41 research outputs found
New Colour-Mass to Light Relations: the role of the Asymptotic Giant Branch phase and of interstellar dust
Colour-M/L (mass-to-light) relations are a popular recipe to derive stellar
mass in external galaxies. Stellar mass estimates often rely on near infrared
(NIR) photometry, considered an optimal tracer since it is little affected by
dust and by the "frosting" effect of recent star formation episodes. However,
recent literature has highlighted that theoretical estimates of the NIR M/L
ratio strongly depend on the modelling of the Asymptotic Giant Branch (AGB)
phase. We use the latest Padova isochrones, with detailed modelling of the
Thermally Pulsing AGB phase, to update theoretical colour-M/L relations in the
optical and NIR and discuss the consequences for the estimated stellar masses
in external galaxies. We also discuss the effect of attenuation by interstellar
dust on colour-M/L relations in the statistical case of large galaxy samples.Comment: 17 pages, 13 figures. MNRAS in pres
The colours of the Sun
We compile a sample of Sun-like stars with accurate effective temperatures,
metallicities and colours (from the UV to the near-IR). A crucial improvement
is that the effective temperature scale of the stars has recently been
established as both accurate and precise through direct measurement of angular
diameters obtained with stellar interferometers. We fit the colours as a
function of effective temperature and metallicity, and derive colour estimates
for the Sun in the Johnson/Cousins, Tycho, Stromgren, 2MASS and SDSS
photometric systems. For (B-V)_Sun, we favour the ``red'' colour 0.64 versus
the ``blue'' colour 0.62 of other recent papers, but both values are consistent
within the errors; we ascribe the difference to the selection of Sun-like stars
versus interpolation of wider colour-Teff-metallicity relations.Comment: 5 pages, 2 figures, accepted by MNRA
Solar analogues and solar twins in the HARPS archive
We present 63 solar analogues and twins for which high signal-to-noise ratio (S/N) archival data are available for the High Accuracy Radial velocity Planet Searcher (HARPS) high-resolution spectrograph at the European Southern Observatory (ESO) 3.6-m telescope. We perform a differential analysis of these stellar spectra relative to the solar spectrum, similar to previous work using ESO 2.2-m/fiber-fed extended range optical spectrograph (FEROS) data, and expand our analysis by introducing a new method to test the temperature and metallicity calibration of Sun-like stars in the Geneva-Copenhagen Survey (GCS). The HARPS data are significantly better than the FEROS data, with improvements in S/N, spectral resolution and number of lines we can analyse. We confirm the offsets to the photometric scale found in our FEROS study. We confirm three solar twins found in the FEROS data as solar twins in the HARPS data, as well as identify six new twins.</p
Spectroscopic study of solar twins and analogues
Context. Many large stellar surveys have been and are still being carried
out, providing huge amounts of data, for which stellar physical parameters will
be derived. Solar twins and analogues provide a means to test the calibration
of these stellar catalogues because the Sun is the best-studied star and
provides precise fundamental parameters. Solar twins should be centred on the
solar values. Aims. This spectroscopic study of solar analogues selected from
the Geneva-Copenhagen Survey (GCS) at a resolution of 48,000 provides effective
temperatures and metallicities for these stars. We test whether our
spectroscopic parameters, as well as the previous photometric calibrations, are
properly centred on the Sun. In addition, we search for more solar twins in our
sample. Methods. The methods used in this work are based on literature methods
for solar twin searches and on methods we developed in previous work to
distinguish the metallicity-temperature degeneracies in the differential
comparison of spectra of solar analogues versus a reference solar reflection
spectrum. Results. We derive spectroscopic parameters for 148 solar analogues
(about 70 are new entries to the literature) and verify with a-posteriori
differential tests that our values are well-centred on the solar values. We use
our dataset to assess the two alternative calibrations of the GCS parameters;
our methods favour the latest revision. We show that the choice of spectral
line list or the choice of asteroid or time of observation does not affect the
results. We also identify seven solar twins in our sample, three of which are
published here for the first time. Conclusions. Our methods provide an
independent means to differentially test the calibration of stellar catalogues
around the values of a well-known benchmark star, which makes our work
interesting for calibration tests of upcoming Galactic surveys.Comment: 12 pages, 11 figures. Published versio
New solar twins and the metallicity and temperature scales of the Geneva Copenhagen Survey
We search for "solar twins" in the Geneva-Copenhagen Survey (GCS) using high
resolution optical spectroscopy. We initially select Sun-like stars from the
GCS by absolute magnitude, b-y colour and metallicity close to the solar
values. Our aim is to find the stars which are spectroscopically very close to
the Sun using line depth ratios and the median equivalent widths and depths of
selected lines with a range of excitation potentials. We present the ten best
stars fulfilling combined photometric and spectroscopic criteria, of which six
are new twins. We use our full sample of Sun-like stars to examine the
calibration of the metallicity and temperature scale in the GCS. Our results
give rise to the conclusion that the GCS may be offset from the solar
temperature and metallicity for sun-like stars by 100K and 0.1dex,
respectively.Comment: 14 pages, 15 figures and 7 tables. Published versio
Management of the pelvic floor disfunctions: combined versus single surgical procedure in a multidisciplinary approach: a retrospective study
The objective of this study was to compare the outcome of combined surgical treatment of multicompartmental pelvic floor defects versus single procedures
within a multidisciplinary path in order to try to clarify what is
the most effective surgical approach
Simulating galaxy Clusters -II: global star formation histories and galaxy populations
Cosmological (LambdaCDM) TreeSPH simulations of the formation and evolution
of galaxy groups and clusters have been performed. The simulations invoke star
formation, chemical evolution with non-instantaneous recycling, metal dependent
radiative cooling, strong star burst and (optionally) AGN driven galactic super
winds, effects of a meta-galactic UV field and thermal conduction. The
properties of the galaxy populations in two clusters, one Virgo-like (T~3 keV)
and one (sub) Coma-like (T~6 keV), are discussed. The global star formation
rates of the cluster galaxies are found to decrease very significantly with
time from redshift z=2 to 0, in agreement with observations. The total K-band
luminosity of the cluster galaxies correlates tightly with total cluster mass,
and for models without additional AGN feedback, the zero point of the relation
matches the observed one fairly well. The match to observed galaxy luminosity
functions is reasonable, except for a deficiency of bright galaxies (M_B <
-20), which becomes increasingly significant with super-wind strength. Results
of a high resolution test indicate that this deficiency is not due to
``over--merging''. The redshift evolution of the luminosity functions from z=1
to 0 is mainly driven by luminosity evolution, but also by merging of bright
galaxies with the cD. The colour--magnitude relation of the cluster galaxies
matches the observed "red sequence" very well and, on average, galaxy
metallicity increases with luminosity. As the brighter galaxies are essentially
coeval, the colour--magnitude relation results from metallicity rather than age
effects, as observed.Comment: 15 pages, 10 figures. Final version accepted by MNRAS, presenting new
simulations and major changes. Printing in colour recommende
Revisiting Delta Y/Delta Z from multiple main sequences in Globular Clusters: insight from nearby stars
For nearby K dwarfs, the broadening of the observed Main Sequence at low
metallicities is much narrower than expected from isochrones with the standard
helium-to-metal enrichment ratio DY/DZ=2. Though the latter value fits well the
Main Sequence around solar metallicity, and agrees with independent
measurements from HII regions as well as with theoretical stellar yields and
chemical evolution models, a much higher DY/DZ~10 is necessary to reproduce the
broadening observed for nearby subdwarfs. This result resembles, on a milder
scale, the very high DY/DZ estimated from the multiple Main Sequences in Omega
Cen and NGC 2808. Although not "inverted" as in omega Cen, where the metal-rich
Main Sequence is bluer than the metal-poor one, the broadening observed for
nearby subdwarfs is much narrower than stellar models predict for a standard
helium content. We use this empirical evidence to argue that a revision of
lower Main Sequence stellar models, suggested from nearby stars, could
significantly reduce the helium content inferred for the subpopulations of
those globular clusters. A simple formula based on empirically calibrated
homology relations is constructed, for an alternative estimate of DY/DZ in
multiple main sequences. We find that, under the most favourable assumptions,
the estimated helium content for the enriched populations could decrease from
Y~0.4 to as low as Y~0.3.Comment: 15 pages, 12 figures, in press on MNRA
Galaxy Formation: CDM, Feedback and the Hubble Sequence
TreeSPH simulations of galaxy formation in a LCDM cosmology, with star
formation, energetic stellar feedback and a meta-galactic UV field have been
performed, resulting in realistic disk, S0 and E galaxies at z=0. The disk
galaxies are deficient in angular momentum by only about a factor of two
compared to observations for runs where fairly strong star-bursts in early,
proto-galactic clouds lead to "blow-away" of the remaining gas. The surface
density profiles of the stellar disks are approximately exponential and those
of the bulges range from exponential to r^(1/4). B/D ratios and integrated B-V
colours are consistent with observations. The observed I-band TF relation can
be matched with M/L_I ~ 0.8, in fair agreement with recent determinations. The
(E/S0)s have approximately r^(1/4) profiles, non disk-like kinematics and are
flattened due to non-isotropic stellar velocity distributions. We predict hot
halo gas to cool out and accrete onto the Galactic disk at 0.5-1 Msun/yr at
z=0, consistent with upper limits from FUSE observations of O VI. We analyzed
two Milky Way-sized galaxies and find accretion rates, and X-ray halo
luminosities, 6-7 times larger at z=1 than at z=0. The gas infall declines
nearly exponentially with time, supporting the approximation often used in
chemical evolution models. The infall time-scales are comparable to what is
used to solve the "G-dwarf problem". One disk forms "inside-out", the other
"outside-in". For both, the mean stellar ages in the outskirts agree with
findings for the disk of M31. The amount of hot gas in disk galaxy haloes is
consistent with observational upper limits, as are dispersion measures to
pulsars in the globular cluster M53 and the LMC, which were "inserted" in the
disk galaxy haloes.Comment: Version published on ApJ. 20 pages in emulateapj.cls, incl. 23 figs.
Pictures of some of the galaxies can be seen at
http://www.tac.dk/~jslarsen/Hubble_Sequence/index.htm