Identifying gaps in flaring Herbig Ae/Be disks using spatially resolved mid-infrared imaging

Abstract

Context. The evolution of young massive protoplanetary disks toward planetary systems is expected to correspond to structural changes in observational appearance, which includes the formation of gaps and the depletion of dust and gas. Aims. A special group of disks around Herbig Ae/Be stars do not show prominent silicate emission features, although they still bear signs of flaring disks, the presence of gas, and small grains. We focus our attention on four key Herbig Ae/Be stars to understand the structural properties responsible for the absence of silicate feature emission. Methods. We investigate Q-and N-band images taken with Subaru/COMICS, Gemini South/T-ReCS, and VLT/VISIR. We perform radiative transfer modeling to examine the radial distribution of dust and polycyclic aromatic hydrocarbons (PAHs). Our solutions require a separation of inner-and outer-disks by a large gap. From this, we characterize the radial density structure of dust and PAHs in the disk. Results. The inner edge of the outer disk has a high surface brightness and a typical temperature between ∼100-150 K and therefore, dominates the emission in the Q-band. All four disks are characterized by large gaps. We derive radii of the inner edge of the outer disk of 34 +4−4 AU for HD 97048, HD 169142, HD 135344 B, and Oph IRS 48, respectively. For HD 97048 this is the first detection of a disk gap. The large gaps deplete the entire population of silicate particles with temperatures suitable for prominent midinfrared feature emission, while small carbonaceous grains and PAHs can still show prominent emission at mid-infrared wavelengths. The continuum emission in the N-band is not due to emission in the wings of PAHs. This continuum emission can be due to very small grains or to thermal emission from the inner disk. We find that PAH emission is not always dominated by PAHs on the surface of the outer disk. Conclusions. The absence of silicate emission features is due to the presence of large gaps in the critical temperature regime. Many, if not all Herbig disks with spectral energy distribution classification "group I", are disks with large gaps and can be characterized as (pre-) transitional. An evolutionary path from the observed group I to the observed group II sources seems no longer likely. Instead, both might derive from a common ancestor.

Five steps in the evolution from protoplanetary to debris disk

“…The relatively low level of nearinfrared emission in HAe disks suggest that most have inner regions that are relatively empty of dust (Yasui et al 2014), an interpretation which is confirmed for the disks that have been directly imaged in scattered light (Fukagawa et al 2006), polarised light (Quanz et al 2012) or submillimetre emission (Brown et al 2012). While group II sources tend to have larger grains than those in group I, group II disks are not necessarily at a later evolutionary stage, since there is also a high incidence of transition disks in group I (Maaskant et al 2013).…”

Accretion disks in luminous young stellar objects

“…Such spatially resolved mid-IR observations also provide an increase in the number of HAeBe disks with evidence for gaps, i.e., near discontinuous jumps in the radial density distribution (Panić et al, 2014;Matter et al, 2014). A correlation is emerging between the presence of gaps and the flared geometry of the disk (Maaskant et al, 2013). This is furthermore supported by near-IR scattered light images of the outer disk, where the group II disks are rather featureless, while the group I sources show the hallmarks of ongoing dispersal, like multiple spiral arms, disk gaps, pericentre offsets, and asymmetric shadowing of the outer disk favouring dynamical clearing by sub-stellar objects (see the overview by Grady et al, 2015).…”

Mass constraints for 15 protoplanetary discs from HD 1–0

“…Starlight impinges at a shallow angle on flat disks, and heating is inefficient compared to that above the same midplane location in a flaring disk. In addition, among the Herbig Ae/Be systems flaring, group I disks have resolved cavities or gaps 10-100 au scales in their millimetre dust emission (Maaskant et al 2013;Kama et al 2015).…”