25 research outputs found
A Survey of IUE Spectra of the Active Binary System UX Arietis
To investigate the ultraviolet (UV) activity of the bright, non-eclipsing,
double-lined spectroscopic binary system UX Ari, IUE spectra (194 images) were
taken from the IUE archive. The spectra, obtained during the period 1978-1996,
show emission lines originating in the chromosphere and transition region. The
long-wavelength low dispersion spectra were examined for ultraviolet excess by
comparing the UV continuum level of UX Ari with the levels of Cet and
Cep in the spectral range 2100 - 3200 . The individual MgII
h and k emission-line fluxes of component stars show that the contributions to
the activity of the system for G5 V and K0 IV are about 20% and 80%,
respectively. Apart from the flare event observed on 1979 Jan 1, there are some
flux enhancements in the years 1987, 1991 and 1994 which may suggest existence
of a periodicity of about 7-9 years. Examination of the ultraviolet excess in
the system showed that there is some UV excess in UX Ari, which varies from 1%
up to 24% with the exception of two images which showed no UV excess. The
results revealed that there was an agreement on the source of emission regions
which could be attributed to the magnetic activity. The contribution of G5 V
and K0 IV components to the MgII activity of the system suggests a need to take
into consideration the spot distribution not only on the surface of K0 IV but
also on the surface of G5 V component of UX Ari.Comment: 15 pages, 10 figure
A New Method for Classifying Flares of UV Ceti Type Stars: Differences Between Slow and Fast Flares
In this study, a new method is presented to classify flares derived from the
photoelectric photometry of UV Ceti type stars. This method is based on
statistical analyses using an independent samples t-test. The data used in
analyses were obtained from four flare stars observed between 2004 and 2007.
The total number of flares obtained in the observations of AD Leo, EV Lac, EQ
Peg, and V1054 Oph is 321 in the standard Johnson U band. As a result flares
can be separated into two types, slow and fast, depending on the ratio of flare
decay time to flare rise time. The ratio is below 3.5 for all slow flares,
while it is above 3.5 for all fast flares. Also, according to the independent
samples t-test, there is a difference of about 157 s between equivalent
durations of slow and fast flares. In addition, there are significant
differences between amplitudes and rise times of slow and fast flares.Comment: 46 pages, 7 figures, 4 tabels, 2010AJ....140..483
The first close-up of the "flip-flop" phenomenon in a single star
We present temperature maps of the active late-type giant FK Com which
exhibit the first imagining record of the ``flip-flop'' phenomenon in a single
star. The phenomenon, in which the main part of the spot activity shifts 180
degrees in longitude, discovered a decade ago in FK Com, was reported later
also in a number of RS CVn binaries and a single young dwarf. With the surface
images obtained right before and after the ``flip-flop'', we clearly show that
the ``flip-flop'' phenomenon in FK Com is caused by changing the relative
strengths of the spot groups at the two active longitudes, with no actual spot
movements across the stellar surface, i.e. exactly as it happens in other
active stars.Comment: 4 pages, accepted by A&A Letter
The statistical analyses of flares detected in B band photometry of UV Ceti type stars
In this study, we present the unpublished flare data collected from 222
flares detected in the B band observations of five stars and the results
derived by statistical analysis and modeling of these data. Six basic
properties have been found with a statistical analysis method applied to all
models and analyses for the flares detected in the B band observation of UV
Ceti type stars. We have also compared the U and B bands with the analysis
results. This comparison allowed us to evaluate the methods used in the
analyses. The analyses provided the following results. (1) The flares were
separated into two types, fast and slow flares. (2) The mean values of the
equivalent durations of the slow and the fast flares differ by a factor of 16.2
\pm 3.7. (3) Regardless of the total flare duration, the maximum flare energy
can reach a different Plateau level for each star. (4) The Plateau values of EV
Lac and EQ Peg are higher than the others. (5) The minimum values of the total
flare duration increase toward the later spectral types. This value is called
the Half-Life value in models. (6) Both the maximum flare rise times and the
total flare duration obtained from the observed flares decrease toward the
later spectral types.Comment: 17 pages, 10 figures, 8 table
Statistical Analysis and the OPEA Model of the White-Light Flares Occurring on Kr\"uger 60B (DO Cep)
In this study, new observations and some results of statistical analyses are
presented. The largest flare data set of DO Cep in the literature has been
obtained with 89 flares detected in 67.61 hours of U-band flare patrol. First
of all, the observations demonstrated that the star is one of the most active
flare stars in respect to the computed flare frequency. Secondly, using the
independent samples t-test, the detected flares were classified into two
subtypes, and then they were modelled. Analysing the models demonstrated that
the fast and slow flares occurring on the star can be separated with a critical
value of the ratio of their decay time to rise time. The critical value was
computed as 3.40. According to this value, the fast flare rate is 20.22%, while
the slow flare rate is 79.78%. Besides, there is a 39.282 times difference
between the energies of these two types of flares. However, the flare
equivalent durations versus the flare rise times increase in similar ways for
both groups. In addition, all all the flares were modelled with the one-phase
exponential association function. Analysing this model, the plateau value was
found to be 2.810. Moreover, the half-life value was computed as 433.1s from
the model. The maximum flare rise time was found to be 1164s, while the maximum
flare total duration was found to be 3472s. The results of the flare timescales
indicate that the geometry of the flaring loop on the surface of the star might
be similar to those seen on analogues of DO Cep. Consequently, considering both
the half-life value and flare timescales, the flares detected on the surface of
DO Cep get maximum energy in longer times, while the geometries of the flaring
loops or areas get smaller.Comment: 18 pages, 8 figures, 5 tables, 2011PASA...28..365D. arXiv admin note:
substantial text overlap with arXiv:1206.579
The periodic variations of a white-light flare observed with ULTRACAM
High time resolution observations of a white-light flare on the active star EQ PegB show evidence of intensity variations with a period of ≈10 s. The period drifts to longer values during the decay phase of the flare. If the oscillation is interpreted as an impulsively-excited,
standing-acoustic wave in a flare loop, the period implies a loop length of ≈3.4 Mm and ≈6.8 Mm for the case of the fundamental mode and the second harmonic, respectively. However, the small loop lengths imply a very high modulation depth making the acoustic interpretation unlikely. A more realistic interpretation may be that of a fast-MHD wave, with the modulation of the emission being
due to the magnetic field. Alternatively, the variations could be due to a series of reconnection events. The periodic signature may then arise as a result of the lateral separation of individual flare loops or current sheets with oscillatory dynamics (i.e., periodic reconnection)
High-Level Magnetic Activity on a Low-Mass Close Binary: GSC 02038-0293
Taking into account results obtained from light-curve analysis and
out-of-eclipse analyses, we discuss the nature of GSC 02038-00293 and also its
magnetic activity behaviour. We obtained light curves of the system during
observing seasons 2007, 2008 and 2011. We obtained its secondary minimum
clearly in I-band observations in 2008 for the first time. Analysing this light
curve, we found the physical parameters of the components. The light-curve
analysis indicates that the possible mass ratio of the system is 0.35. We
obtained the remaining V-band light curves, extracting the eclipses. We
modelled these remaining curves using the spotmodel program and found possible
spot configurations of the magnetically active component for each observing
season. The models demonstrated that there are two active longitudes for the
active component. The models reveal that both active longitudes migrate in the
direction of decreasing longitude. We also examined the light curves in
out-of-eclipse phases with respect to minimum and maximum brightness,
amplitude, etc. The amplitude of the curves during out-of-eclipse phases varies
in a sinusoidal way with a period of ~8.9yr the mean brightness of the system
is dramatically decreasing. The phases of the deeper minimum during
out-of-eclipse periods exhibit a migration toward decreasing phase.Comment: 15 pages, 8 figures, 5 table
Quiescent and flare analysis for the chromospherically active star Gl355 (LQHya)
We discuss ROSAT and ASCA observations of the young active star Gl355}.
During the ROSAT observation a strong flare was detected with a peak flux more
than an order of magnitude larger than the quiescent level. Spectral analysis
of the data allows us to study the temperature and emission measure
distribution, and the coronal metal abundance, for the quiescent phase and, in
the case of ROSAT, also during the evolution of the flare. The global coronal
metallicity derived from both ROSAT and ASCA data is
much lower than solar and presumably also much lower than the photospheric
abundance expected for this very young star. The temperature structure of the
quiescent corona was about the same during the various observations, with a
cooler component at MK and a hotter component (to which only ASCA
was sensitive) at MK. During the flare, the low temperature
component remained approximately constant and equal to the quiescent value,
while the high-temperature component was the only one that varied. We have
modeled the flare with the hydrodynamic-decay sustained-heating approach of
Reale at al. (1997) and we have derived a loop semi--length of the order of
stellar radii, i.e. much larger than the dimensions of flares on the
Sun, but comparable with the typical dimensions inferred for other stellar
flares. We have compared the derived loop size with that estimated with a
simpler (but physically inconsistent) approach, finding that for this, as well
for several other stellar flares, the two methods give comparable loop sizes.
Possible causes and consequences of this result are discussed.Comment: A&A, in pres
UV habitable zones around M stars
During the last decade, there was a paradigm-shift in order to consider
terrestrial planets within liquid-water habitable zones (LW-HZ) around M stars,
as suitable places for the emergence and evolution of life. Here we analyze the
influence of UV boundary conditions to three planetary systems around dM (HIP
74995, HIP 109388 and HIP 113020). We apply our model of UV habitable zone
(UV-HZ) (Buccino et al. 2006) to these cases and show that during the quiescent
UV output there would not be enough UV radiation within the LW-HZ in order to
trigger biogenic processes. We also analyze the cases of two other M flare
stars and show that the flares of moderate intensity could provide the
necessary energy to trigger those biogenic processes, while the strong flares
not necessary rule-out the possibility of life-bearing planets.Comment: 17 pages, 3 figures, accepted for publication in Icaru
Multiband Photometry of the Blazar PKS 0537-441: A Major Active State in December 2004 - March 2005
Multiband VRIJHK photometry of the Blazar PKS 0537-441 obtained with the REM
telescope from December 2004 to March 2005 is presented. A major period of
activity is found with more than four magnitudes variability in the V filter in
50 days and of 2.5 in 10 days. In intensity and duration the activity is
similar to that of 1972 reported by Eggen (1973), but it is much better
documented. No clear evidence of variability on time-scale of minutes is found.
The spectral energy distribution is roughly described by a power-law, with the
weaker state being the softer.Comment: 4 pages, 3 figures, A&A letters, in pres