The composite-spectrum binary hypothesis does not explain theλBootis stars

Abstract

The existence of the λ Boo type as a class of chemically-peculiar stars in its own right has taxed numerous researchers, and has challenged spectroscopists to produce a model which is plausible, comprehensive and predictive. Stars which are recognized as members of the λ Boo class have late-B to early-F spectral types, and exhibit (often substantially) low abundances of Fe-peak elements although elements such as C, N, O and S may have more nearly solar abundances. Since less than 2% of objects within the relevant spectral-type domain appear to be λ Boo stars their existence has demanded rather special conditions, and has triggered opinion that this group may not in fact exist but that each case can be explained as an unrecognized binary. In this paper we examine those claims by monitoring 10 stars, listed in the literature as possible λ Boo stars but said to be "likely candidates" for composite-spectrum binaries, by employing high-dispersion spectroscopy in an intermittent observing programme designed to reveal the sort of line-profile changes that should be detectable if each object were really a pair of similar stars in an SB2 system. We also monitor two other stars: HR 7903, said to be a binary (but is more like an Ap star), and λ Boo itself. The sample includes 1 possible, 1 marginal and 4 definite λ Boo classifications. In addition, we derive the physical properties of the 12 stars by photometric and spectroscopic synthesis, and measure their radial velocities. Three of the sample show small line-profile variations, but not of the sort that can be attributed to the presence of a companion star; they are the suspected Ap star HR 7903, HR 6878 (which exhibits spectrum peculiarities very similar to those of HR 7903 but has not previously been classified as Bp or Ap), and λ Aql, whose rapid spectrum variations resemble those observed in spotted or CP stars. None of the stars shows any evidence to suggest that it could be a composite-spectrum binary.

The Discovery of λ Bootis Stars: The Southern Survey I

“…With a sufficiently high accretion rate ( M 10 yr 13 1  --☉ ) coupled with a lack of a deep surface convection zone, that accreted gas can lead to observable under-abundances (Charbonneau 1993). Indeed, a sizeable fraction of λ Boo stars show discrete, sharp, variable absorption components in the cores of the Ca II K-line and the Na I D lines, similar to those observed in the spectrum of the protoplanetary-disk star β Pic (Gray 1988;Holweger & Stürenburg 1991;Holweger & Rentzsch-Holm 1995;Griffin et al 2012). These components are often interpreted as arising from Falling Evaporating Bodies (cf.…”

Gaia's view of the λ Boo star puzzle

“…The hypothesis was advocated in a series of papers by the same authors (e.g. Faraggiana et al 2001), but was refuted in critical independent analyses (Stütz & Paunzen 2006;Griffin et al 2012).…”

An Evaluation of the Membership Probability of 212 λ Boo Stars. I. A Catalogue

“…The extent of the heterogeneity of the λ Boo group was articulated by (and actually exacerbated by) Gerbaldi, Faraggiana, & Lai (2003), after which interest in the λ Boo stars dropped. That work has been heavily criticised (Stütz & Paunzen 2006; Griffin, Gray, & Corbally 2012) and we discuss it in more detail in Section 1.4.…”

“…First and foremost, we look for a spectral classification of ‘λ Boo’, particularly if performed by Gray or Gray & Corbally, who have been authorities on the classification of λ Boo stars for the last 25 years (Gray 1988; Gray & Corbally 1993) and equally for A stars as a whole (Gray & Garrison 1987, 1989a, 1989b; Gray & Corbally 2009). We take a conservative approach to λ Boo classifications made by Abt (Abt 1984a, 1985; Abt & Morrell 1995), who was a little lenient when admitting stars to the λ Boo group, having based some inclusions on a weak Mg ii 4481 line only, when in fact that star could have belonged to any of a wider set of groups with that characteristic, such as classical shell stars, field horizontal branch (FHB) stars, or generally metal-weak stars (see Griffin et al 2012).…”

“…The binary hypothesis requires that the radial velocity difference between the two stars in the binary causes the continuum of one star to ‘veil’ the lines of the other, implying that the radial velocity difference should be comparable to the v sin i of the stars. Griffin et al (2012) sought to test the veiling mechanism and found it could produce the appearance of metal weakness by only a factor of about 2, i.e. dex; it certainly could not produce metal weaknesses on the order of − 2.0 dex that is seen in some λ Boo stars.…”