85 research outputs found
Probing the LMC age gap at intermediate cluster masses
The LMC has a rich star cluster system spanning a wide range of ages and
masses. One striking feature of the LMC cluster system is the existence of an
age gap between 3-10 Gyrs. But this feature is not as clearly seen among field
stars. Three LMC fields containing relatively poor and sparse clusters whose
integrated colours are consistent with those of intermediate age simple stellar
populations have been imaged in BVI with the Optical Imager (SOI) at the
Southern Telescope for Astrophysical Research (SOAR). A total of 6 clusters, 5
of them with estimated initial masses M < 10^4M_sun, were studied in these
fields. Photometry was performed and Colour-Magnitude Diagrams (CMD) were built
using standard point spread function fitting methods. The faintest stars
measured reach V ~ 23. The CMD was cleaned from field contamination by making
use of the three-dimensional colour and magnitude space available in order to
select stars in excess relative to the field. A statistical CMD comparison
method was developed for this purpose. The subtraction method has proven to be
successful, yielding cleaned CMDs consistent with a simple stellar population.
The intermediate age candidates were found to be the oldest in our sample, with
ages between 1-2 Gyrs. The remaining clusters found in the SOAR/SOI have ages
ranging from 100 to 200 Myrs. Our analysis has conclusively shown that none of
the relatively low-mass clusters studied by us belongs to the LMC age-gap.Comment: 9 pages, 8 figures. Accepted to MNRA
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
Deriving star formation histories from photometry using energy balance spectral energy distribution modelling
Panchromatic spectral energy distribution (SED) fitting is a critical tool for determining the physical properties of distant galaxies, such as their stellar mass and star formation rate. One widely used method is the publicly available MAGPHYS code. We build on our previous analysis (Hayward & Smith 2015) by presenting some modifications which enable MAGPHYS to automatically estimate galaxy star formation histories (SFHs), including uncertainties, based on ultra-violet to far-infrared photometry. We use state-of-the art synthetic photometry derived by performing three-dimensional dust radiative transfer on hydrodynamic simulations of isolated disc and merging galaxies to test how well the modified MAGPHYS is able to recover SFHs under idealised conditions, where the true SFH is known. We find that while the SFH of the model with the best fit to the synthetic photometry is a poor representation of the true SFH (showing large variations with the line-of-sight to the galaxy and spurious bursts of star formation), median-likelihood SFHs generated by marginalising over the default MAGPHYS libraries produce robust estimates of the smoothly-varying isolated disk simulation SFHs. This preference for the median-likelihood SFH is quantitatively underlined by our estimates of (analogous to the goodness-of-fit estimator) and (the integrated absolute mass discrepancy between the model and true SFH) that strongly prefer the median-likelihood SFHs over those that best fit the UV-to-far-IR photometry. In contrast, we are unable to derive a good estimate of the SFH for the merger simulations (either best-fit or median-likelihood) despite being able to obtain a reasonable fit to the simulated photometry, likely because the analytic SFHs with bursts superposed in the standard MAGPHYS library are insufficiently general/realistic.Peer reviewe
Calcium II Triplet Spectroscopy of LMC Red Giants. I. Abundances and Velocities for a Sample of Populous Clusters
Abridged Abstract -
Utilizing the FORS2 instrument on the VLT, we have obtained near infrared
spectra for more than 200 stars in 28 populous LMC clusters. This cluster
sample spans a large range of ages (~ 1-13 Gyr) and metallicities (-0.3 >
[Fe/H] > -2.0) and has good areal coverage of the LMC disk. The strong
absorption lines of the Calcium II triplet are used to derive cluster radial
velocities and abundances. We determine mean cluster velocities to typically
1.6 km/s and mean metallicities to 0.04 dex (random error). For eight of these
clusters, we report the first spectroscopically determined metallicities based
on individual cluster stars, and six of these eight have no published radial
velocity measurements.
(continued in paper)Comment: 26 pages of text plus 14 figures and 6 tables. Accepted for
publication in AJ. Scheduled for Vol. 132, No. 4 (October 2006
Reddening and metallicity maps of the Milky Way bulge from VVV and 2MASS II. The complete high resolution extinction map and implications for Bulge studies
We use the Vista Variables in the Via Lactea (VVV) ESO public survey data to
measure extinction values in the complete area of the Galactic bulge covered by
the survey at high resolution. We derive reddening values using the method
described in Paper I. This is based on measuring the mean (J-Ks) color of red
clump giants in small subfields of 2' to 6' in the following bulge area:
-10.3<b<+5.1 and -10<l<+10.4. To determine the reddening values E(J-Ks) for
each region, we measure the RC color and compare it to the (J-Ks) color of RC
stars measured in Baade's window, for which we adopt E(B-V)=0.55. This allows
us to construct a reddening map sensitive to small scale variations minimizing
the problems arising from differential extinction. The significant reddening
variations are clearly observed on spatial scales as small as 2'. We find a
good agreement between our extinction measurements and Schlegel maps in the
outer bulge, but, as already stated in the literature the Schlegel maps are not
reliable for regions within |b| < 6. In the inner regions we compare our
results with maps derived from DENIS and Spitzer surveys. While we find good
agreement with other studies in the corresponding overlapping regions, our
extinction map has better quality due to both higher resolution and a more
complete spatial coverage in the Bulge. We investigate the importance of
differential reddening and demonstrate the need for high resolution extinction
maps for detailed studies of Bulge stellar populations and structure. The
extinction variations on scales of up to 2'-6', must be taken into account when
analysing the stellar populations of the Bulge.Comment: Accepted for publication in A&
The puzzling dredge-up pattern in NGC 1978
Low-mass stars are element factories that efficiently release their products
in the final stages of their evolution by means of stellar winds. Since they
are large in number, they contribute significantly to the cosmic matter cycle.
To assess this contribution quantitatively, it is crucial to obtain a detailed
picture of the stellar interior, particularly with regard to nucleosynthesis
and mixing mechanisms. We seek to benchmark stellar evolutionary models of
low-mass stars. In particular, we measure the surface abundance of ^{12}C in
thermally pulsing AGB stars with well-known mass and metallicity, which can be
used to infer information about the onset and efficiency of the third
dredge-up. We recorded high-resolution near-infrared spectra of AGB stars in
the LMC cluster NGC 1978. The sample comprised both oxygen-rich and carbon-rich
stars, and is well-constrained in terms of the stellar mass, metallicity, and
age. We derived the C/O and ^{12}C/^{13}C ratio from the target spectra by a
comparison to synthetic spectra. Then, we compared the outcomes of stellar
evolutionary models with our measurements. The M stars in NGC 1978 show values
of C/O and ^{12}C/^{13}C that can best be explained with moderate extra-mixing
on the RGB coupled to a moderate oxygen enhancement in the chemical
composition. These oxygen-rich stars do not seem to have undergone third
dredge-up episodes (yet). The C stars show carbon-to-oxygen and carbon isotopic
ratios consistent with the occurrence of the third dredge-up. We did not find S
stars in this cluster. None of the theoretical schemes that we considered was
able to reproduce the observations appropriately. Instead, we discuss some
non-standard scenarios to explain the puzzling abundance pattern in NGC 1978.Comment: 16 pages, 9 figures, 4 tables, accepted for publication in A&A,
language revise
The Nature of Starbursts: I. The Star Formation Histories of Eighteen Nearby Starburst Dwarf Galaxies
We use archival HST observations of resolved stellar populations to derive
the star formation histories (SFHs) of 18 nearby starburst dwarf galaxies. In
this first paper we present the observations, color-magnitude diagrams, and the
SFHs of the 18 starburst galaxies, based on a homogeneous approach to the data
reduction, differential extinction, and treatment of photometric completeness.
We adopt a star formation rate (SFR) threshold normalized to the average SFR of
the individual system as a metric for classifying starbursts in SFHs derived
from resolved stellar populations. This choice facilitates finding not only
currently bursting galaxies but also "fossil" bursts increasing the sample size
of starburst galaxies in the nearby (D<8 Mpc) universe. Thirteen of the
eighteen galaxies are experiencing ongoing bursts and five galaxies show fossil
bursts. From our reconstructed SFHs, it is evident that the elevated SFRs of a
burst are sustained for hundreds of Myr with variations on small timescales. A
long >100 Myr temporal baseline is thus fundamental to any starburst definition
or identification method. The longer lived bursts rule out rapid
"self-quenching" of starbursts on global scales. The bursting galaxies' gas
consumption timescales are shorter than the Hubble time for all but one galaxy
confirming the short-lived nature of starbursts based on fuel limitations.
Additionally, we find the strength of the H{\alpha} emission usually correlates
with the CMD based SFR during the last 4-10 Myr. However, in four cases, the
H{\alpha} emission is significantly less than what is expected for models of
starbursts; the discrepancy is due to the SFR changing on timescales of a few
Myr. The inherently short timescale of the H{\alpha} emission limits
identifying galaxies as starbursts based on the current characteristics which
may or may not be representative of the recent SFH of a galaxy.Comment: 53 pages, 11 figure
Close binary stars in the solar-age Galactic open cluster M67
We present multi-colour time-series CCD photometry of the solar-age galactic
open cluster M67 (NGC 2682). About 3600 frames spread over 28 nights were
obtained with the 1.5 m Russian-Turkish and 1.2 m Mercator telescopes.
High-precision observations of the close binary stars AH Cnc, EV Cnc, ES Cnc,
the Scuti type systems EX Cnc and EW Cnc, and some long-period
variables belonging to M67 are presented. Three full multi-colour light curves
of the overcontact binary AH Cnc were obtained during three observing seasons.
Likewise we gathered three light curves of EV Cnc, an EB-type binary, and two
light curves of ES Cnc, a blue straggler binary. Parts of the light change of
long-term variables S1024, S1040, S1045, S1063, S1242, and S1264 are obtained.
Period variation analysis of AH Cnc, EV Cnc, and ES Cnc were done using all
times of mid-eclipse available in the literature and those obtained in this
study. In addition, we analyzed multi-colour light curves of the close binaries
and also determined new frequencies for the Scuti systems. The
physical parameters of the close binary stars were determined with simultaneous
solutions of multi-colour light and radial velocity curves. Finally we
determined the distance of M67 as 857(33) pc via binary star parameters, which
is consistent with an independent method from earlier studies.Comment: 12 pages, 9 Figures, 13 Table
CORS Baade-Wesselink distance to the LMC NGC 1866 blue populous cluster
We used Optical, Near Infrared photometry and radial velocity data for a
sample of 11 Cepheids belonging to the young LMC blue populous cluster NGC 1866
to estimate their radii and distances on the basis of the CORS Baade-Wesselink
method. This technique, based on an accurate calibration of the surface
brightness as a function of (U-B), (V-K) colors, allows us to estimate,
simultaneously, the linear radius and the angular diameter of Cepheid
variables, and consequently to derive their distance. A rigorous error estimate
on radius and distances was derived by using Monte Carlo simulations. Our
analysis gives a distance modulus for NGC 1866 of 18.51+/-0.03 mag, which is in
agreement with several independent results.Comment: 21 pages, 10 figures, Accepted for publication in the Astrophysical
Journa
Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy
We review HB stars in a broad astrophysical context, including both variable
and non-variable stars. A reassessment of the Oosterhoff dichotomy is
presented, which provides unprecedented detail regarding its origin and
systematics. We show that the Oosterhoff dichotomy and the distribution of
globular clusters (GCs) in the HB morphology-metallicity plane both exclude,
with high statistical significance, the possibility that the Galactic halo may
have formed from the accretion of dwarf galaxies resembling present-day Milky
Way satellites such as Fornax, Sagittarius, and the LMC. A rediscussion of the
second-parameter problem is presented. A technique is proposed to estimate the
HB types of extragalactic GCs on the basis of integrated far-UV photometry. The
relationship between the absolute V magnitude of the HB at the RR Lyrae level
and metallicity, as obtained on the basis of trigonometric parallax
measurements for the star RR Lyrae, is also revisited, giving a distance
modulus to the LMC of (m-M)_0 = 18.44+/-0.11. RR Lyrae period change rates are
studied. Finally, the conductive opacities used in evolutionary calculations of
low-mass stars are investigated. [ABRIDGED]Comment: 56 pages, 22 figures. Invited review, to appear in Astrophysics and
Space Scienc
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