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Spatially resolved optical spectroscopy of close companions to Herbig Ae/Be and T Tauri stars
AndrÈs Carmona M.E van den Acker , and Th. Henning
1,2 1
(1) European Southern Observatory, Karl Schwarzschild Strasse 2 , 75748 Garching bei MÝnchen, Germany (2) MaxPlanckInstitut for Astronomy, KÆnigstuhl 17, 69117 Heidelberg, Germany

2

Abstract
We present spatially resolved VLTFORS2 optical (5700 ­ 9400 å) spectroscopy of close companions (d < 5") to four nearby (d < 200 pc) Herbig Ae/Be stars (HD 100546, HD 144432, HD150193, KK Oph, HD 163296) and one T Tauri star (S CrA). We report the detection of Li I (6707 å) in absorption and emission lines (H, Ca II) in several companions. The observations suggest that the companions are physically associated PMS stars. They are most likely surrounded by disks. Localizing primaries and companions in the HR diagram and assuming a contemporaneous formation history, we determined empirical isochrones ranging from 0.2 to about 2.5 M .

1. Herbig Ae/Be stars and planet formation
Herbig Ae/Be stars (HaeBes) are intermediate mass premainsequence stars of spectral type B, G, or F. Their spectral energy distributions show near and midinfrared excess. Their optical spectra present H, H and Ca II in
Herbig Ae/Be stars are intermediate mass (2 ­ 5 M) Young Stellar Objects. They are a natural laboratory for studying planet formation

4. Results: The companions are emission line stars
Hin emission was detected in almost all the companions. LiI (6707 å ) in absorption was detected in HD 144432B, HD 150193B and KKOph B. The sources were classified spectrally using the method outlined by Hernandez et al. 2004. The deduced spectral type combined with photometric measurements from the literature allowed us to localize the primaries and the companions in the HR diagram. We assumed that the companion is at the same distance as the primary and has the same extinction.
Star HD 100546 B HD 100546 D HD 144432 B HD 150193 B KK Oph B HD 163296 B HD 163296 C HD 163296 D S CrA B SpT F1 e F0 e K2 e F0 e G5 e A0e F0e A0e F2e A0 ­ F2 F8eG0e

emission. These observational characteristics suggest that HAeBes are young stellar objects surrounded by an accreting circumstellar disk of gas and dust. HAeBes and their low mass counterparts, the T Tauri stars, are important because they are natural laboratories for studying the process of planet formation.

2. Imaging revealed close companions to HAeBe and TTauri stars, the nature of these companions is unknown.
Near and midinfrared imaging (PÈrez et al. 2004, Pirzkal et al 1997), HST optical coronography (Grady et al. 2000, Augereau et al. 2001) and speckle interferometry (Baier et al. 1985) revealed the presence of nearby companions in the 5" vicinity of nearby HAeBe and TTauri stars. The nature of these companions is poorly constrained.

H



Examples of the spectra obtained in the range 5700 to 7200 å (GRISM 1200R). The position of important diagnostics is indicated. All the companions presented in H in emission. We concluded the companions are PMS stars physically associated to the primary.

emission abs



Ca II

Li I

emission absorption abs abs abs abs abs abs emi

Star
HD 144432 HD 100546

r [arc sec] B D B B B 4 5 1 1 1 4 .5 .1 .4 .1 .5 .1

P.A 198 156 0 236 257 312 147

V 16.4 16.8 12.7 8.7 11.6 16 12

Imaging revealed that several nearby HAeBes have close (r< 5") companions. Their spectral type is unknown.

B S CrA

HD 100546 HD 100546 HD 144432 HD 150193 KK Oph B HD 163296

1.4

Reference Augereau et al. 2001 Augereau et al. 2001 Perez et al. 2004 Pirzkal et al. 1997 Pirzkal et al. 1997 Grady et al. 2000 Baier et al. 1985

5. Discussion


Given that (i) the companions present emission lines (H, Ca II) characteristic of classical T Tauri stars, (ii) a number of systems present Li I in absorption, and (iii) several sources have early spectral types (unlikely for field background sources), we conclude that the companions are PMS stars physically associated to the primary. Within the errors, according to the theoretical evolutionary tracks (Palla & Stahler 1993), only HD 150193, KK Oph and SCr A are coeval objects. In the case of HD 144432 there is no isochrone that connects primary and secondary. Theoretical tracks of intermediatemass and lowmass stars, do not predict the same age for this system. The companions of HD 100546 and HD 163296 lie in unphysical positions in the HR diagram. Since photometry and spectral type are robust diagnostics measured observationally, the uncertain factor is the distance and/or the extinction. The primary is nearby, therefore, it is likely that the secondary is in the background. But, since the secondary is a early type emission line star, it is not a field star. If the companion is in the background, it suffers of a high extinction. That could be due to the dense circumstellar material of the primary.



3. Observations.
In order do determine the nature of the companions optical spectroscopy is needed. Given their close separation to the parent star (r < 5") and their magnitude (V ~ 16), for obtain good quality spectra a large aperture telescope is required. In the night of 17 May 2005 we obtained medium resolution (R~3200) VLTFORS2 optical spectroscopy (5700 ­ 9400 å) of the companions to the Herbig Ae/Be stars: KK Oph , HD 100546, HD 150193 and HD 163296, and to the classical T Tauri star S CrA.
th
HD163296



HR diagrams of the primary and its companion(s). Within the errors HD150193, KK Oph and S CrA are coeval objects. Theoretical tracks do not predict the same age for HD144432 and its companion.. The companions of HD100546 and HD163296 lie in a unphysical position, they suffer a high extinction , probably due to the circumstellar material of the primary.

We

obtained

FORS2VLT

medium

resolution (R~3200) optical spectra of the close companions (r < 5") to four nearby HaeBes and one T Tauri star. The slit was oriented perpendicular to the line connecting the companion to the central star.

6. References
Augereau et al. 2001, A&A, 365,78A ; Baier et al. 1985, A&A, 153, 278; Grady et al. 2000, ApJ, 544, 895; Hernandez et al. 2004, AJ, 127, 1682; Palla & Staler, 1993, ApJ, 418, 414; Perez et al. 2004, A&A, 416, 647P; Pirzkal et al. 1997, ApJ, 481,329

To minimize the contamination in companion's spectra, the slit was oriented perpendicular to the line connecting the companion to the central star.