590 research outputs found
The different origins of magnetic fields and activity in the Hertzsprung gap stars, OU Andromedae and 31 Comae
Context: When crossing the Hertzsprung gap, intermediate-mass stars develop a
convective envelope. Fast rotators on the main sequence, or Ap star
descendants, are expected to become magnetic active subgiants during this
evolutionary phase. Aims: We compare the surface magnetic fields and activity
indicators of two active, fast rotating red giants with similar masses and
spectral class but diferent rotation rates - OU And (Prot=24.2 d) and 31 Com
(Prot=6.8 d) - to address the question of the origin of their magnetism and
high activity.
Methods: Observations were carried out with the Narval spectropolarimeter in
2008 and 2013.We used the least squares deconvolution technique to extract
Stokes V and I profiles to detect Zeeman signatures of the magnetic field of
the stars. We provide Zeeman-Doppler imaging, activity indicator monitoring,
and a precise estimation of stellar parameters. We use stellar evolutionary
models to infer the evolutionary status and the initial rotation velocity on
the main sequence.
Results: The detected magnetic field of OU And is a strong one. Its
longitudinal component Bl reaches 40 G and presents an about sinusoidal
variation with reversal of the polarity. The magnetic topology of OU And is
dominated by large scale elements and is mainly poloidal with an important
dipole component, and a significant toroidal component. The detected magnetic
field of 31 Com is weaker, with a magnetic map showing a more complex field
geometry, and poloidal and toroidal components of equal contributions. The
evolutionary models show that the progenitors of OU And and 31 Com must have
been rotat
Conclusions: OU And appears to be the probable descendant of a magnetic Ap
star, and 31 Com the descendant of a relatively fast rotator on the main
sequence.Comment: 16 pages, 12 figure
Search for surface magnetic fields in Mira stars. First detection in chi Cyg
In order to complete the knowledge of the magnetic field and of its influence
during the transition from Asymptotic Giant Branch to Planetary Nebulae stages,
we have undertaken a search for magnetic fields at the surface of Mira stars.
We used spectropolarimetric observations, collected with the Narval instrument
at TBL, in order to detect - with Least Squares Deconvolution method - a Zeeman
signature in the visible part of the spectrum. We present the first
spectropolarimetric observations of the S-type Mira star chi Cyg, performed
around its maximum light. We have detected a polarimetric signal in the Stokes
V spectra and we have established its Zeeman origin. We claim that it is likely
to be related to a weak magnetic field present at the photospheric level and in
the lower part of the stellar atmosphere. We have estimated the strength of its
longitudinal component to about 2-3 Gauss. This result favors a 1/r law for the
variation of the magnetic field strength across the circumstellar envelope of
chi Cyg. This is the first detection of a weak magnetic field at the stellar
surface of a Mira star and we discuss its origin in the framework of shock
waves periodically propagating throughout the atmosphere of these radially
pulsating stars. At the date of our observations of chi Cyg, the shock wave
reaches its maximum intensity, and it is likely that the shock amplifies a weak
stellar magnetic field during its passage through the atmosphere. Without such
an amplification by the shock, the magnetic field strength would have been too
low to be detected. For the first time, we also report strong Stokes Q and U
signatures (linear polarization) centered onto the zero velocity (i.e., at the
shock front position). They seem to indicate that the radial direction would be
favored by the shock during its propagation throughout the atmosphere.Comment: 9 pages, 4 figures accepted by Astronomy and Astrophysics (21
November 2013
Magnetic fields in single late-type giants in the Solar vicinity: How common is magnetic activity on the giant branches?
We present our first results on a new sample containing all single G,K and M
giants down to V = 4 mag in the Solar vicinity, suitable for
spectropolarimetric (Stokes V) observations with Narval at TBL, France. For
detection and measurement of the magnetic field (MF), the Least Squares
Deconvolution (LSD) method was applied (Donati et al. 1997) that in the present
case enables detection of large-scale MFs even weaker than the solar one (the
typical precision of our longitudinal MF measurements is 0.1-0.2 G). The
evolutionary status of the stars is determined on the basis of the evolutionary
models with rotation (Lagarde et al. 2012; Charbonnel et al., in prep.) and
fundamental parameters given by Massarotti et al. (1998). The stars appear to
be in the mass range 1-4 M_sun, situated at different evolutionary stages after
the Main Sequence (MS), up to the Asymptotic Giant Branch (AGB). The sample
contains 45 stars. Up to now, 29 stars are observed (that is about 64 % of the
sample), each observed at least twice. For 2 stars in the Hertzsprung gap, one
is definitely Zeeman detected. Only 5 G and K giants, situated mainly at the
base of the Red Giant Branch (RGB) and in the He-burning phase are detected.
Surprisingly, a lot of stars ascending towards the RGB tip and in early AGB
phase are detected (8 of 13 observed stars). For all Zeeman detected stars v
sin i is redetermined and appears in the interval 2-3 km/s, but few giants with
MF possess larger v sin i.Comment: 4 pages, 3 figures, Proceedings IAU Symposium No. 302, 201
The influence silicon dioxide nanoparticles on mechanical properties of erythrocyte and platelet membranes estimated by atomic force microscopy method
The investigation performed within the Programs of State Research “Energy systems, process and technologies”, project 2.2
Magnetic Field Structure and Activity of the He-burning Giant 37 Comae
We present the first magnetic map of the late-type giant 37 Com. The Least Squares Deconvolution (LSD) method and Zeeman Doppler Imaging (ZDI) inversion technique were applied. The chromospheric activity indicators Hα, S-index, Ca ii IRT and the radial velocity were also measured. The evolutionary status of the star has been studied on the basis of state-of-the-art stellar evolutionary models and chemical abundance analysis. 37 Com appears to be in the core Helium-burning phas
THROMBOLYTIC THERAPY IN PATIENTS WITH ACUTE MYOCARDIAL INFARCTION
It is reported that the implementation in clinical practice thrombolytic (fibrinolytic) therapy resulted inmortality reduction during firstmonth after myocardial infarction from 17-18% to 5-8%. Different details of this therapy are considered: terms of thrombolysis since the beginning of myocardial infarction, alternative methods of coronary blood flow recovery , indications and contraindications, complications and side effects, estimation of thrombolysis efficacy. Fibrin-selective and fibrin-non-selective drugs are presented. Different fibrinolytics are described: streptokinase, anistreplase, alteplase, reteplase, tenekteplase. The results of large randomized clinical trials devoted to fibrinolytic therapy of myocardial infarction are analyzed: GISSI, ISSIS, TIMI, GUSTO, INJECT, ASSENT. The possibility to increase in efficacy and safety of fibrinolytics by their combination with acetylsalicylic acid, IIb/IIIa receptor inhibitors and heparins are discussed
The Magnetic Fields at the Surface of Active Single G-K Giants
We investigate the magnetic field at the surface of 48 red giants selected as
promising for detection of Stokes V Zeeman signatures in their spectral lines.
We use the spectropolarimeters Narval and ESPaDOnS to detect circular
polarization within the photospheric absorption lines of our targets and use
the least-squares deconvolution (LSD) method. We also measure the classical
S-index activity indicator, and the stellar radial velocity. To infer the
evolutionary status of our giants and to interpret our results, we use
state-of-the-art stellar evolutionary models with predictions of convective
turnover times. We unambiguously detect magnetic fields via Zeeman signatures
in 29 of the 48 red giants in our sample. Zeeman signatures are found in all
but one of the 24 red giants exhibiting signs of activity, as well as 6 out of
17 bright giant stars.The majority of the magnetically detected giants are
either in the first dredge up phase or at the beginning of core He burning,
i.e. phases when the convective turnover time is at a maximum: this corresponds
to a 'magnetic strip' for red giants in the Hertzsprung-Russell diagram. A
close study of the 16 giants with known rotational periods shows that the
measured magnetic field strength is tightly correlated with the rotational
properties, namely to the rotational period and to the Rossby number Ro. Our
results show that the magnetic fields of these giants are produced by a dynamo.
Four stars for which the magnetic field is measured to be outstandingly strong
with respect to that expected from the rotational period/magnetic field
relation or their evolutionary status are interpreted as being probable
descendants of magnetic Ap stars. In addition to the weak-field giant Pollux, 4
bright giants (Aldebaran, Alphard, Arcturus, eta Psc) are detected with
magnetic field strength at the sub-gauss level.Comment: 34 pages, 22 Figures, accepted for publication in Astronomy &
Astrophysic
Direct detection of a magnetic field in the photosphere of the single M giant EK Boo: How common is magnetic activity among M giants?
We study the fast rotating M5 giant EK Boo by means of spectropolarimetry to
obtain direct and simultaneous measurements of both the magnetic field and
activity indicators, in order to infer the origin of the activity in this
fairly evolved giant. We used the new spectropolarimeter NARVAL at the Bernard
Lyot Telescope (Observatoire du Pic du Midi, France) to obtain a series of
Stokes I and Stokes V profiles for EK Boo. Using the Least Square Deconvolution
technique we were able to detect the Zeeman signature of the magnetic field. We
measured its longitudinal component by means of the averaged Stokes V and
Stokes I profiles. The spectra also permitted us to monitor the CaII K&H
chromospheric emission lines, which are well known as indicators of stellar
magnetic activity. From ten observations obtained between April 2008 and March
2009, we deduce that EK Boo has a magnetic field, which varied in the range of
-0.1 to -8 G. We also determined the initial mass and evolutionary stage of EK
Boo, based on up-to-date stellar evolution tracks. The initial mass is in the
range of 2.0-3.6 M_sun, and EK Boo is either on the asymptotic giant branch
(AGB), at the onset of the thermal pulse phase, or at the tip of the first (or
red) giant branch (RGB). The fast rotation and activity of EK Boo might be
explained by angular momentum dredge-up from the interior, or by the merging of
a binary. In addition, we observed eight other M giants, which are known as
X-ray emitters, or to be rotating fast for their class. For one of these, beta
And, presumably also an AGB star, we have a marginal detection of magnetic
field, and a longitudinal component Bl of about 1G was measured. More
observations like this will answer the question whether EK Boo is a special
case, or whether magnetic activity is, rather, more common among M giants than
expected.Comment: Accepted for publication in Astronomy & Astrophysics, 10 pages, 8
figure
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