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

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 1 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 components 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 )=(9.6, 21.8, 7.7) km s1. 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. 1999), that is, below the limiting luminosity ¨ related to the evolutionary status of the object. However, adopted here. The origin of X-ray emission in late B and this proposal appears not to be commonly adopted, and A stars remains unclear, since they have neither high- some of the active stars may have different assignments in velocity winds able to generate energetic shocks as in O the literature. If we stick to the new definition, some of the to B2 stars, nor significant coronal activity. It is not RS-type objects in Table 1 should be referred to as binary unlikely that all X-ray–emitting B and A stars are BY Dra, that is, XO-type. These controversial cases were binaries whose dimmer companions are responsible for marked with RS: (uncertain RS) in Table 1. the emission. It is noted that all stars earlier than F2 in It must be noted that the type XO includes detached the present sample, with the exception of the RS CVn main-sequence binaries as well as semidetached eclipsing binary Per, are visual or astrometric binaries. However, binaries (Algols). While the WU-type of contact binaries is according to Bergho¨fer et al. (1997), ROSAT HRI obser- clearly separable, the difference between the close BY vations of well-separated visual pairs with late B primar- binaries and classical Algols may be somewhat artificial and ies showed that the detected X-ray emission was not due often difficult to establish. Similar difficulties meet attempts to the secondary later-type companions. to use the distribution of orbital or rotation periods for dif- Based largely on the information in the literature on ferentiating activity types. Although most of BY Dra–type active stars, I classified the 100 most luminous X-ray objects binaries have periods less than 5 days, a lot of RS CVn into the following categories: systems with giant primary components have periods in 1. RS: short-period spectroscopic binaries of RS CVn excess of 14 days, being nonetheless active because of that type, mostly stars with rapid rotation synchronous with (Fernańdez-Figueroa et al. 1994). Giants in close binaries the orbital motion, often with at least one evolved simply have to have longer periods lest they reach the component (giant or subgiant); mass-transfer stage. 2. XO: other short-period spectroscopic or eclipsing The rate of rotation has important implications for stellar binaries, typically with both components on the main activity. The level of X-ray activity in coronal sources, in sequence, including binary stars of BY Dra–type, semi- particular, is expected to be driven by the interplay between detached binaries (Algols), eclipsing binaries of Lyr the depth of the outer convection zone and the velocity of type; rotation. A clear correlation was found between chromo- 3. XY: pre–main-sequence (PMS) stars, post–T Tauri spheric activity, as measured by C iv flux or Rossby stars and very young main-sequence stars, typically numbers, and rotation periods (Gunn et al. 1998). Similar younger than the Pleiades (’60 Myr); correlations between LX or LX/Lbol and rotation were 4. XG: evolved (spectral types IV and III) single or sought for in nearby open clusters, but they turned out to be long-period binary stars; less obvious (e.g., Stauffer et al. 1994, for the Pleiades). The TABLE 1 The 100 Brightest X-Ray Stars within 50 Parsecs

Std. Prop. Rad. Hipparcos HD Alt. Activity BV Parallax Error LX Binarity Motion Vel. ROSAT Name No. No. Name Type Spec. Type, Class V Mag. Color (mas) (mas) (1029 ergs) HR1 Type (mas yr1) (km s1) (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15)

J001229.4+143358...... 999 LN Peg RS G8+ 8.44 0.74 24.7 1.2 16.72 0.02 SB2,O 321.1, 72.1 15.3 J003025.8481245 ...... 2383 2726 X? F2V 5.67 0.37 22.2 0.7 49.02 0.13 A 134.2, 84.5 +2.0 J003452.2615506 ...... 2729 3221 XY K4V 9.56 1.05 21.8 1.0 9.80 0.13 88.0, 50.5 1.0 J004335.3175911 ...... 3419 4128 Cet XG K0III 2.04 1.02 34.0 0.8 26.66 0.27 232.8 32.7 +13.0 J005655.5515232 ...... 4448 5578 BW Phe XY K3/4V 8.99 1.02 23.0 2.2 14.09 0.09 V 90.8 11.7 J012256.6+072505...... 6454 8357 AR Psc RS K1IV+G7V 7.30 0.83 22.1 1.0 85.23 0.16 SB2 94.9 229.7 +18.2 J012320.9572853 ...... 6485 8558 XY G6V 8.53 0.70 20.3 0.9 13.10 0.15 90.2, 36.4 +9.2 J013232.8493138 ...... 7183 9528 AE Phe WU G1/2IV/V 7.78 0.64 20.5 0.8 11.21 0.06 151.1, 53.6 J013500.7295430 ...... 7372 9770 BB Scl XO (K1V+K:V)+ 7.11 0.91 42.3 1.5 12.68 0.19 E,V 85.6 96.6 +31.5 (K3V+K3/4V) J020355.4452446 ...... 9642 12759 XY: G3V 7.30 0.69 20.3 1.0 11.08 0.11 V 328.7 54.3 J020506.8+771651...... 9727 12230 47 Cas XY F0V 5.27 0.34 29.8 0.6 23.62 0.05 O 122.7, 60.2 26.0 J020718.6531155 ...... 9892 13183 XY G5/6V+ 8.64 0.70 19.9 0.8 16.84 0.04 SB1 84.4, 22.5 +9.9 J024326.2375540 ...... 12716 17084 UX For RS: G6V+K2:V 8.04 0.75 24.8 0.8 30.42 0.03 SB2,A 83.7, 65.4 +20.3 J024843.0+310701...... 13118 17433 VY Ari RS K3/4IV+ 6.94 0.96 22.7 0.9 124.09 0.08 SB1 216.9, 170.2 2.8 J025153.5613704 ...... 13359 18134 VZ Hor BY K1V(p) 8.86 0.85 23.4 0.9 17.52 0.01 146.9 131.2 J030810.0+405727...... 14576 18134 Per XO B7.7V+G8III 2.09 0.00 35.1 0.9 80.70 0.06 E,V 2.4, 1.4 +4.0

1998 J031225.6442511 ...... 14913 20121 X? F6III 5.92 0.44 22.8 0.8 24.79 0.06 V 92.0, 6.2 +17.0 J031640.7033142 ...... 15247 20385 X? F5 7.48 0.53 20.0 0.9 10.59 0.09 77.7, 45.8 J032635.1+284302...... 16042 21242 UX Ari RS K0IV+G5V 6.47 0.88 19.9 1.2 142.33 0.05 SB2,V 46.4, 100.9 +27.2 J033313.4+461530...... 16563 21845 V577 Per XY K2 8.15 0.80 29.6 1.4 12.06 0.03 V 67.0, 176.0 2.0 J033647.2+003518...... 16846 22468 V711 Tau RS G5IV+K1IV 5.82 0.88 34.5 0.9 151.28 0.07 SB2,V 21.4, 162.3 15.0 J033711.0+255934...... 16879 22403 V837 Tau RS: G2V+K5V 7.28 0.70 26.8 0.9 56.48 0.03 SB2 235.8, 270.9 17.7 J034823.5+520210...... 17782 23524 X? G6IV 8.75 0.77 19.8 3.0 21.95 0.00 V 56.1, 72.8 0.7 J035025.0+171455...... 17962 V471 Tau RS K2V+DA 9.46 0.78 21.4 1.6 18.51 0.71 E,O 127.5, 22.9 +40.0 J040729.6523413 ...... 19248 26354 AG Dor RS: K0V+K4V 8.59 0.94 28.7 0.8 9.96 0.03 SB2,A 147.2, 233.9 +69.0 J042620.9+153646...... 20713 28052 V777 Tau X? F0V 4.48 0.26 20.9 0.8 20.47 0.05 A 111.2, 26.6 +38.3 J045817.4+002718...... 23105 31738 V1198 Ori RS: G5IV+ 7.20 0.70 29.9 1.0 16.20 0.10 SB2,A 156.6, 34.6 +6.6 J045915.4+375330...... 23179 31647 ! Aur X? A1V 4.93 0.04 20.5 0.9 16.57 0.10 V 46.3, 98.1 +5.0 J051642.2+460001...... 24608 31647 Aur RS G8III+G0III 0.08 0.80 77.3 0.9 41.92 0.11 SB2,V 46.3, 98.1 +29.2 J052038.3394517 ...... 24947 35114 X? F6V 7.39 0.51 21.9 0.6 11.30 0.30 37.9 12.6 J052704.7115400 ...... 25486 35850 XY F7V: 6.30 0.55 37.3 0.8 22.57 0.24 17.5, 49.8 +18.8 J052844.7652700 ...... 25647 36705 AB Dor XY K1V 6.88 0.83 66.9 0.5 15.22 0.08 A 48.9 137.6 +28.0 J054513.2595527 ...... 27134 XZ Pic XO K0V+M0V 9.28 0.85 20.2 0.8 13.96 0.05 SB1 22.6 121.3 J060446.6482728 ...... 28796 41824 XO G5V+K7:V 6.60 0.71 33.6 0.8 18.10 0.13 SB1,V 105.7, 26.5 +11.4 J061828.8720242 ...... 29964 45081 AO Men XY K6/7V 9.95 0.96 26.0 1.0 14.43 0.07 8.5 75.7 +16.2 J061908.2032625 ...... 30030 43989 V1358 Ori XY G0 7.95 0.59 20.1 1.0 12.36 0.04 10.6, 43.7 +23.0 J063800.7613156 ...... 31711 48189 XY G1.5V 6.15 0.62 46.2 0.6 9.82 0.15 V 26.0 72.4 +32.3 J074318.7+285306...... 37629 62044 Gem RS K1III+dG/K 4.23 1.12 26.7 0.8 119.41 0.09 SB1 62.6, 230.8 +44.3 J082551.4162244 ...... 41322 X? 10.24 1.34 20.5 2.7 10.14 0.25 V 182.5 16.7 J084646.9+062518...... 43109 74874 Hya XG G5III+F0V 3.38 0.69 24.1 1.3 28.11 0.18 SB1,V 199.4, 53.2 +34.4 J090817.3370649 ...... 44851 78644 XO G3V+M0:V 8.22 0.64 19.1 1.0 27.49 0.10 SB1,E 56.8, 4.3 J092226.2+401213...... 45963 80715 BF Lyn XO K2V+dK 7.69 0.99 41.2 1.1 16.94 0.01 SB2 340.7, 359.3 3.2 J092536.0531508 ...... 46236 81734 XY F7V 6.98 0.51 22.5 1.0 23.32 0.27 V 31.5, 77.7 J093428.7+694950...... 46977 82210 DK UMa XG G4IIIIV 4.54 0.78 30.9 0.6 21.69 0.04 63.0 77.4 27.4 TABLE 1—Continued

J094344.9+555724...... 47727 83950 W UMa WU G2V 7.85 0.62 20.2 1.0 10.31 0.13 E 15.4, 30.0 J100002.3+243313...... 49018 86590 DH Leo XO K0V+K7V 7.86 0.87 30.8 1.3 21.22 0.02 SB3,V 233.2, 33.1 +9.8 J104645.7492518 ...... 52727 93497 l Vel XG G5III+G2V 2.69 0.90 28.2 0.5 32.12 0.15 V 74.2, 54.9 +5.6 J112205.4244632 ...... 55505 98800 TV Crt XY K4V+ 8.89 1.15 21.4 2.9 14.76 0.06 SB1+SB2,V 91.7, 31.1 +9.5 J114439.2492458 ...... 57269 102077 V838 Cen XY K0/K1Vp 8.91 0.91 20.5 2.4 14.86 0.20 V 128.9, 43.0 +15.9 J120921.0275854 ...... 59259 105575 QY Lyr XO K2V+ 8.98 0.94 19.8 1.2 15.95 0.08 SB2,E 110.7 30.2 J121723.7210324 ...... 59914 106855 UV Crv XO K1V+ 9.37 0.95 22.9 2.4 13.42 0.26 SB2,V 115.2, 164.3 J131255.3594852 ...... 64478 114630 HR 4980 XO G0V+G0V 6.18 0.59 25.1 0.7 11.93 0.00 SB2,V 4.6, 105.9 +15.5 J133241.6+223007...... 66072 X? KV:e 9.64 1.03 21.5 2.0 16.58 0.13 V 133.9, 69.2 J133447.5+371100...... 66257 118216 BH CVn RS F2IV+K2IV 4.91 0.40 22.5 0.6 52.14 0.07 SB1 85.0, 9.6 +6.4 J140503.9+100055...... 68801 123034 X? G5 8.77 0.86 20.0 1.2 16.39 0.11 190.8 44.4 J142812.3021341 ...... 70755 126868 Vir XG G2IV+ 4.81 0.69 24.1 1.0 21.58 0.07 SB:,V 140.4, 1.0 +14.4 J143859.9+641731...... 71631 129333 EK Dra XY G1.5V 7.60 0.63 29.5 0.6 10.24 0.14 A 135.9, 25.3 20.6 J150757.9+761214...... 74045 135363 XY G5V/IV 8.72 0.95 34.0 0.7 13.74 0.09 130.5 163.7 4.7 J153859.3574225 ...... 76629 139084 V343 Nor XY K0V 8.14 0.82 25.1 1.1 21.38 0.07 A 46.2, 97.9 +0.5 J154547.5302100 ...... 77199 140637 KW Lup XY K2V 9.37 1.04 24.4 1.4 17.15 0.04 V 69.5, 100.9 4.8 J154935.4+260358...... 77512 141714 CrB XG G5IIIIV 4.59 0.79 19.7 0.7 18.15 0.09 78.1, 64.3 19.1 J160330.5574637 ...... 78662 143474 HR 5961 X? A7IV 4.63 0.25 23.3 1.0 35.68 0.08 V 116.1, 78.6 14.4

1999 J160403.4215545 ...... 78708 143937 V1044 Sco XO G9V+M0:V 8.64 0.91 23.7 2.1 18.01 0.05 SB2,E,V 295.7, 137.3 J161441.0+335125...... 79607 146361 TZ CrB RS: F6V+G0V 5.23 0.60 46.1 1.0 46.08 0.06 SB2,V 265.4, 83.9 12.2 J162517.7490855 ...... 80448 147633 XO G2V+(G4V+k5:V) 7.33 0.30 22.0 2.8 15.43 0.03 SB1,V 92.3, 83.7 3.2 J163329.7785341 ...... 81065 147675 Aps XG G9III 3.86 0.92 20.4 0.5 16.07 0.24 125.6, 77.9 +5.4 J171725.5665707 ...... 84586 155555 V824 Ara RS,Y: G5IV+K0V 6.87 0.80 31.8 0.7 44.93 0.06 SB2,V 21.6, 136.4 +2.7 J172012.8700246 ...... 84827 155875 ? G0.5IV: 6.53 0.60 25.8 0.8 10.05 0.10 V 47.2, 196.4 5.0 J174043.7074610 ...... 86509 V2383 Oph BY K7V 10.32 1.07 22.1 2.0 11.20 0.00 17.5 28.4 J175524.7+361122...... 87746 163621 V835 Her XO G8V+K7V 7.92 0.86 32.4 0.7 17.35 0.04 SB2 136.7, 20.8 19.9 J175745.7+291453...... 87933 163993 Her XG G8III 3.70 0.94 24.1 0.5 29.09 0.20 83.8, 19.7 1.5 J180549.9+212638...... 88637 165590 V772 Her RS: G0V+M1V 7.07 0.65 26.5 1.4 41.89 0.04 SB1,V 29.2, 40.3 22.8 J180749.6+260558...... 88817 166046 X? A3V 5.79 0.13 19.6 4.6 11.82 0.09 V 10.2 26.4 J180815.9+294127...... 88848 166181 V815 Her RS:,XY: G5V+M1/2V 7.70 0.73 30.7 2.1 31.83 0.05 SB1,A 108.5, 25.4 13.4 J183420.0+184126...... 91043 171488 V889 Her XY G0V 7.39 0.62 26.9 0.9 23.83 0.13 20.2, 49.6 24.3 J185306.0501045 ...... 92680 174429 PZ Tel XY K0V 8.43 0.78 20.1 1.2 24.84 0.02 15.8, 84.1 0.1 J190619.7522028 ...... 93815 177171 Tel XY F7V 5.17 0.53 19.1 0.8 65.76 0.24 A 31.0, 116.0 +2.0 J190621.4+274247...... 93817 337518 V511 Lyr RS: G8V+K3V 8.95 0.95 19.8 1.6 23.36 0.16 SB2,V 83.8, 197.1 +2.9 J190732.5+301513...... 93926 178450 V478 Lyr XO G8V+dK/dM 7.78 0.76 35.7 0.8 15.83 0.05 SB1 109.6 104.8 20.2 J191812.8382305 ...... 94863 180445 XO G8V+K5:V 8.46 0.81 24.0 1.2 16.59 0.04 SB2 99.6, 90.8 J203719.4+753554...... 101750 197433 VW Cep WU F8V 7.46 0.86 36.2 1.0 10.50 0.04 E,V 331.6 548.3 J204744.8363539 ...... 102626 197890 BO Mic XY K0V 9.44 0.94 22.5 1.6 116.78 0.04 A 18.4, 80.0 6.5 J205547.8170657 ...... 103311 199143 XY F8V 7.27 0.54 21.0 1.0 36.52 0.24 V 62.2, 65.4 9.0 J210225.7+274829...... 103833 200391 ER Vul RS: G0V+G5V 7.33 0.61 20.1 0.9 61.47 0.12 SB2,E 87.3 6.2 24.6 J210442.9770124 ...... 104043 199532 Oct XO F4III+F5III 5.13 0.49 22.1 0.6 22.78 0.14 SB2 13.4, 369.3 45.0 J211123.3522016 ...... 104604 201427 BR Ind XO F8V 6.98 0.57 20.5 2.1 25.12 0.03 E,V 24.3 29.6 J212050.3530200 ...... 105388 202917 XY G5V 8.65 0.69 21.8 1.2 11.89 0.13 30.3, 96.5 0.9 J212059.3522834 ...... 105404 202947 BS Ind XY K0V 8.89 0.85 21.7 1.5 15.76 0.17 E,A 35.4, 101.2 +6.0 J213359.4+453530...... 106481 205435 Cyg XG G8IIICN 3.98 0.88 26.2 0.5 10.26 0.03 25.2, 95.0 +6.7 J215821.0+825218...... 108456 209942 XG F6IVV 6.92 0.52 25.4 1.9 39.23 0.04 V 133.1, 46.8 24.4 TABLE 1—Continued

J215821.0+825218...... 108461 209943 V376 Cep RS F5+ 7.45 0.70 26.0 4.6 37.30 0.04 SB2,V 137.4, 40.4 17.0 J220840.9+454432...... 109303 210334 AR Lac RS G2IV+K0IV 6.11 0.76 23.8 0.6 137.72 0.01 SB2,E 51.8 49.1 34.5 J221534.8390053 ...... 109901 211087 CS Gru BY G8/K0V 9.35 0.83 19.4 3.3 14.46 0.05 A 90.7, 38.0 J230957.6+475756...... 114379 218738 XY:,XO K2V+K2V 7.93 0.88 39.6 7.7 15.22 0.12 SB2,V 157.3, 9.6 6.4 J230957.6+475732...... 114385 218739 XY G5 7.11 0.66 34.1 2.3 15.27 0.12 V 157.3, 9.6 5.7 J231922.8+790014...... 115147 220140 V368 Cep XY G9V 7.53 0.89 50.7 0.6 10.40 0.10 202.7 72.1 16.7 J233733.3+462736...... 116584 222107 And RS G8III/IV+ 3.81 0.98 38.7 0.7 62.61 0.06 SB1 159.1, 421.6 +6.8 J233939.1691148 ...... 116748 222259 DS Tuc XY G5/6V 8.17 0.78 21.6 1.3 17.71 0.04 V 82.1, 88.3 +7.3 J234244.1143244 ...... 116971 222661 X? B9V 4.49 0.03 21.2 0.9 11.96 0.12 V 98.9, 66.5 +3.8 J235503.5+283802...... 117915 224085 II Peg RS K2IV+M0/3V 7.51 1.01 23.6 0.9 175.75 0.15 SB1 574.9 34.5 20.5 J235540.8+250845...... 117972 224168 X? G5 9.01 0.92 20.2 1.3 21.57 0.06 A 196.1, 40.6

Notes.—Col. (5): RS for RS CVn–type stars, XO for other types of short-period active binaries, XG for active evolved stars without spectroscopic binarity, XY for young stars, BY for apparently single BY Dra–type variable stars, WU for WU UMa–type binaries, X? for unknown type; col. (7): from Hipparcos; col. (8): from Hipparcos; col. (9): from Hipparcos; col. (10): formal standard error of parallax from Hipparcos; col. (12): hardness ratio HR1 (from ROSAT); col. (13): SB for spectroscopic binaries, O for astrometric binaries with orbital solutions, A for other astrometric binaries, E for eclipsing, V for resolved visual; and col. (14): components of proper motion l*, l in milliarcseconds per year (usually from Tycho-2). Uncertainties are marked with a colon. THE 100 BRIGHTEST X-RAY STARS 2001

this star, located at the base of the giant branch and under- going dramatic structural changes, inherited its strong magnetic ﬁeld from the original Ap status, that is, possibly, from much stronger primordial intergalactic magnetic ﬁelds. 31.0 The origin of X-ray activity in single evolved stars (XG- type in this paper) is still an open issue, while more data and clues are being collected. We note the large fraction of XG-

erg / s type emitters (10%) among the nearest stars. Despite the X 30.5 presumed relatively short duration of that stage of stellar log L evolution, the phenomenon may actually be quite common. Four of the XG-type stars in the present sample have measured periods of rotation, three of them long (167.8, 71.7,

30.0 and 120.8 days for HIP 3419, 43109, and 87933, respectively) and one very short (0.088 days for HIP 46977).

-1 0 1 2 3 log P days 3. X-RAY PROPERTIES

Fig. 1.—X-ray luminosity, log LX, against photometric or orbital The most luminous X-ray source in the sample is II Peg, 30 1 period, log P, for 60 active stars. XY-type objects are marked with filled an RS-type star, at LX = 17.6 10 ergs s . Could there circles, RS-type with triangles, XO-type with crosses, XG-type with be a more luminous object in the solar vicinity that is miss- squares, and WU-type with asterisks. Orbital periods for binaries were used when photometric periods were not available. ing in the sample? The original selection was based on Hipparcos stars with parallaxes larger or approximately equal to 20 mas. But the Hipparcos catalog is not complete with stars out to 50 pc; it is possible that an inconspicuous collected data in Figure 1 for 60 stars show no correlation, star at a distance between 25 and 50 pc is even brighter in except, possibly, at periods larger than 10 days, where only X-rays than II Peg. The distance of 25 pc is the limit of the some RS CVn stars, single giants, and very young stars are Catalogue of Nearby Stars (CNS), which is fairly complete found. Among the most X-ray–luminous nearby stars, the down to late K stars. The Hipparcos catalog is fairly com- level of X-ray activity seems to be much better correlated plete down to V 9 mag (Turon et al. 1992), which corre- with the evolutionary status, as discussed in the next section, sponds to early G dwarfs at 50 pc. Thus, only late G and K than with the rate of rotation. Single giants are often slow dwarfs at distances between 25 and 50 pc may be missing in rotators, but still very luminous in X-ray. Besides, not all of the sample, and M dwarfs closer in. On the other hand, M the spectroscopic binaries in the sample have rotation rates dwarfs, although commonly active, are relatively dim X-ray matching the orbital periods. Adopting several new deter- sources (Hu¨nsch et al. 1999) and perhaps would not make it minations of photometric periods based on Hipparcos into the sample even if we knew them all. Young K dwarfs, epoch photometry (Koen & Eyer 2002), I found the follow- it appears, are relatively moderate X-ray emitters, with a ing mismatching periods (in days): HIP 6454 (Pphot = 3.636, notable exception of BO Mic (Table 1). It cannot be pre- Porb = 12.245), HIP 13118 (Pphot = 16.58, Porb = 13.198), cluded that a few nearby K dwarfs luminous in X-ray HIP 16879 (Pphot = 24.53, Porb = 1.930), HIP 19248 remain unknown. They could be distinguished by large (Pphot = 0.72, Porb = 2.562), HIP 84586 (Pphot = 1.946, proper motions and by significant parallaxes, if the latter Porb = 1.682), HIP 116584 (Pphot = 54.33, Porb = 20.521), were available. The completeness of the present sample with and HIP 104043 (Pphot = 2.88, Porb = 9.073). X-ray–emitting stars within 25 pc was confirmed by a Coronal X-ray emission is an unambiguous indicator of perusal of the X-ray data compilation for stars in the CNS activity for giants and other evolved stars (Schro¨der, by Hu¨nsch et al. (1999). Hu¨nsch, & Schmitt 1998). When a star more massive than X-ray luminosity distributions for different types of the Sun begins to cross the Hertzsprung gap, it develops a objects are summarized in Table 2. Most of the XY-type relatively deep convection zone, as witnessed spectroscopi- objects (70%) and XO+BY objects (75%) are weaker than cally by a solar-type granulation velocity field. This happens 20. At the same time, RS-type emitters are markedly stron- between F0 and G0 spectral types, well before the onset of a ger, with only 17% of the sample under 20. Hence, the limit 30 1 helium flash. Hence, in terms of convection-related mecha- of LX =2 10 ergs s appears to be a good statistical nisms of coronal activity, an evolving star lives through a criterion to differentiate RS-type objects and XY, XO, and relatively short and violent period of ’’youth,’’ followed by BY objects. It is noted that some of the RS–type stars in a sudden braking of the original rotation (Gray 1992, his Table 1 would possibly end up in the XO class (and two, Fig. 18.19). Schro¨der et al. (1998) noted that a few late-type HIP 88848 and 84586, may be even in XY) if we strictly giants in their sample under investigation with especially apply the requirement that at least one of the components high degrees of coronal emission were either stars of higher should be evolved in a RS CVn system. That would blur mass or they were still crossing the gap between the main somewhat the dividing line of LX = 20 between the classes, sequence and the giant branch. Makarov (2002) noted that because a few stars with LX between 20 and 40 would be most X-ray luminous giants in visual binaries are located in gone in the RS class, but the distribution for RS would the blue half of the giant clump, implying that they are more become even more biased toward very large luminosities. massive than the average stars and, possibly, are at a stage Main-sequence active binaries and young stars are practi- prior to a helium flash. An interesting example of magneti- cally indistinguishable by X-ray luminosities. The single cally active but slowly rotating evolved stars, EK Eri, was evolved stars (XG-type), however, seem to have somewhat discussed by Strassmeier et al. (1999). They proposed that larger luminosities than even young stars. The three contact 2002 MAKAROV Vol. 126

TABLE 2 Distributions of X-Ray Luminosities

LX2 1029 ergs s1 XY RS XO+BY XG WU ?

[9.8, 20.0[ ...... 21 (70%) 4 (17%) 15 (75%) 3 (30%) 3 (100%) 9 (60%) [20.0, 40.0[ ...... 7 (24%) 4 (17%) 4 (20%) 7 (70%) 0 5 (33%) [40.0, 80.0[ ...... 1 (3%) 8 (35%) 0 0 0 1 (7%) [80.0, +1[ ...... 1 (3%) 7 (31%) 1 (5%) 0 0 0 Total ...... 30 (100%) 23 (100%) 20 (100%) 10 (100%) 3 (100%) 15 (100%) binaries (WU) in the sample are all at the lower boundary of the present sample. Suchkov et al. (2003) found a consider- the sample. This is the opposite of the expectation that able population of very hard (and very X-ray luminous) F active mass transfer between the components should lend stars at distances beyond 100 pc. It follows that spectral some strength to X-ray activity. Singh, Drake, & White properties of strong X-ray emitters, in particular, of young (1996) found that Algol-type binaries (with mass exchange stars, in the solar vicinity and at larger distances are signifi- between the components) are 3–4 times less luminous than cantly different. This is further bolstered by the remarkable RS CVn systems (without mass transfer). In the present difference between the hardness ratios of spectroscopically sample the three WU stars have luminosities about 10–11, identified young stars (10 Myr) in and around the CrA and three other eclipsing binaries of Algol and Lyr type, SFR at 120–150 pc (Stelzer & Neuhaüser 2001), which are QY Lyr, BR Ind, and BS Ind, have luminosities between 16 mostly very hard, and those of the nearby mini-associations and 25, much smaller than most of the RS stars. In fact, Tucanae and TWA (ages 30 and 10 Myr, respectively), binaries of all kinds with rotational periods less than 1 day which are soft (Stelzer & Neuhaüser 2000). are weaker X-ray sources than RS binaries with longer Hardness ratios for different spectral classes (dwarfs, sub- periods (Fig. 1). giants and giants, reckoning by primary components in The hardness ratio HR1 is a measured parameter that binary or multiple systems) are summarized in Table 3. The tells how the energy of X-ray emission is distributed across dwarfs, which constitute the majority, and subgiants have the ROSAT passband. It is defined as very similar distributions with the median values just below 0 and confined to the interval (0.2, +0.2). The giants, how- H S HR1 ¼ ; ð1Þ ever, have appreciably harder X-ray emission, with the H þ S median HR1 at +0.11. Half of the giants are harder than where H and S are source counts in the hard (0.5–2.0 keV) +0.1. One binary system in the sample is very soft, V471 and soft (0.1–0.4 keV) passbands, respectively. HR1 may Tau at HR1 = 0.71. Although it is tentatively identified as take values between 1 (very soft) and +1 (very hard). The an RS-type object, this binary star with an orbital period of overall distribution of the nearby emitters is quite narrow 0.521 days may belong to a different class of X-ray emitters, and centered almost exactly on HR1 = 0. This is appreci- i.e., binaries with a hot white dwarf companion. The black- ably different from both the HR1 distributions for volume- body type of radiation from the white dwarf may account limited sample of nearby weak sources and the distribution for the super-soft emission. This star is probably member of for X-ray brightness-limited samples of more distant, but the Hyades open cluster (x 5), which constrains its age to more powerful sources. Indeed, the vast majority of X-weak 600 Myr. Gliese stars are very soft, with negative HR1 (Hu¨nsch et al. 1999). On the other hand, in the X-ray-limited sample of RASS-BSC/Tycho stars, dominated by strong emitters at 4. BINARITY larger distances, a dearth of soft sources is observed, while a significant component of very hard sources emerges (Voges At least 40% of the most luminous X-ray sources within et al. 1999, their Fig. 14). There are no such hard emitters in ’50 pc are identified as spectroscopic binaries (Table 1). In fact, the most powerful emitters, RS-type stars (e23% of the sample) are short-period binaries by definition, while TABLE 3 the main-sequence spectroscopic or otherwise short-period Distributions of X-Ray Stars of Different Luminosity Classes binaries (XO-type) contribute another e17%. Although on Hardness Ratio (HR1) there seems to be no particular reason for very young stars not to be found in tight binaries, we find only four such pos- HR1 V IV III sible cases: HIP 9892, 55505, 105404, and 114379. It cannot <0.20 ...... 3 (5%) 0 0 be precluded that a few binary systems identified as XO-type [0.20, 0.10[...... 10 (15%) 2 (13%) 0 sources are actually also very young. For example, the star [0.10, 0.00[...... 26 (39%) 8 (53%) 1 (7%) HIP 80448, which is a triple system with a solar-type pri- [0.00, +0.10[ ...... 12 (18%) 3 (20%) 6 (43%) mary and an SB1 secondary component, has a Pleiades-like [+0.10, +0.20[ ...... 10 (15%) 2 (14%) 4 (29%) lithium abundance (Cutispoto et al. 2002) and may be +0.20 ...... 5 (8%) 0 3 (21%) member of the Pleiades stream (x 5). Total ...... 66 (100%) 15 (100%) 14 (100%) There is a poorly understood relation between wider, vis- Median...... 0.02 0.05 +0.11 ual or astrometric binarity and the ability of stars to radiate Note.—Spectroscopic binaries are assigned to luminosity classes much in X-rays. Makarov (2002) detected a significantly IV or III if at least one of the components belongs to these classes. higher rate of long-period binaries among nearby X-ray No. 4, 2003 THE 100 BRIGHTEST X-RAY STARS 2003 stars than among stars below the ROSAT detectability from scattered papers (see the Appendix for details on indi- threshold. It was suggested that the presence of very young vidual stars). The equatorial components of proper stars that are more frequently found in binary and multiple motions, (l*, l), in Table 1 are mostly from the Tycho-2 systems than old field stars may account for at least part of catalog (Høg et al. 2000), except for HIP 14576 (Algol), the difference. Indeed, the present sample of young stars 24608 (Capella), and 114379, missing in Tycho-2, for which confirms the high rate of long-period binaries for this class the Hipparcos proper motions are given. Generally, Tycho- of objects. The fraction of components (FC) computed as the 2 proper motions are more robust at astrometric binaries total number of components in all binary and multiple sys- with periods larger than 3 yr. A few significant deviations of tems divided by the sum of the total number of components Hipparcos proper motions from the values in Tycho-2 were and single stars, is greater or equal to 0.81. In other terms, used to identify new astrometric binaries. only 10 out of the 30 young objects seem to be single stars. Using the collected radial velocities, proper motions, Only four of the 20 binary or multiple XY-systems contain coordinates, and parallaxes from the Hipparcos catalog, I short-period binaries; the rest are very wide visual or astro- computed heliocentric velocities (U, V, W ) for the majority metric pairs. It is commonly agreed upon that long-period of the stars in Table 1. The right-handed galactic coordinate binarity cannot be the reason for enhanced X-ray emission, system has its U axis pointing toward the Galactic center since the tidal interactions are too small to maintain the (‘ =0), V-axis in the direction of the Galactic rotation original rate of rotation. Rather, this correlation may be (‘ =90), and W-axis toward the north pole. Table 4 lists about the way nearby young stars were formed. Practically the derived velocity components for stars sorted by type of all of them seem to be isolated from open clusters or SFR, activity. Column (5) in Table 4 contains the total magnitude and belong to very sparse, expanding mini-associations. of velocity with respect to the local standard of rest (LSR), The lack of encounters and dynamical interaction taking assuming a solar velocity with respect to the LSR of place inside clusters and dense groups may cause a larger (10.0, 5.3, 7.2) km s1 from Dehnen & Binney (1998). rate of primordial well-separated binaries. Vector endpoints of LSR velocities of the selected types of In light of these dynamical ramifications, it comes as a X-ray stars are shown in Figure 2. surprise that nearby RS-type binaries, too, tend to be in Even a cursory look at Table 5 and Figure 2 reveals a hierarchical triple or multiple systems. Of the 23 tentatively strong uniformity of the motion of young X-ray emitters. identified RS-objects, at least 16 have wider separated com- They are involved in a streaming motion at an average heli- ponents. This does not agree with the idea that RS-type ocentric velocity (9.6, 21.8, 7.7) km s1, computed on stars are dynamically highly evolved, i.e., experienced multi- 23 XY-stars with the closest directions of motion. This ple interactions with perturbing masses in the disk. Allen, streaming motion is ascribed to the so-called Local Associa- Poveda, & Herrera (2000) find statistical evidence that halo tion (LA) or the Pleiades moving group (PMG) (Eggen and thick disk stars in wide binaries, a considerable number 1975, 1992). Eggen determined the motion of the LA at of which are known, retain their original distribution of (11, 21, 11), and Jeffries (1995) at a slightly different orbit sizes. This may imply that, spending most of the time (10, 21, 13) km s1, with a coordinate dispersion of 5.7 away from the dense regions of the disk and coming through km s1. The existence of streaming motion of young stars is the disk at high velocities, these binaries of low binding confirmed by the kinematically unbiased data in this paper. energy have much better chances to survive as such than The average motion of the XY-type stars, as determined population I binaries. But would not that also imply that in this paper, is just the reflex solar motion in U and W, these evolved binaries (and our RS-type triple systems) were while in the V component it is smaller by 15 km s1 than born already with high velocity dispersion, rather than the Dehnen & Binney definition of the solar velocity. It acquired it through dynamical heating? should be noted that most of the previous determinations of The fraction of components in the general sample is 0.90, the solar velocity quoted substantially larger values of V owing in part to the large number of spectroscopic binaries. Not counting short-period binaries, the highest rate is found for the X?-type stars, whose origin of X-ray emission could not be identified. Only three of these objects are single stars, whereas 24 are visual or astrometric binaries (FC = 0.89). Many of these stars have spectral types earlier than F5. This group therefore may be a mixture of binaries with unknown active secondary companions and of still unrecognized very young stars.

5. KINEMATICS Thanks to the proximity and optical brightness, radial velocities are available for the majority of the selected stars (Table 1). For spectroscopic binaries, the center of mass radial velocity is part of the orbital solution that requires a signiﬁcantly long series of accurate spectroscopic measure- ments. A fair number of orbital solutions were found in the Second Catalog of Chromospherically Active Binary Fig. 2.—Distribution of velocities of the brightest X-ray stars with respect to the LSR. Vector endpoints are marked with open circles for XY- Systems (CABS, Strassmeier et al. 1993). A few binaries are type stars, triangles for RS-type stars, squares for XG-type stars and given in the older catalog of spectroscopic binaries by crosses for XO-type stars. Large circle shows the direction of the mean Batten, Fletcher, & Mann (1978), and the rest were culled velocity of the young stars (Y-type). 2004 MAKAROV Vol. 126

TABLE 4 TABLE 4—Continued Kinematics of Nearby X-Ray Stars U V W vLSR 1 1 1 1 U V W vLSR HIP (km s ) (km s ) (km s ) (km s ) HIP (km s1) (km s1) (km s1) (km s1) (1) (2) (3) (4) (5) (1) (2) (3) (4) (5) XO Type XY Type 7372 ...... 17.5 1.6 29.9 24.2 2729 ...... 12.4 17.3 5.8 17.9 28796 ...... 0.1 3.2 18.9 15.6 6485 ...... 10.0 22.3 1.1 18.1 45963 ...... 24.0 42.4 29.7 45.6 9727 ...... 2.5 32.2 10.1 28.1 49018 ...... 32.1 12.5 14.8 24.5 9892 ...... 9.8 20.8 0.7 16.8 64478 ...... 7.6 14.9 19.0 23.2 16563 ...... 8.9 23.9 16.2 20.7 80448 ...... 14.4 22.8 1.6 20.1 25486 ...... 10.2 15.1 8.2 9.8 87746 ...... 4.8 26.9 7.7 26.7 25647 ...... 7.8 25.6 13.3 21.3 93926 ...... 25.4 7.5 10.6 15.9 29964 ...... 10.5 16.4 8.8 11.2 104043 ...... 10.1 90.6 2.6 85.9 30030 ...... 13.4 20.7 5.9 15.8 XG Type 31711 ...... 7.2 28.9 14.8 24.9 55505 ...... 13.2 18.1 6.8 13.2 3419 ...... 30.6 11.4 13.1 22.3 57269 ...... 17.1 27.5 13.6 24.2 43109 ...... 43.0 25.4 18.4 40.2 71631 ...... 7.2 29.1 4.7 24.1 46977 ...... 13.1 3.2 28.4 31.4 74045 ...... 26.3 11.2 7.7 17.3 52727 ...... 16.3 2.3 1.5 27.1 76629 ...... 10.4 14.6 9.7 9.7 70755 ...... 9.7 20.4 21.4 32.3 77199 ...... 9.5 21.0 7.4 15.7 77512 ...... 5.1 30.1 5.0 25.4 91043 ...... 8.9 23.9 5.5 18.7 81065 ...... 17.5 28.6 9.0 29.3 92680 ...... 7.6 16.4 8.9 11.4 87933 ...... 1.7 5.4 16.0 18.1 93815 ...... 10.8 23.9 14.3 20.0 106481 ...... 14.9 6.1 10.1 27.6 102626 ...... 7.1 17.0 0.8 13.7 108456 ...... 33.9 11.9 0.0 44.9 103311 ...... 11.2 16.5 10.0 11.6 105388 ...... 7.8 20.5 0.7 16.7 Notes.—Velocities with respect to LSR, vLSR, are computed 1 105404 ...... 3.8 23.0 6.3 18.8 assuming a peculiar solar motion (10.0, 5.3, 7.2) km s . Heliocentric 114379 ...... 13.6 11.9 6.0 7.6 velocity components U, V, W are given in cols. (2)–(4). Axis U points toward the Galactic center, V toward the direction of Galactic 114385 ...... 16.8 12.6 8.4 10.1 rotation, W toward the north Galactic pole. 115147 ...... 10.2 23.5 5.5 18.3 116748 ...... 9.5 25.6 1.9 22.3

RS Type Branham (2002) arrived at a Sun’s velocity of (14.5, 19.7, 2.8) km s1, yet closer to the reflex motion of 999...... 44.1 47.2 7.2 54.0 the XY-stars in V. The local system of molecular clouds 6454 ...... 45.2 29.7 15.5 54.6 was found to have an overall heliocentric velocity of 12716 ...... 4.3 26.6 10.0 22.2 ( 10.1, 15.6, 7.5) km s1 (Taylor, Dickman, & Scoville 13118 ...... 22.2 52.6 7.2 48.9 16042 ...... 27.1 14.8 22.0 24.5 1987; Ramesh 1994), in excellent agreement with the aver- 1 16846 ...... 23.2 13.8 2.1 34.6 age motion of O–B5 stars and only different by 6 km s in 16879 ...... 1.4 65.9 2.0 61.9 the V component from the streaming motion of the XY- 17962 ...... 45.8 17.7 4.2 38.0 stars. There was a vigorous argument in the 1980s that the 19248 ...... 20.1 76.0 25.9 79.1 LSR moves faster by 7kms1 in the V direction than the 23105 ...... 1.4 8.7 24.8 25.2 ‘‘ true ’’ circular velocity; accepting it would make the bulk 24608 ...... 35.8 14.0 8.9 27.3 of our XY-stars be practically at rest with this ‘‘ true ’’ 37629 ...... 32.9 49.7 14.7 54.6 standard, i.e., be exactly on the circular orbit. There is not 66257 ...... 15.4 10.8 3.6 31.9 enough data to delve into this discussion, but a firm 79607 ...... 6.8 28.8 9.5 29.0 84586 ...... 10.6 15.7 8.4 10.5 88637 ...... 8.5 22.3 5.6 17.1 88848 ...... 5.0 4.0 20.9 14.6 TABLE 5 93817 ...... 45.8 23.2 2.0 58.8 Distributions of Velocities of X-Ray Stars 103833 ...... 21.6 21.9 9.3 20.4 108461 ...... 30.2 6.6 3.8 41.7 vLSR 109303 ...... 5.7 31.7 18.8 40.3 (km s1)XYRSXG 116584 ...... 0.9 8.0 55.0 48.7 117915 ...... 98.3 63.6 8.3 105.8 [0, 10[...... 3 (11%) 0 0 [10, 20[ ...... 16 (59%) 3 (13%) 1 (10%) [20, 30[ ...... 8 (30%) 6 (26%) 5 (50%) [30, 50[ ...... 0 7 (31%) 4 (40%) than the 5.3 km s1 of Dehnen & Binney. Miyamoto & Zhu [50, 80[ ...... 0 6 (26%) 0 (1998), using Hipparcos data, determined a solar velocity 80...... 0 1 (4%) 0 with respect to 1320 O–B5 stars out to 3 kpc of around Total ...... 27 (100%) 23 (100%) 10 (100%) 1 1 1 (11, 14, 7) km s1, which is different by only 8 km s1 in V Median...... 17 km s 38 km s 29 km s from the systemic velocity of our young stars. Using a Note.—Velocities with respect to LSR, vLSR, are computed slightly larger sample of O–B5 stars mostly within 1 kpc, assuming the peculiar solar motion (10.0, 5.3, 7.2) km s1. No. 4, 2003 THE 100 BRIGHTEST X-RAY STARS 2005 conclusion is reached that young X-ray stars within 50 pc, O had different age. The younger group of giants, exhibiting and B stars within 2 kpc, and molecular clouds within 500 slow motions with respect to the LSR, was assigned a kine- pc share the same streaming motion with respect to the matical age of 1–2 Gyr, while the fast moving older group LSR, with the young stars only slightly lagging behind by 6– could be greater than 5 Gyr of age. We also find several RS- 8kms1. In fact, Eggen (1975) equated kinematically the type systems in the current sample whose LSR velocities are Gould Belt and the PMG. below 20 km s1. Some of them, however, may be misidenti- The term ‘‘ association ’’ is unfortunate for this loosely fied, in that their true types may be XO or even XY. For coherent stream, because it is not consistent with the defini- example, HIP 84586 and HIP 88848 may in fact be quite tion given by the discoverer of associations (Ambartsumian; young. Therefore, the existence of a slow-motion group of e.g., Ambartsumian 1962). This extensive complex of clus- active evolved stars is questionable and should be re- ters, OB associations, and dispersed young stars was not examined on a larger sample, taking account of the new formed in a single molecular cloud within a relatively short definitions and new data. We also do not find evidence of a period of time, and it scarcely undergoes expansion from a sizeable difference in the kinematics of SB1 and SB2 active common center, as required of an association. The Pleiades systems suggested by Eker (1992). stream likely includes recently discovered mini-associations of PMS stars, such as the Pictoris group (Zuckerman et al. 2001), Tucana (Zuckerman, Song, & Webb 2001) and 6. DISCUSSION Horologium (Torres et al. 2000), which some of the XY- Despite the remaining uncertainties in classification and objects in our sample have tentatively been identified with lack of observational data for some nearby stars, we have (see the Appendix). reached interesting conclusions about the status and proper- Table 5 summarizes the distribution of stars of three types ties of the brightest X-ray stars. Not less than 40% of X-ray 30 1 of activity (XY, RS, and XG) on LSR velocity. Whereas all emitters with LX e 10 ergs s in the close solar neighbor- the putative young objects have velocities smaller than 30 hood are short-period, mainly spectroscopic binaries. Their km s1, only 37% of the RS objects have such moderate activity is probably underpinned by the fast rotation main- velocities. The median velocity of the XY-stars is only 17 tained by the transfer of the orbital momentum. Some of the km s1; almost all of it is due to the residual motion in the V active short-period binaries include evolved stars, such as component. The median velocity of the RS-type sources is RS CVn–type objects. These stars are often distinguished by 38 km s1, that is, more than double that of the XY-type extremely high X-ray luminosities, accompanied by large stars. Almost a third of the RS-type objects have LSR veloc- velocity dispersions and random directions of motion. ities above 50 km s1. This group moves even faster than the Apparently, RS-type sources are kinematically very old. single evolved stars (XG). It may imply that old spectro- Another sizeable population of nearby active stars is com- scopic binaries have undergone more dynamical heating posed of single or long-period evolved stars (giants and sub- than single stars of comparable age or were formed with a giants), whose activity cannot be explained by dynamical larger velocity dispersion. The median velocity of the XG- interaction between components in binary systems. They type stars is 29 km s1, right in between the typical velocities make up at least 10% of the strongest X-ray emitters. Their of the XY- and RS-type objects. The lack of very fast mov- rates of rotation are often fairly low. Giants, as single active ers (vLSR > 50) among the XG-stars is salient, but it should stars and in RS-systems, are significantly harder as X-ray be confirmed on a larger sample. sources than main-sequence stars. Montes et al. (2001a) determined the velocity of the PMG Young stars, whether single or binary, seem to make up at (11.6, 21.0, 11.4) km s1 and proposed candidate only a third of the X-ray population. It is therefore hazard- members of four more local streams, IC 2391 supercluster ous to rely on just X-ray brightness in selecting possible (age 35–55 Myr), Castor moving group (200 Myr), Ursa young stars, as was also shown by dedicated spectroscopic Majoris group (or Sirius supercluster, 300 Myr) and the observations (e.g., Torres, Neuhaüser, & Guenther 2002). Hyades supercluster (600 Myr). Of our sample, only one We have also seen that main-sequence active binaries have star is likely to belong to the Hyades group, HIP 17962, very similar X-ray properties to young stars. What may which is in fact considered member of the Hyades cluster. really serve as a telltale property is the kinematics of X-ray Two stars, kinematically, seem to belong to the IC 2391 stars. Young stars within 50 pc have very orderly patterns of stream, viz., HIP 114385 and HIP 103833. The latter, ER motion at moderate velocities with respect to the LSR. Vul, is identified as an RS-type object; however, provided it Moreover, the majority of nearby stars seem to be members is younger than 60 Myr, it should rather be listed as a XY- of the Pleiades stream, with the mean velocity of type spectroscopic binary. Figure 2 depicts positions of the (9.6, 21.8, 7.7) km s1, and only a few stars may be LSR velocity vector endpoints in Galactic coordinates. The members of other kinematic groups identified in the broader streaming motion of the young stars shows up as a distinct solar neighborhood. Most of the nearest young stars belong clustering of endpoints around the mean direction indicated to sparse, probably expanding mini-associations like the by a larger circle. The remaining classes of active stars have Tucana or Pic. much more random distribution of velocity directions. Yet, Binarity is a common feature of X-ray stars. The cause of the majority of short-period spectroscopic binaries (RS and the empirical relation between the rate of well-separated, XO) are located to the left of the middle, i.e., their V veloc- long-period binarity and X-ray activity is far from obvious. ities are negative. Some of the XO-type systems may in fact This study confirms the high rate of such binaries among be fairly young and, hence, be involved in the streaming young stars, but it also finds a puzzling large number of hier- motion of the Pleiades moving group or other groups; but archical multiple systems in the RS group. How could these the negative V velocities of the RS-type objects are puzzling. systems survive several gigayears of dynamical evolution in Eker (1992) found two kinematic groups among active the Galaxy? Through what process did these systems attain giants (including RS CVn systems) and proposed that they their high velocities with respect to the LSR? 2006 MAKAROV Vol. 126

At larger distances, 50–200 pc, some of the statistics may HIP 16879: CABS 30 in Strassmeier et al. (1993). change. One would expect a larger fraction of very young HIP 17962: CABS 32 in Strassmeier et al. (1993). stars due to the presence of massive OB associations and Member of the Hyades cluster (600 Myr). Triple; second SFR in the near part of the Gould Belt. More distant X-ray component is a hot white dwarf (DA); third component is a stars seem to be considerably harder, including T Tauri stars brown dwarf of mass M3 sin i3 = 0.0393 0.0038 M. in the vicinity of active SFR. The stellar density of RS- and HIP 19248: orbital solution from Washuettl & XG-type stars is expected to be quite uniform, so the bal- Strassmeier (2001). ance can be tipped toward young stars. At larger distances HIP 24608: CABS 51 in Strassmeier et al. (1993). Orbital we may find more objects of other types, e.g., thermally solution from Pourbaix (2000). emitting hot white dwarfs or super-massive wind-generating HIP 24947: extremely fast rotator, v sin i >60kms1 O stars, barely represented in the current sample. (Cutispoto et al. 1999). HIP 25486: possible member of the Pic mini-association I thank D. Soderblom for helpful comments and the (age 10 Myr) (Zuckerman et al. 2001). Fast rotator, USNO Editorial Board for improvements and corrections v sin i =50kms1. to this paper. This research has made use of the SIMBAD HIP 25647: one of the most enigmatic nearby stars. Origi- database, operated at CDS, Strasbourg, France. nally believed to be an RS CVn star, but radial velocity is constant. Photometric period P = 0.514 days (Rucinski 1983). An effective temperature of 5000 100 K was determined by Vilhu, Gustafssonays, & Edvardsson (1987), and APPENDIX it was proposed that the star is still contracting (age between 1 and 30 Myr). Chugainov & Lovkaya (1989) noted a signif- NOTES TO INDIVIDUAL STARS icant infrared excess at 12 lm from the IRAS data, indica- HIP 999: triple, according to (Fekel et al. 1999), with tive of a dusty disk. The star is an astrometric binary with orbital periods 1.888 and 1564.4 days. an invisible companion of between 0.08 and 0.11 M HIP 2729: putative member of the Tucana mini- (Guirado et al. 1997). Wichmann et al. (1998) placed it on association (Zuckerman et al. 2001); age between 10 and the ZAMS, but its lithium abundance is more common to 30 Myr; lg LX/Lbol = 2.76 (Stelzer & Neuhaüser 2000). PMS stars (Jeffries 1995). It is more likely that the star is HIP 4448: young star but probably not member of the young, about 30 Myr old, surrounded by a probably eccen- Tucana mini-association (Zuckerman et al. 2001) because tric disk from which large comets plunge into it (Go´mez de of deviating kinematics. Tentatively, BY Dra variable Castro 2002). (Kazarovets et al. 1999). HIP 27134: SB1 according to Cutispoto et al. (1999); HIP 6454: orbital solution from (Fekel 1996). v sin i =23 2kms1. HIP 6485: member of the Horologium mini-association; HIP 28796: triple, according to Cutispoto et al. (1999). age 30 Myr (Torres et al. 2000). Fast rotator, v sin i =15 Moderate rotator v sin i =8 2kms1. Photometric km s1. period P = 3.3 days. Not PMS (Soderblom et al. 1998) ˚ HIP 7183: period P = 0.36 days. Discarded as member of because of depleted lithium, WLi < 0.016 A. the Horologium mini-association because of zero lithium HIP 29964: possible member of the Pic mini-association ˚ content (Torres et al. 2000). (age 10 Myr) (Zuckerman et al. 2001). WLi = 0.357 A. HIP 7372: possibly a five-component system in which the HIP 30030: single according to Cutispoto et al. (1999). secondary (B) spectroscopic binary component is active Very fast rotator, v sin i =47 3kms1. Significant ˚ (Watson et al. 2001). Period P = 0.476 days is determined lithium, WLi = 0.181 A (Strassmeier et al. 2000). for the B system. HIP 31711: PMS according to Cutispoto et al. (1999); 1 ˚ HIP 9642: no lithium (Torres et al. 2000). However, v sin i =18kms , WLi = 0.151 A (Torres et al. 2000). 0 extremely chromospherically active, log R HK = 4.30 HIP 37629: orbital solution from Duemmler, Ilyin, & (Henry et al. 1996), and may be very young. Tuominen (1997). HIP 9727: in the Local Association according to Montes HIP 43109: photometric period P = 71.7 days from et al. (2001a). Henry et al. (1996), rather than the contradicting period of HIP 9892: member of the Horologium mini-association 0.088 days from Koen & Eyer (2002). (Torres et al. 2000). Considerable lithium abundance, HIP 44851: SB1; fast rotator, v sin i =59 8kms1 ˚ WLi = 0.241 A. Variable radial velocity, SB1 (Cutispoto (Cutispoto et al. 1999). et al. 2002). Fast rotator, v sin i =23kms1. HIP 45963: CABS 84 in Strassmeier et al. (1993). HIP 12716: orbital solution from Washuettl & HIP 46977: v sin i =5.5 1.0 km s1 (De Medeiros & Strassmeier (2001). Mayor 1999). Not an SB, therefore, classified as XG-type. HIP 13118: orbital solution from Bopp et al. (1989). Photometric period P = 0.92 days is adopted from Bakos & Likely evolved; asynchronously rotating. Tremko (1987). HIP 13359: chromospherically active, but no lithium HIP 49018: CABS 87 in Strassmeier et al. (1993). Triple; (Torres et al. 2000). Eggen (1984) included it in the Hyades v sin i = (45/31)5 km s1; both spectroscopic components supercluster (age 600 Myr?). may be active. HIP 16042: orbital solution from Duemmler & Aarum HIP 57269: another controversial object. Kinematically, (2001). Triple. belongs to the TWA mini-association (Makarov & ˚ HIP 16563: significant lithium, WLi = 0.236 A, but slow Fabricius 2001); thus, may be only 10 Myr old. Song, rotator (Jeffries 1995). Tentatively, BY Dra variable Bessell, & Zuckerman (2002), however, discarded it as a (Kazarovets et al. 1999). PMS star on the grounds of insufficient lithium abundance, ˚ HIP 16846: CABS 29 in Strassmeier et al. (1993). WLi = 0.196 A. But note its fast rotation: v sin i =20km No. 4, 2003 THE 100 BRIGHTEST X-RAY STARS 2007 s1 and high degree of X-ray activity. Member of the HIP 93817: v sin i = (14.2/13.5) km s1 (Fekel 1997). Pleiades moving group (Anders et al. 1991). Orbital solution from Jeffries, Bertram, & Spurgeon (1995). HIP 59914: v sin i = (13/16) km s1 (Strassmeier et al. As a pair of main-sequence stars (Osten & Saar 1998), may 2000). be not RS. HIP 64478: CABS 110, v sin i = (16.8/17.1) km s1 HIP 93926: v sin i = 24.7 km s1 (Fekel 1997). More (Strassmeier et al. 1993). likely an early-type BY Dra than RS (Fekel 1988). See also HIP 66257: CABS 114; orbital period may be variable Fernańdez-Figueroa et al. (1994). (Strassmeier et al. 1993). HIP 94863: SB2 discovered by (Cutispoto et al. 2002). HIP 71631: according to Montes et al. (2001b), ‘‘ signifi- v sin i = 14 + 13 km s1. cantly younger than the Pleiades ’’; v sin i = 17.3 km s1, HIP 101750: contact binary; but also visual binary with a ˚ WLi = 0.195 A. Photometric period P = 2.787 days. companion separated by 1>1, probably an M2 star. Member HIP 74045: Jeffries (1995) determined v sin i =14km of the Pic mini-association (age 10 Myr) (Zuckerman et al. 1 ˚ s , WLi = 0.225 A. Discordant radial velocities are quoted 2001)? in Jeffries (1995), Montes et al. (2001a), and Simbad. Could HIP 103833: CABS 179, v sin i = (85/85) km s1 be binary? (Strassmeier et al. 1993). HIP 76629: possible member of the Pic mini-association HIP 104043: orbital solution from Batten et al. (1978). (age 10 Myr) (Zuckerman et al. 2001); v sin i =11kms1. HIP 105388: putative member of the Tucana mini- HIP 77199: one of the nearest T Tauri stars. Age 11 Myr association (Zuckerman et al. 2001); age between 10 and 30 ˚ 1 according to Neuhaüser & Brandner (1998). Photometric Myr. WLi = 0.225 A; v sin i =14kms (Cutispoto et al. period P = 2.72 days (Cutispoto et al. 1999). Brandner et al. 2002). However, located plump on the main sequence. (1996) detected a faint companion in a 1 lm band at 0>67 HIP 105404: possible member of the Tucana mini-associ- from the primary, although no orbital or acceleration ation (Zuckerman et al. 2001); age 30 Myr. Eclipsing solution is given in Hipparcos. binary of Lyr–type; P = 0.44 days. HIP 78708: v sin i =42 2kms1 (Cutispoto et al. HIP 106481: single; slow rotator, v sin i = 1.9 km s1 (De 1999). Medeiros & Mayor 1999). In Ursa Majoris moving group? HIP 79607: CABS 132; v sin i = (26/25) km s1 HIP 108456: single; v sin i = 5.8 km s1 (De Medeiros & (Strassmeier et al. 1993). Mayor 1999). HIP 84586: CABS 141; v sin i = (37/29) km s1 HIP 108461: orbital solution from (Batten et al. 1978). In (Strassmeier et al. 1993). Components A and B are in spec- common proper motion pair with 108456 and corresponds troscopic binary, and component C is 3000 away (Soderblom to the same ROSAT source. Since the ROSAT position is et al. 1998). Some consider it an RS CVn, but others argue it much closer to this star (200 mismatch against 1600), most of is an SB2 post-TT star. Kinematically, belongs to the the X-ray emission probably comes from HIP 108461. Pleiades moving group. HIP 109303: orbital solution from Marino et al. (1998). ˚ HIP 87746: CABS 146; the secondary is evolved accord- HIP 114379: moderate lithium, WLi = 0.113 A ing to Strassmeier et al. (1993), but Griffin et al. (1994) (Strassmeier et al. 2000). May be youngish, but not very detect a pair of main-sequence stars (G8V+K7V); thus, it is young; age between Pleiades and Hyades. not RS. HIP 115147: photometric period P = 2.74 days; 1 1 ˚ HIP 88637: CABS 149; v sin i =65kms (Strassmeier v sin i = 16.1 km s (Montes et al. 2001b). WLi = 0.207 A. et al. 1993). The tertiary component is a G5V star. Kine- Post–T Tauri or Pleiades age. matically, belongs to the Pleiades moving group; may be not HIP 116584: CABS 204; v sin i =6kms1 (Strassmeier RS. et al. 1993). HIP 88848: CABS 152, v sin i =27kms1 (Strassmeier HIP 116748: nuclear member of the Tucana mini- et al. 1993). Another controversial object. Fekel et al. (1986) association (Zuckerman et al. 2001), but tentatively RS-type describe it as an early BY Dra–type; very young, with strong according to Kazarovets et al. (1999). Visual binary with 500 Li. separation; both components are active (Cutispoto et al. 1 ˚ HIP 91043: single or long-period binary according to 2002). Component A: v sin i =17kms , WLi = 0.230 A. 1 ˚ Cutispoto et al. (2002). Very fast rotator, v sin i =45km Component B is SB2, v sin i =14kms ,WLi = 0.245 A. 1 s . Significant lithium, WLi = 0.220. HIP 117972: orbital solution from Berdyugina et al. HIP 92680: possible member of the Pic mini-association (1998). (age 10 Myr) (Zuckerman et al. 2001)? Or Tucana group (Zuckerman et al. 2001)?

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