The open cluster Havlen-Moffat No. 1 revisited

A deep CCD UBVRI photometric survey combined with UBVRI polarimetric observations of 21 bright stars was carried out in the region of the open cluster Havlen-Moffat No. 1. Our data reveal that the extinction law in this cluster is variable and that six cluster stars show very high polarisation values (>4%), probably because of the presence of a nearby small dust cloud. The cluster is at a distance of d = 3300 pc, it is 2-4 Myr old and the initial mass function of its most massive stars (M > 3 M⊙) has a flat slope of x ≈ 0.7. As an additional result, it was possible to reconcile the absolute magnitudes of the two WN7-type members using the R-values valid in the regions where they are located.Facultad de Ciencias Astronómicas y Geofísica

The open cluster Havlen-Moffat No. 1 revisited

A deep CCD UBVRI photometric survey combined with UBVRI polarimetric observations of 21 bright stars was carried out in the region of the open cluster Havlen-Moffat No. 1. Our data reveal that the extinction law in this cluster is variable and that six cluster stars show very high polarisation values (>4%), probably because of the presence of a nearby small dust cloud. The cluster is at a distance of d = 3300 pc, it is 2-4 Myr old and the initial mass function of its most massive stars (M > 3 M⊙) has a flat slope of x ≈ 0.7. As an additional result, it was possible to reconcile the absolute magnitudes of the two WN7-type members using the R-values valid in the regions where they are located.Facultad de Ciencias Astronómicas y Geofísica

Viewing angles of the Magellanic Clouds obtained via cluster analysis

Realizamos un estudio sobre los ángulos de rotación (o visión) de las Nubes Mayor y Menor de Magallanes (L/SMC), basado en las posiciones y distancias estimadas de 239 cúmulos estelaresWe performed a study of the viewing angles of the Large and Small Magellanic Clouds (L/SMC), based on the estimated position and distance of 239 observed star clusters.Trabajo publicado en Actas de las Terceras Jornadas de Astrofísica Estelar. La Plata, Asociación Argentina de Astronomía, 2018.Facultad de Ciencias Astronómicas y Geofísica

Viewing angles of the Magellanic Clouds obtained via cluster analysis

Realizamos un estudio sobre los ángulos de rotación (o visión) de las Nubes Mayor y Menor de Magallanes (L/SMC), basado en las posiciones y distancias estimadas de 239 cúmulos estelaresWe performed a study of the viewing angles of the Large and Small Magellanic Clouds (L/SMC), based on the estimated position and distance of 239 observed star clusters.Trabajo publicado en Actas de las Terceras Jornadas de Astrofísica Estelar. La Plata, Asociación Argentina de Astronomía, 2018.Facultad de Ciencias Astronómicas y Geofísica

The young open cluster Markarian 50

We report on a deep CCD UBV(RI) C photometric survey in the region of the young open cluster Markarian 50. The new photometric data allow us to extend the cluster membership down to V ≈ 17.5, about 2 mag deeper than any previous investigation. On the basis of these data we derive a distance d = 3460 ± 350 pc (V o - M v = 12.7 ± 0.2), which turns out to be only slightly lower than previous estimates. The cluster presents differential reddening, with E(B-V) values ranging from 0.69 to 1.1. The brightest member (HD 219460) is a double star, which we separate photometrically for the first time, providing individual magnitudes and colours for each component. One of them is a Wolf-Rayet (WR) star and, according to evolutionary models, the mass of its progenitor should be greater than ∼20 M⊙. The age obtained for the cluster is 7.5 ± 2 Myr and the mass function for the most massive stars (M > 1 M⊙) presents a slope x ≈ 1.0.Facultad de Ciencias Astronómicas y Geofísica

Stellar populations in the Canis Major overdensity

We performed a photometric multicolour survey of the core of the Canis Major overdensity at l ≈ 244°, b ≈ —8°.0, reaching V ~ 22 and covering 0.3 x 1.0 arcmin2. The main aim is to unravel the complex mixture of stellar populations toward this Galactic direction, where in the recent past important signatures of an accretion event have been claimed to be detected. While our previous investigations were based on disjointed pointings aimed at revealing the large-scale structure of the third Galactic Quadrant, we now focus on a complete coverage of a smaller field centred on the Canis Major overdensity. A large wavelength baseline, in the UBVRI bands, allows us to build up a suite of colour-colour and colour-magnitude diagrams, providing a much better diagnostic tool to disentangle the stellar populations of the region. In fact, the simple use of one colour-magnitude diagram, widely employed in all the previous studies defending the existence of the Canis Major galaxy, does not allow one to separate the effects of the different parameters (reddening, age, metallicity and distance) involved in the interpretation of data, forcing to rely on heavy modelling. In agreement with our previous studies, in the same general region ofthe Milky Way, we recognize a young stellar population compatible with the expected structure and extension of the Local (Orion) and Outer (NormaCygnus) spiral arms in the Third Galactic Quadrant. Moreover, we interpret the conspicuous intermediate-age metal-poor population as belonging to the Galactic thick disc, distorted by the effect of strong disc warping at this latitude, and to the Galactic halo.Facultad de Ciencias Astronómicas y Geofísica

Blue straggler stars in Galactic open clusters and the effect of field star contamination

Context. We investigate the distribution of blue straggler stars in the field of three open star clusters. Aims. The main purpose is to highlight the crucial role played by general Galactic disk fore-/back-ground field stars, which are often located in the same region of the color magnitude diagram as blue straggler stars. Methods. We analyze photometry taken from the literature of 3 open clusters of intermediate/old age rich in blue straggler stars, which are projected in the direction of the Perseus arm, and study their spatial distribution and the color magnitude diagram. Results. As expected, we find that a large portion of the blue straggler population in these clusters are simply young field stars belonging to the spiral arm. This result has important consequences on the theories of the formation and statistics of blue straggler stars in different population environments: open clusters, globular clusters, or dwarf galaxies. Conclusions. As previously emphasized by many authors, a detailed membership analysis is mandatory before comparing the blue straggler population in star clusters to theoretical models. Moreover, these sequences of young field stars (blue plumes) are potentially powerful tracers of Galactic structure and they require further consideration.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

Photometric distances to young stars in the inner Galactic disk : II. The region towards the open cluster Trumpler 27 at L = 355°

Context. The spiral structure of the Milky Way inside the solar circle is still poorly known because of the high density of the material that causes strong extinction towards the Galactic center. Aims. We present results of the first extensive and deep color-color diagram (CCD) photometric survey carried out in the field of the open cluster Trumpler 27, an object immersed in a region of extremely high visual absorption in the constellation of Sagittarius not far from the Galaxy center. The survey covers almost a quarter of square degree. Methods. We look for young stars clumps that might plausibly be associated with spiral structure. Wide-field UBVI photometry combined with infrared information allows us to reconstruct the distribution in the reddening and distance of young stars in the field using the CCD and color-magnitude diagrams (CMD). Results. The analysis of our data, combined with extensive spectroscopy taken from the literature, shows that the real entity of Trumpler 27 as an open cluster is far from being firmly stated. In fact, instead of finding a relatively compact group of stars confined to a small distance range, we found that stars associated with Trumpler 27 are, indeed, a superposition of early-type stars seen along the line of sight extending over several kiloparsecs beyond even the center of the Galaxy. We demonstrate that at each distance range it becomes possible to generate a CMD resembling that of an open cluster. This way, our analysis indicates that what was considered an open cluster characterized by a significant age spread is a stellar continuum that reaches its maximum number of stars at approximately 3.5 kpc from the Sun, the distance of the Scutum-Crux arm approximately. After analyzing the way early-type stars distribute with distance, we found that some of these stellar groups may be linked, within the distance errors, with other inner spiral arms of our Galaxy, including the Near 3 kpc arm at approximately 5 kpc from the Sun. However, very young stars by themselves do not seem to trace strongly the inner spiral arms since they are distributed evenly across several kiloparsecs toward the center of the Galaxy. This is remarkably different to current HI and CO radio observation maps, which show inner spiral arms composed by discrete structures of gas with a well-defined inter-arm separation.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat

Young and very young stars in NGC 3372, the Carina nebula

Results are presented of a large-scale imaging photometric study of the stellar population in the northern part of NGC 3372 with a w avelength co verage from 0.33 to 2.5 µm. All observations were made at Las Campanas Observatory . The sizes of the three stellar clusters, Tr 14, Tr 15 and Tr 16, were determined b y means of star counts. Two-colour and colour-magnitude diagrams are presented and analyzed for eac h individual cluster. The three clusters were found to b e at a similar distance from the Sun, = 2.7 kpc, but with very large scatter in both AV and d. suggesting drastic variations in intracluster dust densit y . Dust particle size distribution variations are eviden t resulting in wide variations in extinction law. We determined ages bet ween 3 and 60 million years for Tr 15 and bet ween less than 1 and 6 million years for Tr 14 and Tr 16. The Tr 14 cluster is partially em bedded in a dense molecular cloud that extends to wards the south west reaching its highest densit y some three arcmin from the cluster nucleus. The ric h UV field created b y the Tr14 stars ionizes most of the visible HII region in its vicinit y and most of the radio HII region Car I. Deep J H K images of the Car I region reveal the presence of a young, em bedded stellar population that includes several O9–B0 stars and an ultracompact HII region.Se presentan los resultados de un estudio fotométrico de la población estelar en el norte de NGC 3372 en un intervalo espectral que cubre 0.33 a 2.5 µm. Las observaciones se llevaron a cabo en el Observatorio de Las Campanas. Se determinaron los tamaños de los tres cúmulos, Tr 14, Tr 15 y Tr 16, por medio de conteo de estrellas. Diagramas color-color y color-magnitud sugieren edades entre 3 y 60 millones de años para Tr 15 y entre menos de 1 y 6 millones de años para Tr 14 y Tr 16. Los tres cúmulos se encuentran a la misma distancia: = 2.7 kpc aunque con gran dispersión real en AV y en d. Tr 14 es un cúmulo que se encuentra parcialmente inmerso en una nube molecular densa que se extiende hacia el sureste, teniendo su máxima densidad a unos tres minutos de arco del núcleo del cúmulo. Casi todo el gas de la región HII es ionizado por las estrellas de Tr 14. Sin embargo, imágenes en J H K profundas revelan la presencia de una jo ven población de estrellas O y B inmersas en la nube, en donde se encuentra también una región HII ultracompacta.Instituto de Astrofísica de La PlataFacultad de Ciencias Astronómicas y Geofísica

Determinación de la secuencia principal de edad cero y ensanchamiento diferencial de la secuencia principal

El material provisto por 47 cúmulos abiertos muy jóvenes ha sido utilizado para varios estudios que conciernen a la situación evolucionaría de estrellas de más de 10 masas solares. Se hizo una discusión sobre índices de colores y se midió el efecto introducido sobre las Mᵥ al considerar diferentes valores de R. Para estrellas con Mᵥ=-5.0 mag (60 M⊙) se hizo un análisis de sus características (binaridad, pérdida de masa, tipos espectrales, etc.). Posteriormente se compararon los resultados obtenidos con modelos evolucionarios.Asociación Argentina de Astronomí