3,328 research outputs found
Fifteen years in the high-energy life of the solar-type star HD 81809. XMM-Newton observations of a stellar activity cycle
Aims. The data set of the long-term XMM-Newton monitoring program of HD 81809
is analyzed to study its X-ray cycle, to investigate if the latter is related
to the chromospheric one, to infer the structure of the corona of HD 81809, and
to explore if the coronal activity of HD 81809 can be ascribed to phenomena
similar to the solar ones and, therefore, considered an extension of the solar
case. Methods. We analyze the observations of HD 81809 performed with
XMM-Newton with a regular cadence of 6 months from 2001 to 2016 and
representing one of the longest available observational baseline (~yr)
for a solar-like star with a well-studied chromospheric cycle (with a period of
~yr). We investigate the modulation of coronal luminosity and
temperature and its relation with the chromospheric cycle. We interpret the
data in terms of a mixture of solar-like coronal regions, adopting a
methodology originally proposed to study the Sun as an X-ray star. Results. The
observations show a well-defined regular cyclic modulation of the X-ray
luminosity that reflects the activity level of HD 81809. The data covers
approximately two cycles of coronal activity; the modulation has an amplitude
of a factor of (excluding evident flares, as in the June 2002
observation) and a period of ~yr, consistent with that of the
chromospheric cycle. We demonstrate that the corona of HD 81809 can be
interpreted as an extension of the solar case and it can be modeled with a
mixture of solar-like coronal regions along the whole cycle. The activity level
is mainly determined by a varying coverage of very bright active regions,
similar to cores of active regions observed in the Sun. Evidence of unresolved
significant flaring activity is present especially in proximity of cycle
maxima.Comment: 11 pages, 5 Figures, A&A accepte
The Ca II infrared triplet's performance as an activity indicator compared to Ca II H and K
Aims. A large number of Calcium Infrared Triplet (IRT) spectra are expected
from the GAIA- and CARMENES missions. Conversion of these spectra into known
activity indicators will allow analysis of their temporal evolution to a better
degree. We set out to find such a conversion formula and to determine its
robustness.
Methods. We have compared 2274 Ca II IRT spectra of active main-sequence F to
K stars taken by the TIGRE telescope with those of inactive stars of the same
spectral type. After normalizing and applying rotational broadening, we
subtracted the comparison spectra to find the chromospheric excess flux caused
by activity. We obtained the total excess flux, and compared it to established
activity indices derived from the Ca II H & K lines, the spectra of which were
obtained simultaneously to the infrared spectra.
Results. The excess flux in the Ca II IRT is found to correlate well with
and , as well as , if the
-dependency is taken into account. We find an empirical conversion formula
to calculate the corresponding value of one activity indicator from the
measurement of another, by comparing groups of datapoints of stars with similar
B-V.Comment: 16 pages, 15 figures. Accepted for publication in Astronomy &
Astrophysic
Time series of high resolution spectra of SN 2014J observed with the TIGRE telescope
We present a time series of high resolution spectra of the Type Ia supernova
2014J, which exploded in the nearby galaxy M82. The spectra were obtained with
the HEROS echelle spectrograph installed at the 1.2 m TIGRE telescope. We
present a series of 33 spectra with a resolution of R = 20, 000, which covers
the important bright phases in the evolution of SN 2014J during the period from
January 24 to April 1 of 2014. The spectral evolution of SN 2014J is derived
empirically. The expansion velocities of the Si II P-Cygni features were
measured and show the expected decreasing behaviour, beginning with a high
velocity of 14,000 km/s on January 24. The Ca II infrared triplet feature shows
a high velocity component with expansion velocities of > 20, 000 km/s during
the early evolution apart from the normal component showing similar velocities
as Si II. Further broad P-Cygni profiles are exhibited by the principal lines
of Ca II, Mg II and Fe II. The TIGRE SN 2014J spectra also resolve several very
sharp Na I D doublet absorption components. Our analysis suggests interesting
substructures in the interstellar medium of the host galaxy M82, as well as in
our Milky Way, confirming other work on this SN. We were able to identify the
interstellar absorption of M82 in the lines of Ca II H & K at 3933 and 3968 A
as well as K I at 7664 and 7698 A. Furthermore, we confirm several Diffuse
Interstellar Bands, at wavelengths of 6196, 6283, 6376, 6379 and 6613 A and
give their measured equivalent widths.Comment: 11 pages, 10 figures, accepted for publication in MNRA
Simultaneous X-ray and optical spectroscopy of the Oef supergiant lambda Cep
Probing the structures of stellar winds is of prime importance for the
understanding of massive stars. Based on their optical spectral morphology and
variability, the stars of the Oef class have been suggested to feature
large-scale structures in their wind. High-resolution X-ray spectroscopy and
time-series of X-ray observations of presumably-single O-type stars can help us
understand the physics of their stellar winds. We have collected XMM-Newton
observations and coordinated optical spectroscopy of the O6Ief star lambda Cep
to study its X-ray and optical variability and to analyse its high-resolution
X-ray spectrum. We investigate the line profile variability of the He II 4686
and H-alpha emission lines in our time series of optical spectra, including a
search for periodicities. We further discuss the variability of the broadband
X-ray flux and analyse the high-resolution spectrum of lambda Cep using
line-by-line fits as well as a code designed to fit the full high-resolution
X-ray spectrum consistently. During our observing campaign, the He II 4686 line
varies on a timescale of ~18 hours. On the contrary, the H-alpha line profile
displays a modulation on a timescale of 4.1 days which is likely the rotation
period of the star. The X-ray flux varies on time-scales of days and could in
fact be modulated by the same 4.1 days period as H-alpha, although both
variations are shifted in phase. The high-resolution X-ray spectrum reveals
broad and skewed emission lines as expected for the X-ray emission from a
distribution of wind-embedded shocks. Most of the X-ray emission arises within
less than 2R* above the photosphere.Comment: Accepted for publication in Astronomy & Astrophysic
Reaction-diffusion systems and nonlinear waves
The authors investigate the solution of a nonlinear reaction-diffusion
equation connected with nonlinear waves. The equation discussed is more general
than the one discussed recently by Manne, Hurd, and Kenkre (2000). The results
are presented in a compact and elegant form in terms of Mittag-Leffler
functions and generalized Mittag-Leffler functions, which are suitable for
numerical computation. The importance of the derived results lies in the fact
that numerous results on fractional reaction, fractional diffusion, anomalous
diffusion problems, and fractional telegraph equations scattered in the
literature can be derived, as special cases, of the results investigated in
this article.Comment: LaTeX, 16 pages, corrected typo
Solution of generalized fractional reaction-diffusion equations
This paper deals with the investigation of a closed form solution of a
generalized fractional reaction-diffusion equation. The solution of the
proposed problem is developed in a compact form in terms of the H-function by
the application of direct and inverse Laplace and Fourier transforms.
Fractional order moments and the asymptotic expansion of the solution are also
obtained.Comment: LaTeX, 18 pages, corrected typo
Variational method and duality in the 2D square Potts model
The ferromagnetic q-state Potts model on a square lattice is analyzed, for
q>4, through an elaborate version of the operatorial variational method. In the
variational approach proposed in the paper, the duality relations are exactly
satisfied, involving at a more fundamental level, a duality relationship
between variational parameters. Besides some exact predictions, the approach is
very effective in the numerical estimates over the whole range of temperature
and can be systematically improved.Comment: 20 pages, 5 EPS figure
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