The stellar content of a prototype double barred galaxy

High spatial resolution visible and NIR observations of the bar of NGC 5850, a prototype of double barred spirals, together with visible intermediate and high dispersion spectra along the primary bar, are being used, with the aid of simple stellar population synthesis models, to investigate the mean age and metallicity of the different stellar components of the central part of the galaxy. The determination of stellar ages and metallicities would constrain theoretical scenarios for secondary bar formation and the evolution of barred spirals. Unfortunately, we cannot obtain a good fit with simple stellar populations (SSPs) to the spectral indices, so it can not give us, by now, insight into the mean stellar age and metallicity of the real populations in the central region of the galaxy. These preliminary results show a relatively old primary bar with metallicity about solar, although absolute values must be taken with care. The nucleus has a young stellar component, and is very dusty. This revised version was published online in August 2006 with corrections to the Cover Date.     

Outflows from evolved stars with OASIS, NAOMI and GLAS

I discuss the potential of integral-field spectroscopy (IFS) with adaptive optics in the study of the outflows from evolved stars of different masses.With IFS, detailed 3-D spatio-kinematical models of the outflows can be built, providing excellent observational datasets to be confronted with the existing dynamical theories. In addition, if multi-epoch observations are able to resolve the apparent expansion of the nebulae in the plane of the sky, then their dynamics can be further constrained, and other basic quantities like the distance via the expansion parallax, can be determined. The kind of results that can be obtained are illustrated by recent HST and VLT observations of the ring nebula around the symbiotic nova He 2-147.Given the present capabilities of the OASIS integral-field spectrograph of the Isaac Newton Group of Telescopes (ING), classical novae ejecta are the most appealing targets for such kind of studies, provided that its AO system NAOMI is complemented with the forthcoming laser guide star system GLAS.IFS+AO is also a powerful technique to detect faint ionized nebulae around bright stars, like for instance the outflows from luminous blue variables.     

Anisotropic distribution functions for spherical galaxies

A method is presented for finding anisotropic distribution functions for stellar systems with known, spherically symmetric, densities, which depends only on the two classical integrals of the energy and the magnitude of the angular momentum. It requires the density to be expressed as a sum of products of functions of the potential and of the radial coordinate. The solution corresponding to this type of density is in turn a sum of products of functions of the energy and of the magnitude of the angular momentum. The products of the density and its radial and transverse velocity dispersions can be also expressed as a sum of products of functions of the potential and of the radial coordinate. Several examples are given, including some of new anisotropic distribution functions. This device can be extended further to the related problem of finding two-integral distribution functions for axisymmetric galaxies.     

确定恒星表面有效温度的曲面拟合方法

恒星表面有效温度是恒星的一个重要物理参量，是恒星光谱差异的决定因素。本文提出了一种确定恒星表面有效温度的曲面拟合方法，所使用的拟合曲面模型是多项式的指数函数。首先对历史光谱数据进行PCA处理，再根据PCA特征数据与其表面温度的对应关系计算拟合曲面。通过实验，我们发现使用2维PCA数据和指数为3次多项式，根为10的指数函数模型所得到的拟合曲面，不仅有效好的拟合精度而且有很好的鲁棒性。本文的研究结果对恒星表面有效温度的自动测量具有重要的意义。     

ePSF拟合法与Gaussian拟合法的比较

为了解决哈勃空间望远镜的图像欠采样问题,美国学者Anderson和King提出了精确测量星像位置和通量的有效点扩散函数(ePSF—effective point-spread function)拟合法。然而,他们却不加比较地将其应用于地面CCD图像中星像的位置高精度测量。因此,我们试图将ePSF拟合法与经典的高斯函数(Gaussian)拟合法作比较研究。调用CFITSIO库生成模拟的背景图像,并应用不同参数条件下的星像轮廓模型产生非欠采样的星像。最后,分别采用ePSF拟合法与Gaussian拟合法对这些星像进行拟合,并对它们的拟合精度进行比较。实验结果表明,在非欠采样的图像中这两种算法对星像位置的测量几乎是等精度的。     

两种快速星像匹配算法的比较

星像的位置和亮度等信息对天文研究和空间导航具有基础意义。人们常常需要快速匹配从密集星场CCD图像中提取的信息来获取与星表中对应的信息(如位置、光度等),因此开发出了多种星像匹配算法。重点比较了两种最新的星像匹配算法:i)基于向量的方法;ii)基于径向和环向特征的方法。通过对云南天文台1m望远镜观测的CCD图像进行实际匹配,结果表明基于向量的方法更优,它是一种快速的、接近常数时间的匹配算法。     

Spectral Energy Distribution fitting: Application to Lyα-emitting galaxies

Spectral Energy Distribution (SED) fitting is a well-developed astrophysical tool that has recently been applied to high-redshift Lyα-emitting galaxies. If rest-frame ultraviolet through near-infrared photometry is available, it allows the simultaneous determination of the star formation history and dust extinction of a galaxy. Lyα-emitter SED fitting results from the literature find star formation rates 3 M yr⁻¹, stellar masses 10⁹ M for the general population but 10¹⁰ M for the subset detected by IRAC, and very low dust extinction, A_V  0.3, although a couple of outlying analyses prefer significantly more dust and higher intrinsic star formation rates. A checklist of 14 critical choices that must be made when performing SED fitting is discussed.     

Organic matter in space: from star dust to the Solar System

Organic compounds of high degree of complexity are now known to be widespread in the Universe, ranging from objects in our Solar System to distant galaxies. Through the techniques of millimeter-wave spectroscopy, over 140 molecules have been identified through their rotational transitions. Space infrared spectroscopy has detected the stretching and bending modes of compounds with aromatic and aliphatic structures. Analyses of samples of meteorites, comets, asteroids, and interplanetary dust also revealed a rich content of organic substances, some of which could be of extra-solar origin. We review the current state of understanding of the origin, evolution, nature, and distribution of organic matter in space. Also discussed are a number of unexplained astronomical phenomena whose origins could be traced to organic carriers.     

关于仙王座β型变星的脉动激发机制

仙王座β型变星是一类上部主序的短周期内因变星，找到它们及其它早型变星的脉动激发机制是一个长期未能解决的问题．新发表的不透明度表格ＯＰＡＬ比早期的表格有显著的提高，这样不透明度机制似乎就能够解释仙王座β型变星的脉动．本文用线性径向非绝热脉动理论研究了仙王座β型星的脉动稳定性，检查了不透明度机制，用的不透明度是对ＯＰＡＬ表格的一种分析逼近，但计算的十阶以下脉动模式都是稳定的．由于未能证实别人得到的脉动不稳定性，所以我们怀疑不透明度机制可能还不足以激发仙王座β型星，因而给它们的激发机制下一个肯定的结论似乎还为时过早．我们认为对仙王座β型星的脉动起因，有必要进一步研究不透明度机制或别的脉动激发机制．     

Molecular mixing layers in stellar outflows

High velocity jets from young stars interact with the surrounding molecular environment and molecular outflows quite possibly are the result. This interaction can take place through the formation of a turbulent mixing layer. Models have been constructed (following Cant/'o and Raga) of a plane mixing layer in the boundary between a high velocity, atomic wind (i.e., the stellar jet) and a stationary, molecular environment, computed considering a detailed chemical network.The chemical composition of the mixing layer initially corresponds to the direct mixture of the (atomic) jet and (molecular) environmental material. However, we find that the mixing layer is hot (with temperatures exceeding 10⁴ K), and the surprising only partial dissociation of H₂ means that a number of molecules are either created or survive in the high velocity gas. This contrasts with the slower, cooler flows that have tended to be termed a molecular outflow.The emission from such atomic jet/molecular environment mixing layers is dominated by emission in the rotational and vibrational lines of H₂. As a result of the high temperatures and velocities (ranging from zero to the jet velocity) of these mixing layers, the predicted H₂ emission line spectrum has interesting characteristics.