| 1. |
- Abdo, A. A., et al.
(författare)
-
The on-orbit calibration of the Fermi Large Area Telescope
- 2009
-
Ingår i: Astroparticle physics. - : Elsevier BV. - 0927-6505 .- 1873-2852. ; 32:3-4, s. 193-219
-
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.
|
|
| 2. |
- Arroyo-Torres, B., et al.
(författare)
-
VLTI/AMBER studies of the atmospheric structure and fundamental parameters of red giant and supergiant stars
- 2015
-
Ingår i: Why Galaxies Care about AGB Stars III: A Closer Look in Space and Time. Proceedings of ASP Conference Series. - 9781583818794 ; 497, s. 91-96
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We present recent near-IR interferometric studies of red giant and supergiant stars, which are aimed at obtaining information on the structure of the atmospheric layers and at constraining the fundamental parameters of these objects. The observed visibilities of the red supergiants (RSGs) and also of one red giant indicate large extensions of the molecular layers, as those previously observed for Mira stars. These extensions are not predicted by hydrostatic PHOENIX model atmospheres, hydrodynamical (RHD) simulations of stellar convection, or self-excited pulsation models. All these models based on parameters of RSGs lead to atmospheric structures that are too compact compared to our observations. We discuss how alternative processes might explain the atmospheric extensions for these objects. As the continuum appears to be largely free of contamination by molecular layers, we can estimate reliable angular Rosseland radii of our stars. Together with distances and bolometric fluxes, we estimate the effective temperatures and luminosities of our targets, locate them in the HR diagram, and compare their positions to recent evolutionary tracks.
|
|
| 3. |
- Chiavassa, A., et al.
(författare)
-
Optical interferometry and Gaia measurement uncertainties reveal the physics of asymptotic giant branch stars
- 2020
-
Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 640
-
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.
|
|
| 4. |
- Chiavassa, A, et al.
(författare)
-
Radiative hydrodynamics simulations of red supergiant stars I. interpretation of interferometric observations
- 2009
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 506:3, s. 1351-1365
-
Tidskriftsartikel (refereegranskat)abstract
- Context. It has been proposed that convection in red supergiant (RSG) stars produces large-scale granules causing observable surface inhomogeneities. This convection is also extremely vigorous and is suspected to be one of the main causes of mass-loss in RSGs. It should thus be understood in detail. Evidence has accumulated of asymmetries in the photospheres of RSGs, but detailed studies of granulation are still lacking. Interferometric observations provide an innovative way of addressing this question, but they are still often interpreted using smooth symmetrical limb-darkened intensity distributions, or simple, spotted, ad hoc models. Aims. We explore the impact of the granulation on visibility curves and closure phases using the radiative transfer code OPTIM3D. We simultaneously assess how 3D simulations of convection in RSG with (COBOLD)-B-5 can be tested by comparing with these observations. Methods. We use 3D radiative hydrodynamical (RHD) simulations of convection to compute intensity maps at various wavelengths and time, from which we derive interferometric visibility amplitudes and phases. We study their behaviour with time, position angle, and wavelength, and compare them to observations of the RSG alpha Ori. Results. We provide average limb-darkening coefficients for RSGs. We describe the prospects for the detection and characterization of granulation (i.e., contrast, size) on RSGs. We demonstrate that our RHD simulations provide an excellent fit to existing interferometric observations of alpha Ori, in contrast to limb darkened disks. This confirms the existence of large convective cells on the surface of Betelgeuse.
|
|
| 5. |
- Chiavassa, A., et al.
(författare)
-
Radiative hydrodynamics simulations of red supergiant stars II. Simulations of convection on Betelgeuse match interferometric observations
- 2010
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 515, s. A12-
-
Tidskriftsartikel (refereegranskat)abstract
- Context. The red supergiant (RSG) Betelgeuse is an irregular variable star. Convection may play an important role in understanding this variability. Interferometric observations can be interpreted using sophisticated simulations of stellar convection. Aims. We compare the visibility curves and closure phases obtained from our 3D simulation of RSG convection with CO5BOLD to various interferometric observations of Betelgeuse from the optical to the H band to characterize and measure the convection pattern on this star. Methods. We use a 3D radiative-hydrodynamics (RHD) simulation to compute intensity maps in different filters and thus derive interferometric observables using the post-processing radiative transfer code OPTIM3D. The synthetic visibility curves and closure phases are compared to observations. Results. We provide a robust detection of the granulation pattern on the surface of Betelgeuse in both the optical and the H band based on excellent fits to the observed visibility points and closure phases. We determine that the Betelgeuse surface in the H band is covered by small to medium scale (5-15 mas) convection-related surface structures and a large (approximate to 30 mas) convective cell. In this spectral region, H2O molecules are the main absorbers and contribute to both the small structures and the position of the first null of the visibility curve (i.e., the apparent stellar radius).
|
|
| 6. |
- Chiavassa, A., et al.
(författare)
-
VLTI/AMBER spectro-interferometric imaging of VX Sagittarii's inhomogenous outer atmosphere
- 2010
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 511, s. A51-
-
Tidskriftsartikel (refereegranskat)abstract
- Aims. We aim to explore the photosphere of the very cool late-type star VX Sgr and in particular the characterization of molecular layers above the continuum forming photosphere. Methods. We obtained interferometric observations with the VLTI/AMBER interferometer using the fringe tracker FINITO in the spectral domain 1.45-2.50 mu m with a spectral resolution of approximate to 35 and baselines ranging from 15 to 88 m. We performed independent image reconstruction for different wavelength bins and fit the interferometric data with a geometrical toy model. We also compared the data to 1D dynamical models of Miras atmosphere and to 3D hydrodynamical simulations of red supergiant (RSG) and asymptotic giant branch (AGB) stars. Results. Reconstructed images and visibilities show a strong wavelength dependence. The H-band images display two bright spots whose positions are confirmed by the geometrical toy model. The inhomogeneities are qualitatively predicted by 3D simulations. At approximate to 2.00 mu m and in the region 2.35-2.50 mu m, the photosphere appears extended and the radius is larger than in the H band. In this spectral region, the geometrical toy model locates a third bright spot outside the photosphere that can be a feature of the molecular layers. The wavelength dependence of the visibility can be qualitatively explained by 1D dynamical models of Mira atmospheres. The best-fitting photospheric models show a good match with the observed visibilities and give a photospheric diameter of Theta = 8.82 +/- 0.50 mas. The H2O molecule seems to be the dominant absorber in the molecular layers. Conclusions. We show that the atmosphere of VX Sgr seems to resemble Mira/AGB star model atmospheres more closely than do RSG model atmospheres. In particular, we see molecular ( water) layers that are typical of Mira stars.
|
|
| 7. |
- Höfner, Susanne, et al.
(författare)
-
Dynamic Model Atmospheres of Cool Giants
- 2005
-
Ingår i: High Resolution Infrared Spectroscopy in Astronomy. ; , s. 271-282
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Cool giant stars are highly dynamical objects, and complex micro-physical processes play an important role in their extended atmospheres and winds. The interpretation of observations, and in particular of high-resolution IR spectra, requires realistic self-consistent model atmospheres. Current dynamical models include rather detailed micro-physics, and the resulting synthetic spectra compare reasonably well with observations. A transition from qualitative to quantitative modelling is taking place at present. We give an overview of existing dynamical model atmospheres for AGB stars, discussing recent advances and current trends in modelling. When comparing synthetic spectra and other observable properties resulting from dynamical models with observations we focus on the near- and mid-IR wavelength range.
|
|
| 8. |
- Arroyo-Torres, B., et al.
(författare)
-
VLTI/AMBER Studies of the Atmospheric Structure and Fundamental Parameters of Red Giant and Supergiant Stars
- 2015
-
Ingår i: WHY GALAXIES CARE ABOUT AGB STARS III. - : ASTRONOMICAL SOC PACIFIC. - 9781583818794 ; , s. 91-96
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We present recent near-IR interferometric studies of red giant and super giant stars, which are aimed at obtaining information on the structure of the atmospheric layers and constraining the fundamental parameters of these objects. The observed visibilities of six red supergiants (RSGs), and also of one of the five red giants observed, indicate large extensions of the molecular layers, as previously observed for Mira stars. These extensions are not predicted by hydrostatic PHOENIX model atmospheres, hydrodynamical (RED) simulations of stellar convection, or self-excited pulsation models. All these models based on parameters of RSGs lead to atmospheric structures that are too compact compared to our observations. We discuss how alternative processes might explain the atmospheric extensions for these objects. As the continuum appears to be largely free of contamination by molecular layers, we can estimate reliable Rosseland angular radii for our stars. Together with distances and bolometric fluxes, we estimate the effective temperatures and luminosities of our targets, locate them in the HR diagram, and compare their positions to recent evolutionary tracks.
|
|
| 9. |
- Arroyo-Torres, B., et al.
(författare)
-
What causes the large extensions of red supergiant atmospheres? : Comparisons of interferometric observations with 1D hydrostatic, 3D convection, and 1D pulsating model atmospheres
- 2015
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 575
-
Tidskriftsartikel (refereegranskat)abstract
- Aims. This research has two main goals. First, we present the atmospheric structure and the fundamental parameters of three red supergiants (RSGs), increasing the sample of RSGs observed by near-infrared spectro-interferometry. Additionally, we test possible mechanisms that may explain the large observed atmospheric extensions of RSGs. Methods. We carried out spectro-interferometric observations of the.RSGs V602 Car, EID 95687, and EID 183589 in the near-infrared K-band (1.92-2.47 mu m) with the VLTI/AMBER instrument at medium spectral resolution (R similar to 1500). To categorize and comprehend the extended atmospheres, we compared our observational results to predictions by available hydrostatic PHOENIX, available 3D convection, and new 1D self-excited pulsation models of RSGs. Results. Our near-infrared flux spectra of V602 Car, HD 95687, and HD 183589 are well reproduced by the PHOENIX model atmospheres. The continuum visibility values are consistent with a limb-darkened disk as predicted by the PHOENIX models, allowing us to determine the angular diameter and the fundamental parameters of our sources. Nonetheless, in the case of V602 Car and HD 95686, the PHOENIX model visibilities do not predict the large observed extensions of molecular layers, most remarkably in the CO bands. Likewise, the 3D convection models and the ID pulsation models with typical parameters of RSGs lead to compact atmospheric structures as well, which are similar to the structure of the hydrostatic PHOENIX models. They can also not explain the observed decreases in the visibilities and thus the large atmospheric molecular extensions. The full sample of our RSGs indicates increasing observed atmospheric extensions with increasing luminosity and decreasing surface gravity, and no correlation with effective temperature or variability amplitude. Conclusions. The location of our RSG sources in the Hertzsprung-Russell diagram is contirm.ed to be consistent with the red limits of recent evolutionary tracks. The observed extensions of the atmospheric layers of our sample of RSGs are comparable to those of Mira stars. This phenomenon is not predicted by any of the considered model atmospheres including as 311) convection and new 1D pulsation models of.RSGs. This confirms that neither convection nor pulsation alone can levitate the molecular atmospheres of.RSGs. Our observed correlation of atmospheric extension with luminosity supports a scenario of radiative acceleration on Doppler-shifted molecular lines.
|
|
| 10. |
- Battino, U., et al.
(författare)
-
Application Of A Theory And Simulation-Based Convective Boundary Mixing Model For AGB Star Evolution And Nucleosynthesis
- 2016
-
Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 827:1
-
Tidskriftsartikel (refereegranskat)abstract
- The s-process nucleosynthesis in Asymptotic giant branch (AGB) stars depends on the modeling of convective boundaries. We present models and s-process simulations that adopt a treatment of convective boundaries based on the results of hydrodynamic simulations and on the theory of mixing due to gravity waves in the vicinity of convective boundaries. Hydrodynamics simulations suggest the presence of convective boundary mixing (CBM) at the bottom of the thermal pulse-driven convective zone. Similarly, convection-induced mixing processes are proposed for the mixing below the convective envelope during third dredge-up (TDU), where the C-13 pocket for the s process in AGB stars forms. In this work, we apply a CBM model motivated by simulations and theory to models with initial mass M = 2 andM = 3M(circle dot), and with initial metal content Z = 0.01 and Z = 0.02. As reported previously, the He-intershell abundances of C-12 and O-16 are increased by CBM at the bottom of the pulse-driven convection zone. This mixing is affecting the Ne-22(alpha, n)Mg-25 activation and the s-process efficiency in the C-13-pocket. In our model, CBM at the bottom of the convective envelope during the TDU represents gravity wave mixing. Furthermore, we take into account the fact that hydrodynamic simulations indicate a declining mixing efficiency that is already about a pressure scale height from the convective boundaries, compared to mixing-length theory. We obtain the formation of the C-13-pocket with a mass of approximate to 10(-4) M-circle dot. The final s-process abundances are characterized by 0.36<[s/Fe] < 0.78 and the heavy-to-light s-process ratio is -0.23< [hs/ls] < 0.45. Finally, we compare our results with stellar observations, presolar grain measurements and previous work.
|
|
