[Paper Review] The Villafranca catalog of Galactic OB groups. II. From Gaia DR2 to EDR3 and ten new systems with O stars

J. Maíz Apellániz, R. H. Barbá+5 more|arXiv (Cornell University)|Oct 4, 2021

Astronomy and Astrophysical Research6 references4 citations

TL;DR

This study revises the Villafranca catalog of Galactic OB groups using Gaia EDR3 astrometry and photometry combined with high-resolution spectroscopy from GOSSS and LiLiMaRlin, achieving sub-1% distance precision within 1 kpc. It identifies ten new O-star systems, confirms orphan clusters with capping mass functions, and derives precise ages and IMF constraints for four young clusters using pre-main-sequence isochrone fitting.

ABSTRACT

CONTEXT. This is the second paper of a series on Galactic OB groups that uses astrometric and photometric data from Gaia and spectral classifications from GOSSS and LiLiMaRlin. The previous paper was based on Gaia DR2, this is based on Gaia EDR3. AIMS. The two aims of this paper are to revise the results for the sample from paper I using Gaia EDR3 data and to expand the sample of analyzed stellar groups to 26, from Villafranca O-001 to Villafranca O-026. METHODS. We use GOSSS to select Galactic stellar groups with O stars and an updated version of the method in paper 0 of this series, combining Gaia EDR3 G+GBP+GRP photometry, positions, proper motions, and parallaxes to assign memberships and measure distances. We present 99 spectra from GOSSS and 32 from LiLiMaRlin for stars in the analyzed groups or in their foreground. RESULTS. We derive distances to the 26 stellar groups with unprecedented precision and accuracy, with total uncertainties <1% within 1 kpc and of ~3% around 3 kpc, values that are almost 4x better than for Gaia DR2. We provide homogeneous spectral types for 110 stars and correct a number of errors in the literature, especially for objects in the Orion nebula cluster. For each of the groups we discuss its membership and present possible runaway/walkaway stars. At least two groups, Villafranca O-O12 S and Villafranca O-014 NW, are orphan clusters in which the most massive stars have been ejected by dynamical interactions, leaving objects with a capped mass function. The existence of such clusters has important consequences for the study of the IMF, the distribution of SNe across the Galaxy, and the population and dynamics of isolated compact objects. We fit PMS isochrones to four clusters to derive ages of 2.0$\pm$0.5 Ma for the sigma Orionis cluster, 4$\pm$2 Ma for NGC 2264, 5.0$\pm$0.5 Ma for NGC 2362, and 8$\pm$2 Ma for the gamma Velorum cluster. [ABRIDGED]

Motivation & Objective

• To update the Villafranca catalog of Galactic OB groups using improved Gaia EDR3 astrometry and photometry.
• To expand the sample to 26 stellar groups, including 10 new systems with O stars.
• To derive precise distances and spectral types for OB groups using combined astrometric and spectroscopic data.
• To investigate the initial mass function (IMF) and dynamical evolution of young clusters, especially orphan clusters with capping mass functions.
• To assess the impact of stellar ejection and supernova feedback on cluster mass function and compact object populations.

Proposed method

• Applied a refined membership and distance estimation method using Gaia EDR3 G, G_BP, and G_RP photometry, positions, proper motions, and parallaxes.
• Combined data from the Galactic O-Star Spectroscopic Survey (GOSSS) and LiLiMaRlin for spectral classification of 141 stars across 26 groups.
• Fitted pre-main-sequence (PMS) isochrones to color-magnitude diagrams (CMDs) to derive cluster ages with uncertainties.
• Used a Kroupa initial mass function (IMF) as a reference to compare observed numbers of 2–50 M_⊙ and 8–50 M_⊙ stars.
• Identified runaway and walkaway stars via kinematic analysis and astrometric quality flags (e.g., RUWE > 1.4 or σ_ext>0.1 mas).
• Corrected literature spectral types, especially for stars in the Orion Nebula Cluster (Villafranca O-023).

Experimental results

Research questions

• RQ1 What are the updated distances and membership probabilities for 26 Galactic OB groups using Gaia EDR3 data?
• RQ2 How do the observed initial mass functions (IMFs) of young clusters compare to the canonical Kroupa IMF, especially in low-mass systems?
• RQ3 What evidence supports the existence of orphan clusters with capping mass functions, and what are their dynamical implications?
• RQ4 How do pre-main-sequence isochrone fits constrain the ages of young clusters such as σ Ori and NGC 2264?
• RQ5 To what extent do stellar ejection and supernova feedback explain the observed mass function truncation in clusters like NGC 2467 and the North America Nebula?

Key findings

• Distances to the 26 OB groups were measured with total uncertainties below 1% within 1 kpc and ~3% at 3 kpc, representing a ~4× improvement over Gaia DR2.
• The age of the σ Ori cluster (Villafranca O-026) was determined as 2.0 ± 0.5 Ma using PMS isochrone fitting to its CMD.
• The NGC 2264 cluster (Villafranca O-016) was found to be 4 ± 2 Ma old, with a PMS isochrone fit consistent with its observed CMD.
• The NGC 2362 cluster (Villafranca O-021) has an age of 5.0 ± 0.5 Ma, and its IMF counts are consistent with a Kroupa IMF within 1σ for 8–50 M_⊙ stars.
• The γ Velorum cluster (Villafranca O-024) has an age of 8 ± 2 Ma and shows a slight overabundance of massive stars (4 in 8–50 M_⊙ range) compared to the Kroupa IMF, suggesting stochastic IMF effects.
• Two clusters—Villafranca O-012 S in NGC 2467 and O-014 NW in the North America Nebula—are identified as orphan clusters likely formed by dynamical ejection of massive stars, resulting in a capping mass function.