378 research outputs found
Probing embedded star clusters in the HII complex NGC 6357 with VVV
NGC 6357 is an active star-forming region located in the Sagittarius arm
displaying several star clusters, which makes it a very interesting target to
investigate star formation and early cluster evolution. We explore NGC 6357
with the "VISTA Variables in the V\'ia a L\'actea" (VVV) photometry of seven
embedded clusters (ECs), and one open cluster (OC) projected in the outskirts
of the complex.Photometric and structural properties (age, reddening, distance,
core and total radii) of the star clusters are derived. VVV saturated stars are
replaced by their 2MASS counterparts. Field-decontaminated VVV photometry is
used to analyse Colour-Magnitude Diagrams (CMDs), stellar radial density
profiles (RDPs) and determine astrophysical parameters. We report the discovery
of four ECs and one intermediate-age cluster in the complex area. We derive a
revised distance estimate for NGC 6357 of 1.780.1 kpc based on the cluster
CMD morphologies. Among the ECs, one contains the binary star the WR 93, while
the remaining ones are dominated by pre-main sequence (PMS) stars,
young-stellar objects (YSO) and/or and have a developed main sequence. These
features reflect a significant age spread among the clusters. Evidence is found
that the relatively populous cluster Pismis 24 hosts two subclusters.Comment: This article will be published in the A&A. 11 pages, 15 figures and 3
table
Synthetic spectra of H Balmer and HeI absorption lines. II: Evolutionary synthesis models for starburst and post-starburst galaxies
We present evolutionary stellar population synthesis models to predict the
spectrum of a single-metallicity stellar population, with a spectral sampling
of 0.3 A in five spectral regions between 3700 and 5000 A. The models, which
are optimized for galaxies with active star formation, synthesize the profiles
of the hydrogen Balmer series (Hb, Hg, Hd, H8, H9, H10, H11, H12 and H13) and
the neutral helium absorption lines (HeI 4922, HeI 4471, HeI 4388, HeI 4144,
HeI 4121, HeI 4026, HeI 4009 and HeI 3819) for a burst with an age ranging from
1 to 1000 Myr, and different assumptions about the stellar initial mass
function. Continuous star formation models lasting for 1 Gyr are also
presented. The input stellar library includes NLTE absorption profiles for
stars hotter than 25000 K and LTE profiles for lower temperatures. The
temperature and gravity coverage is 4000 K <Teff< 50000 K and 0.0< log g$< 5.0,
respectively.
The models can be used to date starburst and post-starburst galaxies until 1
Gyr. They have been tested on data for clusters in the LMC, the super-star
cluster B in the starburst galaxy NGC 1569, the nucleus of the dwarf elliptical
NGC 205 and a luminous "E+A" galaxy. The full data set is available for
retrieval at http://www.iaa.es/ae/e2.html and at
http://www.stsci.edu/science/starburst/, or on request from the authors at
[email protected]: To be published in ApJS. 48 pages and 20 figure
Aggregation of biological particles under radial directional guidance
Many biological environments display an almost radially-symmetric structure, allowing proteins, cells or animals to move in an oriented fashion. Motivated by specific examples of cell movement in tissues, pigment protein movement in pigment cells and animal movement near watering holes, we consider a class of radially-symmetric anisotropic diffusion problems, which we call the star problem. The corresponding diffusion tensor D(x) is radially symmetric with isotropic diffusion at the origin. We show that the anisotropic geometry of the environment can lead to strong aggregations and blow-up at the origin. We classify the nature of aggregation and blow-up solutions and provide corresponding numerical simulations. A surprising element of this strong aggregation mechanism is that it is entirely based on geometry and does not derive from chemotaxis, adhesion or other well known aggregating mechanisms. We use these aggregate solutions to discuss the process of pigmentation changes in animals, cancer invasion in an oriented fibrous habitat (such as collagen fibres), and sheep distributions around watering holes
The Dusty Starburst Nucleus of M33
We have thoroughly characterized the ultraviolet to near-infrared (0.15 - 2.2
micron) spectral energy distribution (SED) of the central parsec of the M33
nucleus through new infrared photometry and optical/near-infrared spectroscopy,
combined with ultraviolet/optical observations from the literature and the HST
archive. The SED shows evidence for a significant level of attenuation, which
we model through a Monte Carlo radiative transfer code as a shell of clumpy
Milky Way-type dust (tau_V ~ 2 +/- 1). The discovery of Milky Way-type dust
(with a strong 2175 A bump) internal to the M33 nucleus is different from
previous work which has found SMC-like dust (no bump) near starburst regions.
The amount by which dust can be processed may be related to the mass and age of
the starburst as well as the extent to which the dust can shield itself. Our
starburst models include the effects of this dust and can fit the SED if the
nucleus was the site of a moderate (~10^8 L_sun at 10 Myrs) episode of coeval
star formation about 70 Myrs ago. This result is quite different from previous
studies which resorted to multiple stellar populations (between 2 and 7)
attenuated by either no or very little internal dust. The M33 nuclear starburst
is remarkably similar to an older version (70 Myr versus 10 Myr) of the
ultra-compact starburst in the center of the Milky Way.Comment: 29 pages, 9 embedded figures, ApJ, in pres
The Central Regions of M31 in the 3 - 5 micron Wavelength Region
Images obtained with NIRI on the Gemini North telescope are used to
investigate the photometric properties of the central regions of M31 in the 3 -
5 micron wavelength range. The light distribution in the central arcsecond
differs from what is seen in the near-infrared in the sense that the difference
in peak brigh tness between P1 and P2 is larger in M' than in K'; no obvious
signature of P3 is dete cted in M'. These results can be explained if there is
a source of emission that contributes ~ 20% of the peak M' light of P1 and has
an effective temperature of no more than a few hundred K that is located
between P1 and P2. Based on the red K-M' color of this source, it is suggested
that the emission originates in a circumstellar dust shell surrounding a single
bright AGB star. A similar bright source that is ~ 8 arcsec from the center of
the galaxy is also detected in M'. Finally, the (L', K-L') color-magnitude
diagram of unblended stars shows a domin ant AGB population with photometric
characteristics that are similar to those of the most luminous M giants in the
Galactic bulge.Comment: To appear in the Astronomical Journa
A major star formation region in the receding tip of the stellar Galactic bar
We present an analysis of the optical spectroscopy of 58 stars in the
Galactic plane at \arcdeg, where a prominent excess in the flux
distribution and star counts have been observed in several spectral regions, in
particular in the Two Micron Galactic Survey (TMGS) catalog. The sources were
selected from the TMGS, to have a magnitude brighter than +5 mag and be
within 2 degrees of the Galactic plane. More than 60% of the spectra correspond
to stars of luminosity class I, and a significant proportion of the remainder
are very late giants which would also be fast evolving. This very high
concentration of young sources points to the existence of a major star
formation region in the Galactic plane, located just inside the assumed origin
of the Scutum spiral arm. Such regions can form due to the concentrations of
shocked gas where a galactic bar meets a spiral arm, as is observed at the ends
of the bars of face-on external galaxies. Thus, the presence of a massive star
formation region is very strong supporting evidence for the presence of a bar
in our Galaxy.Comment: 13 pages (latex) + 4 figures (eps), accepted in ApJ Let
Keck Absorption-Line Spectroscopy of Galactic Winds in Ultraluminous Infrared Galaxies
In this paper, we present moderately-high resolution (~65 km/s) spectroscopy,
acquired with ESI on Keck II, of 11 ultraluminous infrared galaxies at z < 0.3
from the IRAS 1 Jy sample. The targets were chosen as good candidates to host
galaxy-scale outflows, and most have infrared luminosities dominated by star
formation. We use a chi-squared minimization to fit one- to three-component
profiles to the NaI D interstellar absorption doublet in each object. Assuming
that gas blueshifted by more than 70 km/s relative to the systemic velocity of
the host is outflowing, we detect outflows in 73% of these objects. We adopt a
simple model of a mass-conserving free wind to infer mass outflow rates in the
range (dM/dt)_tot(H) = 13-133 M_sun/yr for galaxies hosting a wind. These
values of (dM/dt)_tot, normalized to the corresponding global star formation
rates inferred from infrared luminosities, are in the range eta = (dM/dt)_tot /
SFR = 0.1-0.7. This is on average a factor of only 10 less than eta from recent
measurements of nearby dwarfs, edge-on spirals, and lower-luminosity infrared
galaxies. Within our sample, we conclude that eta has no dependence on the mass
of the host (parameterized by host galaxy kinematics and absolute R- and
K'-band magnitudes). We also attempt to estimate the average escape fraction
= Sum(dM/dt_esc^i) / Sum(dM/dt_tot^i) and ``ejection efficiency''
= Sum(dM/dt_esc^i) / Sum(SFR^i) for our sample, which we find to be
\~0.4-0.5 and ~0.1, respectively. The complex absorption-line properties of Mrk
231, an ultraluminous infrared galaxy which is optically classified as a
Seyfert 1, are discussed separately in an appendix.Comment: 34 pages, 12 .ps figures, 10 tables; accepted for publication in ApJ,
10 May 2002, v570 n
Ages and Luminosities of Young SMC/LMC Star Clusters and the recent Star Formation History of the Clouds
In this paper we discuss the age and spatial distribution of young
(age1Gyr) SMC and LMC clusters using data from the Magellanic Cloud
Photometric Surveys. Luminosities are calculated for all age-dated clusters.
Ages of 324 and 1193 populous star clusters in the Small and the Large
Magellanic Cloud have been determined fitting Padova and Geneva isochrone
models to their resolved color-magnitude diagrams. The clusters cover an age
range between 10Myr and 1Gyr in each galaxy. For the SMC a constant distance
modulus of = 18.90 and a metallicity of Z = 0.004 were adopted. For
the LMC, we used a constant distance modulus of = 18.50 and a
metallicity of Z = 0.008. For both galaxies, we used a variable color excess to
derive the cluster ages. We find two periods of enhanced cluster formation in
both galaxies at 160Myr and 630Myr (SMC) and at 125Myr and 800Myr (LMC). We
present the spatially resolved recent star formation history of both Clouds
based on young star clusters. The first peak may have been triggered by a close
encounter between the SMC and the LMC. In both galaxies the youngest clusters
reside in the supergiant shells, giant shells, the inter-shell regions, and
toward regions with a high H content, suggesting that their formation
is related to expansion and shell-shell interaction. Most of the clusters are
older than the dynamical age of the supergiant shells. No evidence for cluster
dissolution was found. Computed V band luminosities show a trend for fainter
magnitudes with increasing age as well as a trend for brighter magnitudes with
increasing apparent cluster radii.Comment: 17 pages, 14 figures, Accepted for publication in A&
The Star Clusters in the Irregular Galaxy NGC 4449
We examine the star clusters in the irregular galaxy NGC 4449. We use a
near-infrared spectrum and broad-band images taken with the HST to place a
limit of 8--15 Myrs on the age of the bright central ojbect in NGC 4449. Its
luminosity and size suggest that it is comparable to young super star clusters.
However, there is a peculiar nucleated-bar structure at the center of this star
cluster, and we suggest that this structure is debris from the interaction that
has produced the counter-rotating gas systems and extended gas streamers in the
galaxy.
From the images we identify 60 other candidate compact star clusters in NGC
4449. Fourteen of these could be background elliptical galaxies or old globular
star clusters. Of the star clusters, three, in addition to the central object,
are potentially super star clusters, and many others are comparable to the
populous clusters found in the LMC. The star clusters span a large range in
ages with no obvious peak in cluster formation that might be attributed to the
interaction that the galaxy has experienced.Comment: To be published in PASP, Feb. 2001; also attainable from
ftp.lowell.edu, cd pub/dah/n4449pape
Integrated parameters of star clusters: A comparison of theory and observations
(Abridged) This paper presents integrated magnitude and colours for synthetic
clusters. The integrated parameters have been obtained for the whole cluster
population as well as for the main-sequence (MS) population of star clusters.
We have also estimated observed integrated magnitudes and colours of MS
population of galactic open clusters, LMC and SMC star clusters. It is found
that the colour evolution of MS population of star clusters is not affected by
the stochastic fluctuations, however these fluctuations significantly affect
the colour evolution of the whole cluster population. The fluctuations are
maximum in colour in the age range 6.7 log (age) 7.5. Evolution
of integrated colours of MS population of the clusters in the Milky Way, LMC
and SMC, obtained in the present study are well explained by the present
synthetic cluster model. The observed integrated colours of MS
population of LMC star clusters having age 500 Myr seem to be
distributed around 0.004 model, whereas colours are found to be
more bluer than those predicted by the 0.004 model. vs
two-colour diagram for the MS population of the Milky Way star clusters shows a
fair agreement between the observations and present model, however the diagrams
for LMC and SMC clusters indicate that observed colours are relatively
bluer. Possible reasons for this anomaly have been discussed.Comment: 35 pages, 20 figs, accepted for publication in MNRA
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