Context. The origin of brown dwarfs is still under debate. While some models predict a star-like formation scenario, others invoke a substellar mass embryo ejection, a stellar disk fragmentation, or ...the photo-evaporation of an external core due to the presence of massive stars. Aims. The aim of our work is to characterize the youngest and lowest mass population of the dark cloud Barnard 30, a region within the Lambda Orionis star-forming region. In particular, we aim to identify proto-brown dwarfs and study the mechanism of their formation. Methods. We obtained ALMA continuum observations at 880 μm of 30 sub-mm cores previously identified with APEX/LABOCA at 870 μm. We have complemented part of the ALMA data with sub-mm APEX/SABOCA observations at 350 μm, and with multi-wavelength ancillary observations from the optical to the far-infrared (e.g., Spitzer, CAHA/O2000, WISE, INT/WFC). Results. We report the detection of five (out of 30) spatially unresolved sources with ALMA, with estimated masses between 0.9 and 67 MJup. From these five sources, only two show gas emission. The analysis of multi-wavelength photometry from these two objects, namely B30-LB14 and B30-LB19, is consistent with one Class II- and one Class I low-mass stellar object, respectively. The gas emission is consistent with a rotating disk in the case of B30-LB14, and with an oblate rotating envelope with infall signatures in the case of LB19. The remaining three ALMA detections do not have infrared counterparts and can be classified as either deeply embedded objects or as starless cores if B30 members. In the former case, two of them (LB08 and LB31) show internal luminosity upper limits consistent with Very Low Luminosity objects, while we do not have enough information for LB10. In the starless core scenario, and taking into account the estimated masses from ALMA and the APEX/LABOCA cores, we estimate final masses for the central objects in the substellar domain, so they could be classified as pre-BD core candidates. According to the turbulent fragmentation scenario, the three ALMA pre-BD core candidates should be gravitationally stable based on APEX/LABOCA data. However, this result is not consistent with the presence of compact sources inside the cores, indicative of on-going collapse. As an alternative scenario we propose that these cores could be the result of on-going gravitational contraction. Indeed, we have verified that their estimated masses are consistent with the ones expected within an ALMA beam for a r-2 density profile, which is typical for a collapsing core. Conclusions. ALMA observations have allowed us to detect very low-mass compact sources within three APEX/LABOCA cores. Future observations will help us to unveil their true nature.
We present the discovery of CoRoT 223992193, a double-lined, detached eclipsing binary, comprising two pre-main sequence M dwarfs, discovered by the CoRoT space mission during a 23-day observation of ...the 3 Myr old NGC 2264 star-forming region. Using multi-epoch optical and near-IR follow-up spectroscopy with FLAMES on the Very Large Telescope and ISIS on the William Herschel Telescope we obtain a full orbital solution and derive the fundamental parameters of both stars by modelling the light curve and radial velocity data. The orbit is circular and has a period of 3.8745745 ± 0.0000014 days. The masses and radii of the two stars are 0.67 ± 0.01 and 0.495 ± 0.007 M⊙ and 1.30 ± 0.04 and 1.11-0.05+0.04 R⊙, respectively. This system is a useful test of evolutionary models of young low-mass stars, as it lies in a region of parameter space where observational constraints are scarce; comparison with these models indicates an apparent age of ~3.5–6 Myr. The systemic velocity is within 1σ of the cluster value which, along with the presence of lithium absorption, strongly indicates cluster membership. The CoRoT light curve also contains large-amplitude, rapidly evolving out-of-eclipse variations, which are difficult to explain using starspots alone. The system’s spectral energy distribution reveals a mid-infrared excess, which we model as thermal emission from a small amount of dust located in the inner cavity of a circumbinary disk. In turn, this opens up the possibility that some of the out-of-eclipse variability could be due to occultations of the central stars by material located at the inner edge or in the central cavity of the circumbinary disk.
Context.
Stellar ages are key to improving our understanding of different astrophysical phenomena. However, many techniques to estimate stellar ages are highly model-dependent. The lithium depletion ...boundary (LDB), based on the presence or absence of lithium in low-mass stars, can be used to derive ages in stellar associations of between 20 and 500 Ma.
Aims.
The purpose of this work is to revise former LDB ages in stellar associations in a consistent way, taking advantage of the homogeneous
Gaia
parallaxes as well as bolometric luminosity estimations that do not rely on monochromatic bolometric corrections.
Methods.
We studied nine open clusters and three moving groups characterised by a previous determination of the LDB age. We gathered all the available information from our data and the literature: membership, distances, photometric data, reddening, metallicity, and surface gravity. We re-assigned membership and calculated bolometric luminosities and effective temperatures using distances derived from
Gaia
DR2 and multi-wavelength photometry for individual objects around the former LDB. We located the LDB using a homogeneous method for all the stellar associations. Finally, we estimated the age by comparing it with different evolutionary models.
Results.
We located the LDB for the twelve stellar associations and derived their ages using several theoretical evolutionary models. We compared the LDB ages among them, along with data obtained with other techniques, such as isochrone fitting, ultimately finding some discrepancies among the various approaches. Finally, we remark that the 32 Ori MG is likely to be composed of at least two populations of different ages.
ABSTRACT We present a time-variability study of young stellar objects (YSOs) in the cluster IRAS 20050+2720, performed at 3.6 and 4.5 m with the Spitzer Space Telescope; this study is part of the ...Young Stellar Object VARiability (YSOVAR) project. We have collected light curves for 181 cluster members over 60 days. We find a high variability fraction among embedded cluster members of ca. 70%, whereas young stars without a detectable disk display variability less often (in ca. 50% of the cases) and with lower amplitudes. We detect periodic variability for 33 sources with periods primarily in the range of 2-6 days. Practically all embedded periodic sources display additional variability on top of their periodicity. Furthermore, we analyze the slopes of the tracks that our sources span in the color-magnitude diagram (CMD). We find that sources with long variability time scales tend to display CMD slopes that are at least partially influenced by accretion processes, while sources with short variability timescales tend to display extinction-dominated slopes. We find a tentative trend of X-ray detected cluster members to vary on longer timescales than the X-ray undetected members.
We have conducted a photometric monitoring program of three field late L brown dwarfs (DENIS-P J0255-4700, 2MASS J0908+5032, and 2MASS J2244+2043) looking for evidence of nonaxisymmetric structure or ...temporal variability in their photospheres. The observations were performed using Spitzer IRAC 4.5 and 8 km bandpasses and were designed to cover at least one rotational period of each object; 1 s rms uncertainties of less than 3 mmag at 4.5 km and around 9 mmag at 8 km were achieved. Two out of the three objects studied exhibit some modulation in their light curves at 4.5 km--but not 8 km--with periods of 7.4 hr (DENIS 0255) and 4.6 hr (2MA 2244) and peak-to-peak amplitudes of 10 and 8 mmag. Although the lack of detectable 8 km variation suggests an instrumental origin for the detected variations, the data may nevertheless still be consistent with intrinsic variability, since the shorter wavelength IRAC bandpasses probe more deeply into late L dwarf atmospheres than the longer wavelengths. A cloud feature occupying a small percentage (1%-2%) of the visible hemisphere could account for the observed amplitude of variation. If, instead, the variability is indeed instrumental in origin, then our nonvariable L dwarfs could be either completely covered with clouds or objects whose clouds are smaller and uniformly distributed. Such scenarios would lead to very small photometric variations. Follow-up IRAC photometry at 3.6 and 5.8 km bandpasses should distinguish between the two cases. In any event, the present observations provide the most sensitive search to date for structure in the photospheres of late L dwarfs at mid-IR wavelengths, and our photometry provides stringent upper limits to the extent to which the photospheres of these transition L dwarfs are structured.
ABSTRACT We present radiation transfer models of rotating young stellar objects (YSOs) with hot spots in their atmospheres, inner disk warps, and other three-dimensional effects in the nearby ...circumstellar environment. Our models are based on the geometry expected from magneto-accretion theory, where material moving inward in the disk flows along magnetic field lines to the star and creates stellar hot spots upon impact. Due to rotation of the star and magnetosphere, the disk is variably illuminated. We compare our model light curves to data from the Spitzer YSOVAR project to determine if these processes can explain the variability observed at optical and mid-infrared wavelengths in young stars. We focus on those variables exhibiting "dipper" behavior that may be periodic, quasi-periodic, or aperiodic. We find that the stellar hot-spot size and temperature affects the optical and near-infrared light curves, while the shape and vertical extent of the inner disk warp affects the mid-IR light curve variations. Clumpy disk distributions with non-uniform fractal density structure produce more stochastic light curves. We conclude that magneto-accretion theory is consistent with certain aspects of the multiwavelength photometric variability exhibited by low-mass YSOs. More detailed modeling of individual sources can be used to better determine the stellar hot-spot and inner disk geometries of particular sources.
Context. The Chamaeleon II molecular cloud is an active star-forming region that offers an excellent opportunity to study the formation of brown dwarfs in the southern hemisphere. Aims. Our aims are ...to identify a population of pre- and proto-brown dwarfs (5σ mass limit threshold of ~0.015 M⊙) and provide information on the formation mechanisms of substellar objects. Methods. We performed high sensitivity observations at 870 μm using the LABOCA bolometer at the APEX telescope towards an active star-forming region in Chamaeleon II. The data are complemented by an extensive multiwavelength catalogue of sources, which covers the optical to the far-infrared, to study the nature of the LABOCA detections. Results. We detect 15 cores at 870 μm, and 11 of them show masses in the substellar regime. The most intense objects in the surveyed field correspond to the submillimetre counterparts of the well-known young stellar objects DK Cha and IRAS 12500-7658. We identify a possible proto-brown dwarf candidate (ChaII-APEX-L) with IRAC emission at 3.6 and 4.5 μm. Conclusions. Our analysis indicates that most of the spatially resolved cores are transient, and that the point-like starless cores in the substellar regime (with masses between 0.016 M⊙ and 0.066 M⊙) could be pre-brown dwarfs cores that are gravitationally unstable if they have radii less than 220 AU to 907 AU (1.2′′ to 5′′ at 178 pc), respectively, for different masses. ALMA observations will be key to revealing the energetic state of these pre-brown dwarfs candidates.
We present new high spatial resolution (less than or similar to 0 ''.1) 1-5 mu m adaptive optics images, interferometric 1.3 mm continuum and (CO)-C-12 2-1 maps, and 350 mu m, 2.8 and 3.3 mm fluxes ...measurements of the HV Tau system. Our adaptive optics images unambiguously demonstrate that HV Tau AB-C is a common proper motion pair. They further reveal an unusually slow orbital motion within the tight HV Tau AB pair that suggests a highly eccentric orbit and/or a large deprojected physical separation. Scattered light images of the HV Tau C edge-on protoplanetary disk suggest that the anisotropy of the dust scattering phase function is almost independent of wavelength from 0.8 to 5 mu m, whereas the dust opacity decreases significantly over the same range. The images further reveal a marked lateral asymmetry in the disk that does not vary over a timescale of two years. We further detect a radial velocity gradient in the disk in our (CO)-C-12 map that lies along the same position angle as the elongation of the continuum emission, which is consistent with Keplerian rotation around a 0.5-1 M-circle dot central star, suggesting that it could be the most massive component in the triple system. To obtain a global representation of the HV Tau C disk, we search for a model that self-consistently reproduces observations of the disk from the visible regime up to millimeter wavelengths. We use a powerful radiative transfer model to compute synthetic disk observations and use a Bayesian inference method to extract constraints on the disk properties. Each individual image, as well as the spectral energy distribution, of HV Tau C can be well reproduced by our models with fully mixed dust provided grain growth has already produced larger-than-interstellar dust grains. However, no single model can satisfactorily simultaneously account for all observations. We suggest that future attempts to model this source include more complex dust properties and possibly vertical stratification. While both grain growth and stratification have already been suggested in many disks, only a panchromatic analysis, such as presented here, can provide a complete picture of the structure of a disk, a necessary step toward quantitatively testing the predictions of numerical models of disk evolution.
A new species, Catasetum caquetense from the Caquetá department of the Colombian Amazonia, is described and illustrated. Catasetum caquetense is morphologically distinct from similar species such as ...C. tuberculatum in its rectangular, entire, denticulate, prominent apiculate basal callus surrounded by inconspicuous crown-shaped protrusions with an extra callus towards the apex that partially narrows towards the mid-lip. Information about its distribution, ecology, habitat, and taxonomic comments on morphologically similar species is also provided. With this addition, the genus comprises eight species in the department of Caquetá.
Context . Chance-aligned sources or blended companions can cause false positives in planetary transit detections or simply bias the determination of the candidate properties. In the era of ...high-precision space-based photometers, the need for high spatial resolution images has been demonstrated to be critical for validating and confirming transit signals. This already applied to the Kepler mission, is now applicable to the TESS survey, and will be critical for the PLATO mission. Aims . In this paper we present the results of the AstraLux-TESS survey, a catalog of high spatial resolution images obtained with the AstraLux instrument at the Calar Alto observatory (Almería, Spain) in the context of the TESS Follow-up Observing Program. Methods . We used the lucky imaging technique to obtain high spatial resolution images from planet candidate hosts included mostly in two relevant regimes: exoplanet candidates belonging to the level one requirement of the TESS mission (planets with radii R < 4 R ⊕ ) and TESS planet candidates around intermediate-mass main-sequence stars. Results . Among the 185 planet host candidate stars observed, we found 13 (7%) to be accompanied by additional sources within a separation of 2.2 arcsec. Among them, six are not associated with sources in the Gaia DR3 catalog, thus contaminating the TESS light curve. Even if no contaminants have been detected, we can provide upper limits and probabilities to the possible existence of field contaminants through the sensitivity limits of our images. Among the isolated hosts, we can discard hazardous companions (bright enough to mimic a planetary transit signals) with an accuracy below 1% for all their planets. Conclusions . The results from this catalog are key to the statistical validation of small planets (prime targets of the TESS mission) and planets around intermediate-mass stars in the main sequence. These two populations of planets are difficult to confirm with the radial velocity technique because of the shallow amplitude of small planets and the high rotational velocities and low number of available spectral lines in the intermediate stellar mass regime. Our results also demonstrate the importance of this type of follow-up observation for future transit missions such as PLATO, even in the Gaia era.
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