792 research outputs found
Magnetic properties of the strongly interacting matter
We study the magnetic properties of the strongly interacting matter using Lattice QCD simulations. The QCD medium shows a paramagnetic behavior in the range of temperatures 100–400 MeV, with a sharp increase of the magnetic susceptibility above the deconfinement temperature. We expect a significant magnetic contribution to the pressure of the system in non-central heavy-ion collisions
Anisotropy of the QQ potential in a magnetic field
We study how the static quark-antiquark potential for Nf = 2+1 QCD at the physical point gets modified by the presence of a constant and uniform magnetic field. We observe an anisotropy to appear in the potential: it gets steeper in the directions transverse to the magnetic field than in the longitudinal one. By comparing to the case with zero magnetic field, we show that the string tension increases
(decreases) in the perpendicular (parallel) direction, while the absolute value of the Coulomb coupling and the Sommer parameter show the opposite behavior
Thyroid-specific transcription factors control Hex promoter activity
The homeobox-containing gene Hex is expressed in several cell types, including thyroid follicular cells, in which it regulates the transcription of tissue-specific genes. In this study the regulation of Hex promoter activity was investigated. Using co-transfection experiments, we demonstrated that the transcriptional activity of the Hex gene promoter in rat thyroid FRTL-5 cells is ∼10-fold greater than that observed in HeLa and NIH 3T3 cell lines (which do not normally express the Hex gene). To identify the molecular mechanisms underlying these differences, we evaluated the effect of the thyroid-specific transcription factor TTF-1 on the Hex promoter activity. TTF-1 produced 3-4-fold increases in the Hex promoter activity. Gel-retardation assays and mutagenesis experiments revealed the presence of functionally relevant TTF-1 binding sites in the Hex promoter region. These in vitro data may also have functional relevance in vivo, since a positive correlation between TTF-1 and Hex mRNAs was demonstrated in human thyroid tissues by means of RT-PCR analysis. The TTF-1 effect, however, is not sufficient to explain the difference in Hex promoter activity between FRTL-5 and cells that do not express the Hex gene. For this reason, we tested whether Hex protein is able to activate the Hex promoter. Indeed, co-transfection experiments indicate that Hex protein is able to increase the activity of its own promoter in HeLa cells ∼4-fold. TTF-1 and Hex effects are additive: when transfected together in HeLa cells, the Hex promoter activity is increased 6-7-fold. Thus, the contemporary presence of both TTF-1 and Hex could be sufficient to explain the higher transcriptional activity of the Hex promoter in thyroid cells with respect to cell lines that do not express the Hex gene. These findings demonstrate the existence of direct cross-regulation between thyroid-specific transcription factors
UVES/VLT high resolution absorption spectroscopy of the GRB080330 afterglow: a study of the GRB host galaxy and intervening absorbers
We study the Gamma Ray Burst (GRB) environment and intervening absorbers by
analyzing the optical absorption features produced by gas surrounding the GRB
or along its line of sight. We analyzed high resolution spectroscopic
observations (R=40000, S/N=3 - 6) of the optical afterglow of GRB080330, taken
with UVES at the VLT ~ 1.5 hours after the GRB trigger. The spectrum
illustrates the complexity of the ISM of the GRB host galaxy at z = 1.51 which
has at least four components in the main absorption system. We detect strong
FeII, SiII, and NiII excited absorption lines associated with the bluemost
component only. In addition to the host galaxy, at least two more absorbers
lying along the line of sight to the afterglow have been detected in the
redshift range 0.8 < z < 1.1, each exhibiting MgII absorption. For the bluemost
component in the host galaxy, we derive information about its distance from the
site of the GRB explosion. We do so by assuming that the excited absorption
lines are produced by indirect UV pumping, and compare the data with a time
dependent photo-excitation code. The distance of this component is found to be
280+40-50 pc, which is lower than found for other GRBs (1 - 6 kpc). We identify
two additional MgII absorbers, one of them with a rest frame equivalent width
larger than 1A. The distance between the GRB and the absorber measured in this
paper confirms that the power of the GRB radiation can influence the conditions
of the interstellar medium up to a distance of at least several hundred pc. For
the intervening absorbers, we confirm the trend that on average one strong
intervening system is found per afterglow, as has been noted in studies
exhibiting an excess of strong MgII absorbers along GRB sightlines compared to
quasars.Comment: 8 Pages, 7 ps figures, A&A in pres
X-Shooter spectroscopy of young stellar objects: II. Impact of chromospheric emission on accretion rate estimates
Context. The lack of knowledge of photospheric parameters and the level of
chromospheric activity in young low-mass pre-main sequence stars introduces
uncertainties when measuring mass accretion rates in accreting (Class II) Young
Stellar Objects. A detailed investigation of the effect of chromospheric
emission on the estimates of mass accretion rate in young low-mass stars is
still missing. This can be undertaken using samples of young diskless (Class
III) K and M-type stars. Aims. Our goal is to measure the chromospheric
activity of Class III pre main sequence stars to determine its effect on the
estimates of accretion luminosity (Lacc) and mass accretion rate (Macc) in
young stellar objects with disks. Methods. Using VLT/X-Shooter spectra we have
analyzed a sample of 24 non-accreting young stellar objects of spectral type
between K5 and M9.5. We identify the main emission lines normally used as
tracers of accretion in Class II objects, and we determine their fluxes in
order to estimate the contribution of the chromospheric activity to the line
luminosity. Results. We have used the relationships between line luminosity and
accretion luminosity derived in the literature for Class II objects to evaluate
the impact of chromospheric activity on the accretion rate measurements. We
find that the typical chromospheric activity would bias the derived accretion
luminosity by Lacc,noise< 10-3Lsun, with a strong dependence with the Teff of
the objects. The noise on Macc depends on stellar mass and age, and the typical
values of log(Macc,noise) range between -9.2 to -11.6Msun/yr. Conclusions.
Values of Lacc< 10-3Lsun obtained in accreting low-mass pre main sequence stars
through line luminosity should be treated with caution as the line emission may
be dominated by the contribution of chromospheric activity.Comment: accepted for publication in Astronomy & Astrophysic
X-Shooter spectroscopy of young stellar objects: IV -- Accretion in low-mass stars and sub-stellar objects in Lupus
We present X-Shooter/VLT observations of a sample of 36 accreting low-mass
stellar and sub-stellar objects (YSOs) in the Lupus star forming region,
spanning a range in mass from ~0.03 to ~1.2Msun, but mostly with 0.1Msun <
Mstar < 0.5Msun. Our aim is twofold: firstly, analyse the relationship between
excess-continuum and line emission accretion diagnostics, and, secondly, to
investigate the accretion properties in terms of the physical properties of the
central object. The accretion luminosity (Lacc), and from it the accretion rate
(Macc), is derived by modelling the excess emission, from the UV to the
near-IR, as the continuum emission of a slab of hydrogen. The flux and
luminosity (Ll) of a large number of emission lines of H, He, CaII, etc.,
observed simultaneously in the range from ~330nm to 2500nm, were computed. The
luminosity of all the lines is well correlated with Lacc. We provide empirical
relationships between Lacc and the luminosity of 39 emission lines, which have
a lower dispersion as compared to previous relationships in the literature. Our
measurements extend the Pab and Brg relationships to Lacc values about two
orders of magnitude lower than those reported in previous studies. We confirm
that different methodologies to measure Lacc and Macc yield significantly
different results: Ha line profile modelling may underestimate Macc by 0.6 to
0.8dex with respect to Macc derived from continuum-excess measures. Such
differences may explain the likely spurious bi-modal relationships between Macc
and other YSOs properties reported in the literature. We derive Macc in the
range 2e-12 -- 4e-8 Msun/yr and conclude that Macc is proportional to
Mstar^1.8(+/-0.2), with a dispersion lower by a factor of about 2 than in
previous studies. A number of properties indicate that the physical conditions
of the accreting gas are similar over more than 5 orders of magnitude in Macc
X-Shooter spectroscopy of young stellar objects III. Photospheric and chromospheric properties of Class III objects
We analyzed X-Shooter/VLT spectra of 24 ClassIII sources from three nearby
star-forming regions (sigmaOrionis, LupusIII, and TWHya). We determined the
effective temperature, surface gravity, rotational velocity, and radial
velocity by comparing the observed spectra with synthetic BT-Settl model
spectra. We investigated in detail the emission lines emerging from the stellar
chromospheres and combined these data with archival X-ray data to allow for a
comparison between chromospheric and coronal emissions. Both X-ray and Halpha
luminosity as measured in terms of the bolometric luminosity are independent of
the effective temperature for early-M stars but decline toward the end of the
spectral M sequence. For the saturated early-M stars the average emission level
is almost one dex higher for X-rays than for Halpha: log(L_x/L_bol) = -2.85 +-
0.36 vs. log(L_Halpha/L_bol) = -3.72 +- 0.21. When all chromospheric emission
lines (including the Balmer series up to H11, CaII HK, the CaII infrared
triplet, and several HeI lines) are summed up the coronal flux still dominates
that of the chromosphere, typically by a factor 2-5. Flux-flux relations
between activity diagnostics that probe different atmospheric layers (from the
lower chromosphere to the corona) separate our sample of active pre-main
sequence stars from the bulk of field M dwarfs studied in the literature. Flux
ratios between individual optical emission lines show a smooth dependence on
the effective temperature. The Balmer decrements can roughly be reproduced by
an NLTE radiative transfer model devised for another young star of similar age.
Future, more complete chromospheric model grids can be tested against this data
set.Comment: accepted for publication in Astronomy & Astrophysic
Realization of the farad from the dc quantum Hall effect with digitally-assisted impedance bridges
A new traceability chain for the derivation of the farad from dc quantum Hall
effect has been implemented at INRIM. Main components of the chain are two new
coaxial transformer bridges: a resistance ratio bridge, and a quadrature
bridge, both operating at 1541 Hz. The bridges are energized and controlled
with a polyphase direct-digital-synthesizer, which permits to achieve both main
and auxiliary equilibria in an automated way; the bridges and do not include
any variable inductive divider or variable impedance box. The relative
uncertainty in the realization of the farad, at the level of 1000 pF, is
estimated to be 64E-9. A first verification of the realization is given by a
comparison with the maintained national capacitance standard, where an
agreement between measurements within their relative combined uncertainty of
420E-9 is obtained.Comment: 15 pages, 11 figures, 3 table
Confinement and Chiral Symmetry
We illustrate why color deconfines when chiral symmetry is restored in gauge
theories with quarks in the fundamental representation, and while these
transitions do not need to coincide when quarks are in the adjoint
representation, entanglement between them is still present.Comment: 4 pages, 1 figure, proceedings of Quark Matter 200
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