Astron. Astrophys. 343, 273-280 (1999)

4. Individual stars

4.1. HD 11753 = HR 558 = Phe

Dworetsky et al. (1982) suggested that the radial velocity of the HgMn star HD 11753 is variable with a period longer than 30 days.

By combining our radial velocities of HD 11753 with the measurements of Dworetsky and co-workers, we found that the most probable orbital period is 41.4890.019 days (Fig. 1).

Fig. 1. Radial velocity curve of HD 11753. Squares represent Dworetsky et al. (1982) observations, circles our observations. The full-drawn curve is a least-squares fit of data using Eq. (1). Relative orbital parameters are listed in Table 4.

In the Oblique Rotator Model (ORM) proposed by Stibbs (1950), the common period of the photometric, spectroscopic and magnetic variations for a CP star is the stellar rotational period. The photometric observations obtained by the Hipparcos satellite (SP-ESA 1200, Vol. 17) for HD 11753(=HIP8882) present no evidence of periodic variability: H = 5.1070.008. Thus it is not directly possible to know if the orbital and rotational motions are synchronised.

From our spectra, we measured a projected rotational velocity = 14 km s^-1. For this value, the relation:

(where i is the angle between the rotational axis and the line of sight, is the stellar radius in solar radii, velocities are in km s^-1 and P is the stellar rotational period in days) gives 13 R if P is equal to the orbital period. This value of the stellar radius is too large for a main sequence star with T K and makes it implausible that the rotational and orbital periods for HD 11753 are synchronized.

4.2. HD 15144 = HR 710

Babcock (1958) found that the A5 SrCrEu star HD 15144 presents radial velocity variations with a 2.997814 day period. Even though the few photometric observations of this star obtained by van Genderen (1971) appear to be variable with the orbital period, there is no evidence of periodic variability in the Hipparcos photometric data of HD 15144 (= HIP 11348): H = 5.920.01. This result confirms Adelman & Boyce (1995) statement that HD 15144 is not a photometric variable. According to Bonsack (1981), HD 15144 has a rotational period equal to the 2.997824 day orbital period. Moreover Bonsack concluded that the magnetic field variations, with a 15.88 day period, are intrinsic variations in the field strength or geometry. Tokovinin (1997) has recently determined an orbital period equal to 2.997812 0.000004 days.

The least-squares fit of Bonsack (1981), Tokovinin (1997) and our radial velocities by using Eq. (1) gives the orbital period equal to 2.99781 0.00001 days (Fig 2). The eccentricity is very low (e = 0.04): HD 15144 is one of the few CP stars with circular orbit.

Fig. 2. Radial velocity curve of HD 15144. Triangles represent Bonsack et al. (1981) observations, squares Tokovinin (1997) observations and circles our observations. The full-drawn curve is a least-squares fit of data using Eq. (1). Relative orbital parameters are listed in Table 4.

In the framework of the ORM, the period of the magnetic field variation is the stellar rotational period, thus we should conclude that in spite of the short orbital period and the almost circular orbit the HD 15144 binary system is not synchronised. Anyway as our observations span a seven day interval, they rule out a spectral variability with a 15.88 day period and support the 2.99781 day period. If we consider that the effective magnetic field variation, reported by Bonsack (1981) assuming the 15.88 day period, is not accurately defined it cannot be excluded that the HD 15144 binary system is synchronised. Further spectroscopic observations and measurements of the effective magnetic field should be obtained to check the rotational period of HD 15144.

4.3. HD 25267 = HR 1240 = Eri

The A0 silicon star HD 25267 is the brightest component of a binary system whose orbital period is equal to 5.95367 days (Sahade 1950). According to Borra & Landstreet (1980) this period is also representative of the magnetic field variation. Manfroid et al. (1985) found that the HD 25267 binary system shows two periodicities in the photometric variations. The 1.210005 day period, being also representative of the magnetic variation, is attributed to the CP component. The origin of the variation with the second period (3.8 days) remains uncertain, as the spectral lines of the secondary component are almost invisible.

By combining our radial velocity measurements with those by Sahade (1950), we found that the most probable orbital period for the HD 25267 binary system is 5.9538 0.0001 days (Fig 3).

Fig. 3. Radial velocity curve of HD 25267. Squares represent Sahade (1950) observations, circles our observations. The full-drawn curve is a least-squares fit of the data using Eq. (1). Relative orbital parameters are listed in Table 4.

Our measurement of the projected radial velocity (30 km s^-1), the stellar radius () measured by North (1998) from Hipparcos parallaxes and the measurements by Borra & Landstreet (1980) of the effective magnetic field seem to exclude synchronisation for the HD 25267 system. Eq. (4) gives an inclination for P 1.2 days and for P 5.9 days. As the effective magnetic field changes from 0 to -400 gauss during a rotational period, an inclination angle i close to has to be ruled out.

On the hypothesis that the rotational axis of HD 25267 is perpendicular to the orbital plane we can estimate the mass of the secondary star. North (1998) has determined the mass of HD 25267 as equal to 3.35 ; for this value of and Eq. (3) gives . This value is typical for a main sequence A5 star whose characteristics are consistent with the statements by Manfroid et al. (1985) on the HD 25267 binary system. These authors found that the spectral lines of the secondary star are almost invisible and the MgII line can be attributed to the secondary star.

Jaschek & Jaschek (1976) compared the frequency of for normal and CP stars. They noted that normal stars peak at and that there is an excess of companions of low mass in CP star binary systems. As to HD 25267, we obtain which confirms Jaschek & Jaschek's (1976) conclusion.

4.4. HD 36485 = HR 1851 = Ori C

Morrell & Levato (1991) noted that there is some confusion in the literature concerning the observations of the helium-strong star HD 36485. By combining their observations with data from the literature, Morrell & Levato (1991) found that HD 36485 shows radial velocity variations with a 9.9144 day period and km s^-1. Bohlender (1994) found that HD 36485 shows emission features in the H line with a 1.4778 day period. Because of possible emission also in the helium lines, we have measured the radial velocity of HD 36485 from the two carbon lines at 657.8 and 658.3 nm.

Our radial velocities are incompatible with the 1.4778 day rotational period and they exclude that we are observing the rotating non-homogeneous stellar surface of HD 36485. Moreover the measured radial velocity does not vary with the orbital period given by Morrell & Levato (1991).

Combining our data with the observations by Abt (1970), we found that the most probable orbital period is 25.592 0.001 days. In this case, it appears that the radial velocity amplitude is very small, only 8.0 km s^-1. Such a value is consistent with a constant effective magnetic field (Bohlender et al. 1987) and an inclination of the rotational axis of for HD 36485 (Bohlender 1989).

Assuming T = 19000 K for HD 36485, Bohlender (1989) determined: a ) M = 82 M and R = 62 R on the hypothesis of a helium-rich spot on the stellar surface, b ) M = 113 M and R = 104 R if helium is stratified in the atmosphere. We can thus estimate the mass of the secondary star of the binary system assuming that the HD 36485 rotational axis is orthogonal to the orbital plane. From Eq. (3), for the two previous hypotheses the values for the secondary star mass are 4.0 M and 4.9 M respectively. In this case and HD 36485 provides further confirmation that in binary systems with a primary CP star the secondary star is not massive.

Fig. 4. Radial velocity curve of HD 36485. Squares are Abt (1970) observations, circles our observations. The full-drawn curve is a least-squares fit of data using Eq. (1). Relative orbital parameters are listed in Table 4.

Since the secondary star is at least one V magnitude fainter than HD 36485 and the rotational velocity (= 32 km s^-1 Bohlender 1989) is much larger than the orbital radial velocity, as observed, we are dealing with a single line spectroscopic binary system.

As the rotational period is equal to 1.4778 days and the orbital period to 25.592 days, the binary system of the helium-strong star HD 36485 is not synchronised.

4.5. HD 37017

Blaauw & van Albada (1963) suggested that the helium strong star HD 37017 is a spectroscopic binary with an orbital period of 18.65 days. Morrell & Levato (1991) concluded that the orbital period is 18.622 days.

Bohlender et al. (1987) found that the rotational period is 0.901195 days and that the inclination of the rotational axis is in the range . Moreover, these authors report that according to Dr C.T. Bolton the orbital inclination is between and and that the secondary is approximately 1 mag fainter than HD 37017.

We have observed HD 37017 in six consecutive nights at LaSilla in October 1992 with the 1.5m telescope and in six consecutive nights at CASLEO in December 1995. The radial velocity variation is not periodic with the stellar rotational period determined by Bohlender et al. (1987).

Combining our data and those of Blaauw & van Albada (1963), we found two possible orbital periods: 1.056576 and 18.6556 days (Fig. 5). By matching the H line profile and the visible flux distribution, and taking into account the helium abundance, Leone (1998) has determined the effective temperature of HD 37017 equal to 19000 K. For a main sequence star with this effective temperature the expected mass is 7.6 M. Solving Eq. (3) for the determined values and , for the secondary star we get 1.4 M mass for the shortest period and 4.5 M mass for the longest one. Thus the spectral type of the secondary star, if a main sequence star, should be F5 and B7 respectively. If the secondary star is one magnitude fainter than HD 37017, as stated by Bolton, the shortest period must be ruled out, as a F5 star would be five magnitudes fainter in the visible than HD 37017.

Fig. 5. Radial velocity curve of HD 37017. Squares represent Blaauw & van Albada (1963) observations, circles our observations. The full-drawn curve is a least-squares fit of data using Eq. (1). Relative orbital parameters are listed in Table 4.

Like HD 36485, the helium-strong star HD 37017 also belongs to a non-synchronised binary system.

4.6. HD 142096 = HR 5902 = Lib

Van Hoof et al. (1963) found that the radial velocity of the helium-weak star HD 142096 is variable. Combining our radial velocities with those of van Hoof and co-workers we obtain an orbital period equal to 12.4619 days.

No variability period is known for the helium-weak star HD 142096. Hipparcos photometry of this star (HIP = 77811) gives H = 5.030.01 without any clear evidence of variability.

From our spectra (R = 50,000), the measured projected rotational velocity of HD 142096 is 140 km s^-1. This value is close to the value given by Brown & Verschuren (1997) who measured 146 km s^-1. Assuming that the rotation period is equal to the orbital period (P = 12.4619 days), Eq. (4) gives R R and excludes that the orbital and rotational periods are coincident for the B3 star HD 142096.

Fig. 6. Radial velocity curve of HD 142096. Squares represent van Hoof et al. (1963) observations, circles our observations. The full-drawn curve is a least-squares fit of data using Eq. (1). Relative orbital parameters are listed in Table 4.

4.7. HD 189178 = HR 7628A

This star HD 189178 (= HIP 98194) is given as a suspected He-weak star in the General Catalogue of Ap and Am stars by Renson et al. (1991). Its rotational period has not been determined yet. From Hipparcos observations, it appears that the star is not a photometric variable with H = 5.437 0.006.

Combining our radial velocity measurements and those of Batten et al. (1982), we found that the orbital period is 70.23 0.02 days (Fig. 7).

Fig. 7. Radial velocity curve of HD 189178. Squares represent Batten et al. (1982) observations, circles our observations. The full-drawn curve is a least-squares fit of data using Eq. (1). Relative orbital parameters are listed in Table 4.

We measured a projected rotational velocity of 50 km s^-1 which is much smaller than the Uesugi & Fukuda (1970) value (115 km s^-1). Eq. (4) gives R R even for our value and excludes that orbital and rotational motions are synchronised.

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
helpdesk.link@springer.de