A deep look into the cores of young clusters

Bouy, H.; Huélamo, N.; Martín, E. L.; Marchis, Franck; Barrado, D.; Kolb, Johann; Marchetti, Enrico; Petr-Gotzens, M. G.; Sterzik, M.; Ivanov, V. D.; Köhler, Rainer H.; Nürnberger, D. E. A.

doi:10.1051/0004-6361:200810267

Cited by 42 publications

(44 citation statements)

References 55 publications

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“…noticed by Caballero (2008b), could be explained by the presence of σ Ori, a massive multiple system at the center of the cluster (Caballero 2008a) that would have prevented the survival of the lowest-mass objects in the core of the cluster. However, new potential member candidates were extracted within the central 1.5 × 1.5 arcmin of the cluster from a multi-conjugate adaptive optics survey (Bouy et al 2009), suggesting that this issue is not yet settled. The formation mechanism of brown dwarfs proposed by Whitworth & Zinnecker (2004), where the wind of O and B stars would erode the cores during the formation, would not be the main mechanism at play in σ Orionis.…”

Section: The Distribution Of Low-mass Stars and Brown Dwarfsmentioning

confidence: 99%

A census of very-low-mass stars and brown dwarfs in the σ Orionis cluster

Lodieu

Osorio

Rebolo

et al. 2009

A&A

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113

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Context. The knowledge of the initial mass function (IMF) in open clusters constitutes one way of constraining the formation of low-mass stars and brown dwarfs, along with the frequency of multiple systems and the properties of disks. Aims. The aim of the project is to determine the shape of the mass function in the low-mass and substellar regimes in the σ Orionis cluster (∼3 Myr, ∼352 pc, solar metallicity) as accurately as possible and compare it with the results in other clusters. Methods. We have analysed the near-infrared photometric data from the fourth data release (DR4) of the UKIRT Infrared Deep Sky Suvey (UKIDSS) Galactic clusters survey (GCS) to derive the cluster luminosity and mass functions, evaluate the extent of the cluster, and study the distribution and variability of low-mass stars and brown dwarfs down to the deuterium-burning limit. Results. We have recovered most of the previously published members and found a total of 287 candidate members within the central 30 arcmin in the 0.5-0.009 M mass range, including new objects not previously reported in the literature. This new catalogue represents a homogeneous dataset of brown dwarf member candidates over the central 30 arcmin of the cluster. The expected photometric contamination by field objects with similar magnitudes and colours to σ Orionis members is ∼15%. We present evidence of variability at the 99.5% confidence level over ∼yearly timescales in 10 member candidates that exhibit signs of youth and the presence of disks. The level of variability is low (≤0.3 mag) and does not impact the derivation of the cluster luminosity and mass functions. Furthermore, we find a possible dearth of brown dwarfs within the central five arcmin of the cluster, which is not caused by a lower level of photometric sensitivity around the massive, O-type multiple star σ Ori in the GCS survey. Using state-of-the-art theoretical models, we derived the luminosity and mass functions within the central 30 arcmin from the cluster centre, with completeness down to J = 19 mag, corresponding to masses ranging from 0.5 M down to the deuterium-burning mass boundary (∼0.013 M ). Conclusions. The mass function of σ Orionis in this mass interval shows a power law index α = 0.5 ± 0.2 (when expressed as dN/dM ∝ M −α ), which agrees with the one derived for the 3-5 Myr cluster Upper Sco (based on similar data obtained with the GCS) in the same mass range.

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Section: The Distribution Of Low-mass Stars and Brown Dwarfsmentioning

confidence: 99%

A census of very-low-mass stars and brown dwarfs in the σ Orionis cluster

Lodieu

Osorio

Rebolo

et al. 2009

A&A

Self Cite

113

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“…If the cluster membership census and the individual masses are confirmed, this configuration could be explained by several mechanisms, including e.g., a possible photo-erosion by the central OB stars (Hester et al 1996;Whitworth & Zinnecker 2004) in the deep gravity well. Complementary studies of the dense cluster core (Bouy et al 2009) and other cluster regions could thus help us to understand the formation of low-mass planetary-mass objects. …”

Section: Cluster Membershipmentioning

confidence: 99%

Candidate free-floating super-Jupiters in the young σ Orionis open cluster

Bihain

Rebolo

Osorio³

et al. 2009

A&A

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Context. Free-floating substellar candidates with estimated theoretical masses of as low as ∼5 Jupiter masses have been found in the ∼3 Myr old σ Orionis open cluster. As the overlap with the planetary mass domain increases, the question of how these objects form becomes important. The determination of their number density and whether a mass cut-off limit exists is crucial to understanding their formation. Aims. We propose to search for objects of yet lower masses in the cluster and determine the shape of the mass function at low mass. Methods. Using new-and (re-analysed) published IZJHK s [3.6]−[8.0]-band data of an area of 840 arcmin 2 , we performed a search for LT-type cluster member candidates in the magnitude range J = 19.5-21.5 mag, based on their expected magnitudes and colours. Results. Besides recovering the T type object S Ori 70 and two other known objects, we find three new cluster member candidates, S Ori 72-74, with J ≈ 21 mag and within 12 arcmin of the cluster centre. They have theoretical masses of 4 +3 −2 M Jup and are among the least massive free-floating objects detected by direct imaging outside the Solar System. The photometry in archival Spitzer [3.6]-[5.8]-band images infers that S Ori 72 is an L/T transition candidate and S Ori 73 a T-type candidate, following the expected cluster sequence in the mid-infrared. Finally, the L-type candidate S Ori 74 with lower quality photometry is located at 11.8 arcsec (∼4250 AU) of a stellar member of σ Orionis and could be a companion. After contaminant correction in the area complete to J = 21.1 mag, we estimate that there remain between zero and two cluster members in the mass interval 6-4 M Jup . Conclusions. We present S Ori 73, a new candidate T type and candidate σ Orionis member of a few Jupiter masses. Our result suggests a possible turnover in the substellar mass spectrum below ∼6 Jupiter masses, which could be investigated further by wider and deeper photometric surveys.

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“…All of them have been classified in the same way in other photometric (Wolk 1996;Sherry et al 2004;Scholz & Eislöffel 2004;Caballero 2007b;Hernández et al 2007;Bouy et al 2009) and X-ray (Franciosini et al 2006;Skinner et al 2008) searches in the cluster. Of the young star candidates, there is spectroscopic information only for one.…”

Section: Cluster Members and Candidatesmentioning

confidence: 99%

HRC-I/ChandraX-ray observations towardsσ Orionis

Caballero

Colombo

López‐Santiago

2010

A&A

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Aims. We investigated the X-ray emission from young stars and brown dwarfs in the σ Orionis cluster (τ ∼ 3 Ma, d ∼ 385 pc) and its relation to mass, the presence of circumstellar discs, and separation to the cluster centre by taking advantage of the superb spatial resolution of the Chandra X-ray Observatory. Methods. We used public HRC-I/Chandra data from a 97.6 ks pointing towards the cluster centre and complemented them with X-ray data from IPC/Einstein, HRI/ROSAT, EPIC/XMM-Newton, and ACIS-S/Chandra together with optical and infrared photometry and spectroscopy from the literature and public catalogues. On our HRC-I/Chandra data, we measured count rates, estimated X-ray fluxes, and searched for short-term variability. We also looked for long-term variability by comparing with previous X-ray observations. Results. Among the 107 detected X-ray sources, there were 70 cluster stars with known signposts of youth, two young brown dwarfs, 12 cluster member candidates, four field dwarfs, and two galaxies with optical-infrared counterpart. The remaining sources were extragalactic. Based on a robust Poisson-χ 2 analysis, nine cluster stars displayed flares or rotational modulation during the HRC-I observations, while eight other stars and one brown dwarf showed X-ray flux variations between the HRC-I and IPC, HRI, and EPIC epochs. We constructed a cluster X-ray luminosity function from O9.5 (about 18 M ) to M6.5 (about 0.06 M ). We found: (i) that early-type stars in multiple systems or with spectroscopic peculiarities tend to display X-ray emission; (ii) that the two detected brown dwarfs and the least-massive star are among the σ Orionis objects with the highest L X /L J ratios; and (iii) that a large fraction of known classical T Tauri stars in the cluster are absent in this and other X-ray surveys. Finally, from a spatial distribution analysis, we quantified the impact of sensitivity degradation towards the HRC-I borders on the detection of faint X-ray sources and concluded that dozens X-ray σ Orionis stars and brown dwarfs still need to be detected.

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A deep look into the cores of young clusters

Cited by 42 publications

References 55 publications

A census of very-low-mass stars and brown dwarfs in the σ Orionis cluster

A census of very-low-mass stars and brown dwarfs in the σ Orionis cluster

Candidate free-floating super-Jupiters in the young σ Orionis open cluster

HRC-I/ChandraX-ray observations towardsσ Orionis

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