OMP, University of Cambridge and Brera Astronomical Observatory

A Definitive Orbit for the Visual Binary ADS 8630 = gamma Virginis

(Scardia et al., 2007, Astron. Nachr., 328, 2, 146--153)

Gamma Virginis (Sigma 1670 = ADS 8630) is one of the first binary stars discovered in history. The first measurement of the relative positions of its components goes back to March 1718 by Bradley and Pound, but accurate measurements started only with F. G. W Struve in 1820. Since then, this binary was intensively followed by the observers so much that the Washington Double Star (WDS) Catalogue contained 1515 mean measures of its position angle and separation (version of February 2006). However, despite all those efforts, and the several tentative orbits derived from those data, the position-angle ephemerides computed on the basis of the latest orbit (Söderhjelm, S., 1999, A&A, 341, 121 failed to represent the position angles observed during the periastron passage in mid-2005. This was due to the fact that the period of revolution (estimated between 130 and 200 years) was insufficiently known, both because of the large eccentricity of the orbit (e > 0.88) and the fact that the first observed periastron passage in 1836 was poorly followed because of the lack of adequate telescopic power. To solve this problem, gamma Virginis was regularly observed at the INAF - Osservatorio Astronomico di Brera in Merate between 2004 and 2006. The observations were performed with the Pupil Interferometry Speckle camera and COronograph (PISCO) placed at the Cassegrain focus of the 102cm Zeiss telescope.

By combining 35 speckle measurements made in Merate with all the data available in the WDS Catalogue plus other three recent visual observations, for a total of 1553 measurements, a new orbit was computed, allowing to represent all the observed positions angles and separations with low residuals. The elements of such an orbit, with related errors, are printed in Table 1, where Omega is the position angle of the ascending node, omega the longitude of periastron, i the inclination of the orbit relative to the plane of the sky, e the eccentricity, T the epoch of the last periastron passage, P the period of revolution, and a the semi-mayor axis in arcseconds. The apparent orbit is shown in Fig. 1, with an expanded view in Fig. 2. This last figure shows the importance of the 2004-2006 PISCO observations for determining the orbital elements: they cover an arc of 130° around the periastron.

New orbital elements for gamma Virginis

Omega (2000) (°)	omega (°)	i (°)	e	T (yr)	P (yr)	a (")
35.36	255.03	149.41	0.8815	2005.509	169.101	3.641
± 0.41	± 0.37	± 0.16	± 0.00018	± 0.0019	± 0.011	± 0.008

Fig. 1 - Left panel: the new apparent orbit of gamma Virginis with all the observations utilized for the orbit computation.

Fig. 2 - Right panel: enlarged view of Fig. 1 near the periastron; note that the new orbit fits closely the observations of PISCO.

 

Using the Hipparcos parallax of 0.08453" ± 0.00118" (ESA, 1997, The Hipparcos and Tycho Catalogues, ESA SP-1200), the semi-major axis turns out to be 43.1 AU , and the total masse of the system M_star-A + M_star-B = 2.8 M_Sun with an error of 4.2%, which is consistent with the F0V spectral class of the two components. The ephemeris for the years 2006-2015 is given in Table 2, where the Epoch is in Besselian Years and rho and theta are the predicted separation and position angle, respectively.

New ephemeris of gamma Virginis

The possibility of the presence of a third star in the system, that was proposed by other authors, was also investigated. To this end, high-angular resolution infrared images of gamma Virginis were obtained on June 5th 2006 with the LuckyCam instrument on the 3.6m ESO New Technology Telescope (NTT) at La Silla Observatory, Chile. The images (Fig. 3) do not show any companion as faint as a M0V star at a distance larger than 0.4". This result, combined with the analysis of the residuals of the new orbit, the values found for the masses of the individual components (1.4 M_Sun with an error of 3%) and the published radial velocity measurements, allows to rule out the presence in the system of a third companion with a masse larger than 0.3 M_Sun.

Fig. 3: An SDSS i-band image of gamma Virginis A and obtained with the LuckyCam on the 3.6m ESO NTT on June 5^th 2006, when the angular separation was 0.5". The image is a composite construction using the best 20% of 10,000 short exposure images for an effective exposure time of ~40 sec. Unsharp masking has been applied to facilitate the detection of faint companions. No third star is visible in the 14.6" X 5.7" field, and a lower limit for the magnitude of this companion in the SDSS i-band (used for the Sloan Digital Sky Survey, close to the I band) is estimated to be m_i ~ 7 at 0.4" from the primary.

A detailed paper presenting those results was submitted in August 11th 2006 and published in Astronomische Nachrichten (Scardia et al., 2007, Astron. Nachr.  328, 2,  146--153). A shorter version was presented as a poster at the IAU Symposium 240 held in Prague between 22 and 25 August 2006, and should be published in the Proceedings. 

The present work was made by:

Marco Scardia, INAF- Osservatorio Astronomico di Brera, Merate, Italy,

Robert W. Argyle, Institute of Astronomy, Cambridge, UK,

Jean-Louis Prieur, UMR 5572 d'Astrophysique- Observatoire Midi-Pyrénées, Toulouse, France,

Luigi Pansecchi, - INAF- Osservatorio Astronomico di Brera, Merate, Italy,

Stefano Basso, - INAF- Osservatorio Astronomico di Brera, Merate, Italy,

Nicholas M. Law, - Institute of Astronomy, Cambridge, UK,

Craig D. Mackay, - Institute of Astronomy, Cambridge, UK.