Cepheus Star-Forming Field Revisited

We present a photometric investigation in the uvbyβ system of a 20 ̊ × 20 ̊ field centered at galactic coordinates (l, b) = (105 ̊, 5 ̊). This region contains a relatively isolated structure of numerous HII regions in the direction of some of the OB associations in Cepheus and Cassiopeia. Photoelectric uvbyβ data of more than 220 stars of spectral types O-B9 are available for this field and are used to obtain homogeneous distance estimates for a number of prominent stellar groups, among which are the Cep OB2 and Cep OB3 associations and the open clusters NGC 7243 and NGC 7654.


Introduction
The field between 95˚ and 116˚ galactic longitude and −5˚ and 15˚ galactic latitude contains numerous HII regions. Among them Sh 2-131, Sh 2-132 and Sh 2-157 are the most prominent, associated with some very young open clusters. Several OB associations have been identified in this direction (Cep OB1, Cep OB2, Cep OB3, Cep OB5 and Cas OB2), some of which are well studied in terms of broad-band photometry (see Humphreys [1] and the references therein).
The goal of this investigation is to provide homogeneous distance and reddening estimates for as many young stars as possible in the studied coordinate range. We use the intermediate-band uvbyβ photometric system (Strömgren [2], Crawford & Mander [3]), since it allows reliable distance determination which helps distinguish among closely spaced groups along the line of sight (see for example Kaltcheva & Golev [4]). This study is aimed to provide the basis for a further detailed investigation of the interaction between the OB stars and the interstellar medium for this field.

The Sample
The field is selected to cover the isolated large-scale feature seen in Hα between 95˚ and 116˚ galactic longitude. All uvbyβ data within this coordinate range was extracted from the catalog of Hauck & Mermilliod [5]. The sample contains 222 stars of spectral types O to B9 with complete uvbyβ photometry. The sample stars are presented in Figure 1. They are overplotted on an Hα map smoothed to 4 arcmin resolution to remove star residuals, Gaustad et al. [6], which is obtained via the SkyView VO interface (McGlynn et al. [7]).

Calculation of Interstellar Extinction and Stellar Distances
The color excess E(b-y) for stars of luminosity classes (LC) III, IV and V was obtained via the Crawford's [8] calibration. The calibration by Kilkenny & Whittet [9] was used for LC II, Ib, Iab and Ia. We used R = 3.18 and E(B − V) = E(b − y)/0.74 to obtain the corrected for reddening visual magnitude V 0 . The calibration by Balona & Shobbrook [10] was applied to derive the absolute magnitude M V for each star. For stars indicated as emission-line stars, β calculated from c 0 was used to calculate M V . This procedure has been described in detail in earlier studies of star-forming fields based on uvbyβ photometry (see for example Kaltcheva & Hilditch [11]). Note that the uvbyβ photometric distances derived in this way are in excellent agreement with the recalculated Hipparcos data (Kaltcheva & Makarov [12]). In addition, the by Balona & Shobbrook [10] luminosity calibration has been tested via the Hipparcos data and shown to be reliable (Kaltcheva & Knude [13], Torra et al. [14], Kaltcheva & Golev [15]). Note that in the uvbyβ system both the color excess and absolute magnitude calculations do not rely on a precise determination of spectral type and, to some extent, the luminosity class. This is because the calculations are carried on in the same way for all stars in a given spectral range (e.g. O-B9), depending on the LC for the color excess derivation only. The spectral and luminosity classes used in these calculations were ex-tracted from the SIMBAD database and verified based on the classification [c 1 ] vs. [m 1 ] diagram (Strömgren [2] not shown here). The expected uncertainties in the derived distances are of the order of −13% to +15% for LC III-V, and −21% to +26% for LC I-II. Table 1 (available in full from the authors upon request) summarizes the photometric data and the derived stellar parameters. The table includes stellar identifications, followed by spectral and luminosity type and uvbyβ photometric data, color excess and dereddened N. KALTCHEVA ET AL. 474 Table 1. Distance modulus and color excess for the clusters and OB associations represented in the sample.

Results
The diagrams color excess E(b − y) vs. true distance modulus (DM), dereddened visual magnitude V 0 vs. dereddened color index. (b − y) 0 , and M V vs. (b − y) 0 are presented in Figures 2 and 3 and are used to identify spatially coherent features in the studied coordinate range. Our sample is representative for several associations and young open clusters in this field. At l = 106˚, we divide the field into two parts and consider them separately in the following analysis. The new distance and color excess estimates, as discussed below, are presented in Ta [16] identified Cep OB3 as a group with well defined MS and, based on 29 stars, obtained a true DM of 9.3 mag. Humphreys [1] adopts true DM = 9.7 mag, based on 6 stars. The first two panels of Figure 2 present the sample stars plotted in galactic coordinates and the diagram color excess vs. true distance modulus. The locations of the three open clusters with available uvbyβ photometry present in our sample are also shown. The apparent field stars we indicate with filled symbols of different size, according to the calculated absolute magnitude. All intrinsically bright field stars (M V < −4 mag) are located beyond DM = 9.5. The stars referred to Cep OB3 (open symbols) are all more reddened than E(b − y) = 0.4 mag and very well grouped in terms of distance. There is a previously noticed group of B9-A0 foreground stars located in the direction of Cep OB3 (see Garmany & Stencel [16] and the references therein). This group (marked with x-symbols) is about 1 mag closer in comparison to Cep OB3. The separation evident on the E(b − y) vs. DM diagram can be followed on the rest of the photometric diagrams on Figure 2. Cep OB3 has a tight MS, covering a considerable range in M V . The field stars do not form well defined main sequences, as they do not form obvious groups. The group of B9-A0 stars foreground to Cep OB3 also form a quite well defined MS on the V 0 vs. (b − y) 0 diagram. The distance estimates to Cep OB3 existing in the literature vary between 580 and 850 pc (see Allen et al. [17] for a recent review). The average distance, based on 27 stars, that we provide in this paper is 756 ± 27 pc (DM = 9.36 mag, Table 2). The O7Vn star HD 217086 (LID 900020041) is thought to be the ionizing star of Sh 2-155. We obtain a distance of 738 pc to this star, which is twice less than the one based on the CaII HK lines (1688 pc, Megier et al. [18]). Note that the uvbyβ-based distance to HD 217086 is in excellent agreement with the average distance to the Cep OB3 association as obtained here.
In our sample Cas OB2 is represented by several stars intrinsically brighter than M V = −4 mag. The photometric diagrams (Figure 2 bottom) indicate that these stars are evolved from the MS. They are also quite spread in distance, with an average DM = 11.4 (± 1.3 s.d.) mag. All these stars are included in the previous membership lists of Cas OB2. Thus, despite of the small number of stars with uvbyβ photometry available, there are indications that the existing distance estimates for Cas OB2 maybe slightly overestimated.
The open cluster NGC 7654 is well represented in our sample (14 stars), but only the brightest stars of NGC 7789 and NGC 7380 have uvbyβ photometry presently available. We find NGC 7654 farther at about 0.3 mag in comparison to the true DM listed in WEBDA. For NGC 7789 the true DM in WEBDA is significantly larger (up to 0.7 mag) in comparison to the one obtained here, and also the one provided by Dias et al. [19]. The only star from NGC 7380 with uvbyβ data available is found at a distance at agreement with the estimate for the cluster (WEBDA). Based on the available uvbyβ data, all clus- ters appear located between distance moduli 11.0 mag and 11.4 mag. Within the errors, this range is consistent with the average distance found here for the Cas OB2 association.

Longitude Range 94˚ -106˚: Cep OB1 and Cep OB2
This longitude range is dominated by the very extended Cep OB2 association (Rupreht 27). Garmany & Stencel [16] and Humphreys [1] derive DM of 9.9 mag and 9.6 mag, respectively. Humphreys [1] found that the association extends more than 10˚ × 10˚ in the 97˚ -108˚ longitude range and between 12˚ and −1˚ galactic latitude. The open cluster Tr 37 and the prominent HII region Sh 2-131 are sometimes referred to as the core of Cep OB2. The open clusters NGC 7160, NGC 7235 and NGC 7281 are also thought to be part of the association. Along its southern edge, Cep OB2 is adjacent to Cep OB1. The HII region Sh 2-132 and NGC 7380 projected toward Sh 2-142 are located between the two associations. According to Humphreys [1], who lists DM = 12.7 mag to Cep OB1, the association is about 3 times more extended along galactic longitude than in galactic latitude. Garmany & Stencel [16] obtain DM = 12.2 mag to Cep OB1. The first two panels of Figure 3 present the stars considered in this subsection plotted in galactic coordinates and the diagram color excess vs. true distance modulus. The locations of the four open clusters represented to some extent in the uvbyβ sample are also indicated. We tentatively divide this subsample into two parts according to the calculated intrinsic brightness. Most of the stars intrinsically brighter than −2.5 mag (filled symbols) are located farther than DM = 9.5 mag, while the rest of the stars (open circles) are foreground. Eleven of the intrinsically bright stars are found between longitudes 100˚ to 105˚ and latitudes 1˚ to 8˚, in a DM range from 9.0 to NGC 7243 is represented by 18 stars with uvbyβ data presently available. The average color excess and DM derived here are in good agreement with those listed in WEBDA. The uvbyβ-based color excess and distance modulus of the only star from IC 5146 with such a data available are also consistent with the average parameters of the cluster (see Table 2). Of all 54 stars of NGC 7209 measured by Peña & Peniche [20] we have classified only two stars as earlier than A0 type and calculated individual color excesses and distance moduli in agreement with the latter authors. In general, the open clusters in this subsample (Tr 37, IC 5146, NGC 7243 and NGC 7209) are all found between DM 9.4 and 10 mag.
The Cep OB1 association is represented by several intrinsically bright stars at negative latitudes, quite spread according to their uvbyβ distances. Some of them are located at DM = 12 to 13 mag, thus consistent with previous distance estimates. The large spread in distance, however, supports previous suggestions that Cep OB1 might not be a single association (Moffat [21]). The E (b − y) vs. DM diagram on Figure 3 contains more stars farther than 11 mag, but some of them are located at positive galactic latitude. Based on our sample, we cannot reliably study the classical Cen OB1 association.
It seems possible to identify a previously undetected group of seven young stars (star-symbols) at average galactic coordinates (l, b) = (105˚, 0.5˚). The group is located at true DM = 11.09 (± 0.76 s.d., ± 0.29 s.e.) and contains the following stars with uvby98 identifications: 5702531, 100239955, 5802435, 5702522, 620080025, 5702536 and 100213470. These stars have similar proper motions (where available). The stars mentioned above are all more reddened than A V = 0.9 mag and are all projected near the HII regions Sh 2-138 and Sh 2-139. Several foreground stars (plus-symbols) are also located toward (l, b) = (105˚, 0.5˚), all of them less massive and closer than DM = 9.5 mag.

Concluding Remarks
Based on all O-B9 stars with available uvbyβ photometry in the studied longitude range we obtain homogeneous distance estimates for several stellar groups, as summarized in Table 2, and for a sample of field stars as well. In general, there is a good agreement between the uvbyβ distances and these by Garmany & Stencel [16], which are obtained via MS fitting. However, the uvbyβ estimates, which are based on individual stellar distances should be considered more reliable for groups without well defined MS. The distance obtained here for Cep OB3 is in excellent agreement with previous estimates. There are indications that the currently adopted distance to Cas OB2 may be overestimated, but the uvbyβ sample is not large enough for firm conclusions. Although the uvbyβ sample for Cep OB2 is also relatively small, the derived distances seem to be in agreement with previous estimates. The large spread in distance for Cep OB1 suggests, how-