| 1. |
- Bétrisey, Jérôme, et al.
(författare)
-
Imprint of the magnetic activity cycle on solar asteroseismic characterisation based on 26 years of GOLF and BiSON data
- 2024
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 688
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Context. Asteroseismic modelling will play a key role in future space-based missions, such as PLATO, CubeSpec, and Roman. Despite remarkable achievements, asteroseismology has revealed significant discrepancies between observations and theoretical predictions of the physics used in stellar models, which have the potential to bias stellar characterisation at the precision level demanded by PLATO. The current modelling strategies largely overlook magnetic activity, assuming that its effects are masked within the parametrisation of the so-called ‘surface effects’. Given the presence of activity cycles in multiple solar-like oscillators and activity variations in a significant fraction of Kepler observations of main-sequence stars, it is therefore relevant to investigate systematic errors in asteroseismic characterisations caused by our incomplete understanding of magnetic activity.Aims. Based on 26.5 years of GOLF and BiSON observations, we measured the impact of magnetic activity on the asteroseismic characterisation of the Sun as a star, a reference target for assessing the PLATO mission requirements.Methods. The GOLF and BiSON observations, which fully cover solar cycles 23 and 24, were divided into yearly overlapping snapshots, each delayed by a quarter of a year. For each snapshot, an advanced asteroseismic characterisation, similar to that to be adopted by the PLATO pipeline, was performed with standard prescriptions for the parametrisation of the surface effects. This allowed us to monitor the apparent temporal evolution of fundamental solar parameters such as mass, radius, and age. The correlation of these parameters with the 10.7 cm radio emission flux, a proxy of the solar activity cycle, was then measured.Results. The effects of magnetic activity are partially absorbed into the parametrisation of the surface effects when suitable prescriptions are used, and they do not significantly affect the measured solar mass or radius. However, contrary to literature expectations, we find a significant imprint on the age determination, with variations of up to 6.5% between solar minima and maxima. This imprint persists across both BiSON and GOLF datasets.Conclusions. Considering that the Sun exhibits low levels of activity, our study highlights the looming challenge posed by magnetic activity for future photometry missions, and it prompts a potential reevaluation of the asteroseismic characterisation of the most active Kepler targets.
|
|
| 2. |
|
|
| 3. |
- Melinder, Jens, 1977-, et al.
(författare)
-
The Rate of Supernovae at Redshift 0.1 − 1.0 : the Stockholm VIMOS Supernova Survey IV
- 2012
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 545:A96
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- We present supernova rate measurements at redshift 0.1–1.5 from the Stockholm VIMOS Supernova Survey (SVISS). The sample contains 16 supernovae in total. The discovered supernovae have been classified into core collapse or thermonuclear (Ia) types based on their light curves, colour evolution and host galaxy photometric redshift. The rates we find for the core collapse supernovae are 1.25 (+2.27 +0.85 −0.97 −0.78) - with statistical and systematic errors, respectively - at z = 0.39 and 6.90 (+5.24 +3.04 −3.25 −2.14) at z = 0.73. For the Ia supernovae the rates are 2.02 (+1.57 +0.53−0.96 −0.57) at z = 0.39 and 1.03 (+0.92 +0.31−0.54 −0.36) at z = 0.80. All of these rate estimates have been corrected for host galaxy extinction. Using Monte Carlo simulations we make a thorough study of the systematic effects from assumptions made when calculating the rates and find that the most important errors comes from misclassification, the assumed mix of faint and bright supernova types and uncertainties in redshift. We compare our rates to other observations, to the star formation history for core collapse rates and to different models of the delay time distribution for Ia rates. Overall, our measurements agree quite well with these other rates when using redshift-dependent corrections for extinction. We do not find any evidence of a missing fraction of core collapse supernovae.
|
|
| 4. |
- Rickman, Hans, et al.
(författare)
-
Observations of water in comets with Odin
- 2003
-
Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 402, s. L55-L58
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
|
|
| 5. |
- Gruyters, Pieter, 1986-, et al.
(författare)
-
Atomic diffusion and mixing in old stars VI : Chemical abundance variations in M4
-
Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746.
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Context. Abundance trends in heavier elements with evolutionary phase have been shown to exist in the globular cluster NGC 6121 (M4) ([Fe/H] = −1.1). These trends are a result of atomic diffusion and additional (non-convective) mixing. Studying such trends can provide us with important constraints on the extent to which diffusion modifies the internal structure and surface abundances of solar-type, metal-poor stars.Aims. Taking advantage of a larger data sample, we investigate the reality and the size of these abundance trends and address questions and potential biases associated with the various stellar populations that make up M4. Methods. We perform an abundance analysis by combining photometric and spectroscopic data of 194 stars located between the turnoff point and the base of the red giant branch. Stellar parameters are derived from uvby Strömgren photometry. Using the quantitative-spectroscopy package SME, stellar surface abundances for light elements such as Li, Na, Mg, Al, and Si as well as heavier elements such as Ca, Ti, and Fe are derived in an automated way by fitting synthetic spectra to individual lines in the stellar spectra, obtained with the VLT/FLAMES-GIRAFFE spectrograph.Results. Based on uvby Strömgren photometry, we are able to separate three stellar populations in M4 along the evolutionary sequence from the base of the red giant branch down to the turnoff point. We find weak systematic abundance trends with evolutionary phase for Ca, Ti, and Fe which are best explained by stellar-structure models including atomic diffusion with efficient additional mixing. We derive a new value for the initial lithium abundance of M4 after correcting for the effect of atomic diffusion and additional mixing which falls slightly below the predicted standard BBN value.Conclusions. We find three stellar populations by combining photometric and spectroscopic data of 86 stars in the globular cluster M4. Abundance trends for groups of elements, differently affected by atomic diffusion and additional mixing, are identified. Although the statistical significance of the individual trends is weak, they all support the notion that atomic diffusion is operational along the evolutionary sequence of M4.
|
|
| 6. |
- Nordlander, Thomas, 1985-, et al.
(författare)
-
Atomic diffusion and mixing in old stars VII : Chemical abundance variations in M4 (NGC 6121)
- 2017
-
Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746.
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Context. Variations in chemical abundances with evolutionary phase have been identified in previous papers of this series among stars in three globular clusters, M30 ([Fe/H] = −2.3), NGC 6397 ([Fe/H] = −2.1) and NGC 6752 ([Fe/H] = −1.6). These variations compare well with the predictions from stellar-structure models with atomic diffusion moderated by additional mixing of low and high efficiency at the respective ends of the metallicity scale.Aims. By extending these studies to higher metallicity, for a large number of elements, further empirical constraints can be provided to the nature of the additional mixing mechanism. We therefore investigate whether evolutionary abundance variations are present among stars in the globular cluster NGC 6121 (M4, [Fe/H] = −1.1).Methods. We perform a detailed chemical abundance analysis of 86 stars, ranging from the cluster turnoff point to the red giant branch just above the bump. We determine the abundances of 14 elements consistently by means of spectrum matching, using medium-resolution spectra obtained with VLT/FLAMES-GIRAFFE. Stellar parameters are obtained from UBVI broadband photometry, after correcting for differential reddening effects.Results. We observe the usual C-N-O anticorrelations and confirm the presence of a bimodal population characterised by their N content. We find systematic evolutionary variations in individual chemical abundances of weak statistical significance, but which are rather robust to uncertainties in stellar parameters and modelling assumptions, for magnesium, silicon, calcium, titanium and iron. These variations match predictions from stellar evolution models including atomic diffusion if efficient additional mixing is employed, in line with previous results. Using these models, we derive an initial lithium abundance for the cluster, 2.59 ± 0.10, which is fully compatible with those determined for M30, NGC 6397 and NGC 6752, falling slightly short of the predicted primordial BBN value.Conclusions. The observed abundance patterns of 14 elements investigated here suggest that the second generation stars in M4 were formed out of gas that was polluted by both massive (20−40 solar masses) stars and AGB stars. Element-specific abundance trends are identified in stars along the evolutionary sequence in M4. Although the significance of the individual trends is weak, they all seem to indicate that atomic diffusion is at work in M4. This is the fourth cluster in which atomic diffusion signatures are seen, giving more evidence for the assumption that atomic diffusion produces measurable surface-abundance changes in warm metal-poor stars and hence should be accounted for in stellar-evolution models and studies of Galactic chemical evolution.
|
|
| 7. |
- Rusomarov, Naum, 1986-, et al.
(författare)
-
Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712 : III. Three-dimensional abundance distribution of iron and calcium
- 2016
-
Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746.
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Context. The presence of abundance inhomogeneities in the atmospheres of chemically peculiar Ap stars presents us with an outstand- ing opportunity to investigate three-dimensional abundance distributions of chemical elements with Doppler imaging techniques, and confront these observations with the theory of atomic diffusion in the presence of a magnetic field. Aims. In this paper we present the first three-dimensional abundance distribution of Fe and Ca in the atmosphere of the cool, magnetic, chemically peculiar star HD 24712.Methods. We used a modified version of the magnetic Doppler imaging code invers10 to derive the abundance distribution of Fe and Ca in three directions in a self-consistent way without any simplifying assumptions about the magnetic field structure of HD 24712. The analysis was performed on the basis of phase-resolved observations obtained with the HARPSpol instrument at the 3.6 m ESO telescope.Results. The resulting three-dimensional abundance distributions of Fe and Ca successfully reproduce the observed line profiles of all spectral lines used in the inversion. We reveal a strong correlation between the horizontal and vertical abundance distribution of Fe derived from two and three-dimensional magnetic Doppler imaging, higher effective local abundance corresponds to the stratification profiles in which the transition region is shifted higher in the atmosphere. However, we could not confirm the presence of a similar correlation for Ca.
|
|
| 8. |
- ASPLUND, M
(författare)
-
SPECTROSCOPY OF RY-SAGITTARII DURING THE 1993 MINIMUM
- 1995
-
Ingår i: ASTRONOMY AND ASTROPHYSICS. - : SPRINGER VERLAG. - 0004-6361. ; 294:3, s. 763-772
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The R Coronae Borealis (RCB) star RY Sgr underwent a deep minimum (DELTAV approximately 7.m5) in 1993. Here medium and low resolution observations from approximately 35 days into the decline are presented together with photometry from the early phases unt
|
|
| 9. |
- Asplund, M
(författare)
-
The stability of late-type stars close to the Eddington limit
- 1998
-
Ingår i: ASTRONOMY AND ASTROPHYSICS. - : SPRINGER VERLAG. - 0004-6361. ; 330:2, s. 641-650
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The opacity-modified Eddington limit has been computed for hydrogen-deficient model atmospheres. The R Coronae Borealis (R CrB) stars are found to be located strikingly close to the limit, which suggests that the unknown trigger mechanism for their visual
|
|
| 10. |
|
|
