236 research outputs found
Building Reliable Models of M Dwarf Chromospheres: The Spectral Diagnostics
As part of an ongoing project to study how reliable are the chromospheric models constructed semiempirically to fit the chromospheric spectrum of cold stars, in this paper we study how changes in the assumed chromospheric structure of dM and dMe stars affect the emitted spectrum. In this way, we study if a given set of features can wholly determine the atmospheric structure, how large are the indeterminations in the atmospheric parameters deduced, and to what extent the atmospheric model computed can be considered unique. We find that the profiles of the Ca II K line or the Na D lines can be used to determine the structure of the Tmin region and that the Hα profile can provide good information on the structure of the chromosphere. The Lyα flux, in turn, can be used to constrain the position of the transition region.Fil: Mauas, Pablo Jacobo David. Consejo Nacional de Investigaciónes CientÃficas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de AstronomÃa y FÃsica del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de AstronomÃa y FÃsica del Espacio; Argentin
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
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
Fast temporal correlation between hard X-ray and ultraviolet continuum brightenings
Recent Solar Maximum Mission (SMM) observations have shown fast and simultaneous increases in hard X-rays (HXR, E25 keV) and ultraviolet continuum (UVC, lambda lambda approx. equals 1600 and 1388 A) radiation. A simple and natural explanation is given for this phenomenon to happen, which does not involve extreme conditions for energy transport processes, and confirms earlier results on the effect of XUV photoionization in the solar atmosphere
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
The Na I D resonance lines in main sequence late-type stars
We study the sodium D lines (D1: 5895.92 \AA; D2: 5889.95 \AA) in late-type
dwarf stars. The stars have spectral types between F6 and M5.5 (B-V between
0.457 and 1.807) and metallicity between [Fe/H] = -0.82 and 0.6. We obtained
medium resolution echelle spectra using the 2.15-m telescope at the argentinian
observatory CASLEO. The observations have been performed periodically since
1999. The spectra were calibrated in wavelength and in flux. A definition of
the pseudo-continuum level is found for all our observations. We also define a
continuum level for calibration purposes. The equivalent width of the D lines
is computed in detail for all our spectra and related to the colour index (B-V)
of the stars. When possible, we perform a careful comparison with previous
studies. Finally, we construct a spectral index (R_D') as the ratio between the
flux in the D lines, and the bolometric flux. We find that, once corrected for
the photospheric contribution, this index can be used as a chromospheric
activity indicator in stars with a high level of activity. Additionally, we
find that combining some of our results, we obtain a method to calibrate in
flux stars of unknown colour.Comment: 12 pages, including 14 figures and 4 tables. Accepted for publication
in MNRA
Blue lines as chromospheric diagnostics: The Si I lines at 3906 and 4103 Ã…
We present a complete atomic model for Si I line synthesis. We study how the computed profiles of two blue lines of this atom are influenced by the choice of the atomic parameters and find that, although several cross sections are not known accurately, the line profiles do not depend strongly on them and are therefore useful as diagnostics of the atmospheric structure. We study which transitions need not be included in the model, in order to reduce as much as possible the computing time. We compare the profiles computed for a standard model of the quiet solar atmosphere with the observations and find very good agreement. We confirm that irradiation by UV lines originating in the transition region above sunspot umbrae or plages strongly enhances the continuum between 1300 and 1700 Ã…, which is due to Si I bound-free transitions. If line fluxes typical of the impulsive phase of flares are assumed, the line profiles are also affected.Fil: Cincunegui, Carolina Cecilia. Consejo Nacional de Investigaciónes CientÃficas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de AstronomÃa y FÃsica del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de AstronomÃa y FÃsica del Espacio; ArgentinaFil: Mauas, Pablo Jacobo David. Consejo Nacional de Investigaciónes CientÃficas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de AstronomÃa y FÃsica del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de AstronomÃa y FÃsica del Espacio; Argentin
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&
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