David James | National Optical Astronomy Observatory (original) (raw)
Papers by David James
WASP0 is a prototype for what is intended to become a collection of WASPs whose primary aim is to... more WASP0 is a prototype for what is intended to become a collection of WASPs whose primary aim is to detect transiting extra-solar planets across the face of their parent star. The WASP0 instrument is a wide-field (9-degree) 6.3cm aperture F/2.8 Apogee 10 CCD camera (2Kx2K chip, 16-arcsec pixels). The camera is mounted piggy-back on a commercial 10-inch Meade telescope. We
ABSTRACT WASP0 is a prototype for what is intended to become a collection of WASPs whose primary ... more ABSTRACT WASP0 is a prototype for what is intended to become a collection of WASPs whose primary aim is to detect transiting extra-solar planets across the face of their parent star. The WASP0 instrument is a wide-field (9-degree) 6.3cm aperture F/2.8 Apogee 10 CCD camera (2Kx2K chip, 16-arcsec pixels). The camera is mounted piggy-back on a commercial 10-inch Meade telescope. We present some recent results from the WASP camera, including observations from La Palma of the known transiting planet around HD 209458 and preliminary analysis of other stars located in the same field. We also outline further problems which restrict the ability to achieve photon limited precision with a wide-field commercial CCD.
Two of the most important predictions from standard stellar evolution theory about the lithium de... more Two of the most important predictions from standard stellar evolution theory about the lithium depletion of solar-type F and G dwarfs are that it occurs primarily during the pre-main sequence and that it depends on metallicity. Abundant evidence from star clusters shows that Li depletion does indeed occur during the pre-main sequence, but then also continues during the main sequence,
We have identified a well-populated class of young stellar objects (YSOs) in the Orion Nebula clu... more We have identified a well-populated class of young stellar objects (YSOs) in the Orion Nebula cluster whose light curves show narrow (few day timescale) flux dips. Based on comparison to the only previously well-documented member of this class (AA Tau), we believe these flux dips are due to 'clouds' in the inner circumstellar disk of these stars that pass through
The Astrophysical Journal, 1995
The Astrophysical Journal, 2011
We present initial results from time-series imaging at infrared wavelengths of 0.9 deg 2 in the O... more We present initial results from time-series imaging at infrared wavelengths of 0.9 deg 2 in the Orion Nebula Cluster (ONC). During Fall 2009 we obtained 81 epochs of Spitzer 3.6 and 4.5 μm data over 40 consecutive days. We extracted light curves with ∼3% photometric accuracy for ∼2000 ONC members ranging from several solar masses down to well below the hydrogen-burning mass limit. For many of the stars, we also have time-series photometry obtained at optical (I c ) and/or near-infrared (JK s ) wavelengths. Our data set can be mined to determine stellar rotation periods, identify new pre-main-sequence eclipsing binaries, search for new substellar Orion members, and help better determine the frequency of circumstellar disks as a function of stellar mass in the ONC. Our primary focus is the unique ability of 3.6 and 4.5 μm variability information to improve our understanding of inner disk processes and structure in the Class I and II young stellar objects (YSOs). In this paper, we provide a brief overview of the YSOVAR Orion data obtained in Fall 2009 and highlight our light curves for AA-Tau analogs-YSOs with narrow dips in flux, most probably due to disk density structures passing through our line of sight. Detailed follow-up observations are needed in order to better quantify the nature of the obscuring bodies and what this implies for the structure of the inner disks of YSOs.
Monthly Notices of the Royal Astronomical Society: Letters, 2005
We use Doppler imaging techniques to determine the dependence of starspot rotation rates on latit... more We use Doppler imaging techniques to determine the dependence of starspot rotation rates on latitude in an homogeneous sample of young, rapidly-rotating solar analogues. A solar-like differential rotation law is used, where the rotation depends on sin 2 (θ), where θ is the stellar latitude. By including this term in the image reconstruction process, using starspots as tracers, we are able to determine the magnitude of the shear over more than one rotation cycle. We also consider results from matched filter starspot tracking techniques, where individual starspot rotation rates are determined. In addition we have re-analysed published results and present a new measurement for the K3 dwarf, Speedy Mic.
Monthly Notices of the Royal Astronomical Society, 2000
A rotation rate and X-ray luminosity analysis is presented for rapidly rotating single and binary... more A rotation rate and X-ray luminosity analysis is presented for rapidly rotating single and binary M-dwarf systems. X-ray luminosities for the majority of both single & binary M-dwarf systems with periods below ≃ 5 − 6 days (equatorial velocities, V eq ∼ > 6 km s −1 ) are consistent with the current rotation-activity paradigm, and appear to saturate at about 10 −3 of the stellar bolometric luminosity.
Monthly Notices of the Royal Astronomical Society, 2005
We present results from 30 nights of observations of the open cluster NGC 7789 with the WFC camer... more We present results from 30 nights of observations of the open cluster NGC 7789 with the WFC camera on the INT telescope in La Palma. From ∼900 epochs, we obtained lightcurves and Sloan r ′ − i ′ colours for ∼33000 stars, with ∼2400 stars with better than 1% precision. We expected to detect ∼2 transiting hot Jupiter planets if 1% of stars host such a companion and that a typical hot Jupiter radius is ∼ 1.2R J . We find 24 transit candidates, 14 of which we can assign a period. We rule out the transiting planet model for 21 of these candidates using various robust arguments. For 2 candidates we are unable to decide on their nature, although it seems most likely that they are eclipsing binaries as well. We have one candidate exhibiting a single eclipse for which we derive a radius of 1.81 +0.09 −0.00 R J . Three candidates remain that require follow-up observations in order to determine their nature.
Monthly Notices of the Royal Astronomical Society, 1996
Monthly Notices of the Royal Astronomical Society, 2000
We present Doppler images of the young, late-type star PZ Tel from spectroscopic observations at ... more We present Doppler images of the young, late-type star PZ Tel from spectroscopic observations at two epochs. The 0.95-d period necessitated dual-site observations in order to obtain full phase coverage. The resulting maximum entropy reconstructions based on leastsquares deconvolved profiles derived from ,2800 photospheric absorption lines reveal the presence of starspots at all latitudes. By cross-correlation of constant-latitude strips from the images, we derive a measure of rotation as a function of latitude. The rate of surface shear is found to be in close agreement with the solar value. Ha absorption transients reveal several prominences to be present at the time of observation. We also examine further evidence for the young evolutionary status of PZ Tel through radial velocity, and lithium abundance.
Monthly Notices of the Royal Astronomical Society, 2005
The Wide Angle Search for Planets prototype (WASP0) is a wide-field instrument used to search for... more The Wide Angle Search for Planets prototype (WASP0) is a wide-field instrument used to search for extra-solar planets via the transit method. Here we present the results of a monitoring program which targeted a 9-degree field in Draco. WASP0 monitored 35000 field stars for two consecutive months. Analysis of the lightcurves resulted in the detection of 11 multi-transit candidates and 3 single-transit candidates, two of which we recommend for further follow-up. Monte-Carlo simulations matching the observing parameters estimate the expected number of transit candidates from this survey. A comparison of the expected number with the number of candidates detected is used to discuss limits on planetary companions to field stars.
Monthly Notices of the Royal Astronomical Society, 2004
We undertook a multisite photometric campaign for the β Cephei star ν Eridani. More than 600 hour... more We undertook a multisite photometric campaign for the β Cephei star ν Eridani. More than 600 hours of differential photoelectric uvyV photometry were obtained with 11 telescopes during 148 clear nights.
Monthly Notices of the Royal Astronomical Society, 2002
P Dedicated to the memory of A. W. J. Cousins, discoverer of the variability of g Doradus.
Monthly Notices of the Royal Astronomical Society, 2003
We have used a doppler tomographic analysis to conduct a deep search for the starlight reflected ... more We have used a doppler tomographic analysis to conduct a deep search for the starlight reflected from the planetary companion to HD 75289. In 4 nights on VLT2/UVES in January 2003, we obtained 684 high resolutionéchelle spectra with a total integration time of 26 hours. We establish an upper limit on the planet's geometric albedo p < 0.12 (to the 99.9% significance level) at the most probable orbital inclination i ≃ 60 • , assuming a grey albedo, a Venus-like phase function and a planetary radius R p = 1.6R Jup . We are able to rule out some combinations of the predicted planetary radius and atmospheric albedo models with high, reflective cloud decks.
Using the Blanco 4m+HYDRA, we propose to obtain intermediate resolution spectra of an optically s... more Using the Blanco 4m+HYDRA, we propose to obtain intermediate resolution spectra of an optically selected sample of early-G to mid-M photometric members of the young (220 Myr) open cluster NGC 6475, which lies mid-way in age between the Hyades and Pleiades. These spectra will enable us to measure radial velocities and Li equivalent widths (for abundances). Because NGC 6475 lies in the Galactic plane (b=-4.5^o), we estimate that only 30-40 % of the photometric members are bona fide cluster members, the remainder being contaminating field stars. It is essential to obtain a further membership criterion (radial velocities) to establish a solid cluster membership list of genuine members. Furthermore, we will measure lithium equivalent widths (and determine abundances) for the bona fide cluster members. These abundances will allow us to trace the evolution of lithium depletion during the main sequence at intermediate ages, and allow us to examine whether the spread in Pleiades K-star Li ab...
IAU Colloq. 185: …, 2002
... but this explanation also has its weaknesses, as the star seems chemically normal, has normal... more ... but this explanation also has its weaknesses, as the star seems chemically normal, has normal space motions, and is not an X-Ray source. ... MNRAS, 309, L19 Handler, G. &amp; Shobbrook, RR 2002, MNRAS, in preparation Koen, C., Van Rooyen, R., Van Wyk, F., &amp; Marang, F. 1999 ...
Arxiv preprint astro-ph/ …, 2001
We report the discovery of both intermediate-order gravity mode and low-order pressure mode pulsa... more We report the discovery of both intermediate-order gravity mode and low-order pressure mode pulsation in the same star, HD 209295. It is therefore both a γ Doradus and a δ Scuti star, which makes it the first confirmed member of two classes of pulsating star. This object is located in a close binary system with an unknown, but likely degenerate companion in an eccentric orbit, and some of the γ Doradus pulsation frequencies are exact integer multiples of the orbital frequency. We suggest that these pulsations are tidally excited. HD 209295 may be the progenitor of an intermediate-mass X-Ray binary.
Astronomy and Astrophysics, 2006
We present a study of the young (30-100 Myr) open cluster IC 4665 with the aim to determine the s... more We present a study of the young (30-100 Myr) open cluster IC 4665 with the aim to determine the shape of the mass function well into the brown dwarf regime. We photometrically select 691 low-mass stellar and 94 brown dwarf candidate members over an area of 3.82 square degrees centred on the cluster. K-band follow-up photometry and Two-Micron All-Sky Survey data allow a first filtering of contaminant objects from our catalogues. A second filtering is performed for the brightest stars using proper motion data provided by the Tycho-2 and UCAC2 public catalogues. Contamination by the field population for the lowest mass objects is estimated using same latitude control fields. We fit observed surface densities of various cluster populations with King profiles and find a consistent tidal radius of 1.0 • . The presence of possible mass segregation is discussed. In most respects investigated, IC 4665 is similar to other young open clusters at this age: (1) a power law fit to the mass function between 1 and 0.04 M ⊙ results in best fit for a slope of −0.6; (2) a cusp in the mass function is noticed at about the substellar boundary with respect to the power law description, the interpretation of which is discussed; (3) a fraction between 10-19% for BDs with M ∼ > 0.03 M ⊙ to total members; (4) a best-fit lognormal function to the full mass distribution shows an average member mass of 0.32 M ⊙ , if IC 4665 has an age of 50 Myr.
Astronomy and Astrophysics, 2008
Aims. We conducted a search for brown dwarfs (BDs) and very low mass (VLM) stars in the 625 Myr-o... more Aims. We conducted a search for brown dwarfs (BDs) and very low mass (VLM) stars in the 625 Myr-old Hyades cluster in order to derive the cluster's mass function across the stellar-substellar boundary. Methods. We performed a deep (I=23, z=22.5) photometric survey over 16 deg 2 around the cluster center, followed up with K-band photometry to measure the proper motion of candidate members, and optical and near-IR spectroscopy of probable BD and VLM members. Results. We report the discovery of the first 2 brown dwarfs in the Hyades cluster. The 2 objects have a spectral type early-T and their optical and near-IR photometry as well as their proper motion are consistent with them being cluster members. According to models, their mass is 50 Jupiter masses at an age of 625 Myr. We also report the discovery of 3 new very low mass stellar members of the cluster, and confirm the membership of 16 others. We combine these results with a list of previously known cluster members to build the present-day mass function (PDMF) of the Hyades cluster from 50 Jupiter masses to 3 M ⊙ . We find the Hyades PDMF to be strongly deficient in very low mass objects and brown dwarfs compared to the IMF of younger open clusters such as the Pleiades. We interpret this deficiency as the result of dynamical evolution over the past few 100 Myr, i.e., the preferential evaporation of low mass cluster members due to weak gravitational encounters. Conclusions. We thus estimate that the Hyades cluster currently hosts about 10-15 brown dwarfs, while its initial substellar population may have amounted up to 150-200 members.
WASP0 is a prototype for what is intended to become a collection of WASPs whose primary aim is to... more WASP0 is a prototype for what is intended to become a collection of WASPs whose primary aim is to detect transiting extra-solar planets across the face of their parent star. The WASP0 instrument is a wide-field (9-degree) 6.3cm aperture F/2.8 Apogee 10 CCD camera (2Kx2K chip, 16-arcsec pixels). The camera is mounted piggy-back on a commercial 10-inch Meade telescope. We
ABSTRACT WASP0 is a prototype for what is intended to become a collection of WASPs whose primary ... more ABSTRACT WASP0 is a prototype for what is intended to become a collection of WASPs whose primary aim is to detect transiting extra-solar planets across the face of their parent star. The WASP0 instrument is a wide-field (9-degree) 6.3cm aperture F/2.8 Apogee 10 CCD camera (2Kx2K chip, 16-arcsec pixels). The camera is mounted piggy-back on a commercial 10-inch Meade telescope. We present some recent results from the WASP camera, including observations from La Palma of the known transiting planet around HD 209458 and preliminary analysis of other stars located in the same field. We also outline further problems which restrict the ability to achieve photon limited precision with a wide-field commercial CCD.
Two of the most important predictions from standard stellar evolution theory about the lithium de... more Two of the most important predictions from standard stellar evolution theory about the lithium depletion of solar-type F and G dwarfs are that it occurs primarily during the pre-main sequence and that it depends on metallicity. Abundant evidence from star clusters shows that Li depletion does indeed occur during the pre-main sequence, but then also continues during the main sequence,
We have identified a well-populated class of young stellar objects (YSOs) in the Orion Nebula clu... more We have identified a well-populated class of young stellar objects (YSOs) in the Orion Nebula cluster whose light curves show narrow (few day timescale) flux dips. Based on comparison to the only previously well-documented member of this class (AA Tau), we believe these flux dips are due to 'clouds' in the inner circumstellar disk of these stars that pass through
The Astrophysical Journal, 1995
The Astrophysical Journal, 2011
We present initial results from time-series imaging at infrared wavelengths of 0.9 deg 2 in the O... more We present initial results from time-series imaging at infrared wavelengths of 0.9 deg 2 in the Orion Nebula Cluster (ONC). During Fall 2009 we obtained 81 epochs of Spitzer 3.6 and 4.5 μm data over 40 consecutive days. We extracted light curves with ∼3% photometric accuracy for ∼2000 ONC members ranging from several solar masses down to well below the hydrogen-burning mass limit. For many of the stars, we also have time-series photometry obtained at optical (I c ) and/or near-infrared (JK s ) wavelengths. Our data set can be mined to determine stellar rotation periods, identify new pre-main-sequence eclipsing binaries, search for new substellar Orion members, and help better determine the frequency of circumstellar disks as a function of stellar mass in the ONC. Our primary focus is the unique ability of 3.6 and 4.5 μm variability information to improve our understanding of inner disk processes and structure in the Class I and II young stellar objects (YSOs). In this paper, we provide a brief overview of the YSOVAR Orion data obtained in Fall 2009 and highlight our light curves for AA-Tau analogs-YSOs with narrow dips in flux, most probably due to disk density structures passing through our line of sight. Detailed follow-up observations are needed in order to better quantify the nature of the obscuring bodies and what this implies for the structure of the inner disks of YSOs.
Monthly Notices of the Royal Astronomical Society: Letters, 2005
We use Doppler imaging techniques to determine the dependence of starspot rotation rates on latit... more We use Doppler imaging techniques to determine the dependence of starspot rotation rates on latitude in an homogeneous sample of young, rapidly-rotating solar analogues. A solar-like differential rotation law is used, where the rotation depends on sin 2 (θ), where θ is the stellar latitude. By including this term in the image reconstruction process, using starspots as tracers, we are able to determine the magnitude of the shear over more than one rotation cycle. We also consider results from matched filter starspot tracking techniques, where individual starspot rotation rates are determined. In addition we have re-analysed published results and present a new measurement for the K3 dwarf, Speedy Mic.
Monthly Notices of the Royal Astronomical Society, 2000
A rotation rate and X-ray luminosity analysis is presented for rapidly rotating single and binary... more A rotation rate and X-ray luminosity analysis is presented for rapidly rotating single and binary M-dwarf systems. X-ray luminosities for the majority of both single & binary M-dwarf systems with periods below ≃ 5 − 6 days (equatorial velocities, V eq ∼ > 6 km s −1 ) are consistent with the current rotation-activity paradigm, and appear to saturate at about 10 −3 of the stellar bolometric luminosity.
Monthly Notices of the Royal Astronomical Society, 2005
We present results from 30 nights of observations of the open cluster NGC 7789 with the WFC camer... more We present results from 30 nights of observations of the open cluster NGC 7789 with the WFC camera on the INT telescope in La Palma. From ∼900 epochs, we obtained lightcurves and Sloan r ′ − i ′ colours for ∼33000 stars, with ∼2400 stars with better than 1% precision. We expected to detect ∼2 transiting hot Jupiter planets if 1% of stars host such a companion and that a typical hot Jupiter radius is ∼ 1.2R J . We find 24 transit candidates, 14 of which we can assign a period. We rule out the transiting planet model for 21 of these candidates using various robust arguments. For 2 candidates we are unable to decide on their nature, although it seems most likely that they are eclipsing binaries as well. We have one candidate exhibiting a single eclipse for which we derive a radius of 1.81 +0.09 −0.00 R J . Three candidates remain that require follow-up observations in order to determine their nature.
Monthly Notices of the Royal Astronomical Society, 1996
Monthly Notices of the Royal Astronomical Society, 2000
We present Doppler images of the young, late-type star PZ Tel from spectroscopic observations at ... more We present Doppler images of the young, late-type star PZ Tel from spectroscopic observations at two epochs. The 0.95-d period necessitated dual-site observations in order to obtain full phase coverage. The resulting maximum entropy reconstructions based on leastsquares deconvolved profiles derived from ,2800 photospheric absorption lines reveal the presence of starspots at all latitudes. By cross-correlation of constant-latitude strips from the images, we derive a measure of rotation as a function of latitude. The rate of surface shear is found to be in close agreement with the solar value. Ha absorption transients reveal several prominences to be present at the time of observation. We also examine further evidence for the young evolutionary status of PZ Tel through radial velocity, and lithium abundance.
Monthly Notices of the Royal Astronomical Society, 2005
The Wide Angle Search for Planets prototype (WASP0) is a wide-field instrument used to search for... more The Wide Angle Search for Planets prototype (WASP0) is a wide-field instrument used to search for extra-solar planets via the transit method. Here we present the results of a monitoring program which targeted a 9-degree field in Draco. WASP0 monitored 35000 field stars for two consecutive months. Analysis of the lightcurves resulted in the detection of 11 multi-transit candidates and 3 single-transit candidates, two of which we recommend for further follow-up. Monte-Carlo simulations matching the observing parameters estimate the expected number of transit candidates from this survey. A comparison of the expected number with the number of candidates detected is used to discuss limits on planetary companions to field stars.
Monthly Notices of the Royal Astronomical Society, 2004
We undertook a multisite photometric campaign for the β Cephei star ν Eridani. More than 600 hour... more We undertook a multisite photometric campaign for the β Cephei star ν Eridani. More than 600 hours of differential photoelectric uvyV photometry were obtained with 11 telescopes during 148 clear nights.
Monthly Notices of the Royal Astronomical Society, 2002
P Dedicated to the memory of A. W. J. Cousins, discoverer of the variability of g Doradus.
Monthly Notices of the Royal Astronomical Society, 2003
We have used a doppler tomographic analysis to conduct a deep search for the starlight reflected ... more We have used a doppler tomographic analysis to conduct a deep search for the starlight reflected from the planetary companion to HD 75289. In 4 nights on VLT2/UVES in January 2003, we obtained 684 high resolutionéchelle spectra with a total integration time of 26 hours. We establish an upper limit on the planet's geometric albedo p < 0.12 (to the 99.9% significance level) at the most probable orbital inclination i ≃ 60 • , assuming a grey albedo, a Venus-like phase function and a planetary radius R p = 1.6R Jup . We are able to rule out some combinations of the predicted planetary radius and atmospheric albedo models with high, reflective cloud decks.
Using the Blanco 4m+HYDRA, we propose to obtain intermediate resolution spectra of an optically s... more Using the Blanco 4m+HYDRA, we propose to obtain intermediate resolution spectra of an optically selected sample of early-G to mid-M photometric members of the young (220 Myr) open cluster NGC 6475, which lies mid-way in age between the Hyades and Pleiades. These spectra will enable us to measure radial velocities and Li equivalent widths (for abundances). Because NGC 6475 lies in the Galactic plane (b=-4.5^o), we estimate that only 30-40 % of the photometric members are bona fide cluster members, the remainder being contaminating field stars. It is essential to obtain a further membership criterion (radial velocities) to establish a solid cluster membership list of genuine members. Furthermore, we will measure lithium equivalent widths (and determine abundances) for the bona fide cluster members. These abundances will allow us to trace the evolution of lithium depletion during the main sequence at intermediate ages, and allow us to examine whether the spread in Pleiades K-star Li ab...
IAU Colloq. 185: …, 2002
... but this explanation also has its weaknesses, as the star seems chemically normal, has normal... more ... but this explanation also has its weaknesses, as the star seems chemically normal, has normal space motions, and is not an X-Ray source. ... MNRAS, 309, L19 Handler, G. &amp; Shobbrook, RR 2002, MNRAS, in preparation Koen, C., Van Rooyen, R., Van Wyk, F., &amp; Marang, F. 1999 ...
Arxiv preprint astro-ph/ …, 2001
We report the discovery of both intermediate-order gravity mode and low-order pressure mode pulsa... more We report the discovery of both intermediate-order gravity mode and low-order pressure mode pulsation in the same star, HD 209295. It is therefore both a γ Doradus and a δ Scuti star, which makes it the first confirmed member of two classes of pulsating star. This object is located in a close binary system with an unknown, but likely degenerate companion in an eccentric orbit, and some of the γ Doradus pulsation frequencies are exact integer multiples of the orbital frequency. We suggest that these pulsations are tidally excited. HD 209295 may be the progenitor of an intermediate-mass X-Ray binary.
Astronomy and Astrophysics, 2006
We present a study of the young (30-100 Myr) open cluster IC 4665 with the aim to determine the s... more We present a study of the young (30-100 Myr) open cluster IC 4665 with the aim to determine the shape of the mass function well into the brown dwarf regime. We photometrically select 691 low-mass stellar and 94 brown dwarf candidate members over an area of 3.82 square degrees centred on the cluster. K-band follow-up photometry and Two-Micron All-Sky Survey data allow a first filtering of contaminant objects from our catalogues. A second filtering is performed for the brightest stars using proper motion data provided by the Tycho-2 and UCAC2 public catalogues. Contamination by the field population for the lowest mass objects is estimated using same latitude control fields. We fit observed surface densities of various cluster populations with King profiles and find a consistent tidal radius of 1.0 • . The presence of possible mass segregation is discussed. In most respects investigated, IC 4665 is similar to other young open clusters at this age: (1) a power law fit to the mass function between 1 and 0.04 M ⊙ results in best fit for a slope of −0.6; (2) a cusp in the mass function is noticed at about the substellar boundary with respect to the power law description, the interpretation of which is discussed; (3) a fraction between 10-19% for BDs with M ∼ > 0.03 M ⊙ to total members; (4) a best-fit lognormal function to the full mass distribution shows an average member mass of 0.32 M ⊙ , if IC 4665 has an age of 50 Myr.
Astronomy and Astrophysics, 2008
Aims. We conducted a search for brown dwarfs (BDs) and very low mass (VLM) stars in the 625 Myr-o... more Aims. We conducted a search for brown dwarfs (BDs) and very low mass (VLM) stars in the 625 Myr-old Hyades cluster in order to derive the cluster's mass function across the stellar-substellar boundary. Methods. We performed a deep (I=23, z=22.5) photometric survey over 16 deg 2 around the cluster center, followed up with K-band photometry to measure the proper motion of candidate members, and optical and near-IR spectroscopy of probable BD and VLM members. Results. We report the discovery of the first 2 brown dwarfs in the Hyades cluster. The 2 objects have a spectral type early-T and their optical and near-IR photometry as well as their proper motion are consistent with them being cluster members. According to models, their mass is 50 Jupiter masses at an age of 625 Myr. We also report the discovery of 3 new very low mass stellar members of the cluster, and confirm the membership of 16 others. We combine these results with a list of previously known cluster members to build the present-day mass function (PDMF) of the Hyades cluster from 50 Jupiter masses to 3 M ⊙ . We find the Hyades PDMF to be strongly deficient in very low mass objects and brown dwarfs compared to the IMF of younger open clusters such as the Pleiades. We interpret this deficiency as the result of dynamical evolution over the past few 100 Myr, i.e., the preferential evaporation of low mass cluster members due to weak gravitational encounters. Conclusions. We thus estimate that the Hyades cluster currently hosts about 10-15 brown dwarfs, while its initial substellar population may have amounted up to 150-200 members.