共查询到20条相似文献，搜索用时 15 毫秒
1.
Present Observational Status of the Intermediate Mass Stars: δ Sct Stars, γ Dor Stars and roAp Stars
D.W. Kurtz 《Astrophysics and Space Science》2003,284(1):29-36
The present observational status of the Sct stars, Dor stars and roAp stars is discussed. The Sct stars are the most intensively observed of the three groups, but it has become clear that there are severe problems in extracting asteroseismic information from them. Dozens of frequencies are observed, but hundreds of frequencies are predicted from the models; unique matches of observation and theory still elude us. The Sct stars are observationally complex – some recent `best case' campaigns are discussed. It is possible that substantial observational advances for Sct stars may need to await upcoming satellite missions. New Dor stars are beingdiscovered frequently, and new behaviour is being found for them. They constitutean observationally young field. Their pulsational frequency range is being expanded, their position in the HR diagram is becoming better known (but is yet to be fully constrained), and the possibility exists of hybrid Dor – Sct stars that have greatasteroseismic promise, although it is clear such stars are rare, if they do exist. It has been observationally challenging to extract more than a fewfrequencies for any Dor star so far. Exciting spectroscopic discoveries of new behaviour in roAp stars promise unprecedented information about the structure of the peculiar atmospheres ofthose stars – pulsation amplitude and phase in 3D, magnetic field structurein 3D, abundance stratification in 3D, realistic T- for the most peculiarstars – as well as entirely new information about the interaction of pulsation,rotation and magnetic fields. Recent theoretical work has led to new understandingof the previously inexplicable frequency spacing of HR 1217 with new Whole Earth Telescope observations supporting this theory. An `improved oblique pulsator model' has been developed in which the pulsationaxis is not the magnetic axis; this model has passed several observationaltests and new ones are being devised to examine it further. 相似文献
2.
Michel Breger 《Astrophysics and Space Science》2003,284(1):137-140
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
3.
Vincenzo Ripepi 《Astrophysics and Space Science》2003,284(1):69-72
We present time series observations of the intermediate mass Pre-Main Sequence star H254 belonging to the young star cluster IC 348 and of the Herbig Ae star V351 Ori.Both these stars present light variation on short time scale (a few hours) typical of the Scuti pulsation. The new data are briefly described together with the plan for future observational campaigns on PMS Scuti stars. 相似文献
4.
We show that in evolved stars, even at relatively low surfaceamplitudes, nonradial modes become strongly nonlinear in thehydrogen shell source, where the Brunt-Väisälä frequency hasits absolute maximum. The measure of nonlinearity is the productof horizontal displacement times the radial wavenumber,
. It becomes large already in evolved -Scuti stars.This nonlinearity presents a major problem for interpretations ofamplitude modulation in RR Lyrae stars in terms of nonradial modeexcitation. 相似文献
5.
We briefly summarize the impact of protostellar evolution on the initialconditions of pre-main-sequence (PMS) contraction and identifytwo different kinds of instability in young stars closely connected to thePMS physical conditions. These are the Scuti type pulsations in intermediate-mass PMS stars (Herbig Ae stars) and the pulsation instability of brown dwarfs and very low mass stars. For the former, we review recentobservational and theoretical work and provide the location in the H-Rdiagram of known candidates. For the latter, the possibility that deuteriumcan be an efficient driving mechanism of the instability is explored. 相似文献
6.
P. O. Lagage G. Oloffson S. Cabrit C. Cesarsky L. Nordh J. M. Rodriguez Espinosa 《Experimental Astronomy》1994,3(1-4):163-164
The region around the Herbig Ae/Be star LkH198 was imaged at 10m with the CAMIRAS camera at the Canada-France-Hawaii Telescope and the Nordic Optical Telescope. We discovered a deeply embedded companion 6 north of LkH198, which may be responsible for most of the far-infrared emission in the region and for driving the molecular flow. LkH198 appears point-like at our resolution (FWHM 1.3), excluding an extended envelope of transiently heated small grains as the dominant origin of the mid-infrared excess in that star. 相似文献
7.
We have used the ESO 10 m camera, TIMMI, to image with a very high angular resolution (PFoV: 0.3; FWHM:0.9) several main-sequence star disk candidates. Information on the -Pictoris dust disk has been obtained in a region largely inaccessible to previous observations: 0–80 AU, with a resolution of about 5 AU after deconvolution. Another promising target for 10 m imaging, 51 Ophiuchi, appears point-like.based on data collected at the European Southern Observatory (ESO), La Silla, Chile 相似文献
8.
One of the major goals in VHE--ray astronomy is to open the energy range below 100 GeV with earthbound detectors. This paper demonstrates a new method for analyzing erenkov light of a shower in a erenkov telescope array. This method is successful for showers in this low energy regime where previous techniques (e.g. alpha analysis) are not applicable. A Monte Carlo simulation is applied to a system of 19 Whipple type [3, Cawley 1990] Imaging Atmospheric erenkov telescopes (IAT), each built as a 10 m diameter reflector and equipped with a 109 photomultiplier tube camera. The energy threshold for a single detector of this type is given [5, Kerrick et al. 1995] as 250 GeV. Analysis of simulated coincident events of the system for those events not having enough light to apply a standard imaging analysis [4, Hillas 1985], leads to a considerably lower threshold of 85 GeV. With a new analysis method of these events it is shown that it should be possible to distinguish between -ray induced and proton induced showers. The improvement of sensitivity (Q = figure of merit) of this analysis method is found to be Q=2.9. 相似文献
9.
We have investigated ROSAT X-ray data of OB stars to search for evidence of time variability in the X-ray emission from early-type stars. As an example for such studies we present a detailed variability analysis for our two program stars Ori and Ori which have been multiply observed with ROSAT. The long-term analysis of both stars now covers a time range of 2.5 years and includes six pointed PSPC observations, an additional pointed HRI observation of Ori and the ROSAT all-sky survey data of both stars. Over a long time range the X-ray light curves of Ori and Ori show no evidence for variability. In the case of Ori we detected a moderate increase in X-ray count rate during a period of 2 days which can be explained as a strong shock propagating in the wind of an O-type star. 相似文献
10.
We investigate the production of26Al during hydrogen burning and its ejection by massive single and binary stars. Effects of convection and rotation are studied. We discuss the importance of RSGs, LBVs and WR stars to the total Galactic26Al production, and the detection probability of the26Al decay in individual objects as P Cygni, Velorum and Carinae. 相似文献
11.
The large Area Detectors (LADs) of the BATSE experiment aboard the Compton Gamma-ray Observatory have been used recently as the first hard X-ray all-sky imager at energies between 20 keV and 300 keV. The Earth occultation process is formulated in terms of a curved Radon transform convoluted by a step transform in a selected field of view (FOV) ranging from 5°×5° to 40°×40°. The Maximum Entropy Method is then used to reconstruct an image in the hard X-ray sky. Multiple images of different regions of the sky can be produced simultaneously. A source location accuracy of 0.1° for strong sources and a sensitivity limit of 100 mCrab have been achieved in an one-day integration period.Invited Paper on Imaging in High Energy Astronomy, Sept., 1994, Capri, Italyalso Universities Space Research Association 相似文献
12.
We have started a program of high-resolution (0.4/pixel) 10m imaging of bipolar outflow sources using the 10m camera CAMIRAS. We present recent results obtained at the Canada France Hawaii Telescope which reveal extended emission or IR companions in several luminous objects. The extended emission we detected probably arises from transiently heated very small grains, while the newly discovered companions could contribute significantly to the outflow activity and extended far-IR emission usually attributed to the main optical source. 相似文献
13.
Yu. V. Obrubov 《Earth, Moon, and Planets》1995,68(1-3):443-449
It was shown that the orbit of Virginid meteors is an octuple Earth-crosser so that the stream can produce eight related meteor showers. Of these eight theoretical showers five are identified in the catalogues; one of these is the Cygnids. 相似文献
14.
The existence of escape and nonescape orbits arbitrarily close to the homothetic equilateral triplecollision orbit is considered analytically in the threebody problem with zero initial velocities and equal masses. It is proved that escape orbits in the initial condition space are distributed around three kinds of isosceles orbits. It is also proved that nonescape orbits are distributed in between the escape orbits where different particles escape. In order to show this, it is proved that the homotheticequilateral orbit is isolated from other triplecollision orbits as far as the collision at the first triple encounter is concerned. Moreover, the escape criterion is formulated in the planarisosceles problem and translated into the words of regularizing variables. The result obtained by us explains the orbital structure numerically. 相似文献
15.
Efi Meletlidou 《Celestial Mechanics and Dynamical Astronomy》2000,78(1-4):161-166
We consider a one-dimensional non-degenerate Hamiltonian system perturbed by a periodically time dependent non-Hamiltonian vector field and show that the non-vanishing of the Melnikov subharmonic function is strongly related to the non-existence of an analytic integral in the perturbed system. 相似文献
16.
Following the discovery, by IRAS, of the dust disc around Vega and three other main sequence stars, searches have been made for other candidates. The-Pic-like candidates have 12µm excesses and 100µm fluxes (unlike the Vega-like candidates), so they can be further investigated using ground-based techniques. Data are presented here, comprising 10µm spectroscopy and sub-mm observations, for several candidates from the Walker & Wolstencroft list, showing that the stars have silicate dust, and optically thick dust discs even at 1300µm. 相似文献
17.
M.G. Larionov 《Astrophysics and Space Science》1997,252(1-2):139-149
The possibility of using a Friedmann-Lemaître model with a non-zero cosmological constant is investigated on the basis of analyses of the RATAN-600 and 87GB radio surveys and optical constraints. A zero deceleration parameter and critical matter density are features of the model. The model is in agreement with current observational data, including estimates of the age of the Universe, Hubble constant measurements, and the - z relation. A value for the cosmological constant is determined and its physical nature discussed. 相似文献
18.
P. Armstrong P. M. Chadwick P. J. Cottle J. E. Dickinson M. R. Dickinson N. A. Dipper S. E. Hilton W. Hogg J. Holder T. R. Kendall T. J. L. McComb C. M. Moore K. J. Orford S. M. Rayner I. D. Roberts M. D. Roberts M. Robertshaw S. E. Shaw K. Tindale S. P. Tummey K. E. Turver 《Experimental Astronomy》1999,9(2):51-80
19.
S. I. Plachinda 《Astrophysics》2005,48(1):9-19
Multiyear high precision measurements of the longitudinal component of the magnetic field (Be) of four supergiants are reported: Aqr (G0 Ib), Aqr (G2 Ib), Gem (G8 Ib), and Peg (K2 Ib). The best measurement accuracy, =0.8 G, was achieved for Peg. A Monte Carlo method was used to test the reliability of the derived measurement errors. The differences between the observational errors and the calculated Monte Carlo errors were 3.2%. For Aqr and Aqr no statistically significant value of the magnetic field was recorded when averaged over a night. For eGem the following overnight average values of the magnetic field were recorded on five nights: 11.1±2.7 G, 9.8±2.5 G, –10.5±3.0 G, 38.1±7.4 G, and 5.3±1.5 G. For Peg the magnetic field recorded over two nights was –5.3±0.9 G and – 2.7±0.8 G.Translated from Astrofizika, Vol. 48, No. 1, pp. 15–28 (February 2005). 相似文献
20.
Toshio Fukushima 《Celestial Mechanics and Dynamical Astronomy》1997,68(2):121-137
We developed a method to solve Keplers equation for the hyperboliccase. The solution interval is separated into three regions; F 1, F 1, F 1. For the region F is large, we transformed the variablefrom F to exp F and applied the Newton method to the transformed equation.For the region F is small, we expanded the equation with respect to F andapplied the Newton method to the approximated equations. For the middleregion, we adopted a discretization of the Newton method and used the Taylorseries expansion for the evaluation of transcendental functions (Fukushima,1997). Numerical measurements showed that, in the case of Intel Pentiumprocessor, the new method is more than 3.7 times as fast as the combinationof a fourth order correction formula (Fukushima, 1997) and Roysstarting procedure (Roy, 1988) and others. 相似文献