144 research outputs found
How Universal are the Young Cluster Sequences? - the Cases of LMC, SMC, M83 and the Antennae
Aims.Recently a new analysis of cluster observations in the Milky Way found
evidence that clustered star formation may work under tight constraints with
respect to cluster size and density, implying the presence of just two
sequences of young massive cluster. These two types of clusters each expand at
different rates with cluster age. Methods. Here we investigate whether similar
sequences exist in other nearby galaxies. Results:We find that while for the
extragalactic young stellar clusters the overall trend in the cluster-density
scaling is quite comparable to the relation obtained for Galactic clusters,
there are also possible difference. For the LMC and SMC clusters the densities
are below the Galactic data points and/or the core radii are smaller than those
of data points with comparable density. For M83 and the Antenna clusters the
core radii are possibly comparable to the Galactic clusters but it is not clear
whether they exhibit similar expansion speeds. These findings should serve as
an incentive to perform more systematic observations and analysis to answer the
question of a possible similarity between young galactic and extragalactic star
clusters sequences.Comment: 6 pages, 4 figures, A&A in pres
Spectral Variations of Of?p Oblique Magnetic Rotator Candidates in the Magellanic Clouds
Optical spectroscopic monitoring has been conducted of two O stars in the
Small and one in the Large Magellanic Cloud, the spectral characteristics of
which place them in the Of?p category, which has been established in the Galaxy
to consist of oblique magnetic rotators. All of these Magellanic stars show
systematic spectral variations typical of the Of?p class, further strengthening
their magnetic candidacy to the point of virtual certainty. The spectral
variations are related to photometric variations derived from OGLE data by Naze
et al. (2015) in a parallel study, which yields rotational periods for two of
them. Now circular spectropolarimetry is required to measure their fields, and
ultraviolet spectroscopy to further characterize their low-metallicity,
magnetically confined winds, in support of hydrodynamical analyses.Comment: 18 pages, 6 figures, accepted for publication by A
Metal enrichment in near-IR luminous galaxies at z~2: signatures of proto-ellipticals?
We present the analysis of the coadded rest-frame UV spectrum (1200<z<2000 A)
of five K-luminous galaxies at z~2 from the K20 survey. The composite spectrum
is characterized by strong absorption lines over the UV continuum from C, N, O,
Al, Si, and Fe in various ionization stages. While some of these lines are
interstellar, several among the strongest absorptions are identified with
stellar photospheric lines. Most of the photospheric and interstellar features
are stronger in the K-luminous composite spectrum than in LBGs at z~3. This
suggests higher metallicity and possibly also larger interstellar velocity
dispersion caused by macroscopic motions. The absorption lines and the slope of
the UV continuum is well matched by the spectrum of the nearby luminous
infrared galaxy NGC 6090, which is in the process of merging. A metallicity
higher than solar is suggested by comparing the pure photospheric lines (SiIII,
CIII, FeV) with starburst models. The evidence of high metallicity, together
with the high masses, high star-formation rates, and possibly strong
clustering, well qualify these galaxies as progenitors of local massive
ellipticals.Comment: 4 pages, 3 figures, Accepted ApJ Letter
FIELD TRIP TO THE YPRESIAN/LUTETIAN BOUNDARY AT THE GORRONDATXE BEACH SECTION (BASQUE COUNTRY, W PYRENEES)
One of the Paleogene Stage boundaries still needing official definition is the Ypresian/Lutetain (Early-
Middle Eocene) boundary. With the aim of contributing to attain this definition, a high-resolution multidisciplinary
study, including physical stratigraphy (lithostratigraphy, sequence stratigraphy and
magnetostratigraphy) and biostratigraphy (calcareous nannofossil, planktic foraminifer and larger
foraminifer), has been carried out over the 700 m thick uppermost Ypresian – lower Lutetian Gorrondatxe
section. The results show that the different events traditionally used to place the Ypresian/Lutetian
boundary, hitherto thought to be simultaneous (i.e., the planktic foraminifer P9 (=E7) / P10 (=E8) Zone
boundary; the calcareous nannofossil CP12a / CP12b Subzone boundary; the larger foraminifer SBZ12 /
SBZ13 Zone boundary; and the boundary between magnetic polarity chrons C22n and C21r), actually
occur at very different levels. Therefore, before considering any section to place the Ypresian/Lutetian
boundary stratotype, the criterion to precisely define this boundary should be selected. To this end, the
succession of events pinpointed in the Ypresian/Lutetian boundary interval of the Gorrondatxe beach
section might prove a useful database.
The Gorrondatxe section fulfils most of the requirements demanded of a prospective stratotype section. In
addition, the great sedimentary thickness, which implies a very high deep-marine sedimentation rate,
provides the Gorrondatxe section an additional value, as it offers the opportunity to chronologically order
successive biomagnetostratigraphic events more precisely than elsewhere. Therefore, we consider that,
once the criterion to define the Ypresian/Lutetian boundary is selected, the Gorrondatxe beach section
should be deemed a firm candidate to place the Global Stratotype Section and Point of the base of the
Lutetian Stage
Cyclostratigraphy of the Early/Middle Eocene transition: a Pyrenean perspective
An integrated bio-, magneto- and cyclostratigraphic study of the Ypresian/Lutetian (Early/Middle Eocene)
transition along the Pyrenean Otsakar section (Payros et al., 2011) resulted in the identification of the
C22n/C21r chron boundary and of the calcareous nannofossil CP12a/b zonal boundary; the latter is the
main correlation criterion of the Lutetian Global Stratotype Section and Point (GSSP) recently defined
at Gorrondatxe (Basque Country). By counting precession-related mudstone-marl couplets of 21 ka,
the time lapse between both events was calculated to be of 819 ka. This suggests that the age of the
CP12a/b boundary, and hence that of the Early/Middle Eocene boundary, is 47.76 Ma, 250 ka younger
than previously thought. This age agrees with, and is supported by, estimates from Gorrondatxe based
on the time lapse between the Lutetian GSSP and the C21r/C21n boundary. The duration of Chron
C21r is estimated at 1.326 Ma. Given that the base of the Eocene is dated at 55.8 Ma, the duration of
the Early Eocene is of 8 Ma, 0.8 Ma longer than in current time scales. The Otsakar results further show
that the bases of planktic foraminiferal zones E8 and P10 are younger than the CP12a/b boundary. The
first occurrence of Turborotalia frontosa, being approximately 550 ka older that the CP12a/b boundary,
is the planktic foraminiferal event that lies closest to the Early/Middle Eocene boundary. The larger
foraminiferal SBZ12/13 boundary is located close to the CP12a/b boundary and correlates with Chron
C21r, not with the C22n/C21r boundary
The Interaction between the ISM and Star Formation in the Dwarf Starburst Galaxy NGC 4214
We present the first interferometric study of the molecular gas in the
metal-poor dwarf starburst galaxy NGC 4214. Our map of the 12CO(1-0) emission,
obtained at the OVRO millimeter array, reveals an unexpected structural wealth.
We detected three regions of molecular emission in the north-west (NW),
south-east (SE) and centre of NGC 4214 which are in very different and distinct
evolutionary stages (total molecular mass: 5.1 x 10^6 M_sun). These differences
are apparent most dramatically when the CO morphologies are compared to optical
ground based and HST imaging: massive star formation has not started yet in the
NW region; the well-known starburst in the centre is the most evolved and star
formation in the SE complex started more recently. We derive a star formation
efficiency of 8% for the SE complex. Using high--resolution VLA observations of
neutral hydrogen HI and our CO data we generated a total gas column density map
for NGC 4214 (HI + H_2). No clear correlation is seen between the peaks of HI,
CO and the sites of ongoing star formation. This emphasizes the irregular
nature of dwarf galaxies. The HI and CO velocities agree well, so do the
H-alpha velocities. In total, we cataloged 14 molecular clumps in NGC 4214. Our
results from a virial mass analysis are compatible with a Galactic CO-to-H_2
conversion factor for NGC 4214 (lower than what is usually found in metal-poor
dwarf galaxies).Comment: accepted for publication in the AJ (February 2001), full ps file at:
ftp://ftp.astro.caltech.edu/users/fw/ngc4214/walter_prep.p
A Galactic O-Star Catalog
We have produced a catalog of 378 Galactic O stars with accurate spectral
classifications which is complete for V<8 but includes many fainter stars. The
catalog provides cross-identifications with other sources; coordinates
(obtained in most cases from Tycho-2 data); astrometric distances for 24 of the
nearest stars; optical (Tycho-2, Johnson, and Stromgren) and NIR photometry;
group membership, runaway character, and multiplicity information; and a
web-based version with links to online services.Comment: 76 pages, 13 tables, and 3 figures. Accepted for publication in
Astrophysical Journal. Online version of the catalog available at
http://www.stsci.edu/~jmaiz/GOSmain.htm
The R136 star cluster dissected with Hubble Space Telescope/STIS. I. Far-ultraviolet spectroscopic census and the origin of HeII 1640 in young star clusters
We introduce a HST/STIS stellar census of R136a, the central ionizing star
cluster of 30 Doradus. We present low resolution far-ultraviolet STIS/MAMA
spectroscopy of R136 using 17 contiguous 52x0.2 arcsec slits which together
provide complete coverage of the central 0.85 parsec (3.4 arcsec). We provide
spectral types of 90% of the 57 sources brighter than m_F555W = 16.0 mag within
a radius of 0.5 parsec of R136a1, plus 8 additional nearby sources including
R136b (O4\,If/WN8). We measure wind velocities for 52 early-type stars from CIV
1548-51, including 16 O2-3 stars. For the first time we spectroscopically
classify all Weigelt & Baier members of R136a, which comprise three WN5 stars
(a1-a3), two O supergiants (a5-a6) and three early O dwarfs (a4, a7, a8). A
complete Hertzsprung-Russell diagram for the most massive O stars in R136 is
provided, from which we obtain a cluster age of 1.5+0.3_-0.7 Myr. In addition,
we discuss the integrated ultraviolet spectrum of R136, and highlight the
central role played by the most luminous stars in producing the prominent HeII
1640 emission line. This emission is totally dominated by very massive stars
with initial masses above ~100 Msun. The presence of strong HeII 1640 emission
in the integrated light of very young star clusters (e.g A1 in NGC 3125)
favours an initial mass function extending well beyond a conventional upper
limit of 100 Msun. We include montages of ultraviolet spectroscopy for LMC O
stars in the Appendix. Future studies in this series will focus on optical
STIS/CCD medium resolution observations.Comment: 20 pages plus four Appendices providing LMC UV O spectral templates,
UV spectral atlas in R136, wind velocities of LMC O stars and photometry of
additional R136 source
Spectrophotometric Observations of Blue Compact Dwarf Galaxies: Mrk 370
We present results from a detailed spectrophotometric analysis of the blue
compact dwarf galaxy (BCD) Mrk 370, based on deep UBVRI broad-band and Halpha
narrow-band observations, and long-slit and two-dimensional spectroscopy of its
brightest knots. The spectroscopic data are used to derive the internal
extinction, and to compute metallicities, electronic density and temperature in
the knots. By subtracting the contribution of the underlying older stellar
population, modeled by an exponential function, removing the contribution from
emission lines, and correcting for extinction, we can measure the true colors
of the young star-forming knots. We show that the colors obtained this way
differ significantly from those derived without the above corrections, and lead
to different estimates of the ages and star-forming history of the knots. Using
predictions of evolutionary synthesis models, we estimate the ages of both the
starburst regions and the underlying stellar component. We found that we can
reproduce the colors of all the knots with an instantaneous burst of star
formation and the Salpeter initial mass function with an upper mass limit of
100 solar masses. The resulting ages range between 3 and 6 Myrs. The colors of
the low surface brightness component are consistent with ages larger than 5
Gyr. The kinematic results suggest ordered motion around the major axis of the
galaxy.Comment: 26 pages with 14 figures; accepted for publication in Ap
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