IX Ophiuchi: A High-Velocity Star Near a Molecular Cloud

G. H. Herbig
2005 Astronomical Journal  
The molecular cloud Barnard 59 is probably an outlier of the Upper Sco/ρ Oph complex. B59 contains several T Tauri stars (TTS), but outside its northwestern edge are three other emission-Hα 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 (1. 6) double. The brighter component is a HAeBe star; the fainter a K-type TTS. The complex
more » ... R 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 spectrum 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. Hα and Hβ are the only prominent emission lines. They are broad, with variable central reversals. However, the most unusual characteristic of IX Oph is the very high (heliocentric) radial velocity: about −310 km s −1 , common to all spectrograms, and very different from the radial velocity of B59, about −7 km s −1 . There is no detectable Li I λ6707 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 λ6707 shows 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 = 357 • ). (2) A stronger possibility is that it is a 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 Hα emission like that found in IX Oph, and in one example, emission lines of neutral metals and double absorption lines as in IX Oph.
doi:10.1086/431759 fatcat:zthnazajj5ajzfcf3klohikpvu