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تسجيل مستخدم جديدThe Solar-System-Scale Disk Around AB Aurigae
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Ben R. Oppenheimer
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The young star AB Aurigae is surrounded by a complex combination of gas-rich and dust dominated structures. The inner disk which has not been studied previously at sufficient resolution and imaging dynamic range seems to contain very little gas inside a radius of least 130 astronomical units (AU) from the star. Using adaptive-optics coronagraphy and polarimetry we have imaged the dust in an annulus between 43 and 302 AU from the star, a region never seen before. An azimuthal gap in an annulus of dust at a radius of 102 AU, along with a clearing at closer radii inside this annulus, suggests the formation of at least one small body at an orbital distance of about 100 AU. This structure seems consistent with crude models of mean motion resonances, or accumulation of material at two of the Lagrange points relative to the putative object and the star. We also report a low significance detection of a point source in this outer annulus of dust. This source may be an overdensity in the disk due to dust accreting onto an unseen companion. An alternate interpretation suggests that the objects mass is between 5 and 37 times the mass of Jupiter. The results have implications for circumstellar disk dynamics and planet formation.
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Aims. Our goal is to determine the molecular composition of the circumstellar disk around AB Aurigae (hereafter, AB Aur). AB Aur is a prototypical Herbig Ae star and the understanding of its disk chemistry is of paramount importance to understand the
chemical evolution of the gas in warm disks. Methods. We used the IRAM 30-m telescope to perform a sensitive search for molecular lines in AB Aur as part of the IRAM Large program ASAI (A Chemical Survey of Sun-like Star-forming Regions). These data were complemented with interferometric observations of the HCO+ 1-0 and C17O 1-0 lines using the IRAM Plateau de Bure Interferometer (PdBI). Single-dish and interferometric data were used to constrain chemical models. Results. Throughout the survey, several lines of CO and its isotopologues, HCO+, H2CO, HCN, CN and CS, were detected. In addition, we detected the SO 54-33 and 56-45 lines, confirming the previous tentative detection. Comparing to other T Tauris and Herbig Ae disks, AB Aur presents low HCN 3-2/HCO+ 3-2 and CN 2-1/HCN 3-2 line intensity ratios, similar to other transition disks. AB Aur is the only protoplanetary disk detected in SO thus far. Conclusions. We modeled the line profiles using a chemical model and a radiative transfer 3D code. Our model assumes a flared disk in hydrostatic equilibrium. The best agreement with observations was obtained for a disk with a mass of 0.01 Msun , Rin=110 AU, Rout=550 AU, a surface density radial index of 1.5 and an inclination of 27 deg. The intensities and line profiles were reproduced within a factor of 2 for most lines. This agreement is reasonable taking into account the simplicity of our model that neglects any structure within the disk. However, the HCN 3-2 and CN 2-1 line intensities were predicted more intense by a factor of >10. We discuss several scenarios to explain this discrepancy.
(Abriged) We present the results of millimeter observations and a suitable chemical and radiative transfer model of the AB Aur (HD 31293) circumstellar disk and surrounding envelope. The integral molecular content of this system is studied by observi
ng CO, C$^{18}$O, CS, HCO$^+$, DCO$^+$, H$_2$CO, HCN, HNC, and SiO rotational lines with the IRAM 30-m antenna, while the disk is mapped in the HCO$^+$(1-0) transition with the Plateau de Bure interferometer. Using a flared disk model with a vertical temperature gradient and an isothermal spherical envelope model with a shadowed midplane and two unshielded cones together with a gas-grain chemical network, time-dependent abundances of observationally important molecules are calculated. Then a 2D non-LTE line radiative transfer code is applied to compute excitation temperatures of several rotational transitions of HCO$^+$, CO, C$^{18}$O, and CS molecules. We synthesize the HCO$^+$(1-0) interferometric map along with single-dish CO(2-1), C$^{18}$O(2-1), HCO$^+$(1-0), HCO$^+$(3-2), CS(2-1), and CS(5-4) spectra and compared them with the observations. Our disk model successfully reproduces observed interferometric HCO$^+$(1-0) data, thereby constraining the following disk properties: (1) the inclination angle $iota=17^{+6}_{-3}degr$, (2) the position angle $phi=80pm30degr$, (3) the size $R_mathrm{out}=400pm200$ AU, (4) the mass $M_mathrm{disk}=1.3cdot10^{-2} M_{sun}$ (with a factor of $sim7$ uncertainty), and (5) that the disk is in Keplerian rotation. Furthermore, indirect evidence for a local inhomogeneity of the envelope at $ga600$ AU is found...
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