1,245 research outputs found
A New Association of Post-T Tauri Stars Near The Sun
Observing ROSAT sources in 20 x 25 deg centered at the high latitude active
star ER Eri, we found evidences for a new young nearby association (~30Myr
at~60pc), the Horologium Association (HorA), formed by at least 10 probable and
6 possible members, some being Post-T Tauri stars. We examine several
requirements that characterize a young association and they, together, create a
strong evidence for the reality of the HorA. In fact, the Li line intensities
are between those of the oldest classical T Tauri stars and the ones of the
Local Association stars. The space velocities of the HorA relative to the Sun,
U= -9.5+/-1.0, V = -20.9 +/- 1.1, W = -2.1 +/- 1.9, are not far from those of
the Local Association. We suggest that some hotter and non-X-ray active stars,
with similar space velocities, could be massive members of the HorA, among
them, the nearby Be star Achernar. The maximum of the mass distribution
function of the HorA is around 0.8 solar masses. At its distance, the projected
size of the HorA, ~50 pc, would be larger than our surveyed area and many other
members could have been missed. We also observed 3 control regions, two at
northern and southern galactic latitudes and a third one in the known TW Hya
Association (TWA), and the properties and distribution of their young stars
strengthen the reality of the HorA. Contrary to the TWA, the only known
binaries in the HorA are 2 very wide systems. The HorA is much more isolated
from clouds and older than the TWA and could give some clues about the lifetime
of the disks around T Tauri stars. Actually, none of the proposed members is an
IRAS source indicating an advanced stage of the evolution of their accreting
disks. ER Eri itself was found to be a RS CVn-like system.Comment: 25 pages, 5 eps figures, to appear in Astron.
Many-flavor electron gas approach to electron-hole drops
A many-flavor electron gas (MFEG) is analyzed, such as could be found in a
multi-valley semiconductor or semimetal. Using the re-derived polarizability
for the MFEG an exact expression for the total energy of a uniform MFEG in the
many-flavor approximation is found; the interacting energy per particle is
shown to be -0.574447E_h a_0^3/4 m*^3/4 n^1/4 with E_h being the Hartree
energy, a_0 Bohr radius, and m^* particle effective mass. The short
characteristic length-scale of the MFEG motivates a local density
approximation, allowing a gradient expansion in the energy density, and the
expansion scheme is applied to electron-hole drops, finding a new form for the
density profile and its surface scaling properties.Comment: 11 pages, 5 figure
Incoherent non-Fermi liquid scattering in a Kondo lattice
One of the most notorious non-Fermi liquid properties of both archetypal
heavy-fermion systems [1-4] and the high-Tc copper oxide superconductors [5] is
an electrical resistivity that evolves linearly with temperature, T. In the
heavy-fermion superconductor CeCoIn5 [5], this linear behaviour was one of the
first indications of the presence of a zero-temperature instability, or quantum
critical point. Here, we report the observation of a unique control parameter
of T-linear scattering in CeCoIn5, found through systematic chemical
substitutions of both magnetic and non-magnetic rare-earth, R, ions into the Ce
sub-lattice. We find that the evolution of inelastic scattering in Ce1-xRxCoIn5
is strongly dependent on the f-electron configuration of the R ion, whereas two
other key properties -- Cooper-pair breaking and Kondo-lattice coherence -- are
not. Thus, T-linear resistivity in CeCoIn5 is intimately related to the nature
of incoherent scattering centers in the Kondo lattice, which provides insight
into the anomalous scattering rate synonymous with quantum criticality [7].Comment: 4 pages, 3 figures (published version
The Gaia -ESO Survey : Empirical determination of the precision of stellar radial velocities and projected rotation velocities
Context. The Gaia-ESO Survey (GES) is a large public spectroscopic survey at the European Southern Observatory Very Large Telescope. Aims. A key aim is to provide precise radial velocities (RVs) and projected equatorial velocities (v sin i) for representative samples of Galactic stars, which will complement information obtained by the Gaia astrometry satellite. Methods. We present an analysis to empirically quantify the size and distribution of uncertainties in RV and v sin i using spectra from repeated exposures of the same stars. Results. We show that the uncertainties vary as simple scaling functions of signal-to-noise ratio (S/N) and v sin i, that the uncertainties become larger with increasing photospheric temperature, but that the dependence on stellar gravity, metallicity and age is weak. The underlying uncertainty distributions have extended tails that are better represented by Student's t-distributions than by normal distributions. Conclusions. Parametrised results are provided, which enable estimates of the RV precision for almost all GES measurements, and estimates of the v sin i precision for stars in young clusters, as a function of S/N, v sin i and stellar temperature. The precision of individual high S/N GES RV measurements is 0.22-0.26 kms-1, dependent on instrumental configuration.Peer reviewedFinal Accepted Versio
Precise study of the resonance at Q0=(1,0,0) in URu2Si2
New inelastic neutron scattering experiments have been performed on URu2Si2
with special focus on the response at Q0=(1,0,0), which is a clear signature of
the hidden order (HO) phase of the compound. With polarized inelastic neutron
experiments, it is clearly shown that below the HO temperature (T0 = 17.8 K) a
collective excitation (the magnetic resonance at E0 \approx 1.7 meV) as well as
a magnetic continuum co-exist. Careful measurements of the temperature
dependence of the resonance lead to the observation that its position shifts
abruptly in temperature with an activation law governed by the partial gap
opening and that its integrated intensity has a BCS-type temperature
dependence. Discussion with respect to recent theoretical development is made
Late-Type Near-Contact Eclipsing Binary [HH97] FS Aur-79
The secondary photometric standard star #79 for the FS Aur field (Henden &
Honeycutt 1997) designated as [HH97] FS Aur-79 (GSC 1874 399) is a short period
(0.2508 days) eclipsing binary whose light curve is a combination of the
Lyr and BY Dra type variables. High signal-to-noise multi-color
photometry were obtained using the USNO 1-m telescope. These light curves show
asymmetry at quadrature phases (O'Connell effect), which can be modeled with
the presence of star spots. A low resolution spectrum obtained with the 3.5-m
WIYN telescope at orbital phase 0.76 is consistent with a spectral type of dK7e
and dM3e. A radial velocity curve for the primary star was constructed using
twenty-four high resolution spectra from the 9.2 m HET. Spectra show H-alpha
and H-beta in emission confirming chromospheric activity and possibly the
presence of circumstellar material. Binary star models that simultaneously fit
the U, B, V, R and RV curves are those with a primary star of mass 0.59+-0.02
Msun, temperature 4100+-25 K, mean radius of 0.67 Rsun, just filling its Roche
lobe and a secondary star of mass 0.31+-0.09 Msun, temperature 3425+-25 K, mean
radius of 0.48 Rsun, just within its Roche lobe. An inclination angle of 83+-2
degrees with a center of mass separation of 1.62 Rsun is also derived. Star
spots, expected for a rotation period of less than a day, had to be included in
the modeling to fit the O'Connell effect
The Gaia-ESO Survey: radial metallicity gradients and age-metallicity relation of stars in the Milky Way disk
We study the relationship between age, metallicity, and alpha-enhancement of
FGK stars in the Galactic disk. The results are based upon the analysis of
high-resolution UVES spectra from the Gaia-ESO large stellar survey. We explore
the limitations of the observed dataset, i.e. the accuracy of stellar
parameters and the selection effects that are caused by the photometric target
preselection. We find that the colour and magnitude cuts in the survey suppress
old metal-rich stars and young metal-poor stars. This suppression may be as
high as 97% in some regions of the age-metallicity relationship. The dataset
consists of 144 stars with a wide range of ages from 0.5 Gyr to 13.5 Gyr,
Galactocentric distances from 6 kpc to 9.5 kpc, and vertical distances from the
plane 0 < |Z| < 1.5 kpc. On this basis, we find that i) the observed
age-metallicity relation is nearly flat in the range of ages between 0 Gyr and
8 Gyr; ii) at ages older than 9 Gyr, we see a decrease in [Fe/H] and a clear
absence of metal-rich stars; this cannot be explained by the survey selection
functions; iii) there is a significant scatter of [Fe/H] at any age; and iv)
[Mg/Fe] increases with age, but the dispersion of [Mg/Fe] at ages > 9 Gyr is
not as small as advocated by some other studies. In agreement with earlier
work, we find that radial abundance gradients change as a function of vertical
distance from the plane. The [Mg/Fe] gradient steepens and becomes negative. In
addition, we show that the inner disk is not only more alpha-rich compared to
the outer disk, but also older, as traced independently by the ages and Mg
abundances of stars.Comment: accepted for publication in A&
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