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Identifying Gaps in Flaring Herbig Ae/Be Disks Using Spatially Resolved Mid-Infrared Imaging

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Identifying Gaps in Flaring Herbig Ae/Be Disks Using Spatially Resolved Mid-Infrared Imaging A&A 555, A64 (2013) Astronomy DOI: 10.1051/0004-6361/201321300 & c ESO 2013 Astrophysics Identifying gaps in flaring Herbig Ae/Be disks using spatially resolved mid-infrared imaging Are all group I disks transitional?? K. M. Maaskant1,2, M. Honda3, L. B. F. M. Waters4,2, A. G. G. M. Tielens1, C. Dominik2,5, M. Min2, A. Verhoeff2, G. Meeus6, and M. E. van den Ancker7 1 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: [email protected] 2 Anton Pannekoek Astronomical Institute, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands 3 Department of Mathematics and Physics, Faculty of Science, Kanagawa University, 2946 Tsuchiya, Hiratsuka, Kanagawa 259-1293, Japan 4 SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands 5 Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands 6 Universidad Autonoma de Madrid, Dpt. Fisica Teorica, Campus Cantoblanco, 28049 Madrid, Spain 7 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching b. München, Germany Received 15 February 2013 / Accepted 7 May 2013 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 +4 +3 +5 +4 of 34−4, 23−5, 30−3 and 63−4 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 mid- infrared 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. Key words. protoplanetary disks – circumstellar matter – planet-disk interactions – stars: variables: T Tauri, Herbig Ae/Be – stars: pre-main sequence 1. Introduction such as in Fomalhaut (Boley et al. 2012), β Pictoris (Lagrange et al. 2010), or HR8799 (Marois et al. 2010). Characterization Planetary systems form in the dusty disks around pre-main- of pathways toward mature planetary systems are key in under- sequence stars. The structure and evolution of these disks depend standing protoplanetary disks evolution. upon a wide range of parameters, such as the initial mass, size, The evolution of intermediate-mass pre-main-sequence stars, and chemical characteristics of the disk and the properties of the known as Herbig Ae/Be stars, is different from that of lower- and central star. In addition, evolutionary processes such as grain higher-mass stars due to the differences in stellar and circumstel- growth, grain settling, photo-evaporation and planet formation lar physics (Natta et al. 2007). Meeus et al.(2001) classified the can take place in disks. Their interplay will eventually transform Herbig Ae/Be stars into two groups: group I sources, which re- gas rich disks into debris disks with possibly planetary systems, quire blackbody components up to far-infrared wavelengths to ? Tables A.1–A.4 are only available at the CDS via anonymous ftp fit the spectral energy distribution (SED), and group II sources, to cdsarc.u-strasbg.fr (130.79.128.5) or via which can be well fitted with only a single power law at mid- to http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/555/A64 far-infrared wavelengths. Article published by EDP Sciences A64, page 1 of 18 A&A 555, A64 (2013) Table 1. Star and disk parameters used in this study. Object RA (J2000) Dec (J2000) Teff [K] L∗ Av d M∗ i ◦ 0 00 ◦ h m s [K] [L ] [pc] [M ][ ] +16 ◦ HD 97048 11 08 03.32 –77 39 17.48 10 010 40.70 ± 5.9 1.21 158−14 1.84 43 +15 ◦ HD 169142 18 24 29.79 –29 46 49.22 8200 15.33 ± 2.17 0.46 145−15 2.28 13 +14 ◦ HD 135344 B 15 15 48.40 –37 08 55.86 6590 8.26 ± 1.17 0.32 140−14 3.30 11 +5 ◦ Oph IRS 48 16 27 37.19 –24 30 34.80 9000 14.3 ± 5 11.50 120−5 2.0 48 Several explanations have been proposed to understand set for these objects, which consists of Spitzer IRS spectra and the evolutionary link between group I and group II objects. photometry. See AppendixA for an overview of available pho- Dullemond & Dominik(2004b, 2005) showed that SEDs of tometric UV, optical, infrared, and millimeter data and their ref- group I sources can be interpreted by hydrostatic disks with a erences. Spitzer IRS spectra are taken from Juhász et al.(2010). flaring geometry while group II sources are thought to be the The Spitzer IRS spectrum of Oph IRS 48 has been reduced with evolved version of group I sources: as dust grains coagulate and the latest calibration version (S18.18.0, Sturm et al. 2013). These settle to the mid-plane, the disk becomes flatter producing the four sources are representative for the sample of flaring Herbig steeper, bluer mid- to far-infrared SEDs. Additionally, a decrease Ae/Be objects, which lack the silicate features in their spectra. of mid-to far-infrared flux in the SED can be explained by a We have chosen these stars, since we could acquire a homo- puffed-up inner disk that shadows large parts of the outer disk geneous set of mid-infrared (MIR) data for study. The four se- (Dullemond & Dominik 2004a). lected stars are listed in Table1 with the adopted spectral types, Gaps have been found in an increasing number of group I temperatures, luminosities, visual extinctions, distances, stellar sources such as in AB Aur (Honda et al. 2010), HD 142527 masses, and disk inclinations. The SEDs are bright at mid- to (Fukagawa et al. 2006; Fujiwara et al. 2006), HD 135344 (Brown far-infrared (FIR) and sub-mm wavelengths and all show a simi- et al. 2009), HD 36112 (Isella et al. 2010), HD 169142 (Honda lar “bump” of excess emission at ∼20 micron. Detailed radiative et al. 2012), Oph IRS 48 (Geers et al. 2007a) and HD 100546 transfer modeling suggests that this kind of SEDs are consistent (Bouwman et al. 2003; Benisty et al. 2010). Recently, Honda with disks with partially depleted inner region (i.e., a cavity or et al.(2012) proposed that the classification as a group I source a gap), while a wall-like structure at the outer edge of this re- could mean a disk geometry with a strongly depleted inner disk gion can be responsible for the abrupt rise of the SED at MIR (i.e. a transitional disk). Due to gaps and inner holes, huge verti- (Bouwman et al. 2003; Calvet et al. 2005; Espaillat et al. 2007). cal walls arise at the edge of the outer disk and significantly con- In the remainder of this paper, we present and discuss the tar- tribute to the mid-to far-infrared flux. In view of the possibility gets ordered by their sub-mm emission above the photosphere, that all group I disks are transitional disks, the relation between which is strongest for HD 97048, followed by HD 169142, group I and group II sources may have to be revisited. HD 135344 B and Oph IRS 48. With all other properties being To get a better understanding of the geometry of group I equal, the sub-mm excess can be used as a crude estimate of the disks and the use of polycyclic aromatic hydrocarbons (PAHs) total disk mass. Thought we are aware that the sub-mm excess to trace the geometry of disks, we examine a representative set is sensitive for some other properties, such as the differences in of four group I objects: HD 97048, HD 169142, HD 135344 B stellar temperature, luminosity and the temperatures and opac- and IRS48 for which we have obtained a homogeneous set of N- ities in the outer disk, we use the sub-mm excess as a rough and Q-band data.
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