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.     

A photometric study of the bright stars in M33

TheUBV photometry of 105 bright stars and theU magnitudes of 36 bright stars in M33 taken by the 2 m RCC telescope of the Rozhen Observatory at the Bulgarian Academy of Sciences were used to obtain colour-magnitude and colour-colour diagrams. A criterion forV, B-V, andU-B is suggested which facilitates the separation of the member stars from the foreground stars. The vast majority of the bright blue stars with strong negativeU-B are M33 members. The existence of objects with an unusual combination of colours (B-V>0.5 and strong negativeU-B) has been confirmed.     

The luminosity of red supergiants in the andromeda galaxy

A blink survey of M31 undertaken by means ofB andV plates taken at the 2 m Rozhen observatory telescope gave the possibility to select ten brightest red supergiant candidates. There are a few bright red supergiants in M31 in comparison the LMC, M33, and our Galaxy. They are more frequent in the northern part of M31 where the higher rate of star formation occurs. Our search indicatedM _V –8.0 for the brightest red supergiants. Their occurrence in a galaxy depends not only on luminosity of the latter but also on the rate of star formation in it.     

A photometric study of three star fields in the southern region of M33

The 2 m RCC telescope of the Rozhen National Observatory (Bulgaria) was used to obtain the colour-magnitude and colour-colour diagrams of OB110 and OB112 associations which form the largest star grouping in the southern region of M33. The stars withV andU20.4 andB21 were measured. TheV magnitude, the coloursB - V, U - B and the respective identification charts are given. This allowed to construct the luminosity function of OB110+OB112. The group has an integrated magnitudeM _VT=–11.8 which is by 0 ^m .5 fainter than OB78 (M31). The mean absorption and the foreground were examined as well.     

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.     

The brightest blue stars in M33 galaxy as unresolved star groups

The brightest blue stars in the M33 galaxy have peculiar images. A method to resolve them into components was applied. The brightest members stars atV15–16 mag were recognized as multiple ones. Some stars up toV=17 mag have multiple structures, too. The image of the brightest blue stars consist of two or more components and probably some of them, in fact, resemble dense stellar groups such of R 136 in 30 Dor.     

Schmidt survey in the Galactic anticentre direction. 1. Investigation of open clusters

A study of four open clusters in the direction of the Galactic anticentre (l = 186°, b = +2°) is presented. In a field of 8.32 square degrees proper motions and B magnitudes for about 79 000 stars down to 19.5 were determined on Tautenburg Schmidt plates. For more than 15 500 of them U magnitudes down to 17.3 could be obtained. Additionally, OCA Schmidt plates were used to determine V, R magnitudes in a larger field of 24.45 square degrees for 271 000 stars down to V = 18.2. For stars brighter than V = 15.5 an accuracy of about 1.5 mas/yr has been estimated for proper motions. The rms errors of stellar magnitudes and colour indices are 0.09 – 0.12 mag. Several open clusters have been already known in this direction of the sky, e.g. NGC 2168 or M 35 (C 0605+243), NGC 2158 (C 0604+241) and IC 2157 (C 0601+240). Inspecting the plates and analysing the colour-magnitude diagrams and published data, we could identify an additional anonymous cluster C 0605+242 with a projection on the sky near the centre of M 35 but at a larger distance from the Sun. The cluster membership determination was carried out using information on spatial and proper motion distributions of stars in the field. The colour-magnitude diagrams were derived down to the limiting stellar magnitude. For each cluster the interstellar extinction A_v, the diameters of the core and corona, the ages and spatial velocity components (V,W) relative to the LSR in the Y,Z – Galactic directions were determined. The distances to the clusters of 960 pc, 2 600 pc, 2 520 pc and 3 700 pc were obtained for M 35, IC 2157, C 0605+242 and NGC 2158. They show the loci of the clusters in the Local and Perseus spiral arms and at external border of Perseus arm, respectively.     

Star counts in M15 onU,B andV plates

We present new counts of stars in M15, using plates inB, V andU. We are able to explore relatively close to the central parts of the cluster (0.1 pc) and we derive the best fitting parameters for the star distribution.     

Proper motions and CCD photometry of stars in the region of the open cluster NGC 6866

We present the results of our comprehensive study of the Galactic open star cluster NGC 6866. The positions of stars in the investigated region have been obtained with the “Fantasy” automatic measuring machine from 10 plates of the normal astrograph at the Pulkovo Astronomical Observatory. The size of the investigated field is 40′ × 40′, the limiting magnitude is B ∼ 16_· ^m 6, and the maximum epoch difference is 79 yr. For 1202 field stars, we have determined the relative proper motions with an rms error of 2.5 mas yr⁻¹. Out of them, 423 stars may be considered cluster members with a probability P > 70% according to the astrometric criterion. Photometric diagrams have been used as an additional criterion. We have performed two-color BV CCD photometry of stars with the Pulkovo ZA-320M mirror astrograph. The U magnitudes from the literature have also been used to construct the two-color diagrams. A total of 267 stars have turned out to be members of NGC 6866 according to the two criteria. We present refined physical parameters of the cluster and its age estimate (5.6 × 10⁸ yr). The cluster membership of red and blue giants, variable, double, and multiple stars is considered. We have found an almost complete coincidence of the positions of one of the stars in the region (a cluster nonmember) and a soft X-ray source in the ROSAT catalog. The “Fantasy” automatic measuring machine is described in the Appendix.     

Investigation of flare stars in the pleiades by the method of stellar tracks. I

Observations of the Pleiades cluster by the method of stellar tracks, carried out on the 40-inch Schmidt telescope of Byurakan Astrophysical Observatory, have resulted in the detection of 49 flares from 38 stars, 17 of which were not previously known to be flare stars. It is shown that for bright stars (U ≤ 16.0) the detection of flares in observations by the method of stellar tracks is at least three times more efficient than for observations by the method of stellar chains. Another advantage of the first method is that one can detect brief flares that last less than 6 min. The visual stellar magnitude at the minimum for the brightest of the flare stars that we found is 11.92. This raised the upper luminosity limit of known flare stars in the Pleiades by 0.21 magnitude. A comparison of the expected number of bright flare stars in the Pleiades with the number of all bright members of the cluster (falling in the range from V ≈ 12.0 toV ≈16.0) suggested that all these stars evidently must be flare stars. Translated from Astrofizika, Vol. 42, No. 3, pp. 351–358, July–September, 1999.     

The stellar content of binary star clusters in the LMC

The bright stellar content for fifteen binary star clusters and their adjoining fields in the Large Magellanic Cloud (LMC) were studied here. Film copies of plates taken with the 1.2 U.K. Schmidt telescope were used for deriving the spectral types of the stars in the studied regions. All classified stars are brighter thanV=17.5 mag and situated in large areas around each pair and in a neighbouring field. Seven of the pairs, the brightest and most populous are young clusters (located mainly at the north part of the parent galaxy). The derived distributions of spectral types of their stars give strong evidence that each pair consists of similar stellar content with ages 0.6–8×10⁷ yr.Eight more binary star clusters were studied as well, selected among the rest of the binaries in the LMC. It is found that their stars were faint for our limit of detection so the poor statistics did not allow a comparison among the two cluster members of each pair. However the bright limit of their stars implies ages >6×10⁸ yr. Considering that these objects were randomly selected it is unlikely that all are projected pairs. So it seems that binarity in star clusters is a phenomenon (favourable in the LMC) which did not happen only once in the life of this galaxy.     

A search for the optical and near-infrared counterpart of the accreting millisecond X-ray pulsar XTE J1751−305

We have obtained optical and near-infrared images of the field of the accreting millisecond X-ray pulsar XTE J1751−305. There are no stars in the 0.7-arcsec error circle (0.7 arcsec is the overall uncertainty arising from tying the optical and X-ray images and from the intrinsic uncertainty in the Chandra X-ray astrometric solution). We derive limiting magnitudes for the counterpart of   R > 23.1, I > 21.6, Z > 20.6, J > 19.6  and   K > 19.2  . We compare these upper limits with the magnitudes one would expect for simple models for the possible donor stars and the accretion disc subject to the reddening observed in X-rays for XTE J1751−305 and when put at the distance of the Galactic Centre (8.5 kpc). We conclude that our non-detection does not constrain any of the models for the accretion disc or possible donor stars. Deep, near-infrared images obtained during quiescence will, however, constrain possible models for the donor stars in this ultracompact system.     

The structure of the spiral arm S4 in andromeda galaxy

TheUBV photometry of 690 stars in the spiral arm S4 and the U magnitudes of 120 stars in the spiral arm S6 with the help of the 2 m RCC telescope of the Rozhen Observatory at the Bulgarian Academy of Sciences, has been used to obtain the colour-magnitude and colour-colour diagrams across the arms. Our age estimations are compared with van den Bergh's (1964). The age gradient across the S4 arm has been found. The colour excessE _B-V is highest at the inner edge of the arm S4. From the age we have evaluated the velocity of star formation propagation across the arm S4 60 km s^–1 , pattern frequency _p 14 km s^–1 kpc^–1 and corotation radiusR _c20 kpc. The structure of S4 along the arm is complicated. In the OB 82 region an age gradient is absent. The young associationOB 79b is located near the outer edge of S4 and it has a large absorptionA _v2^m.5 contrary to the density wave prediction. This association bears no relation to the spiral density wave and it is probably, supernova events that stimulated the star formation in it. The colour excessE _R-V is randomly distributed and the youngest stars are concentrated in the middle of the S6 arm. A value of pattern frequency _p = 12km s^–1 kpc^–1 andR _c=12 kpc of our Galaxy has been obtained from the age distribution of the open clusters and cepheids across the Carina-Sagittarius arm. The spiral structure of M31 is compared with that of the galaxy. There is a similarity between S4 in M31 and Carina-Sagittarius in the Galaxy, and also between the S6 and Perseus arms. The Orion arm in the Galaxy bears no relation to the wave density.     

Gamma-ray burst energy spectra: theoretical models,old and new

The M, S and C stars may be placed in an evolutionary sequence on the basis of direct observation of the spectroscopic transitions on the AGB of rich intermediate-age clusters in the Magellanic Clouds, but some S and C stars cannot be accounted for in this way. The S stars in Centauri owe their peculiarity to a primordial enrichment in s-process elements. The J-type (¹³C-rich) carbon stars originate in a different way to the ordinary cool N-type carbon stars. Some of them have silicate-rich circumstellar dust, contrary to expectation. Some of the carbon-rich RV Tauri stars also have silicate-rich dust and in both cases it may be organised in a disc. Observational evidence for gas and dust ejection by ordinary N-type carbon stars has been found and may be inferred for some RV Tauri stars.     

L' and M' standard stars for the Mauna Kea Observatories Near-Infrared system

We present L ' and M ' photometry, obtained at the United Kingdom Infrared Telescope (UKIRT) using the Mauna Kea Observatories Near-Infrared (MKO-NIR) filter set, for 46 and 31 standard stars, respectively. The L ' standards include 25 from the in-house 'UKIRT Bright Standards' with magnitudes deriving from Elias et al. and observations at the Infrared Telescope Facility in the early 1980s, and 21 fainter stars. The M ' magnitudes derive from the results of Sinton and Tittemore. We estimate the average external error to be 0.015 mag for the bright L ' standards and 0.025 mag for the fainter L ' standards, and 0.026 mag for the M ' standards. The new results provide a network of homogeneously observed standards, and establish reference stars for the MKO system, in these bands. They also extend the available standards to magnitudes which should be faint enough to be accessible for observations with modern detectors on large and very large telescopes.     

First Byurakan Spectral Sky Survey. Stars of Late-Spectral Types. XIV. Zone +13° ≤ δ ≤ +33°

We present the fourteenth list of faint late M and carbon type stars detected on the plates of the First Byurakan Spectral Sky Survey in the zone +13° +33° covering about 4736 sq. degrees. From 260 stars, 118 are newly detected objects: they are 19 carbon stars, 5 carbon star candidates, and 94 M-type stars. Among 118 detected objects 73( 57 PSC + 16 FSC ) are unclassified IRAS sources. Accurate positions, spectral classes, red magnitudes, color indices, and near-infrared J, H and K photometry are given, using several astronomical databases. Finding charts from DSS are given for the most interesting objects.     

CCD photometry of BM Orionis and other trapezium stars

New light curves have been obtained for the eclipsing binary BM Orionis in theuvbyRI filter regions. The observations were made from 1988 to 1990 using the CCD photometer of the Behlen Observatory of the University of Nebraska. Reduction of all image data to magnitudes for BM Orionis and for each of the five other brightest stars in the ¹ Orionis group known as the Trapezium was accomplished by use of Stetson's DAOPHOT crowded-field photometry program. Differential magnitudes and light curves for BM Orionis and V1016 Orionis have been constructed using star D of the group as the primary comparison star. Possible indications of variability have also been looked for in the member stars C, E, and F with negative results for stars C and F and positive results for star E. The same CCD frames also contained the stars ² Orionis A and B. A light curve of the possibly-eclipsing spectroscopic binary ² Orionis A folded on its spectroscopic period does show some variability, but no obvious eclipses.     

Field optimization and CCD data simulation for the antarctic International Concordia Explorer Telescope (ICE‐T)

We performed extensive data simulations for the planned ultra‐wide‐field, high‐precision photometric telescope ICE‐T (International Concordia Explorer Telescope). ICE‐T consists of two 60 cm‐aperture Schmidt telescopes with a joint field of view simultaneously in two photometric bandpasses. Two CCD cameras, each with a single 10.3k × 10.3k thinned back‐illuminated device, would image a sky field of 65 square degrees. Given a location of the telescope at Dome C on the East Antarctic Plateau, we searched for the star fields that best exploit the technical capabilities of the instrument and the site. We considered the effects of diurnal air mass and refraction variations, solar and lunar interference, interstellar absorption, overexposing of bright stars and ghosts, crowding by background stars, and the ratio of dwarf to giant stars in the field. Using NOMAD, SSA, Tycho‐2 and 2MASS‐based stellar positions and BVIJH magnitudes for these fields, we simulated the effects of the telescope's point‐spread‐function, the integration, and the co‐addition times. Simulations of transit light curves are presented for the selected star fields and convolved with the expected instrumental characteristics. For the brightest stars, we showed that ICE‐T should be capable of detecting a 2 R_Earth Super Earth around a G2 solar‐type star, as well as an Earth around an M0‐star – if these targets were as abundant as hot Jupiters. Simultaneously, the telescope would monitor the host star's surface activity in an astrophysically interpretable manner (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

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.