Chemical abundances of magnetic and non-magnetic Herbig Ae/Be stars

Abstract

The photospheres of about 10–20 per cent of main‐sequence A‐ and B‐type stars exhibit a wide range of chemical peculiarities, often associated with the presence of a magnetic field. It is not exactly known at which stage of stellar evolution these chemical peculiarities develop. To investigate this issue, in this paper we study the photospheric compositions of a sample of Herbig Ae and Be stars, which are considered to be the pre‐main‐sequence progenitors of A and B stars. We have performed a detailed abundance analysis of 20 Herbig stars (three of which have confirmed magnetic fields), and one dusty young star. We have found that half the stars in our sample show λ Boötis (λ Boo) chemical peculiarities to varying degrees, only one star shows weak Ap/Bp peculiarities and all the remaining stars are chemically normal. The incidence of λ Boo chemical peculiarities we find in Herbig stars is much higher than what is seen on the main sequence. We argue that a selective accretion model for λ Boo star formation is a natural explanation for the remarkably large number of λ Boo stars in our sample. We also find that the magnetic Herbig stars do not exhibit a range of chemical compositions remarkably different from the normal stars: one magnetic star displays λ Boo chemical peculiarities (HD 101412), one displays weak Ap/Bp peculiarities (V380 Ori A) and one (HD 190073) is chemically normal. This is completely different from what is seen on the main sequence, where all magnetic A and cool B stars show Ap/Bp chemical peculiarities, and this is consistent with the idea that the magnetic field precedes the formation of the chemical peculiarities typical of Ap and Bp stars.

Spiral Arms in Disks: Planets or Gravitational Instability?

“…Second, early-type stars with metal-poor photospheres, in particular the λBoo stars, show an ultraviolet excess (Murphy & Paunzen 2017). Folsom et al (2012) and Kama et al (2015) found a relatively high fraction of HAeBes in their samples (up to ∼50%) showing the λBoo pattern to varying degrees. If this effect is common among the targets in our sample, it is possible that M  has been systematically overestimated.…”

“…The SED only reveals an infrared excess longward of 20 μm, leading them to conclude that this is a Vega-type system. Folsom et al (2012) included this source in their survey of abundances among Herbig Ae/Be stars, noted the lack of strong optical emission lines, and found that the star had a solar abundance pattern (see also Fossati et al 2009). We infer the luminosity for this source from the published photometry (Høg et al 2000), adopting the distance from Gaia Collaboration et al (2016), and we arrive at L å =87 L ☉ .…”

Fingerprints of giant planets in the photospheres of Herbig stars

“…We adopt the photospheric abundances from Acke & Waelkens (2004) and Folsom et al (2012). For overlapping sources, the surveys are largely consistent, but we adopt the Folsom et al values as the study is more consistent in the stellar parameter determination.…”

“…The abundances for HD 100546 are from Kama et al (2015) and for HD 34282 from Merín et al (2004). HD 101412 was identified as a λ Boö star by Cowley et al (2010), we use newer abundances from Folsom et al (2012). The solar abundances shown in Fig.…”

“…The peculiarity affects up to 2% of all late B through early F stars (e.g., Gray & Corbally 1998;Paunzen 2001). Among the young Herbig Ae/Be stars, which host protoplanetary disks, the fraction is at least 33% (Folsom et al 2012).…”