1,010 research outputs found
The stellar content of the Local Group dwarf galaxy Phoenix
We present new deep ground-based photometry of the Local Group dwarf
galaxy Phoenix. Our results confirm that this galaxy is mainly dominated by red
stars, with some blue plume stars indicating recent (100 Myr old) star
formation in the central part of the galaxy. We have performed an analysis of
the structural parameters of Phoenix based on an ESO/SRC scanned plate, in
order to search for differentiated component. The results were then used to
obtain the color-magnitude diagrams for three different regions of Phoenix in
order to study the variation of the properties of its stellar population. The
young population located in the central component of Phoenix shows a clear
asymmetry in its distribution, that could indicate a propagation of star
formation across the central component. The HI cloud found at 6 arcmin
Southwest by Young & Lo (1997) could have been involved in this process.
We also find the presence of a substantial intermediate-age population in the
central region of Phoenix that would be less abundant or absent in its outer
regions. This result is also consistent with the gradient found in the number
of horizontal branch stars, whose frequency relative to red giant branch stars
increases towards the outer part of the galaxy. These results, together with
those of our morphological study, suggest the existence of an old, metal-poor
population with a spheroidal distribution surrounding the younger inner
component of Phoenix. This two-component structure may resemble the halo-disk
structure observed in spirals, although more data, in particular on kinematics,
are necessary to confirm this.Comment: 46 pages, 21 figures, 9 Tables, to be published in AJ, August 9
Old Main-Sequence Turnoff Photometry in the Small Magellanic Cloud. I. Constraints on the Star Formation History in Different Fields
We present ground-based B and R-band color-magnitude diagrams (CMDs),
reaching the oldest main-sequence turnoffs with good photometric accuracy for
twelve fields in the Small Magellanic Cloud (SMC). Our fields, located between
~1 and ~4 degrees from the center of the galaxy, are situated in different
parts of the SMC such as the "Wing'' area, and towards the West and South. In
this paper we perform a first analysis of the stellar content in our SMC fields
through comparison with theoretical isochrones and color functions (CFs). We
find that the underlying spheroidally distributed population is composed of
both intermediate-age and old stars and that its age composition does not show
strong galacto-centric gradients. The three fields situated toward the east, in
the Wing region, show very active current star formation. However, only in the
eastern field closest to the center do we find an enhancement of recent star
formation with respect to a constant SFR(t). The fields corresponding to the
western side of the SMC present a much less populated young MS, and the CF
analysis indicates that the SFR(t) greatly diminished around 2 Gyr ago in these
parts. Field smc0057, the closest to the center of the galaxy and located in
the southern part, shows recent star formation, while the rest of the southern
fields present few bright MS stars. The structure of the red clump in all the
CMDs is consistent with the large amount of intermediate-age stars inferred
from the CMDs and color functions. None of the SMC fields presented here are
dominated by old stellar populations, a fact that is in agreement with the lack
of a conspicuous horizontal branch in all these SMC CMDs. This could indicate
that a disk population is ruling over a possible old halo in all the observed
fields.Comment: To appear in A
Hydrological processes and their seasonal controls in a small Mediterranean mountain catchment in the Pyrenees
The Vallcebre catchments are located in a middle mountain area of the Pyrenean ranges, built up by sedimentary rocks and loamy soils. The vegetation cover is pastures and forests of <i>Pinus sylvestris</i>, mostly occupying former agricultural terraces. Some relatively small, heavily eroded landscapes (badlands) occur in the catchments, playing a relevant hydrological and geomorphic role. Annual precipitation is 924 mm and potential (reference) evapotranspiration is about 700 mm. Rainfall interception in forests represents about 24% of precipitation; interception rates were similar throughout the seasons because of a compensation between rainfall intensities and atmospheric conditions. Soil moisture showed a temporal pattern characterised by the occurrence of marked deficit periods in summer and also, but less pronounced, in winter. During most of the year, subsurface flows on hillslopes drove the spatial organisation of soil moisture and the occurrence of saturated areas. Nevertheless, this spatial organisation was also controlled by the patterns of vegetation cover. During dry periods, subsurface flow ceased, saturated areas disappeared and the spatial patterns of soil moisture changed. Stream flow from these catchments was dominated by storm flow, and the runoff generating mechanisms showed a clear seasonal pattern, controlled mainly by the soil moisture and the extent of saturated areas. During the dry periods, runoff was produced only on impervious areas and badlands. At the end of the dry periods, some large rainfall events generated significant runoff because of the perched saturation of the shallow soil horizons. Thereafter, runoff generation was dominated by the role of saturated areas. Stream waters in catchments with badlands had very high suspended sediment concentrations. The seasonal pattern of erosion processes in badlands was characterised by physical weathering during winter, regolith breakdown and vigorous hillslope erosion during spring and summer, and efficient transport of sediments in autumn
Monitoring soil water content variability in the Cal Parisa basin (Alt Llobregat) with TDR. Experimental design and first results
A small catchment on abandoned fields in the Pyrenees was monitored with the TDR method to evaluate the temporal and spatial variations of soil water content in relation with the hydrological response. The first results obtained over one year show a series of three periods of increasing wetness and three periods of decreasing wetness. Frequently saturated zones show high moisture values and stability over several rnonths, mesophile grassland zones show a wide and variable moisture range, and forested zones show the lowest soil moisture content
Stellar Populations in the Outskirts of the Small Magellanic Cloud: No Outer Edge Yet
We report the detection of intermediate-age and old stars belonging to the
SMC at 6.5 kpc from the SMC center in the southern direction. We show, from the
analysis of three high quality 34\arcmin 33\arcmin CMDs, that the age
composition of the stellar population is similar at galactocentric distances of
4.7 kpc, 5.6 kpc, and 6.5 kpc. The surface
brightness profile of the SMC follows an exponential law, with no evidence of
truncation, all the way out to 6.5 kpc. These results, taken together, suggest
that the SMC `disk' population is dominating over a possible old Milky Way-like
stellar halo, and that the SMC may be significantly larger than previously
thought.Comment: Accepted for publication in ApJ Letters. High resolution figures are
available at ftp://ftp.iac.es/out/noe
Evolución de la respuesta hidrológica de un área arcillosa acarcavada, en relación con el estado físico superficial mediante experiencias de simulación de lluvia
[Resumen] Las geomorfologías acarcavadas desarrolladas en áreas de montaña, suelen mostrar condiciones físicas superficiales altamente dinámicas. Como consecuencia, la producción de escorrentía y de transporte sedimento muestran una marcada variabilidad estacional. Para adquirir un mayor conocimiento al respecto, fueron realizadas periódicamente simulaciones de lluvia, durante tres años y en dos litologías representativas del área de Vallcebre. Los resultados confirmaron claramente esta estacionalidad y su dependencia del estado de alteración física del regolito, más determinante que las condiciones hídricas previas al experimento.[Abstract] Badland geomorphologies developed on mountainous areas usually show a high dynamics of the surface. In consequence, runoff and sediment production rates show clear seasonal changes. In order to increase knowledge on this subject, periodical rainfall simulations were carryed out during three years in the Vallcebre badlands area. Results clearly confirm the seasonality and its relationship with antecedent regolith physical conditions, more determinant that previous hydric conditions
Properties of RR Lyrae stars in the inner regions of the Large Magellanic Cloud. II. The extended sample
All galaxies that have been adequately examined so far have shown an extended
stellar halo. To search for such a halo in the LMC we have obtained
low-resolution spectra for 100 LMC RR Lyrae stars, of which 87 are in the field
and 13 in the clusters NGC1835 and NGC2019. We measured radial velocities for
87 LMC RR Lyrae stars, and metallicities for 78 RR Lyrae stars, nearly tripling
the previous sample. These targets are located in 10 fields covering a wide
range of distances, out to 2.5 degrees from the center of the LMC. Our main
result is that the mean velocity dispersion for the LMC RR Lyrae stars is
50+-2km/s. This quantity does not appear to vary with distance from the LMC
center. The metallicity shows a Gaussian distribution, with mean
[Fe/H]=-1.53+-0.02dex, and dispersion 0.20 dex in the Harris metallicity scale,
confirming that they represent a very homogeneous metal-poor population. There
is no dependence between the kinematics and metallicity of the field RR Lyrae
star population. Using good quality low-resolution spectra from FORS1, FORS2
and GEMINI-GMOS we have found that field RR Lyrae stars in the LMC show a large
velocity dispersion and that this indicate the presence of old and metal-poor
stellar halo. All the evidence so far for the halo, however, is from the
spectroscopy of the inner LMC regions, similar to the inner flattened halo in
our Galaxy. Further study is necessary to confirm this important result.Comment: 12 pages, 4 figure
Revealing the tidal scars of the Small Magellanic Cloud
Due to their close proximity, the Large and Small Magellanic Clouds (SMC/LMC)
provide natural laboratories for understanding how galaxies form and evolve.
With the goal of determining the structure and dynamical state of the SMC, we
present new spectroscopic data for 3000 SMC red giant branch stars
observed using the AAOmega spectrograph at the Anglo-Australian Telescope. We
complement our data with further spectroscopic measurements from previous
studies that used the same instrumental configuration and proper motions from
the \textit{Gaia} Data Release 2 catalogue. Analysing the photometric and
stellar kinematic data, we find that the SMC centre of mass presents a
conspicuous offset from the velocity centre of its associated \mbox{H\,{\sc
i}} gas, suggesting that the SMC gas is likely to be far from dynamical
equilibrium. Furthermore, we find evidence that the SMC is currently undergoing
tidal disruption by the LMC within 2\,kpc of the centre of the SMC, and
possibly all the way in to the very core. This is evidenced by a net outward
motion of stars from the SMC centre along the direction towards the LMC and
apparent tangential anisotropy at all radii. The latter is expected if the SMC
is undergoing significiant tidal stripping, as we demonstrate using a suite of
-body simulations of the SMC/LMC system disrupting around the Milky Way.
These results suggest that dynamical models for the SMC that assume a steady
state will need to be revisited.Comment: Revised version submitted to MNRAS after referee report, 18 pages, 18
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