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A Library of Near-Infrared Integral Field Spectra of Young M–L Dwarfs⋆⋆⋆

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A Library of Near-Infrared Integral Field Spectra of Young M–L Dwarfs⋆⋆⋆ A&A 562, A127 (2014) Astronomy DOI: 10.1051/0004-6361/201118270 & c ESO 2014 Astrophysics A library of near-infrared integral field spectra of young M–L dwarfs?;?? M. Bonnefoy1, G. Chauvin2;1, A.-M. Lagrange2, P. Rojo3, F. Allard4, C. Pinte2, C. Dumas5, and D. Homeier4 1 Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany e-mail: [email protected] 2 UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, Grenoble 38041, France 3 Departamento de Astronomia, Universidad de Chile, 36-D Casilla, Santiago, Chile 4 CRAL-ENS, 46 allée d’Italie, 69364 Lyon Cedex 7, France 5 European Southern Observatory, Casilla 19001, 19 Santiago, Chile Received 13 October 2011 / Accepted 13 June 2013 ABSTRACT Context. At young ages, low surface gravity affects the atmospheric properties of ultracool dwarfs. The impact on medium-resolution near-infrared (NIR) spectra has only been slightly investigated at the M–L transition so far. Aims. We present a library of NIR (1.1–2.45 µm) medium-resolution (R ∼ 1500–2000) integral field spectra of 15 young M6−L0 dwarfs. We aim at deriving updated NIR spectral type, luminosity, and physical parameters (Teff , log g, M, L=L ) for each source. This work also aims at testing the latest generation of BT-SETTL atmospheric models. Methods. We estimated spectral types using spectral indices and the spectra of young objects classified in the optical. We used the 2010 and 2012 releases of the BT-SETTL synthetic spectral grid and cross-checked the results with the DRIFT-PHOENIX models to derive the atmospheric properties of the sources. Results. We do not find significant differences between the spectra of young companions and those of young isolated brown dwarfs +0:5 in the same spectral type range. We derive infrared spectral types L0 ± 1, L0 ± 1, M9.5 ± 0.5, M9.5 ± 0.5, M9.25 ± 0.25, M8−0:75, and M8.5 ± 0.5 for AB Pic b, Cha J110913-773444, USco CTIO 108B, GSC 08047-00232 B, DH Tau B, CT Cha b, and HR7329B, respectively. The BT-SETTL and DRIFT-PHOENIX models yield close Teff and log g estimates for each source. The models seem +600 to show a 600−300 K drop in the effective temperature at the M–L transition. Assuming the former temperatures are correct, we then derive new mass estimates that confirm that DH Tau B, USco CTIO 108B, AB Pic b, KPNO Tau 4, OTS 44, and Cha1109 lie inside or at the boundary of the planetary mass range. We combine the empirical luminosities of the M9.5–L0 sources to the Teff to derive semi-empirical radii estimates that do not match “hot-start” evolutionary models predictions at 1–3 Myr. We use complementary data to demonstrate that atmospheric models are able to reproduce the combined optical and infrared spectral energy distribution, together with the NIR spectra of these sources simultaneously. But the models still fail to represent the dominant features in the optical. This issue raises doubts on the ability of these models to predict effective temperatures from NIR spectra alone. Conclusions. The library provides templates for characterizing other young and late type objects. The study advocates the use of pho- tometric and spectroscopic information over a broad range of wavelengths to study the properties of very low-mass young companions to be detected with the planet imagers (Subaru/SCExAO, LBT/LMIRCam, Gemini/GPI, VLT/SPHERE). Key words. stars: low-mass – brown dwarfs – planetary systems – techniques: spectroscopic 1. Introduction (Geballe et al. 2002; Burgasser et al. 2002). In the NIR, spectra of late-M and L field dwarfs are dominated by a mix of atomic Since the discovery of the first bound substellar objects lines and molecular bands (Jones et al. 1994; Ali et al. 1995; GD 165 B (Becklin & Zuckerman 1988) and Gl 229 B Leggett et al. 1996, 2001; Reid et al. 2001). At low resolution, (Nakajima et al. 1995), the development of large infrared sur- the variations in strengths of these features were followed to de- veys has led to an explosion of discoveries of very low-mass fine a classification scheme coherent with what is found at opti- stars and mature brown dwarfs in the field. Many of these cal wavelengths (Reid et al. 2001; Testi et al. 2001; Geballe et al. “ultra-cool dwarfs” failed to enter the MK spectroscopic clas- 2002). More recently, McLean et al.(2003, ML03) and Cushing sification scheme and required the creation of two new classes et al.(2005, C05) have built medium-resolution ( R ∼ 2000) NIR “L” (Kirkpatrick et al. 1999; Martín et al. 1999) and “T” libraries and quantified the variations of narrow features that can ? Based on observations with ESO telescopes at the La Silla Paranal give additional constraints on spectral types. Models aiming at Observatory under programs 076.C-0379, 078.C-0510, 078.C-0800, reproducing the emergent flux of ultracool atmospheres showed 080.C-0590, 279.C-5010, and 083.C-0595 collected at the European that these changes are best explained by a decrease in the ef- Organization for Astronomical Research in the Southern Hemisphere, fective temperature that leads in turn to the formation of a dusty Chile. cloud deck below . 2700 K (Tsuji et al. 1996; Allard et al. 2001; ?? The library of spectra can be downloaded at Helling et al. 2008a). http://ipag.osug.fr/~gchauvin/addmaterial.html and is also available at the CDS via anonymous ftp to Evolutionary models (Chabrier et al. 2000; Burrows et al. cdsarc.u-strasbg.fr (130.79.128.5) or via 2001) predict that substellar objects form and contract with http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/562/A127 age and are hotter, larger, and more luminous at very young . (.100 Myr) ages. As a consequence, a population of young Article published by EDP Sciences A127, page 1 of 26 A&A 562, A127 (2014) very low-mass objects that would be too faint to study once medium resolution (R ∼ 1500–2000). In contrast with other they have aged were rapidly discovered in young clusters studies, our spectra are less sensitive to chromatic slit losses that and in young nearby (<100 pc) associations (see the re- occur on AO-fed spectrographs with narrow entrance slits (Goto view of Torres et al. 2008) during deep infrared surveys et al. 2002; Chauvin et al. 2005a). Our original source sample (e.g., Robberto et al. 2010), observation campaigns with adap- (see Table1) is composed of 15 objects close to the planetary tive optics devices (e.g. Chauvin et al. 2003, 2010; Ireland mass range, found isolated (Cha J110913-773444, hereafter Cha et al. 2011) and with space observatories (Hubble, Spitzer: 1109; OTS 44; KPNO Tau 4) or as companions to star (AB Pic b, e.g Lowrance et al. 1999, 2000, 2005; Luhman et al. 2007; DH Tau B, GSC 08047-00232 B, hereafter GSC08047 B, Todorov et al. 2010). Among these discoveries, planetary mass TWA 5B, Gl 417B, USco CTIO 108B, and HR7329B). We objects (613.6 MJup, see the definition of the International observed in addition 2MASS J03454316+2540233 (2M0345), Astronomical Union) were found to be free-floating (e.g. Oasa a member of the Ursa Major moving group (300 to 600 Myr et al. 1999; Lucas & Roche 2000; Zapatero Osorio et al. 2000), Castellani et al. 2002; King et al. 2003; Bannister & Jameson orbiting brown dwarfs (2MASSW J1207334-393254, USCO 2007), to get the reference spectrum of a mature L0 dwarf. CTIO 108, 2MASS J04414489+2301513; Chauvin et al. 2004; Apart from Gl 417 B (80–300 Myr; Kirkpatrick et al. 2000), Béjar et al. 2008; Todorov et al. 2010) or as wide (>5 AU) our young objects are all members of young nearby associa- companions to stars (AB Pic, DH Tau, CHXR 73, GQ Lup, tions (TW Hydrae, age ∼ 8 Myr; Tucana-Horologium, age ∼ CT Cha, Fomalhaut, HR8799, β Pictoris, GSC 06214-00210, 30 Myr; Colomba, age ∼ 30 Myr; Carina, age ∼ 30 Myr) and 1RXS J235133.3+312720, κ Andromedae; Chauvin et al. 2005b; young clusters/star-forming regions (Chameleon I, age ∼ 1– Itoh et al. 2005; Luhman et al. 2006; Neuhäuser et al. 2005; 3 Myr; Taurus, age ∼ 1 Myr; Upper-Sco, age = 5–11 Myr). Schmidt et al. 2008; Kalas et al. 2008; Marois et al. 2008, We also reduced and re-analyzed spectra of the tight binary 2010; Lagrange et al. 2010; Ireland et al. 2011; Bowler et al. TWA 22AB (see Bonnefoy et al. 2009), of the young compan- 2012; Carson et al. 2013), or even binaries (SR 12 AB, 2MASS ion CT Cha b (Schmidt et al. 2008), of the young field dwarf J01033563-5515561 AB; Kuzuhara et al. 2011; Delorme et al. 2MASS J01415823-4633574 (2M0141; Kirkpatrick et al. 2006), 2013). This variety of configurations offers precious bench- and 2MASSW J1207334-393254 A (2M1207 A; Gizis 2002). In marks for planet and brown-dwarf formation models (e.g. Boley Sect. 2, we describe the observations and the associated data re- 2009; Kratter et al. 2010; Mordasini et al. 2012; Lambrechts & duction. We explain our spectral analysis and derive the physi- Johansen 2012; Bate 2012). cal parameters of each object in Sect. 3. Finally, in Sect. 4, we Together with the temperature, the reduced surface gravity discuss our results and try to identify and quantify the different of these objects modifies the chemical and physical properties biases that could have affected the analysis. of their atmospheric layers. This translates into peculiar spectro- scopic features that have been identified in the optical spectra of late-M dwarfs (Martin et al.
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