INAUGURAL{DISSERTATION zur Erlangung der Doktorwurde¨ der Naturwissentschaftlich{Mathematischen Gesamtfakult¨at der Ruprecht{Karls{Universit¨at Heidelberg vorgelegt von Dipl.{Phys. Agn`es D.F. Metanomski aus Paris Tag der mundl.¨ Prufung:¨ 22. April 1998 Optical study of F, G and K stars in the ROSAT All-Sky Survey Gutachter: Herr Prof. Dr. Joachim Krautter Herr Prof. Dr. Reiner Wehrse Zusammenfassung Ich untersuche eine Stichprobe von 107 Sudhimmels-Sternen¨ von Spektraltyp A bis K. Diese Sterne sind als die optischen Gegenstuc¨ ke zu R¨ontgen-Quellen, die der ROSAT Satellit w¨ahrend seines All-Sky Survey (RASS) entdeckte, iden- tifiziert wurden. Die Untersuchung wird mit Hilfe optischen Beobachtungen, sowohl photometrischer als auch spektroskopischer, durchgefuhrt.¨ Verschiedene Parameter werden fur¨ die untersuchten Objekte bestimmt, wie z.B. Spektraltyp und Leuchtkraftklasse, absolute Helligkeit, Entfernung, Radialgeschwindigkeit, projezierte Rotationsgeschwindigkeit, Effektivtemperatur, Lithium H¨aufigkeit. Ich vergleiche die R¨ontgen-Parameter meiner Stichproben-Sterne mit denen aus einer ahnlic¨ hen Stichprobe von Sternen, die vom Einstein Observatory w¨ahrend dessen Medium Sensitivity Survey (EMSS) entdeckt wurden. Ich suche auch nach Korrelationen zwischen verschiedenen Parametern, die fur¨ meine Stichproben- Sterne bestimmt wurden und vergleiche die Ergebnisse mit denen, die fur¨ die EMSS Stichprobe erhalten wurden, sowie mit den Ergebnissne anderer, fruherer¨ Studien. Abstract I analyze a sample of 107 southern stars of spectral types A to K, which were identified as the optical counterparts of X-ray sources detected by ROSAT during its All-Sky Survey. The study is conducted using optical observations, photometric as well as spectroscopic (high-resolution). Various parameters are determined for the sample objects, mainly spectral types and luminosity classes, absolute magnitudes, distances, radial velocities, projected rotational velocities, effective temperatures, Lithium abundances. I compare the X-ray parameters of my sample stars with those of a similar sample, detected by the Einstein Observatory during its Medium Sensitivity Sur- vey (EMSS). I also look for correlations between the different stellar parameters determined for my sample, and compare those results to those obtained for the EMSS sample, and also to some results obtained from other, earlier studies. Contents 1 Introduction 1 1.1 Stellar X-ray emission . 1 1.1.1 Pre-Einstein Era . 1 1.1.2 Einstein Era . 2 1.2 Results from the Einstein Observatory . 2 1.3 Dissertation project . 5 2 The ROSAT All-Sky Survey 7 2.1 The survey . 7 2.2 The sample . 7 3 Observations 10 3.1 Photometry . 10 3.2 High-resolution spectroscopy . 12 4 Analysis 14 4.1 Photometry . 14 4.1.1 Spectral classification . 14 4.1.2 The (U B) colour index . 14 4.1.3 Effective−temperature . 17 4.1.4 Absolute magnitude, distance and X-ray luminosity . 18 4.1.5 Hipparcos data . 20 4.2 Spectroscopy . 20 4.2.1 Radial and rotational velocity . 20 4.2.2 Lithium abundance . 25 5 Results 29 5.1 Distribution of spectral types . 29 5.2 X-ray luminosity as a function of spectral type . 30 5.3 Luminosity function . 36 5.4 Distribution with distance . 43 5.5 Lithium abundances . 53 5.6 Rotational velocity . 59 6 Discussion 61 A X-ray data of the sample 70 B Stellar parameters 75 C Spectroscopic Data 84 D Standards 89 E Northern Sample 95 2 List of Figures 1.1 Representation of X-ray activity on the Hertzsprung-Russel diagram. 3 1.2 Comparison of the Einstein spectrum of Ori with the shock model of Lucy (1982). The best fit to the observations requires strong, but infrequent shocks (image taken from Cassinelli and Swank 1983). 4 1.3 The relation between X-ray luminosity and projected rotation rate. 5 2.1 Schematic view of the ROSAT satellite and X-ray telescope. 8 2.2 Location of the fields studied in the southern and northern identi- fication programs. 9 4.1 Color-color plots for the southern sample. 15 4.2 Spectrum, solar template and cross-correlation function with fit, for a slow rotating single star, RXJ 1121.8-2411, v sin i = 5 km/s. 21 4.3 Spectrum, solar template and cross-correlation function with fit, for a single fast rotator, RXJ 0135.8-3956, v sin i = 33 km/s. 22 4.4 Spectrum, solar template and cross-correlation function with fit, for one of the triple systems in the sample, RXJ 2331.4-4209. 23 4.5 The fits for the rotaional velocity determination. 24 4.6 Examples of synthetic spectra. 27 5.1 log(LX =LV ) vs. the (B V ) color index. 31 5.2 Luminosity distribution−for the single stars and binaries. 31 5.3 Luminosity distribution for the RASS sample, by spectral type. 33 5.4 Luminosity distribution for the RASS, EMSS and EXOSAT samples. 33 5.5 Luminosity function for the RASS single stars and binaries. 37 5.6 Luminosity functions for the southern sample. 37 5.7 X-ray luminosity as a function of distance for the RASS and EMSS samples. 39 5.8 Histrogram of the distance distribution for the RASS and EMSS samples. 39 5.9 Histogram of the distance distribution for single stars and binaries. 45 5.10 Lithium abundances for single stars as a function of effective tem- perature. 45 5.11 Lithium abundances for the binaries and multiple systems as a function of effective temperature. 47 5.12 Lithium abundance vs rotational velocity. 47 5.13 Lithium abundance vs X-ray luminosity for the single stars. 49 5.14 Lithium abundance vs. rotational velocity for the single stars, by spectral type. 49 5.15 X-ray luminosity vs. rotational velocityfor the single stars. 51 5.16 X-ray luminosity vs. rotational velocity for the multiple systems. 51 5.17 X-ray luminosity vs. radius. 57 List of Tables 3.1 Observing periods, for photometry. 11 3.2 The ESO Cousins filters. 11 3.3 The references used for the brightest sources in the sample. 11 3.4 Observing periods, for spectroscopy. The observing period be- tween Sept. 23d abd 29th corresponds to reserved time for the 1.4 m CAT and CES. 12 4.1 The sources of the southern sample for which Hipparcos data was found. 19 5.1 Distribution of the X-ray sources among the spectral types studied, for my sample, the EMSS sample (Stocke et al. 1991) and the EXOSAT sample (Cutispoto et al. 1996) . 29 5.2 Results of the χ2 tests for the X-ray luminosity distributions. 36 5.3 Results of the χ2 tests for the distance distributions. 44 5.4 Results of the correlation tests Pearson's r and Spearman's rs tests for the lithium abundance. 56 5.5 Results of the correlation tests Pearson's r and Spearman's rs tests for correlations between the X-ray luminosity and the rotational velocity or the radius. 59 A.1 ROSAT X-ray data for the sources in the studied southern sample. 71 B.1 Data for the counterparts of the sample sources. 76 C.1 Velocities, Li abundances for the southern hemisphere sample. 85 D.1 The photometric standards from the E-regions used in the pho- tometry for the southern sample. Data from Vogt et al. (1981) . 89 D.2 The open clusters used for standard stars in the photometry for the northern sample. The standards from the clusters and their colors are taken from: 1: Christian et al. (1985), 2: Odewahn et al. (1992). 91 D.3 The radial velocity standards used for the southern sample. The standards were taken from 1: Duquennoy and Mayor (1991) 2: The Astronomical Almanach (1996) . 91 D.4 The v sin i standards used for the southern sample. The standards were taken from 1: Soderblom (1982), 2: Sletteback et al. (1975), 3: Randich et al. (1993), 4: Soderblom et al. (1989) . 92 D.5 The radial velocity standards used for the northern sample.The standards were taken from Duquennoy and Mayor (1991). 93 D.6 The v sin i standards used for the northern sample.The standards were taken from 1: Soderblom (1982), 2: Sletteback et al. (1975), 3: Randich et al. (1993). 93 D.7 A- and B-type stars used in the Ca II H & K and He I D3 observations. 93 E.1 The northern sample stars. 96 5 1 Introduction 1.1 Stellar X-ray emission From observations of the Sun during solar eclipses, it was known that the atmosphere extends itself well beyond the normally visible solar \surface", into regions that only have a thin, but very hot plasma. Through models and ob- servations the atmosphere was divided into three zones: the photosphere, where most absorption lines are created, the chromosphere and the corona. The chro- mosphere appears were the temperature inversion occurs, ie. after the point at which the temperature of the atmospheric plasma starts rising again. The corona has a very low density but high temperatures, of the order 106 107 − K. At such temperatures the plasma emits the most in the higher energy bands. If the plasma can be approximated to a black body, then the maximum of the emission lies in the range around 0.1 keV, that is in the soft X-ray range. Studies made of the solar corona have shown that its luminosity varies strongly over time. The X-ray luminosity of the Sun has a minimal value of 1025erg s−1 − ∼ and a maximum around 1027erg s 1. As a matter of fact, the Sun shows variations in many different features, the most visible being the variation of the number and position of solar spots. The variations extend over a period of 11 years, which is considered now as the activity cycle of the Sun.