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D. W. Kurtz

Publications and source records attributed to D. W. Kurtz.

97 records · Page 6Linked to original sources
arXiv2001

Discover of the "missing" mode in HR 1217 by the Whole Earth Telescope

HR 1217 is a prototypical rapidly oscillating Ap star that has presented a test to the theory of nonradial stellar pulsation. Prior observations showed a clear pattern of five modes with alternating frequency spacings of 33.3 microHz and 34.6 microHz, with a sixth mode at a problematic spacing of 50.0 microHz (which equals 1.5 times 33.3 microHz) to the high-frequency side. Asymptotic pulsation theory allowed for a frequency spacing of 34 microHz, but Hipparcos observations rule out such a spacing. Theoretical calculations of magnetoacoustic modes in Ap stars by Cunha (2001) predicted that there should be a previously undetected mode 34 microHz higher than the main group, with a smaller spacing between it and the highest one. In this Letter, we present preliminary results from a multi-site photometric campaign on the rapidly oscillating Ap star HR 1217 using the "Whole Earth Telescope". While a complete analysis of the data will appear in a later paper, one outstanding result from this run is the discovery of a newly detected frequency in the pulsation spectrum of this star, at the frequency predicted by Cunha (2001).

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arXiv2001

HD 12098 - A new norhtern hemisphere roAp star

We present the analysis of 65 hours of high speed photometric observations of HD 12098 taken from State Observatory, Naini Tal and Gurushikhar Observatory, Mt.Abu on sixteen nights spanning from November 1999 to November 2000. HD 12098 is the first rapidly oscillating Ap star discovered from the `Naini Tal-Cape survey for northern hemisphere roAp stars'. It is the 32nd in the complete list. HD 12098 exhibits one predominant mode of oscillation at nu_1 = 2.1738 mHz. The second-most significant frequency in our data is at nu_2 = 2.1641 mHz with a 1 cycle/day alias ambiguity. We argue that nu_2 is a rotational sidelobe of nu_1, rather than an independent pulsation mode. Evidence for the presence of two other frequencies at 2.1807 and 2.3056 mHz is also presented.

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arXiv2000

The MACHO Project LMC Variable Star Inventory. IX. Frequency Analysis of the First Overtone RR Lyrae Stars and the Indication for Nonradial Pulsations

More than 1300 variables classified provisionally as first overtone RR Lyrae pulsators in the MACHO variable star database of the Large Magellanic Cloud (LMC) have been subjected to standard frequency analysis. Based on the remnant power in the prewhitened spectra, we found 70% of the total population to be monoperiodic. The remaining 30% (411 stars) are classified as one of 9 types according to their frequency spectra. Several types of RR Lyrae pulsational behavior are clearly identified here for the first time. Together with the earlier discovered double-mode (fundamental & first overtone) variables this study increased the number of the known double-mode stars in the LMC to 181. During the total 6.5yr time span of the data, 10% of the stars show strong period changes. We also discovered two additional types of multifrequency pulsators with low occurrence rates of 2% for each. In the first type there remains one closely spaced component after prewhitening by the main pulsation frequency. In the second type the number of remnant components is two, they are also closely spaced, and, in addition, they are symmetric in their frequency spacing relative to the central component. This latter type of variables is associated with their relatives among the fundamental pulsators, known as Blazhko variables. Their high frequency (~20%) among the fundamental mode variables versus the low occurrence rate of their first overtone counterparts makes it more difficult to explain Blazhko phenomenon by any theory depending mainly on the role of aspect angle or magnetic field. Current theoretical models invoke nonradial pulsation components in these stars.

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arXiv1999

Cepheid Variables in the LMC and SMC

In this paper, we will review major new results regarding classical Cepheids, in the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC). Specifically, we discuss recent work regarding multimode Cepheids and describe new observations of a W Vir star (HV 5756) and a Cepheid which are each in eclipsing binary systems. An additional interesting pulsating supergiant in an eclipsing system is also identified. Ephemerides for eclipses for the three systems are provided.

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arXiv1998

Some recent results for the roAp stars

Three topics are discussed: 1) Photometric observations of the rapidly oscillating Ap stars have shown that the pulsation amplitude drops dramatically as a function of wavelength from the blue to the red. A theoretical derivation, plus modelling, indicates that this is because the vertical wavelength of the pulsation mode is short compared to the scale height of the atmosphere; in fact, it indicates that we are seeing a pulsation node in the observable atmosphere. Radial velocity observations, and theoretical calculations now support this. The implication for other research on CP stars is that this can provide observational constraints on the atmospheric structure independent of traditional spectral analysis. 2) Luminosities of roAp stars can be determined from asteroseismology. A recent comparison of such asteroseismic luminosities with HIPPARCOS luminosities is shown. This suggests that roAp stars have lower temperatures and/or smaller radii than previous models have used, or that the magnetic fields in these stars alter the frequency separations. 3) The latest results of our long-term monitoring of the pulsation frequencies in certain roAp stars are discussed. There is a clear cyclic variability to the pulsation cavity, hence the sound speed and/or sound travel time (radius) of these stars. This might be indicative of magnetic cycles at a level that magnetic measurements cannot currently detect, although there is no theoretical support for such an idea.

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arXiv1997

Understanding the Cool DA White Dwarf, G29-38

The white dwarfs are promising laboratories for the study of cosmochronology and stellar evolution. Through observations of the pulsating white dwarfs, we can measure their internal structures and compositions, critical to understanding post main sequence evolution, along with their cooling rates, allowing us to calibrate their ages directly. The most important set of white dwarf variables to measure are the oldest of the pulsators, the cool DAVs, which have not previously been explored through asteroseismology due to their complexity and instability. Through a time-series photometry data set spanning ten years, we explore the pulsation spectrum of the cool DAV, G29-38 and find an underlying structure of 19 (not including multiplet components) normal-mode, probably l=1 pulsations amidst an abundance of time variability and linear combination modes. Modelling results are incomplete, but we suggest possible starting directions and discuss probable values for the stellar mass and hydrogen layer size. For the first time, we have made sense out of the complicated power spectra of a large-amplitude DA pulsator. We have shown its seemingly erratic set of observed frequencies can be understood in terms of a recurring set of normal-mode pulsations and their linear combinations. With this result, we have opened the interior secrets of the DAVs to future asteroseismological modelling, thereby joining the rest of the known white dwarf pulsators.

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