8,189 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
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&
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
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
Habitability of Super-Earth Planets around Main-Sequence Stars including Red Giant Branch Evolution: Models based on the Integrated System Approach
In a previous study published in Astrobiology, we focused on the evolution of
habitability of a 10 M_E super-Earth planet orbiting a star akin to the Sun.
This study was based on a concept of planetary habitability in accordance to
the integrated system approach that describes the photosynthetic biomass
production taking into account a variety of climatological, biogeochemical, and
geodynamical processes. In the present study, we pursue a significant
augmentation of our previous work by considering stars with zero-age main
sequence masses between 0.5 and 2.0 M_sun with special emphasis on models of
0.8, 0.9, 1.2 and 1.5 M_sun. Our models of habitability consider again
geodynamical processes during the main-sequence stage of these stars as well as
during their red giant branch evolution. Pertaining to the different types of
stars, we identify so-called photosynthesis-sustaining habitable zones (pHZ)
determined by the limits of biological productivity on the planetary surface.
We obtain various sets of solutions consistent with the principal possibility
of life. Considering that stars of relatively high masses depart from the
main-sequence much earlier than low-mass stars, it is found that the biospheric
life-span of super-Earth planets of stars with masses above approximately 1.5
M_sun is always limited by the increase in stellar luminosity. However, for
stars with masses below 0.9 M_sun, the life-span of super-Earths is solely
determined by the geodynamic time-scale. For central star masses between 0.9
and 1.5 M_sun, the possibility of life in the framework of our models depends
on the relative continental area of the super-Earth planet.Comment: 25 pages, 6 figures, 2 tables; submitted to: International Journal of
Astrobiolog
Scaling of the magnetic entropy and magnetization in YbRh_2(Si_{0.95}Ge_{0.05})_2
The magnetic entropy of YbRh_2(Si_{0.95}Ge_{0.05})_2 is derived from
low-temperature ( mK) specific heat measurements. Upon field-tuning
the system to its antiferromagnetic quantum critical point unique temperature
over magnetic field scaling is observed indicating the disintegration of heavy
quasiparticles. The field dependence of the entropy equals the temperature
dependence of the dc-magnetization as expected from the Maxwell relation. This
proves that the quantum-critical fluctuations affect the thermal and magnetic
properties in a consistent way.Comment: 6 pages, 2 figures, manuscript submitted to SCES2004 conferenc
Magnetic fields in single late-type giants in the Solar vicinity: How common is magnetic activity on the giant branches?
We present our first results on a new sample containing all single G,K and M
giants down to V = 4 mag in the Solar vicinity, suitable for
spectropolarimetric (Stokes V) observations with Narval at TBL, France. For
detection and measurement of the magnetic field (MF), the Least Squares
Deconvolution (LSD) method was applied (Donati et al. 1997) that in the present
case enables detection of large-scale MFs even weaker than the solar one (the
typical precision of our longitudinal MF measurements is 0.1-0.2 G). The
evolutionary status of the stars is determined on the basis of the evolutionary
models with rotation (Lagarde et al. 2012; Charbonnel et al., in prep.) and
fundamental parameters given by Massarotti et al. (1998). The stars appear to
be in the mass range 1-4 M_sun, situated at different evolutionary stages after
the Main Sequence (MS), up to the Asymptotic Giant Branch (AGB). The sample
contains 45 stars. Up to now, 29 stars are observed (that is about 64 % of the
sample), each observed at least twice. For 2 stars in the Hertzsprung gap, one
is definitely Zeeman detected. Only 5 G and K giants, situated mainly at the
base of the Red Giant Branch (RGB) and in the He-burning phase are detected.
Surprisingly, a lot of stars ascending towards the RGB tip and in early AGB
phase are detected (8 of 13 observed stars). For all Zeeman detected stars v
sin i is redetermined and appears in the interval 2-3 km/s, but few giants with
MF possess larger v sin i.Comment: 4 pages, 3 figures, Proceedings IAU Symposium No. 302, 201
The ultraviolet limit and sum rule for the shear correlator in hot Yang-Mills theory
We determine a next-to-leading order result for the correlator of the shear
stress operator in high-temperature Yang-Mills theory. The computation is
performed via an ultraviolet expansion, valid in the limit of small distances
or large momenta, and the result is used for writing operator product
expansions for the Euclidean momentum and coordinate space correlators as well
as for the Minkowskian spectral density. In addition, our results enable us to
confirm and refine a shear sum rule originally derived by Romatschke, Son and
Meyer.Comment: 16 pages, 2 figures. v2: small clarifications, one reference added,
published versio
Basal Chromospheric Flux and Maunder Minimum-type Stars: The quiet-Sun Chromosphere as a Universal Phenomenon
Aims: We demonstrate the universal character of the quiet-Sun chromosphere
among inactive stars (solar-type and giants). By assessing the main physical
processes, we shed new light on some common observational phenomena. Methods:
We discuss measurements of the solar Mt. Wilson S-index, obtained by the
Hamburg Robotic Telescope around the extreme minimum year 2009, and compare the
established chromospheric basal Ca II K line flux to the Mt. Wilson S-index
data of inactive ("flat activity") stars, including giants. Results: During the
unusually deep and extended activity minimum of 2009, the Sun reached S-index
values considerably lower than in any of its previously observed minima. In
several brief periods, the Sun coincided exactly with the S-indices of inactive
("flat", presumed Maunder Minimum-type) solar analogues of the Mt. Wilson
sample; at the same time, the solar visible surface was also free of any plages
or remaining weak activity regions. The corresponding minimum Ca II K flux of
the quiet Sun and of the presumed Maunder Minimum-type stars in the Mt. Wilson
sample are found to be identical to the corresponding Ca II K chromospheric
basal flux limit. Conclusions: We conclude that the quiet-Sun chromosphere is a
universal phenomenon among inactive stars. Its mixed-polarity magnetic field,
generated by a local, "fast" turbulent dynamo finally provides a natural
explanation for the minimal soft X-ray emission observed for inactive stars.
Given such a local dynamo also works for giant chromospheres, albeit on larger
length scales, i.e., l ~ R/g, with R and g as stellar radius and surface
gravity, respectively, the existence of giant spicular phenomena and the
guidance of mechanical energy toward the acceleration zone of cool stellar
winds along flux-tubes have now become traceable.Comment: 6 pages, 4 figures; Astronomy & Astrophysics (Research Note), in
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