[Paper Review] Massive open star clusters using the VVV survey III: A young massive cluster at the far edge of the Galactic bar

S. Ramírez Alegría, J. Borissova+15 more|Americanae (AECID Library)|Mar 13, 2014

Stellar, planetary, and galactic studies44 references14 citations

TL;DR

This study presents the physical characterization of VVV CL086, a young massive cluster located at the far edge of the Galactic bar using near-infrared photometry and spectroscopy from the VVV survey. It confirms the cluster's distance at 11⁺⁵₋₆ kpc, estimates a lower limit of (2.8⁺¹.⁶₋₁.₄)×10³ M☉ for its total mass, and constrains its age to 1.0–5.0 Myr, marking the second such cluster identified in this highly obscured region of the Milky Way.

ABSTRACT

Context: Young massive clusters are key to map the Milky Way's structure, and near-IR large area sky surveys have contributed strongly to the discovery of new obscured massive stellar clusters. Aims: We present the third article in a series of papers focused on young and massive clusters discovered in the VVV survey. This article is dedicated to the physical characterization of VVV CL086, using part of its OB-stellar population. Methods: We physically characterized the cluster using $JHK_S$ near-infrared photometry from ESO public survey VVV images, using the VVV-SkZ pipeline, and near-infrared $K$-band spectroscopy, following the methodology presented in the first article of the series. Results: Individual distances for two observed stars indicate that the cluster is located at the far edge of the Galactic bar. These stars, which are probable cluster members from the statistically field-star decontaminated CMD, have spectral types between O9 and B0V. According to our analysis, this young cluster ($1.0$ Myr $

Motivation & Objective

• To physically characterize VVV CL086, a newly discovered massive cluster in the far edge of the Galactic bar, using near-infrared data.
• To determine the cluster's distance, age, and mass by analyzing photometric and spectroscopic data from the VVV survey.
• To investigate the presence of early-type OB stars and constrain the cluster's initial mass function (IMF) in a highly obscured region.
• To assess the completeness of the observed stellar population and identify potential missing massive stars.
• To improve understanding of massive star formation in the outer regions of the Galactic bar, a region previously underexplored due to high extinction.

Proposed method

• Near-infrared $JHK_S$ photometry was extracted from VVV survey images using the VVV-SkZ pipeline, which applies PSF photometry with ALLFRAME.
• The cluster's color-magnitude diagram (CMD) was constructed using decontaminated field-star statistics to isolate cluster members.
• K-band spectroscopy of bright stars in the cluster field was used to classify stellar types and confirm membership, focusing on early-B and O-type stars.
• Cluster age was estimated by fitting pre-main sequence and main sequence isochrones to the CMD, particularly using the pre-main sequence turn-on point.
• The total cluster mass was derived by integrating the Kroupa initial mass function (IMF) fitted to the observed present-day mass function, with uncertainties from fitting errors only.
• Distance was inferred from individual star parallaxes and photometric distance moduli, with a final estimate of 11⁺⁵₋₆ kpc.

Experimental results

Research questions

• RQ1 What is the distance to VVV CL086, a newly discovered massive cluster at the far edge of the Galactic bar?
• RQ2 What is the age of VVV CL086, and how does it compare to the evolutionary stage of its earliest-type stars?
• RQ3 What is the total mass of VVV CL086, and what is the lower limit based on the observed OB population?
• RQ4 Are there more massive or earlier-type stars in VVV CL086 than those spectroscopically detected?
• RQ5 How does the cluster's IMF compare to the standard Kroupa IMF, and what does this imply about its initial mass function?

Key findings

• VVV CL086 is located at a distance of 11⁺⁵₋₆ kpc, placing it at the far edge of the Galactic bar, consistent with the expected symmetry of massive cluster formation.
• The cluster's age is constrained to 1.0 Myr < age < 5.0 Myr, based on fitting pre-main sequence isochrones to the CMD's turn-on point.
• The lower limit for the cluster's total mass is (2.8⁺¹.⁶₋₁.₄)×10³ M☉, derived from integrating the Kroupa IMF to the observed present-day mass function.
• Two observed stars are confirmed as early-B dwarfs (spectral types O9–B0 V), indicating the presence of a massive population, with likely earlier and more massive stars still undetected.
• The cluster exhibits a mean reddening of A_K = 1.5⁺⁰.⁰³₋₀.¹ mag, consistent with its location in a highly obscured region of the Galactic disk.
• VVV CL086 is the second known massive cluster in the far edge of the Galactic bar, following Mercer 81, highlighting the importance of near-infrared surveys in probing obscured star formation regions.