1,064 research outputs found
Single top-quark production by strong and electroweak supersymmetric flavor-changing interactions at the LHC
(Abridged) We report on a complete study of the single top-quark production
by direct supersymmetric flavor-changing neutral-current (FCNC) processes at
the LHC. The total cross section for pp(gg)->t\bar{c}+\bar{t}c is computed at
the 1-loop order within the unconstrained Minimal Supersymmetric Standard Model
(MSSM). The present study extends the results of the supersymmetric strong
effects (SUSY-QCD), which were advanced by some of us in a previous work, and
includes the computation of the full supersymmetric electroweak corrections
(SUSY-EW). Our analysis of pp(gg)->t\bar{c}+\bar{t}c in the MSSM has been
performed in correspondence with the stringent low-energy constraints from b->s
gamma. In the most favorable scenarios, the SUSY-QCD contribution can give rise
to production rates of around 10^5 events per 100 fb^{-1} of integrated
luminosity. Furthermore, we show that there exist regions of the MSSM parameter
space where the SUSY-EW correction becomes sizeable. In the SUSY-EW favored
regions, one obtains lower, but still appreciable, event production rates that
can reach the 10^3 level for the same range of integrated luminosity. We study
also the possible reduction in the maximum event rate obtained from the full
MSSM contribution if we additionally include the constraints from
B^0_s-\bar{B}^0_s. In view of the fact that the FCNC production of heavy quark
pairs of different flavors is extremely suppressed in the SM, the detection of
a significant number of these events could lead to evidence of new physics --
of likely supersymmetric origin.Comment: LaTex, 35 pages, typos corrected. Version accepted in JHE
Discovery of a Low-Mass Brown Dwarf Companion of the Young Nearby Star G196-3
A substellar-mass object in orbit at about 300 astronomical units (AU) from
the young low-mass star G196-3 was detected by direct imaging. Optical and
infrared photometry and low- and intermediate-resolution spectroscopy of the
faint companion, hereafter referred to as G196-3B, confirms its cool atmosphere
and allows its mass to be estimated at 25^{+15}_{-10} Jupiter masses. The
separation between both objects and their mass ratio suggest the fragmentation
of a collapsing cloud as the most likely origin for G196-3B, but alternatively
it could have originated from a proto-planetary disc which has been dissipated.
Whatever the formation process was, the young age of the primary star (about
100 Myr) demonstrates that substellar companions can form in short time scales.Comment: Published in Science (13 Nov). One color figur
VLT X-shooter spectroscopy of the nearest brown dwarf binary
The aim of the project is to characterise both components of the nearest
brown dwarf sytem to the Sun, WISE J104915.57-531906.1 (=Luhman16AB) at optical
and near-infrared wavelengths. We obtained high signal-to-noise
intermediate-resolution (R~6000-11000) optical (600-1000 nm) and near-infrared
(1000-2480nm) spectra of each component of Luhman16AB, the closest brown dwarf
binary to the Sun, with the X-Shooter instrument on the Very Large Telescope.
We classify the primary and secondary of the Luhman16 system as L6-L7.5 and
T0+/-1, respectively, in agreement with previous measurements published in the
literature. We present measurements of the lithium pseudo-equivalent widths,
which appears of similar strength on both components (8.2+/-1.0 Angstroms and
8.4+/-1.5 Angstroms for the L and T components, respectively). The presence of
lithium (Lithium 7) in both components imply masses below 0.06 Msun while
comparison with models suggests lower limits of 0.04 Msun. The detection of
lithium in the T component is the first of its kind. Similarly, we assess the
strength of other alkali lines (e.g. pseudo-equivalent widths of 6-7 Angstroms
for RbI and 4-7 Angstroms for CsI) present in the optical and near-infrared
regions and compare with estimates for L and T dwarfs. We also derive effective
temperatures and luminosities of each component of the binary: -4.66+/-0.08 dex
and 1305(+180)(-135) for the L dwarf and -4.68+/-0.13 dex and 1320(+185)(-135)
for the T dwarf, respectively. Using our radial velocity determinations, the
binary does not appear to belong to any of the well-known moving group. Our
preliminary theoretical analysis of the optical and J-band spectra indicates
that the L- and T-type spectra can be reproduced with a single temperature and
gravity but different relative chemical abundances which impact strongly the
spectral energy distribution of L/T transition objects.Comment: 12 pages, 9 figure, 3 tables, accepted to A&
2MASS J154043.42-510135.7: a new addition to the 5 pc population
The aim of the project is to find the stars nearest to the Sun and to
contribute to the completion of the stellar and substellar census of the solar
neighbourhood. We identified a new late-M dwarf within 5 pc, looking for high
proper motion sources in the 2MASS-WISE cross-match. We collected astrometric
and photometric data available from public large-scale surveys. We complemented
this information with low-resolution optical and near-infrared spectroscopy
with instrumentation on the ESO NTT to confirm the nature of our candidate. We
also present a high-quality medium-resolution VLT/X-shooter spectrum covering
the 400 to 2500 nm wavelength range. We classify this new neighbour as an
M7.00.5 dwarf using spectral templates from the Sloan Digital Sky Survey
and spectral indices. Lithium absorption at 670.8 nm is not detected in the
X-shooter spectrum, indicating that the M7 dwarf is older than 600 Myr and more
massive than 0.06 M. We also derive a trigonometric distance of 4.4
pc, in agreement with the spectroscopic distance estimate, making
2MASS\,J154043.42510135.7 the nearest M7 dwarf to the Sun. This
trigonometric distance is somewhat closer than the 6 pc distance reported
by the ALLWISE team, who independently identified this object recently. This
discovery represents an increase of 25\% in the number of M7--M8 dwarfs already
known at distances closer than 8\,pc from our Sun. We derive a density of
\,=\,1.90.910\,pc for M7 dwarfs in the 8 pc
volume, a value similar to those quoted in the literature. This new ultracool
dwarf is among the 50 nearest systems to the Sun, demonstrating that our
current knowledge of the stellar census within the 5 pc sample remains
incomplete. 2M1540 represents a unique opportunity to search for extrasolar
planets around ultracool dwarfs due to its proximity and brightness.Comment: 8 pages, 5 figures. Acepted in Astronomy & Astrophysics (15/05/2005
Temporal changes of the flare activity of Proxima Cen
We study temporal variations of the emission lines of Halpha, Hepsilon, H and
K Ca II, D1 and D2 Na I, 4026 and 5876 A He I in the HARPS spectra of Proxima
Centauri across an extended time of 13.2 years, from May 27, 2004, to September
30, 2017.
Aims. We analyse the common behaviour and differences in the intensities and
profiles of different emission lines in flare and quiet modes of Proxima
activity.
Methods. We compare the pseudo-equivalent widths (pEW) and profiles of the
emission lines in the HARPS high-resolution (R ~ 115,000) spectra observed at
the same epochs.
Results. All emission lines show variability with a timescale of at least 10
min. The strength of all lines except He I 4026 A correlate with \Halpha.
During strong flares the `red asymmetry' appears in the Halpha emission line
indicating the infall of hot condensed matter into the chromosphere with
velocities greater than 100 km/s disturbing chromospheric layers. As a result,
the strength of the Ca II lines anti-correlates with Halpha during strong
flares. The He I lines at 4026 and 5876 A appear in the strong flares. The
cores of D1 and D2 Na I lines are also seen in emission. During the minimum
activity of Proxima Centauri, Ca II lines and Hepsilon almost disappear while
the blue part of the Na I emission lines is affected by the absorption in the
extending and condensing flows.
Conclusions. We see different behaviour of emission lines formed in the flare
regions and chromosphere. Chromosphere layers of Proxima Cen are likely heated
by the flare events; these layers are cooled in the `non-flare' mode. The
self-absorption structures in cores of our emission lines vary with time due to
the presence of a complicated system of inward and outward matter flows in the
absorbing layers.Comment: 22 pages, 12 Figures, accepted by A
Binary frequency of planet-host stars at wide separations: A new brown dwarf companion to a planet-host star
The aim of the project is to improve our knowledge on the multiplicity of
planet-host stars at wide physical separations.
We cross-matched approximately 6200 square degree area of the Southern sky
imaged by the Visible Infrared Survey Telescope for Astronomy (VISTA)
Hemisphere Survey (VHS) with the Two Micron All Sky Survey (2MASS) to look for
wide common proper motion companions to known planet-host stars. We
complemented our astrometric search with photometric criteria.
We confirmed spectroscopically the co-moving nature of seven sources out of
16 companion candidates and discarded eight, while the remaining one stays as a
candidate. Among these new wide companions to planet-host stars, we discovered
a T4.5 dwarf companion at 6.3 arcmin (~9000 au) from HIP70849, a K7V star which
hosts a 9 Jupiter mass planet with an eccentric orbit. We also report two new
stellar M dwarf companions to one G and one metal-rich K star. We infer stellar
and substellar binary frequencies for our complete sample of 37 targets of
5.4+/-3.8% and 2.7+/-2.7% (1 sigma confidence level), respectively, for
projected physical separations larger than ~60-160 au assuming the range of
distances of planet-host stars (24-75 pc). These values are comparable to the
frequencies of non planet-host stars. We find that the period-eccentricity
trend holds with a lack of multiple systems with planets at large
eccentricities (e > 0.2) for periods less than 40 days. However, the lack of
planets more massive than 2.5 Jupiter masses and short periods (<40 days)
orbiting single stars is not so obvious due to recent discoveries by
ground-based transit surveys and space missions.Comment: Accepted for publication in A&A, 13 pages, 5 figures, 3 tables,
optical spectra will be available at CDS Strasbour
Oxygen-vacancy-related giant permittivity and ethanol sensing response in SrTiO3-δ ceramics
The ethanol sensing properties of SrTiO3-δ (δ = 0.075 and 0.125) ceramics was analyzed by dielectric measurements. The ceramics were prepared by solid state reaction method followed by the creation of oxygen vacancies-δ, through a thermal activated process. The crystal symmetry, space group and unit cell dimensions were derived from the X-ray diffraction (XRD) data using FullProf software whereas grain's size distribution was assessed by scanning electron microscopy (SEM). The prepared samples have been analyzed by impedance spectroscopy over the frequency range from 100 Hz to 1 MHz and temperature range from 240 to 340 K. The dielectric properties of SrTiO3-δ ceramics showed a quite remarkable stability of giant permittivity (>104) as well as a low dielectric loss, which open ways for several applications such as over voltage protections of electronic devices. A low-frequency dielectric relaxation behavior was found, and the carriers for electrical conduction result from the first-ionization of oxygen vacancies. The conductivity and gas sensitivity of SrTiO3-δ-based sensors were investigated. Results demonstrated that the conductivity decreases after the introduction of the ethanol gas, and p-type semiconductor gas-sensing materials were obtained. Both characteristics present higher responses at lower optimal operating temperatures.publishe
Optical Linear Polarization of Late M- and L-Type Dwarfs
(Abridged). We report on the linear polarimetric observations in the Johnson
I filter of 44 M6-L7.5 ultracool dwarfs (2800-1400 K). Eleven (10 L and 1 M)
dwarfs appear to have significant linear polarization (P = 0.2-2.5%). We have
compared the M- and L-dwarf populations finding evidence for a larger frequency
of high I-band polarization in the coolest objects, supporting the presence of
significant amounts of dust in L-dwarfs. The probable polarizing mechanism is
related to the presence of heterogeneous dust clouds nonuniformly distributed
across the visible photospheres and the asymmetric shape of the objects. In
some young ultracool dwarfs, surrounding dusty disks may also yield
polarization. For polarimetric detections, a trend for slightly larger
polarization from L0 to L6.5 may be present in our data, suggesting changes in
the distribution of the grain properties, vertical height of the clouds,
metallicity, age, and rotation speed. One of our targets is the peculiar brown
dwarf (BD) 2MASS J2244+20 (L6.5), which shows the largest I-band polarization
degree. Its origin may lie in a surrounding dusty disk or rather large
photospheric dust grains. The M7 young BD CFHT-BD-Tau 4 and the L3.5 field
dwarf 2MASS J0036+18 were also observed in the Johnson R filter. Our data
support the presence of a circum(sub)stellar disk around the young accreting
BD. Our data also support a grain growth in the submicron regime in the visible
photosphere of J0036+18 (1900 K). The polarimetric data do not obviously
correlate with activity or projected rotational velocity. Three polarized
early- to mid-L dwarfs display I-band light curves with amplitudes below 10
mmag.Comment: Accepted for publication in ApJ (March 2005), 35 pages, 5 figure
Discovery of a wide companion near the deuterium burning mass limit in the Upper Scorpius association
We present the discovery of a companion near the deuterium burning mass limit
located at a very wide distance, at an angular separation of 4.6+/-0.1 arcsec
(projected distance of ~ 670 AU) from UScoCTIO108, a brown dwarf of the very
young Upper Scorpius association. Optical and near-infrared photometry and
spectroscopy confirm the cool nature of both objects, with spectral types of M7
and M9.5, respectively, and that they are bona fide members of the association,
showing low gravity and features of youth. Their masses, estimated from the
comparison of their bolometric luminosities and theoretical models for the age
range of the association, are 60+/-20 and 14^{+2}_{-8} MJup, respectively. The
existence of this object around a brown dwarf at this wide orbit suggests that
the companion is unlikely to have formed in a disk based on current planet
formation models. Because this system is rather weakly bound, they did not
probably form through dynamical ejection of stellar embryos.Comment: 10 pages, including 4 figures and 2 table
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