A&A 375, 387–396 (2001) Astronomy DOI: 10.1051/0004-6361:20010767 & c ESO 2001 Astrophysics An optical study of X-ray sources in the old open clusters NGC 752 and NGC 6940? M. van den Berg and F. Verbunt Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands Received 20 April 2001 / Accepted 22 May 2001 Abstract. We observed the optical counterparts of X-ray sources in the old open clusters NGC 752 and NGC 6940 to search for the origin of the X-rays. The photometric variability reported earlier for the blue straggler H 209 is not confirmed by our light curves, nor is an indication for variability seen in the spectra; thus its X-rays remain unexplained. The X-rays of VR 111 and VR 114 are likely not a result of magnetic activity as these stars lack strong Ca II H&K emission, while in VR 108 the level of activity could be enhanced. The short-period binary H 313 is a photometric variable; this supports the interpretation that it is a magnetically active binary. From the detection of the Li I 6707.8 A˚ line, we classify the giant in VR 84 as a first-ascent giant; this leaves its circular orbit unexplained. As a side-result we report the detection of Li I 6707.8 A˚ in the spectrum of the giant H 3 and the absence of this line in the spectrum of the giant H 11; this classifies H 3 as a first-ascent giant and H 11 as a core-helium-burning clump star, and confirms the faint extension of the red-giant clump in NGC 752. Key words. stars: activity – stars: blue stragglers – stars: variables: general – open clusters and associations: individual: NGC 752, NGC 6940 – X-rays: stars 1. Introduction peculiar X-ray sources in two other old clusters, NGC 752 and NGC 6940, to look for optical signatures for the X-ray X-ray observations of open clusters older than 1 Gyr led emission and to compare their properties with those of the to the detection of many magnetically active binaries (e.g. ones found in M 67. The properties of the X-ray sources Belloni 1997). This is not unexpected: at these high ages in the two clusters are summarised in Table 1. single late-type stars are believed to rotate too slowly to ∼ emit detectable X-rays, and tidal interaction in a close NGC 752 is a nearby ( 450 pc, Daniel et al. 1994) binary is therefore necessary to maintain rotation at a open cluster with an estimated age of 2 Gyr (Dinescu higher rate than is typical for stars of that age. However, et al. 1995). Seven X-ray sources were identified with our interest is mainly in the stars whose X-rays are not optical cluster members (Belloni & Verbunt 1996; not as easily explained: binaries with orbital periods too long counting the probable detection of the short-period binary for tidal interaction, and stars whose evolutionary statuses DS And). Our study is focused on the blue straggler H 209, are poorly understood. Such peculiar X-ray sources were the only X-ray source in NGC 752 that is not located along pointed out in M 67 (Belloni et al. 1998) and were se- the cluster’s main sequence (see Fig. 1). H 209 is a spectro- lected by us for an optical follow-up study (van den Berg scopic binary with a long orbital period (Latham et al. pri- et al. 1999). This revealed interesting results (e.g. the blue vate communication). Magnetic activity in the early-type straggler S 1082 turned out to be a complex triple sys- star is not expected, but also the orbital period is too long tem, van den Berg et al. 2001a) but in several cases the to generate magnetic activity in a possible late-type sec- X-rays remain unexplained. The failure to explain the ori- ondary. Low-amplitude (0.05 mag) photometric variabil- gin and properties of these stars demonstrates that our ity was noted by Hrivnak (1977). Three of the remaining understanding of the X-rays and stellar interactions in X-ray sources (H 205, H 313 and the contact binary H 235) old open clusters is still limited. We have observed similar are binaries with orbital periods less than 2 days; hence their X-rays are likely a result of magnetic activity. The Send offprint requests to: M. van den Berg, same could be the case for the rapid rotator H 214 (Daniel e-mail: [email protected] ? et al. 1994). The radial-velocity measurements of H 182 by Based on observations made with the Jacobus Kapteyn Daniel et al. (1994) show no indication of binarity while Telescope and the William Herschel Telescope operated on the no measurements are made of H 156. island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de NGC 6940 is somewhat younger than NGC 752. It is Astrofisica de Canarias. considered to be an old cluster (Janes & Phelps 1994) Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20010767 388 M. van den Berg and F. Verbunt: An optical study of X-ray sources in NGC 752 and NGC 6940 Table 1. Properties of the stars in NGC 752 and NGC 6940 discussed in this paper. We list the star identification number (Heinemann 1926; Vasilevskis & Rach 1957), V magnitude and B − V colour (Daniel et al. 1994; Larsson-Leander 1964), the −1 orbital period Porb (see Belloni & Verbunt 1996 for references on NGC 752; rot = rapid rotator vrot sin i>30 km s ,Daniel et al. 1994; see Mermilliod & Mayor 1989 for NGC 6940), the X-ray count rate in PSPC channels 41-240 corresponding to 0.4–2.4 keV (Belloni & Verbunt 1996; Belloni & Tagliaferri 1997), the spectral type from literature (Garrison 1972; Rebeirot 1970; Larsson-Leander 1964), the spectral type as derived from our low-resolution spectra, the flux in the Ca II H&K lines FCa, the heliocentric radial velocity vrad and projected rotational velocity vrot sin i. ID VB−VPorb ctrate sp.type sp.type FCa vrad vrot sin i (days) (s−1) (literature) (erg s−1 cm−2)(kms−1)(kms−1) NGC 752 H 156 13.77 0.87 0.0036(6) G9-K0V H 182 12.24 0.61 0.0038(6) F5-7V H 205 9.92 0.42 1.45 0.0045(6) F3IV-Vp H 209 9.74 0.061 1574a 0.0018(5) B9.5V 10(3) H 214 10.47 0.38 rot 0.0039(6) F2IV 6.1(4) 64(5) H 235 11.47 0.48 0.4118 0.0020(4) F5 H 313 13.39 0.83 1.95 0.0062(7) H3b 9.57 1.00 <0.0084 K0III gG4-G8 8.7(2.2) 10−14 4.7(2) – H11b 9.29 0.97 <0.0014 gG 5.2(2) – NGC 6940 VR 84 10.91 0.83 54.2 0.0020(5) F8 F4-F6 1.4(4) 4–5 VR 100 10.43 0.74 82.5 0.0011(4) F5c F4-F6 –15.8(1) 1–2 VR 108 11.19 1.04 0.0018(5) gG5 gG4-G8 1.3(0.3) 10−14 7.7(1) 5–6 VR 111 11.56 1.05 3595 0.0011(4) gG5 gG5-G9 5.8(1.5) 10−15 10.0(6) 3–4 VR 114 11.17 1.27 0.0012(5) gG8 gG9-K0 2.4(0.6) 10−15 –9.1(7) 1–2 VR 92b 11.77 1.20 549.2 <0.0010 gG8 gG5-G9 6.4(1.6) 10−15 8.1(4) 2–3 VR 152b 10.84 1.07 <0.0005 gG8 gG5-G9 8.2(5) 1–2 aPreliminary solution by Latham et al. (private communication). bComparison stars, 2σ-upper limit to the PSPC count rate determined from re-analysis of ROSAT data). cSpectrum is composite (Bidelman suggests G8II+A7, see Walker 1958). with age-estimates ranging from 0.6 Gyr (Carraro & Chiosi 1994) to 1.1 Gyr (van den Bergh & McClure 1980). Four X-ray sources were identified with cluster members (Belloni & Tagliaferri 1997, Fig. 2). VR 111 is a spectroscopic binary with a 3595-day eccentric orbit, too wide for tidal interaction (Verbunt & Phinney 1995). For VR 108 the limit on the radial-velocity variations ex- cludes orbital periods shorter than 4000 days (Mermilliod & Mayor 1989). VR 114 is not a proper-motion member (Sanders 1972) and therefore not included in photomet- ric and radial-velocity studies of the cluster. However, Belloni & Tagliaferri derived that it is located at a sim- ilar distance as the cluster (∼900 pc, Larsson-Leander 1964) and suggested that the star could somehow be re- lated to the cluster. We are mainly interested in these three stars. The other sources, VR 100 and VR 84, are spectroscopic binaries with circular orbits shorter than 100 days, indicative for tidal interaction. VR 100 is a double-lined spectroscopic binary which explains its posi- tion in the colour-magnitude diagram between the turnoff stars and the giants. Fig. 1. Colour-magnitude diagram of NGC 752 showing the We have obtained low- and high-resolution spectra to member stars according to Daniel et al. (1994). X-ray sources look for optical signatures of X-ray activity.