کوتوله قهوه‌ای کوچک

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مقایسه‌ای میان خورشید، یک کوتولهٔ قهوه‌ای کوچک جوان و سیارهٔ هرمز. با گذر زمان کوتولهٔ قهوه‌ای کوچک، سردتر و کوچکتر می‌شود.

کوتولهٔ قهوه‌ای کوچک، یک جرم سیاره‌گون است که به دور یک ستاره نمی‌گردد ولی آن را یک کوتولهٔ قهوه‌ای نیز در نظر نمی‌گیرند؛ چون جرمش کمتر از ۱۳ برابر جرم هرمز، جرم یک کوتولهٔ قهوه‌ای است.[۱]

کوتوله‌های قهوه‌ای ناکام[ویرایش]

کوتوله‌های قهوه‌ای کوچک، به همان ترتیبی که ستاره‌ها تشکیل می‌شوند، ایجاد شده‌اند یعنی بوسیلهٔ فروپاشی ابری از گازها و نه از راه فروپاشی هسته یک دیسک گازی. تفاوت در روش تشکیل یک سیاره و یک کوتولهٔ قهوه‌ای کوچک، در میان همهٔ ستاره شناسان مورد پذیرش نیست.[۲]

کمینهٔ جرم[ویرایش]

کمترین جرم لازم از ابر گازی، که بتواند از راه فروپاشی یک کوتولهٔ قهوه‌ای کوچک ایجاد کند، تقریباً برابر با ۱ Mj است. به این دلیل که برای فروپاشی نیاز به نیروی گرانش و در مقابل تولید گرما و انرژی است.[۳]

جستارهای وابسته[ویرایش]

منابع[ویرایش]

  • ویکی‌پدیای انگلیسی
  1. Working Group on Extrasolar Planets - Definition of a "Planet" POSITION STATEMENT ON THE DEFINITION OF A "PLANET" (IAU)
  2. What is a Planet? Debate Forces New Definition, by Robert Roy Britt, 02 November 2000
  3. Nomenclature: Brown Dwarfs, Gas Giant Planets, and ?, Brown Dwarfs, Proceedings of IAU Symposium #211, held 20–24 May 2002 at University of Hawaii, Honolulu, Boss, A. P., Basri, G., Kumar, S. S., Liebert, J., Martín, E. L., Reipurth, B
Comparison: the Sun, a young sub-brown dwarf, and Jupiter. As the sub-brown dwarf ages, it will gradually cool and shrink.

A sub-brown dwarf or planetary-mass brown dwarf is an astronomical object that formed in the same manner as stars and brown dwarfs (i.e. through the collapse of a gas cloud) but that has a mass below the limiting mass for thermonuclear fusion of deuterium (about 13 MJ).[1] Some researchers call them free-floating planets[2] whereas others call them planetary-mass brown dwarfs.[3]

Description

Sub-brown dwarfs are formed in the manner of stars, through the collapse of a gas cloud (perhaps with the help of photo-erosion) but there is no consensus amongst astronomers on whether the formation process should be taken into account when classifying an object as a planet.[4] Free-floating sub-brown dwarfs can be observationally indistinguishable from rogue planets which originally formed around a star and were ejected from orbit, whilst on the other hand, a sub-brown dwarf formed free-floating in a star cluster may get captured into orbit around a star. A definition for the term "sub-brown dwarf" was put forward by the IAU Working Group on Extra-Solar Planets (WGESP), which defined it as a free-floating body found in young star clusters below the lower mass cut-off of brown dwarfs.[5]

Lower mass limit

The smallest mass of gas cloud that could collapse to form a sub-brown dwarf is about 1 Jupiter mass (MJ).[6] This is because to collapse by gravitational contraction requires radiating away energy as heat and this is limited by the opacity of the gas.[7] A 3 MJ candidate is described in a 2007 paper.[8]

List of possible sub-brown dwarfs

Orbiting one or more stars

There is no consensus whether these companions of stars should be considered sub-brown dwarfs or planets.

Orbiting a brown dwarf

There is no consensus whether these companions of brown dwarfs should be considered sub-brown dwarfs or planets.

Free-floating

Also called Rogue planets:

See also

References

  1. ^ Working Group on Extrasolar Planets – Definition of a "Planet" Archived 16 September 2006 at the Wayback Machine POSITION STATEMENT ON THE DEFINITION OF A "PLANET" (IAU)
  2. ^ Delorme, P.; et al. (December 2012). "CFBDSIR2149-0403: a 4–7 Jupiter-mass free-floating planet in the young moving group AB Doradus ?". Astronomy & Astrophysics. 548: A26. arXiv:1210.0305. Bibcode:2012A&A...548A..26D. doi:10.1051/0004-6361/201219984.
  3. ^ Luhman, K. L. (21 April 2014). "Discovery of a ~250 K Brown Dwarf at 2 pc from the Sun". The Astrophysical Journal Letters. 786 (2): L18. arXiv:1404.6501. Bibcode:2014ApJ...786L..18L. doi:10.1088/2041-8205/786/2/L18.
  4. ^ What is a Planet? Debate Forces New Definition, by Robert Roy Britt, 02 November 2000
  5. ^ IAU WGESP, 'Position Statement on the Definition of "Planet"', 28 February 2003
  6. ^ Boss, Alan P.; Basri, Gibor; Kumar, Shiv S.; Liebert, James; Martín, Eduardo L.; Reipurth, Bo; Zinnecker, Hans (2003), "Nomenclature: Brown Dwarfs, Gas Giant Planets, and ?", Brown Dwarfs, 211: 529, Bibcode:2003IAUS..211..529B
  7. ^ Scholz, Alexander; Geers, Vincent; Jayawardhana, Ray; Fissel, Laura; Lee, Eve; Lafreniere, David; Tamura, Motohide (2009), "Substellar Objects in Nearby Young Clusters (Sonyc): The Bottom of the Initial Mass Function in Ngc 1333", The Astrophysical Journal, 702 (1): 805–822, arXiv:0907.2243v1, Bibcode:2009ApJ...702..805S, doi:10.1088/0004-637X/702/1/805
  8. ^ Scholz, Aleks; Jayawardhana, Ray (2007), "Dusty disks at the bottom of the IMF", The Astrophysical Journal, 672 (1): L49–L52, arXiv:0711.2510v1, Bibcode:2008ApJ...672L..49S, doi:10.1086/526340