[en] Aims. Near-infrared imaging polarimetry at high-angular resolutions has revealed an intriguing distribution of circumstellar dust toward FU Ori-type objects (FUors). These dust grains are probably associated with either an accretion disk or an infalling envelope. Follow-up observations in the mid-infrared would lead us to a better understanding of the hierarchy of the mass accretion processes onto FUors (that is envelope and disk accretion), which hold keys for understanding the mechanism of their accretion outbursts and the growth of low-mass young stellar objects (YSOs) in general. Methods. We developed a semi-analytic method to estimate the mid-infrared intensity distributions using the observed polarized intensity (PI) distributions in the H band (λ = 1.65 μm). This new method allows us to estimate the intensity levels with an order-of-magnitude accuracy, assuming that the emission is a combination of scattered and thermal emission from circumstellar dust grains illuminated and heated by a central source, but the radiation heating through the inner edge of the dust disk is negligible due to the obscuration by an optically thick compact disk. We have derived intensity distributions for two FUors, FU Ori and V1735 Cyg, at three wavelengths (λ = 3.5, 4.8, and 12 μm) for various cases, with a star or a flat compact self-luminous disk as an illuminating source; an optically thick disk or an optically thin envelope for circumstellar dust grains; and three different dust models. The calculations were carried out for typical aspect ratios of the disk surface and the envelope z/r of ∼0.1, ∼0.2, and ∼0.4. Results. We have been able to obtain self-consistent results for many cases and regions, in particular when the viewing angle of the disk or envelope is zero (face-on). Our calculations suggest that the mid-infrared extended emission at the above wavelengths is dominated by the single scattering process. The contribution of thermal emission is negligible unless we add an additional heating mechanism such as adiabatic heating in spiral structures and/or fragments. The uncertain nature of the central illuminating source, the distribution of circumstellar dust grains and the optical properties of dust grains yield uncertainties in the intensity levels on orders of magnitude, for example, 20-800, for the aspect ratio of the disk or the envelope of ∼0.2 and λ = 3-13 μm. Conclusions. The new method we have developed is useful for estimating the detectability of the extended mid-infrared emission. Observations with the forthcoming extremely large telescopes, with a telescope diameter of 24-39 m, would yield a breakthrough for the above research topic at angular resolutions comparable to the existing near-infrared observations. The new semi-analytic method is complementary to full radiative transfer simulations, which offer more accurate calculations but only with specific dynamical models and significant computational time.
Disciplines :
Space science, astronomy & astrophysics
Author, co-author :
Takami, Michihiro ; Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan
Gu, Pin-Gao ; Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan
Otten, Gilles; Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan
Delacroix, Christian ; Université de Liège - ULiège > Département d'astrophysique, géophysique et océanographie (AGO) > Planetary & Stellar systems Imaging Laboratory
Liu, Sheng-Yuan ; Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan
Wang, Shiang-Yu ; Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan
Karr, Jennifer L.; Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan
Language :
English
Title :
Semi-analytic calculations for extended mid-infrared emission associated with FU Ori-type objects
We thank Professor Oliver Absil for useful discussion and the anonymous referee for useful comments. M.T. is supported by the National Science and Technology Council (NSTC) of Taiwan (grant nos. 109-2112-M-001-019, 110-2112-M-001-044-, 111-2112-M-001-059-). M.T. and S.-Y.L. are supported by NSTC Taiwan 108-2923-M-001-006-MY3 for the Taiwanese-Russian collaboration project. P.-G.G. is supported by NSTC Taiwan 111-2112-M-001-037. This work has made use of data from the European Space Agency (ESA) mission Gaia ( https://www.cosmos.esa.int/gaia ), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium ). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This research made use of the Simbad database operated at CDS, Strasbourg, France, and the NASA’s Astrophysics Data System Abstract Service.
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