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The Astrophysical Journal, 571:L155­L159, 2002 June 1
2002. The American Astronomical Society. All rights reser ved. Printed in U.S.A.

A YOUNG VERY LOW MASS OBJECT SURROUNDED BY WARM DUST L. Testi,1 A. Natta,1 E. Oliva,1,2 F. D'Antona,3 F. Comeron,4 C. Baffa, G. Comoretto,1 and S. Gennari1
Received 2002 February 28; accepted 2002 April 23; published 2002 May 6
1

ABSTRACT We present a complete low-resolution (R 100 ) near-IR spectrum of the substellar object GY11, a member of the r Ophiuchi young association. The object is remarkable because of its low estimated mass and age and because it is associated with a mid-IR source, an indication of a surrounding dusty disk. Based on the comparison of our spectrum with similar spectra of field M dwarfs and atmospheric models, we obtain revised estimates of the spectral type, effective temperature, and luminosity of the central object. These parameters are used to place the object on a H-R diagram and to compare it with the predictions of pre­main-sequence evolutionar y models. Our analysis suggests that the central object has a ver y low mass, probably below the deuterium-burning limit and in the range of 8­12 MJ, and a young age of less than 1 Myr. The IR excess is shown to be consistent with the emission of a flared, irradiated disk similar to those found in more massive brown dwarf and T Tauri systems. This result suggests that substellar objects, even the so-called isolated planetar y mass objects, found in young stellar associations are produced in a similar fashion as stars, by core contraction and gravitational collapse. Subject headings: infrared: stars -- stars: atmospheres -- stars: fundamental parameters -- stars: low-mass, brown dwarfs On-line material: color figures

1. INTRODUCTION

The discover y of brown dwarfs (BDs) and objects with masses comparable to those of giant planets (well below the deuteriumburning limit M ! 13 MJ) "free floating" in young stellar clusters (Zapatero-Osorio et al. 2000; Lucas & Roche 2000) has opened an interesting debate on their origin. Do they form like ordinar y stars from the collapse of molecular cores (Shu, Adams, & Lizano 1987)? If so, the ver y existence of ver y low mass objects and their mass function put strong constraints on star formation theories. Alternatively, BDs and isolated planetar y mass objects may be stellar "embr yos" ejected from multiple forming systems before reaching a stellar mass (Reipurth & Clarke 2001), or they may form like planets by coagulation of dust particles and subsequent gas accretion within circumstellar protoplanetar y disks (Lin et al. 1998). Young (proto)planets could then be extracted or ejected by dynamical interactions from the forming planetar y systems. In this scenario, isolated BDs and planetar y mass objects are intrinsically different from stars; their study sheds no light on star formation theories but provides instead a chance of studying the early evolution of giant planets where they can be obser ved in isolation, rather than ver y close to a much brighter star. One way to shed light on the origins of ver y low mass objects is to ascertain their association with circumstellar disks, which are characteristic of stellar formation from the contraction of a molecular core. Deep images in the L band (Muench et al. 2001) and in the mid-IR (Persi et al. 2000; Bontemps et al. 2001) have shown that many ver y low luminosity objects have
1 INAF­Osser vatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, I-50125 Firenze, Italy. 2 INAF­Telescopio Nazionale Galileo and Centro Galileo Galilei, P.O. Box 565, E-38700, Santa Cruz de La Palma, Spain. 3 INAF­Osser vatorio Astronomico di Roma, Via Frascati 33, I-00044 Monteporzio Catone, Rome, Italy. 4 European Southern Obser vator y, Karl-Schwarzschild-Strasse 2, Garching D-85748, Munchen, Germany. ¨

excess emission at these wavelengths, and detailed studies of some of them have proved that the central objects are bona fide BDs. Their IR excess is consistent with the presence of a surrounding disk similar to those found around more massive pre­main-sequence stars (Comeron et al. 1998; Comeron, Neu´ ´ hauser, & Kaas 2000; Natta & Testi 2001). These initial find¨ ings, albeit limited, seem to suggest that indeed BDs form like ordinar y stars. We report here the first results of a project aimed on one hand to improve our understanding of disks around BDs, following the approach of in Natta & Testi (2001), and on the other hand to find evidence of a circumstellar disk around bona fide isolated planetar y mass objects. We have used the Near-Infrared Camera and Spectrograph (NICS) on the 3.56 m Telescopio Nazionale Galileo to acquire low-resolution (near-IR) spectra of a sample of mid-IR sources located within the r Ophiuchi cloud (Bontemps et al. 2001). Our target list included objects detected at 6.7 and 14.3 mm with the ISOCAM on board the Infrared Space Observatory (ISO; Kessler et al. 1996; Cesarsky et al. 1996), having low effective temperature (Tef f), luminosity (L ), and extinction (AV) based on photometric or limited 2.2 mm spectroscopic estimates. The goal of our new obser vations was to obtain improved determinations of these parameters and to derive accurate values of masses and ages of the targets by comparison with theoretical evolutionar y tracks. The results for the complete sample will be discussed in a future paper (Natta et al. 2002). In this Letter we report on one of the sources, source 33 of the ISOCAM list of Bontemps et al. (2001), which is associated with the near-IR source GY11 (Greene & Young 1992). The substellar nature of this object was already proposed by Rieke & Rieke (1990) and confirmed by Comeron et al. ´ (1998) and Wilking, Greene, & Meyer (1999). We derive a new accurate spectral classification of GY11 and of its effective temperature and luminosity. Our data suggest that GY11 is a planetar y mass object, with an IR excess that can be roughly L155


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pared with the source photometr y, and colors were found to be consistent with those of 2MASS to within 10%, as expected from typical uncertainties. To better constrain the values of the extinction, we also obtained optical i-band (0.77 mm) photometr y at the ESO La Silla 1.54 m Danish Telescope using the Danish Faint Object Spectrograph and Camera. Photometric calibration was ensured by obser vations of a set of Landolt (1992) standard stars, converted into the Gunn system using the transformations given by Fukugita et al. (1996).
3. CENTRAL SOURCE PARAMETERS

Fig. 1.--(a) Spectrum of GY11 (solid lines) is compared with reddened spectra of field M dwarfs (dotted lines) as labeled (from L. Testi et al. 2002, in preparation); all spectra are normalized at the mean flux in the 1.6­1.7 mm range and shifted with constant offsets for clarity. Spectrum of GY11 is reproduced at ever y offset to ease the comparison. (b) Similar to (a), but the dotted spectra are reddened theoretical atmospheric models (Allard et al. 2000); Teff is as labeled. In both panels the obser ved spectra have a lower signal-tonoise ratio in the region corresponding to the strong telluric absorption (1.35­1.45 and 1.82­1.95 mm).

modeled as due to a circumstellar disk similar to those associated to T Tauri stars. These obser vations provide the first evidence that objects of such small mass actually form in a starlike manner and thus that they are genetically different from "planets."
2. OBSERVATIONS

A near-IR low-resolution spectrum of GY11 was obtained with the Telescopio Nazionale Galileo (TNG) on La Palma on 2001 July 9, using a 0 . 5 slit and the high-throughput lowresolution prism-based disperser unique to NICS (Baffa et al. 2001), the Amici device (Oliva 2000); this setup offers a complete near-IR spectrum, 0.85­2.35 mm, at R 100 across the entire range, and it allows an accurate classification of faint and cool dwarfs (Testi et al. 2001). Instrumental and telluric correction was ensured by obser vations of A0 stars. The shape of the final spectrum was checked using near-IR photometric measurements from the Two Micron All Sky Sur vey (2MASS) second incremental data release; synthetic magnitudes were computed using the appropriate transmission cur ves and com-

Bontemps et al. (2001) associated the ISOCAM source 33 with a class II object member of the r Oph young stellar cluster known as GY11 (Greene & Young 1992). The BD nature of GY11 has been suspected for some time (Rieke & Rieke 1990). However, there is a large uncertainty in the literature as to the exact value of its photospheric parameters and mass. Bontemps et al. (2001) estimated bolometric luminosity and extinction to be L 0.001 L ,, AV p 2.7 mag from near-IR photometr y. Based on multiband IR photometr y and a 2.2 mm low-resolution spectrum, Wilking et al. (1999) estimated a spectral type M6.5, AV 5 mag, L 0.002 L ,, and Tef f 2650 K. These authors noted that owing to veiling caused by dust emission, the spectral type could easily be some 2­3 subclasses later, and the extinction would be significantly underestimated. In fact, Comeron et al. (1998) derive a higher value of AV p 10 mag from ´ broadband photometric measurements that include optical and IR bands. Our complete near-IR spectrum offers the possibility of a better estimate of the source parameters, as it allows us to use the global spectrum shape below 2 mm, a region that is less affected by the continuum veiling due to the dust emission. Given the expectation that the surface gravity of ver y young BDs should be similar to that of subgiants, we derive the photospheric parameters by comparison with field dwarf spectra and model atmospheres with appropriate surface gravity, as suggested by the evolutionar y models (Comeron et al. 2000). ´ We first derive extinction and spectral type by matching the obser ved GY11 spectrum with that of field dwarfs in the solar neighborhood (L. Testi et al. 2002, in preparation), obtained with the same instrumental setup and reddened using the Cardelli, Clayton, & Mathis (1989) extinction law most appropriate for Ophiucus (RV p 4.2; Fig. 1a). We tr y to provide the best fit to the global shape of the spectrum, with particular attention to the H-band shape, the J-band features, and the drop due to water vapor absorption at the red edge of the J band. Overall, the best spectral match is found with the field dwarf with spectral type M8.5 and extinction AV 7.0 mag. Lower values of AV (by 1 mag) offer a better match of the spectrum with later dwarf spectra (M9­L0) but are not consistent with the broadband optical measurements (see Fig. 4, inset). A higher value of the extinction causes a too steep rise of the spectrum below 1.2 mm. Field dwarfs with spectral types earlier than M7.5 show large deviations from the obser ved shape of the H band and the drop at 1.3 mm. Given the uncertainties of a classification based on objects with ver y different surface gravity, we expect our classification to be accurate within one spectral subclass and the visual extinction estimate to be accurate within 1 mag. As a second step, in order to derive an estimate of the photospheric effective temperature (Fig. 1a), we compare the obser ved GY11 spectrum with reddened, appropriate surface


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Fig. 2.--H-R diagram for three sets of evolutionar y tracks (D'Antona & Mazzitelli 1997; Chabrier et al. 2000; Burrows et al. 1997). Position of GY11 is shown as a blue circle. Tracks are labeled with the appropriate mass, hydrogen-burning stars are shown in red, deuterium-burning BDs in green, and objects below the deuterium-burning limit in cyan. Isochrones are shown as dotted lines and are labeled with the appropriate ages. [See the electronic edition of the Journal for a color version of this figure.]

gravity (log g p 3.5) model atmospheres (Allard, Hauschildt, & Schweitzer 2000; Chabrier et al. 2000). The best estimate of the effective temperature is 2 400 100 K. Higher temperature models offer a better match of the H-band shape but underestimate the drop near 1.3 mm and the global shape of the spectrum at the J band. The derivation of the effective temperature of young dwarfs based on theoretical synthesis of the near-IR spectrum is ver y uncertain (Lodieu et al. 2002); however, our value of Teff for an object of spectral type M8.5 is consistent with the spectral type versus effective temperature scale discussed by Wilking et al. (1999) and marginally higher than the latest effective temperature scales derived for field dwarfs (Leggett et al. 2001). To estimate the luminosity (L ) of the object, we used the 2MASS J-band magnitude, dereddened by AV 7.0 mag, and the bolometric correction derived from the best-fitting (2400 K) atmospheric model. The value of this "theoretical" bolometric correction is nearly identical to the empirical value adopted by Wilking et al. (1999). We derive a value of L 0.008 L , 30%. Using the L and Teff values derived above, we can compare the position of GY11 in the H-R diagram with the predictions of theoretical pre­main-sequence evolutionar y models. In Figure 2 we show this comparison for the latest release of evolutionar y tracks from three leading groups in the theor y of pre­main-sequence evolution of substellar objects. In spite of the relatively large uncertainties on L and Teff and the limited

accuracy of pre­main-sequence evolutionar y tracks at ages and masses, we confirm that GY11 is a young (t ! 1 ver y low mass object, probably below or ver y close deuterium-burning limit, with a best mass estimate in the 8­12 MJ.
4. INFRARED EXCESS AND DISK MODELS

these Myr), to the range

GY11 is the lowest mass object with a clearly detected IR excess. It is detected by ISOCAM in the two broadband filters LW2 and LW3 (leff p 6.7 and 14.3 mm, respectively) used by Bontemps et al. (2001) in their r Oph sur vey, as well as in the three intermediate-band filters, SW1, LW1, LW4 (l ef f p 3.6, 4.5, and 6.0 mm, respectively), used by Comeron et al. (1998) ´ in their pointed obser vations of optically identified candidate BDs. The ISOCAM measurements in the broad and narrow bands are in good agreement within the flux calibration uncertainties, which we assume to be 20%. As usual with ISOCAM, the 6 beam includes multiple sources seen in higher resolution near-IR images, which may contribute to the obser ved fluxes. Figure 3 compares a K s image of the region around GY11 extracted from the Ver y Large Telescope (VLT) ESO archive (originally obser ved as part of ESO proposal 63.I-0691) and the ISOCAM LW1 (l ef f p 4.5 mm) image from the ISO archive (Comeron et al. 1998). The mid-IR ´ emission peaks ver y close to the position of GY11. Although a small contamination from the near-IR source 8 to the east is


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Fig. 3.--ISOCAM LW1 (contours;4.5 mm) image of the region surrounding GY11 is overlaid on the VLT Ks (gray scale; 2.2 mm) image. ISOCAM image has been aligned with the VLT image by matching the position of GY10 (2320.8­1708; Comeron et al. 1998) with the associated mid-IR source. ´

possible, we think that most of the mid-IR obser ved flux comes from GY11; a similar conclusion was also reached by Comero ´n et al. (1998). In Figure 4 we show the spectral energy distribution (SED) of GY11 at all wavelengths and compare it to that of an irradiated, flared disk similar to those that reproduce the obser ved characteristics of T Tauri systems (Chiang & Goldreich 1997). The disk has a dust mass of 1 M (3% of the mass of the central object, for an assumed gas-to-dust mass ratio of 100) and is heated by a central source with the GY11 temperature, luminosity, and mass. We show the predicted SED when the disk extends inward to the stellar surface (solid line) and when it has an inner hole of 3R (dotted line). More details on the disk models can be found in Natta & Testi (2001) and Natta et al. (2002). The agreement between obser ved and predicted fluxes is rather good, especially for the disk with the large inner hole. In particular, both models predict total (star disk) fluxes that in the J, H, and K bands are smaller than the calibrated TNG fluxes by 15% at most. As an independent check, we computed optical and near-IR broadband magnitudes from the model-predicted SED. They are compared in the inset of Figure 4 with the obser ved de´ reddened magnitudes in i (this Letter), R, I, L (Comeron et al. 1998), J, H, and Ks (2MASS) bands. The agreement is again quite good. The ISOCAM measurements, especially that at 14.3 mm, have large error bars, and one should not overinterpret them. However, it is of some interest to point out that if indeed the mid-IR excess is due to disk emission, the disk must be flared. Therefore, dust and gas must be well mixed, as in the majority of pre­mainsequence stars, and no major settling of the dust onto the disk midplane has occurred during the lifetime of GY11. The disk must be optically thick to mid-IR radiation; this, however, sets only a lower limit to the disk mass of roughly 10 5 to 10 6 M,, depending on the exact value of the dust mid-IR opacity and surface density profile. Note that the disk mass has to be ver y small; if we assume the ratio of the disk mass to the mass of the central object typical of T Tauri stars (0.03), then

Fig. 4.--Disk and photosphere model of the GY11 SED. Red circles with error bars show the mid-IR fluxes from ISOCAM (Comeron et al. 1998; Bon´ temps et al. 2001). Black line is our near-IR dereddened Amici spectrum. Green jagged line is the photospheric model for Teff p 2400 K and log g p 3.5 (Allard et al. 2000). Blue lines show the combined photosphere plus disk emission computed as in the text. In one case (solid line) the disk inner radius is equal to 1R, in the other (dotted line) to 3R. In the inset we show the comparison between the dereddened broadband photometr y (from R to L ; red squares) and the models (practically coincident): photosphere is shown as a dashed line, photosphere plus disk as a solid line. [See the electronic edition of the Journal for a color version of this figure.]

the disk mass is about 3 # 10 4 M,, and the disk contains only 1 M of dust. As a consequence, the accretion rate (if any) is also likely to be low, with an average value over the lifetime of the object that cannot exceed 3 # 10 10 M, yr 1 (for an age of 1 Myr). The accretion luminosity is also small, about 40 times lower than the luminosity of the photosphere. A direct determination of the disk mass can be derived from (sub)millimeter wavelength obser vations. We predict for GY11 a 1.3 mm flux of about 0.6 mJy, which is well below the upper limit set by the sur vey of Motte, Andre, & Neri (1998), but within the expected ´ capabilities of the Atacama Large Millimeter Array.
5. CONCLUSIONS

The results presented in this Letter show evidence that a young isolated planetar y mass object in r Oph, with a mass of about 10 MJ, is surrounded by warm dust, possibly distributed on a disk similar in properties to those around young BDs and T Tauri stars. The implications of this finding, that should be confirmed by higher spatial resolution mid-IR obser vations and should be extended to a large sample of similar objects, are profound, since they give a clear indication that isolated BDs and even planetar y mass objects form like stars and are not produced in a planet-like fashion within protoplanetar y disks around more massive objects and later ejected by dynamical interactions. Isolated BDs and planetar y mass objects are thus an extension of the stellar and substellar sequence to ver y low masses and have different origins from "planets." This work is partly based on obser vations collected at the Italian Telescopio Nazionale Galileo, Canar y Islands, Spain, at


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the European Southern Obser vator y telescopes on La Silla and Paranal obser vatories, Chile, and on data obtained by the European Space Agency Infrared Space Observatory. This publication makes use of data products from the Two Micron All Sky Sur vey, which is a joint project of the University of Massachu-

setts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. This work was partly supported by ASI grant ARS 1/R/27/00 to the Osser vatorio di Arcetri.

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