arXiv:1310.1936v1 [astro-ph.SR] 7 Oct 2013 otenOsraoya aaa,Ciei rga 80.C-059 program in Chile Paranal, at Observatory Southern fte lot rv uflw eg,Wea ta.20;Phan- 2005; al. et Whelan (e.g., ha outflows a brown drive and 2012), these to al. also of et d them Many Rigliaco the of (e.g., through object 2013). material central accrete the al. actively onto et to found Ricci ( were 2012; wavelengths dwarfs mm al. and et (far-IR) Harvey far-infrared at disks stellar objects. low-mass as very such for formation, outflows, explori and and accretion, of detecting for- characteristic the by features dwarf that main produce brown mass the can and minimum process star the formation define understanding star observationally to to key is A et mation (Machida 2009). dwarfs brown become al. high and experience outflows that in Inutsu di- cores self-erosion low-mass (e.g., by form collapse mode might filament 1992) isolated example, Miyama an For in collapse. form dis rect nearb could circumstellar a dwarfs in instabilities of brown disk natively, radiation (iii) through and p envelope star, (ii) hot gas interactions, the dynamical of through evaporation ar core mass the stellar to of accreting ejection from environment subs s dense nec- a a very prevent in is to create core phase scenarios to Proposed high-density fragmentation cores. A formation gravitational Jeans-unstable star form. the of dwarfs for theory brown essary the do in How questions open is: main the of One Introduction 1. coe ,2018 9, October Astronomy ⋆ ae nosrain tteVr ag eecp fteEuro the of Telescope Large Very the at observations on Based on rw wrswr hw ohv usata circum- substantial have to shown were dwarfs brown Young hti niaieo cieacein ohtePa the Both accretion. active of indicative is that edsoe httevr o-asbondafOS4 (M9.5, 44 OTS dwarf brown low-mass very the that discover We accepted Received; eosrtsta h rcse htacmaycnncls VLT canonical in accompany that discover processes the that demonstrates asof mass a H strong detect also We border. planetary the at aeinaayi.W n htOS4 a ihyflrdds ( disk flared highly a has 44 OTS that find fl We of modeling analysis. dow transfer Bayesian radiative masses solar through several 44 OTS of surrounding s for its found considering trend rate decreasing mass-accretion high relatively a has eso httertoo ikt-eta-aso bu 10 about of disk-to-central-mass of ratio the that show We about e words. Key 4 3 2 1 5 7 6 ntttfrTertsh hskudAtohsk Univers Astrophysik, und Physik Theoretische für Institut upeMuti bevtr e aoaoyfrRdoAstr Radio for Laboratory Key & Observatory Mountain Purple nvriä edleg etu ü srnme nt für Inst. Astronomie, für Zentrum Heidelberg, Universität a-lnkIsiu ü srnme öisul1,617H 69117 17, Königstuhl Astronomie, für Institut Max-Planck uoenSuhr bevtr,Aos eCroa30,Vit 3107, Córdova de Alonso Observatory, Southern European eatmnod srnma nvria eCie Casilla Chile, de Universidad Astronomia, de Departamento nt fPaeooy&Atohsc rnbe 1,Red la de Rue 414, Grenoble, Astrophysics & Planetology of Inst. ± & 0 ms km 200 ∼ T 4 ikadaceina h lntr border planetary the at accretion and Disk 44: OTS Astrophysics 0.01 rw wrs-Sas r-ansqec icmtla mat circumstellar - sequence pre-main Stars: - dwarfs brown .Joergens, V. M − / IFN pcr htOS4 a tog ra,advral P variable and broad, strong, has 44 OTS that spectra SINFONI 1 ⊙ edtrietems crto aeo T 4bsdo H on based 44 OTS of rate accretion mass the determine We . u bevtosaei iewt nioae trlk mod star-like isolated an with line in are observations Our . aucitn.ots44_v6 no. manuscript 1 , 2 .Bonnefoy, M. α β 1 msino T 4i ieauesetu n eemn nH an determine and spectrum literature a in 44 OTS of emission n H and .Liu, Y. L s Alter- ks. 0(A) te othe to etter alcnrlms.Ti asrt snvrhls consisten nevertheless is rate mass This mass. central mall − α disks, 2 ndful hoto- tellar a& ka mall ABSTRACT pean e.g., obondaf.Frhroe edtrieteproperties the determine we Furthermore, dwarfs. brown to n a omto cu ont eta aso e uie mas few a of mass central a to down occur formation tar msinlnshv ra rfie ihwnsetnigt ve to extending wings with profiles broad have lines emission ttKe,Linzt.1,218Ke,Germany Kiel, 24118 15, Leibnizstr. Kiel, ität (i) e 3 on o bet ewe 0.03 between objects for found ho.Atohsk letUbreSr ,610Heidelb 69120 2, Albert-Ueberle-Str. Astrophysik, Theor. xmaueetfo h pia otefrI Hrce)b a by (Herschel) far-IR the to optical the from measurement ux , isk ng 4 6D atao Chile Santiago, 36-D, y iebr,Gray e-mail: Germany, eidelberg, icn,DmieUiestie 80 an-atnd’Hér Saint-Martin 38400 Universitaire, Domaine Piscine, .Bayo, A. cr,Snig,Chile Santiago, acura, > β nm,CieeAaeyo cecs ajn 108 China 210008, Nanjing Sciences, of Academy Chinese onomy, ∼ 99.I a ofimdt eavr o-asbondafof dwarf brown low-mass of very mass estimated a an a be with et to M9.5 imag- spectral-type (Oasa confirmed near-IR region was star-forming deep It I a 1999). in the candidate in survey dwarf ing brown a as identified n T 4( 44 OTS and umne l 20b n nRbn antd f23.5 of magnitude R-band data an I-band and use (2005b) we al. optical, et the Luhman measure In flux far-IR literature. the to from optical ments compiled we 44, OTS of tosphere distribution. energy spectral The Observations 2. 4 OTS of disk the data. of Herschel (SED) on distribution based modelin energy detailed spectral a and the (2007), of Luhman from spectrum a in sion 05)adHrce Hre ta.21)idctdta T 4 SINFONI OTS present that We indicated disk. 2012) a al. has et et (Harvey (Luhman Herschel Spitzer and with 2005b) detected emission excess far-IR and 02;Mnne l 03.Aogtelws-asisolated 2012a, lowest-mass ( the al. 110913-773444 Cha et Among are disk Joergens a 2013). 2011; harbor to found al. al. objects et et Bacciotti Monin 2008; 2012b; al. et Bao htrvassrn Paschen strong reveals that umn20) onfye l 21)rcnl ofimdi a ( in regime planetary confirmed the recently in (2013) mass a al. study near-IR et 2 Bonnefoy al. et 2007). (Luhman Luhman spectra optical and near-IR low-resolution umne l 05) Oi16( 156 LOri 2005a), al. et Luhman 2M 12 .)wt aso 9.1 of mass a with 1.2) E T 4 hc stesbeto h rsn ok a first was work, present the of subject the is which 44, OTS ditor Jup e crto tr:frain-sas niiul(T 4 (OTS individual stars: - formation Stars: - Accretion - ter 1 a infiataceinadasbtnilds,which disk, substantial a and accretion significant has ) , 5 ftefraino rw wrsdw o0.01 to down dwarfs brown of formation the of e .Wolf, S. α ∼ a o7.6 to 2M 12 β msinta seiec o cieaccretion active for evidence is that emission 4 × Jup .Chauvin, G. 10 M [email protected] − + umne l 2005b). al. et Luhman , 5 1 ⊙ − . . 5 7 12 n 14 and × M β 10 ⊙ msin naayi fH of analysis an emission, yr − 5 M − M 1 ⊙ / hc hw htOS44 OTS that shows which , 6 ⊙ ∼ L pcrsoyo T 44 OTS of spectroscopy VLT sas ai o T 4at 44 OTS for valid also is ( 23 .Rojo P. omdlteds n pho- and disk the model To ∼ 0.1 M ril ubr ae1o 6 of 1 page number, Article Jup M ihtegeneral the with t Jup aoe l 2012), al. et Bayo , 7 α ∼ ∼ r,Germany erg, r30 or ⋆ W(11Å) (-141 EW -7M 6-17 5M 15 ftedisk the of s France es, oiisof locities pyn a pplying

e.We ses. c M Jup S 2018 ESO Earth Jup M ae on based ∼ ± α ⊙ 4) .Mid- ). 8 ). . (K. 0.1 . emis- M from 004; Jup al. l. 4 4 g - , Luhman, pers. comm.). The near-IR (JHK) regime is covered by Table 1. Photospheric properties of OTS 44. observations of 2MASS and WISE. We note that the WISE W4 photometry of OTS44 was not used because of contamination Teff L∗ AV log(g) Reference by a bright spike. Mid-IR photometry of OTS44 was obtained by Spitzer using IRAC (3.6, 4.5, 5.8, 8.0 µm) and MIPS (24 µm, [K] [L⊙] [mag] Luhman et al. 2008). Recently, OTS44 was observed in the far- Set 1 2300 0.00077 0.0 Luhman 2007 IR (70, 160 µm) by PACS/Herschel (Harvey et al. 2012). Set 2 1700 0.0024 2.6 3.5 this work Near-infrared SINFONI integral field spectroscopy. We observed OTS44 with the Spectrograph for INtegral Field Ob- Notes. The errors for set 2 are ∆Teff=140 K, ∆L∗=0.00054 L⊙, and servations in the Near Infrared (SINFONI) at the VLT on De- AV <3.0 with 90% probability. log(g) is taken from Bonnefoy et al. cember 14 and 21, 2007. The observations were conducted as (2013). Teff in set 2 agrees with the value found by Bonnefoy et al. part of a program designed to provide a library of near-IR spec- (2013), while L∗ slightly deviates between these two works. tra of young late-type objects (Bonnefoy et al. 2013). The instrument was operated with pre-optics and gratings enabling medium-resolution (R=λ/∆λ∼2000) J-band spectroscopy (1.1- 1.4 µm) with a spatial sampling of 125×250 mas/pixel. We re- geneous throughout the system. This model has been suc- duced the data with the SINFONI data reduction pipeline ver- cessfully used to explain the observed SEDs of a large sam- sion 1.9.8 and custom routines (cf. Bonnefoy et al. 2013 for ple of young stars and brown dwarfs (e.g., Wolf et al. 2003; details). The pipeline reconstructs datacubes with a field of view Harvey et al. 2012). For the dust in the disk we assume of 1.125 ×1.150" from bi-dimentional raw frames. Telluric ab- a density structure with a Gaussian vertical profile ρdust = α 2 2 sorptions features were calibrated and removed based on the ob- ρ0(R∗/̟) exp(−z /2h (̟)), and a power-law distribution for p servations of B5V stars. The wavelength calibration based on the surface density Σ(̟) = Σ0(R∗/̟) , where ̟ is the radial arc lamp spectra has an accuracy of about 30kms−1. A value of distance from the central star measured in the disk midplane, and −1 h(̟) is the scale height of the disk. The outer disk radius R V0=15.2kms was adopted as rest velocity of OTS44 through- out out the paper, which is the average radial velocity of T Tauri stars is set to 100 AU. To allow flaring, the scale height follows the β and brown dwarfs in ChaI (Joergens 2006). power law h(̟)=h100(̟/100 AU) , with the flaring exponent β Optical spectroscopy. An optical spectrum covering the describing the extent of flaring and the scale height h100 at Rout. range 0.5-1 µm with a resolution R=900 was taken by Luhman Dust properties. We consider the dust grains to be homo- (2007) with IMACS at the MagellanI telescope on January 6, geneous spheres, which is a valid approximation to describe the 2005. This spectrum is used to analyze the Hα emission of scattering behavior as compared to a more complex description OTS44 and to provide an additional constraint to the photo- with fractal grain structures. The dust grain ensemble incorpo- sphere model fit. We calculate the heliocentric radial velocities rates both astronomical silicate (62.5%) and graphite (37.5%) relative to V0. material. The grain size distribution is given by the standard power law n(a) ∝ a−3.5 with minimum and maximum grain sizes of 0.005 µm and 0.25 µm, respectively. 3. Photospheric properties of OTS 44 Heating sources. We consider a passive disk with only stel- We find that it is not possible to fit the SED of OTS44 when us- lar irradiation, but no viscous heating (e.g., Chiang & Goldreich ing the properties reported for this in the literature 1997). Radiation heating of the dust from the accretion lumi- (Teff=2300K, L∗=0.00077L⊙, AV =0, Luhman 2007). There- nosity can be neglected because Lacc < 0.2% L∗ (Sect.5). For fore, we performed a thorough modeling of the photosphere of Teff and L∗ of the central source, we use the values derived here OTS44 by applying the BT-Settl models (Allard et al. 2012). (Table1, set 2) and d=162.5pc. As said, an SED model using These models incorporate a sophisticated treatment of photo- the parameters from Luhman (2007; set 1 in Table1) cannot re- spheric dust, which is likely to affect the cool atmosphere of produce the observations. The incident substellar spectrum is OTS44. We use broad-band photometry, a narrow grid of flux taken from the BT-Settl atmosphere database with log(g)=3.5 points calculated from the optical spectrum (Luhman 2007), and (Allard et al. 2012). The radiative transfer problem is solved a surface gravity log(g) of 3.5, as determined from gravity- self-consistently considering 100 wavelengths, logarithmically sensitive lines (Bonnefoy et al. 2013). We derive an effective distributed in the range of [0.05 µm, 2000 µm]. temperature of Teff=1700K, a of L∗=0.0024 L⊙, and Fitting results. The SED fitting is performed with a hybrid an extinction of AV =2.6mag (Table1). Bonnefoy et al. (2013) strategy that combines the database method and the simulated previously found indications for a lower Teff of OTS44. The annealing (SA) algorithm (Liu et al. 2013). We first run a large probability distributions of the parameter values in our modeling grid of disk models with a broad range of disk parameters. Then approach are relatively broad, which hints at a remaining de- SA is used to improve upon the results returned by the model screpancy with the models. We conclude that the photospheric grid and to calculate local confidence intervals. The best-fit properties of OTS44 may still need to be refined in the future. model is shown in Fig.1 and the corresponding disk parameter values are listed in Table2. The best-fit model is not a unique so- 4. SED modeling of the disk of OTS44 lution due to model degeneracies between different parameters, for example mdisk/Rout. We therefore conduct a Bayesian anal- We model the SED of OTS44 using the radiative transfer code ysis to estimate the validity range for each parameter (Pinte et MC3D (Wolf 2003) to characterize its circumstellar environment. al. 2008). We find that the best-fit and the most probable values Because the disk parameters are strongly degenerate in the fit- (cf. Sect.A) agree well with each other, in particular the disk ting procedure, we employ a passive-disk model consisting ofa mass, which demonstrates that the modeling efforts place good central substellar source surrounded by a parametrized disk. constraints on the mass and structure of the disk of OTS44. We Dust distribution in the disk. We introduce a parametrized note that strong grain growth would remain undetected in our flared disk in which dust and gas are well mixed and homo- data (≤160 µm) and could affect mdisk. The values of most disk

Article number, page 2 of 6 Joergens et al.: OTS 44: Disk and accretion at the planetary border

Table 2. Disk parameter values of the best-fit SED model.

Rin Rout p β h100 mdisk mdisk i −5 ◦ AU] [AU] [AU] [10 M⊙] [M⊕][ ] 0.023 100 1.136 1.317 17.42 9.06 30.2 58

Notes. For the photosphere, the values of set 2 of Table 1 were used. See Sect. A for confidence intervals of the disk parameters.

Table 3. Observed emission lines of OTS 44.

line date Vcenter FWHM EW Fig. 1. SED of OTS 44. Shown are photometric measurements (red / / diamonds) with errors if larger than the symbol, an upper limit for the [km s] [km s] [Å] 160 µm flux (black triangle), the mid-IR Spitzer/IRS spectrum (light H α 20050106 -30 283 -141 ± 14 gray), the best-fit SED model (dashed line), and the input BT-Settl pho- + ± tosphere model (gray dotted line). Pa β 2007 12 14 44 265 -6.7 0.3 Pa β 2007 12 21 +48 251 -4.2 ± 0.3 parameters derived here are largely consistent with the results of Harvey et al. (2012, R =0.01 AU, m = 5.0×10−5 M , β=1.15, in disk ⊙ Mass accretion rate. We determine the mass accretion rate of h =15 AU, i= 60◦), who used a coarser grid resolution and 100 OTS44 based on the Hα line by assuming that the Hα emission slighlty different dust properties and photospheric parameters. is entirely formed by accretion processes. For this purpose, we An SED model by Bonnefoy et al. (2013) aimed at understand- calculate the Hα line luminosity from the Hα EW using broad- ing whether the disk could create a near-IR excess that would band photometry and converte it into an accretion luminosity by bias the temperature estimate showed that this is not the case. applying the empirical relation of Fang et al. (2009, cf. also Jo- ergens et al. 2012b). We use a dereddened R-band magnitude, a distance of 162.5pc, the set 2 parameters(Table1) for the extinc- 5. Paschen β and Hα emission tion and for calculating a radius via the Stefan-Boltzmann law, and a mass of 11.5 MJup, which is an intermediate value of the We discover a strong and broad Paschen β (Pa β) emission line in estimated range of 6-17 M (Luhman et al. 2005b; Bonnefoy et our near-IR SINFONI spectra of OTS44 (Fig.2). Furthermore, a Jup al. 2013). We derive an Hα line luminosity of 6.9+1.4×10−7L , prominent Hα emission line is visible in the optical spectrum of −1.2 ⊙ +17.4× −6 Luhman (2007, see Fig.2). Both of these Hydrogen emission an accretion luminosity of 3.7−3.1 10 L⊙ (0.15% of L∗), and −1 +36 −12 −1 lines exhibit a broad profile with velocities of ±200kms or a mass accretion rate of 7.6−6.4 × 10 M⊙ yr . We take into more. We investigate the properties and origin of these lines account errors in EW, Rmag, and in the empirical relation. We through a line profile analysis. We determine the equivalent furthermore attempt to estimate the mass accretion rate based on width (EW) by directly integrating the flux within the line re- the Pa β line by using the empirical relation of Rigliaco et al. gion and the EW errors following Sembach & Savage (1992). (2012), a dereddened J-band magnitude, and again set 2 of Ta- Furthermore, we measure the line center and full-width-at-half- ble1. This yields a mass rate that is two orders of magnitude −9 −1 maximum (FWHM) based on a Gaussian fit to the profiles. Ta- higher than that derived from Hα, namely 1.7×10 M⊙ yr for −10 −1 ble 3 lists the results. 1 (December 14) and 8.5×10 M⊙ yr for epoch 2 (De- The Hα line has a symmetrically shaped profile with an EW cember 21). This descrepancy lets us speculate that part of the ff of -141Å, demonstrating that OTS44 is actively accreting (e.g. Pa β line might come from a di erent origin than accretion. Barrado & Martin 2003). The line appears to be blueshifted with its center located at -30kms−1. Higher resolution spectroscopy 6. Conclusions is needed to determine whether this shift is real. The shape of the Pa β line appears to be slightly asymmetric, We have discovered strong, broad, and variable Pa β emission with the red wing being more pronounced. The profile is signif- of the young very low-mass brown dwarf OTS44 (M9.5) in icantly variable between the two observing epochs separated by VLT/SINFONI spectra, which is evidence for active accretion a few days. We measure an EW of -6.7 and -4.2Å for the spec- at the planetary border. We determined the properties of the disk tra from December 14 and 21, respectively. The line has a peak that surrounds OTS44 through MC3D radiative transfer modeling at redshifted velocities at about 40-50kms−1. We see a redshift of flux measurements from the optical to the far-IR (Herschel). of similar order also in photospheric lines of OTS44. While the We found that OTS44 has a highly flared disk (β >1.2) with a +1.7 −5 Pa β emission of T Tauri stars is mostly attributed to magneto- massof 9.1 −5.5×10 M⊙, that isabout0.1 MJup or30 MEarth. We spheric accretion and winds (e.g., Rigliaco et al. 2012), there also investigated the Hα line of OTS44 in a spectrum from Luh- is observational evidence that part of the Pa β emission, in par- man (2007) and found strong Hα emission with an EW of -141Å ticular the broad line wings, can be formed by other processes, indicative of active accretion. Both the Pa β and Hα emission such as outflows (e.g., Whelan et al. 2004). We conducted lines of OTS44 have broad profiles with the wings extending a spectro-astrometric analysis of the Pa β line in the SINFONI to velocities of about ±200kms−1. The Pa β emission is signifi- 3D cube data to locate the formation site of this emission. We cantly variable on timescales of a few days, indicating variability find no spectro-astrometric signal in this line that exceeds 5mas in accretion-related processes of OTS44. We estimated the mass +36 −12 −1 (0.8 AU). accretion rate of OTS44 to 7.6−6.4 × 10 M⊙ yr by using the Article number, page 3 of 6 10-6 10-7 10-8

-9 / yr] 10 • O

-10 [M 10 acc -11 M 10 OTS44 (this work) Mohanty et al. (2005) -12 10 Fang et al. (2009) Herczeg et al. (2009) 10-13 Rigliaco et al. (2011,2012) 0.01 0.10 1.00 10.00

Mstar [MO •] Fig. 4. Mass accretion rate versus central mass of stars and brown dwarfs including OTS 44 (red diamond).

obvious from this figure that OTS44 has a relatively high mass accretion rate considering its small mass. These observations show that the processes that accompany canonical star forma- tion, disks and accretion, are present down to a central mass of a few Jupiter masses. OTS 44 plays a key role in the study of disk Fig. 2. Pa β emission of OTS 44 in SINFONI/VLT spectra and Hα evolution and accretion at an extremely low mass and, therefore, emission of OTS 44 based on a spectrum from Luhman (2007). The in constraining the minimum mass that star formation can pro- dashed lines are Gaussian fits to the profiles. duce. It will be the target of our future observational efforts. Acknowledgements. We thank K. Luhman for providing the optical spectrum of 0 OTS 44 and related information, C. Dumas and A.-M. Lagrange for help with 10 the SINFONI data reduction, the ESO staff at Paranal for executing the SIN- FONI observations in service mode, and the anonymous referee for very helpful

-2 comments. This work made use of the NIST database (Kramida et al. 2012) and star 10 VOSA (Bayo et al. 2008). MB acknowledges funding from Agence Nationale pour la Recherche, France through grant ANR10-BLANC0504-01. / M

disk 10-4 M OTS44 (this work) Osterloh & Beckwith (1995) Natta et al. (2000) Scholz et al. (2006) References 10-6 Harvey et al. (2012) Joergens et al. (2012a) Ricci et al. (2012,2013) [1] Allard, F., Homeier, D., Freytag, B., & Sharp, C. M. 2012, in EAS Publications Series, Vol. 57, ed. C. Reyle, C. Charbonnel, & M. Schultheis, 3–43 0.01 0.10 1.00 10.00 [2] Bacciotti, F., Whelan, E. T., Alcalá, J. M. et al. 2011, ApJ, 737, L26 Mstar [MO •] [3] Barrado y Navascués, D., & Martín, E. L. 2003, AJ, 126, 2997 [4] Bayo, A., Barrado, D., Huelamo, N. et al. 2012, A&A, 547, A80 Fig. 3. Relative disk mass versus central mass of stars and brown [5] Bonnefoy, M., Chauvin, G., Lagrange, A.-M. et al. 2013, A&A, in press [6] Chiang, E. I., & Goldreich, P. 1997, ApJ, 490, 368 dwarfs including OTS 44 (red diamond). We note that for ρ Oph102 we [7] Fang, M., van Boekel, R., Wang, W. et al. 2009, A&A, 504, 461 use M∗=0.13 M⊙ (M5.5, K. Luhman, pers. comm.), different from Ricci [8] Harvey, P.M., Henning, Th., Liu Y. et al. 2012, 755, 67 [9] Herczeg, G. J., Cruz, K. L., Hillenbrand, L. A. 2009, ApJ, 696, 1589 et al. (2012, 0.06 M⊙). [10] Inutsuka S. & Miyama S. M. 1992, ApJ, 388, 392 [11] Joergens, V. 2006, A&A, 448, 655 [12] Joergens, V., Pohl, A., Sicilar-Aguilar, A., & Henning, Th. 2012a, A&A, 543, A151 Hα line. A mass accretion rate based on the Pa β line gives a [13] Joergens, V., Kopytova, T. & Pohl, A. 2012b, A&A, 548, A124 significantly higher value, and we speculate that part of the Pa β [14] Liu, Y., Madlener, D., Wolf, S., & Wang, H.-C. 2013, Research in Astronomy emission might come from other processes related to accretion, and Astrophysics, 13, 420 [15] Luhman, K. L., Peterson, D. E., & Megeath, S. T. 2004, ApJ, 617, 565 such as outflows. Furthermore,in the course of studyingOTS44,[16] Luhman, K. L., Adame, L., D’Alessio, P. et al. 2005a, ApJ, 635, L93 we fitted a photospheric BT-Settl model to its optical and near-[17] Luhman, K. L., D’Alessio, P., Calvet, N. et al. 2005b, ApJ, 620, L51 [18] Luhman, K. L. 2007, ApJS, 173, 104 IR SED and derived a lower effective temperature and higher[19] Luhman, K. L., Allen, L. E., Allen, P. R. et al. 2008, ApJ, 675, 1375 [20] Machida, M. N., Inutsuka, S., & Matsumoto, T. 2009, ApJ, 699, L157 extinction than was previously found (Luhman 2007). [21] Mohanty, S., Jayawardhana, R. & Basri G. 2005, ApJ, 626, 498 We have presented the first detection of Pa β emission for[22] Monin, J.-L., Whelan, E. T., Lefloch, B., Dougados, C., & Alves de Oliveira, an object at the deuterium-burning limit. Our analysis of Pa β C. 2013, A&A, 551, L1 [23] Natta, A., Grinin, V., & Mannings, V. 2000, Protostars and IV, Tucson, and Hα emission of OTS44 demonstrates that objects of a few University of Arizona Press, 559 Jupiter masses can be active accretors. Furthermore, OTS44 is [24] Oasa, Y., Tamura, M., & Sugitani, K. 1999, ApJ, 526, 336 [25] Osterloh, M., & Beckwith, S. V. W. 1995, ApJ, 439, 288 the lowest-mass object to date for which the disk mass is deter-[26] Phan-Bao, N., Riaz, B., Lee, C.-F. et al. 2008, ApJ, 689, L141 mined based on far-IR data. Our detections therefore extend the[27] Pinte, C., Padgett, D. L., Ménard, F., et al. 2008, A&A, 489, 633 explorationof disks and accretion during the T Tauri phase down[28] Ricci, L., Testi, L., Natta, A., Scholz, A., & de Gregorio-Monsalvo, I. 2012, ApJ, 761, L20 to the planetary mass regime. Plotting the relative disk masses of[29] Ricci, L., Isella, A., Carpenter, J. M., & Testi, L. 2013, ApJ, 764, L27 stars and brown dwarfs including OTS44 (Fig.3) shows that the[30] Rigliaco, E., Natta, A., Randich S. et al. 2011, A&A, 526, L6 − [31] Rigliaco, E., Natta, A., Testi et al. 2012, A&A, 548, A56 ratio of the disk-to-central-mass of about 10 2 found for objects[32] Scholz, A., Jayawardhana, R., & Wood, K. 2006, ApJ, 645, 1498 [33] Sembach, K. R., & Savage, B. D. 1992, ApJS, 83, 147 between 0.03 M⊙ and 14 M⊙ is also validfor OTS44 at a mass of[34] Whelan, E. T., Ray, T. P., & Davis, C. J. 2004, A&A, 417, 247 about 0.01 M⊙. Furthermore, the mass accretion rate of OTS44[35] Whelan, E. T., Ray, T. P., Bacciotti, F. et al. 2005, Nature, 435, 652 [36] Wolf, S. 2003, Computer Physics Communications, 150, 99 is consistent with a decreasing trend from stars of several so-[37] Wolf, S., Padgett, D. L., & Stapelfeldt, K. R. 2003, ApJ, 588, 373 lar masses to brown dwarfs down to 0.01 M⊙ (Fig.4). It is also

Article number, page 4 of 6 A&A–ots44_v6, Online Material p 5

Appendix A: Bayesian probability analysis

Table A.1. Confidence intervals of the disk parameter values of OTS 44.

Parameter bestmodel validrange +0.018 Rin [AU] 0.023 −0.013 0.01-0.04 Rout [AU] 100 fixed +0.051 p 1.136 −0.025 0.62-1.27 +0.017 β 1.317 −0.042 1.16-1.32 +0.68 h100 [AU] 17.42 −2.55 9.0-18.5 −5 +1.72 mdisk [10 M⊙] 9.06 −5.46 0.32-56.2 ◦ +6 i [ ] 58−9 18-65 Notes. Parameter values of the best-fit SED model with local errors and the valid ranges for each parameter. The local error is deduced based on simulated annealing by probing the direct environment of the best fit with a Markov chain. The valid range for each parameter gives a 68% confidence interval based on the Bayesian probability distribution, as shown in Fig. A.1. A&A–ots44_v6, Online Material p 6

Fig. A.1. Bayesian probability distributions of selected disk parameters.