| 1. |
- Hubrig, Swetlana, et al.
(författare)
-
Magnetic fields in O-type stars measured with FORS 1 at the VLT
- 2009
-
Ingår i: Cosmic Magnetic Fields: From Planets, to Stars and Galaxies, Proceedings of the International Astronomical Union. ; 259, s. 381-382, s. 381-382
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The presence of magnetic fields in O-type stars has been suspected for a long time. The discovery of such fields would explain a wide range of well documented enigmatic phenomena in massive stars, in particular cyclical wind variability, Hα emission variations, chemical peculiarity, narrow X-ray emission lines and non-thermal radio/X-ray emission. Here we present the results of our studies of magnetic fields in O-type stars, carried out over the last years.
|
|
| 2. |
- Wheelwright, H. E., et al.
(författare)
-
The close Be star companion of β Cephei
- 2009
-
Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 497, s. 487-495
-
Tidskriftsartikel (refereegranskat)abstract
- Context: The prototype of the β Cephei class of pulsating stars, β Cep, rotates relatively slowly, and yet displays episodic Hα emission. Such behaviour is typical of a rapidly rotating, classical Be star. For some time this posed a contradiction to our understanding of the Be phenomena as rapid rotation is thought to be a prerequisite for the characteristic emission phases of Be stars. Recent work has demonstrated that the Hα emission is in fact due to a close companion (separation ≈ 0.25 arcsec) of the star. This resolves the apparent enigma if this close companion is indeed a classical Be star, as has been proposed. Aims: We aim to test the hypothesis that this close companion is a valid Be star by determining properties such as its spectral type and v sin i. Methods: We employed the technique of spectroastrometry to investigate the close binary system. Using the spectroastrometric signatures observed, we split the composite binary spectra into its constituent spectra in the B band (4200-5000 Å) and R band (6200-7000 Å). Results: The spectroastrometrically split spectra allow us to estimate spectral types of the binary components. We find that the primary of the close binary system has a spectral type of B2III and the secondary a spectral type of B5Ve. From the relationship between mass and spectral type, we determine the masses of the binary components to be M_pri = 12.6 ± 3.2 Msun and M_sec = 4.4 ± 0.7 Msun respectively. The spectroastrometric data allow some constraint on the orbit, and we suggest a moderate revision to the previously determined orbit. We confirm that the primary of the system is a slow rotator (v sin i = 29+43-29 km s-1), while the secondary rotates significantly faster, at a v sin i = 230 ± 45 km s-1 Conclusions: We show that the close companion to the β Cephei primary is certainly a valid classical Be star. It has a spectral type of B5Ve and is a relatively fast rotator. We confirm that the β Cephei system does not contradict our current understanding of classical Be stars.
|
|
| 3. |
- Henrichs, Huib F., et al.
(författare)
-
Search for the magnetic field of the O7.5 III star ξ Persei
- 2009
-
Ingår i: Cosmic Magnetic Fields: From Planets, to Stars and Galaxies, Proceedings of the International Astronomical Union. ; 259, s. 383-384, s. 383-384
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Cyclical wind variability is an ubiquitous but as yet unexplained feature among OB stars. The O7.5 III(n)((f)) star ξ Persei is the brightest representative of this class on the Northern hemisphere. As its prominent cyclical wind properties vary on a rotational time scale (2 or 4 days) the star has been already for a long time a serious magnetic candidate. As the cause of this enigmatic behavior non-radial pulsations and/or a surface magnetic field are suggested. We present a preliminary report on our attempts to detect a magnetic field in this star with high-resolution measurements obtained with the spectropolarimeter Narval at TBL, France during 2 observing runs of 5 nights in 2006 and 5 nights in 2007. Only upper limits could be obtained, even with the longest possible exposure times. If the star hosts a magnetic field, its surface strength should be less than about 300 G. This would still be enough to disturb the stellar wind significantly. From our new data it seems that the amplitude of the known non-radial pulsations has changed within less than a year, which needs further investigation.
|
|
| 4. |
- Henrichs, Huib F., et al.
(författare)
-
The magnetic field of the B3V star 16 Pegasi
- 2009
-
Ingår i: Proceedings of the International Astronomical Union. ; 259, s. 393-394, s. 393-394
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The slowly pulsating B3V star 16 Pegasi was discovered by Hubrig et al. (2006) to be magnetic, based on low-resolution spectropolarimetric observations with FORS1 at the VLT. We have confirmed the presence of a magnetic field with new measurements with the spectropolarimeters Narval at TBL, France and Espadons at CFHT, Hawaii during 2007. The most likely period is about 1.44 d for the modulation of the field, but this could not be firmly established with the available data set. No variability has been found in the UV stellar wind lines. Although the star was reported once to show Hα in emission, there exists at present no confirmation that the star is a Be star.
|
|
| 5. |
- Ruediger, G., et al.
(författare)
-
Cross helicity at the solar surface by simulations and observations
- 2012
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 546, s. A23-
-
Tidskriftsartikel (refereegranskat)abstract
- A result of the quasilinear mean-field theory for driven magnetohydrodynamic (MHD) turbulence is that the observed cross helicity < u . b > may directly yield the magnetic eddy diffusivity eta(T) of the quiet Sun. In order to model the cross helicity at the solar surface, magnetoconvection under the presence of a vertical large-scale magnetic field is simulated with the nonlinear MHD code Nirvana. The very robust result of the calculations is that < u(z)b(z)> similar or equal to 2 < u.b > independent of the applied magnetic field amplitude. The correlation coefficient for the cross helicity is about 10%. Of similar robustness is the finding that the rms value of the magnetic perturbations exceeds the mean-field amplitude (only) by a factor of five. The characteristic helicity speed u(eta) as the ratio of the eddy diffusivity and the density scale height for an isothermal sound velocity of 6.6 kms(-1) prove to be 1 kms(-1) for weak fields. This value coincides well with empirical results obtained from the data of the Hinode satellite and the Swedish 1-m Solar Telescope (SST) providing the cross-helicity component < u(z)b(z)>. Both simulations and observations thus lead to the numerical value of eta(T) similar or equal to 10(12) cm(2)/s as characteristic for the surface of the quiet Sun.
|
|
| 6. |
- Schnerr, R. S., et al.
(författare)
-
The brightness of magnetic field concentrations in the quiet Sun
- 2011
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 532, s. A136-
-
Tidskriftsartikel (refereegranskat)abstract
- In addition to the facular brightening of active regions, the quiet Sun also contains a small scale magnetic field with associated brightenings in continuum radiation. We measure this contribution of quiet regions to the Sun's brightness from high spatial resolution (0 ''.16-0 ''.32) observations of the Swedish 1-m Solar Telescope (SST) and Hinode satellite. The line-of-sight magnetic field and continuum intensity near Fe I 6302.5 angstrom are used to quantify the correlation between field strength and brightness. The data show that magnetic flux density contains a significant amount of intrinsically weak fields that contribute little to brightness. We show that with data of high spatial resolution a calibration of magnetic flux density as a proxy for brightness excess is possible. In the SST data, the magnetic brightening of a quiet region with an average (unsigned) flux density of 10 G is about 0.15%. In the Hinode data, and in SST data reduced to Hinode resolution, the measured brightening is some 40% lower. With appropriate correction for resolution, magnetic flux density can be used as a reliable proxy in regions of small scale mixed polarity. The measured brightness effect is larger than the variation of irradiance over a solar cycle. It is not clear, however, if this quiet Sun contribution actually varies significantly.
|
|
