The detailed spiral structure of the outer Galactic disk remains poorly constrained, and for several line of sights in the Galaxy we depend on model extrapolations. One of these regions is the fourth Galactic quadrant, between Vela and Carina (270{deg}<=l<=300{deg}), where, apart from the conspicuous Carina branch of the Carina Sagittarius arm, no spiral arms have been detected so far in the optical beyond l~270{deg}. By means of deep UBVI photometry, we search for spiral features in known low absorption windows. Although observationally demanding, U photometry constitutes a powerful tool for detecting and characterizing distant aggregates of young stars, and allows firmer distance estimates to be derived. We study a direction close to the tangent (l~290{deg}) of the Carina arm, in an attempt to detect optical spiral tracers far beyond the Carina branch, where radio observations and model predictions appear to indicate the presence of an extension of the Perseus and Norma-Cygnus spiral arms in the fourth quadrant. Along this line of sight, we detect three distinct groups of young stars. Two of them, at distances of ~2.5 and ~6.0kpc, belong to the Carina spiral arm, which is traversed twice in this particular direction. Interestingly, the latter is detected for the first time. The third group, at a distance of ~12.7kpc, is probably related to the Perseus arm which lies beyond the Carina arm, and constitutes the first optical detection of this arm in the fourth Galactic quadrant. The position of this feature is consistent with both HI observations and model predictions. We also present evidence that this extremely distant group, formerly assumed to be a star cluster (Shorlin 1), is a diffuse, young population, typically found in spiral galaxies. In addition, our data-set does not support, as claimed in the literature, the possible presence of the Monoceros Ring in this direction. This study highlights how multicolor optical studies can be effective in probing the spiral structure of the outer Galactic disk. More fields need to be studied in this region of the Galaxy to constrain the spiral structure in the fourth Galactic quadrant more accurately, in particular, the shape and extent of the Perseus arm, and, possibly, to detect the even more distant Norma-Cygnus arm.
We present UBV photometry of the highly reddened and poorly studied open cluster Berkeley 55 (= C 2115+515), revealing an important population of B-type stars and several evolved stars of high luminosity. Intermediate-resolution far-red spectra of several candidate members confirm the presence of one F-type supergiant and six late supergiants or bright giants. The brightest blue stars are mid-B giants. Spectroscopic and photometric analyses indicate an age 50+/-10Myr. The cluster is located at a distance d~4kpc, consistent with other tracers of the Perseus Arm in this direction. Berkeley 55 is thus a moderately young open cluster with a sizable population of candidate red (super)giant members, which can provide valuable information about the evolution of intermediate-mass stars.
UBV photometry is presented for stars from AGK3R, NPZT, the Yale Bright Star Catalog (HR), and the FK5 faint extension. Stars with previously published UBV data were not selected. Observations were taken at the Kvistaberg Observatory and La Palma.
We report on Planetary Camera observations of the central region of 30 Doradus in the Large Magellanic Cloud. These images of 30 Doradus are the first "deep" HST exposures that have appropriate photometric calibration. The B band (F439W) image, which shows R136a at the center of the PC6 CCD chip, reveals over 200 stars within 3" of the center of R136a, and over 800 stars in a 35"x35" area. We used Malumuth et al.'s [The First Year of HST Observations, edited by A. L. Kinney and J. C. Blades (ST ScI, Baltimore) (1991)] PSF-fitting method to measure the magnitudes of all stars on the PC6 chip. These new B magnitudes, along with U and V magnitudes from archival PC images, yield a luminosity function, mass density profile, and initial mass function of the 30 Doradus ionizing cluster. The mass distribution is well fit by a King model with a core radius, Rc=0.96" (0.24pc), a tidal radius, Rt=110" (28pc), and a total mass, M=16800M_{sun}_. Both the luminosity function and initial mass function show evidence for mass segregation, in the sense that the central region has a higher fraction of massive stars than the outer regions. This is the first observational evidence for mass segregation in a very young cluster (age ~3 million years). The observations admit the hypothesis that the mass segregation occurred in the process of star formation and/or that the mass segregation is the result of dynamical evolution.
We present simultaneous UBV linear polarization measurements for 361 A- and F-type stars with accurate colour excess and distance determination. These stars are distributed in 35 Kapteyn's Selected Areas, covering the third and fourth quadrants of the galactic plane (|b|<=30{deg}). The obtained polarization and the known colour excess are compared. An analysis of the polarization distribution as a function of the stellar distance is also performed.
We present results from a continuing campaign to secure deep multi-colour CCD sequences for photoelectric calibration in UK Schmidt fields with galactic latitudes |b|>50{deg}. In this paper, we present UBVR photometry in 12 fields and BR photometry in a further 14 fields observed within UK Schmidt survey fields centered at {delta}=-30{deg}. Our observations were carried out with the 0.9m Telecope at the Cerro-Tololo Inter-American Observatory.
We present photometry and a preliminary interpretation of a UBVR survey of the Large and Small Magellanic Clouds, which covers 14.5 deg^2^ and 7.2deg^2^, respectively. This study is aimed at obtaining well-calibrated data on the brighter, massive stars, complementing recent, deeper CCD surveys. Our catalog contains 179,655 LMC and 84,995 SMC stars brighter than V~18.0, and is photometrically complete to U~B~V~15.7 and R~15.2, although stars in crowded regions are selectively missed. We compare our photometry to that of others, and describe the need for gravity-dependent corrections to our CCD U-band photometry. We provide a preliminary cross-reference between our catalog stars and the stars with existing spectroscopy.
We present precision CCD light curves, a period study, photometrically derived standard magnitudes, and a five-color simultaneous Wilson code solution of the totally eclipsing, yet shallow amplitude (Av~0.4mag) eclipsing, binary V1853 Orionis. It is determined to be an extreme mass ratio, q=0.20, W-type W UMa overcontact binary. From our standard star observations, we find that the variable is a late-type F spectral-type dwarf, with a secondary component of about 0.24 solar masses (stellar type M5V). Its long eclipse duration (41 minutes) as compared to its period, 0.383-days, attests to the small relative size of the secondary. Furthermore, it has reached a Roche lobe fill-out of ~50% of its outer critical lobe as it approaches its final stages of binary star evolution, that of a fast spinning single star. Finally, a summary of about 25 extreme mass ratio solar-type binaries is given.
