ANCHORING THE DISTANCE SCALE VIA X-RAY/INFRARED DATA FOR CEPHEID CLUSTERS: SU Cas

The positions for stars r < 35' from SU Cas that feature UCAC3 (Third U.S. Naval Observatory CCD Astrograph Catalog; Zacharias et al. 2010) proper motions and 2MASS photometry are shown in Figure 1 (top panel). The stellar overdensity near the proper motion for SU Cas (μ_α = 1.0 ± 0.7 and μ_δ = −8.6 ± 0.8 mas yr⁻¹) represents the cluster Alessi 95. The analysis was subsequently expanded and 45 stars with similar proper motions to SU Cas (μ_α = −0.5 → 5.0 and μ_δ = −6.0 → −10.5 mas yr⁻¹, σ < 5 mas yr⁻¹) were detected within ∼1° of the Cepheid. The positions and proper motions of those objects are displayed in Figure 1 (bottom panel), and the cluster core is discernible in that diagram. Of the proper motion selected stars within r ≲ 40' of SU Cas (Figure 1), 27 form a distinct main sequence in the JH color–magnitude diagram (Figure 3). Only one star from the proper motion selected sample (Figure 1) exhibiting r ≲ 40' appears to be a non-member, reaffirming that the proper motion data efficiently separate cluster members from field stars (Figures 1 and 3). An inability to distinguish cluster members from field stars hinders isochrone fitting and exacerbates uncertainties tied to the derived distance. For example, the host cluster for ζ Gem proved more difficult to assess owing to the small offset of its proper motion relative to the field, particularly given the uncertainties (Majaess et al. 2012b). That shortcoming may be addressed by acquiring XMM-Newton/Chandra observations to identify cluster members, as later-type stars associated with ζ Gem (log τ = 7.85 ± 0.15; Majaess et al. 2012a) may emit X-rays owing to their comparative youth (Randich et al. 1996; Evans et al. 2010; Evans 2011). Independent confirmation of the ζ Gem cluster (Majaess et al. 2012a) is desirable.

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X-ray data for the SU Cas field (PI: Guinan) were obtained from the XMM-Newton public data archive (XSA). Point sources identified via the XMM-Newton Processing Pipeline Subsystem (PPS; Figure 2) were correlated with 2MASS photometry (closest source within 01). The majority of the X-ray sources lie directly on the cluster main sequence (Figure 3), and the field stars are readily discernible in the diagram (e.g., objects redder than M-type cluster dwarfs). X-ray observations are a pertinent means for identifying cluster members in harmony with other methods. The brightest star in Figure 2 (BD+68°201, B9III-IV; Turner et al. 2012) may exhibit a later-type companion which is the source of the X-ray emission, since later-type B stars are typically X-ray quiet (Evans 2011 and references therein). BD+68°201 and SU Cas exhibit analogous UCAC3 proper motions, to within the uncertainties. The faintest star in Figure 2 is likely an M-type cluster dwarf, as implied by its JHK_s photometry. A list of the X-ray sources and their corresponding 2MASS data are available via the XMM-Newton public data archive (PPS products).

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2.1.1. Mean Reddening for Alessi 95

Deeper JHK_s images were obtained for the SU Cas field from the OMM. The OMM houses a 1.6 m telescope equipped with a near-infrared wide-field imager (CPAPIR⁹). The images were acquired on 2012 January 11. Point spread function (PSF) photometry was performed using DAOPHOT (Stetson 1987). The instrumental photometry was subsequently tied to 2MASS secondary standards in the field. A color–magnitude diagram was tabulated for stars within r < 4' of the cluster center to mitigate field contamination. OMM JHK_s photometry for stars near the core of Alessi 95 shall be tabulated online at CDS or WEBDA (Paunzen 2008).

Comparison fields displayed in Figure 3 reaffirm the existence of Alessi 95. However, the analysis is complicated by the cluster's extent, high latitude position (ℓ, b ∼ 133°, 9°) and inhomogeneous reddening. The first XMM-Newton comparison field lies beyond the coronal radius (02:05:55 +64:56:33, r ∼ 6°); however, hierarchical clustering observed in giant molecular clouds implies that members may still be sampled. The second XMM-Newton comparison field encompasses another classical Cepheid (β Dor). The 2MASS comparison field (Figure 3, far right panel) is equal in area to the r ∼ 4' OMM sampling, but traces a symmetric annulus r ∼ 5° away. Field contamination will be most acute for the fainter OMM data.

No low-mass stars are visible in HST WFC3 images for SU Cas (HST Proposal 12215; Evans 2009). The saturation from the Cepheid was treated by subtracting (normalized) the image from a master, which was constructed (median combine) using numerous Cepheids observed for proposal 12215 (see also Evans 2011). The late B-type (B9.5V) companion discovered by Evans (1991, their Figure 4) via IUE observations was not detected, indicating that the star is in rather close proximity to the Cepheid. A comprehensive analysis is forthcoming (N. R. Evans et al., in preparation).

A precise distance may be established for the cluster via main-sequence fitting. The infrared (2MASS/NOMAD) calibration presented by Majaess et al. (2011b) was employed since it offers numerous advantages. First, the JHK_s calibration is comparatively insensitive to variations in [Fe/H] and age (Majaess et al. 2011b; see also Straižys & Lazauskaitė 2009). Second, the deep JHK_s photometry extends into the low-mass regime (≃ 0.4M_☉) where J − K_s remains constant with increasing magnitude (M_J ≳ 6) for low-mass M-type dwarfs, and J − H exhibits an inversion (see Figure 3; Majaess et al. 2011b and references therein). The trends ensure precise JHK_s main-sequence fits by providing distinct anchor points in color–magnitude and color–color diagrams (Figure 3). Third, the calibration's zero point is tied to seven benchmark open clusters (d < 250 pc) which exhibit matching JHK_s and revised Hipparcos distances (the Hyades, α Per, Praesepe, Coma Ber, IC 2391, IC 2609, and NGC 2451; van Leeuwen 2009; Majaess et al. 2011b; see also McArthur et al. 2011). The scale is anchored using clusters where consensus exists, rather than the discrepant case (i.e., the Pleiades). The objective is to avoid deriving distances to Cepheid clusters using a single benchmark cluster, and prevent the propagation of ambiguity into the Cepheid calibration. The revised Hipparcos distance for the Pleiades is d = 120.2 ± 1.9 pc (van Leeuwen 2009), whereas JHK_s data implied d = 138 ± 6 pc (Majaess et al. 2011b), and Soderblom et al. (2005) employed HST to deduce d = 134.6 ± 3.1 pc. By contrast, van Leeuwen (2009) obtained d = 172.4 ± 2.7 pc for α Per, which matches that established via the JHK_s analysis (Majaess et al. 2011b, their Table 1). Fourth, potential variations in the JHK_s reddening and extinction laws are predicted to be comparatively smaller than in the optical. Obscuration by dust is less significant in the infrared (E_{J − H} ∼ 0.3 × E_{B − V}; Majaess et al. 2008; Bonatto et al. 2004 and references therein), which consequently mitigates the impact of variations in R_λ (J₀ = J − E_{J − H} × R_J). The ratio of total to selective extinction R_J was adopted from Majaess et al. (2011c; see also Bonatto et al. 2004). The offset from the calibration yields a distance to Alessi 95 of d = 405 ± 15 pc.

A mean distance to SU Cas may be derived from the new JHK_s cluster parameters established here (the Cepheid lies near cluster center), and the following estimates: the distance inferred from UBV photometry and optical spectra for cluster members (d = 429 ± 8 pc; Turner et al. 2012), the mean Hipparcos parallax for cluster stars (420 ± 33 pc; van Leeuwen 2007; Turner et al. 2012), the ISB distance for SU Cas (d = 414 ± 12 pc; Storm et al. 2011; Turner et al. 2012), the original Hipparcos Cepheid parallax (d = 433 ± 116 pc; Perryman & ESA 1997), and the revised Hipparcos Cepheid parallax (d = 395 ± 50 pc; van Leeuwen 2007). A weighted mean of $d=414\pm 5(\sigma _{\bar{x}}) \pm 10 (\sigma)$ pc was established after assigning w = 3 to the ISB, JHK_s, and UBV distances, w = 2 for the revised Hipparcos distances, and w = 1 for the original Hipparcos distance. $\sigma _{\bar{x}}$ is the standard error, σ is the standard deviation, and w is the weight. Lastly, the absolute Wesenheit magnitude ($W_{VI_c}$) for SU Cas implied by that distance should be corrected for contamination from the companion (Evans 1991, 1992; see also Turner et al. 2012).