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An Optical Survey of the Partially Embedded Young Cluster in NGC

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An Optical Survey of the Partially Embedded Young Cluster in NGC An Optical Survey of the Partially Embedded Young Cluster in NGC 7129 S. E. Dahm1 and L. A. Hillenbrand2 ABSTRACT NGC 7129 is a bright reflection nebula located in the molecular cloud complex near l =105◦.4, b =+9◦.9, about 1.15 kpc distant. Embedded within the reflection nebula is a young cluster dominated by a compact grouping of four early-type stars: BD+65◦1638 (B3V), BD+65◦1637 (B3e), SVS 13 (B5e), and LkHα 234 (B8e). About 80 Hα emission sources brighter than V ∼23 are identified in the region, many of which are presumably T Tauri star members of the cluster. We also present deep (V ∼23), optical (V RCIC ) photometry of a field centered on the reflection nebula and spectral types for more than 130 sources determined from low dispersion, optical spectroscopy. The narrow pre-main sequence evident in the color-magnitude diagram suggests that star formation was rapid and coeval. A median age of about 1.8 Myr is inferred for the Hα and literature-identified X-ray emission sources having established spectral types, using pre- main sequence evolutionary models. Our interpretation of the structure of the molecular cloud and the distribution of young stellar objects is that BD+65◦1638 is primarily responsible for evacuating the blister-like cavity within the molecular cloud. LkHα 234 and several embedded sources evident in near infrared adaptive optics imaging have formed recently within the ridge of compressed molecular gas. The compact cluster of low-mass stars formed concurrently with the early-type members, concentrated within a central radius of ∼0.7 pc. Star formation is simultaneously occurring in a semi-circular arc some ∼3 pc in radius that outlines remaining dense regions of molecular gas. High dispersion, optical spectra are presented for BD+65◦1638, BD+65◦1637, SVS 13, LkHα 234, and V350 Cep. These spectra are discussed in the context of the circumstellar environments inferred for these stars. Subject headings: stars: formation, pre-main sequence - Galaxy: open clusters and associations: individ- ual(NGC 7129) 1. Introduction a tight grouping of B-type stars that includes BD+65◦1638 (B3), BD+65◦1637 (B3e), SVS 13 The bright reflection nebula NGC 7129 (l = ◦ ◦ (B5e), and the Herbig Be star LkHα 234 (B8e). arXiv:1503.06230v1 [astro-ph.SR] 20 Mar 2015 105 ,b =+9.9) is illuminated by the early-type An elongated cavity one parsec in diameter has members of a young stellar cluster emerging from been carved out of L1181, presumably by the stel- a molecular cloud complex composed of the dark lar winds and ultraviolet flux emanating from the nebulae Lynds 1181 (L1181), L1183, and TDS central grouping of B-stars. Shown in Figure 1 395 (Taylor et al. 1987; Yonekura et al. 1997). is a three-color (V RC IC -band) image of the re- Signatures of star formation activity are present flection nebula obtained by G. H. Herbig in 1999 throughout the region including two dozen Herbig- October using the University of Hawaii (UH) 2.2 Haro (HH) objects, two large molecular outflows, m telescope on Mauna Kea. The image clearly numerous nebulous filaments, and embedded in- depicts the dense molecular ridge to the east and frared sources. The cluster is dominated by south, the evacuated region around the four lumi- 1 nous central stars, and the emerging young cluster W. M. Keck Observatory, Kamuela, HI 96743 lying within the reflection nebula. The distance of 2California Institute of Technology 1 NGC7129 was estimated to be ∼1 kpc by Racine cluster core (defined as a 0.5 pc radius arc ex- (1968), but more recent estimates include 1.26 tending from the peak of the local stellar surface kpc (Shevchenko & Yakubov 1989) and 1.15 kpc density). From a star count analysis, Gutermuth (Straiˇzys et al. 2014), which is adopted in this et al. (2004) estimate that there are about 80 clus- analysis. Kun et al. (2008) present a comprehen- ter members within the core, down to a limiting sive literature review of the Cepheus star forming magnitude of KS=16 mag, or approximately 0.055 region including NGC 7129 and its embedded clus- M⊙ for unattenuated sources. In the peripheral ter. Below we summarize only pertinent studies areas Gutermuth et al. (2004) estimate that the that are directly relevant to the present survey of extended cluster population is roughly equal to the young stellar population. that of the core. NGC 7129 and its associated molecular clouds Muzerolle et al. (2004) present 24 µm Multi- have been studied extensively at millimeter wave- band Imaging Photometer for Spitzer (MIPS) lengths by Loren (1975, 1977); Bechis et al. (1978); imaging of NGC 7129 and provide a census of the Bertout (1987); and Miskolczi et al. (2001). These cluster population outside of the reflection neb- 12CO and 13CO surveys have identified multi- ula. Of the 36 sources detected at 24 µm and in ple density enhancements in the region, a semi- at least three IRAC passbands, 13 were classified circular ridge of CO emission to the north, east, as Class 0/I protostars with envelopes including and south of the reflection nebula, and the large FIRS 2 (Class 0), another 18 as Class II sources evacuated cavity centered near BD+65◦1637. In (classical T Tauri stars, CTTS) with relatively low the infrared, Bechis et al. (1978) detected two 100 infrared luminosity, and six were classified as Class µm sources in the area: LkHα 234 and far infrared III sources with emission consistent with that of source 2 (FIRS 2), which has no optical coun- pure stellar photospheres. The majority of these terpart, located some 2′ south of BD+65◦1638. sources are located in an arc sweeping north-east At least two molecular outflows have been iden- to south of the reflection nebula, effectively trac- tified in the region (Edwards & Snell 1983; Liseau ing the densest regions of the remnant molecular & Sandell 1983, Bechis et al. 1978); one cen- cloud. Muzerolle et al. (2004) conclude that star tered on FIRS 2 and the other originating near formation is not concentrated in the cluster core, LkHα234. Yonekura et al. (1997) estimate the to- but rather dispersed over a substantial area, some tal mass of the molecular cloud group associated ∼3 pc in scale. with NGC7129 to be ∼2900 M⊙. Stelzer & Scholz (2009) combined the mid- A single-channel 2.2 µm survey of the region infrared Spitzer surveys of Gutermuth et al. (2004) was carried out by Strom et al. (1976), here- and Muzerolle et al. (2004) with a shallow 22 ks after SVS, in a search for an embedded popula- Chandra Advanced CCD Imaging Spectrometer tion of newly formed stars. Although none were (ACIS) X-ray observation of the region. From the identified, the detection limit of the survey was Spitzer infrared excess sources and the 59 Chandra marginally close to the expected brightness of an X-ray detections, they define a sample of 26 Class average T Tauri star. Cohen & Schwartz (1983) II and 25 Class III cluster members, estimated to identified four new infrared sources in the region, be complete to ∼0.5 M⊙. The disk fraction of one of which was V350 Cep, an M2-type T Tauri the least biased sub-sample composed of lightly +24 star that brightened dramatically between 1970 extincted sources is ∼33−19%. and 1976. Ibryamov et al. (2014) present a photo- While NGC 7129 has been studied extensively metric study for this source that includes a long- at millimeter, micron, and X-ray wavelengths, term light curve constructed from archival data. it remains poorly characterized in the optical. Gutermuth et al. (2004) used the Two Micron Hartigan & Lada (1985) obtained V RC IC and All Sky Survey (2MASS), the FLAMINGOS spec- narrowband Hα CCD imaging of two regions in trograph on the Multiple Mirror Telescope, and NGC 7129 which include most sources from the the Infrared Array Camera (IRAC) onboard the infrared surveys of Cohen & Schwartz (1983) and Spitzer Space Telescope to complete an extensive SVS. V RC IC photometry is provided for about 30 1.2 to 8.0 µm survey of the cluster, ultimately de- sources in these two fields. Magakian et al. (2004) termining a disk fraction of 54±14% within the identified 22 Hα emission stars in the central and 2 northeast cluster regions, complete to V ∼20 us- V350 Cep; in Section 4 the stellar population of ing slitless spectroscopy. V RI− band photometry NGC 7129 is examined to include the early-type for about 100 sources was also obtained, but not stars, the Hα emission sources, and the X-ray de- provided in the published analysis. Magakian et tected sources. We present in tabular form pho- al. (2004) concluded that most of the detected Hα tometry and spectral types determined at optical emission sources were T Tauri stars. Vilnius 7- wavelengths for two subsets of stars: 1) those ex- color photometry down to V =18.8 is provided by hibiting Hα and/or X-ray emission, and 2) others Straiˇzys et al. (2014) for 159 stars in the cluster in the field, some of which exhibit infrared excess. region. The photometric data was used to clas- In Section 5 we discuss the color-magnitude di- sify about half of the detected sources, assign- agram of the pre-main sequence population, the ing spectral types, luminosity classes and deriv- average extinction suffered by those stars with as- ing extinction estimates. The extinction across a signed spectral types, the ages and masses of the 23′×23′ area varies between 0.6 and 2.8 mag, but cluster members, and the infrared color-color dia- in the densest parts of the molecular cloud, AV grams.
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