arXiv subjects

Z. H. Zhang

Publications and source records attributed to Z. H. Zhang.

At least 55 records · Page 3Linked to original sources

arXiv2017

VIRAC: The VVV Infrared Astrometric Catalogue

We present VIRAC version 1, a near-infrared proper motion and parallax catalogue of the VISTA VVV survey for 312,587,642 unique sources averaged across all overlapping pawprint and tile images covering 560 deg$^2$ of the bulge of the Milky Way and southern disk. The catalogue includes 119 million high quality proper motion measurements, of which 47 million have statistical uncertainties below 1 mas yr$^{-1}$. In the 11$<K_s<$14 magnitude range the high quality motions have a median uncertainty of 0.67 mas yr$^{-1}$. The catalogue also includes 6,935 sources with quality-controlled 5 $σ$ parallaxes with a median uncertainty of 1.1 mas. The parallaxes show reasonable agreement with the TYCHO-Gaia Astrometric Solution (TGAS), though caution is advised for data with modest significance. The SQL database housing the data is made available via the web. We give example applications for studies of Galactic structure, nearby objects (low mass stars and brown dwarfs, subdwarfs, white dwarfs) and kinematic distance measurements of YSOs. Nearby objects discovered include LTT 7251 B, an L7 benchmark companion to a G dwarf with over 20 published elemental abundances, a bright L sub-dwarf, VVV 1256-6202, with extremely blue colours and nine new members of the 25 pc sample. We also demonstrate why this catalogue remains useful in the era of Gaia. Future versions will be based on profile fitting photometry, use the Gaia absolute reference frame and incorporate the longer time baseline of the VVV extended survey (VVVX).

astro-ph.SR↗

arXiv2017

Primeval very low-mass stars and brown dwarfs - II. The most metal-poor substellar object

SDSS J010448.46+153501.8 has previously been classified as an sdM9.5 subdwarf. However, its very blue $J-K$ colour ($-0.15 \pm 0.17$) suggests a much lower metallicity compared to normal sdM9.5 subdwarfs. Here, we re-classify this object as a usdL1.5 subdwarf based on a new optical and near-infrared spectrum obtained with X-shooter on the Very Large Telescope. Spectral fitting with BT-Settl models leads to $T_{\rm eff}$ = 2450 $\pm$ 150 K, [Fe/H] = $-$2.4 $\pm$ 0.2 and log $g$ = 5.5 $\pm$ 0.25. We estimate a mass for SDSS J010448.46+153501.8 of 0.086 $\pm$ 0.0015 M$_{\odot}$ which is just below the hydrogen-burning minimum mass at [Fe/H] = $-$2.4 ($\sim$0.088 M$_{\odot}$) according to evolutionary models. Our analysis thus shows SDSS J0104+15 to be the most metal-poor and highest mass substellar object known to-date. We found that SDSS J010448.46+153501.8 is joined by another five known L subdwarfs (2MASS J05325346+8246465, 2MASS J06164006$-$6407194, SDSS J125637.16$-$022452.2, ULAS J151913.03$-$000030.0 and 2MASS J16262034+3925190) in a 'halo brown dwarf transition zone' in the $T_{\rm eff}-$[Fe/H] plane, which represents a narrow mass range in which unsteady nuclear fusion occurs. This halo brown dwarf transition zone forms a 'substellar subdwarf gap' for mid L to early T types.

astro-ph.SR↗

arXiv2016

Primeval very low-mass stars and brown dwarfs. I. Six new L subdwarfs, classification and atmospheric properties

We have conducted a search for L subdwarf candidates within the photometric catalogues of the UKIRT Infrared Deep Sky Survey and Sloan Digital Sky Survey. Six of our candidates are confirmed as L subdwarfs spectroscopically at optical and/or near infrared wavelengths. We also present new optical spectra of three previously known L subdwarfs (WISEA J001450.17-083823.4, 2MASS J00412179+3547133, ULAS J124425.75+102439.3). We examined the spectral types and metallicity subclasses classification of known L subdwarfs. We summarised the spectroscopic properties of L subdwarfs with different spectral types and subclasses. We classify these new L subdwarfs by comparing their spectra to known L subdwarfs and L dwarf standards. We estimate temperatures and metallicities of 22 late type M and L subdwarfs by comparing their spectra to BT-Settl models. We find that L subdwarfs have temperatures between 1500 K and 2700 K, which are higher than similarly-typed L dwarfs by around 100-400 K depending on different subclasses and subtypes. We constrained the metallicity ranges of subclasses of M, L and T subdwarfs. We also discussed the spectral type and absolute magnitude relationships for L and T subdwarfs.

astro-ph.SR↗

arXiv2015

Probing M subdwarf metallicity with an esdK5+esdM5.5 binary

We present a spectral analysis of the binary G 224-58 AB that consists of the coolest M extreme subdwarf (esdM5.5) and a brighter primary (esdK5). This binary may serve as a benchmark for metallicity measurement calibrations and as a test-bed for atmospheric and evolutionary models for esdM objects. We determine abundances primarily using high resolution optical spectra of the primary. Other parameters were determined from the fits of synthetic spectra computed with these abundances to the observed spectra from 0.4 to 2.5 microns for both components. We determine \Tef =4625 $\pm$ 100 K, \logg = 4.5 $\pm$ 0.5 for the A component and \Tef = 3200 $\pm$ 100 K, \logg = 5.0 $\pm$ 0.5, for the B component. We obtained abundances of [Mg/H]=$-$1.51$\pm$0.08, [Ca/H]=$-$1.39$\pm$0.03, [Ti/H]=$-$1.37$\pm$0.03 for alpha group elements and [CrH]=$-$1.88$\pm$0.07, [Mn/H]=$-$1.96$\pm$0.06, [Fe/H]=$-$1.92$\pm$0.02, [Ni/H]=$-$1.81$\pm$0.05 and [Ba/H]W=$-$1.87$\pm$0.11 for iron group elements from fits to the spectral lines observed in the optical and infrared spectral regions of the primary. We find consistent abundances with fits to the secondary albeit at lower signal-to-noise. Abundances of elements in \ga and \gb atmospheres cannot be described by one metallicity parameter. The offset of $\sim$ 0.4 dex between the abundances derived from alpha element and iron group elements corresponds with our expectation for metal-deficient stars. We thus clarify that some indices used to date to measure metallicities for establishing esdM stars based on CaH, MgH and TiO band system strength ratios in the optical and H$_2$O in the infrared relate to abundances of alpha-element group rather than to iron peak elements. For metal deficient M dwarfs with [Fe/H] < -1.0, this provides a ready explanation for apparently inconsistent "metallicities" derived using different methods.

astro-ph.SR↗

arXiv2015

A large spectroscopic sample of L and T dwarfs from UKIDSS LAS: peculiar objects, binaries, and space density

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.

astro-ph.SR↗

arXiv2014

Multiple chiral doublet bands of identical configuration in 103Rh

Three sets of chiral doublet band structures have been identified in the 103Rh nucleus. The properties of the observed chiral doublet bands are in good agreement with theoretical results obtained using constrained covariant density functional theory and particle rotor model calculations. Two of them belong to an identical configuration, and provide the first experimental evidence for a novel type of multiple chiral doublets, where an "excited" chiral doublet of a configuration is seen together with the "yrast" one. This observation shows that the chiral geometry in nuclei can be robust against the increase of the intrinsic excitation energy.

nucl-ex↗

arXiv2014

The extremely red L dwarf ULAS J222711$-$004547 - dominated by dust

We report the discovery of a peculiar L dwarf from the UKIDSS LAS, ULAS J222711-004547. The very red infrared photometry (MKO J-K = 2.79$\pm$0.06, WISE W1-W2 = 0.65$\pm$0.05) of ULAS J222711-004547 makes it one of the reddest brown dwarfs discovered so far. We obtained a moderate resolution spectrum of this target, and classify it as L7pec, confirming its very red nature. Comparison to theoretical models suggests that the object could be a low-gravity L dwarf with a solar or higher than solar metallicity. Nonetheless, the match of such fits to the spectrum is rather poor and this and other peculiar red L dwarfs pose new challenges for the modeling of ultracool atmospheres. We determined the proper motion of ULAS J222711-004547 using the data available in the literature, and we find that its kinematics do not suggest membership of any of the known young associations. We show that applying a simple de-reddening curve to its spectrum allows it to resemble the spectra of the L7 spectroscopic standards. Given the negligible interstellar reddening of the field containing our target, we conclude that the reddening of the spectrum is mostly due to an excess of dust in the photosphere of the target. De-reddening the spectrum using extinction curves for different dust species gives surprisingly good results and suggests a characteristic grain size of $\sim$0.5 $μ$m. We show that by increasing the optical depth, the same extinction curves allow the spectrum of ULAS J222711-004547 to resemble the spectra of unusually blue L dwarfs and slightly metal-poor L dwarfs. Grains of similar size yield very good fits when de-reddening other unusually red L dwarfs in the L5 to L7.5 range. These results suggest that the diversity in near infrared colours and spectra seen in late-L dwarfs could be due to differences in the optical thickness of the dust cloud deck.

astro-ph.SR↗

arXiv2013

A spectroscopic and proper motion search of Sloan digital Sky Survey: red subdwarfs in binary systems

Red subdwarfs in binary systems are crucial for both model calibration and spectral classification. We search for red subdwarfs in binary systems from a sample of high proper motion objects with Sloan digital Sky Survey spectroscopy. We present here discoveries from this search, as well as highlight several additional objects of interest. We find 30 red subdwarfs in wide binary systems including: two with spectral type of esdM5.5, 6 companions to white dwarfs and 3 carbon enhanced red subdwarfs with normal red subdwarf companions. 15 red subdwarfs in our sample are partially resolved close binary systems. With this binary sample, we estimate the low limit of the red subdwarf binary fraction of ~ 10%. We find that the binary fraction goes down with decreasing masses and metallicities of red subdwarfs. A spectroscopic esdK7 subdwarf + white dwarf binary candidate is also reported. 30 new M subdwarfs have spectral type of >= M6 in our sample. We also derive relationships between spectral types and absolute magnitudes in the optical and near-infrared for M and L subdwarfs, and we present an M subdwarf sample with measured U, V, W space velocities.

astro-ph.GA↗

arXiv2013

Two new ultra-cool benchmark systems from WISE+2MASS

We have used the 2MASS all-sky survey and the WISE to look for ultracool dwarfs that are part of multiple systems containing main sequence stars. We cross-matched L dwarf candidates from the surveys with Hipparcos and Gliese stars, finding two new systems. The first system, G255-34AB is an L2 dwarf companion to a K8 star, at a distance of 36 pc. We estimate its bolometric luminosity as log (L/Lsolar) = -3.78 +/- 0.045 and Teff = 2080 +/- 260 K. The second system, GJ499ABC, is a triple, with an L5 dwarf as a companion to a binary with an M4 and K5 star. These two new systems bring the number of L dwarf plus main sequence star multiple systems to twenty four, which we discuss. We consider the binary fraction for L dwarfs and main sequence stars, and further assess possible unresolved multiplicity within the full companion sample. This analysis shows that some of the L dwarfs in this sample might actually be unresolved binaries themselves, since their MJ appears to be brighter than the expected for their spectral types.

astro-ph.GA↗

arXiv2013

The sub-stellar birth rate from UKIDSS

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 unresolved binaries, which form the first complete sub-set from our program, covering 495 sq degrees of sky, complete to J=18.1. We compare our results for this sub-sample with simulations of differing birth rates for objects of mass 0.10-0.03Msol and ages 1-10Gyrs. We find that the more extreme birth rates (e.g. a halo type form) can likely be excluded as the true form of the birth rate. In addition we find that although there is substantial scatter we find a preference for a mass function, with a power-law index, alpha in the range -1 < alpha < 0 that is consistent (within the errors) with the studies of late T dwarfs.

astro-ph.SR↗

arXiv2012

The brightest pure-H ultracool white dwarf

We report the identification of LSR J0745+2627 in the United Kingdom InfraRed Telescope Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS) as a cool white dwarf with kinematics and age compatible with the thick-disk/halo population. LSR J0745+2627 has a high proper motion (890 mas/yr) and a high reduced proper motion value in the J band (H_J=21.87). We show how the infrared-reduced proper motion diagram is useful for selecting a sample of cool white dwarfs with low contamination. LSR J0745+2627 is also detected in the Sloan Digital Sky Survey (SDSS) and the Wide-field Infrared Survey Explorer (WISE). We have spectroscopically confirmed this object as a cool white dwarf using X-Shooter on the Very Large Telescope. A detailed analysis of its spectral energy distribution reveals that its atmosphere is compatible with a pure-H composition model with an effective temperature of 3880+-90 K. This object is the brightest pure-H ultracool white dwarf (Teff<4000 K) ever identified. We have constrained the distance (24-45 pc), space velocities and age considering different surface gravities. The results obtained suggest that LSR J0745+2627 belongs to the thick-disk/halo population and is also one of the closest ultracool white dwarfs.

astro-ph.SR↗

arXiv2012

Theoretical studies of 63Cu Knight shifts of the normal state of YBa2Cu3O7

The 63Cu Knight shifts and g factors for the normal state of YBa2Cu3O7 in tetragonal phase are theoretically studied in a uniform way from the high (fourth-) order perturbation formulas of these parameters for a 3d9 ion under tetragonally elongated octahedra. The calculations are quantitatively correlated with the local structure of the Cu2+(2) site in YBa2Cu3O7. The theoretical results show good agreement with the observed values, and the improvements are achieved by adopting fewer adjustable parameters as compared to the previous works. It is found that the significant anisotropy of the Knight shifts is mainly attributed to the anisotropy of the g factors due to the orbital interactions.

cond-mat.supr-con↗

arXiv2012

Discovery of the benchmark metal poor T8 dwarf BD+01 2920B

We have searched the WISE first data release for widely separated (<10,000AU) late T dwarf companions to Hipparcos and Gliese stars. We have discovered a new binary system containing a K-band suppressed T8p dwarf WISEP J1423+0116 and the mildly metal poor ([Fe/H]=-0.38+-0.06) primary BD+01 2920 (Hip 70319), a G1 dwarf at a distance of 17.2pc. This new benchmark has Teff=680+-55K and a mass of 20-50 Mjup. Its spectral properties are well modelled except for known discrepancies in the Y and K bands. Based on the well determined metallicity of its companion, the properties of BD+01 2920B imply that the currently known T dwarfs are dominated by young low-mass objects. We also present an accurate proper motion for the T8.5 dwarf WISEP J075003.84+272544.8.

astro-ph.SR↗

arXiv2011

Theoretical studies of the spin Hamiltonian parameters for the two tetragonal Cu$^{2+}$ centers in the calcined catalysts CuO-ZnO

The spin Hamiltonian parameters for the two Cu$^{2+}$ centers $A1$ and $A2$ in the calcined catalysts CuO-ZnO are theoretically investigated using the high order perturbation formulas of these parameters for a $3d^{9}$ ion in tetragonally elongated octahedra. In the above formulas, the tetragonal field parameters $Ds$ and $Dt$ are determined from the superposition model, by considering the relative axial elongation of the oxygen octahedron around the Cu$^{2+}$ due to the Jahn-Teller effect. Based on the calculations, the relative elongation ratios of about 5% and 3% are obtained for the tetragonal Cu$^{2+}$ centers $A1$ and $A2$, respectively. The theoretical spin Hamiltonian parameters are in good agreement with the observed values for both systems. The larger axial elongation in center $A1$ is ascribed to the more significant low symmetrical (tetragonal) distortion of the Jahn-Teller effect. The local structures characterized by the above axial elongations are discussed.

cond-mat.mtrl-sci↗

arXiv2011

Nuclear pairing reduction due to rotation and blocking

Nuclear pairing gaps of normally deformed and superdeformed nuclei are investigated using the particle-number conserving (PNC) formalism for the cranked shell model, in which the blocking effects are treated exactly. Both rotational frequency $ω$-dependence and seniority (number of unpaired particles) $ν$-dependence of the pairing gap $\tildeΔ$ are investigated. For the ground-state bands of even-even nuclei, PNC calculations show that in general $\tildeΔ$ decreases with increasing $ω$, but the $ω$-dependence is much weaker than that calculated by the number-projected Hartree-Fock-Bogolyubov approach. For the multiquasiparticle bands (seniority $ν> 2$), the pairing gaps keep almost $ω$-independent. As a function of the seniority $ν$, the bandhead pairing gaps $\tildeΔ(ν,ω=0)$ decrease slowly with increasing $ν$. Even for the highest seniority $ν$ bands identified so far, $\tildeΔ(ν,ω=0)$ remains greater than 70% of $\tildeΔ(ν=0,ω=0)$.

nucl-th↗

arXiv2011

The properties of the T8.5p dwarf Ross 458C

We present near-infrared photometry and spectroscopy, and warm-Spitzer IRAC photometry of the young very cool T dwarf Ross 458C, which we have typed as T8.5p. By applying the fiducial age constraints (< 1Gyr) imposed by the properties of the active M dwarf Ross 458A, we have used these data to determine that Ross 458C has Teff = 695 \pm 60K, log g = 4.0 - 4.7 and an inferred mass of 5-20 MJup. We have compared fits of the near-infrared spectrum and IRAC photometry to the BT Settl and Saumon & Marley model grids, and have found that both sets provide best fits that are consistent with our derived properties, whilst the former provide a marginally closer match to the data for all scenarios explored here. The main difference between the model grids arises in the 4.5μm region, where the BT Settl models are able to better predict the flux through the IRAC filter, suggesting that non-equilibrium effects on the CO-CO2 ratio are important for shaping the mid-infrared spectra of very cool T dwarfs. We have also revisited the issue of dust opacity in the spectra of Ross 458C that was raised by Burgasser et al (2010). We have found that the BT Settl models which also incorporate a condensate cloud model, provide a better match to the near-infrared spectrum of this target than the Saumon & Marley model with fsed = 2, and we briefly discuss the influence of condensate clouds on T dwarf spectra.

astro-ph.SR↗

arXiv2011

The discovery of the T8.5 dwarf UGPS J0521+3640

We have carried out a search for late-type T dwarfs in the UKIDSS Galactic Plane Survey 6th Data Release. The search yielded two persuasive candidates, both of which have been confirmed as T dwarfs. The brightest, UGPS J0521+3640 has been assigned the spectral type T8.5 and appears to lie at a distance of 7-9 pc. The fainter of the two, UGPS J0652+0324, is classified as a T5.5 dwarf, and lies at an estimated distance of 28-37 pc. Warm-Spitzer observations in IRAC channels 1 and 2, taken as part of the GLIMPSE360 Legacy Survey, are available for UGPS J0521+3640 and we used these data with the near-infrared spectroscopy to estimate its properties. We find best fitting solar metallicity BT-Settl models for Teff = 600K and 650K and log g = 4.5 and 5.0. These parameters suggest a mass of between 14 and 32 MJup for an age between 1 and 5 Gyr. The proximity of this very cool T dwarf, and its location in the Galactic plane makes it an ideal candidate for high resolution adaptive optics imaging to search for cool companions.

astro-ph.SR↗

arXiv2011

Blue not brown: UKIDSS T dwarfs with suppressed K-band flux

We have used blue near-infrared colours to select a group of 12 spectroscopically-confirmed UKIDSS T dwarfs later than T4. From amongst these we identify the first two kinematic halo T-dwarf candidates. Blue near-infrared colours have been attributed to collisionally-induced hydrogen absorption, which is enhanced by either high surface gravity or low metallicity. Proper motions are measured and distances estimated, allowing the determination of tangential velocities. U and V components are estimated for our objects by assuming radial velocity = 0 km/s. From this, ULAS J0926+0835 is found to have U=62 km/s and V=-140km/s and ULAS J1319+1209 is found to have U=192 km/s and V=-92 km/s. These values are consistent with potential halo membership. However, these are not the bluest objects in our selection. The bluest is ULAS J1233+1219, with J-K=-1.16 +/- 0.07, and surprisingly this object is found to have young disc-like U and V. Our sample also contains Hip 73786B, companion to the metal-poor K5 dwarf Hip 73786. Hip 73786 is a metal-poor star, with [Fe/H]=-0.3 and is located at a distance of 19 pc. U, V, W space velocity components are calculated for Hip 73786A and B, finding that U=-48 +/- 7 km/s, V=-75 +/- 4 km/s and W=-44 +/- 8 km/s. From the properties of the primary, Hip 73786B is found to be at least 1.6 Gyr old. As a metal poor object, Hip 73786B represents an important addition to the sample of known T dwarf benchmarks.

astro-ph.SR↗