The magnetic field of the pre-main sequence Herbig Ae star HD 190073 (original) (raw)
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Recent advances in the observation and analysis of stellar magnetic fields
Lecture Notes in Physics, 1991
A b s t r a c t : There has been considerable progress recently in the study of magnetic fields on late-type stars. Advances in the theory include investigation of systematic effects, new and improved methods of analysis for unpolarized and circularly polarized lines, new models of stellar dynamos and of broadband linear polarization, and the first studies of the thermodynamic nature of stellar magnetic regions and their vertical structure. Observationally, there have been new measurements, particularly of young and active stars (including the first detection of a field on a pre-main sequence object), a new monitoring campaign, the first indications of the relative temperatures in stellar plages, and the first measurements of fields in a single stellar active region. I discuss the results in the framework of stellar activity and surface structure.
Methods and Results for Detecting Magnetic Fields on Late-Type Stars
Solar and Stellar Magnetic Fields: Origins and Coronal Effects, 1983
We discuss our program to detect and measure magnetic flux on the surfaces of late-type stars. We adopt a novel technique to deconvolve magnetically insensitive lines from similar, magnetically sensitive lines to infer the degree of Zeeman splitting in the latter lines. These measurements yield values for the magnetic field strength and filling factor (flux). To illustrate our approach we present multiple observations of the RS CVn star X And. At the epoch of observation 26 April 1981 we find a field strength of 1290 ± 50 gauss covering 48 ± 2 percent of this star's surface. Observations at other epochs clearly demonstrate magnetic flux variability on X And. Standard polarization methods for measuring stellar magnetic fields are inappropriate for solar-type field topologies where the field polarities are tangled, and where polarization effects cancel.
Weak magnetic fields of intermediate-mass stars
We present the result of a highly sensitive spectropolarimetric study dedicated to intermediate mass stars. We report the detection of sub-gauss surface magnetic fields on the normal, rapidly-rotating A-type star Vega and on the moderately-rotating Am star Sirius A. These magnetic detections constitute the first evidence that tepid stars that do not belong to the class of Ap/Bp stars can also host magnetized photospheres, suggesting that a significant fraction of stars in this mass regime are magnetic. We present here the observational clues gathered so far to progress towards understanding the physical processes at the origin of this newly identified Vega-like magnetism.
Astronomy & Astrophysics, 2011
Context. Our knowledge of the presence and the role of magnetic fields in intermediate-mass pre-main-sequence stars remains very poor. Aims. We present the magnetic properties of four Herbig Ae/Be stars that have not been previously studied in detail. Methods. Our results for the three Herbig Ae/Be stars HD 97048, HD 150193, and HD 176386 are based on multi-epoch lowresolution spectra obtained in spectropolarimetric mode with FORS 2 mounted on the VLT. New high-resolution polarimetric spectra were obtained for MWC 480 with the SOFIN spectrograph installed at the Nordic Optical Telescope. We discuss 41 FORS 2 lowresolution observations of the Herbig Ae/Be stars HD 97048, HD 150193, and HD 176386 and determine their rotational periods. Using stellar fundamental parameters and the longitudinal magnetic field phase curves, we place constraints on the magnetic field geometry. Three high-resolution circularly polarised SOFIN spectra obtained for MWC 480 were measured using the moment technique where wavelength shifts between right-and left-hand side circularly polarised spectra are interpreted in terms of a longitudinal magnetic field B z. Results. Our search for periodicities resulted in P = 0.693 d for HD 97048, P = 1.317 d for HD 150193, and P = 0.899 d for HD 176386. The magnetic field geometry can likely be described by a centred dipole with a polar magnetic field strength B d of several hundred Gauss. The longitudinal magnetic-field measurements of MWC 480 reveal the presence of a strong kG field, which was undetected in our previous low-resolution polarimetric observations with FORS 1. A weak magnetic field was detected in the circumstellar components of the Ca ii H&K lines and the Na i D lines, indicating a complex interaction between the stellar magnetic field and the circumstellar environment.
Magnetism of Herbig Ae/Be stars
2009
Observations of magnetic fields of stars at the pre-main sequence phase can provide important new insights into the complex physics of the late stages of star formation. This is especially true at intermediate stellar masses, where magnetic fields are strong and globally organised, and therefore most amenable to direct study. Recent circularly-polarised spectroscopic observations of pre-main sequence Herbig Ae/Be stars have revealed the presence of organised magnetic fields in the photospheres of a small fraction of these objects. To date, 9 magnetic HAeBe stars have been detected, and those detections confirmed by repeated observations. The morphology and variability of their Stokes V signatures indicates that their magnetic fields have important dipole components of kG strength, and that the dipole is stable on timescales ofat least years. These magnetic stars exhibit a large range of stellar mass, from about 2-13 solar masses, and diverse rotational properties, with vsini from a ...
Magnetism in pre-MS intermediate-mass stars and the fossil field hypothesis
Today, one of the greatest challenges concerning the Ap/Bp stars is to understand the origin of their slow rotation and their magnetic fields. The favoured hypothesis for the latter is the fossil field, which implies that the magnetic fields subsist throughout the different evolutionary phases, and in particular during the pre-main sequence phase. The existence of magnetic fields at the pre-main sequence phase is also required to explain the slow rotation of Ap/Bp stars. However, until recently, essentially no information was available about the magnetic properties of intermediate-mass pre-main sequence stars, the so-called Herbig Ae/Be stars. The new high-resolution spectropolarimeter ESPaDOnS, installed in 2005 at the Canada-France-Hawaii telescope, provided the capability necessary to perform surveys of the Herbig Ae/Be stars in order to investigate their magnetism and rotation. These investigations have resulted in the detection and/or confirmation of magnetic fields in 8 Herbig Ae/Be stars, ranging in mass from 2 to nearly 15 solar masses. In this contribution I will present the results of our survey, as well as their implications for the origin and evolution of the magnetic fields and rotation.
Characterization of the magnetic field of the Herbig Be star HD 200775★
Monthly Notices of the Royal Astronomical Society, 2008
The origin of the magnetic fields observed in some intermediate mass and high mass main sequence stars is still a matter of vigorous debate. The favoured hypothesis is a fossil field origin, in which the observed fields are the condensed remnants of magnetic fields present in the original molecular cloud from which the stars formed. According to this theory a few percent of the PMS Herbig Ae/Be star should be magnetic with a magnetic topology similar to that of main sequence intermediate-mass stars.
Magnetic fields in Herbig Ae�stars
Astronomy and Astrophysics, 2004
Herbig Ae stars are young A-type stars in the pre-main sequence evolutionary phase with masses of ∼1.5-3 M. They show rather intense surface activity (Dunkin et al. 1997, MNRAS, 290, 165) and infrared excess related to the presence of circumstellar disks. Because of their youth, primordial magnetic fields inherited from the parent molecular cloud may be expected, but no direct evidence for the presence of magnetic fields on their surface, except in one case (Donati et al. 1997, MNRAS, 291, 658), has been found until now. Here we report observations of optical circular polarization with FORS 1 at the VLT in the three Herbig Ae stars HD 139614, HD 144432 and HD 144668. A definite longitudinal magnetic field at 4.8 σ level, B z = −450 ± 93 G, has been detected in the Herbig Ae star HD 139614. This is the largest magnetic field ever diagnosed for a Herbig Ae star. A hint of a weak magnetic field is found in the other two Herbig Ae stars, HD 144432 and HD 144668, for which magnetic fields are measured at the ∼1.6 σ and ∼2.5 σ level respectively. Further, we report the presence of circular polarization signatures in the Ca K line in the V Stokes spectra of HD 139614 and HD 144432, which appear unresolved at the low spectral resolution achievable with FORS 1. We suggest that models involving accretion of matter from the disk to the star along a global stellar magnetic field of a specific geometry can account for the observed Zeeman signatures.
The magnetic field in the photospheric and circumstellar components of Herbig Ae stars
Astronomy and Astrophysics, 2007
Aims. We investigate the photospheric and circumstellar (CS) magnetic field components separately in seven Herbig Ae stars. Methods. The study is based on low-resolution (R ∼ 2000 and 4000) spectropolarimetric data collected from 2003 to 2005 at the Very Large Telescope (ESO, Chile) with the multi-mode instrument FORS1. Results. We show that the spectropolarimetric results strongly depend on the level of CS contribution to the stellar spectra. We have improved the determination accuracy of magnetic fields up to the 7σ level in the two Herbig Ae stars HD 139614 and HD 144432, observed in 2005 when these objects were at a low level state in their CS activity. We have established that at a higher level state of CS activity the polarisation signatures are mainly related to the CS matter. The presence of CS polarisation signatures formed in the stellar wind supports the assumption that the magnetic centrifuge is one of the main mechanisms of wind acceleration. Conclusions. We conclude that the most effective way to investigate the magnetism of Herbig Ae stars is to monitor their spectropolarimetric behaviour at different stages of CS activity. Obviously, higher-resolution spectropolarimetric observations would extend the sample of spectral lines which can be used to measure magnetic fields at different levels in the stellar atmosphere and CS envelope. These observations will give a more complete insight into the magnetic topology in Herbig Ae stars.