Astron. Astrophys. 354, 881-891 (2000)

4. Results for Hyades

Six Am stars in the Hyades had already published orbits. Our elements (Table 4) confirm the previous ones (Abt 1961, 1985; Conti 1969), except for vB 83 and vB 131.

Table 4. Orbital elements of Am spectroscopic binaries in the Hyades

4.1. vB 38 (HD 27628)

vB 38 classified Am (A5/F0/F2) by Abt (1995), was already observed as a single-lined spectroscopic binary by Abt (1961). Our orbital parameters are in good agreement with Abt's (1961) elements. The period is short, 2.143 days, and the eccentricity is e = 0.00.

vB 38 is one of the two Am stars in the Hyades that has been detected in X-ray from ROSAT All-Sky Survey (RASS) (Stern et al. 1995). However its luminosity is about 20 times fainter than that of KW 224 or KW 229 and 3 times lower than that of KW 40. The luminosity is compatible with a solar-type companion or a M-type dwarf. The mass function ( = 0.04860) favours the first solution: the minimum mass is 0.75 for a 1.5 primary. It may also happen that the X-ray emission is enhanced by the short binary period. No evidence for a secondary dip has been seen in CORAVEL observations.

Fig. 6. Radial-velocity curve of vB 38. The period is 2.14361  0.00006 days.

4.2. vB 45 (HD 27749)

Abt (1961) had already detected vB 45 (A2/F0/F2, Abt 1995) as a single-line spectroscopic binary and determined a first orbit, which was later improved (Abt 1985). The observations give a short period of , which agrees well with Abt's (1985) value, and an eccentricity equal to 0.000 0.007. A circular orbit with a period of is compatible either with the cut-off limit for the Hyades based on solar-type stars (, Mathieu et al. 1992) or with the results of Matthews & Mathieu (1992): stars with an A-type primary may have circular orbits with periods up to .

The mass function gives a minimum mass for the secondary of 0.70 , but no evidence for the presence of a secondary has been seen in our observations. vB 45 has not been detected in X-ray (Stern et al. 1995). This is surprising because nearly all solar-type stars in the Hyades have been detected by ROSAT.

Fig. 7. Radial-velocity curve of vB 45. The period is 8.4177  0.0001 days.

4.3. vB 83 (HD 28546)

vB 83, classified A8/A8/F2 by Abt & Levy (1985) is a single-lined spectroscopic binary with an orbital period of and a small amplitude of 2.53 km s^-1. The mean errors on the radial velocities reflect the limit inherent to CORAVEL for measuring Am stars and the effect of rotation. But the orbit is well defined (see Table 4 for the orbital parameters) and has a (O-C) of 0.95 km s^-1, lower than the mean error of each radial-velocity and we assume that the orbital motion is real. During the last 20 years, the mean radial velocity did not change. vB 83 belongs to the small group of Am stars having orbital periods more than 30 days. vB 83 has not been detected in X-ray (Stern et al. 1995).

Fig. 8. Radial-velocity curve of vB 83. The period is 58.37  0.03 days.

4.4. vB 130 (HD 33254)

vB 130, classified Am (A2/A7/F2) by Gray & Garrisson (1989), was not taken into account by Abt (1961) during his research of duplicity on Am stars. However, Conti (1969) measured vB 130 during his observations of magnetic Am stars. He detected a magnetic field through the Zeeman effect and also detected a variation in radial velocity. He determined an orbit of this single-lined spectroscopic binary with a period of and e = 0.67.

CORAVEL observations began in 1979 and ended in 1993 with a 10-year gap between 1983 and 1993, without observation. Fig. 9 shows the orbital curve with the parameters listed in Table 4. The eccentricity is not well constrained by our observations and the differences observed between our elements and Conti's elements are probably not significant.

Fig. 9. Radial-velocity curve of vB 130. The period is 155.497  0.099 days.

vB 130 has not been detected in X-ray (Stern et al. 1995).

4.5. vB 131 (HD 33204)

vB 131 is classified Am (A9/A9/F2) by Abt (1995), and is the primary component of the triple system ADS 3730. The secondary is vB 132 (ADS 3730 BC). The separation AxBC is 11 and BxC, 04. Heintz (1976) derived a visual orbit of 32 yrs for the visual binary ADS 3730 BC. vB 131 shows a radial-velocity dispersion which is larger than the standard radial-velocity error and 0.000. A Fourier analysis gives a possible period of . No long-term variation is obviously seen in a simple plot of the radial-velocity in function of time (from 1979 to 1999). The orbital solution shown in Fig. 10 is fitted with the value of the Fourier period. The orbital parameters listed in Table 4 represent a possible solution, but due to the small amplitude of the orbit, and to the radial-velocity errors, the orbital solution is not absolutely certain.

Fig. 10. Radial-velocity curve of vB 131. The period is 32.528  0.009 days.

During some observing runs an observation of vB 132 was obtained. vB 132 does not show any radial-velocity variation either on short or long time scales. The separation between vB 131 and vB 132 leads us to predict a long period system and a small radial-velocity variation, which CORAVEL is not able to measure.

vB 131 has been detected in X-ray: = 3.9 (Stern et al. 1995), which could be produced by a low-main-sequence star, which is in good agrement with the orbital parameters.

4.6. vB 169 (HD 40932)

vB 169, classified Am (A4/A5/A7) by Abt (1995), is a visual hierarchical quadruple system. Frost (1906) was the first to observe the single-lined binary system. Then Aitken (1914) observed the visual companion. Alden (1942) computed a visual orbit by using some of the spectroscopic elements. He remarked that the visual secondary contains nearly half of the mass of the system (Osvalds 1964).

Fekel (1980) performed a detailed analysis of the whole system. He found that each visual component is itself a spectroscopic binary. The components are noted Aa, Ab and Ba, Bb. A is SB1 with a period of 4.45 days and B is SB2 with a period of 4.48 days. The visual orbit has a period of 18.2 years. Fekel (1980) has answered another important question: which component is the Am star? He observed that Aa is the Am star. Unfortunately the Am star belongs to the SB1 system, which contributes little information about the secondary.

CORAVEL observations began in 1979 and ended in 1993. Due to the 19-year binary motion, a simple analysis of this system will not provide a good orbital solution. To perform a global solution of the two periods simultaneously, we had to include the radial velocities of Fekel (1980), who was the only observer who measured the visual secondary's radial-velocity, and also the speckle observations listed in the CHARA interferometric catalogue (Hartkopf et al. 1999). Fig. 11 shows the short period () SB1 Aa, corrected for the perturbation from the visual companion. Fig. 12 shows the radial-velocity orbital solution for the visual binary. The triangles (55 observations) represent the Aa radial velocities values and the squares the Bab values from Fekel (1980). Note that the B sample is very poor due to the difficulty of detecting the corresponding lines. The observation of the radial-velocities were not obtained in a favorable phase, so the secondary amplitude is not well defined. The radial-velocity orbital elements are listed in Table 4. Fekel's (1980) radial-velocity solution for B is not reproduced here. Fig. 13 shows the visual orbit. The dotted line represents the motion of the secondary around the primary. Due to the large eccentricity of the visual system, the highest precision is needed and we used only the latest data given on CHARA web site (Hartkopf et al. 1999).

Fig. 11. Radial-velocity curve of vB 169 A. The period is 4.44757  0.00005 days.

Fig. 12. Radial-velocity curve of vB 169 AB. The period is 6952.7  202.0 days. The radial-velocities of the secondary Bab (square) were measured by Feckel (1980) at a phase where they are close to the systemic radial-velocity of AB.

Fig. 13. Speckle orbit of vB 169 AB. The period is 6952.7  202.0 days.

The combined analysis of the vB 169 system gives a good estimation of the masses of each visual component, for the primary M(Aa + Ab) = 3.07 0.95 and M(Ba + Bb) = 2.40 0.28 for the secondary.

Assuming that the primary Aa is the Am star of the system, and a mass of about 2 , the mass of the secondary is about 1 . Thus the inclination of the orbit is about 30^o, which is different from the inclination of the visual orbit (i=60^o). Although it is often stated (Batten 1973) that orbital planes, in visual system, are not coplanar. The mass ratio () is in agreement with the observation that the secondary Ab was never seen in the radial-velocity correlations.

For the secondary visual component B, the mass ratio between Ba and Bb is 0.984 Fekel (1980), which allows the determination of the masses of each stars, 1.21 for Ba and 1.19 for Bb.

ROSAT X-ray emission has been observed from vB 169, with a luminosity of = 29.25 (Hünsch et al. 1998). The detection of this system seems to be normal because there are two or three lower-main-sequence stars in this system. But the flux is not abnormally large owing to the number of possible emitters and the short periods of both binary sub-systems.

© European Southern Observatory (ESO) 2000

Online publication: February 25, 2000
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