Analysis of the far ultraviolet silicon lines in G dwarf stars

The structure of the outer stellar regions is investigated for four G type dwarfs observed with the IUE satellite. Line fluxes of the Si II lines at 1817 Å, 1808 A, and 1309 Å and Si III at 1206 Å are used to obtain temperatures and electronic densities. A temperature of 16,000 K is found from the lines at 1817 Å and 1808 Å, 26,000 K from the 1309 A line and 50,000 K from that at 1206 A. Predicted fluxes are compared with the observed ones. The significance of the results is discussed in terms of line formation regions

The relationship between soft X-rays and the 1640 Å feature fluxes in late-type stars

The λ 1640 feature has been observed in a sample of late­ type stars of different luminosity classes. The intensity was measured from IUE low dispersion spectra, and it has been compared with the observed X-ray fluxes, finding a relationship between both quantities for "solar type" stars. The X-ray fluxes derived from this relationship for a reduced sample of stars are consistent with the observed ones in the case of "solar type" stars. "Non solar type" stars exhibit discrepancies that could be explained assuming that the λ 1640 feature is formed by contri­buters other than He II, which supply an important fraction of this emission in "solar type" stars. The obtained empirical relationship has been used to derive the X-ray flux for some stars that have not been observed in the X-ray range

Emissions from the transition regions and coronae of three cool dwarf stars

Ultraviolet emissions of τCet, δPav, and 61 Cyg A have been analysed to determine the structure of their outer atmospheres. Emission line fluxes are used to find the emission measure distributions. Using two boundary values of the electron pressure, models of the transition region have been derived. In the two models of τCet the net conductive flux is less than the radiation losses at all temperatures. However, for δPav and 61 Cyg A the model with the upper boundary value of the electron pressure has a temperature range where the radiative losses are less than the net conductive fluxes. Measured X-ray fluxes are used to test the coronal temperature values derived from the ultraviolet observations

The transition region structure of κ ceti

Observations of κ Ceti (HD 20630), G5 V, obtained with the IUE satellite have been analysed to determine the structure of its outer atmosphere. This analysis is used to derive a model of transition region and to examine the terms of energy balance equation

An interpretation of the line-strength indexes in old stellar populations using an evolutionary synthesis approach

Evolutionary population synthesis models with dif­ferent metallicities have been computed in order to interpret the observed Mg_2 and Hβ, line-strength indices in old stellar popu­lations. Both indices are found to be quite insensitive to changes in the slope of the IMF and the upper mass limit. These models have been applied to three different cases, with the following results: a) Galactic globular clusters do not exhibit a significant dis­persion in age, and the metallicities of the computed models resemble those estimated for the clusters. The indices for the metal-poor globulars ([Fe/H] < -1) cannot be attained with these models due to the lack of low metallicity evolutionary tracks and stellar spectra libraries. b) The line-strength gradients observed in the elliptical gal­axy NGC 5813 are due, essentially, to intrinsic variations in metallicity, and they cannot be explained just from changes in the remaining parameters of the stellar population. c) In order to synthesize the M 32 indices we must introduce a star formation elapsed for a long time-scale , the star forrnation being still significant ≈ 5 Gyr ago

Analysis of the far ultraviolet emission lines in late type stars

Far ultraviolet emission lines in a sample of seven stars of spectral type later than F5 and different luminosity class have been analyzed to calculate the electron density and pressure in the formation region of the lines. Three different methods were used. We obtain electron pressures larger than solar for all dwarf stars of the sample. The low resolution observations were made with the IUE satellite

Study of a complete sample of Hα emission-line galaxies from the UCM survey

The Universidad Complutense de Madrid survey is a long-term project with the aim of finding and analyzing star forming galaxies using the Hα line as the tracer for star formation processes. In order to obtain a representative and complete sample of the population detected, spectroscopic observations were carried out for the full sample of Hα emission-line galaxy (ELG) candidates of the UCM lists 1 and 2. The ELGs types most commonly found (47%) are intermediate to low-luminosity objects with a very intense star-formation region which dominates the optical energy output of the galaxy. This kind of ELGs is similar to the galaxy population detected in the blue objective-prism surveys. And what is more important, a second population (43%) of star-forming galaxies with low ionization or high extinction properties has been found. This ELGs group is detected neither in the blue (University of Michigan survey, Case survey) nor in other surveys (Kiso, IRAS, Markarian) using other selection techniques

Automatic selection of new Hα emission-line galaxies using MAMA

A full set of algorithms for the automatic analysis of low-resolution objective-prism spectra has been developed with the aim of finding new emission-line galaxies (ELGs) candidates from the Universidad Complutense de Madrid (UCM) survey prism plates. An objective-prism plate and a direct plate, used for pairing objects and to perform star-galaxy separation, have been scanned with the MAMA machine. The plates were taken in the red region of the spectrum, using the Hα+[NII] blend in emission as selection criterion. The procedure, applied to digitized spectra, is able to automatically select ELGs candidates. Density to intensity transformation has not been used in our method; the detection of the emission is performed applying three independent criteria over the one-dimensional spectra in raw data. The automatically selected sample is compared with that obtained after a careful visual scan. Spectroscopic observations at moderate spectral resolution are presented for the whole sample of candidates in order to study the drawbacks and the biases of both methods

Barium and iron abundances in red giants

An intermediate-dispersion abundance analysis has been carried out on a sample of 21 barium and 14 comparison stars. The excess of barium over iron has been used as the most representative indicator of peculiarity. These excesses are higher in the peculiar stars than in the nonpeculiar stars. Particularly interesting is the case of HD 67447, included in the comparison stars, with an excess Ba/Fe abundance = 1.61, probably a new barium star. A trend indicating a possible anticorrelation between barium overabundance and metallicity favors the suggestion that the "barium strong" group is older than the "barium weak" one

Discovery of an emitting ring in the seyfert 1 galaxy UCM 2329+2500

We analyze a rather interesting galaxy discovered during the UCM survey for new Hα emission-line galaxies. The new galaxy UCM 2329+2500 presents a compact core that hosts a Seyfert 1 nucleus. The spectrum of the Seyfert nucleus presents broad components (FWHM=8000 km s^-1) and asymmetric profiles at the Balmer lines showing a secondary peak more apparent on the Hβ line and also observed in the Hα deblended line. Only traces of profile variability have been detected during spectroscopic observations at four different dates spanning 29 months. Eight kpc away from the nucleus, a ring-like structure that surrounds the core is observed. A long slit spectrum at PA 45° has revealed emission lines coming from both sides of the ring. The emission detected is prominent at a condensation observed on the red image. Star formation is the most plausible explanation. A high obscuration is also observed. All this information is coherent with a galactic encounter scenario