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Ix Ophiuchi: a High-Velocity Star Near a Molecular Cloud G

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Ix Ophiuchi: a High-Velocity Star Near a Molecular Cloud G The Astronomical Journal, 130:815–824, 2005 August # 2005. The American Astronomical Society. All rights reserved. Printed in U.S.A. IX OPHIUCHI: A HIGH-VELOCITY STAR NEAR A MOLECULAR CLOUD G. H. Herbig Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822 Received 2005 March 24; accepted 2005 April 26 ABSTRACT The molecular cloud Barnard 59 is probably an outlier of the Upper Sco/ Oph complex. B59 contains several T Tauri stars (TTSs), but outside its northwestern edge are three other H -emission objects whose nature has been unclear: IX, KK, and V359 Oph. This paper is a discussion of all three and of a nearby Be star (HD 154851), based largely on Keck HIRES spectrograms obtained in 2004. KK Oph is a close (1B6) double. The brighter component is an HAeBe star, and the fainter is a K-type TTS. The complex BVR variations of the unresolved pair require both components to be variable. V359 Oph is a conventional TTS. Thus, these pre-main-sequence stars continue to be recognizable as such well outside the boundary of their parent cloud. IX Oph is quite different. Its absorption spec- trum is about type G, with many peculiarities: all lines are narrow but abnormally weak, with structures that depend on ion and excitation level and that vary in detail from month to month. It could be a spectroscopic binary of small amplitude. H and H are the only prominent emission lines. They are broad, with variable central reversals. How- ever, the most unusual characteristic of IX Oph is the very high (heliocentric) radial velocity: about À310 km sÀ1, common to all spectrograms, and very different from the radial velocity of B59, about À7kmsÀ1. There is no detectable Li i k6707 line. There is reason to believe that IX Oph is actually a background object, only aligned with B59. Several conceivable interpretations are discussed: (1) It is unlikely that it is a high-velocity ejectee from the Upper Sco or Upper Cen-Lup associations (the lack of detectable k6707 shows that it is not the product of a very recent event, and the proper motion points in the wrong direction) or that it was born in or ejected from one of the distant high-velocity CO clouds at this longitude (l ¼ 357). (2) A stronger possibility is that it is simply a metal- poor high-velocity G- or K-type giant (but such stars are not irregularly variable in light and do not have such strong Balmer emission lines). More likely, (3) IX Oph is a member of the high-velocity, low-metallicity SRd class of semiregular variables found in the field and in some globular clusters. At some phases, those stars show H emission like that found in IX Oph and, in one example, emission lines of neutral metals and double absorption lines as in IX Oph. Key words: ISM: individual (Barnard 59) — stars: emission-line, Be — stars: formation — stars: individual (IX Ophiuchi, KK Ophiuchi, V359 Ophiuchi, HD 154851) 1. INTRODUCTION find pre-main-sequence (PMS) stars. As seen below, IX Oph is The molecular cloud Barnard 59 is an irregular mass of clumps a most unusual high-velocity object, and little is known about the others; so, in what follows, the spectra and circumstances of and streamers, largely enclosed within an area of about 300 ; 150 all three, and of a nearby Be star (HD 154851), are described in (Fig. 1). It is located between the Upper Sco association and the Galactic plane and is the northwestern extremity of the ‘‘fila- some detail. Of the four, only KK Oph is an IRAS point source, and it is the only one that lies in the IR excess region in the mentary massive dark cloud’’ called the Pipe Nebula by Onishi (J H, H K ) plane; the others fall within the conventional et al. (1999). Unlike the active star-forming region around Oph À À reddening band. about 12 to the northwest, B59 contains no bright stars. For the present purpose, the distance of B59 is assumed to be 140 pc, a 2. IX OPHIUCHI compromise between that of the Upper Sco association (about 0 145 pc; see Preibisch & Zinnecker 1999) and that of the star- IX Oph is located in a relatively unobscured area about 6 from the edge of B59. Possibly for that reason, and the fact of its forming region behind Oph (about 130 pc; Bertout et al. 1999). 8 Several faint variable stars were found in and around B59 by relatively weak H emission (W ¼ 1:3 , according to Cohen Swope (1928). Swope’s HV 4410 is now IX Oph (range 11.8– & Kuhi 1979), it has received relatively little attention from 12.7 pg) and HV 4413 is KK Oph (11.9–12.7 pg), both noted as PMS investigators. Several low-resolution Lick spectrograms of IX Oph were obtained in the 1970s and 1980s, but this paper ‘‘very near edge of dark nebulous cloud.’’ Years later, Merrill & 1 rests largely on three Keck HIRES spectrograms, two obtained Burwell (1950) found H emission in KK Oph (and called it 8 AS 220); in the 1950s and 1960s, these and several other H emit- on 2004 June 13 and July 24 that cover the 4350–6740 region (with interorder gaps) at a resolution of about 44,000 and a third ters were found in the area at Lick Observatory and by Stephenson 8 & Sanduleak (1977) and The (1964), and, more recently, another on 2004 September 24 covering 4400–8800 (also with gaps) (KW 002) was found by Kohoutek & Wehmeyer (2003). These at slightly higher resolution. H and H were in emission on all objects are identified in Figure 1, reproduced from the Digitized Sky Survey red image, and are listed in Table 1, which gives 1 The W. M. Keck Observatory is operated as a scientific partnership among the additional information. California Institute of Technology, the University of California, and NASA. The Three of these objects (KK, IX, and V359 Oph) lie in a clear Observatory was made possible by the generous financial support of the W. M. area well off the edge of B59, where one would not expect to Keck Foundation. 815 816 HERBIG Vol. 130 Fig. 1.—Barnard 59 region, from the red exposures of the Digitized Sky Survey. The stars discussed in this paper are indicated. The coordinates are for equinox J2000.0, but the coordinate marks along the margins are only approximate. occasions. Herbig (1962) at very low photographic dispersion overlaps IS D2. Note that there are two additional sets of much classified the absorption spectrum as type Fpe, while Cohen & weaker IS Na i lines at about À40 (itself with a weaker, blended Kuhi (1979) at somewhat higher scanner resolution called it component slightly longward) and À97 km sÀ1. The splitting of type G, but at high resolution the spectrum now appears to be stellar D2 (at 5883 8) is discussed in x 2.1. (The synthetic very peculiar. In particular, all lines are quite narrow, are weaker telluric absorption spectrum in this region by Lundstro¨m et al. than in normal G-type dwarfs, and show a variety of structures [1991] was appropriately scaled and used to remove the many depending on ion and excitation level, the details of which atmospheric H2O lines.) change between the HIRES exposures. All the HIRES spectrograms confirm this same large nega- The most striking feature of the spectrum, however, is the tive velocity of IX Oph. The only other relevant information large negative radial velocity; all lines agree that the average (known to the writer) comes from a low-resolution (R about velocity2 is near À310 km sÀ1. Figure 2 (from the HIRES spec- 4700) CCD spectrogram of IX Oph obtained with the coude´ trogram of 2004 June 13) shows this very well. The strong pair spectrograph of the 120 inch (3 m) telescope at the Lick Ob- of Na i lines at 5889 (D2) and 5895 (D1) 8 are interstellar (IS) servatory on 1987 June 7. It covered only the 6400–6800 8 at a (heliocentric) velocity of about À3kmÀ1, but the stellar region, so contained only six unblended absorption lines, from À1 Na i lines are shifted shortward by 5 8, so that the stellar D1 which the mean velocity is À326 Æ 6kms .Thesamesix lines measured on the HIRES spectrogram of 2004 June 13 give À1 2 All velocities in this paper are heliocentric. At the position of IX Oph, the À310 Æ 3kms . The difference is only about 2 ,hardly correction to LSR is +10.4 km sÀ1. enough to demonstrate that the velocity was different in 1987, TABLE 1 H -Emission Stars In and Near B59 Star Other (J2000.0) (J2000.0) V V359 Oph .................................... TH 27-1a 17 08 54.29 À27 12 32.1 12.2 var HD 154851 .................................. ... 17 09 00.93 À27 25 02.6 10.4 IX Oph ......................................... ... 17 09 48.16 À27 16 59.3 10.9–11.5 KK Oph AB................................. AS 220 17 10 08.04 À27 15 18.9 9.4–12.9 LkH 345 Oph ............................ ... 17 10 48.06 À27 40 51.1 13.0 var LkH 346 Oph AB ..................... TH 27-4 17 11 04.01 À27 22 57.5 (14.7–15.4 pg) KW 002........................................ ... 17 11 16.29 À27 25 14.3 ... TH 27-5 ..................................... ... 17 11 41.80 À27 25 48.0 13.6 LkH 347 Oph ............................ TH 27-6 17 12 00.20 À27 20 17.8 (15.8 to <16.5 pg) Note.—Units of right ascension are hours, minutes, and seconds, and units of declination are degrees, arc- minutes, and arcseconds.
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