24 research outputs found
The magnetic field of zeta Orionis A
Zeta Ori A is a hot star claimed to host a weak magnetic field, but no clear
magnetic detection was obtained so far. In addition, it was recently shown to
be a binary system composed of a O9.5I supergiant and a B1IV star. We aim at
verifying the presence of a magnetic field in zeta Ori A, identifying to which
of the two binary components it belongs (or whether both stars are magnetic),
and characterizing the field.Very high signal-to-noise spectropolarimetric data
were obtained with Narval at the Bernard Lyot Telescope (TBL) in France.
Archival HEROS, FEROS and UVES spectroscopic data were also used. The data were
first disentangled to separate the two components. We then analyzed them with
the Least-Squares Deconvolution (LSD) technique to extract the magnetic
information. We confirm that zeta Ori A is magnetic. We find that the
supergiant component zeta Ori Aa is the magnetic component: Zeeman signatures
are observed and rotational modulation of the longitudinal magnetic field is
clearly detected with a period of 6.829 d. This is the only magnetic O
supergiant known as of today. With an oblique dipole field model of the Stokes
V profiles, we show that the polar field strength is ~ 140 G. Because the
magnetic field is weak and the stellar wind is strong, zeta Ori Aa does not
host a centrifugally supported magnetosphere. It may host a dynamical
magnetosphere. Its companion zeta Ori Ab does not show any magnetic signature,
with an upper limit on the undetected field of 300 G
Detection of ultra-weak magnetic fields in Am stars: beta UMa and theta Leo
An extremely weak circularly polarized signature was recently discovered in
spectral lines of the chemically peculiar Am star Sirius A. A weak surface
magnetic field was proposed to account for the observed polarized signal, but
the shape of the phase-averaged signature, dominated by a prominent positive
lobe, is not expected in the standard theory of the Zeeman effect. We aim at
verifying the presence of weak circularly polarized signatures in two other
bright Am stars, beta UMa and theta Leo, and investigating the physical origin
of Sirius-like polarized signals further. We present here a set of deep
spectropolarimetric observations of beta UMa and theta Leo, observed with the
NARVAL spectropolarimeter. We analyzed all spectra with the Least Squares
Deconvolution multiline procedure. To improve the signal-to-noise ratio and
detect extremely weak signatures in Stokes V profiles, we co-added all
available spectra of each star (around 150 observations each time). Finally, we
ran several tests to evaluate whether the detected signatures are consistent
with the behavior expected from the Zeeman effect. The line profiles of the two
stars display circularly polarized signatures similar in shape and amplitude to
the observations previously gathered for Sirius A. Our series of tests brings
further evidence of a magnetic origin of the recorded signal. These new
detections suggest that very weak magnetic fields may well be present in the
photospheres of a significant fraction of intermediate-mass stars. The strongly
asymmetric Zeeman signatures measured so far in Am stars (featuring a dominant
single-sign lobe) are not expected in the standard theory of the Zeeman effect
and may be linked to sharp vertical gradients in photospheric velocities and
magnetic field strengths
Discovery of starspots on Vega - First spectroscopic detection of surface structures on a normal A-type star
The theoretically studied impact of rapid rotation on stellar evolution needs
to be confronted with the results of high resolution spectroscopy-velocimetry
observations. A weak surface magnetic field had recently been detected in the
A0 prototype star Vega, potentially leading to a (yet undetected) structured
surface. The goal of this article is to present a thorough analysis of the line
profile variations and associated estimators in the early-type standard star
Vega (A0) in order reveal potential activity tracers, exoplanet companions and
stellar oscillations. Vega was monitored in high-resolution spectroscopy with
the velocimeter Sophie/OHP. A total of 2588 high S/N spectra was obtained
during 5 nights (August 2012) at R = 75000 and covering the visible domain. For
each reduced spectrum, Least Square Deconvolved (LSD) equivalent photospheric
profiles were calculated with a Teff = 9500 and logg = 4.0 spectral line mask.
Several methods were applied to study the dynamic behavior of the profile
variations (evolution of radial velocity, bisectors, vspan, 2D profiles,
amongst others). We present the discovery of a starspotted stellar surface in
an A-type standard star with faint spot amplitudes Delta F/Fc ~5 10^{-4}. A
rotational modulation of spectral lines with a period of rotation P = 0.68 d
has clearly been exhibited, confirming the results of previous
spectropolarimetric studies. Either a very thin convective layer can be
responsible for magnetic field generation at small amplitudes, or a new
mechanism has to be invoked in order to explain the existence of activity
tracing starspots. This first strong evidence that standard A-type stars can
show surface structures opens a new field of research and asks the question
about a potential link with the recently discovered weak magnetic field
discoveries in this category of stars.Comment: accepted for publication by Astronomy & Astrophysics (23rd of March
2015
The Gaia-ESO survey: A spectroscopic study of the young open cluster NGC 3293
We present a spectroscopic analysis of the GIRAFFE and UVES data collected by
the Gaia-ESO survey for the young open cluster NGC 3293. Archive spectra from
the same instruments obtained in the framework of the `VLT-FLAMES survey of
massive stars' are also analysed. Atmospheric parameters, non-LTE chemical
abundances for six elements, or variability information are reported for a
total of about 160 B stars spanning a wide range in terms of spectral types (B1
to B9.5) and rotation rate (up to 350 km/s). We take advantage of the
multi-epoch observations to detect several binary systems or intrinsically
line-profile variables. A deconvolution algorithm is used to infer the current,
true (deprojected) rotational velocity distribution. We find a broad,
Gaussian-like distribution peaking around 200-250 km/s. Although some stars
populate the high-velocity tail, most stars in the cluster appear to rotate far
from critical. We discuss the chemical properties of the cluster, including the
low occurrence of abundance peculiarities in the late B stars and the paucity
of objects showing CN-cycle burning products at their surface. We argue that
the former result can largely be explained by the inhibition of diffusion
effects because of fast rotation, while the latter is generally in accord with
the predictions of single-star evolutionary models under the assumption of a
wide range of initial spin rates at the onset of main-sequence evolution.
However, we find some evidence for a less efficient mixing in two quite rapidly
rotating stars that are among the most massive objects in our sample. Finally,
we obtain a cluster age of ~20 Myrs through a detailed, star-to-star correction
of our results for the effect of stellar rotation. This is significantly older
than previous estimates from turn-off fitting that fully relied on classical,
non-rotating isochrones. [abridged]Comment: 29 pages, 24 figures, accepted for publication in A&
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
The magnetic properties of Am stars
International audienceWe present the results of a spectropolarimetric study of three Am stars: β UMa, θ Leo and Alhena. Two of the three stars of this study showed peculiar magnetic signatures with prominent positive lobes, like the one of Sirius A, that are not expected in the standard theory of the Zeeman effect. Alhena, contrary to Sirius A, β UMa and θ Leo, exhibits normal signatures. The follow-up spectropolarimetric observations of Alhena allowed us to determine the magnetic properties of this star
Discovery of starspots on Vega
Context. The theoretically studied impact of rapid rotation on stellar evolution needs to be compared with these results of high-resolution spectroscopy-velocimetry observations. Early-type stars present a perfect laboratory for these studies. The prototype A0 star Vega has been extensively monitored in recent years in spectropolarimetry. A weak surface magnetic field was detected, implying that there might be a (still undetected) structured surface. First indications of the presence of small amplitude stellar radial velocity variations have been reported recently, but the confirmation and in-depth study with the highly stabilized spectrograph SOPHIE/OHP was required. Aims. The goal of this article is to present a thorough analysis of the line profile variations and associated estimators in the early-type standard star Vega (A0) in order to reveal potential activity tracers, exoplanet companions, and stellar oscillations. Methods. Vega was monitored in quasi-continuous high-resolution echelle spectroscopy with the highly stabilized velocimeter SOPHIE/OHP. A total of 2588 high signal-to-noise spectra was obtained during 34.7 h on five nights (2 to 6 of August 2012) in high-resolution mode at R = 75 000 and covering the visible domain from 3895−6270 Å. For each reduced spectrum, least square deconvolved equivalent photospheric profiles were calculated with a Teff = 9500 and log g = 4.0 spectral line mask. Several methods were applied to study the dynamic behaviour of the profile variations (evolution of radial velocity, bisectors, vspan, 2D profiles, amongst others). Results. We present the discovery of a spotted stellar surface on an A-type standard star (Vega) with very faint spot amplitudes ΔF/Fc ~ 5 × 10-4. A rotational modulation of spectral lines with a period of rotation P = 0.68 d has clearly been exhibited, unambiguously confirming the results of previous spectropolarimetric studies. Most of these brightness inhomogeneities seem to be located in lower equatorial latitudes. Either a very thin convective layer can be responsible for magnetic field generation at small amplitudes, or a new mechanism has to be invoked to explain the existence of activity tracing starspots. At this stage it is difficult to disentangle a rotational from a stellar pulsational origin for the existing higher frequency periodic variations. Conclusions. This first strong evidence that standard A-type stars can show surface structures opens a new field of research and ask about a potential link with the recently discovered weak magnetic field discoveries in this category of stars.</p