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Y矮星

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Y矮星是具有Y型光谱的天体。要么是棕矮星或自由漂浮的行星质量天体。它们的温度比T矮星更低，大约在500 K（227°C；440°F）左右[1][2]。与巨行星木星比较，Y矮星的光谱与木星相似[3]。

早期理论和发现

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詹姆斯·韦伯太空望远镜 的图像，显示了最冷的Y矮星WISE 0855-0714的运动。

在L矮星和T矮星中提到字母Y可用于额外的光谱类别[4]。在2000年代初期，已经有理论认为在 T矮星之外应该存在温度更低的天体，并且这些天体将弥合T矮星与太阳系的巨行星之间的空隙。比T矮星冷的天体主要会发射红外线作为热辐射，因此使用红外望远镜进行观测和发现，例如WISE， Spitzer和JWST是意料之中的。对这种寒冷物体的建模预测了钠（Na D）和钾（K I）特征在500 K左右消失，而水云在400-500 K左右出现。预测氨云存在于160 K左右以下。[5]。这些云的形成在萨达斯基气态巨行星分类法中得到了早期的理论化[6]。

在2010年和2011年提出了一些候选者之后[7][8][9]，用WISE发现了更大的Y矮星样本，并使用UGPS 0722-05做为T9的标准和 WISE 1738+2732做为Y0的标准[1][2]。一个重大发现是发现了 WISE J0855−0714，它仍然是发现的最冷、最接近的Y矮星[10]。它的温度为285 K（12 °C；53 °F），具有最新的 Y4光谱型[11]。

Y型光谱

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WISE 0359−5401的JWST光谱，显示不同的分子吸收带。

Y矮星的特征是其深甲烷（CH4）和水蒸气 （H2O）波段，以及较窄的J波段峰值的峰值。 如果光谱类型晚于T8，则J波段峰值会变得更窄。早期观测还显示了近红外光谱中存在氨（NH3）的证据[1][2]。使用JWST的现代观测检测到Y矮星大气中的 CH4、H2O、NH3、一氧化碳（CO）和 二氧化碳（CO2）。尽管预测存在磷化氢（PH3），但却从大气中消失[12][13]。JWST的观测表明，模型低估了CO2的丰度，而高估了晚期T和Y矮星的 PH3。对缺乏PH3的解释是，对磷化学的理解不完整或大气的不同混合，它凝结成磷酸二氢铵（NH4H2PO4）的云。通过更好地了解 CO2化学成分与CO的化学关系[14]。 CH4、H2O和NH3吸收特性随著温度的降低而加深。5 μm峰没有显示出这种相关性，而是显示出很大的多样性。该区域受多种分子的影响，包括CO和CO2，这些分子在不同来源之间的差异很大。这种变化的原因可能是由于不同的表面重力或由于金属量的差异[13][15]。注意到CO2从T矮星到Y矮星略有下降，但CO没有[15]。硫化氢 （H2S）用于提高T矮星和Y矮星的光谱拟合。目前被检测到 H2S的Y矮星只有WISE 1828+26和WISE 0359−5401[16]。

较冷的低层大气

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通常棕矮星具有绝热形式的压力–温度（P–T）剖面，这意味著压力和温度随深度的增加而增加。JWST的光谱学和光度测定表明Y矮星具有非标准绝热形式的P-T剖面。这意味著大气的上层温度较高，而大气的下层温度较低。这可以通过这些孤立天体的快速旋转来解释。 快速旋转导致动力学、热和化学变化，从而破坏了热量从低层大气到高层大气的对流传输。这种不同的P-T分布会影响光谱的形状，并影响Y矮星大气中含碳和含氮分子的组成[17]。

云和可变性

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自2000年代初以来，Y矮星就存在著水云的理论[6][5]。然而，Y矮星也可能有由其它凝聚物组成的云，如硫化物、氯化钾（KCl）和可能的磷酸二氢铵（NH4H2PO4）。这些云将存在于较冷的 Y 矮星的任何水云下方[18]。一些Y矮星可能太热而无法形成水云，但可能还有其它可观测的云[19]。第一颗发现的Y矮星变星是WISE 1405+5534（Y0），它的变率是用一个亮点建模的[20]。另一颗Y矮星变星是WISE 1738+2732（Y0）及其变化可以用KCl和硫化钠 （Na2S）云分解成斑驳的云层来解释[19]。 WISE 0855−0714（Y4）被怀疑有水冰云，这应该会产生较大的振幅变化。它确实显示出4-5%的相对较小的变化，这可能意味著两个半球都有相似的云层覆盖[21]。

奇特的Y矮星

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通过甲烷排放检测到CWISEP J1935-1546中的极光。

目前，只有后缀pec代表“奇特（peculiar）”或“不寻常（unusual）”，存在于Y矮星中。任何光谱特性都以这种方式表示，例如WISE 1639−6847（Y0pec）的Y波段峰值和 Y-J颜色，与其它Y矮星不同[22]。在某些情况下，这种特性可以用非太阳的金属量或不寻常的表面重力来解释。一个例子是CWISE J1055+5443，为此，研究人员发现低重力模型更适合光谱，这可能是由于年龄较小[23]。JWST观测发现了两颗Y矮星，具有含碳分子的不寻常光谱特征。CWISEP J1047+54显示异常强的CO和CO2，并且可能较弱的CH4[13]。在WISE J1206+8401中发现了类似的强CO和CO2吸收特征[15]。另一方面，WISE J0535−75显示没有可辨别的CO2和几乎无法检测到的CO，但与具有相似温度的Y矮星相比，它也显示出更强的NH3吸收[13]。JWST另一个值得注意的发现是CWISEP J1935-1546光谱中甲烷的排放，这可以用极光的存在来解释[24]。最早被怀疑的Y型次矮星之一是WISEA J1534−1043，它显示出不寻常的蓝色。然而，需要光谱观测来证实这一假设[25]。

系外行星和伴星

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Y矮星的质量估计在3-29MJ之间，但更常见的是低于21 MJ。这使它们类似于大质量系外行星[26][27][28]。

只有一颗已确认的Y矮星与一颗称为WD 0806−661 B的白矮星共同移动[29]。尽管质量与行星相同，但罗德里格斯（英语：Rodriguez）等人认为它不太可能以与行星相同的方式形成[8]。此外，还有围绕著一颗主序星运行的T/Y伴侣，罗斯19B[30]。存在一小部分温度低于500 K的系外行星样本（候选者），将来可以通过光谱学证实为Y矮星。这些系外行星是 印地安座εAb（275 K）[31]、WD 1202–232b、WD 2105–82b（均为低温）[32]、GALEX J071816.4+373139b（400 K）[33]和WD 0310–688b（248+84
−61 K）[34]。

联星

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在发现Y矮星的早期，由晚期T矮星主星和Y矮星伴星组成的CFBDS J1458+10是发现的第二对联星[9]。其它此类联星包括WISEPC J1217+1626[35]和WISE J0146+4234[36]。第一对Y+Y联星是JWST在2023年发现的WISE J0336−0143[37]。

个别发现

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参见：Y矮星列表

Y矮星发现时程表：

  • 2011年2月：卢曼等人报导了WD 0806−661 B的发现，这是一颗与附近白矮星相伴的棕矮星，温度为c.300 K（27 °C；80 °F），质量为7 MJ[29]。
  • 2011年2月：不久之后，刘等人发表了一篇关于一颗“非常冷”（c.370 K（97 °C；206 °F））的棕矮星围绕另一颗质量非常低的棕矮星运行的报导，并指出：“鉴于其低光度、非典型颜色和低温，CFBDS J1458+10B是假设的Y光谱类的有希望的候选者[9]。”
  • 2011年8月：科学家利用美国国家航空暨太空总署（NASA）的广角红外巡天探测器（WISE）的数据发现了六颗温度低至25 °C（298 K；77 °F）天体，他们将其归类为Y矮星[38][39]。这些发表在两篇论文中[1][2]。
  • 2012年7月：发现了7颗新的Y矮星，使确认的Y矮星总数达到14颗[40][41]。其中一颗名为WISE 1828+2650的Y矮星，截至2011年8月，是最冷棕矮星的记录保持者——它根本不发射可见光，这种天体更像是一颗自由漂浮的行星，而不是一颗恒星。 最初估计WISE 1828+2650的大气温度低于300 K（27 °C；80 °F）[42]。之后，它的温度被修正了，新的估计将其范围定在250至400 K（−23至127 °C；−10至260 °F）[43]。
  • 2012年11月：发现WISE J1639−6847。截至2024年2月，它是已知距离地球第二近的Y矮星。[44]
  • 2014年4月：WISE 0855−0714据估计，温度分布约为225至260 K（−48至−13 °C；−55至8 °F），质量为3至10 MJ[10]。这也是不寻常的，因为观察到它的视差意味著与太阳系的距离接近至7.2 ± 0.7光年。
  • 2015年5月：哈伯太空望远镜发现了三颗Y矮星，使确认的Y矮星总数达到21颗[49]。
  • 2019年8月：对CatWISE星表的搜索显示CWISP J1935-1546，它是最冷的棕矮星之一，估计温度为270至360 K（−3至87 °C；26至188 °F）[50]。2023年，宣布CWISEP J1935-1546因极光而产生甲烷排放[51]。
  • 2020年2月：CatWISE目录结合了美国国家航空暨太空总署的WISE和NEOWISE调查[53]。它扩大了微弱源的数量，因此被用来寻找最微弱的棕矮星，包括Y矮星。CatWISE研究人员发现了17颗Y矮星候选者。史匹哲太空望远镜的初始颜色表明，CW1446是最红、最冷的Y矮星之一[54]。史匹哲的其它资料显示，CW1446是第五红的棕矮星，温度约为310至360 K（37至87 °C；98至188 °F），距离约为10秒差距[55]。
  • 2020年8月：通过后院世界项目发现了五颗Y矮星候选者[56]。
  • 2021年4月：CatWISE和后院世界团队在一篇合作论文中发表了新的Y矮星候选者[57]。
  • 2021年8月：罗斯19B，后院世界团队在T/Y边界附近发现了一颗围绕M矮星运行的古老天体[30]。
  • 2023年4月：WISE J0336−0143被JWST确认为Y矮星联星[58]。B次矮星可能是截至2023年12月确认的最冷的Y矮星之一，估计温度为246至404 K（−27至131 °C；−17至268 °F）[59]。
  • 2023年11月：CWISE J1055+5443，先前被归类为T矮星的天体被确认为附近的Y矮星[60]。
  • 2023年12月：发表三颗新的Y矮星候选者[59]。截至2024年2月，已确认的Y矮星总数为27颗，另有30颗Y矮星候选者。

参考资料

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