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L矮星

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按光谱类型划分的L矮星特性[1]
子类 温度 发光度
M8 2300 ~0.0007 Lʘ
L0 2100 K ~0.00035 Lʘ
L2 1950 K ~0.0001 Lʘ
L4 1800 K ~0.000075 Lʘ
L6 1700 K ~0.000065 Lʘ
L8 1500 K ~0.000045 Lʘ

L矮星是光谱类型L(也称为 D矮星)的天体,可以是低质量的恒星 [2]棕矮星[3]或年轻的自由漂流的行星质量天体[4]。如果通过直接成像检测到年轻的系外行星或行星质量伴星,它也可能具有L光谱类型,例如仙女座κb[5]

光谱特性

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与M6矮星相比,科鲁-1英语Kelu-1(L型联星,底部的线)的光谱显示出更强的TiO和钠吸收。

2MASS之前,已知光谱类型晚于M9.5V的天体只有六颗。随着20个新的晚型天体的发现,有必要定义L型和T型光谱类型。柯克帕特里克等人在1999年定义了这两种光谱类型。在这些L-矮星中,存在于晚期M-矮星中的金属氧化物TiOVO)被金属氢化物(例如CrHFeH)和中性碱金属(如KRbCs所取代。L矮星和T矮星之间的跃迁是通过光谱中出现甲烷(CH4)来定义的[6]。M矮星在其近红外光谱中显示出由水蒸气(H2O)引起的吸收。这种吸收特征随着L光谱类型的后期类型而增强,而一氧化碳(CO)的吸收在光谱类型上几乎没有变化[7]。在T矮星,CO被CH4取代[8]。最初估计最热的L0矮星的温度约为2000K,最冷的L8矮星的温度为1500K左右[6]。现代估计范围从L9的1100 K到L0的最高2500 K[9][10]

L矮星具有红色紫罗兰紫紫色的颜色,这是由于D线的吸收,该线的中心位于5890Å,与绿色重叠[8]。后来的研究将L矮星描述为紫色[11]

次矮星

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次矮星是金属丰度低的天体。这些天体通常很老,它们的金属性会影响不同的吸收特性。特别是,氢分子碰撞诱导吸收导致H-K波段的抑制,这导致L型次矮星具有蓝色近红外颜色。2MASS J0532+8246是发现的第一颗L型次矮星。前缀sd、esd和usd表示次矮星、极次矮星和超次矮星。 带有usd首码的天体金属丰度最低[12]

主序星

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对于具有太阳金属量的天体,氢燃烧最小质量为0.075M(78.5MJ[13]。超冷基本参数表列出了几个红外光谱类型为L0至L4、质量高于78.5MJ的天体。该表中质量最高的L矮星之一是G 239-25B(L0),他们发现其质量为88.9±0.59MJ[9][10]。氢燃烧极限取决于金属量,金属量低的天体可能具有高的氢燃烧极限。另一个因素是较低的金属量会使大气更加透明。因此,较老的天体具有更高的温度[14]。具有早期L光谱类型的老L次矮星可能是主序星[15]。例如,棕矮星SDSS J0104+1535(usdL1.5,0.086±0.0015M)刚好低于约0.088M的氢燃烧极限,它的金属量 [Fe/H] = -2.4 ± 0.2.[14]。同一团队也发现,已知的L-次矮星中有三分之一是次恒星,三分之二是低质量恒星[2]CWISE J1249+3621(sdL1, 0.082+0.002
−0.003
M)是另一个恒星的例子,因为对[M/H]=-1的恒星,氢燃烧的极限在0.080左右。这颗恒星也是一颗超高速星[15]

棕矮星

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大多数L矮星是棕矮星。棕矮星是质量低于78.5MJ的天体[13]。质量低于14MJ的天体通常被称为行星质量天体[16],但根据它们的形成机制,它们也被称为行星质量棕矮星[17]

在超冷基本参数表中,目前有422个红外光谱类型为L、质量范围为14-78.5MJ的物体[9][10]。 此外,已知有数十颗L型棕矮星与恒星、白矮星或棕矮星共同运动[3]。发现的第一颗L型棕矮星是GD 165B,它围绕着一颗白矮星运行[18]。它的质量后来被确定为62.58±15.57MJ[19]

行星质量天体和系外行星

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自由漂浮的行星质量天体PSO J318.5−22,是一颗L矮星。

行星质量天体通常被定义为质量低于14MJ的天体。这些天体可以自由浮动[16]或与恒星或棕矮星共同运动(例如HD 106906 b[20][21]。如果一个这样的天体围绕一颗恒星运行的距离在约100天文单位内,它就被称为系外行星。超过100 AU时,它被称为行星质量伴星,因为理论预测这些天体是自己形成的,而不是由原行星盘的材料形成的[22]。靠近这个 100 天文单位边界的一颗系外行星是Delorme 1 (AB)b,它可能是通过拱星盘的碎裂形成的,因此被认为是一颗系外行星[23]。 更近的行星,例如HR 8799周围的行星[24]仙女座κ b也类似于L矮星或具有L光谱型[5]

这些天体通常通过它们的年轻的年龄来识别。例如,一个天体可以出现在一个年轻的星团中(例如NGC 1333[25]或年轻的星协 (参见附近星协列表清单)。研究人员可以使用温度-年龄或光度-年龄关系来确定其质量是否低于13MJ[16]。对于非常年轻的星团(<1百万年),即使是L0光谱类型也对应于行星质量,因此这样一个星团中的所有L矮星都有一个行星质量[25]

另一种方法是确定年轻的其他指标。例如,质样较低的天体具有较低的表面重力,这会导致更广阔的大气层和更多的垂直混合。这将影响某些光谱特征的深度,并可能导致红色近红外颜色。低重力L矮星通常用后缀 β、γ 和 δ 表示,表示中等(β)、低(γ)和非常低(δ)的重力。低重力的L3-L5矮星也可以显示吸收。但所谓的“锂测试”对于确定低质量的年轻L矮星不太可靠[26]。低重力天体的一个例子是CWISE J0506+0738, 它的光谱类型介于L8γ和T0γ之间,质量可能为7±2MJ[16]

可变性和云

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自2000年代初以来,云及其顶部有硅酸盐云被认为是L矮星的的理论[27]。L矮星中存在的硅酸盐已经被史匹哲太空望远镜观察到。特别是L4-L6矮星经常表现出硅酸盐吸收。但任何L矮星也可能不存在硅酸盐吸收[28]。变率通常与L矮星和T矮星中云的存在有关。然而,还有其它可能的解释,例如热点、温度变化和 极光。特别是年轻的天体表现出可变性[29]。最易变的L矮星之一是行星质量伴星VHS J1256–1257b (L7),振幅为33-38%[30]

磁场和极光

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可以在L矮星中检测到电波发射,这种电波发射有时显示旋转的周期性电波脉冲。此外,在早期L矮星中常见的H-α发射被解释为色球日冕,但在后来的光谱类型中,它越来越成为一个极光特征。因此,具有H-α发射的L4-T8天体通常也是电波源。来自棕矮星的无线电脉冲是高度圆极化的,可能来自与极光相连的电子回旋加速器不稳定性(ECMI)。目前尚不清楚是什么为棕矮星的极光无线电发射提供动力。一个建议是与等离子盘的同向旋转的分解,这也为木星上的主极光提供动力。另一个建议的能量来源是与棕矮星周围的岩石行星的相互作用,类似于Io和木星之间的交互作用。电子与氢分子的撞击会产生三氢阳离子(H+
3
)。这可以在2 μm和4 μm的红外线中用JWST检测到。H+
3
被气体破坏,例如H2O和CH4,可能意味着它在棕矮星中没有显著积累[31]。在凯克天文台,可能是由于极光电子更深入地渗透到棕矮星大气层中并被气体破坏,在任何M、L或T矮星中检测到H+
3
[32][33]。第一颗有无线电发射的L矮星是2MASS J00361617+1821104(L3.5)[34]

联星

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L型联星CWISE J0146-0508AB (L4+L8 blue)[35]

通常L矮星比M矮星更缺乏联星。以 L矮星为主要系统的联星分数为 24+6
−2
%,典型间隔为5–8 天文单位(AU)[36]。也有一些距离更宽的L矮星,例如WISE 2150−7520 (L1+T8),它的距离间隔为341 AU[37]。最靠近太阳系的L矮星是主星为卢曼16 AB联星。它的光谱类型为 L8[38]

相关条目

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参考资料

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