252 research outputs found
The third dredge-up and the carbon star luminosity functions in the Magellanic Clouds
We investigate the formation of carbon stars as a function of the stellar
mass and parent metallicity. Theoretical modelling is based on an improved
scheme for treating the third dredge-up in synthetic calculations of thermally
pulsing asymptotic giant branch (TP-AGB) stars. In this approach, the usual
criterion (based on a constant minimum core mass for the occurrence of
dredge-up, M_c^min) is replaced by one on the minimum temperature at the base
of the convective envelope, T_b^dred, at the stage of the post-flash luminosity
maximum. Envelope integrations then allow determination of M_c^min as a
function of stellar mass, metallicity, and pulse strength (see Wood 1981), thus
inferring if and when dredge-up first occurs. Moreover, the final possible shut
down of the process is predicted. Extensive grids of TP-AGB models were
computed using this scheme. We present and discuss the calibration of the two
dredge-up parameters (lambda and T_b^dred) aimed at reproducing the carbon star
luminosity function (CSLF) in the LMC. It turns out that the faint tail is
almost insensitive to the history of star formation rate (SFR) in the parent
galaxy (it is essentially determined by T_b^dred), in contrast to the bright
wing which may be more affected by the details of the recent SFR. Once the
faint end is reproduced, the peak location is a stringent calibrator of lambda.
The best fit to the observed CSLF in the LMC is obtained with Z=0.008,
lambda=0.50, log(T_b^dred)=6.4, and a constant SFR up to 5x10^8 yr ago. A good
fit to the CSLF in the SMC is then easily derived from the Z=0.004 models, with
a single choice of parameters, and a constant SFR over the entire significant
age interval. The results are consistent with the theoretical expectation that
the third dredge-up is more efficient at lower Zs.Comment: 22 pages with 15 figures, to appear in A&
Motif counting beyond five nodes
Counting graphlets is a well-studied problem in graph mining and social network analysis. Recently, several papers explored very simple and natural algorithms based on Monte Carlo sampling of Markov Chains (MC), and reported encouraging results. We show, perhaps surprisingly, that such algorithms are outperformed by color coding (CC) [2], a sophisticated algorithmic technique that we extend to the case of graphlet sampling and for which we prove strong statistical guarantees. Our computational experiments on graphs with millions of nodes show CC to be more accurate than MC; furthermore, we formally show that the mixing time of the MC approach is too high in general, even when the input graph has high conductance. All this comes at a price however. While MC is very efficient in terms of space, CC’s memory requirements become demanding when the size of the input graph and that of the graphlets grow. And yet, our experiments show that CC can push the limits of the state-of-the-art, both in terms of the size of the input graph and of that of the graphlets
The Limits of Popularity-Based Recommendations, and the Role of Social Ties
In this paper we introduce a mathematical model that captures some of the
salient features of recommender systems that are based on popularity and that
try to exploit social ties among the users. We show that, under very general
conditions, the market always converges to a steady state, for which we are
able to give an explicit form. Thanks to this we can tell rather precisely how
much a market is altered by a recommendation system, and determine the power of
users to influence others. Our theoretical results are complemented by
experiments with real world social networks showing that social graphs prevent
large market distortions in spite of the presence of highly influential users.Comment: 10 pages, 9 figures, KDD 201
Dust and Nebular Emission in Star Forming Galaxies
Star forming galaxies exhibit a variety of physical conditions, from
quiescent normal spirals to the most powerful dusty starbursts. In order to
study these complex systems, we need a suitable tool to analyze the information
coming from observations at all wavelengths. We present a new
spectro-photometric model which considers in a consistent way starlight as
reprocessed by gas and dust. We discuss preliminary results to interpret some
observed properties of VLIRGs.Comment: 8 pages, to be published in "The link between stars and cosmology",
26-30 March, 2001, Puerto Vallarta, Mexico, by Kluwer, eds. M. Chavez, A.
Bressan, A. Buzzoni, and D. Mayy
Formation of black holes in the pair-instability mass gap: evolution of a post-collision star
The detection of GW190521 by the LIGO-Virgo collaboration revealed the
existence of black holes (BHs) in the pair-instability (PI) mass gap. Here, we
investigate the formation of BHs in the PI mass gap via star -- star collisions
in young stellar clusters. To avoid PI, the stellar-collision product must have
a relatively small core and a massive envelope. We generate our initial
conditions from the outputs of a hydro-dynamical simulation of the collision
between a core helium burning star ( M) and a main-sequence
star ( M). The hydro-dynamical simulation allows us to take
into account the mass lost during the collision ( M) and to
build the chemical composition profile of the post-collision star. We then
evolve the collision product with the stellar evolution codes PARSEC and MESA.
We find that the post-collision star evolves through all the stellar burning
phases until core collapse, avoiding PI. At the onset of core collapse, the
post-collision product is a blue super-giant star. We estimate a total mass
loss of about 1 M during the post-collision evolution, due to stellar
winds and shocks induced by neutrino emission in a failed supernova. The final
BH mass is M. Therefore, we confirm that the collision
scenario is a suitable formation channel to populate the PI mass gap.Comment: 9 pages, 6 figures, comments welcome
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