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| 2. |
- Bollard, J., et al.
(författare)
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Early formation of planetary building blocks inferred from Pb isotopic ages of chondrules.
- 2017
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Ingår i: Science Advances. - : American Association for the Advancement of Science (AAAS). - 2375-2548. ; 3
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Tidskriftsartikel (refereegranskat)abstract
- The most abundant components of primitive meteorites (chondrites) are millimeter-sized glassy spherical chondrules formed by transient melting events in the solar protoplanetary disk. Using Pb-Pb dates of 22 individual chondrules, we show that primary production of chondrules in the early solar system was restricted to the first million years after the formation of the Sun and that these existing chondrules were recycled for the remaining lifetime of the protoplanetary disk. This finding is consistent with a primary chondrule formation episode during the early high-mass accretion phase of the protoplanetary disk that transitions into a longer period of chondrule reworking. An abundance of chondrules at early times provides the precursor material required to drive the efficient and rapid formation of planetary objects via chondrule accretion.
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| 4. |
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| 5. |
- Rieutord, M, et al.
(författare)
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Are granules good tracers of solar surface velocity fields?
- 2001
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 377, s. 14-17
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Tidskriftsartikel (refereegranskat)abstract
- Using a numerical simulation of compressible convection with radiativetransfer mimicking the solar photosphere, we compare the velocity fieldderived from granule motions to the actual velocity field of the plasma.We thus test the idea that granules may be used to trace large-scalevelocity fields at the sun's surface. Our results show that this isindeed the case provided the scale separation is sufficient. We thusestimate that neither velocity fields at scales less than 2500 km nortime evolution at scales shorter than 0.5 hr can be faithfully describedby granules. At larger scales the granular motions correlate linearlywith the underlying fluid motions with a slope of ≲2 reachingcorrelation coefficients up to ~ 0.9.
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| 6. |
- van Eck, S., et al.
(författare)
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A Grid of MARCS Model Atmospheres for S Stars
- 2011
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Ingår i: Why Galaxies Care about AGB Stars II.
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Konferensbidrag (refereegranskat)abstract
- S-type stars are late-type giants whose atmospheres are enriched in carbon and s-process elements because of either extrinsic pollution by a binary companion or intrinsic nucleosynthesis and dredge-up on the thermally-pulsing AGB. A large grid of S-star model atmospheres has been computed covering the range 2700 \le T$_eff$(K) \le 4000 with 0.5 \le C/O \le 0.99. ZrO and TiO band strength indices as well as VJHKL photometry are needed to disentangle T$_eff$, C/O and [s/Fe]. A \ldquobest-model finding tool\rdquo has been developed using a set of well-chosen indices and checked against photometry as well as low- and high-resolution spectroscopy. It is found that applying M-star model atmospheres (i.e., with a solar C/O ratio) to S stars can lead to errors in T$_eff$ up to 400 K. We constrain the parameter space occupied by the S stars of the vast Henize sample in terms of T$_eff$, [C/O] and [s/Fe].
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| 7. |
- Van Eck, S., et al.
(författare)
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A grid of S stars MARCS model atmospheres
- 2011
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Ingår i: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 328, s. 012009-
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Tidskriftsartikel (refereegranskat)abstract
- S stars are cool stars of temperatures similar to those of M giants, but their atmospheres are enriched in carbon and s-process elements because of either extrinsic pollution by a binary companion or intrinsic nucleosynthesis and dredge-up on the thermally-pulsing AGB. Despite numerous attempts to link phenomenological spectral classification criteria to physical parameters (T$_eff$, gravity, C/O, [s/Fe], [Fe/H]), the parameter space of S stars is poorly known and this has prevented accurate abundance analysis of S stars until now. Here we present a large grid of S-star model atmospheres. ZrO and TiO band strength indices as well as VJHKL photometry are needed to disentangle the effective temperature, C/O and [s/Fe]. The stellar parameters derived on the basis of low-resolution spectra and photometry are shown to be fairly accurate when compared to high-resolution data of the same stars. The C/O ratio of S stars is found to be between the solar value (0.5) and 0.99, and not 1 as often claimed in the literature. Consistently with stellar evolution expectations, the C/O ratio increases as the effective temperature decreases.
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| 8. |
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