Secondly, a fundamental role of binary studies is to allow the direct determination of physical parameters. Noticeably, the stellar mass is only accessible through the observation of gravitationally bound pairs of stars, by straight application of gravitational law.
Main-Sequence (MS) binary stars are overall quite well studied. However, as orbits of binary systems evolve with time, it is mandatory to derive the properties of the systems during the pre-Main Sequence (PMS) phase. A major issue is to quantify the binary frequency fb (the probability that a given object is multiple, Reipurth & Zinnecker 1993) for young multiple objects, their separation distribution as well as their mass ratio.
Recent studies have shown that more than half of T Tauri stars, young
stars having a mass 
, are members of a binary or
multiple system
(Mathieu 1992;
Leinert et al. 1993;
Reipurth & Zinnecker 1993;
Prosser et al. 1994;
Simon et al. 1995;
,Ghez et al. 1993, 1997;
Brandner et al. 1996;
Padgett et al. 1997).
Whether
there is an overabundance of low-mass PMS binaries versus MS binaries is
still a matter of debate, due to the difficulties to compare both
statistics obtained with different approaches. Not only the employed
techniques are different, (MS stars were mainly spectroscopically searched
for, while PMS binaries were searched with high angular resolution
imaging), but also the wavelength domains are different (optical
observations predominate for MS stars, while infrared (IR) surveys of the
young objects, mostly embedded in dark regions, were used ipso
facto). The only systematic effort aimed at finding new low-mass pre-Main
Sequence binaries with visible spectroscopy dates back from
Mathieu (1992).
The question is then whether the stellar density may have an effect on the formation of multiple systems. MS stars are thought to have been formed in OB associations or in dense clusters (Miller & Scalo 1978; Lada et al. 1991), while PMS stars in T associations are formed in lower density environments which might enhance the production of binaries. However, Brandner et al. (1996) observed T Tauri stars in OB and T associations and concluded that the binary frequency is the same for MS and PMS low mass stars in the range of separation 120 - 1800 AU (except for the Taurus-Auriga star forming region). Moreover, Padgett et al. (1997) using HST observations of PMS stars in dense clusters recently found a comparable binary frequency for dense clusters and low-density star-forming regions. They thus claim that multiplicity is not influenced by the local stellar density (at least in regions with densities ranging between 40 and 5000 stars pc-3).
Since the pioneering work of Mathieu (1992), selective mass determinations for some low-mass PMS binaries have been obtained (Padgett & Stapelfeldt 1994; Welty 1995; Figueiredo 1997), but the sample of young multiple systems with orbital and physical parameters determined must be enlarged in order to test the early stages of stellar evolutionary models.
As pointed out by Hillebrand (1994), the intermediate-mass PMS stars, namely Herbig Ae/Be (HAeBe) stars are found in various environments: in dense star forming regions (containing few tens of HAeBe plus a myriad of T Tauri stars), in lower density groups of young stellar objects (2 to 5 HAeBe stars sharing the same birth place) and in isolated molecular cores (where a central HAeBe star and embedded young lower mass stars are found). Studying the binary frequency among HAeBe objects may help to better understand the relation between the direct environment and the multiplicity status of the stars during their earlier formation stages.
Besides reinforcing the binary frequency estimate, HAeBe binaries study is of great interest because direct mass determination are fervently required to test the stellar evolution models for young intermediate-mass stars.
To date, however, the binarity status of Ae/Be Herbig stars has been far less surveyed, probably because these stars form a class less homogeneous than T Tauri stars (Thé et al. 1994). Few recent studies using infrared imaging (Li et al. 1994; Leinert et al. 1997b; Pirzkal et al. 1997) have found a binary frequency (although based on limited samples) in excess by a factor 2 versus A/B type MS stars, (by considering G type MS stars degree of multiplicity fb, spectroscopically determined by Duquennoy and Mayor 1991 identical through the Main Sequence, as explained by Leinert et al. 1993).
Up to now, the pilot study made on the eclipsing and spectroscopic triple system TY CrA (Lagrange et al. 1993; ,Corporon et al. 1994, 1996; Beust et al. 1997, see also ,,Casey et al. 1993, 1995, 1998) is the only work that led to the first direct determination of masses for a HAeBe multiple system.
The present paper is structured as follows: in Sect. 2, we describe our sample, the instruments used as well as our observing strategy. We present in Sect. 3 the spectra of some known and new spectroscopic HAeBe binaries. Notes on individual sources are given in Sect. 4 and in Sect. 5 we discuss our results.
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