arXiv subjects

Ben Burningham

Publications and source records attributed to Ben Burningham.

At least 55 records · Page 3Linked to original sources

arXiv2015

Uniform Atmospheric Retrieval Analysis of Ultracool Dwarfs I: Characterizing Benchmarks, Gl570D and HD3651B

Interpreting the spectra of brown dwarfs is key to determining the fundamental physical and chemical processes occurring in their atmospheres. Powerful Bayesian atmospheric retrieval tools have recently been applied to both exoplanet and brown dwarf spectra to tease out the thermal structures and molecular abundances to understand those processes. In this manuscript we develop a significantly upgraded retrieval method and apply it to the SpeX spectral library data of two benchmark late T-dwarfs, Gl570D and HD3651B, to establish the validity of our upgraded forward model parameterization and Bayesian estimator. Our retrieved metallicities, gravities, and effective temperature are consistent with the metallicity and presumed ages of the systems. We add the carbon-to-oxygen ratio as a new dimension to benchmark systems and find good agreement between carbon-to-oxygens ratio derived in the brown dwarfs and the host stars. Furthermore, we have for the first time unambiguously determined the presence of ammonia in the low-resolution spectra of these two late T-dwarfs. We also show that the retrieved results are not significantly impacted by the possible presence of clouds, though some quantities are significantly impacted by uncertainties in photometry. This investigation represents a watershed study in establishing the utility of atmospheric retrieval approaches on brown dwarf spectra.

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arXiv2015

Results from the Wide-field Infrared Survey Explorer (WISE) Future Uses Session at the WISE at 5 Meeting

During the "WISE at 5: Legacy and Prospects" conference in Pasadena, CA -- which ran from February 10 - 12, 2015 -- attendees were invited to engage in an interactive session exploring the future uses of the Wide-field Infrared Survey Explorer (WISE) data. The 65 participants -- many of whom are extensive users of the data -- brainstormed the top questions still to be answered by the mission, as well as the complementary current and future datasets and additional processing of WISE/NEOWISE data that would aid in addressing these most important scientific questions. The results were mainly bifurcated between topics related to extragalactic studies (e.g. AGN, QSOs) and substellar mass objects. In summary, participants found that complementing WISE/NEOWISE data with cross-correlated multiwavelength surveys (e.g. SDSS, Pan-STARRS, LSST, Gaia, Euclid, etc.) would be highly beneficial for all future mission goals. Moreover, developing or implementing machine-learning tools to comb through and understand cross-correlated data was often mentioned for future uses. Finally, attendees agreed that additional processing of the data such as co-adding WISE and NEOWISE and extracting a multi-epoch photometric database and parallax and proper motion catalog would greatly improve the scientific results of the most important projects identified. In that respect, a project such as MaxWISE which would execute the most important additional processing and extraction as well as make the data and catalogs easily accessible via a public portal was deemed extremely important.

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arXiv2015

Methane and Ammonia in the near-infrared spectra of late T dwarfs

Analysis of T dwarfs using model atmospheres has been hampered by the absence of reliable line lists for methane and ammonia. Newly computed high temperature line lists for both of these important molecules are now available, so it is timely to investigate the appearance of the various absorption features in T dwarfs in order to better understand their atmospheres and validate the new line lists. We present high quality R~5000 Gemini/NIFS 1.0-2.4 microns spectra of the T8 standard 2MASS 0415-0935 and the T9 standard UGPS 0722-0540. We use these spectra to identify numerous methane and ammonia features not previously seen and we discuss the implications for our understanding of T dwarf atmospheres. Among our results, we find that ammonia is the dominant opacity source between ~1.233-1.266 microns in UGPS 0722-0540, and we tentatively identify several absorption features in this wavelength range in the T9's spectrum which may be due entirely to ammonia opacity. Our results also suggest that water rather than methane is the dominant opacity source in the red half of the J-band of the T8 dwarf. Water appears to be the main absorber in this wavelength region in the T9 dwarf until ~1.31 microns, when methane starts to dominate.

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arXiv2014

The discovery of a T6.5 subdwarf

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 luminosity both in specific bands and bolometrically. As a result, it is likely that its velocity is somewhat smaller, and we conclude it is a likely thick disc or halo member. This object represents the only T dwarf earlier than T8 to be classified as a subdwarf, and is a significant addition to the currently small number of known unambiguously substellar subdwarfs.

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arXiv2013

Seventy six T dwarfs from the UKIDSS LAS: benchmarks, kinematics and an updated space density

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 bolometric luminosity. We compare the colours and properties of known benchmark T dwarfs to the latest model atmospheres and draw two principal conclusions. Firstly, it appears that the H - [4.5] and J - W2 colours are more sensitive to metallicity than has previously been recognised, such that differences in metallicity may dominate over differences in Teff when considering relative properties of cool objects using these colours. Secondly, the previously noted apparent dominance of young objects in the late-T dwarf sample is no longer apparent when using the new model grids and the expanded sample of late-T dwarfs and benchmarks. This is supported by the apparently similar distribution of late-T dwarfs and earlier-type T dwarfs on reduced proper motion diagrams that we present. Finally, we present updated space densities for the late-T dwarfs, and compare our values to simulation predictions and those from WISE.

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arXiv2013

The Kinematic Age of the Coolest T Dwarfs

Surprisingly, current atmospheric models suggest that the coolest T dwarfs (T8.5 to T10) are young and very low mass (0.06-2Gyr, 5-20Mjup, Leggett et al.2009, 2010, 2012). Studies of population kinematics offer an independent constraint on the age of the population. We present kinematic data of a sample of 75 mid to late T dwarfs drawn from a variety of sources. We define our samples, T5.5 to T8 and T8.5 to T10, as mid and late T respectively. UKIDSS LAS kinematics were derived from our automated LAS proper motion pipeline and distance estimates derived from spectral types and photometry for the minority of sources that lack parallaxes. Our results show that the mid and late T populations do not have distinctly separate tangential velocity distributions to 95% probability. They also give an approximate mean kinematic age equal to that of a population with B-V colour 0.51-0.54, and a spectral type late F, which corresponds to an age of about 2 Gyr. However the median and modal ages are greater. This indicates that while model atmospheres correctly predict some trends in colour with gravity and age, reliable ages cannot yet be inferred from them. More benchmark objects are needed to anchor the models.

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arXiv2012

The kinematics of very low mass dwarfs: splinter session summary

Kinematic investigations are being increasingly deployed in studies of the lowest mass stars and brown dwarfs to investigate their origins, characterize their atmospheres, and examine the evolution of their physical parameters. This article summarizes the contributions made at the Kinematics of Very Low Mass Dwarfs Splinter Session. Results discussed include analysis of kinematic distributions of M, L and T dwarfs; theoretical tools for interpreting these distributions; identifications of very low mass halo dwarfs and wide companions to nearby stars; radial velocity variability among young and very cool brown dwarfs; and the search and identification of M dwarfs in young moving groups. A summary of discussion points at the conclusion of the Splinter is also presented.

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arXiv2012

The Properties of the 500 K Dwarf UGPS J072227.51-054031.2, and a Study of the Far-Red Flux of Cold Brown Dwarfs

We present i and z photometry for 25 T dwarfs and one L dwarf. Combined with published photometry, the data show that the i - z, z - Y and z - J colors of T dwarfs are very red, and continue to increase through to the late-type T dwarfs, with a hint of a saturation for the latest types with T_eff ~ 600 K. We present new 0.7-1.0 um and 2.8-4.2 um spectra for the very late-type T dwarf UGPS J072227.51-054031.2, as well as improved astrometry for this dwarf. Examination of the spectral energy distribution using the new and published data, with Saumon & Marley models, shows that the dwarf has T_eff = 505 +/- 10 K, a mass of 3-11 M_Jupiter and an age between 60 Myr and 1 Gyr. This young age is consistent with the thin disk kinematics of the dwarf. The mass range overlaps with that usually considered to be planetary, despite this being an unbound object discovered in the field near the Sun. This apparently young rapid rotator is also undergoing vigorous atmospheric mixing, as determined by the IRAC and WISE-2 4.5 um photometry and the Saumon & Marley models. The optical spectrum for this 500 K object shows clearly detected lines of the neutral alkalis Cs and Rb, which are emitted from deep atmospheric layers with temperatures of 900-1200 K.

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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.

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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.

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arXiv2010

Spitzer Mid-Infrared Photometry of 500 - 750 K Brown Dwarfs

Mid-infrared data, including Spitzer warm-IRAC [3.6] and [4.5] photometry, is critical for understanding the cold population of brown dwarfs now being found, objects which have more in common with planets than stars. As effective temperature (T_eff) drops from 800 K to 400 K, the fraction of flux emitted beyond 3 microns increases rapidly, from about 40% to >75%. This rapid increase makes a color like H-[4.5] a very sensitive temperature indicator, and it can be combined with a gravity- and metallicity-sensitive color like H-K to constrain all three of these fundamental properties, which in turn gives us mass and age for these slowly cooling objects. Determination of mid-infrared color trends also allows better exploitation of the WISE mission by the community. We use new Spitzer Cycle 6 IRAC photometry, together with published data, to present trends of color with type for L0 to T10 dwarfs. We also use the atmospheric and evolutionary models of Saumon & Marley to investigate the masses and ages of 13 very late-type T dwarfs, which have H-[4.5] > 3.2 and T_eff ~ 500 K to 750 K.

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arXiv2010

The discovery of a very cool, very nearby brown dwarf in the Galactic plane

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 the coolest brown dwarf presently known -- the only known T dwarf that is redder in H-[4.5] is the peculiar T7.5 dwarf SDSS J1416+13B, which is thought to be warmer and more luminous than UGPS 0722-05. Our measurement of the luminosity, aided by Gemini/T-ReCS N band photometry, is L = 9.2 +/- 3.1x10^{-7} Lsun. Using a comparison with well studied T8.5 and T9 dwarfs we deduce Teff=520 +/- 40 K. This is supported by predictions of the Saumon & Marley models. With apparent magnitude J=16.52, UGPS 0722-05 is the brightest T dwarf discovered by UKIDSS so far. It offers opportunities for future study via high resolution near-infrared spectroscopy and spectroscopy in the thermal infrared.

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arXiv2010

Properties of the T8.5 Dwarf Wolf 940 B

We present 7.5-14.2um low-resolution spectroscopy, obtained with the Spitzer Infrared Spectrograph, of the T8.5 dwarf Wolf 940 B, which is a companion to an M4 dwarf with a projected separation of 400 AU. We combine these data with previously published near-infrared spectroscopy and mid-infrared photometry, to produce the spectral energy distribution for the very low-temperature T dwarf. We use atmospheric models to derive the bolometric correction and obtain a luminosity of log L/Lsun = -6.01 +/- 0.05. Evolutionary models are used with the luminosity to constrain the values of effective temperature (T_eff) and surface gravity, and hence mass and age for the T dwarf. We further restrict the allowed range of T_eff and gravity using age constraints implied by the M dwarf primary, and refine the physical properties of the T dwarf by comparison of the observed and modelled spectroscopy and photometry. This comparison indicates that Wolf 940 B has a metallicity within 0.2 dex of solar, as more extreme values give poor fits to the data - lower metallicity produces a poor fit at lambda > 2um while higher metallicity produces a poor fit at lambda < 2um. This is consistent with the independently derived value of [m/H] = +0.24 +/- 0.09 for the primary star, using the Johnson & Apps (2008) M_K:V-K relationship. We find that the T dwarf atmosphere is undergoing vigorous mixing, with an eddy diffusion coefficient K_zz of 10^4 to 10^6 cm^2 s^-1. We derive an effective temperature of 585 K to 625 K, and surface gravity log g = 4.83 to 5.22 (cm s^-2), for an age range of 3 Gyr to 10 Gyr, as implied by the kinematic and H alpha properties of the M dwarf primary. The lower gravity corresponds to the lower temperature and younger age for the system, and the higher value to the higher temperature and older age. The mass of the T dwarf is 24 M_Jupiter to 45 M_Jupiter for the younger to older age limit.

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arXiv2010

Forty seven new T dwarfs from the UKIDSS Large Area Survey

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 a sole explanation for both types of peculiarity, and suggest that they may represent hitherto unrecognised tracers of composition and/or gravity. Clear trends in z'(AB)-J and Y-J are apparent for our sample, consistent with weakening absorption in the red wing of the KI line at 0.77microns with decreasing effective temperature. We have used our sample to estimate space densities for T6-T9 dwarfs. By comparing our sample to Monte-Carlo simulations of field T dwarfs for various mass functions of the form phi(M) \propto M^-alpha, we have placed weak constraints on the form of the field mass function. Our analysis suggests that the substellar mass function is declining at lower masses, with negative values of alpha preferred. This is at odds with results for young clusters that have been generally found to have alpha > 0.

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arXiv2010

The discovery of a very cool binary system

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 for classifying T dwarfs. UKIDSS and follow-up UKIRT/WFCAM photometry shows the T dwarf to have the bluest near infrared colours yet seen for such an object with H-K = -1.31+/-0.17. Warm Spitzer IRAC photometry shows the T dwarf to have extremely red H-[4.5] = 4.86+/-0.04, which is the reddest yet seen for a substellar object. The lack of parallax measurement for the pair limits our ability to estimate parameters for the system. However, applying a conservative distance estimate of 5-15 pc suggests a projected separation in range 45-135 AU. By comparing H-K:H-[4.5] colours of the T dwarf to spectral models we estimate that Teff = 500 K and [M/H]~-0.30, with log g ~ 5.0. This suggests a mass of ~30 MJupiter for the T dwarf and an age of ~10 Gyr for the system. The primary would then be a 75MJupiter object with log g ~ 5.5 and a relatively dust-free Teff ~ 1500K atmosphere. Given the unusual properties of the system we caution that these estimates are uncertain. We eagerly await parallax measurements and high-resolution imaging which will constrain the parameters further.

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arXiv2010

Mid-Infrared Photometry of Cold Brown Dwarfs: Diversity in Age, Mass and Metallicity

We present Spitzer IRAC photometry of twelve very late-type T dwarfs; nine have [3.6], [4.5], [5.8] and [8.0] photometry and three have [3.6] and [4.5] photometry only. We investigate trends with type and color for the planning and interpretation of surveys for the coldest T or Y dwarfs. Brown dwarfs with effective temperature (T_eff) below 700K emit more than half their flux at wavelengths beyond 3um, and the ratio of the mid-infrared flux to the near-infrared flux becomes very sensitive to T_eff. The color H-[4.5] is a good indicator of T_eff with a relatively weak dependence on metallicity and gravity. Conversely, the colors H-K and [4.5]-[5.8] are sensitive to metallicity and gravity. Thus near- and mid-infrared photometry provide useful indicators of the fundamental properties of brown dwarfs, and if temperature and gravity are known, then mass and age can be determined from evolutionary models. There are twelve dwarfs known with H-[4.5]>3.0, and 500< T_eff K < 800, which we examine in detail. The ages of these dwarfs range from very young (0.1-1.0 Gyr) to relatively old (3-12 Gyr). The mass range is possibly as low as 5 Jupiter masses to up to 70 Jupiter masses. The metallicities also span a large range, from [m/H] -0.3 to [m/H] +0.3. The small number of T8-T9 dwarfs found in the UKIRT Infrared Deep Sky Survey to date appear to be mostly young low-mass dwarfs. Extensions to the mid-infrared space missions warm-Spitzer and WISE are desirable in order to obtain the vital mid-infrared data for cold brown dwarfs, and to discover more of these rare objects. (Abridged.)

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arXiv2009

The discovery of an M4+T8.5 binary system

We report the discovery of a T8.5 dwarf, which is a companion to the M4 dwarf Wolf 940. At a distance of 12.50 (+0.75,-0.67) pc, the angular separation of 32arcsec corresponds to a projected separation of 400 AU. The M4 primary displays no Halpha emission, and we apply the age-activity relations of West et al. to place a lower limit on the age of the system of 3.5 Gyr. Weak Halpha absorption suggests some residual activity and we estimate an upper age limit of 6 Gyr. We apply the relations of Bonfils et al for V-Ks and M_Ks to determine the metallicity, [Fe/H] = -0.06 +/- 0.20 for Wolf~940A, and by extension the T8.5 secondary, Wolf 940B. We have obtained JHK NIRI spectroscopy and JHKL' photometry of Wolf 940B, and use these data, in combination with theoretical extensions, to determine its bolometric flux, Fbol = 1.75 +/- 0.18 E-16 Wm^-2 and thus its luminosity log(L*/Lsun) = -6.07 +/- 0.04. Using the age constraints for the system, and evolutionary structural models of Baraffe et al. we determine Teff = 570 +/- 25K and log g = 4.75-5.00 for Wolf940B, based on its bolometric luminosity. This represents the first determination of these properties for a T8+ dwarf that does not rely on the fitting of T-dwarf spectral models. This object represents the first system containing a T8+ dwarf for which fiducial constraints on its properties are available, and we compare its spectra with those of the latest very cool BT-Settl models. This clearly demonstrates that the use of the (W_J,K/J) spectral ratios (used previously to constrain Teff and log g) would have over-estimated Teff by ~100K.

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arXiv2008

Exploring the substellar temperature regime down to ~550K

We report the discovery of three very late T dwarfs in the UKIRT Infrared Deep Sky Survey (UKIDSS) Third Data Release: ULAS J101721.40+011817.9 (ULAS1017), ULAS J123828.51+095351.3 (ULAS1238) and ULAS J133553.45+113005.2 (ULAS1335).We detail optical and near-infrared photometry for all three sources, and mid-infrared photometry for ULAS1335. We use near-infrared spectra of each source to assign spectral types T8p (ULAS1017), T8.5 (ULAS1228) and T9 (ULAS1335) to these objects. We estimate that ULAS1017 has 750 < Teff < 850K, and 5.0 < log g < 5.5, assuming solar metallicity, an age of 1.6-15 Gyr, a mass of 33-70 MJ and lies at a distance of 31-54 pc. We extend the unified scheme of Burgasser et al. (2006) to the type T9 and suggest the inclusion of the WJ index to replace the now saturated J-band indices. ULAS1335 is the same spectral type as ULAS J003402.77-005206.7 and CFBDS J005910.90-011401.3. Comparison of model spectra with that of ULAS1335 suggest a temperature below 600K. We find ULAS1335 to be extremely red in near to mid-infrared colours, with H-[4.49]=4.34+/-0.04. This is the reddest near to mid-infrared colour yet observed for a T dwarf, which supports Teff < 600K, and we estimate Teff ~550-600K for ULAS1335. We estimate that ULAS1335 has an age of 0.6-5.3 Gyr, a mass of 15-31 MJ and lies at a distance of 8-12 pc.

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