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تسجيل مستخدم جديدالعمر والتراكيز العالية للعناصر من النجوم الضعيفة المعدنية في جريمة الصقر الصغير
Ages and heavy element abundances from very metal-poor stars in the Sagittarius dwarf galaxy
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تعد سجيتاريوس (Sgr) كوكبة دورفية محطمة كبيرة في حوض المجرة الكبيرة التي تعرضت لعدة أحداث تجذيب. كانت الدراسات الكيميائية السابقة مقيدة بشكل أساسي ببضعة نجوم رشيقة في الحديد ([Fe/H] ~ -1) التي أشارت إلى وجود دالة توزيع مؤشرات الشحنة الأولية (IMF) ذات عرض خفيف. هنا نقدم عينة أولى عالية الدقة من النجوم العظمى الضعيفة في الحديد ([Fe/H] = -1 إلى -3) من الجسم الرئيسي لسجيتاريوس. نحتسب غلاظات 13 عناصر وهي الكربون، الكالسيوم، الكوبالت، الحديد، السرب، الباريوم، اللانتيوم، السيريوم، النديوم، اليورو، الدايسيوم، الببليوم والثوريوم، التي تشكل تحدياً للتفسير الذي تم عليه بناءً على الدراسات السابقة. غلاظاتنا من سجيتاريوس تشبه تلك الخاصة بالحوض الضعيف في الحديد، وتشير أكثر النجوم الضعيفة في الحديد ([Fe/H] ~ -3) إلى تلوث صاروخي نقي. نقدم غلاظات السرب والببليوم والثوريوم لأول مرة في سجيتاريوس، مما يسمح لتحديد العمر باستخدام العلوم الفلكية النووية. نحسب أعماراً تتراوح بين 9$pm$2.5 مليار عام. وقد تم تغذية معظم النجوم المشار إليها في العينة بمجموعة من النجوم الشائعة العظمى المصابة بالحالات الأخيرة (AGB) ذات الأحجام بين 1.3 و 5 M$_{odot}$. تظهر نجمة سجيتاريوس J190651.47-320147.23 فوق الغلاظة الكبيرة في الببليوم (2.05dex) ونمط غلاظات مختلف عن الآخرين الذي يمكن أن يتناسب بشكل أفضل مع نجمة AGB بحجم 3 M$_{odot}$. وبناءً على الانحراف النظري والأنماط الملاحظة للغلاظات، فإن الخلط بين النجوم العظمى الضعيفة والعظمى الكبيرة المصابة بالحالات الأخيرة (AGB) والنيون الشعاعية (15-25 M$_{odot}$) من الضروري لشرح هذه الأنماط. ويشير المستوى العالي (0.29$pm$0.05 dex) للكالسيوم إلى وجود نيون شعاعية كبيرة يجب أن تكون قد تلوثت الفضاء الأوسع في سجيتاريوس قبل فقدانه للغاز. وهذا النتيجة متناقض مع دالة توزيع مؤشرات الشحنة الأولية (IMF) ذات عرض خفيف وعدم وجود نجوم كبيرة تلوث سجيتاريوس.
Sagittarius (Sgr) is a massive disrupted dwarf spheroidal galaxy in the Milky Way halo that has undergone several stripping events. Previous chemical studies were restricted mainly to a few, metal- rich ([Fe/H]~ -1) stars that suggested a top-light initial mass function (IMF). Here we present the first high-resolution, very metal-poor ([Fe/H]=-1 to -3) sample of 13 giant stars in the main body of Sgr. We derive abundances of 13 elements namely C, Ca, Co, Fe, Sr, Ba, La, Ce, Nd, Eu, Dy, Pb, and Th which challenge the interpretation based on previous studies. Our abundances from Sgr mimic those of the metal-poor halo and our most metal-poor star ([Fe/H]~ -3) indicates a pure r-process pollution. Abundances of Sr, Pb, and Th are presented for the first time in Sgr, allowing for age determination using nuclear cosmochronology. We calculate ages of 9$pm$2.5 Gyr. Most of the sample stars have been enriched by a range of asymptotic giant branch (AGB) stars with masses between 1.3 and 5 M$_{odot}$. Sgr J190651.47-320147.23 shows a large overabundance of Pb (2.05dex) and a peculiar abundance pattern best fit by a 3 M$_{odot}$ AGB star. Based on star-to-star scatter and observed abundance patterns a mixture of low- and high-mass AGB stars and supernovae (15-25 M$_{odot}$) are necessary to explain these patterns. The high level (0.29$pm$0.05 dex) of Ca indicates that massive supernovae must have existed and polluted the early ISM of Sgr before it lost its gas. This result is in contrast with a top-light IMF with no massive stars polluting Sgr.
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