| 1. |
- Abdo, A. A., et al.
(författare)
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The on-orbit calibration of the Fermi Large Area Telescope
- 2009
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Ingår i: Astroparticle physics. - : Elsevier BV. - 0927-6505 .- 1873-2852. ; 32:3-4, s. 193-219
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Tidskriftsartikel (refereegranskat)abstract
- The Large Area Telescope (LAT) on-board the Fermi Gamma-ray Space Telescope began its on-orbit operations on June 23, 2008. Calibrations, defined in a generic sense, correspond to synchronization of trigger signals, optimization of delays for latching data, determination of detector thresholds, gains and responses, evaluation of the perimeter of the South Atlantic Anomaly (SAA), measurements of live time, of absolute time, and internal and spacecraft boresight alignments. Here we describe on-orbit calibration results obtained using known astrophysical sources, galactic cosmic rays, and charge injection into the front-end electronics of each detector. Instrument response functions will be described in a separate publication. This paper demonstrates the stability of calibrations and describes minor changes observed since launch. These results have been used to calibrate the LAT datasets to be publicly released in August 2009.
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| 2. |
- Hait, A. S., et al.
(författare)
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Defects in LC3B2 and ATG4A underlie HSV2 meningitis and reveal a critical role for autophagy in antiviral defense in humans
- 2020
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Ingår i: Science Immunology. - : American Association for the Advancement of Science (AAAS). - 2470-9468. ; 5:54
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Tidskriftsartikel (refereegranskat)abstract
- Recurrent herpesvirus infections can manifest in different forms of disease, including cold sores, genital herpes, and encephalitis. There is an incomplete understanding of the genetic and immunological factors conferring susceptibility to recurrent herpes simplex virus 2 (HSV2) infection in the central nervous system (CNS). Here, we describe two adult patients with recurrent HSV2 lymphocytic Mollaret's meningitis that each carry a rare monoallelic variant in the autophagy proteins ATG4A or LC3B2. HSV2-activated autophagy was abrogated in patient primary fibroblasts, which also exhibited significantly increased viral replication and enhanced cell death. HSV2 antigen was captured in autophagosomes of infected cells, and genetic inhibition of autophagy by disruption of autophagy genes, including ATG4A and LC3B2, led to enhanced viral replication and cell death in primary fibroblasts and a neuroblastoma cell line. Activation of autophagy by HSV2 was sensitive to ultraviolet (UV) irradiation of the virus and inhibited in the presence of acyclovir, but HSV2-induced autophagy was independent of the DNA-activated STING pathway. Reconstitution of wild-type ATG4A and LC3B2 expression using lentiviral gene delivery or electroporation of in vitro transcribed mRNA into patient cells restored virus-induced autophagy and the ability to control HSV2 replication. This study describes a previously unknown link between defective autophagy and an inborn error of immunity that can lead to increased susceptibility to HSV2 infection, suggesting an important role for autophagy in antiviral immunity in the CNS.
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| 3. |
- Bonifacio, P., et al.
(författare)
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Using CO5BOLD models to predict the effects of granulation on colours .
- 2017
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Ingår i: MEMORIE della Società Astronomica Italiana. - 0037-8720 .- 1824-016X. ; 88
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Abstract.In order to investigate the effects of granulation on fluxes and colours, we computedthe emerging fluxes from the models in theCO5BOLDgrid with metallicities [M/H]=0.0,–1.0,–2.0 and –3.0. These fluxes have been used to compute colours in different photometric systems.We explain here how our computations have been performed and provide some results.Key words.Convection – Hydrodynamics - Stars: atmosphere
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| 4. |
- Bonifacio, P., et al.
(författare)
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Using the CIFIST grid of CO5BOLD 3D model atmospheres to study the effects of stellar granulation on photometric colours : I. Grids of 3D corrections in the UBVRI, 2MASS, HIPPARCOS, Gaia, and SDSS systems
- 2018
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 611:9
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Tidskriftsartikel (refereegranskat)abstract
- Context. The atmospheres of cool stars are temporally and spatially inhomogeneous due to the effects of convection. The influence of this inhomogeneity, referred to as granulation, on colours has never been investigated over a large range of effective temperatures and gravities.Aim. We aim to study, in a quantitative way, the impact of granulation on colours.Methods. We use the CIFIST (Cosmological Impact of the FIrst Stars) grid of CO5BOLD (COnservative COde for the COmputation of COmpressible COnvection in a BOx of L Dimensions, L = 2, 3) hydrodynamical models to compute emerging fluxes. These in turn are used to compute theoretical colours in the UBV RI, 2MASS, HIPPARCOS, Gaia and SDSS systems. Every CO5BOLD model has a corresponding one dimensional (1D) plane-parallel LHD (Lagrangian HydroDynamics) model computed for the same atmospheric parameters, which we used to define a “3D correction” that can be applied to colours computed from fluxes computed from any 1D model atmosphere code. As an example, we illustrate these corrections applied to colours computed from ATLAS models.Results. The 3D corrections on colours are generally small, of the order of a few hundredths of a magnitude, yet they are far from negligible. We find that ignoring granulation effects can lead to underestimation of Teff by up to 200 K and overestimation of gravity by up to 0.5 dex, when using colours as diagnostics. We have identified a major shortcoming in how scattering is treated in the current version of the CIFIST grid, which could lead to offsets of the order 0.01 mag, especially for colours involving blue and UV bands. We have investigated the Gaia and HIPPARCOS photometric systems and found that the (G − Hp), (BP − RP) diagram is immune to the effects of granulation. In addition, we point to the potential of the RVS photometry as a metallicity diagnostic.Conclusions. Our investigation shows that the effects of granulation should not be neglected if one wants to use colours as diagnostics of the stellar parameters of F, G, K stars. A limitation is that scattering is treated as true absorption in our current computations, thus our 3D corrections are likely an upper limit to the true effect. We are already computing the next generation of the CIFIST grid, using an approximate treatment of scattering.
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| 5. |
- Chiavassa, A., et al.
(författare)
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Optical interferometry and Gaia measurement uncertainties reveal the physics of asymptotic giant branch stars
- 2020
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Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 640
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Tidskriftsartikel (refereegranskat)abstract
- Context. Asymptotic giant branch (AGB) stars are cool luminous evolved stars that are well observable across the Galaxy and populating Gaia data. They have complex stellar surface dynamics, which amplifies the uncertainties on stellar parameters and distances.Aims. On the AGB star CL Lac, it has been shown that the convection-related variability accounts for a substantial part of the Gaia DR2 parallax error. We observed this star with the MIRC-X beam combiner installed at the CHARA interferometer to detect the presence of stellar surface inhomogeneities.Methods. We performed the reconstruction of aperture synthesis images from the interferometric observations at different wavelengths. Then, we used 3D radiative hydrodynamics (RHD) simulations of stellar convection with CO5BOLD and the post-processing radiative transfer code OPTIM3D to compute intensity maps in the spectral channels of MIRC-X observations. Then, we determined the stellar radius using the average 3D intensity profile and, finally, compared the 3D synthetic maps to the reconstructed ones focusing on matching the intensity contrast, the morphology of stellar surface structures, and the photocentre position at two different spectral channels, 1.52 and 1.70 mu m, simultaneously.Results. We measured the apparent diameter of CL Lac at two wavelengths (3.299 0.005 mas and 3.053 +/- 0.006 mas at 1.52 and 1.70 mu m, respectively) and recovered the radius (R = 307 +/- 41 and R = 284 +/- 38 R-circle dot) using a Gaia parallax. In addition to this, the reconstructed images are characterised by the presence of a brighter area that largely affects the position of the photocentre. The comparison with 3D simulation shows good agreement with the observations both in terms of contrast and surface structure morphology, meaning that our model is adequate for explaining the observed inhomogenities.Conclusions. This work confirms the presence of convection-related surface structures on an AGB star of Gaia DR2. Our result will help us to take a step forward in exploiting Gaia measurement uncertainties to extract the fundamental properties of AGB stars using appropriate RHD simulations.
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| 6. |
- Chiavassa, A., et al.
(författare)
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The extended atmosphere and circumstellar environment of the cool evolved star VX Sagittarii as seen by MATISSE star
- 2022
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 658
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Tidskriftsartikel (refereegranskat)abstract
- Context. VX Sgr is a cool, evolved, and luminous red star whose stellar parameters are difficult to determine, which affects its classification.Aims. We aim to spatially resolve the photospheric extent as well as the circumstellar environment.Methods. We used interferometric observations obtained with the MATISSE instrument in the L (3-4 mu m), M (4.5-5 mu m), and N (8-13 mu m) bands. We reconstructed monochromatic images using the MIRA software. We used 3D radiation-hydrodynamics simulations carried out with (COBOLD)-B-5 and a uniform disc model to estimate the apparent diameter and interpret the stellar surface structures. Moreover, we employed the radiative transfer codes OPTIM3D and RADMC3D to compute the spectral energy distribution for the L, M, and N bands, respectively.Results. MATISSE observations unveil, for the first time, the morphology of VX Sgr across the L, M, and N bands. The reconstructed images show a complex morphology with brighter areas whose characteristics depend on the wavelength probed. We measured the angular diameter as a function of the wavelength and showed that the photospheric extent in the L and M bands depends on the opacity through the atmosphere. In addition to this, we also concluded that the observed photospheric inhomogeneities can be interpreted as convection-related surface structures. The comparison in the N band yielded a qualitative agreement between the N-band spectrum and simple dust radiative transfer simulations. However, it is not possible to firmly conclude on the interpretation of the current data because of the difficulty in constraing the model parameters using the limited accuracy of our absolute flux calibration.Conclusions. MATISSE observations and the derived reconstructed images unveil the appearance of VX Sgr's stellar surface and circumstellar environment across a very large spectral domain for the first time.
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| 7. |
- Cukanovaite, E., et al.
(författare)
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Calibration of the mixing-length theory for structures of helium-dominated atmosphere white dwarfs
- 2019
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 490:1, s. 1010-1025
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Tidskriftsartikel (refereegranskat)abstract
- We perform a calibration of the mixing-length parameter at the bottom boundary of the convection zone for helium-dominated atmospheres of white dwarfs. This calibration is based on a grid of 3D DB (pure-helium) and DBA (helium-dominated with traces of hydrogen) model atmospheres computed with the CO5BOLD radiation-hydrodynamics code, and a grid of 1D DB and DBA envelope structures. The 3D models span a parameter space of hydrogen-to-helium abundances in the range -10.0 <= log (H/He) <= -2.0, surface gravities in the range 7.5 <= log g <= 9.0, and effective temperatures in the range 12 000K less than or similar to T-eff less than or similar to 34 000 K. The 1D envelopes cover a similar atmospheric parameter range, but are also calculated with different values of the mixing-length parameter, namely 0.4 <= ML2/alpha <= 1.4. The calibration is performed based on two definitions of the bottom boundary of the convection zone: the Schwarzschild and the zero convective flux boundaries. Thus, our calibration is relevant for applications involving the bulk properties of the convection zone including its total mass, which excludes the spectroscopic technique. Overall, the calibrated ML2/alpha is smaller than what is commonly used in evolutionary models and theoretical determinations of the blue edge of the instability strip for pulsating DB and DBA stars. With calibrated ML2/alpha we are able to deduce more accurate convection zone sizes needed for studies of planetary debris mixing and dredge-up of carbon from the core. We highlight this by calculating examples of metal-rich 3D DBAZ models and finding their convection zone masses. Mixing-length calibration represents the first step of in-depth investigations of convective overshoot in white dwarfs with helium-dominated atmospheres.
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| 8. |
- Tremblay, P. -E, et al.
(författare)
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Convective overshoot and metal accretion onto white dwarfs.
- 2017
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Ingår i: MEMORIE della Società Astronomica Italiana. - 0037-8720 .- 1824-016X. ; 88, s. 104-
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- A large fraction of white dwarfs host evolved planetary systems and show evidence of accretion from planetary debris. The accretion-diffusion model is the preferred method to understand the metal pollution in these otherwise hydrogen- and helium-rich white dwarf atmospheres. In this scenario, the accreted material first settles on the atmosphere. If the outer stellar layers are unstable to convection, the metals are then rapidly mixed up within the convection zone. In the classical 1D approach, it is generally assumed that the convection zone has a sharp bottom boundary, below which microscopic diffusion is unhampered and slowly removes metals from the visible layers. More realistic 3D radiation-hydrodynamics simulations of white dwarfs with CO5BOLD demonstrate, however, that the bottom of the convection zone does not have a sharp boundary, and that velocities decay exponentially below the unstable convective layers with a velocity scale height of the order of one pressure scale height. This has a potentially dramatic effect on the inferred mass of accreted materiel, hence on the chemical composition and size of planetary debris around white dwarfs.
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