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Structures and Evolution of the Circumstellar Environments around Classical T Tauri Stars in nearby Star Forming Regions
https://ir.soken.ac.jp/records/437
https://ir.soken.ac.jp/records/43769d2f16f-c215-4533-bb9f-a3be6aaf997b
| 名前 / ファイル | ライセンス | アクション |
|---|---|---|
要旨・審査要旨 (383.4 kB)
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| Item type | 学位論文 / Thesis or Dissertation(1) | |||||
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| 公開日 | 2010-02-22 | |||||
| タイトル | ||||||
| タイトル | Structures and Evolution of the Circumstellar Environments around Classical T Tauri Stars in nearby Star Forming Regions | |||||
| タイトル | ||||||
| タイトル | Structures and Evolution of the Circumstellar Environments around Classical T Tauri Stars in nearby Star Forming Regions | |||||
| 言語 | en | |||||
| 言語 | ||||||
| 言語 | eng | |||||
| 資源タイプ | ||||||
| 資源タイプ識別子 | http://purl.org/coar/resource_type/c_46ec | |||||
| 資源タイプ | thesis | |||||
| 著者名 |
塚越, 崇
× 塚越, 崇 |
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| フリガナ |
ツカゴシ, タカシ
× ツカゴシ, タカシ |
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| 著者 |
TSUKAGOSHI, Takashi
× TSUKAGOSHI, Takashi |
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| 学位授与機関 | ||||||
| 学位授与機関名 | 総合研究大学院大学 | |||||
| 学位名 | ||||||
| 学位名 | 博士（理学） | |||||
| 学位記番号 | ||||||
| 内容記述タイプ | Other | |||||
| 内容記述 | 総研大甲第1190号 | |||||
| 研究科 | ||||||
| 値 | 物理科学研究科 | |||||
| 専攻 | ||||||
| 値 | 09 天文科学専攻 | |||||
| 学位授与年月日 | ||||||
| 学位授与年月日 | 2008-09-30 | |||||
| 学位授与年度 | ||||||
| 値 | 2008 | |||||
| 要旨 | ||||||
| 内容記述タイプ | Other | |||||
| 内容記述 | It is fundamental to investigate the origin of low-mass stars and planets in order to answer how the sun or the earth was formed. In the widely accepted star and planet formation scenario, a dispersion process of the circumstellar material around young stellar object is recognized as one of the most important mechanisms to control the mass input to the star-disk system and the nature of the planet. In the protostar phase, although most of the systematic studies restricted their sample to the young stellar objects in Taurus, we have had some reasonable understanding of the dispersion process. However, our knowledge about the dispersal mechanism and time scale of the circumstellar gas has been poor yet in the phase of Classical T Tauri Star (CTTS). Therefore, more systematic observational data are required in order to understand the dispersion process of the circumstellar gas material in an envelope as well as a disk. The main purpose of this study presented in this thesis is to reveal the circumstellar environments and evolution of CTTSs in nearby star forming regions. We have performed two kinds of molecular line observations with radio telescopes, Atacama Submillimeter Telescope Experiment (ASTE) and Nobeyama Millimeter Array (NMA), for the purposes.<br /> One of observations is the <sup>12</sup>CO and <sup>13</sup>CO J=3-2 line survey of CTTSs in nearby star forming regions, Taurus, Ophiuchus, and Lupus, using ASTE. We selected 26 CTTS s (22 for <sup>12</sup>CO observations and l3 for <sup>13</sup>CO observations) in the regions which have been reported to have strong 1.3mm continuum emission. six targets in Ophiuchus were selected as our NMA observations. In the observations with ASTE, we employed five-points observations in which spectra at stellar position and surrounding. four-points are obtained, which enable us to compare the central spectrum and other four surrounding ones. By subtracting an ambient cloud component, the excess emission toward the star which is embedded in their parent cloud can be evaluated (residual spectrurm). The purposes of the observations are to characterize the molecular environment around CTTSs (～3000 AU scale) and to investigate the dispersion time scale of gas around CTTSs. Additionally, it is important to search for gas-rich disks suitable for future follow-up observations with interferometers.<br /> Another observation is high-resolution interferometric <sup>12</sup>CO(1-0), <sup>13</sup>CO(l-0), and millimeter continuum observations toward eight CTTSs in the Ophiuchus molecular cloud. The six of eight targets were also observed in the five-points method with ASTE. The aims of the NMA observations are to reveal the structures and kinematics of the residual spectra obtained with ASTE by the high-resolution aperture synthesis observations (～500 AU scale), and to investigate that the external environments affect the dissipation of the circumstellar gas by observing CTTSs in two clouds of Ophiuchus which have different characters ( ρ -Oph and North-Oph).<br /> From the ASTE survey observations, <sup>12</sup>CO and <sup>13</sup>CO J=3-2 spectra over the five positions were obtained toward 26 and 13 sources, respectively. We found that all of them have spatially extended emission over the five points. These extended emissions indicate that the all CTTSs observed here are likely to be associated with the ambient gas in the parent clouds. On the other hand, the residual spectra were found for 22 and 7 sources in <sup>12</sup>CO and <sup>13</sup>CO, respectively. Almost all the residual spectra showed the positive profile, but four sources, Sz 83, Sz 88, EL 24, and SR 21, also have the negative above 3σ in <sup>12</sup>CO. We found an evolutional trend in the total intensity of the residual spectra in <sup>l2</sup>CO and <sup>13</sup>CO after the correction of the distance to the source. Although the problems about the optical depth of the ambient cloud still remains, the total intensity of 12CO seems to decrease with increasing stellar age. In addition, we found that there is variation in the corrected total intensities of the residual spectra around an age of 10<sup>6</sup> yr. Two sources in North-Oph, AS 209 and V2508 Oph, conspicuously show the variation, and the circumstellar material has been dissipated obviously in comparison with the sources in the other regions. This diversity probably reflects to the characteristics of the Oph-North cloud, such as the low density of the cloud and/or the effect of the nearby O type star, ζ Oph.<br /> Our NMA observations reveal the circumstellar environments around CTTSs in ρ-Oph and North-Oph as follows. We found point-like continuum emission associated with the star toward seven of the eight targets. The spectral index, β, of 0.1-0.5, the mass of 10<sup>-3</sup>-10<sup>-2</sup>M⊗ and the beam-deconvolved size of 400-1000 AU suggest that the emission originates from the circumstellar dust disk. Our <sup>12</sup>CO and <sup>13</sup>Co J=1-0 maps show that a large amount of the remnant envelope (and a molecular outflow) still remains around the ρ-Oph sources, EL 24, SR 21, DoAr 25, and SR 24. Surrounding gas in most case is gravitationally unbound from the central stars, suggesting that it never accretes to the star-disk system. Although we found Kepler rotating gas disk with a radius of ～800 AU around AS 205 in <sup>12</sup>CO J=l-0, we did not detect significant emission of the circumstellar envelope in <sup>13</sup>CO J=1-0 except for RNO 9l, which is thought to be younger than the other sources. From the ASTE observations in <sup>12</sup>CO J=3-2, we obtained the five-points spectra for six sources in ρ-Oph and North-Oph, and the significant residual spectra were found toward four sources, AS 205, EL 24, SR 21, and DoAr 25. The important point is that circumstellar structures, gas envelope or disk, were found by our NMA observations around these sources. The spectra of <sup>12</sup>CO and <sup>13</sup>CO J=1-0 by the NMA observations were well consistent with the <sup>12</sup>CO J=3-2 spectra in terms of velocity and shape, These facts suggest that the residual spectra originate from the remnant envelope, the molecular outflow, or the gas disk.<br /> On the basis of our results by ASTE and NMA as well as some literature, the circumstellar environments around CTTSs were categorized in the following three cases: (1) envelope with/without outflow, (2) gas disk, and (3) no clear evidence for disk / envelope. The category l has the intensity of the <sup>12</sup>CO residual spectra is large so that it is difficult to reproduce by the emission from the disk with a size of a few hundreds AU. In addition, the <sup>13</sup>CO J=3-2 residual spectra are also seen or the spatially extended gas is found by the high-resolution observations. The category 2 has the <sup>12</sup>CO J=3-2 residual spectra whose intensity is reproducible of the disk with a few hundreds AU radius, and another evidences of the existence of the circumstellar disk are suggested, such as a kepler rotating motion or a scattered disk. The other sources belong to the category 3.<br /> In order to examine the evolutional trend of the categories, a plot of the total intensity after the distance correction v.s. stellar age was made. Our analysis revealed the time scale of the evolution of the circumsterllar structure especially an envelope as follows. It seems that the envelope with/without outflow(category 1) remains at least 1 Myr, which is larger than the free-fall time scale (～10<sup>5</sup>yr). Although we did not find an evolutional trend, the sources with a gas disk (category 2) were seen from 0.7 to 10 Myr. The category 3 was also seen up to 10 Myr. The remnant envelope of category 1 sources in most cases is likely to be gravitationally unbound from the central star, indicating that the material is never supplied to the star-disk system. These results imply the initial condition of a protoplanetary disk is achieved by the age of 1 Myr. | |||||
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| 値 | 有 | |||||
