ABSTRACT
Ultracool dwarfs (UCDs) encompass a wide variety of compact stellar-like objects with spectra classified as late-M, L, T, and Y. Most of them have been discovered using wide-field imaging ...surveys. The Virtual Observatory (VO) has proven to be of great utility to efficiently exploit these astronomical resources. We aim to validate a VO methodology designed to discover and characterize UCDs in deep extragalactic surveys like Advance Large Homogeneous Area Medium-Band Redshift Astronomical (ALHAMBRA) and Cosmological Evolution Survey (COSMOS). Three complimentary searches based on parallaxes, proper motions and colours, respectively, were carried out. A total of 897 candidate UCDs were found, with only 16 previously reported in SIMBAD. Most of the new UCDs reported here are likely late-M and L dwarfs because of the limitations imposed by the utilization of optical (Gaia DR2 and r-band) data. We complement ALHAMBRA and COSMOS photometry with other catalogues in the optical and infrared using VOSA, a VO tool that estimates effective temperatures from the spectral energy distribution fitting to collections of theoretical models. The agreement between the number of UCDs found in the COSMOS field and theoretical estimations together with the low false-negative rate (known UCDs not discovered in our search) validates the methodology proposed in this work, which will be used in the forthcoming wide and deep surveys provided by the Euclid space mission. Simulations of Euclid number counts for UCDs detectable in different photometric passbands are presented for a wide survey area of 15 000 deg2, and the limitations of applicability of Euclid data to detect UCDs using the methods employed in this paper are discussed.
Lupus DANCe Galli, P A B; Bouy, H; Olivares, J ...
Astronomy & astrophysics,
11/2020, Letnik:
643
Journal Article
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Context. Lupus is recognised as one of the closest star-forming regions, but the lack of trigonometric parallaxes in the pre-Gaia era hampered many studies on the kinematic properties of this region ...and led to incomplete censuses of its stellar population. Aims. We use the second data release of the Gaia space mission combined with published ancillary radial velocity data to revise the census of stars and investigate the 6D structure of the Lupus complex. Methods. We performed a new membership analysis of the Lupus association based on astrometric and photometric data over a field of 160 deg2 around the main molecular clouds of the complex and compared the properties of the various subgroups in this region. Results. We identified 137 high-probability members of the Lupus association of young stars, including 47 stars that had never been reported as members before. Many of the historically known stars associated with the Lupus region identified in previous studies are more likely to be field stars or members of the adjacent Scorpius-Centaurus association. Our new sample of members covers the magnitude and mass range from G ≃ 8 to G ≃ 18 mag and from 0.03 to 2.4 M⊙, respectively. We compared the kinematic properties of the stars projected towards the molecular clouds Lupus 1–6 and showed that these subgroups are located at roughly the same distance (about 160 pc) and move with the same spatial velocity. Our age estimates inferred from stellar models show that the Lupus subgroups are coeval (with median ages ranging from about 1 to 3 Myr). The Lupus association appears to be younger than the population of young stars in the Corona-Australis star-forming region recently investigated by our team using a similar methodology. The initial mass function of the Lupus association inferred from the distribution of spectral types shows little variation compared to other star-forming regions. Conclusions. In this paper, we provide an updated sample of cluster members based on Gaia data and construct the most complete picture of the 3D structure and 3D space motion of the Lupus complex.
Transitional discs are protoplanetary discs with opacity gaps/cavities in their dust distribution, a feature that may be linked to planet formation. We perform Bayesian modelling of the three ...transitional discs SZ Cha, CS Cha, and T25 including photometry from the Herschel Space Observatory to quantify the improvements added by these new data. We find disc dust masses between 2 × 10−5 and 4 × 10−4 M⊙ and gap radii in the range of 7–18 au, with uncertainties of ∼ one order of magnitude and ∼4 au, respectively. Our results show that adding Herschel data can significantly improve these estimates with respect to mid-infrared data alone, which have roughly twice as large uncertainties on both disc mass and gap radius. We also find weak evidence for different density profiles with respect to full discs. These results open exciting new possibilities to study the distribution of disc masses for large samples of discs.
Aims. Proper motion measurements of the cool and ultracool populations in the Upper Scorpius OB association are crucial to confirm membership and to identify possible run-away objects. Methods. We ...cross-match samples of photometrically selected and spectroscopically confirmed cool and ultracool (K5 < SpT < M8.5) candidate members in the Upper Scorpius OB association using the literature and the USNO-B and the UCAC2 catalogues. 251 of these objects have a USNO-B and/or UCAC2 counterpart with proper motion measurements. Results. A significant fraction (19 objects, 7.6 ± 1.8%) of spectroscopically confirmed young objects show discrepant proper motion. They must either belong to unidentified coincident foreground associations, or originate from neighboring star forming regions or have recently experienced dynamical interactions within the association. The observed accretor and disc frequencies are lower among outliers, but with only 19 objects it is unreliable to draw firm statistical conclusions. Finally, we note that transverse velocities of very low mass members are indistinguishable from those of low mass members within ≈4 km s-1.
We used the OSIRIS camera at the 10.4 m Gran Telescopio Canarias (GTC) to monitor the astrometric motion of the L4.5 dwarf 2M1821+14 over 17 months. The astrometric residuals of 11 epochs have an rms ...dispersion of 0.4 mas, which is larger than the average precision of 0.23 mas per epoch and hints towards an additional signal or excess noise. Comparison of the point-spread functions in OSIRIS and FORS2/VLT images reveals no differences critical for high-precision astrometry, despite the GTC's segmented primary mirror. We attribute the excess noise to an unknown effect that may be uncovered with additional data. For 2M1821+14, we measured a relative parallax of 106.15 plus or minus 0.18 mas and determined a correction of 0.50 plus or minus 0.05 mas to absolute parallax, leading to a distance of 9.38 plus or minus 0.03 pc. We excluded at 3... confidence the presence of a companion to 2M1821+14 down to a mass ratio of 0.1 (...5 M...) with a period of 50-1000 d and a separation of 0.1-0.7 au. The accurate parallax allowed us to estimate the age and mass of 2M1821+14 of 120-700 Myr and ... M..., thus confirming its intermediate age and substellar mass. We complement our study with a parallax and proper motion catalogue of 587 stars (i' ... 15.5-22) close to 2M1821+14, used as astrometric references. This study demonstrates submas astrometry with the GTC, a capability applicable for a variety of science cases including the search for extrasolar planets and relevant for future astrometric observations with E-ELT and TMT. (ProQuest: ... denotes formulae/symbols omitted.)
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 have obtained projected rotation velocities (v sub(rot) sin i) of a sample of 19 ultracool dwarfs with spectral types in the interval M6.5-T8 using high-resolution, near-infrared spectra obtained ...with NIRSPEC and the Keck II telescope. Among our targets there are two young brown dwarfs, two likely field stars, and 15 likely brown dwarfs (30-72 M sub(Jup)) in the solar neighborhood. Our results indicate that the T-type dwarfs are fast rotators in marked contrast to M-type stars. We have derived v sub(rot) sin i velocities between ,15 and 40 km s super(-1) for them and have found no clear evidence for T dwarfs rotating strongly faster than L dwarfs. However, there is a hint for an increasing lower envelope on moving from mid-M to L spectral types in the v sub(rot)sin i-spectral-type diagram that was previously reported in the literature; our v sub(rot) sin i results extend it to even cooler types. Assuming that field brown dwarfs have a size of 0.08-0.1 R , we can place an upper limit of 12.5 hr on the equatorial rotation period of T-type brown dwarfs. In addition, we have compared our v sub(rot) sin i measurements to spectroscopic rotational velocities of very young brown dwarfs of similar mass available in the literature. The comparison, although model dependent, suggests that brown dwarfs lose some angular momentum during their contraction; however, their spin-down time seems to be significantly longer than that of solar-type to early M stars.
We present observations of 36 late M dwarfs obtained with the Keck II/NIRSPEC in the J band at a resolution of ~20,000. We have measured projected rotational velocities, absolute radial velocities, ...and pseudo-equivalent widths of atomic lines. Twelve of our targets did not have previous measurements in the literature. For the other 24 targets, we confirm previously reported measurements. We find that 13 stars from our sample have v sin i below our measurement threshold (12 km s super(-1)) whereas four of our targets are fast rotators (v sin i > 30 km s super(-1)). As fast rotation causes spectral features to be washed out, stars with low projected rotational velocities are sought for radial velocity surveys. At our intermediate spectral resolution, we have confirmed the identification of neutral atomic lines reported in McLean et al. We also calculated pseudo-equivalent widths of 12 atomic lines. Our results confirm that the pseudo-equivalent width of K I lines is strongly dependent on spectral types. We observe that the pseudo-equivalent width of Fe I and Mn I lines remains fairly constant with later spectral type. We suggest that these lines are particularly suitable for deriving metallicities for late M dwarfs.
With the PACS instrument on Herschel, 134 low mass members of the Taurus star-forming region spanning the M4-L0 spectral type range and covering the transition from low mass stars to brown dwarfs ...were observed. Combining the new Herschel results with other Herschel programs, a total of 150 of the 154 M4-L0 Taurus members have observations, and we have added an additional 3 targets from Spitzer to form the 153-object TBOSS (Taurus Boundary of Stellar/Substellar) sample, a 99% complete study. Among the 150 targets, 70 μm flux densities were measured for 7 of the 7 Class I objects, 48 of the 67 Class II objects, and 3 of the 76 Class III objects. For the detected Class II objects, the median 70 μm flux density level declines with spectral type; however, the distribution of excess relative to central object flux density does not change across the stellar/substellar boundary in the M4-L0 range. Connecting the 70 μm TBOSS values with the results from K0-M3 Class II members results in the first comprehensive census of far-IR emission across the full mass spectrum of the stellar and substellar population of a star-forming region, and the median flux density declines with spectral type in a trend analogous to the flux density decline expected for the central objects. Spectral energy distributions (SEDs) were constructed for all TBOSS targets covering the optical to far-IR range and extending to the submm/mm for a subset of sources with longer wavelength data. Based on an initial exploration of the impact of different physical parameters on the Herschel flux densities, geometrical factors such as inclination and structural parameters such as scale height and flaring have the largest influence on the flux densities in the PACS bands. From the 24 μm to 70 μm spectral index of the SEDs, 5 new candidate transition disks were identified. Considering the previously known and new candidate transition disks, the spectral indices over longer wavelengths (≥70 μm) are not distinct from those of the full Class II population, suggesting that the outer regions of the transition disks are similar to Class II disks. The steep 24 μm to 70 μm slope for a subset of 8 TBOSS targets may be an indication of truncated disks in these systems, however additional measurements are required to establish the outer radii of these disks conclusively. From existing high angular resolution companion search observations, two examples of mixed pair systems that include secondaries with disks were measured in the Herschel data. Finally, comparing the TBOSS results with a Herschel study of Ophiuchus brown dwarfs reveals a lower fraction of disks around the Taurus substellar population with flux densities comparable to the Ophiuchus disks.
Messier 35 (NGC 2168) DANCe Bouy, H; Berlin, E; Barrado, D ...
Astronomy and astrophysics (Berlin),
03/2015, Letnik:
575
Journal Article
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Messier 35 (NGC 2168) is an important young nearby cluster. Its age, richness and relative proximity make it an ideal target for stellar evolution studies. The Kepler K2 mission recently observed it ...and provided a high accuracy photometric time series of a large number of sources in this area of the sky. Identifying the cluster's members is therefore of high importance to optimize the interpretation and analysis of the Kepler K2 data. We aim to identify the cluster's members by deriving membership probabilities for the sources within 1degree of the cluster's center, which is farther away than equivalent previous studies. We measure accurate proper motions and multiwavelength photometry using groundbased archival images of the cluster. We use these measurements to compute membership probabilities. The final catalog includes 338 892 sources with multiwavelength photometry. Our study, nevertheless, provides a coherent and quantitative membership analysis of Messier 35 based on a large fraction of the best ground-based data sets obtained over the past 18 years.
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