83 research outputs found
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
Dynamics of gene expression and the regulatory inference problem
From the response to external stimuli to cell division and death, the
dynamics of living cells is based on the expression of specific genes at
specific times. The decision when to express a gene is implemented by the
binding and unbinding of transcription factor molecules to regulatory DNA.
Here, we construct stochastic models of gene expression dynamics and test them
on experimental time-series data of messenger-RNA concentrations. The models
are used to infer biophysical parameters of gene transcription, including the
statistics of transcription factor-DNA binding and the target genes controlled
by a given transcription factor.Comment: revised version to appear in Europhys. Lett., new titl
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
Optical spectrum of the post-AGB star HD56126 in the region 4010-8790 AA
We studied in detail the optical spectrum of the post-AGB star HD56126
(IRAS07134+1005). We use high resolution spectra (R=25000 and 60000) obtained
with the echelle spectrographs of the 6-m telescope. About one and a half
thousand absorptions of neutral atoms and ions, absorption bands of C_2, CN,
and CH molecules, and interstellar bands (DIBs) are identified in the 4010 to
8790 AA wavelength region, and the depths and radial velocities of these
spectral features are measured. Differences are revealed between the variations
of the radial velocities measured from spectral features of different
excitation. In addition to the well-known variability of the Halpha profile, we
found variations in the profiles of a number of FeII, YII, and BaII lines. We
also produce an atlas of the spectrum of HD56126 and its comparison staralpha
Per. The full version of the atlas is available in electronic form from
Web-address: http://www.sao.ru/hq/ssl/Atlas/Atlas.htmlComment: 42 pages, 6 figure
The nature of V39: an LBV candidate or LBV impostor in the very low metallicity galaxy IC 1613?
[abridged]
Context: Very few examples of luminous blue variable (LBV) stars or LBV
candidates (LBVc) are known, particularly at metallicities below the SMC. The
LBV phase is crucial for the evolution of massive stars, and its behavior with
metallicity is poorly known. V39 in IC 1613 is a well-known photometric
variable, with B-band changes larger than 1mag. over its period. The star,
previously proposed to be a projection of a Galactic W Virginis and an IC 1613
red supergiant, shows features that render it a possible LBVc.
Method: We investigate mid-resolution blue and red VLT-VIMOS spectra of V39,
covering a time span of 40 days, and perform a quantitative analysis of the
combined spectrum using the model atmosphere code CMFGEN.
Results: We identify strong Balmer and FeII P-Cygni profiles, and a hybrid
spectrum resembling a B-A supergiant in the blue and a G-star in the red. No
significant Vrad variations are detected, and the spectral changes are small
over the photometric period. Our analysis places V39 in the low-luminosity part
of the LBV and LBVc region, but it is also consistent with a sgB[e] star.
Conclusions: The radial velocity indicates that V39 belongs to IC 1613. The
lack of Vrad changes and spectroscopic variations excludes binary scenarios.
The features observed are not consistent with a W Virginis star, and this
possibility is also discarded. We propose that the star is a B-A LBVc or sgB[e]
star surrounded by a thick disk precessing around it.
If confirmed, V39 would be the lowest metallicity resolved LBV candidate
known to date. Alternatively, it could represent a new transient phase of
massive star evolution, an LBV impostor.Comment: In press at A&A. 10 pages, 11 figure
Magnetic field structure in single late-type giants: The effectively single giant V390 Aur
We have studied the active giant V390 Aur using spectropolarimetry to obtain
direct and simultaneous measurements of the magnetic field and the activity
indicators in order to get a precise insight of its activity. We used the
spectropolarimeter NARVAL at the Bernard Lyot Telescope (Observatoire du Pic du
Midi, France) to obtain a series of Stokes I and Stokes V profiles. The Least
Square deconvolution (LSD) technique was applied to detect the Zeeman signature
of the magnetic field in each of our 13 observations and to measure its
longitudinal component. We could also monitor the CaII K & H and IR triplet, as
well as the H_alpha lines which are activity indicators. In order to
reconstruct the magnetic field geometry of V390 Aur, we applied the Zeeman
Doppler Imaging (ZDI) inversion method and present a map for the magnetic
field. Based on the obtained spectra, we also refined the fundamental
parameters of the star and the Li abundance. The ZDI revealed a structure in
the radial magnetic field consisting of a polar magnetic spot of positive
polarity and several negative spots at lower latitude. A high latitude belt is
present on the azimuthal field map, indicative of a toroidal field close to the
surface. It was found that the photometric period cannot fit the behaviour of
the activity indicators formed in the chromosphere. Their behaviour suggests
slower rotation compared to the photosphere, but our dataset is too short to be
able to estimate the exact periods for them.Accepted for publication in A&A All
these results can be explained in terms of an \alpha-\omega dynamo operation,
taking into account the stellar structure and rotation properties of V390 Aur
that we study using up to-date stellar models computed at solar metallicity.
The calculated Rossby number also points to a very efficient dynamoComment: To appear in Astronomy & Astrophysics, 8 pages, 5 figure
Lithium-rich giants in the Galactic thick disk
Context: Lithium is a fragile element, which is easily destroyed in the
stellar interior. The existence of lithium-rich giants still represents a
challenge for stellar evolution models. Aims: We have collected a large
database of high-resolution stellar spectra of 824 candidate thick-disk giants
having 2\,MASS photometry and proper motions measured by the Southern
Proper-Motion Program (SPM). In order to investigate the nature of Li-rich
giants, we searched this database for giants presenting a strong Li\,I
resonance line. Methods: We performed a chemical abundance analysis on the
selected stars with the MOOG code along with proper ATLAS-9 model atmospheres.
The iron content and atmospheric parameters were fixed by using the equivalent
width of a sample of Fe lines. We also derive abundances for C, N, and O and
measure or derive lower limits on the C/C isotopic ratios, which
is a sensible diagnostic of the stars evolutionary status. Results: We detected
five stars with a lithium abundance higher than 1.5, i.e. Li-rich according to
the current definition. One of them (SPM-313132) has A(Li)3.3 and, because
of this, belongs to the group of the rare super Li-rich giants. Its kinematics
makes it a likely thin-disk member and its atmospheric parameters are
compatible with it being a 4\,M star either on the red giant branch
(RGB) or the early asymptotic giant branch. This object is the first super
Li-rich giant detected at this phase. The other four are likely low-mass
thick-disk stars evolved past the RGB luminosity bump, as determined from their
metallicities and atmospheric parameters. The most evolved of them lies close
to the RGB-tip. It has A(Li)2.7 and a low C/C isotopic ratio,
close to the cool bottom processing predictions.Comment: 11 pages, 7 tables, 7 figures. Accepted for publication in A&
An exploration of ambigrammatic sequences in narnaviruses
Narnaviruses have been described as positive-sense RNA viruses with a remarkably simple genome of ~3 kb, encoding only a highly conserved RNA-dependent RNA polymerase (RdRp). Many narnaviruses, however, are 'ambigrammatic' and harbour an additional uninterrupted open reading frame (ORF) covering almost the entire length of the reverse complement strand. No function has been described for this ORF, yet the absence of stops is conserved across diverse narnaviruses, and in every case the codons in the reverse ORF and the RdRp are aligned. The >3 kb ORF overlap on opposite strands, unprecedented among RNA viruses, motivates an exploration of the constraints imposed or alleviated by the codon alignment. Here, we show that only when the codon frames are aligned can all stop codons be eliminated from the reverse strand by synonymous single-nucleotide substitutions in the RdRp gene, suggesting a mechanism for de novo gene creation within a strongly conserved amino-acid sequence. It will be fascinating to explore what implications this coding strategy has for other aspects of narnavirus biology. Beyond narnaviruses, our rapidly expanding catalogue of viral diversity may yet reveal additional examples of this broadly-extensible principle for ambigrammatic-sequence development
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