The refinement of the brown dwarf binary fraction may contribute to the understanding of the subs... more The refinement of the brown dwarf binary fraction may contribute to the understanding of the substellar formation mechanisms. Peculiar brown dwarf spectra or discrepancy between optical and near-infrared spectral type classification of brown dwarfs may indicate unresolved brown dwarf binary systems. We obtained medium-resolution spectra of 22 brown dwarfs of potential binary candidates using X-Shooter at the VLT. We aimed to select brown dwarf binary candidates. We also tested whether BT-Settl 2014 atmospheric models reproduce the physics in the atmospheres of these objects. To find different spectral type spectral binaries, we used spectral indices and we compared the selected candidates to single spectra and composition of two single spectra from libraries, to try to reproduce our X-Shooter spectra. We also created artificial binaries within the same spectral class, and we tried to find them using the same method as for brown dwarf binaries with different spectral types. We compar...
4 We present the discovery of what appears to be both a uniquely cool and old white dwarf within ... more 4 We present the discovery of what appears to be both a uniquely cool and old white dwarf within 5 30 parsecs of the sun. DES J214756.46-403529.3 is detected in four separate surveys, 50 degrees away 6 from the Galactic Plane. The combination of its very low luminosity and spectral energy distribution 7 suggests an object unlike any other astrophysical body currently known. Among 8,000 of the nearest 8 single objects in the immediate solar neighbourhood, it appears completely isolated in multiple colour9 magnitude diagrams. The data seem compatible with an extremely old and cool white dwarf with a 10 helium dominated atmosphere and a mass around 0.7-0.8 M but spectroscopic follow-up is required 11 to confirm its nature. 12
ABSTRACT We will use Spitzer/IRAC to confirm and characterise a new deep sample of very late type... more ABSTRACT We will use Spitzer/IRAC to confirm and characterise a new deep sample of very late type objects identified using an optimised method to probe the faint limits of the WISE All-Sky survey. WISE has previously revealed a new spectral class beyond the T dwarfs, with 14 of these Y dwarfs currently known. However, recently measured parallax distances have shown that this population has Teff=400-450 K, and that the observed diversity amongst the known Y dwarfs must be due to other properties (e.g. gravity and/or metallicity). The surprisingly small Teff range of the known Y dwarfs suggests that cooler objects (300-400 K) may be significantly fainter than the existing sample, with a possible cause being the (recently modelled) emergence of water clouds and a resulting decrease in mid-infrared flux due to water cloud absorption. Our new WISE search probes to W2~16.5 (~1.5 magnitudes deeper than the WISE team's search), and we are thus exploring this fainter parameter space (Mw2<16.5) out to distances of ~10pc. The increased depth (and greater volume) of our new sample has already led to the identification of 2 halo/thick-disk T dwarfs (T8 and T9) just accepted for publication, and as we follow-up candidates with redder J-W2 color we expect to confirm intrinsically fainter and/or rarer Y dwarfs. IRAC [3.6] and [4.5] photometry of a 77 strong target list will allow us to confirm and characterise (through color and proper motion) the reddest objects from our programme, as well as the most challenging objects that are undetected by our near-infrared follow-up. Spitzer offers the capability to fully exploit our new search, and thus identify the faintest objects in the WISE sky.
Using 2MASS photometry of Gaia DR2 sources, we present a technique for selecting upper main seque... more Using 2MASS photometry of Gaia DR2 sources, we present a technique for selecting upper main sequence stars and giants without the need for individual extinction estimates, to a distance of 7 kpc from the Sun. The spatial distribution of the upper main sequence stars clearly shows the nearest spiral arms, while the large-scale kinematics of the two populations perpendicular to the Galactic plane both show for the first time a clear signature of the warp of the Milky Way.
Context. Gaia Early Data Release 3 (Gaia EDR3) provides accurate astrometry for about 1.6 million... more Context. Gaia Early Data Release 3 (Gaia EDR3) provides accurate astrometry for about 1.6 million compact (QSO-like) extragalactic sources, 1.2 million of which have the best-quality five-parameter astrometric solutions. Aims. The proper motions of QSO-like sources are used to reveal a systematic pattern due to the acceleration of the solar system barycentre with respect to the rest frame of the Universe. Apart from being an important scientific result by itself, the acceleration measured in this way is a good quality indicator of the Gaia astrometric solution. Methods. The effect of the acceleration is obtained as a part of the general expansion of the vector field of proper motions in Vector Spherical Harmonics (VSH). Various versions of the VSH fit and various subsets of the sources are tried and compared to get the most consistent result and a realistic estimate of its uncertainty. Additional tests with the Gaia astrometric solution are used to get a better idea on possible syst...
We use the multi-epoch K$\rm _S$ band observations, covering a $\sim$5 years baseline to obtain m... more We use the multi-epoch K$\rm _S$ band observations, covering a $\sim$5 years baseline to obtain milli and sub-milli arcsec precision astrometry for a sample of eighteen previously known high proper motion sources, including precise parallaxes for these sources for the first time. In this study we show the capability of the VVV project to measure high precision trigonometric parallaxes for very low mass stars (VLMS) up to distances of $\sim$250\,pc reaching farther than most other ground based surveys or space missions for these types of stars. Additionally, we used spectral energy distribution to search for evidence of unresolved binary systems and cool sub-dwarfs. We detected five systems that are most likely VLMS belonging to the Galactic halo based on their tangential velocities, and four objects within 60 pc that are likely members of the thick disk. A more comprehensive study of high proper motion sources and parallaxes of VLMS and brown dwarfs with the VVV is ongoing , includi...
Locating ultra-cool companions to M dwarfs is important for constraining low-mass formation model... more Locating ultra-cool companions to M dwarfs is important for constraining low-mass formation models, the measurement of sub-stellar dynamical masses and radii, and for testing ultra-cool evolutionary models. We present an optimised method for identifying M dwarfs which may have unresolved ultra-cool companions. We construct a catalogue of 440,694 M dwarf candidates, from WISE, 2MASS and SDSS, based on optical and near-infrared colours and reduced proper motion. With strict reddening, photometric and quality constraints we isolate a subsample of 36,898 M dwarfs and search for possible mid-infrared M dwarf + ultra-cool dwarf candidates by comparing M dwarfs which have similar optical/near-infrared colours (chosen for their sensitivity to effective temperature and metallicity). We present 1,082 M dwarf + ultracool dwarf candidates for follow-up. Using simulated ultra-cool dwarf companions to M dwarfs, we estimate that the occurrence of unresolved ultra-cool companions amongst our M dwarf + ultra-cool dwarf candidates should be at least four times the average for our full M dwarf catalogue. We discuss possible contamination and bias and predict yields of candidates based on our simulations.
We present 27 new L subdwarfs and classify five of them as esdL and 22 as sdL. Our L subdwarf can... more We present 27 new L subdwarfs and classify five of them as esdL and 22 as sdL. Our L subdwarf candidates were selected with the UKIRT Infrared Deep Sky Survey and Sloan Digital Sky Survey. Spectroscopic follow-up was carried out primarily with the OSIRIS spectrograph on the Gran Telescopio Canarias. Some of these new objects were followed up with the X-shooter instrument on the Very Large Telescope. We studied the photometric properties of the population of known L subdwarfs using colourspectral type diagrams and colour-colour diagrams, by comparison with L dwarfs and main-sequence stars, and identified new colour spaces for L subdwarf selection/study in current and future surveys. We further discussed the brown dwarf transition-zone and the observational stellar/substellar boundary. We found that about one-third of 66 known L subdwarfs are substellar objects, with two-thirds being very low-mass stars. We also present the Hertzsprung-Russell diagrams, spectral type-absolute magnitude corrections, and tangential velocities of 20 known L subdwarfs observed by the Gaia astrometry satellite. One of our L subdwarf candidates, ULAS J233227.03+123452.0, is a mildly metal-poor spectroscopic binary brown dwarf: a ∼L6p dwarf and a ∼T4p dwarf. This binary is likely a thick disc member according to its kinematics.
Monthly Notices of the Royal Astronomical Society: Letters
Using Gaia DR2 astrometry, we map the kinematic signature of the Galactic stellar warp out to a d... more Using Gaia DR2 astrometry, we map the kinematic signature of the Galactic stellar warp out to a distance of 7 kpc from the Sun. Combining Gaia DR2 and 2MASS photometry, we identify, via a probabilistic approach, 599 494 upper main sequence stars and 12 616 068 giants without the need for individual extinction estimates. The spatial distribution of the upper main sequence stars clearly shows segments of the nearest spiral arms. The large-scale kinematics of both the upper main sequence and giant populations show a clear signature of the warp of the Milky Way, apparent as a gradient of 5-6 km/s in the vertical velocities from 8 to 14 kpc in Galactic radius. The presence of the signal in both samples, which have different typical ages, suggests that the warp is a gravitationally induced phenomenon.
Context. The second Gaia data release (Gaia DR2) provides precise five-parameter astrometric data... more Context. The second Gaia data release (Gaia DR2) provides precise five-parameter astrometric data (positions, proper motions, and parallaxes) for an unprecedented number of sources (more than 1.3 billion, mostly stars). This new wealth of data will enable the undertaking of statistical analysis of many astrophysical problems that were previously infeasible for lack of reliable astrometry, and in particular because of the lack of parallaxes. However, the use of this wealth of astrometric data comes with a specific challenge: how can the astrophysical parameters of interest be properly inferred from these data? Aims. The main focus of this paper, but not the only focus, is the issue of the estimation of distances from parallaxes, possibly combined with other information. We start with a critical review of the methods traditionally used to obtain distances from parallaxes and their shortcomings. Then we provide guidelines on how to use parallaxes more efficiently to estimate distances ...
Proceedings of the International Astronomical Union
We test two contemporary low-mass atmospheric models using three L dwarfs with distances and publ... more We test two contemporary low-mass atmospheric models using three L dwarfs with distances and published spectra. We find that the two models do not predict the same trends for temperature, gravity and metallicity in absorption lines. We find that one model appears to better reflect the temperature, but this sample is too small to investigate the other parameters in depth.
Proceedings of the International Astronomical Union
To make GSC-II more accurate and useful, it is necessary to re-calibrate GSC-II when a better ref... more To make GSC-II more accurate and useful, it is necessary to re-calibrate GSC-II when a better reference catalogue (UCAC2) is available. With UCAC2 as the reference, preliminary re-calibration of some sample plates from GSC2.3 were carried out with different methods, such as Global, Mask and Filter. The results indicate that a 7th-order polynomial is sufficient to account for the influence of Schmidt plate deformation on the measured coordinates of stars. The magnitude equation can be eliminated after correcting for a common magnitude equation. The RMS of the re-calibrated data is around ±0.2~0.3 arcsec.
Proceedings of the International Astronomical Union
We discuss the role of distances for understanding brown dwarfs and estimate the contribution exp... more We discuss the role of distances for understanding brown dwarfs and estimate the contribution expected by Gaia. We show that Gaia will only observe 25% of L and T dwarfs within 50pc which, at a conservative estimate, amounts to less than 400 objects. We discuss how Gaia results will nevertheless aid the ground-based programs providing reliable, bias free constraints for the calculation of parallaxes in an absolute system. We list the current ground-based programs underway and the possibilities for future all sky survey programs.
Context. The mechanism responsible for the warp of our Galaxy, as well as its dynamical nature, c... more Context. The mechanism responsible for the warp of our Galaxy, as well as its dynamical nature, continues to remain unknown. With the advent of high precision astrometry, new horizons have been opened for detecting the kinematics associated with the warp and for constraining possible warp formation scenarios for the Milky Way. Aims. The aim of this contribution is to establish whether the first Gaia data release (DR1) shows significant evidence of the kinematic signature expected from a long-lived Galactic warp in the kinematics of distant OB stars. As the first paper in a series, we present our approach for analyzing the proper motions and apply it to the subsample of Hipparcos stars. Methods. We select a sample of 989 distant spectroscopically-identified OB stars from the new reduction of Hipparcos, of which 758 are also in the first Gaia data release (DR1), covering distances from 0.5 to 3 kpc from the Sun. We develop a model of the spatial distribution and kinematics of the OB stars from which we produce the probability distribution functions of the proper motions, with and without the systematic motions expected from a long-lived warp. A likelihood analysis is used to compare the expectations of the models with the observed proper motions from both Hipparcos and Gaia DR1. Results. We find that the proper motions of the nearby OB stars are consistent with the signature of a kinematic warp, while those of the more distant stars (parallax <1 mas) are not. Conclusions. The kinematics of our sample of young OB stars suggests that systematic vertical motions in the disk cannot be explained by a simple model of a stable long-lived warp. The warp of the Milky Way may either be a transient feature, or additional phenomena are acting on the gaseous component of the Milky Way, causing systematic vertical motions that are masking the expected warp signal. A larger and deeper sample of stars with Gaia astrometry will be needed to constrain the dynamical nature of the Galactic warp.
We explore the potential of Gaia for the field of benchmark ultracool/brown dwarf companions, and... more We explore the potential of Gaia for the field of benchmark ultracool/brown dwarf companions, and present the results of an initial search for metal-rich/metal-poor systems. A simulated population of resolved ultracool dwarf companions to Gaia primary stars is generated and assessed. Of the order of ∼24 000 companions should be identifiable outside of the Galactic plane (|b| > 10 deg) with large-scale ground-and space-based surveys including late M, L, T and Y types. Our simulated companion parameter space covers 0.02 ≤ M/M ≤ 0.1, 0.1 ≤ age/Gyr ≤ 14 and −2.5 ≤ [Fe/H] ≤ 0.5, with systems required to have a false alarm probability <10 −4 , based on projected separation and expected constraints on common distance, common proper motion and/or common radial velocity. Within this bulk population, we identify smaller target subsets of rarer systems whose collective properties still span the full parameter space of the population, as well as systems containing primary stars that are good age calibrators. Our simulation analysis leads to a series of recommendations for candidate selection and observational follow-up that could identify ∼500 diverse Gaia benchmarks. As a test of the veracity of our methodology and simulations, our initial search uses UKIRT Infrared Deep Sky Survey and Sloan Digital Sky Survey to select secondaries, with the parameters of primaries taken from Tycho-2, Radial Velocity Experiment, Large sky Area Multi-Object fibre Spectroscopic Telescope and Tycho-Gaia Astrometric Solution. We identify and follow up 13 new benchmarks. These include M8-L2 companions, with metallicity constraints ranging in quality, but robust in the range −0.39 ≤ [Fe/H] ≤ +0.36, and with projected physical separation in the range 0.6 < s/kau < 76. Going forward, Gaia offers a very high yield of benchmark systems, from which diverse subsamples may be able to calibrate a range of foundational ultracool/sub-stellar theory and observation.
Monthly Notices of the Royal Astronomical Society, 2015
We present the discovery of 49 new photometrically classified T dwarfs from the combination of la... more We present the discovery of 49 new photometrically classified T dwarfs from the combination of large infrared and optical surveys combined with follow-up Telescopio Nazionale Galileo photometry. We used multiband infrared and optical photometry from the United Kingdom Infrared Telescope and Sloan Digital Sky Surveys to identify possible brown dwarf candidates, which were then confirmed using methane filter photometry. We have defined a new photometric conversion between CH 4 s − CH 4 l colour and spectral type for T4-T8 brown dwarfs based on a part of the sample that has been followed up using methane photometry and spectroscopy. Using methane differential photometry as a proxy for spectral type for T dwarfs has proved to be a very efficient technique. Of a subset of 45 methane selected brown dwarfs that were observed spectroscopically, 100 per cent were confirmed as T dwarfs. Future deep imaging surveys will produce large samples of faint brown dwarf candidates, for which spectroscopy will not be feasible. When broad wavelength coverage is unavailable, methane imaging offers a means to efficiently classify candidates from such surveys using just a pair of near-infrared images.
We discuss the two main methods adopted for estimating the correction from relative to absolute (... more We discuss the two main methods adopted for estimating the correction from relative to absolute (CRA) parallax. The error in this correction is now an unacceptably large part of the overall error budget for ground based parallax programs and for realistic situations the methods do not agree at a reasonable level. For the spectroscopic determination of the correction care must be applied in finding the luminosity class.
The refinement of the brown dwarf binary fraction may contribute to the understanding of the subs... more The refinement of the brown dwarf binary fraction may contribute to the understanding of the substellar formation mechanisms. Peculiar brown dwarf spectra or discrepancy between optical and near-infrared spectral type classification of brown dwarfs may indicate unresolved brown dwarf binary systems. We obtained medium-resolution spectra of 22 brown dwarfs of potential binary candidates using X-Shooter at the VLT. We aimed to select brown dwarf binary candidates. We also tested whether BT-Settl 2014 atmospheric models reproduce the physics in the atmospheres of these objects. To find different spectral type spectral binaries, we used spectral indices and we compared the selected candidates to single spectra and composition of two single spectra from libraries, to try to reproduce our X-Shooter spectra. We also created artificial binaries within the same spectral class, and we tried to find them using the same method as for brown dwarf binaries with different spectral types. We compar...
4 We present the discovery of what appears to be both a uniquely cool and old white dwarf within ... more 4 We present the discovery of what appears to be both a uniquely cool and old white dwarf within 5 30 parsecs of the sun. DES J214756.46-403529.3 is detected in four separate surveys, 50 degrees away 6 from the Galactic Plane. The combination of its very low luminosity and spectral energy distribution 7 suggests an object unlike any other astrophysical body currently known. Among 8,000 of the nearest 8 single objects in the immediate solar neighbourhood, it appears completely isolated in multiple colour9 magnitude diagrams. The data seem compatible with an extremely old and cool white dwarf with a 10 helium dominated atmosphere and a mass around 0.7-0.8 M but spectroscopic follow-up is required 11 to confirm its nature. 12
ABSTRACT We will use Spitzer/IRAC to confirm and characterise a new deep sample of very late type... more ABSTRACT We will use Spitzer/IRAC to confirm and characterise a new deep sample of very late type objects identified using an optimised method to probe the faint limits of the WISE All-Sky survey. WISE has previously revealed a new spectral class beyond the T dwarfs, with 14 of these Y dwarfs currently known. However, recently measured parallax distances have shown that this population has Teff=400-450 K, and that the observed diversity amongst the known Y dwarfs must be due to other properties (e.g. gravity and/or metallicity). The surprisingly small Teff range of the known Y dwarfs suggests that cooler objects (300-400 K) may be significantly fainter than the existing sample, with a possible cause being the (recently modelled) emergence of water clouds and a resulting decrease in mid-infrared flux due to water cloud absorption. Our new WISE search probes to W2~16.5 (~1.5 magnitudes deeper than the WISE team&#39;s search), and we are thus exploring this fainter parameter space (Mw2&lt;16.5) out to distances of ~10pc. The increased depth (and greater volume) of our new sample has already led to the identification of 2 halo/thick-disk T dwarfs (T8 and T9) just accepted for publication, and as we follow-up candidates with redder J-W2 color we expect to confirm intrinsically fainter and/or rarer Y dwarfs. IRAC [3.6] and [4.5] photometry of a 77 strong target list will allow us to confirm and characterise (through color and proper motion) the reddest objects from our programme, as well as the most challenging objects that are undetected by our near-infrared follow-up. Spitzer offers the capability to fully exploit our new search, and thus identify the faintest objects in the WISE sky.
Using 2MASS photometry of Gaia DR2 sources, we present a technique for selecting upper main seque... more Using 2MASS photometry of Gaia DR2 sources, we present a technique for selecting upper main sequence stars and giants without the need for individual extinction estimates, to a distance of 7 kpc from the Sun. The spatial distribution of the upper main sequence stars clearly shows the nearest spiral arms, while the large-scale kinematics of the two populations perpendicular to the Galactic plane both show for the first time a clear signature of the warp of the Milky Way.
Context. Gaia Early Data Release 3 (Gaia EDR3) provides accurate astrometry for about 1.6 million... more Context. Gaia Early Data Release 3 (Gaia EDR3) provides accurate astrometry for about 1.6 million compact (QSO-like) extragalactic sources, 1.2 million of which have the best-quality five-parameter astrometric solutions. Aims. The proper motions of QSO-like sources are used to reveal a systematic pattern due to the acceleration of the solar system barycentre with respect to the rest frame of the Universe. Apart from being an important scientific result by itself, the acceleration measured in this way is a good quality indicator of the Gaia astrometric solution. Methods. The effect of the acceleration is obtained as a part of the general expansion of the vector field of proper motions in Vector Spherical Harmonics (VSH). Various versions of the VSH fit and various subsets of the sources are tried and compared to get the most consistent result and a realistic estimate of its uncertainty. Additional tests with the Gaia astrometric solution are used to get a better idea on possible syst...
We use the multi-epoch K$\rm _S$ band observations, covering a $\sim$5 years baseline to obtain m... more We use the multi-epoch K$\rm _S$ band observations, covering a $\sim$5 years baseline to obtain milli and sub-milli arcsec precision astrometry for a sample of eighteen previously known high proper motion sources, including precise parallaxes for these sources for the first time. In this study we show the capability of the VVV project to measure high precision trigonometric parallaxes for very low mass stars (VLMS) up to distances of $\sim$250\,pc reaching farther than most other ground based surveys or space missions for these types of stars. Additionally, we used spectral energy distribution to search for evidence of unresolved binary systems and cool sub-dwarfs. We detected five systems that are most likely VLMS belonging to the Galactic halo based on their tangential velocities, and four objects within 60 pc that are likely members of the thick disk. A more comprehensive study of high proper motion sources and parallaxes of VLMS and brown dwarfs with the VVV is ongoing , includi...
Locating ultra-cool companions to M dwarfs is important for constraining low-mass formation model... more Locating ultra-cool companions to M dwarfs is important for constraining low-mass formation models, the measurement of sub-stellar dynamical masses and radii, and for testing ultra-cool evolutionary models. We present an optimised method for identifying M dwarfs which may have unresolved ultra-cool companions. We construct a catalogue of 440,694 M dwarf candidates, from WISE, 2MASS and SDSS, based on optical and near-infrared colours and reduced proper motion. With strict reddening, photometric and quality constraints we isolate a subsample of 36,898 M dwarfs and search for possible mid-infrared M dwarf + ultra-cool dwarf candidates by comparing M dwarfs which have similar optical/near-infrared colours (chosen for their sensitivity to effective temperature and metallicity). We present 1,082 M dwarf + ultracool dwarf candidates for follow-up. Using simulated ultra-cool dwarf companions to M dwarfs, we estimate that the occurrence of unresolved ultra-cool companions amongst our M dwarf + ultra-cool dwarf candidates should be at least four times the average for our full M dwarf catalogue. We discuss possible contamination and bias and predict yields of candidates based on our simulations.
We present 27 new L subdwarfs and classify five of them as esdL and 22 as sdL. Our L subdwarf can... more We present 27 new L subdwarfs and classify five of them as esdL and 22 as sdL. Our L subdwarf candidates were selected with the UKIRT Infrared Deep Sky Survey and Sloan Digital Sky Survey. Spectroscopic follow-up was carried out primarily with the OSIRIS spectrograph on the Gran Telescopio Canarias. Some of these new objects were followed up with the X-shooter instrument on the Very Large Telescope. We studied the photometric properties of the population of known L subdwarfs using colourspectral type diagrams and colour-colour diagrams, by comparison with L dwarfs and main-sequence stars, and identified new colour spaces for L subdwarf selection/study in current and future surveys. We further discussed the brown dwarf transition-zone and the observational stellar/substellar boundary. We found that about one-third of 66 known L subdwarfs are substellar objects, with two-thirds being very low-mass stars. We also present the Hertzsprung-Russell diagrams, spectral type-absolute magnitude corrections, and tangential velocities of 20 known L subdwarfs observed by the Gaia astrometry satellite. One of our L subdwarf candidates, ULAS J233227.03+123452.0, is a mildly metal-poor spectroscopic binary brown dwarf: a ∼L6p dwarf and a ∼T4p dwarf. This binary is likely a thick disc member according to its kinematics.
Monthly Notices of the Royal Astronomical Society: Letters
Using Gaia DR2 astrometry, we map the kinematic signature of the Galactic stellar warp out to a d... more Using Gaia DR2 astrometry, we map the kinematic signature of the Galactic stellar warp out to a distance of 7 kpc from the Sun. Combining Gaia DR2 and 2MASS photometry, we identify, via a probabilistic approach, 599 494 upper main sequence stars and 12 616 068 giants without the need for individual extinction estimates. The spatial distribution of the upper main sequence stars clearly shows segments of the nearest spiral arms. The large-scale kinematics of both the upper main sequence and giant populations show a clear signature of the warp of the Milky Way, apparent as a gradient of 5-6 km/s in the vertical velocities from 8 to 14 kpc in Galactic radius. The presence of the signal in both samples, which have different typical ages, suggests that the warp is a gravitationally induced phenomenon.
Context. The second Gaia data release (Gaia DR2) provides precise five-parameter astrometric data... more Context. The second Gaia data release (Gaia DR2) provides precise five-parameter astrometric data (positions, proper motions, and parallaxes) for an unprecedented number of sources (more than 1.3 billion, mostly stars). This new wealth of data will enable the undertaking of statistical analysis of many astrophysical problems that were previously infeasible for lack of reliable astrometry, and in particular because of the lack of parallaxes. However, the use of this wealth of astrometric data comes with a specific challenge: how can the astrophysical parameters of interest be properly inferred from these data? Aims. The main focus of this paper, but not the only focus, is the issue of the estimation of distances from parallaxes, possibly combined with other information. We start with a critical review of the methods traditionally used to obtain distances from parallaxes and their shortcomings. Then we provide guidelines on how to use parallaxes more efficiently to estimate distances ...
Proceedings of the International Astronomical Union
We test two contemporary low-mass atmospheric models using three L dwarfs with distances and publ... more We test two contemporary low-mass atmospheric models using three L dwarfs with distances and published spectra. We find that the two models do not predict the same trends for temperature, gravity and metallicity in absorption lines. We find that one model appears to better reflect the temperature, but this sample is too small to investigate the other parameters in depth.
Proceedings of the International Astronomical Union
To make GSC-II more accurate and useful, it is necessary to re-calibrate GSC-II when a better ref... more To make GSC-II more accurate and useful, it is necessary to re-calibrate GSC-II when a better reference catalogue (UCAC2) is available. With UCAC2 as the reference, preliminary re-calibration of some sample plates from GSC2.3 were carried out with different methods, such as Global, Mask and Filter. The results indicate that a 7th-order polynomial is sufficient to account for the influence of Schmidt plate deformation on the measured coordinates of stars. The magnitude equation can be eliminated after correcting for a common magnitude equation. The RMS of the re-calibrated data is around ±0.2~0.3 arcsec.
Proceedings of the International Astronomical Union
We discuss the role of distances for understanding brown dwarfs and estimate the contribution exp... more We discuss the role of distances for understanding brown dwarfs and estimate the contribution expected by Gaia. We show that Gaia will only observe 25% of L and T dwarfs within 50pc which, at a conservative estimate, amounts to less than 400 objects. We discuss how Gaia results will nevertheless aid the ground-based programs providing reliable, bias free constraints for the calculation of parallaxes in an absolute system. We list the current ground-based programs underway and the possibilities for future all sky survey programs.
Context. The mechanism responsible for the warp of our Galaxy, as well as its dynamical nature, c... more Context. The mechanism responsible for the warp of our Galaxy, as well as its dynamical nature, continues to remain unknown. With the advent of high precision astrometry, new horizons have been opened for detecting the kinematics associated with the warp and for constraining possible warp formation scenarios for the Milky Way. Aims. The aim of this contribution is to establish whether the first Gaia data release (DR1) shows significant evidence of the kinematic signature expected from a long-lived Galactic warp in the kinematics of distant OB stars. As the first paper in a series, we present our approach for analyzing the proper motions and apply it to the subsample of Hipparcos stars. Methods. We select a sample of 989 distant spectroscopically-identified OB stars from the new reduction of Hipparcos, of which 758 are also in the first Gaia data release (DR1), covering distances from 0.5 to 3 kpc from the Sun. We develop a model of the spatial distribution and kinematics of the OB stars from which we produce the probability distribution functions of the proper motions, with and without the systematic motions expected from a long-lived warp. A likelihood analysis is used to compare the expectations of the models with the observed proper motions from both Hipparcos and Gaia DR1. Results. We find that the proper motions of the nearby OB stars are consistent with the signature of a kinematic warp, while those of the more distant stars (parallax <1 mas) are not. Conclusions. The kinematics of our sample of young OB stars suggests that systematic vertical motions in the disk cannot be explained by a simple model of a stable long-lived warp. The warp of the Milky Way may either be a transient feature, or additional phenomena are acting on the gaseous component of the Milky Way, causing systematic vertical motions that are masking the expected warp signal. A larger and deeper sample of stars with Gaia astrometry will be needed to constrain the dynamical nature of the Galactic warp.
We explore the potential of Gaia for the field of benchmark ultracool/brown dwarf companions, and... more We explore the potential of Gaia for the field of benchmark ultracool/brown dwarf companions, and present the results of an initial search for metal-rich/metal-poor systems. A simulated population of resolved ultracool dwarf companions to Gaia primary stars is generated and assessed. Of the order of ∼24 000 companions should be identifiable outside of the Galactic plane (|b| > 10 deg) with large-scale ground-and space-based surveys including late M, L, T and Y types. Our simulated companion parameter space covers 0.02 ≤ M/M ≤ 0.1, 0.1 ≤ age/Gyr ≤ 14 and −2.5 ≤ [Fe/H] ≤ 0.5, with systems required to have a false alarm probability <10 −4 , based on projected separation and expected constraints on common distance, common proper motion and/or common radial velocity. Within this bulk population, we identify smaller target subsets of rarer systems whose collective properties still span the full parameter space of the population, as well as systems containing primary stars that are good age calibrators. Our simulation analysis leads to a series of recommendations for candidate selection and observational follow-up that could identify ∼500 diverse Gaia benchmarks. As a test of the veracity of our methodology and simulations, our initial search uses UKIRT Infrared Deep Sky Survey and Sloan Digital Sky Survey to select secondaries, with the parameters of primaries taken from Tycho-2, Radial Velocity Experiment, Large sky Area Multi-Object fibre Spectroscopic Telescope and Tycho-Gaia Astrometric Solution. We identify and follow up 13 new benchmarks. These include M8-L2 companions, with metallicity constraints ranging in quality, but robust in the range −0.39 ≤ [Fe/H] ≤ +0.36, and with projected physical separation in the range 0.6 < s/kau < 76. Going forward, Gaia offers a very high yield of benchmark systems, from which diverse subsamples may be able to calibrate a range of foundational ultracool/sub-stellar theory and observation.
Monthly Notices of the Royal Astronomical Society, 2015
We present the discovery of 49 new photometrically classified T dwarfs from the combination of la... more We present the discovery of 49 new photometrically classified T dwarfs from the combination of large infrared and optical surveys combined with follow-up Telescopio Nazionale Galileo photometry. We used multiband infrared and optical photometry from the United Kingdom Infrared Telescope and Sloan Digital Sky Surveys to identify possible brown dwarf candidates, which were then confirmed using methane filter photometry. We have defined a new photometric conversion between CH 4 s − CH 4 l colour and spectral type for T4-T8 brown dwarfs based on a part of the sample that has been followed up using methane photometry and spectroscopy. Using methane differential photometry as a proxy for spectral type for T dwarfs has proved to be a very efficient technique. Of a subset of 45 methane selected brown dwarfs that were observed spectroscopically, 100 per cent were confirmed as T dwarfs. Future deep imaging surveys will produce large samples of faint brown dwarf candidates, for which spectroscopy will not be feasible. When broad wavelength coverage is unavailable, methane imaging offers a means to efficiently classify candidates from such surveys using just a pair of near-infrared images.
We discuss the two main methods adopted for estimating the correction from relative to absolute (... more We discuss the two main methods adopted for estimating the correction from relative to absolute (CRA) parallax. The error in this correction is now an unacceptably large part of the overall error budget for ground based parallax programs and for realistic situations the methods do not agree at a reasonable level. For the spectroscopic determination of the correction care must be applied in finding the luminosity class.
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Papers by Ricky Smart