[vsnet-be 37] (fwd) Delta Sco (Bedding)

[Taichi Kato <tkato>]

(fwd) Delta Sco (Bedding)

Date: Thu, 21 Sep 2000 14:02:48 +1100 (EST)
From: Tim Bedding <bedding@Physics.usyd.edu.au>
Subject: [vsnet-be 0] (No Subject in original)

<M.H.vanKerkwijk@phys.uu.nl>, Waters Rens <rensw@astro.uva.nl>,
l.b.f.m.waters@astro.rug.nl, Jacqueline Cote <jcote@sron.guspace.nl>, Lex
Kaper <lexk@astro.uva.nl>, Petr Harmanec <hec@sunstel.asu.cas.cz, Alan
MacRobert <amacrobert@skypub.com>, Joshua Roth <jroth@skypub.com>
Subject: delta Sco (second attempt)
BCC: bedding@physics.usyd.edu.au

Hi Juan and Pablo (and everyone else interested in this star),

I was very excited to hear about your discovery of Halpha emission in delta
Sco (IAUC 7461), especially since I had predicted it!  When you read the
following, you will understand what I mean.

======================================================================

  The orbit of the binary star Delta Scorpii
    Bedding 1993, Astron. J., 106, 768
 
 
 Abstract: Although delta Sco is a bright and well-studied star, the details
 of its multiplicity have remained unclear.  Here we present the first
 diffraction-limited image of this 0.12 arcsec binary star, made using
 optical interferometry, and resolve the confusion that has existed in the
 literature over its multiplicity.  Examining published speckle
 measurements, together with the present result, reveals a periodicity of
 10.5 yr and allows calculation of the orbital parameters.  The orbit has a
 high eccentricity (e = 0.82) and large inclination (i = 70 deg), making it
 a favourable target for radial velocity measurements during the next
 periastron (in 2000).
 
 [... text of paper ...]
 
 Note added in proof.  Cot\'e & van Kerkwijk (A&A, in press) have discovered
 delta Sco to be a Be star, with the Halpha line showing emission on the
 flanks of an absorption core.  This makes delta Sco one of the brightest
 known Be stars (the Bright Star Catalogue lists only four Be stars that are
 brighter).  Previous observations in the literature have shown no
 indication of Be behaviour in delta Sco and, interestingly, the Cot\'e &
 van Kerkwijk spectrum was taken only ten months after the last periastron
 of the system.  It is tempting to speculate that the approach of the
 companion (about 600 Rsun = about 85 stellar radii) triggered the mechanism
 responsible for Be emission.  I am grateful to Rens Waters for bringing
 this new result to my attention.
 
======================================================================

Since I wrote that paper, a revised (and better) orbit has been calculated
by Hartkopf et al. (1996, AJ 111, 370).  They got a period of P = 10.58 y
and epoch of periastron of T = 1979.41, only slightly different from my
values.  Their periastron prediction is 2000.6 (i.e. July 2000).  The main
difference is that the eccentricity is even higher (e = 0.92), so that
periastron was even closer than I thought.  I am sure you are seeing Be
activity triggered by the close approach of the companion.  Very exciting!
I wonder how long the Be emission will last.

By the way, in calculating his orbit, Hartkopf et al. remarked that my 1993
observation is discrepant and was therefore given zero weight.  My
observation was actually made in 1991, not in 1993 -- he assigned it the
wrong date and has since apologised.  The addition of the corrected point
does not affect the orbit very much.

Cheers,

	Tim Bedding

-- 
Tim Bedding   School of Physics A28, University of Sydney 2006, AUSTRALIA
              Phone: +61 2 9351 2680            Fax: +61 2 9351 7726

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