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1.
Helium-rich subdwarf-B (He-sdB) stars are extremely rare hot subluminous stars found in the field of our Galaxy as well as in some globular clusters. The existence of these hot helium stars cannot be explained by canonical stellar evolution theories nor can it be explained by normal sdB evolution. We discuss the existing evolutionary models for the formation of He-sdB stars—the flash mixing model and the binary white dwarf merger model in the light of new observational results. Spectral classification of objects as He-sdB stars by various authors has resulted in a range of objects, including white dwarfs, being classified as He-sdB stars. We propose a homogeneous definition for this class of objects based on the original classification scheme used in the PG catalogue. Spectral analysis of He-sdB stars in the last 15 years is also briefly reviewed. 相似文献
2.
《New Astronomy》2020
We have compiled infrared photometric data from the literature of practically all T Tauri stars found up to date including 444 classical T Tauri stars (CTTSs), 1698 weak-line T Tauri stars (WTTSs) and 1258 not classified T Tauri stars (3400 in total) in addition to 196 post-T Tauri stars (PTTSs). From this data bank we extract the infrared characteristics of the different groups and discuss different origins of the infrared radiation. The observational data are taken from the AKARI, IRAS, WISE and 2MASS missions. We show that in the wavelength range 1–140 μm, all T Tauri stars have infrared excesses. CTTSs have more infrared excess than WTTSs, while PTTSs have little or no infrared excess. We found that in the 1–3 μm wavelength range the infrared emission of T Tauri stars is mainly due to thermal radiation from the photosphere and hot dust grains from circumstellar envelopes. In the 3–140 μm wavelength range the infrared emission of T Tauri stars is mainly due to radiation from dusty/gaseous disks surrounding the stars. In addition, we also make a comparison between T Tauri stars and Herbig AeBe stars (HAeBe). There are some differences between these two kinds of objects in that for HAeBe stars the infrared radiation as a rule originates in dusty/gaseous disks in the 1–3 μm wavelength range, while in the range 3–12 μm it is possibly due to PAH emission for about half of HAeBe stars. In other wavelength ranges both kinds of stars have similar infrared characteristics indicating emission from dusty/gaseous disks. 相似文献
3.
M. H. Jones † 《Monthly notices of the Royal Astronomical Society》2001,327(1):191-198
The infrared properties of barium stars are studied using published data in the K band and from IRAS . At 12 and 25 μm the emission from barium stars shows no excess over photospheric emission. Thus the claim made by Hakkila that some barium stars show evidence of the presence of warm (∼300 K ) circumstellar material is not supported. The 60-μm properties of barium stars are studied using survival analysis methods, and it is found that very few (3.7 ± 2.6 per cent) barium stars exhibit far-infrared excesses. Furthermore, it is found that the incidence of excess emission at 60 μm is lower in barium stars than for normal G and K giants. This may indicate that the mass-transfer event that is assumed to have taken place in barium stars has removed any cool circumstellar material that may have existed in these systems. Alternatively, it is suggested that the incidence of infrared excesses in normal G and K giants may have been over-estimated as a result of not fully accounting for foreground contamination by interstellar cirrus. 相似文献
4.
The spectral signatures of circumstellar dust shells (CS) of late-type stars cannot be tied in a simple manner to their optical spectral features. The spectral indices of featureless low-resolution spectrometer (LRS) spectra of K and early-M (M0-M5) stars are consistent with that of the Rayleigh-Jeans law and those of late-M (M6 and later) stars tend to have lower values showing CS radiation. There is no correlation observed between the optical sub-type and the 10 m emission strength for M stars. The absence of a simple relationship is also evident for C stars where the temperature and abundance indices show no correlation with the 11 m emission strength. The sample is confined to the IRC stars identified with accurate spectra observed by the LRS of IRAS. The IRC stars are the whole sky survey done with the 2 m band, where late-type stars are bright and interstellar extinction is one-tenth of the visual colour band. 相似文献
5.
We collected 55 galactic extreme carbon stars from the published literature in this paper. Observational data from IRAS, 2MASS
and ISO were analyzed. The results show that the infrared properties of extreme carbon stars are quite different to those
for ordinary visual carbon stars. It is shown from IRAS and 2MASS photometric data that extreme carbon stars have much redder
infrared colors not only in the far infrared, but also in the near infrared, hence they have much thicker ciucumstellar envelopes
and mass loss. It is also indicated from IRAS Low-Resolution Spectra (LRS) and ISO Short Wavelength Spectra (SWS) that they
have much redder infrared spectra from 2 μm to 45 μm. The above results are believed to be the signature of undergoing the
last stages of AGB evolution for extreme carbon stars. 相似文献
6.
Yu. V. Glagolevskij 《Astrophysics》2003,46(3):319-328
The low rotation velocities of magnetic CP stars are discussed. Arguments against the involvement of the magnetic field in the loss of angular momentum are given: (1) the fields are not strong enough in young stars in the stage of evolution prior to the main sequence; (2) there is no significant statistical correlation between the magnetic field strength and the rotation period of CP stars; (3) stars with short periods have the highest fields; (4) a substantial number of stars with very low magnetic fields (B
e < 500 G) have rotation speeds that are typical of other CP stars; (5) simulations of the magnetic fields by Leroy and the author show that the orientation of dipoles inside rotating stars, both slow and fast, is consistent with an arbitrary orientation of the dipoles; and, (6) slow rotators with P>25 days, which form 12% of the total, probably lie at the edge of the velocity distribution for low mass stars. All of these properties conflict with the hypothesis of magnetic braking of CP stars. 相似文献
7.
We collected almost all Galactic Wolf-Rayet (hereafter WR) stars found so far from the literature. 578 WR stars are gathered in this paper. 2MASS counterparts with good quality magnitudes in all JHK bands are listed for 364 WR stars. In addition, WISE counterparts for these sources are also identified. It is found that free-free emission is the main dominant source for the infrared excess in most WR stars up to 3.4 μm. However at the longer wavelengths the thermal radiation is dominant. In addition, WR stars in Clusters of the Galactic center region have the strong infrared excess in the near infrared due to the dust thermal emission from the strong star forming activity in the Galactic center region. For some WR stars with the WC spectral type, in particular, with WCd type, the dust thermal emission is important radiation source while many WR stars with the WC spectral type have the near infrared flux enhancement from the broad line emission in the K band. It is also shown that many single WC stars with different spectral sub-types have different locations in the near infrared two-color diagram, in particular, WC6 and WC9d stars can be separated respectively from other spectral type stars while single WN stars with different spectral sub-types can not be separated in the near infrared two-color diagram. 相似文献
8.
Near-infrared photometry of 35 SiO maser stars (v = 1, J = 1 - 0) of various types is presented in this paper. Combining the JHK flux densities from IRAS, the distributions of the stars on two color-color diagrams are obtained. The spectral slopes, equivalent black-body temperatures, maser luminosities and mass-loss rates of the central stars are calculated from the infrared or radio data. These parameters are then analysed together with the integrated SiO fluxes. The results show that SiO masers are common in AGB stages. The integrated SiO maser flux is only weakly correlated with the mass-loss rate, and increase slowly along the sequence from Mira stars to OH/IR stars. It is more closely correlated with the near-infrared colors and the equivalent black-body temperatures. Also, its upper limit is correlated with 12μm and 25μm fluxes. We also briefly discusses the general function and specific role of SiO maser in the evolution of AGB stars. 相似文献
9.
Chen Pei-sheng Gao Heng Bao Meng-xian Xiong Guang-zhu 《Chinese Astronomy and Astrophysics》1992,16(4):399-406
The JHKL' photometry and 2.3–4 μm spectrophotometry of some M giants, S type stars and carbon stars are presented in this paper. It is found that in combination with IRAS data, the energy spectra in 1–100 μm of S type stars are intermediate between those of M giants and carbon stars, which are obviously different. The spectrophotometry in the near infrared shows that, besides carbon stars, which have HCN and C2H2 strong absorptions at 3.1 μm, some S type stars have the similar but weaker absorption in the same spectral region. However, no trace of any absorption at 3.1 μm can be seen in M giants. These results probably provide more evidence for the M-S-C sequence in the late stage of stellar evolution. 相似文献
10.
M.D. Reed 《Astrophysics and Space Science》2003,284(1):73-84
The goal of asteroseismology is to discern the physical conditions of stars by comparing observed pulsations with models.To obtain this goal, the observed pulsation periods and the spherical harmonics (n, , and m) need to match the theoretical model.Typically the most difficult part in this process is the identification of the pulsation modes in the observations.Multicolour photometry is one method that has proven useful for identifying pulsation modes.By observing stars through various wavebands, and comparing the amplitudes and phases, it is possible to determine the spherical harmonics.This contribution will emphasize the work of Watson (1988), which has since been applied to many different types of variable stars including Scuti (Garrido et al., 1990), Doradus (Breger et al., 1997), Cepheid (Cugier et al., 1994), and EC 14026 (Koen, 1998) stars. I will also discuss the technique of summing spectra (especially UV) into various wavebands which are then used to identify modes as pioneered by Robinson, Kepler, and Nather (1982) and applied to white dwarf stars (Kepler et al., 2000). 相似文献
11.
By using the 2.16 m telescope of Xinglong Observing Station of National Astronomical Observatories and its high-dispersion spectrograph, the high-dispersion spectroscopic observations of six T Tauri-type stars with weak emission lines (i.e., weak-line T Tauri-type stars, abbreviated as WTTS) were carried out. The lithium abundances of these weak-line T Tauri-type stars are calculated and the relationships of the lithium abundances with the rotation periods as well as the amplitudes of light variations of these stars are discussed. It is found by this study that the lithium abundance for the weak-line T Tauri-type stars with fast rotations tends to be less than that of those with slow rotations. However, for all these weak-line T Tauri-type stars, the lithium abundances have no conspicuous correlation with the amplitudes of light variations of these stars in the V waveband. 相似文献
12.
Spitzer observations of acetylene bands in carbon-rich asymptotic giant branch stars in the Large Magellanic Cloud 总被引：1，自引：0，他引：1
M. Matsuura P. R. Wood G. C. Sloan A. A. Zijlstra J. Th. van Loon M. A. T. Groenewegen J. A. D. L. Blommaert M.-R. L. Cioni M. W. Feast H. J. Habing S. Hony E. Lagadec C. Loup J. W. Menzies L. B. F. M. Waters P. A. Whitelock 《Monthly notices of the Royal Astronomical Society》2006,371(1):415-420
We investigate the molecular bands in carbon-rich asymptotic giant branch (AGB) stars in the Large Magellanic Cloud (LMC), using the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope ( SST ) over the 5–38 μm range. All 26 low-resolution spectra show acetylene (C2 H2 ) bands at 7 and 14 μm. The hydrogen cyanide (HCN) bands at these wavelengths are very weak or absent. This is consistent with low nitrogen abundances in the LMC. The observed 14 μm C2 H2 band is reasonably reproduced by an excitation temperature of 500 K. There is no clear dilution of the 14 μm C2 H2 band by circumstellar dust emission. This 14-μm band originates from molecular gas in the circumstellar envelope in these high mass-loss rate stars, in agreement with previous findings for Galactic stars. The C2 H2 column density, derived from the 13.7 μm band, shows a gas mass-loss rate in the range 3 × 10−6 to 5 × 10−5 M⊙ yr−1 . This is comparable with the total mass-loss rate of these stars estimated from the spectral energy distribution. Additionally, we compare the line strengths of the 13.7 μm C2 H2 band of our LMC sample with those of a Galactic sample. Despite the low metallicity of the LMC, there is no clear difference in the C2 H2 abundance among LMC and Galactic stars. This reflects the effect of the third dredge-up bringing self-produced carbon to the surface, leading to high carbon-to-oxygen ratio at low metallicity. 相似文献
13.
Unidentified infrared emission bands (UIR bands) have been attributed to polycyclic aromatic hydrocarbons (PAHs), which are
believed to require ultraviolet radiation in order for the UIR bands to be excited. If, in addition to amorphous carbon and
hydrogenated amorphous carbon (HAC) particles, PAHs are able to form in the outflows of cool carbon-rich stars (Cherchneff
et al. 1991), then the weak UV radiation field from such stars would be unlikely to be able to excite the UIR bands and so
the PAH species could remain undetected in the spectra of C-stars. However, cool carbon stars with hot companions might be
exposed to strong enough UV radiation fields for UIR-band emission to be excited from PAHs. Buss et al. (1991) reported the
detection of the 8 μm UIR-band (C-C stretch) in the IRAS LRS spectrum of HD 38218 (TU Tau), a carbon star with a hotter A2III
companion. To investigate the phenomenon further, we have therefore obtained UKIRT CGS3 10 μm spectra of three carbon stars
with hot companions, TU Tau, UV Aur and CS776. It was found that TU Tau showed the 11.25 μm and 8.6 μm UIR-bands (both attributed
to C-H bend modes) at good contrast, while UV Aur clearly exhibited the 11.25 μm UIR band. No narrow UIR-band emission was
detected in the spectrum of CS776. We have fitted these 10 μm region spectra using a χ2-minimization program equipped to fit stellar and dust emission continua together with the broad SiC feature and the narrow
UIR-bands. The features seen in the spectra of TU Tau and UV Aur can be well fitted by a narrow 11.25 μm UIR-band sitting
on top of a broad, self-absorbed 11.3 μm silicon carbide feature. Our results therefore provide strong support for the supposition
that PAHs can form in carbon star outflows.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
14.
Yu.N. Gnedin 《Astrophysics and Space Science》1997,252(1-2):95-106
Astronomical methods of searching for light Goldstone bosons (axions and arions), which are candidates for dark matter, are briefly discussed. Three processes for the coupling between axions and photons are considered: (a) the decay of axions into two photons; (b) the conversion of photons into axions in the magnetic fields of stars and interstellar and intergalactic media; (c) the inverse transformation of axions generated inside the cores of stars. The intergalactic light of clusters of galaxies and the brightness of the night sky are good candidates for searches for an emission line arising from the radiative decay of axions. The results of observations made with the 6 m telescope of the Special Astrophysical Observatory do not provide any evidence for the existence of the hadronic axion decay line. The best upper limit for the photon-axion coupling constant is derived from linear polarization observations of magnetic Ap stars. 相似文献
15.
We present velocity observations, obtained with the E. W. Fick Observatory 0.6 m telescope, of the 7th magnitudeK1 star HR 7112. Our observations show that HR 7112 is a double-line spectroscopic binary system composed of almost equal mass
stars with a period of 215.6 days. The orbit has an eccentricitye = 0.142 and the stars orbit each other with a mean semi-major axis distance of approximately 0.4sin(i) AU. The masses of
the stars are consistent with the classification as giant stars. Tidal effects are known to circularise the orbits of giant
stars and this process is apparently underway in this system. 相似文献
16.
T. Montmerle 《Astronomische Nachrichten》2008,329(2):210-213
Star-forming regions have been observed in X-rays since the first generation of satellites in the late 70s. They are very rich in magnetically-controlled X-ray phenomena: stellar flares and star-disk interactions in hundreds of T Tauri stars, confined winds in massive stars, etc. More recently, in a few low-mass stars, X-ray evidence has been found for accretion shocks. Even if it is not dominant, when it is found the influence of the circumstellar environment on X-ray emission gives precious clues on the magnetic structure in the vicinity of young stars. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
17.
A new spectrograph (FIASCO) is in operation at the 0.9 m telescope of the University Observatory Jena. This article describes the characterization of the instrument and reports its first astronomical observations, among those lithium (6708 Å) detection in the atmosphere of young stars, and the simultaneous photometric and spectroscopic monitoring of variable stars (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
18.
B. N. G. Guthrie 《Astrophysics and Space Science》1969,3(4):542-551
On the theory that peculiar A stars were once secondaries in binary systems in which the primaries exploded as type II supernovae, the nucleosynthesis during the final stages of evolution of massive stars is investigated. For heavy elements (Z>30) the observed abundances in peculiar. A stars reflect the composition of material ejected by the exploding primaries. Peculiar A stars are divided into two groups, the main group and the Mn group, and abundances in each group are summarised. During the explosions of the primaries, rapid (n, ) or (, n) reactions operate on the abundance peaks previously formed by the s-process during the giant phase. In the main group primaries (n, ) reactions predominate, and rare-earths are formed from the Ba peak. In the Mn group primaries (, n) reactions operate on the Sr, Ba and Pb peaks to form Kr, Xe and Hg. 相似文献
19.
Abstract— Infrared spectra of mineral grains from primitive meteorites could be useful for comparison with astronomical infrared spectra since some of their grains might be similar to those formed in the planet‐forming disks around young stars or in the envelopes surrounding late‐type stars. To assess the usefulness of meteorite spectra, olivine grains separated from primitive meteorites have been analyzed using FTIR microscope techniques in the 2–16 μm wavelength range. The sub‐micron sizes of the grains made a complex preparation process necessary. Five characteristic bands were measured near 11.9, 11.2, 10.4, 10.1, and 10.0 μm. The results of 59 analyses allow the calculation of band positions for meteoritic olivines as a function of their iron and magnesium contents. Comparison of the meteoritic results with astronomical data for comets and dust around young and old stars, which exhibit bands similar to the strongest infrared bands observed in the grains (at 11.2 μm), show that the spectral resolution of the astronomical observations is too low to ascertain the exact iron and magnesium (Mg: Fe) ratio of the dust in the 8–13 μm wavelength range. 相似文献
20.
Cornelis De Jager 《Solar physics》1990,126(1):201-205
The granulation boundary is a line running in the HR diagram from about F0V stars to G1Ib. It divides the HR diagram in two regions in which the spectral line bisector behaves differently. To the right (cooler stars) the Fraunhofer lines in stellar spectra show the characteristics typical for convective motions. For hot stars it indicates the presence of large velocities involving a large part of the line-forming region. We give evidence that the opposite behaviour shown by spectra of hotter stars can be explained by gravity waves. 相似文献