Abstract
ABSTRACT We present spectroscopic follow-up observations of 68 red, faint candidates from our multi-epoch, multiwavelength, previously published survey of NGC 2264. Using near-infrared spectra from VLT/KMOS, we measure spectral types and extinction for 32 young low-mass sources. We confirm 13 as brown dwarfs in NGC 2264, with spectral types between M6 and M8, corresponding to masses between 0.02 and 0.08 M⊙. These are the first spectroscopically confirmed brown dwarfs in this benchmark cluster. 19 more objects are found to be young M-type stars of NGC 2264 with masses of 0.08–0.3 M⊙. 7 of the confirmed brown dwarfs as well as 15 of the M-stars have IR excess caused by a disc. Comparing with isochrones, the typical age of the confirmed brown dwarfs is <0.5–5 Myr. More than half of the newly identified brown dwarfs and very low-mass stars have ages <0.5 Myr, significantly younger than the bulk of the known cluster population. Based on the success rate of our spectroscopic follow-up, we estimate that NGC 2264 hosts 200–600 brown dwarfs in total (in the given mass range). This would correspond to a star-to-brown dwarf ratio between 2.5:1 and 7.5:1. We determine the slope of the substellar mass function as $\alpha = 0.43^{+0.41}_{-0.56}$; these values are consistent with those measured for other young clusters. This points to a uniform substellar mass function across all star-forming environments.
Highlights
As one of the youngest and richest star-forming regions accessible to deep surveys, NGC 2264 provides a perfect laboratory to study star and brown dwarf formation
We find that the stellar density and the presence or absence of observing blocks (OBs) stars do not have a significant effect on brown dwarf numbers, a result that provides constraints on formation scenarios for brown dwarfs (Muzicet al. 2019)
For 33 out of 68 objects, we find a robust fit, which is visually convincing, with a clear structure in the H band resembling the expected spectral shape of M dwarfs. 13 of those have derived spectral types of M6 or later (Fig. 5), Baraffe et al (2015) suggest that the hydrogen-burning mass limit occurs near M6 (Teff ∼ 2925 K) at ages of ≤10 Myr
Summary
As one of the youngest and richest star-forming regions accessible to deep surveys, NGC 2264 provides a perfect laboratory to study star and brown dwarf formation. The prerequisite for making progress on these issues is to have large and well-characterized samples of young brown dwarfs in diverse star-forming regions. NGC 2264 is a diverse star-forming region that has the potential to provide this large sample but currently lacks the spectroscopic observations needed for detailed characterization. We present spectra for the first confirmed brown dwarfs in NGC 2264 and measure the overall success rate of our candidates This will allow us to quantify the substellar inventory of this cluster, building an essential sample of young brown dwarfs, as well as lay the groundwork for extending the MF to the substellar domain.
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