126 research outputs found
Multiwavelength interferometric observations and modeling of circumstellar disks
We investigate the structure of the innermost region of three circumstellar
disks around pre-main sequence stars HD 142666, AS 205 N, and AS 205 S. We
determine the inner radii of the dust disks and, in particular, search for
transition objects where dust has been depleted and inner disk gaps have formed
at radii of a few tenths of AU up to several AU. We performed interferometric
observations with IOTA, AMBER, and MIDI in the infrared wavelength ranges
1.6-2.5um and 8-13um with projected baseline lengths between 25m and 102m. The
data analysis was based on radiative transfer simulations in 3D models of young
stellar objects (YSOs) to reproduce the spectral energy distribution and the
interferometric visibilities simultaneously. Accretion effects and disk gaps
could be considered in the modeling approach. Results from previous studies
restricted the parameter space. The objects of this study were spatially
resolved in the infrared wavelength range using the interferometers. Based on
these observations, a disk gap could be found for the source HD 142666 that
classifies it as transition object. There is a disk hole up to a radius of
R_in=0.30AU and a (dust-free) ring between 0.35AU and 0.80AU in the disk of HD
142666. The classification of AS 205 as a system of classical T Tauri stars
could be confirmed using the canonical model approach, i. e., there are no
hints of disk gaps in our observations.Comment: accepted by Astronomy & Astrophysic
Mid-infrared interferometric variability of DG Tau: implications for the inner-disk structure
Context. DG Tau is a low-mass pre-main sequence star, whose strongly
accreting protoplanetary disk exhibits a so-far enigmatic behavior: its
mid-infrared thermal emission is strongly time-variable, even turning the 10
m silicate feature from emission to absorption temporarily. Aims. We look
for the reason for the spectral variability at high spatial resolution and at
multiple epochs. Methods. We study the temporal variability of the mid-infrared
interferometric signal, observed with the VLTI/MIDI instrument at six epochs
between 2011 and 2014. We fit a geometric disk model to the observed
interferometric signal to obtain spatial information about the disk. We also
model the mid-infrared spectra by template fitting to characterize the profile
and time dependence of the silicate emission. We use physically motivated
radiative transfer modeling to interpret the mid-infrared interferometric
spectra. Results. The inner disk (r<1-3 au) spectra exhibit a 10 m
absorption feature related to amorphous silicate grains. The outer disk (r>1-3
au) spectra show a crystalline silicate feature in emission, similar to the
spectra of comet Hale-Bopp. The striking difference between the inner and outer
disk spectral feature is highly unusual among T Tauri stars. The mid-infrared
variability is dominated by the outer disk. The strength of the silicate
feature changed by more than a factor of two. Between 2011 and 2014 the
half-light radius of the mid-infrared-emitting region decreased from 1.15 to
0.7 au. Conclusions. For the origin of the absorption we discuss four possible
explanations: a cold obscuring envelope, an accretion heated inner disk, a
temperature inversion on the disk surface and a misaligned inner geometry. The
silicate emission in the outer disk can be explained by dusty material high
above the disk plane, whose mass can change with time, possibly due to
turbulence in the disk.Comment: 16 pages, 13 figure
V1647 Orionis: One Year into Quiescence
We present new optical, near-IR, and mid-IR observations of the young
eruptive variable star V1647 Orionis that went into outburst in late 2004 for
approximately two years. Our observations were taken one year after the star
had faded to its pre-outburst optical brightness and show that V1647Ori is
still actively accreting circumstellar material. We compare and contrast these
data with existing observations of the source from both pre-outburst and
outburst phases. From near-IR spectroscopy we identify photospheric absorption
features for the first time that allow us to constrain the classification of
the young star itself. Our best fit spectral type is M0+-2 sub-classes with a
visual extinction of 19+-2 magnitudes and a K-band veiling of rK~1.5+-0.2. We
estimate that V1647Ori has a quiescent bolometric luminosity of ~9.5Lsun and a
mass accretion rate of ~1.10^-6Msun yr^-1. Our derived mass and age, from
comparison with evolutionary models, are 0.8+-0.2 Msun and ~0.5Myrs,
respectively. The presence towards the star of shock excited optical [S II] and
[Fe II] emission as well as near-IR H2 and [Fe II] emission perhaps suggests
that a new Herbig-Haro flow is becoming visible close to the star.Comment: 22 pages, 19 Figures, accepted AJ 13 October 200
Low-Dose Rivaroxaban to Prevent Recurrences of Venous Thromboembolism in Cancer: A Real-Life Experience with a Focus on Female Patients
Background: The way in which to prevent recurrent venous thromboembolism (VTE) is an unmet clinical need in cancer patients. International guidelines only provide conditional recommendations and do not specify which anticoagulant and dose should be used. In the last 2 years, we have been using low-dose rivaroxaban to prevent VTE recurrences in cancer patients. The results of this real-life experience are presented in this study. Methods: All patients had cancer and had previously completed a cycle of at least six months of full-dose anticoagulation for the treatment of a VTE index event, before receiving a prescription of low-dose rivaroxaban (10 mg once daily) for secondary prevention of VTE. Effectiveness and safety of this therapeutic regimen were evaluated in terms of VTE recurrences, major bleedings (MB), and clinically relevant non-major bleedings (CRNMB). Results: The analysis included 106 cancer patients. Their median age was 60 years (IQR 50–69). Metastatic cancer was present in 87 patients (82.1%). Six patients (5.7%) had brain metastases. Over a median follow-up time of 333 days (IQR 156–484), the incidence of VTE recurrences was 3.8% (95%CI 1.0–9.4), with a recurrence rate of 4.0 per 100 person-years (95%CI 1.1–10.2). We observed no MB (0.0%) and three CRNMB (2.8%) (95%CI 0.6–8.1). Conclusions: Low-dose rivaroxaban is potentially effective and safe in cancer patients that require prevention of recurrent VTE. Large-scale studies are needed to confirm these findings
The 2008 outburst of EX Lup - silicate crystals in motion
EX Lup is the prototype of the EXor class of eruptive young stars. These
objects show optical outbursts which are thought to be related to runaway
accretion onto the star. In a previous study we observed in-situ crystal
formation in the disk of EX Lup during its latest outburst in 2008, making the
object an ideal laboratory to investigate circumstellar crystal formation and
transport. This outburst was monitored by a campaign of ground-based and
Spitzer Space Telescope observations. Here we modeled the spectral energy
distribution of EX Lup in the outburst from optical to millimeter wavelengths
with a 2D radiative transfer code. Our results showed that the shape of the SED
at optical wavelengths was more consistent with a single temperature blackbody
than a temperature distribution. We also found that this single temperature
component emitted 80-100 % of the total accretion luminosity. We concluded that
a thermal instability, the most widely accepted model of EXor outbursts, was
likely not the triggering mechanism of the 2008 outburst of EX Lup. Our
mid-infrared Spitzer spectra revealed that the strength of all crystalline
bands between 8 and 30 um increased right after the end of the outburst. Six
months later, however, the crystallinity in the 10 um silicate feature complex
decreased. Our modeling of the mid-infrared spectral evolution of EXLup showed
that, although vertical mixing should be stronger during the outburst than in
the quiescent phase, fast radial transport of crystals (e.g., by stellar/disk
wind) was required to reproduce the observed mid-infrared spectra.Comment: Accepted for publication in ApJ, 37 pages, 11 figures, 2 table
Fundamental Vibrational Transition of CO During the Outburst of EX Lupi in 2008
We report monitoring observations of the T Tauri star EX Lupi during its outburst in 2008 in the CO fundamental
band at 4.6–5.0 μm. The observations were carried out at the Very Large Telescope and the Subaru Telescope at six
epochs from 2008 April to August, covering the plateau of the outburst and the fading phase to a quiescent state.
The line flux of CO emission declines with the visual brightness of the star and the continuum flux at 5 μm, but
composed of two subcomponents that decay with different rates. The narrow-line emission (50 kms^(−1) in FWHM) is
near the systemic velocity of EX Lupi. These emission lines appear exclusively in v =1–0. The line widths translate
to a characteristic orbiting radius of 0.4 AU. The broad-line component (FWZI ~ 150 km s^(−1)) is highly excited up
to v ≤ 6. The line flux of the component decreases faster than the narrow-line emission. Simple modeling of the
line profiles implies that the broad-line emitting gas is orbiting around the star at 0.04–0.4 AU. The excitation state, the decay speed of the line flux, and the line profile indicate that the broad-line emission component is physically distinct from the narrow-line emission component, and more tightly related to the outburst event
Pulmonary Embolism in COVID-19 Patients: Which Diagnostic Algorithm Should We Use?
Introduction: Although pulmonary embolism (PE) is a frequent complication of the clinical course of COVID-19, there is a lack of explicit indications regarding the best algorithm for diagnosing PE in these patients. In particular, it is not clear how to identify subjects who should undergo computed tomography pulmonary angiography (CTPA), rather than simply X-ray and/or high resolution computed tomography (HRCT) of the chest. Methods: We retrospectively analyzed COVID-19 patients who presented to the Emergency Department (ED) of our University hospital with acute respiratory failure, or that developed acute respiratory failure during hospital stay, to determine how many of them had a theoretical indication to undergo CTPA for suspected PE according to current guidelines. Next, we looked for differences between patients who underwent CTPA and those who only underwent X-ray and/or HRCT of the chest. Finally, we determined whether patients with a confirmed diagnosis of PE had specific characteristics that made them different from those with a CTPA negative for PE. Results: Out of 93 subjects with COVID-19 and acute respiratory failure, 73 (78.4%) had an indication to undergo CTPA according to the revised Geneva and Wells scores and the PERC rule-out criteria, and 54 (58%) according to the YEARS algorithm. However, in contrast with these indications, only 28 patients (30.1%) underwent CTPA. Of note, they were not clinically different from those who underwent X-ray and/or HRCT of the chest. Among the 28 subjects who underwent CTPA, there were 10 cases of PE (35.7%). They were not clinically different from those with CTPA negative for PE. Conclusions: COVID-19 patients with acute respiratory failure undergo CTPA, X-ray of the chest, or HRCT without an established criterion. Nonetheless, when CTPA is performed, the diagnosis of PE is anything but rare. Validated tools for identifying COVID-19 patients who require CTPA for suspected PE are urgently needed
VLTI/MIDI atlas of disks around low- and intermediate-mass young stellar objects
Context. Protoplanetary disks show large diversity regarding their morphology
and dust composition. With mid-infrared interferometry the thermal emission of
disks can be spatially resolved, and the distribution and properties of the
dust within can be studied. Aims. Our aim is to perform a statistical analysis
on a large sample of 82 disks around low- and intermediate-mass young stars,
based on mid-infrared interferometric observations. We intend to study the
distribution of disk sizes, variability, and the silicate dust mineralogy.
Methods. Archival mid-infrared interferometric data from the MIDI instrument on
the VLTI are homogeneously reduced and calibrated. Geometric disk models are
used to fit the observations to get spatial information about the disks. An
automatic spectral decomposition pipeline is applied to analyze the shape of
the silicate feature. Results. We present the resulting data products in the
form of an atlas, containing N band correlated and total spectra, visibilities,
and differential phases. The majority of our data can be well fitted with a
continuous disk model, except for a few objects, where a gapped model gives a
better match. From the mid-infrared size--luminosity relation we find that
disks around T Tauri stars are generally colder and more extended with respect
to the stellar luminosity than disks around Herbig Ae stars. We find that in
the innermost part of the disks (~au) the silicate feature is
generally weaker than in the outer parts, suggesting that in the inner parts
the dust is substantially more processed. We analyze stellar multiplicity and
find that in two systems (AB Aur and HD 72106) data suggest a new companion or
asymmetric inner disk structure. We make predictions for the observability of
our objects with the upcoming MATISSE instrument, supporting the practical
preparations of future MATISSE observations of T Tauri stars.Comment: 54 pages, 14 figures, 6 tables, accepted for publication in A&
MATISSE, the VLTI mid-infrared imaging spectro-interferometer
Context. Optical interferometry is at a key development stage. The Very Large Telescope Interferometer (VLTI) has established a stable, robust infrastructure for long-baseline interferometry that is usable by general astronomical observers. The present second-generation instruments offer a wide wavelength coverage and improved performance. Their sensitivity and measurement accuracy lead to data and images of high reliability. Aims: We have developed the Multi AperTure mid-Infrared SpectroScopic Experiment (MATISSE) to access, for the first time, high resolution imaging in a wide spectral domain. Many front-line topics are explored with this new equipment, including: stellar activity and mass loss; planet formation and evolution in the gas and dust disks around young stars; and environment interaction and accretion processes around super massive black holes in active galactic nuclei. Methods: The instrument is a spectro-interferometric imager in the transmission windows called L, M, and N, from 2.8 to 13.0 microns, combining four optical beams from the VLTI's unit or auxiliary telescopes. Its concept, related observing procedure, data reduction, and calibration approach, is the product of 30 years of instrumental research and has benefitted from the expertise developed in the frame of the VLTI's first generation instruments. The instrument utilises a multi-axial beam combination that delivers spectrally dispersed fringes. The signal provides the following quantities at several spectral resolutions: photometric flux, coherent fluxes, visibilities, closure phases, wavelength differential visibilities and phases, and aperture-synthesis imaging. Results: This article provides an overview of the physical principle of the instrument and its functionalities. The motivation of the choice of the instrumental concept and the characteristics of the delivered signal are detailed with a description of the observing modes and of their performance limit. MATISSE offers four spectral resolutions in L&M bands, namely 30, 500, 1000 and 3400, and 30 and 220 in the N band, and it provides an angular resolution down to 3 mas for the shortest wavelengths. The MATISSE stand-alone sensitivity limits are 60 mJy in L and 300 mJy in N. The paper gives details of the sensitivity limits for the different measurables and their related precision criteria, considering telescope configurations and spectral resolutions. We also discuss the gain provided with the GRA4MAT fringe tracker. An ensemble of data and reconstructed images illustrate the first acquired key observations. Conclusions: The instrument has been in operation at Cerro Paranal, ESO, Chile, since 2018, and has been open for science use by the international community since April 2019. The first scientific results are being published now
Interferometric Observations of the Hierarchical Triple System Algol
Algol is a triple stellar system consisting of a close semidetached binary
orbited by a third object. Due to the disputed spatial orientation of the close
pair, the third body perturbation of this pair is a subject of much research.
In this study, we determine the spatial orientation of the close pair orbital
plane using the CHARA Array, a six-element optical/IR interferometer located on
Mount Wilson, and state-of-the-art e-EVN interferometric techniques. We find
that the longitude of the line of nodes for the close pair is
\Omega_1=48\degr\pm2\degr and the mutual inclination of the orbital planes of
the close and the wide pairs is 95\degr\pm3\degr. This latter value differs
by 5\degr from the formerly known 100\degr which would imply a very fast
inclination variation of the system, not borne out by the photometric
observations. We also investigated the dynamics of the system with numerical
integration of the equations of motions using our result as an initial
condition. We found large variations in the inclination of the close pair (its
amplitude \sim 170\degr) with a period of about 20 millennia. This result is
in good agreement with the photometrically observed change of amplitude in
Algol's primary minimum.Comment: ApJ, in press. This is the accepted version; will be changed with the
final version later (minor language corrections
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