12,752 research outputs found
Measuring stellar magnetic fields with the low resolution spectropolarimeter of the William Herschel Telescope
Despite the influence of magnetic fields on the structure and evolution of
stars is largely demonstrated from the theoretical point of view, their
observational evidence in non-degenerated stars is still rather scanty and
mainly circumscribed to bright objects (V<10). Stellar magnetic fields are
commonly measured on the basis of circular spectropolarimetry at high/middle
resolution across the profile of metal lines. The present sensitivity of
telescopes and spectrographs makes this still an almost prohibitive method for
faint stars. In principle, stellar magnetic fields can be also measured on the
basis of low resolution spectropolarimetry, with very important results
obtained at the 8 m ESO telescopes with FORS1. The trade off between S/N and
spectral resolution in measuring stellar magnetic fields justify an attempt,
here presented, to perform these measurements at the 4.5 m William Herschel
Telescope. HD3360, one of the stars with the weakest known magnetic field, and
the magnetic chemically peculiar stars HD10783, HD74521 and HD201601 have been
observed with the spectropolarimeter ISIS in the 3785-4480 A range. Measured
stellar magnetic fields, from Stokes I and V spectra with S/N > 600, show an
internal error of <50 G selecting the whole interval and <200 G within a Balmer
line. Ripples in the Stokes V spectra of HD3360 result in an instrumental
positive magnetic field certainly not larger than 80 G.Comment: 7 pages, 4 figures, MNRAS in pres
Variability of the HeI5876 A line in early type chemically peculiar stars
Chemically peculiar stars present spectral and photometric variability with a
single period. In the oblique rotator model, the non homogeneous distribution
of elements on the stellar surface is at the origin of the observed variations.
As to helium weak stars, it has been suggested that photometric and helium line
equivalent width variations are out of phase. To understand the behaviour of
helium in CP stars, we have obtained time resolved spectra of the HeI5876 A
line for a sample of 16 chemically peculiar stars in the spectral range B3 --
A1 and belonging to different sub-groups. The HeI5876 A line is too weak to be
measured in the spectra of the stars HD 24155, HD 41269, and HD 220825. No
variation of the equivalent width of the selected He line has been revealed in
the stars HD 22920, HD 24587, HD 36589, HD 49606, and HD 209515. The equivalent
width variation of the HeI5876 A line is in phase with the photometric
variability for the stars HD 43819, HD 171247 and HD 176582. On the contrary it
is out of phase for the stars HD 28843, HD 182255 and HD 223640. No clear
relation has been found for the stars HD 26571 and HD 177003.Comment: 8 pages, 9 figures. 1998, A&AS in pres
The origin of the radio emission from beta Lyrae
In this paper we present new observational evidence that supports the
presence of an extra source of continuum emission in the binary system beta
Lyrae. New VLA and IRAM observations, together with published data from the
literature and ISO archive data, allow us to build the Spectral Energy
Distribution of the binary between 5x10^9 Hz and 5x10^15Hz. The
radio-millimeter part of the spectrum is consistent with free-free emission
from a symbiotic-like wind associated with the primary component and ionized by
the radiation field of the hidden companion. Furthermore, we also consider the
possibility that the observed radio flux originates from collimated radio
structures associated with the mass gaining component and its disk (Conical
thermal jets). An extrapolation of this emission to the far-IR part of the
spectrum indicates that in both cases the free-free contribution at these
frequencies cannot explain the observations and that the observed infrared
excess flux is due principally to the secondary component and its associated
disk.Comment: 8 pages, 3 figures, A&A in pres
The radio lighthouse CU Virginis: the spindown of a single main sequence star
The fast rotating star CU Virginis is a magnetic chemically peculiar star
with an oblique dipolar magnetic field. The continuum radio emission has been
interpreted as gyrosyncrotron emission arising from a thin magnetospheric
layer. Previous radio observations at 1.4 GHz showed that a 100% circular
polarized and highly directive emission component overlaps to the continuum
emission two times per rotation, when the magnetic axis lies in the plane of
the sky. This sort of radio lighthouse has been proposed to be due to cyclotron
maser emission generated above the magnetic pole and propagating
perpendicularly to the magnetic axis. Observations carried out with the
Australia Telescope Compact Array at 1.4 and 2.5 GHz one year after this
discovery show that this radio emission is still present, meaning that the
phenomenon responsible for this process is steady on a timescale of years. The
emitted radiation spans at least 1 GHz, being observed from 1.4 to 2.5 GHz. On
the light of recent results on the physics of the magnetosphere of this star,
the possibility of plasma radiation is ruled out. The characteristics of this
radio lighthouse provides us a good marker of the rotation period, since the
peaks are visible at particular rotational phases. After one year, they show a
delay of about 15 minutes. This is interpreted as a new abrupt spinning down of
the star. Among several possibilities, a quick emptying of the equatorial
magnetic belt after reaching the maximum density can account for the magnitude
of the breaking. The study of the coherent emission in stars like CU Vir, as
well as in pre main sequence stars, can give important insight into the angular
momentum evolution in young stars. This is a promising field of investigation
that high sensitivity radio interferometers such as SKA can exploit.Comment: Accepted to MNRAS, 8 pages, 7 figures, updated versio
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