冷磁DAZ白矮星NLTT 10480
The cool magnetic DAZ white dwarf NLTT 10480
摘要：
我们在新的Luyten十分之二（NLTT）星表中发现了一颗新的冷磁白矮星。高自行恒星NLTT 10480（mu=0.5”/年）显示出微弱的钙塞曼分裂线，以及特征性的H α和β塞曼三重态。用VLT X射线能谱测得表面平均磁场B_S ~ 0.5 MG。钙线和氢线的π和σ分量的相对强度意味着高倾角（i > 60度）。光学-红外V-J色指数和CaI/CaII电离平衡表明温度在4900和5200 K之间，而Balmer线轮廓有利于5400 K的更高温度。这种差异可能通过将金属丰度增加到0.03倍太阳能来解决，从而增加电子压力。然而，测得的钙丰度和丰度上限为其他元素（钠，铝，硅。和Fe）表明光球金属丰度< 10^{-4}太阳。假设扩散处于稳定状态，则需要log（dM/dt）（g/s）=5.6+/-0.3的钙吸积速率来维持白矮星大气中log（n（Ca）/n（H））=-10.30+/-0.30的钙丰度。我们研究这一发现的影响，在冷白矮星行星碎片和弱磁场的发病率。
英文摘要：
We have identified a new, cool magnetic white dwarf in the New Luyten Two-Tenths (NLTT) catalogue. The high proper-motion star NLTT 10480 (mu=0.5"/year) shows weak Zeeman-split lines of calcium, as well as characteristic Halpha and beta Zeeman triplets. Using VLT X-shooter spectra we measured a surface-averaged magnetic field B_S ~ 0.5 MG. The relative intensity of the pi and sigma components of the calcium and hydrogen lines imply a high inclination (i > 60 deg). The optical-to-infrared V-J colour index and the CaI/CaII ionization balance indicate a temperature between 4900 and 5200 K, while the Balmer line profiles favour a higher temperature of 5400 K. The discrepancy is potentially resolved by increasing the metallicity to 0.03x solar, hence increasing the electron pressure. However, the measured calcium abundance and abundance upper limits for other elements (Na, Al, Si. and Fe) imply a low photospheric metallicity < 10^{-4} solar. Assuming diffusion steady-state, a calcium accretion rate of log(dM/dt)(g/s)=5.6+/-0.3 is required to sustain a calcium abundance of log(n(Ca)/n(H))=-10.30+/-0.30 in the white dwarf atmosphere. We examine the implications of this discovery for the incidence of planetary debris and weak magnetic fields in cool white dwarf stars.
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