The young, wide and very low mass visual binary LOri167

We look for wide, faint companions around members of the 5 Myr Lambda Orionis open cluster. We used optical, near-infrared, and Spitzer/IRAC photometry. We report the discovery of a very wide very low mass visual binary, LOri167, formed by a brown dwarf and a planetary-mass candidate located at 5 arcsec, which seems to belong to the cluster. We derive Teff of 2125 and 1750 K. If they are members, comparisons with theoretical models indicate masses of 17 (20-15) Mjup and 8 (13-7) Mjup, with a projected separation of 2000 AU. Such a binary system would be difficult to explain in most models, particularly those where substellar objects form in the disks surrounding higher mass stars.Comment: Astronomy & Astrophysics Letters, in pres

XMM-Newton investigations of the Lambda Orionis star-forming region (XILO). I. The young cluster Collinder 69

This is the first paper of a series devoted to the Lambda Orionis star-forming region, from the X-ray perspective, which will provide a comprehensive view of this complex region. In this paper we focus in uncovering the population of the central, young cluster Collinder 69 (C69), and in particular those diskless members not identified by previous near- and mid-infrared surveys, and to establish the X-ray luminosity function for the association. We have combined two exposures taken with the XMM-Newton satellite with an exhaustive data set of optical, near- and mid-infrared photometry to assess the membership of the X-ray sources based on color-color and color-magnitude diagrams, as well as other properties, such as effective temperatures, masses and bolometric luminosities. We detected a total of 164 X-ray sources, of which 66 are probable and possible cluster members. A total of 16 are newly identified probable members. The two XMM-Newton pointings east and west of the cluster center have allowed us to verify the heterogeneous spatial distribution of young stars, probably related to the large scale structure of the region. The disk fraction of the X-ray detected cluster sample is very low, close to 10%, in remarkable contrast to the low-mass stellar and substellar population (mostly undetected in X-rays) where the disk fraction reaches about 50%. The X-ray luminosity function of C69 provides support for an age of several Myr when compared with other well known young associations. With our improved cluster census we confirm previous reports on the untypically low disk fraction compared to other clusters of several Myr age. The different disk fractions of X-ray detected (essentially solar-like) and undetected (mostly low-mass stars and brown dwarfs) members can be understood as a consequence of a mass-dependence of the time-scale for disk evolution.Comment: 38 pages, 16 figure

The lower mass function of the young open cluster Blanco 1: from 30 M_(Jup) to 3 M_☉

Aims. We performed a deep wide field optical survey of the young (~100−150 Myr) open cluster Blanco 1 to study its low mass population well down into the brown dwarf regime and estimate its mass function over the whole cluster mass range. Methods. The survey covers 2.3 square degrees in the I and z-bands down to I ≃ z ≃ 24 with the CFH12K camera. Considering two different cluster ages (100 and 150 Myr), we selected cluster member candidates on the basis of their location in the (I, I − z) CMD relative to the isochrones, and estimated the contamination by foreground late-type field dwarfs using statistical arguments, infrared photometry and low-resolution optical spectroscopy. Results. We find that our survey should contain about 57% of the cluster members in the 0.03−0.6 M_☉ mass range, including 30–40 brown dwarfs. The candidate’s radial distribution presents evidence that mass segregation has already occured in the cluster. We took it into account to estimate the cluster mass function across the stellar/substellar boundary. We find that, between 0.03 M_☉ and 0.6 M_☉, the cluster mass distribution does not depend much on its exact age, and is well represented by a single power-law, with an index α = 0.69 ± 0.15. Over the whole mass domain, from 0.03 M_☉ to 3 M_☉, the mass function is better fitted by a log-normal function with m_0 = 0.36 ± 0.07 M_☉ and σ = 0.58 ± 0.06. Conclusions. Comparison between the Blanco 1 mass function, other young open clusters’ MF, and the galactic disc MF suggests that the IMF, from the substellar domain to the higher mass part, does not depend much on initial conditions. We discuss the implications of this result on theories developed to date to explain the origin of the mass distribution

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

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.Comment: Accepted for publication in A&

A mid-infrared study of very low mass stars and brown dwarfs in Upper Scorpius

We report the results of mid-IR observations with VISIR at the VLT of 10 ultracool dwarfs members of the nearby Upper Scorpius OB association in four filters ranging between 8.59 (PAH1) to 12.8 $\mu$m (Ne II), and one brown dwarf with Spitzer between 3.6 and 24 $\mu$m. Seven of our targets are detected in at least one of the bands, and we derive upper limits on the fluxes of the remaining 4. These results combined with previous studies from the literature lead to an improved disk frequency of 50$\pm$12%. This frequency is significantly higher than that of accretors (16.3%$\pm$6.2%). Only one object showing mid-IR excess also has H$\alpha$ emission at a level indicating that it must be accreting. Four of the detected targets are multiple system candidates. The observed disk frequency for sub-stellar objects in the Upper Scorpius association is similar to that of stars, consistent with a common formation scenario. It is also similar to the disk fractions observed in younger clusters, suggesting that the disk lifetimes might be longer for ultracool dwarfs than for higher-mass stars.Comment: 10 pages, 4 figures, accepted for A&

The effect of stellar activity on the LiI 6708, NaI 5896 and KI 7699 \AA lines: A comparison with the Pleiades, field stars and the Sun

An analytical model has been developed to empirically study the effects of stellar spots and faculae on the observed equivalent widths of Li {\sc i} 6708, Na {\sc i} 5896 and K {\sc i} 7699 \AA lines (and abundances in the case of lithium) in late-type stars, taking into account the changes in the observed magnitudes and colors. Solar spectra corresponding to different active regions are used as input data and a range of filling factors are applied to simulate the surfaces of stars with different levels of activity. Detailed comparisons between predicted and observed photometric colors and equivalent widths are made for late-type stars of the Pleiades and the field. The observed dispersions in K {\sc i} and Li {\sc i} equivalent widths for Pleiades stars can be partially accounted by the simultaneous effects of activity on colors and the line formation, indicating that the lithium-rotation connection suggested for ~0.7-0.9 Msun Pleiades stars could be due in part to the stellar activity. However, under realistic values for the filling factors, only a small portion of the observed spread could be explained by these effect