The intermediate-age open cluster NGC 2112

We report on $BVI$ CCD photometry of a field centered on the region of the intermediate-age open cluster NGC 2112 down to V=21. Due to the smaller field coverage, we are able to limit the effect of field star contamination which hampered in the past precise determinations of the cluster age and distance. This way, we provide updated estimates of NGC 2112 fundamental parameters. Having extended the photometry to the $I$ pass-band, we are able to construct a colour-colour diagram, from which we infer a reddening $E_{B-V}= 0.63\pm0.14$ mag. The comparison of the Colour-Magnitude Diagram (CMD) with theoretical isochrones leads to a distance of $850 \pm 100$ pc, and an age of $2.0 \pm 0.3$ Gyr. While the distance is in agreement with previous determinations, the age turns out to be much better constrained and significantly lower than previous estimates.Comment: 7 pages, 7 eps figures, in press in MNRA

Galactic Extinction from Colors and Counts of Field Galaxies in WFPC2 Frames: An Application to GRB 970228

We develop the ``simulated extinction method'' to measure average foreground Galactic extinction from field galaxy number-counts and colors. The method comprises simulating extinction in suitable reference fields by changing the isophotal detection limit. This procedure takes into account selection effects, in particular, the change in isophotal detection limit (and hence in isophotal magnitude completeness limit) with extinction, and the galaxy color--magnitude relation. We present a first application of the method to the HST WFPC2 images of the gamma-ray burster GRB 970228. Four different WFPC2 high-latitude fields, including the HDF, are used as reference to measure the average extinction towards the GRB in the F606W passband. From the counts, we derive an average extinction of A_V = 0.5 mag, but the dispersion of 0.4 mag between the estimates from the different reference fields is significantly larger than can be accounted by Poisson plus clustering uncertainties. Although the counts differ, the average colors of the field galaxies agree well. The extinction implied by the average color difference between the GRB field and the reference galaxies is A_V = 0.6 mag, with a dispersion in the estimated extinction from the four reference fields of only 0.1 mag. All our estimates are in good agreement with the value of 0.81\pm0.27 mag obtained by Burstein & Heiles, and with the extinction of 0.78\pm0.12 measured by Schlegel et al. from maps of dust IR emission. However, the discrepancy between the widely varying counts and the very stable colors in these high-latitude fields is worth investigating.Comment: 14 pages, 2 figures; submitted to the Astrophysical Journa

Trumpler 20 - an old and rich open cluster

We show that the open cluster Trumpler 20, contrary to the earlier findings, is actually an old Galactic open cluster. New CCD photometry and high-resolution spectroscopy are used to derive the main parameters of this cluster. At [Fe/H]=-0.11 for a single red giant star, the metallicity is slightly subsolar. The best fit to the color-magnitude diagrams is achieved using a 1.3 Gyr isochrone with convective overshoot. The cluster appears to have a significant reddening at E(B-V)=0.46 (for B0 spectral type), although for red giants this high reddening yields the color temperature exceeding the spectroscopic T_eff by about 200 K. Trumpler 20 is a very rich open cluster, containing at least 700 members brighter than M_V=+4. It may extend over the field-of-view available in our study at 20'x20'.Comment: 7 pages, 5 figures; accepted for publication in MNRA

Chandra X-ray Sources in M101

A deep (98.2 ks) Chandra Cycle-1 observation has revealed a wealth of discrete X-ray sources as well as diffuse emission in the nearby face-on spiral galaxy M101. From this rich dataset we have created a catalog of the 110 sources from the S3 chip detected with a significance of >3 sigma, corresponding to a flux of ~1.0E-16 ergs/cm/cm/s and a luminosity of 1.0E36 ergs/s for a distance to M101 of 7.2 Mpc. The sources display a distinct correlation with the spiral arms and include a variety of X-ray binaries, supersoft sources, supernova remnants, and other objects of which only ~27 are likely to be background sources. There are only a few sources in the interarm regions, and most of these have X-ray colors consistent with that of background AGNs. The derived log N-log S relation for the sources in M101 (background subtracted) has a slope of -0.80+/-0.05 over the range of 1.0E36 - 1.0E38 ergs/s. The nucleus is resolved into 2 nearly identical X-ray sources, each with a 0.5-2.0 keV flux of 4.0E37 ergs/s. One of these sources coincides with the optical nucleus, and the other coincides with a cluster of stars 110 pc to the south.Comment: 39 pages including 13 figures and 4 tables; ApJ, in pres

A deep wide-field sub-mm survey of the Carina Nebula complex

The Great Nebula in Carina is a superb location in which to study the physics of violent massive star-formation and the resulting feedback effects, including cloud dispersal and triggered star-formation. In order to reveal the cold dusty clouds in the Carina Nebula complex, we used the Large APEX Bolometer Camera LABOCA at the APEX telescope to map a 1.25 deg x 1.25 deg (= 50 x 50 pc^2) region at 870 micrometer. From a comparison to Halpha images we infer that about 6% of the 870 micrometer flux in the observed area is likely free-free emission from the HII region, while about 94% of the flux is very likely thermal dust emission. The total (dust + gas) mass of all clouds for which our map is sensitive is ~ 60 000 Msun, in good agreement with the mass of the compact clouds in this region derived from 13CO line observations. We generally find good agreement in the cloud morphology seen at 870 micrometer and the Spitzer 8 micrometer emission maps, but also identify a prominent infrared dark cloud. Finally, we construct a radiative transfer model for the Carina Nebula complex that reproduces the observed integrated spectral energy distribution reasonably well. Our analysis suggests a total gas + dust mass of about 200000 Msun in the investigated area; most of this material is in the form of molecular clouds, but a widely distributed component of (partly) atomic gas, containing up to ~ 50% of the total mass, may also be present. Currently, only some 10% of the gas is in sufficiently dense clouds to be immediately available for future star formation, but this fraction may increase with time owing to the ongoing compression of the strongly irradiated clouds and the expected shockwaves of the imminent supernova explosions.Comment: Accepted for publication in Astronomy & Astrophysics; high-quality pre-prints can be obtained from http://www.usm.uni-muenchen.de/people/preibisch/publications.htm

The OSACA Database and a Kinematic Analysis of Stars in the Solar Neighborhood

We transformed radial velocities compiled from more than 1400 published sources, including the Geneva--Copenhagen survey of the solar neighborhood (CORAVEL-CfA), into a uniform system based on the radial velocities of 854 standard stars in our list. This enabled us to calculate the average weighted radial velocities for more than 25~000 HIPPARCOS stars located in the local Galactic spiral arm (Orion arm) with a median error of +-1 km/s. We use these radial velocities together with the stars' coordinates, parallaxes, and proper motions to determine their Galactic coordinates and space velocities. These quantities, along with other parameters of the stars, are available from the continuously updated Orion Spiral Arm CAtalogue (OSACA) and the associated database. We perform a kinematic analysis of the stars by applying an Ogorodnikov-Milne model to the OSACA data. The kinematics of the nearest single and multiple main-sequence stars differ substantially. We used distant (r\approx 0.2 kpc) stars of mixed spectral composition to estimate the angular velocity of the Galactic rotation -25.7+-1.2 km/s/kpc, and the vertex deviation,l=13+-2 degrees, and detect a negative K effect. This negative K effect is most conspicuous in the motion of A0-A5 giants, and is equal to K=-13.1+-2.0 km/s/kpc.Comment: 16 pages, 8 figure

Kinematics of Tycho-2 Red Giant Clump Stars

Based on the Ogorodnikov-Milne model, we analyze the proper motions of 95 633 red giant clump (RGC) stars from the Tycho-2 Catalogue. The following Oort constants have been found: A = 15.9+-0.2 km/s/kpc and B = -12.0+-0.2 km/s/kpc. Using 3632 RGC stars with known proper motions, radial velocities, and photometric distances, we show that, apart from the star centroid velocity components relative to the Sun, only the model parameters that describe the stellar motions in the XY plane differ significantly from zero. We have studied the contraction (a negative K-effect) of the system of RGC stars as a function of their heliocentric distance and elevation above the Galactic plane. For a sample of distant (500--1000 pc) RGC stars located near the Galactic plane (|Z|<200 pc) with an average distance of d=0.7 kpc, the contraction velocity is shown to be Kd= -3.5+-0.9 km/s; a noticeable vertex deviation, lxy = 9.1+-0.5 degrees, is also observed for them. For stars located well above the Galactic plane (|Z|>=200 pc), these effects are less pronounced, Kd = -1.7+-0.5 km/s and lxy = 4.9+-0.6 degrees. Using RGC stars, we have found a rotation around the Galactic X axis directed toward the Galactic center with an angular velocity of -2.5+-0.3 km/s/kpc, which we associate with the warp of the Galactic stellar-gaseous disk.Comment: 23 pages, 7 figures, 4 table

The Mass and Structure of the Pleiades Star Cluster from 2MASS

We present the results of a large scale search for new members of the Pleiades star cluster using 2MASS near-infrared photometry and proper motions derived from POSS plates digitized by the USNO PMM program. The search extends to a 10 degree radius around the cluster, well beyond the presumed tidal radius, to a limiting magnitude of R ~ 20, corresponding to ~ 0.07 M_sun at the distance and age of the Pleiades. Multi-object spectroscopy for 528 candidates verifies that the search was extremely effective at detecting cluster stars in the 1 - 0.1 M_sun mass range using the distribution of H_alpha emission strengths as an estimate of sample contamination by field stars. When combined with previously identified, higher mass stars, this search provides a sensitive measurement of the stellar mass function and dynamical structure of the Pleiades. The degree of tidal elongation of the halo agrees well with current N body simulation results. Tidal truncation affects masses below ~ 1 M_sun. The cluster contains a total mass ~ 800 M_sun. Evidence for a flatter mass function in the core than in the halo indicates the depletion of stars in the core with mass less than ~ 0.5 M_sun, relative to stars with mass \~1 - 0.5 M_sun, and implies a preference for very low mass objects to populate the halo or escape. The overall mass function is best fitted with a lognormal form that becomes flat at ~ 0.1 M_sun. Whether sufficient dynamical evaporation has occurred to detectably flatten the initial mass function, via preferential escape of very low mass stars and brown dwarfs, is undetermined, pending better membership information for stars at large radial distances.Comment: 19 pages, 14 figures, 2 tables, accepted by AJ, to appear April 200

Visual/infrared interferometry of Orion Trapezium stars: Preliminary dynamical orbit and aperture synthesis imaging of the Theta 1 Orionis C system

Located in the Orion Trapezium cluster, Theta 1 Orionis C is one of the youngest and nearest high-mass stars (O5-O7) and also known to be a close binary system. Using new multi-epoch visual and near-infrared bispectrum speckle interferometric observations obtained at the BTA 6 m telescope, and IOTA near-infrared long-baseline interferometry, we trace the orbital motion of the Theta 1 Ori C components over the interval 1997.8 to 2005.9, covering a significant arc of the orbit. Besides fitting the relative position and the flux ratio, we apply aperture synthesis techniques to our IOTA data to reconstruct a model-independent image of the Theta 1 Ori C binary system. The orbital solutions suggest a high eccentricity (e approx. 0.91) and short-period (P approx. 10.9 yrs) orbit. As the current astrometric data only allows rather weak constraints on the total dynamical mass, we present the two best-fit orbits. From these orbital solutions one can be favoured, implying a system mass of 48 M_sun and a distance to the Trapezium cluster of 434 pc. When also taking the measured flux ratio and the derived location in the HR-diagram into account, we find good agreement for all observables, assuming a spectral type of O5.5 for Theta 1 Ori C1 (M=34.0 M_sun) and O9.5 for C2 (M=15.5 M_sun). We find indications that the companion C2 is massive itself, which makes it likely that its contribution to the intense UV radiation field of the Trapezium cluster is non-negligible. Furthermore, the high eccentricity of the preliminary orbit solution predicts a very small physical separation during periastron passage (approx. 1.5 AU, next passage around 2007.5), suggesting strong wind-wind interaction between the two O stars.Comment: 13 pages, 9 figures, Accepted for publication in Astronomy & Astrophysic

The Distances to Open Clusters from Main-Sequence Fitting. IV. Galactic Cepheids, the LMC, and the Local Distance Scale

We derive the basic properties of seven Galactic open clusters containing Cepheids and construct their period-luminosity (P-L) relations. For our cluster main-sequence fitting we extend previous Hyades-based empirical color-temperature corrections to hotter stars using the Pleiades as a template. We use BVI_{C}JHK_{s} data to test the reddening law, and include metallicity effects to perform a more comprehensive study for our clusters than prior efforts. The ratio of total to selective extinction R_V that we derive is consistent with expectations. Assuming the LMC P-L slopes, we find = -3.93 +/- 0.07 (statistical) +/- 0.14 (systematic) for 10-day period Cepheids, which is generally fainter than those in previous studies. Our results are consistent with recent HST and Hipparcos parallax studies when using the Wesenheit magnitudes W(VI). Uncertainties in reddening and metallicity are the major remaining sources of error in the V-band P-L relation, but a higher precision could be obtained with deeper optical and near-infrared cluster photometry. We derive distances to NGC4258, the LMC, and M33 of (m - M)_0 = 29.28 +/- 0.10, 18.34 +/- 0.06, and 24.55 +/- 0.28, respectively, with an additional systematic error of 0.16 mag in the P-L relations. The distance to NGC4258 is in good agreement with the geometric distance derived from water masers [\Delta (m - M)_0 = 0.01 +/- 0.24]; our value for M33 is less consistent with the distance from an eclipsing binary [\Delta (m - M)_0 = 0.37 +/- 0.34]; our LMC distance is moderately shorter than the adopted distance in the HST Key Project, which formally implies an increase in the Hubble constant of 7% +/- 8%.Comment: 28 pages, 21 figures; accepted for publication in the Ap