| 摘要: | 傳統觀點認為，銀河盤面中較高的天體及星際介質密度，使重力束縛系統難以長期存續，因此，主要分布在盤面的疏散星團大多會在形成後的十億年內瓦解。然而，年老卻仍然擁有豐富成員星的疏散星團挑戰了這一觀點。我們研究了八個符合此特徵的疏散星團：Collinder 261、NGC 2158、NGC 2477、NGC 2506、NGC 6791、NGC 6819、NGC 7789、Trumpler 5，這些疏散星團的年齡皆與球狀星團相當，且仍維持著超過一千顆成員星受其重力束縛範圍。利用Gaia DR3的光度及天體測量學數據，我們確定了每個目標星團迄今最可靠的成員星列表，並根據主序後的成員星，透過等時線模型的擬合推導它們的年齡。以NGC 6791 為例，其星團半徑20.6 角分內，蘊含超過5000顆恆星，距離地球4.4千秒差距，且年齡高達85億年，證
實了這些離群的疏散星團不僅長壽，且仍懷有大量成員星。此外，透過「偽陽性」比較樣本——即空間分布遠離星團區域，但與成員星擁有相同自行運動及視差的恆星——我們推導出星團的光度函數及質量函數。再將Gaia（極限星等約21.5 等）與Pan-STARRS（極限星等約23.5 等）的數據比對後發現，我們的目標星團皆呈現頂端較重的質量函數，這並非數據的不完整所致，而是由低質量成員星的匱乏所造成，明確顯示了這些疏散星團的瓦解過程。最後，我們提出這些難以捉摸的星團能夠倖存的可能解釋，即它們的軌道相對孤立，使其免受銀河盤面擾動的影響。;The presence of old open clusters but remaining rich in members challenges the conventional view that such systems dissociate within 1 Gyr in the typically inhospitable Galactic disk. We present a study of eight such clusters—Collinder 261,
NGC 2158, NGC 2477, NGC 2506, NGC 6791, NGC 6819, NGC 7789, and Trumpler 5—each with an age comparable to those of globular clusters yet harboring thousands of members. Using Gaia DR3, we determined the most reliable member list so far for these clusters, hereafter fitting the post-main sequence members with isochrone models to derive their ages. For example, NGC 6791, determined to have a cluster radius of 20.6 arcmin and more than 5000 members, lies 4.4 kpc away
and has an age of 8.5 Gyr, confirming its longevity and rich membership. With a comparison sample of "false positives", i.e., stars way outside the cluster region yet sharing the same proper motion and parallax ranges, the luminosity function and hence the mass function are derived. All eight star clusters show a relative depletion of low-mass members, signifying an advanced member ejection. A comparison of the Gaia catalog (limiting mag of 21.5) against Pan-STARRS 1 (limiting mag of 23.3) suggests that the top-heavy mass function is not due to data incompleteness, lending clear evidence of cluster dissociation. We propose a possible explanation for the survival of these elusive clusters to be their relative orbital isolation from the Galactic disk perturbation. |
