Abstract
HD 166191 has been identified by several studies as hosting a rare and extremely bright warm debris disc with an additional outer cool disc component. However, an alternative interpretation is that the star hosts a disc that is currently in transition between a full gas disc and a largely gas-free debris disc. With the help of new optical to mid-IR spectra and Herschel imaging, we argue that the latter interpretation is supported in several ways: i) we show that HD 166191 is co-moving with the ~4 Myr-old Herbig Ae star HD 163296, suggesting that the two have the same age, ii) the disc spectrum of HD 166191 is well matched by a standard radiative transfer model of a gaseous protoplanetary disc with an inner hole, and iii) the HD 166191 mid-IR silicate feature is more consistent with similarly primordial objects. We note some potential issues with the debris disc interpretation that should be considered for such extreme objects, whose lifetime at the current brightness is mush shorter than the stellar age, or in the case of the outer component requires a mass comparable to the solid component of the Solar nebula. These aspects individually and collectively argue that HD 166191 is a 4-5 Myr old star that hosts a gaseous transition disc. Though it does not argue in favour of either scenario, we find strong evidence for 3-5 um disc variability. We place HD 166191 in context with discs at different evolutionary stages, showing that it is a potentially important object for understanding the protoplanetary to debris disc transition.
Highlights
As nurseries in which planets form, protoplanetary discs are allimportant in setting the boundary conditions for planet formation
The very large disc luminosity (Ldisc/L⋆ ≈ 10%), the fact that the excess spectrum extends over a wide range of wavelengths (Fig 1), and the probable variability suggests at first glance that the emission is probably from a young ( 10 Myr old) star that hosts a gas-rich protoplanetary disc
It is known that some debris disc brightnesses vary (e.g. Meng et al 2012; Melis et al 2012; Kennedy & Wyatt 2013), and an alternative interpretation could be that HD 166191 hosts an unusually bright debris disc with both warm and cool components (Fujiwara et al 2013; Schneider et al 2013), in essence an extreme analogue of the two-component debris disc around η Corvi (Wyatt et al 2005; Smith et al 2009)
Summary
As nurseries in which planets form, protoplanetary discs are allimportant in setting the boundary conditions for planet formation. The possibility of confusion of the IRAS emission with another nearby source, which is visible to the South-West in higher resolution images, meant that the excess was only detected unambiguously from 8-20 μm This system was suggested to have a mid-IR excess with both WISE (Kennedy & Wyatt 2013) and AKARI (Fujiwara et al 2013), which was confirmed with high resolution mid-IR imaging (Schneider et al 2013). While these studies went in search of warm terrestrial-zone debris discs, only Schneider et al (2013) used the Multi-band Imaging Photometer for Spitzer (MIPS, Werner et al 2004; Rieke et al 2004) to show that the far-IR emission is probably associated with the star, and that the disc emission is significant over a wide range of wavelengths.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
