arXiv subjects

M. H. Montgomery

Publications and source records attributed to M. H. Montgomery.

At least 73 records · Page 4Linked to original sources

arXiv2007

Strong limits on the DFSZ axion mass with G117-B15A

We compute rates of period change (Pdots) for the 215s mode in G117-B15A and the 213s mode in R548, first for models without axions, and then for models with axions of increasing mass. We use the asteroseismological models for G117-B15A and R548 we derived in an earlier publication . For G117-B15A, we consider two families of solutions, one with relatively thick hydrogen layers and one with thin hydrogen layers. Given the region of parameter space occupied by our models, we estimate error bars on the calculated Pdots using Monte Carlo simulations. Together with the observed Pdot for G117-B15A, our analysis yields strong limits on the DFSZ axion mass. Our thin hydrogen solutions place an upper limit of 13.5 meV on the axion, while our thick hydrogen solutions relaxes that limit to 26.5 meV.

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arXiv2007

Fine Grid Asteroseismology of R548 and G117-B15A

We now have a good measurement of the cooling rate of G117-B15A. In the near future, we will have equally well determined cooling rates for other pulsating white dwarfs, including R548. The ability to measure their cooling rates offers us a unique way to study weakly interacting particles that would contribute to their cooling. Working toward that goal, we perform a careful asteroseismological analysis of G117-B15A and R548. We study them side by side because they have similar observed properties. We carry out a systematic, fine grid search for best fit models to the observed period spectra of those stars. We freely vary 4 parameters: the effective temperature, the stellar mass, the helium layer mass, and the hydrogen layer mass. We identify and quantify a number of uncertainties associated with our models. Based on the results of that analysis and fits to the periods observed in R548 and G117-B15A, we clearly define the regions of the 4 dimensional parameter space ocuppied by the best fit models.

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arXiv2006

Mean ZZ Ceti pulsation period gauges stellar temperature

The mean pulsation period of ZZ Ceti stars increases with decreasing effective temperature as we traverse from the blue to the red edge of the instability strip. This well-established correlation between the mean period and spectroscopic temperature suggests that the mean period could be utilized as a tool to measure the relative temperature of the star independent of spectroscopy. Measuring the pulsation periods of a ZZ Ceti star is a simple, model-independent, and straight forward process as opposed to a spectroscopic determination of its temperature. Internal uncertainties in determining the spectroscopic temperature of a ZZ Ceti star are at least 200K, 15% of the 1350K width of the instability strip. The uncertainties in determining the mean period arise mostly from amplitude modulation in the pulsation spectrum and are smaller than 100s for 91% of the ZZ Ceti stars, <8% of the 1300s width of the instability strip. In principle this implies that for 90% of the ZZ Ceti stars, the average uncertainty in determining the location of a ZZ Ceti star within the instability strip decreases by a factor of two in utilizing the mean period of the ZZ Ceti star as a temperature indicator rather than conventional spectroscopy. Presently we only claim that the relative temperatures of ZZ Ceti stars derived by using the mean pulsation period are certainly as good as and perhaps about 15% better than spectroscopy.

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arXiv2005

Ensemble Characteristics of the ZZ Ceti stars

We present the observed pulsation spectra of all known non-interacting ZZ Ceti stars (hydrogen atmosphere white dwarf variables; DAVs) and examine changes in their pulsation properties across the instability strip. We confirm the well established trend of increasing pulsation period with decreasing effective temperature across the ZZ Ceti instability strip. We do not find a dramatic order of magnitude increase in the number of observed independent modes in ZZ Ceti stars, traversing from the hot to the cool edge of the instability strip; we find that the cool DAVs have one more mode on average compared to the hot DAVs. We confirm the initial increase in pulsation amplitude at the blue edge, and find strong evidence of a decline in amplitude prior to the red edge. We present the first observational evidence that ZZ Ceti stars lose pulsation energy just before pulsations shut down at the empirical red edge of the instability strip.

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arXiv2005

Exploring uncharted territory in particles physics using pulsating white dwarfs: Prospects

Pulsating white dwarfs, especially DBVs, can be used as laboratories to study elusive particles such as plasmon neutrinos and axions. In the degenerate interiors of DBVs, plasmon decay is the dominant neutrino producing process. We can measure the neutrino luminosity using asteroseismology and constrain plasmon neutrino rates. In the same way, we can measure any additional loss of energy due to other weakly interacting particles, such as axions. Depending upon their (theoretically largely unconstrained) mass, axions could be a significant source of energy loss for DAVs as well. We are looking at what the uncertainties in the observables are, and what mass and temperature range minimizes them.

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arXiv2005

Driving in ZZ Ceti stars - Problem solved?

There is a fairly tight correlation between the pulsation periods and effective temperatures of ZZ Ceti stars (cooler stars have longer periods). This seems to fit the theoretical picture, where driving occurs in the partial ionization zone, which lies deeper and deeper within the star as it cools. It is reasonable to assume that the pulsation periods should be related to the thermal timescale in the region where driving occurs. As that region sinks further down below the surface, that thermal timescale increases. Assuming this connection, the pulsation periods could provide an additional way to determine effective temperatures, independent of spectroscopy. We explore this idea and find that in practice, things are not so simple.

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arXiv2005

A New Technique for Probing Convection in Pulsating White Dwarf Stars

In this paper we demonstrate how pulsating white dwarfs can be used as an astrophysical laboratory for empirically constraining convection in these stars. We do this using a technique for fitting observed non-sinusoidal light curves, which allows us to recover the thermal response timescale of the convection zone (its "depth") as well as how this timescale changes as a function of effective temperature. We also obtain constraints on mode identifications for the pulsation modes, allowing us to use asteroseismology to study the interior structure of these stars. Aspects of this approach may have relevance for other classes of pulsators, including the Cepheids and RR Lyrae stars.

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arXiv2004

White Dwarf Light Curves: Constraining Convection and Mode Identification Using Non-sinusoidal Light Curves

In this paper we show how the formalism of Wu & Goldreich developed for weakly nonlinear white dwarf pulsations can be extended to the fully nonlinear case. Due to the computational simplification this affords over the simulations of Brickhill (1992) and Ising & Koester (2001), we are able to obtain the first ever fits of observed light curves. These fits allow us to obtain not only mode identifications but also to constrain the average depth of the convection zone and its sensitivity to changes in effective temperature. We are thus able to probe the fundamental physics of convection in these objects.

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arXiv2004

Whole Earth Telescope observations of BPM 37093: a seismological test of crystallization theory in white dwarfs

BPM 37093 is the only hydrogen-atmosphere white dwarf currently known which has sufficient mass (~ 1.1 M_sun) to theoretically crystallize while still inside the ZZ Ceti instability strip (T_eff ~ 12,000 K). As a consequence, this star represents our first opportunity to test crystallization theory directly. If the core is substantially crystallized, then the inner boundary for each pulsation mode will be located at the top of the solid core rather than at the center of the star, affecting mainly the average period spacing. This is distinct from the "mode trapping" caused by the stratified surface layers, which modifies the pulsation periods more selectively. In this paper we report on Whole Earth Telescope observations of BPM 37093 obtained in 1998 and 1999. Based on a simple analysis of the average period spacing we conclude that a large fraction of the total stellar mass is likely to be crystallized.

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arXiv2004

The Crystal Method: Asteroseismology of BPM 37093

More than 40 years have passed since Ed Salpeter and others predicted that the carbon/oxygen cores of the coolest white dwarf stars in our Galaxy will theoretically crystallize. This effect has a dramatic impact on the calculated ages of cool white dwarfs, but until recently we have had no way of testing the theory. In 1992, pulsations were discovered in the massive potentially crystallized white dwarf BPM 37093, and in 1999 the theoretical effects of crystallization on the pulsation modes were determined. Observations from two Whole Earth Telescope campaigns in 1998 and 1999, combined with a new model-fitting method using a genetic algorithm, are now giving us the first glimpse inside of a crystallized star.

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arXiv2004

New evolutionary models for massive ZZ Ceti stars. II. The effects of crystallization on their pulsational properties

We present in this work new pulsational calculations for improved carbon-oxygen DA white dwarf models suitable for the study of massive ZZ Ceti stars. The background models employed in this study, presented in detail in a recent paper by Althaus et al. (2003), are the result of the complete evolution of massive white dwarf progenitors from the zero-age main sequence through the Asymptotic Giant Branch (AGB) and mass loss phases to the white dwarf regime. Abundance changes are accounted for by means of a full coupling between nuclear evolution and time-dependent mixing due to convection, salt fingers, and diffusive overshoot. In addition, time-dependent element diffusion for multicomponent gases has been considered during the white dwarf evolution. Crystallization and chemical rehomogenization due to phase separation upon crystallization in the core of our models have been fully considered. The effects of crystallization on the period spectrum of these massive white dwarf models are assessed by means of a detailed pulsational analysis. We find that the theoretical pulsation spectrum is strongly modified when crystallization is considered, in particular concerning the mode trapping properties of the equilibrium models. We also discuss at some length the implications of our study for BPM 37093, the most massive ZZ Ceti star presently known. We find that if BPM 37093 has a stellar mass of $\approx 1.00$ \msun its observed spectrum could bear the signature of overshoot episodes during the helium core burning.

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arXiv2004

Re-defining the Empirical ZZ Ceti Instability Strip

We use the new ZZ Ceti stars (hydrogen atmosphere white dwarf variables; DAVs) discovered within the Sloan Digital Sky Survey (Mukadam et al. 2004) to re-define the empirical ZZ Ceti instability strip. This is the first time since the discovery of white dwarf variables in 1968 that we have a homogeneous set of spectra acquired using the same instrument on the same telescope, and with consistent data reductions, for a statistically significant sample of ZZ Ceti stars. The homogeneity of the spectra reduces the scatter in the spectroscopic temperatures and we find a narrow instability strip of width ~950K, from 10850--11800K. We question the purity of the DAV instability strip as we find several non-variables within. We present our best fit for the red edge and our constraint for the blue edge of the instability strip, determined using a statistical approach.

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arXiv2004

Testing White Dwarf Crystallization Theory with Asteroseismology of the Massive Pulsating DA Star BPM 37093

It was predicted more than 40 years ago that the cores of the coolest white dwarf stars should eventually crystallize. This effect is one of the largest sources of uncertainty in white dwarf cooling models, which are now routinely used to estimate the ages of stellar populations in both the Galactic disk and the halo. We are attempting to minimize this source of uncertainty by calibrating the models, using observations of pulsating white dwarfs. In a typical mass white dwarf model, crystallization does not begin until the surface temperature reaches 6000-8000 K. In more massive white dwarf models the effect begins at higher surface temperatures, where pulsations are observed in the ZZ Ceti (DAV) stars. We use the observed pulsation periods of BPM 37093, the most massive DAV white dwarf presently known, to probe the interior and determine the size of the crystallized core empirically. Our initial exploration of the models strongly suggests the presence of a solid core containing about 90% of the stellar mass, which is consistent with our theoretical expectations.

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arXiv2004

White dwarf envelopes: further results of a non-local model of convection

We present results of a fully non-local model of convection for white dwarf envelopes. We show that this model is able to reproduce the results of numerical simulations for convective efficiencies ranging from very inefficient to moderately efficient; this agreement is made more impressive given that no closure parameters have been adjusted in going from the previously reported case of A-stars to the present case of white dwarfs; for comparison, in order to match the peak convective flux found in numerical simulations for both the white dwarf envelopes discussed in this paper and the A-star envelopes discussed in our previous work requires changing the mixing length parameter of commonly used local models by a factor of 4. We also examine in detail the overshooting at the base of the convection zone, both in terms of the convective flux and in terms of the velocity field: we find that the flux overshoots by approximately 1.25 H_P and the velocity by approximately 2.5 H_P. Due to the large amount of overshooting found at the base of the convection zone the new model predicts the mixed region of white dwarf envelopes to contain at least 10 times more mass than local mixing length theory (MLT) models having similar photospheric temperature structures. This result is consistent with the upper limit given by numerical simulations which predict an even larger amount of mass to be mixed by convective overshooting. Finally, we attempt to parametrise some of our results in terms of local MLT-based models, insofar as is possible given the limitations of MLT

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arXiv2004

A Strong Test of Electro-Weak Theory using Pulsating DB White Dwarf Stars as Plasmon Neutrino Detectors

We demonstrate that plasmon neutrinos are the dominant form of energy loss in model white dwarf stars down to T_eff ~ 25,000 K, depending on the stellar mass. The lower end of this range overlaps the observed temperatures for the V777 Her star (DBV) instability strip. The evolution of white dwarfs at these temperatures is driven predominantly by cooling, so this directly affects the stellar evolutionary timescale in proportion to the ratio of the neutrino energy loss to the photon energy loss. This evolutionary timescale is observable through the time rate of change of the pulsation periods. Although the unified electro-weak theory of lepton interactions that is crucial for understanding neutrino production has been well tested in the high energy regime, the approach presented here should result in an interesting low-energy test of the theory. We discuss observational strategies to achieve this goal.

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arXiv2003

Probing the core and envelope structure of DBV white dwarfs

We investigate the global pulsation properties of DBV white dwarf models that include both the double-layered envelope structure expected from time-dependent diffusion calculations, as well as a non-uniform C/O core expected from prior nuclear burning. We compare these models to otherwise identical models containing a pure C core to determine whether the addition of core structure leads to any significant improvement. Our double-layered envelope model fit to GD 358 that includes an adjustable C/O core is significantly better than our pure C core fit (7sigma improvement). We find a comparable improvement from fits to a second DBV star, CBS 114, though the values of the derived parameters may be more difficult to reconcile with stellar evolution theory. We find that our models are systematically cooler by 1,900 K relative to the similar models of Fontaine & Brassard (2002). Although a fit to their model reproduces the mass and envelope structure almost exactly, we are unable to reproduce the absolute quality of their fit to GD 358. Differences between the constitutive physics employed by the two models may account for both the temperature offset and the period residuals.

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arXiv2003

The core/envelope symmetry in pulsating stars

We demonstrate that there is an inherent symmetry in the way high-overtone stellar pulsations sample the core and the envelope, which can potentially lead to an ambiguity in the asteroseismologically derived locations of internal structures. We provide an intuitive example of the source of this symmetry by analogy with a vibrating string. For the stellar case, we focus on the white dwarf stars, establishing the practical consequences for high-order white dwarf pulsations both analytically and numerically. In addition, we verify the effect empirically by cross-fitting two different structural models, and we discuss the consequences that this approximate symmetry may have for past and present asteroseismological fits of the pulsating DBV, GD 358. Finally, we show how the signatures of composition transition zones that are brought about by physically distinct processes may be used to help alleviate this potential ambiguity in our asteroseismological interpretation of the pulsation frequencies observed in white dwarf stars.

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arXiv2003

New evolutionary models for massive ZZ Ceti stars: I. First results for their pulsational properties

We present new and improved evolutionary calculations for carbon-oxygen white dwarf (WD) stars appropriate for the study of massive ZZ Ceti stars. We follow the complete evolution of massive WD progenitors from the zero-age main sequence through the thermally pulsing and mass loss phases to the WD regime. Abundance changes are accounted for by means of a full coupling between nuclear evolution and time-dependent mixing due to diffusive overshoot, semiconvection and salt fingers. In addition, time-dependent element diffusion for multicomponent gases has been considered during the WD stage. We find that before the ZZ Ceti stage is reached, element diffusion has strongly smoothed out the chemical profile to such a degree that the resulting internal abundance distribution does not depend on the occurrence of overshoot episodes during the thermally pulsing phase. The mass of the hydrogen envelope left at the ZZ Ceti domain amounts to $M_H \approx 2.3 \times 10^{-6}$ \msun. This is about half as large as for the case when element diffusion is neglected. The implications of our new models for the pulsational properties of massive ZZ Ceti stars are discussed. In this regard, we find that the occurrence of core overshooting during central helium burning leaves strong imprints on the theoretical period spectrum of massive ZZ Ceti stars. Finally, we present a simple new prescription for calculating the He/H profile which goes beyond the trace element approximation.

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