We present the spectroscopic analysis of a large sample of late-M, L, and T dwarfs from UKIDSS. Using the YJHK photometry from ULAS and the red-optical photometry from SDSS we selected a sample of 262 brown dwarf candidates and we followed-up 196 of them using X-shooter on the VLT. The large wavelength coverage (0.30-2.48 $μ$m) and moderate resolution (R~5000-9000) of X-shooter allowed us to identify peculiar objects including 22 blue L dwarfs, 2 blue T dwarfs, and 2 low gravity M dwarfs. Using a spectral indices-based technique we identified 27 unresolved binary candidates, for which we determined the spectral type of the potential components via spectral deconvolution. The spectra allowed us to measure the equivalent width of the prominent absorption features and to compare them to atmospheric models. Cross-correlating the spectra with a radial velocity standard, we measured the radial velocity for our targets, and we determined the distribution of the sample, which is centred at -1.7$\pm$1.2 km s$^{-1}$ with a dispersion of 31.5 km s$^{-1}$. Using our results we estimated the space density of field brown dwarfs and compared it with the results of numerical simulations. Depending on the binary fraction, we found that there are (0.85$\pm$0.55) x 10$^{-3}$ to (1.00$\pm$0.64) x 10$^{-3}$ objects per cubic parsec in the L4-L6.5 range, (0.73$\pm$0.47) x 10$^{-3}$ to (0.85$\pm$0.55) x 10$^{-3}$ objects per cubic parsec in the L7-T0.5 range, and (0.74$\pm$0.48) x 10$^{-3}$ to (0.88$\pm$0.56) x 10$^{-3}$ objects per cubic parsec in the T1-T4.5 range. There seem to be an excess of objects in the L/T transition with respect to the late T dwarfs, a discrepancy that could be explained assuming a higher binary fraction than expected for the L/T transition, or that objects in the high-mass end and low-mass end of this regime form in different environments, i.e. following different IMFs.
We aim at developing an efficient method to search for late-type subdwarfs (metal-depleted dwarfs with spectral types >M5) to improve the current statistics. Our objectives are: improve our knowledge of metal-poor low-mass dwarfs, bridge the gap between the late-M and L types, determine their surface density, and understand the impact of metallicity on the stellar and substellar mass function. We carried out a search cross-matching the SDSS, 2MASS, and UKIDSS using STILTS, Aladin, and Topcat. We considered different photometric and proper motion criteria for our selection. We identified 29 and…
The aim of the project is to improve our knowledge on the low-mass and low-metallicity population to investigate the influence of metallicity of the stellar (and substellar) mass function. We present the results of a photometric and proper motion search aimed at unearthing ultracool subdwarfs in large-scale surveys. We employed and combined the UKIDSS LAS DR5 and the SDSS DR7 complemented with ancillary data from 2MASS, DENIS and SuperCOSMOS. The SDSS DR7 vs UKIDSS LAS DR5 search returned a total of 32 ultracool subdwarf candidates, only two being recognised as a subdwarf in the literature. Tw…
Benchmark brown dwarfs are those objects for which fiducial constraints are available, including effective temperature, parallax, age, metallicity. We searched for new cool brown dwarfs in 186 sq.deg. of the new area covered by the data release DR5+ of the UKIDSS Large Area Survey. Follow-up optical and near-infrared broad-band photometry, and methane imaging of four promising candidates, revealed three objects with distinct methane absorption, typical of mid- to late-T dwarfs, and one possibly T4 dwarf. The latest-type object, classified as T8-9, shares its large proper motion with Ross 458 (…
The UKIDSS Large Area Survey (LAS) began in 2005, with the start of the UKIDSS program as a 7 year effort to survey roughly 4000 deg$^2$ at high galactic latitudes in Y, J, H and K bands. The survey also included a significant quantity of 2-epoch J band observations, with an epoch baseline greater than 2 years to calculate proper motions. We present a near infrared proper motion catalogue for the 1500 deg$^2$ of the 2 epoch LAS data, which includes 135,625 stellar sources and a further 88,324 with ambiguous morphological classifications, all with motions detected above the 5$σ$ level. We devel…
We report the discovery of 76 new T dwarfs from the UKIDSS Large Area Survey (LAS). Near-infrared broad and narrow-band photometry and spectroscopy are presented for the new objects, along with WISE and warm-Spitzer photometry. Proper motions for 128 UKIDSS T dwarfs are presented from a new two epoch LAS proper motion catalogue. We use these motions to identify two new benchmark systems: LHS 6176AB, a T8+M4 pair and HD118865AB, a T5.5+F8 pair. Using age constraints from the primaries and evolutionary models to constrain the radii we have estimated their physical properties from their bolometri…
We report the discovery of 47 new T dwarfs in the Fourth Data Release (DR4) from the Large Area Survey (LAS) of the UKIRT Infrared Deep Sky Survey with spectral types ranging from T0 to T8.5. These bring the total sample of LAS T dwarfs to 80 as of DR4. In assigning spectral types to our objects we have identified 8 new spectrally peculiar objects, and divide 7 of them into two classes. H2O-H-early have a H2O-H index that differs with the H2O-J index by at least 2 sub-types. CH4-J-early have a CH4-J index that disagrees with the H20-J index by at least 2 subtypes. We have ruled out binarity as…
We report the discovery of a very cool d/sdL7+T7.5p common proper motion binary system, SDSS J1416+13AB, found by cross-matching the UKIDSS Large Area Survey Data Release 5 against the Sloan Digital Sky Survey Data Release 7. The d/sdL7 is blue in J-H and H-K and has other features suggestive of low-metallicity and/or high gravity. The T7.5p displays spectral peculiarity seen before in earlier type dwarfs discovered in UKIDSS LAS DR4, and referred to as CH4-J-early peculiarity. We suggest that CH4-J-early peculiarity arises from low-metallicity and/or high-gravity, and speculate as to its use …
We present a new sample of mid L to mid T dwarfs with effective temperatures of 1100 to 1700K selected from the UKIDSS Large Area Survey and confirmed with infrared spectra from X-Shooter/VLT. This effective temperature range is especially sensitive to the formation history of Galactic brown dwarfs and allows us to constrain the form of the sub-stellar birth rate, with sensitivity to differentiate between a flat (stellar like) birth rate, and an exponentially declining form. We present the discovery of 63 new L and T dwarfs from the UKIDSS LAS DR7, including the identification of 12 likely unr…
We analyze the last 125 years optical light curve of the symbiotic star AX Per through some remarkable correlations that we discovered in its power spectrum. The data were assembled from the literature and from the AAVSO database. A series of 6 major outbursts dominate the light curve. They are presented in the power spectrum as 13 harmonics of the fundamental frequency fa=1/Pa=1/23172 day-1. We refer to them as the "red" frequencies. Oscillations with the binary periodicity of the system Pb=1/fb=681.48 d are also seen in the light curve, with particularly large amplitudes during outbursts. Th…
We present a homogeneous sample of 1361 L and T dwarfs brighter than J = 17.5 (of which 998 are new), from an effective area of 3070 deg2, classified by the photo-type method to an accuracy of one spectral sub-type using izYJHKW1W2 photometry from SDSS+UKIDSS+WISE. Other than a small bias in the early L types, the sample is shown to be effectively complete to the magnitude limit, for all spectral types L0 to T8. The nature of the bias is an incompleteness estimated at 3% because peculiar blue L dwarfs of type L4 and earlier are classified late M. There is a corresponding overcompleteness becau…
The upper main sequence stars CU Virginis is the most enigmatic object among magnetic chemically peculiar (mCP) stars. It is an unusually fast rotator showing strictly periodic light variations in all regions of the electromagnetic spectrum, as well as spectroscopic and spectropolarimetric changes. At same time, it is also the first radio main-sequence pulsar. Exploiting information hidden in phase variations, we monitored the secular oscillation of the rotational period during the last 53 years. Applying own phenomenological approach, we analyzed 37\,975 individual photometric and spectroscop…
We have developed a method "photo-type" to identify and accurately classify L and T dwarfs, onto the standard system, from photometry alone. We combine SDSS, UKIDSS and WISE data and classify point sources by comparing the izYJHKW1W2 colours against template colours for quasars, stars, and brown dwarfs. In a sample of $6.5\times10^6$ bright point sources, J$<$17.5, from 3150 deg$^2$, we identify and type 898 L and T dwarfs, making this the largest homogeneously selected sample of brown dwarfs to date. The sample includes 713 (125) new (previously known) L dwarfs and 21 (39) T dwarfs. For the p…
We report the discovery of ULAS J131610.28+075553.0, a sdT6.5 dwarf in the UKIDSS Large Area Survey 2 epoch proper motion catalogue. This object displays significant spectral peculiarity, with the largest yet seen deviations from T6 and T7 templates in the Y and K bands for this subtype. Its large, ~1 arcsec/yr, proper motion suggests a large tangential velocity of Vtan = 240 - 340km/s, if we assume its MJ lies within the typical range for T6.5 dwarfs. This makes it a candidate for membership of the Galactic halo population. However, other metal poor T dwarfs exhibit significant under luminosi…
We report the discovery of a very cool, isolated brown dwarf, UGPS 0722-05, with the UKIDSS Galactic Plane Survey. The near-infrared spectrum displays deeper H2O and CH4 troughs than the coolest known T dwarfs and an unidentified absorption feature at 1.275 um. We provisionally classify the object as a T10 dwarf but note that it may in future come to be regarded as the first example of a new spectral type. The distance is measured by trigonometric parallax as d=4.1{-0.5}{+0.6} pc, making it the closest known isolated brown dwarf. With the aid of Spitzer/IRAC we measure H-[4.5] = 4.71. It is th…
A CCD photometry of the dwarf nova MASTER OT J172758.09 +380021.5 was carried out in 2019 during 134 nights. Observations covered three superoutbursts, five normal outbursts and quiescence between them. The available ASASSN and ZTF data for 2014-2020 were also examined. Spectral observations were done in 2020 when the object was in quiescence. Spectra and photometry revealed that the star is an H-rich active ER UMa-type dwarf nova with a highly variable supercycle of ~50-100 d that implies a high and variable mass-transfer rate. This object demonstrated peculiar behaviour: short-lasted superou…
We have identified eight to ten new cool white dwarfs from the Large Area Survey (LAS) Data Release 9 of the United Kingdom InfraRed Telescope (UKIRT) Infrared Deep Sky Survey (UKIDSS). The data set was paired with the Sloan Digital Sky Survey (SDSS) to obtain proper motions and a broad ugrizYJHK wavelength coverage. Optical spectroscopic observations were secured at Gemini Observatory and confirm the degenerate status for eight of our targets. The final sample includes two additional white dwarf candidates with no spectroscopic observations. We rely on improved 1D model atmospheres and new mu…
The DQ white dwarfs are stars whose atmosphere is enriched with carbon, which for cool stars ($T_{\rm eff}<8000\rm \, K$) is indicated by the Swan bands of $\rm C_2$ in the optical part of their spectra. With decreasing effective temperature these molecular bands undergo a significant blueshift ($\sim 100-300 Å$). The origin of this phenomenon has been disputed over the last two decades and has remained unknown. We attempt to address this problem by investigating the impact of dense helium on the spectroscopic properties of molecular carbon under the physical conditions encountered inside heli…
The number of known variable stars has increased by several magnitudes over the last decade, and automated classification routines are becoming increasingly important to cope with this development. Here we show that the "upside-down CBH variables", which were proposed as a potentially new class of variable stars by Heinze et al. (2018) in the ATLAS First Catalogue of Variable Stars, are, at least to a high percentage, made up of alpha2 Canum Venaticorum (ACV) variables - that is, photometrically variable magnetic chemically peculiar (CP2/He-peculiar) stars - with distinct double-wave light cur…
The frequency of multiple systems and their properties are key constraints of stellar formation and evolution. Formation mechanisms of very low-mass (VLM) objects are still under considerable debate and an accurate assessment of their multiplicity and orbital properties are essential for constraining current theoretical models. Taking advantage of the Virtual Observatory capabilities, we looked for comoving low and VLM binary (or multiple) systems using the Large Area Survey of the UKIDSS LAS DR10, SDSS DR9, and the 2MASS Catalogues. Other catalogues (WISE, GLIMPSE, SuperCosmos ...) were used …
Context: We present results of an extensive world-wide observing campaign of MN Draconis. Aims: MN Draconis is a poorly known active dwarf nova in the period gap and is one of the only two known cases of period gap SU UMa objects showing the negative superhumps. Photometric behaviour of MN Draconis poses a challenge for existing models of the superhump and superoutburst mechanisms. Therefore, thorough investigation of peculiar systems, such as MN Draconis, is crucial for our understanding of evolution of the close binary stars. Methods: To measure fundamental parameters of the system, we colle…
We present the first spectroscopic verification of a bona fide chemically peculiar (CP) star in the Large Magellanic Cloud. CP stars reside on the upper main sequence and are characterized by strong global stellar magnetic fields with a predominant dipole component oriented at random with respect to the stellar rotation axis and displaced from the star's centre. Overabundances with respect to the Sun for heavy elements such as silicon, chromium, strontium and europium are also a common phenomenon. These objects are excellent astrophysical laboratories by which to investigate many of the proces…
With the advent of high cadence, all-sky automated surveys, supernovae (SNe) are now discovered closer than ever to their dates of explosion. However, young pre-maximum light follow-up spectra of Type Ic supernovae (SNe Ic), probably arising from the most stripped massive stars, remain rare despite their importance. In this paper we present a set of 49 optical spectra observed with the Las Cumbres Observatory through the Global Supernova Project for 6 SNe Ic, including a total of 17 pre-maximum spectra, of which 8 are observed more than a week before V-band maximum light. This dataset increase…
Spin properties and spectral variations of high mass X-ray binary 4U 2206+54 are studied with long-term hard X-ray monitoring observations by INTEGRAL. A long-period X-ray pulsar of P_spin\sim 5558 s has been identified in 4U 2206+54. The spin evolution of the neutron star in 4U 2206+54 is detected with the INTEGRAL/IBIS data. From 2005 to 2011, the spin period of the neutron star in 4U 2206+54 varies from \sim 5558 s to 5588 s. The average spin-down rate in the last 20 years is derived as \sim 5\times 10^{-7} s s^{-1}. 4U 2206+54 is a variable source with luminosities of \sim 10^{35} - 10^{36…
