AS 209
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| AS 209 | |
|---|---|
| Name | AS 209 |
| Type | T Tauri star |
| Constellation | Ophiuchus |
| Epoch | J2000 |
| Distance | ~121 pc |
| Mass | ~0.9 M☉ |
| Spectral type | K5 |
| Age | ~0.5–1.0 Myr |
| Coordinates | Right Ascension: 16h 49m 15s; Declination: −14° 22′ 09″ |
AS 209 is a young pre-main-sequence K-type T Tauri star in the Ophiuchus star-forming region. It is notable for a massive, structured protoplanetary disk that has become a benchmark for studies in protoplanetary disk morphology, planet formation processes, and disk chemistry. The target has been observed by major facilities such as Atacama Large Millimeter/submillimeter Array, Hubble Space Telescope, and Karl G. Jansky Very Large Array, and figures in comparative studies alongside objects like HL Tauri, TW Hydrae, and HD 163296.
Introduction
AS 209 lies in the southern Ophiuchus Molecular Cloud near star-forming clusters including Rho Ophiuchi, L1688, and sources cataloged in surveys by IRAS, 2MASS, and Spitzer Space Telescope. It was included in early T Tauri compilations alongside members like DG Tauri and RY Tauri and later became a high-priority target for millimeter interferometry programs led by teams at NRAO, ESO, and the Max Planck Institute for Astronomy. AS 209 is often referenced in reviews of disks such as those by Phil Armitage, Paolo Pinilla, and Cathie Clarke.
Star and System Properties
The central star in AS 209 has been classified near spectral type K5 with parameters estimated from evolutionary tracks by Baraffe et al., Siess et al., and D'Antona & Mazzitelli. Mass and age estimates reference pre-main-sequence isochrones used by Palla & Stahler and groups at Harvard–Smithsonian Center for Astrophysics. Its distance is tied to parallax measurements from Gaia and earlier ground-based estimates from HIPPARCOS. Photometric and spectroscopic monitoring by observers at European Southern Observatory, Subaru Telescope, and Calar Alto Observatory informed measurements of accretion indicators used by teams led by Nuria Calvet, Lee Hartmann, and Gullbring. AS 209’s X-ray activity has been measured with Chandra X-ray Observatory and XMM-Newton and compared to samples studied by Eric Feigelson and Kevin Luhman.
Protoplanetary Disk Structure
The AS 209 disk displays concentric rings and gaps detected in continuum and line emission; structures have been analyzed using models from Andrews et al. and Zhu et al.. Surface density profiles are compared with viscous evolution predictions from Lynden-Bell & Pringle and dust evolution frameworks by Birnstiel et al.. Radial structures in AS 209 have been contextualized with observations of ringed disks like HL Tauri, TW Hydrae, and HD 163296 and theoretical mechanisms proposed by Rice et al., Dong et al., and Flock et al.. Vertical structure and flaring are interpreted with radiative transfer codes developed by groups at Max Planck Institute for Astronomy and Instituto de Astrofísica de Canarias, building on methods by Dullemond & Dominik and Pinte et al..
Observational Studies and Imaging
High-resolution continuum imaging of AS 209 has been produced by ALMA programs such as DSHARP and other large surveys coordinated with NRAO and ESO. Scattered light images were obtained with instruments on VLT including SPHERE and on Gemini Observatory with GPI, while near-infrared photometry came from 2MASS and UKIDSS. Radio continuum and molecular line mapping were obtained with the VLA and SMA (Submillimeter Array), and aperture-mask interferometry observations were performed with Keck Observatory. Imaging results have been discussed in papers by teams including Andrews, Isella, Huang (ALMA DSHARP team), and Anderson et al. and compared to surveys such as the Disk Substructures at High Angular Resolution Project and [observational] compilations by Ansdell et al..
Disk Chemistry and Kinematics
Molecular-line observations of AS 209 include CO isotopologues (e.g., 12CO, 13CO, C18O), CN, HCO+, and CS, analyzed using excitation and chemical models from Bruderer, Bergin, and Aikawa. Studies used non-LTE radiative transfer tools like LIME and RADEX and thermo-chemical codes developed by teams at Leiden Observatory and KOSMA. Kinematic analysis leveraged techniques from Teague et al. and Pinte et al. to search for deviations from Keplerian rotation indicative of embedded companions, using velocity channel maps produced with pipelines from CASA and MIRIAD. Isotopic ratios and deuteration constraints were compared to chemical histories studied by Oberg et al. and van Dishoeck.
Planet Formation Evidence
Gaps, rings, and localized kinematic signatures in AS 209 have been interpreted as potential evidence for planet–disk interactions as modeled by Kley & Nelson, Zhu & Stone, and Dong & Fung. Hydrodynamic simulations and dust-trapping scenarios by Pinilla et al., Rice et al., and Lyra & Lin provide frameworks for inferring planet masses and migration behavior. Comparisons to candidate planet detections in disks such as PDS 70, HD 100546, and LkCa 15 inform discussions of direct imaging limits from SPHERE, GPI, and MagAO. ALMA kinematic perturbation analyses used methods developed for disks like HD 163296 and applied by Pinte, Teague, and Yen to estimate possible embedded body masses.
Theoretical Modeling and Simulations
Theoretical work on AS 209 integrates viscous disk evolution models by Hartmann et al. with dust coagulation frameworks from Birnstiel, planet-disk interaction studies by Crida et al., and magnetohydrodynamic instability analyses from Balbus & Hawley. Numerical simulations have been run using codes like FARGO3D, PLUTO, and Athena++ by groups including Zhu, Dong, Bae, and Lesur. Radiative transfer post-processing used tools by RADMC-3D and MCFOST to produce synthetic observations compared with ALMA and VLT data. Population synthesis and core accretion models by Mordasini and disk instability scenarios from Boss provide broader context for possible evolutionary pathways in systems analogous to AS 209.