The Binarity of Herbig Ae/Be Stars Observed with Adaptive Optics

Patrice Corporon

Université de Montréal, C.P. 6128, Succ A,
Montréal, QC H3C 3J7, CANADA
Electronic-mail: corporon@astro.umontreal.ca

Jérome Bouvier

Laboratoire d'Astrophysique de Grenoble,
BP 53, 38041 CEDEX, FRANCE
Electronic-mail: bouvier@obs.ujf-grenoble.fr

and

Jean-Luc Beuzit

Canada-France-Hawaii Telescope,
P.O. Box 1597, Kamuela, HI 96743, USA
Electronic-mail: beuzit@cfht.hawaii.edu

Abstract:

We have done a large systematic search for HAeBe binaries using CFH (and ESO) Adaptive Optics (AO) systems. Among the 68 HAeBe stars observed, 30 binaries (18 discovered) have been detected. Based on our multi-color AO images, spectral types of twenty-two visual companions have been determined. A trend is found such that companions of Ae stars are low-mass T Tauri stars (spectral type K-M), while companions of Be stars are intermediate mass stars (A-F). Companions usually have no infrared excess, nor do primaries with massive companions. Furthermore, X-ray emission in some HAeBe stars may well be explained by the presence of a T Tauri companion.

Introduction

  Multiplicity is a major issue in stellar astrophysics. First, any stellar formation theory must explain the large abundance of multiple systems among Main Sequence and young low-mass T Tauri stars. Second, binary studies allow the direct determination of physical parameters.

In the case of Herbig Ae/Be (HAeBe) stars, the binarity status is not well known. Until our work, few single color speckle images have been made on a relatively small number of binaries (Leinert et al. 1997, Pirzkal et al.1997). Our detailed study provides new information on the multiplicity status of Ae/Be Herbig stars.

Observations

  Observations have been carried out in both hemispheres, using PUEO (Rigaut et al. 1998) at CFHT (Hawaii) and ADONIS (ESO 3.6m) from December 1993 to December 1996. Our sample consists of 68 HAeBe stars selected from tables 1 & 2 of Thé et al.the94 (1994), with m_V <14-15.

In total, 30 visual binaries have been detected, 18 of which are new detections ($\rho_{lim}=0.13'', \Delta\,m_{{K}_{max}}=6.5$). Figure 1 shows some sample PUEO binaries (Corporon 1998, Bouvier et al. 1998).

 

Figure 1:   HAeBe binaries observed with PUEO at 2.2$\mu\mathrm{m}$ and view in 3D projection. Smallest peak is the infrared companion. Separation and magnitude difference $\Delta\,m_{{K}}$ are given for each system

Analysis

  Based on our multi-colors AO images (JHK, but also VRI thanks to the high capabilities of PUEO in the visible domain), spectral type of 22 visual companions have been determined for the first time. The trend is found that companions of Ae stars are low-mass T Tauri stars (spectral type K-M), while companions of Be stars are intermediate mass stars (A-F) (see Figure 2).

 

Figure 2:   Spectral energy distribution of 2 typical HAeBe binaries observed with PUEO. The vertical axis is the flux (erg s^-1 cm^-2) and the horizontal axis is the wavelength (cm) (both scales are logarithmic). Circles represent the primary star, triangles represent the companion. Dotted line are best fitted stellar models.

Within our restricted HAeBe sample, there are more X-ray emitters among binary stars compared to single stars, while non-X-ray emitters are more probably single stars. This suggests that X-ray emission in HAeBe stars may well be explained, in some cases, by the presence of a (T Tauri) companion.

The observed visual binary frequency for HAeBe stars is of the order of 50%. The excess of binary compared to Main Sequence distribution (Duquennoy & Mayor 1991) is real, and seems comparable (in first approximation) to T Tauri binary excess in Taurus (Mathieu et al. 1992). Considering observational bias effects, our HAeBe binary frequency estimates are lower limits for the true HAeBe binary frequency (completeness study is in progress).