Massive Stellar Content of the Galactic Supershell GSH 305+01-24

The distribution of OB stars along with that of H$\alpha$, $^{12}$CO, dust infrared emission, and neutral hydrogen is carried out in order to provide a more complete picture of interactions of the young massive stars and the observed supershell GSH 305+01-24. The studied field is located between $299^\circ \le l \le 311^\circ$ and $-5^\circ \le b \le 7^\circ$. The investigation is based on nearly 700 O-B9 stars with $uvby\beta$ photometry currently available. The derived stellar physical parameters were used to establish a homogeneous scale for the distances and extinction of light for major apparent groups and layers of foreground and background stars in Centaurus and study the interaction with the surrounding interstellar medium. The distance to the entire Centaurus star-forming complex is revised and a maximum of the OB-star distance distribution is found at 1.8$\pm$0.4 (r.m.s) kpc. The massive star component of GSH 305+01-24 is identified at about 85-90 % completeness up to 11.5-12 mag. The projected coincidence of the OB stars with the shell and the similarities between the shell's morphology and the OB-star distribution indicate a strong interaction of the stellar winds with the superbubble material. We demonstrate that these stars contribute a sufficient wind injection energy in order to explain the observed size and expansion velocity of the supershell. The derived stellar ages suggest an age gradient over the Coalsack Loop. A continuous star-formation might be taking place within the shell with the youngest stars located at its periphery and the open cluster NGC 4755 being the oldest. A layer of very young stars at 1 kpc is detected and its connection to both GSH 305+01-24 and the foreground GSH 304-00-12 H I shells is investigated.Comment: Accepted for publication in A&A. Paper consists of 11 pages, 3 tables and 9 figures. Table 1 and Table 3 will only be available from CD

The Structure and the Distance of Collinder 121 from Hipparcos and Photometry: Resolving the Discrepancy

We present further arguments that the Hipparcos parallaxes for some of the clusters and associations represented in the Hipparcos catalog should be used with caution in the study of the Galactic structure. It has been already shown that the discrepancy between the Hipparcos and ground based parallaxes for several clusters including the Pleiades, Coma Ber and NGC 6231 can be resolved by recomputing the Hipparcos astrometric solutions with an improved algorithm diminishing correlated errors in the attitude parameters. Here we present new parallaxes obtained with this algorithm for another group of stars with discrepant data - the galactic cluster Cr 121. The original Hipparcos parallaxes led de Zeeuw et al. to conclude that Cr 121 and the surrounding association of OB stars form a relatively compact and coherent moving group at a distance of 550 -- 600 pc. Our corrected parallaxes reveal a different spatial distribution of young stellar populace in this area. Both the cluster Cr 121 and the extended OB association are considerably more distant (750 -- 1000 pc), and the latter has a large depth probably extending beyond 1 kpc. Therefore, not only are the recalculated parallaxes in complete agreement with the photometric uvbybeta parallaxes, but the structure of the field they reveal is no longer in discrepancy with that found by the photometric method.Comment: 7 pages, 1 figure, ApJL accepte

OB Stars in the Solar Neighborhood I: Analysis of their Spatial Distribution

We present a newly-developed, three-dimensional spatial classification method, designed to analyze the spatial distribution of early type stars within the 1 kpc sphere around the Sun. We propose a distribution model formed by two intersecting disks -the Gould Belt (GB) and the Local Galactic Disk (LGD)- defined by their fundamental geometric parameters. Then, using a sample of about 550 stars of spectral types earlier than B6 and luminosity classes between III and V, with precise photometric distances of less than 1 kpc, we estimate for some spectral groups the parameters of our model, as well as single membership probabilities of GB and LGD stars, thus drawing a picture of the spatial distribution of young stars in the vicinity of the Sun.Comment: 28 pages including 9 Postscript figures, one of them in color. Accepted for publication in The Astronomical Journal, 30 January 200

On the progenitor of V838 Monocerotis

We summarize and analyze the available observational data on the progenitor and the enviroment of V838 Mon. From the available photometric data for the progenitor of V838 Mon we exclude the possibility that the object before eruption was an evolved red giant star (AGB or RGB star). We find that most likely it was a main sequence or pre-main sequence star of ~ 5-10 M_sun. From the light echo structure and evolution we conclude that the reflecting dust is of interstellar nature rather than blown by V838 Mon in the past. We discuss the IRAS and CO data for interstellar medium observed near the position of V838 Mon. Several interstellar molecular regions have radial velocities similar to that of V838 Mon, so dust seen in the light echo might be related to one of them.Comment: 12 pages, 7 figures, accepted for publication in Astronomy & Astrophysic

Galactic Structure Toward the Carina Tangent

This investigation presents a photometric study of the Galactic structure toward the Carina arm tangent. The field is located between 280 deg and 286 deg galactic longitude and -4 deg to 4 deg galactic latitude. All currently available uvbybeta data is used to obtain homogeneous color excesses and distances for more than 260 stars of spectral types O to G. We present revised distances and average extinction for the open clusters and cluster candidates NGC 3293, NGC 3114, Loden 46 and Loden 112. The cluster candidate Loden 112 appears to be a very compact group at a true distance modulus of 11.06 +\- 0.11 (s.e.) (1629 +84,-80 pc), significantly closer than previous estimates. We found other OB stars at that same distance and, based on their proper motions, suggest a new OB association at coordinates 282 deg < l < 285 deg, -2 deg < b < 2 deg. Utilizing BV photometry and spectral classification of the known O-type stars in the very young open cluster Wd 2 we provide a new distance estimate of 14.13 +\-0.16 (s.e.) (6698 +512,-475 pc), in excellent agreement with recent distance determinations to the giant molecular structures in this direction. We also discuss a possible connection between the HII region RCW 45 and the highly-reddened B+ star CPD -55 3036 and provide a revised distance for the luminous blue variable HR Car.Comment: accepted to PAS

AO Velorum: a young quadruple system with a ZAMS eclipsing BpSi primary

Using recent spectroscopic observations, we show that the triple system AO Vel with an eclipsing BpSi primary is in fact a remarkable quadruple system formed by two double-lined spectroscopic binaries with components close to the ZAMS. All available data have been reanalyzed to derive proper orbital parameters for both binary systems and to calculate absolute parameters of the eclipsing system. For the first time, direct determination of the radius and the mass have been obtained for a BpSi star. The study of the physical parameters of this unique system is especially important since it can be used as a test of evolutionary models for very young stars of intermediate mass.Comment: 9 pages, 5 figures, accepted for publication in A&

HI Shells Behind the Coalsack

We report the discovery of two new large HI shells in the direction of the Coalsack nebula. Both shells were observed with the Parkes Radio Telescope as part of the Southern Galactic Plane Survey. The largest shell, GSH 304-00-12, is at a distance of ~1.2 kpc and has derived physical dimensions of 280 x 200 pc. The second shell, GSH 305+01-24, is at a distance of ~2.2 kpc and has derived dimensions of 280 x 440 pc. We present a simple numerical model to show that GSH 305+01-24 most likely formed from stellar winds in the Centaurus OB1 stellar association. There is associated radio, infrared and H-alpha continuum emission. Both shells are situated in the Sagittarius-Carina arm, with GSH 305+01-24 more distant. The far edge of GSH 304-00-12 is at the near side of the arm and opens into the interarm region. We find no evidence for closure at the near side of the shell and therefore describe the geometry as conical. Emission from the near side of the shell may be lost in absorption by the Coalsack Nebula.Comment: 12 pages, 5 embedded EPS figures, uses emulateapj5.sty. To appear in the Astrophysical Journal, vol. 56

New OB star candidates in the Carina Arm around Westerlund 2 from VPHAS+

Date of Acceptance: 10/04/2015O and early B stars are at the apex of galactic ecology, but in the Milky Way, only a minority of them may yet have been identified. We present the results of a pilot study to select and parametrise OB star candidates in the Southern Galactic plane, down to a limiting magnitude of $g=20$. A 2 square-degree field capturing the Carina Arm around the young massive star cluster, Westerlund 2, is examined. The confirmed OB stars in this cluster are used to validate our identification method, based on selection from the $(u-g, g-r)$ diagram for the region. Our Markov Chain Monte Carlo fitting method combines VPHAS+ $u, g, r, i$ with published $J, H, K$ photometry in order to derive posterior probability distributions of the stellar parameters $\log(\rm T_{\rm eff})$ and distance modulus, together with the reddening parameters $A_0$ and $R_V$. The stellar parameters are sufficient to confirm OB status while the reddening parameters are determined to a precision of $\sigma(A_0)\sim0.09$ and $\sigma(R_V)\sim0.08$. There are 489 objects that fit well as new OB candidates, earlier than $\sim$B2. This total includes 74 probable massive O stars, 5 likely blue supergiants and 32 reddened subdwarfs. This increases the number of previously known and candidate OB stars in the region by nearly a factor of 10. Most of the new objects are likely to be at distances between 3 and 6 kpc. We have confirmed the results of previous studies that, at these longer distances, these sight lines require non-standard reddening laws with $3.5R_VPeer reviewe

Identifying birth places of young isolated neutron stars

Young isolated radio-quiet neutron stars are still hot enough to be detectable at X-ray and optical wavelengths due to their thermal emission and can hence probe cooling curves. An identification of their birth sites can constrain their age. For that reason we try to identify the parent associations for four of the so-called Magnificent Seven neutron stars for which proper motion and distance estimates are available. We are tracing back in time each neutron star and possible birth association centre to find close encounters. The associated time of the encounter expresses the kinematic age of the neutron star which can be compared to its characteristic spin-down age. Owing to observational uncertainties in the input data, we use Monte-Carlo simulations and evaluate the outcome of our calculations statistically. RX J1856.5-3754 most probably originated from the Upper Scorpius association about 0.3 Myr ago. RX 0720.4-3125 was either born in the young local association TWA about 0.4 Myr ago or in Tr 10 0.5 Myr in the past. Also RX J1605.3+3249 and RBS 1223 seem to come from a nearby young association such as the Sco-Cen complex or the extended Corona-Australis association. For RBS 1223 also a birth in Sct OB2 is possible. We also give constraints on the observables as well as on the radial velocity of the neutron star. Given the birth association, its age and the flight time of the neutron star, we estimate the mass of the progenitor star. Some of the potential supernovae were located very nearby (<100pc) and thus should have contributed to the 10Be and 60Fe material found in the Earth's crust. In addition we reinvestigate the previously suggested neutron star/ runaway pair PSR B1929+10/ zeta Ophiuchi and conclude that it is very likely that both objects were ejected during the same supernova event.Comment: 14 figures, 13 table