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W B Landsman - One of the best experts on this subject based on the ideXlab platform.
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flash mixing on the white dwarf Cooling Curve spectroscopic confirmation in ngc 2808
The Astrophysical Journal, 2012Co-Authors: Thomas M Brown, Thierry Lanz, Allen V Sweigart, Misty Cracraft, I Hubeny, W B LandsmanAbstract:We present new Hubble Space Telescope far-UV spectroscopy of two dozen hot evolved stars in NGC 2808, a massive globular cluster with a large population of 'blue-hook' (BHk) stars. The BHk stars are found in ultraviolet color-magnitude diagrams of the most massive globular clusters, where they fall at luminosities immediately below the hot end of the horizontal branch (HB), in a region of the H-R diagram unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that these subluminous HB stars are very likely the progeny of stars that undergo extensive internal mixing during a late He-core flash on the white dwarf Cooling Curve. This flash mixing leads to hotter temperatures and an enormous enhancement of the surface He and C abundances; these hotter temperatures, together with the decrease in H opacity shortward of the Lyman limit, make the BHk stars brighter in the extreme UV while appearing subluminous in the UV and optical. Our far-UV spectroscopy demonstrates that, relative to normal HB stars at the same color, the BHk stars of NGC 2808 are hotter and greatly enhanced in He and C, thus providing unambiguous evidence of flash mixing in the subluminous population.more » Although the C abundance in the BHk stars is orders of magnitude larger than that in the normal HB stars, the atmospheric C abundance in both the BHk and normal HB stars appears to be affected by gravitational settling. The abundance variations seen in Si and the Fe-peak elements also indicate that atmospheric diffusion is at play in our sample, with all of our hot subdwarfs at 25,000-50,000 K exhibiting large enhancements of the iron-peak elements. The hottest subdwarfs in our BHk sample may be pulsators, given that they fall in the temperature range of newly discovered pulsating subdwarfs in {omega} Cen. In addition to the normal hot HB and BHk stars, we also obtain spectra of five blue HB stars, a post-HB star, and three unclassified stars with unusually blue UV colors.« less
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new observational evidence of flash mixing on the white dwarf Cooling Curve
arXiv: Solar and Stellar Astrophysics, 2012Co-Authors: Thomas M Brown, Thierry Lanz, Allen V Sweigart, Misty Cracraft, I Hubeny, W B LandsmanAbstract:Blue hook stars are a class of subluminous extreme horizontal branch stars that were discovered in UV images of the massive globular clusters omega Cen and NGC 2808. These stars occupy a region of the HR diagram that is unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that the blue hook stars are very likely the progeny of stars that undergo extensive internal mixing during a late helium-core flash on the white dwarf Cooling Curve. This "flash mixing" produces hotter-than-normal EHB stars with atmospheres significantly enhanced in helium and carbon. The larger bolometric correction, combined with the decrease in hydrogen opacity, makes these stars appear subluminous in the optical and UV. Flash mixing is more likely to occur in stars born with a high helium abundance, due to their lower mass at the main sequence turnoff. For this reason, the phenomenon is more common in those massive globular clusters that show evidence for secondary populations enhanced in helium. However, a high helium abundance does not, by itself, explain the presence of blue hook stars in massive globular clusters. Here, we present new observational evidence for flash mixing, using recent HST observations. These include UV color-magnitude diagrams of six massive globular clusters and far-UV spectroscopy of hot subdwarfs in one of these clusters (NGC 2808).
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flash mixing on the white dwarf Cooling Curve spectroscopic confirmation in ngc 2808
arXiv: Solar and Stellar Astrophysics, 2012Co-Authors: Thomas M Brown, Thierry Lanz, Allen V Sweigart, Misty Cracraft, I Hubeny, W B LandsmanAbstract:[Abridged] We present new HST FUV spectroscopy of 24 hot evolved stars in NGC2808, a massive globular cluster with a large population of "blue-hook" (BHk) stars. The BHk stars are found in UV color-magnitude diagrams of the most massive globular clusters, where they fall at luminosities immediately below the hot end of the horizontal branch (HB), in a region of the HR diagram unexplained by canonical stellar evolution theory. Using new evolutionary and atmospheric models, we have shown that these subluminous HB stars are very likely the progeny of stars that undergo extensive internal mixing during a late He-core flash on the white dwarf Cooling Curve. This flash mixing leads to hotter temperatures and an enormous enhancement of the surface He and C; these hotter temperatures, together with the decrease in H opacity shortward of the Lyman limit, make the BHk stars brighter in the EUV while appearing subluminous in the UV and optical. Our FUV spectroscopy demonstrates that, relative to normal HB stars at the same color, the BHk stars of NGC2808 are hotter and greatly enhanced in He and C, thus providing unambiguous evidence of flash mixing. Although the C abundance in the BHk stars is orders of magnitude larger than that in the normal HB stars, the C abundance in both the BHk and normal HB stars appears to be affected by gravitational settling. The variations seen in Si and the Fe-peak elements also indicate that atmospheric diffusion is at play in our sample, with all of our hot subdwarfs at 25,000 to 50,000 K exhibiting large enhancements of the Fe-peak elements. The hottest subdwarfs in our BHk sample may be pulsators, given that they fall in the temperature range of newly-discovered pulsating subdwarfs in omega Cen. In addition to the normal hot HB and BHk stars, we obtain spectra of 5 blue HB stars, a post-HB star, and 3 unclassified stars with unusually blue UV colors.
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new observational evidence of flash mixing on the white dwarf Cooling Curve
Fifth Meeting on Hot Subdwarf Stars and Related Objects, 2011Co-Authors: Thomas M Brown, Thierry Lanz, Allen V Sweigart, Misty Cracraft, I Hubeny, W B LandsmanAbstract:Blue hook stars are a class of subluminous extreme horizontal branch stars that were discovered in UV images of the massive globular clusters w Cen and NGC 2808. These stars occupy a region of the HR diagram that is unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that the blue hook stars are very likely the progeny of stars that undergo extensive internal mixing during a late helium-core flash on the white dwarf Cooling Curve. This "flash mixing" produces hotter-than-normal EHB stars with atmospheres significantly enhanced in helium and carbon. The larger bolometric correction, combined with the decrease in hydrogen opacity, makes these stars appear sub luminous in the optical and UV. Flash mixing is more likely to occur in stars born with a high helium abundance, due to their lower mass at the main sequence turnoff. For this reason, the phenomenon is more common in those massive globular clusters that show evidence for secondary populations enhanced in helium. However, a high helium abundance does not, by itself, explain the presence of blue hook stars in massive globular clusters. Here, we present new observational evidence for flash mixing, using recent HST observations. These include UV color-magnitude diagrams of six massive globular clusters and far-UV spectroscopy of hot subdwarfs in one of these clusters (NGC 2808).
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flash mixing on the white dwarf Cooling Curve far ultraviolet spectroscopic explorer observations of three he rich sdb stars
The Astrophysical Journal, 2004Co-Authors: Thomas M Brown, Thierry Lanz, Allen V Sweigart, I Hubeny, W B LandsmanAbstract:We present Far-Ultraviolet Spectroscopic Explorer (FUSE) spectra of three He-rich sdB stars. Two of these stars, PG 1544+488 and JL 87, reveal extremely strong C III lines at 977 and 1176 A, while the carbon lines are quite weak in the third star, LB 1766. We have analyzed the FUSE data using TLUSTY non-LTE line-blanketed model atmospheres and find that PG 1544+488 has a surface composition of 96% He, 2% C, and 1% N. JL 87 shows a similar surface enrichment of carbon and nitrogen, but some significant fraction of hydrogen still remains in its atmosphere. Finally, LB 1766 has a surface composition devoid of hydrogen and strongly depleted of carbon, indicating that its surface material has undergone CN-cycle processing. We interpret these observations with new evolutionary calculations which suggest that He-rich sdB stars with C-rich compositions are the progeny of stars which underwent a delayed helium-core flash on the white-dwarf Cooling Curve. During such a flash the interior convection zone will penetrate into the hydrogen envelope, thereby mixing the envelope with the He- and C-rich core. Such "flash-mixed" stars will arrive on the extreme horizontal branch (EHB) with He- and C-rich surface compositions and will be hotter than the hottest canonical (i.e., unmixed) EHB stars. Two types of flash mixing are possible: "deep" and "shallow," depending on whether the hydrogen envelope is mixed deeply into the site of the helium flash or only with the outer layers of the core. Based on both their stellar parameters and surface compositions, we suggest that PG 1544+488 and JL 87 are examples of "deep" and "shallow" flash mixing, respectively. Flash mixing may therefore represent a new evolutionary channel for producing the hottest EHB stars. However, flash mixing cannot explain the abundance pattern in LB 1766, which remains a challenge to current evolutionary models.
Thierry Lanz - One of the best experts on this subject based on the ideXlab platform.
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flash mixing on the white dwarf Cooling Curve spectroscopic confirmation in ngc 2808
The Astrophysical Journal, 2012Co-Authors: Thomas M Brown, Thierry Lanz, Allen V Sweigart, Misty Cracraft, I Hubeny, W B LandsmanAbstract:We present new Hubble Space Telescope far-UV spectroscopy of two dozen hot evolved stars in NGC 2808, a massive globular cluster with a large population of 'blue-hook' (BHk) stars. The BHk stars are found in ultraviolet color-magnitude diagrams of the most massive globular clusters, where they fall at luminosities immediately below the hot end of the horizontal branch (HB), in a region of the H-R diagram unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that these subluminous HB stars are very likely the progeny of stars that undergo extensive internal mixing during a late He-core flash on the white dwarf Cooling Curve. This flash mixing leads to hotter temperatures and an enormous enhancement of the surface He and C abundances; these hotter temperatures, together with the decrease in H opacity shortward of the Lyman limit, make the BHk stars brighter in the extreme UV while appearing subluminous in the UV and optical. Our far-UV spectroscopy demonstrates that, relative to normal HB stars at the same color, the BHk stars of NGC 2808 are hotter and greatly enhanced in He and C, thus providing unambiguous evidence of flash mixing in the subluminous population.more » Although the C abundance in the BHk stars is orders of magnitude larger than that in the normal HB stars, the atmospheric C abundance in both the BHk and normal HB stars appears to be affected by gravitational settling. The abundance variations seen in Si and the Fe-peak elements also indicate that atmospheric diffusion is at play in our sample, with all of our hot subdwarfs at 25,000-50,000 K exhibiting large enhancements of the iron-peak elements. The hottest subdwarfs in our BHk sample may be pulsators, given that they fall in the temperature range of newly discovered pulsating subdwarfs in {omega} Cen. In addition to the normal hot HB and BHk stars, we also obtain spectra of five blue HB stars, a post-HB star, and three unclassified stars with unusually blue UV colors.« less
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new observational evidence of flash mixing on the white dwarf Cooling Curve
arXiv: Solar and Stellar Astrophysics, 2012Co-Authors: Thomas M Brown, Thierry Lanz, Allen V Sweigart, Misty Cracraft, I Hubeny, W B LandsmanAbstract:Blue hook stars are a class of subluminous extreme horizontal branch stars that were discovered in UV images of the massive globular clusters omega Cen and NGC 2808. These stars occupy a region of the HR diagram that is unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that the blue hook stars are very likely the progeny of stars that undergo extensive internal mixing during a late helium-core flash on the white dwarf Cooling Curve. This "flash mixing" produces hotter-than-normal EHB stars with atmospheres significantly enhanced in helium and carbon. The larger bolometric correction, combined with the decrease in hydrogen opacity, makes these stars appear subluminous in the optical and UV. Flash mixing is more likely to occur in stars born with a high helium abundance, due to their lower mass at the main sequence turnoff. For this reason, the phenomenon is more common in those massive globular clusters that show evidence for secondary populations enhanced in helium. However, a high helium abundance does not, by itself, explain the presence of blue hook stars in massive globular clusters. Here, we present new observational evidence for flash mixing, using recent HST observations. These include UV color-magnitude diagrams of six massive globular clusters and far-UV spectroscopy of hot subdwarfs in one of these clusters (NGC 2808).
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flash mixing on the white dwarf Cooling Curve spectroscopic confirmation in ngc 2808
arXiv: Solar and Stellar Astrophysics, 2012Co-Authors: Thomas M Brown, Thierry Lanz, Allen V Sweigart, Misty Cracraft, I Hubeny, W B LandsmanAbstract:[Abridged] We present new HST FUV spectroscopy of 24 hot evolved stars in NGC2808, a massive globular cluster with a large population of "blue-hook" (BHk) stars. The BHk stars are found in UV color-magnitude diagrams of the most massive globular clusters, where they fall at luminosities immediately below the hot end of the horizontal branch (HB), in a region of the HR diagram unexplained by canonical stellar evolution theory. Using new evolutionary and atmospheric models, we have shown that these subluminous HB stars are very likely the progeny of stars that undergo extensive internal mixing during a late He-core flash on the white dwarf Cooling Curve. This flash mixing leads to hotter temperatures and an enormous enhancement of the surface He and C; these hotter temperatures, together with the decrease in H opacity shortward of the Lyman limit, make the BHk stars brighter in the EUV while appearing subluminous in the UV and optical. Our FUV spectroscopy demonstrates that, relative to normal HB stars at the same color, the BHk stars of NGC2808 are hotter and greatly enhanced in He and C, thus providing unambiguous evidence of flash mixing. Although the C abundance in the BHk stars is orders of magnitude larger than that in the normal HB stars, the C abundance in both the BHk and normal HB stars appears to be affected by gravitational settling. The variations seen in Si and the Fe-peak elements also indicate that atmospheric diffusion is at play in our sample, with all of our hot subdwarfs at 25,000 to 50,000 K exhibiting large enhancements of the Fe-peak elements. The hottest subdwarfs in our BHk sample may be pulsators, given that they fall in the temperature range of newly-discovered pulsating subdwarfs in omega Cen. In addition to the normal hot HB and BHk stars, we obtain spectra of 5 blue HB stars, a post-HB star, and 3 unclassified stars with unusually blue UV colors.
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new observational evidence of flash mixing on the white dwarf Cooling Curve
Fifth Meeting on Hot Subdwarf Stars and Related Objects, 2011Co-Authors: Thomas M Brown, Thierry Lanz, Allen V Sweigart, Misty Cracraft, I Hubeny, W B LandsmanAbstract:Blue hook stars are a class of subluminous extreme horizontal branch stars that were discovered in UV images of the massive globular clusters w Cen and NGC 2808. These stars occupy a region of the HR diagram that is unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that the blue hook stars are very likely the progeny of stars that undergo extensive internal mixing during a late helium-core flash on the white dwarf Cooling Curve. This "flash mixing" produces hotter-than-normal EHB stars with atmospheres significantly enhanced in helium and carbon. The larger bolometric correction, combined with the decrease in hydrogen opacity, makes these stars appear sub luminous in the optical and UV. Flash mixing is more likely to occur in stars born with a high helium abundance, due to their lower mass at the main sequence turnoff. For this reason, the phenomenon is more common in those massive globular clusters that show evidence for secondary populations enhanced in helium. However, a high helium abundance does not, by itself, explain the presence of blue hook stars in massive globular clusters. Here, we present new observational evidence for flash mixing, using recent HST observations. These include UV color-magnitude diagrams of six massive globular clusters and far-UV spectroscopy of hot subdwarfs in one of these clusters (NGC 2808).
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flash mixing on the white dwarf Cooling Curve far ultraviolet spectroscopic explorer observations of three he rich sdb stars
The Astrophysical Journal, 2004Co-Authors: Thomas M Brown, Thierry Lanz, Allen V Sweigart, I Hubeny, W B LandsmanAbstract:We present Far-Ultraviolet Spectroscopic Explorer (FUSE) spectra of three He-rich sdB stars. Two of these stars, PG 1544+488 and JL 87, reveal extremely strong C III lines at 977 and 1176 A, while the carbon lines are quite weak in the third star, LB 1766. We have analyzed the FUSE data using TLUSTY non-LTE line-blanketed model atmospheres and find that PG 1544+488 has a surface composition of 96% He, 2% C, and 1% N. JL 87 shows a similar surface enrichment of carbon and nitrogen, but some significant fraction of hydrogen still remains in its atmosphere. Finally, LB 1766 has a surface composition devoid of hydrogen and strongly depleted of carbon, indicating that its surface material has undergone CN-cycle processing. We interpret these observations with new evolutionary calculations which suggest that He-rich sdB stars with C-rich compositions are the progeny of stars which underwent a delayed helium-core flash on the white-dwarf Cooling Curve. During such a flash the interior convection zone will penetrate into the hydrogen envelope, thereby mixing the envelope with the He- and C-rich core. Such "flash-mixed" stars will arrive on the extreme horizontal branch (EHB) with He- and C-rich surface compositions and will be hotter than the hottest canonical (i.e., unmixed) EHB stars. Two types of flash mixing are possible: "deep" and "shallow," depending on whether the hydrogen envelope is mixed deeply into the site of the helium flash or only with the outer layers of the core. Based on both their stellar parameters and surface compositions, we suggest that PG 1544+488 and JL 87 are examples of "deep" and "shallow" flash mixing, respectively. Flash mixing may therefore represent a new evolutionary channel for producing the hottest EHB stars. However, flash mixing cannot explain the abundance pattern in LB 1766, which remains a challenge to current evolutionary models.
S. G. Shabestari - One of the best experts on this subject based on the ideXlab platform.
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investigation on the effect of sn on solidification and microstructure of az91 magnesium alloy using Cooling Curve thermal analysis
Thermochimica Acta, 2018Co-Authors: O Sedighi, S. G. Shabestari, F. YavariAbstract:Abstract Thermal analysis technique has been used to study the effect of Sn addition on solidification characteristics and microstructure of AZ91 magnesium alloy. Solidification parameters have been extracted from Cooling Curves, first and second derivative Curves. Different ranges of Sn between 0.1 to 1.0 wt%. have been added to the alloy. The results illustrated that the alloy containing 0.5 wt.% Sn displayed the best solidification behavior such as high nucleation temperature ( T N . α ), low recalescence underCooling (ΔTR), low dendrite coherency temperature and more solid fraction at dendrite coherency point. Microstructure of the alloys has been studied through optical microscopy (OM) and scanning electron microscopy (SEM). The phases were characterized by use of X-ray diffraction (XRD). Microstructural analysis showed that by adding Sn up to 0.5 wt.% into AZ91 alloy, refinement of α-Mg primary dendrites and β-Mg17Al12 took place due to the formation of new Mg2Sn intermetallic compound.
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Effect of Cooling rate and Al content on solidification characteristics of AZ magnesium alloys using Cooling Curve thermal analysis
Journal of Thermal Analysis and Calorimetry, 2017Co-Authors: F. Yavari, S. G. ShabestariAbstract:The solidification behavior of AZ Magnesium alloys in various Cooling conditions was investigated using a computer-aided Cooling Curve thermal analysis method. In each case, the Cooling Curve and its first and second derivative Curves have been plotted using accurate thermal analysis equipment and solidification characteristics were recognized from these Curves. The Cooling rates used in the present study range from 0.22 to 8.13 °C s^−1. The results of thermal analysis show that the solidification parameters of AZ alloys such as nucleation temperature ( T _N,α), nucleation underCooling (∆ T _N,α), recalescence underCooling (∆ T _R,α), range of solidification temperature (∆ T _S) and total solidification time ( t _f) are influenced by variation of Cooling rate. Also, the effect of Al content on these parameters was studied. Microstructural evaluation was carried out to determine the correlation between the Cooling rate and secondary dendrite arm spacing.
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Study on the eutectic and post-eutectic reactions in LM13 aluminum alloy using Cooling Curve thermal analysis technique
Journal of Thermal Analysis and Calorimetry, 2016Co-Authors: V. A. Hosseini, S. G. ShabestariAbstract:Effect of non-equilibrium solidification conditions on the eutectic and post-eutectic reactions temperature and percentage of the phases were investigated using computer-aided Cooling Curve thermal analysis. In addition, hardness, secondary dendrite arm spacing, and maximum pore size were studied at different Cooling conditions. Cooling Curves were determined by setting thermocouples in the center of the molds. Solid fractions were calculated by Newtonian baseline technique. Results showed that increasing the Cooling rate shifted the temperature of post-eutectic reaction upward, except final reaction. Higher Cooling rate increased eutectic percentage about 4 %, but reduced total percentage of post-eutectic phases. Additionally, increasing the Cooling rate shortened the maximum porosity diameter and secondary dendrite arm spacing and increased the hardness of the alloy.
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investigation on solidification conditions in functionally si gradient al alloys using simulation and Cooling Curve analysis methods
Journal of Thermal Analysis and Calorimetry, 2014Co-Authors: F Rikhtegar, S. G. ShabestariAbstract:During recent years, functionally graded alloys have been widely produced by melt processing routes to achieve advanced material properties. In this paper, the new cast-decant-cast (CDC) method has been used to produce the gradient in concentration of Si particles in cross section of final Al–Si castings. For this purpose, the hypereutectic LM28 and hypoeutectic LM25 alloys were selected for each step casting into the steel mold. Cooling Curve thermal analysis and simulation methods were applied to investigate the Cooling behavior of first poured LM28 alloy and improve the accuracy of CDC process by determining the Curves of solid fraction and temperature profiles. The final products were studied through optical microscopy, image analysis, and Brinell hardness measurement. The results showed that the silicon concentration decreased along transition zone between two alloys by increasing the decanting time in the order of 25, 40, and 50 s. This can be due to the lower temperature of exterior LM28 alloy in semi-solid state and shorter solidification time of interior LM25 alloy. This can lead a to reduction of the diffusion rate of elemental silicon along the transition zone. The microscopic scale of transition zone between two alloys developed the maximum thickness of 438 μm and hardness value of 83 HB comparing with the hardness of 88 and 62 HB for external and internal alloys, respectively. The microscopic observations and hardness evaluations confirmed the creation of functionally Si-gradient products.
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computer aided Cooling Curve thermal analysis of near eutectic al si cu fe alloy effect of silicon modifier refiner and solidification conditions on the nucleation and growth of dendrites
Journal of Thermal Analysis and Calorimetry, 2013Co-Authors: Saeed Farahany, Ali Ourdjini, Mohd. Hasbullah Idris, S. G. ShabestariAbstract:The effects of bismuth (Bi), antimony (Sb) and strontium (Sr) additions on the characteristic parameters of the evolution of aluminium dendrites in a near eutectic Al–11.3Si–2Cu–0.4Fe alloy during solidification at different Cooling rates (0.6–2 °C) were investigated by computer-aided Cooling Curve thermal analysis (CA-CCTA). Nucleation temperature (\( T_{\text{N}}^{{\alpha {\text{ - Al}}}} \)) is defined with a new approach based on second derivative Cooling Curve. The results showed that \( T_{\text{N}}^{{\alpha {\text{ - Al}}}} \) increased with increasing Cooling rate but both the growth temperature (\( T_{\text{G}}^{{\alpha {\text{ - Al}}}} \)) and the coherency temperature (TDCP) decreased. Increase in the temperature difference for dendrite coherency (\( T_{\text{N}}^{{\alpha {\text{ - Al}}}} - T_{\text{DCP}} \)) with increasing Cooling rate indicate a wider range of temperature before the dendrite can impinge on each other and higher fraction solid (\( f_{\text{S}}^{\text{DCP}} \)). Additions of Bi, Sb and Sr to the base alloy produced only a minor effect on \( T_{\text{N}}^{{\alpha {\text{ - Al}}}} \). Additions of Bi and Sb resulted in an increase in fraction solid and an increase of 30 % in the value of \( T_{\text{N}}^{{\alpha {\text{ - Al}}}} \, - \,T_{\text{G}}^{{\alpha {\text{ - Al}}}} \) to almost 13 °C.
Thomas M Brown - One of the best experts on this subject based on the ideXlab platform.
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flash mixing on the white dwarf Cooling Curve spectroscopic confirmation in ngc 2808
The Astrophysical Journal, 2012Co-Authors: Thomas M Brown, Thierry Lanz, Allen V Sweigart, Misty Cracraft, I Hubeny, W B LandsmanAbstract:We present new Hubble Space Telescope far-UV spectroscopy of two dozen hot evolved stars in NGC 2808, a massive globular cluster with a large population of 'blue-hook' (BHk) stars. The BHk stars are found in ultraviolet color-magnitude diagrams of the most massive globular clusters, where they fall at luminosities immediately below the hot end of the horizontal branch (HB), in a region of the H-R diagram unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that these subluminous HB stars are very likely the progeny of stars that undergo extensive internal mixing during a late He-core flash on the white dwarf Cooling Curve. This flash mixing leads to hotter temperatures and an enormous enhancement of the surface He and C abundances; these hotter temperatures, together with the decrease in H opacity shortward of the Lyman limit, make the BHk stars brighter in the extreme UV while appearing subluminous in the UV and optical. Our far-UV spectroscopy demonstrates that, relative to normal HB stars at the same color, the BHk stars of NGC 2808 are hotter and greatly enhanced in He and C, thus providing unambiguous evidence of flash mixing in the subluminous population.more » Although the C abundance in the BHk stars is orders of magnitude larger than that in the normal HB stars, the atmospheric C abundance in both the BHk and normal HB stars appears to be affected by gravitational settling. The abundance variations seen in Si and the Fe-peak elements also indicate that atmospheric diffusion is at play in our sample, with all of our hot subdwarfs at 25,000-50,000 K exhibiting large enhancements of the iron-peak elements. The hottest subdwarfs in our BHk sample may be pulsators, given that they fall in the temperature range of newly discovered pulsating subdwarfs in {omega} Cen. In addition to the normal hot HB and BHk stars, we also obtain spectra of five blue HB stars, a post-HB star, and three unclassified stars with unusually blue UV colors.« less
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new observational evidence of flash mixing on the white dwarf Cooling Curve
arXiv: Solar and Stellar Astrophysics, 2012Co-Authors: Thomas M Brown, Thierry Lanz, Allen V Sweigart, Misty Cracraft, I Hubeny, W B LandsmanAbstract:Blue hook stars are a class of subluminous extreme horizontal branch stars that were discovered in UV images of the massive globular clusters omega Cen and NGC 2808. These stars occupy a region of the HR diagram that is unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that the blue hook stars are very likely the progeny of stars that undergo extensive internal mixing during a late helium-core flash on the white dwarf Cooling Curve. This "flash mixing" produces hotter-than-normal EHB stars with atmospheres significantly enhanced in helium and carbon. The larger bolometric correction, combined with the decrease in hydrogen opacity, makes these stars appear subluminous in the optical and UV. Flash mixing is more likely to occur in stars born with a high helium abundance, due to their lower mass at the main sequence turnoff. For this reason, the phenomenon is more common in those massive globular clusters that show evidence for secondary populations enhanced in helium. However, a high helium abundance does not, by itself, explain the presence of blue hook stars in massive globular clusters. Here, we present new observational evidence for flash mixing, using recent HST observations. These include UV color-magnitude diagrams of six massive globular clusters and far-UV spectroscopy of hot subdwarfs in one of these clusters (NGC 2808).
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flash mixing on the white dwarf Cooling Curve spectroscopic confirmation in ngc 2808
arXiv: Solar and Stellar Astrophysics, 2012Co-Authors: Thomas M Brown, Thierry Lanz, Allen V Sweigart, Misty Cracraft, I Hubeny, W B LandsmanAbstract:[Abridged] We present new HST FUV spectroscopy of 24 hot evolved stars in NGC2808, a massive globular cluster with a large population of "blue-hook" (BHk) stars. The BHk stars are found in UV color-magnitude diagrams of the most massive globular clusters, where they fall at luminosities immediately below the hot end of the horizontal branch (HB), in a region of the HR diagram unexplained by canonical stellar evolution theory. Using new evolutionary and atmospheric models, we have shown that these subluminous HB stars are very likely the progeny of stars that undergo extensive internal mixing during a late He-core flash on the white dwarf Cooling Curve. This flash mixing leads to hotter temperatures and an enormous enhancement of the surface He and C; these hotter temperatures, together with the decrease in H opacity shortward of the Lyman limit, make the BHk stars brighter in the EUV while appearing subluminous in the UV and optical. Our FUV spectroscopy demonstrates that, relative to normal HB stars at the same color, the BHk stars of NGC2808 are hotter and greatly enhanced in He and C, thus providing unambiguous evidence of flash mixing. Although the C abundance in the BHk stars is orders of magnitude larger than that in the normal HB stars, the C abundance in both the BHk and normal HB stars appears to be affected by gravitational settling. The variations seen in Si and the Fe-peak elements also indicate that atmospheric diffusion is at play in our sample, with all of our hot subdwarfs at 25,000 to 50,000 K exhibiting large enhancements of the Fe-peak elements. The hottest subdwarfs in our BHk sample may be pulsators, given that they fall in the temperature range of newly-discovered pulsating subdwarfs in omega Cen. In addition to the normal hot HB and BHk stars, we obtain spectra of 5 blue HB stars, a post-HB star, and 3 unclassified stars with unusually blue UV colors.
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new observational evidence of flash mixing on the white dwarf Cooling Curve
Fifth Meeting on Hot Subdwarf Stars and Related Objects, 2011Co-Authors: Thomas M Brown, Thierry Lanz, Allen V Sweigart, Misty Cracraft, I Hubeny, W B LandsmanAbstract:Blue hook stars are a class of subluminous extreme horizontal branch stars that were discovered in UV images of the massive globular clusters w Cen and NGC 2808. These stars occupy a region of the HR diagram that is unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that the blue hook stars are very likely the progeny of stars that undergo extensive internal mixing during a late helium-core flash on the white dwarf Cooling Curve. This "flash mixing" produces hotter-than-normal EHB stars with atmospheres significantly enhanced in helium and carbon. The larger bolometric correction, combined with the decrease in hydrogen opacity, makes these stars appear sub luminous in the optical and UV. Flash mixing is more likely to occur in stars born with a high helium abundance, due to their lower mass at the main sequence turnoff. For this reason, the phenomenon is more common in those massive globular clusters that show evidence for secondary populations enhanced in helium. However, a high helium abundance does not, by itself, explain the presence of blue hook stars in massive globular clusters. Here, we present new observational evidence for flash mixing, using recent HST observations. These include UV color-magnitude diagrams of six massive globular clusters and far-UV spectroscopy of hot subdwarfs in one of these clusters (NGC 2808).
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flash mixing on the white dwarf Cooling Curve far ultraviolet spectroscopic explorer observations of three he rich sdb stars
The Astrophysical Journal, 2004Co-Authors: Thomas M Brown, Thierry Lanz, Allen V Sweigart, I Hubeny, W B LandsmanAbstract:We present Far-Ultraviolet Spectroscopic Explorer (FUSE) spectra of three He-rich sdB stars. Two of these stars, PG 1544+488 and JL 87, reveal extremely strong C III lines at 977 and 1176 A, while the carbon lines are quite weak in the third star, LB 1766. We have analyzed the FUSE data using TLUSTY non-LTE line-blanketed model atmospheres and find that PG 1544+488 has a surface composition of 96% He, 2% C, and 1% N. JL 87 shows a similar surface enrichment of carbon and nitrogen, but some significant fraction of hydrogen still remains in its atmosphere. Finally, LB 1766 has a surface composition devoid of hydrogen and strongly depleted of carbon, indicating that its surface material has undergone CN-cycle processing. We interpret these observations with new evolutionary calculations which suggest that He-rich sdB stars with C-rich compositions are the progeny of stars which underwent a delayed helium-core flash on the white-dwarf Cooling Curve. During such a flash the interior convection zone will penetrate into the hydrogen envelope, thereby mixing the envelope with the He- and C-rich core. Such "flash-mixed" stars will arrive on the extreme horizontal branch (EHB) with He- and C-rich surface compositions and will be hotter than the hottest canonical (i.e., unmixed) EHB stars. Two types of flash mixing are possible: "deep" and "shallow," depending on whether the hydrogen envelope is mixed deeply into the site of the helium flash or only with the outer layers of the core. Based on both their stellar parameters and surface compositions, we suggest that PG 1544+488 and JL 87 are examples of "deep" and "shallow" flash mixing, respectively. Flash mixing may therefore represent a new evolutionary channel for producing the hottest EHB stars. However, flash mixing cannot explain the abundance pattern in LB 1766, which remains a challenge to current evolutionary models.
Saeed Farahany - One of the best experts on this subject based on the ideXlab platform.
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Cooling Curve thermal analysis of al mg 2 si cu x sr composite
Journal of Thermal Analysis and Calorimetry, 2020Co-Authors: Saeed Farahany, Nur Azmah Nordin, Hamidreza GhandvarAbstract:In situ composites are today being considered for industrial use, owing to the fewer production steps involved, lower production cost, and better wetting of reinforcements. This study emphasises the characteristic features of an Al–Mg2Si–Cu in situ composite, with the addition of different amounts of Sr (0.01–0.1 mass%) as a modifier reagent, by employing computer-aided Cooling Curve thermal analysis. The identification of microstructures and phases was carried out using a scanning electron microscope equipped with an energy dispersive spectrometer. The results show that the nucleation temperature of the primary Mg2Si, eutectic Mg2Si, and Al5FeSi phases initially increased with the addition of 0.01 mass% Sr, and subsequently decreased with further addition of the element. Two new Sr-containing phases were detected after the precipitation of primary Mg2Si phase and prior to the formation of eutectic Mg2Si phase. A relationship between the Cooling rate (CR) and solidification rate (SR) was established. Based on cell coherency point, it was found that the eutectic Al–Mg2Si cell required a longer time to grow with the increment of Sr. The solid fraction of Al5FeSi and Al5Cu2Mg8Si6 + Al2Cu phases remained constant at 8 ± 1% and 3 ± 1%, respectively. The increase in the terminal freezing range and the cracking susceptibility coefficient, by 182% and 16%, respectively, shows that Sr increases the probability of hot tearing.
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computer aided Cooling Curve thermal analysis and microstructural evolution of mg 5zn x y cast alloys
Journal of Thermal Analysis and Calorimetry, 2017Co-Authors: Hassan Jafari, Maryam Khalilnezhad, Saeed FarahanyAbstract:Computer-aided Cooling Curve thermal analysis technique was used to investigate microstructure and phase evolutions in Mg–5Zn alloy containing 0–1.5 mass% yttrium (Y) solidified at 0.7 °C s−1 Cooling rate. Optical and field emission scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction analysis were performed to characterise microstructure of the cast alloys. The observations revealed that a considerable refinement, calculated as over 74%, was occurred in the alloys with increasing Y content to 1.5 mass%. The first derivative Curves revealed two peaks corresponding to α-Mg and Mg7Zn3 for the binary Mg–5Zn alloy. The results also showed the evolution of three peaks related to the α-Mg, W-phase (Mg3Zn3Y2) and I-phase (Mg3Zn6Y) in the Mg–5Zn–xY alloys. The nucleation temperature of both α-Mg and I-phase decreased slightly by increasing Y content. In addition, the solidification range and total solidification time increased by 44.4 and 50% with increasing Y content.
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Evaluation of the effect of grain refiners on the solidification characteristics of an Sr-modified ADC12 die-casting alloy by Cooling Curve thermal analysis
Journal of Thermal Analysis and Calorimetry, 2015Co-Authors: Saeed Farahany, Ali Ourdjini, Mohd. Hasbullah Idris, Fadi Faris, Hamidreza GhandvarAbstract:The influence of grain refinement—with the addition of 6 mass% Al–5Ti–B and 10 mass% Al–3Ti–B master alloys—on the temperature-based parameters of a commercial ADC12 die-casting alloy, modified with 0.04 mass% Sr, was investigated using computer-aided Cooling Curve thermal analysis. Results show that the addition of grain refiners caused the reaction area of the aluminium dendrite to shift up. With the addition of refiner reagents, the nucleation temperature and growth temperature for the evolution of the primary Al dendrite increased, while the recalescence temperature decreased. The dendrite coherency temperature ( T _DCP), time ( t _DCP), temperature ( T _N– T _DCP), and time ( t _N– t _DCP) interval for dendrite coherency and solid fraction at coherency point increased with the addition of Al–3Ti–B and Al–5Ti–B grain refiners. Nevertheless, the growth temperature of the eutectic aluminium–silicon phase remained at the modified range (563.5–565 °C) even after the addition of grain refiners. A grain-refined and modified structure was obtained after the addition of refiners, but no obvious mutual effect was found in terms of the coarsening of grain size or silicon demodification in the range of concentrations used in this study.
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computer aided Cooling Curve thermal analysis of near eutectic al si cu fe alloy effect of silicon modifier refiner and solidification conditions on the nucleation and growth of dendrites
Journal of Thermal Analysis and Calorimetry, 2013Co-Authors: Saeed Farahany, Ali Ourdjini, Mohd. Hasbullah Idris, S. G. ShabestariAbstract:The effects of bismuth (Bi), antimony (Sb) and strontium (Sr) additions on the characteristic parameters of the evolution of aluminium dendrites in a near eutectic Al–11.3Si–2Cu–0.4Fe alloy during solidification at different Cooling rates (0.6–2 °C) were investigated by computer-aided Cooling Curve thermal analysis (CA-CCTA). Nucleation temperature (\( T_{\text{N}}^{{\alpha {\text{ - Al}}}} \)) is defined with a new approach based on second derivative Cooling Curve. The results showed that \( T_{\text{N}}^{{\alpha {\text{ - Al}}}} \) increased with increasing Cooling rate but both the growth temperature (\( T_{\text{G}}^{{\alpha {\text{ - Al}}}} \)) and the coherency temperature (TDCP) decreased. Increase in the temperature difference for dendrite coherency (\( T_{\text{N}}^{{\alpha {\text{ - Al}}}} - T_{\text{DCP}} \)) with increasing Cooling rate indicate a wider range of temperature before the dendrite can impinge on each other and higher fraction solid (\( f_{\text{S}}^{\text{DCP}} \)). Additions of Bi, Sb and Sr to the base alloy produced only a minor effect on \( T_{\text{N}}^{{\alpha {\text{ - Al}}}} \). Additions of Bi and Sb resulted in an increase in fraction solid and an increase of 30 % in the value of \( T_{\text{N}}^{{\alpha {\text{ - Al}}}} \, - \,T_{\text{G}}^{{\alpha {\text{ - Al}}}} \) to almost 13 °C.
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The usage of computer-aided Cooling Curve thermal analysis to optimise eutectic refiner and modifier in Al–Si alloys
Journal of Thermal Analysis and Calorimetry, 2012Co-Authors: Saeed Farahany, Ali Ourdjini, M. H. IdrisAbstract:Bismuth, antimony and strontium concentrations were optimised to alter the eutectic Al–Si phase in a commercial Al–Si–Cu–Mg alloy by way of computer-aided Cooling Curve thermal analysis. The results show that the eutectic growth temperature shifted to lower temperatures for all three inoculants. However, addition of Sr resulted in more depression of growth temperature compared with Bi and Sb. No further significant changes were observed with increasing the concentrations to more than 1, 0.5 and 0.04 wt% of Bi, Sb and Sr, respectively. The recalescence of these concentrations, meanwhile, showed a significant increase of magnitude. A good correlation was found between the results of thermal and microstructural analysis. For Bi and Sb, the eutectic depression temperature can be used as an individual criterion to gauge optimal levels of content in the refinement of Si, whereas for Sr, both depression temperature and recalescence magnitude must be considered. Based on the observed depression in eutectic growth temperature and recalescence, it can be concluded that the optimal concentrations to refine the eutectic Al–Si phase with Bi and Sb and to modify it with Sr at the given solidification conditions were 1, 0.5 and 0.04 wt%, respectively.