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Studies of the presence of magnetic fields in Herbig Ae/Be stars are extremely important because they enable us to improve our insight into how the magnetic fields of these stars are generated and how they interact with their environment, including their impact on the planet formation process and the planet-disk interaction. We report new detections of weak mean longitudinal magnetic fields in the close Herbig Ae double-lined spectroscopic binary AK Sco and in the presumed spectroscopic Herbig Ae binary HD 95881 (Järvinen et al. 2018) based on observations obtained with HARPSpol attached to ESO’s 3.6 m telescope. Such studies are important because only very few close spectroscopic binaries with orbital periods below 20 d are known among Herbig Ae stars. Our detections favour the conclusion that the previously suggested low incidence (5-10%) of magnetic Herbig Ae stars can be explained by the weakness of these fields and the limited accuracy of the published measurements. The search for magnetic fields and the determination of their geometries in close binary systems will play an important role for understanding the mechanisms that are responsible for the magnetic field generation.
Herbig Ae/Be-type stars are analogs of T Tauri stars at higher masses. Since the confirmation of magnetospheric accretion using Balmer and sodium line profiles in the Herbig Ae star UX Ori, a number of magnetic studies have been attempted, indicating that about 20 Herbig Ae/Be stars likely have globally organized magnetic fields. The low detection rate of magnetic fields in Herbig Ae stars can be explained by the weakness of these fields and rather large measurement uncertainties. The obtained density distribution of the root mean square longitudinal magnetic field values revealed that only a few stars have magnetic fields stronger than 200 G, and half of the sample possesses magnetic fields of about 100 G or less. We report on the results of our analysis of a sample of presumably single Herbig Ae/Be stars based on recent observations obtained with HARPSpol attached to ESO’s 3.6m telescope. Knowledge of the magnetic field structure combined with the determination of the chemical composition are indispensable to constrain theories on star formation and magnetospheric accretion in intermediate-mass stars. As of today, magnetic phase curves have been obtained only for two Herbig Ae/Be stars, HD 101412 and V380 Ori.
. A fraction of high-mass X-ray binaries are supergiant fast X-ray transients. These systems have on average low X-ray luminosities, but display short flares during which their X-ray luminosity rises by a few orders of magnitude. The leading model for the physics governing this X-ray behaviour suggests that the winds of the donor OB supergiants are magnetized. In agreement with this model, the first spectropolarimetric observations of the SFXT IGR J11215-5952 using the FORS 2 instrument at the Very Large Telescope indicate the presence of a kG longitudinal magnetic field. Based on these results, it seems possible that the key difference between supergiant fast X-ray transients and other high-mass X-ray binaries are the properties of the supergiant’s stellar wind and the physics of the wind’s interaction with the neutron star magnetosphere.
Previous circular polarization observations obtained with the ESO FOcal Reducer low dispersion spectrograpgh at the VLT in 2007–2008 revealed the presence of a weak longitudinal magnetic field on the surface of the optical component of the X-ray binary Cyg X-1, which contains a black hole and an O9.7Iab supergiant on a 5.6 d orbit. In this contribution we report on recently acquired FORS 2 spectropolarimetric observations of Cyg X-1 along with measurements of a few additional high-mass X-ray binaries.
The B fields in OB stars (BOB) survey is an ESO large programme collecting spectropolarimetric observations for a large number of early-type stars in order to study the occurrence rate, properties, and ultimately the origin of magnetic fields in massive stars. As of July 2014, a total of 98 objects were observed over 20 nights with FORS2 and HARPSpol. Our preliminary results indicate that the fraction of magnetic OB stars with an organised, detectable field is low. This conclusion, now independently reached by two different surveys, has profound implications for any theoretical model attempting to explain the field formation in these objects. We discuss in this contribution some important issues addressed by our observations (e.g., the lower bound of the field strength) and the discovery of some remarkable objects.
The control of stored-product moths in bagged commodities is difficult because the developmental stages of the moths are protected by the bagging material from control measures such as the application of contact insecticides. Studies were carried out to assess the ability of Hymenopteran parasitoids to locate their hosts inside jute bags in the laboratory. The ability of different parasitoids to penetrate jute bags containing rice was investigated in a controlled climate chamber. Few Habrobracon hebetor (Say) (Hymenoptera: Braconidae) passed through the jute material while a high percentage of Lariophagus distinguendus (Förster), Anisopteromalus calandrae (Howard) (Hymenoptera: Pteromalidae), Theocolax elegans (Westwood) (Hymenoptera: Pteromalidae) and Trichogramma evanescens Westwood (Hymenoptera: Trichogrammatidae) were able to enter the Petri-dishes. Significantly more L. distinguendus and T. elegans entered compared to H. hebetor. There was significant difference in the mean percentage parasitoids invading depending on species. Head capsules and/or thorax widths were measured in order to determine whether the opening in the jute material would be large enough for entry of the parasitoids. These morphometric data differed depending on parasitoid species and sex. The parasitoid Venturia canescens (Gravenhorst) (Hymenoptera: Ichneumonidae) did not enter the bags, but located host larvae inside the jute bags and parasitized rice moths Corcyra cephalonica larvae by stinging through the jute material. Venturia canescens significantly reduced the number of C. cephalonica adults emerging from the bagged rice; therefore, it could be released in storage rooms containing bagged rice for biological control of C. cephalonica. The use of parasitoids to suppress stored-product insect pests in bagged commodities could become a valuable supplement to the use of synthetic pesticides.
We present the results of the continuation of our magnetic survey with FORS 1 at the VLT of a sample of B-type stars consisting of confirmed or candidate β Cephei stars and Slowly Pulsating B stars. Roughly one third of the studied β Cephei stars have detected magnetic fields. The fraction of magnetic Slowly Pulsating B and candidate Slowly Pulsating B stars is found to be higher, up to 50%. We find that the domains of magnetic and non-magnetic pulsating stars in the H-R diagram largely overlap, and no clear picture emerges as to the possible evolution of the magnetic field across the main sequence.
We present the results of a new magnetic field survey of Herbig Ae/Be and A debris disk stars. They are used to determine whether magnetic field properties in these stars are correlated with the mass-accretion rate, disk inclinations, companion(s), Silicates, PAHs, or show a more general correlation with age and X-ray emission as expected for the decay of a remnant dynamo.
We report the results of our study of magnetic fields in a sample of Be stars using spectropolarimetric data obtained at the European Southern Observatory with the multi-mode instrument FORS 1 installed at the 8m Kueyen telescope. The detected magnetic fields are rather weak, not stronger than ~150G. A few classical Be stars display cyclic variability of the magnetic field with periods of tens of minutes.
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.
The Very Large Telescope Interferometer on Cerro Paranal in Northern Chile
is one of the largest optical facilities used in astronomy.
It can combine two or three of the four 8.2 m and four movable 1.8 m telescopes,
which span baselines between 8 and 202 m.
Observations are carried out in the near- and mid-infrared, covering the
wavelength range from 1 to 13 μm.
We present and discuss the results of a comprehensive analysis of magnetic field measurements for a group of seven young Herbig Ae/Be stars obtained using low-resolution spectropolarimetry with FORS1 at the VLT. The first definite evidence for the presence of surface magnetic fields has been found only recently in the Herbig Ae stars HD 139614, HD 31648 and HD 144432 (Hubrig et al. 2004; Hubrig et al. 2006) from the measurement of circular polarization in Ca II lines and hydrogen lines located in the spectral region from 3900 to 4900 Å. The unusual Herbig Ae star HD 190073 shows distinctive Zeeman features in the Ca II doublet which displays several components in both H and K lines. The most recent analysis carried out separately for different lines has shown that Zeeman features appear in metallic lines in all seven objects of our programme. The most prominent of them are seen in lines of the CaII doublet possibly of both photospheric and circumstellar origin. The circumstellar Balmer line components demonstrate the existence of a magnetic field in HD 144432 and HD 31648. The photospheric components of these lines with signs of magnetic field are present in HD 144668.
In this article, we discuss the instrumental requirements for planet searches from the ground. We review several methods and their fundamental limitations comparing the direct measurement of the visibility function (both modulus and phase) to astrometry and nulling interferometry. In particular, we will take the Very Large Telescope Interferometer (VLTI) as an example. The VLTI is an excellent facility for planet searches from the ground since it has both large apertures and long baselines, providing high SNR and milli-arcsecond resolution.
The Very Large Telescope (VLT) Observatory on Cerro Paranal (2635
m) in Northern Chile is approaching completion. After the four 8-m
Unit Telescopes (UT) individually saw first light in the last years,
two of them were combined for the first time on October 30, 2001 to
form a stellar interferometer, the VLT Interferometer. The remaining
two UTs will be integrated into the interferometric array later this
year, so that any two UTs can be used for interferometry. In this
article, we will describe the subsystems of the VLTI and the planning
for the following years.
We describe the first long-baseline interferometric measurement ever made of the luminous blue variable η Carinae. η Car was observed with the Very Large Telescope Interferometer (VLTI) with the 8.2 m Unit Telescopes as well as with the 40 cm test siderostats on three different baselines. We will detail the measurements and data analysis and discuss briefly the implications of the derived visibility on the morphology of the central source. Furthermore we will discuss the future possibilities of interferometric observations and will show that η Car is one of the prime candidates for imaging with the VLTI.
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