35 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
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
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
The Helium abundance and Delta Y / Delta Z in Lower Main Sequence stars
We use nearby K dwarf stars to measure the helium-to-metal enrichment ratio,
a diagnostic of the chemical history of the Solar Neighbourhood. Our sample of
K dwarfs has homogeneously determined effective temperatures, bolometric
luminosities and metallicities, allowing us to fit each star to the appropriate
stellar isochrone and determine its helium content indirectly. We use a newly
computed set of Padova isochrones which cover a wide range of helium and metal
content. Our theoretical isochrones have been checked against a congruous set
of main sequence binaries with accurately measured masses, to discuss and
validate their range of applicability. We find that the stellar masses deduced
from the isochrones are usually in excellent agreement with empirical
measurements. Good agreement is also found with empirical mass-luminosity
relations. Despite fitting the masses of the stars very well, we find that
anomalously low helium content (lower than primordial helium) is required to
fit the luminosities and temperatures of the metal poor K dwarfs, while more
conventional values of the helium content are derived for the stars around
solar metallicity. We have investigated the effect of diffusion in stellar
models and LTE assumption in deriving metallicities. Neither of these is able
to resolve the low helium problem alone and only marginally if the cumulated
effects are included, unless we assume a mixing-length which is strongly
decreasing with metallicity. Further work in stellar models is urgently needed.
The helium-to-metal enrichment ratio is found to be Delta Y / Delta Z = 2.1 +/-
0.9 around and above solar metallicity, consistent with previous studies,
whereas open problems still remain at the lowest metallicities. Finally, we
determine the helium content for a set of planetary host stars.Comment: 29 pages, 13 figures, replaced to match published version in MNRA