THE CEPHEID DISTANCE TO NGC 0247

In 1983, we began a ground-based project to provide a more secure calibration of the zero point for secondary distance indicators (such as the Tully–Fisher relation) by building up a database of accurate Cepheid distances to nearby spiral galaxies. In due course, the Hubble Space Telescope was launched and other activities took precedence over the ground-based effort. Preliminary mention of work on Cepheids discovered in NGC 0247 was given in Freedman et al. (1988) and again in Catanzarite et al. (1994) but details were never published until now. Here we present the photometric data and provide a brief analysis leading to a distance determination to NGC 0247 based on nine Cepheids discovered in NGC 0247 some 25 years ago. We then go on to compare it with new observations published by Garcia-Varela et al. (2008, hereafter [GV08]).

Observations for this program were carried out over a span of eight years (giving a time baseline of almost 3000 days). Observation began first at the Cerro Tololo 4 m, and was completed using the 2.5 m du Pont telescope at Las Campanas, Chile. Three fields of NGC 0247 were surveyed in B, V, R, and I filters at the Cerro Tololo 4 m telescope during 1984 November to 1988 November. Figure 1 shows a photograph of NGC 0247 with the three fields delineated. The coordinates of the CCD field centers are given in Table 1. From the second through the fourth year of the program, data were obtained in the service-observing mode offered at Cerro Tololo Inter-American Observatory (CTIO); those data were taken by M. Navarrete. For most of the runs, the 512 × 320 RCA chip 5 (having a scale of 0.60 arcsec pixel⁻¹ and a total field of view of 3' × 5' at the prime focus) was used. In 1988, a different RCA chip (4) with similar characteristics was substituted. Exposure times for these frames were typically 400 s in B and 300 s in V, R, and I. The frames were bias-subtracted, flat-fielded, and defringed using standard data-reduction packages available at Cerro Tololo. Beginning in 1990, the observing program for the Sculptor galaxies shifted to the du Pont 2.5 m telescope at the Las Campanas Observatory. BVRI CCD observations covering the same three selected fields were obtained in 1990 December, 1991 September, and 1992 October, November, and December. Exposure times at this telescope were generally 900 s in B and 600 s in V, R, and I. Most of these observations were electronically binned (2 × 2) at the telescope. For the 1990 and 1991 runs, the FORD1 CCD chip was used. For the 1992 October and November runs, a Tektronix CCD chip (TEK4) was used; for the 1992 December runs, Tektronix CCD chips were also used (TEK3 and TEK4). These chips each had dimensions of 2048 × 2048 pixels; the image scales obtained were as follows—FORD1: 0.16 arcsec pixel⁻¹; TEK3: 0.23 arcsec pixel⁻¹; and TEK4: 0.26 arcsec pixel⁻¹. The survey totaled about 250 exposures on 29 different nights over a span of eight years. Table 2 gives a journal of the observations.

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Photometric calibration of the CTIO frames was accomplished using E-region standards in E1, E2, E3, E7, E8, and E9 (Graham 1984) and in SA 98 Landoldt (1983). BVRI standards were taken on 20 independent photometric nights. As described in Freedman et al. (1992), a check on the external accuracy of the photometric calibration for these runs was made by individually calibrating the frames for NGC 0300. The magnitudes for the brightest stars were in agreement to within 0.01–0.03 ± 0.03 mag of the average for all filter/field combinations.

The CCD frames from CTIO were reduced using both DoPHOT (Schechter et al. 1993) and DAOPHOT (Stetson 1987), and cross checked. The unresolved background level in NGC 0247 is highly nonuniform, and is characterized both by regions in which there are strong spiral arms as well as relatively blank, interarm regions. In order to maximize the detection limits of the algorithm FIND in DAOPHOT, the frames were first median smoothed using a 7 × 7 pixel boxcar averaging scheme, and subtracted from the original frames.

Details of the calibration process are discussed in Freedman et al. (1992). The LCO data were reduced using a variant of the DoPHOT package (Mateo & Schechter 1989). This version of DoPHOT uses median smoothing to construct an initial model of the background sky before searching for objects. The sky model is refined after objects at the next-to-the-lowest threshold have been found and subtracted. The refined sky model is then adopted as the baseline, and objects are again found down-to-the-lowest threshold and their PSF parameters are remeasured. LCO frames were brought onto the CTIO-calibrated magnitude system by the following process: the (B − V) color term for the CCD chips used at LCO relative to RCA chip used in the CTIO observations was measured and a correction was applied to the LCO B photometry (the LCO V photometry had no significant (B − V) color term). Next, the magnitude zero point of each LCO frame was offset to the instrumental zero point of a fiducial CTIO frame, for the corresponding field and filter. The calibration transformation derived for the fiducial CTIO frame was then applied to the LCO data.

For Fields 1 and 3, all of the observations were tied to the photometric zero point for 1988 October 7. Observations on this night were taken under excellent seeing conditions and had the best photometric calibration available to us. For Field 2, 1988 October 13 was used to calibrate the data. To put all of the stars in each field on the same coordinate system, all frames from each field were spatially registered to the 1988 October 7 V frame for that field (since that was the best or close to the best V night for all three fields). Coordinate transformations produced matches with an rms scatter of ±0.30 pixels or better. Calibrated, matched photometry files containing the entire set of observations for each field/bandpass combination were produced. Stars with high internal V magnitude dispersion were then identified as described in Freedman et al. (1994). These variable candidates were then subjected to a further test: a star was flagged as a Cepheid candidate only if the histogram of its magnitudes was consistent with a uniform magnitude histogram, as expected for Cepheids. All three fields were searched for variables down to a signal-to-noise level of 1.5σ. The V photometric data for each candidate was then phased to the 12 periods (in the range of 1–100 days) with the lowest phase dispersions, using a routine based on the Lafler–Kinman algorithm (Lafler & Kinman 1965). The V light curves were then visually inspected and the best period was selected. Calibrated B, R, and I observations were then phased to this adopted period and the multiwavelength light curves were inspected for consistency. Candidates with strong correlation of phase and amplitude between their BVRI light curves, having well-determined periods, mean colors, and well-sampled light curves characteristic of known Cepheids, were then identified as Cepheids. Each of these stars was then visually inspected in the best image frame to check for nearby companions. Nine Cepheids in total made it through the selection procedure. All were found in BVRI, with the exception of NGC 0247:[MF09] C9, which was too faint to be recovered on the I frames. The positions for the nine Cepheids in our sample are given in Table 3, the first six of which are mapped over from [GV08], with the positions for C7, C8, and C9 being on that system but having lower precision. The individual Cepheid observations are presented in Tables 4–12. The light curves are shown in Figure 2. The time-averaged properties of the individual Cepheids are listed in Table 4.

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4.1. Other Variable Stars Found in NGC 0247

A comprehensive review of previously published distance estimates to NGC 0247 is given in [GV08].⁴ In that paper, the authors also present their new VI observations of 23 Cepheids in the period range 17–131 days. Based on those two colors, they derive a true distance modulus of 27.80 ± 0.09 mag (3.6 Mpc) tied to an LMC true distance modulus of 18.50 mag, as also adopted in this paper. Another important independent distance measurement to NGC 0247 worth noting here, because of its comparably high precision, is the tip of the red giant branch (TRGB) distance modulus (μ_o = 27.81 mag or 3.65 Mpc) published by Karachentsev et al. (2006).

5.1. Discussion of Data

5.2. PL Relations and Apparent Distance Moduli

To determine apparent VRI distance moduli, residuals about the PL relations for NGC 0247 Cepheids were minimized relative to the mean LMC PL relations given in Madore (1985), and updated to the VI calibration of Madore & Freedman (1991). For a given bandpass, the LMC PL relation was iteratively shifted relative to the NGC 0247 PL relation until the χ² of the fit was minimized. The offset determined in this way is then the apparent distance modulus (for that bandpass) of NGC 0247 with respect to the LMC. The results of the PL fits are shown in Figure 3. In the absence of other physical effects, determination of the true distance modulus and reddening can obtained by fitting the apparent moduli in different filters to an interstellar extinction law (e.g., Cardelli et al. 1989)⁵ originally discussed in Freedman (1988).

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In Figure 4, the apparent distance moduli at VRI for the Cepheid sample in NGC 0247 are plotted with respect to inverse wavelength. The solid line gives a fit to a standard Cardelli et al. (1989) Galactic extinction law flanked by 1σ error curves (dashed lines). The VRI data are very well fitted by an extinction curve (with a small positive reddening equivalent to E(V − I) = 0.07 mag)⁶ having an intercept corresponding to a true distance modulus of μ_o = 27.81 ± 0.10 mag (3.65 ± 0.16 Mpc). The solution using only V and I gives essentially the same numbers (μ_o = 27.79 ± 0.13 mag; 3.61 ± 0.23 Mpc).

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We have made a positional cross-correlation of our Cepheids with those discovered by [GV08]. Six of our nine variables were recovered by the Araucania Project, and the correct identification of these stars across the two studies is reinforced by the independently derived periods which agree to better than 10% in most cases. We now combine the two data sets, revise the periods when possible, and update the V and I intensity–mean magnitudes. The results of that update are given in Table 15. The combined light curves are shown in Figure 5.

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The updated VRI [MF09] sample alone gives μ_V = 27.97 ± 0.05 mag, μ_R = 27.98 ± 0.06 mag, μ_I = 27.90 ± 0.06 mag, E(V − I) = 0.11 ±  0.03 mag resulting in μ_o = 27.81 ±  0.05 mag or 3.65 ±  0.08 Mpc for the three-band fit, and μ_o = 27.79 ± 0.13 mag (3.61 ± 0.23 Mpc) with E(V − I) = 0.07 ± 0.04 mag for the VI fit alone.

We consider a progressive merger of the two data sets. We first apply our standard fitting techniques to the [GV08] preferred subset of 17 Cepheids omitting, as they did, the longest and shortest period Cepheids in their sample. We get μ_V = 28.21 ± 0.05 mag, μ_I = 28.05 ± 0.06 mag, E(V − I) = 0.15 ± 0.03 mag resulting in μ_o = 27.82 ± 0.08 mag or 3.66 ± 0.14 Mpc. This differs from the [GV08] solution by +0.025 mag in the true modulus.

If we now update the [GV08] sample with the revised periods and magnitudes for NGC 0247:[MF09] C2 through C6, we get μ_V = 28.15 ± 0.06 mag, μ_I = 28.03 ± 0.06 mag, E(V − I) = 0.11 ± 0.03 mag resulting in μ_o = 27.87 ± 0.09 mag or 3.75 ± 0.15 Mpc. Augmenting the [GV08] sample with NGC 0247:[MF09] C7 and NGC 0247:[MF09] C8, plus reintroducing NGC 0247:[MF09] C1 with its first-epoch period and magnitude, in addition to its evolved values from [GV08] as described in Section 7, we get μ_V = 28.13 ± 0.05 mag, μ_I = 28.01 ± 0.06 mag, E(V − I) = 0.11 ± 0.03 mag resulting in μ_o = 27.85 ± 0.09 mag or 3.72 ± 0.15 Mpc. The above results are summarized in Table 16.

In an attempt to update the period and combine the photometry for the longest period Cepheid in our sample, NGC 0247:[MF09] C1, we quickly found that the mean magnitudes and colors derived from our data did not correspond to data published for it in the [GV08] study. Figure 6 shows the differences. In that plot, our data are shown as circled solid symbols, phased to our adopted period of 69.9 days. Below those light curves are the data from [GV08], shown as open circles, phased to their period of 65.862 days (with an arbitrarily added phase shift of 0.6 to align the light curves for ease of visual comparison). The time-averaged V magnitudes differ by 0.8 mag, with the most recent epoch being fainter; while the (V−I) colors differ by 0.23 mag, with the most recent data being bluer. The sense of the change eliminates a self-shrouding event as the possible cause. A remaining explanation is that the structure of the star itself may have systematically changed in the intervening quarter century: in the face of a rising surface temperature (indicated by the decrease in the (V−I) color) and the resulting increased surface brightness, the overall radius of this star may have decreased significantly. In the process the period dropped by 6%, from 70 to 66 days.

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A simple linear decrease of the period with time (ΔP/P = 0.0075 day/day) fails to phase the light curves over the total baseline (and, in fact, destroys coherence within the individual observing campaigns). Without undertaking more sophisticated modeling, we default to the next simplest conclusion that the period change was a discontinuous event. Further monitoring of this star could reveal interesting aspects of the structure of Cepheids in general if this behavior persists.

Nine Cepheids have been identified in the Galaxy NGC 0247. Six of these variable stars have been independently found by [GV08].

The period and magnitude-updated VRI [MF09] sample alone gives apparent moduli of μ_V = 27.97 ± 0.05 mag, μ_R = 27.98 ± 0.06 mag, μ_I = 27.90 ± 0.06 mag, and E(V − I) = 0.11 ± 0.03 mag resulting in μ_o = 27.81 ± 0.05 mag or 3.65 ± 0.08 Mpc for the three-band fit. This is (fortuitously) identical to the TRGB distance modulus recently published by Karachentsev et al. (2006) further reinforcing the consistency of these two distance scales, which are based on largely independent assumptions, and have very different systematics.

Combining our observations with newly published data from [GV08] in the V and I bands, and updating the periods accordingly, results in a reddening of E(V − I) = 0.11 ±  0.03 mag and a (preferred) true modulus of μ_o = 27.85 ±  0.09 mag (3.72 ±  0.15 Mpc).

The 70-day Cepheid NGC 0247:[MF09] C1 deserves followup observations to see if the extraordinary changes in its magnitude, period, and color found between these epochs (first 1984–1992 and then 2002–2005) is an ongoing phenomenon.

During the initial period in which these observations were made, W.L.F.'s research was supported in part by NSF grants AST 87-13889 and 9116496 on the extragalactic distance scale. We thank Bob Williams who provided us with Director's Discretion Time in 1988, and Irwin Horowitz who participated in the early stages of reducing the Las Campanas data. We also thank Jose Garcia-Varela, Grzegorz Pietrzynski, and Wolfgang Gieren for providing their more recently acquired Cepheid data in advance of publication. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, Caltech, under contract with the National Aeronautics and Space Administration.