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THE 100 BRIGHTEST X-RAY STARS WITHIN 50 PARSECS of the SUN Valeri V

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THE 100 BRIGHTEST X-RAY STARS WITHIN 50 PARSECS of the SUN Valeri V The Astronomical Journal, 126:1996–2008, 2003 October # 2003. The American Astronomical Society. All rights reserved. Printed in U.S.A. THE 100 BRIGHTEST X-RAY STARS WITHIN 50 PARSECS OF THE SUN Valeri V. Makarov Universities Space Research Association, 1101 17th Street, NW, Washington, DC 20024; and US Naval Observatory, 3450 Massachusetts Avenue, NW, Washington, DC 20392; [email protected] Received 2003 April 21; accepted 2003 July 2 ABSTRACT Based on the Hipparcos and Tycho-2 astrometric catalogs and the ROSAT surveys, a sample of 100 stars most luminous in X-rays within or around a distance of 50 pc is culled. The smallest X-ray luminosity in the 29 À1 sample, in units of 10 ergs s ,isLX = 9.8; the strongest source in the solar neighborhood is II Peg, a RS CVn star, at LX = 175.8. With respect to the origin of X-ray emission, the sample is divided into partly overlapping classes of pre–main-sequence, post–T Tauri, and very young ZAMS objects (type XY), RS CVn–type binary stars (type RS), other active short-period binaries, including binary BY Dra–type objects (type XO), apparently single or long-period binary active evolved stars (type XG), contact binaries of WU UMa kind (type WU), apparently single or long-period binary variable stars of BY Dra kind (type BY), and objects of unknown nature (type X?). Chromospherically active, short-period binaries (RS and XO) make up 40% of the brightest X-ray emitters, followed by young stars (XY) at 30% and unknown sources (X?) at 15%. The fraction of spectroscopically single evolved X-ray emitters of spectral classes IV and III is quite large (10%). The sources identified as RS CVn–type stars (RS, 23 objects) are considerably stronger in X-ray than the XY-objects and the other active binaries (XO and WU, 20 objects). Seven objects have LX > 100, all RS except one XY, viz., BO Mic. Only five (22%) RS objects have LX < 25, while only three (10%) XY stars have LX > 25. Formally, the limit of LX = 25 could serve as a statistical criterion to differentiate RS and XY stars. However, the other short-period binaries (including eclipsing stars of Algol and Lyr type) have a distribution of LX very similar to the XY objects. The contact binaries (WU) appear to be much weaker in X-rays than their detached counter- parts of RS type, but the sample of the former is too small (three objects) to reach a firm conclusion. Sources matched with giants (either single or in binaries) are found to be significantly harder, with only 7% of hardness ratios below 0, than subgiants (66% of HR1 < 0) and dwarfs (59% of HR1 < 0). Almost all objects in the sample are binary or multiple stars; the fraction of components (FC), defined as the total number of compo- nents in all binary and multiple systems divided by the sum of the total number of components and single stars, is at least 0.90. The FC for the XY objects reaches 0.81, and for the unknown type 0.89. About 70% of RS objects have also visual or astrometric companions, which makes them hierarchical multiple systems. The RS objects (mostly old, evolved stars) and the XY stars have quite different kinematics. While the RS objects move at considerable velocities in apparently random directions with respect to the local standard of rest, the young stars have smaller and orderly velocities and tend to comprise expanding mini-associations such as the Pic and the Tucana groups. The majority of the young X-ray active stars belong to the Pleiades stream with the mean heliocentric velocity (U, V, W )=(À9.6, À21.8, À7.7) km sÀ1. Key words: binaries: general — stars: activity — stars: kinematics — stars: statistics — X-rays 1. INTRODUCTION telltale characteristics for different categories of emitters, based on X-ray, kinematics, and binarity parameters. In Thousands of stars tabulated in astrometric and photo- particular, one of the objectives is to find out if very young metric catalogs have been detected by the Einstein stars in the solar neighborhood can be selected, in a statisti- Observatory and, later, by ROSAT as X-ray–emitting cal sense, from other types without labor-costly and time- sources. Many of these stars are 103–104 times more lumi- consuming spectroscopic observations. Most of the 100 nous in X-rays than the Sun in a quiescent state. Being stars are relatively well studied and supplied with very accu- undoubtedly a viable sign of activity, X-ray emission alone rate astrometric data, which allows me to draw reasonably does not single out a well-defined class of stars of even reliable conclusions about their properties. approximately the same basic characteristics or evolution- The situation with bright X-ray stars is reminiscent of the ary status. Indeed, stars from the most massive O super- one with chromospherically active stars. Chromospheric giants to the dimmest late M dwarfs and from the youngest activity, as measured by the level of Ca ii H and K emission, contracting T Tauri objects to the dynamically oldest RS is known to decline with age. The survey of chromospheric CVn binaries may be strong X-ray emitters. This multitude activity in G dwarfs (Henry et al. 1996) identified a group of of types and populaces is a hindrance in studies of young very active stars that could be very young or close binaries. stars, short-period binaries, and other important kinds of The later high-resolution spectroscopic examination of 18 active stars, in that no simple criteria based on the easily very active stars (Soderblom, King, & Henry 1998) showed available X-ray properties can be established to tell them that most of them belonged to active binaries and that only apart. In this paper, I collect data on the 100 most luminous five were probably quite young. The high level of chromo- X-ray stars within approximately 50 pc and try to find spheric activity (as well as X-ray activity) may be related to 1996 THE 100 BRIGHTEST X-RAY STARS 1997 high velocities of rotation. Fast rotation of young stars is 5. BY: single or long-period variable stars of BY Dra inherited from the previous contraction stage, whereas stars type, rapidly rotating, chromospherically active; in close binaries retain large angular momentum due to tidal 6. WU: contact binaries of WU UMa type; synchronization with the orbital motion. 7. X?: emitters of unknown nature. The selection of 100 X-ray sources is based on the ROSAT All-Sky Survey (RASS)/Tycho-2 sample described The border line between the RS and BY types is fuzzy. In elsewhere (Makarov & Urban 2000; Suchkov, Makarov, & the classification given in Table 1 I usually relied on the Voges 2003). The sample was checked against the ROSAT types of activity indicated in the Simbad Database and in all-sky survey of the nearby stars by Hu¨nsch et al. (1999) in the up-to-date literature. The definition of the BY Dra type order to verify its completeness with stars within 25 pc. The has in fact changed since their first introduction as rapidly X-ray data in Table 1 were computed directly from the data rotating, spotty G5V–KV dwarfs (Chugainov 1976; Bopp & in the RASS-BSC (Voges et al. 1999) and the Hipparcos par- Fekel 1977) to include also F and early-G dwarfs (Fekel, allaxes (ESA 1997). Binary stars in Table 1 were not re- Moffet, & Henry 1986). These stars may be short-period solved with the ROSAT instrument; their luminosities binary or single and are characterized by strong Ca ii Hand correspond therefore to the combined X-ray flux from the K lines in emission and by small-amplitude quasi-sinusoidal components. The number of entries in Table 1 is in fact 101, light variation due to a fast rotation and presence of cool because the stars HIP 108456 and 108461 are resolved com- spots. The definition of RS CVn–type stars also evolved ponents of a wide multiple system in the Hipparcos catalog, from binary late-type stars with strong chromospheric activ- corresponding to the same ROSAT source. The positional ity and periods between 1 and 14 days to a broader class of offsets between the ROSAT source and the two stars are 1600 spectroscopic binaries with periods shorter than 1 day and 200, respectively; hence, the latter star, V376 Cep, through longer than 14 days, including primaries of F, G, is likely the true X-ray source in this pair. The star and K type. The differences between the two types are that HIP 108456 is also counted in the following statistics, RS CVn stars are always short-period binaries, while nonetheless. BY Dra stars may be single, and that BY Dra stars are always dwarfs, while RS CVn stars may be evolved. This 2. TYPES OF X-RAY ACTIVITY certainly leaves a lot of room for overlap between the types. Fekel et al. (1986) suggested a more distinct definition of RS Counting by primary components in binaries, the CVn class requiring that at least one of the components be sample contains two B stars, three A stars, 16 F stars, 48 evolved (giant or subgiant). This concept is of considerable G stars, and 30 K stars. M dwarfs are absent, except for value, since (1) it is practically possible to find out the status possibly HIP 41322, despite their well-known activity in of binary components by, e.g., putting them on the HR dia- X-rays. The reason for this is that X-ray luminosities of gram (Gunn, Mitrou, & Doyle 1998) and eliminate the over- the most active M dwarfs are between 1029 and 1030 (see lap between the types; and (2) types of activity are now Hunsch et al.
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