32 research outputs found
No evidence of a strong magnetic field in the Blazhko star RR Lyrae
Astronomy and Astrophysics, v. 413, p. 1087-1093, 2004. http://dx.doi.org/10.1051/0004-6361%3a20031600International audienc
Magnetism, rotation and accretion in Herbig Ae-Be stars
Studies of stellar magnetism at the pre-main sequence phase can provide
important new insights into the detailed physics of the late stages of star
formation, and into the observed properties of main sequence stars. This is
especially true at intermediate stellar masses, where magnetic fields are
strong and globally organised, and therefore most amenable to direct study.
This talk reviews recent high-precision ESPaDOnS observations of pre-main
sequence Herbig Ae-Be stars, which are yielding qualitatively new information
about intermediate-mass stars: the origin and evolution of their magnetic
fields, the role of magnetic fields in generating their spectroscopic activity
and in mediating accretion in their late formative stages, and the factors
influencing their rotational angular momentum.Comment: 8 page
Magnetic Field Generation in Stars
Enormous progress has been made on observing stellar magnetism in stars from
the main sequence through to compact objects. Recent data have thrown into
sharper relief the vexed question of the origin of stellar magnetic fields,
which remains one of the main unanswered questions in astrophysics. In this
chapter we review recent work in this area of research. In particular, we look
at the fossil field hypothesis which links magnetism in compact stars to
magnetism in main sequence and pre-main sequence stars and we consider why its
feasibility has now been questioned particularly in the context of highly
magnetic white dwarfs. We also review the fossil versus dynamo debate in the
context of neutron stars and the roles played by key physical processes such as
buoyancy, helicity, and superfluid turbulence,in the generation and stability
of neutron star fields.
Independent information on the internal magnetic field of neutron stars will
come from future gravitational wave detections. Thus we maybe at the dawn of a
new era of exciting discoveries in compact star magnetism driven by the opening
of a new, non-electromagnetic observational window.
We also review recent advances in the theory and computation of
magnetohydrodynamic turbulence as it applies to stellar magnetism and dynamo
theory. These advances offer insight into the action of stellar dynamos as well
as processes whichcontrol the diffusive magnetic flux transport in stars.Comment: 41 pages, 7 figures. Invited review chapter on on magnetic field
generation in stars to appear in Space Science Reviews, Springe
Magnetism in Herbig Ae/Be stars and the link to the Ap/Bp stars
Among the A/B stars, about 5% host large-scale organised magnetic fields.
These magnetic stars show also abundance anomalies in their spectra, and are
therefore called the magnetic Ap/Bp stars. Most of these stars are also slow
rotators compared to the normal A and B stars. 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
that the fields are fosils, 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.
During the last 3 years we performed a spectropolarimetric survey of the Herbig
Ae/Be stars in the field and in young clusters, 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 paper I will present the results of
our survey, as well as their implications for the origin and evolution of the
magnetic fields and rotation of the A and B stars.Comment: To appear in the proceedings of "Ecole d'astronomie du CNRS et du
PNPS : Les champs magnetiques stellaires", editors: C. Neiner and J.-P. Zahn,
EAS Publications Serie
Evidence for mass accretion driven by spiral shocks onto the white dwarf in SDSS J123813.73–033933.0
We present high-time-resolution photometry and phase-resolved spectroscopy of the short-period (Porb=80.52min) cataclysmic variable SDSS J123813.73–033933.0, observed with the Hubble Space Telescope (HST), the Kepler/K2 mission, and the Very Large Telescope (VLT). We also report observations of the first detected superoutburst. SDSS J1238–0339 shows two types of variability: quasi-regular brightenings recurring every ≃8.5 h during which the system increases in brightness by ≃0.5mag, and a double-hump quasi-sinusoidal modulation at the orbital period. The detailed K2 light curve reveals that the amplitude of the double-humps increases during the brightenings and that their phase undergoes a ≃90° phase shift with respect to the quiescent intervals. The HST data unambiguously demonstrate that these phenomena both arise from the heating and cooling of two relatively large regions on the white dwarf. We suggest that the double-hump modulation is related to spiral shocks in the accretion disc resulting in an enhanced accretion rate heating two localized regions on the white dwarf, with the structure of the shocks fixed in the binary frame explaining the period of the double humps. The physical origin of the 8.5 h brightenings is less clear. However, the correlation between the observed variations of the amplitude and phase of the double-humps with the occurrence of the brightenings is supportive of an origin in thermal instabilities in the accretion disc
A planetesimal orbiting within the debris disc around a white dwarf star
Many white dwarf stars show signs of having accreted smaller bodies, implying that they may host planetary systems. A small number of these systems contain gaseous debris discs, visible through emission lines. We report a stable 123.4-minute periodic variation in the strength and shape of the Ca ii emission line profiles originating from the debris disc around the white dwarf SDSS J122859.93+104032.9. We interpret this short-period signal as the signature of a solid-body planetesimal held together by its internal strength
Observing and Modelling Stellar Magnetic Fields
In this chapter, we examine the question of how spectropolarimetric observations of magnetic stars may be modelled, and how modelling techniques may be used to extract much detailed information about the stellar magnetic field and other characteristics of the magnetized stellar atmosphere
Observations of stellar magnetism and associated phenomena
This paper first surveys how stellar magnetic fields are measured, and especially the impact of the most recent spectropolarimeters on our measurement capabilities. The two main types of stellar magnetism, dynamo and fossil fields, are discussed in a general way. This is followed by a broad survey of the current level of knowledge of magnetic fields in various classes of stars, from the pre-main sequence through main sequence stars, giants, and white dwarfs. Some of the hotter magnetic stars show clear abundance anomalies relative to the solar abundance ratios. The connection between magnetism and the atomic diffusion believed to be responsible for these anomalies is discussed, especially in the context of some new results from cluster A and B magnetic stars
Observing and Modelling Stellar Magnetic Fields
This and the following two chapters present a general introduction to the subject of observing magnetic fields in stars using spectropolarimetry. The three chapters will consider (1) the basic physics of the Zeeman and related effects, direct deductions about stellar magnetic fields that may be made where this effect is detected, and how such measurements have provided much information about the Sun and other magnetic stars; (2) how spectropolarimetric observations of magnetic stars may be modelled, and how such techniques may be used to extract much detailed information about the stellar magnetic field and other characteristics of the magnetized stellar atmosphere; and (3) how other magnetic effects such as the Hanle effect and continuum polarisation may be used to detect fields in a variety of stellar types such as white dwarfs, and how the current body of observational knowledge about magnetic stars may be integrated into a roughly coherent, provisional scenario of field origin and evolution.
In this chapter we survey the physics of an atom in a magnetic field (the Zeeman effect and its relatives), and identify ways in which magnetic effects on the atomic energy level structure may be used to detect and characterise stellar magnetic fields. We survey some of the basic features of magnetism in the Sun. We then describe how the available data may be used to deduce simple averages of various magnetic quantities over the stellar surface (for example, the “mean longitudinal field”) and how such simple field measurements are used to develop and explore the “oblique (dipole) rotator model” of magnetism for a group of middle main sequence stars called the “magnetic Ap stars”