| 1. |
- Azulay, R., et al.
(författare)
-
Dynamical masses of the low-mass stellar binary AB Doradus B
- 2015
-
Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 578, s. Art. no. A16-
-
Tidskriftsartikel (refereegranskat)abstract
- Context. AB Doradus is the main system of the AB Doradus moving group. It is a quadruple system formed by two widely separated binaries of pre-main-sequence (PMS) stars: AB Dor A/C and AB Dor Ba/Bb. The pair AB Dor A/C has been extensively studied and its dynamical masses have been determined with high precision, thus making AB Dor C a benchmark for calibrating PMS stellar models. If the orbit and dynamical masses of the pair AB Dor Ba/Bb could be determined, they could play a similar role to that of AB Dor C in calibrating PMS models, and would also help to better understand the dynamics of the whole AB Doradus system. Aims. We aim to determine the individual masses of the pair AB Dor Ba/Bb using VLBI observations and archive infrared data as part of a larger program that monitors binary systems in the AB Doradus moving group. Methods. We observed the system AB Dor B between 2007 and 2013 with the Australian Long Baseline Array (LBA) at a frequency of 8.4 GHz in phase-reference mode. Results. We detected, for the first time, compact radio emission from both stars in the binary, AB Dor Ba and AB Dor Bb. This result allowed us to determine the orbital parameters of both the relative and absolute orbits and, consequently, their individual dynamical masses: 0.28 ± 0.05 M⊙ and 0.25 ± 0.05 M⊙, respectively. Conclusions. Comparisons of the dynamical masses with the prediction of PMS evolutionary models show that the models underpredict the dynamical masses of the binary components Ba and Bb by 10-30% and 10-40%, respectively, although they still agree at the 2σ level. Some of the stellar models considered favor an age between 50 and 100 Myr for this system, while others predict older ages. We also discuss the evolutionary status of AB Dor Ba/Bb in terms of an earlier double-double star scenario that might explain the strong radio emission detected in both components.
|
|
| 2. |
- Belonoshko, Anatoly B., et al.
(författare)
-
High-pressure melting of MgSiO3
- 2005
-
Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 94:19
-
Tidskriftsartikel (refereegranskat)abstract
- The melting curve of MgSiO3 perovskite has been determined by means of ab initio molecular dynamics complemented by effective pair potentials, and a new phenomenological model of melting. Using first principles ground state calculations, we find that the MgSiO3 perovskite phase transforms into post perovskite at pressures above 100 GPa, in agreement with recent theoretical and experimental studies. We find that the melting curve of MgSiO3, being very steep at pressures below 60 GPa, rapidly flattens on increasing pressure. The experimental controversy on the melting of the MgSiO3 perovskite at high pressures is resolved, confirming the data by Zerr and Boehler.
|
|
| 3. |
- Belonoshko, Anatoly B., et al.
(författare)
-
High-pressure melting of molybdenum
- 2004
-
Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 92:19
-
Tidskriftsartikel (refereegranskat)abstract
- The melting curve of the body-centered cubic (bcc) phase of Mo has been determined for a wide pressure range using both direct ab initio molecular dynamics simulations of melting as well as a phenomenological theory of melting. These two methods show very good agreement. The simulations are based on density functional theory within the generalized gradient approximation. Our calculated equation of state of bcc Mo is in excellent agreement with experimental data. However, our melting curve is substantially higher than the one determined in diamond anvil cell experiments up to a pressure of 100 GPa. An explanation is suggested for this discrepancy.
|
|
| 4. |
- Belonoshko, Anatoly B., et al.
(författare)
-
Molybdenum at High Pressure and Temperature : Melting from Another Solid Phase
- 2008
-
Ingår i: Physical review letters / publ. by the American Physical Society. ; 100:13, s. 135701-
-
Tidskriftsartikel (refereegranskat)abstract
- The Gibbs free energies of bcc and fcc Mo are calculated from first principles in the quasiharmonic approximation in the pressure range from 350 to 850 GPa at room temperatures up to 7500 K. It is found that Mo, stable in the bcc phase at low temperatures, has lower free energy in the fcc structure than in the bcc phase at elevated temperatures. Our density-functional-theory-based molecular dynamics simulations demonstrate that fcc melts at higher than bcc temperatures above 1.5 Mbar. Our calculated melting temperatures and bcc-fcc boundary are consistent with the Mo Hugoniot sound speed measurements. We find that melting occurs at temperatures significantly above the bcc-fcc boundary. This suggests an explanation of the recent diamond anvil cell experiments, which find a phase boundary in the vicinity of our extrapolated bcc-fcc boundary.
|
|
| 5. |
- Belonoshko, Anatoly, et al.
(författare)
-
High-pressure melting of MgSiO3
- 2005
-
Ingår i: Phys. Rev. Lett.. ; 94, s. 195701-
-
Tidskriftsartikel (refereegranskat)
|
|
| 6. |
- Belonoshko, Anatoly, et al.
(författare)
-
Xenon melting: Density functional theory versus diamond anvil cell experiments
- 2006
-
Ingår i: Physical Review B Condensed Matter. - : American Physical Society. - 0163-1829 .- 1095-3795. ; 74:5, s. 054114-
-
Tidskriftsartikel (refereegranskat)abstract
- We performed two-phase ab initio density functional theory based molecular dynamics simulations of Xe melting and demonstrated that, contrary to claims in the recent literature, the pressure dependence of the Xe melting curve is consistent with the corresponding-states theory as well as with the melting curve obtained earlier from classical molecular dynamics with a Xe pair potential. While at low pressure the calculated melting curve is in perfect agreement with reliable experiments, our calculated melting temperatures at higher pressures are inconsistent with those from the most recent diamond anvil cell experiment. We discuss a possible explanation for this inconsistency.
|
|
| 7. |
- Burakovsky, L., et al.
(författare)
-
Ab initio phase diagram of iridium
- 2016
-
Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 94:9, s. 094112-
-
Tidskriftsartikel (refereegranskat)abstract
- The phase diagram of iridium is investigated using the Z methodology. The Z methodology is a technique for phase diagram studies that combines the direct Z method for the computation of melting curves and the inverse Z method for the calculation of solid-solid phase boundaries. In the direct Z method, the solid phases along the melting curve are determined by comparing the solid-liquid equilibrium boundaries of candidate crystal structures. The inverse Z method involves quenching the liquid into the most stable solid phase at various temperatures and pressures to locate a solid-solid boundary. Although excellent agreement with the available experimental data (to less than or similar to 65 GPa) is found for the equation of state (EOS) of Ir, it is the third-order Birch-Murnaghan EOS with B-0 = 5 rather than the more widely accepted B-0 = 4 that describes our ab initio data to higher pressure (P). Our results suggest the existence of a random-stacking hexagonal close-packed structure of iridium at high P. We offer an explanation for the 14-layer hexagonal structure observed in experiments by Cerenius and Dubrovinsky.
|
|
| 8. |
- Burakovsky, L., et al.
(författare)
-
High-Pressure-High-Temperature Polymorphism in Ta : Resolving an Ongoing Experimental Controversy
- 2010
-
Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 104:25, s. 255702-
-
Tidskriftsartikel (refereegranskat)abstract
- Phase diagrams of refractory metals remain essentially unknown. Moreover, there is an ongoing controversy over the high-pressure melting temperatures of these metals: results of diamond anvil cell (DAC) and shock wave experiments differ by at least a factor of 2. From an extensive ab initio study on tantalum we discovered that the body-centered cubic phase, its physical phase at ambient conditions, transforms to another solid phase, possibly hexagonal omega phase, at high temperature. Hence the sample motion observed in DAC experiments is very likely not due to melting but internal stresses accompanying a solid-solid transformation, and thermal stresses associated with laser heating.
|
|
| 9. |
- Burakovsky, L., et al.
(författare)
-
Recent ab initio phase diagram studies: Iridium
- 2017
-
Ingår i: JOINT AIRAPT-25TH and EHPRG-53RD INTERNATIONAL CONFERENCE ON HIGH PRESSURE SCIENCE AND TECHNOLOGY, 2015. - : IOP PUBLISHING LTD.
-
Konferensbidrag (refereegranskat)abstract
- The phase diagram of iridium is investigated using the Z methodology in conjunction with the VASP ab initio molecular dynamics package. The Z methodology is a novel technique for phase diagram studies which combines the direct Z method for the computation of melting curves and the inverse Z method for the calculation of solid-solid phase boundaries. We compare our results to the available experimental data on iridium. We offer explanation for the 14-layer hexagonal structure observed in experiments by Cerenius and Dubrovinsky.
|
|
| 10. |
- Cricchio, F., et al.
(författare)
-
High-pressure melting of lead
- 2006
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 73:14
-
Tidskriftsartikel (refereegranskat)abstract
- The melting curve of the hexagonal close-packed (hcp) phase of lead (Pb) has been determined over a wide pressure range using both ab initio molecular dynamics (AIMD) simulations and classical molecular dynamics (CMD) employing an effective pair potential. The AIMD simulations are based on a density functional theory (DFT) in the generalized gradient approximation (GGA). The Pb melting curve, constructed using a well-established theoretical scheme, is in excellent agreement with the AIMD results. Our calculated equation of state (EOS) of hcp Pb is in excellent agreement with experimental data up to 40 GPa. Our melting curve agrees very well with melting temperatures obtained in both shock-wave and diamond-anvil cell (DAC) experiments, but at higher pressures our curve lies between the two data sets.
|
|
