3.1. OB supergiants
4U0900-40/HD77581, 4U1700-37/HD153919, Cyg XR-1/HDE226868, and HD152667 are all binary systems with an OB supergiant primary (Dolan & Tapia 1988; 1984; 1989). No member of this group has a measurable parallax (Table 1). If each of these systems is at a distance consistent with its spectroscopic parallax, then 4U0900-40, 4U1700-37 and Cyg XR-1 have peculiar tangential velocities t km s-1, while the 2 upper limit on the peculiar tangential velocity of HD152667 is t km s-1. The major difference between these systems is that 4U0900-40, 4U1700-37 and Cyg XR-1 have a collapsed secondary, while HD152667 apparently does not (Dolan & Tapia 1984). The commonly accepted evolutionary path of neutron stars and black holes in binary systems requires them to be formed in a supernova event (Canal et al. 1990). If the event occurs in a binary system that remains bound afterwards, then a large space velocity of the system, generally defined to be km s-1, should result (Blaauw 1961; Hills 1983). Each system's peculiar tangential velocity agrees with this prediction of the Blaauw scenario: the three systems with a secondary formed in a supernova event have a large peculiar tangential velocity, while the system without such a secondary does not.
If we locate 4U0900-40 at its spectroscopic-parallax distance, then the peculiar tangential velocity we derive from its proper motion as measured by Hipparcos (40 km s-1) agrees both in magnitude and direction with that derived by Kaper et al. (1997a) from observations of the bow shock wave caused by the motion of the system through the interstellar medium. Kaper et al. (1997b) estimate a space velocity of 50 km s-1 (no uncertainty stated) for 4U0900-40 relative to the velocity of the nearby Vel OB1 association.
3.2. Be/X-ray sources
Three of the sources in our sample, X Per, A0535 +26/HDE245770, and 4U1145-619(= Cen X-5)/HD102567, are X-ray binary systems with Be star primaries. In each case, a compact secondary is the X-ray source (Dolan et al. 1998). 4U1145-619 has a measured parallax that is significant at the 2 level (Table 1). Only a lower limit to the distance to X Per and A0535+26 can be obtained from the Hipparcos observations.
Chevalier & Ilovaisky (1998, hereafter CI) included all three sources in a survey of 13 Be/X-ray binary systems. CI found a distance of 830 (+2870, -330) pc to X Per, 330 (+450, -120) pc to A0535+26, and 500 (+500, -200) pc to 4U1145-619. These distances are consistent with the distance we find for 4U1145-619 and the lower limits we find for X Per and A0535+26. CI interpret the Hipparcos results for A0535+26 and X Per as measurements of their parallax because they apparently selected a significance criterion C . (The Hipparcos parallax measurements for these two sources have R = 1.3 and 1.7, respectively.)
The distances CI derive for several Be/X-ray binary systems are significantly smaller than the previously accepted spectroscopic-parallax distances for these systems. CI concluded that Be/X-ray binaries are relatively nearby, low-velocity systems (d kpc, t km s-1). Steele et al. (1998) and Neguerela (1998) disagreed with the distances CI derived to A0535+26 and LSI +61o 303, and argued from other lines of evidence that the larger spectroscopic-parallax distance to these systems was correct. Our interpretation of the Hipparcos parallax results as lower limits allows the astrometric distances to X Per and A0535+26 to be consistent with the spectroscopic-parallax distances.
The 90% (1.6) confidence interval on the Hipparcos parallax measurement of 4U1145-619 extends to 0.43 arcsec, which corresponds to a distance of 2.3 kpc. (The best estimate of its distance given in Table 1 is 510 pc. The uncertainty on the best estimate means that if the measurement were repeated many times using the same technique, the distribution of the values obtained for the best estimate would lie in the range 120 pc to 900 pc 90% of the time.) Neguerela (1998) gives an estimate of 3.1 kpc (no uncertainty stated) for the spectroscopic-parallax distance of 4U1145-619. The distance ranges corresponding to the 90% confidence intervals on the spectroscopic and astrometric parallaxes of 4U1145-619 probably overlap.
The hot DA white dwarf HZ43 (Margon et al. 1976), a soft X- ray source and high proper motion spectroscopic binary, was included in the trigonometric parallax program of the U. S. Naval Observatory. Dahn et al. (1982) found the system's parallax to be 15.5 mas and its proper motion 176 mas toward 235.8o o. The Hipparcos and USNO parallaxes differ by 1.8 times their the combined standard error, which is not significantly different under the criterion C = 2. The proper motions are consistent with each other.
© European Southern Observatory (ESO) 1999
Online publication: October 14, 1999