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1.
C.S. Jeffery R.L.C. Starling P.W. Hill D. Pollacco 《Astrophysics and Space Science》2002,279(1-2):115-122
Speculative connections have been made between Sakurai's Object andother hydrogen-deficient stars, principally the RCB stars and [WC]central stars of planetary nebulae. RCBs have also been postulated as the precursors of extreme helium stars (EHes). The question arises to whether Sakurai's Object will evolve down the [WC]-PG1159 evolution track, or the RCB-EHe-HesdO track. From a number of IUE observations, we have measured thesecular contraction rates and pulsation masses of several EHes. These are inconsistent with the predicted contraction rates for helium-shell burning giants produced by a final helium-shell flash (the [WC]-PG1159 track). Although there may be some similarities between Sakurai's Object and some RCBs, if the RCB-EHe conection is valid then these must be superficial rather than structural. 相似文献
2.
Albert A. Zijlstra 《Astrophysics and Space Science》2001,275(1-2):79-90
A number of late [WC] stars have unique infrared properties, not foundamong the non-[WC] planetary nebulae, and together define a class of IR-[WC]stars. They have unusual IRAS colours, resembling stars in theearliest post-AGB evolution and possibly related to PAH formation.Most or all show a double chemistry, with both a neutral (molecular)oxygen-rich and an inner carbon-rich region. Their dense nebulae indicaterecent evolution from the AGB, suggesting a fatal-thermal-pulse (FTP)scenario. Although both the colours and the stellar characteristicspredict fast evolution, it is shown that this phase must last for104 yr. The morphologies of the nebulae are discussed. Forone object in Sgr, the progenitor mass (1.3 M⊙) is known.The stellar temperatures of the IR-[WC] stars appear much higher inlow metallicity systems (LMC, Sgr). This may be indicative of anextended `pseudo' photosphere. It is proposed that re-accretion ofejected gas may slow down the post-AGB evolution and so extend the lifetime of the IR-[WC] stars. 相似文献
3.
Falk Herwig 《Astrophysics and Space Science》2002,279(1-2):103-113
Sakurai's Object is a born again AGBstar of the very late thermal pulse flavor. In thiscontribution I will discuss new models of stellar evolution andnucleosynthesis models of this phase. Two most intriguing properties ofSakurai's Object have so far not been understood theoretically: the peculiar chemical appearance, in particular the high lithiumabundance, and the short time scale of only a few years on which thetransition from the dwarf configuration into the born again giantappearance has occurred. A new nucleosynthesis mode of hot hydrogen-deficient
3He burning can explain the extraordinarylithium abundance. During the thermal pulse 3He is ingested fromthe envelope together with the protons into the hot He-flashconvection zone. The first network calculations show that, due to thelarge 12C abundance protons are captured rather by carbon, thandestroy newly formed 7Be and ultimately 7Li. Moreover, the shortevolution time scale has been reproduced by making the assumption that the convective efficiency forelement mixing is smaller by two to three orders of magnitude thanpredicted by the mixing-length theory. As a result the main energygeneration from fast convective proton capture will occur at a largermass coordinate, closer to the surface and the expansion to the giantstate is accelerated to a few years, in excellent agreement with thebehavior of Sakurai's Object. This result represents an independent empiricalconstraint on the poorly known efficiency of element mixing inconvective zones of the stellar interior. 相似文献
4.
Orsola De Marco 《Astrophysics and Space Science》2002,279(1-2):157-166
[WC] central stars of planetary nebulae are members of the larger class of hydrogen-deficient central stars. The whole class constitutes about20% of all spectroscopically-known central stars. Observational connections between [WC] central stars and the born-again phenomenon show that at least a fraction of the [WC] stars can be createdthrough this scenario. However, it is unlikely that the class as a wholeevolved through this channel.In this paper the arguments against a born-again origin for the whole class of [WC] central stars of planetary nebula are outlined. It is suggested that the roleof the H-deficient weak emission lines stars might be crucial in explaining the origin of [WC]stars. It is also demonstrated how difficult it isto pin down the exact stellar parameters of these objects (which help toposition them on the HR diagram). This is due to the largely unknown distancesand to the fact that small changesin the model assumptions can have large repercussions on the derived parameters.This difficultyhampers our efforts to determine the true evolutionary position of individual [WC] central stars, as well as their relationship to one another, andtherefore pin down their origin. 相似文献
5.
M. Cohen 《Astrophysics and Space Science》2001,275(1-2):103-111
The rather rare class of central stars of planetary nebulae thatshow Wolf-Rayet spectra have been a subject of great interest,particularly in the infrared, since their discovery in the late1960s. I will focus on further peculiarities found within thepast 1-2 years with the advent of infrared spectroscopy fromISO. Notably, these stars simultaneously betray the presenceof regions of carbon-rich and oxygen-rich dust chemistry. Icompare and contrast complete ISO spectra from 2 to 200 micronsof a small sample of [WC9] to [WC11] central stars. 相似文献
6.
We review elemental abundances derived for planetary nebula (PN) WCcentral stars and for their nebulae. Uncertainties in the abundances of[WC] stars are still too large to enable an abundance sequenceto be constructed. In particular it is not clear why the hotter [WCE]stars have C and O abundances which are systematically lower than those oftheir supposed precursors, the [WCL] stars. This abundance differencecould be real or it may be due to unaccounted-for systematic effects inthe analyses. Hydrogen might not be present in [WC] star winds asoriginallysuggested, since broad pedestals observed at the base of nebular lines canplausibly be attributed to high velocity nebular components. It isrecommended that stellar abundance analyses should be carried out withnon-LTE model codes, although recombination line analyses can provideuseful insights. In particular, C II dielectronic recombinationlines provide a unique means to determine electron temperatures in cool[WC] star winds. We then compare the abundances found for PNe which have [WC] central starswith those that do not. Numerous abundance analyses of PNe have beenpublished, but comparisons based on non-uniform samples and methods arelikely to lack reliability. Nebular C/H ratios, which might be expected todistinguish between PNe around H-poor and H-rich stars, are rather similarfor the two groups, with only a small tendency towards larger values fornebulae around H-deficient stars. Nebular abundances should be obtainedwith photoionization models using the best-fitting non-LTE modelatmosphere for the central star as the input. Heavy-metal line blanketingstill needs to be taken into consideration when modeling the central star,as its omission can significantly affect the ionizing fluxes as well asthe abundance determinations. We discuss the discrepancies between nebularabundances derived from collisionally excited lines and thosederived from optical recombination lines, a phenomenon that may havelinks with the presence of H-deficient central stars. 相似文献
7.
D. H. Morgan Q. A. Parker D. Russeil 《Monthly notices of the Royal Astronomical Society》2001,322(4):877-884
The central stars of two of the new planetary nebulae found during scans of the AAO/UKST H α Survey of the Milky Way have been found to exhibit Wolf–Rayet (WR) emission features. One (PMR 1) is an early-type star of class either [WO4] or [WC4]. The other (PMR 2) is a late [WC] star which, depending on the classification scheme used, is either intermediate in class between [WC9] and [WC10] or the sole member of the [WC10] class. Both stars exhibit unusual spectral features which may be attributed to enhanced nitrogen in their atmospheres and could be indicative of unusual stellar evolution. 相似文献
8.
S. P. S. Eyres B. Smalley T. R. Geballe A. Evans M. Asplund V. H. Tyne 《Monthly notices of the Royal Astronomical Society》1999,307(1):L11-L15
We report the appearance and evolution during 1998 of strong neutral helium 3 S–3 Po absorption at ∼10 830 Å in Sakurai's Object (V4334 Sgr), which is believed to be a planetary nebula nucleus (PNN) undergoing a final helium shell-flash. First detected on 1998 March 18, the profile of the He i feature is P Cygni-like. The absorption depth has increased in three subsequent spectra in 1998. If this is owing to a wind, the profile indicates a wind velocity of ∼670±50 km s−1 . The strong C i 10 690-Å line seen prior to the appearance of the helium feature has disappeared; however Sr ii and CN absorption features remain present. We tentatively identify several new features as Si i . Taken together with other observations we suggest that the data are consistent with Sakurai's Object entering a phase in which it seems to have become a member of the R Coronae Borealis-type class of stars. 相似文献
9.
We create a grid of evolutionary models which include models thatexhibit born-again behavior; that is a very late helium pulse. Ourmodels include metalicities between Z = 0.001 and Z = 0.020. Massloss is varied beginning at the peak of the last thermal pulse onthe AGB. By doing this, we determine the range of helium mass atAGB departure that later produces a very late helium flash. Wepresent a direct comparison between our models, Sakurai's Object andFG Sge. Based on our comparisons, we make an observable predictionfor the future of Sakurai's Object: We expect it to increase intemperature and decrease slightly in luminosity within the next 20to 70 years and then to cool and brighten a second time with alonger time scale of roughly 200 to 500 years. It will become asFG Sge is now. 相似文献
10.
Martin Cohen M. J. Barlow X.-W. Liu A. F. Jones 《Monthly notices of the Royal Astronomical Society》2002,332(4):879-890
The rather rare class of central stars of planetary nebulae that show very low-excitation Wolf–Rayet spectra has been a subject of great interest, particularly in the infrared, since its discovery in the late 1960s. Further peculiarities have been found with the advent of infrared spectroscopy from ISO . Notably, these objects simultaneously betray the presence of regions of carbon-rich and oxygen-rich dust chemistry. We compare and contrast complete ISO spectra between 2 and 200 μm of a sample of six [WC8] to [WC11] central stars, finding many similarities. Among this sample, one star provides strong evidence of quasi-periodic light variations, suggestive of a dust cloud orbiting in a plane from which we view the system. 相似文献
11.
Geoffrey C. Clayton 《Astrophysics and Space Science》2002,279(1-2):167-170
The R Coronae Borealis (RCB) stars are rare hydrogen-deficient carbon-rich supergiants which undergo spectaculardeclines in brightness of up to 8 magnitudes at irregular intervals as dust forms along the line of sight.Understanding the RCB stars is a key test for any theory whichaims to explain hydrogen deficiency in post-Asymptotic Giant Branch (AGB) stars. There are two major evolutionary models for the origin of RCB stars: the Double Degenerate and the Final Helium ShellFlash. In the final flashmodel, there is a close relationship between RCB stars and Planetary Nebulae (PNe). The connection between RCB stars and PNe has recentlybecome stronger, since the central stars of three old PNe (Sakurai's Object, V605 Aql and FG Sge) have had observedoutbursts that transformed them from hot evolved central stars into cool giants with the spectral properties of an RCB star. 相似文献
12.
Rémi Fahed Anthony F. J. Moffat Alceste Z. Bonanos 《Monthly notices of the Royal Astronomical Society》2009,392(1):376-382
Do some Wolf–Rayet stars owe their strong winds to something else besides radiation pressure? The answer to this question is still not entirely obvious, especially in certain Wolf–Rayet subclasses, mainly WN8 and WC9. Both of these types of Wolf–Rayet stars are thought to be highly variable, as suggested by observations, possibly due to pulsations. However, only the WN8 stars have so far been vigorously and systematically investigated for variability. We present here the results of a systematic survey during three consecutive weeks of 19 Galactic WC9 stars and one WC8 star for photometric variability in two optical bands, V and I . Of particular interest are the correlated variations in brightness and colour index in the context of carbon dust formation, which occurs frequently in WC9 and some WC8 stars. In the most variable case, WR76, we used this information to derive a typical dust grain size of ∼ 0.1 μm . However, most photometric variations occur at surprisingly low levels, and in fact almost half of our sample shows no significant variability at all above the instrumental level ( σ∼ 0.005– 0.01 mag). 相似文献
13.
P. Bartzakos A. F. J. Moffat †‡ V. S. Niemela ‡§ 《Monthly notices of the Royal Astronomical Society》2001,324(1):33-50
A search for evidence of colliding winds is undertaken among the four certain Magellanic Cloud WC/WO spectroscopic binaries found in the companion Paper I, as well as among two Galactic WC/WO binaries of very similar subtype. Two methods of analysis, which allow the determination of orbital inclination and parameters relating to the shock cone from spectroscopic studies of colliding winds, are attempted. In the first method, Lührs' spectroscopic model is fitted to the moderately strong C iii 5696-Å excess line emission arising in the shock cone for the stars Br22 and WR 9. The four other systems show only very weak C iii 5696-Å emission. Lührs' model follows well the mean displacement of the line in velocity space, but is unable to reproduce details in the line profile and fails to give a reliable estimate of the orbital inclination. In the second method, an alternative attempt is also made to fit the variation of more global quantities, full width at half-maximum and radial velocity of the excess emission, with phase. This method also gives satisfactory results in a qualitative way, but shows numerical degeneracy with orbital inclination. Colliding wind effects on the very strong C iv 5808-Å Wolf–Rayet emission line, present in all six binaries, are also found to behave qualitatively as expected. After allowing for line enhancement in colliding wind binaries, it now appears that all Magellanic Cloud WC/WO stars occupy a very narrow range in spectral subclass: WC4/WO3. 相似文献
14.
The R Coronae Borealis (RCB) stars represent a rare, or short-lived stage of low and intermediate mass stellar evolution. They are important in the context of mass-loss anddust formation on the Asymptotic Giant Branch.These stars are defined by their large declines in brightness at irregular intervals caused by dust formation.There are two major evolutionary models for the origin of RCB stars, the Double Degenerate and the Final Helium Shell Flash.In the final flash model, RCB stars lie at the centers of old Planetary Nebulae (PN). Here I discuss possible relationships with the [WC] stars. 相似文献
15.
We present new near-infrared observations of Sakurai's Object obtainedduring 1998–99 when this final helium shell flash object was in the dustcondensation phase. The infrared colours have reddened compared to earlierepochs, indicating increased dust condensation. The infrared spectrareveal all the features of a carbon star superposed on a dust continuum. 相似文献
16.
Hans Van Winckel 《Astrophysics and Space Science》2001,275(1-2):159-167
In this contribution the observational characteristicsof F to G type post-AGB stars, showing both O-rich andC-rich circumstellar chemistry, are reviewed. It turnsout that binarity and the presence of a stablecircumbinary dusty disc are fundamental properties of these objects. The possible common origin of the mixed chemistry observed in these systems and in IR-bright [WC] stars is discussed. 相似文献
17.
Michaël De Becker 《Astronomy and Astrophysics Review》2007,14(3-4):171-216
In this paper, I present a general discussion of several astrophysical processes likely to play a role in the production of
non-thermal emission in massive stars, with emphasis on massive binaries. Even though the discussion will start in the radio
domain where the non-thermal emission was first detected, the census of physical processes involved in the non-thermal emission
from massive stars shows that many spectral domains are concerned, from the radio to the very high energies. First, the theoretical
aspects of the non-thermal emission from early-type stars will be addressed. The main topics that will be discussed are respectively
the physics of individual stellar winds and their interaction in binary systems, the acceleration of relativistic electrons,
the magnetic field of massive stars, and finally the non-thermal emission processes relevant to the case of massive stars.
Second, this general qualitative discussion will be followed by a more quantitative one, devoted to the most probable scenario
where non-thermal radio emitters are massive binaries. I will show how several stellar, wind and orbital parameters can be
combined in order to make some semi-quantitative predictions on the high-energy counterpart to the non-thermal emission detected
in the radio domain. These theoretical considerations will be followed by a census of results obtained so far, and related
to this topic. These results concern the radio, the visible, the X-ray and the γ-ray domains. Prospects for the very high energy γ-ray emission from massive stars will also be addressed. Two particularly interesting examples—one O-type and one Wolf-Rayet
binary—will be considered in details. Finally, strategies for future developments in this field will be discussed. 相似文献
18.
Vito Francesco Polcaro Corinne Rossi Roberto Viotti Laura Norci 《Astrophysics and Space Science》1996,238(1):59-62
Long slit spectra of the planetary nebula NGC 5189 with its [WC2] type central object and the G2.4+1.4 wind bubble around the evolved Pop. I WO star Sand 4 were obtained in order to investigate the observable effects of different initial masses in stars with otherwise similar spectra. Our preliminary results of a comparative spectroscopic study of these two objects shows that the chemical composition of the two nebuale is completely different, even though their morphology is most probably quite similar. In addition, the PN appears much more chemically homogeneous. These features are clearly associated with the evolutionary paths of the stars. 相似文献
19.
Garrelt Mellema 《Astrophysics and Space Science》2001,275(1-2):147-157
This review discusses the physics of the formation of planetarynebulae around low mass WR stars, or [WR] stars. It especially focuseson the differences which can be expected due to the differentcharacter of the fast winds from these [WR] stars. Their fast windsare more massive and are highly H deficient and metal enrichedcompared to the winds of normal central stars of planetarynebulae. This is expected to lead to faster expansion velocities forthe nebulae and a longer momentum-driven phase in the evolution of thewind-driven bubble, leading to more turbulent nebulae. Theobservational evidence also shows that the process which produces the[WR] stars is unlikely to influence the onset of aspherical mass loss,something which can be used as a test for models for aspherical massloss from AGB and post-AGB stars. Finally it is shown that thenebular characteristics rule out a very late He shell flash as theorigin of most [WR] stars. 相似文献
20.
Allan J. Willis 《Astrophysics and Space Science》1996,237(1-2):145-168
This paper reviews the current status of knowledge regarding the basic physical and chemical properties of Wolf-Rayet stars;
their overall mass loss and stellar wind characteristics and current ideas about their evolutionary status. WR stars are believed
to be the evolved descendents of massive O-type stars, in which extensive mass loss reveals successive stages of nuclear processed
material: WN stars the products of interior CNO-cycle hydrogen burning, and WC and WO stars the products of interior helium
burning. Recent stellar evolution models, particularly those incorporating internal mixing, predict results which are in good
accord with the different chemical compositions observationally inferred for WN, WC and WO stars. WR stars exhibit the highest
levels of mass loss amongst earlytype stars: mass loss rates, typically, lie in the range [1–10]×10−5
M
⊙yr−1. Radiation pressure-driven winds incorporating multi-scattering in high ionisation-stratified winds may cause these levels,
but additional mechanisms may also be needed. 相似文献