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4. Results

A total of 602 stars with tex2html_wrap_inline1111 larger than 10 was considered as possible candidates. The light curves of these stars were individually inspected and a variety of variable stars and possible variables was found. To identify the Cepheid variables in the sample, we searched for stars with light curves similar in appearance to known Galactic and LMC Cepheids. Of the 602 significantly variable stars, 228 were identified as Cepheid candidates. The remaining variable stars did not have light curves clearly similar to Cepheid light curves: e.g., short period variables with periods < 1 day or erratic variables with only a small number of points far from the mean. Rough estimates of their periods were made by eye. We did not attempt to fit model light curves rigorously, partly because of the poor sampling, but also because exact periods are not crucial to our goals. We also determined the V magnitude at maximum light, tex2html_wrap_inline1153, for each candidate. The error on tex2html_wrap_inline1153 varied considerably, from tex2html_wrap_inline1015 0.02 mag to 0.2, depending on whether the peak of the light curve was observed or not.

Figure 2:   Histogram of period ratios for M 31 Cepheids observed by us and by Baade & Swope (1962, 1964) or Gaposhkin (1963)

Figure 3:   Comparison of peak magnitudes for M 31 Cepheids observed by us and by Baade & Swope (1962, 1964) or Gaposhkin (1963)

Of the 228 Cepheid candidates, 97 were previously identified by Baade & Swope (1962, 1964) and Gaposhkin (1963), particularly in our south-most field. Because they have a much larger amount of data covering a larger baseline, their period determinations are far more accurate than ours. Figure 2 (click here) shows a histogram of the ratio between their measurement of the periods and ours for each star found in common. The central peak has a 1tex2html_wrap_inline1159 width of about 15%. A peak can also be seen at 0.5 due to aliasing affecting our determination. Most of the error in our period determination is due to the poor sampling, since the full period is not covered for many of the variables. Figure 3 (click here) shows a comparison of the magnitude at maximum light for the same Cepheids, as determined by us and by Baade & Swope (1962, 1964) and Gaposhkin (1963). The general correlation is clear, though a significant offset can be seen. This offset is due to the fact that the magnitudes reported by Baade and collaborators are photoelectric magnitudes, which are close to B, while ours are V magnitudes. The offset of tex2html_wrap_inline1015 0.7 magnitudes is consistent with the typical colors of these stars.

Figure 4:   Period -- Luminosity (PL) diagram. The open squares show the measurements of M 31 Cepheids from Baade's group (Baade & Swope 1962, 1964; Gaposhkin 1963). The filled squares show our new Cepheid candidates. The solid line is the PL relationship determined for a set of Baade M 31 Cepheids by Freedman & Madore (1990)

In Fig. 4 (click here) we present a period-luminosity relationship for the Cepheid candidates we have identified, along with those identified by Baade & Swope (1962, 1964) and Gaposhkin (1963). To bring the Baade magnitudes in line with the V magnitudes, we have simply added the 0.7 magnitude offset, which is sufficiently accurate for our purposes here. Included in this diagram is a line representing the position of the PL relationship found by Freedman & Madore (1990) using improved observations of the Baade & Swope (1964) Cepheids, from their Field IV, where the extinction is insignificant. It is clear that these variables fall generally along the P-L relationship, lending credence to their identification as Cepheids. One variable which was identified by the light curve turned out to fall far above the P-L relationship and has been rejected as a true Cepheid. This variable is probably of Galactic origin, possibly a binary system in the Galactic halo. There is a small tendency for the points in Fig. 4 (click here) to fall below the line for tex2html_wrap_inline1173, most likely due to the range of extinction for the sample. For tex2html_wrap_inline1175, there is a small tendancy for the points to land above the line. This may be due to Malmquist's bias, to the second track due to Cepheids oscillating at the overtone (e.g., Böhm-Vitense 1994), or because of tendancy for us to underestimate the period (see Fig. 2 (click here)).

We present lightcurves of all 130 new Cepheid candidates in Figs. 5-10 sorted in order of their Right Ascension. Our estimates for the upper and lower limits on tex2html_wrap_inline1153 are shown in these light curves by two dashed lines. We also tabulate observed quantities of these candidates in Table 1. The candidate rejected on the basis of its location in the P-L diagram is not included in the light curves or table, nor are those Cepheids which were already identified by Baade & Swope (1962, 1964) or Gaposhkin (1963). An estimate of the contamination of our sample can be obtained from the number of non-Cepheid variables identified by both Baade's group and our analysis, which we mis-identified as a Cepheid. This number is very low: Of 97 objects which we considered to be Cepheids, none were found to be another kind of variable by Baade's group. This suggests that our contamination is < 1%, but a more conservative estimate might be 3%, because of the low number statistics. The completeness can be estimated in a similar way, by comparing the number of Cepheids which were found in our fields by Baade's group, but which we failed to find. By doing this comparison, we find that our completeness is 53%, while the completeness of the Baade dataset is 88%. The magnitude distributions of both samples are similar, and suggest significant lack of completeness due to the magnitude limit for tex2html_wrap_inline1185.

Figure 1 (click here) shows the distribution of our newly identified Cepheid variables along with those identified by previous researchers (Baade & Swope 1962, 1964; Gaposhkin 1963). The reader is cautioned that some of the large-scale structure apparent in this image is due to the limited coverage. The small inset shows the coverage of both our observations and those of Baade & Swope (1962, 1964) and Gaposhkin (1963). We have also included in this figure a contour map from the HI survey of Unwin (1980). This allows the reader to compare the current location of active star formation with the location of the Cepheids, which have a typical age of roughly tex2html_wrap_inline1187. In a companion paper, we will discuss in detail the implications the observed distribution has for the star formation history.

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