59 research outputs found
Excitation and Propagation of Eccentricity Disturbances in Planetary Systems
The high eccentricities of the known extrasolar planets remain largely
unexplained. We explore the possibility that eccentricities are excited in the
outer parts of an extended planetary disk by encounters with stars passing at a
few hundreds of AU. After the encounter, eccentricity disturbances propagate
inward due to secular interactions in the disks, eventually exciting the
innermost planets. We study how the inward propagation of eccentricity in
planetary disks depends on the number and masses of the planets and spacing
between them and on the overall surface-density distribution in the disk. The
main governing factors are the large-scale surface-density distribution and the
total size of the system. If the smeared-out surface density is approximated by
a power-law \Sigma(r)\propto r^{-q}, then eccentricity disturbances propagate
inward efficiently for flat density distributions with q < 1. If this condition
is satisfied and the size of the planetary system is 50 AU or larger, the
typical eccentricities excited by this mechanism by field star encounters in
the solar neighborhood over 5 Gyr are in the range 0.01-0.1. Higher
eccentricities (> 0.1) may be excited in planetary systems around stars that
are formed in relatively dense, long-lived open clusters. Therefore, this
mechanism may provide a natural way to excite the eccentricities of extrasolar
planets.Comment: 23 pages including 4 b/w figures and 1 color figure, accepted to A
HST observations of star clusters in NGC 1023: Evidence for three cluster populations?
Using HST images we have carried out a study of cluster populations in the
nearby S0 galaxy NGC 1023. In two WFPC2 pointings we have identified 221
cluster candidates. The small distance (~9 Mpc) combined with deep F555W and
F814W images allows us to reach about two magnitudes below the expected
turn-over of the globular cluster luminosity function. NGC 1023 appears to
contain at least three identifiable cluster populations: the brighter clusters
show a clearly bimodal color distribution with peaks at V-I = 0.92 and at V-I =
1.15 and in addition there are a number of fainter, more extended objects with
predominantly red colors. Among the brighter clusters, we find that the blue
clusters have somewhat larger sizes than the red ones with mean effective radii
of R(eff) ~ 2 and R(eff) ~ 1.7 pc, respectively. These clusters have luminosity
functions (LFs) and sizes consistent with what is observed for globular
clusters in other galaxies. Fitting Gaussians to the LFs of the blue and red
compact clusters we find turn-over magnitudes of M(TO,blue)=-7.58 and
M(TO,red)=-7.37 in V and dispersions of sigma(V,blue)=1.12 and
sigma(V,red)=0.97. The fainter, more extended clusters have effective radii up
to R(eff) ~ 10-15 pc and their LF appears to rise at least down to M(V) ~ -6,
few of them being brighter than M(V) = -7. We suggest that these fainter
objects may have a formation history distinct from that of the brighter GCs.Comment: 27 pages, 13 figures. Accepted for publication in the Astronomical
Journa
Photometry of a Galactic field at l = 232, b = -6. The old open cluster Auner 1, the Norma-Cygnus spiral arm and the signature of the warped Galactic Thick Disk
We perform a detailed photometric study of the stellar populations in a
Galactic Field at l = 232, b = -6 in the Canis Major (CMa) constellation. We
present the first U,B,V,I photometry of the old open cluster Auner1 and
determine it to be 3.25 Gyr old and to lie at 8.9 kpc from the Sun. In the
background of the cluster, at more than 9 kpc, we detect a young population
most probably associated to the Norma Cygnus spiral arm. Furthermore, we detect
the signature of an older population and identify its Turn Off and Red Giant
Branch. This population is found to have a mean age of 7 Gyrs and a mean
metallicity of Z = 0.006 . We reconstruct the geometry of the stellar
distribution and argue that this older population - often associated to the
Canis Major {\it galaxy}- belongs in fact to the warped old thin/thick disk
component along this line of sight.Comment: 19 pages, 7 eps figures (some degraded), accepted for publication in
the Astronomical Journa
The Nature and Nurture of Star Clusters
Star clusters have hierarchical patterns in space and time, suggesting
formation processes in the densest regions of a turbulent interstellar medium.
Clusters also have hierarchical substructure when they are young, which makes
them all look like the inner mixed parts of a pervasive stellar hierarchy.
Young field stars share this distribution, presumably because some of them came
from dissolved clusters and others formed in a dispersed fashion in the same
gas. The fraction of star formation that ends up in clusters is apparently not
constant, but may increase with interstellar pressure. Hierarchical structure
explains why stars form in clusters and why many of these clusters are
self-bound. It also explains the cluster mass function. Halo globular clusters
share many properties of disk clusters, including what appears to be an upper
cluster cutoff mass. However, halo globulars are self-enriched and often
connected with dwarf galaxy streams. The mass function of halo globulars could
have initially been like the power law mass function of disk clusters, but the
halo globulars have lost their low mass members. The reasons for this loss are
not understood. It could have happened slowly over time as a result of cluster
evaporation, or it could have happened early after cluster formation as a
result of gas loss. The latter model explains best the observation that the
globular cluster mass function has no radial gradient in galaxies.Comment: to be published in IAUS266: Star Clusters Basic Galactic Building
Blocks Throughout Time And Space, eds. Richard de Grijs and Jacques Lepine,
Cambridge University Press, 11 page
Binary coalescence from case A evolution -- mergers and blue stragglers
We constructed some main-sequence mergers from case A binary evolution and
studied their characteristics via Eggleton's stellar evolution code. Both total
mass and orbital angular momentum are conservative in our binary evolutions.
Some mergers might be on the left of the ZAMS as defined by normal surface
composition on a CMD because of enhanced surface helium content. The study also
shows that central hydrogen content of the mergers is independent of mass. As a
consequence, we fit the formula of magnitude and B-V of the mergers when they
return back to thermal equilibrium with maximum error 0.29 and 0.037,
respectively. Employing the consequences above, we performed Monte Carlo
simulations to examine our models in NGC 2682 and NGC 2660. In NGC 2682, binary
mergers from our models cover the region with high luminosity, but its
importance is much less than that of AML. Our results are well-matched to the
observations of NGC2660 if there is about 0.5Mo of mass loss in the merger
process.Comment: 14 pages, 12 figures. accepted by MNRA
The intermediate-age open clusters Ruprecht 61, Czernik 32, NGC 2225 and NGC 2262
We present the first CCD photometry to of 4 fields centered on
the region of the southern Galactic star clusters Ruprecht~61, Czernik~32, NGC
2225 and NGC 2262 and of 4 displaced control fields. These clusters were never
studied before, and we provide for the first time estimates of their
fundamental parameters, namely radial extent, age, distance and reddening. We
find that the four clusters are all of intermediate age (around 1 Gyr), close
to the Sun and possess lower than solar metal abundance.Comment: 10 pages, 14 figures, in press in MNRA
Photometry of seven overlooked open clusters in the First and Fourth Galactic Quadrants
CCD BVI photometry is presented for 7 previously unstudied star clusters
projected toward the inner side of the Galaxy: Trumpler 23, Lynga 3, Collinder
307, Ruprecht 134, ESO552SC16, AL 5 and Kronberger 3. Color magnitude diagrams
of the cluster regions allow us to conclude that Lynga 3 and ESO552SC16, are
not clusters, but groups of bright stars probably located in the
Carina-Sagittarius spiral arm. AL 5 and Kronberger 3 are so embedded in a dense
stellar field that we cannot confirm their nature. Trumpler~ 3 and Ruprecht 134
are two intermediate-age open clusters located well inside the solar ring which
deserve further attention. Finally, Collinder 307 is an obscured younger
cluster (250 Myr) located in the Carina-Sagittarius spiral arm. Our results
emphasize the difficulty to search for open clusters in the inner regions of
the Galaxy due to the richness of the field and the patchy nature of the
interstellar absorption, but at the same time significantly contribute to a
better understanding of this complicated regions of the Milky Way.Comment: 11 pages, 16 degraded eps figures, in press in MNRA
Morphology of Galactic Open Clusters
We analyzed the shapes of Galactic open clusters by the star counting
technique with the 2MASS star catalog database. Morphological parameters such
as the ellipticity and size have been derived via stellar density distribution,
weighed by clustering probability. We find that most star clusters are
elongated, even for the youngest star clusters of a few million years old,
which are located near to the Galactic disk. The shapes of young star clusters
must reflect the conditions in the parental molecular clouds and during the
cluster formation process. As an open cluster ages, stellar dynamics cause the
inner part of the cluster to circularize, but the overall radius gets larger
and the stellar density becomes sparser. We discuss how internal relaxation
process competes with Galactic external perturbation during cluster evolution.Comment: 13 pages; 10 pages; accepted Astronomical Journa
Statistics of Stellar Populations of Star Clusters and Surrounding Fields in the Outer Disk of the Large Magellanic Cloud
A comparative analysis of Washington color-magnitude diagrams (CMDs) for 14
star clusters and respective surrounding fields in the Large Magellanic Cloud
(LMC) outer disk is presented. Each CCD frame including field and respective
cluster covers an area of 185 arcmin^2. The stellar population sampled is of
intermediate age and metallicity. CMD radial analysis involving star count
ratios, morphology and integrated light properties are carried out. Luminosity
functions (LFs) are also presented. Two main results are: (i) Within the range
4<R(kpc)<8, the distance from the LMC center is well correlated with the
average age in the sense that inner fields are younger and; (ii) Beyond
approximately 8kpc the outer fields do not show evidence of a significant
intermediate-age component in their stellar populations, as inferred from red
giant clump star counts.Comment: 27 pages, 4 tables, 11 figures; accepted by the A
Velocity Dispersion of Dissolving OB Associations Affected by External Pressure of Formation Environment
This paper presents a possible way to understand dissolution of OB
associations (or groups). Assuming rapid escape of parental cloud gas from
associations, we show that the shadow of the formation environment for
associations can be partially imprinted on the velocity dispersion at their
dissolution. This conclusion is not surprising as long as associations are
formed in a multiphase interstellar medium, because the external pressure
should suppress expansion caused by the internal motion of the parental clouds.
Our model predicts a few km s as the internal velocity dispersion.
Observationally, the internal velocity dispersion is km s which
is smaller than our prediction. This suggests that the dissipation of internal
energy happens before the formation of OB associations.Comment: 6 pages. AJ accepte
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