Poésie et musique — une expérience

Berio, Luciano

doi:10.4000/books.contrechamps.1731

Cited by 15 publications

(21 citation statements)

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“…This action prevents the matter within this region from being lost during the maximum of the dynamo cycle. In this context we note that Berio et al (1999) find evidence for a density maximum at a radius of about 2.5 R * . Further, Smith & Robinson (1999) report on drifting spectral features near the Si iv resonance lines that can be interpreted as corotating clouds near 2.5 R * .…”

Section: A Possible Dynamomentioning

confidence: 59%

“…If this simulation is applicable to Cas, then the observed 70 day cycle implies that the dynamo is operating in the inner disk, at a radius of about 2.5 R * (assuming M Ã ¼ 17 M and R Ã ¼ 7 R ). This is just slightly outside the Keplerian corotation radius, R K ¼ 1:7 R * , and is near the location where the disk density seems to be highest (Berio et al 1999). …”

Section: A Possible Dynamomentioning

confidence: 90%

“…These variations are now widely believed to be due to a dynamical instability in the disk that excites a one-armed density wave that precesses around the star on a timescale of 5-7 yr. This feature has been imaged interferometrically in H light and has been shown to rotate around the star on this timescale (Berio et al 1999). During the year 2000 the V =R ratio decreased from a value of about 1.3 to 1.0 (G. J. Peters 2001, private communication).…”

Section: Short-term Variations In the Optical Fluxmentioning

confidence: 95%

“…The authors concluded that more observations were needed in order to quantify the timescale and form of these variations. It was also hoped that the nature of the variations could be better understood by comparing the X-ray results with other parameters of the star, such as the one-armed density enhancement in the Be disk, which is known to be responsible for the 5-7 yr cyclic variations of the H emission profile (Telting & Kaper 1994;Berio et al 1999). Thus, an observing program using the Automated Photometric Telescope (APT) was initiated to monitor the optical variability of Cas in the Johnson B and V bands.…”

Section: Introductionmentioning

confidence: 99%

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X‐Ray and Optical Variations in the Classical Be Star γ Cassiopeia: The Discovery of a Possible Magnetic Dynamo

Robinson

Smith

Henry

2002

ApJ

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The classical B0.5e star Cassiopeia is known to be a unique X-ray source by virtue of its moderate L X (10 33 ergs s À1 ), hard X-ray spectrum, and light curve punctuated by ubiquitous flares and slow undulations. The peculiarities of this star have led to a controversy concerning the origin of these emissions: whether they are from wind infall onto a putative degenerate companion, as in the case of normal Be/X-ray binaries, or from the Be star itself. Recently, much progress has been made to resolve this question: (1) the discovery that Cas is a moderately eccentric binary system (P ¼ 203:59 days) with unknown secondary type, (2) the addition of RXTE observations at six epochs in 2000, adding to three others in 1996-1998, and (3) the collation of robotic telescope (Automated Photometric Telescope) B-and V-band photometric observations over four seasons that show a 3%, cyclical flux variation with cycle lengths of 55-93 days. We find that X-ray fluxes at all nine epochs show random variations with orbital phase, thereby contradicting the binary accretion model, which predicts a substantial modulation. However, these fluxes correlate well with the cyclical optical variations. In particular, the six flux measurements in 2000, which vary by a factor of 3, closely track the interpolated optical variations between the 2000 and 2001 observing seasons. The energy associated with the optical variations greatly exceeds the energy in the X-rays, so that the optical variability cannot simply be due to reprocessing of X-ray flux. However, the strong correlation between the two suggests that they are driven by a common mechanism. We propose that this mechanism is a cyclical magnetic dynamo excited by a BalbusHawley instability located within the inner part of the circumstellar disk. According to our model, variations in the field strength directly produce the changes in the magnetically related X-ray activity. Turbulence associated with the dynamo results in changes to the density (and therefore the emission measure) distribution within the disk and creates the observed optical variations.

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Section: A Possible Dynamomentioning

confidence: 59%

Section: A Possible Dynamomentioning

confidence: 90%

Section: Short-term Variations In the Optical Fluxmentioning

confidence: 95%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

X‐Ray and Optical Variations in the Classical Be Star γ Cassiopeia: The Discovery of a Possible Magnetic Dynamo

Robinson

Smith

Henry

2002

ApJ

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“…The SPR instrument is not the first spectro-interferometer: the technique was first demonstrated in the visible with the GI2T interferometer (Mourard et al 1989;Vakili et al 1997Vakili et al , 1998Berio et al 1999). Application to the infrared followed, but without fringe tracking and therefore with limited sensitivity.…”

Section: Introductionmentioning

confidence: 99%

Self-Phase-Referenced Spectro-Interferometry on the Keck Interferometer

Woillez¹,

Akeson²,

Colavita³

et al. 2012

Publications of the Astronomical Society of the Pacific

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ABSTRACT. As part of the astrometric and phase-referenced astronomy (ASTRA) project, three new science modes are being developed for the Keck Interferometer that extend the science capabilities of this instrument to include higher spectral resolution, fainter magnitudes, and astrometry. We report on the successful implementation of the first of these science modes, the self-phase-referencing mode, which provides a K-band (λ ¼ 2:2 μm) spectral resolution of R ∼ 1000 on targets as faint as 7.8 mag with spatial resolution as fine as λ=B ¼ 5 mas in the K band, with the 85 m interferometer baseline. This level of spectral resolution would not have been possible without a phase-referencing implementation extending the integration time limit imposed by atmospheric turbulence. For narrow spectral features, we demonstrate a precision of AE0:01 on the differential V 2 ðλÞ, and AE1:7 mrad on the differential phase ΦðλÞ, equivalent to a differential astrometry precision of AE1:45 μas. This new Keck Interferometer instrument is typically used to study the geometry and location of narrow spectral features at high angular resolution, referenced to a continuum. By simultaneously providing spectral and spatial information, the geometry of velocity fields (e.g., rotating disks, inflows, outflows, etc.) larger than 150 km s À1 can also be explored.

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Strange Among the Strange: The B-emission Star γ Cassiopeiæ

Harmanec

2002

International Astronomical Union Colloquium

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Abstractγ Cas (HD 5394) is an important representative of Be stars, hot B stars exhibiting Balmer and other emission lines in their spectra at certain epochs. Its emission spectrum was observed as early as in 1863 and has undergone spectacular changes since then. γ Cas is also extremely interesting because it qualifies for several phenomenologically defined groups: it is a light and colour variable, long-term radial-velocity variable, X-ray source, IRAS source, rapid line-profile variable, a single-line spectroscopic binary and a central star of a reflection nebula.A critical compilation of the wealth of observational data on the star, with emphasis on its time variability on several time scales, as well as an estimate of the most probable values of all basic physical properties of the object is presented. It is pointed out that in spite of a large quantity of data, systematic and well-calibrated observations are still rather rare. The present understanding of the object is put into perspective of the more general – and as yet unsolved – problem of the formation of Be envelopes.

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Poésie et musique — une expérience

Cited by 15 publications

References 0 publications

X‐Ray and Optical Variations in the Classical Be Star γ Cassiopeia: The Discovery of a Possible Magnetic Dynamo

X‐Ray and Optical Variations in the Classical Be Star γ Cassiopeia: The Discovery of a Possible Magnetic Dynamo

Self-Phase-Referenced Spectro-Interferometry on the Keck Interferometer

Strange Among the Strange: The B-emission Star γ Cassiopeiæ

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