We present the results of a search for brown dwarfs (BDs) and very low mass
(VLM) stars in the 625 Myr-old, metal-rich ([Fe/H]=0.14) Hyades cluster. We
performed a deep (I=23, z=22.5) photometric survey over 16 deg$^2$ around the
cluster center. We report the discovery of the first 2 BDs in the Hyades
cluster, with a spectral type T1 and T2, respectively. 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 about 50 Jupiter
masses at an age of 625 Myr. We also report the discovery of 3 new very low
mass stellar members and confirm the membership of 15 others

Eric Stempels

G. Meeus

J. Bouvier

T. Kendall

English

arXiv

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Metal-rich T-dwarfs in the Hyades cluster

We present the results of a search for brown dwarfs (BDs) and very low mass (VLM) stars in the 625 Myr-old, metal-rich ([Fe/H]=0.14) Hyades cluster. We performed a deep (I=23, z=22.5) photometric survey over 16 deg$^2$ around the cluster center. We report the discovery of the first 2 BDs in the Hyades cluster, with a spectral type T1 and T2, respectively. 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 about 50 Jupiter masses at an age of 625 Myr. We also report the discovery of 3 new very low mass stellar members and confirm the membership of 15 others

Bouvier, Jerome

Kendall, Tim

Meeus, Gwendolyn

Hal - Université Grenoble Alpes

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Metal-rich T-dwarfs in the Hyades clusterJerome Bouvier, Tim Kendall, Gwendolyn MeeusTo cite this version:Jerome Bouvier, Tim Kendall, Gwendolyn Meeus. Metal-rich T-dwarfs in the Hyades cluster.Cool Stars 15, Jul 2008, United Kingdom. <hal-00330643>HAL Id: hal-00330643https://hal.archives-ouvertes.fr/hal-00330643Submitted on 15 Oct 2008HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.Metal-rich T-dwarfs in the Hyades clusterJ. Bouvier∗, T. Kendall† and G. Meeus∗∗∗Laboratoire d’Astrophysique, Observatoire de Grenoble, Université J. Fourier, CNRS, France†Centre for Astrophysics Research, Univ. Hertfordshire, College Lane, Hatfield AL10 9AB, UK∗∗Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D-14482 Potsdam, GermanyAbstract. We present the results of a search for brown dwarfs (BDs) and very low mass (VLM)stars in the 625 Myr-old, metal-rich ([Fe/H]=0.14) Hyades cluster. We performed a deep (I=23,z=22.5) photometric survey over 16 deg2 around the cluster center. We report the discovery ofthe first 2 BDs in the Hyades cluster, with a spectral type T1 and T2, respectively. Their optical andnear-IR photometry, as well as their proper motion, are consistent with them being cluster members.According to models, their mass is about 50 Jupiter masses at an age of 625 Myr. We also report thediscovery of 3 new very low mass stellar members and confirm the membership of 15 others.THE HYADES CLUSTERThe Hyades (Melotte 25, α2000=04h26m54s, δ2000=+15o52′; l=180.05o, b=-22.40o) isone of the richest open clusters and the closest to the Sun. Perryman et al. (2008) derivedits main structural and kinematical properties based on Hipparcos measurements : adistance of 46.3±0.27 pc, an age of 625±50 Myr, a metallicity [Fe/H] of 0.14±0.05,a present-day total mass of about 400 M⊙, a tidal radius of 10.3 pc, a core radius of2.5-3.0 pc and negligible extinction on the line of sight. The large proper motion ofthe cluster (µ ≃100 mas yr−1) can easily be measured from imaging surveys over atimeframe of only a few years, which helps in assessing cluster’s membership.THE CFHT SURVEYWide-field optical images were obtained in the I and z bands with the CFHT 12K camera,a mosaic of 12 CCD arrays with a pixel size of 0.21” which provides a FOV of 42′×28′.The survey consists of 53 mosaic fields covering a total of 16 square degrees. It extendssymmetrically around the cluster’s center, along a 4 deg-wide stripe of constant galacticlatitude, and up to 3 degrees away from the cluster center in galactic longitude. Thesurvey is at least 90% complete down to I∼23.0 and z∼22.5, a limit which varies onlyslightly with seeing conditions (0.6-0.8 arcsec).NextGenDustyCondNextGenDustyCondFIGURE 1. Left : (I, I-z) and (I, I-K) CMDs of optically selected candidated followed up with CFHTIR in the K-band. Small dots : 17 optically selected candidates without follow up IR photometry. Largedots : optically selected candidates whose proper motion is inconsistent with Hyades membership (cf.right panel). Triangles : candidates whose proper motion is consistent with Hyades membership. Thestellar/substellar boundary occurs at I≃17.8 mag. The 2 most promising substellar cluster candidates areshown by large triangles. NextGen (0.07-0.3 M⊙), Dusty (0.04-0.07 M⊙), and Cond (0.015-0.05 M⊙)600 Myr isochrones are shown and labelled with mass (Baraffe et al. 1998; Chabrier et al. 2000). In the(I, I-K) CMD, the dotted line indicates the locus of M8-T5 field dwarfs (from Dahn et al. 2002). Therms photometric error is shown as bars. Right : Proper motion vector diagram for 107 optically selectedcandidates followed up in the K-band (see text). The expected proper motion for Hyades members isshown by the (red) box (Bryja et al. 2004). Within these boundaries, 23 optically selected candidates(empty circles) are found to share the proper motion of the cluster, including 2 BDs (large triangles).Typical rms errors on the ppm measurements are shown by a cross.CANDIDATE MEMBER SELECTIONPSF photometry was performed on the I and z-band images with a modified versionof SExtractor (Bertin & Arnouts 1996) from a PSF model computed with the PSFExsoftware. The (I, I-z) color magnitude diagram (CMD) is shown in Fig 1. A total of125 possible Hyades members were selected in this CMD from their location relative tomodel isochrones. Follow up K-band imaging was obtained for 108 of the 125 opticallyselected candidate members using the 1k×1k CFHT IR camera. The (I, I-K) CMD forthe 108 candidates followed up in the K-band is shown in Fig 1. In addition, the propermotion of optically selected Hyades candidates was computed from pairs of optical (I, z)and infrared (K) images obtained 2 or 3 years apart. The proper motion vector diagramof 107 optically selected Hyades candidate members is shown in Figure 1.TABLE 1. The lowest mass Hyades members : photometry and proper motion.CFHT-Hy-# RA(2000) Dec(2000) I I-z I-K µα cosδ µδ Mass(mas.yr−1) (M⊙)CFHT-Hy-19 4 17 24.8 16 34 36 17.49 1.18 4.59 99 -28 0.08CFHT-Hy-20 4 30 38.7 13 09 57 21.58 1.79 5.50 135 -9 0.05CFHT-Hy-21 4 29 22.7 15 35 29 22.16 1.36 5.57 79 -18 0.05FIGURE 2. Near-infrared Amici low resolution spectra of CFHT-Hy-19, 20 and 21 (solid lines fromtop to bottom). In each panel we also show the closest matching field dwarf spectrum (dotted line) fromthe low resolution Amici spectral library (Testi et al. 2001; Testi 2004).BROWN DWARFS IN THE HYADES CLUSTERBased on photometry and astrometry, we eventually identified 20 candidates which con-sistently qualify as probable Hyades members on the basis of their optical photometry,(I-K) color and proper motion. Of these, 15 were already listed as possible or probableHyades members in Prosser & Stauffer’s Open Cluster Database. The remaining 5 prob-able members we report here are new. They include 3 low mass stars (∼0.14 M⊙) and 2objects well within the substellar regime (∼0.050 M⊙).Our survey thus identifies the first 2 Hyades BD candidates (CFHT-Hy-20, 21) as wellas a previously detected very low mass star (CFHT-Hy-19) close to the stellar-substellarboundary, that had originally been considered as a non-member by Gizis et al. (1999).The properties of these lowest mass members are listed in Table 1. The 2 BDs are wellwithin the substellar domain with an estimated mass of about 50 Jupiter masses whilethe lowest mass star has an estimated mass around 0.08 M⊙. Low resolution infraredspectra were obtained for these 3 objects using TNG/NICS and are shown in Figure 2.Fitting the observed spectra with those of template field dwarfs observed with the sameinstrument, we derive a spectral type of M8, T2 and T1 for CFHT-Hy-19, 20 and 21,respectively.The 2 T-dwarfs we report here are strong candidate Hyades members based on theirconsistent photometry and proper motion. Nevertheless, we proceed in estimating theprobability that they could be unrelated field T dwarfs projected onto the Hyades cluster.From the combination of the 2MASS and SDSS DR1 surveys, Metchev et al. (2008)derived an upper limit of 0.9×10−3 pc−3 on the space density of T0-T2.5 dwarfs inthe solar neighborhood. Combining the area of our survey with the range of distancesfor possible field contaminants, the corresponding volume is 65 pc3. We thus expect≤0.06 early field T dwarf to contaminate our survey. This estimate further strengthensthe likelihood that the 2 candidates we report here are indeed the first BDs and the lowestmass members of the Hyades cluster known to date.CONCLUSIONOur survey is complete in the mass range from less than 50 Jupiter masses up to 0.20 M⊙.In this mass range, we identified 18 very low mass stars, down to the stellar-substellarlimit, as well as 2 brown dwarfs with a spectral type T1 and T2. These are the first T-dwarfs identified in the Hyades cluster at an age of 625 Myr, and also the only knowninstances of metal-rich ([Fe/H]=0.14) methane dwarfs. A full account of these results isgiven in Bouvier et al. (2008)1. Additional spectroscopy is planned on Gemini duringthe fall of 2008 in order to investigate the spectral characteristics of metal-rich T-dwarfsin more detail and confront them with model predictions.REFERENCES1. Baraffe, I., Chabrier, G., Allard, F., & Hauschildt, P. H. 1998, A&A, 337, 4032. Bertin, E., & Arnouts, S. 1996, A&AS, 117, 3933. Bouvier, J., et al. 2008, A&A, 481, 6614. Bryja, C., Humphreys, R. M., & Jones, T. J. 1994, AJ, 107, 2465. Chabrier, G., Baraffe, I., Allard, F., & Hauschildt, P. 2000, ApJ, 542, 4646. Dahn, C. C., et al. 2002, AJ, 124, 11707. Dobbie, P. D., Kenyon, F., Jameson, R. F., et al. 2002, MNRAS, 329, 5438. Gizis, J. E., Reid, I. N., & Monet, D. G. 1999, AJ, 118, 9979. Hogan, E., Jameson, R. F., Casewell, S. L., et al. 2008, MNRAS, 388, 49510. Metchev, S. A., Kirkpatrick, J. D., Berriman, G. B., & Looper, D. 2008, ApJ, 676, 128111. Perryman, M. A. C., et al. 1998, A&A, 331, 8112. Testi, L. 2004, Mem. Sc. Astr. It., 75, 891 More recently, Hogan et al. (2008) reported the discovery of 12 L-dwarfs in the cluster.13. Testi, L., D’Antona, F., Ghinassi, F., Licandro, J., Magazzù, A., et al. 2001, ApJ 552, L147

Metal-rich T-dwarfs in the Hyades cluster

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