The Discovery of λ Bootis Stars: The Southern Survey I

Abstract

The λ Boo stars are a class of chemically peculiar Population I A-type stars characterized by under-abundances of the refractory elements, but near-solar abundances of carbon, nitrogen, oxygen, and sulfur. There is some evidence that λ Boo stars have higher frequencies of “bright” debris disks than normal A-type stars. The discovery of four exoplanets orbiting HR 8799, a λ Boo star with a resolved debris disk, suggests that the λ Boo phenomenon may be related to the presence of a dynamic debris disk, perhaps perturbed by migrating planets. However, only 64 λ Boo stars are known, and those stars were discovered with different techniques, making it problematic to use that sample for statistical purposes, including determining the frequency of debris disks. The purpose of this paper is to derive a new sample of λ Boo stars using a technique that does not lead to biases with respect to the presence of infrared excesses. Through spectroscopic observations in the southern hemisphere, we have discovered 33 λ Boo stars and have confirmed 12 others. As a step toward determining the proportion of λ Boo stars with infrared excesses, we have used WISE data to examine the infrared properties of this sample out to 22 μm. On this basis, we cannot conclude that λ Boo stars have a greater tendency than normal A-type stars to show infrared excesses. However, observing this sample at longer wavelengths may change that conclusion, as many λ Boo debris disks are cool and do not radiate strongly at 22 μm.

Stellar photospheric abundances as a probe of discs and planets

“…The λ Boö abundance pattern is seen in young, disk-hosting stars with ages ∼ 0.1 to 10 Myr (Section 4.1) and in stars as old as 1 Gyr, on the order of their main-sequence lifetimes (Murphy & Paunzen 2017). Some λ Boö stars possess debris disks (Draper et al 2016), but the fraction of confirmed debris disks was found to be statistically indistinguishable from that in nonλ Boö sources (Gray et al 2017). Although the fraction of stars with the λ Boö phenomenon among protoplanetary disk hosts ( 30 %, Folsom et al 2012) is much higher than among the total B to mid-F sample, the refractory element depletion in any given star does not correlate with its age (Iliev & Barzova 1995).…”

Testing the accretion scenario of lambda Boo stars

“…They comprise main-sequence late-B to early-F stars, where a maximum of about 2% of all objects are believed to be λ Boo stars (Gray & Corbally 1998;Paunzen et al 2001b). Classification-resolution spectroscopy shows promising λ Boo candidates (e.g., Murphy et al 2015;Gray et al 2017), and a more detailed abundance determination, especially including the lighter elements, is considered a ultimate test to confirm that a candidate is indeed a bonafide member of the class (e.g., Andrievsky et al 2002;Heiter et al 2002).…”

The Eclipsing δ Scuti Star EPIC 245932119

“…Wraight et al (2011) identified the variability of this star as an EB with a period of 1.59430 days and a primary eclipse depth of 0.15 mag using the STEREO observations (Kaiser et al 2008). On the other hand, Gray et al (2017) conducted a survey in the southern hemisphere to increase the number of λ Boo stars, which are a rare class of Population I metal-weak A-type stars. They reported that the infrared excesses detected in EPIC 245932119 are not the λ Boo phenomena but arise from the presence of the cooler component.…”