HST/STIS ULTRAVIOLET SPECTROSCOPY OF THE COMPONENTS OF THE MASSIVE TRIPLE STARδORI A

Abstract

The multiple star system of δ Orionis is one of the closest examples of a system containing a luminous O-type, bright giant star (component Aa1). It is often used as a spectral-type standard and has the highest observed X-ray flux of any hot-star binary. The main component Aa1 is orbited by two lower mass stars, faint Aa2 in a 5.7 day eclipsing binary, and Ab, an astrometric companion with an estimated period of 346 years. Generally the flux from all three stars is recorded in ground-based spectroscopy, and the spectral decomposition of the components has proved difficult. Here we present Hubble Space Telescope/Space Telescope Imaging Spectrograph ultraviolet spectroscopy of δ Ori A that provides us with spatially separated spectra of Aa and Ab for the first time. We measured radial velocities for Aa1 and Ab in two observations made near the velocity extrema of Aa1. We show tentative evidence for the detection of the Aa2 component in cross-correlation functions of the observed and model spectra. We discuss the appearance of the UV spectra of Aa1 and Ab with reference to model spectra. Both stars have similar effective temperatures, but Ab is fainter and is a rapid rotator. The results will help in the interpretation of ground-based spectroscopy and in understanding the physical and evolutionary parameters of these massive stars.

Lucky Spectroscopy, an equivalent technique to Lucky Imaging

“…The differences between the spectral types of Aa (Aa1+Aa2) and Ab are consistent with the T eff and log g differences measured by Shenar et al (2015) using spectral disentangling (as opposed to the spatial deconvolution used here). Richardson et al (2015) also spatially resolved Aa and Ab in the UV using HST/STIS and obtained very similar values of T eff around 31 kK for Aa1 and Ab, with error bars close to 2000 K, which is also consistent with our spectral classifications. That paper using UV data is the only one we have found where δ Ori Aa+Ab was spatially deconvolved, making our result the first time it has been done in the optical.…”

A COORDINATED X-RAY AND OPTICAL CAMPAIGN OF THE NEAREST MASSIVE ECLIPSING BINARY,δORIONIS Aa. III. ANALYSIS OF OPTICAL PHOTOMETRIC (MOST) AND SPECTROSCOPIC (GROUND-BASED) VARIATIONS

“…After this paper was accepted, we were informed of the results of Harmanec et al (2013), who report on some success in determining the secondary's radial velocity curve, with a semi-amplitude of K 263 km s 2 1 = -. Further, the mentioned HST/STIS spectra were collected and the results have been published by Richardson et al (2015). The HST results seem to confirm the semi-amplitude reported by Harmanec et al (2013), leading to the low-mass (LM) solution presented in Table 3 to match most of the spectroscopic observations presented here and elsewhere.…”

Orbits, Distance, and Stellar Masses of the Massive Triple Star Σ Orionis

“…Two other bright members of the Orion OB1b association are also known triples, ζ Ori (Hummel et al 2013) and δ Ori (Richardson et al 2015). The outer tertiary star appears to be a rapid rotator in each of σ Ori (V sin i = 250 km s −1 ; Simón-Díaz et al 2015), ζ Ori (V sin i = 350 km s −1 ; Hummel et al 2013), and δ Ori (V sin i = 252 km s −1 ; Richardson et al 2015).…”

“…The outer tertiary star appears to be a rapid rotator in each of σ Ori (V sin i = 250 km s −1 ; Simón-Díaz et al 2015), ζ Ori (V sin i = 350 km s −1 ; Hummel et al 2013), and δ Ori (V sin i = 252 km s −1 ; Richardson et al 2015). This suggests that the angular momentum of the natal cloud was transformed mainly into orbital angular momentum for the stars of the inner binary and into spin angular momentum for the outer tertiary star.…”