نسبت جرم در نواختر کوتوله‌ی OT J002656.6+284933

نویسنده

استادیار، دانشکده فیزیک، دانشگاه دامغان، دامغان، ایران

چکیده

OT J002656.6+284933 یک سیستم ستاره­ای دوتایی است که متشکل از یک ستاره کوتوله سفید (ستاره اولیه) و یک ستاره کم جرم (ستاره ثانویه) می­باشد. مشاهدات تأیید کرده­اند که این جسم می‌بایست یک نواختر کوتوله باشد که در دوره ابرفوران، برآمدگی­های متناوب به نام سوپرهامپ را در منحنی نوری‌اش نشان می­دهد. مشاهدات اخیر بر روی نواختر کوتوله­ی OT J002656.6+284933 دلالت بر دوره تناوب مداری 0.13d، مقدار جرم ستاره ثانویه  و سوپرهامپ با دوره تناوب 0.13225d دارد. متأسفانه تحلیل داده­های مشاهداتی موفق در محاسبه دقیق نسبت جرم سیستم (که حاصل تقسیم جرم ستاره ثانویه به جرم کوتوله ­سفید می­باشد) نبوده و عدم قطعیت قابل‌ملاحظه‌ای برابر با 0.1-0.15 را برای نسبت جرم این سیستم ستاره­ای تخمین زده­اند. به جهت رفع مشکل تخمین نسبت جرم و همچنین به جهت مدل‌سازی این سیستم ستاره­ای، در این مقاله با استفاده از روش هیدرودینامیک ذرات همواره شده، این نواختر کوتوله را با توجه به دوره تناوب مداری مشاهداتی 0.13d و نسبت جرمی­ بین 0.14 تا 0.18 شبیه­سازی نموده­ایم. نتایج شبیه­سازی­ها حاکی از آن است که به‌ازای نسبت جرمی تقریباً 0.145 میتوان به دوره تناوب سوپرهامپ مشاهداتی 0.13225d دست یافت.

کلیدواژه‌ها

موضوعات


عنوان مقاله [English]

The mass ratio of dwarf nova OT J002656.6+284933

نویسنده [English]

  • Kazem Faghei
Assistant Professor, school of physics, Damghan University, Damghan, Iran
چکیده [English]

Dwarf novae are a class of cataclysmic variables that consist of a white dwarf star (as primary star) and a low-mass donor star (as secondary star).  The white dwarf and the secondary star orbit each other once every few hours. When the low-mass secondary star fills its Roche-lobe, it becomes tidally distorted due to its vicinity to the white dwarf. Thus, the secondary star matter is transferred into the white dwarf Roche-lobe. The matter being transferred has high angular momentum in respect to the white dwarf and it forms an accretion disk around the white dwarf. The angular momentum of the accretion disk is transferred by viscous torques from the inner regions of the disk to the outer regions. The viscosity is also responsible for heat generation in the accretion disk. The energy generated by viscous dissipation inside the disk is radiated away from the surface of it. That is why the accretion disk has been found to be the source of the luminosity in the dwarf novae. The light curve of a dwarf nova shows the suddenly increase of brightness which is known as outburst. The SU UMa stars, which are a subclass of dwarf novae, exhibit two distinct modes of outburst, normal outburst and superoutburst.  The normal outbursts have amplitude of approximately equal to three magnitude and last typically from one to four days. However, the superoutbursts are approximately one magnitude brighter than normal outbursts and last as long as a couple of few weeks rather than just a few days. During superoutburst, the periodic humps with the name of superhump appear in the light curve of SU UMa stars. The superhump is an additional variation of the brightness which has a period that is a few percent longer than the orbital period of binary star system.  The superhumps are seen in systems with the mass ratio smaller than 0.3, with being the mass ratio as the ratio of masses of secondary star to primary star. In such systems, the disk grows to a size a where and a tidal instability that induces the accretion disc to become eccentric and starts to process in the corotating frame. The superhumps are appeared in the light curve, because of the tidal effects of donor star on the disk and also the viscous dissipation is large when the bulk of the eccentric disc passes the donor star. The observations of OT J002656.6+284933 have confirmed that this object should be a dwarf nova of SU UMa type which the superhumps appear in its light curve in during superoutburst. The recent observations imply that dwarf nova OT J002656.6+284933 has the orbital period 0.13d, the secondary star with massand the superhump period 0.13225d. The analysis of observational data has not been successful to calculate an exact value for the mass ratio (which is the ratio of masses of secondary star to primary star); their estimation for the mass ratio has the uncertainty between 0.1 and 0.15. To solve the problem regarding mass ratio estimation and to simulate this object, we have simulated this dwarf nova in a two dimensional approach using the smoothed particle hydrodynamics method. As mentioned before, the superhumps are the distinctive humps on the light curve and its period can be estimated accurately. Thus, we applied the different values of the mass ratio in the simulations that for which one of them, we could reach to the observational superhump period value.  In the simulations, we assumed that the observational orbital period is 0.13d and applied the mass ratio between 0.14 and 0.18. The simulations imply that the superhump period 0.13225d can be obtained approximately for the mass ratio 0.145.

کلیدواژه‌ها [English]

  • accretion
  • accretion disks
  • stars: cataclysmic variables
  • nova
  • dwarf nova
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