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 compared our spectra to the BT-Settl models 2014. We selected six possible candidates to be combination of L plus T brown dwarfs. All candidates, except one, are better reproduced by a combination of two single brown dwarf spectra than by a single spectrum. The one-sided F-test discarded this object as a binary candidate. We found that we are not able to find the artificial binaries with components of the same spectral type using the same method used for L plus T brown dwarfs. Best matches to models gave a range of effective temperatures between 950 K and 1900 K, a range of gravities between 4.0 and 5.5. Some best matches corresponded to supersolar metallicity

ALCALA', JUAN MANUEL

Allard, F.

Bonnefoy, M.

Béjar, B. J. S.

Goldman, B.

Henning, T.

Homeier, D.

Manjavacas, E.

SMART, Richard Laurence

Zapatero-Osorio, M. R.

OA@INAF - Istituto Nazionale di Astrofisica

2015Publication Year2020-05-04T13:39:11ZAcceptance in OA@INAFHunting for brown dwarf binaries with X-ShooterTitleManjavacas, E.; Goldman, B.; ALCALA', JUAN MANUEL; Zapatero-Osorio, M. R.; Béjar, B. J. S.; et al.Authorshttp://hdl.handle.net/20.500.12386/24436Handle	  	  Highlights	  of	  Spanish	  Astrophysics	  VIII,	  Proceedings	  of	  the	  XI	  Scientific	  Meeting	  of	  the	  Spanish	  Astronomical	  Society	  held	  on	  September	  8–12,	  2014,	  in	  Teruel,	  Spain.	  A.	  J.	  Cenarro,	  F.	  Figueras,	  C.	  Hernández-­‐Monteagudo,	  J.	  Trujillo	  Bueno,	  and	  L.	  Valdivielso	  (eds.)	  Hunting for brown dwarf binaries with X-ShooterE. Manjavacas1, B. Goldman1,2, J. M. Alcalá3, M. R. Zapatero-Osorio4, V. J. S.Béjar5,6, D. Homeier7, M. Bonnefoy8,9, R. L. Smart10, T. Henning1, and F.Allard 71 Max Planck Institute für Astronomie Königstuhl, 17. D-69117 Heidelberg, Germany2 Observatoire astronomique de Strasbourg, Université de Strasbourg, 11 rue de lUniversité,F-67000 Strasbourg, France3 INAF - Osserv. Astronomico di Capodimonte, Via Moiariello 16, 80131, Napoli, Italy4 Centro de Astrobioloǵıa (CSIC/INTA). Instituto Nacional de Técnica Aeroespacial. Ctrade Torrejón a Ajalvir, km 4. 28850 Torrejón de Ardoz, Madrid Spain5 Instituto de Astrof́ısica de Canarias, C/ Vı́a Láctea, s/n, E38205 - La Laguna (Tenerife).Spain.6 Universidad de La Laguna, Dpto. Astrof́ısica, E-38206 La Laguna, Tenerife, Spain7 CRAL-ENS, 46, Allée dItalie , 69364 Lyon Cedex 07, France8 Univ. Grenoble Alpes, IPAG, F-38000 Grenoble, France.9 CNRS, IPAG F-38000 Grenoble, France10 Instituto Nazionale di Astrofisica, Osservatorio Astrofisico di Torino, Strada Osservatorio20, 10025 Pino Torinese, ItalyAbstractThe refinement of the brown dwarf binary fraction may contribute to the understanding ofthe substellar formation mechanisms. Peculiar brown dwarf spectra or discrepancy betweenoptical and near-infrared spectral type classification of brown dwarfs may indicate unre-solved brown dwarf binary systems. We obtained medium-resolution spectra of 22 browndwarfs of potential binary candidates using X-Shooter at the VLT. We aimed to select browndwarf binary candidates. We also tested whether BT-Settl 2014 atmospheric models repro-duce the physics in the atmospheres of these objects. To find different spectral type spectralbinaries, we used spectral indices and we compared the selected candidates to single spectraand composition of two single spectra from libraries, to try to reproduce our X-Shooterspectra. We also created artificial binaries within the same spectral class, and we tried tofind them using the same method as for brown dwarf binaries with different spectral types.We compared our spectra to the BT-Settl models 2014. We selected six possible candidatesto be combination of L plus T brown dwarfs. All candidates, except one, are better repro-duced by a combination of two single brown dwarf spectra than by a single spectrum. Theone-sided F-test discarded this object as a binary candidate. We found that we are notable to find the artificial binaries with components of the same spectral type using the samemethod used for L plus T brown dwarfs. Best matches to models gave a range of effectivetemperatures between 950 K and 1900 K, a range of gravities between 4.0 and 5.5. Somebest matches corresponded to supersolar metallicity.500E. Manjavacas et al. 2014 5011 IntroductionStars are believed to be born in nurseries of several objects. After childhood they leavetheir birth place and settle on the main sequence. A majority of stars remain in binary orhierarchical systems. Multiplicity provides constraints on fundamental parameters, such asdynamical masses, essential to test atmospheric models. It is well known that the binaryfraction decreases when decreasing mass. This fraction decreases from 80%-60% for O, A, Band G, K stars to 40% for the M dwarfs [9, 17] and 20% for brown dwarfs[9, 21, 17]The peak of the separation distribution of brown dwarfs is ∼ 3 au, which is very closeto the limit of the high resolution imaging surveys [9].Atmospheric models allow us to disentangle the effect of varying effective temperature,gravity, and metallicity on the spectral features. Below a effective temperature of ∼ 2600 K,models predict that clouds of iron and silicate grains begin to form, changing the opacity.Self-consistent atmospheric models, such as the BT-Settl models [4] and the Drift-PHOENIX models, [19], use cloud models where the dust properties do not require thedefinition of any other additional free parameters other than gravity, effective temperatureand metallicity. Synthetic spectra for a specific set of atmospheric parameters can be com-pared to empirical spectra. These models are just starting to be tested on spectra of younglate-type objects ([8], [25], [24], [7], [22]). They were used on spectra of L3-T7 dwarfs in [7]to reveal the behavior of these models over this spectral range (500-2500 nm) and test if theyare able to reproduce spectra of unresolved brown dwarf binaries.2 The sample2.1 Sample selectionWe selected a sample of 22 brown dwarfs found in the literature with optical spectral typesbetween L3 and T7 which have discrepant optical and near-infrared classification, or, peculiarspectra. Optical subtypes are typically earlier than the near infrared subypes. These objectsare candidates for unresolved binaries. Furthermore, to calibrate our results and confirmthe reliability of our method, we added some known brown dwarfs systems, LHS 102B [18],formed by a L4.5 plus a L4.5, and SDSS J042348.56-041403.4 [12], formed by a L6±1 and aT2±1.3 Empirical analysis3.1 Finding L plus T brown dwarf binariesSpectra of L plus T brown dwarf binary systems have been widely studied in the past years[9], [10], [6]. The combined spectra of these type of binary systems are expected to showpeculiar characteristics, as they are a mix of two quite different spectra, with different atomicand molecular absorptions [15].502 Hunting for brown dwarf binaries with X-ShooterBurgasser et al. 2010 [10] described differences of L plus T binary systems in comparisonwith single template brown dwarf spectra: in general, spectra of L plus T binary systemsshow bluer spectral energy distribution in the near-infrared, and in particular, some spectralfeatures vary, like the CH4 and H2O features at 1.1 µm which are deeper for binaries andthe CH4 feature at 1.6 µm is stronger in comparison to the 2.2 µm CH4 band. Using suchdifferences, [11], [10] and [6], defined spectral indices which help to detect L plus T browndwarf binaries.By calculating these indices we select those objects in our sample that are candidatesL plus T binary systems. These indices selected two strong binary candidates and four weakcandidates.The next step in the empirical analysis consists on comparing our spectra with librariesof well characterized brown dwarf template spectra. First, we degrade the resolution of ourX-Shooter spectra to the resolution of each template. Then we re-interpolate the library ofbrown dwarf template spectra and X-Shooter spectra to the same grid.As template brown dwarf spectra we use McLean et al. (2003) [23] and Cushing etal. (2005) [15] spectra and SpeX Prism Spectral Library spectra1. In total we consider 462spectra from SpeX Spectral Library, plus 14 from [15] library and 47 spectra from [23] library.In our analysis we search for the best matches to single template spectra from librariesand to combinations of two template spectra from those libraries. Before comparing our X-Shooter spectra to combination of two template spectra coming from the libraries, we calibratetheir fluxes to the same distance using the absolute magnitude-color relation published byDupuy & Liu (2012) [16]. To identify the best matches to our spectra, we use a χ2 approachas explained in Cushing et al. (2008) [14], as well as visual inspection. We test if the fit toa binary template is significantly better than the fit to a single template using a a one-sidedF-test statistic. The F-statistic rejected one of our candidates, namely: SIMP0136.We show the best matches to 2M1341 with a single and a composite spectra in Figure1.3.2 Finding L plus L or T plus T brown dwarf binariesSpectroscopic identification of brown dwarf L plus L and T plus T binaries pairs has notbeen as developed as that for L plus T binaries. Differences within L or T brown dwarfsof different sub-types reside mainly in smooth changes of the spectral energy distribution.Therefore the search for binaries with similar spectral types is challenging.Before comparing to the set of libraries following the same method as in Section 3.1,we first test the efficiency of this method to search for L plus L and T plus T binaries usingsynthetic binaries. To this aim we have chosen several L and T single and not peculiar browndwarfs spectra from the SpeX library. Then, we calibrated their fluxes to the same distance,using the absolute magnitude-color relation published by [16]. We combined different pairsof L brown dwarfs spectra with L brown dwarf and different pairs of T brown dwarfs spectrawith T brown dwarfs, creating L plus L and T plus T synthetic spectra of binaries.1http://pono.ucsd.edu/∼adam/browndwarfs/spexprism/E. Manjavacas et al. 2014 503Figure 1: Best matches for object 2M1341 (L2, peculiar) to single and composite spectrafrom SpeX. Our spectra is shown in black. In the upper plot, we show the best match toa single spectra in blue. In the lower plot, we show in red the best match to a compositespectra. in green SpeX primary and in blue the SpeX secondary. The flux is F(λ).We compared these artificial binary pairs to single SpeX templates and composites oftwo SpeX template spectra. Our objective is to test whether synthetic L plus L and T plusT binaries can be identified by comparing to spectral library spectra.From this analysis, we conclude that L plus L and T plus T brown dwarfs binaries arenot straightforward to detect just by comparing with single or composite spectral libraries.Therefore, additional data will be necessary to find these binaries, i.e. parallaxes, highresolution imaging or high resolution spectra.4 Comparison to synthetic spectraIn this section, we compare the optical and near-infrared spectra of our objects, to predictionsfrom BT-Settl atmospheric models [2, 1, 3], excepting brown dwarf binary candidates andobjects with low signal to noise. We derive atmospheric parameters of the objects and toreveal non-reproducibilities of the models. The models are described in [3, 4, 5].We selected subgrids of synthetic spectra with 400 K≤ Teff ≤ 3000 K, 3.5 ≤ log g ≤ 5.5and metallicities of +0.0 and +0.3, which are the metallicities for which the latest version ofthe BT-Settl models are available. The solar metallicity is based on metallicities calculatedby [13]. The spacing of the model grid is 50 K and 0.5 dex in log g. Effective temperature,gravity, metallicity and alpha element enhancement are described in the model name stringsas lte-LOGG+[M/H]a+[ALPHA/H].The BT-Settl 2014 synthetic spectra were smoothed to the resolution of X-Shooter.The models were then reinterpolated on the X-Shooter wavelength grid. The spectra werenormalized using the same method than in Section 3 and explained in [14]. The parametersTeff , log g, and [M/H] have minimum uncertainties of 50 K, and 0.5 dex respectively. Theseerrors correspond to the sampling of the atmospheric parameters of the model grids. InFigure 2 we show the best fit of the synthetic spectra to two of our targets.504 Hunting for brown dwarf binaries with X-ShooterFigure 2: Best matches to BT-Settl models 2014. Effective temperature, gravity,metallicity and alpha element enhancement are described in the model name strings aslte-LOGG+[M/H]a+[ALPHA/H]. The flux is F(λ).The CH4 and the FeH molecules opacities are still incomplete in the new BT-Settl 2014models. Methane line opacities are based on the semi-empirical list of Homeier et al. 2003[20], which is highly incomplete in the H band and only supplemented with a small set ofroom-temperature transitions for the Y and J bands. This explains that the H-band is notwell reproduced for any of the L or T brown dwarf spectra, and also the J band in the caseof T brown dwarfs. For three of the L brown dwarfs, the best match is found for log g= 5.5,with solar metallicity, four of the L brown dwarfs have best matches with log g= 5.0, but[M/H] = +0.3.5 ConclusionsWe observed and analyzed 22 optical and near infrared medium resolution VLT/X-Shooterspectra of brown dwarfs. Two objects from our sample are known binaries, that allow us totest our analysis.Using [11, 10] and [6] method, we selected six objects as potential L plus T binarycandidates. We compared these six objects with spectral libraries of field brown dwarfs ([23],[15] and Spex) and composite spectra of these libraries.We also examined the possibility to find L plus L or T plus T brown dwarfs binariesusing a comparison to spectral libraries. As they are not remarkable differences in the spectralcharacteristics within subtypes of the same spectral type, apart from the SED, we are notable to find L plus L or T plus T brown dwarfs binary systems using this method. Additionaldata, such as parallax measurements, high-resolution imaging or high resolution spectra arenecessary in order to find these systems.We find that 27±11% of the L and T objects in our sample may be unresolved binarycandidates with one L and one T possible members, which corresponds to a mass ratio ofq ≥ 0.2 for an age of a few Gyr (expected for most investigated objects). This percentageagrees with previous results. Our work does not explore smaller mass ratios.BT-Settl models 2014 are able to reproduce the majority of the SEDs of our objectsE. Manjavacas et al. 2014 505in the optical and in the near infrared. Nonetheless, these models usually fail to reproducethe shape of the H-band, due to incomplete opacities for the FeH molecule in BT-Settl 2014models.References[1] Allard, F., Allard, N. F., Homeier, D. et al. 2007, A&A, 474, 21[2] Allard, F., Guillot, T., Ludwig, H.-G. et al. 2003, IAUS, 211, 325A[3] Allard, F., Homeier, D., Freytag, B. 2011, ASPC, 448, 91[4] Allard, F., Homeier, D., Freytag, B. 2012, 370, 2765[5] Allard, F., Homeier, D., Freytag, B. et al. 2012, EAS, 57, 3[6] Bardalez Gagliuffi, D. C., Burgasser, A. J., Gelino, C. R. at al. 2014, ApJ, 794, 143[7] Bonnefoy, M., Boccaletti, A., Lagrange, A.-M. et al. 2013, A&A, 555, 107[8] Bonnefoy, M., Chauvin, G., Rojo, P. et al. 2010, A&A, 512A, 52B[9] Burgasser, A. 2007, ApJ, 659, 655B[10] Burgasser, A. J., Cruz, K. L., Cushing, M. et al. 2010, ApJ, 710,1142[11] Burgasser, A. J., Geballe, T. R., Leggett, S. K. et al. 2006, ApJ, 637, 1067[12] Burgasser, A. J., Reid, I. N., Leggett, S. K. et al. 2005, ApJ, 634, 177[13] Caffau, E., Ludwig, H.-G., Steffen, M. et al. 2011, SoPh, 268, 255[14] Cushing, M. C., Marley, M. S., Saumon, D. et al. 2008, ApJ, 678, 1372[15] Cushing, M. C., Rayner, J. T., Vacca, W. D. 2005, ApJ, 623, 1115[16] Dupuy, T. J. & Liu, M. C. 2012, ApJS, 201, 19[17] Goldman, B., Bouy, H., Zapatero Osorio, M. R. et al. 2008, A&A, 490, 763G[18] Golimowski, D. A., Leggett, S. K., Marley, M. S. et al. 2004, AJ, 127, 3516[19] Helling, C., Dehn, M., Woitke, P. et al. 2008, ApJ, 675, 105[20] Homeier, D., Hauschildt, P.H., Allard, F. ASP Conference Series, 2003[21] Luhman, K. L., Allers, K. N., Jaffe, D. T. et al. 2007, ApJ, 659, 1629[22] Manjavacas, E., Bonnefoy, M., Schlieder, J. E. et al. 2014, A&A, 564, 55[23] McLean, I. S., McGovern, M. R., Burgasser, A. J. et al. 2003, ApJ, 596, 561[24] Patience, J., King, R. R., De Rosa, R. J. et al. 2012, A&A, 540, 85[25] Witte, S., Helling, C., Barman, T. et al. 2011, A&A, 529A, 44

Hunting for brown dwarf binaries with X-Shooter

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Hunting for brown dwarf binaries with X-Shooter

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