This paper analyses the Whole Earth Telescope observations of HL Tau 76, the first discovered pulsating DA white dwarf. The star was observed during two Whole Earth Telescope campaigns. It was a ...second priority target during the XCOV13 campaign in 1996 and the first priority one during the XCOV18 campaign in 1999. The 1999 campaign reached 66.5% duty cycle. With a total duration of 18 days, the frequency resolution achieved is 0.68 μHz. With such a frequency resolution, we were able to find as many as 78 significant frequencies in the power spectrum, of which 34 are independent frequencies after removal of all linear combinations. In taking into account other frequencies present during the 1996 WET campaign and those present in earlier data, which do not show up in the 1999 data set, we find a total of 43 independent frequencies. This makes HL Tau 76 the richest ZZ Ceti star in terms of number of observed pulsation modes. We use those pulsation frequencies to determine as much as possible of the internal structure of HL Tau 76. The pulsations in HL Tau 76 cover a wide range of periods between 380 s and 1390 s. We propose an identification for 39 of those 43 frequencies in terms of $\ell=1$ and $\ell=2$ non-radial g-modes split by rotation. We derive an average rotation period of 2.2 days. The period distribution of HL Tau 76 is best reproduced if the star has a moderately “thick” hydrogen mass fraction log $q_{\rm H} \geq -7.0$. The results presented in this paper constitute a starting point for a detailed comparison of the observed periods with the periods calculated for models as representative as possible of HL Tau 76.
Since pulsating subdwarf B (sdBV or EC14026) stars were first discovered, observational efforts have tried to realize their potential for constraining the interior physics of extreme horizontal ...branch stars. Difficulties encountered along the way include uncertain mode identifications and a lack of stable pulsation mode properties. Here we report on Feige 48, an sdBV star for which follow-up observations have been obtained spanning more than four years. These observations show some stable pulsation modes. We resolve the temporal spectrum into five stable pulsation periods in the range 340–380 s with amplitudes less than 1 per cent, and two additional periods that appear in one data set each. The three largest amplitude periodicities are nearly equally spaced, and we explore the consequences of identifying them as a rotationally split ℓ= 1 triplet by consulting a representative stellar model. The general stability of the pulsation amplitudes and phases allows us to use the pulsation phases to constrain the time-scale of evolution for this sdBV star. Additionally, we are able to place interesting limits on any stellar or planetary companion to Feige 48.
We report 323 hours of nearly uninterrupted time series photometric observations of the DBV star GD 358 acquired with the Whole Earth Telescope (WET) during May 23rd to June 8th, 2000. We acquired ...more than 232 000 independent measurements. We also report on 48 hours of time-series photometric observations in Aug 1996. We detected the non-radial g-modes consistent with degree $\ell=1$ and radial order 8 to 20 and their linear combinations up to 6th order. We also detect, for the first time, a high amplitude $\ell=2$ mode, with a period of 796 s. In the 2000 WET data, the largest amplitude modes are similar to those detected with the WET observations of 1990 and 1994, but the highest combination order previously detected was 4th order. At one point during the 1996 observations, most of the pulsation energy was transferred into the radial order $k=8$ mode, which displayed a sinusoidal pulse shape in spite of the large amplitude. The multiplet structure of the individual modes changes from year to year, and during the 2000 observations only the $k=9$ mode displays clear normal triplet structure. Even though the pulsation amplitudes change on timescales of days and years, the eigenfrequencies remain essentially the same, showing the stellar structure is not changing on any dynamical timescale.
PG 0014+067 is one of the most promising pulsating subdwarf B stars for seismic analysis, as it has a rich pulsation spectrum. The richness of its pulsations, however, poses a fundamental challenge ...to understanding the pulsations of these stars, as the mode density is too complex to be explained only with radial and nonradial low-degree (l < 3) p-modes without rotational splittings. One proposed solution, suggested by Brassard et al. in 2001 for the case of PG 0014+067 in particular, assigns some modes with high degree (l= 3). On the other hand, theoretical models of sdB stars suggest that they may retain rapidly rotating cores, and so the high mode density may result from the presence of a few rotationally split triplet (l = 1) and quintuplet (l = 2) modes, along with radial (l = 0) p-modes. To examine alternative theoretical models for these stars, we need better frequency resolution and denser longitude coverage. Therefore, we observed this star with the Whole Earth Telescope for two weeks in 2004 October. In this paper we report the results of Whole Earth Telescope observations of the pulsating subdwarf B star PG 0014+067. We find that the frequencies seen in PG 0014+067 do not appear to fit any theoretical model currently available; however, we find a simple empirical relation that is able to match all of the well-determined frequencies in this star.
The central regions of many interacting and early-type spiral galaxies are actively forming stars. This process affects the physical and chemical properties of the local interstellar medium, as well ...as the evolution of the galaxies. We observed near-infrared H sub(2) emission lines: v = 1-0 S(1), 3-2 S(3), 1-0 S(0), and 2-1 S(1) from the central approx1 kpc regions of the archetypical starburst galaxies M82 and NGC 253 and the less dramatic but still vigorously star-forming galaxies NGC 6946 and IC 342. Like the far-infrared continuum luminosity, the near-infrared H sub(2) emission luminosity can directly trace the amount of star formation activity because the H sub(2) emission lines arise from the interaction between hot and young stars and nearby neutral clouds. The observed H sub(2) line ratios show that both thermal excitation and nonthermal excitation are responsible for the emission lines but that the great majority of the near-infrared H sub(2) line emission in these galaxies arises from energy states excited by ultraviolet fluorescence. The derived physical conditions, e.g. far-ultraviolet radiation field and gas density, from C II and O I lines and far-infrared continuum observations when used as inputs to photodissociation models also explain the luminosity of the observed H sub(2) 1-0 S(1) line. The ratio of the H sub(2) 1-0 S(1) line to far-IR continuum luminosity is remarkably constant over a broad range of galaxy luminosities: Limage/L sub(FIR) ~ 10 super(-5), in normal late- type galaxies (including the Galactic center), in nearby starburst galaxies, and in luminous IR galaxies (LIRGs: L sub(FIR) > 10 super(11) L sub(o)). Examining this constant ratio in the context of photodissociation region models, we conclude that it implies that the strength of the incident UV field on typical molecular clouds follows the gas density at the cloud surface.
92 h of new Whole Earth Telescope observations have been acquired for the δ Scuti star CD-24 7599. All the seven pulsation modes reported by Handler et al. are confirmed. However, significant ...amplitude variations which are not caused by beating of closely spaced frequencies occurred within two years. Analysing the combined data of both WET runs, we detect six further pulsation modes, bringing the total number up to 13. We also examine our data for high-frequency pulsations similar to those exhibited by rapidly oscillating Ap stars, but we do not find convincing evidence for variability in this frequency domain.
We present a detailed mode identification performed on the 1994 Whole Earth Telescope (WET) run on GD 358. The results are compared with that obtained for the same star from the 1990 WET data. The ...two temporal spectra show very few qualitative differences, although amplitude changes are seen in most modes, including the disappearance of the mode identified as k=14 in the 1990 data. The excellent coverage and signal-to-noise ratio obtained during the 1994 run lead to the secure identification of combination frequencies up to fourth order, i.e. peaks that are sums or differences of up to four parent frequencies, including a virtually complete set of second-order frequencies, as expected from harmonic distortion. We show how the third-order frequencies are expected to affect the triplet structure of the normal modes by back-interacting with them. Finally, a search for ℓ=2 modes was unsuccessful, not verifying the suspicion that such modes had been uncovered in the 1990 data set.
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