High-resolution observations in T Cha: the ATCA and ALMA · PDF file High-resolution...
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High-resolution observations in T Cha: the ATCA and ALMA view
Transitional protoplanetary disks are considered as the "missing link" between disks surrounding young stars and planetary systems. These disks show little or no excess emission at λ < 10 μm and a signi?icant excess at λ ≥ 10 μm. This lack of near-infrared excess was interpreted as a diagnostic of inner disk clearing possibly connected to the early stages of planet formation (e.g., Calvet et al. 2002). Dust clearing can be related with gaps or holes within the disks, which can be created by several mechanisms, such as a close companion, disk photoevaporation, or giant planet formation. Until recently, no planet had been detected around stars with transitional disks, but this has changed with the recent ?irst direct detection of a young accreting planet around LkCa 15 (Kraus & Ireland, 2012; Sallum et al. 2015) and a yet uncon?irmed substellar companion inside the gap of the transitional disk that surrounds T Chamaleontis (T Cha, hereafter; Huelamo et al. 2011). This opens a very interesting research line for studying in-situ formation of planets in transitional disks, being a unique opportunity to capture a key moment of on-going planetary formation.
I. de Gregorio-Monsalvo1, N.Huelamo2, M. Ireland3,4, C. Pinte5, E. Macias6, P. Tuthill3, H. Bouy2, S. Lacour7, A. Kraus8
(1) ESO-ALMA (Chile), (2) LAEX-Astrobiology Center (INTA-CSIC) (Spain), (3) Macquarie U. (Australia), (4) Australian Astronomical Observatory (Australia), (5) UMI-FCA, CNRS/INSU, France , (6) IAA-CSIC (Spain), (7) Observatoire de Paris (France), (8) Hawaii U. (USA)
REFERENCES: Calvet et al. 2002, ApJ, 568, 1008 Cieza et al. 2011, ApJL, 471, L25 De Gregorio-Monsalvo et al. 2013, A&A, 557, 133 Dullemond & Dominik, 2005, A&A, 434, 971 Huelamo et al. 2011, A&A, 528, L7 Huelamo et al. 2015, A&A, 575, L5 Kraus & Ireland, 2012, ApJ, 742, L5 Sallum et al. 2015, Nature 527, 324
Fig 2. ATCA map at 7 mm. The spatial resolution is 0.2”x 0.2“
The case of T Chamaleontis: T Cha is a young (~7 Myr) nearby (108 pc) T Tauri star with a Spectral Energy Distribution (SED) typical of a transitional disk. Huelamo et al. (2011) reported the presence of a substellar companion candidate within its gap (T Cha b). T Cha is surrounded by an outer disk whose main properties have been inferred from the modeling of its spectral energy distribution (Cieza et al. 2011). Those models are highly degenerate and can ?it the SED equally well either with a very compact (few AUs) outer disk or much wider tenuous disk with a very steep surface density pro?ile. To shed light on the peculiar disk properties of T Cha, we performed high angular resolution observations in the submillimeter, millimeter, and in the centimeter regimes.
Unveiling the outer disk properties using ALMA High spatial resolution and high sensitivity ALMA observations in continuum and in CO(3-2) at 850 μm show T Cha is surrounded by a compact dusty disk and a three times larger gaseous disk (see Fig 1).
Using the CO(3−2) image we derive an outer radius of the gaseous disk of 230 AU, an inclination of 67±5 degrees and a position angle of PA=113±6 degrees. The gas emission is in Keplerian rotation, and the estimated dynamical mass of the central object is 1.5±0.2 Msun, in good agreement with previous estimations based on evolutionary tracks.
The dusty disk is resolved at 850 μm and it shows a similar inclination and P.A. to the gaseous disk. The continuum intensity pro?ile displays two emission bumps separated by 40 AU, suggesting for the ?irst time the presence of an inner dust gap as predicted by SED modeling, and an outer radius of ~80 AU. These data allow us to rule out both the very small and large Rout families predicted by SED models (Huelamo et al. 2015).
Comparison with radiative transfer models We used the radiative transfer code MCFOST with a tapered exponential edge in the surface density distribution for reproducing simultaneously the gas and the dust pro?iles (see Fig. 3). The best model provides values of γ=0.5, and critical radius of 50 AU, which is consistent with having most of the disk mass within the inner 50 AU. Our best model does not perfectly ?it the CO line pro?iles, which can be related either to the underlying chemistry (we assumed ISM abundances and very simple CO freeze-out) or to the model prescriptions. In fact, tapered edge models sometimes fail to reproduce simultaneously the observed dust and gas pro?iles obtained from very high spatial resolution and sensitivity observations (e.g. de Gregorio-Monsalvo et al. 2013), and suggests that other processes such as grain growth and radial migration should be taken into account.
Fig 1. ALMA CO(3-2) (colors) + continuum at 850 μm (contours)
Resolving the central gap using ATCA High-angular resolution continuum observations using the Australia Telescope Compact Array at 7 mm show dust emission asymmetric in both axis with a clear gap at the center (see Fig 2). We also performed continuum observation at 1.7 cm. It shows faint emission barely resolved and tracing a similar structure in the main axis as observed at 7 mm.
The spectral index between 3.3 mm (from literature) and 7 mm is ~2.6, consistent with thermal dust emission. This provides a dust opacity law β=(2-α)