Massive open star clusters using the VVV survey

Abstract

Context. The ESO Public Survey "VISTA Variables in the Vía Láctea" (VVV) provides deep multi-epoch infrared observations for unprecedented 562 sq. degrees of the Galactic bulge, and adjacent regions of the disk. Aims. The VVV observations will foster the construction of a sample of Galactic star clusters with reliable and homogeneously derived physical parameters (e.g., age, distance, and mass, etc.). In this first paper in a series, the methodology employed to establish cluster parameters for the envisioned database are elaborated upon by analysing four known young open clusters: Danks 1, Danks 2, RCW 79, and DBS 132. The analysis offers a first glimpse of the information that can be gleaned from the VVV observations for clusters in the final database. Methods. Wide-field, deep JHK s VVV observations, combined with new infrared spectroscopy, are employed to constrain fundamental parameters for a subset of clusters. Results. Results are inferred from VVV near-infrared photometry and numerous low resolution spectra (typically more than 10 per cluster). The high quality of the spectra and the deep wide-field VVV photometry enables us to precisely and independently determine the characteristics of the clusters studied, which we compare to previous determinations. An anomalous reddening law in the direction of the Danks clusters is found, specifically E(J − H)/E(H − Ks) = 2.20 ± 0.06, which exceeds published values for the inner Galaxy. The G305 star forming complex, which includes the Danks clusters, lies beyond the Sagittarius-Carina spiral arm and occupies the Centaurus arm. Finally, the first deep infrared colour-magnitude diagram of RCW 79 is presented, which reveals a sizeable pre-main sequence population. A list of candidate variable stars in G305 region is reported. Conclusions. This study demonstrates the strength of the dataset and methodology employed, and constitutes the first step of a broader study which shall include reliable parameters for a sizeable number of poorly characterised and/or newly discovered clusters.Key words. open clusters and associations: general -infrared: stars -surveys IntroductionIt is commonly accepted that the majority of stars with masses ≥0.50 M form in clustered environments (e.g. than half of these clusters have actually been studied, and this subset suffers from further selection biases. Extending this current sample towards the Milky Way's highly obscured central region would provide unique insight into the formation, evolution, and dissipation of open clusters in the Galactic environment. To achieve this goal, we are using the unprecedented deep infrared data from the VISTA Variables in the Vía Láctea (VVV) survey (Minniti et al. 2010;Saito et al. 2012), one of the six ESO Public Surveys operating on the new 4-m Visible and Infrared Survey Telescope for Astronomy (VISTA). We are in the process of building a large sample of star clusters (including many discovered by our group; Borissova et al. 2011; Bonatto et al., in prep.), that are practically invisible in th...

The embedded clusters DBS 77, 78, 102, and 160−161, and their link with the interstellar medium

“…The methods employed to determine the fundamental parameters of the young stellar clusters, (sizes, extinctions, distances, ages, members, and masses) have already been described in several previous papers (see e.g., Chené et al 2012Chené et al , 2013Baume et al 2014). Here, we provide a brief summary and highlight what we did differently in this study.…”

“…As a measure of atmospheric absorption, we observed bright stars of spectral type G. We used the standard procedures to reduce the data. For more details of our reduction procedures see Moorwood et al (1998) and Chené et al (2012Chené et al ( , 2013.…”

“…Our goal is to obtain the characteristic parameters of a large sample of embedded clusters using homogeneous data and analysis methods. Results for four known clusters have been presented by Chené et al (2012) and for six of the new VVV cluster candidates with Wolf-Rayet (WR) stars in Chené et al (2013). In this paper we follow similar analytical methods to those papers, but we applied these methods to a sample of clusters of the DBS catalogue (Dutra et al 2003) and we complemented them with optical and radio data.…”

Proper motions of the optically visible open clusters based on the UCAC4 catalog

“…For open clusters Danks 1 and Danks 2, the radius was small which did not allow a sufficient UCAC4 number of stars in the field for the application of statistical method. We prefer to keep the results obtained by Chené et al (2012), although with large estimated errors, based on a selection of members using photometric (J, H, K) data, PPMXL proper motion and radial velocity. For the cluster Feigelson 1, our results were not satisfactory given the limitation of the method for nearby clusters, which typically have large proper motion and small number of member stars.…”

Deriving metallicities from calcium triplet spectroscopy in combination with near-infrared photometry

“…We performed photometry through fitting of point spread function using the VVV-SkZ_pipeline code (Mauro et al 2013), which is based on DAOPHOT suite (Stetson 1987(Stetson , 1994, on the single 2048 × 2048 pixel chips extracted from the stacked VVV pawprints (Saito et al 2012). The photometry was tied to the 2MASS system, as described in Moni Bidin et al (2011) and Chené et al (2012). The use of the 2MASS system as the standard photometric system permitted us to integrate our photometric database with the 2MASS PSC.…”