4,114 research outputs found
Calcium Triplet Synthesis
We present theoretical equivalent widths for the sum of the two strongest
lines of the Calcium Triplet, CaT index, in the near-IR, using evolutionary
techniques and the most recent models and observational data for this feature
in individual stars. We compute the CaT index for Single Stellar Populations
(instantaneous burst, standard Salpeter-type IMF) at four metallicities,
Z=0.004, 0.008, 0.02 (solar) and 0.05, and ranging in age from very young
bursts of star formation (few Myr) to old stellar populations, up to 17 gyr,
representative of globular clusters, elliptical galaxies and bulges of spirals.
The interpretation of the observed equivalent widths of CaT in different
stellar systems is discussed. Composite-population models are also computed as
a tool to interpret the CaT detections in star-forming regions, in order to
disantangle between the component due to Red Supergiants stars, RSG, and the
underlying, older, population. CaT is found to be an excellent
metallicity-indicator for populations older than 1 Gyr, practically independent
of the age. We discuss its application to remove the age- metallicity
degeneracy, characteristic of all studies of galaxy evolution based on the
usual integrated indices (both broad band colors and narrow band indices). The
application of the models computed here to the analysis of a sample of
elliptical galaxies will be discussed in a forthcoming paper (Gorgas et al.
1998).Comment: 17 pages, 7 figures, to be published in A&
Moving constraints as stabilizing controls in classical mechanics
The paper analyzes a Lagrangian system which is controlled by directly
assigning some of the coordinates as functions of time, by means of
frictionless constraints. In a natural system of coordinates, the equations of
motions contain terms which are linear or quadratic w.r.t.time derivatives of
the control functions. After reviewing the basic equations, we explain the
significance of the quadratic terms, related to geodesics orthogonal to a given
foliation. We then study the problem of stabilization of the system to a given
point, by means of oscillating controls. This problem is first reduced to the
weak stability for a related convex-valued differential inclusion, then studied
by Lyapunov functions methods. In the last sections, we illustrate the results
by means of various mechanical examples.Comment: 52 pages, 4 figure
The impact of metallicity and dynamics on the evolution of young star clusters
The early evolution of a dense young star cluster (YSC) depends on the
intricate connection between stellar evolution and dynamical processes. Thus,
N-body simulations of YSCs must account for both aspects. We discuss N-body
simulations of YSCs with three different metallicities (Z=0.01, 0.1 and 1
Zsun), including metallicity-dependent stellar evolution recipes and
metallicity-dependent prescriptions for stellar winds and remnant formation. We
show that mass-loss by stellar winds influences the reversal of core collapse.
In particular, the post-collapse expansion of the core is faster in metal-rich
YSCs than in metal-poor YSCs, because the former lose more mass (through
stellar winds) than the latter. As a consequence, the half-mass radius expands
more in metal-poor YSCs. We also discuss how these findings depend on the total
mass and on the virial radius of the YSC. These results give us a clue to
understand the early evolution of YSCs with different metallicity.Comment: to appear in "Massive Young Star Clusters Near and Far: From the
Milky Way to Reionization", 2013 Guillermo Haro Conference, Eds. Y. D. Mayya,
D. Rosa-Gonzalez & E. Terlevich, INAOE and AMC. 4 pages, 2 figure
On Core Collapse Supernovae in Normal and in Seyfert Galaxies
This paper estimates the relative frequency of different types of
core-collapse supernovae, in terms of the ratio f between the number of type
Ib--Ic and of type II supernovae. We estimate f independently for all normal
and Seyfert galaxies whose radial velocity is <=14000 km/s, and which had at
least one supernova event recorded in the Asiago catalogue from January 1986 to
August 2000. We find that the ratio f is approx. 0.23+/-0.05 in normal
galaxies. This value is consistent with constant star formation rate and with a
Salpeter Initial Mass Function and average binary rate approx. 50 %. On the
contrary, Seyfert galaxies exceed the ratio f in normal galaxies by a factor
approx. 4 at a confidence level >= 2 sigma. A caveat is that the numbers for
Seyferts are still small (6 type Ib-Ic and 6 type II supernovae discovered as
yet). Assumed real, this excess of type Ib and Ic with respect to type II
supernovae, may indicate a burst of star formation of young age (<= 20 Myr), a
high incidence of binary systems in the inner regions (r <= 0.4 R25) of Seyfert
galaxies, or a top-loaded mass function.Comment: Accepted for Publication in MNRA
Star Formation History and Extinction in the central kpc of M82-like Starbursts
We report on the star formation histories and extinction in the central kpc
region of a sample of starburst galaxies that have similar far infrared (FIR),
10 micron and K-band luminosities as those of the archetype starburst M82. Our
study is based on new optical spectra and previously published K-band
photometric data, both sampling the same area around the nucleus. Model
starburst spectra were synthesized as a combination of stellar populations of
distinct ages formed over the Hubble time, and were fitted to the observed
optical spectra and K-band flux. The model is able to reproduce simultaneously
the equivalent widths of emission and absorption lines, the continuum fluxes
between 3500-7000 Ang, the K-band and the FIR flux. We require a minimum of 3
populations -- (1) a young population of age < 8 Myr, with its corresponding
nebular emission, (2) an intermediate-age population (age < 500 Myr), and (3)
an old population that forms part of the underlying disk or/and bulge
population. The contribution of the old population to the K-band luminosity
depends on the birthrate parameter and remains above 60% in the majority of the
sample galaxies. Even in the blue band, the intermediate age and old
populations contribute more than 40% of the total flux in all the cases. A
relatively high contribution from the old stars to the K-band nuclear flux is
also apparent from the strength of the 4000 Ang break and the CaII K line. The
extinction of the old population is found to be around half of that of the
young population. The contribution to the continuum from the relatively old
stars has the effect of diluting the emission equivalent widths below the
values expected for young bursts. The mean dilution factors are found to be 5
and 3 for the Halpha and Hbeta lines respectively.Comment: 20 pages, uses emulateapj.cls. Scheduled to appear in ApJ Jan 1, 200
The role of the synchrotron component in the mid infrared spectrum of M 87
We study in detail the mid-infrared Spitzer-IRS spectrum of M 87 in the range
5 to 20 micron. Thanks to the high sensitivity of our Spitzer-IRS spectra we
can disentangle the stellar and nuclear components of this active galaxy. To
this end we have properly subtracted from the M 87 spectrum, the contribution
of the underlying stellar continuum, derived from passive Virgo galaxies in our
sample. The residual is a clear power-law, without any additional thermal
component, with a zero point consistent with that obtained by high spatial
resolution, ground based observations. The residual is independent of the
adopted passive template. This indicates that the 10 micron silicate emission
shown in spectra of M 87 can be entirely accounted for by the underlying old
stellar population, leaving little room for a possible torus contribution. The
MIR power-law has a slope alpha ~ 0.77-0.82 (S),
consistent with optically thin synchrotron emission.Comment: 5 pages, 4 figures, accepted for publication in ApJ main journa
Mid-infrared colour gradients and the colour-magnitude relation in Virgo early-type galaxies
We make use of Spitzer imaging between 4 and 16 micron and near-infrared data
at 2.2 micron to investigate the nature and distribution of the mid-infrared
emission in a sample of early-type galaxies in the Virgo cluster. These data
allow us to conclude, with some confidence, that the emission at 16 micron in
passive ETGs is stellar in origin, consistent with previous work concluding
that the excess mid-infrared emission comes from the dusty envelopes around
evolved AGB stars. There is little evidence for the mid-infrared emission of an
unresolved central component, as might arise in the presence of a dusty torus
associated with a low-luminosity AGN. We nonetheless find that the 16 micron
emission is more centrally peaked than the near-infrared emission, implying a
radial stellar population gradient. By comparing with independent evidence from
studies at optical wavelengths, we conclude that a metallicity that falls with
increasing radius is the principal driver of the observed gradient. We also
plot the mid-infrared colour-magnitude diagram and combine with similar work on
the Coma cluster to define the colour-magnitude relation for absolute K-band
magnitudes from -26 to -19. Because a correlation between mass and age would
produce a relation with a gradient in the opposite sense to that observed, we
conclude that the relation reflects the fact that passive ETGs of lower mass
also have a lower average metallicity. The colour-magnitude relation is thus
driven by metallicity effects. In contrast to what is found in Coma, we do not
find any objects with anomalously bright 16 micron emission relative to the
colour-magnitude relation. Although there is little overlap in the mass ranges
probed in the two clusters, this may suggest that observable ``rejuvenation''
episodes are limited to intermediate mass objects.Comment: 8 pages, 4 figure
Ultra-luminous X-ray sources and remnants of massive metal-poor stars
Massive metal-poor stars might form massive stellar black holes (BHs), with
mass 25<=mBH/Msun<=80, via direct collapse. We derive the number of massive BHs
(NBH) that are expected to form per galaxy through this mechanism. Such massive
BHs might power most of the observed ultra-luminous X-ray sources (ULXs). We
select a sample of 64 galaxies with X-ray coverage, measurements of the star
formation rate (SFR) and of the metallicity. We find that NBH correlates with
the number of observed ULXs per galaxy (NULX) in this sample. We discuss the
dependence of our model on the SFR and on the metallicity. The SFR is found to
be crucial, consistently with previous studies. The metallicity plays a role in
our model, since a lower metallicity enhances the formation of massive BHs.
Consistently with our model, the data indicate that there might be an
anticorrelation between NULX, normalized to the SFR, and the metallicity. A
larger and more homogeneous sample of metallicity measurements is required, in
order to confirm our results.Comment: 21 pages, 8 figures, accepted for publication in MNRA
SPITZER IRS spectra of Virgo early type galaxies: detection of stellar silicate emission
We present high signal to noise ratio Spitzer Infrared Spectrograph
observations of 17 Virgo early-type galaxies. The galaxies were selected from
those that define the colour-magnitude relation of the cluster, with the aim of
detecting the silicate emission of their dusty, mass-losing evolved stars. To
flux calibrate these extended sources we have devised a new procedure that
allows us to obtain the intrinsic spectral energy distribution and to
disentangle resolved and unresolved emission within the same object. We have
found that thirteen objects of the sample (76%) are passively evolving galaxies
with a pronounced broad silicate feature which is spatially extended and likely
of stellar origin, in agreement with model predictions. The other 4 objects
(24%) are characterized by different levels of activity. In NGC 4486 (M 87) the
line emission and the broad silicate emission are evidently unresolved and,
given also the typical shape of the continuum, they likely originate in the
nuclear torus. NGC 4636 shows emission lines superimposed on extended (i.e.
stellar) silicate emission, thus pushing the percentage of galaxies with
silicate emission to 82%. Finally, NGC 4550 and NGC 4435 are characterized by
polycyclic aromatic hydrocarbon (PAH) and line emission, arising from a central
unresolved region. A more detailed analysis of our sample, with updated models,
will be presented in a forthcoming paper.Comment: 6 pages; ApJ Letters, accepte
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