The nearby Galaxy structure toward the Vela Gum nebula

We report on $UBVI$ photometry and spectroscopy for MK classification purposes carried out in the fields of five open clusters projected against the Vela Gum in the Third Galactic Quadrant of the Galaxy. They are Ruprecht 20, Ruprecht 47, Ruprecht 60, NGC 2660 and NGC 2910. We could improve/confirm the parameters of these objects derived before. The spectroscopic parallax method has been applied to several stars located in the fields of four out of the five clusters to get their distances and reddenings. With this method we found two blue stars in the field of NGC 2910 at distances that make them likely members of Vela OB1 too. Also, projected against the fields of Ruprecht 20 and Ruprecht 47 we have detected other young stars favoring not only the existence of Puppis OB1 and OB2 but conforming a young stellar group at $\sim1$ kpc from the Sun and extending for more than 6 kpc outward the Galaxy. If this is the case, there is a thickening of the thin Galactic disk of more than 300 pc at just 2-3 kpc from the Sun. Ruprecht 60 and NGC 2660 are too old objects that have no physical relation with the associations under discussion. An astonishing result has been the detection in the background of Ruprecht 47 of a young star at the impressive distance of 9.5 kpc from the Sun that could be a member of the innermost part of the Outer Arm. Another far young star in the field of NGC 2660, at near 6.0 kpc, may become a probable member of the Perseus Arm or of the inner part of the Local Arm. The distribution of young clusters and stars onto the Third Galactic Quadrant agrees with recent findings concerning the extension of the Local Arm as revealed by parallaxes of regions of star formation. We show evidences too that added to previous ones found by our group explain the thickening of the thin disk as a combination of flare and warp.Comment: Accepted for publication in New Astronom

Optical spectroscopy of X-Mega targets in the Carina nebula - VII On the multiplicity of Tr 16-112, HD 93343 and HD 93250

We present the results of a spectroscopic monitoring campaign devoted to three O-type stars in the Carina nebula. We derive the full SB2 orbital solution of the binary system Tr 16-112, an exceptional dissymmetrical system consisting of an O5.5-6V((f$^+$?p)) primary and a B2V-III secondary. We also report on low-amplitude brightness variations in Tr 16-112 that are likely due to the ellipsoidal shape of the O5.5-6 primary revolving in an eccentric orbit around the system's centre of mass. We detect for the first time a clear SB2 binary signature in the spectrum of HD 93343 (O8 + O8), although our data are not sufficient to establish an orbital solution. This system also displays low amplitude photometric modulations. On the other hand, no indication of multiplicity is found in the optical spectra of HD 93250. Finally, we discuss the general properties of multiple massive stars in the Carina OB1 association.Comment: Accepted for publication in MNRA

The first orbital solution for the massive colliding-wind binary HD93162 (=WR25)

Since the discovery, with the EINSTEIN satellite, of strong X-ray emission associated with HD93162 (=WR25), this object has been predicted to be a colliding-wind binary system. However, radial-velocity variations that would prove the suspected binary nature have yet to be found. We spectroscopically monitored this object to investigate its possible variability to address this discordance. We compiled the largest available radial-velocity data set for this star to look for variations that might be due to binary motion. We derived radial velocities from spectroscopic data acquired mainly between 1994 and 2006, and searched these radial velocities for periodicities using different numerical methods. For the first time, periodic radial-velocity variations are detected. Our analysis definitively shows that the Wolf-Rayet star WR25 is an eccentric binary system with a probable period of about 208 days.Comment: 7 pages, 4 figures, accepted by A+

NGC 2401: A template of the Norma-Cygnus Arm's young population in the Third Galactic Quadrant

Based on a deep optical CCD (UBV(RI)_C) photometric survey and on the Two-Micron All-Sky-Survey (2MASS) data we derived the main parameters of the open cluster NGC 2401. We found this cluster is placed at 6.3 $\pm$ 0.5 kpc (V_O - M_V = 14.0 \pm 0.2) from the Sun and is 25 Myr old, what allows us to identify NGC 2401 as a member of the young population belonging to the innermost side of the extension of the Norma-Cygnus spiral--arm in the Third Galactic Quadrant. A spectroscopic study of the emission star LSS 440 that lies in the cluster area revealed it is a B0Ve star; however, we could not confirm it is a cluster member. We also constructed the cluster luminosity function (LF) down to $V \sim 22$ and the cluster initial mass function (IMF) for all stars with masses above M \sim 1-2 M_{\sun}. It was found that the slope of the cluster IMF is $x \approx 1.8 \pm 0.2$. The presence of a probable PMS star population associated to the cluster is weakly revealed.Comment: 10 paginas, 11 eps figures, accepted for publication in MNRA

Observational templates of star cluster disruption : The stellar group NGC 1901 in front of the Large Magellanic Cloud

Context. Observations indicate that present-day star formation in the Milky Way disk takes place in stellar ensembles or clusters rather than in isolation. Bound, long-lived stellar groups are known as open clusters. They gradually lose stars and are severely disrupted in their final evolutionary stages, leaving an open cluster remnant made up of a few stars. Aims. In this paper, we study in detail the stellar content and kinematics of the poorly populated star cluster NGC 1901. This object appears projected against the Large Magellanic Cloud. The aim of the present work is to derive the current evolutionary status, binary fraction, age, and mass of this stellar group. These are fundamental quantities to compare with those from N-body models in order to study the most general topic of star cluster evolution and dissolution. Methods. The analysis is performed using wide-field photometry in the UBVI pass-band, proper motions from the UCAC.2 catalog, and 3 epochs of high-resolution spectroscopy, as well as results from extensive N-body calculations. Results. The star group NGC 1901 is found to be an ensemble of solar metallicity stars, 400 ± 100 Myr old, with a core radius of 0.23 pc, a tidal radius of 1.0 pc, and a location at 400 ± 50 pc from the Sun. Out of 13 confirmed members, only 5 single stars have been found. Its estimated present-day binary fraction is at least 62%. The calculated heliocentric space motion of the cluster is not compatible with possible membership in the Hyades stream. Conclusions. Our results show that NGC 1901 is a clear prototype of an open cluster remnant characterized by a high value of the binary fraction and a significant depletion of low-mass stars. In light of numerical simulations, this is compatible with NGC 1901 being what remains of a larger system initially made of 500-750 stars.Facultad de Ciencias Astronómicas y Geofísica

The Effective Temperatures of Hot Stars II. The Early-O Types

We derive the stellar parameters of a sample of Galactic early-O type stars by analysing their UV and Far-UV spectra from FUSE (905-1187A), IUE, HST-STIS and ORFEUS (1200-2000A). The data have been modeled with spherical, hydrodynamic, line-blanketed, non-LTE synthetic spectra computed with the WM-basic code. We obtain effective temperatures ranging from Teff = 41,000 K to 39,000 K for the O3-O4 dwarf stars, and Teff = 37,500 K for the only supergiant of the sample (O4 If+). Our values are lower than those from previous empirical calibrations for early-O types by up to 20%. The derived luminosities of the dwarf stars are also lower by 6 to 12%; however, the luminosity of the supergiant is in agreement with previous calibrations within the error bars. Our results extend the trend found for later-O types in a previous work by Bianchi & Garcia.Comment: Accepted for publication in The Astrophysical Journal. 38 pages (including 9 figures and 4 tables

The Vela-Puppis open clusters Pismis 8 and Pismis 13

CCD UBVI imaging photometry was carried out in the fields of the open clusters Pismis 8 and Pismis 13, located in the Vela-Puppis region in our Galaxy. MK spectral types have also been determined for a number of stars located in the fields of these two clusters which were used to secure membership among the brightest stars. Since our photometry goes to a fainter limit than previous studies we could provide better reddening, distance and age determinations. Both clusters are located close to the edge of the local arm in the third quadrant. Pismis 8 is a cluster about 5–7 My old located at 2000 pc from the Sun, while Pismis 13 was found at 2750 pc with a probable age of about 100 My. The estimate of the slopes of the mass functions in both cases yielded $x\approx 1.7$ and $x\approx 2.1$ for Pismis 8 and 13 respectively