Massive stars are fundamental in determining the appearance and evolution of galaxies. Throughout their life cycle, they produce a dominant fraction of the heavy elements, generate significant amounts of energy and momentum with stellar winds, molecular outflows, UV radiation, and eventually supernova explosions. Massive stars and their associated molecular clouds trace the spiral arms of The Galaxy. In principle, one can construct a simple model of the rotation speed of stars and gas as a function of distance with respect to the center of the Galaxy. Since the Galactic rotation depends on the distribution of mass, rotation curves of galaxy, plotted with rotation velocities as functions of distance from galaxy centers, play a very important role in studies of the mass distributions in disk galaxies. For instance, rigid-body rotation near the Galactic center implies that the mass must be roughly spherically distributed and the density nearly constant. On the other hand, flat rotation curves suggest that the bulk of the mass in the outer region of the Galaxy are spherically distributed with a density law that is proportional to γ(-2) On the other hand, observations of rotation curves in external galaxies revealed that they are basically flat within optical disks of spiral galaxies. These flat rotation curves observed in external galaxies provided strong evidences for the dark matter, which can not detect with observations, in outer regions of galaxy. In contrast, although a large number of observations and simulations have been made to determine it of the Galaxy, the outer rotation curve is still crude because first, in case of our galaxy, it is difficult to determine its spiral structure because of our view from the interior of the Galaxy. Second, if the observations are performed with 1-dimension, only the line of sight component of spatial velocity, it is difficult to determine accurate distances (that is, a kinematic distance) of the Galactic objects which can be trace the rotation curve, such as bright stars and molecular clouds enough to observable at the far distance. Another reason is a difficulty to measure proper motions of Galactic objects, a low spatial resolution of observation system gives large uncertainties to study of the Galaxy. Besides, the large scatter in the data from observation, not only caused by the different methods but also due to the different parameters can be reason of difficulties to determine the rotation velocities.

In spite of numbers of observational studies on massive star-formation processes, the formation of massive stars is poorly understood. There are two scenarios to explain the formation of massive stars. The first is the accretion scenario, similar to process for low-mass star, of which a accreting process result in a formation disk and outflow then stellar core is formed with these process. Another scenario, the coalescence scenario proposes that massive stars are formed by merging of stars with low or intermediate masses. Massive stars are born in the dense cores of giant molecular clouds and deeply embedded. In addition, massive stars are usually born in clusters, and hence their individual studies are usually affected by confusion, which is more serious for distant massive star forming regions than for nearby low-mass sources. Moreover, the time scale of high-mass stars which much shorter than low-mass stars also cause the lack of knowledge in massive star forming process. The presence of maser emission represents the physical conditions of star forming region, in all case, the maser spots are not only extraordinarily bright, but also small in size and narrowly confined in frequency. Therefore masers act as powerful signposts of active star formation and as unique tool to probe the physical conditions and kinematics of these regions.

The distance to source is one of the most fundamental physical properties, if it is estimated with high accuracy then can be applied to study a structure of the Galaxy, to luminosities and masses of stars in massive star forming regions, and eventually one can determine the type of stars. The direct method to obtain distance to objects is to measure the annual parallax. As the Earth orbits around the Sun with a period of one year, star positions show seasonal modulations. By measuring this apparent displacement of star positions (annual parallax) which nearby objects have a larger parallax than more distant objects, one can determine the distance. For instance, the Hipparcos satellite has used this technique for over 100,000 nearby stars, and has reached distances of ~300 pc by parallax measurements. However, this is much smaller than the size of the Galaxy which radius of the Galactic disk is thought about 15 kpc. Hence, present astrometry in the optical domain also has the difficulties to study a distant stars and structure of the Galaxy. At radio wavelengths, however, high precision astrometry have been made with VLBI (Very Long Baseline Interferometry), while VLBI observations suffer from the fluctuation of atmosphere, which mainly due to the water vapor in the troposphere, this problem can be eliminated by phase-referencing method which has been developed to cancel out the tropospheric fluctuations. In phase-referencing observations, one target and nearby reference sources of one or more are observed by rapidly switching the telescope, where reference sources have been known their absolute position already. After correcting for the influence of troposphere, relative position of the target source with respect to the reference sources can be determined. By performing VLBI observation with phase-referencing method, therefore one can measure the absolute positions, parallaxes, and proper motions of target sources with high accuracy of sub milliarcsecond (mas) or a few μas. VERA (VLBI Exploration of Radio Astrometry) is a VLBI array aimed for obtaining 3-dimensional map of the Galaxy. Most unique aspect of VERA is 'dual-beam' receiver system, which can observe two nearby sources at the same time. VERA's dual-beam observations more effectively cancel out the atmospheric fluctuations than single-beam VLBI system, and can measure the absolute positions of target sources with high accuracy of ~10μas.

In this thesis, we report the studies of the Galactic rotation curve and massive star-forming regions performed H2O maser (22GHz) observations with phase-referencing by VERA. Three massive star-forming region of IRAS 06058+2138, IRAS 19213+1723, and AFGL 2789 are selected, these are thought that locate in the different position of the Galaxy and different distance with respect to the Galactic center, and trace slightly different evolution phase and stellar type. The galactic coordinates of IRAS 06058+2138, IRAS 19213+1723 and AFGL 2789, (l, b)=(188°.95, 0°.89), (52°.10, 1°.04) and (94°.60, -1°.80), respectively. For these massive star forming regions, the first astrometric observations are performed, therefore we can understand the importance of this study, as defining the kinematics of sources in massive star-forming region and the Galactic plane as well as determining the absolute position of the celestial sources on the sky plane. In previous studies for IRAS 06058+2138, IRAS 19213+1723 and AFGL 2789, kinematic distances have uncertainties significantly greater than their parallax distances. However, by performing VLBI observation with phase-referencing method of VERA, the absolute positions of target sources are measured with high accuracy of sub-mas or several μas. Therefore our parallaxes and proper motions yield full space motions accurate to the order of a few to sub km s(-1) The parallaxes of IRAS 19213+1723 and AFGL 2789 are 0.569±0.0344 mas (1.76±0.106 kpc) and 0.326±0.0314 mas (3.07±0.295 kpc), and these are located in the Perseus spiral arm. The parallax of IRAS 19213+1723 is 0.569±0.0344 mas (1.76±0.106 kpc), placing it in the Carina-Sagittarius arm.

In chapter 6, we present the studies of the rotation curve of outer Galactic plane and earlier phase of stellar evolution in massive star forming regions by using distributions and proper motions of H2O masers. The rotation curves of the Galaxy are determined with 3-dimensional velocity components of H2O maser features, we show that the flat rotation curve in the range of 6.4 kpc≦ R ≦ 13 kpc as combining with S269 which observed with VERA, and overall rotation curves of the Galaxy are not depending on the rotation velocity of LSR, Θ0. However, if one pay attention to detailed profile of rotation curve, there are some inconsistency from flat rotation curve. Our results as well as the previous observations of HI and CO molecular emission show a dip profile from ~8.5 to 11 kpc although those show the flat rotation curve as overall range of outer region. Dip profile, due to slower than the Galactic rotation, can be explain either that another structure except the galactic bulge, disk and dark halo in outer region of the Galaxy is exist and provides gravitational influence, or dip profile trace the peculiar motion of the Perseus arm itself. In addition, IRAS 06058+2138, IRAS 19213+1723, and AFGL 2789 are moving systematically toward the Galactic center, it is difficult to explain as their random motions described by the Schwarzschild distribution function. Thus we suggest that this peculiar motions of IRAS 06058+2138 and AFGL 2789 located in the Perseus arm may be trace the peculiar motion of the Perseus arm, and IRAS 19213+1723 in the inner Galaxy are affected by the gravitational potential of the central bar. Alternatively, this peculiar motion toward the Galactic center can be explain that LSR is moving to the opposite of Galactic center direction.

On the other hand, from the studies of the kinematics and earlier phase of stellar evolution in massive star forming regions, we determined the luminosities of IRAS 06058+2138 (MM1, MM2), IRAS 19213+1723 and AFGL 2789 with new distances determined by phase-referencing VLBI observations, and estimated stellar type are early B type for all sources. From proper motions and LSR velocities of the H2O masers, we found that MM1 of IRAS 06058+2138 accompanied with an outflow and a rotating expanding disk. Then, we have calculated the disk size to be 90 mas (~160 AU at the distance of 1.76 kpc) and the mass of the central star to be about 11 MΘ. The estimated stellar mass is comparable to early B type star, that is consistent with the stellar type determined with the luminosity. The H2O masers in AFGL 2789 and IRAS 19213+1723 may trace the bipolar outflow with wide angle and outflow with expanding, respectively. In the case of IRAS 19213+1723, the maser features are concentrated in compact region of less than only 1 mas and have high velocity difference between the H2O emission and CO emission, we suggest that these high velocity maser features may be appeared from relatively less dense region of ambient gas. And finally, by comparing the previous observational results of MM1 and MM2 in IRAS 06058+2138, IRAS 19213+1723 and AFGL 2789 to our results of phase-referencing VLBI observations, the evolutionary status can be determined and suggested that the H2O masers of MM2 trace the earlier evolutionary phase than the methanol masers, therefore MM2 represents the earliest evolutionary phase among these sources, and IRAS 19213+1723 represents the latest one.

本論文は、3つの大質量星形成領域を選び国立天文台VERA 観測網によって水メーザー天体の高精度位置天文観測をまとめたものである。年周視差および固有運動を高精度で測定し、それらから銀河系回転のパラメータやダークマターの存在および銀河系の太陽外周の特異構造などについて重要な知見を与えた。その後、これらの大質量星形成領域において生成されている天体がB 型星であることを距離計測から明らかにし、相互比較によって進化過程におけるメーザー発生や赤外線および電波放射の順序についても重要な知見が得られたものである。

本論文は5章からなる。第1章では銀河系の回転とダークマター、大質量星形成領域の特性および各種メーザー放射の特性について、およびVLBI(超長基線干渉計)による位置天文計測について概説し、最後に本論文の目的について要約している。

第2章は観測に使用されたVERA(VLBI Exploration of Radio Astrometry)システムについて述べ、観測期間中に測定された開口能率、アンテナビーム半値幅、指向および受信機特性などの基本特性を示している。

第3章では、観測対象となった大質量星形成領域IRAS 06058+2138、IRAS 19213+1723、AFGL 2789 についてそれぞれ、光赤外および電波観測についてのまとめと、VERA による2006 年から2008 年にかけての9ないし10 エポックの水メーザー観測結果を示している。各天体は数個から数十個の水メーザー放射領域(以下、水メーザー源と呼ぶ)が存在する。VERA システムの特長である、参照天体による位相準拠法を利用して、これら水メーザー源個々の位置の変化を追跡する。各エポック間の水メーザー源の同定は視線速度の情報も考慮して行ない、固有運動と年周視差を求める。年周視差は天体毎に平均して、天体までの距離を決定する。固有運動もそれぞれ天体毎に平均し、これらを基に再度各水メーザー源の固有運動と年周視差を決定する手法でイタレーションを行ない、収束する事を確認している。このように求めた、それぞれの天体の水メーザー源の固有運動は、次章での銀河系内回転運動や天体の性質の議論に利用する。上記3天体の年周視差はそれぞれ、0.57±0.03 mas、0.25±0.04 mas、0.33±0.03 mas で、対応する距離はそれぞれ1.8±0.1 kpc、4.0±0.6 kpc、3.1±0.3 kpc である。ここで2天体の位置精度は10%以下の高精度で決定された。これらは全て、これまでに求められた最高の距離精度を与えたものとなった。

第4章の議論では、まず観測系の誤差を検討し、絶対位置計測について大気補正の残差が最大の誤差要因で0.3-0.5 mas となる事を示した。以降は前章の結果を踏まえて、いくつかの議論を行なっている。最初に各天体の固有運動と位置とを使いそれぞれの銀河系回転速度を導き、2007 年に同様にVERA によって求められたS269 のそれと合わせて、太陽系近傍で銀河中心からの距離が6 kpc から14 kpc に渡って銀河系回転速度が基本的に200 km/s とフラットである事を明らかにした。太陽より外側の銀河系回転速度の測定はこれまでに中性水素線や分子線の観測などで行なわれて来た。これらの視線速度の計測では、原理的に太陽より外側天体の距離の測定には不定性が大きく、これまではフラットであること、即ちダークマターの存在に対して明確な結論が出せなかった。本論文では年周視差の計測による回転運動をVERA により直接高精度で計測した。高精度で決められた点はこれまで1点であったが、それに今回複数の点を加えてフラットな回転曲線であることを初めて明確に示した。本論文により示されたこの意義は高く評価する。さらに、太陽の外側2 kpc 程度にわたって回転速度の減少を示すことを指摘し、リング構造等の質量分布の存在を示唆した。また銀河系中心方向に向かう固有運動成分が3天体ともにあることに着目し、ペルセウス腕の影響、またヒッパルコス衛星の観測により決められた太陽系近傍の基準値LSR の系統的な誤差の可能性などについて検討した。次に、3天体までの距離が正確に決定された事により、それぞれの光度から、これらは物理的性質が似ているB 型星を持つ大質量星形成領域であることを明らかにした。水およびメタノールメーザー放射、電波、中間赤外、および遠赤外放射の有無を比較し、進化過程での放射について以下の議論をした。最も初期段階のIRAS 06058+2138 中にあるMM2 で水メーザー放射および遠赤外放射が始まり、次に同MM1 およびAFGL2789 がほぼ同一の進化段階としてメタノールメーザー放射および中間赤外放射を示す。その後IRAS 19213+1723 で電波放射が始まる一方、メタノールメーザー放射は止まるという描像を提案した。このような大質量星形成領域の各進化段階での特徴的な放射を明らかにする事は、進化の研究を進める上で重要な手がかりを与えるものであり、本論文はそれに大きく寄与することとなる。

第5章には以上のまとめが記されている。

以上、本論文は数kpc 離れた大質量星形成領域の距離を高精度で系統的に複数観測した研究として高く評価出来る。

なお、本論文の一部は小林秀行・本間希樹・柴田克典・廣田朋也・小山友明との共同研究であるが、論文提出者が主体となって解析及び論証を行ったもので、論文提出者の寄与が十分であると判断する。

したがって、博士(理学)の学位を授与できるものと認める。