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DIRECT项目:混合对造父变星距离尺度的影响。II。M33中的造父变星

The DIRECT Project: Influence of Blending on the Cepheid Distance Scale. II. Cepheids in M33

B. J. Mochejska, L. M. Macri, D. D. Sasselov, K. Z. Stanek

摘要:

我们调查的造父变星距离尺度的影响。混合是造父变星与一颗或多颗本身发光的恒星的紧密联系。高分辨率的HST图像与我们的地面数据进行了比较,这些数据是作为DIRECT项目的一部分获得的,用于M33星系中102颗造父变星的样本。在B、V和I波段(S_B、S_V、S_I)上,来自地面图像上未分辨的亮伴星的平均(中值)通量贡献约为造父变星通量的29%(15%)、24%(14%)、30%(21%)。64颗周期大于10天的造父变星的S_B、S_V、S_I的平均值(中位值)分别为20%(10%)、16%(7%)、23%(14%)。这表明,根据所选物体的样本,从我们的地面V波段震级获得的M33造父变星的距离将被系统地低估约8%-11%(3%-6%)。使用人造星测试,我们研究拥挤和混合作为单独的现象。这些测试表明,在表面亮度低于21.6 mag/arcsec ^[2]的较低区域,混合效应可能比拥挤效应更显著。我们还发现,在长周期造父变星(P> 10 ^d)的情况下,混合产生的伴星平均比拥挤产生的伴星更蓝。我们在M33的地基分辨率对应于HST的分辨率约为11 Mpc。我们估计的影响,在相应的距离范围内观察到的HST使用HST M33数据作为模板未混合人口的分辨率混合。我们将我们的结果与其他小组的结果进行了比较。

英文摘要:

We investigate the influence of blending on the Cepheid distance scale. Blending is the close association of a Cepheid with one or more intrinsically luminous stars. High-resolution HST images are compared to our ground-based data, obtained as part of the DIRECT project, for a sample of 102 Cepheids in the M33 galaxy. The average (median) flux contribution from luminous companions not resolved on the ground-based images in the B, V and I bands (S_B, S_V, S_I) is about 29% (15%), 24% (14%), 30% (21%) of the flux of the Cepheid. For 64 Cepheids with periods in excess of 10 days the average (median) values of S_B, S_V, S_I are 20% (10%), 16% (7%), 23% (14%). This shows that, depending on the sample of objects chosen, the distance derived from our ground-based V-band magnitudes for the M33 Cepheids would be systematically underestimated by about 8%-11% (3%-6%). Using artificial star tests we study crowding and blending as separate phenomena. These tests indicate that the effect of blending could be more significant than crowding in regions of lower surface brightness, below 21.6 mag/arcsec^2. We also find indications that in the case of long period Cepheids (P>10^d) companions from blending are on average bluer than companions introduced by crowding. Our ground-based resolution in M33 corresponds to the HST resolution at about 11 Mpc. We estimate the effect of blending at resolutions corresponding to a range of distances as observed with the HST using the HST M33 data as the template unblended population. We compare our results to those obtained by other groups.

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