New stellar associations in M33 galaxy

Extending the numeration of Humphreys and Sandage (1980) 54 new associations are identified in the galaxy M33 by comparing the plates in theU andB system. The larger part of the associations are located in the spiral arms. The young associations are prevailing in proximity to the nucleus. A large part of the newly identified associations are connected withHii regions. Judging by the farthest associations M33 diameter is estimated 20 kpc. The associations outline two strong and two faint spiral arms up to 4 kpc away from the nucleus. Their possible behaviour is observed in the further parts.     

Stellar associations and complexes in M33

About 460 OB associations were selected by a comparison of theUBV plates. TheUBV photographic photometry of 1944 blue stars in the associations was made. The new associations appear like cores within Humphreys and Sandage's associations. Their star content, size distribution, and mean size 80 pc confirm their identity with the OB associations in the Galaxy and in the Magellanic clouds. The boundaries of the associations are delineated mainly by the density of the resolved stars on theU plates. It is impossible to divide them into smaller areas. The genuine OB associations form groups of two or more members with a length scale of 250 pc. Their boundaries were delineated independently, but they coincide with the OB associations of Humphreys and Sandage (1980). These groups represent real concentration of blue massive stars with a large age dispersion. The star complexes unify a group of associations,Hii regions, andHi peak distribution. Their mean size is 570 pc. The extensiveHi clouds with a mean size of 1.2 kpc contain two or more star complexes. The questions related to star formation are briefly discussed.     

On the connection between radial systems of dark globules and stellar associations

We study the connection between radial systems of dark globules and stellar associations. It is shown that of the 17 systems of type 1 in Table 1 of [1] 16 radial systems are connected with known associations. A new association is found (missing from the catalogs) connected with the remaining system (System No. 2). Four systems of the six systems of type 2 (Table 2 of [1]) are connected with known associations. A new method of determining the distance to associations is proposed, using the mean linear thickness of dark globules of radial systems connected with these associations as the criterion for distance. Using this method we make the distance to the association Cyg OB 9 more precise and answer the question whether several radial systems belong to the corresponding stellar associations.Translated fromAstrofizika, Vol. 37, No. 4, 1994.The authors are grateful to Academician V. A. Ambartsumyan for constant attention to this work.     

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.     

Polarimetric Investigations of Stellar Associations

The degree of polarization of light from stars in 44 O B associations as a function of interstellar absorption is investigated. It is shown that the character of the dependence of P on A _V for stars in associations and stars not in associations depends on the value of A _V: for A _V 2 ^m .5 it has a linear character and is the same for both groups of stars. For A _V > 2 ^m .5 the dependence of P on A _V for stars in and not in associations departs from linearity and for A _V = 4 ^m .5 it reaches P _ass = 1.8% and P _nonass = 1%, respectively. Such a difference is explained by the additional depolarization in stellar associations. Such strong depolarization in associations may be due to the overall magnetic field of the Galaxy and to physical peculiarities in the association itself.     

Observational approach to star formation

The observational approach to the early stages of stellar evolution has been applied to some problems relating to the formation and dissipation of stellar associations, the origin of OB field stars, and low-mass star formation in OB associations. The OB field stars ejected from parent associations are older on the average than the OB stars in the associations. The average duration of active OB-star formation in associations is evaluated. It is suggested that, under the conditions in OB associations, low-mass stars may be formed from dense protostellar objects.Translated fromAstrofizika, Vol. 39, No. 3, pp. 393–406, July–September, 1996.     

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.     

Photometry of the bright stars inside and outside associations in the spiral arm S5 of the andromeda galaxy

Photometry of more than 1000 bright stars of 20 associations and some fields in its vicinity is made, usingUBV plates taken with a 2-m Ritchey-Chrétien telescope of the National Observatory at the Bulgarian Academy of Sciences with limited magnitudes of about 20.6 inU andV, 21.6 inB. The colour-magnitudes of about 20.6 inU andV, 21.6 inB. The colour-magnitude and colour-colour diagrams of 12 associations and 8 fields are constructed and the brightest blue and red stars colour excesses, ages, and integrated luminosities of the associations are determined. The composite diagrams for all association and nonassociation measured stars are constructed too. Inside associations bright stars as well as non-stellar objects occur more often than outside. The brightest stars in S5 are withM _v7 and do not differ considerably from the brightest ones in the spiral arm S4 of M31. Approximately the same are the absolute magnitudes of the brightest stars in our Galaxy and M33. A small difference between the brightest stars of the latter galaxy with those of M31 is possible because the stars in M33 are younger. This is in agreement with the concept that Sc galaxies (for example M33) are younger than Sb ones (for example M31). The same could be the reason for the smaller ratio of blue to red supergiants in M33 than in M31. The age estimations of the associations in S5 together with those in S4 in M31 show that they are older than the associations in M33 which is in agreement with the above mentioned concept.     

Statistics of apparent distances between QSO's and bright galaxies

Two experiments with points randomly distributed over the sky were carried out in order to check the significance of the apparent associations between bright galaxies and quasistellar objects found by Burbidgeet al. (1971). The experiments confirm the statistical relevance of these associations.     

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)     

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)     

Finding the most variable stars in the Orion Belt with the All Sky Automated Survey

We look for high‐amplitude variable young stars in the open clusters and associations of the Orion Belt. We use public data from the ASAS‐3 Photometric V ‐band Catalogue of the All Sky Automated Survey, infrared photometry from the 2MASS and IRAS catalogues, proper motions, and the Aladin sky atlas to obtain a list of the most variable stars in a survey area of side 5° centred on the bright star Alnilam (ε Ori) in the centre of the Orion Belt. We identify 32 highly variable stars, of which 16 had not been reported to vary before. They are mostly variable young stars and candidates (16) and background giants (8), but there are also field cataclysmic variables, contact binaries, and eclipsing binary candidates. Of the young stars, which typically are active Herbig Ae/Be and T Tauri stars with Hα emission and infrared flux excess, we discover four new variables and confirm the variability status of another two. Some of them belong to the well known σ Orionis cluster. Besides, six of the eight giants are new variables, and three are new periodic variables (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Distribution of luminous blue stars in M33

M₃₃ luminous stars with M_V ≲ -7 and U--B < o are selected in this paper. The spatial distribution of the stars shows a good agrement with the associations and HII regions. However HII regions are absent in the three youngest associations in M₃₃ (t ≈ 5 ÷ 7 × 10yr). It is supposed that the true distance modulus of M₃₃ derived from the brightest blue stars is 24.2. An age gradient across the arms is absent. The spiral arms outlined by the luminous stars differ form those in M₃₁. They are diffuse and wide. The multiarm features of M₃₃ support the stochastic self-propagating model.     

Associations of very high energy gamma-ray sources discovered by H.E.S.S. with pulsar wind nebulae

The H.E.S.S. array of imaging Cherenkov telescopes has discovered a number of previously unknown γ-ray sources in the very high energy (VHE) domain above 100 GeV. The good angular resolution of H.E.S.S. (∼0.1° per event), as well as its sensitivity (a few percent of the Crab Nebula flux) and wide 5° field of view, allow a much better constrained search for counterparts in comparison to previous instruments. In several cases, the association of the VHE sources revealed by H.E.S.S. with pulsar wind nebulae (PWNe) is supported by a combination of positional and morphological evidence, multi-wavelength observations, and plausible PWN model parameters. These include the plerions in the composite supernova remnants G 0.9+0.1 and MSH 15–52, the recently discovered Vela X nebula, two new sources in the Kookaburra complex, and the association of HESS J1825–137 with PSR B1823–13. The properties of these better-established associations are reviewed. A number of other sources discovered by H.E.S.S. are located near high spin-down power pulsars, but the evidence for association is less complete. These possible associations are also discussed, in the context of the available multi-wavelength data and plausible PWN scenarios. For the H.E.S.S. Collaboration     

UBV photometry of bright stars in the spiral arm S5 of M31

A photometric study inUBV of about 1000 bright stars outside and inside associations in S5 spiral arm is presented briefly. Characteristics of the associations (such as absorption, age and brightest stars are compared with the same characteristics of the associations in S4 spiral arm of M31, as well as in M33) are discussed.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

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.     

Structure and origin of the cygnus superbubble

This paper summarizes and analyzes the results of radio optical, infrared, and X-ray observations of a large sector of the sky in the constellation Cygnus (19^h20^m-22^h, =30–50°;l ^II=65–90°, |b ^II|10°). This region is associated with an extended X-ray source referred to as the Cygnus superbubble. About a quarter of the superbubble region is occupied by the extensively investigated multicomponent thermal radio source Cyg X. The region contains eight OB-associations which, when projected on the sky, duplicate the outline of the X-ray superbubble. These associations contains 110 stars of high luminosity (about 40 Wolf-Rayet and Of stars). The observations suggest that the X-ray superbubble is not a single object. Between 50 and 75% of its X-ray emission can be ascribed to discrete sources, the rest being probably due to regions of coronal gas about 100 pc in diameter, created by stellar winds and, possibly, supernova explosions in individual associations. The objects that produce the X-ray and optical radiation of the presumed superbubble are located at distances from 0.5 to 2.5 kpc from the Sun in the Carina-Cygnus spiral arm. The eastern portion of the region presumed superbubble contains the associations Cyg OB7 and Cyg OB4 and is generally less than 1 kpc distant, while the western portion contains the associations Cyg OB1, 2, 3, 8, and 9 and is 1 to 2 kpc distant.     

On the observation of scattered radio emission from sources in the solar corona

The observations of a brief flaring region between two plages on the eastern limb of the Sun and the subsequent coronal transient are reported for June 16, 1972 by Koomen et al. (1974). Both of these events have unambiguous and closely timed associations with the solar noise bursts observed at 2800 and 2700 MHz and are also accompanied with good X-ray and SID effects but faint subflare (Solar-Geophysical Data). The two frequencies are those monitored at widely separated stations operated by the Astrophysics Branch of the National Research Council at Lake Traverse, Ontario and at Penticton, B.C.     

The flare-associated magnetic changes in an active region

The associations of flares to flux emergence and cancellation have been further examined and clarified with the aid of complete time sequences of vector magnetograms of an active region for a 4-day period around the central meridian passage.It is found that the emergence of new flux and its driven flux cancellation with existing flux is a wholly inseparable, elementary process in the active region, favorable for flare occurence. The early discovery ofstructures magnetique evolutive (Martreset al., 1968) is confirmed and identified to be the net result of this process.All events of flux cancellation appear in the interface of two topologically separated magnetic loops. Direct indications of magnetic reconnection between two cancelling components in the photospheric layer are identified. The cancellation is most likely a slow reconnection in the lower atmosphere of the Sun. The quite popular view of interpreting flux cancellation as a pure flux submergence could not fit the magnetic topology learned from alignments of the transverse magnetic field. In this sense, the association of flares to flux cancellation seems to represent a coupling of the slow reconnection in the lower atmosphere to the fast reconnection higher in the corona.This slow reconnection can even take place below the photosphere. In one case, an inferred sub-photospheric reconnection eventually prevents one pole of an emerging flux region with the polarity opposite to the background from showing up at the photospheric level.Six of all eight flares which appeared in this period are spatially and temporally associated with the emergence of new flux and its driven cancellation. They might be divided into two groups. The first group of flares appears at the early phase of flux emergence and in close proximity to the cancelling site between new and old flux; the second ones appear after several hours of flux cancellation, centering around the cancelling site.     

Trapezium systems containing T tauri stars

The results of a spectroscopic investigation of four stars comprising a Trapezium system in the nebula Sh 155 are presented. The stars have a number of emission lines typical of red emission stars and T Tau stars. This system is one of the 13 systems which we have found in the Palomar atlas. It is shown that all of these systems are members of OB associations.Translated fromAstrofizika, Vol. 38, No. 1, pp. 17–24, January–March, 1995.