1,155 research outputs found
X-Shooter study of accretion in -Ophiucus: very low-mass stars and brown dwarfs
We present new VLT/X-Shooter optical and NIR spectra of a sample of 17
candidate young low-mass stars and BDs in the rho-Ophiucus cluster. We derived
SpT and Av for all the targets, and then we determined their physical
parameters. All the objects but one have M*<0.6 Msun, and 8 have mass below or
close to the hydrogen-burning limit. Using the intensity of various emission
lines present in their spectra, we determined the Lacc and Macc for all the
objects. When compared with previous works targeting the same sample, we find
that, in general, these objects are not as strongly accreting as previously
reported, and we suggest that the reason is our more accurate estimate of the
photospheric parameters. We also compare our findings with recent works in
other slightly older star-forming regions to investigate possible differences
in the accretion properties, but we find that the accretion properties for our
targets have the same dependence on the stellar and substellar parameters as in
the other regions. This leads us to conclude that we do not find evidence for a
different dependence of Macc with M* when comparing low-mass stars and BDs.
Moreover, we find a similar small (1 dex) scatter in the Macc-M* relation as in
some of our recent works in other star-forming regions, and no significant
differences in Macc due to different ages or properties of the regions. The
latter result suffers, however, from low statistics and sample selection biases
in the current studies. The small scatter in the Macc-M* correlation confirms
that Macc in the literature based on uncertain photospheric parameters and
single accretion indicators, such as the Ha width, can lead to a scatter that
is unphysically large. Our studies show that only broadband spectroscopic
surveys coupled with a detailed analysis of the photospheric and accretion
properties allows us to properly study the evolution of disk accretion rates.Comment: accepted for publication in Astronomy & Astrophysics. Abstract
shortened to fit arXiv constraint
Opioid-Induced Constipation in Advanced Illness: Safety and Efficacy of Methylnaltrexone Bromide
Constipation, one of the major side effects of opiates used in palliative care, can impair patients’ quality of life to a point where it prevents sufficient pain control. Methylnaltrexone is a novel μ-receptor antagonist, which does not pass the blood brain barrier. It is licensed to treat opiate induced constipation for patients with advanced diseases. This review article presents an overview of pharmacology and safety of its application, evidence of its efficacy and economic aspects of its use in clinical practice. Available data are limited but strongly suggest that methylnaltrexone causes laxation in less than 24 hours for at least half of those patients over the first two weeks of usage without impairing pain control or causing serious adverse effects. To avoid danger of gastrointestinal perforation it is contraindicated for patients at risk for that complication. More research is needed to evaluate its long-term efficacy and economic impact
On the gas content of transitional disks: a VLT/X-Shooter study of accretion and winds
Transitional disks (TDs) are thought to be a late evolutionary stage of
protoplanetary disks with dust depleted inner regions. The mechanism
responsible for this depletion is still under debate. To constrain the models
it is mandatory to have a good understanding of the properties of the gas
content of the inner disk. Using X-Shooter broad band -UV to NIR- medium
resolution spectroscopy we derive the stellar, accretion, and wind properties
of a sample of 22 TDs. The analysis of these properties allows us to put strong
constraints on the gas content in a region very close to the star (<0.2 AU)
which is not accessible with any other observational technique. We fit the
spectra with a self-consistent procedure to derive simultaneously SpT,Av,and
mass accretion rates (Macc) of the targets. From forbidden emission lines we
derive the wind properties of the targets. Comparing our findings to values for
cTTs, we find that Macc and wind properties of 80% of the TDs in our sample,
which is strongly biased towards strongly accreting objects, are comparable to
those of cTTs. Thus, there are (at least) some TDs with Macc compatible with
those of cTTs, irrespective of the size of the dust inner hole.Only in 2 cases
Macc are much lower, while the wind properties are similar. We do not see any
strong trend of Macc with the size of the dust depleted cavity, nor with the
presence of a dusty optically thick disk close to the star. In the TDs in our
sample there is a gas rich inner disk with density similar to that of cTTs
disks. At least for some TDs, the process responsible of the inner disk
clearing should allow for a transfer of gas from the outer disk to the inner
region. This should proceed at a rate that does not depend on the physical
mechanism producing the gap seen in the dust emission and results in a gas
density in the inner disk similar to that of unperturbed disks around stars of
similar mass.Comment: Accepted on Astronomy & Astrophysics. Abstract shortened to fit arXiv
constraint
An extensive VLT/X-Shooter library of photospheric templates of pre-main sequence stars
Studies of the formation and evolution of young stars and their disks rely on
the knowledge of the stellar parameters of the young stars. The derivation of
these parameters is commonly based on comparison with photospheric template
spectra. Furthermore, chromospheric emission in young active stars impacts the
measurement of mass accretion rates, a key quantity to study disk evolution.
Here we derive stellar properties of low-mass pre-main sequence stars without
disks, which represent ideal photospheric templates for studies of young stars.
We also use these spectra to constrain the impact of chromospheric emission on
the measurements of mass accretion rates. The spectra in reduced,
flux-calibrated, and corrected for telluric absorption form are made available
to the community. We derive the spectral type for our targets by analyzing the
photospheric molecular features present in their VLT/X-Shooter spectra by means
of spectral indices and comparison of the relative strength of photospheric
absorption features. We also measure effective temperature, gravity, projected
rotational velocity, and radial velocity from our spectra by fitting them with
synthetic spectra with the ROTFIT tool. The targets have negligible extinction
and spectral type from G5 to M8. We perform synthetic photometry on the spectra
to derive the typical colors of young stars in different filters. We measure
the luminosity of the emission lines present in the spectra and estimate the
noise due to chromospheric emission in the measurements of accretion luminosity
in accreting stars. We provide a calibration of the photospheric colors of
young PMS stars as a function of their spectral type in a set of standard
broad-band optical and near-infrared filters. For stars with masses of ~
1.5Msun and ages of ~1-5 Myr, the chromospheric noise converts to a limit of
measurable mass accretion rates of ~ 3x10^-10 Msun/yr.Comment: Accepted for publication on Astronomy & Astrophysics. The spectra of
the photospheric templates will be uploaded to Vizier, but are already
available on request. Abstract shortened for arxiv constraints. Language
edited versio
Innovative Breakthroughs for the Treatment of Advanced and Metastatic Synovial Sarcoma
Simple Summary Synovial sarcoma (SyS) is a rare malignant soft tissue sarcoma bearing the chromosomal translocation t(X;18), which encodes the fusion oncoprotein SS18::SSX. More than 80% of the patients, mainly young in age, are initially diagnosed with localized disease with a 5-year survival rate of 70-80%. Metastatic relapse occurs in 50% of the cases. Advanced, unresectable, or metastatic disease shows a poor prognosis with a 5-year survival rate below 10%, representing an urgent clinical issue. This review will focus on: (i) current front-line therapies; (ii) alternative treatments in second line and beyond settings; and (iii) new epigenetic and immunological strategies. The improved understanding of the SyS molecular biology coupled with the recent development of innovative technologies, such as proteolysis targeting chimera (PROTAC) protein degraders or adoptive transfer of engineered immune cells, is offering new promising tools. Clinical trial results underline the need for accurate patient selection based on genetic and tumor immune microenvironment signatures. Synovial sarcoma (SyS) is a rare aggressive soft tissue sarcoma carrying the chromosomal translocation t(X;18), encoding the fusion transcript SS18::SSX. The fusion oncoprotein interacts with both BAF enhancer complexes and polycomb repressor complexes, resulting in genome-wide epigenetic perturbations and a unique altered genetic signature. Over 80% of the patients are initially diagnosed with localized disease and have a 5-year survival rate of 70-80%, but metastatic relapse occurs in 50% of the cases. Advanced, unresectable, or metastatic disease has a 5-year survival rate below 10%, representing a critical issue. This review summarizes the molecular mechanisms behind SyS and illustrates current treatments in front line, second line, and beyond settings. We analyze the use of immune check point inhibitors (ICI) in SyS that do not behave as an ICI-sensitive tumor, claiming the need for predictive genetic signatures and tumor immune microenvironment biomarkers. We highlight the clinical translation of innovative technologies, such as proteolysis targeting chimera (PROTAC) protein degraders or adoptive transfer of engineered immune cells. Adoptive cell transfer of engineered T-cell receptor cells targeting selected cancer/testis antigens has shown promising results against metastatic SyS in early clinical trials and further improvements are awaited from refinements involving immune cell engineering and tumor immune microenvironment enhancement
Gaia DR2 view of the Lupus V-VI clouds: the candidate diskless young stellar objects are mainly background contaminants
Extensive surveys of star-forming regions with Spitzer have revealed
populations of disk-bearing young stellar objects. These have provided crucial
constraints, such as the timescale of dispersal of protoplanetary disks,
obtained by carefully combining infrared data with spectroscopic or X-ray data.
While observations in various regions agree with the general trend of
decreasing disk fraction with age, the Lupus V and VI regions appeared to have
been at odds, having an extremely low disk fraction. Here we show, using the
recent Gaia data release 2 (DR2), that these extremely low disk fractions are
actually due to a very high contamination by background giants. Out of the 83
candidate young stellar objects (YSOs) in these clouds observed by Gaia, only
five have distances of 150 pc, similar to YSOs in the other Lupus clouds, and
have similar proper motions to other members in this star-forming complex. Of
these five targets, four have optically thick (Class II) disks. On the one
hand, this result resolves the conundrum of the puzzling low disk fraction in
these clouds, while, on the other hand, it further clarifies the need to
confirm the Spitzer selected diskless population with other tracers, especially
in regions at low galactic latitude like Lupus V and VI. The use of Gaia
astrometry is now an independent and reliable way to further assess the
membership of candidate YSOs in these, and potentially other, star-forming
regions.Comment: Accepted for publication on Astronomy&Astrophysics Letter
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
Simulation of the Melting Behavior of the UO2-Zircaloy Fuel Cladding System by Laser Heating
The current research focuses on laser melting and successive analysis of laboratory-scale uranium dioxide nuclear fuel samples in direct contact with Zircaloy-4 cladding. The goal was to characterize the melted and refrozen interfaces, in particular, observing local changes of the melting point and interdiffusion of fuel and cladding materials under inert gas (Ar), in the presence of hydrogen (Ar + 6% H2) or in air. Results obtained by laser heating UO2 pellets clad in a Zircaloy ring were interpreted in light of reference tests performed on pellets in which UO2 and zirconium were blended in a series of given compositions. The sample composition was analyzed by scanning electron microscopy to verify the occurrence of diffusion and segregation phenomena during the laser-heating cycles. Laser-melting experiments were performed on pellets of uranium dioxide clad in Zircaloy-4 rings to simulate the configuration of a light water reactor fuel rod. Under inert gas, the material interdiffusion resulted in consistent melting point depression (of up to 200 K below the melting point of pure UO2) at the interface between the fuel and the cladding. Experiments carried out in the presence of H2 displayed a more limited effect on the melting temperature, but they resulted in a remarkable embrittlement of the whole structure, with large fragmentation of the Zircaloy cladding. This was probably due to the formation of brittle and highly volatile Zr hydrides. The observed melting point decrease was even more pronounced (up to over 400 K) under air in uranium-rich samples, due to the change in the stoichiometry of UO2 in UO2+x
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