Rotation, Expansion and Duplicity of Be Stars

Harmanec, P.

doi:10.1017/s0074180900037955

Cited by 6 publications

(5 citation statements)

References 51 publications

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“…1, together with the values of Table 2, marked by crosses. All phases are from the ephemeris derived by Harmanec & Scholz (1993). As one can see, the values of Table 2 are in an excellent agreement with the expected ones following from the binary motion.…”

Section: Astronomical Observationssupporting

confidence: 76%

TRAFICOS – an Échelle Zeeman spectrograph

et al. 1997

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We describe the TRAnsportable FIbre Coupled kchelle Spectrograph (TRAFICOS) equipped with a Zeeman analyzer and manufactured mainly for the observation of stellar magnetic fields. The IntroductionMagnetic fields are observed on stars with quite different temperatures, masses, and states of evolution. In stars with masses of about one solar mass and smaller and the so-called T-Tauri variable stars the magnetic fields are an important indicator for confirmation of the dynamo theory. in stars with higher masses, especially in stars belonging to the chemically peculiar stars, CP stars, the connection between the magnetic field and the distribution of the chemical elements on the stellar surface should provide decisive arguments for the diffusion theory.In the past our investigations of stellar magnetic fields were mainly concentrated on CP stars and based on observations carried out photographically at the coudk focus of the 2 m telescope in Tautenburg. In most C P stars the temporal variations of the magnetic field observed are periodical and most frequently interpreted by the rotation of the star possessing a magnetic field, which has a lack of symmetry with regard to the axis of rotation. The widespread evidence observed of one extremum more prominent than the other requires a field geometry which differs from that of a centred or decentred dipole even with arbitrary obliquity. In order to continue the investigations on magnetic fields with an up-to-date device a fibre-fed kchelle spectrograph, equipped with a Zeeman analyzer for measurement of the circularly polarized light, was designed and constructed and is at the moment installed at the Nasmyth focus of the reflector in Tautenburg, but it can also be used at the focus of other telescopes with f/8 to f/12. For the determination of magnetic fields the observations with the new device at the Nasmyth focus should be more suitable than the previous ones at the coudC focus. The reason is that the number of light reflexions in the telescope is only two (instead of five in the coudk system), so that, on the one hand, the speed at the Nasmyth focus should be higher than at the coudC and, on the other hand, the problems of the instrumental polarization are not so difficult to handle as in the path of rays of the coudk system. Furthermore, the achromatic quarter wave plate of the Zeeman analyzer allows the magnetic field from lines of a wide spectral region to be derived, especially so for spectral lines in the red domain.

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Section: Astronomical Observationssupporting

confidence: 76%

TRAFICOS – an Échelle Zeeman spectrograph

et al. 1997

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“…He argued that the disk precession is caused by the attractive force of the secondary in the 218-d binary. Harmanec (1982) compiled the majority of at that time available RVs of BU Tau and averaged them over about 100 days. This resulted in a smooth RV curve with a period of about 13 000 days (35.6 years), in phase with the recorded shell episodes.…”

Section: Introductionmentioning

confidence: 99%

“…This resulted in a smooth RV curve with a period of about 13 000 days (35.6 years), in phase with the recorded shell episodes. Harmanec (1982) speculated that BU Tau could be a long-periodic binary with shell phases occurring always at the same orbital phases. A more distant companion with an angular distance of 0.…”

Section: Introductionmentioning

confidence: 99%

Properties and nature of Be stars

Nemravová¹,

Harmanec²,

Kubát

et al. 2010

A&A

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Radial-velocity variations of the Hα emission measured on the steep wings of the Hα line, prewhitened for the long-time changes, vary periodically with a period of 218. d 025 ±0. d 022, confirming the suspected binary nature of the bright Be star BU Tau, a member of the Pleiades cluster. The orbit seems to have a high eccentricity over 0.7, but we also briefly discuss the possibility that the true orbit is circular and that the eccentricity is spurious owing to the phase-dependent effects of the circumstellar matter. The projected angular separation of the spectroscopic orbit is large enough to allow the detection of the binary with large optical interferometers, provided the magnitude difference primary − secondary is not too large. Since our data cover the onset of a new shell phase up to development of a metallic shell spectrum, we also briefly discuss the recent long-term changes. We confirm the formation of a new envelope, coexisting with the previous one, at the onset of the new shell phase. We find that the full width at half maximum of the Hα profile has been decreasing with time for both envelopes. In this connection, we briefly discuss Hirata's hypothesis of precessing gaseous disk and possible alternative scenarios of the observed long-term changes.

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“…As pointed out by Johnson (1958), a simple explanation of why such a disk revolves is an analogy to apsidal motion in a binary system. Quantitative modelling of V/R changes are discussed in e.g., Harmanec (1982Harmanec ( , 2001; Mennickent et al (1997) and references given there. Here, we shall concentrate on one particular explanation of the long-term V/R variations -the model of one-armed non-radial oscillations in Be star disks.…”

Section: Introductionmentioning

confidence: 99%

Is the model of one-armed oscillations able to explain the long-termV/Rchanges of Be stars?

Fiřt

Harmanec

2006

A&A

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Context. Many scientists studying Be stars currently adopt the model of one-armed oscillations as the correct explanation of the cyclic longterm V/R variations observed for a number of Be stars. We test the ability of this model to be used for the predictions of V/R variations in real observed Be stars. Aims. The behavior of the one-armed oscillations can be described as a solution of linearized hydrodynamical equations with the presence of "distorted" gravitational potential and a radiation force. Methods. We developed a new computer program to model one-armed oscillations in Be star disks, which includes both the pressure force and the quadrupole term in the gravitational potential, related to the obliquity of a rapidly rotating star inside the disk. Moreover, we slightly improved the model in an effort to decrease the number of input parameters with the help of NLTE stellar atmosphere models. Results. We carried out detailed tests of the dependence of V/R "periods" predicted by the model on all individual input parameters. We arrived at the following results: (1) the model has great potential to explain not only the cause of the cyclic long-term V/R changes but also some of the observed statistical properties of the phenomenon. (2) The model in its present linear form cannot be considered as proven. Its ability to predict the duration of V/R cycles for individual well observed Be stars is insufficient. Changing some of the input parameters of the model, which are still loosely constrained by observations and/on current understanding of the disks, like the radial density distribution in the disk, one can easily arrive at any desired cycle length from, say, 1 to 20 years. Conclusions. Clearly, a much more sophisticated non-linear and self-consistent model of disk structure and its oscillations will be needed before a truly quantitative test of a one-armed model vs. observations will be possible.

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Rotation, Expansion and Duplicity of Be Stars

Cited by 6 publications

References 51 publications

TRAFICOS – an Échelle Zeeman spectrograph

TRAFICOS – an Échelle Zeeman spectrograph

Properties and nature of Be stars

Is the model of one-armed oscillations able to explain the long-termV/Rchanges of Be stars?

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