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

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 $\sim7$ years and a less-significant shorter one of $\sim2$ 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 ($R^{\prime}_D$), H$\alpha$ and Ca II K fluxes at different activity levels of AD Leo, including flares.Comment: 21 pages, 3 figures.Accepted for publication in ApJ

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 $\sim$4 yr and $\sim$7 yr for Gl 229 A and Gl 752 A respectively.Comment: Accepted for publication by Astronomical Journal (AJ

Ultraviolet Radiation Constraints around the Circumstellar Habitable Zones

Ultraviolet radiation is known to inhibit photosynthesis, induce DNA destruction and cause damage to a wide variety of proteins and lipids. In particular, UV radiation between 200-300 nm becomes energetically very damaging to most of the terrestrial biological systems. On the other hand, UV radiation is usually considered one of the most important energy source on the primitive Earth for the synthesis of many biochemical compounds and, therefore, essential for several biogenesis processes. In this work, we use these properties of the UV radiation to define the bounderies of an ultraviolet habitable zone. We also analyze the evolution of the UV habitable zone during the main sequence stage of the star. We apply these criteria to study the UV habitable zone for those extrasolar planetary systems that were observed by the International Ultraviolet Explorer (IUE). We analyze the possibility that extrasolar planets and moons could be suitable for life, according to the UV constrains presented in this work and other accepted criteria of habitability (liquid water, orbital stability, etc.).Comment: 34 pages, 8 figures Accepted for publication by Icaru