321 research outputs found
Relationship of type III radio bursts with quasi-periodic pulsations in a solar flare
We studied a solar flare with pronounced quasi-periodic pulsations detected in the microwave, X-ray, and radio bands. We used correlation, Fourier, and wavelet analyses methods to examine the temporal fine structures and relationships between the time profiles in each wave band. We found that the time profiles of the microwaves, hard X-rays, and type III radio bursts vary quasi-periodically with a common period of 40 – 50 s. The average amplitude of the variations is high, above 30 % of the background flux level, and reaches 80 % after the flare maximum. We did not find this periodicity in either the thermal X-ray flux component or in the source size dynamics. Our findings indicate that the detected periodicity is probably associated with periodic dynamics in the injection of non-thermal electrons, which can be produced by periodic modulation of magnetic reconnection
Mass-luminosity relation for FGK main sequence stars: metallicity and age contributions
The stellar mass-luminosity relation (MLR) is one of the most famous
empirical "laws", discovered in the beginning of the 20th century. MLR is still
used to estimate stellar masses for nearby stars, particularly for those that
are not binary systems, hence the mass cannot be derived directly from the
observations. It's well known that the MLR has a statistical dispersion which
cannot be explained exclusively due to the observational errors in luminosity
(or mass). It is an intrinsic dispersion caused by the differences in age and
chemical composition from star to star. In this work we discuss the impact of
age and metallicity on the MLR. Using the recent data on mass, luminosity,
metallicity, and age for 26 FGK stars (all members of binary systems, with
observational mass-errors <= 3%), including the Sun, we derive the MLR taking
into account, separately, mass-luminosity, mass-luminosity-metallicity, and
mass-luminosity-metallicity-age. Our results show that the inclusion of age and
metallicity in the MLR, for FGK stars, improves the individual mass estimation
by 5% to 15%.Comment: 7 pages, 4 figures, 1 table, accepted in Astrophysics and Space
Scienc
Hot Subdwarfs in Resolved Binaries
In the last decade or so, there have been numerous searches for hot subdwarfs
in close binaries. There has been little to no attention paid to wide binaries
however. The advantages of understanding these systems can be many. The stars
can be assumed to be coeval, which means they have common properties. The
distance and metallicity, for example, are both unknown for the subdwarf
component, but may be determinable for the secondary, allowing other properties
of the subdwarf to be estimated. With this in mind, we have started a search
for common proper motion pairs containing a hot subdwarf component. We have
uncovered several promising candidate systems, which are presented here.Comment: 6 pages, 4 figures. Proceedings of The Fourth Meeting on Hot Subdwarf
Stars and Related Objects held in China, 20-24 July 2009. Accepted for
publication in Astrophysics and Space Scienc
The Empirical Mass-Luminosity Relation for Low Mass Stars
This work is devoted to improving empirical mass-luminosity relations and
mass-metallicity-luminosity relation for low mass stars. For these stars,
observational data in the mass-luminosity plane or the
mass-metallicity-luminosity space subject to non-negligible errors in all
coordinates with different dimensions. Thus a reasonable weight assigning
scheme is needed for obtaining more reliable results. Such a scheme is
developed, with which each data point can have its own due contribution.
Previous studies have shown that there exists a plateau feature in the
mass-luminosity relation. Taking into account the constraints from the
observational luminosity function, we find by fitting the observational data
using our weight assigning scheme that the plateau spans from 0.28 to 0.50
solar mass. Three-piecewise continuous improved mass-luminosity relations in K,
J, H and V bands, respectively, are obtained. The visual
mass-metallicity-luminosity relation is also improved based on our K band
mass-luminosity relation and the available observational metallicity data.Comment: 8 pages, 2 figures. Accepted for publication in Astrophysics & Space
Scienc
Interactions between brown-dwarf binaries and Sun-like stars
Several mechanisms have been proposed for the formation of brown dwarfs, but
there is as yet no consensus as to which -- if any -- are operative in nature.
Any theory of brown dwarf formation must explain the observed statistics of
brown dwarfs. These statistics are limited by selection effects, but they are
becoming increasingly discriminating. In particular, it appears (a) that brown
dwarfs that are secondaries to Sun-like stars tend to be on wide orbits, a\ga
100\,{\rm AU} (the Brown Dwarf Desert), and (b) that these brown dwarfs have a
significantly higher chance of being in a close (a\la 10\,{\rm AU}) binary
system with another brown dwarf than do brown dwarfs in the field. This then
raises the issue of whether these brown dwarfs have formed {\it in situ}, i.e.
by fragmentation of a circumstellar disc; or have formed elsewhere and
subsequently been captured. We present numerical simulations of the purely
gravitational interaction between a close brown-dwarf binary and a Sun-like
star. These simulations demonstrate that such interactions have a negligible
chance () of leading to the close brown-dwarf binary being captured by
the Sun-like star. Making the interactions dissipative by invoking the
hydrodynamic effects of attendant discs might alter this conclusion. However,
in order to explain the above statistics, this dissipation would have to favour
the capture of brown-dwarf binaries over single brown-dwarfs, and we present
arguments why this is unlikely. The simplest inference is that most brown-dwarf
binaries -- and therefore possibly also most single brown dwarfs -- form by
fragmentation of circumstellar discs around Sun-like protostars, with some of
them subsequently being ejected into the field.Comment: 10 pages, 8 figures, Accepted for publication in Astrophysics and
Space Scienc
The DA+dMe eclipsing binary EC13471-1258: its cup runneth over...just
EC13471-1258 is a detached eclipsing binary with Porb = 3h37m, comprising a
DA white dwarf and a dMe dwarf. Total eclipses of the white dwarf lasting 14
min, and a large amplitude ellipsoidal variation are seen in the light curve.
Flares from the dMe star occur regularly. Each star contributes roughly equal
amounts of light at 5500 Ang.
HST STIS spectra show strong Ly alpha with weak metal lines, and yield Teff =
14220 K, log g = 8.34, Z = 1/30th solar, K = 138 km/s and V sin i = 400 km/s
for the white dwarf. Optical spectra yield the spectral type (M3.5-4.0), Teff =
3100 K, Z = solar, K = 266 km/s and V sin i = 140 km/s for the dMe star. The H
alpha emission line comprises 2 or more components and implies that very weak
mass transfer is occurring. The dynamical solution also implies that the dMe
star just fills its Roche lobe. Accurate masses and radii for each star were
derived: the dMe values favour the Clemens et al. (1998) mass-radius relation.
The large rotational velocity of the white dwarf (400 km/s) suggests that the
system has transferred mass in the past so that it is presently a hibernating
cataclysmic variable. The metallicity contrast between the component stars
provides an opportunity for tests of diffusion theory.Comment: 25 pages, 18 figures, accepted for publication in MNRA
Building the cosmic distance scale: from Hipparcos to Gaia
Hipparcos, the first ever experiment of global astrometry, was launched by
ESA in 1989 and its results published in 1997 (Perryman et al., Astron.
Astrophys. 323, L49, 1997; Perryman & ESA (eds), The Hipparcos and Tycho
catalogues, ESA SP-1200, 1997). A new reduction was later performed using an
improved satellite attitude reconstruction leading to an improved accuracy for
stars brighter than 9th magnitude (van Leeuwen & Fantino, Astron. Astrophys.
439, 791, 2005; van Leeuwen, Astron. Astrophys. 474, 653, 2007).
The Hipparcos Catalogue provided an extended dataset of very accurate
astrometric data (positions, trigonometric parallaxes and proper motions),
enlarging by two orders of magnitude the quantity and quality of distance
determinations and luminosity calibrations. The availability of more than 20000
stars with a trigonometric parallax known to better than 10% opened the way to
a drastic revision of our 3-D knowledge of the solar neighbourhood and to a
renewal of the calibration of many distance indicators and age estimations. The
prospects opened by Gaia, the next ESA cornerstone, planned for launch in June
2013 (Perryman et al., Astron. Astrophys. 369, 339, 2001), are still much more
dramatic: a billion objects with systematic and quasi simultaneous astrometric,
spectrophotometric and spectroscopic observations, about 150 million stars with
expected distances to better than 10%, all over the Galaxy. All stellar
distance indicators, in very large numbers, will be directly measured,
providing a direct calibration of their luminosity and making possible detailed
studies of the impacts of various effects linked to chemical element
abundances, age or cluster membership. With the help of simulations of the data
expected from Gaia, obtained from the mission simulator developed by DPAC, we
will illustrate what Gaia can provide with some selected examples.Comment: 16 pages, 16 figures, Conference "The Fundamental Cosmic Distance
scale: State of the Art and the Gaia perspective, 3-6 May 2011, INAF,
Osservatorio Astronomico di Capodimonte, Naples. Accepted for publication in
Astrophysics & Space Scienc
The initial mass function : from Salpeter 1955 to 2005
Fifty years after Ed Salpeter's seminal paper, tremendous progress both on
the observational and theoretical sides allow a fairly accurate determination
of the Galactic IMF not only down to the hydrogen-burning limit but into the
brown dwarf domain. The present review includes the most recent observations of
low-mass stars and brown dwarfs to determine this IMF and the related Galactic
mass budget. The IMF definitely exhibits a similar behaviour in various
environments, disk, young and globular clusters, spheroid. Small scale
dissipation of large scale compressible MHD turbulence seems to be the
underlying triggering mechanism for star formation. Modern simulations of
compressible MHD turbulence yield an IMF consistent with the one derived from
observations.Comment: 10 pages. Invited review "IMF@50" in honor of E. Salpete
Microlensing as a probe of the Galactic structure; 20 years of microlensing optical depth studies
Microlensing is now a very popular observational astronomical technique. The
investigations accessible through this effect range from the dark matter
problem to the search for extra-solar planets. In this review, the techniques
to search for microlensing effects and to determine optical depths through the
monitoring of large samples of stars will be described. The consequences of the
published results on the knowledge of the Milky-Way structure and its dark
matter component will be discussed. The difficulties and limitations of the
ongoing programs and the perspectives of the microlensing optical depth
technique as a probe of the Galaxy structure will also be detailed.Comment: Accepted for publication in General Relativity and Gravitation.
General Relativity and Gravitation in press (2010) 0
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