Spectroscopic determination of photospheric parameters and chemical abundances of 6 K-type stars

in Astronomy and Astrophysics (2005), 433

High resolution, high -S/N- ratio optical spectra have been obtained for a sample of 6 K-type dwarf and subgiant stars, and have been analysed with three different LTE methods in order to derive detailed ... [more ▼]

High resolution, high -S/N- ratio optical spectra have been obtained for a sample of 6 K-type dwarf and subgiant stars, and have been analysed with three different LTE methods in order to derive detailed photospheric parameters and abundances and to compare the characteristics of analysis techniques. The results have been compared with the aim of determining the most robust method to perform complete spectroscopic analyses of K-type stars, and in this perspective the present work must be considered as a pilot study. In this context we have determined the abundance ratios with respect to iron of several elements. In the first method the photospheric parameters (T_eff, log g, and ξ) and metal abundances are derived using measured equivalent widths and Kurucz LTE model atmospheres as input for the MOOG software code. The analysis proceeds in an iterative way, and relies on the excitation equilibrium of the ion{Fe}{i} lines for determining the effective temperature and microturbulence, and on the ionization equilibrium of the ion{Fe}{i} and ion{Fe}{ii} lines for determining the surface gravity and the metallicity. The second method follows a similar approach, but discards the ion{Fe}{i} low excitation potential transitions (which are potentially affected by non-LTE effects) from the initial line list, and relies on the B-V colour index to determine the temperature. The third method relies on the detailed fitting of the 6162 à ion{Ca}{i} line to derive the surface gravity, using the same restricted line list as the second method. Methods 1 and 3 give consistent results for the program stars; in particular the comparison between the results obtained shows that the ion{Fe}{i} low-excitation potential transitions do not appear significantly affected by non-LTE effects (at least for the subgiant stars), as suggested by the good agreement of the atmospheric parameters and chemical abundances derived. The second method leads to systematically lower T_eff and log g values with respect to the first one, and a similar trend is shown by the chemical abundances (with the exception of the oxygen abundance). These differences, apart from residual non-LTE effects, may be a consequence of the colour-T_eff scale used. The α-elements have abundance ratios consistent with the solar values for all the program stars, as expected for â normalâ disk stars. The first method appears to be the most reliable one, as it is self-consistent, it always leads to convergent solutions and the results obtained are in good agreement with previous determinations in the literature. Based on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roche de los Muchachos of the Instituto de Astrofisica de Canarias. Table 6 is only available in electronic form at http://www.edpsciences.org [less ▲]

Near-infrared identification of the counterpart to X1908+075: a new OB-supergiant X-ray binary

in Monthly Notices of the Royal Astronomical Society (2005), 356

We report the near-infrared (near-IR) identification of the likely counterpart to X1908+075, a highly absorbed Galactic X-ray source recently suspected to belong to the rare class of OB supergiant-neutron ... [more ▼]

We report the near-infrared (near-IR) identification of the likely counterpart to X1908+075, a highly absorbed Galactic X-ray source recently suspected to belong to the rare class of OB supergiant-neutron star binary systems. Our JHK[SUB]s[/SUB]-band imaging of the field reveals the existence within the X-ray error boxes of a near-IR source consistent with an early-type star lying at d~ 7 kpc and suffering A[SUB]V[/SUB]~ 16 mag of extinction, the latter value being in good agreement with the hydrogen column density derived from modelling of the X-ray spectrum. Our follow-up, near-IR spectroscopic observations confirm the nature of this candidate and lead to a late O-type supergiant classification, thereby supporting the identification of a new Galactic OB-supergiant X-ray binary. [less ▲]

The European Large Area ISO Survey - VIII. 90-microns final analysis and source counts

in Monthly Notices of the Royal Astronomical Society (2004), 354

We present a re-analysis of the European Large Area Infrared Space Observatory (ISO) Survey (ELAIS) 90-μm observations carried out with ISOPHOT, an instrument on board the ISO of the European Space ... [more ▼]

We present a re-analysis of the European Large Area Infrared Space Observatory (ISO) Survey (ELAIS) 90-μm observations carried out with ISOPHOT, an instrument on board the ISO of the European Space Agency. With more than 12 deg[SUP]2[/SUP], the ELAIS survey is the largest area covered by ISO in a single programme and is about one order of magnitude deeper than the IRAS 100-μm survey. The data analysis is presented and was mainly performed with the PHOT interactive analysis software but using the pairwise method of Stickel et al. for signal processing from edited raw data to signal per chopper plateau. The ELAIS 90-μm catalogue contains 237 reliable sources with fluxes larger than 70 mJy and is available in the electronic version of this article. Number counts are presented and show an excess above the no-evolution model prediction. This confirms the strong evolution detected at shorter (15 μm) and longer (170 μm) wavelengths in other ISO surveys. The ELAIS counts are in agreement with previous works at 90 μm and in particular with the deeper counts extracted from the Lockman hole observations. Comparison with recent evolutionary models show that the models of Franceschini et al. and Guiderdoni et al. (which includes a heavily extinguished population of galaxies) give the best fit to the data. Deeper observations are nevertheless required to discriminate better between the model predictions in the far-infrared, and are scheduled with the Spitzer Space Telescope, which has already started operating, and will also be performed by ASTRO-F. [less ▲]

On the determination of oxygen abundances in chromospherically active stars

in Astronomy and Astrophysics (2004), 423

We discuss oxygen abundances derived from [O I] λ6300s and the O I triplet in stars spanning a wide range in chromospheric activity level, and show that these two indicators yield increasingly discrepant ... [more ▼]

We discuss oxygen abundances derived from [O I] λ6300s and the O I triplet in stars spanning a wide range in chromospheric activity level, and show that these two indicators yield increasingly discrepant results with higher chromospheric/coronal activity measures. While the forbidden and permitted lines give fairly consistent results for solar-type disk dwarfs, spuriously high O I triplet abundances are observed in young Hyades and Pleiades stars, as well as in individual components of RS CVn binaries (up to 1.8 dex). The distinct behaviour of the [O I]-based abundances which consistently remain near-solar suggests that this phenomenon mostly results from large departures from LTE affecting the O I triplet at high activity level that are currently unaccounted for, but also possibly from a failure to adequately model the atmospheres of K-type stars. These results suggest that some caution should be exercised when interpreting oxygen abundances in active binaries or young open cluster stars. Based on observations collected at the European Southern Observatory, Chile (Proposals 64.L-0249 and 071.D-0260). Table \ref{tab_data} is only available in electronic form at http://www.edpsciences.org [less ▲]

Large-scale wind structures in OB supergiants: a search for rotationally modulated Halpha variability

in Monthly Notices of the Royal Astronomical Society (2004), 351

We present the results of a long-term monitoring campaign of the Hα line in a sample of bright OB supergiants (O7.5-B9) which aims at detecting rotationally modulated changes potentially related to the ... [more ▼]

We present the results of a long-term monitoring campaign of the Hα line in a sample of bright OB supergiants (O7.5-B9) which aims at detecting rotationally modulated changes potentially related to the existence of large-scale wind structures. A total of 22 objects were monitored during 36 nights spread over six months in 2001-2002. Coordinated broad-band photometric observations were also obtained for some targets. Conspicuous evidence for variability in Hα is found for the stars displaying a feature contaminated by wind emission. Most changes take place on a daily time-scale, although hourly variations are also occasionally detected. Convincing evidence for a cyclical pattern of variability in Hα has been found in two stars: HD 14134 and HD 42087. Periodic signals are also detected in other stars, but independent confirmation is required. Rotational modulation is suggested from the similarity between the observed recurrence time-scales (in the range 13-25 d) and estimated periods of stellar rotation. We call attention to the atypical case of HD 14134, which exhibits a clear 12.8-d periodicity, both in the photometric and in the spectroscopic data sets. This places this object among a handful of early-type stars where one may observe a clear link between extended wind structures and photospheric disturbances. Further modelling may test the hypothesis that azimuthally-extended wind streams are responsible for the patterns of spectral variability in our target stars. [less ▲]

The near-IR counterpart to the high-mass X-ray binary X1908+075

in The Astronomer's Telegram (2004), 291

<html> We report the near-IR identification of the likely counterpart to X1908+075, a highly-absorbed Galactic X-ray source recently suspected to belong to the rare class of OB supergiant-neutron star ... [more ▼]

<html> We report the near-IR identification of the likely counterpart to X1908+075, a highly-absorbed Galactic X-ray source recently suspected to belong to the rare class of OB supergiant-neutron star binary systems (Wen et al. 2000, ApJ, 532, 1119; Levine et al. 2004, ApJ, submitted, astro-ph/0404428). The <em>Einstein</em> and <em>HEAO 1</em> satellites have localized this X-ray source with an uncertainty of about 50''. [less ▲]

The photospheric abundances of active binaries. II. Atmospheric parameters and abundance patterns for 6 single-lined RS CVn systems

in Astronomy and Astrophysics (2003), 412

Photospheric parameters and abundances are presented for a sample of single-lined chromospherically active binaries from a differential LTE analysis of high-resolution spectra. Abundances have been ... [more ▼]

Photospheric parameters and abundances are presented for a sample of single-lined chromospherically active binaries from a differential LTE analysis of high-resolution spectra. Abundances have been derived for 13 chemical species, including several key elements such as Li, Mg, and Ca. Two methods have been used. The effective temperatures, surface gravities and microturbulent velocities were first derived from a fully self-consistent analysis of the spectra, whereby the temperature is determined from the excitation equilibrium of the Fe I lines. The second approach relies on temperatures derived from the (B-V) colour index. These two methods give broadly consistent results for the stars in our sample, suggesting that the neutral iron lines are formed under conditions close to LTE. We discuss the reliability in the context of chromospherically active stars of various colour indices used as temperature indicators, and conclude that the (V-R) and (V-I) colours are likely to be significantly affected by activity processes. Irrespective of the method used, our results indicate that the X-ray active binaries studied are not as metal poor as previously claimed, but are at most mildly iron-depleted relative to the Sun (-0.41protect <~ [Fe/H]protect la +0.11). A significant overabundance of several chemical species is observed (e.g., the alpha -synthezised elements). These abundance patterns are discussed in relation to stellar activity. Based on observations collected at ESO (La Silla, Chile). Table A.1 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/412/495 [less ▲]

Large-scale Wind Structures in OB Supergiants: a Search for Rotationally Modulated Halpha variability

in Bulletin of the Astronomical Society of India (2003), 31

We present preliminary results of a long-term spectroscopic monitoring of a sample of bright OB-supergiants aimed at establishing the incidence of co-rotating, large-scale wind structures by detecting ... [more ▼]

We present preliminary results of a long-term spectroscopic monitoring of a sample of bright OB-supergiants aimed at establishing the incidence of co-rotating, large-scale wind structures by detecting rotationally modulated variability in H. Dramatic line-profile variations operating on a daily (and in some cases on a hourly) timescale are observed. A detailed period analysis has been so far carried out for 2 stars, and revealed in both cases the existence of cyclical H variations consistent with rotational modulation. In the case of HD 14134, the same periodicity is found in the contemporaneous light curve. [less ▲]

Near- and mid-infrared colours of star-forming galaxies in European Large Area ISO Survey fields

in Monthly Notices of the Royal Astronomical Society (2002), 337

We present J- and K-band near-infrared (near-IR) photometry of a sample of mid-infrared (mid-IR) sources detected by the Infrared Space Observatory (ISO) as part of the European Large Area ISO Survey ... [more ▼]

We present J- and K-band near-infrared (near-IR) photometry of a sample of mid-infrared (mid-IR) sources detected by the Infrared Space Observatory (ISO) as part of the European Large Area ISO Survey (ELAIS) and study their classification and star-forming properties. We have used the Preliminary ELAIS Catalogue for the 6.7-μm (LW2) and 15-μm (LW3) fluxes. All of the high-reliability LW2 sources and 80 per cent of the LW3 sources are identified in the near-IR survey reaching K~ 17.5 mag. The near-IR/mid-IR flux ratios can effectively be used to separate stars from galaxies in mid-IR surveys. The stars detected in our survey region are used to derive a new accurate calibration for the ELAIS ISOCAM data in both the LW2 and LW3 filters. We show that near- to mid-IR colour-colour diagrams can be used to classify galaxies further, as well as to study star formation. The ELAIS ISOCAM survey is found mostly to detect strongly star-forming late-type galaxies, possibly starburst-powered galaxies, and it also picks out obscured active galactic nuclei. The ELAIS galaxies yield an average mid-IR flux ratio LW2/LW3 = 0.67 +/- 0.27. We discuss the f[SUB]ν[/SUB](6.7 μ m)/f[SUB]ν[/SUB](15 μ m) ratio as a star formation tracer using ISO and IRAS data of a local comparison sample. We find that the f[SUB]ν[/SUB](2.2 μ m)/f[SUB]ν[/SUB](15 μ m) ratio is also a good indicator of activity level in galaxies and conclude that the drop in the f[SUB]ν[/SUB](6.7 μ m)/f[SUB]ν[/SUB](15 μ m) ratio seen in strongly star-forming galaxies is a result of both an increase of 15-μm emission and an apparent depletion of 6.7-μm emission. Near-IR together with the mid-IR data make it possible to estimate the relative amount of interstellar matter in the galaxies. [less ▲]

Near-infrared [Fe II] emission from supernova remnants and the supernova rate of starburst galaxies

in Monthly Notices of the Royal Astronomical Society (2002), 329

In an effort better to calibrate the supernova rate of starburst galaxies as determined from near-infrared [FeII] features, we report on a [FeII] λ1.644μm line-imaging survey of a sample of 42 optically ... [more ▼]

In an effort better to calibrate the supernova rate of starburst galaxies as determined from near-infrared [FeII] features, we report on a [FeII] λ1.644μm line-imaging survey of a sample of 42 optically selected supernova remnants (SNRs) in M33. A wide range of [FeII] luminosities are observed within our sample (from less than 6 to 695L[SUB]solar[/SUB]). Our data suggest that the bright [FeII] SNRs are entering the radiative phase and that the density of the local interstellar medium (ISM) largely controls the amount of [FeII] emission. We derive the following relation between the [FeII] λ1.644μm line luminosity of radiative SNRs and the electronic density of the post-shock gas, n[SUB]e[/SUB]: L[SUB][FeII][/SUB] (L[SUB]solar[/SUB])~1.1n[SUB]e[/SUB] (cm[SUP]-3[/SUP]). We also find a correlation in our data between L[SUB][FeII][/SUB] and the metallicity of the shock-heated gas, but the physical interpretation of this result remains inconclusive, as our data also show a correlation between the metallicity and n[SUB]e[/SUB]. The dramatically higher level of [FeII] emission from SNRs in the central regions of starburst galaxies is most likely due to their dense environments, although metallicity effects might also be important. The typical [FeII]-emitting lifetime of a SNR in the central regions of starburst galaxies is found to be of the order of 10[SUP]4[/SUP]yr. On the basis of these results, we provide a new empirical relation allowing the determination of the current supernova rate of starburst galaxies from their integrated near-infrared [FeII] luminosity. [less ▲]