124 research outputs found
Helium line formation and abundance during a C-class flare
During a coordinated campaign which took place in May 2001, a C-class flare
was observed both with SOHO instruments and with the Dunn Solar Telescope of
the National Solar Observatory at Sacramento Peak. In two previous papers we
have described the observations and discussed some dynamical aspects of the
earlier phases of the flare, as well as the helium line formation in the active
region prior to the event. Here we extend the analysis of the helium line
formation to the later phases of the flare in two different locations of the
flaring area. We have devised a new technique, exploiting all available
information from various SOHO instruments, to determine the spectral
distribution of the photoionizing EUV radiation produced by the corona
overlying the two target regions. In order to find semiempirical models
matching all of our observables, we analyzed the effect on the calculated
helium spectrum both of A(He) (the He abundance) and of the uncertainties in
the incident EUV radiation (level and spectral distribution). We found that the
abundance has in most cases (but not in all) a larger effect than the coronal
back-radiation. The result of our analysis is that, considering the error of
the measured lines, and adopting our best estimate for the coronal EUV
illumination, the value A(He)=0.075 +/- 0.010 in the chromosphere (for T>6300
K) and transition region yields reasonably good matches for all the observed
lines. This value is marginally consistent with the most commonly accepted
photospheric value: A(He)=0.085.Comment: 34 pages + 13 figures; to be published in Ap
Possible chromospheric activity cycles in II Peg, UX Ari and V711 Tau
We study the Mount Wilson indices we obtained indirectly from IUE high and
low resolution spectra of the RS CVn-type systems II Peg (K2IV), UX Ari
(K0IV+G5V) and V711 Tau (K1IV+G5V), extensively observed by IUE from 1978 to
1996. We analyze the activity signatures, which correspond to the primary star,
with the Lomb-Scargle periodogram. From the analysis of V711 Tau data, we found
a possible chromospheric cycle with a period of 18 years and a shorter near
3-year cycle, which could be associated to a chromospheric flip-flop cycle. The
data of II Peg also suggest a chromospheric cycle of near 21 years and a
flip-flop cycle of 9 years approximately. Finally, we obtained a possible
chromospheric cycle of near 6 years for UX Ari.Comment: 4 pages, to appear in proceedings of Cool Stars 1
Measuring helium abundance difference in giants of NGC 2808
Multiple populations have been detected in several globular clusters (GC)
that do not display a spread in metallicity. Unusual features of their CMD can
be interpreted in terms of differences in the Helium content of the stars
belonging to the sub-populations. Differences in He abundance have never been
directly observed. We attempt to measure these differences in two giant stars
of NGC 2808 with very similar parameters but different Na and O abundances,
hence that presumably belong to different sub-populations, by directly
comparing their He I 10830 {\AA} lines. The He 10830 {\AA} line forms in the
upper chromosphere. Our detailed models derive the chromospheric structure
using the Ca II and H, and simulate the corresponding He I 10830 line
profiles. We show that, at a given value of He abundance, the He I 10830
equivalent width cannot significantly change without a corresponding much
larger change in the Ca II lines. We have used the VLT-CRIRES to obtain
high-resolution spectra in the 10830 {\AA} region, and the VLT-UVES to obtain
spectra of the Ca II and H lines of our target stars. The two target
stars have very similar Ca II and H lines, but different appearances in
the He region. One line, blueshifted by 17 km s with respect to the He
10830 rest wavelength, is detected in the spectrum of the Na-rich star, whereas
the Na-poor star spectrum is consistent with a non-detection. The difference in
the spectra is consistent and most closely explained by an He abundance
difference between the two stars of 0.17.We provide direct
evidence of a significant He line strength difference in giant stars of NGC
2808 belonging to different sub-populations, which had been previously detected
by other photometric and spectroscopic means.Comment: Accepted for publication in A&
Long-term chromospheric activity in southern M dwarfs: Gl 229 A and Gl 752 A
Several late-type stars present activity cycles similar to that of the Sun.
However, these cycles have been mostly studied in F to K stars. Due to their
small intrinsic brightness, M dwarfs are not usually the targets of long-term
observational studies of stellar activity, and their long-term variability is
generally not known. In this work, we study the long-term activity of two M
dwarf stars: Gl 229 A (M1/2) and Gl 752 A (M2.5). We employ medium resolution
echelle spectra obtained at the 2.15 m telescope at the Argentinian observatory
CASLEO between the years 2000 and 2010 and photometric observations obtained
from the ASAS database. We analyzed Ca \II K line-core fluxes and the mean V
magnitude with the Lomb-Scargle periodogram, and we obtain possible activity
cycles of 4 yr and 7 yr for Gl 229 A and Gl 752 A respectively.Comment: Accepted for publication by Astronomical Journal (AJ
Long-term solar activity influences on South American rivers
River streamflows are excellent climatic indicators since they integrate
precipitation over large areas. Here we follow up on our previous study of the
influence of solar activity on the flow of the Parana River, in South America.
We find that the unusual minimum of solar activity in recent years have a
correlation on very low levels in the Parana's flow, and we report historical
evidence of low water levels during the Little Ice Age. We also study data for
the streamflow of three other rivers (Colorado, San Juan and Atuel), and snow
levels in the Andes. We obtained that, after eliminating the secular trends and
smoothing out the solar cycle, there is a strong positive correlation between
the residuals of both the Sunspot Number and the streamflows, as we obtained
for the Parana. Both results put together imply that higher solar activity
corresponds to larger precipitation, both in summer and in wintertime, not only
in the large basin of the Parana, but also in the Andean region north of the
limit with Patagonia.Comment: Accepted to publication by Journal of Atmospheric and
Solar-Terrestrial Physic
Solar Forcing of the Streamflow of a Continental Scale South American River
Solar forcing on climate has been reported in several studies although the
evidence so far remains inconclusive. Here, we analyze the stream flow of one
of the largest rivers in the world, the Parana in southeastern South America.
For the last century, we find a strong correlation with the sunspot number, in
multidecadal time scales, and with larger solar activity corresponding to
larger stream flow. The correlation coefficient is r=0.78, significant to a 99%
level. In shorter time scales we find a strong correlation with El Nino. These
results are a step toward flood prediction, which might have great social and
economic impacts.Comment: 4 pages, 3 figures, published in Physics and Review Letter
Possible chromospheric activity cycles in AD Leo
AD Leo (GJ 388) is an active dM3 flare star extensively observed both in the
quiescent and flaring states. Since this active star is near the
fully-convective boundary, to study in detail its long-term chromospheric
activity could be an appreciable contribution for the dynamo theory. Here, we
analyze with the Lomb-Scargle periodogram the Ca II K line-core fluxes derived
from CASLEO spectra obtained between 2001 and 2013 and the V magnitude from the
ASAS database between 2004 and 2010. From both totally independent time-series,
we obtain a possible activity cycle of period years and a
less-significant shorter one of years. A tentative interpretation is
that a dynamo operating near the surface could be generating the longer cycle,
while a second dynamo operating in the deep convection zone could be
responsible for the shorter one. Based on the long duration of our observing
program at CASLEO and the fact that we observe simultaneously different
spectral features, we also analyze the relation between simultaneous
measurements of the Na I index (), H and Ca II K fluxes
at different activity levels of AD Leo, including flares.Comment: 21 pages, 3 figures.Accepted for publication in ApJ
Tasas colisionales no-térmicas y fulguraciones solares
Aplicando un formalismo de Fokker-Planck para estudiar la función de distribución que describen los haces de electrones acelerados que inciden en la cromosfera solar durante fulguraciones, se calculan las tasas de excitación e ionización debidas a colisiones con partÃculas del haz. Se utilizan estas tasas no térmicas para el cálculo de modelos auto-consistentes de atmósfera, y se estudia cómo este efecto altera los perfiles de las lÃneas cromosféricas durante una fulguración. En particular, se muestra que si estas tasas son tenidas en cuenta en el cálculo de la atmósfera, las lÃneas de Balmer aparecen considerablemente ensanchadas, lo que coincide con las observaciones.Asociación Argentina de AstronomÃ
- …