Direct Distances to Cepheids in the Large Magellanic Cloud: Evidence for a Universal Slope of the Period-Luminosity Relation up to Solar Abundance
Direct Distances to Cepheids in the Large Magellanic Cloud: Evidence for a Universal Slope of the Period-Luminosity Relation up to Solar Abundance
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大麦哲伦星云中造父变星的直接距离：周期-光度关系与太阳丰度之间普遍斜率的证据
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发表时间：
2005
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通讯作者：
F. Kienzle
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作者：
W. Gieren;J. Storm;Thomas G. Barnes III;P. Fouqué;G. Pietrzyński;F. Kienzle
我们利用红外表面亮度（ISB）技术推导了LMC中13个造父变星的距离，这些变星的周期从3天到42天不等。从这些距离计算的变量的绝对星等来看，我们发现LMC造父变星在V、I、W、J和K波段定义了紧密的周期-光度（PL）关系，这些关系与用相同技术得出的相应的银河系PL关系非常吻合，并且明显比OGLE-II项目在V、I、I和K波段观测到的LMC PL关系更陡峭。和W，以及Persson及其同事J和K.我们发现，在校正了LMC柱的倾斜后，我们得出的LMC造父变星的距离模量在很大程度上取决于恒星的周期，即最短周期造父变星的距离模量在18.3附近，而最长周期造父变星的距离模量在18.6附近。由于倾斜校正LMC距离模量的周期依赖性不应该存在，因此在ISB技术中必须存在以前工作中未发现的系统的周期依赖性误差。我们认为最可能的罪魁祸首是p因子，它用于将观测到的造父变星的径向速度转换为它们的脉动速度。通过要求(1)距离模量与周期图的斜率为零，(2)ISB和ZAMS拟合的一个成熟的造父变星星团样本的距离模量之间的平均差为零，我们发现p = 1.58（±0.02）- 0.15（±0.05）log p，其中p因子对造父变星周期（因此光度）的依赖程度比过去的理论计算结果更强。当我们用修正后的p因子定律重新计算LMC造父变星的距离时，我们不仅得到了所有恒星的一致的距离模量，而且还将LMC各PL关系（特别是在与变红无关的K和W波段）的斜率降低到与OGLE-II和Persson及其同事观测到的值一致。从我们的13颗造父变星中，我们确定了LMC距离模量为18.56±0.04等，另外估计的系统不确定度为~0.1等。使用相同的修正p因子定律重新确定了星系造父变星的距离，发现新的星系PL关系也与观测到的LMC光学和近红外PL关系一致。我们从LMC造父变星样本的ISB分析中得出的主要结论是，在当前的不确定性范围内，银河系和LMC中PL关系的斜率似乎没有显著差异。有了更多金属贫乏系统的文献数据，现在似乎可以得出这样的结论：造父变星PL关系的斜率与金属丰度无关，在[Fe/H]从-1.0指数到太阳丰度的广泛范围内，不确定性很小。对LMC造父变星的大量样本进行的第一次ISB分析的新证据表明，以前通过这种技术获得的更陡峭的星系PL关系是由于低估了以前工作中使用的基于模型的p因子定律中的周期依赖性而引起的。然而，我们强调，我们目前的结果必须得到新的理论模型的证实，这些理论模型能够解释p因子定律的更陡峭的周期依赖性，我们还需要更多的LMC领域造父变星的数据，以排除对我们目前解释有效性的剩余担忧。
We have applied the infrared surface brightness (ISB) technique to derive distances to 13 Cepheid variables in the LMC that span a period range from 3 to 42 days. From the absolute magnitudes of the variables calculated from these distances, we find that the LMC Cepheids define tight period-luminosity (PL) relations in the V, I, W, J, and K bands that agree exceedingly well with the corresponding Galactic PL relations derived from the same technique and are significantly steeper than the LMC PL relations in these bands observed by the OGLE-II Project in V, I, and W and by Persson and coworkers in J and K. We find that the LMC Cepheid distance moduli we derive, after correcting them for the tilt of the LMC bar, depend significantly on the period of the stars, in the sense that the shortest period Cepheids have distance moduli near 18.3, whereas the longest period Cepheids are found to lie near 18.6. Since such a period dependence of the tilt-corrected LMC distance moduli should not exist, there must be a systematic, period-dependent error in the ISB technique not discovered in previous work. We identify as the most likely culprit the p-factor, which is used to convert the observed Cepheid radial velocities into their pulsational velocities. By demanding (1) a zero slope on the distance modulus versus period diagram and (2) a zero mean difference between the ISB and ZAMS fitting distance moduli of a sample of well-established Galactic cluster Cepheids, we find that p = 1.58(±0.02) - 0.15(±0.05) log P, with the p-factor depending more strongly on Cepheid period (and thus luminosity) than indicated by past theoretical calculations. When we recalculate the distances of the LMC Cepheids with the revised p-factor law suggested by our data, we not only obtain consistent distance moduli for all stars but also decrease the slopes in the various LMC PL relations (and particularly in the reddening-independent K and W bands) to values that are consistent with the values observed by OGLE-II and Persson and coworkers. From our 13 Cepheids, we determine the LMC distance modulus to be 18.56 ± 0.04 mag, with an additional estimated systematic uncertainty of ~0.1 mag. Using the same corrected p-factor law to redetermine the distances of the Galactic Cepheids, the new Galactic PL relations are also found consistent with the observed optical and near-infrared PL relations in the LMC. Our main conclusion from the ISB analysis of the LMC Cepheid sample is that, within current uncertainties, there seems to be no significant difference between the slopes of the PL relations in the Milky Way and LMC. With literature data on more metal-poor systems, it seems now possible to conclude that the slope of the Cepheid PL relation is independent of metallicity in the broad range in [Fe/H] from -1.0 dex to solar abundance, within a small uncertainty. The new evidence from the first ISB analysis of a sizable sample of LMC Cepheids suggests that the previous, steeper Galactic PL relations obtained from this technique were caused by an underestimation of the period dependence in the model-based p-factor law used in the previous work. We emphasize, however, that our current results must be substantiated by new theoretical models capable of explaining the steeper period dependence of the p-factor law, and we will also need data on more LMC field Cepheids to rule out remaining concerns about the validity of our current interpretation.