arXiv subjects

Th. Rivinius

Publications and source records attributed to Th. Rivinius.

At least 73 records · Page 4Linked to original sources

arXiv2012

Basic parameters and properties of the rapidly rotating magnetic helium strong B star HR7355

The spectral and magnetic properties and variability of the B2Vnp emission-line magnetosphere star HR7355 were analyzed. The object rotates at almost 90% of the critical value, meaning it is a magnetic star for which oblateness and gravity darkening effects cannot be ignored any longer. A detailed modeling of the photospheric parameters indicate that the star is significantly cooler than suggested by the B2 spectral type, with T_eff=17500K atypically cool for a star with a helium enriched surface. The spectroscopic variability of helium and metal lines due to the photospheric abundance pattern is far more complex than a largely dipolar, oblique magnetic field of about 11 to 12kG may suggest. Doppler imaging shows that globally the most He enriched areas coincide with the magnetic poles and metal enriched areas with the magnetic equator. While most of the stellar surface is helium enriched with respect to the solar value, some isolated patches are depleted. The stellar wind in the circumstellar environment is governed by the magnetic field, i.e. the stellar magnetosphere is rigidly corotating with the star. The magnetosphere of HR7355 is similar to the well known σOri E: the gas trapped in the magnetospheric clouds is fairly dense, and at the limit to being optically thick in the hydrogen emission. Apart from a different magnetic obliquity, HR7355 and the more recently identified HR5907 have virtually identical stellar and magnetic parameters.

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arXiv2012

A Magnetic Confinement vs. Rotation Classification of Massive-Star Magnetospheres

Building on results from the Magnetism in Massive Stars (MiMeS) project, this paper shows how a two-parameter classification of massive-star magnetospheres in terms of the magnetic wind confinement (which sets the Alfvén radius RA) and stellar rotation (which sets the Kepler co-rotation radius RK) provides a useful organisation of both observational signatures and theoretical predictions. We compile the first comprehensive study of inferred and observed values for relevant stellar and magnetic parameters of 64 confirmed magnetic OB stars with Teff > 16 kK. Using these parameters, we locate the stars in the magnetic confinement-rotation diagram, a log-log plot of RK vs. RA. This diagram can be subdivided into regimes of centrifugal magnetospheres (CM), with RA > RK, vs. dynamical magnetospheres (DM), with RK > RA. We show how key observational diagnostics, like the presence and characteristics of Halpha emission, depend on a star's position within the diagram, as well as other parameters, especially the expected wind mass-loss rates. In particular, we identify two distinct populations of magnetic stars with Halpha emission: namely, slowly rotating O-type stars with narrow emission consistent with a DM, and more rapidly rotating B-type stars with broader emission associated with a CM. For O-type stars, the high mass-loss rates are sufficient to accumulate enough material for line emission even within the relatively short free-fall timescale associated with a DM: this high mass-loss rate also leads to a rapid magnetic spindown of the stellar rotation. For the B-type stars, the longer confinement of a CM is required to accumulate sufficient emitting material from their relatively weak winds, which also lead to much longer spindown timescales. [abbreviated]

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arXiv2012

Be Stars: Rapidly Rotating Pulsators

I will show that Be stars are, without exception, a class of rapidly rotating stars, which are in the majority of cases pulsating stars as well, while none of them does possess a large scale (i.e. with significant dipolar contribution) magnetic field.

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arXiv2012

Spectroscopic Observations of δSco Through the 2011 Periastron Passage

We present prelimiary results from a coordinated spectroscopic campaign in 2011, centered on the δSco periastron passage in July. Data have mostly been obtained with the FEROS/2.2 m at La Silla and ESPaDOnS/CFHT at Mauna Kea echelle instruments. Main results include the absence of tidally induced disturbance to the main βCephei pulsation mode and the absence of tidally triggered mass-ejection at time of periastron proper. The observed (as far as yet analyzed) variations are compatible with the picture of a disk that is disturbed on its outer radius, with the disturbance propagating inwards after the periastron.

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arXiv2012

PUCHEROS Early Science: A New Be+sdO Candidate

We report on the first scientific results with the recently commissioned PUCHEROS spectrograph, mounted at the 50cm telescope of the Pontificia Universidad Catolica near Santiago, Chile. A hitherto unknown candidate Be+sdO binary was identified, omicron Pup. If confirmed, it would be the fourth member of this class. Such stars have obtained their rapid rotation through binary mass transfer and now consist of a Be star and a hot subdwarf.

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arXiv2012

On the CQE Phase Signature Feature in High-Resolution Spectro-Interferometry of Be Stars

We analyse a feature that is detectable at the differential phases of high-resolution spectro-interferometry of Be stars. The origin of this feature (dubbed CQE-PS, Central Quasi Emission Phase Signature) lies in the differential absorption of photospheric radiation by the circumstellar disk, which is spectroscopically detected as a CQE line profile in shell stars. This phenomenon has great diagnostic potential for Be star disks, revealing properties of these disks on the scale of a few stellar radii.

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arXiv2012

Investigating the spectroscopic, magnetic and circumstellar variability of the O9 subgiant star HD 57682

The O9IV star HD 57682, discovered to be magnetic within the context of the MiMeS survey in 2009, is one of only eight convincingly detected magnetic O-type stars. Among this select group, it stands out due to its sharp-lined photospheric spectrum. Since its discovery, the MiMeS Collaboration has continued to obtain spectroscopic and magnetic observations in order to refine our knowledge of its magnetic field strength and geometry, rotational period, and spectral properties and variability. In this paper we report new ESPaDOnS spectropolarimetric observations of HD 57682, which are combined with previously published ESPaDOnS data and archival Hα spectroscopy. This dataset is used to determine the rotational period (63.5708 \pm 0.0057 d), refine the longitudinal magnetic field variation and magnetic geometry (dipole surface field strength of 880\pm50 G and magnetic obliquity of 79\pm4\circ as measured from the magnetic longitudinal field variations, assuming an inclination of 60\circ), and examine the phase variation of various lines. In particular, we demonstrate that the Hα equivalent width undergoes a double-wave variation during a single rotation of the star, consistent with the derived magnetic geometry. We group the variable lines into two classes: those that, like Hα, exhibit non-sinusoidal variability, often with multiple maxima during the rotation cycle, and those that vary essentially sinusoidally. Based on our modelling of the Hα emission, we show that the variability is consistent with emission being generated from an optically thick, flattened distribution of magnetically-confined plasma that is roughly distributed about the magnetic equator. Finally, we discuss our findings in the magnetospheric framework proposed in our earlier study.

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arXiv2012

Dynamical Evolution of Viscous Disks around Be Stars. I: photometry

Be stars possess gaseous circumstellar disks that modify in many ways the spectrum of the central B star. Furthermore, they exhibit variability at several timescales and for a large number of observables. Putting the pieces together of this dynamical behavior is not an easy task and requires a detailed understanding of the physical processes that control the temporal evolution of the observables. There is an increasing body of evidence that suggests that Be disks are well described by standard $α$-disk theory. This paper is the first of a series that aims at studying the possibility of inferring several disk and stellar parameters through the follow-up of various observables. Here we study the temporal evolution of the disk density for different dynamical scenarios, including the disk build-up as a result of a long and steady mass injection from the star, the disk dissipation that occurs after mass injection is turned off, as well as scenarios in which active periods are followed by periods of quiescence. For those scenarios, we investigate the temporal evolution of continuum photometric observables using a 3-D non-LTE radiative transfer code. We show that lightcurves for different wavelengths are specific of a mass loss history, inclination angle and $α$ viscosity parameter. The diagnostic potential of those lightcurves is also discussed.

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arXiv2011

HR 5907: Discovery of the most rapidly rotating magnetic B-type star by the MiMeS Collaboration

We report the discovery and analysis of a very strong magnetic field in the rapidly rotating early B-type star HR 5907, based on observations obtained as part of the Magnetism in Massive Stars (MiMeS) project. We infer a rotation period of 0.508276 +0.000015/-0.000012 d from photometric and Hα EW measurements, making this the shortest period, non-degenerate, magnetic massive star known to date. From the comparison of IUE UV and optical spectroscopy with LTE BRUCE/KYLIE models we find a solid-angle integrated, uniform black-body temperature of 17 000 \pm 1000 K, a projected rotational velocity of 290 \pm 10 km/s, an equatorial radius of 3.1 \pm 0.2 R_sun, a stellar mass of 5.5 \pm 0.5 M_sun, and an inclination angle of the rotation axis to our line-of-sight of 70 \pm 10\circ. Our measurements of the longitudinal magnetic field, which vary between -500 and -2000 G, phase coherently with the rotation period and imply a surface dipole field strength of \sim15.7 kG. On the other hand, from fits to mean Least-Squares Deconvolved Stokes V line profiles we infer a dipole field strength of \sim10.4 kG. This disagreement may result from a magnetic configuration more complex than our model, and/or from the non-uniform helium surface abundance distribution. In either case we obtain a magnetic obliquity nearly aligned with the rotation axis (β = 7+2/-1\circ). Our optical spectroscopy also shows weak variability in carbon, silicon and nitrogen lines. The emission variability in hydrogen Balmer and Paschen lines indicates the presence of a dense, highly structured magnetosphere, interpreted as a centrifugally supported, magnetically confined circumstellar disk.

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arXiv2010

The rapid magnetic rotator HR7355

For early type magnetic stars slow, at most moderate rotational velocities have been considered an observational fact. The detection of a multi-kilogauss magnetic field in the B2Vpn star with P approx. 0.52d and v sin approx. 300km/s has brought down this narrative. We have obtained more than 100 high-resolution, high-S/N echelle spectra in 2009. These spectra provide the most detailed description of the variability of any He-strong star to date. The circumstellar environment is dominated by a rotationally locked magnetosphere out to several stellar radii, causing hydrogen emission. The photosphere is characterized by surface chemical abundance inhomogeneities, with much stronger amplitudes, at least for helium, than slower rotating stars like σOri E. The highly complex rotational line profile modulations of metal lines are probably a consequence the equatorial gravity darkening of HR 7355, and thus may offer an independent measurement of the von Zeipel parameter β.

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arXiv2010

Magnetism of the He-weak star HR2949

A magnetic field and rotational line profile variability (lpv) is found in the He-weak star HR2949. The field measured from metallic lines varies in a clearly non-sinusoidal way, and shows a phase lag relative to the morphologically similar HeI equivalent width variations. The surface abundance patterns are strong and complex, and visible even in the hydrogen lines.

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arXiv2010

Masses of the astrometric SB2 ζOri A

We report the first dynamic mass for an O-type supergiant, the interferometrically resolved SB2 system ζOri A (O9.5Ib+B0/1). The separation of the system excludes any previous mass-transfer, ensuring that the derived masses can be compared to single star evolutionary tracks.

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arXiv2010

The resonant B1II+B1II binary BI108

BI108 is a luminous variable star in the Large Magellanic Cloud classified B1II. The variability consists of two resonant periods (3:2), of which only one is orbital, however. We discuss possible mechanisms responsible for the second period and its resonant locking.

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arXiv2010

The (B0+?)+O6 system FN CMa: A case for tidal-pulsational interaction?

FN CMa is visually double with a separation of about 0.6arcsec. Sixty high-cadence VLT/UVES spectra permit the A and B components to be disentangled, as the relative contribution of each star to the total light entering the spectrograph fluctuates between exposures due to changes in seeing. Component A exhibits rapid line-profile variations, leading us to attribute the photometric variability seen by HIPPARCOS (with a derived P=0.08866d) to this component. From a total of 122 archival and new echelle spectra it is shown that component A is an SB1 binary with an orbital period of 117.55 days. The eccentricity of 0.6 may result in tidal modulation of the pulsation(s) of component Aa.

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arXiv2010

Magnetic field detection in the B2Vn star HR 7355

The B2Vn star HR 7355 is found to be a He-rich magnetic star. Spectropolarimetric data were obtained with FORS1 at UT2 on Paranal observatory to measure the disk-averaged longitudinal magnetic field at various phases of the presumed 0.52 d cycle. A variable magnetic field with strengths between B_z = -2200 and +3200G was found, with confidence limits of 100 to 130G. The field topology is that of an oblique dipole, while the star itself is seen about equator-on. In the intensity spectra the HeI-lines show the typical equivalent width variability of He-strong stars, usually attributed to surface abundance spots. The amplitudes of the equivalent width variability of the HeI lines are extraordinarily strong compared to other cases. These results not only put HR 7355 unambiguously among the early-type magnetic stars, but confirm its outstanding nature: With v sin i = 320 km/s the parameter space in which He-strong stars are known to exist has doubled in terms of rotational velocity.

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arXiv2009

Cyclic variability of the circumstellar disk of the Be star zetaTau - I. Long-term monitoring observations

Emission lines formed in decretion disks of Be stars often undergo long-term cyclic variations, especially in the violet-to-red (V/R) ratio of their primary components. From observations of the bright Be-shell star zeta Tau, the possibly broadest and longest data set illustrating the prototype of this behaviour was compiled from our own and archival observations. It comprises optical and infrared spectra, broad-band polarimetry, and interferometric observations. From 3 V/R cycles between 1997 and 2008, a mean cycle length in H alpha of 1400-1430 days was derived. After each minimum in V/R, the shell absorption weakens and splits into two components, leading to 3 emission peaks. This phase makes the strongest contribution to the variability in cycle length. V/R curves of different lines are shifted in phase. Lines formed on average closer to the central star are ahead of the others. The shell absorption lines fall into 2 categories differing in line width, ionization/excitation potential, and variability of the equivalent width. The interferometry has resolved the continuum and the line emission in Br gamma and He I 2.06. The phasing of the Br gamma emission shows that the photocenter of the line-emitting region lies within the plane of the disk but is offset from the continuum source. The plane of the disk is constant throughout the observed V/R cycles. The self-consistent, one-armed, disk-oscillation model is developed in Paper II.

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arXiv2009

Magnetic survey of emission line B-type stars with FORS1 at the VLT

We report the results of our search for magnetic fields in a sample of 16 field Be stars, the binary emission-line B-type star upsilon Sgr, and in a sample of fourteen members of the open young cluster NGC3766 in the Carina spiral arm. The sample of cluster members includes Be stars, normal B-type stars and He-strong/He-weak stars. Nine Be stars have been studied with magnetic field time series obtained over ~1 hour to get an insight into the temporal behaviour and the correlation of magnetic field properties with dynamical phenomena taking place in Be star atmospheres. The spectropolarimetric data were obtained at the European Southern Observatory with the multi-mode instrument FORS1 installed at the 8m Kueyen telescope. We detect weak photospheric magnetic fields in four field Be stars, HD62367, mu Cen, o Aqr, and epsilon Tuc. The strongest longitudinal magnetic field, =117+-38G, was detected in the Be star HD62367. Among the Be stars studied with time series, one Be star, lambda Eri, displays cyclic variability of the magnetic field with a period of 21.12min. The binary star upsilon Sgr, in the initial rapid phase of mass exchange between the two components with strong emission lines in the visible spectrum, is a magnetic variable star, probably on a timescale of a few months. The maximum longitudinal magnetic field =-102+-10G at MJD54333.018 was measured using hydrogen lines. The cluster NGC3766 seems to be extremely interesting, where we find evidence for the presence of a magnetic field in seven early B-type stars out of the observed fourteen cluster members.

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arXiv2009

Cyclic Variability of the Circumstellar Disc of the Be Star $ζ$ Tau. II. Testing the 2D Global Disc Oscillation Model

Aims. In this paper we model, in a self-consistent way, polarimetric, photometric, spectrophotometric and interferometric observations of the classical Be star $ζ$ Tauri. Our primary goal is to conduct a critical quantitative test of the global oscillation scenario. Methods. We have carried out detailed three-dimensional, NLTE radiative transfer calculations using the radiative transfer code HDUST. For the input for the code we have used the most up-to-date research on Be stars to include a physically realistic description for the central star and the circumstellar disc. We adopt a rotationally deformed, gravity darkened central star, surrounded by a disc whose unperturbed state is given by a steady-state viscous decretion disc model. We further assume that disc is in vertical hydrostatic equilibrium. Results. By adopting a viscous decretion disc model for $ζ$ Tauri and a rigorous solution of the radiative transfer, we have obtained a very good fit of the time-average properties of the disc. This provides strong theoretical evidence that the viscous decretion disc model is the mechanism responsible for disc formation. With the global oscillation model we have successfully fitted spatially resolved VLTI/AMBER observations and the temporal V/R variations of the H$α$ and Br$γ$ lines. This result convincingly demonstrates that the oscillation pattern in the disc is a one-armed spiral. Possible model shortcomings, as well as suggestions for future improvements, are also discussed.

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