100 research outputs found
The intermediate-age open cluster NGC 2112
We report on 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 pass-band, we are able to construct a
colour-colour diagram, from which we infer a reddening
mag. The comparison of the Colour-Magnitude Diagram (CMD) with theoretical
isochrones leads to a distance of pc, and an age of
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
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