The interferometric view of Herbig Ae/Be stars

Abstract

In this contribution I review how optical interferometry has contributed to shaping our understanding of the class of Herbig Ae/Be stars and of their associated circumstellar disks. I discuss the evidence for an optically-thin cavity in the inner few astronomical units (au) and a "puffed-up rim" near the dust sublimation radius and how these observations helped to establish the current generation of irradiated disk models. Multi-wavelength interferometric observations also revealed systems with clear signatures of grain growth and dynamically-cleared disk gaps, tracing important stages of disk evolution and of ongoing planet formation. I discuss the new opportunities provided by spectro-interferometry, which enables detailed studies on the gas distribution and velocity field on sub-au scales, resulting in constrains on the accretion properties of the system and the disk excitation structure. Finally, I outline some of the open questions and loose ends in current interferometric studies, and how these might point towards new disk physics.

The Dispersal of Protoplanetary Disks

“…This wide variety of observational tracers allows us to build up a broad picture of protoplanetary disk evolution. In the youngest clusters ( 1Myr) the IR excess fraction for single stars is close to 100%, but this declines dramatically with age and is typically 10% or less for ages 5Myr (e.g., Haisch et al, 2001;Mamajek, 2009;Kraus et al, 2012). A similar decline in disk fraction is seen in accretion signatures (Fedele et al, 2010), and the mass of cold dust and gas in outer disks is also substantially depleted in older clusters (e.g., Mathews et al, 2012).…”

“…As with the gas tracers, there is a substantial population of young stars (up to 40% at ∼2Myr) that exhibit no near-IR excess from warm dust in an inner disk, and a small (but not negligible, 10%) sample of older (∼10-20Myr) pre-main sequence stars that retain their dust (and gas) signatures (e.g., TW Hya; see Section 3.5). However, recent high-resolution imaging surveys revealed that significant fraction of the youngest "diskless" stars and WTTs are in fact close (< 40AU) binaries (Ireland and Kraus, 2008;Kraus et al, 2008Kraus et al, , 2011. In this case dynamical clearing is expected to suppress most inner disk tracers, and the "corrected" disk fraction for single stars in young clusters (with ages 1-2Myr) is close to 100% (Kraus et al, 2012).…”

“…However, recent high-resolution imaging surveys revealed that significant fraction of the youngest "diskless" stars and WTTs are in fact close (< 40AU) binaries (Ireland and Kraus, 2008;Kraus et al, 2008Kraus et al, , 2011. In this case dynamical clearing is expected to suppress most inner disk tracers, and the "corrected" disk fraction for single stars in young clusters (with ages 1-2Myr) is close to 100% (Kraus et al, 2012). Finally, although in most cases there is a correspondence between accretion and dust signatures (e.g., Hartigan et al, 1990;Fedele et al, 2010), recent studies have identified a small but significant population of young stars that show weak (or very red) dust emission but no hints of accretion (e.g., Lada et al, 2006;Cieza et al, 2007Cieza et al, , 2013: this may be evidence for substantial radial evolution in the disk at late times (see Section 3.4, and the chapter by Espaillat et al).…”

“…Near-IR interferometry (Pott et al, 2010), adaptive optics imaging (Cieza et al, 2012), and aperture masking observations (Kraus et al, 2011) have shown that the observed inner holes and gaps in transitional disks are rarely due to close stellar companions or brown dwarfs. Similarly, grain growth models have difficulties explaining the large (sub)-mm cavities observed in resolved images of transitional disks (Birnstiel et al, 2012).…”

The innermost astronomical units of protoplanetary disks

“…The majority of these findings are already described in recent reviews. 1,2 In this paper we will focus on the new discoveries that were not discussed in previous reviews. Our review is divided in three sections.…”