天山雪岭云杉个体生物量分配及其变化规律的研究

为了探讨天山雪岭云杉林生物量在个体组织中的分配情况及其变化规律,在研究区进行了大量的野外测量,利用已有的雪岭云杉林估算方程,分析了天山雪岭云杉林生物量在各器官（干、枝、叶、皮、根）中的分配及其变化规律。结果表明：（1） 研究区雪岭云杉林的平均生物量为388.74 t·hm^-2,树木各器官中,干、枝、根、叶和皮分别占生物量的43.65%、28.60%、13.49%、11.08%和3.18%。（2）各径级生物量所占百分比为：33.53%（40~50 cm）、20.13%（20~30 cm）、19.59%（30~40 cm）、18.19%（50~60 cm）和2.05%（10~20 cm）;树木生物量在不同树高中的分配表现为：48.78%（20~30 m）＞35.27%（10~20 m）＞14.70%（30~40 m）＞1.25%（0~10 m）;地上和地下生物量的分配比例为：87.54%和12.46%,分别为340.30 t·hm^-2和48.44 t·hm^-2。（3） 随海拔升高,天山雪岭云杉林生物量呈“单峰”变化,在海拔2 100~2 400 m处达到最大值611.58 t·hm^-2;干、皮生物量所占比例随海拔升高而减小,枝生物量逐渐增加,叶、根生物量呈先减小后增加的趋势;径级20~30 cm、30~40 cm和50~60 cm的生物量随海拔升高均呈“单峰型”变化趋势,都在海拔2 100~2 400 m处达到最大;雪岭云杉林不同树高生物量随海拔的升高呈现的趋势不同。天山雪岭云杉林生物量和年均降水量随经纬度的升高均呈降低变化,研究区林分生物量自西向东总体呈现逐渐降低的趋势;林分密度、海拔和降水共同决定了森林生物量的大小及其变化规律,海拔2 100~2 400 m是本研究区雪岭云杉林生长的最适宜场所。结果可为雪岭云杉林生态系统的恢复和重建提供基础资料,对研究区进行综合管理与生态健康分析具有重要意义。     

A multiple shooting approach for the numerical treatment of stellar structure and evolution

We present a new numerical method for solving the system of partial differential equations describing the structure and evolution of a spherically symmetric star. As usual, we employ the transversal method of lines in order to split the equations into a coupled spatial and temporal part. The novel features of the algorithm are the following: (a) Instead of using the Lagrangian picture we formulate the system of partial differential equations in the Eulerian picture. (b) We reformulate the equations of stellar structure as a multipoint boundary-value problem. By means of this reformulation the rather clumsy iterative matching procedure of stellar atmosphere and interior is avoided. (c) The multipoint boundary-value problem is solved by the multiple shooting method. This approach not only ensures a high accuracy of the stellar models calculated at each time step but also allows the free boundaries inside the star due to different energy transport mechanisms to be located exactly. (d) The time derivatives involved in the stellar-structure equations are discretized implicitly to second order accuracy. Moreover, at each time step, the chemical abundances are determined by using a sophisticated update procedure. In this way, a high accuracy is achieved with respect to the integration in time. The algorithm has turned out to be exceedingly reliable and numerically accurate. This is shown by the evolution of a 1 M_⊙ star up to the hydrogen-shell burning phase. In this example, the virial theorem, the law of mass conservation, and the law of energy conservation is fulfilled to a hitherto unattainable degree of accuracy. Since the multiple shooting method, which is at the heart of our approach, is a perfect example of a parallel algorithm, the computational speed of the algorithm might be substantially improved provided easy-to-program, high-performance parallel computers with sufficiently many processors become available in the near future.     

A uvbyβ photometric analysis of the Monoceros star‐forming field

This investigation presents a new analysis of the spatial distribution of the bright early‐type stars in the field of Northern Monoceros. A database of all O–B9 stars with available uvbyβ photometry is collated and a homogeneous distance scale is established for the clusters and layers of field stars. We provide revised distances for NGC 2264 and NGC 2244 of 833±38 (s.e.) pc and 1585±60 (s.e.) pc, respectively. We present arguments that there might be substructures in the clusters projected along the line of sight. According to the present sample the classical Mon OB2 association at 1.6 kpc is represented by a relatively compact group at 1.26 kpc in the vicinity of NGC 2244 and a layer of massive stars located between 1.5 and 3 kpc (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The symbiotic star CH Cygni – I. Non-thermal bipolar jets

Very Large Array surface brightness and spectral index maps of the evolving extended emission of the triple symbiotic star CH Cygni are presented. These are derived from observations at 4.8, 8.4 and 14 GHz between 1985 and 1999. The maps are dominated by thermal emission around the central bright peak of the nebula, but we also find unambiguous non-thermal emission associated with the extended regions. Our observations confirm that this is a jet. The central region has been associated with the stellar components through Hubble Space Telescope imaging. If the jets are the result of ejection events at outburst, expansion velocities are consistent with those from other measurement methods. We propose that the non-thermal emission is caused by material ejected in the bipolar jets interacting with the circumstellar wind envelope. The resulting shocks lead to local enhancements in the magnetic field from the compact component of the order of 3 mG.     

Can late-type active stars be explained by a dipole magnetic trap?

In this paper we review four different types of X-ray and/or radio observations of active late-type stars. We then consider if a single magnetic source configuration – a toroidal dipole magnetic trap – can possibly explain these various different observations. We conclude that, indeed, dipole magnetic confinement (similar to the magnetic configurations of the Earth's radiation belts and the case of Jupiter and the Io torus) can explain all the diverse observational data. We take this to be very strong observational support for this type of magnetic confinement scheme. We also consider that this magnetic configuration is only likely to be established and maintained in the most active stars.     

Massive stars in the UV

We emphasize in this paper the importance of the UV range for our knowledge of massive stars and the fundamental role played by past and present space-based UV capabilities (IUE, HST, FUSE and others). Based on a review of the work developed in the last years and the state of the art situation for quantitative spectroscopy of massive stars, we present crucial advances which could be addressed by hypothetical future space-based UV missions. Advantages and unique data that these missions could provide are explained in the context of our present knowledge and theories on massive stars in the Milky Way and nearby galaxies. It is argued that these studies are our key to a correct interpretation of observations of more distant objects.     

Variability of long-period pulsating stars. II. Additional parameters for classifying stars

Results are presented from an analysis of observations in a sample of 53 long-period pulsating stars (44 of which are Mira-type and 9 are semiregular variables). A catalog of the characteristics of the individual pulsation cycles of the light curves is obtained for these stars. A correlation analysis of the characteristics of the individual pulsation cycles is carried out for each star. A number if diagrams relating the variability of the characteristics of the individual cycles to the average parameters of the variability of the stars and to their spectral classes. New criteria for classifying long-period variables are proposed on the basis of these diagrams. __________ Translated from Astrofizika, Vol. 49, No. 4, pp. 595–612 (November 2006).     

Statistical properties of H II regions in the disc of M100

From a new mosaic image in the Hα line of the complete disc of the spiral galaxy M100, a catalogue is composed listing 1948 individual H II regions. I give details of the data collection and reduction procedure, and of the production of the H II region catalogue. For each H II region, the catalogue gives its position relative to the centre of the galaxy, its deprojected distance to the centre, its radius and its calibrated luminosity. An indication is included as to whether the H II region is located in the arms, between them, or in the circumnuclear star-forming region. I present the results of a statistical study of properties of the H II regions. The luminosity function of the complete ensemble of H II regions shows a characteristic shape well fitted by a power-law slope in the higher luminosity range, and complying with literature values for galaxies like M100. Luminosity function slopes for arm and interarm H II region populations separately are found to be equal within the errors of the fits, indicating that whereas the density wave accumulates material into the arm regions, and may trigger star formation there, it does not in fact change the mass distribution of the star-forming clouds, nor the statistical properties of the H II region population. Diameter distributions and the radial number density distribution are discussed. The latter indicates those areas where most star formation occurs: the circumnuclear region and the spiral arms. The huge number of H II regions allowed the construction of a number of independent luminosity functions at different distances to the nucleus. The slope of the luminosity function shows a marginal decrease with increasing distance from the centre, which could indicate a gradual change towards shallower IMF slopes with increasing galactocentric distance, or an evolutionary effect.     

The space density of cataclysmic variables: constraints from the ROSAT North Ecliptic Pole survey

We use the ROSAT North Ecliptic Pole (NEP) survey to construct a small, but purely X-ray flux-limited sample of cataclysmic variable stars (CVs). The sample includes only four systems, two of which (RX J1715.6+6856 and RX J1831.7+6511) are new discoveries. We present time-resolved spectroscopy of the new CVs and measure orbital periods of 1.64 ± 0.02 and 4.01 ± 0.03 h for RX J1715.6+6856 and RX J1831.7+6511, respectively. We also estimate distances for all the CVs in our sample, based mainly on their apparent brightness in the infrared. The space density of the CV population represented by our small sample is  1.1^+2.3_−0.7× 10⁻⁵ pc⁻³  . We can also place upper limits on the space density of any subpopulation of CVs too faint to be included in the NEP survey. In particular, we show that if the overall space density of CVs is as high as  2 × 10⁻⁴ pc⁻³  (as has been predicted theoretically), the vast majority of CVs must be fainter than   L _X≃ 2 × 10²⁹ erg s⁻¹  .