In spite of its importance for the study of star formation at all mass domains, the nearby young sigma Orionis cluster still lacks a comprehensive survey for multiplicity. We try to fill that observational gap by looking for wide resolved binaries with angular separations between 0.4 and 4.0 arcsec. We search for companions to 331 catalogued cluster stellar members and candidates in public K-band UKIDSS images outside the innermost 1 arcmin, which is affected by the glare of the bright, eponymous sigma Ori multiple system, and investigate their cluster membership with colour-magnitude diagrams and previous knowledge of youth features. Of the 18 identified pairs, ten have very low individual probabilities of chance alignment (< 1 %) and are considered here as physical pairs. Four of them are new, while the other six had been discovered previously, but never investigated homogeneously and in detail. Projected physical separations and magnitude differences of the ten probably bound pairs range from 180 to 1220 au, and from 0.0 to 3.4 mag in K, respectively. Besides, we identify two cluster stars with elongated point spread functions. We determine the minimum frequency of wide multiplicity in the interval of projected physical separations s = 160-1600 au in sigma Orionis at 3.0^{+1.2}_{-1.1} %. We discover a new Lindroos system, find that massive and X-ray stars tend to be in pairs or trios, conclude that multiplicity truncates circumstellar discs and enhances X-ray emission, and ascribe a reported lithium depletion in a young star to unresolved binarity in spectra of moderate resolution. When accounting for all know multiples, including spectroscopic binaries, the minimum frequency of multiplicity increases to about 10 %, which implies that of the order of 80-100 unknown multiple systems still await discovery in sigma Orionis.
Parallactic distances and proper motions of virtually all stars in the sigma Orionis cluster or: How I learned to get the most out of TOPCAT and love Gaia DR2
The photoevaporation of circumstellar disks is a powerful process in the disk dissipation at the origin of the Orion proplyds. This Letter reports the first detection of a photoevaporating disk in the final but long-lasting phase of its evolution. The disk is associated to a low-mass T Tauri member of the sigma Orionis Cluster. It is characterized by a very low (if any) accretion rate and by a tenuous (Mloss ~ 10^{-9} Msun/yr) photoevaporation wind, which is unambiguously detected in the optical spectrum of the object. The wind emits strong forbidden lines of [SII] and [NII] because the low-ma…
We present a deep I,Z photometric survey covering a total area of 1.12 deg^{2} of the Sigma Orionis cluster (Icompl=22 and Zcompl=21.5mag). From I, I-Z color-magnitude diagrams we have selected 153 candidates that fit the previously known sequence of the cluster. Using J-band photometry, we find that 124 of the 151 candidates follow the previously known infrared photometric sequence of the cluster and are probably members. We have studied the spatial distribution of these candidates and found that there are objects located at distances greater than 30 arcmin to the north and west of Sigma Orio…
We present interferometric observations of the sigma Orionis triple system using the CHARA Array, NPOI, and VLTI. Using these measurements, we spatially resolve the orbit of the close spectroscopic binary (Aa,Ab) for the first time and present a revised orbit for the wide pair (A,B). Combining the visual orbits with previously published radial velocity measurements and new radial velocities measured at CTIO, we derive dynamical masses for the three massive stars in the system of M_Aa = 16.99 +/- 0.20 Msun, M_Ab = 12.81 +/- 0.18 Msun, and M_B = 11.5 +/- 1.2 Msun. The inner and outer orbits in t…
The sigma Orionis cluster is to date the star-forming region with the largest number of confirmed brown dwarfs and substellar objects below the deuterium burning mass limit. The most massive star, sigma Ori Aa, just in the cluster centre, is the \sim20Msol-mass O9.5V star that illuminates the Horsehead Nebula, while the least massive object yet reported, S Ori 70, is only around 3 MJup. In the middle, there is a continuum of stars and substellar objects of all types (including magnetically active B2Vp stars, Herbig-Haro objects, FU Ori stars or T Tauri brown dwarfs) that makes the cluster a co…
We present a spectroscopic survey of the stellar population of the Sigma Orionis cluster. We have obtained spectral types for 340 stars. Spectroscopic data for spectral typing come from several spectrographs with similar spectroscopic coverage and resolution. More than a half of stars of our sample are members confirmed by the presence of lithium in absorption, strong H$α$ in emission or weak gravity-sensitive features. In addition, we have obtained high resolution (R~34000) spectra in the H$α$ region for 169 stars in the region. Radial velocities were calculated from this data set. The radial…
With the new Tycho-Gaia Astrometric Solution, I derive a new parallactic distance to the young sigma Orionis open cluster, which is a cornerstone region for studying the formation and evolution of stars and substellar objects from tens of solar masses to a few Jupiter masses. I started with the list of the 46 brightest cluster stars of Caballero (2007). After identifying the 24 TGAS stars in the 30 arcmin-radius survey area and accounting for 11 FGKM-type dwarfs and giants in the fore- and background, I got a list of 13 cluster members and candidates with new parallaxes. Of them, I discarded f…
Context. Although many studies have been performed so far, there are still dozens of low-mass stars and brown dwarfs in the young sigma Orionis open cluster without detailed spectroscopic characterisation. Aims. We look for unknown strong accretors and disc hosts that were undetected in previous surveys. Methods. We collected low-resolution spectroscopy (R ~ 700) of ten low-mass stars and brown dwarfs in sigma Orionis with OSIRIS at the Gran Telescopio Canarias under very poor weather conditions. These objects display variability in the optical, infrared, Halpha, and/or X-rays on time scales o…
The Virtual Observatory is useful. I summarise some of my works where I extensively use the Aladin sky atlas. Topics cover from the search and common proper motion confirmation of very low-mass stars and brown dwarfs in wide (rho > 1000 AU) binaries and multiple systems, to the identification and characterisation of stellar and substellar populations in young open clusters and OB associations at heliocentric distances of up to 1000 pc. I present three practical examples of what one can do with Aladin in one morning: a fruitful proper-motion search of objects with available ugrizJHKs photometry…
We report on our analysis of the VISTA Orion ZYJHKs photometric data (completeness magnitudes Z=22.6 and J=21.0mag) focusing on a circular area of 2798.4 arcmin^2 around the young sigma Orionis star cluster (~3Myr, ~352pc, solar metallicity). The combination of the VISTA photometry with optical, WISE and Spitzer data allows us to identify a total of 210 cluster member candidates with masses in the interval 0.25-0.004Msun, 23 of which are new planetary-mass object findings. These discoveries double the number of cluster planetary-mass candidates known so far. One object has colors compatible wi…
We present new U-band photometry of the magnetic Helium-strong star Sigma Ori E, obtained over 2004-2009 using the SMARTS 0.9-m telescope at Cerro Tololo Inter-American Observatory. When combined with historical measurements, these data constrain the evolution of the star's 1.19 d rotation period over the past three decades. We are able to rule out a constant period at the p_null = 0.05% level, and instead find that the data are well described (p_null = 99.3%) by a period increasing linearly at a rate of 77 ms per year. This corresponds to a characteristic spin-down time of 1.34 Myr, in good a…
Aims: We investigated the X-ray emission from young stars and brown dwarfs in the sigma Orionis cluster (tau~3 Ma, d~385 pc) and its relation to mass, 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 …
We used the Aladin Virtual Obsrvatory tool and High Resolution Imager ROSAT archival data to search for X-ray variability in scale of days in 23 young stars in the sigma Orionis cluster and a background galaxy. Five stars displayed unambiguous flares and had probabilities p_var >> 99% of being actual variables. Two of the detected flares were violent and long-lasting, with maximum duration of six days and amplitude of eight times above the quiescent level. We classified another four stars as possible X-ray variables, including the binary system formed by the B2Vp star sigma Ori E and its close…
Context. Only a few open clusters are as important for the study of stellar and substellar objects, and their formation and evolution, as the young sigma Orionis cluster. However, a complete spectroscopic characterisation of its whole stellar population is still missing. Aims. We filled most of that gap with a large spectroscopic and astrometric survey of targets towards sigma Orionis. Eventually, it will be one of the open clusters with the lowest proportion of interlopers and the largest proportion of confirmed cluster members with known uncontrovertible youth features. Methods. We acquired …
We present high-precision photometry on 107 variable low-mass stars and brown dwarfs in the ~3 Myr Sigma Orionis open cluster. We have carried out I-band photometric monitoring within two fields, encompassing 153 confirmed or candidate members of the low-mass cluster population, from 0.02 to 0.5 M_\odot. We are sensitive to brightness changes on time scales from 10 minutes to two weeks with amplitudes as low as 0.004 magnitudes, and find variability on these time scales in nearly 70% of cluster members. We identify both periodic and aperiodic modes of variability, as well as semi-periodic rapi…
Aims: I seek to fully know the stellar and substellar populations in the young sigma Orionis open cluster, which is a benchmark for star-forming studies. Because of the very low proper motion of the cluster, late-type dwarfs with appreciable proper motion in the foreground of sigma Orionis can be easily discarded for expensive spectroscopic follow-up. Methods: I use the Aladin sky atlas, USNO-B1, public astrometric catalogues, and photographic plate digitisations to identify stars with proper motions inconsistent with cluster membership in a circular area of radius 30 arcmin centred on the ear…
(Abridged) We aim to: i) confirm the presence of methane absorption in S Ori 73 (a T-type member candidate of the sig Orionis cluster, 3 Myr, 352 pc) through methane imaging; ii) study S Ori 70 and 73 cluster membership via photometric colors and accurate proper motion analysis; iii) perform a new search to identify additional T-type sig Orionis member candidates with likely masses below 7 Mjup. We obtained HAWK-I (VLT) J, H, and CH4off photometry of an area of 119.15 sq. arcmin in sig Orionis down to Jcomp = 21.7 and Hcomp = 21 mag. Near-infrared data were complemented with optical photometry…
(ABRIDGED) 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. 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 Msun mass range, including new objects not previously reported in the lit…
We report optical and infrared imaging spectroscopy observations of the young binary object Sigma Orionis IRS1 A/B. The brighter component (Sigma Ori IRS1 A) of this binary system has M1 spectral type and a mass in the range of 0.3 to 0.8 solar masses. The fainter component (Sigma Ori IRS1 B) has a unique morphology and spectrum. The unresolved stellar object is surrounded by an extended envelope that is slightly offset from the position of this star. The envelope's spectrum shows strong emission lines of H and HeI but no shock-excited emission from molecular hydrogen or from [FeII]. The embed…
(Abridged) A total of three T type candidates (SOri70, SOri73, and SOriJ0538-0213) lying in the line of sight towards Sigma Orionis were characterized by means of near-infrared photometric, astrometric, and spectroscopic studies. H-band methane images were collected for all three sources and an additional sample of 15 field T type dwarfs using LIRIS/WHT. J-band spectra of resolution of ~500 were obtained for SOriJ0538-0213 with ISAAC/VLT, and JH spectra of resolution of ~50 acquired with WFC3/HST were employed for the spectroscopic classification of SOri70 and 73. Proper motions with a typical…
We present an optical through sub-millimetre multi-wavelength study of two very low-luminosity Class I/Flat systems, Mayrit 1701117 and Mayrit 1082188, in the sigma Orionis cluster. We performed moderate resolution (R ~ 1000) optical (~0.4-0.9mu) spectroscopy with the TWIN spectrograph at the Calar Alto 3.5-m telescope. The spectra for both sources show prominent emission in accretion- and outflow-associated lines. The mean accretion rate measured from multiple line diagnostics is 6.4x10^{-10} Msun/yr for Mayrit 1701117, and 2.5x10^{-10} Msun/yr for Mayrit 1082188. The outflow mass loss rates …
The continuous temporal coverage and high photometric precision afforded by space observatories has opened up new opportunities for the study of variability processes in young stellar cluster members. Of particular interest is the phenomenon of deuterium-burning pulsation in brown dwarfs and very-low-mass stars, whose existence on 1-4 hours timescales has been proposed but not yet borne out by observations. To investigate short-timescale variability in young, low-mass objects, we carried out high-precision, high-cadence time series monitoring with the Warm Spitzer mission on 14 low mass stars …
Deep optical imaging is used to study time-domain properties of young brown dwarfs in sigma Orionis over typical rotational time-scales and to search for new substellar and planetary-mass cluster members. We used VIMOS at the VLT to monitor a 24 arcmin x 16 arcmin field in the I-band. Using the individual images from this run we investigated the photometric time series of nine substellar cluster members with masses from 10 to 60 Mj. The deep stacked image shows cluster members down to $\approx$ 5 Mj. We search for new planetary mass objects by combining our deep I-band photometry with public J…
We have obtained low-resolution optical (0.7-0.98 micron) and near-infrared (1.11-1.34 micron and 0.8-2.5 micron) spectra of twelve isolated planetary-mass candidates (J = 18.2-19.9 mag) of the 3-Myr sigma Orionis star cluster with a view to determining the spectroscopic properties of very young, substellar dwarfs and assembling a complete cluster mass function. We have classified our targets by visual comparison with high- and low-gravity standards and by measuring newly defined spectroscopic indices. We derived L0-L4.5 and M9-L2.5 using high- and low-gravity standards, respectively. Our targ…
Free-floating substellar candidates with estimated theoretical masses of as low as ~5 Jupiter masses have been found in the ~3 Myr old sigma 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. We propose to search for objects of yet lower masses in the cluster and determine the shape of the mass function at low mass. Using new- and (re-analysed) published IZJHKs[3.6]-[8.0]-band data of…
We present the results from a large 850 micron survey of the sigma Orionis cluster using the SCUBA-2 camera on the James Clerk Maxwell Telescope. The 0.5-degree diameter circular region we surveyed contains 297 young stellar objects with an age estimated at about 3Myr. We detect 9 of these objects, 8 of which have infrared excesses from an inner disc. We also serendipitously detect 3 non-stellar sources at > 5sigma that are likely background submillimetre galaxies. The 9 detected stars have inferred disc masses ranging from 5 to about 17MJup, assuming similar dust properties as Taurus discs an…
We present the results of a proper motion wide-field near-infrared survey of the entire Upper Sco (USco) association (~160 square degrees) released as part of the UKIRT Infrared Deep Sky (UKIDSS) Galactic Clusters Survey (GCS) Data Release 10 (DR10). We have identified a sample of ~400 astrometric and photometric member candidates combining proper motions and photometry in five near-infrared passbands and another 286 with HK photometry and 2MASS/GCS proper motions. We also provide revised membership for all previously published USco low-mass stars and substellar members based on our selection …
We studied the X-ray properties of the young (~1-8 Myr) open cluster around the O8III star lambda Ori using XMM-Newton. We detected 167 X-ray sources, of which 58 are identified with known cluster members, from massive stars down to low-mass stars with spectral types M5.5. Another 23 sources were identified with new possible photometric candidates. Late-type stars have a median log LX/Lbol ~ -3.3, close to the saturation limit. Variability was observed in 35% of late-type members, including six flaring sources. The emission from the central hot star lambda Ori is dominated by plasma at 0.2-0.3…
The young O9.5 V spectroscopic binary Theta 2 Ori A shows moderately hard X-ray emission and relatively narrow X-ray lines, suggesting that it may be a Magnetically Confined Wind Shock (MCWS) source, similar to its more massive analogue Theta 1 Ori C. X-ray flares occurring near periastron led to the suggestion that the flares are produced via magnetic reconnection as magnetospheres on both components of the Theta 2 Ori A binary interact at closest approach. We use a series of high-resolution spectropolarimetric observations of Theta 2 Ori A to place an upper limit on the magnetic field streng…
This paper presents the results of an U band survey with FORS1/VLT of a large area in the sigma Orionis star-forming region. We combine the U-band photometry with literature data to compute accretion luminosity and mass accretion rates from the U-band excess emission for all objects (187) detected by Spitzer in the FORS1 field and classified by Hernandez et al. (2007) as likely members of the cluster. The sample stars range in mass from ~0.06 to ~1.2 Msun; 72 of them show evidence of disks and we measure mass accretion rates Macc between <10^{-11} and few 10^{-9} Msun/yr, using the colors of t…
We report the discovery of a new Herbig-Haro jet, HH 1165, in SOAR narrow-band imaging of the vicinity of the sigma Orionis cluster. HH 1165 shows a spectacular extended and collimated spatial structure, with a projected length of 0.26 pc, a bent C-shaped morphology, multiple knots, and fragmented bow-shocks at the apparent ends of the flow. The Halpha image shows a bright halo with a clumpy distribution of material seen around the driving source, and curved reflection nebulosity tracing the outflow cavities. The driving source of HH 1165 is a Class I proto-brown dwarf, Mayrit 1701117 (M170111…
The evolution of young stars and disks is driven by the interplay of several processes, notably accretion and ejection of material. Critical to correctly describe the conditions of planet formation, these processes are best probed spectroscopically. About five-hundred orbits of the Hubble Space Telescope (HST) are being devoted in 2020-2022 to the ULLYSES public survey of about 70 low-mass (M<2Msun) young (age<10 Myr) stars at UV wavelengths. Here we present the PENELLOPE Large Program that is being carried out at the ESO Very Large Telescope (VLT) to acquire, contemporaneous to HST, optical E…
We look for high-amplitude variable young stars in the open clusters and associations of the Orion Belt. We use public data from the ASAS-3 Photometric V-band Catalogue of the All Sky Automated Survey, infrared photometry from the 2MASS and IRAS catalogues, proper motions, and the Aladin sky atlas to obtain a list of the most variable stars in a survey area of side 5 deg centred on the bright star Alnilam (eps Ori) in the centre of the Orion Belt. We identify 32 highly-variable stars, of which 16 had not been reported to vary before. They are mostly variable young stars and candidates (16) and…
The reflection nebula NGC 7129 has long been known to be a site of recent star formation as evidenced, e.g., by the presence of deeply embedded protostars and HH objects. However, studies of the stellar population produced in the star formation process have remained rudimentary. At a presumed age of ~3 Myr, NGC7129 is in the critical range where disks around young stars disappear. We make use of Chandra X-ray and Spitzer and 2MASS IR imaging observations to identify the pre-main sequence stars in NGC7129. We define a sample of Young Stellar Objects based on color-color diagrams composed from I…
