3,235 research outputs found
Spatially resolved LMC star formation history: I. Outside in evolution of the outer LMC disk
We study the evolution of three fields in the outer LMC disk Rgc=3.5-6.2 Kpc.
Their star formation history indicates a stellar populations gradient such that
younger stellar populations are more centrally concentrated. We identify two
main star forming epochs, separated by a period of lower activity between ~7
and ~4 Gyr ago. Their relative importance varies from a similar amount of stars
formed in the two epochs in the innermost field, to only 40% of the stars
formed in the more recent epoch in the outermost field. The young star forming
epoch continues to the present time in the innermost field, but lasted only
till ~0.8 and 1.3 Gyr ago at Rgc=5.5 degrees and 7.1 degrees, respectively.
This gradient is correlated with the measured HI column density and implies an
outside-in quenching of the star formation, possibly related to a variation of
the size of the HI disk. This could either result from gas depletion due to
star formation or ram-pressure stripping, or from to the compression of the gas
disk as ram-pressure from the Milky Way halo acted on the LMC interstellar
medium. The latter two situations may have occurred when the LMC first
approached the Milky Way.Comment: 15 pages, 13 figures, 4 tables. MNRAS, in pres
The Chemical Enrichment History of the Large Magellanic Cloud
Ca II triplet spectroscopy has been used to derive stellar metallicities for
individual stars in four LMC fields situated at galactocentric distances of
3\arcdeg, 5\arcdeg, 6\arcdeg\@ and 8\arcdeg\@ to the north of the Bar. Observed
metallicity distributions show a well defined peak, with a tail toward low
metallicities. The mean metallicity remains constant until 6\arcdeg\@
([Fe/H]-0.5 dex), while for the outermost field, at 8\arcdeg, the mean
metallicity is substantially lower than in the rest of the disk
([Fe/H]-0.8 dex). The combination of spectroscopy with deep CCD
photometry has allowed us to break the RGB age--metallicity degeneracy and
compute the ages for the objects observed spectroscopically. The obtained
age--metallicity relationships for our four fields are statistically
indistinguishable. We conclude that the lower mean metallicity in the outermost
field is a consequence of it having a lower fraction of intermediate-age stars,
which are more metal-rich than the older stars. The disk age--metallicity
relationship is similar to that for clusters. However, the lack of objects with
ages between 3 and 10 Gyr is not observed in the field population. Finally, we
used data from the literature to derive consistently the age--metallicity
relationship of the bar. Simple chemical evolution models have been used to
reproduce the observed age--metallicity relationships with the purpose of
investigating which mechanism has participated in the evolution of the disk and
bar. We find that while the disk age--metallicity relationship is well
reproduced by close-box models or models with a small degree of outflow, that
of the bar is only reproduced by models with combination of infall and outflow.Comment: 45 pages, 10 figures, accepted for publication in Astronomical
Journa
Recent Star Formation in Sextans A
We investigate the relationship between the spatial distributions of stellar
populations and of neutral and ionized gas in the Local Group dwarf irregular
galaxy Sextans A. This galaxy is currently experiencing a burst of localized
star formation, the trigger of which is unknown. We have resolved various
populations of stars via deep UBV(RI)_C imaging over an area with diameter \sim
5.'3. We have compared our photometry with theoretical isochrones appropriate
for Sextans A, in order to determine the ages of these populations. We have
mapped out the history of star formation, most accurately for times \lesssim
100 Myr. We find that star formation in Sextans A is correlated both in time
and space, especially for the most recent (\lesssim 12 Myr) times. The youngest
stars in the galaxy are forming primarily along the inner edge of the large H I
shell. Somewhat older populations, \lesssim 50 Myr, are found inward of the
youngest stars. Progressively older star formation, from \sim 50--100 Myr,
appears to have some spatially coherent structure and is more centrally
concentrated. The oldest stars we can accurately sample appear to have
approximately a uniform spatial distribution, which extends beyond a surface
brightness of \mu_B \simeq 25.9 mag arcsec^{-2} (or, a radius r \simeq 2.'3$).
Although other processes are also possible, our data provides support for a
mechanism of supernova-driven expansion of the neutral gas, resulting in cold
gas pileup and compression along the H I shell and sequential star formation in
recent times.Comment: 64 pages, 22 figures, to appear in A
The Stellar Structures around Disk Galaxies
We present a brief summary of our current results on the stellar distribution
and population gradients of the resolved stars in the surroundings of ~50
nearby disk galaxies, observed with space- (Hubble & Spitzer) and ground-based
telescopes (Subaru, VLT, BTA, Palomar, CFHT & INT). We examine the radial
(in-plane) and vertical (extraplanar) distributions of resolved stars as a
function of stellar age and metallicity by tracking changes in the
color-magnitude diagram of face-on and edge-on galaxies. Our data show, that
the scale length and height of a stellar population increases with age, with
the oldest detected stellar populations identified at a large galactocentric
radius or extraplanar height, out to typically a few kpc. In the most massive
of the studied galaxies there is evidence for a break in number density and
color gradients of evolved stars, which plausibly correspond to the thick disk
and halo components of the galaxies. The ratio of intermediate-age to old stars
in the outermost fields correlate with the gas fraction, while relative sizes
of the thick-to-thin disks anticorrelate with galactic circular velocity.Comment: To appear in the proceedings for the IAUS 241 'Stellar Populations as
Building Blocks of Galaxies' held in La Palma, Spain, December 10-16 200
Phase Diagram of Multilayer Magnetic Structures
Multilayer "ferromagnet-layered antiferromagnet" (Fe/Cr) structures
frustrated due to the roughness of layer interfaces are studied by numerical
modeling methods. The "thickness-roughness" phase diagrams for the case of thin
ferromagnetic film on the surface of bulk antiferromagnet and for two
ferromagnetic layers separated by an antiferromagnetic interlayer are obtained
and the order parameter distributions for all phases are found. The phase
transitions nature in such systems is considered. The range of applicability
for the "magnetic proximity model" proposed by Slonczewski is evaluated.Comment: 8 pages, 8 figure
The Carina Project VII: Towards the breaking of the age-metallicity degeneracy of red giant branch stars using the c_UBI index
We present an analysis of photometric and spectroscopic data of the Carina
dSph galaxy, testing a new approach similar to that used to disentangle
multiple populations in Galactic globular clusters (GCs). We show that a proper
colour combination is able to separate a significant fraction of the red giant
branch (RGB) of the two main Carina populations (the old one, \sim 12 Gyr, and
the intermediate-age one, 4-8 Gyr). In particular, the c_UBI=(U-B)-(B-I)
pseudo-colour allows us to follow the RGB of both populations along a relevant
portion of the RGB. We find that the oldest stars have more negative c_UBI
pseudo-colour than intermediate-age ones. We correlate the pseudo-colour of RGB
stars with their chemical properties, finding a significant trend between the
iron content and the c_UBI. Stars belonging to the old population are
systematically more metal poor ([Fe/H]=-2.32\pm0.08 dex) than the
intermediate-age ones ([Fe/H]=-1.82\pm0.03 dex). This gives solid evidence on
the chemical evolution history of this galaxy, and we have a new diagnostic
that can allow us to break the age-metallicity degeneracy of H-burning advanced
evolutionary phases. We compared the distribution of stars in the c_UBI plane
with theoretical isochrones, finding that no satisfactory agreement can be
reached with models developed in a theoretical framework based on standard
heavy element distributions. Finally, we discuss possible systematic
differences when compared with multiple populations in GCs.Comment: 11 pages, 11 figures, 1 table, accepted for publication in Ap
A technique using a membrane flow cell to determine average mass transfer coefficients and tortuosity factors in biofilms
Average mass transfer coefficients of an inert compound (LiCl) within denitrifying biofilms were
monitored during biofilm growth in a membrane flow cell under different flow conditions, until the biofilm
reached (pseudo-) steady state. Average effective diffusivities were found to increase with the decrease in
tortuosity factors of the biofilm matrix. The lowest tortuosity factor corresponded to the biofilm formed under
the highest liquid velocity.União Europeia - BioToBio - ERBFMRXCT97-0114.
Centro de Investigación Científica y Tecnológica (CICYT) - AMB 98-0288, PR404E
2000/6-
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