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- Mattsson, Lars, 1974-, et al.
(författare)
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Dust Driven Mass Loss from Carbon Stars as Function of Stellar Parameters II : Effects of Relaxing the Small Particle Approximation
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Context. It is well-established that the winds of carbon-rich AGB stars (carbon stars) can be driven by radiation pressure on grains of amorphous carbon and collisional transfer of momentum to the gas. This has been demonstrated by different numerical wind models including time-dependent dust formation, where it has been assumed that the dust grains that form never grow to sizes comparable to wavelengths around the stellar flux maximum (or beyond), which simplifies the treatment of grain opacities considerably. It is not clear, however, if this small particle approximation (SPA) is always valid. Aims. In this paper we explore the effects of relaxing the SPA by considering a few less severe approximations for the radiation pressure efficiency, which include the effects of grain sizes. The purpose of the study is mainly to establish when the SPA can be applied and to quantify the possible errors that may occur when it does not hold. Methods. We have computed wind models with time-dependent dust formation and grain-size dependent opacities, where (1) the radiation pressure efficiency is approximated using grain sizes based on various means of the grain size distribution, and (2) where the problem is simplified by assuming a single dust-grain size. Results. It is shown that in critical cases, the effect of grain sizes can be significant. Mass-loss rates may increase by a factor of two, or more, and wind speeds may be an order of magnitude higher. Furthermore, all models with grain-size dependent opacities that have resultant winds appear to have much lower degrees of dust condensation, compared to corresponding SPA models. Consequently, the "dust-loss rates" are much lower in the new models. However, for well-developed dust-driven winds, where the dust formation has saturated, the effect of grain sizes on the mass-loss rate and wind speed is almost negligible. Conclusions. We conclude that the SPA is, under many circumstances, a reasonable simplification in models of carbon star mass loss. However, critical cases do exist, where especially the effects of momentum transfer due to scattering become significant. It is therefore uncertain whether previous results for winds in the transition regions (between no dust-driven mass loss and well-developed winds) are quantitatively correct. However, we argue that the SPA is a reasonable simplification for strong dust-driven winds. Furthermore, we note that there are other effects (not yet included in the model) than those of grain-size dependent opacities, which may become important in the critical wind regime.
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| 2. |
- Mattsson, Lars, 1974-, et al.
(författare)
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Effects of Carbon-excess dependent Mass Loss and Molecular Opacities on Models of C-star Evolution
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We present models of stellar evolution for a 2Msun-star of initial metallicity Z = 0.01 (scaled solar), with focus on the carbon-star phase, the effects of increasing carbon excess on mass loss and molecular opacities. We employ a new, state-of-the-art theoretical mass-loss prescription for dust-driven winds, which takes effects of the carbon excess into account and implement a set of composition-dependent molecular opacities, which incorporates the changes due to the evolution of the carbon excess. This work should be regarded as an exploratory and comparative study. The stellar evolution models were computed using the MESA code-package (http://mesa.sourceforge.net), which includes a new fast and efficient code for stellar evolution, but still needs further calibration before quantitative comparisons with observations can be made. We find that the development of the carbon excess is controlling much of the carbon-star evolution. A very pronounced superwind is forming and the termination of the AGB happens soon after the star becomes carbon rich, in fact, after only 4-5 thermal pulses, which is much less than in existing models.
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| 6. |
- Fynbo, J. P. U., et al.
(författare)
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Galaxy counterparts of metal-rich damped Lyman-alpha absorbers - II. A solar-metallicity and dusty DLA at z_abs=2.58
- 2010
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Annan publikation (populärvet., debatt m.m.)abstract
- [Abridged]. Here, we report on the discovery of the galaxy counterpart of the z_abs=2.58 DLA on the line-of-sight to the z=3.07 quasar SDSS J091826.16+163609.0. The galaxy counterpart of the DLA is detected in the OIII 5007 and OII 3726,3729 emission lines redshifted into the NIR at an impact parameter of 16 kpc. Ly-alpha emission is not detected. The upper limit implies that Ly-alpha emission from this galaxy is suppressed by more than an order of magnitude. The DLA is amongst the most metal-rich DLAs studied so far at comparable redshifts. We find evidence for substantial depletion of refractory elements onto dust grains. Fitting the main metal line component of the DLA, which is located at z_abs=2.5832 and accounts for at least 85% of the total column density of low-ionisation species, we measure metal abundances from ZnII, SII, SiII, CrII, MnII, FeII and NiII of -0.12, -0.26, -0.46, -0.88, -0.92, -1.03 and -0.78, respectively. In addition, we detect absorption in the Lyman and Werner bands of hydrogen, which represents the first detection of H_2 molecules with X-shooter. The background quasar Q0918+1636 is amongst the reddest QSOs at redshifts 3.02<3.12 from the SDSS catalogue. Its UV to NIR spectrum is well fitted by a composite QSO spectrum reddened by SMC/LMC-like extinction curves at z_abs=2.58 with a significant amount of extinction given by A_V = 0.2 mag. This supports previous claims that there may be more metal-rich DLAs missing from current samples due to dust reddening of the background QSOs. The fact that there is evidence for dust both in the central emitting regions of the galaxy (as evidenced by the lack of Ly-alpha emission) and at an impact parameter of 16 kpc (as probed by the DLA) suggests that dust is widespread in this system.
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| 7. |
- Mencia Trinchant, Laia, et al.
(författare)
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Deep UBVRI observations of a field within ELAIS-S1 : the Stockholm VIMOS Supernova Survey III
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Context. The Stockholm VIMOS Supernova Survey (SVISS) is an optical broad-band photometry survey, aimed at the search fordistant (redshifts z~0.7) supernovae, which are detected by means of monthly imaging in R and I bands with the VIMOS instrumenton the ESO VLT. The distances of the supernovae are estimated using photometric redshifts facilitated by additional U, B and V imaging also with VIMOS. In this paper we present the deep stacked imaging data in all these bands and a catalogue of galaxiesextending to very faint magnitudes. Aims. We discuss the various steps followed to create a galaxy catalog, including the reduction and stacking of the data, astrometricand photometric calibration and source detections.We derived completeness, corrected number counts and color selected Lyman breakgalaxies. Raw number counts are also calculated. Finally, making use of a template fitting code, photometric redshifts are derived. Methods. UBVRI deep imaging of a 4x56 sq. arcmin field within the ELAIS-S1 field is presented in this paper. The data reduction has been done with a dedicated MIDAS pipeline. The final images have been weighted by their seeing, which was found to optimise depth and spatial resolution. Source detection and phortometry was done using SExtractor. The photometric accuracy was tested by comparing the colors of the sources to stellar libraries and redshifted galaxy templates. A mixture of synthetic point-like andextended sources are simulated in order to estimate completeness. Derived number counts have been compared to the results ofsimilar surveys. Lyman break galaxy candidates were selected using color criteria: UBR, BRI, BVI and VRI, and their numbers computed and compared to other surveys.We have obtained photometric redshifts by spectral energy distribution template fitting. The photometric redshifts were calibrated using a sample of galaxies in the HDF-S with available spectroscopic redshifts that have also been observed with the same filter set as used here. Results. Using a seeing weighted method to combine the science frames resulted in an increase of the number of faint objects detectedand minimizing the photometric errors. The astrometric accuracy is 0.400 arcsec with respect to 2MASS and 5 sigma limiting magnitudes for UBVRI are 28.2, 29.3, 28.3, 28.5 and 27.2 respectively (Vega). The computed galaxy number counts are in good agreement with previous works, with the U band counts of this survey being one of the deepest ever obtained. The raw number counts from broadband color selected Lyman break galaxies show a good agreement for the U and B dropouts with other surveys. Photometric redshifts have a scatter of dz=0.085 with a 9.1% fraction of outliers. Conclusions. We derive deep number counts which are found to be consistent with previous works, reproducing the bright and faintends. Especially, the stacked deep U and I band data are found to present a comptetetive combination of depth and area, comparableto HDF observations. As well as V band, comparable to Subaru and the Lockman hole observations. Our U and B dropouts are within the expected redshift range, with mean values of 3 and 3.8 respectively.
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| 8. |
- Wehrhahn, Ansgar, et al.
(författare)
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CRIRES+ detection of H2O and CO in the transmission spectra of WASP-107 b
- 2022
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- WASP-107 b is a warm super-Neptune with a large extended atmosphere orbiting a bright K dwarf of solar composition. The planet's size and the brightness of its stellar host make it highly amenable to atmospheric characterisation, with previous detections of both He and H2O. We observe it here for the first time in high-resolution spectroscopy in the K-band. To obtain a transmission spectrum of the exoplanet atmosphere, we observe a transit event of WASP-107 b for a total of 5.4 hours (2.7 hours in transit) in the K-band with an average SNR = 64 for each exposure using the high-resolution infrared CRIRES+ spectrograph on the Very Large Telescope on Cerro Paranal, Chile. We aim to demonstrate the capabilities of this new instrument for exoplanet characterisation by searching for the presence of molecular features of key chemical species in the atmosphere with spectral lines detectable in this band, such as CO, CO2, H2O, and CH4. The search for particular chemical species is conducted by using the cross-correlation method on the observed spectra. The transit spectroscopy observations are first cleaned from the stellar and telluric features using the SYSREM algorithm, and the remaining spectral information is then compared to a model transmission spectrum. The model transmission spectrum is obtained using the petitRADTRANS code based on expectations for a warm Neptune-class planet and the observed orbital parameters. We report the detection of H2O in the atmosphere of WASP-107 b with 4.71 sigma significance, as well as a detection of CO at 3.46 sigma. The velocities of these detections show signs of atmospheric weather patterns, such as day-to-night-side temperature gradients. We do not detect the presence of CO2 or CH4. Our results confirm the chemical composition of the WASP-107 b's atmosphere and pave the way for efficient atmospheric studies of other planets of the same class. They show that the newly upgraded high-resolution spectrograph CRIRES+ is suitable for exoplanet transit observations. All detections and non-detections are in agreement with previous transmission studies of WASP-107 b based on low-resolution spectroscopy.
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| 9. |
- Schweitz, Jan-Åke, 1945-
(författare)
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Expansion of the universe : Back to basics (updated version)
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The ambition of this article is to establish a fundamental model of our expanding universe in a minimalistic spirit, and show that a simple source flux model actually agrees just as well with observed expansion data as General Relativity and its offspring Standard Cosmology do. The theory is based on three constitutive equations, relating three basic expansion parameters to each other. Vacuum energy is interpreted as kinetic expansion energy, thereby eliminating the need for a cosmological constant corresponding to dark energy with abnormal negative pressure. Expansion parameters in good agreement with WMAP and Planck results are derived from one single experimental input value (the present Hubble parameter). A strong relationship between vacuum energy and other energy species in the universe is revealed. The model eliminates the so-called flatness, horizon and coincidence issues, and yields singular and non-singular Big Bang solutions; the Planck unit system being closely linked to the non-singular primordial state of the universe. Furthermore, the ‘worst theoretical prediction in the history of science’ (error factor 10120 ) may be explained by this theory.
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