702 research outputs found
Detection of Surface Brightness Fluctuations in Elliptical Galaxies imaged with the Advanced Camera for Surveys. B- and I-band measurements
Taking advantage of the exceptional capabilities of ACS on board of HST, we
derive Surface Brightness Fluctuation (SBF) measurements in the B and I bands
from images of six elliptical galaxies with . Given the
low S/N ratio of the SBF signal in the blue band images, the reliability of the
measurements is verified both with numerical simulations and experimental data
tests.
This paper presents the first published B- and I-band SBF measurements for
distant ( 20 Mpc) galaxies, essential for the comparisons of the models
to observations of normal ellipticals. By comparing I-band data with our new
Simple Stellar Population (SSP) models we find an excellent agreement and we
confirm that I-band SBF magnitudes are mainly sensitive to the metallicity of
the dominant stellar component in the galaxy, and are not strongly affected by
the contribution of possible secondary stellar components. As a consequence
I-band fluctuations magnitudes are ideal for distance studies. On the other
hand, we show that standard SSP models do not reproduce the B-band SBF
magnitudes of red ((B-I)_0 \gsim 2.1) galaxies in our sample. We explore the
capability of two non--canonical models in properly reproducing the high
sensitivity of B SBF to the presence of even small fractions of bright, hot
stars (metal poor stars, hot evolved stars, etc.). The disagreement is solved
both by taking into account hot (Post--AGB) stars in SSP models and/or by
adopting Composite Stellar Population models. Finally, we suggest a limit value
of the S/N for the B-band SBF signal required to carry out a detailed study of
stellar population properties based on this technique.Comment: ApJ accepte
Evolution of progenitor stars of Type Ibc supernovae and long gamma-ray bursts
We discuss how rotation and binary interactions may be related to the
diversity of type Ibc supernovae and long gamma-ray bursts. After presenting
recent evolutionary models of massive single and binary stars including
rotation, the Tayler-Spruit dynamo and binary interactions, we argue that the
nature of SNe Ibc progenitors from binary systems may not significantly differ
from that of single star progenitors in terms of rotation, and that most long
GRB progenitors may be produced via the quasi-chemically homogeneous evolution
at sub-solar metallicity. We also briefly discuss the possible role of magnetic
fields generated in the convective core of a massive star for the transport of
angular momentum, which is potentially important for future stellar evolution
models of supernova and GRB progenitors.Comment: 6 pages, 4 figures, to appear in IAU Symp. 250, Massive Stars as
Cosmic Engines, Kauai (HI), 12/2007, ed. F. Bresolin, P. Crowther, & J. Pul
On the origin of microturbulence in hot stars
We present results from the first extensive study of convection zones in the
envelopes of hot massive stars, which are caused by opacity peaks associated
with iron and helium ionization. These convective regions can be located very
close to the stellar surface. Recent observations of microturbulence in massive
stars from the VLT-Flames survey are in good agreement with our predictions
concerning the occurrence and the strength of sub-surface convection in hot
stars. We argue further that convection close to the surface may trigger
clumping at the base of the stellar wind of massive stars.Comment: to appear in Comm. in Astroseismology - Proceedings of the 38th
LIAC/HELAS-ESTA/BAG, 200
Thermohaline mixing in low-mass giants: RGB and beyond
Thermohaline mixing has recently been proposed to occur in low mass red
giants, with large consequence for the chemical yields of low mass stars. We
investigate the role of thermohaline mixing during the evolution of stars
between 1 Msun and 3 Msun. We use a stellar evolution code which includes
rotational mixing and internal magnetic fields. We confirm that thermohaline
mixing has the potential to destroy most of the helium 3 which is produced
earlier on the main sequence during the red giant stage, in stars below
1.5Msun. We find this process to continue during core helium burning and
beyond. We find rotational and magnetic mixing to be negligible compared to the
thermohaline mixing in the relevant layers, even if the interaction of
thermohaline motions with the differential rotation may be essential to
establish the time scale of thermohaline mixing in red giants.Comment: Proceedings of the Conference "Unsolved problems in stellar physics"
- Cambridge, July 200
An Ultra diffuse Galaxy in the NGC 5846 group from the VEGAS survey
Many ultra diffuse galaxies (UDGs) have now been identified in clusters of
galaxies. However, the number of nearby UDGs suitable for detailed follow-up
remain rare. Our aim is to begin to identify UDGs in the environments of nearby
bright early-type galaxies from the VEGAS survey. Here we use a deep g band
image of the NGC 5846 group, taken as part of the VEGAS survey, to search for
UDGs. We found one object with properties of a UDG if it associated with the
NGC 5846 group, which seems likely. The galaxy, we name NGC 5846UDG1, has
an absolute magnitude of M = -14.2, corresponding to a stellar mass of
10 M. It also reveals a system of compact sources which are
likely globular clusters. Based on the number of globular clusters detected we
estimate a halo mass that is greater than 810 M for
UDG1.Comment: 5 pages, 4 figures, accepted for publication in A&
Surface Brightness Fluctuations from archival ACS images: a stellar population and distance study
We derive Surface Brightness Fluctuations (SBF) and integrated magnitudes in
the V- and I-bands using Advanced Camera for Surveys (ACS) archival data. The
sample includes 14 galaxies covering a wide range of physical properties:
morphology, total absolute magnitude, integrated color. We take advantage of
the latter characteristic of the sample to check existing empirical
calibrations of absolute SBF magnitudes both in the I- and V-passbands.
Additionally, by comparing our SBF and color data with the Teramo-SPoT simple
stellar population models, and other recent sets of population synthesis
models, we discuss the feasibility of stellar population studies based on
fluctuation magnitudes analysis. The main result of this study is that
multiband optical SBF data and integrated colors can be used to significantly
constrain the chemical composition of the dominant stellar system in the
galaxy, but not the age in the case of systems older than 3 Gyr.
SBF color gradients are also detected and analyzed. These SBF gradient data,
together with other available data, point to the existence of mass dependent
metallicity gradients in galaxies, with the more massive objects showing a
non--negligible SBF versus color gradient. The comparison with models suggests
that such gradients imply more metal rich stellar populations in the galaxies'
inner regions with respect to the outer ones.Comment: ApJ Accepte
Long GRBs from binary stars: runaway, Wolf-Rayet progenitors
The collapsar model for long gamma-ray bursts requires a rapidly rotating
Wolf-Rayet star as progenitor. We test the idea of producing rapidly rotating
Wolf-Rayet stars in massive close binaries through mass accretion and
consecutive quasi-chemically homogeneous evolution; the latter had previously
been shown to provide collapsars below a certain metallicity threshold for
single stars. The binary channel presented here may provide a means for massive
stars to obtain the high rotation rates required to evolve quasi-chemically
homogeneous and fulfill the collapsar scenario. Moreover, it suggests that a
possibly large fraction of long gamma-ray bursts occurs in runaway stars.Comment: To appear in the proceedings of the conference "Unsolved problems in
stellar physics" - Cambridge, July 200
Disentangling age and metallicity in distant unresolved stellar systems
We present some results of an observational and theoretical study on
unresolved stellar systems based on the Surface Brightness Fluctuations (SBF)
technique. It is shown that SBF magnitudes are a valuable tracer of stellar
population properties, and a reliable distance indicator. SBF magnitudes,
SBF-colors, and SBF-gradients can help to constrain within relatively narrow
limits the metallicity and age of the dominant stellar component in distant
stellar systems, especially if coupled with other spectro-photometric
indicators.Comment: A contributed paper to the Cefalu' (Italy) "Probing Stellar
Populations out to the Distant Universe", 4 pages. To appear as an AIP
Conference Proceedin
Suppression of Quadrupole and Octupole Modes in Red Giants Observed by Kepler
An exciting new theoretical result shows that observing suppression of dipole oscillation modes in red giant stars can be used to detect strong magnetic fields in the stellar cores. A fundamental facet of the theory is that nearly all the mode energy leaking into the core is trapped by the magnetic greenhouse effect. This results in clear predictions for how the mode visibility changes as a star evolves up the red giant branch, and how that depends on stellar mass, spherical degree, and mode lifetime. Here, we investigate the validity of these predictions with a focus on the visibility of different spherical degrees. We find that mode suppression weakens for higher degree modes with a reduction in the quadrupole mode visibility of up to 49%, and no detectable suppression of octupole modes, in agreement with theory. We find evidence for the influence of increasing mode lifetimes on the visibilities along the red giant branch, in agreement with previous independent observations. These results support the theory that strong internal magnetic fields cause suppression of non-radial modes in red giants. We also find preliminary evidence that stars with suppressed dipole modes on average have slightly lower metallicity than normal stars
A Measurement of Stellar Surface Gravity Hidden in Radial Velocity Differences of Co-moving Stars
The gravitational redshift induced by stellar surface gravity is notoriously
difficult to measure for non-degenerate stars, since its amplitude is small in
comparison with the typical Doppler shift induced by stellar radial velocity.
In this study, we make use of the large observational data set of the Gaia
mission to achieve a significant reduction of noise caused by these random
stellar motions. By measuring the differences in velocities between the
components of pairs of co-moving stars and wide binaries, we are able to
statistically measure gravitational redshift and nullify the effect of the
peculiar motions of the stars. For the subset of stars considered in this
study, we find a positive correlation between the observed differences in Gaia
radial velocities and the differences in surface gravity inferred from
effective temperature and luminosity measurements. This corresponds to the
first ever measurement of extra-Solar surface gravity induced gravitational
redshift in non-degenerate stars. Additionally, we study the sub-dominant
effects of convective blueshifting of emission lines, effects of binary motion,
and possible systematic errors in radial velocity measurements within Gaia.
Results from the technique presented in this study are expected to improve
significantly with data from the next Gaia data release. Such improvements
could be used to constrain the mass-luminosity relation and stellar models
which predict the magnitude of convective blueshift.Comment: 7 pages, 3 figure
- …