10,913 research outputs found
Galactic archaeology: IMF and depletion in the "thin disk"
We determine the initial mass function (IMF) of the ``thin disk'' by means of
a direct comparison between synthetic stellar samples (for different matching
choices of IMF, star formation rate SFR and depletion) and a complete
(volume-limited) sample of single stars near the galactic plane (|z| < 25pc),
selected from the Hipparcos catalogue (d < 100pc, M_v < +4.0). Our synthetic
samples are computed from first principles: stars are created with a random
distribution of mass M_* and age t_* which follow a given (genuine) IMF and
SFR(t_*). They are then placed in the HR diagram by means of a grid of
empirically well-tested evolution tracks. The quality of the match (synthetic
versus observed sample) is assessed by means of star counts in specific regions
in the HR diagram. 7 regions are located along the MS (main sequence, mass
sensitive), while 4 regions represent different evolved (age-sensitive) stages
of the stars. The counts of evolved stars, in particular, give valuable
evidence of the history of the ``thin disk'' (apparent) star formation and lift
the ambiguities in models restricted to MS star counts.Comment: 10 pages, 3 figures, submitted to MNRA
A critical test of empirical mass loss formulae applied to individual giants and supergiants
To test our new, improved Reimers-type mass-loss relation, given by Schroder
& Cuntz in 2005 (ApJL 630, L73), we take a look at the best studied galactic
giants and supergiants - particularly those with spatially resolved
circumstellar shells and winds, obtained directly or by means of a companion
acting as a probing light source. Together with well-known physical parameters,
the selected stars provide the most powerful and critical observational venues
for assessing the validity of parameterized mass-loss relations for cool winds
not driven by molecules or dust.
In this study, star by star, we compare our previously published relation
with the original Reimers relation (1975), the Lamers relation (1981), and the
two relations by de Jager and his group (1988, 1990). The input data,
especially the stellar masses, have been constrained using detailed stellar
evolution models. We find that only the relationship by Schroder & Cuntz
agrees, within the error bars, with the observed mass-loss rates for all giants
and supergiants.Comment: 11 pages, 5 Figs. accepted by Astronomy & Astrophysic
High-precision epsilon expansions of single-mass-scale four-loop vacuum bubbles
In this article we present a high-precision evaluation of the expansions in
\e=(4-d)/2 of (up to) four-loop scalar vacuum master integrals, using the
method of difference equations developed by S. Laporta. We cover the complete
set of `QED-type' master integrals, i.e. those with a single mass scale only
(i.e. ) and an even number of massive lines at each vertex.
Furthermore, we collect all that is known analytically about four-loop
`QED-type' masters, as well as about {\em all} single-mass-scale vacuum
integrals at one-, two- and three-loop order.Comment: 25 pages, uses axodraw.st
Revisiting the connection between magnetic activity, rotation period, and convective turnover time for main-sequence stars
The connection between stellar rotation, stellar activity, and convective
turnover time is revisited with a focus on the sole contribution of magnetic
activity to the Ca II H&K emission, the so-called excess flux, and its
dimensionless indicator R in relation to other stellar
parameters and activity indicators. Our study is based on a sample of 169
main-sequence stars with directly measured Mount Wilson S-indices and rotation
periods. The R values are derived from the respective S-indices
and related to the rotation periods in various -colour intervals. First,
we show that stars with vanishing magnetic activity, i.e. stars whose excess
flux index R approaches zero, have a well-defined,
colour-dependent rotation period distribution; we also show that this rotation
period distribution applies to large samples of cool stars for which rotation
periods have recently become available. Second, we use empirical arguments to
equate this rotation period distribution with the global convective turnover
time, which is an approach that allows us to obtain clear relations between the
magnetic activity related excess flux index R, rotation
periods, and Rossby numbers. Third, we show that the activity versus Rossby
number relations are very similar in the different activity indicators. As a
consequence of our study, we emphasize that our Rossby number based on the
global convective turnover time approaches but does not exceed unity even for
entirely inactive stars. Furthermore, the rotation-activity relations might be
universal for different activity indicators once the proper scalings are used.Comment: 13 pages, 7 figures, accepted for publication in A&
Four-Loop Decoupling Relations for the Strong Coupling
We compute the matching relation for the strong coupling constant within the
framework of QCD up to four-loop order. This allows a consistent five-loop
running (once the function is available to this order) taking into
account threshold effects. As a side product we obtain the effective coupling
of a Higgs boson to gluons with five-loop accuracy.Comment: 11 page
Three-dimensional physics and the pressure of hot QCD
We update Monte Carlo simulations of the three-dimensional SU(3) + adjoint
Higgs theory, by extrapolating carefully to the infinite volume and continuum
limits, in order to estimate the contribution of the infrared modes to the
pressure of hot QCD. The sum of infrared contributions beyond the known 4-loop
order turns out to be a smooth function, of a reasonable magnitude and specific
sign. Unfortunately, adding this function to the known 4-loop terms does not
improve the match to four-dimensional lattice data, in spite of the fact that
other quantities, such as correlation lengths, spatial string tension, or quark
number susceptibilities, work well within the same setup. We outline possible
ways to reduce the mismatch.Comment: 14 page
The spillover effects of monitoring:A field experiment
Published Online: March 13, 2015We provide field experimental evidence of the effects of monitoring in a context where productivity is multidimensional and only one dimension is monitored and incentivized. We hire students to do a job for us. The job consists of identifying euro coins. We study the direct effects of monitoring and penalizing mistakes on work quality and evaluate spillovers on unmonitored dimensions of productivity (punctuality and theft). We find that monitoring improves work quality only if incentives are harsh, but substantially reduces punctuality irrespectively of the associated incentives. Monitoring does not affect theft, with 10% of participants stealing overall. Our findings are supportive of a reciprocity mechanism, whereby workers retaliate for being distrusted
Recurrent proofs of the irrationality of certain trigonometric values
We use recurrences of integrals to give new and elementary proofs of the
irrationality of pi, tan(r) for all nonzero rational r, and cos(r) for all
nonzero rational r^2. Immediate consequences to other values of the elementary
transcendental functions are also discussed
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