昴星团平均视差与运动学参数的最大似然估计

本文利用最大似然原理求解移动星团问题，同时确定了昂星团的平均距离和运动学参数及其相应的弥散度·归算结果得到昂星团距离为d=135．6±0．7pc，相应弥散度。σd=7.7±08pc；昂星团平均空间速度为V=25.9±0．1km／s，相应弥散度σv=0．6±0．1km／s，以及昴星团会聚点坐标（A，D）＝（6h47m24s±2m00s-47°．4±0°.3.）。     

Hipparcos后的昴星团距离

详细介绍了Hipparcos测定的昴星团距离与以往地面测定结果比较所出现的差异、近年来对昴星团距离新的测定和对Hipparcos数据作的重新处理以及对造成不一致的原因所进行的探讨，最后对有关进展加以评述。     

疏散星团中的耀星及其演化学意义

耀星是红矮星早期演化的一个必经阶段，这个阶段的持续时间取决于恒星的质量。在年轻和比较年轻的疏散星闭中，都含有大量耀星，它们的光度分布是疏散星闭年龄的标志。在昴生闭中已发现大量耀星，对它们的自行，测光和分光资料进行深入的统计研究，对于解决疏散星闭及矮星起源和演化问题，有着极其重要的价值。     

Detailed kinematic study of the ionized and neutral gas in the complex of star formation in the galaxy IC 1613

We carried out detailed kinematic studies of the complex of multiple H I and H II shells that represent the only region of ongoing star formation in the dwarf irregular galaxy IC 1613. We investigated the ionized-gas kinematics by using Fabry—Perot Hα observations with the 6-m Special Astrophysical Observatory telescope and the neutral-gas kinematics by using VLA 21-cm radio observations. We identified three extended (300–350 pc) neutral shells with which the brightest H II shells in the complex of star formation are associated. The neutral-gas kinematics in the complex has been studied for the first time and the H I shells were found to expand at a velocity of 15–18 km s^?1. We constructed velocity ellipses for all H II shells in the complex and refined (increased) the expansion velocities of most of them. The nature of the interacting ionized and neutral shells is discussed.     

The mass of the Pleiades

A membership catalogue for the Pleiades is divided into four mass bins, and a tidally truncated King profile is fitted to each bin with good agreement with the data. The tidal radius of the cluster is found to be 13.1 pc, and the total mass of the cluster down to the stellar limit is calculated to be 735 M. The spread of stars in each bin, as well as the relaxation and crossing times, shows the Pleiades to be an approximately relaxed cluster with equilibrium between the density and velocity distributions. The cluster kinetic energy and binding energy are consistent with the virial theorem, indicating no large unseen population of brown dwarfs. However, the 1 σ errors in the cluster parameters provide an upper limit to the mass of any brown dwarf population of 131 M, which would show up in deep CCD surveys as ≤ 5.5 brown dwarfs per 10 × 10 arcminute field in the cluster centre.     

Proper motion L and T dwarf candidate members of the Pleiades

We present the results of a deep optical–near-infrared (optical–NIR) multi-epoch survey covering 2.5 deg² of the Pleiades open star cluster to search for new very low-mass brown dwarf members. A significant (∼5 yr) epoch difference exists between the optical (CFH12k I -, Z -band) and NIR (UKIRT WFCAM J -band) observations. We construct I , I − Z and Z , Z − J colour–magnitude diagrams to select candidate cluster members. Proper motions are computed for all candidate members and compared to the background field objects to further refine the sample. We recover all known cluster members within the area of our survey. In addition, we have discovered nine new candidate brown dwarf cluster members. The seven faintest candidates have red Z − J colours and show blue NIR colours. These are consistent with being L- and T-type Pleiads. Theoretical models predict their masses to be around 11 M _Jup.     

The formation of a bound star cluster: from the Orion nebula cluster to the Pleiades

Direct N -body calculations are presented of the formation of Galactic clusters using GasEx , which is a variant of the code Nbody6 . The calculations focus on the possible evolution of the Orion nebula cluster (ONC) by assuming that the embedded OB stars explosively drove out 2/3 of its mass in the form of gas about 0.4 Myr ago. A bound cluster forms readily and survives for 150 Myr despite additional mass loss from the large number of massive stars, and the Galactic tidal field. This is the very first time that cluster formation is obtained under such realistic conditions. The cluster contains about 1/3 of the initial 10⁴ stars, and resembles the Pleiades cluster to a remarkable degree, implying that an ONC-like cluster may have been a precursor of the Pleiades. This scenario predicts the present expansion velocity of the ONC, which will be measurable by upcoming astrometric space missions. These missions should also detect the original Pleiades members as an associated expanding young Galactic-field subpopulation. The results arrived at here suggest that Galactic clusters form as the nuclei of expanding OB associations.
The results have wide implications, also for the formation of globular clusters and the Galactic-field and halo stellar populations. In view of this, the distribution of binary orbital periods and the mass function within and outside the model ONC and Pleiades is quantified, finding consistency with observational constraints. Advanced mass segregation is evident in one of the ONC models. The calculations show that the primordial binary population of both clusters could have been much the same as is observed in the Taurus–Auriga star-forming region. The computations also demonstrate that the binary proportion of brown dwarfs is depleted significantly for all periods, whereas massive stars attain a high binary fraction.     

Determination of the Proper Motions and Membership of the Globular Cluster M3 and of its Orbit in the Galaxy

We examine 14 plates of the globular cluster M3 (NGC 5272) taken with the 40 cm refractor at the Sheshan station of Shanghai Astronomical Observatory. The plates span over a period of about 77 years. The positions and absolute proper motions of eight stars in the Hipparcos Catalogue and of 49 stars in the Tycho-2 Catalogue are used as the reference frame. The astrometric reduction is made with the central overlapping principle. The absolute proper motions of 534 stars in a region of about 100' × 100' around the cluster are measured. With the new proper motion data the membership probabilities of the stars are determined. The average absolute proper motion obtained for the cluster is -0.06@0.30 mas yr-1 in R.A. and -2.6@0.30 mas yr-1 in Decl. By combining this result with the known distance and radial velocity of the cluster, we also obtained the Galactic orbit of M3 for a chosen three-component Galactic potential.     

A calculation of structural parameters of spherical stellar systems by means of characteristic functions method. I. Theory

To calculate structural parameters of stellar systems such as an effective radius and central space (or surface) density, the method of characteristic functions is suggested. The characteristic function of the system is a Fourier image of their normalized space density profile f₃(r). In the case of spherical symmetry the probability distribution of r (Q₃(r) = (3/a³)r²f₃(r)) and its orthogonal projections have the same characteristic functions. This fact is used to calculate the effective radii of a few star cluster models (King law, Plummer model and Gausian profile). It is shown, that the characteristic function for King law clusters tends to a finite generalised function if the concentration parameter c is large. The expression for the effective radius (at c ≫ 1) is given. The formula of the effective radius in the Plummer model as well as the relation between the one-dimensional central velocity dispersion and the root mean square velocity are obtained. It is shown, that in the Gaussian model and for King law clusters the effective radius (half-mass visual radius) can differ from the effective (harmonic) radius a few times. This fact should be taken into account in estimating the mass-to-light ratio from the virial mass of such systems using the King radius.     

Chromospheric activity in the late A- and early F-type stars of open clusters – II. Pleiades and Alpha Persei

We report observations of the He  i λ 5876 (D3) line in the late A- and early F-type stars in the Pleiades and Alpha Persei star clusters used to determine chromospheric activity levels. This represents the first sample of young stars in this temperature range with chromospheric activity measurements. We find the same average activity level in the young early F stars as in Hyades-age stars and field stars. In addition, the young star sample shows the same large star-to-star variation in activity as seen in the older stars. Thus, as a whole, chromospheric activity in this photospheric temperature range remains the same over nearly a factor of 100 in stellar age (50 Myr to 3 Gyr), in striking contrast to the behaviour of later-type stars. In the five late A stars we find three certain detections of D3 and one likely detection. This includes the bluest star yet observed with a chromospheric D3 line, Pleiades star HII 1362 at ( B − V )₀=0.22, making it one of the earliest stars with an observed chromosphere. The late A stars have D3 equivalent widths comparable to the weakest early F stars. However, when comparing D3 measurements in the young late A stars with older late A stars, we find evidence for a slight decrease in activity with age based on the large number of non-detections in the older stars. We find an apparently linear relationship between the activity upper limit and B − V over our entire range of B − V . Extrapolated blueward, this relationship predicts that the chromospheric D3 line would disappear for all stars at B − V ≈0.13.     

Resolving the kinematic distance ambiguity of southern massive young stellar object candidates

We investigate the use of H  i data to resolve the near/far ambiguity in kinematic distances of massive young stellar object (MYSO) candidates. Kinematic distances were obtained from ¹³CO 1-0 (and N₂H⁺) spectral-line observations with the Mopra Telescope towards 94 candidates selected from the Red MSX Source (RMS) survey in the fourth Galactic quadrant  (282° < l < 350°)  . H  i data from the Southern Galactic Plane Survey (SGPS) were used in conjunction with the H  i self-absorption (SA) technique to determine the near or far distance. We resolved the kinematic distance ambiguity to 70 per cent of the sources. We can also simultaneously solve for any multiple line-of-sight component sources. We discuss the advantages and disadvantages of this technique in comparison with other methods, and also perform confidence checks on the reliability of using the H  i SA technique.
We examined the projected location of these sources in both the Galactic plane and longitude–velocity ( l – v ) diagrams to ascertain any recognizable spiral arm pattern. Although no obvious spiral pattern was found when compared to that proposed by Cordes and Lazio, far distance sources tended to lie on or near spiral arm loci. Near distance sources, however, had peculiar velocity uncertainties consistent with the separation between the spiral arms themselves. The l – v plot shows a more consistent picture, with tangent points of the spiral arm loci easily seen.
We conclude that using the H  i SA technique to determine kinematic distance ambiguities is a quick and reliable method in most cases, with an 80 per cent success rate in determining the correct designation of whether an object is at the near or far distance.