[Paper Review] Spectroscopy of Very Low Mass Stars and Brown Dwarfs in the Lambda Orionis Star Forming Region. I. Enlarging the census down to the planetary mass domain in Collinder 69

A. Bayo, D. Barrado+8 more|arXiv (Cornell University)|Sep 22, 2011

Astrophysics and Star Formation Studies71 references52 citations

TL;DR

This study presents the most complete spectroscopically confirmed census of young very low-mass stars and brown dwarfs in the Collinder 69 cluster of the Lambda Orionis Star Forming Region, using optical and near-infrared spectroscopy to identify members down to 0.016 M⊙ (~16.5 MJup). It establishes a robust initial mass function (IMF) from 0.016 to 20 M⊙ and finds spatial distribution patterns that challenge the ejection formation scenario for brown dwarfs in this environment.

ABSTRACT

Whilst there is a generally accepted evolutionary scheme for the formation of low-mass stars, the analogous processes when moving down in mass to the brown dwarfs regime are not yet well understood. In this first paper we try to build the most complete and unbiased spectroscopically confirmed census of the population of Collinder 69, the central cluster of the Lambda Orionis star forming region, as a first step in addressing the question of how brown dwarfs and planetary mass objects form. We have studied age dependent features in optical and near-infrared spectra of candidate members to the cluster (such as alkali lines and accretion associated indicators). In addition, we have complemented that study with the analysis of other youth indicators like X-ray emission or mid-infrared excess. We have confirmed the membership to Collinder 69 of \sim90 photometric candidate members. As a byproduct we have determined a temperature scale for young M, very low-mass stars and brown dwarfs. We have assembled one of the most complete Initial Mass Functions from 0.016 to 20 M\odot. And, finally, we have studied the implications of the spatial distribution of the confirmed members on the proposed mechanisms of brown dwarfs formation.

Motivation & Objective

• To build the most complete and unbiased spectroscopically confirmed census of very low-mass stars and brown dwarfs in the Collinder 69 cluster.
• To determine the initial mass function (IMF) from 0.016 M⊙ to 20 M⊙ using spectroscopically confirmed members.
• To assess the formation mechanisms of brown dwarfs by analyzing the spatial distribution of confirmed members.
• To establish a temperature scale for young M dwarfs, very low-mass stars, and brown dwarfs using spectral features.
• To evaluate the viability of the ejection formation scenario for brown dwarfs in the context of the cluster's spatial structure.

Proposed method

• Acquired optical and near-infrared spectra of ~170 candidate members using multiple telescopes, including Keck, VLT, Magellan, and Subaru.
• Used alkali lines and accretion indicators (e.g., Hα emission) as youth diagnostics to confirm cluster membership.
• Combined photometric data from CFHT, XMM-Newton, and IRAC with spectroscopic results to cross-verify membership and derive physical parameters.
• Applied spectral type classification based on molecular absorption bands and temperature scales from Basri et al. (2000).
• Calculated masses using 5 Myr composite isochrones (SIESS + COND) based on effective temperature and bolometric luminosity.
• Constructed the IMF using three independent mass estimates per source and two spatial regions (overlap of XMM, CFHT, IRAC and full CFHT field).

Experimental results

Research questions

• RQ1 What is the most complete and unbiased spectroscopically confirmed census of very low-mass stars and brown dwarfs in the Collinder 69 cluster?
• RQ2 How does the initial mass function (IMF) derived from spectroscopically confirmed members compare to photometric IMFs in similar-age young associations?
• RQ3 What do the spatial distributions of confirmed members reveal about the formation mechanisms of brown dwarfs?
• RQ4 To what extent do youth indicators such as alkali lines and Hα emission support the presence of accretion and activity in planetary-mass objects?
• RQ5 Is the ejection formation scenario for brown dwarfs viable in the Collinder 69 environment based on spatial and mass distribution data?

Key findings

• The authors confirmed the membership of 90 photometric candidate members to Collinder 69, with 9 additional possible members, using spectroscopic youth indicators such as alkali lines and Hα emission.
• The study provides a temperature scale for young M dwarfs, very low-mass stars, and brown dwarfs, with spectral types L0–L2 corresponding to effective temperatures of ~2000 K.
• The derived initial mass function (IMF) spans from 0.016 M⊙ to 20 M⊙, making it one of the most complete spectroscopically confirmed IMFs for a young association.
• The IMF derived from spectroscopically confirmed members shows no significant differences when compared to photometric IMFs in similar-age associations such as NGC6611, supporting the robustness of the spectroscopic census.
• The spatial distribution of confirmed members shows features inconsistent with a pure ejection scenario for brown dwarf formation, suggesting that in-situ formation may be more plausible in this region.
• The survey reaches completeness down to 0.016 M⊙ (~16.5 MJup), corresponding to the lowest mass limit for confirmed cluster members in this study.