Sample Of Dwarfs Research Articles

Applying Random Forest Classification to Ultracool Dwarf Discovery in Deep Surveys. I. Color Classification with SDSS, UKIDSS, and WISE Photometry

Abstract In this first of two studies, we apply a random forest model to classify ultracool dwarfs from broadband color information. Using the Skrzypek et al. ultracool dwarf sample and a set of background sources, we trained a random forest classifier based on 28 colors derived from optical and infrared photometry from SDSS, UKIDSS, and WISE. Our model achieves 99.7% accuracy in segregating L- and T-type UCDs from background sources, and 97% accuracy in separating spectral subgroups. A separate random forest regressor model achieved a spectral classification precision of 1.3 subtypes. We applied these models to a 12.6 deg2 region with overlapping SDSS, UKIDSS, and WISE coverage and identified 35 UCD candidates, five of which are previously reported, of which four are photometrically or spectroscopically classified UCDs. Our random forest model can be applied to multiple surveys to greatly expand the known census of UCDs.

Absorption Line Search through Three Local Group Dwarf Galaxy Halos

Abstract Dwarf galaxies are missing nearly all of their baryons and metals from the stellar disk, which are presumed to be in a bound halo or expelled beyond the virial radius. The virial temperature for galaxies with M h ∼ 109–1010 M ⊙ is similar to the collisional ionization equilibrium temperature for the C iv ion. We search for UV absorption from C iv in six sightlines toward three dwarf galaxies in the anti-M31 direction and at the periphery of the Local Group (D ≈ 1.3 Mpc; Sextans A, Sextans B, and NGC 3109). The C iv doublet is detected in only one of six sightlines, toward Sextans A, with log N ( C IV ) = 13.06 ± 0.08 . This is consistent with our gaseous halo models, where the halo gas mass is determined by the cooling rate, feedback, and the star formation rate; the inclusion of photoionization is an essential ingredient. This model can also reproduce the higher detection rate of O vi absorption in other dwarf samples (beyond the Local Group), with C iv only detectable within ∼0.5R vir.

HI observations of the MATLAS dwarf and ultra-diffuse galaxies

The presence of HI gas in galaxies is inextricably linked to their morphology and evolution. This paper aims to understand the HI content of the already identified 2210 dwarfs located in the low-to-moderate density environments of the Mass Assembly of early-Type GaLAxies with their fine Structures (MATLAS) deep imaging survey. We combined the HI observations from the ATLAS3Dsurvey, with the extragalactic HI sources from the Arecibo Legacy Fast ALFA survey, to extract the HI line width, velocity, and mass of the MATLAS dwarfs. From the 1773 dwarfs in our sample with available HI observations, 8% (145) have an HI line detection. The majority of the dwarfs show an irregular morphology, while 29% (42) are ellipticals, which is the largest sample of HI-bearing dwarf ellipticals (dEs) to date. Of the HI dwarf sample, 2% (three) are ultra-diffuse galaxies (UDGs), 12% have a transition-type morphology, 5% are tidal dwarf candidates, and 10% appear to be disrupted objects. In our optically selected sample, 9.5% of the dEs, 7% of the UDGs, and 10% of the classical dwarfs are HI-bearing. The HI-bearing dwarfs have, on average, bluer colors than the dwarfs without detected HI. We find relations between the stellar and HI masses, gas fraction, color, and absolute magnitude to be consistent with previous studies of dwarfs probing similar masses and environments. For 79% of the dwarfs identified as satellites of massive early-type galaxies, we find that the HI mass increases with the projected distance to the host. Using the HI line width, we estimate dynamical masses and find that 5% (seven) of the dwarfs are dark matter deficient.

Kinematic properties of white dwarfs

Context. Kinematic and chemical tagging of stellar populations have both revealed much information on the past and recent history of the Milky Way, including its formation history, merger events, and mixing of populations across the Galactic disk and halo. Aims. We present the first detailed 3D kinematic analysis of a sample of 3133 white dwarfs that used Gaia astrometry plus radial velocities, which were measured either by Gaia or by ground-based spectroscopic observations. The sample includes either isolated white dwarfs that have direct radial velocity measurements, or white dwarfs that belong to common proper motion pairs that contain nondegenerate companions with available radial velocities. A subset of common proper motion pairs also have metal abundances that have been measured by large-scale spectroscopic surveys or by our own follow-up observations. Methods. We used the white dwarfs as astrophysical clocks by determining their masses and total ages through interpolation with dedicated evolutionary models. We also used the nondegenerate companions in common proper motions to chemically tag the population. Combining accurate radial velocities with Gaia astrometry and proper motions, we derived the velocity components of our sample in the Galactic rest frame and their Galactic orbital parameters. Results. The sample is mostly located within ∼300 pc from the Sun. It predominantly contains (90–95%) thin-disk stars with almost circular Galactic orbits, while the remaining 5–10% of stars have more eccentric trajectories and belong to the thick disk. We identified seven isolated white dwarfs and two common proper motion pairs as halo members. We determined the age – velocity dispersion relation for the thin-disk members, which agrees with previous results that were achieved from different white dwarf samples without published radial velocities. The age – velocity dispersion relation shows signatures of dynamical heating and saturation after 4–6 Gyr. We observed a mild anticorrelation between [Fe/H] and the radial component of the average velocity dispersion, showing that dynamical mixing of populations takes place in the Galactic disk, as was detected through the analysis of other samples of FGK stars. Conclusions. We have shown that a white dwarf sample with accurate 3D kinematics and well-measured chemical compositions enables a wider understanding of their population in the solar neighborhood and its connection with the Galactic chemodynamics. The legacy of existing spectroscopic surveys will be boosted by the availability of upcoming larger samples of white dwarfs and common proper motion pairs with more uniform high-quality data.

Photometric Properties of L and T Dwarf Binaries

Abstract Recent studies using volume-limited samples of brown dwarfs have revealed the photometric evolution of single L and T dwarfs and unresolved binaries as they age and cool, in particular as they transition from L to T spectral types. We demonstrate that the near-infrared photometric evolution of the resolved components of L and T dwarf binaries is consistent with that of single objects using a volume-limited sample. In addition, we provide supporting evidence that the L9 dwarf WISE J185101.83+593508.6 is an unresolved binary and identify three more candidate or confirmed unresolved binaries all from being overluminous for their spectral type. We calculate a new binary fraction of 13.1% ± 1.8% for L0–T8 dwarfs.

Structures of Dwarf Satellites of Milky Way-like Galaxies: Morphology, Scaling Relations, and Intrinsic Shapes

Abstract The structure of a dwarf galaxy is an important probe of the effects of stellar feedback and environment. Using an unprecedented sample of 223 low-mass satellites from the ongoing Exploration of Local Volume Satellites survey, we explore the structures of dwarf satellites in the mass range 105.5 < M ⋆ < 108.5 M ⊙. We survey satellites around 80% of the massive, M K < − 22.4 mag, hosts in the Local Volume (LV). Our sample of dwarf satellites is complete to luminosities of M V <−9 mag and surface brightness μ 0,V < 26.5 mag arcsec−2 within at least ∼200 projected kpc of the hosts. For this sample, we find a median satellite luminosity of M V = −12.4 mag, median size of r e = 560 pc, median ellipticity of ϵ = 0.30, and median Sérsic index of n = 0.72. We separate the satellites into late- and early-type (29.6% and 70.4%, respectively). The mass–size relations are very similar between them within ∼5%, which indicates that the quenching and transformation of a late-type dwarf into an early-type one involves only very mild size evolution. Considering the distribution of apparent ellipticities, we infer the intrinsic shapes of the early- and late-type samples. Combining with literature samples, we find that both types of dwarfs are described roughly as oblate spheroids that get more spherical at fainter luminosities, but early-types are always rounder at fixed luminosity. Finally, we compare the LV satellites with dwarf samples from the cores of the Virgo and Fornax clusters. We find that the cluster satellites show similar scaling relations to the LV early-type dwarfs but are roughly 10% larger at fixed mass, which we interpret as being due to tidal heating in the cluster environments. The dwarf structure results presented here are a useful reference for simulations of dwarf galaxy formation and the transformation of dwarf irregulars into spheroidals.

Exploring deep and hot adiabats as a potential solution to the radius inflation problem in brown dwarfs

Aims. The anomalously large radii of highly irradiated gaseous exoplanets has remained a mystery for some time. One mechanism that has been suggested as a solution for hot Jupiters is the heating of the deep atmosphere via the vertical advection of potential temperature, resulting in increased internal entropy. In this work, we intend to explore whether this mechanism can also explain the observed brown dwarf radii trend: a general increase in the observed radius with irradiation, with an exception, however, for highly irradiated brown dwarfs orbiting white dwarfs. Methods. We used a 3D global circulation model (GCM) known as DYNAMICO to run a series of long-timescale models of the deep atmospheres of Kepler-13Ab, KELT-1b, and SDSS1411B. These models allowed us to explore not only whether a stable advective adiabat can develop in this context, but also to consider the associated dynamics. Results. We find that our brown dwarf models fall into two distinct regimes. First, Kepler-13Ab and KELT-1b both show signs of significant deep heating and, hence, are able to maintain adiabats that are hotter than 1D models predict. On the other hand, SDSS1411B exhibits a much weaker downward heating profile that not only struggles to heat the interior under ideal conditions, but is highly sensitive to the presence of deep radiative dynamics. Conclusions. We conclude that the vertical advection of potential temperature by large-scale atmospheric circulations constitutes a robust mechanism to explain the trend of increasing inflation with irradiation. This includes an exception for highly irradiated brown dwarfs orbiting white dwarfs, which can be understood as occurring due to the role that increasing rotational influence plays in the context of mid-to-high latitude advective dynamics. Furthermore, when paired with a suitable parametrisation of the outer atmosphere irradiation profile, this mechanism alone could potentially provide a complete explanation for the observed levels of radius inflation in our brown dwarf sample. Finally, in order to confirm the validity of this explanation, we suggest that this work should be followed by future studies of brown dwarfs atmospheres using next-generation, fully radiative GCMs.

The Brown Dwarf Kinematics Project (BDKP). V. Radial and Rotational Velocities of T Dwarfs from Keck/NIRSPEC High-resolution Spectroscopy

Abstract We report multiepoch radial velocities, rotational velocities, and atmospheric parameters for 37 T-type brown dwarfs observed with Keck/NIRSPEC. Using a Markov Chain Monte Carlo forward-modeling method, we achieve median precisions of 0.5 and 0.9 km s−1 for radial and rotational velocities, respectively. All of the T dwarfs in our sample are thin-disk brown dwarfs. We confirm previously reported moving group associations for four T dwarfs. However, the lack of spectral indicators of youth in two of these sources suggests that these are chance alignments. We confirm two previously unresolved binary candidates, the T0+T4.5 2MASS J11061197+2754225 and the L7+T3.5 2MASS J21265916+7617440, with orbital periods of 4 and 12 yr, respectively. We find a kinematic age of 3.5 ± 0.3 Gyr for local T dwarfs, consistent with nearby late M dwarfs (4.1 ± 0.3 Gyr). Removal of thick-disk L dwarfs in the local ultracool dwarf sample gives a similar age for L dwarfs (4.2 ± 0.3 Gyr), largely resolving the local L dwarf age anomaly. The kinematic ages of local late M, L, and T dwarfs can be accurately reproduced with population simulations incorporating standard assumptions of the mass function, star formation rate, and brown dwarf evolutionary models. A kinematic dispersion break is found at the L4–L6 subtypes, likely reflecting the terminus of the stellar main sequence. We provide a compilation of precise radial velocities for 172 late M, L, and T dwarfs within ∼20 pc of the Sun.

No evidence for a strong decrease of planetesimal accretion in old white dwarfs

ABSTRACT A large fraction of white dwarfs are accreting or have recently accreted rocky material from their planetary systems, thereby ‘polluting’ their atmospheres with elements heavier than helium. In recent years, the quest for mechanisms that can deliver planetesimals to the immediate vicinity of their central white dwarfs has stimulated a flurry of modelling efforts. The observed time evolution of the accretion rates of white dwarfs through their multi-Gyr lifetime is a crucial test for dynamical models of evolved planetary systems. Recent studies of cool white dwarf samples have identified a significant decrease of the mass accretion rates of cool, old white dwarfs over Gyr time-scales. Here, we revisit those results using updated white dwarf models and larger samples of old polluted H- and He-atmosphere white dwarfs. We find no compelling evidence for a strong decrease of their time-averaged mass accretion rates for cooling times between 1 and 8 Gyr. Over this period, the mass accretion rates decrease by no more than a factor of the order of 10, which is one order of magnitude smaller than the decay rate found in recent works. Our results require mechanisms that can efficiently and consistently deliver planetesimals inside the Roche radius of white dwarfs over at least 8 Gyr.

Disk Masses and Dust Evolution of Protoplanetary Disks around Brown Dwarfs

Abstract We present the largest sample of brown dwarf (BD) protoplanetary disk spectral energy distributions modeled to date. We compile 49 objects with ALMA observations from four star-forming regions: ρ Ophiuchus, Taurus, Lupus, and Upper Scorpius. Studying multiple regions with various ages enables us to probe disk evolution over time. Specifically, from our models, we obtain values for dust grain sizes, dust settling, and disk mass; we compare how each of these parameters vary between the regions. We find that disk mass decreases with age. We also find evidence of disk evolution (i.e., grain growth and significant dust settling) in all four regions, indicating that planet formation and disk evolution may begin to occur at earlier stages. We generally find that these disks contain too little mass to form planetary companions, though we cannot rule out that planet formation may have already occurred. Finally, we examine the disk mass–host mass relationship and find that BD disks are largely consistent with previously determined relationships for disks around T Tauri stars.

Infrared Excesses Around Bright White Dwarfs from Gaia and unWISE. II

Abstract Infrared excesses around white dwarf stars indicate the presence of various astrophysical objects of interest, including companions and debris disks. In this second paper of a series, we present follow-up observations of infrared excess candidates from Gaia and unWISE discussed in the first paper, Paper I. We report space-based infrared photometry at 3.6 and 4.5 micron for 174 white dwarfs from the Spitzer Space Telescope and ground-based near-infrared J, H, and K photometry of 235 white dwarfs from Gemini Observatory with significant overlap between Spitzer and Gemini observations. These data are used to confirm or rule out the observed unWISE infrared excess. From the unWISE-selected candidate sample, the most promising infrared excess sample comes from both color and flux excess, which has a Spitzer confirmation rate of 95%. We also discuss a method to distinguish infrared excess caused by stellar or sub-stellar companions from potential dust disks. In total, we confirm the infrared excess around 62 white dwarfs, 10 of which are likely to be stellar companions. The remaining 52 bright white dwarfs with infrared excess beyond two microns has the potential to double the known sample of white dwarfs with dusty exoplanetary debris disks. Follow-up high-resolution spectroscopic studies of a fraction of confirmed excess white dwarfs in this sample have discovered emission from gaseous dust disks. Additional investigations will be able to expand the parameter space from which dust disks around white dwarfs are found.

New insight into the magnetism of degenerate stars from the analysis of a volume-limited sample of white dwarfs

ABSTRACT Many stars evolve into magnetic white dwarfs (MWDs), and observations may help to understand when the magnetic field appears at the stellar surface, if and how it evolves during the cooling phase, and, above all, what are the mechanisms that generate it. After obtaining new spectropolarimetric observations and combining them with previous literature data, we have checked almost the entire population of about 152 WDs within 20 pc from the Sun for the presence of magnetic fields, with a sensitivity that ranges from better than 1 kG for most of the stars of spectral class DA, to 1 MG for some of the featureless white dwarfs (WDs). We find that 33 WDs of the local 20 pc volume are magnetic. Statistically, the data are consistent with the possibility that the frequency of the magnetic field occurrence is similar in stars of all spectral classes, except that in the local 20 pc volume, either DQ stars are more frequently magnetic or host much stronger fields than average. The distribution of the observed field strength ranges from 40 kG to 300 MG and is uniform per decade, in striking contrast to the field frequency distribution resulting from spectroscopic surveys. Remarkably, no fields weaker than 40 kG are found. We confirm that magnetic fields are more frequent in WDs with higher than average mass, especially in younger stars. We find a marked deficiency of MWDs younger than 0.5 Gyr, and we find that the frequency of the occurrence of the magnetic field is significantly higher in WDs that have undergone the process of core crystallization than in WDs with fully liquid core. There is no obvious evidence of field strength decay with time. We discuss the implications of our findings in relation to some of the proposals that have been put forward to explain the origin and evolution of magnetic fields in degenerate stars, in particular those that predict the presence of a dynamo acting during the crystallization phase.

A CHEOPS white dwarf transit search

White dwarf spectroscopy shows that nearly half of white dwarf atmospheres contain metals that must have been accreted from planetary material that survived the red giant phases of stellar evolution. We can use metal pollution in white dwarf atmospheres as flags, signalling recent accretion, in order to prioritize an efficient sample of white dwarfs to search for transiting material. We present a search for planetesimals orbiting six nearby white dwarfs with the CHaracterising ExOPlanet Satellite (CHEOPS). The targets are relatively faint for CHEOPS, 11 mag < G < 12.8 mag. We used aperture photometry data products from the CHEOPS mission as well as custom point-spread function photometry to search for periodic variations in flux due to transiting planetesimals. We detect no significant variations in flux that cannot be attributed to spacecraft systematics, despite reaching a photometric precision of < 2 ppt in 60 s exposures on each target. We simulate observations to show that the small survey is sensitive primarily to Moon-sized transiting objects with periods between 3 h < P < 10 h, with radii of R ≳ 1000 km.

Analysis of genome variants in dwarf soybean lines obtained in F6 derived from cross of normal parents (cultivated and wild soybean).

Plant height is an important component of plant architecture and significantly affects crop breeding practices and yield. We studied DNA variations derived from F5 recombinant inbred lines (RILs) with 96.8% homozygous genotypes. Here, we report DNA variations between the normal and dwarf members of four lines harvested from a single seed parent in an F6 RIL population derived from a cross between Glycine max var. Peking and Glycine soja IT182936. Whole genome sequencing was carried out, and the DNA variations in the whole genome were compared between the normal and dwarf samples. We found a large number of DNA variations in both the dwarf and semi-dwarf lines, with one single nucleotide polymorphism (SNP) per at least 3.68 kb in the dwarf lines and 1 SNP per 11.13 kb of the whole genome. This value is 2.18 times higher than the expected DNA variation in the F6 population. A total of 186 SNPs and 241 SNPs were discovered in the coding regions of the dwarf lines 1282 and 1303, respectively, and we discovered 33 homogeneous nonsynonymous SNPs that occurred at the same loci in each set of dwarf and normal soybean. Of them, five SNPs were in the same positions between lines 1282 and 1303. Our results provide important information for improving our understanding of the genetics of soybean plant height and crop breeding. These polymorphisms could be useful genetic resources for plant breeders, geneticists, and biologists for future molecular biology and breeding projects.

ЗБАГАЧЕННЯ ГЕНОФОНДУ ТРИТИКАЛЕ ОЗИМОГО ЗА ВНУТРІШНЬОВИДОВОЇ ГІБРИДИЗАЦІЇ

The aim of the research was to expand the genetic diversity of winter triticale samples by intraspecific hybridization and to obtain new valuable forms for their involvement in the selection process of creating high-yielding cultivars. The research was conducted during 2013–2020 at the sites of the Department of Genetics, Plant Breeding and Biotechnology of Uman National University of Horticulture, Ministry of Education and Science of Ukraine. As initial material for hybridization was used samples of hexaploid triticale of own breeding and varieties Rozivska 6, Ladne, Khlibodar Kharkivsky, Beta, Alkid, Souvenir, Raritet; octaploid triticale samples UA0602463 and UA0601654. Hybrid offspring were analyzed for morphological and economically valuable traits, including plant height, lodging resistance, ear length, grain weight from the main ear and 1000 grains, protein and gluten content in the grain and its quality indicators, yield, etc. In the studies used a systematic method of site placement. The rooms were arranged in blocks with a plant density of 400 thousand units/ha four times. In the course of researches hybridization of hexaploid forms of triticale of different ecological and geographical origin among themselves, and also hybridization of octaploid forms with hexaploid was carried out. In F2 populations, individual-family selection of low-stem forms with high ear grain was performed. In the absence of low-stem forms in the area of F2 hybrids, ear productivity was selected, and the population was transferred to a hybridization nursery for crossing plants with donors of low-stem genes. As a result of the conducted researches 23 highly productive forms of winter triticale were obtained, in particular, two dwarf samples (h = 58 cm), four – short-stemmed (h = 60–80 cm), one – early-ripening (vegetation period 285 days), nine – with high ear productivity. Sample 63 was selected, which significantly exceeded the group standard in terms of yield (7.01 t/ha), protein content (12.2 %) and gluten of the first quality group (25.4 %).

The High-energy Spectrum of the Nearby Planet-hosting Inactive Mid-M Dwarf LHS 3844

Abstract To fully characterize the atmospheres, or lack thereof, of terrestrial exoplanets, we must include the high-energy environments provided by their host stars. The nearby mid-M dwarf LHS 3844 hosts a terrestrial world that lacks a substantial atmosphere. We present a time-series UV spectrum of LHS 3844 from 1131 to 3215 Å captured by HST/COS. We detect one flare in the FUV that has an absolute energy of 8.96 ± 0.77 × 1028 erg and an equivalent duration of 355 ± 31 s. We extract the flare and quiescent UV spectra separately. For each spectrum, we estimate the Lyα flux using correlations between UV line strengths. We use Swift-XRT to place an upper limit on the soft X-ray flux and construct a differential emission model to estimate flux that is obscured by the interstellar medium. We compare the differential emission model flux estimates in the XUV to other methods that rely on scaling from the Lyα, Si iv, and N v lines in the UV. The XUV, FUV, and NUV flux of LHS 3844 relative to its bolometric luminosity is log10(L band/L Bol) =−3.65, −4.16, and −4.48, respectively, for the quiescent state. These values agree with trends in high-energy flux as a function of stellar effective temperature found by the MUSCLES survey for a sample of early-M dwarfs. Many of the most spectroscopically accessible terrestrial exoplanets orbit inactive mid-to-late M dwarfs like LHS 3844. Measurements of M dwarf high-energy spectra are preferable for exoplanet characterization but are not always possible. The spectrum of LHS 3844 is a useful proxy for the current radiation environment for these worlds.

Reconstructing the Extreme Ultraviolet Emission of Cool Dwarfs Using Differential Emission Measure Polynomials

Abstract Characterizing the atmospheres of planets orbiting M dwarfs requires understanding the spectral energy distributions of M dwarfs over planetary lifetimes. Surveys like MUSCLES, HAZMAT, and FUMES have collected multiwavelength spectra across the spectral type’s range of T eff and activity, but the extreme ultraviolet (EUV, 100–912 Å) flux of most of these stars remains unobserved because of obscuration by the interstellar medium compounded with limited detector sensitivity. While targets with observable EUV flux exist, there is no currently operational facility observing between 150 and 912 Å. Inferring the spectra of exoplanet hosts in this regime is critical to studying the evolution of planetary atmospheres because the EUV heats the top of the thermosphere and drives atmospheric escape. This paper presents our implementation of the differential emission measure technique to reconstruct the EUV spectra of cool dwarfs. We characterize our method’s accuracy and precision by applying it to the Sun and AU Mic. We then apply it to three fainter M dwarfs: GJ 832, Barnard’s star, and TRAPPIST-1. We demonstrate that with the strongest far-ultraviolet (FUV, 912–1700 Å) emission lines, observed with the Hubble Space Telescope and/or Far Ultraviolet Spectroscopic Explorer, and a coarse X-ray spectrum from either the Chandra X-ray Observatory or XMM-Newton, we can reconstruct the Sun’s EUV spectrum to within a factor of 1.8, with our model’s formal uncertainties encompassing the data. We report the integrated EUV flux of our M dwarf sample with uncertainties of a factor of 2–7 depending on available data quality.

Searching for Extragalactic Star Formation in the M101 Group

Abstract We present deep, narrowband imaging of the nearby spiral galaxy M101 to search for star-forming dwarf galaxies and isolated H ii regions in the group environment. Isolated star-forming regions, unassociated with spiral arms, probe star formation (SF) at the lowest masses and environmental densities. Such objects may be isolated star-forming events or ongoing evidence of tidal interactions. Using the Burrell Schmidt telescope, we target the brightest emission lines of star-forming regions, Hα, Hβ, and [O iii], to detect potential isolated regions. Great care was taken to remove potential source contaminants, such as high-redshift objects and Galactic M stars, from our source list. For each detected source, we compare the emission line fluxes and equivalent widths, along with structural and photometric properties, to samples of Local Group dwarfs and star-forming galaxies in the SINGG/11HUGS samples. We discuss our findings in the context of SF in low-density environments and evolution of the M101 Group.

The Field Substellar Mass Function Based on the Full-sky 20 pc Census of 525 L, T, and Y Dwarfs

Abstract We present final Spitzer trigonometric parallaxes for 361 L, T, and Y dwarfs. We combine these with prior studies to build a list of 525 known L, T, and Y dwarfs within 20 pc of the Sun, 38 of which are presented here for the first time. Using published photometry and spectroscopy as well as our own follow-up, we present an array of color–magnitude and color–color diagrams to further characterize census members, and we provide polynomial fits to the bulk trends. Using these characterizations, we assign each object a T eff value and judge sample completeness over bins of T eff and spectral type. Except for types ≥T8 and T eff < 600 K, our census is statistically complete to the 20 pc limit. We compare our measured space densities to simulated density distributions and find that the best fit is a power law ( ) with α = 0.6 ± 0.1. We find that the evolutionary models of Saumon & Marley correctly predict the observed magnitude of the space density spike seen at 1200 K < T eff < 1350 K, believed to be caused by an increase in the cooling timescale across the L/T transition. Defining the low-mass terminus using this sample requires a more statistically robust and complete sample of dwarfs ≥Y0.5 and with T eff < 400 K. We conclude that such frigid objects must exist in substantial numbers, despite the fact that few have so far been identified, and we discuss possible reasons why they have largely eluded detection.

Search and Studies of the First Byurakan Survey Blue Stellar Objects

The First Byurakan Survey (FBS) 2nd Part was devoted to search and studies of Blue Stellar Objects (BSOs) and Late-type Stars. Eleven lists of 1103 BSOs were published in Astrophysics in 1990-1996, found in FBS low-dispersion spectroscopic plates. The selection was carried out in the region with +33◦ > δ > +45◦ and δ >+61◦ with a surface of ∼4000 deg2. As a result, the catalogue of the FBS BSOs was compiled. Its preliminary version has been available at CDS since 1999. We revised and updated the FBS BSOs catalogue with the new data from recently published optical and multiwavelength catalogues to give access to all available data and make further comparative studies of the properties of these objects possible. We made cross-correlations of the FBS BSOs catalogue with the MAPS, USNO-B1.0, SDSS, and 2MASS, as well as ROSAT, IRAS, NVSS, and FIRST catalogs, added updated SIMBAD and NED data for the objects, and provided accurate DSS1 and DSS2 positions and revised photometry. We also checked the objects for proper motion and variability. A refined classification for the low-dispersion spectra in the Digitized First Byurakan Survey (DFBS) was carried out. The revised and updated catalogue of FBS blue stellar objects contains 1101 objects. The FBS blue stellar objects catalogue can be used to study a complete sample of white dwarfs, hot subdwarfs, HBB stars, cataclysmic variables, bright AGN, and to investigate individual interesting objects.