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1.
We briefly review the recent advances that have been made on the front of pulsating subdwarf B (sdB) stars. The first family of sdB pulsators, the EC 14026 stars, was discovered a few years ago and consists of short-period (～100?200 s) p-mode variables. The second type of pulsating sdB’s consists of the PG 1716+426 stars, a group of variables showing long-period (～1 h) g-mode pulsations. The existence of the latter was first reported less than a year ago. While the two types of sdB pulsators differ markedly in their observational characteristics, we recently found a unifying property in the sense that the observed modes in these objects are excited through the same driving process, a classic kappa mechanism associated with the radiative levitation of iron in the stellar envelope.  相似文献   

2.
Rapid oscillations in the sdB star Feige 48 have been discovered. The frequency spectrum reveals at least four periods in a narrow interval from 340 to 380 s. The oscillation amplitude is typically a few per cent, but this star shows perhaps the most dramatic amplitude variability from night to night of any of the known sdB pulsators (EC 14026 stars). Analysis of multicolour absolute photometry, as well as low- and intermediate-dispersion spectroscopy, yields an effective temperature of 28 900 ± 300 K and log g  = 5.45 ± 0.05. Feige 48 is thus the coolest EC 14026 star. Its intermediate gravity and intermediate period suggest the existence of a period–gravity correlation, and unite the majority of the EC 14026 stars with the extreme object, PG 1605+072. The narrow frequency intervals in which the pulsations of Feige 48 and other EC 14026 stars fall suggest a narrow bandpass for the excitation mechanism.  相似文献   

3.
Horizontal branch stars should show significant differential rotation with depth. Models that assume systematic angular momentum exchange in the convective envelope and local conservation of angular momentum in the core produce HB models that preserve a rapidly rotating core. A direct probe of core rotation is available. The nonradial pulsations of the EC14026 stars frequently show rich pulsation spectra. Thus their pulsations probe the internal rotation of these stars, and should show the effects of rapid rotation in their cores. Using models of sdB stars that include angular momentum evolution, we explore this possibility and show that some of the sdB pulsators may indeed have rapidly rotating cores.  相似文献   

4.
Recent observational efforts and theoretical breakthroughs have encouraged the development of detailed asteroseismic analyses of rapidly oscillating sdB stars (the so-called EC14026 stars). This led to the first seismic determinations of the fundamental parameters that define the structure of EHB stars. We briefly review the current status of these analyses, discussing some of the properties of acoustic modes in EHB models that affect the asteroseismology of these stars. We then recall the basic ideas behind the method we developed in an attempt to objectively extract, from models, asteroseismic solutions suitable to any given sdB pulsator. A preliminary application of this method to the pulsating sdB star Feige 48 is also presented.  相似文献   

5.
We report the discovery of large-amplitude (∼0.25 mag) pulsations in the bright ( V =12.8) sdB star, PG 1605+072. The dominant period is 480 s, but more than 20 periods were present on at least three separate occasions. Frequency analysis of the complete data set yields more than 30 periods. A few of these are harmonics or linear combinations of the strongest modes. Excluding the latter, the periods span a range of almost 400 s, which contrasts with the typical range <20 s for most other EC 14026 stars.
Analysis of multicolour photometry limited any cool companion to being a main-sequence star of type M0 or later. Balmer line profile fitting yielded an effective temperature of 32 100±1000 K and a log g of 5.25±0.10, significantly smaller than in the other stars of the EC 14026 class, and possibly indicative of a more evolved state. The lower gravity is probably responsible for the fact that the pulsation periods and amplitudes are respectively much longer and larger than in other stars of the class. This star is an obvious target for asteroseismological investigation using a multilongitude photometric campaign.  相似文献   

6.
We present a FUSE abundance analysis of EC14026 stars. We compare the abundances of heavy elements in the atmospheres of EC14026 stars to non-pulsating stars with similar atmospheric parameters, and investigate whether weak stellar winds could explain the coexistence of variable and non-variable sdB stars in the log g – Teff diagram. We also present preliminary results on time-dependent diffusion calculations of iron in presence of radiative levitation and mass loss, and show how weak stellar winds can affect the diffusive equilibrium between gravitational settling and radiative support.  相似文献   

7.
During our campaign of acquiring follow-up photometric data to resolve short period pulsating sdB (EC14026 or V361 Hya) stars, we obtained data on the known pulsator KUV 04421+1416 and discovered that it is also in a reflection-effect binary. Here we present preliminary results of the pulsation analysis and provide some constraints on the companion, which is most likely an MV star. This makes KUV 04421+1416 only the second known system with an EC14026-type pulsator in a reflection-effect binary.  相似文献   

8.
We report the discovery of very rapid pulsations in three sdB stars from the Edinburgh–Cape blue object survey. The short periods, small amplitudes and multi-periodicity clearly establish these stars as members of the EC 14026 class. EC 11583−2708 has pulsation periods near 149, 144 and 114 s, though evidence is presented that the 149-s period is resolved into two periods at 148.87 and 148.55 s by the full photoelectric data set. The amplitudes of the detected variations are in the range 0.002–0.006 mag. The light variation of EC 20338−1925 is dominated by a period near 147 s with a very large amplitude for a variable sdB star (0.025 mag), though four other frequencies are detected with periods near 168, 151, 141 and 135 s and amplitudes in the range 0.002–0.005 mag. The third star, EC 09582−1137, displays a light curve which is virtually a textbook example of frequency beating, being produced by two pulsations of almost equal amplitude (∼0.008 mag) and periods near 136.0 and 151.2 s.  相似文献   

9.
This paper examines the observational evidence for the detectionof gravity modes in Scuti stars, which are p-mode pulsators.Low-order gravity modes have also been found in at least one star (FG Vir).Some reports of gravity modes may be due to systematic errors in theabsolute magnitude calibrations for slowly rotating stars. Furthermore,many detected low frequencies are not high-order gravity modes, but linear combinations,f i -f j , of the main pulsation modes. Other low frequencies are caused bya close binary companion leading to tidal deformation as well as tidallyexcited gravity modes.  相似文献   

10.
PB 8783 is a member of the newly discovered class of pulsating sdB stars, the EC 14026 stars. The potential of the pulsations of these stars as diagnostic probes of their interiors can only be realized fully by intensive campaigns of multisite time series photometry. To this end we have undertaken a campaign spanning 2 weeks and involving data from five different sites. 183 h of data were obtained. Frequency analysis yielded at least 11 pulsation periods in seven discrete groups between 94 and 136 s. Three of the periods in one specific group are equally spaced in frequency, with a spacing of 0.94 μHz. The frequency spacings amongst the members of other groups are similar to this. A rotation period of ∼12 d is implied. A general scheme for identifying the observed modes is suggested. The periods from a detailed model are in good overall agreement with the observed periods, but some difficulties remain. Three additional periods in the range 61–63 s, and with very low amplitude, were also found. No convincing interpretation of these could be devised.  相似文献   

11.
The sdB star PG 1336−018 is found to be a very short-period eclipsing binary system, remarkably similar to the previously unique system HW Vir. In addition, and unlike HW Vir, the sdB star in the PG 1336 system shows rapid oscillations of the type found in the recently discovered sdB pulsators, or EC 14026 stars. The orbital period, 0.101 0174 d, is one of the shortest known for a detached binary. Analysis of photoelectric and CCD photometry reveals pulsation periods near 184 and 141 s, with semi-amplitudes of ∼0.01 and ∼0.005 mag respectively. Both oscillations might have variable amplitude, and it is probable that other frequencies are present with amplitudes ∼0.003 mag or less. The 184- and 141-s pulsations are in the range of periods predicted by models for hot horizontal-branch stars. Analysis of medium-dispersion spectrograms yields T eff=33 000±1000 K and log g =5.7±0.1 for the sdB primary star, a radial velocity semi-amplitude K 1=78±3 km s−1 and a system velocity γ=6±2 km s−1. Spectrograms from the IUE Final Archive give T eff=33 000±3000 K and E ( B − V )=0.05 for log g =6.0 models. The derived angular radius leads to a distance of 710±50 pc for the system, and an absolute magnitude for the sdB star of +4.1±0.2. A preliminary analysis of U , V and R light curves indicates the orbital inclination to be near 81° and the relative radii to be r 1=0.19 and r 2=0.205. Assuming the mass of the sdB primary to be 0.5 M⊙ leads to a mass ratio q =0.3 for the system, and indicates that the secondary is a late-type dwarf of type ∼M5. As with HW Vir, it is necessary to invoke small limb-darkening coefficients and high albedos for the secondary star to obtain reasonable fits to the observed light curves.  相似文献   

12.
We report the discovery of rapid oscillations in the sdB star PG 1047+003. The oscillations are multiperiodic, with nine periods in the range 104–162 s. The optical spectrum of PG 1047+003 is consistent with that of a single sdB star. Line profile fitting yields an effective temperature of 35 000±1000 K and log g =5.9±0.1, although optical photometry and IUE spectrophotometry may indicate a cooler effective temperature. These properties demonstrate that PG 1047+003 is an EC 14026 star, a recently discovered class of sdB pulsators. Optical and infrared photometry constrains any cool companion to the sdB star to be a main-sequence star of spectral type M0 or later. With V =13.47 and a relatively rich pulsation spectrum, PG 1047+003 is an attractive target for an intensive photometric campaign to extract more periods from the light curve which, along with a suitable grid of pulsation models, will probe the interior structure of the star.  相似文献   

13.
We present the first results from the MultiSite Spectroscopic Telescope (MSST) observations of the sdBV star PG 1605+072. Pulsating sdB stars (also known as EC 14026 stars) offer the chance to gain new insights into the formation and evolution of extreme Horizontal Branch stars using the tools of asteroseismology. PG 1605+072 is an outstanding object in its class, with the richest frequency spectrum, the longest periods, and the largest variations. The MSST campaign took place in May/June 2002 immediately following the Whole Earth Telescope Xcov22 run, which observed PG 1605+072 as an alternate target. We will first give an overview of the project and its feasibility, after which we will present the massive data set, made up of 399 h of photometry and 151 h of spectroscopy. The overall aims of the project are to examine light/velocity amplitude ratios and phase differences, changes in equivalent width/line index, and λ-dependence of photometric amplitudes, and to use these properties for mode identification.  相似文献   

14.
We present the first results of an observational campaign aimed at detecting rapid extreme horizontal branch (EHB) pulsators in globular clusters. So far, we have observed multi-frequency luminosity variations for three EHB stars in ω Cen, with typical periods in the 100–120 s range. This is towards the short end, but comparable to, the periodicities measured for rapidly pulsating subdwarf B (sdB) stars in the field. Given that the effective temperatures of the variables discovered seem to be compatible with the instability strip for fast sdB pulsators, we believe we have uncovered the first such variables in a globular cluster.  相似文献   

15.
High-speed spectroscopy of two pulsating subdwarf B stars, KPD 2109+4401 and PB 8783, is presented. Radial motions are detected with the same frequencies as reported from photometric observations and with amplitudes of ∼2 km s−1 in two or more independent modes. These represent the first direct observations of surface motion arising from multimode non-radial oscillations in subdwarf B stars. In the case of the sdB+F binary PB 8783, the velocities of both components are resolved; high-frequency oscillations are found only in the sdB star and not the F star. There also appears to be evidence for mutual motion of the binary components. If confirmed, it implies that the F-type companion is ≳1.2 times more massive than the sdB star, while the amplitude of the F-star acceleration over 4 h would constrain the orbital period to lie between 0.5 and 3.2 d.  相似文献   

16.
We summarize recent results of quantitative spectral analyses using NLTE and metal line-blanketed LTE model atmospheres. Temperatures and gravities derived for hundreds of sdB stars are now available and allow us to investigate systematic uncertainties of T e ff, log g scales and to test the theory of stellar evolution and pulsations. Surface abundance patterns of about two dozen sdB stars are surprisingly homogenous. In particular the iron abundance is almost solar for most sdBs. We highlight one iron-deficient and three super metal-rich sdBs, a challenge to diffusion theory. sdB stars are slowly rotating stars unless they are in close binary systems, which is hard to understand if the sdB stars were formed in merger events. The only exception is the pulsator PG 1605+072 rotating at vsin i= 39 km/s. Signatures of stellar winds from sdB stars may have been found.  相似文献   

17.
Rapidly oscillating Ap stars constitute a unique class of pulsators with which to study non-radial oscillations under some — even for stars — unusual physical conditions. These stars are chemically peculiar, they have strong magnetic fields and they often pulsate in several high-order acoustic modes simultaneously. We discuss here an excitation mechanism for short-period oscillation modes based on the classical κ mechanism. We particularly stress the conditions that must be fulfilled for successful driving. Specifically, we discuss the roles of the chemical peculiarity and strong magnetic field on the oscillation modes and what separates these pulsators from δ Scuti and Am-type stars.  相似文献   

18.
We present FUSE spectra of three He-rich sdB stars. Two of these stars, PG1544+488 and JL87, reveal extremely strong C III lines, suggesting that they have mixed triple-α carbon from the deep interior out to their surfaces. Using TLUSTY NLTE line-blanketed model atmospheres, we find that PG1544+488 has a surface composition of 96% He, 2% C, and 1% N. JL87 shows a similar surface enrichment of C and N but still retains a significant amount of hydrogen. In contrast, the third star, LB1766, is devoid of hydrogen and strongly depleted of carbon, indicating that its surface material has undergone CN-cycle processing. We interpret these observations with new evolutionary calculations which suggest that He-rich sdB stars with C-rich compositions arise from a delayed helium-core flash on the white-dwarf cooling curve. During such a flash the interior convection zone will penetrate into the stellar envelope, thereby mixing the envelope with the He- and C-rich core. Such “flash-mixed” stars will arrive on the extreme horizontal branch (EHB) with He- and C-rich surface compositions and will be hotter than the hottest canonical EHB stars. Two types of flash mixing are possible:“deep” and “shallow”, depending on whether the hydrogen envelope is mixed deeply into the site of the helium flash or only with the outer layers of the core. Based on both their stellar parameters and surface compositions, we suggest that PG1544+488 and JL87 are examples of “deep” and “shallow” flash mixing, respectively.  相似文献   

19.
We present light curves of four binary subdwarf B stars (sdB), Ton 245, Feige 11, PG 1432+159 and PG 1017−086. We also present new spectroscopic data for PG 1017−086 from which we derive its orbital period,   P =0.073 d  , and the mass function,   f m=0.0010±0.0002 M⊙.  This is the shortest period for an sdB binary measured to date. The values of P and f m for the other sdB binaries have been published elsewhere. We are able to exclude the possibility that the unseen companion stars to Ton 245, Feige 11 and PG 1432+159 are main-sequence stars or subgiant stars from the absence of a sinusoidal signal, which would be caused by the irradiation of such a companion star, i.e. they show no reflection effect. The unseen companion stars in these binaries are likely to be white dwarf stars. In contrast, the reflection effect in PG 1017−086 is clearly seen. The lack of eclipses in this binary combined with other data suggests that the companion is a low-mass M-dwarf or, perhaps, a brown dwarf.  相似文献   

20.
HS0702+6043 (DW Lyn) is one of the very few known sdB pulsators which exhibit both pressure- and gravity-mode oscillations. Given its high potential for asteroseismology, we carried out an extensive campaign in integrated light photometry at the Steward Observatory 1.6 m Kuiper Telescope using the new CCD camera Mont4K, a joint venture between the Université de Montréal and the University of Arizona. Over a period starting November 1st 2007 up to March 14th 2008, we were able to gather some 424.5 hours of useful data. To our knowledge, never before a pulsating sdB star has been observed at such high frequency resolution and sensitivity. We present the results of a preliminary analysis of this unique data set.  相似文献   

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