180 research outputs found
On the white dwarf cooling sequence with extremely large telescopes
We present new diagnostics of white dwarf (WD) cooling sequences and
luminosity functions (LFs) in the near-infrared (NIR) bands that will exploit
the sensitivity and resolution of future extremely large telescopes. The
collision-induced absorption (CIA) of molecular hydrogen causes a clearly
defined blue turn-off along the WD (WDBTO) cooling sequences and a bright
secondary maximum in the WD LFs. These features are independent of age over a
broad age range and are minimally affected by metal abundance. This means that
the NIR magnitudes of the WDBTO are very promising distance indicators. The
interplay between the cooling time of progressively more massive WDs and the
onset of CIA causes a red turn-off along the WD (WDRTO) cooling sequences and a
well defined faint peak in the WD LFs. These features are very sensitive to the
cluster age, and indeed the K-band magnitude of the faint peak increases by
0.2--0.25 mag/Gyr for ages between 10 and 14 Gyr. On the other hand, the faint
peak in the optical WD LF increases in the same age range by 0.17 (V band) and
0.15 (I band) mag/Gyr. Moreover, we also suggest to use the difference in
magnitude between the main sequence turn-off and either the WDBTO (optical) or
the WDRTO (NIR). This age diagnostic is also independent of distance and
reddening. Once again the sensitivity in the K band (0.15-0.20 mag/Gyr) is on
average a factor of two higher than in the optical bands (0.10 [V band], 0.07
[I band] mag/Gyr). Finally, we also outline the use of the new diagnostics to
constrain the CO phase separation upon crystallization.Comment: 6 pages, 5 figures, accepted on A&
Using the Ca II Triplet to Trace Abundance Variations in Individual Red Giant Branch stars in Three Nearby Galaxies
Spectroscopic abundance determinations for stars spanning a Hubble time in
age are necessary in order to unambiguously determine the evolutionary
histories of galaxies. Using FORS1 in Multi-Object Spectroscopy mode on ANTU
(UT1) at the ESO-VLT on Paranal we obtained near infrared spectra from which we
measured the equivalent widths of the two strongest Ca II triplet lines to
determine metal abundances for a sample of Red Giant Branch stars, selected
from ESO-NTT optical (I, V-I) photometry of three nearby, Local Group,
galaxies: the Sculptor Dwarf Spheroidal, the Fornax Dwarf Spheroidal and the
Dwarf Irregular NGC 6822. The summed equivalent width of the two strongest
lines in the Ca II triplet absorption line feature, centered at 8500A, can be
readily converted into an [Fe/H] abundance using the previously established
calibrations by Armandroff & Da Costa (1991) and Rutledge, Hesser & Stetson
(1997). We measured metallicities for 37 stars in Sculptor, 32 stars in Fornax,
and 23 stars in NGC 6822, yielding more precise estimates of the metallicity
distribution functions for these galaxies than it is possible to obtain
photometrically. In the case of NGC 6822, this is the first direct measurement
of the abundances of the intermediate-age and old stellar populations. We find
metallicity spreads in each galaxy which are broadly consistent with the
photometric width of the Red Giant Branch, although the abundances of
individual stars do not always appear to correspond to their colour. This is
almost certainly predominantly due to a highly variable star formation rate
with time in these galaxies, which results in a non-uniform,
non-globular-cluster-like, evolution of the Ca/Fe ratio.Comment: Accepted for publication in MNRA
Resolved stellar population of distant galaxies in the ELT era
The expected imaging capabilities of future Extremely Large Telescopes (ELTs)
will offer the unique possibility to investigate the stellar population of
distant galaxies from the photometry of the stars in very crowded fields. Using
simulated images and photometric analysis we explore here two representative
science cases aimed at recovering the characteristics of the stellar
populations in the inner regions of distant galaxies. Specifically: case A) at
the center of the disk of a giant spiral in the Centaurus Group, (mu B~21,
distance of 4.6 Mpc); and, case B) at half of the effective radius of a giant
elliptical in the Virgo Cluster (mu~19.5, distance of 18 Mpc). We generate
synthetic frames by distributing model stellar populations and adopting a
representative instrumental set up, i.e. a 42 m Telescope operating close to
the diffraction limit. The effect of crowding is discussed in detail showing
how stars are measured preferentially brighter than they are as the confusion
limit is approached. We find that (i) accurate photometry (sigma~0.1,
completeness >90%) can be obtained for case B) down to I~28.5, J~27.5 allowing
us to recover the stellar metallicity distribution in the inner regions of
ellipticals in Virgo to within ~0.1 dex; (ii) the same photometric accuracy
holds for the science case A) down to J~28.0, K~27.0, enabling to reconstruct
of the star formation history up to the Hubble time via simple star counts in
diagnostic boxes. For this latter case we discuss the possibility of deriving
more detailed information on the star formation history from the analysis of
their Horizontal Branch stars. We show that the combined features of high
sensitivity and angular resolution of ELTs may open a new era for our knowledge
of the stellar content of galaxies of different morphological type up to the
distance of the Virgo cluster.Comment: 21 pages, 17 figures, PASP accepted in pubblicatio
Massive stellar systems: observational challenges and perspectives in the E-ELT era
We introduce the empirical framework concerning optical and near-infrared (NIR) photometry of crowded stellar fields. In particular, we address the impact that linear detectors and analytical PSF played in improving the accuracy and the precision of multi-band color-magnitude diagrams (CMDs). We focus our attention on recent findings based on deep NIR images collected with Adaptive Optics (AO) systems at the 8-10m class telescopes and discuss pros and cons of the different approaches. We also discuss the estimate of the absolute age of globular clusters using a well defined knee along the lower main sequence. We mention the role which the current AO-assisted instruments will have in addressing longstanding astrophysical problems of the Galactic center. Finally, we outline the role of first generation of E-ELT instruments upon photometry and spectroscopy of crowded stellar fields
Redshift Drift in LTB Void Universes
We study the redshift drift, i.e., the time derivative of the cosmological
redshift in the Lema\^itre-Tolman-Bondi (LTB) solution in which the observer is
assumed to be located at the symmetry center. This solution has often been
studied as an anti-Copernican universe model to explain the acceleration of
cosmic volume expansion without introducing the concept of dark energy. One of
decisive differences between LTB universe models and Copernican universe models
with dark energy is believed to be the redshift drift. The redshift drift is
negative in all known LTB universe models, whereas it is positive in the
redshift domain in Copernican models with dark energy. However,
there have been no detailed studies on this subject. In the present paper, we
prove that the redshift drift of an off-center source is always negative in the
case of LTB void models. We also show that the redshift drift can be positive
with an extremely large hump-type inhomogeneity. Our results suggest that we
can determine whether we live near the center of a large void without dark
energy by observing the redshift drift.Comment: 16 pages, 2 figure
Stellar populations in the ELT perspective
We discuss the impact that the next generation of Extremely Large Telescopes
will have on the open astrophysical problems of resolved stellar populations.
In particular, we address the interplay between multiband photometry and
spectroscopy.Comment: 6 pages, 3 figures. To appear in the proceedings of "EWASS 2013
Symposium 5: Local Group, Local Cosmology," (8-9 July 2013, Turku, Finland),
Mem. S.A.It, M. Monelli and S. Salvadori Ed
Carbon Monoxide in type II supernovae
Infrared spectra of two type II supernovae 6 months after explosion are
presented. The spectra exhibit a strong similarity to the observations of SN
1987A and other type II SNe at comparable epochs. The continuum can be fitted
with a cool black body and the hydrogen lines have emissivities that are
approximately those of a Case B recombination spectrum. The data extend far
enough into the thermal region to detect emission by the first overtone of
carbon monoxide. The molecular emission is modeled and compared with that in
the spectra of SN 1987A. It is found that the flux in the CO first overtone is
comparable to that found in SN 1987A. We argue that Carbon Monoxide forms in
the ejecta of all type II SNe during the first year after explosion.Comment: 6 pages, 6 figures, accepted for publications in A&
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