ON THE POSSIBLE EXISTENCE OF SHORT-PERIODg-MODE INSTABILITIES POWERED BY NUCLEAR-BURNING SHELLS IN POST-ASYMPTOTIC GIANT BRANCH H-DEFICIENT (PG1159-TYPE) STARS (original) (raw)
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HS 0702+6043: a star showing both short-period p-mode and long-period g-mode oscillations
Astronomy & Astrophysics, 2006
Context. The hot subdwarf B star HS 0702+6043 is known as a large-amplitude, short-period p-mode pulsator of the EC 14026 type. Its atmospheric parameters place it at the common boundary between the empirical instability regions of the EC 14026 variables and the typically cooler long-period g-mode pulsators of the PG 1716 kind. Aims. We analyse and interpret the photometric variability of HS 0702+6043 in order to explore its asteroseismological potential. Methods. We report on rapid wide band CCD photometric observations to follow up on and confirm the serendipitous discovery of multiperiodic long-period luminosity variations with typical time scales of ∼1 h in HS 0702+6043, in addition to the two previously known pulsations at 363 s and 383 s. In particular, we isolate a relatively low-amplitude (∼4 mmag), long-period (3538±130 s) light variation. Results. We argue that the most likely origin for this luminosity variation is the presence of an excited g-mode pulsation. If confirmed, HS 0702+6043 would constitute a rare addition to the very select class of pulsating stars showing simultaneously parts of their pressure and gravity mode pulsation spectra. The asteroseismological potential of such stars is immense, and HS 0702+6043 thus becomes a target of choice for future investigations. While our discovery appears consistent with the location of HS 0702+6043 at the common boundary between the two families of pulsating sdB stars, it does challenge theory's current description of stability and driving mechanisms in pulsating B subdwarfs.
Astronomy & Astrophysics, 2006
Aims. We reexamine the theoretical instability domain of pulsating PG 1159 stars (GW Vir variables). Methods. We performed an extensive g-mode stability analysis on PG 1159 evolutionary models with stellar masses ranging from 0.530 to 0.741 M , for which the complete evolutionary stages of their progenitors from the ZAMS, through the thermally pulsing AGB and born-again phases to the domain of the PG 1159 stars have been considered. Results. We found that pulsations in PG 1159 stars are excited by the κ-mechanism due to partial ionization of carbon and oxygen, and that no composition gradients are needed between the surface layers and the driving region, much in agreement with previous studies. We show, for the first time, the existence of a red edge of the instability strip at high luminosities. We found that all of the GW Vir stars lay within our theoretical instability strip. Our results suggest a qualitative good agreement between the observed and the predicted ranges of unstable periods of individual stars. Finally, we found that generally the seismic masses (derived from the period spacing) of GW Vir stars are somewhat different from the masses suggested by evolutionary tracks coupled with spectroscopy. Improvements in the evolution during the thermally pulsing AGB phase and/or during the core helium burning stage and early AGB could help to alleviate the persisting discrepancies.
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 > ∼ T eff > ∼ 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 M env > ∼ 0.0010 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.
Probable nonradial g-mode pulsation in early A-type stars
Astronomy & Astrophysics, 2006
A survey for line profile variability in early A-type stars has been performed in order to detect nonradial pulsation signatures. The star HR 6139, with spectral type A2V and estimated T_eff=8800 K, shows evident line profile variations that can be explained by oscillations in prograde g-modes. This feature and the known photometric variability are similar to those observed in the Slowly Pulsating B-type stars. However HR 6139 is much cooler than the cool border of the instability strip of such variables, and it is hotter than the blue edge of the delta Scuti instability strip. There are indications of a tiny variability also in other four objects, whose nature is not yet clear.
Progress and problems in massive star pulsation theory
Proceedings of the International Astronomical Union, 2015
Massive stars pulsate in various modes; radial and nonradial p-modes, g-modes, and strange modes including oscillatory convective (non-adiabatic g−) modes. Those modes are responsible for the light and velocity variations of β Cephei stars, slowly pulsating B (SPB) stars, and α Cyg variables. The instability mechanisms for these pulsations are discussed. We also discuss the relation between the evolution of massive stars and the excitation of strange modes, which are considered responsible for the pulsation in most of the α Cyg variables. The surface He and CNO abundances of hotter α Cyg variables seem to indicate that the Ledoux criterion of convection is better than the Schwarzschild criterion, although the latter is extensively used in stellar evolution computations.
New Astronomy, 2007
The results of a long-term UBV photometric monitoring of the red supergiant (RSG) star V424 Lac are presented. V424 Lac shows multiperiodic brightness variations which can be attributed to pulsational oscillations. A much longer period (P = 1601 d), that allows us to classify this star as a long secondary period variable star (LSPV) has been also detected. The B À V and U À B color variations related to the long secondary period (LSP) are similar to those related to the shorter periods, supporting the pulsational nature of LSP. The long period brightness variation of V424 Lac is accompanied by a near-UV (NUV) excess, which was spectroscopically detected in a previous study . ApJ 634, 1286 and which is now found to be variable from photometry. On the basis of the results found for V424 Lac, the NUV excess recently found in a number of RSGs may be due not solely to circumstellar dust but may also have a contribution from a still undetected LSP variability.
A- and B-type star pulsations in the Kepler and CoRoT era: theoretical considerations
Cornell University - arXiv, 2013
Among A-type main-sequence variables, pulsations of δ Sct and γ Dor variables are driven in the He II ionization zone, while H-ionization zone and strong magnetic fields seem to play roles in the excitation of high-order pmodes in rapidly oscillating Ap (roAp) stars. Pulsations in B-type variables, β Cephei and slowly pulsating B (SPB) stars are excited by the κ-mechanism at the Fe-opacity bump at T ≈ 2 × 10 5 K. In addition, the strange-mode instability seems responsible for the excitation of pulsations in luminous AB-supergiants (α Cygni variables). We discuss excitation mechanisms for pulsations in A-and B-type variables stars.
On the Challenging Variability of LS IV-14°116: Pulsational Instabilities Excited by the Ε-Mechanism
The Astrophysical Journal, 2011
We investigate the pulsation driving mechanism responsible for the long-period photometric variations observed in LS IV-14 • 116, a subdwarf B star showing a He-enriched atmospheric composition. To this end, we perform detailed nonadiabatic pulsation computations over fully evolutionary post-He-core-flash stellar structure models, appropriate for hot subdwarf stars at evolutionary phases previous to the He-core burning stage. We found that the variability of LS IV-14 • 116 can be attributed to non-radial g-mode pulsations excited by the ²-mechanism acting in the He-burning shells that appear before the star settles in the He-core burning stage. Even more interestingly, our results show that LS IV-14 • 116 could be the first known pulsating star in which the ²-mechanism of mode excitation is operating. Last but not the least, we find that the period range of destabilized modes is sensitive to the exact location of the burning shell, something that might help in distinguishing between the different evolutionary scenarios proposed for the formation of this star.
Pulsation in Intermediate-Mass Stars
Frontiers in Astronomy and Space Sciences
A new perspective of pulsation in stars within the δ Scuti instability strip has recently emerged as a result of Kepler observations. The majority of stars within the instability strip do not pulsate and practically all δ Scuti stars contain low frequencies. Because γ Doradus stars co-exist with δ Sct stars in the same region of the instability strip, it follows that γ Dor stars are driven by the same mechanism as δ Sct stars. The difference must be due to different mode selection processes. The search for an unknown damping factor which is missing from the models will be essential for further progress. Maia variables and hot γ Dor stars are briefly discussed. Luminosities of roAp stars obtained from Gaia DR2 parallaxes and spectroscopic effective temperatures show that the roAp stars are slightly evolved with temperatures in the range 6,300-8,300 K, considerably cooler than predicted by the models. The roAp stars and stars with solar-like oscillations share the same mass-temperature-luminosity relation, but with frequencies which are about 50 percent higher. This suggests that roAp frequencies are determined by the critical acoustic frequency, but this frequency is larger than in standard models, perhaps as a result of a temperature inversion in the atmosphere.
KIC 1718290: A HELIUM-RICH V1093-Her-LIKE PULSATOR ON THE BLUE HORIZONTAL BRANCH
The Astrophysical Journal, 2012
We introduce the first g-mode pulsator found to reside on the classical blue horizontal branch. One year of Kepler observations of KIC 1718290 reveals a rich spectrum of low-amplitude modes with periods between one and twelve hours, most of which follow a regular spacing of 276.3 s. This mode structure strongly resembles that of the V1093 Her pulsators, with only a slight shift towards longer periods. Our spectroscopy, however, reveals KIC 1718290 to be quite distinct from the sdB stars that show V1093 Her pulsations, which all have surface gravities higher than log g = 5.1 and helium abundances depleted by at least an order of magnitude relative to the solar composition. We find that KIC 1718290 has T eff = 22 100 K, log g = 4.72, and a super-solar helium abundance (log N He /N H =-0.45). This places it well above the extreme horizontal branch, and rather on the very blue end of the classical horizontal branch, where shell hydrogen burning is present. We conclude that KIC 1718290 must have suffered extreme mass loss during its first giant stage, but not sufficient to reach the extreme horizontal branch.