A. M. Orsatti

The Puppis region and the last crusade for faint OB stars

A search for faint OB stars over an area of 43 sq. deg in the Puppis region resulted in the discovery of 397 OB stars and 5 early type supergiants; the blue magnitude range of the survey is about 12.0 < B < 15.0 mag. About 73% of the stars found had not been previously recognized as OB stars; about 12% of them show hydrogen line emission. UBV photoelectric and photographic measures of OB stars from this list and from the Luminous Stars Survey are presented. The galactic distribution of the OB stars in the region shows concentrations around the open clusters Ruprecht 44 and Ruprecht 55, and the presence of an important grouping of young stars located far below the plane

Polarimetry in the Outskirts of NGC 6611

We present new polarimetric UBVRI observations of 25 stars in the direction of the halo of NGC 6611, the rich stellar open cluster embedded in an ionized hydrogen complex (M16). Our plan is to characterize the interstellar material (ISM) associated with halo stars in order to make a comparison with the ISM dusty core characteristics that resulted from a previous investigation by the same authors. Of the halo stars, 47% (8 out of 17) show indications of intrinsic polarization in their light, similar to what was found for core stars (50%). We have identified the presence of nearby dust clouds located on the Local arm that produce a mean polarization of about 1%; a value = 0.61 ± 0.07 μm, which is slightly larger than that of the average ISM; and a mean direction of the polarization vectors of = 819 ± 18. The ISM associated with the halo region has similar to the general interstellar medium (0.55 ± 0.07 μm). The observed polarizations show a gradual increase from halo ( = 1.93% ± 0.3%) to core ( = 3.19% ± 0.63%). Position angles of the e-vector for both groups are generally similar, but there exists a slight difference in mean direction between them that is within the errors. We have also found that the halo stars are possibly represented by Whittet & van Bredas relationship, while in the clusters core the dust does not fulfill the above-mentioned relationship. As a conclusion, we cannot find any clear difference between core and halo dust characteristics, with the exception of , which may suggest a change in dust size.

Polarimetry of an intermediate-age open cluster: NGC 5617

Aims. We present polarimetric observations in the UBVRI bands of 72 stars located in the direction of the medium age open cluster NGC 5617. Our intention is to use polarimetry as a tool in membership identification, by building on previous investigations intended mainly to determine the cluster’s general characteristics rather than provide membership suitable for studies such as stellar content and metallicity, as well as study the characteristics of the dust lying between the Sun and the cluster. Methods. The obsevations were carried out using the five-channel photopolarimeter of the Torino Astronomical Observatory attached to the 2.15 m telescope at the Complejo Astronomico El Leoncito (CASLEO; Argentina). Results. We are able to add 32 stars to the list of members of NGC 5617, and review the situation for others listed in the literature. In particular, we find that five blue straggler stars in the region of the cluster are located behind the same dust as the member stars are and we confirm the membership of two red giants. The proposed polarimetric memberships are compared with those derived by photometric and kinematical methods, with excellent results. Among the observed stars, we identify 10 with intrinsic polarization in their light. NGC 5617 can be polarimetrically characterized with Pmax = 4.40% and θv = 73. ◦ 1. The spread in polarization values for the stars observed in the direction of the cluster seems to be caused by the uneven distribution of dust in front of the cluster’s face. Finally, we find that in the direction of the cluster, the interstellar medium is apparently free of dust, from the Sun’s position up to the Carina-Sagittarius arm, where NGC 5617 seems to be located at its farthest border.

Revisiting the region of the open cluster NGC 5606

We present polarimetric observations in the UBVRI bands corresponding to 54 stars located in the direction of NGC 5606. Our intention is to analyze the dust characteristics between the Sun and the cluster, as well as to confirm doubtful memberships using polarimetric tools. We also want to determine if a group of 11 B and A stars identified in the past by Vazquez & Feinstein (1991, AA and we identified 17 (out of the 54 observed stars) with intrinsic polarization in their light. The group of late B- and A-type stars could be physically related, with characteristics of an open cluster of intermediate age (between 0.79 and 1. × 10 8 yr). For this group we obtained representative values of P = 2.12% and θv = 70. 9. The new cluster, which we have provisionally named Anon (La Plata 1), extends from NE to SW across the face of NGC 5606 covering about 10 � . The fitting procedure of Schmidt-Kaler’s ZAMS (1982, in Landolt/Bornstein, Neue Series VI/2b) was used to derive the distance to the group, obtaining a distance modulus of Vo − Mv = 9.90 (649 pc from the Sun). The mean color excess associated with the small group is EB−V = 0.27 ± 0.08 mag and the dust distribution shows great variability across its face. We think that the small open cluster has gone undetected due to the lack of bright member stars in it.

Pismis 20: One or two clusters?

We present UBVRI polarimetric observations of stars in the direction of the open cluster Pismis 20 and WR 67. It is found that the observed members segregate polarimetrically into two groups, with different associated mean

A polarimetric view of the open cluster Lyngå6 and the Cepheid TW Normæ

P_{\max}

Polarimetry of the Highly Reddened Open Clusters Hogg 15 and Lyngå 14

(4.31 and 6.08% respectively). Although in the literature these stars are considered as members of the same cluster, the polarized light from these groups clearly differs. There are two possible explanations: very efficient polarizing dust particles exists between certain members of the cluster; or else Pismis 20 is composed of different groupings superimposed along the line of sight. WR 67 belongs to the group more affected by the dust.

Dust properties in NGC 6611

We present UBVRI polarimetric observations of stars belonging to the small open cluster Lynga6 and of TW Nor, a Cepheid which can be seen near the center of the grouping. The wavelength of maximum polarization is computed and then analyzed in the context of its relation to the optical properties and characteristic particle size distribution of the grains responsible for the polarization. The amount and direction of linear polarization is also computed. Results indicate a mean polarization efficiency close to the value found for the interstellar medium. Two out of seven members of Lynga6 present indications of intrinsic polarization in their measures. We conclude that part of the polarization detected in the light from members of the cluster comes from a dust cloud located in the Local arm, at a distance of about 1 kpc. In general, polarization measurements do not support the membership of TW Nor to Lynga6.