[Paper Review] Low mass star formation and subclustering in the HII regions RCW 32, 33 and 27 of the Vela Molecular Ridge. A photometric diagnostics to identify M-type stars

L. Prisinzano, F. Damiani+5 more|arXiv (Cornell University)|Jun 18, 2018

Astrophysics and Star Formation StudiesPhysics and Astronomy38 references4 citations

TL;DR

This study identifies low-mass pre-main sequence stars down to 0.1 M⊙ in the HII regions RCW 32, 33, and 27 within the Vela Molecular Ridge using multi-wavelength photometry from Gaia, VPHAS, 2MASS, and Spitzer. By introducing a novel reddening-based diagnostic to distinguish M-type pre-main sequence stars from main sequence and giant stars, the authors reveal three distinct young clusters with differing star formation histories—showing a declining rate in RCW 33 and triggered formation in RCW 32 and RCW 27—supported by spatial correlation with dust structures and age gradients.

ABSTRACT

Most stars born in clusters and recent results suggest that star formation (SF) preferentially occurs in subclusters. Studying the morphology and SF history of young clusters is crucial to understanding early SF. We identify the embedded clusters of young stellar objects (YSOs) down to M stars, in the HII regions RCW33, RCW32 and RCW27 of the Vela Molecular Ridge. Our aim is to characterise their properties, such as morphology and extent of the clusters in the three HII regions, derive stellar ages and the connection of the SF history with the environment. Through public photometric surveys such as Gaia, VPHAS, 2MASS and Spitzer/GLIMPSE, we identify YSOs with IR, Halpha and UV excesses, as signature of circumstellar disks and accretion. In addition, we implement a method to distinguish M dwarfs and giants, by comparing the reddening derived in several optical/IR color-color diagrams, assuming suitable theoretical models. Since this diagnostic is sensitive to stellar gravity, the procedure allows us to identify pre-main sequence stars. We find a large population of YSOs showing signatures of circumstellar disks with or without accretion. In addition, with the new technique of M-type star selection, we find a rich population of young M stars with a spatial distribution strongly correlated to the more massive population. We find evidence of three young clusters, with different morphology. In addition, we identify field stars falling in the same region, by securely classifying them as giants and foreground MS stars. We identify the embedded population of YSOs, down to about 0.1 Msun, associated with the HII regions RCW33, RCW32 and RCW27 and the clusters Vela T2, Cr197 and Vela T1, respectively, showing very different morphologies. Our results suggest a decreasing SF rate in Vela T2 and triggered SF in Cr197 and Vela T1.

Motivation & Objective

• To identify embedded low-mass YSOs down to 0.1 M⊙ in the HII regions RCW 32, 33, and 27 of the Vela Molecular Ridge.
• To characterize the morphology, spatial distribution, and star formation history of young clusters in these regions.
• To develop and apply a new photometric diagnostic to distinguish M-type pre-main sequence stars from main sequence and giant stars using reddening in color-color diagrams.
• To determine individual distances to the clusters using TGAS Gaia data and assess the role of environmental triggers in star formation.
• To disentangle field star contaminants (giants and foreground MS stars) from genuine cluster members in the M-type regime.

Proposed method

• Combines photometric data from Gaia, VPHAS, 2MASS, and Spitzer/GLIMPSE to detect IR, Hα, and UV excesses as signatures of circumstellar disks and accretion in YSOs.
• Applies a novel reddening-based diagnostic in optical/IR color-color diagrams to distinguish M-type pre-main sequence stars from main sequence and giant stars, leveraging the higher intrinsic luminosity of PMS stars.
• Uses unreddened color-magnitude diagrams (CMDs) compared to PMS isochrones at cluster distances to reject foreground and background M-type contaminants.
• Performs spatial analysis of M-type YSOs and compares their distribution with WISE 12 μm images to identify dust concentrations and BRCs (barnacle-like structures).
• Splits the M-type YSO population into age bins using derived ages to infer time-resolved star formation rates and identify triggering mechanisms.
• Validates the method on a subsample of 72 M-type stars selected via both classical and new techniques, confirming consistency and enhancing completeness.

Experimental results

Research questions

• RQ1 What is the spatial distribution and morphology of low-mass YSOs in the HII regions RCW 32, 33, and 27 of the Vela Molecular Ridge?
• RQ2 How can M-type pre-main sequence stars be reliably identified and separated from main sequence stars and giants in dusty, high-reddening environments?
• RQ3 What is the star formation history in the three clusters Vela T1, Vela T2, and Cr 197, and how does it vary across the region?
• RQ4 Is there evidence of triggered star formation in RCW 32 and RCW 27, and how does it compare to the declining star formation in RCW 33?
• RQ5 To what extent do dust structures and BRCs correlate with the age distribution and spatial clustering of low-mass YSOs?

Key findings

• The authors identify a total population of 2,035 YSOs in the three HII regions, with a significant fraction being low-mass stars down to 0.1 M⊙, confirmed via a new photometric diagnostic for M-type pre-main sequence stars.
• Three distinct young clusters—Vela T1, Vela T2, and Cr 197—are identified, all at a similar distance (~0.85 kpc), but with markedly different morphologies and star formation histories.
• The star formation rate is found to be decreasing in RCW 33 (associated with Vela T2), while it is likely triggered in RCW 32 (Cr 197) and RCW 27 (Vela T1), as indicated by age gradients and spatial correlation with dust structures.
• The spatial distribution of M-type YSOs strongly correlates with that of YSOs identified via classical methods, validating the new diagnostic and confirming the presence of embedded populations in clumps and elongated features.
• The method successfully rejects field star contaminants, including foreground M-type main sequence stars and background giants, by leveraging individual reddening estimates and comparison with PMS isochrones.
• Stellar ages for the M-type YSOs range from 0.5 to 13 Myr, with age gradients indicating sequential or triggered star formation, consistent with external triggering by ionizing radiation from massive stars.