Nonlinear Dynamics of Ionization Fronts in HII Regions

Hydrodynamic instability of an accelerating ionization front (IF) is investigated with 2D hydrodynamic simulations, including absorption of incident photoionizing photons, recombination in the HII region, and radiative molecular cooling. When the amplitude of the perturbation is large enough, nonlinear dynamics of the IF triggered by the separation of the IF from the cloud surface is observed. This causes the second harmonic of the imposed perturbation to appear on the cloud surfaces, whereas the perturbation in density of ablated gas in the HII region remains largely single mode. This mismatch of modes between the IF and the density perturbation in the HII region prevents the strong stabilization effect seen in the linear regime. Large growth of the perturbation caused by Rayleigh-Taylor-like instability is observed late in time.     

On a new cluster of H<Subscript>2</Subscript>O maser spots in the source ON2

We present the results of our observations of the H₂O maser emission toward the complex source ON2 associated with an active star-forming region. The observations were performed in a wide range of radial velocities, from ?75 to 90 km s^?1. We have detected an emission with flux densities of 9.2, 4, and 26 Jy at radial velocities of ?33.5, ?24.4, and ?18.8 km s^?1, respectively, at which no emission has been observed previously. The detected emission is most likely associated with a hitherto unknown cluster of maser spots located between the northern (N) and southern (S) components of the source ON2 (closer to the northern one). This cluster may be associated with one of the three CO molecular outflows in the ON2 region. We have also detected an emission at ?22 and ?14.5 km s^?1 in N and at 12.6 km s^?1 in S, which has extended significantly the velocity ranges of the maser emission in these sources and allowed their models to be improved.     

Evidence of Evolution in the Dense Cores in Massive Star Forming Regions

The excitation of H2O masers usually needs very high density gas, hence it can serve as a marker of dense gas in HII region. We selected a sample of H2O maser sources from Plume et al. (four with, and four without detected CS(J = 7-6) emission), and observed them in 13CO(J=1-0) and C18O (J=1-0). C18O (J=1-0) emission was detected only in three of the sources with detected CS(J=7-6) emission. An analysis combined with some data in the literature suggests that these dense cores may be located at different evolutionary stages. Multi-line observation study may provide us clues on the evolution of massive star forming regions and the massive stars themselves.     

PNE: The Key to Chemical Evolution in BCDGs

Planetary Nebulae have proven to be an essential key to understand the long term chemical enrichment of the interstellar medium due to low mass stars. They allow to study the original abundances of the star, and the effect of the star on the interstellar medium. Blue compact dwarf galaxies are known to host violent star formation in very heavy element depleted environments. They also show traces of past star formation (Doublier et al., 1999; 2001), including AGB starsand red giants which are responsible for long term enrichment. However, models fails to reproduce the low metallicities observed if those stars are taken into account. We observed PNe in BCDGs, and made a comparative study of the abundances in the HII regions and of the PNe. This revised version was published online in September 2006 with corrections to the Cover Date.     

The spectral investigation of seven HII regions in Kazarian galaxies

Spectra from SDSS DR5 are used for a spectral study of seven HII regions in Kazarian galaxies. The abundances of helium and heavy elements, and also the quantity of ionizing stars and the star formation rate in these galaxies, are determined. The oxygen abundance, 12+log(O/H) lies between 7.94 and 8.35. The mean abundance ratios log(S/O), log(Ar/O), and log(Ne/O) are equal to −1.63, −2.37, and −0.78, respectively. For these HII regions, log(N/O) lies between −0.63 and −1.37. On an N/O-O/H diagram they occupy the same area as high excitation HII regions. In all likelihood the ages of the HII regions studied here exceed the 100–300 million years, required for the enrichment in nitrogen by intermediate-mass stars.. The star formation rate is one order as in the HII regions of spiral and irregular galaxies and ranges from 0.05 ÷ 0.81 M_⊙. year⁻¹. __________ Translated from Astrofizika, Vol. 51, No. 1, pp. 75–87 (February 2008).     

Observational appearance of circumstellar ionized regions and ultracompact HII regions

Im Rahmen eines entwickelten theoretischen Modells, das die Dynamik von Protosternhüllen während der Entstehung massereicher Sterne dar-stellt, werden die Hauptentwicklungsstufen von HII-Gebieten bestimmt. Es sollen charakteristische Besonderheiten der Entwicklung von HII-Gebieten um entstehende Sterne mit Leuchtkräften von und besprochen werden.     

Magnetic Fields in Star-Forming Regions: Theoretical Aspects

We review the basic theoretical elements leading to our current understanding of the role of magnetic fields in the process of star formation. In particular, we concentrate on: (i) the relevance of the mass-to-flux ratio for the stability of molecular clouds; (ii) the consequences of magnetic flux leakage for the evolution of cloud cores; (iii) the phase of anisotropic dynamical collapse following the formation of strongly peaked density distributions; (iv) the mechanism of magnetic braking as a possible solution to the angular momentum problem in star formation.     

Galaxy Evolution in the Northern HDF

The history of star formation in the Northern Hubble Deep Field is probed using a combination of optical and near infrared images taken with WFPC2 and NICMOS on the Hubble Space Telescope. These images cover more than a factor of five in wavelength. This broad wavelength coverage allows accurate photometric determinations of redshift, extinction and intrinsic spectral energy distribution for each galaxy. From these parameters the star formation rate for each galaxy is determined by relating the 1500 angstrom flux to the net star formation rate. We then correct the rates at high redshift for the effects of surface brightness dimming by using a standard form of the star formation intensity distribution. Our measurements show that the star formation rate in the Northern HDF is roughly constant from a redshift of 1 through 6. This revised version was published online in September 2006 with corrections to the Cover Date.     

近红外偏振观测在恒星形成研究中的应用

近红外偏振是研究恒星形成的有效工具.该文介绍了近红外偏振器的工作原理,然后分几个方面介绍了近红外偏振在恒星形成研究中的应用.红外反射云能很好地示踪年轻星天体及分子外流,通过分析偏振矢量的方法确定红外反射云的偏振对称中心,从而确定它的照亮源;偏振波长相关曲线包含了年轻星天体的星周物质的很多信息;年轻星的分子外流导致了红外反射云的形成,因此红外反射云的照亮源通常与年轻星天体成协,并是分子外流的驱动源;一些年轻星天体埋藏得很深,一般在近红外波段无法直接探测到,人们称之为深埋源,通过分析偏振矢量的方法可以找到深埋源;一般认为比较年轻的年轻星天体都是有尘埃盘的,尘埃盘的存在会导致它的偏振形态出现偏振盘,偏振盘町以用来研究尘埃盘;恒星形成区里成员星的偏振主要是由尘埃的二色性消光产生的,这样偏振方向会平行于致使尘埃排列的磁场的方向,从而能够揭示磁场的结构.最后进行了总结,并论述了中远红外偏振研究的优势和意义.     

Chemical Evolution of Protostars

This review discusses the evolution of the chemical composition of the molecular gas throughout the process of star formation. It covers the initial stages of cloud-core formation, gravitational collapse and the formation of the star, the effects of stellar radiation and outflows on the surrounding cloud, and the composition of the circumstellar disk. A brief overview of relevant chemical processes sets the stage. The ensuing evolution of the chemistry is governed by several competing timescales and by energy input by the young star(s). Special attention is given to the role of molecular freeze-out on dust grains and to deuterium fractionation in cold gas.     

Cosmological Evolution of Supergiant Star-Forming Clouds

We present the results of very high resolution CDM simulations of galaxy formation designed to follow the formation and evolution of self-gravitating, supergiant star-forming clouds. We find that the mass spectrum of these clouds is identical to that of globular clusters and GMCs; dN/dM ∝ M ^-1.7 ± 0.1. This revised version was published online in September 2006 with corrections to the Cover Date.     

From discs to planetesimals: Evolution of gas and dust discs

I review the processes that shape the evolution of protoplanetary discs around young, solar-mass stars. I first discuss observations of protoplanetary discs, and note in particular the constraints these observations place on models of disc evolution. The processes that affect the evolution of gas discs are then discussed, with the focus in particular on viscous accretion and photoevaporation, and recent models which combine the two. I then discuss the dynamics and growth of dust grains in discs, considering models of grain growth, the gas–grain interaction and planetesimal formation, and review recent research in this area. Lastly, I consider the so-called “transitional” discs, which are thought to be observed during disc dispersal. Recent observations and models of these systems are reviewed, and prospects for using statistical surveys to distinguish between the various proposed models are discussed.     

Evidence for Co-Propagation of 4765- and 1720-MHz OH Masers in Star-Forming Regions

Two star-forming regions Cepheus A and W75N, were searched for the 4765-MHz OH maser emission using the multi-element radio linked interferometer network (MERLIN). The excited OH emission has an arc-like structure of 40 mas in Cep A and a linear structure of size 45 mas in W75N. We also found the 1720-MHz line in Cep A and Hutawarakorn [MNRAS 330 (2002) 349] reported the 1720-MHz emission in W75N. The 1720- and 4765-MHz OH spots coincided in space within 60 mas and in velocity within 0.3 km s^–1 in both targets implying that both maser transitions arise from the same region. According to the modelling by Gray [MNRAS 252 (1991) 30] the 1720/4765-MHz co-propagation requires a low density, warm environment. The masers lie at the edges of H II regions where such conditions are expected.     

早型星系的恒星形成活动演化研究

从观测上测定早型星系中恒星形成活动随红移的演化有助于理解这类星系的形成演化.结合GEMS（Galaxy Evolution from Morphology and SEDs）巡天的HST/ACS（Hubble Space Telescope/Advanced Camera for Surveys）高分辨图像和CDFS（ChandraDeep Field South）天区Spitzer、GALEX（Galaxy Evolution Explorer）等多波段数据,基于形态、颜色和恒星质量选出一个0.2≤z≤1.0红移范围的包含456个早型星系的完备样本.利用stacking技术测量了样本星系紫外与红外平均光度,估计早型星系的恒星形成率.结果显示,早型星系中的恒星形成率较低(<3 M_⊙·yr^-1),随红移递减而降低.在红移z=1以来的恒星形成贡献的质量小于15%.星族分析亦肯定大质量早型星系的主体星族形成于宇宙早期（z>2）.     

Atmospheric Eclipses in Wr+o Binaries: From Kopal and Shapley to Present Days

Kopal and Shapley (1946) were the first to suggest to solve an integral equation for interpretation of atmospheric eclipses in WR+O binary systems. In our studies, this idea was developed in two ways: solution of two integral equations; development of efficient methods of solution of ill-posed problems. Application of the new method to the interpretation of the light curve of the WN5+O6 eclipsing binary V444 Cyg allowed us to determine the radius and the temperature of the WN5 star core. Some astrophysical applications of these results are presented.     

The Kinematics, Physical Condition and Magnetic Field of the W3 Irs5 Region

We have independently investigated water masers associated with the star-forming region W3 IRS5 with the VLBA. Imai et al., found that the maser 3-D motions exhibit outflows, which likely originate from two of the hyper-compact HII regions in this source. Sarma et al., have detected the Zeeman effect in water masers toward W3 IRS5 and measured line-of-sight magnetic field strengths of between 14 and 42 mG. The directions of maser linear polarization are well aligned in the whole maser region and perpendicular to the estimated magnetic field. These polarimetric results are consistent with an hourglass model of the magnetic field in W3 IRS5. Imai et al., also have analyzed the microstructures found in the individual maser features (1AU), which exhibit the fractal fashion and express turbulence on very small scales. Together, we have demonstrated that observations of water masers enable us to comprehensively enhance our understanding of the early stages of the formation of massive stars in clusters.     

银河系化学演化研究进展

银河系的形成与演化是天体物理学研究的重大前沿课题,银河系的化学演化在其中更具有极其重要的地位。随着观测资料的不断积累和理论工作的不断深入,银河系化学演化的研究取得了一系列进展。在观测方面,从太阳附近区域,整个银盘,银晕和核球等方面简要回顾了银河系化学演化模型主要观测约束的近期结果;在化学演化模型方面,回顾了银河系化学演化研究的发展历程和近期进展,并对未来的研究进行了展望。     

Stability of Relativistic Hydrodynamical Planar Jets: Linear and Nonlinear Evolution of Kelvin-Helmholtz Modes

Some aspects about the stability of relativistic flows against Kelvin-Helmholtz (KH) perturbations are studied by means of relativistic, hydrodynamical simulations. In particular, we analyze the transition to the fully nonlinear regime and the long-term evolution of two jet models with different specific internal energies.     

COSMOS场中星系恒星形成的演化研究

基于COSMOS(Cosmic Evolution Survey)/Ultra VISTA(Ultra-deep Visible and Infrared Survey Telescope for Astronomy)场中多波段测光数据,利用质量限选取了红移分布在0z3.5的星系样本.通过UVJ(U-V和V-J)双色图分类判据将星系分类成恒星形成星系(SFGs)和宁静星系(QGs).对于红移分布在0z1.5范围内且M*1011M⊙的QGs来说,该星系在样本中所占比例高于70%.在红移0z3.5范围内,恒星形成星系的恒星形成率(SFR)与恒星质量(M*)之间有着很强的主序(MS)关系.对于某一固定的恒星质量M*来说,星系的SFR和比恒星形成率(s SFR)会随着红移增大而增大,这表明在高红移处恒星形成星系更加活跃,有激烈的恒星形成.相对于低质量的星系来说,高质量的SFGs有较低的s SFR,这意味着低质量星系的增长更多的是通过星系本身的恒星形成.通过结合来自文献中数据点信息,发现更高红移(2z8)星系的s SFR随红移的演化趋势变弱,其演化关系是s SFR∝(1+z)0.94±0.17.