning. V. Shell Convection Sustained by Helium Burning, Transient Neon Burning, Dredge‐out, Urca Cooling, and Other Properties of an 11M⊙Population I Model Star

A stellar model of mass 11 M☉ and Population I composition is evolved from the hydrogen-burning main sequence through the core carbon-burning phase. In contrast with 9, 10, and 10.5 M☉ models studied in earlier papers of this series, carbon burning is ignited at the center of the 11 M☉ model. Like the 10.5 M☉ model, the 11 M☉ model experiences a dredge-out episode at the end of the carbon-burning phase. At the beginning of this episode, a semiconvective zone forms at the base of the hydrogen-rich envelope and carries hydrogen inward in mass toward the outer edge of a fully convective zone that is sustained by helium burning at its base. Hydrogen diffuses into the helium-rich convective zone untila hydrogen shell flash occurs. Helium burning dies out and the outer edge of the convective layer,sustained by fluxes due to hydrogen burning, extends outward in mass through hydrogen-rich material, mixing freshly synthesized nuclei outward. Then, hydrogen burning dies out and the outer edge of the convective shell, now sustained primarily by fluxes due to the release of gravothermal energy, moves outward until it reaches the inner edge of the convective envelope. Freshly synthesized material is then convected to the surface. Mixing during the final phase of homogenization in the convective envelope is maintained by fluxes due to the release of gravothermal energy. At the end of the dredge-out phase, the surface nitrogen abundance has decreased and the C/N ratio has changed from less than unity to larger than unity, showing that mixing has extended into regions where helium burning has manufactured substantial quantities of 12C and destroyed 14N. Prior to the dredge-out phase, neon burning is narrowly averted, and, after the dredge-out phase, neutrino losses due to electron capture and decay reactions between A=25 and A=23 isotopes in and above convective Urca shells cool the inner portions of the electron-degenerate oxygen-neon (ONe) core. Ultimately, the model becomes a thermally pulsing super-asymptotic giant branch (TPSAGB) star with an ONe core of mass ~1.368 M☉. Hydrogen and helium burning over a period of ~1.4×104 yr of TPSAGB evolution add a carbon-oxygen layer of mass ~0.014 M☉ to the electron-degenerate core. Then, electron captures on products of carbon burning lead to the collapse of the core into a neutron star and expulsion of the envelope in a weak Type II supernova explosion. The ratio of helium to hydrogen in the ejecta is approximately twice solar.

Recent advances in studies of the nova outburst

The nova outburst is a consequence of the accretion of hydrogen-rich material onto a white dwarf in a close binary system and observations show that the accreting material must have mixed with core material at some phase of the outburst. Abundance determinations of novae ejecta show that the core material is either carbon and oxygen or oxygen, neon, and magnesium. I report on the methods and results of a project to determine the ejecta abundances for a number of recent Galactic and LMC novae using combined optical and ultraviolet data. The abundance determinations also provide a value for amount of mass ejected in the outburst. There are large discrepancies between the observed and predicted values of the ejecta mass in that the observed ejecta masses are much larger than predicted for both carbon–oxygen and oxygen–neon–magnesium novae. I also review the X-ray observations and present new evolutionary calculations done to try to understand the missing ejecta mass problem and suggest a possible solution to this problem.

A New Estimation of Mass Accumulation Efficiency in Helium Shell Flashestoward Type I[CLC]a[/CLC] Supernova Explosions

We have calculated the mass accumulation efficiency during helium shell flashes to examine whether or not a carbon-oxygen white dwarf (C+O WD) grows up to the Chandrasekhar mass limit to ignite a Type Ia supernova (SN Ia) explosion. It has been frequently argued that luminous supersoft X-ray sources (SSSs) and symbiotic stars are progenitors of SNe Ia. In such systems, a C+O WD accretes hydrogen-rich matter from a companion and burns hydrogen steadily on its surface. The WD develops a helium layer underneath the hydrogen-rich envelope and undergoes periodic helium shell flashes. Using the OPAL opacity, we have reanalyzed a full cycle of helium shell flashes on a 1.3 M? C+O WD and confirmed that the helium envelope of the WD expands to blow a strong wind. A part of the accumulated matter is lost by the wind. The mass accumulation efficiency in helium shell flashes is estimated as ?He=-0.175(log+5.35)2+1.05 for -7.3<log < -5.9 and ?He=1 for -5.9 ? log -5, where the mass accretion rate is in units of M? yr-1. In relatively high mass accretion rates, as expected in recent SN Ia progenitor models, the mass accumulation efficiency is large enough for C+O WDs to grow to the Chandrasekhar mass, i.e., ?He=0.9 for log=-6.3 and ?He=0.57 for log=-7.0. The wind velocity (~1000 km s-1) is much faster than the orbital velocity of the binary (300 km s-1), and therefore the wind cannot be accelerated further by the companion's motion. We suggest observational counterparts of helium shell flashes in relation to long-term variations in supersoft X-ray fluxes of SSSs and symbiotic stars.

Gravity Modes in ZZ Ceti Stars. I. Quasi‐adiabatic Analysis of Overstability

We analyze the stability of g-modes in white dwarfs with hydrogen envelopes. All relevant physical processes take place in the outer layer of hydrogen-rich material, which consists of a radiative layer overlaid by a convective envelope. The radiative layer contributes to mode damping, because its opacity decreases upon compression and the amplitude of the Lagrangian pressure perturbation increases outward. The convective envelope is the seat of mode excitation, because it acts as an insulating blanket with respect to the perturbed flux that enters it from below. A crucial point is that the convective motions respond to the instantaneous pulsational state. Driving exceeds damping by as much as a factor of 2 provided ωτ_c≥1, where ω is the radian frequency of the mode and τ_c≈4τ_(th), with τ_(th) being the thermal time constant evaluated at the base of the convective envelope. As a white dwarf cools, its convection zone deepens, and lower frequency modes become overstable. However, the deeper convection zone impedes the passage of flux perturbations from the base of the convection zone to the photosphere. Thus the photometric variation of a mode with constant velocity amplitude decreases. These factors account for the observed trend that longer period modes are found in cooler DA variables. Overstable modes have growth rates of order γ~1/(nτ_ω), where n is the mode's radial order and τ_ω is the thermal timescale evaluated at the top of the mode's cavity. The growth time, γ^(−1), ranges from hours for the longest period observed modes (P≈20 minutes) to thousands of years for those of shortest period (P≈2 minutes). The linear growth time probably sets the timescale for variations of mode amplitude and phase. This is consistent with observations showing that longer period modes are more variable than shorter period ones. Our investigation confirms many results obtained by Brickhill in his pioneering studies of ZZ Cetis. However, it suffers from two serious shortcomings. It is based on the quasiadiabatic approximation that strictly applies only in the limit ωτ_c » 1, and it ignores damping associated with turbulent viscosity in the convection zone. We will remove these shortcomings in future papers.