Carlos Feinstein

Massive open star clusters using the VVV survey - I. Presentation of the data and description of the approach

AA: U. de ConcepciA3n, U. de ValparaA-so; AB: U. de ValparaA-so, The Milky Way Millennium Nucleus; AC: U. de ValparaA-so, U. of Hertfordshire; AD: U. Federal do Rio Grande do Sul; AE: Saint Marys University; AF: U. de ConcepciA3n; AG: U. de ValparaA-so, PUC de Chile; AH: U. de ConcepciA3n; AI: U. de ValparaA-so; AJ: IALP; AK: IALP; AL: ESO; AM: U. de ConcepciA3n; AN: The Milky Way Millennium Nucleus, PUC de Chile; AO: The Milky Way Millennium Nucleus, PUC de Chile, Vatican Observatory, Princeton University; AP: U. of Hertfordshire; AQ: Kavli Institute for Astronomy and Astrophysics

Massive open star clusters using the VVV survey - II. Discovery of six clusters with Wolf-Rayet stars

Context. The ESO Public Survey “VISTA Variables in the Via Lactea” (VVV) provides deep multi-epoch infrared observations for an unprecedented 562 sq. degrees of the Galactic bulge, and adjacent regions of the disk. Nearly 150 new open clusters and cluster candidates have been discovered in this survey. Aims. This is the second in a series of papers about young, massive open clusters observed using the VVV survey. We present the first study of six recently discovered clusters. These clusters contain at least one newly discovered Wolf-Rayet (WR) star. Methods. Following the methodology presented in the first paper of the series, wide-field, deep JHKs VVV observations, combined with new infrared spectroscopy, are employed to constrain fundamental parameters for a subset of clusters. Results. We find that the six studied stellar groups are real young (2–7 Myr) and massive (between 0.8 and 2.2 × 10 3 M� ) clusters. They are highly obscured (AV ∼ 5−24 mag) and compact (1–2 pc). In addition to WR stars, two of the six clusters also contain at least one red supergiant star, and one of these two clusters also contains a blue supergiant. We claim the discovery of 8 new WR stars, and 3 stars showing WR-like emission lines which could be classified WR or OIf. Preliminary analysis provides initial masses of 30

Revisiting the area of the open cluster Stock¿16

Deep CCD UBV(RI)C photometry has been carried out in the area of the open cluster Stock 16. The different photo- metric diagrams indicate the presence of three superimposed stellar groups, all composed of early type stars. The first of them, the cluster Stock 16, suffers from slightly variable reddening with a mean color excessEB−V � = 0.51 . The second group, reddened byEB−V � = 0.84 , includes the Wolf-Rayet star WR 50 (a WC7+OB), and the third one, with a meanEB−V � = 1.18 , also includes another Wolf-Rayet star, WR 51 (a WN4+OB?). Our results confirm the distance and reddening already stated for Stock 16 but include new members. Concerning the other two highly reddened groups, we show that the first one - located at a distance of 3600 pc - is probably related to an anonymous OB association behind Cen OB1 also at 3600 pc already mentioned in the literature; the other seems to be a distant OB association - at more than 9000 pc - which may belong to the Scutum-Crux spiral arm. In these two groups we find evidence that the absorption law, AV /EB−V = R amounts to 4.0 approximately. The ages of the different star populations in the zone range from 5 × 10 6 yr to 6.4 × 10 6 yr for Stock 16, 8 × 10 6 for the second most reddened group and 5 × 10 6 for the probable association in Scutum-Crux. A determination of the IMF slope of Stock 16 was also carried out for stars in the mass range 1.8 < M < 14 M� , giving a normal value x = 1.3.

Multicolor Polarization Study of Ara OB1

We present the results of a multicolor polarimetric study of stars in the association Ara OB1. Several objects belonging to the clusters NGC 6204, 6193, and 6167 and the local field were observed as part of a global study of the region. The polarimetry shows that the orientation of the polarimetric vectors of each star is very similar within each cluster. The average values are 355 ± 151 and 522 ± 16° for NGC 6204 and 6193, respectively. An average value of 1065 ± 99 is found for NGC 6167, but the angle distribution is asymmetric, and a second component can be fitted to the angle histogram (P.A. ~ 1208 ± 116), showing a behavior not observed in NGC 6204 and 6193. So, we suggest that some of the observed stars perhaps belong to another stars grouping, located behind NGC 6167 and between a dust layer with a different orientation of the grains. The large difference in the polarization angle between NGC 6167 and the other two clusters could also be explained because NGC 6167 is supposed to be at the center of a gas-expanding structure and it is possible that this cluster was the origin of the star formation process in the Ara OB1 association, triggered by stellar winds or supernova explosions.

Optical Polarization Observations of NGC 6231: Evidence for a Past Supernova Fingerprint*

We present the first linear multicolor polarization observations for a sample of 35 stars in the direction of the Galactic cluster NGC 6231. We have found a complex pattern in the angles of the polarimetric vectors. Near the core of this cluster the structure shows a semicircular pattern that we have interpreted as a reorientation of the dust particles showing the morphology of the magnetic field. We propose that a supernova event occurred some time ago and produced a shock on the local ISM. We discuss in this paper independent confirmations of this event, both from the studies on the diffuse interstellar absorptions and the results of the pre-main-sequence stars. We also show that a supernova is supported by the evolutionary status of the cluster.

Optical polarization observations in the region of Stock 16

We present ( UBVRI ) multicolor linear polarimetric data for 26 of the brightest stars in the area of the open cluster Stock 16 that were considered to study the properties of the ISM (interstellar medium) towards the cluster. Our data yield a mean polarization percentage of

The Highly Polarized Open Cluster Trumpler 27

P\sim2.5\%

Optical multicolor polarization observations in the region of the open cluster NGC 5749

, close to the polarization value produced by the ISM with normal efficiency (

Optical polarization observations in the Scorpius region: NGC 6124★

P_{\lambda_{\rm max}} \sim 5 E_{B-V}

Optical polarization observations in Hogg 22 and NGC 6204

) undergoing a color excess of