G-mode Instability Research Articles

Formation of Extremely Low-mass White Dwarf Binaries

Motivated by the discovery of a handful of pulsating, extremely low mass white dwarfs (ELM WDs, mass $M \lesssim 0.17\, M_\odot$) which likely have WD companions, this paper discusses binary formation models for these systems. Formation of an ELM WD by unstable mass transfer (MT) or a common envelope (CE) event is unlikely. Stable Roche-lobe overflow with conservative MT produces only $M \gtrsim 0.2\, M_\odot$. This paper discusses the formation of ELM WD using angular momentum losses by magnetic braking. The initially more massive star becomes the companion WD through a CE event. The initially less massive star becomes the ELM WD as an evolved donor in a Cataclysmic Variable binary. Evolutionary models are constructed using the Modules for Experiments in Stellar Astrophysics (MESA), with ELM WD progenitors in the range $M_d=1.0-1.5\, M_\odot$ and WD companions in the range $M_{\rm a} \simeq 0.4-0.9\, M_\odot$. A prescription to reduce magnetic braking for thin surface convection zones is included. Upon the thinning of the evolved donor's envelope, the donor star shrinks out of contact and MT ceases, revealing the ELM WD. Systems with small masses have previously been suggested as possible AM CVN's. Systems with large masses, up to the limit $M \simeq 0.18\, M_\odot$ at which shell flashes occur on the WD cooling track, will tend to expand out to orbital periods $P_{\rm orb} \gtrsim 15\, {\rm hr}$. In between this range, ELM WD may become pulsators both as pre-WD and on the WD cooling track. Brickhill's criterion for convective mode driving is used to estimate the location of the blue edge of the g-mode instability strip.

LS IV — 14°116 : A Time-Resolved Spectroscopic Study

Abstract LSIV-14 116 is a very unusual subdwarf B star. It pulsates non-radially with high-order g-modes, these pulsations are unexpected and unexplained, as the effective temperature is 6 000K hotter than the blue edge of the hot subdwarf g-mode instability strip. Its spectrum is enriched in helium which is not seen in either the V361 Hya (p-mode pulsators) or the V1093 Her stars (g-mode pulsators). Even more unusual is the 4 dex overabundance of zirconium, yttrium, and strontium. It is proposed that these over-abundances are a result of extreme chemical stratification driven by radiative levitation. We have over 20hrs of VLT/UVES spectroscopy from which we have obtained radial velocity curves for individual absorption lines. We are currently exploring ways in which to resolve the photospheric motion as a function of optical depth.

The blue-edge problem of the V1093 Herculis instability strip revisited using evolutionary models with atomic diffusion

We have computed a new grid of evolutionary subdwarf B star (sdB) models from the start of central He burning, taking into account atomic diffusion due to radiative levitation, gravitational settling, concentration diffusion, and thermal diffusion. We have computed the non-adiabatic pulsation properties of the models and present the predicted p-mode and g-mode instability strips. In previous studies of the sdB instability strips, artificial abundance enhancements of Fe and Ni were introduced in the pulsation driving layers. In our models, the abundance enhancements of Fe and Ni occur naturally, eradicating the need to use artificial enhancements. We find that the abundance increases of Fe and Ni were previously underestimated and show that the instability strip predicted by our simulations solves the so-called blue edge problem of the subdwarf B star g-mode instability strip. The hottest known g-mode pulsator, KIC 10139564, now resides well within the instability strip {even when only modes with low spherical degrees (l<=2) are considered.

Dipole low-order g-mode instability of metal-poor low-mass main-sequence stars due to the mechanism

We analyzed vibrational stability of metal-poor low-mass main-sequence stars due to the $\varepsilon$-mechanism. Since outer convection zones of the metal-poor stars is limited only to the very outer layers, the uncertainty in treatment of convection does not affect the result significantly. We found that the dipole g$_1$- and g$_2$-modes certainly become unstable due to the $\varepsilon$-mechanism for $Z\la 6\times 10^{-4}$. Besides that, we found that as the metallicity decreases the mass range of the $\varepsilon$-mechanism instability extends toward higher mass.

Perturbation analysis of a general polytropic homologously collapsing stellar core

For dynamic background models of Goldreich & Weber and Lou & Cao, we examine three-dimensional perturbation properties of oscillations and instabilities in a general polytropic homologously collapsing stellar core of a relativistically hot medium with a polytropic index γ= 4/3. Perturbation behaviours, especially internal gravity g modes, depend on the variation of specific entropy in the collapsing core. Among possible perturbations, we identify acoustic p modes and surface f modes as well as internal gravity g+ and g− modes. As in stellar oscillations of a static star, we define g+ and g− modes by the sign of the Brunt–Väisälä buoyancy frequency squared for a collapsing stellar core. A new criterion for the onset of instabilities is established for a homologous stellar core collapse. We demonstrate that the global energy criterion of Chandrasekhar is insufficient to warrant the stability of general polytropic equilibria. We confirm the acoustic p-mode stability of Goldreich & Weber, even though their p-mode eigenvalues appear in systematic errors. Unstable modes include g− modes and sufficiently high-order g+ modes, corresponding to core instabilities. Such instabilities occur before the stellar core bounce, in contrast to instabilities in other models of supernova (SN) explosions. The breakdown of spherical symmetry happens earlier than expected in numerical simulations so far. The formation and motion of the central compact object are speculated to be much affected by such g-mode instabilities. By estimates of typical parameters, unstable low-order l= 1 g-modes may produce initial kicks of the central compact object. Other high-order and high-degree unstable g modes may shred the nascent neutron core into pieces without an eventual compact remnant (e.g. SN 1987A). Formation of binary pulsars and planets around neutron stars might originate from unstable l= 2 g-modes and high-order high-degree g modes, respectively.

Impact of helium diffusion and helium-flash-induced carbon production on gravity-mode pulsations in subdwarf B stars

Realistic stellar models are essential to the forward modelling approach in asteroseismology. For practicality however, certain model assumptions are also required. For example, in the case of subdwarf B stars, one usually starts with zero-age horizontal branch structures without following the progenitor evolution. We analyse the effects of common assumptions in subdwarf B models on the g-mode pulsational properties. We investigate if and how the pulsation periods are affected by the H-profile in the core-envelope transition zone. Furthermore, the effects of C-production and convective mixing during the core helium flash are evaluated. Finally, we reanalyse the effects of stellar opacities on the mode excitation in subdwarf B stars. We find that helium settling causes a shift in the theoretical blue edge of the g-mode instability domain to higher effective temperatures. This results in a closer match to the observed instability strip of long-period sdB pulsators, particularly for l<=3 modes. We show further that the g-mode spectrum is extremely sensitive to the H-profile in the core-envelope transition zone. If atomic diffusion is efficient, details of the initial shape of the profile become less important in the course of evolution. Diffusion broadens the chemical gradients, and results in less effective mode trapping and different pulsation periods. Furthermore, we report on the possible consequences of the He-flash for the g-modes. The outer edge of a flash-induced convective region introduces an additional chemical transition in the stellar models, and the corresponding spike in the Brunt-Vaisala frequency produces a complicated mode trapping signature in the period spacings.

The Pulsating White Dwarf Stars

ABSTRACTWe present a summary of what is currently known about the three distinct families of isolated pulsating white dwarfs. These are the GW Vir stars (He/C/O-atmosphere stars with Teff ≃ 120,000 K), the V777 Her stars (He-atmosphere, Teff ≃ 25,000 K), and the ZZ Ceti stars (H-atmosphere, Teff ≃ 12,000 K), all showing multiperiodic luminosity variations caused by low-order and low-degree g-mode instabilities. We also provide, in an , a very brief overview of the newly found evidence in favor of the existence of a fourth category of oscillating white dwarfs bearing strong similarities with these families of pulsators. We begin our survey with a short historical introduction, followed by a general discussion of pulsating white dwarfs as compact pulsators. We then discuss the class properties of these objects, including an updated census. We next focus on the instability domains for each family of pulsators in the log g - Teff diagram, and present their time-averaged properties in more detail. This is followed by a section on excitation physics, i.e., the causes of the pulsational instabilities, with emphasis on the common properties of the different types of pulsator. We then discuss the time-dependent properties of the pulsating white dwarfs featuring, among other things, a brief “picture tour” across the ZZ Ceti instability strip. We next review the methods used to infer or constrain the angular geometry of a pulsation mode in a white dwarf. These include multicolor photometry and time-resolved spectroscopy, the exploitation of the nonlinear features in the observed light curves, and rotational splitting. We also consider basic adiabatic asteroseismology starting with a discussion of the reaction of the period spectrum to variations of model parameters. We next review the various asteroseismological inferences that have so far been claimed for white dwarfs. We also discuss the potential of exploiting the rates of period change. We finally provide some concluding remarks, including a list with several suggestions for future progress in the field.

Fe-bump instability: the excitation of pulsations in subdwarf B and other low-mass stars

We consider the excitation of radial and non-radial oscillations in low-mass B stars by the iron-bump opacity mechanism. The results are significant for the interpretation of pulsations in subdwarf B stars, helium-rich subdwarfs and extreme helium stars, including the EC14026 and PG1716 variables. We demonstrate that, for radial oscillations, the driving mechanism becomes effective by increasing the contrast between the iron-bump opacity and the opacity from other sources. This can be achieved either by increasing the iron abundance or by decreasing the hydrogen abundance. The location of the iron-bump instability boundary is found to depend on the mean molecular weight in the envelope and also on the radial order of the oscillation. A bluer instability boundary is provided by increasing the iron abundance alone, rather than the entire metal component, and is required to explain the observed EC14026 variables. A bluer instability boundary is also provided by higher radial order oscillations. Using data for observed and theoretical period ranges, we show that the coolest EC14026 variables may vary in the fundamental radial mode, but the hottest variables must vary in modes of higher radial order. In considering non-radial oscillations, we demonstrate that g-modes of high radial order and low spherical degree (l < 4) may be excited in some blue horizontal branch stars with near-normal composition (Z= 0.02). Additional iron enhancement extends the g-mode instability zone to higher effective temperatures and also creates a p-mode instability zone. The latter is essentially contiguous with the radial instability zone. With sufficient iron, the p- and g-mode instability zones overlap, allowing a small region where the EC14026- and PG1716-type variability can be excited simultaneously. The overlap zone is principally a function of effective temperature and only weakly a function of luminosity. However, its location is roughly 5000 K, too low compared with the observed boundary between EC14026 and PG1716 variables. The discrepancy cannot be resolved by simply increasing the iron abundance.

G Modes in F Stars – A Non-adiabatic Investigation of the Stability of g Modes in ϒ Doradus Stars

AbstractWe investigate a sequence of 1.5 M⊙ models for g-mode instabilities. For the computation of the nonradial, non-adiabatic oscillations we make use of the Riccati method introduced by Glatzel &amp; Gautschy (1992) to integrate the pulsation equations as given by Unno et al. (1989).

Gravity-mode Instabilities in Models of Post–Extreme Horizontal Branch Stars: Another Class of Pulsating Stars?

We present new results of a stability analysis of realistic models of post-extreme horizontal branch stars. We find that g-mode instabilities develop in some of these models as a result of a potent -mechanism associated with the presence of an active H-burning shell. The -process drives low-order and low-degree g modes with typical periods in the range 40-125 s. The unstable models populate a broad instability strip covering the interval 76,000 K Teff 44,000 K and are identified with low-mass DAO white dwarfs. They descend from stars that reach the zero-age extended horizontal branch with H-rich envelope masses Menv0.0010 M☉. Our computations indicate that some DAO stars should show luminosity variations resulting from pulsational instabilities. We suggest looking for brightness variations in six particularly promising candidates.

G-Mode instability in the main sequence B-type stars

We show that the OPAL opacities, in addition to explaining the origin of β Cep stars pulsations, also predict existence of a large region in the Main Sequence band at lower luminosities, where high-order g-modes of low harmonic degrees, l, are unstable. The excitation mechanism remains the same and is due to the usual κ-effect acting in the metal opacity bump (T ≈ 2 × 105K). The new instability domain nearly bridges the gap in spectral types between δ Set and β Cep stars. Periods of unstable modes are in the range 0.4–3.5 days for l = 1 and l = 2. We propose that this excitation mechanism causes photometric variability in the Slowly Pulsating B-type stars (SPB stars, Waelkens 1991) and perhaps in other B stars whose variability in the same period range has been reported.

The hydrogen shell game - Pulsational instabilities in hydrogen shell-burning planetary nebula nuclei

view Abstract Citations (51) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Hydrogen Shell Game: Pulsational Instabilities in Hydrogen Shell--Burning Planetary Nebula Nuclei Kawaler, Steven D. Abstract The author reports results of fully nonadiabatic g-mode pulsation calculations for evolutionary models of planetary nebula nuclei (PNNs) with active hydrogen-burning shells. The author finds g-mode instabilities that are driven by the ɛ-mechanism at the position of the nuclear burning shells in stadard PNN models. In all hot high-gravity models, the presence of an active nuclear burning shell demands that some g-modes are unstable; if hot high-gravity PNNs contain hydrogen-burning shells, then they should show such pusations. The absence of observed pulsations suggests that the central stars do not retain enough hydrogen to support nuclear burning following ejection of the planetary nebula. Publication: The Astrophysical Journal Pub Date: November 1988 DOI: 10.1086/166832 Bibcode: 1988ApJ...334..220K Keywords: Astronomical Models; Hydrogen; Planetary Nebulae; Stellar Oscillations; Gravity Waves; Nitrogen Isotopes; Oxygen Isotopes; Stellar Interiors; Astrophysics; NEBULAE: PLANETARY; STARS: INTERIORS; STARS: PULSATION full text sources ADS |

The helium shell game - Nonradial g-mode instabilities in hydrogen-deficient planetary nebula nuclei

view Abstract Citations (43) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Helium Shell Game: Nonradial g-Mode Instabilities in Hydrogen-deficient Planetary Nebula Nuclei Kawaler, S. D. ; Winget, D. E. ; Hansen, C. J. ; Iben, I., Jr. Abstract Preliminary results of fully nonadiabatic g-mode pulsation calculations for evolutionary models of hydrogen-deficient planetary nebula nuclei (PNNs) with active helium-burning shells are reported. Instabilities are found that are driven by the epsilon-mechanism at the position of the helium-burning shell. The periods of the unstable modes range from 50 to 214 s. In all models, the presence of an active nuclear burning shell demands that some g-modes are unstable. It is suggested that if hydrogen-deficient PNNs contain helium-burning shells, then they should show such pulsations. Publication: The Astrophysical Journal Pub Date: July 1986 DOI: 10.1086/184701 Bibcode: 1986ApJ...306L..41K Keywords: Helium Hydrogen Atmospheres; Magnetohydrodynamic Stability; Planetary Nebulae; Stellar Oscillations; Nuclear Fusion; Stellar Evolution; White Dwarf Stars; Astrophysics; NEBULAE: PLANETARY; STARS: EVOLUTION; STARS: PULSATION full text sources ADS | data products SIMBAD (1)

The pulsation properties of DB white dwarfs - A preliminary analysis

view Abstract Citations (52) References (13) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The pulsation properties of DB white dwarfs : a preliminary analysis. Winget, D. E. ; van Horn, H. M. ; Tassoul, M. ; Hansen, C. J. ; Fontaine, G. Abstract Preliminary numerical results are given for the nonradial g-mode pulsation properties of evolutionary DB white dwarf models, where the solutions include the fully nonadiabatic equations for modes corresponding to the spherical harmonic index l = 1-3. For each of the model sequences examined, a nonradial g-mode instability strip approximately 3000 K wide is found which, for models with standard ML1 convection, lies in the 16,000-19,000 effective temperature range. The instability strip boundaries are so sensitive to the assumed convection efficiency that, for sequences with more efficient convection, the instability strip lies between 26,000 and 29,000 K effective temperatures. The extrapolation of these 0.6 solar mass calculations to 0.4 and 0.9 solar masses indicates that the instability strip boundaries are insensitive to uncertainties in the total stellar mass. Publication: The Astrophysical Journal Pub Date: May 1983 DOI: 10.1086/184024 Bibcode: 1983ApJ...268L..33W Keywords: Stellar Evolution; Stellar Models; Stellar Oscillations; Stellar Temperature; White Dwarf Stars; Convection; Gas Ionization; Helium; Nonadiabatic Theory; Spherical Harmonics; Stellar Mass; Variable Stars; Astrophysics full text sources ADS | data products SIMBAD (1)