New OB-association in Pup — CMa

A new method of determination of distances to stellar associations is proposed. This method is based on the measurements of mean thicknesses of globules of the systems of dark globules connected with that association. It is shown that the method is in good agreement with other known methods. A new grouping of OB stars in Pup-CMa is found. It is shown that this grouping has properties characteristic for OB-associations: the stars of that grouping have similar distances, similar radial velocities, the grouping is connected with molecular clouds, with Herbig-Haro objects, and cometary nebulae. All these results are in favour of this grouping to be a new OB-association. We named that association OB-association Pup-CMa. The preliminary results of¹²CO observations of molecular clouds connected with that association are also given. The radial velocities of these clouds are in good agreement with the mean radial velocity of stars in the association in Pup-CMa. Translated from Astrofizika, Vol. 42, No. 2, pp. 179–188, April–June, 1999.     

Relationship of groups of submillimeter starless condensations to radial systems of dark globules

The recently discovered groups of submillimeter starless condensations at the junction of HII regions and molecular clouds, which are invisible in the optical and near IR ranges, are similar in many ways to visible radial systems of dark globules, specifically, in their densities of molecular hydrogen, sizes, proximity to bright stars of early classes (which may be responsible for their formation), etc. It is proposed that the groups of submm starless condensations are radial systems of dark globules that are optically invisible because they lie behind dark clouds. Thus, it is shown that these groups of condensations are not a new type of object.     

Stellar associations

This article is intended to remind us of the history of the origins of one of the fundamental discoveries of the twentieth century, stellar associations, which are connected with the name of V. A. Ambartsumyan. It is not a complete discussion of all the published work, for this is, of course, impossible. Research on stellar associations began immediately after their discovery and continues to this day. Observations are now also carried out by automatic satellites and a vast amount of observational data has been obtained. Many stellar associations have been discovered, both in our galaxy and in other galaxies. Catalogs of stellar associations have been compiled and many problems relating to stellar associations have been solved. Translated from Astrofizika, Vol. 52, No. 2, pp. 171–184 (May 2009).     

Physical parameters of multiple systems like the trapezium of early spectral types, derived fromuvbyβ photometry. I

The results of electrophotometric observations of 51 stars in multiple systems of the Trapezium type in the Strömgren-Crawford six-color photometric system are given. The following Trapezia were selected from the Abastumani Catalog for observations: ABAO 2, 8, 34, 48, 51, 62, 94, 313, 316, 348, 356, 359, 363, 387, 396. The observations were made on the 125-cm, automatically computer controlled, reflecting telescope of Abastumani Astrophysical Observatory. The results of the observations are given in Table 1. Most of the components of the observed Trapezia belong to the early “B ” group based on Stromgren ’s criteria. The absolute stellar magnitudes of the Trapezia components are determined and their distance moduli are calculated (Table 2). It is concluded that these Trapezia are physical systems with a high probability.     

The stellar content of associations and star complexes in M33

A list of 289 stellar associations and 47 star complexes has been compiled. This study is based on a photographicUBV photometry carried out with the 3.6 m CFHT by W. L. Freedman and the 2 m Bulgarian telescope. The identification charts of stellar associations and star complexes are presented. The photographs reveal new appearance for associations. They are seen as clustering within the classical OB associations. The boundaries of the associations were delineated using a cluster analysis. The problems related to the age and size distribution of associations are discussed. The stellar associations provide a good observational test for shock wave across the southern spiral arm. The star complexes in the central region and in the southern spiral arm are rich in WR, O and luminous stars and poor in red supergiants while in the outer region is quite contrary. There is a brief discussion whether the different stellar content of star complexes could be explained with radial metal abundance, in M33 or with variations of the luminosity function from one complex to another. M33 is compared with other nearby galaxies.     

Open Cluster NGC6613 in SerOB1 A Association

We present the original technique to solve the problems of stellar astronomy using the highresolution spectroscopy and we describe the result of implementation of this technique to determine if the stellar clusterNGC6613 belongs to the SerOB1A association. From the only one estimation available, the distance toNGC6613 equals 1.3 kpc. It is not clear if the cluster linkedwith the SerOB1A association, the distance to which is 2.1 kpc. For nine stars in the region of NGC6613, the radial velocities were measured from the spectra obtained with the NES spectrometer of the 6-m SAORAS telescope, as well as conducted the two-dimensional spectral classification and found their distances with themethod of spectral parallaxes. Seven of nine stars are referred to the cluster members; the average distance to them is 2.2 ± 0.1 kpc. The distance estimates were controlled using the Na I (1) interstellar line profiles. Thus, it was understood that the cluster NGC6613 is a member of the SerOB1A association and its influence should be taken into account, when describing the star formation in the association.     

A revisit to agglomerates of early‐type Hipparcos stars

We study the spatial structure and sub‐structure of regions rich in Hipparcos stars with blue B_T – V_T colours. These regions, which comprise large stellar complexes, OB associations, and young open clusters, are tracers of on‐going star formation in the Galaxy. The DBSCAN (Density‐Based Spatial Clustering of Applications with Noise) data clustering algorithm is used to look for spatial overdensities of early‐type stars. Once an overdensity, “agglomerate”, is identified, we carry out a data and bibliographic compilation of their star member candidates. The actual membership in agglomerate of each early‐type star is studied based on its heliocentric distance, proper motion, and previous spectro‐photometric information. We identify 35 agglomerates of early‐type Hipparcos stars. Most of them are associated to previously known clusters and OB associations. The previously unknown P Puppis agglomerate is subject of a dedicated study with Virtual Observatory tools. It is actually a new, nearby, young open cluster (d ∼ 470 pc, age ∼ 20 Ma) with a clear radial density gradient.We list P Puppis and other six agglomerates (including NGC 2451 A, vdBH 23, and Trumpler 10) as new sites for substellar searches because of their youth, closeness, and spatial density. We investigate in detail the sub‐structure in the Orion, CMa‐Pup and Pup‐Vel OB complexes (“super‐agglomerates”). We confirm or discover some stellar overdensities in the Orion complex, like the 25 Ori group, the Horsehead region (including the σ Orionis cluster), and the η Orionis agglomerate. Finally, we derive accurate parallactic distances to the Pleiades, NGC 2451 A, and IC 2391, describe several field early‐type stars at d < 200 pc, and discuss the incompleteness of our search. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Unstable objects in the galaxy and the star-formation process

Using observational data, we consider the problem of instability of astronomical objects and systems of objects and its close connection with the star-formation process. We show that it is only due to the discovery and study of stellar associations that the enormous role of stellar instability in the formation of stars in the Galaxy (and in other galaxies) has been definitively explained. Many kinematic properties of stellar associations are inexplicable from the old point of view. This gives grounds for doubting the correctness of the classical hypothesis that stars form from diffuse matter.Translated fromAstrofizika, Vol. 38, No. 3, 1995.     

Astrophysical supplements to the ASCC‐2.5: Ia. Radial velocities of ∼55000 stars and mean radial velocities of 516 Galactic open clusters and associations

We present the 2nd version of the Catalogue of Radial Velocities with Astrometric Data (CRVAD‐2). This is the result of the cross‐identification of stars from the All‐Sky Compiled Catalogue of 2.5 Million Stars (ASCC‐2.5) with the General Catalogue of Radial Velocities and with other recently published radial velocity lists and catalogues. The CRVAD‐2 includes accurate J2000 equatorial coordinates, proper motions and trigonometric parallaxes in the Hipparcos system, B, V photometry in the Johnson system, spectral types, radial velocities (RVs), multiplicity and variability flags for 54907 ASCC‐2.5 stars. We have used the CRVAD‐2 for a new determination of mean RVs of 363 open clusters and stellar associations considering their established members from proper motions and photometry in the ASCC‐2.5. For 330 clusters and associations we compiled previously published mean RVs from the literature, critically reviewed and partly revised them. The resulting Catalogue of Radial Velocities of Open Clusters and Associations (CRVOCA) contains about 460 open clusters and about 60 stellar associations in the Solar neighbourhood. These numbers still represent less than 30% of the total number of about 1820 objects currently known in the Galaxy. The mean RVs of young clusters are generally better known than those of older ones. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Star formation and molecular clouds

The hypothesis advanced by V. A. Ambartsumyan according to which stars are formed from prestellar superdense objects-- protostars-- was an alternative to the hypothesis of the 1950's (and even now, not much changed) according to which stars are formed by accretion with subsequent collapse (in various modifications). Ambartsumyan's basic inferences were based on an analysis of the observational data available at that time. This paper presents both Ambartsumyan's pioneering ideas and some modern hypotheses of star formation. Some results from studies of molecular clouds and star formation regions are also discussed. One of the distinctive features of young stellar objects (YSO) is the outflow of matter from these objects (molecular, in the form of jets, etc.), a phenomenon whose importance for the evolution of stars was noted by Ambartsumyan as long ago as 1937. Radial systems of dark globules are examined, as well as H-H objects associated with star formation regions, cometary nebulae, and close Trapeziumtype systems (consisting of YSO). Translated from Astrofizika, Vol. 52, No. 2, pp. 185–202 (May 2009).     

Relationship between pulsars and stellar associations

The spatial distribution of pulsars and stellar associations in the solar neighborhood is investigated on the basis of recent data. The distribution of young pulsars and stellar associations indicates fragments of the spiral structure of the galaxy. The similar spatial distributions of young pulsars and stellar associations can be considered as confirmation of the hypothesis that pulsars originate in stellar associations. Translated fromAstrofizika, Vol. 40, No. 1, pp. 60–65, February, 1997.     

Slow modes in stellar systems with nearly harmonic potentials: I. Spoke approximation,radial orbit instability

Using a consistent perturbation theory for collisionless disk-like and spherical star clusters, we construct a theory of slow modes for systems having an extended central region with a nearly harmonic potential due to the presence of a fairly homogeneous (on the scales of the stellar system) heavy, dynamically passive halo. In such systems, the stellar orbits are slowly precessing, centrally symmetric ellipses (2: 1 orbits). Depending on the density distribution in the system and the degree of halo inhomogeneity, the orbit precession can be both prograde and retrograde, in contrast to systems with 1: 1 elliptical orbits where the precession is unequivocally retrograde. In the first paper, we show that in the case where at least some of the orbits have a prograde precession and the stellar distribution function is a decreasing function of angular momentum, an instability that turns into the well-known radial orbit instability in the limit of low angular momenta can develop in the system. We also explore the question of whether the so-called spoke approximation, a simplified version of the slow mode approximation, is applicable for investigating the instability of stellar systems with highly elongated orbits. Highly elongated orbits in clusters with nonsingular gravitational potentials are known to be also slowly precessing 2: 1 ellipses. This explains the attempts to use the spoke approximation in finding the spectrum of slow modes with frequencies of the order of the orbit precession rate. We show that, in contrast to the previously accepted view, the dependence of the precession rate on angular momentum can differ significantly from a linear one even in a narrow range of variation of the distribution function in angular momentum. Nevertheless, using a proper precession curve in the spoke approximation allows us to partially “rehabilitate” the spoke approach, i.e., to correctly determine the instability growth rate, at least in the principal (O(α _T^−1/2) order of the perturbation theory in dimensionless small parameter α _T, which characterizes the width of the distribution function in angular momentum near radial orbits.     

Distribution of early-type stars and dusty matter in the direction of the stellar cluster NGC 1893

The distribution of 255 O-B9-A2, K-G stars and interstellar dust in the direction of the stellar cluster NGC 1893 is studied using V, B-V, and U-B photometric data. Sixteen groups of stars (associations) are discovered at various distances. The first group includes 9 stars of different spectral classes of later types in the sun’s neighborhood lying at a distance of 110 pc. The next 3 groups, at distances of 420, 890, and 14300 pc, are type B associations and the remaining twelve groups are OB associations. They are designated as Aur 0.11, Aur B 0.43, Aur B 0.89, Aur OB 1.4, Aur OB 2.6, AurOB 3.8, Aur OB 4.6, Aur OB 5.4, Aur OB 6.1, Aur OB 7.4, Aur OB 9.3, Aur OB11.6, Aur OB14.3, Aur OB 17.9, Aur OB 25.9, and Aur OB 31.3. For most of these stars the absorption lies within the range from 0m.45 to 5m.41. Such high absorption may be caused by circumstellar absorption as well as by the diffuse nebula IC 410. The dusty matter is distributed nonuniformly in the Aur 0.11, Aur B 0.43, and Aur B 0.89 associations. There is no dust in the space between the associations. There is essentially no dust within the groups (associations) at distances greater than 0.9 kpc. (See Table 2.) __________ Translated from Astrofizika, Vol. 50, No. 2, pp. 243–251 (May 2007).     

On the mass and density of the stellar disk of M33

The disk surface density of the nearby spiral galaxy M33 is estimated assuming that it is marginally stable against gravitational perturbations. For this purpose we used the radial profile of line-of-sight velocity dispersion of the disk planetary nebulae obtained by Ciardullo et al. (2004). The surface density profile we obtained is characterized by the radial scalelength which is close to the photometrical one and is in a good agreement with the rotation curve of M33 and with the mass-to-light ratio which follows from the stellar population model. However at the galactocentric distance r > 7 kpc the dynamical overheating of the disk remains quite possible. The thickness of the stellar disk of M33 should increase outwards from the center. The dark halo to total mass ratio is estimated as a function of r. The effective oxygen yield obtained in the frame of instantaneous recycling approximation using the disk surface density and the observed gradient of O/H increases with radius. It may indicate that the role of accretion of metalpoor gas in the chemical evolution of interstellar medium decreases outwards.     

Photometric distances to nine dark globules

Distances to nine dark globules are determined by a method using optical ( VRI ) and near-infrared (near-IR) ( JHK ) photometry of stars projected towards the field containing the globules. In this method, we compute intrinsic colour indices of stars projected towards the direction of the globule by dereddening the observed colour indices using various trial values of extinction   A _V   and a standard extinction law. These computed intrinsic colour indices for each star are then compared with the intrinsic colour indices of normal main-sequence stars and a spectral type is assigned to the star for which the computed colour indices best match with the standard intrinsic colour indices. Distances ( d ) to the stars are determined using the   A _V   and absolute magnitude  ( M_V )  corresponding to the spectral types thus obtained. A distance versus extinction plot is made and the distance at which   A _V   undergoes a sharp rise is taken to be the distance to the globule. All the clouds studied in this work are in the distance range 160–400 pc. The estimated distances to dark globules LDN 544, LDN 549, LDN 567, LDN 543, LDN 1113, LDN 1031, LDN 1225, LDN 1252 and LDN 1257 are  180 ± 35, 200 ± 40, 180 ± 35, 160 ± 30, 350 ± 70, 200 ± 40, 400 ± 80, 250 ± 50  and 250 ± 50 pc, respectively. Using the distances determined, we have estimated the masses of the globules and the far-IR luminosity of the IRAS sources associated with them. The mass of the clouds studied are in the range  10–200 M_⊙  .     

Spectral study of BIG objects on the 2.6-m telescope at the Byurakan astrophysical observatory

Results of spectral observations of 45 objects from the BIG sample (corresponding to 35 IRAS sources) obtained on the 2.6-m telescope at the BAO are reported. Emission lines are observed for 42 galaxies, 1 object is an absorption galaxy, and 2 turned out to be stars. The red shifts are determined, the radial velocities, distances, sizes, and absolute stellar magnitudes are calculated, the parameters of the spectral lines are determined, the objects are classified according to their activity type, and the IR and far IR luminosities are calculated. Of the 42 emission galaxies, 1 was type Sy2, 2 were LINERs, 1 was an AGN (Sy2 or LINER), 4 were composite, 25 were HII (including 6 with nuclear star-formation activity), and 9 were Em of undetermined type (3 of which may be AGN). Calculations show that 23 of the objects are LIG. A physical coupling is discovered for 9 multiple systems. Spectra of some of the galaxies are shown. __________ Translated from Astrofizika, Vol. 49, No. 1, pp. 23–40 (February 2006).     

Globules and star formation

The principal properties of dark globules are reviewed. Optical and radio (CO) data suggest masses in the range 20 to 750M , radii 0.15 to 1.2 parsecs. There is no evidence for fragmentation. The dust density inside near-spherical globules varies inversely as the cube of the distance from the center. There is evidence to show that several globules are in a state of collapse.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978.Much of the work was done while the author was a Visiting Scientist at Cerro Tololo Interamerican Observatory in Chile, supported by the National Science Foundation under Contract No. NSF-C866.     

Hydrodynamic equations of differentially rotating stellar systems and the velocity ellipsoid

Anisotropic hydrodynamic equations for differentially rotating collisionless stellar systems are derived. These equations can describe the evolution of the systems in a time span longer than their rotation periods.As a by-product of derivation of hydrodynamic equations, the well-known relation that the ratio of the principal axes of the velocity ellipse in a differentially rotating stellar disk is [B/(B-A)]^1/2 is re-found if the system is in a purely circular rotation, whereA andB are the Oort's constants. In addition, we find a systematic mean motion superposed on a purely circular differential rotation makes the directions of axes of the velocity ellipse deviate from the radial and the transverse direction. The observed deviation of directions of axes in our neighbourhood in the Galaxy can be explained if in the mean motion superposed on a purely circular differential rotatin the gas of stars near us is compressed in the radial direction or rarefied in the transverse directions, with irregularities of the order of 5 km/sec in amplitude of velocity and 1 kpc in size. These magnitudes of irregularities agree with those actually observed or with those anticipated from other theoretical considerations.     

A high-precision radial-velocity survey for other planetary systems

The precise measurement of variations in stellar radial velocities provides one of several promising methods of surveying a large sample of nearby solar type stars to detect planetary systems in orbit around them. The McDonald Observatory Planetary Search (MOPS) was started in 1987 September with the goal of detecting other nearby planetary systems. A stabilized I₂ gas absorption cell placed in front of the entrance slit to the McDonald Observatory 2.7 m telescope coudé spectrograph serves as the velocity metric. With this I₂ cell we can achieve radial velocity measurement precision better than 10 m s^–1 in an individual measurement. At this level we can detect a Jupiter-like planet around a solar-type star, and have some hope of detecting Saturn-like planets in a long-term survey. The detectability of planets is ultimately limited by stellar pulsation modes and photospheric motions. Monthly MOPS observing runs allow us to obtain at least 5 independent observations per year of the 33 solar-type (F5-K7) stars on our observing list. We present representative results from the first five years of the survey.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

Southern YSO GRV10 and its environment

In this paper the southern cometary nebula GRV10 and its environment are investigated. The object is connected with a molecular cloud (its dimensions are ~4 pc). 12CO (1-0) observations toward GRV10 revealed the presence of two molecular outflows (a bipolar outflow), a red and a blue-shifted outflow, which coincide in position, indicating that they are moving along the line of sight in opposite directions. Near GRV10 a bright IR star, GRV10 IR, is discovered (near the southern edge of the nebula) and since its spectra and near IR colors favor it to be a Mira type star, a distance of ~4.4 kpc can be estimated. Star GRV10 IR appears connected with the IRAS point source IRAS 10406-6256. The grouping of stars (YSOs - young stellar objects) connected with GRV10 and embedded in a dark nebulosity is also discussed. This grouping is associated with the IRAS point source IRAS 10406-6253, which has FIR colors typical for embedded, not evolved, YSOs.