A Study of 2MASS Low Surface Brightness Galaxies

A study is being made of a sample of about 4,000 Low Surface Brightness (LSB) galaxies detected in the 2MASS all-sky near-infrared survey. These objects have similar central K-band surface brighnesses as optically selected LSBs. About 1,000 of them were observed in HI, and one third of these were detected. BVRI CCD photometry was obtained of 65 objects. These multi-wavelength data provide us with important information on a large, homogenous sample of LSB galaxies (e.g., colors, gas content). They will be used to construct models of the star formation histories and evolution of LSBs, and for a study of their Tully-Fisher (TF) relation, particularly to search for deviations from the ‘classical’ TF relation. This revised version was published online in September 2006 with corrections to the Cover Date.     

星际尘埃研究现状与进展

由于星际尘埃的广泛存在和其在恒星与行星系统的形成、星系以及整个宇宙演化中的重要作用,星际尘埃的研究成为当今天体物理领域的热点前沿课题。该文从尘埃与电磁场相互作用的观测证据出发,系统地介绍了星际消光(包括吸收和散射)、星际红外辐射、星际偏振等的研究现状,讨论了星际元素减损,以及行星际尘埃和陨石中的前太阳尘埃等问题。从相应的观测证据中,可以得到关于星际尘埃的丰度、化学组成、尺寸和形状的信息。该文还对当前比较流行的三种尘埃模型(硅酸盐-石墨-PAHs模型、硅酸盐核-碳有机耐熔质壳层模型和多孔尘埃模型)进行了讨论与比较,对该研究领域中待解决的问题也作了简要的概括。     

Blueshifted diffuse interstellar bands in the spectrum of HD 34078

In this paper, we report the very first observation of diffuse interstellar bands (DIBs) that, in the spectrum of HD 34078 (AE Aur), are blueshifted with respect to the normal position that they have in other objects, where the rest-wavelength velocity frame is determined using very sharp interstellar atomic lines or molecular features. Only reasonably narrow DIBs seemingly show this effect, which is absent in broader ones. The result is confirmed independently using three different spectrographs attached to two different telescopes.     

Low-temperature single crystal reflection spectra of forsterite

The infrared reflectivities of crystalline forsterite (Mg₂SiO₄) were measured for the temperature range 295–50 K for each crystal axis, between wavenumber 5000 and 100 cm⁻¹. The reflection spectra show clear dependence of temperature; most of the bands become more intense, sharper and their peak positions shift to higher wavenumber with decreasing temperature. Reflection spectra were fitted with dispersion formula of damped oscillator model of the dielectric constants and the oscillator parameters in the model were derived. The absorption spectra of forsterite particle are calculated with the derived dielectric constants to show that the forsterite features are good thermal indicator for cold temperature range below 295 K.     

Lines formed in rotating and expanding atmospheres

Spectral lines formed in a rotating and expanding atmosphere have been computed in the frame of the observer at infinity. Two kinds of velocity laws are employed: (i) a uniform radial velocity of the gas and (ii) velocity increasing with radius (i.e. velocity gradients). The atmosphere has been assumed to be rotating with constant velocity. We have considered maximum radial and rotational velocities to be 10 and 3 mean thermal units respectively in an atmosphere whose geometrical thickness is 10 times the stellar radius. The total radial optical depth at line centre is taken to be about 100. In all cases, Doppler profile and a source function which is varying as 1/r ² have been used. Generally, the lines are broadened when rotation is introduced. However, when radial motion is also present, broadening becomes asymmetric and the red emission and blue absorption are enhanced.