Cepheus Star-Forming Field Revisited

Nadia Kaltcheva, Vincent Fabbri, Timothy Conard, Valeri Golev

Department of Astronomy, Faculty of Physics, St Kliment Ohridski University of Sofia, Sofia, Bulgaria.
Department of Physics and Astronomy, University of Wisconsin, Oshkosh, USA.

DOI: 10.4236/ijaa.2013.34054                

Abstract

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.

Keywords

Open Clusters and Associations: Individual (Cep OB2); Open Clusters and Associations: Individual (Cep OB3); Open Clusters and Associations: Individual (NGC 7243); Open Clusters and Associations: Individual (NGC 7654)

1. 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.

2. 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]).

3. 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 V₀. 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 c₀ 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

Figure 1. All stars in the studied field with uvbyβ photometry available in the Hauck & Mermilliod catalog [5] plotted in galactic coordinates. The stars are superimposed on a Hα map obtained via the SkyView VO interface (see text for details).

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 extracted from the SIMBAD database and verified based on the classification [c₁] vs. [m₁] 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

Conflicts of Interest

The authors declare no conflicts of interest.

References

[1]	R. M. Humphreys, “Studies of Luminous Stars in Nearby Galaxies. I. Supergiants and O Stars in the Milky Way,” The Astrophysical Journal Supplement, Vol. 38, 1978, pp. 309-350. http://dx.doi.org/ 10.1086/190559
[2]	B. Stromgren, “Spectral Classification through Photo-Electric Narrow-Band Photometry,” Annual Review of Astronomy and Astrophysics, Vol. 4, 1966, pp. 433-473. http://dx.doi.org/10.1146/ annurev.aa.04.090166.002245
[3]	D. L. Crawford and J. Mander, “Standard Stars for Photoelectric Hβ Photometry,” The Astronomical Journal, Vol. 71, 1966, pp. 114-118. http://dx.doi.org/10.1086/109865
[4]	N. Kaltcheva and V. Golev, “Galactic Structure toward the Carina Tangent,” Publications of the Astronomical Society of the Pacific, Vol. 124, No. 912, 2012, pp. 128-139. http://dx.doi.org/ 10.1086/664697
[5]	B. Hauck and M. Mermilliod, “uvbyβ Photoelectric Photometric Catalogue,” Astronomy and Astrophysics Supplement Series, Vol. 129, No. 3, 1998, pp. 431-433. http://dx.doi.org/10.1051/ aas:1998195
[6]	J. E. Gaustad, P. R. McCullough, W. Rosing and D. Van Buren, “A Robotic Wide-Angle Hα Survey of the Southern Sky,” Publications of the Astronomical Society of the Pacific, Vol. 113, No. 789, 2001, pp. 1326-1348. http://dx.doi.org/10.1086/323969
[7]	T. McGlynn, K. Scollick and N. White, “SkyView: The Multi-Wavelength Sky on the Internet,” Proceedings of the 179th Symposium of the International Astronomical Union, Baltimore, 26-30 August 1996, pp. 465-466.
[8]	D. L. Crawford, “Empirical Calibrations of the uvbyβ Systems. II. The B-Type Stars,” Astronomical Journal, Vol. 83, No. 1, 1978, pp. 48-63. http://dx.doi.org/10.1086/112176
[9]	D. Kilkenny and D. C. B. Whittet, “On the Intrinsic uvby Colours of Early-Type Supergiants,” Monthly Notices of the Royal Astronomical Society, Vol. 216, 1985, pp. 127-137.
[10]	L. A. Balona and R. R. Shobbrook, “A Re-Calibration of the Luminosities of Early-Type Stars—Its Effect on the Cepheid Luminosity Scale,” Monthly Notices of the Royal Astronomical Society, Vol. 211, 1984, pp. 375-390.
[11]	N. T. Kaltcheva and R. W. Hilditch, “The Distribution of Bright OB Stars in the Canis Major-Puppis-Vela Region of the Milky Way,” Monthly Notices of the Royal Astronomical Society, Vol. 312, No. 4, 2000, pp. 753-768. http://dx.doi.org/10.1046/j.1365-8711.2000.03170.x
[12]	N. Kaltcheva and V. Makarov, “The Structure and the Distance of Collinder 121 from Hipparcos and Photometry: Resolving the Discrepancy,” The Astrophysical Journal Letters, Vol. 667, No. 2, 2007, pp. L155-L157. http://dx.doi.org/10.1086/522495
[13]	N. Kaltcheva and J. Knude, “A Test of B-Type Star Hβ Photometric Distances via the HIPPARCOS Parallaxes,” Astronomy and Astrophysics, Vol. 337, 1998, pp. 178-182.
[14]	J. Torra, D. Fernández and F. Figueras, “Kinematics of Young Stars. I. Local Irregularities,” Astronomy and Astrophysics, Vol. 359, 2000, pp. 82-102.
[15]	N. Kaltcheva and V. Golev, “Improved Distances to Several Galactic OB Associations,” Stellar Clusters and Associations: A RIA Workshop on GAIA, Granada, 23-27 May 2011, pp. 299-303. http://sca.iaa. es/content/proceedings
[16]	C. D. Garmany and R. E. Stencel, “Galactic OB Associations in the Northern Milky Way Galaxy. I—Longitudes 55° to 150°,” Astronomy and Astrophysics Supplement Series, Vol. 94, No. 2, 1992, pp. 211-244.
[17]	T. S. Allen, R. A. Gutermuth, E. Kryukova, S. T. Megeath, J. L. Pipher, et al., “Spitzer Imaging of the Nearby Rich Young Cluster, Cep OB3b,” The Astrophysical Journal, Vol. 750, No. 2, 2012, p. 125. http://dx.doi.org/10.1088/0004-637X/750/2/125
[18]	A. Megier, A., A. Strobel, G. A. Galazutdinov and J. Krelowski, “The Interstellar Ca II Distance Scale,” Astronomy and Astrophysics, Vol. 507, No. 2, 2009, pp. 833-840. http://dx.doi.org/10.1051/0004-6361/20079144
[19]	W. S. Dias, B. S. Alessi, A. Moitinho and J. R. Lépine, “New Catalogue of Optically Visible Open Clusters and Candidates,” EAS Publications Series, Vol. 10, 2003, p. 195. http://dx.doi.org/ 10.1051/eas:2003148
[20]	J. H. Pena and R. Peniche, “Uvby-β Photometry of Open Clusters. IV. NGC 1444, NGC 1662, NGC 2129, NGC 2169, and NGC 7209,” Revista Mexicana de Astronomia y Astrofisica, Vol. 28, No. 2, 1994, pp. 139-152.
[21]	A. F. J. Moffat, “Photometry and Structure of the Young Open Cluster NGC 7380,” Astronomy and Astrophysics, Vol. 13, 1971, pp. 30-39.